Huaixun Lim,Kunli Goh,Miao Tian,Rong Wang,4,

1 Interdisciplinary Graduate School,Nanyang Technological University,Singapore 637335,Singapore

2 Singapore Membrane Technology Centre,Nanyang Environmental and Water Research Institute,Nanyang Technology University,Singapore 637141,Singapore

3 School of Ecology and Environment,Northwestern Polytechnical University,1 Dongxiang Road,Chang’an District,Xi’an 710129,China

4 School of Civil and Environmental Engineering,Nanyang Technological University,Singapore 639798,Singapore

Keywords:Membrane dehumidification Membrane contactor H2O/N2 selectivity Permeability Adsorption

ABSTRACT This review compares the different types of membrane processes for air dehumidification.Three main categories of membrane-based dehumidification are identified–membrane contactors using porous membranes with concentrated liquid desiccants,separative membranes using dense membrane morphology with a pressure gradient to drive the separation of moisture from air,and adsorptive membranes using nanofibrous membranes which adsorb and capture moisture to realise dehumidification.Drawing upon the importance of dehumidification and humidity control for urban sustainability and energy efficacy,this review critically analyses and recognizes the three unique categories of membrane-based air dehumidification technologies.Essentially,the discussion is broken into three sections-one for each category-discriminating in terms of the driving force,membrane structure and properties,and its performance indicators.Readers will notice that despite having the same objective to dehumidify air,the polymers used amongst each category differs to suit the operating requirements and optimize dehumidification performance.At the end of each section,a performance table or summary of dehumidifying membranes in its class is provided.The final section concludes with a comparative review of the three categories on membrane-based air dehumidification technologies and draw inspiration from parallel research to rationalise the potential and innovative use of promising materials in membrane fabrication for air dehumidification.

1.Introduction

Humidity control is crucial for a building’s air quality,ensuring the indoor comfort of occupants,protecting valuable electronics,preserving quality of moisture sensitive materials and preventing degradation of natural products[1].The need for dehumidification exacerbates energy demand as traditional dehumidification systems with the use of solid desiccant [2],liquid desiccants [3],or heat exchangers,seem to be yesterdays’ technology as they are either low in moisture uptake capacity,slow rate of humidity control or highly energy intensive and environmentally unsustainable.In addition,with the increasingly challenging task for water treatment,humidity in the air can also contentiously be another clean water resource,which has potential for augmenting our freshwater supply [4–8].From the psychrometric chart as shown in Fig.1,it shows that there are many ways to achieve the ASHRAE standard 55 thermal environmental conditions for human comfort.Besides the adsorption driven processes like the desiccant wheel,temperature driven systems such as the conventional air conditioning(Fig.1(a)) achieves dehumidification by an energy-intensive condensation step via cooling of compressed air to reach its saturation curve followed by subsequent heating to desirable temperatures.Desiccant wheels (Fig.1(b)) are only suitable for regions that fall along the adiabatic enthalpy lines to reach the green zone.Heat exchangers (Fig.1(c)),on the other hand,cycle and recirculate heated or cooled air to achieve dehumidification given that a specific air volume at a higher temperature can hold more moisture,and hence reflect lower relative humidity as compared to an air volume at lower temperature.

Fig.1.(a)Conventional air conditioning[3],(b)desiccant wheel,(c)heat exchanger,(d)process movements on the psychrometric humidity chart at 0.1 MPa with the green highlighted region illustrating ASHRAE standard 55 indoor comfort zone for human occupancy.

Ultimately,conventional dehumidification processes involve the play of temperature or with the use of adsorption and desorption processes.Traditional ways of dehumidification may be welladopted and cost-advantageous,but also face challenges in regeneration and space constraints.These challenges could be overcome with innovative strategies and implementation of new technologies.One such new technology is membrane-based dehumidification.The technology is considered a viable substitute or complement to existing dehumidification systems and is growing rapidly among dehumidification research as evidenced by the increasing number of publications and patents as shown in Fig.2.These numerical values evidently show that membrane-based dehumidification is gaining traction,with the percentage of membrane-related dehumidification papers growing to approximately 42.5% in 2021.Hence,considering this strong potential,we narrow our scope of discussion to membrane-based technology for dehumidification.

Fig.2.Number of publications and patents on membrane-based dehumidification for the last 31 years (1990–2021).Data is calculated based on the percentage of publications on membrane-based dehumidification over the total publications on dehumidification.｜

Currently,membrane-based dehumidification leverage membranes in three different ways:(1) As a contactor,the membrane provides the surface area for two phases to come into contact for water vapour stripping without the phases mixing into one another [9,10];(2) As a separator,the membrane serves as a permselective barrier to offer targeted H2O/N2selectivity at an attractive H2O permeability through the use of rationally designed synthetic polymers [11];and (3) As an adsorbent,intelligently engineered membrane structure [12] creates high water uptake capacities or adsorptions at a lower relative humidity.A crucial key to success lies in the versatility and customisability of membrane-based dehumidification.The key driver is the membrane itself.From basic physicochemical properties of the membrane to complex membrane engineering,the ability to tailor the membranes’properties and capabilities greatly increases the technology to tackle unique applications,operating conditions and achieve targeted outcomes.Table 1 tabulates the differences between membrane-based dehumidification to other existing dehumidification technologies and highlights the merits and limitations of each technology.

As shown in Table 1,conventional dehumidification technology was used in industries via desiccant wheels,heat exchangers or as air conditioners.Each of which has its disadvantages of large footprint,high energy consumption or need for frequent maintenance.Membrane technology proves to be a potential alternative [13,14]to traditional dehumidification for its higher throughput and modular design for potentially higher dehumidification performances.Recent reviews on membrane-based liquid desiccant air dehumidification [15] and isothermal membrane-based dehumidification[16] have provided a good overview and deep insights on membrane contactors and separative membranes.However,there remains gaps in reviewing recent advances of adsorptive membranes as well as delivering a broad context to allow readers to understand the overarching technological roadmap of membrane-based air dehumidification.

Table 1 Comparison amongst conventional dehumidification technologies and membrane-based dehumidification

Hence,in this review,we offer readers a big picture perspective,looking into comparing membrane-based dehumidification via three state-of-the-art membrane processes,namely contacting,separation,and adsorption.While these processes have the same overall objective of reducing the humidity in air,each of them is distinctively different in terms of transport mechanism,driving force,membrane structures,materials used for membrane fabrica-tion and performance indicators.This paper thus aims to help readers critically evaluate these differences so that end users recognize their merits and limitations,which allow them to make informed choices when selecting the appropriate membrane process for its intended application and purpose.

2.Membrane Contactor Dehumidification

Past reviews focused on solid and liquid desiccants as the material for dehumidification [17–21] and discussed on dehumidification systems on its energy efficiency and air quality of the building.A recent review on membrane contactors by Liu et al.[15] provided a good summary for membrane-based liquid desiccant air dehumidification and focused on the configurations of the membrane modules.It is also important to note that there are also reviews on energy harnessing on the heating and cooling effects of adsorption and evaporation processes [22,23] which is closely related as the systems used were similar while focusing on a different outcome.

Our review categorized membrane contactors under Section 2 and dug deep into the transport mechanisms of moisture through the membranes and focused on the membranes’moisture removal performance for dehumidification to understand the key membrane characteristics for high moisture transport.

In membrane contactors,there are two flows–humid feed gas to be dehumidified and the liquid desiccant for absorption–with a semipermeable membrane which acts as a medium to prevent the two flows from mixing.The driving force across the membrane is the concentration gradient between these flows.Moisture from the humid feed air side will transfer through the membrane towards the highly concentrated liquid desiccant solution.The structure of the membrane affects the dehumidification performance largely as it determines the mass transport resistance of moisture across the semipermeable membrane.

2.1.Membrane contactor structure

Membranes for membrane contactor may exist as symmetric or asymmetric structures or fabricated in different configurations such as hollow fibre,flat sheet membranes.Symmetric membranes contactors are made up of one material and are homogeneous in structures,usually porous in nature,while asymmetric membranes contain a thin top layer that provides anti-wetting and/or antifouling properties while the porous support layer provides the surface area for mass transfer.

In one of the pioneering studies for humidity control,Scovazzo and colleagues[24,25]concluded that desired membranes have to be high in porosity,highly hydrophilic,mechanically strong and long-lasting.Hence,symmetric membrane contactors are porous(Fig.3(a)) and mostly hydrophobic to prevent membrane wetting while porous asymmetrical membrane contactors (Fig.3(b)) can be hydrophilic to improve mass transport but coupled with a thin hydrophobic skin layer to prevent membrane wetting [26].

2.2.Membrane contactor mechanism

The membranes in membrane contactors are not selective.It merely acts as an interface to provide surface areas for two phases to contact one another while preventing them from mixing.The driving force for the separation is provided by the concentrated liquid desiccant,which induces a concentration gradient across the membrane to draw out moisture from the humid feed side.

The mass transfer of moisture from humid feed gas occurs from the gas-membrane interface,through the membrane pores to reach the liquid-membrane interface,where the absorbent liquid desiccant takes in the H2O molecules and diffuses them into the bulk liquid phase.The mass transfer in a membrane gas–liquid contactor is generally controlled by the cumulative of three transport resistances[29],where K is the overall transport resistance,Kgis the resistance in the gas phase,Kmis the resistance from the membrane,Klis the resistance the liquid phase,as shown in Eq.(1):

Fig.3.Membrane contactor structure (a) Symmetric porous membrane schematic,(ai) SEM photo of porous membrane [27],(b) Asymmetric porous membrane with hydrophobic layer,(bi) SEM photo of asymmetric porous membrane with hydrophobic layer [28].

2.2.1.Mass transfer resistances

Firstly,Kgis influenced by the density of air(ρa)and the convective mass transfer coefficient (ks) between air stream and membrane.This ksis a function of Sherwood number (Sh),the height of the airstream (Hd) and is inversely related to the vapour diffusion coefficient (Dv),as shown in Eqs.(2) and (3):

In sync with Eqs.(2)and(3),Bui and colleagues[30]found that moisture removal is linearly related to the feed velocity applied which can be explained by the increase in Sherwood number by narrowing the flow of humid air,inducing a larger ksand hence a smaller gas transfer resistance Kg.

Secondly,for a porous membrane contactor,the transport resistance across the membrane is determined by the membrane thickness (δ) and effective diffusivity (Deff),which encompasses the membrane’s pore size (dp),porosity (ε) and tortuosity (τ) [15].The relationship between these elements is depicted in Eqs.(4)and (5).

The membrane thickness refers to the length in which moisture must diffuse through from the air side to the liquid side.The effective diffusivity is relative to the membrane porous structure which can be quantified by the three parameters and it provides the understanding of how easy it is for moisture to diffuse through the membrane.For high diffusion and low membrane resistance,pore size and porosity should be large,while tortuosity should be 1,indicating a direct straight pathway through the pore[10,13,31,32].

Thirdly,on the liquid side,Klis determined by Eq.(6),where L is the air gap distance from membrane to liquid,if any,and the vapour diffusion coefficient (Dv) characterises the extent of moisture affinity of the liquid desiccant.

To achieve low membrane resistance,a thin membrane should have large pore size,high porosity but a low tortuosity factor for faster transport through the membrane.

2.2.2.Membrane wetting

With understanding of each transport resistance,finding the optimal membrane properties is still a challenging feat.A thinner membrane may have a low transport resistance but follows with concerns on its durability and long-term performance.To improve the permeability of moisture through the membrane,the membrane should ideally be hydrophilic and has a large pore size.However,that also increases the likelihood of pore breakthrough,resulting in a lower liquid entry pressure and higher chance of membrane wetting as shown in Fig.4.Membrane wetting is the phenomenon where there is penetration of liquid into the micropores of a membrane,greatly introducing additional resistance to the overall mass transfer process.

Fig.4.Schematic illustration of a (a) nonwetted,and (b) wetted membrane.Nonwetted membrane has a lower mass transfer resistance as compared to wetted membranes.The arrows illustrated cross-flow direction flow of the feed gas and liquid absorbent.

Hence,membrane wetting should be avoided through various membrane fabrication methods or careful selection of materials.In the next sub-section,the materials used to fabricate these membranes will be discussed and the selection of liquid desiccant will be analysed.

2.3.Type of materials

With a plethora of materials available,careful selection of materials for the fabrication of membranes is critical and the liquid desiccant used to provide the concentration gradient for the mass transfer of water vapor from humid air to concentrated liquid desiccants will be discussed and analysed.

2.3.1.Polymers

Polymeric materials are most widely used due to its low cost and easy processability.From past literatures,some polymers are more prevalent for the fabrication of membranes for membrane contactors.Hydrophobic polymers,such as polypropylene (PP),polyethylene (PE),or polytetrafluoroethylene (PTFE),are suitable candidates for its high chemical stability,and good mechanical strength,but most importantly hydrophobicity which provides greater resistance against membrane wetting.For example,membrane wetting would usually occur for membranes with slit-like pores of average pore size of approximate 200 nm but can prevented if the membrane is fabricated with hydrophobic PP [33].

In order not to be limited to just hydrophobic polymers,research explored membrane surface modifications and managed to prepare asymmetric membranes with porous hydrophilic support layer and thin hydrophobic top layer to prevent membrane wetting with a lower overall transport resistance.Some examples of hydrophilic polymers include polyetherimide(PEI),polyvinylalcohol (PVA),polyethersulfone (PES) or polyvinylidenefluoride(PVDF).For example,porous PVDF flat sheet membrane was coated with a layer of silicone[34]or a PEI hollow fibre membrane with a selective Polydimethylsiloxane (PDMS) layer [35] were employed as membrane contactors for air dehumidification.

Surface modifications are also employed to alter surface roughness to give a higher roughness coefficient of the membrane surface to enhance the hydrophobicity of the membrane surface for reducing the occurrence of membrane wetting [36–38].

Apart from porous asymmetric composite membranes,nonporous membranes may also be an alternative for membrane absorbers if we leverage the high intrinsic permeability property of polyether block amide (PEBAX).Results showed that the nonporous PEBAX hollow fibre membrane was effective in removing 15.7 g of water per kg of dry air [39].This showed the potential of dehumidification with a non-porous hydrophilic membrane.

These examples show the versatility of membrane technology,ranging from the polymers to choose from amongst hydrophilic,hydrophobic or polymers with useful intrinsic properties,to the fabrication of unique membrane structures such as the composite asymmetric structure,which allows rational engineering of the membrane to perform in an optimum manner for its desired applications.

2.3.2.Liquid desiccants

The next important component in the membrane contactor is the liquid desiccant.The choice of liquid desiccant must also be tactfully chosen to suit a project’s budget,system lifespan and running operating costs.

The ability to absorb water vapour from the air lies in the substance’s affinity to water vapour.The earliest liquid desiccant used for air dehumidification was tri-ethylene glycol (TEG).TEG had shown its effectiveness for removing moisture from humid air through hollow fibre and flat sheet membranes [40].However,due to its high viscosity and volatility affecting its stability and the quality of the dehumidified air,TEG was later replaced by halide salts as they showed better thermodynamic properties and low volatility.The aqueous solutions of halide salts such as LiBr,LiCl,MgCl2and CaCl2have since became the most widely used liquid desiccants.The critical properties that affect the dehumidification capability of a liquid desiccant will be discussed,namely desiccant crystallisation phenomenon,surface vapour pressure,corrosivity,ease of regeneration and its price.

2.3.2.1.Desiccant crystallisation.Desiccant crystallisation is a phenomenon whereby a high concentration liquid desiccant solution is forced to crystallise by a drop in temperature[41].This is important as the regeneration of salt solution involves reducing temperature of the liquid desiccant for energy exchange and cooled further after desorption occurs.If the reduction in temperature and concentration of desiccant salt solution results in salt crystallisation,it may cause detrimental snowball effects from membrane fouling,channel blockage,or improper desiccant pumping [42],resulting in poorer performance from the membrane contactor.

2.3.2.2.Corrosivity.In terms of chemical stability of desiccants,the use of halide salt solutions tends to be corrosive to metals at high temperatures,which is another hurdle to overcome when longterm sustainable options are sought after.Alternatives that are less corrosive includes ionic liquid desiccants [43] or H2PO3[44] or HCO2K [45] or Ca(NO3)2[46].

Ionic liquid desiccants are synthesized salts in the liquid phase consisting of an organic cation and inorganic anion.There are four main types of ionic liquid desiccants–imidazole salts,pyridine salts,quaternary ammonium salts and quaternary phosphonium salts.They generally have negligible vapour pressures,making them good candidates for air dehumidification [47].Some examples such as 1,3-dimethylimidazolium acetate [dmim]OAc [48] or 1-ethyl-3-methylimidazolium tetrafluoroborate [emim]BF4[49]with concentrations of 87% and 83% respectively,exhibit similar vapour pressure as a 45% (mass) LiCl solution,and could possibly dehumidify air to a relative humidity of 11.6%.Another promising ionic liquid desiccant for dehumidification is 1-ethyl-3-methylimidazolium acetate [emim]OAc,which could dehumidify air to approximately 20% relative humidity with a 75% concentrated solution[50].Hence,a highly concentrated ionic liquid desiccants could serve as alternative dehumidifying agents which are non-corrosive in nature.

2.3.2.3.Regeneration ability.The regeneration of liquid desiccants is of paramount importance if the system is to be continuous by reusing liquid desiccants.Regeneration of diluted desiccant solutions can be performed by a few methods.First,thermal energy from the sun or recycled heat energy can be used to preheat weak solutions,raising the vapour pressure of the solution and reducing the driving force required to remove water content[51].To enable this strategy to work,it is critical to develop a desiccant that can be regenerated at low temperatures.Second,regeneration with electrodialysis works by driving ions through a selective membrane under an electric field,providing a high desiccant regeneration flow rate [52] but at the cost of electrical energy.Third,regeneration method by the use of applied pressure to drive reverse osmosis for much faster regeneration and smaller footprint [53] but again at a larger energy cost as compared to using solar heat.

2.3.2.4.Cost.The overall capital expenditure cost of the system includes the purchase price of liquid desiccants,operating and regeneration energy costs.Generally,a higher cost of liquid desiccants usually exhibits better dehumidifying extent.For example,LiCl and LiBr are stronger desiccants as compared to MgCl2or CaCl2,but are more expensive[44,51].Hence,users must optimize their applications with the cost in mind to be economically practical and time resilient.

2.3.3.Comparison of liquid desiccants

Besides the above discussed points on liquid desiccants,there may be other criterion and properties to consider for determining if a desiccant is suitable for air dehumidification and long-term suitability for dehumidification operations.An ideal liquid desiccant should have large absorption capacity,low viscosity,high heat transfer,non-volatile,odourless,non-toxic,non-flammable and chemically stable [17].The multitude of factors makes the choice of liquid desiccant not as straightforward.The following Table 2 provides a tabulation and comparison amongst liquid desiccants for an overview of how each desiccant fares with its alternatives in terms of the discussed attributes.

Despite the high cost and high corrosivity,the use of LiBr or LiCl would provide for the best dehumidification capability with its lowest value of relative humidity at equilibrium.Alternatives,such as CaCl2or HCO2K,could translate to cost savings in the beginning but may be more costly in the long run due to its higher viscosity and regeneration temperature requiring escalated pumping and regeneration energy.There is always a compromise amongst the criterion and critical pick of the desiccant material is necessary for its intended application.

A potential way to optimize parameters could be to create synergistic effects of a mix of different desiccants at different mass ratios to obtain a liquid desiccant blend that maximizes the merits of each desiccant.The combination of 50%CaCl2with 50%LiCl provides benefits of 29%savings ratio for regeneration and a 30%lower cost as CaCl2is much cheaper as compared to its equivalent 100%LiCl required for similar dehumidification effects.The mixture is also lower in viscosity which translates to operating at higher flow rates.

A newly developed mixed liquid desiccant with formula of 25% LiCl,39% hydoxyethyl urea and 36% water shows smaller corrodibility,better absolute moisture removal improvement of 6.2%–14.3% and dehumidification effectiveness improvement of 5.8%–15.2% as compared to 35% LiCl solution [55].Similar strategies could be further explored to magnify benefits and downplay disadvantages of singular desiccants.

2.4.Membrane contactor performance

With membrane contactors being utilised in diverse applications,for dehumidification,energy exchange,with or without internal cooling.It is impractical to correlate their performances with one another as the focus of each system is different and tackles different pain points.In this section,we will focus on membrane contactors used for dehumidifying air,where two heat exchangers between the refrigerant cycle and desiccant cycle can be paired in conjunction with two membrane systems for the dehumidification of air and regeneration of desiccant.

Dehumidification of air via hydrophobic membranes was first reported by Ito[40]where a hydrophobic microporous membrane was fabricated to support a hygroscopic liquid for air dehumidification under a vacuum pressure.It spurred research into liquid membranes [56] and membrane contactors to separate a liquid desiccant from the feed medium.The next sub-section discusses Membrane-based liquid desiccant air dehumidification (MLDAD)with the help of examples in deeper details.

Focusing on the performance of membrane based liquid desiccants for air dehumidification,Table 3 can be derived with the role of the three main components in a membrane-based liquid desiccant unit.The membrane contactor which acts as a semipermeable layer between humid air and the liquid desiccant,are characterised for the material used,configuration and factors which affect the membrane transfer resistance as discussed in Section 2.2.1.Table 3 lists down the mass flow rate of air and desiccant with the membrane’s characteristics to have an overall comparative sense of its dehumidification extent.

There are several influencers which can affect dehumidification performance.Table 3 shows the differences in experimental setups and environmental conditions amongst past literature.Table 3 categorises membrane contactors into flat sheet and hollow fiber configurations with the membrane material for comparison.It is challenging to observe a clear distinction to compare amongst the dehumidification performance of different researches due to the impact of multiple factors such as incorporation of internal cooling of liquid desiccant [57],differences in flow patterns [63],incoming air velocity,temperature and humidity and desiccant solution concentration [64].

Table 2 Comparison between liquid desiccants

Table 3 Summary of liquid desiccant membrane-based dehumidification performances

Most research uses LiCl with concentration ranging from 35%to 42%,and with operating temperature of between 25 °C to 35 °C.Membrane material used is mostly PP or PVDF for its hydrophobicity to prevent membrane wetting except for the asymmetric porous hollow fiber PEI/PDMS [35] and PVDF/PDMS [62] research.Although the PEI hydrophilic porous layer enhanced permeance,a hydrophobic coating of 1.1 μm thick will result in a 20% loss inpermeance as compared to an uncoated fiber with permeance of 0.64 g·m-2·h-1·Pa-1[35].Both researches seek to have the porous structure to provide for mechanical strength and allow high permeance of moisture from the feed air to the liquid desiccant while the PDMS layer provides the hydrophobic layer to prevent wetting or carryover of LiCl liquid desiccant.Any further reduction in coating layer is challenging due to practical reasons such as reduced durability,defects and risk of pinholes [35] which may result in membrane wetting.

In order to further improve the surface contact between desiccants and humid air for dehumidification,an unique hollow fibre structure with triple bore PVDF hollow fibre membrane was explored [9] for its high desiccant volume to surface ratios,high mechanical stability and ease of handling.It would be interesting and could be anticipated for a dehumidification performance comparison between single bore hollow fibre modules with triple bore hollow fibre modules with similar calculated surface areas.

Therefore,the above examples showed the possibility of using asymmetric structures to provide for dehumidification outcomes with most research done to improve permeance but there is little work to date that discusses on the long-term stability or the effectiveness of the prevention of desiccant carryover.

2.5.Applications

MLDAD technology can be configured for different purposes[65,66]and are usually coupled with heat exchangers to capitalize on the heat gained from absorption and convert waste heat into useful energy[67]whilst applying similar processes and transport mechanisms[68].For example,Composite polyvinyl alcohol(PVA)with PVDF hollow fibre membranes were usually fabricated for humidification of air which also consequently lowers the outlet air temperature [69].However,these membranes will not be specially covered in this review as the focus is on air dehumidification.

In a typical membrane contactor set up with heat transfer,there are two flow cycles of the liquid desiccant and refrigerant.The liquid desiccant cycle performs the dehumidification of air and regeneration of desiccant.The refrigerant cycle is necessary for heat exchange and provides the relative heating and cooling of desiccant by retaining valuable heat energy within dehumidification zone.

The dehumidification cycle consists of two hollow fibre membrane contactors,one for the dehumidification of air and the other for the regeneration of liquid desiccant as shown in Fig.5(a).When the weak desiccant solution (diluted and warm) leaves the dehumidifier,it goes into the condenser (part of the refrigerant cycle)which cools the solution down in preparation for desorption at the regenerator where warm air enters to take moisture from the weak desiccant solution.The concentrated desiccant solution now enters the evaporator which cools it further for subsequent enhanced dehumidification efficiency when it enters the dehumidifier.

3.Separative Membranes Dehumidification

Apart from membrane contactors,membranes can also achieve dehumidification through separation.Isothermal dehumidification occurs when the membrane is selective against moisture from air or allow permeation of moisture through the dense layer of the membrane,driven by pressure or concentration gradient [70].For separative behaviour to take place,the membrane structure is distinctively different from membrane contactors.

Fig.5.(a) Hollow fibre dehumidification cycle with cooling and regeneration of liquid desiccant [14,34] and SEM images of a PEI hollow fibre membrane [35].(b) Inner surface,(c) cross-section,(d) inner PDMS hydrophobic layer on porous support,(e) outer surface.

Most papers acknowledged and built on the works of Metz et al.[71,72],Sijbesma et al.[11],and Ingole et al.[73–75],in search for new materials to improve permeance and selectivity.Past reviews related to dehumidification using separative membranes directed towards the dehydration of natural gas,compressed air,flue gas,ethanol,isopropanol,acetonitrile or steam recovery [76] and in the membrane system configurations [16].

This review section examined the separative membranes carefully,from the materials’ intrinsic properties and selection,to the membrane structure and understanding of its underlying transport mechanisms to provide for the dehumidification outcome and performance.

3.1.Separative membrane structure

Structure for separative membranes commonly is symmetrically dense (Fig.6(a)) or asymmetric with a dense selective layer over a porous support (Fig.6(b)).Its dense structure provides the selectivity towards moisture.It is recognized that the separation factor for gas pairs are inversely correlated to the permeability of the more permeable gas of the specific pair [79,80].However,H2O/N2pair does not follow such phenomenon as high selectivity and permeance is possible [71,72,80].Asymmetric membranes,coated with a thin selective layer over a support layer,can be more versatile,owing to the ability to independently tune both layers.

It may be observed on the similarities in the structure of an asymmetric membrane with selective layer with porous support,and a membrane contactor with a porous membrane and hydrophobic layer.The difference is in the role of the thin selective layer for moisture selectivity while the hydrophobic layer to prevent membrane wetting.

3.2.Separative membrane mechanism

Across dense separative membranes,the main driving force will be the partial pressure gradient.A vacuum can be applied on the permeate side of the separative membrane or a positive pressure can be applied on the feed side to drive gas solutes to permeate through the membrane.Good mechanical strength of the membrane is needed to withstand applied pressures,safety and energy cost in the operation of high-pressured equipment pose limitations for separative membrane dehumidification.

The permeability and selectivity are two critical performance indicators that determine the dehumidification performance of a separative membrane.An intricate balance between these two parameters is required to obtain the desired flowrate and extent of dehumidification in the output supply air with minimum energy consumption.

3.2.1.Solution-diffusion

A dense separative membrane separates via solution-diffusion mechanism where the molecules solubilize and diffuse through the membrane.Gases are separated by their solubility towards the polymer matrix and their diffusivity through the free volumes within the dense layer matrix.The solution-diffusion model assumes that the gas at the high-pressure side diffuses into the polymer,diffuses down a concentration gradient to the lowpressure side,and finally desorbs into the bulk at the permeate side.The permeability of moisture (P) is the product of its solubility (S) and diffusivity (D) with the membrane as shown in Eq.(7):

As for the solubility of moisture,it will depend on the membrane material,whether it has strong affinity towards moisture by being hydrophilic to increase solubility,and thus,increase the permeability of moisture.

From Eq.(7),the volumetric flow rate can be influenced by membrane fabrication–selecting materials with high intrinsic moisture permeability such as increasing membrane area and/or reducing the thickness of the selective layer using composite membranes;or by system process alterations such as increasing the pressure gradient to raise the driving force applied to the membrane.

3.2.2.Molecular sieving

Molecular sieving occurs where separation is based on size exclusion.The kinetic diameter of N2is 0.364 nm;for O2is 0.346 nm;and H2O is even smaller,at 0.265 nm [81,82].Hence,for effective moisture separation via molecular sieving,the nominal pore size (dp) should be between 0.265 nm and 0.346 nm.

Fig.6.Separative membranes structure,(a) symmetric dense membrane schematic,(ai) SEM image of the cross section of dense flat sheet membrane [77],(b) asymmetric membrane with dense selective layer schematic,(bi) SEM image of the cross section of a asymmetric membrane [78].

For example,Wang and colleagues [83] confirmed that the addition of zeolite,with pore sizes of 0 nm,0.3 nm and 0.4 nm,in casted PVA membranes,selectively prevented the transfer of both CO2(0.33 nm) and H2O through the membranes based on their pore sizes.Accordingly,the addition of 0.4 nm zeolite allowed the permeance of both gases,whereas the addition of 0.3 nm zeolite only allowed the transfer of H2O but not CO2.Such findings showed the importance of a tight pore size distribution between the kinetic diameter size of H2O molecules and O2or N2molecules if molecular sieving were to be explored.

Interestingly,molecular sieving may also be achieved even when the pore opening of the zeolite channels are 0.40 nm,which is slightly bigger than the kinetic diameter of water and/or nitrogen molecules [82].This is possible with intelligent use of the membrane’s affinity for moisture.Water molecules are adsorbed onto the pore surface,resulting in narrower membrane pores and blockage towards nitrogen molecules or subsequent bigger molecules.The NaA zeolite membrane managed water vapor permeance of 6.8×10-6mol·m-2·s-1·Pa-1and the H2O/N2separation factor is 178,respectively,which observed stable performance over a 200 hour test.

Having a dense membrane which is highly selective to moisture has its challenges and will inevitably lead to concentration polarization at the feed boundary layer [84] and lowers water vapour permeability.In order to circumvent the challenge of concentration polarisation,optimizing the module design was proposed and it was reported that incorporating an external sweep in the feed side can accomplish the same separation as a more permeable module while still needing up to four times less membrane area.

Therefore,by intelligently embracing high moisture adsorptive materials and optimizing module design,researchers could circumvent the challenge to assemble a defect-free pristine dense membrane but opt for a more forgiving membrane that has a tight pore size distribution porous structure,improving on its water permeability,while at the same time,maintain the membrane selectivity and low pressure drop.

3.3.Types of materials

With the understanding of the two mechanisms for separative dehumidification,this section will discuss the materials used for separation membranes for dehumidification.

The types of suitable materials for membrane fabrication are usually polymeric due to their high processability and low costs[85].For air dehumidification by separation,a common parameter used to evaluate a materials’ performance is its H2O/N2selectivity and its moisture permeability.Research went into polymer science[86–88] to improve moisture affinity and improve vapour permeance while having high selectivity.

3.3.1.Polymeric materials

Amongst different types of polymers,there are polymers that have strong affinity to moisture or chemically structured to provide unique permeance of water vapour or selectivity towards water vapour.The two properties,namely permeance and selectivity,of different polymers had been summarised and plotted in Fig.7.

The green region being highly selective polymers and the blue region being polymers that are interestingly both selective and permeable towards water vapour.

3.3.1.1.Highly selective polymers(green region).For most polymeric materials,the combination of extremely high selectivity and water vapour permeability usually do not occur at the same time.As referenced in the green region in Fig.7,the two polymers with high H2O/N2selectivity are polyimide (PI),which has extremely high selectivity of 5,330,000 but low water vapour permeability of 640 barrer [71],and polyacrylonitrile (PAN) also has a high H2O/N2selectivity of 1,880,000 but low water vapour permeability of 300 barrer.With high selectivity,PI and PAN membranes have good dehumidification performance but are plagued with poor permeability.Besides increasing the driving force or surface area to achieve a higher throughput,some post-processing methods could be applied onto PI membranes to improve its permeability and shift it closer towards the blue region.Modifications to the polymer structure such as the possibility of sulfonation could potentially exhibit higher permeance,similar to the sulfonated polymers in the blue region.The polymer structures of PI and PAN also provide hydrogen bonding sites and has potential to positively enhance moisture solubility [89].

Fig.7.H2O/N2 selectivity and H2O permeability of various polymers.(1 Barrer=7.52 × 10–18m3(STP)·m2·m·s–1·Pa–1)

PVA is another commercially available polymer,which is widely used for dehumidification due to its robustness for chemical modifications.PVA has a high concentration of hydroxyl groups for the formation of hydrogen bonds,hence exhibits high moisture affinity and crosslink potential with carboxylic acid groups [90].In food packaging,polymer-based moisture absorbers such as PVA was used for its excellent film processability,biodegradability and acceptable mechanical strength [62].PVA’s suitability for food application opens the door for wider use of PVA due to its safety and environmental aspect[91].With good stability and robustness for modifications,multiple studies had shown promising alterations to PVA membranes for the improvement of moisture permeance.For example,PVA was used as the active layer of the membrane but due to its poor moisture permeation,a small amount of 2.3% (mass) LiCl was added,resulting in an improvement of permeation rates by 70%[92],as compared to a PVA membrane with no LiCl additive.This was attributed to an increase in membrane hydrophilicity and moisture affinity [78].

3.3.1.2.Highly permeable and selective polymers (blue region).The blue region houses polymers which showcase H2O/N2selectivity of at least 50,000 and,at the same time,a water vapour permeability of at least 10,000 barrer.This strict definition resulted in many conventional polymers falling out of the region,leaving behind polymers of interest for high-performance dehumidification.

One most promising polymer is Sulfonated Poly(ether ether ketone) (SPEEK) [93],which distinctively differentiates itself by positioning at the attractive region (top right-hand corner) of Fig.7,showing both extremely high selectivity and moisture permeability [11,71,94].SPEEK,being a glassy polymer,has low gas permeabilities in its dry state.However,due to its hydrophilic nature and a high swelling degree in the presence of moisture,SPEEK,in turn,shows high gas permeabilities in its wet state [11].This distinct property puts SPEEK as a potential candidate for separative dehumidifying membrane.With its high water uptake and very fast kinetics,SPEEK can also be an effective membrane in the enthalpy heat exchanger systems and provide a good alternative for high moisture removal and heat exchanger[94].The high selectivity and permeability of SPEEK could be related to sulfonation.It was exemplified by Jia and colleagues [86] when they demonstrated sulfonated polyetherethersulfone (SPES-C) membranes[93],which exhibited a linear increase in permeability of water vapour with higher degrees of sulfonation whereas the permeability of nitrogen decreased.Sulfonated polyamide-imide (SPAI) was also reported to show high H2O/N2selectivity of 4.9×105and high water vapour permeability of 9767 barrer[95].The introduction of strongly polar sulfonate groups into the polymer improves hydrophilicity which favours solubility of water vapour molecules,and thus permeability.However,there is an optimal sulfonation extent as the sulfonate groups interact with polymer chains and increase the packing density of polymers,decreasing free volume and hence eventually impeding diffusion of gas molecules.

Commercial sulfonated pentablock copolymer NEXARTMMD9200(NEXAR)also ranked well with its amphiphilic properties,with a good balance between mechanical properties and good dehumidification performance as the middle sulfonated block can create water selective channels while two of the end blocks-tertbutyl styrene and ethylene-co-propylene-provide the mechanical stability and chain flexibility [96].Akhtar and colleagues also explored polybenzimidazole (PBI) which is an aromatic heterocyclic polymer with a long history of applications in gas separations,water treatment,pervaporation and fuel cells [97].It was shortlisted due to its hydrophilicity and ability to sustain high thermal and pressure conditions [98].Another block copolymer in the right most of the blue region with extremely high permeability is PEBAX 1074.It is made up of soft,rubbery,hydrophilic polyethylene oxide (PEO) blocks with rigid polyamide groups to provide high mechanical strength while the malleable PEO parts of the polymer permits high gas permeabilities[99].Another study also uses PEO poly(butylene) terephthalate (PBT) multi-block copolymer which also exhibits high permeability of 85,500 barrer and high H2O/N2selectivity of 40,500 due to the synergistic effects of both block copolymers [71].The increase in the amount of PEO enhances permeability due to higher solubility,while the increase in the amount of PBT boosts the H2O/N2selectivity,suggesting restriction of swelling of the PEO polymer [72].

With the above discussed polymeric material examples,it is evident that for separative membranes to have a good selectivity with high permeance,the selection of polymers is crucial.Various polymer modification methods such as sulfonation or formation of copolymers for unique properties may be explored to obtain both high H2O/N2selectivity and water vapour permeability.

3.3.2.Inorganic materials

Besides organic polymers,another family of membrane materials are inorganic materials,mainly zeolites and some ceramic membranes,focusing on dehumidification.Inorganic zeoliticbased materials have shown promise for dehumidification in several research studies.

Composite membranes of PVA with the addition of zeolites of different pore sizes[83]show increased water vapour transmission due to changes in structure and transport properties of the PVA membrane.The addition of zeolites brings about multiple benefits.Firstly,it reduces the proportion of PVA on the surfaces and cross sections of the membrane which reduces solubility and diffusion.Secondly,the additional internal porosity of zeolitic particles also provided additional transport pathways with lower resistance to accelerate gas permeability.Thirdly,surface roughness is higher,influencing a higher surface hydrophilicity and hence favouring moisture affinity and increasing solubility for higher permeability.

3.4.Separative membranes

With the selection of polymeric and inorganic materials narrowed down to the two regions,this section critically examines separative membranes using these polymers and compares amongst current state-of-the-art membranes’ H2O/N2selectivity and water vapour permeance.

3.4.1.Highly selective membranes

The first criteria for separative membranes are its selectivity.With reference to Fig.7,polymers which falls in the green region such as PVA,PI,PAN are fabricated into thin flat sheet membranes or hollow fibre membranes.

PVA has low intrinsic water vapour permeability and so it is often used as the selective layer which can be made ultra thin[28,100].For example,thin film composite PVA/LiCl membranes were fabricated via consecutive dip coating and drying on strong stainless steel scaffold support [92].Dip coating in LiCl lowers the surface contact angle and enhances water vapour permeability and the membrane showed high water vapor permeability of 300,000 barrer and H2O/N2selectivity of 2800,representing a 50% increase in permeability compared to the single dip membrane.

PI hollow fibre membranes were fabricated and modified with tetrabutylammoniumnaphthalenesulfonate (BAN) and showed improved water vapour permeation rates [101].BAN increases the diffusion coefficients by promoting flexibility of the polymeric chains for better diffusion of water vapour.The sulfonate groups in BAN also gave a hydrophilic boost for better moisture affinity.

PAN hollow fibre membranes was also dip coated with PDMS to produce a composite membrane with high water vapor permeance of 11,227 GPU(1GPU=3.35×10-10mol·(m2·s·Pa)–1)with a H2O/N2selectivity of 220 [102].The membranes were tested over a 150 hour test and found to be stable,showing benefits of a separative membrane operating isothermally as compared to a membrane contactor which requires regeneration of its liquid desiccant solution to maintain dehumidification effects.

Hence,by using a highly selective PVA,PI or PAN polymeric membrane,its permeance can be improved with additives such as LiCl,BAN or PDMS,potentially shifting these polymers in the green region in Fig.7 towards the blue region.In the next section,highly permeable membranes are explored and how it may also be desirable for specific applications.

3.4.2.Highly permeable membranes

Although membranes with high permeability is generally plagued by its low selectivity,it enjoys low operational pressures,and a higher surface area of membrane could be explored to achieve the desired dehumidification results.

Dense polyamide(PA)hydrophilic flat sheet thin film composite membranes were synthesized from 3,5-diaminobenzoic acid (BA)for water vapor separation.With optimized crosslink reaction time between Trimesoyl chloride(TMC)and the membrane,a high permeance of 2160 GPU and 23 selectivity was achieved [103].The key factor that assisted in the high permeance is the hydrophilicity of the membrane when excess acyl chloride groups of TMC was hydrolysed to the carboxylic groups.

Besides solution diffusion,molecular sieving mechanism was the other prominent mechanism in separative membranes.One example was inorganic zeolitic membranes which were also developed through sequential seed coating of different zeolite sizes of 1.4 μm followed by 0.3 μm seeding crystals[82].The zeolitic membrane showed a H2O/N2selectivity of 178 with a H2O permeance of 6.8×10-6mol·m-2·Pa-1·s-1with excellent stability and negligible decline in separation performance over a continuous 8-day test.

Hence,polymeric membranes or zeolitic membranes may provide high permeance by improving its hydrophilicity and through strategic use of zeolite pore sizes to realize precise molecular sieving.In the next sub-section,polymers within the blue zone are explored and fabricated into unique polymeric membranes to showcase both its high moisture selectivity and permeability.

3.4.3.Both highly selective and highly permeable membranes

The trade-off between selectivity and permeance pushed research towards new materials with superior intrinsic properties or optimized membrane structures to exceptionally perform apart from its peers[104].Within the blue zone,novel polymers such as SPEEK and copolymers with unique properties were used to fabricate into membranes for air dehumidification.Some of these membranes were fabricated in the hollow fibre and some in its flat sheet configuration.

Composite hollow fibre SPEEK membranes were fabricated by multiple dip coating onto PES microfiltration support and tested in both lab and real-field tests,maintaining a water vapor permeance of 28,700 GPU and H2O/N2selectivity above 100,000 for 1325 hours of operations [11].Besides SPEEK coated hollow fibre PES membranes,SPEEK films was compared against PEBAX 1074 films with both film membranes showing promising dehydration of nitrogen gas with permeabilities of 49,000 Barrer and 20,000 barrer,respectively [11].

Flat sheet PEO-PBT multi-block copolymer membrane was solution casted with different compositions and showed selectivity of 93,000 and permeance of 65,000 barrer when 40% (mass) of PEO was used,and 50,000 selectivity and 140,000 barrer with 75%(mass)PEO in the PBT copolymer matrix[72].The water vapor permeability increases exponentially with the block copolymer with the largest PEO composition due to the rubbery nature of the PEO segment.On the other hand,increments in PBT weightage restrict the swelling of the PEO phase,lowering solubility of water vapor and hence,a higher selectivity.Such fine-tuning of composition between PEO and PBT allows the control of the selectivity and permeance of the ensuing polymeric membrane.

Apart from thin film composite membranes,mixed matrix membranes could also be explored.PBI mixed matrix membranes was fabricated with the addition of 0.5%(mass)of titanium dioxide nanotubes and the performance almost doubled for water vapor permeability and H2O/N2selectivity to 554,000 Barrer and 1,500,000 as compared to the pristine PBI membranes,respectively.The TiO2nanotubes dispersed within the polymer matrix disrupt polymer chains,resulting in an improvement of water vapour separation properties [96].

Graphene oxide was first demonstrated by Y.Shin et al.,for water vapor separation,showing the outstanding water vapor permeability (182,000 Barrer) and H2O/N2selectivity (＞10,000) of GO membranes[105]at lower test cell temperatures of 22.5°C.It was found that water vapour permeance is higher at lower test cell temperatures.Thereafter,Petukhov et al.,fabricated mediumflake graphene oxide (MFGO) was spin-coated onto anodic aluminum oxide (AAO) and was found to have a relatively high H2O/N2separation factor of 13,260 with a water vapor permeability of 106,100 when tested in a relative humidity of 80%[106].Both studies showed the promise of graphene oxide to produce both highly selective and highly permeable membrane for water air separation.

Despite the high selectivity of separative membranes,these membranes do have their limitations.The requirement for a highly pressured environment translates to high energy consumption and the yield of low flux at lower pressures might be a hindrance to its applicability in some applications.

3.5.Separative membranes performance

Table 4 summarizes air dehumidification performances of polymeric as well as inorganic membranes in terms of water vapour permeance or permeability and its respective H2O/N2selectivity.It was organised with papers reporting in different carrier gas for moisture.

Table 4 attempts to organise past literature related to dehumidification with separative membranes in terms of its testing conditions with different temperatures and feed gases.The significance of a 20°C difference in testing conditions has an impact on solubility,diffusivity and permeabilities of different gases [71].The constituents of the carrier gas may also be different,leading to discrepancies in the permeance of gases with water vapour.Membranes were also made with unique materials and structures which also inevitably resulted in different reporting standards in H2O permeability or permeance.

Table 4 shows close relation between materials used as shown in Fig.7 for membrane fabrication and its intrinsic properties to dehumidification.Careful and intelligent modifications with additives such as metal organic frameworks or modifications via sulfonation or coatings to alter surface hydrophilicity are potential techniques to improve membranes’dehumidification performance.

Table 4 Summary of air dehumidification separative membranes in terms of its H2O permeance or permeability and its respective H2O/N2 selectivity

Apart from high selective and highly permeable membranes,there were also hydrogel membranes which does not have as high a water permeance and has somewhat lower selectivity like the Cuprophan hollow fiber membrane [114].It is hydrophilic by nature and swells into a hydrogel and shrinks as the ambient relative humidity levels change.The water-containing openings in the polymer matrix becomes larger and swells at high relative humidity,removing moisture from the environment and releases moisture as the environment is dry,acting as a smart membrane,regulating indoor humidity.

Separative membranes for air dehumidification are relatively new with high potential for growth and innovation.With meaningful innovation and development into novel polymers and materials showing improved intrinsic properties of selectivity or permeability,it will then increase the likelihood of breakthroughs for new materials to emerge and fuel membrane research with bespoke modifications and skilful fabrication to flaunt better dehumidification capabilities and performances.The next section focuses on adsorptive membranes which provides another potential breakthrough in dehumidification technologies.

4.Adsorption Membrane Dehumidification

Amidst the crowded research on membrane contactors and separative membranes for air dehumidification,adsorptive membrane research is still in its infancy stage and there are limited reviews published in this category.The reason could be due to the overlap of research in materials development such as hydrogels and novel materials for adsorptive materials for dehumidification.

Building on the understanding of membrane contactors and separative membranes,this section aims to explore dehumidification via adsorption with adsorptive polymers and additives of novel materials for the fabrication of super adsorptive membranes.The desirable properties of the adsorptive membrane with high water uptake capacity and ideal hysteresis loop in the adsorption curves for air dehumidification will be discussed and key fabrication techniques from previous literature will be shared in the later sections.

4.1.Adsorption membrane structure

Unlike membrane contactors,where liquid desiccants drive an absorptive process,adsorptive membranes are mostly fibrous membranes with intertwining fibres,forming porous structures.They are produced through melt spinning [115],blow spinning,or electrospinning process,with the latter producing thinner fibres,closer to the nanofiber range.

The nanofibrous membrane is highly porous and allows air permeation through the membrane (Fig.8(a)) while the selection of polymer with good moisture affinity is key for the nanofibrous structure to be selectively adsorptive to H2O [117].It may also be in the form of mixed-matrix membranes where porous high surface area filler materials are added into the continuous polymer matrix (Fig.8(b)) to enhance the adsorptive capacity of the membranes.These adsorptive membranes follow physisorption or chemisorption principles for adsorption,depending on the membrane materials.

Fig.8.Adsorptive membrane structures,(a) nanofibrous membrane schematic,(ai) SEM image of a nanofibrous membrane,(b) mixed matrix nanofibrous membrane schematic,(bi) SEM image of nanofibrous membrane with nanomaterials attached to fibers [116].

A potentially high dehumidification performing adsorptive membrane should have a pore structure that is in the range of mesoporous to microporous decorated with hydrophilic groups[12].Hydrophilicity is crucial as it ensures that the material has a high moisture affinity,and when coupled with high surface areas and many adsorption sites,can effectively provide high contact possibilities to capture moisture from air,yet porous enough to allow higher air permeance as compared to dense separative membranes.To enable high adsorption,the material should exhibit a high surface area with high affinity to moisture.The fabrication of nanofibrous membranes with unique morphologies can be fabricated via electrospinning [118–120],wet electrospinning[121,122] or solution blow spinning [123].

The beauty of electrospinning lies in its versatility where all parameters play a measurable part in the eventual membrane’s morphology.Adsorptive materials can be fabricated into a 3-dimensional nanofibrous scaffolds via electrospinning [124] or similar techniques to achieve tuneable porosity and the ability to manipulate nanofiber composition to obtain desired properties and functions through alterations in its environmental and solution variables (see Fig.9(a)).The production of nanofibrous membranes are known for its high surface area to volume ratio [126]which provide notable advantages in sorption capacity,sorptionrates and lower temperature desorption rates as compared to solution-casted membranes [127].Electro-spraying is a method which can be explored to embed additives onto nanofibrous membranes via liquid atomization by means of electrical forces.The liquid particles are forced by the strong electric field to be ejected from the capillary nozzle,similar to electrospinning,to be dispersed into fine droplets and attaching onto nanofibers [128] as shown in Fig.8bii.

Fig.9.(a)Typical electrospinning setup;SEM images of superabsorbent 7-SAN nanofibers;(b)top view of dry sample,(c)top view of wet sample,(d)cross section showing difference dry and wet sample [125].

4.2.Adsorption membrane mechanism

The forces of attraction between interacting molecules/particles and membrane polymer matrices can be either chemical or physical adsorption.Chemisorption occurs when a chemical reaction takes place,resulting in formation of chemical bonds.Such bonds tend to be strong,rendering bond breaking to reverse the adsorption process.Physisorption,on the other hand,can include van der Waals forces,electrostatic interaction or hydrogen bonds.This weaker energetics allow for reversible regeneration and thus are considered a desired mechanism if membrane regeneration is crucial for the application.Adsorption and desorption are adiabatic processes along the enthalpy lines of the psychrometric humidity chart,depicted with diagonal movements on the chart (Fig.1(d)),given that adsorption is an exothermic process,while desorption is an endothermic process.

According to the International Union of Pure and Applied Chemistry(IUPAC)report,there are six types of isotherms and four hysteresis types [129] (see Fig.10).Each type of isotherm gives information on the pore structure and relationship with its adsorbent-adsorbate interactions [130].

Type I isotherm is related to its accessible pore volume or commonly known as micropore filling for porous materials with pore diameters of less than 2.0 nm.Type II isotherm is associated with either macro-porous or non-porous materials and the adsorption is unrestricted from monolayer to multilayer adsorptions.Type III adsorption isotherms are less common and thus indicative that the adsorbent may be of low interest for air dehumidification.Type IV isotherm tends to be irreversible and its hysteresis loops are commonly associated with capillary condensation occurring in mesopores with pores more than 2.0 nm in diameter.Type V are uncommon and are determined by weak adsorbate-adsorbent interactions.Type VI adsorption shows an elaborated stepwise multilayer adsorption on uniform non-porous surfaces.

For hysteresis types,the two extreme hysteretic behaviours are the H1 and H4 types.The H1 type hysteresis has adsorption and desorption occurring specifically at a certain relative pressure with its curve showing a near vertical profile.These profiles can be approximately parallel to one another,depending on the material’s adsorption capacity.At the other extreme,is the H4 type,where the hysteresis loop spreads over a range of relative pressure,with the desorption curve remaining diagonally parallel to its adsorption capacity.Other hysteresis curves such as the H2 or H3 could exhibit both or either of the extremes,and hence has a loop which falls in between H1 and H4.In cases where the hysteresis loops do not give a complete cycle but show degradation or a lowered adsorption capacity.In such cases,external energy,usually in the form of temperature,may be employed to fully desorb the adsorbents.Such phenomenon may also be associated with the swelling of non-rigid porous structures after adsorption and its incomplete or irreversible desorption of molecules in pores that are close to the adsorbent molecular size.

Fig.10.IUPAC classification of adsorption isotherm types (left) [129] and hysteresis types (right) [42].

With understanding of the adsorption fundamentals and mechanisms,the following subsections will delve into adsorptive materials and then the membranes’main performance parameters such as moisture uptake capacity with regards to the adsorption–desorption isotherm and hysteresis curve.

4.3.Types of materials

In this section,we discuss novel materials with good adsorptive properties that are suitable for air dehumidification.Fundamentally,adsorption is a moisture affinity process,which mandates the need to bring sparse amounts of water molecules in the air to come sufficiently close to or in contact with the surfaces of an adsorbent.Hence,the membrane should be hydrophilic by nature to enhance moisture affinity,drawing water molecules from the air,and capturing them for dehumidification.

4.3.1.Polymers

The typical characteristic of good moisture adsorbing and or even absorbing polymers are its high moisture affinity,high specific surface area and meso to microporous network structures for efficient transport of water vapour to the active adsorption sites.The hydrophilicity property of the polymers also plays a strong influence on the water uptake at a lower relative pressure for the adsorptive material.Super absorbent polymers (SAPs) [131] are another innovative class of materials proposed over common materials for its incredible water uptake capacity.They are water-insoluble,but water-swellable or gelling,with a high H2O absorbent value per gram of SAP making up its gel volume.Sodium polyacrylate salt is one common material and is known to be able to absorb 100–1000 times of its mass.The rate of absorption and swelling capacity increases with decreasing SAP particle size[132].

Hydrophilic groups in the polymeric chain such as -COOH,-SO3H,-NH2and -OH contribute to the high moisture uptake capacity [133].Hence,polymers such as Polyacrylicacid (PAA),Polyacrylamide (PAM),Polyvynlalcohol (PVA) are commonly used for the synthesis and preparation of hydrogels.For example,PAM can be highly efficient solid desiccant that is fabricated as a super porous hydrogel(SPH)incorporating crystalline microporous silico aluminophosphate zeolite[134].These polymeric chains also have the potential to crosslink with ions to form stronger bonds and exhibit unique properties such as elasticity and resilience to compression,tension and twists[135]as seen in acrylamide-based SPH which was synthesized in the presence of sodium alginate.Conventional free radical polymerisation method where a crosslinking agent can be introduced to instigate the formation of covalent bonds between groups of polymers [136].PAA-PAM super absorbent hydrogel was also prepared by an in-situ free radical solution polymerization with graphite oxide homogeneously dispersed for improved swelling capacities and swelling rates [137].

Besides high uptake capacities,thermoresponsive polymers such as Poly(N-isopropyl acrylamide) (PNIPAM) was extensively studied for its luminescence transitions for sensory devices [138–140]and low critical solution temperature(LCST)which was plausible for moisture adsorption and regeneration near its critical solution temperature.This unique thermo-switch-ability effect where the polymer is hydrophilic at a temperature below its LCST of 34 °C and hydrophobic above this LCST [141].Besides these unique properties,PNIPAM was also found to have a water uptake capacity of 138% (mass) when functionalized with mesoporous carbon [12],which increased its favourability as a polymer for adsorption and desorption.

4.3.2.Adsorbents

In the earlier section under membrane contactors,liquid desiccants had been critically discussed while in this section,we focus on two types of materials:(1) hygroscopic or even deliquescent materials,and (2) porous materials for adsorption of moisture and dehumidification of air.A material is considered hygroscopic when it is capable of drawing water molecules from the air by physisorption.Some materials are so hygroscopic that they dissolve themselves with the moisture they adsorb,turning into a liquid form.These materials are commonly known as deliquescent.Novel porous materials are also known for their high surface area and affinity to moisture for capture.

4.3.2.1.Conventional solid adsorbents.Conventional solid adsorbents such as silica gel,activated carbon and activated alumina exhibit low maximum adsorbent capacity while calcium chloride,a common commercially used deliquescent,have higher water uptake capacity [142].Graphene oxide (GO) was also explored to produce moisture adsorbing papers,and managed a water adsorption capacity of 0.6 g·g-1[143].

A crucial and common property linking these solid desiccants is their high surface area driven by their mesoporous structures.Several researches ventured into mesoporous compounds [144],such as KIT-1,SBA-1,SBA-15,MCM-41,MCM-48 and FSM-16,because of their large surface areas of more than 500 m2·g-1,and explored their uptake capacities and adsorption isotherms (Fig.11).

Silanol chemistry helps explain the hydrophobic-hydrophilic characteristics of silica mesoporous materials where isolated silanols exhibit weak or no interactions with water due to its hydrophobicity,while terminal silanols strongly attract water via hydrogen bonding.Aluminium containing MCM-41 is a hydrophilic sorbent [146] which shows high moisture affinity due to the presence of strong electrostatic field created by the anionic sites.By increasing the aluminium content in the framework,it could enhance hydrophilicity and water sorption capacity but to a certain extent as it might result in clogging of pores by water clusters formed around the Al centres [145],indicating a potential cap for enhancement.

Besides leveraging on mesoporous materials’ high surface area for moisture adsorption,modifications may be employed to amplify hydrophilicity and encourage an even higher moisture sorption capacity.Composites were also explored [147,148] to improve uptake capacity or regeneration ability as compared to its conventional desiccants descendants.

4.3.2.2.Composite desiccants.Solely with activated carbon or silica gel or calcium chloride,each of them has its drawbacks and merits.A composite made of 66% activated carbon,13% silica gel and 21%CaCl2was synthesized and optimized to provide the best adsorption rate with considerable capacity [149].Activated carbon and silica gel provided the surface area while calcium chloride provided the high adsorption rate.

Sodium alginate (NaAlg) is a well-known water-soluble polysaccharide extracted from seaweed which is highly hydrophilic due to the abundance of carboxyl and hydroxyl groups.However,NaAlg exhibits a high swelling degree as water molecules diffuse into the polymer matrix.Such a swelling phenomenon must be controlled to a moderate degree before NaAlg can be considered as a candidate suitable for dehumidification membrane material.Lowering of the swelling degree of NaAlg can be achieved by adding 20%–30%(mass)PVA[113].A hydrogel composite made of crosslinked sodium alginate and calcium chloride in a polymer matrix,as the deliquescent salt,exhibits moisture adsorbance capacity of 225% gain in mass at 3 kPa,28 °C and readily regenerates and desorbs at 100 °C [4].

Fig.11.Adsorption isotherms of mesoporous materials [145].In Stage I where relative pressure is low,water molecules face difficulty in adsorbing on pore surfaces at the beginning due to surface hydrophobicity,where water-surface interactions predominate.Once the water molecules are adsorbed onto the surface at higher relative pressures,water-water interactions take place via hydrogen bonding,forming clusters and the first layer (Stage II).Water clusters continue to grow until capillary condensation occur(Stage III)and subsequently filling of pores with water (Stage IV).

4.3.2.3.Porous frameworks.Metal Organic Frameworks (MOFs) are an extraordinary class of porous materials where organic bridging ligands are connected to metal ions nodes to form ordered three dimensional coordinated networks [150–160].Although MOFs tend to be expensive,challenging to synthesize,and many of them faced instability in water[161],its unparalleled surface area makes MOFs highly attractive for moisture adsorption.The prevalent properties for moisture adsorbent MOFs would be to tap into its large specific surface area for high moisture adsorptive sites and rigid network structure for water capture within the structure.For the application of air dehumidification,designing water stable MOFs [162–166] is extremely important.

Some significant key advantages of MOFs as compared to inorganic microporous structures such as zeolites are its tuneable composition,achieved by using different metal ions and changing organic linkers,and very high surface areas with extremely high capacity for gas capture [167].Mechanisms of water adsorption in this class of materials includes pore filling due to flexibility and possible structural modifications of the host material [168].

Fig.12 serves as an overview of novel water-stable MOFs and their respective composites,showing their moisture uptake capacity and its adsorption isotherm with extremely promising uptake capacities.A promising MOF adsorbent which stands out for its uptake capacity is Cr-SOC-MOF-1,which has a surface area of 4500 m2·g-1and a total pore volume of 2.1 cm3·g-1[169].It has a sharp adsorption isotherm at 60%–75% relative humidity and an uptake capacity of 1.95 g of adsorbed water per gram of adsorbent.Remarkably,Cr-soc-MOF-1 also shows good recyclability and regenerates at a low relative humidity of below 40%without heating for good indoor humidity control between 45% and 65%.

Another exciting water-stable MOF with attractive characteristics for moisture adsorption is MIL-101(Cr).It presents a type IV adsorption isotherm with two adsorption steps due to two different sizes of mesoporous channels and shows a water adsorption capacity of 0.56 g·g-1at 25 °C and 50% relative humidity [170].No significant degradation was also indicated after 10 cycles over a 28-day experiment,showing good reuse performance.

Y-shp-MOF-5 is a hybrid microporous highly connected rareearth-based MOF for moisture control within 45%and 65%relative humidity with exceptional structural integrity and robustness,even after large number of water vapour adsorption–desorption cycles [162].Despite having a low uptake capacity at 0.45 g·g-1,Y-shp-MOF-5 has its adsorption–desorption cycle optimally positioned within the ASHRAE 55 comfort relative humidity zone and hence positioned strongly as a prospective MOF for humidity control in confined spaces or for air-conditioning.

Fig.12.Moisture adsorption isotherms of water stable MOFs and COFs.

Besides MOFs,another innovative porous material with reticular structures and high porosity is the covalent organic frameworks(COFs)[171].COF-432 showed exceptional hydrolytic stability but showed a low uptake capacity of 0.23 g·g-1.Its adsorption and desorption cycles are between low relative humidity range of 20% to 40% [7].

4.3.3.Synergistic effects of adsorbents

Various materials had been introduced into membranes as a third component through multiple methods to induce significant improvements in separation or performance for the membranes’intended purpose.Such phenomenon was unsurprisingly exploited,with unique additives or fillers used for synergistic effects for improved dehumidification performance.

On top of the synthesis of PAM and PAA together into a SPH,researchers also capitalize on the high hygroscopic ability of CaCl2and good water binding of PAM.Hybrid gels of PAM and PAA with CaCl2was optimized for its weightage and made to be sensitive to environmental temperature where it adsorbs and swells at different extents in different temperatures[172].A promising maximum swelling degree of 196.6% was concluded at 30 °C,prompting its suitability of use in a hot humid environment.In a separate study,PAM-carbon nanotube (CNT) composite hydrogel was also combined with CaCl2for water harvesting with regeneration via solar heat.An optimal CaCl2loading provided a sorption capacity of 110% (mass) in an environmental condition of 25 °C and 60% relative humidity [8].

By combining the benefits of the moisture absorbing hygroscopic polymer such as chloride-doped polypyrrole (PPy-Cl) and the water storing hydrophilic gel such as PNIPAM,Zhao and colleagues leveraged the synergistic effects and designed a super moisture absorbent gel (SMAG) [173].PNIPAM’s unique property of a low LCST enabled the switch for rapid water release,an added benefit and capability for the intended application for moisture harvesting.

The synergistic effects of the high specific surface area of MOFs and high adsorbent properties of CaCl2was also explored.MIL101(Cr) was immersed in CaCl2and showed good adsorption rates,even at a low relative humidity of below 40%,that were significantly better than pristine MIL-101(Cr) [174].The composite’s uptake capacity was similar to its parent but exhibited better adsorption capability at a low relative humidity without sacrificing the total uptake capacity.Adsorption curves of MOFs can also be shifted to a lower relative humidity by having a hydrophilic polymer [175] or through modifications [176] to enhance adsorptive performance.

Besides enhancing the water uptake capacity,synergising adsorbents could also offer lower regeneration temperature of composites as shown in Fig.13 where the combination of adsorptive materials plotted and grouped together against its adsorption quantity and regeneration temperatures.From Fig.13,the selection of adsorptive materials could be chosen with regards to its adsorptive capacity and temperature required for regeneration.The grouped metal organic frameworks stood out for its higher adsorption capacity of up to 1.7 g·g-1with forgiving regeneration temperatures of between 65 °C and 90 °C.This illustrates the potential of novel materials as compared to silica-based or claybased composites.

With an overview of the landscape of adsorbent materials with its dehumidification adsorption capabilities,researchers are also able to select the material that is best suited for the eventual application.The critical process of adding MOFs into membrane is also challenging as doing so can negatively alter the sorption properties of the MOF composite membrane.Adding additives such as CaCl2also requires optimization as it can lead to blockage of pores.The preparation must be carefully conceptualised and executed to achieve a favourable outcome.Therefore,by intelligently embracing high moisture adsorptive materials,researchers must skilfully fabricate a super adsorptive membrane that is highly porous for high air flux,while maintaining the membrane moisture uptake capacity and fine-tune its desired adsorption rates.

4.4.Adsorption membranes

With the understanding and selection of composite adsorbents with its synergistic effects and high moisture affinity polymers shared,adsorptive membranes should build on the high surface area of its materials and keep it accessible for adsorption while ensuring a high throughput for the flow of humid feed gas.Hence,adsorptive membranes are mostly fibrous in nature with high porosity and surface area to volume ratio.

Past literature had shown MOF-incorporated membranes exhibit much better uptake capacities along with high rates of adsorption due to higher number of adsorption sites,higher surface roughness and high surface area [74].HKUST-1 nanoparticles was conjugated onto nanofibers via electrospraying,producing necklace crystal-like nanofibers[116]as shown in Fig.8bii,exposing the incorporated MOFs and keeping its adsorption sites available to capture moisture from humid air.

Electrospun PVA membranes with sodium polyacrylate was successfully crosslinked and showed high moisture absorbent performance of 3.9 g·g-1of its mass [128].The nanofibers swelled 134%after water absorption.The absorption rate of the nanofibers was also faster than the SAPs in general due to the highly porous nature of the nanofibrous membrane and higher surface area for adsorption.

A super desiccant polymer was developed by ion modification of sodium acrylate salt [177],coated onto a fibrous membrane and showed satisfactory 1.28 g·g-1sorption capacity at 67% relative humidity.A super hygroscopic nanofibrous membrane consisting of two layers of electrospun PAN base showcased an unparalleled moisture absorption capacity of 3.01 g·g-1with fast adsorption rates and good moisture permeability [178].The first layer included MIL-101(Cr),which had exceptional moisture uptake capacity,followed by impregnation with LiCl which acted as the desiccant layer which was responsible for moisture adsorption.The second layer consisted of PAN with a heat absorbing carbon black layer which transferred the heat to desorb the adsorbed water molecules on the first layer.Such intelligent play requires a deep and clear fundamental understanding of the materials’ roles within the membrane matrix.The fact that membranes with nanofibrous structures can show high moisture transport rates illustrates the phenomenal versatility and potential of adsorptive membrane-based solutions for air dehumidification and possibly other applications.

4.5.Adsorption membranes performance

It is notable that membrane research into adsorptive membranes is still in its infancy and a consolidated performance data of adsorptive membranes is still unavailable as performance data of adsorptive membranes remains scarce at the time of writing.

Due to the diversified research on moisture adsorption in materials,energy saving,humidity control[179]or moisture harvesting,it is common for papers to compare the materials’ absorbent performance by its uptake capacity or its saturation point,and the slope of the adsorbing curve which gives an indication of the rate of adsorption.Performance evaluations are usually tested via gravimetric method to measure the gains in mass from adsorption of moisture over the range of humidity level to observe its adsorption at different relative humidity and elucidate the final adsorption capacity.

Fig.13.Adsorptive materials with its adsorption quantity and regeneration temperature [15,21].

From a broad perspective,adsorptive membranes incorporating materials that are capable of adsorbing moisture at high relative humidity is of high interest and there is a constant competition for materials with significantly higher uptake capacity.However,there seems to be a gap in a material with a high adsorption rate at a low relative humidity.This will bring about new opportunities where the membrane can effectively bring down relative humidity down significantly to a lower level.In addition,at low humidity,the absolute amount of moisture in air is low,which skews the priority to have strong adsorption at low levels as compared to having high uptake capacity.

Furthermore,it poses an optimisation challenge for researchers to balance the adsorption ability and the regeneration capability.This is because a stronger adsorptive strength also means more resistance and energy required during desorption and hence,a tougher job for subsequent regeneration.An adsorptive membrane which is made for singular use,may possess strong chemisorption driven by materials with good moisture affinity for high adsorption rates and adsorption capacity,and should be low in cost to justify its single use application.This also brings up the usefulness of the adsorptive and desorptive nature as the membrane swells and shrinks to regulate humidity and collect moisture must be of high value such as within confined space or submarine operations.

The absolute adsorption capacity is also dependent on the quantity of fibrous membrane spun.Despite the challenges for adsorptive membranes to exhibit similar dehumidification capacities or throughput as membrane contactors or separative membranes,the desired high porosity characteristic ensures high permeance of the membrane,which is a unique property that is missing from the previous two categories of membrane-based dehumidification.

5.Outlook and Perspectives

In the beginning of the review,we set down the scope of this review and addressed the fundamentals and transport mechanisms of each type of membranes for air dehumidification.The subsequent three sections explored into the key elements of membrane contactors,separative membranes and adsorptive membranes,delving specifically into the materials and dehumidification performances of the membranes.

Membrane contactors was further classified into the membrane itself and the liquid desiccants that provided the driving force for separation;while separative membranes focused on two key parameters-H2O/N2selectivity against moisture permeance;whereas for adsorptive membranes,moisture uptake capacities and the hysteresis adsorption desorption isotherms at its respective relative humidity was discussed.

The three categories of membranes are fabricated into unique configurations with operations in various flow patterns[180].Past literature had demonstrated and experimented their membrane modules via co-current flow,counter flow,cross flow,countercross flow but only a minority with plug flow.There may be other variants to the above configurations which includes mixing on the non-lumen side,the retentate or the permeate side to enhance homogeneity.Counter-current flow pattern tends to be the most efficient with flatsheet membranes can be modulated into spiral wound modules and may show lower pressure drops than hollow fiber modules [181].Under the cross flow filtration configuration,experimental and modelling analysis had been studied specifically for the purpose of air dehumidification [31].It was found that dehumidification performance is independent to the feed air humidity but trades off with higher selectivity.There are also mathematical analysis of the different flow patterns formulated or the effects of module arrangements on performance of hollow fiber membrane contactors [182].Plug flow,however,was often less researched due to foreseeable fouling issues,high energy requirements,or degradation of performance.Past configurations and its proven advances,suggests research to re-examine plug flow for adsorptive membranes.

With distinct transport mechanisms and different parameters used to measure membrane performances,it is impractical to compare across the three types of membrane dehumidification.Indeed,each type of membrane and the dehumidification process serves a different purpose,and hence in this section,we put everything into perspectives for the reader,informing them on how to choose the right type of membrane dehumidification as well as addressing future challenges and sharing insights on the directions for future research.

5.1.Critical evaluation of each type of membrane dehumidification

On the basis of clear differentiation amongst each type of membrane dehumidification,readers can objectively ascertain the type of membrane that is most suitable for their targeted dehumidifying application.Choosing between dehumidifying air by coupling to a HVAC unit,or for a small device,the type of membrane dehumidification should be carefully chosen for optimal performance to meet the desired outcome.Here,we appraise the three types of membrane dehumidification based on the supply air throughput,dehumidification purity requirement,desiccant regeneration and its sustainability/disposability,footprint,and overall energy consumption and cost of operations.Then we propose the potential area of application for each type of membrane dehumidification(Table 5).

As summarised in Table 5,each type of membrane dehumidification offers its unique merits and limitations.Membrane contactors,being the most widely studied dehumidification process,has the greatest potential for commercial application.Also,owing to the good dehumidification capacity,low membrane cost driven by availability of commercial membranes,and versatility for coupling to other processes such as energy exchanger,membrane contactors are the most suitable among the three for integration to a larger HVAC system that includes a dehumidification unit.Comparatively,separative membranes,with a lower air throughput,may need a significant surface area to dehumidify air at the scale of membrane contactors.Hence,given current state-of-the-art technology for dense membrane fabrication,separative membranes are more suited for application in small-scale devices that require high humidity control.The adsorptive membranes,on the other hand,are plagued by poor humidity control as the dehumidification capacity is capped by the adsorptive ability of the materials used.Being nanofibrous also suggests that the membrane is highly porous,which means that the residence time of air through the membrane is short.Hence,the adsorption rate is limited regardless the adsorption kinetics of the membranes or adsorbents.We reckon that adsorptive membranes are thus more pertinent for applications that require low humidity control such as a simple room ventilation system.

Table 5 Comparison between membrane contactors,separative membranes and adsorptive membranes

5.2.Next-generation materials

Through the course of this review,materials and its transport properties have demonstrated a significant role towards the performance of air dehumidification.The key considerations for a good membrane design include good hydrolytic stability,high hydrophilicity,channels for vapour transport,and high surface area to volume ratio for better adsorption,etc.Herein,we present suggestions to the reader using examples from other membranebased applications to stimulate ideas for brainstorming of potential improvement to membranes for air dehumidification.

5.2.1.Polymers of high intrinsic permeabilities

Moving away from conventional dense polymeric membranes,the use of novel polymeric materials with inherent porosity can be transformative for separative dehumidification membranes.Conventional dense membranes separate using the solutiondiffusion mechanism.Hence,increasing the free volumes within polymer matrices can increase the diffusion coefficient of water vapor for enhancing its permeability.One group of polymers with large free volumes is the polymers of intrinsic microporosity(PIMs).These polymers are named because their microporosity arises intrinsically from their unique molecular structures.The molecular structures contain sites of contortion such as spirocentres to impart the backbone with a rigid structure that prevents space-efficient packing of the polymer chains.PIMs and its variants had been presented in past literature to have apparent BET surface areas of over 750 m2·g-1,and controllable surface areas of up to 1730 m2·g-1[183].With such high surface areas coupled with large microporosity,it is natural that the large free volumes make PIMs an attractive polymer for membrane-based gas separation,including water vapour removal.

5.2.2.Microporous polymers

Recently,other polymers of high intrinsic permeability such as metal-induced ordered microporous polymers (MMPs) had also been reported.These MMPs are constructed from amine-rich polymers,divalent metal ions and small organic linkers containing both halogen and carboxylic acid groups.They exhibited tailorable frameworks to give well-defined pore sizes and high processability to be assembled into ultrathin selective layers with thickness less than 50 nm,and a large-area of more than 100 cm3,forming defect-free membranes [184].More importantly,MMPs showed excellent hydrolytic stability.Its three-dimensional structures showed that the large pore sizes can potentially open the doors for high permeance and testing with water vapour saturated gases.The key issue with using these polymers of high intrinsic permeability is its selectivity.Hence,researchers looking at using these polymers of high intrinsic permeabilities for separative dehumidification membranes must take special care of this trade-off limitation,and evaluate if the compromised H2O/N2selectivity remains sufficiently competitive to meet the desired dehumidification outcome.

5.3.Other challenges

Apart from advancing membrane development,we believe that other areas of research must grow in tandem to see an overall progress in membrane-based dehumidification.An area of research that is hardly investigated for air dehumidification is membrane fouling.

Membrane fouling is a process where the surface of the membrane is deposited with foulants comprising organic matters,particulates,salts and biological substances that increase membrane transport resistance with dynamic separation[185].No membrane to date can evade membrane fouling,although processes such as gas separation see relatively less serious membrane fouling as compared with water treatment processes such as microfiltration and ultrafiltration.Membrane fouling in air dehumidification process can stem from suspended aerosols,dusts,salts,and biological substances such as fungi and pollens in air.Different types of membrane dehumidification deal with different fouling mechanisms and type of foulants.Membrane contactors generally have greater issues with aerosol fouling from the air side and scaling from the liquid desiccant side [15].

Separative and adsorptive membranes,on the other hand,are more prone to particulate and biological fouling with adsorptive membranes experiencing relatively more serious fouling,owing to the significantly higher air throughput.Fouling can bring about many detrimental effects to the dehumidification performances.These effects include increasing the pressure drop and membrane transport resistance,reducing the water vapour permeance,as well as shortening the lifespan of the membranes,which will then incur increased energy consumption and costs of the dehumidification process.Hence,efforts in understanding membrane fouling and developing antifouling membranes should be given greater attention,regardless the type of membrane dehumidification.

Another area of research that can create an impact is the overall process design optimization for each type membrane dehumidification,which can include module designs,and the number of stages and their process configuration,as well as energy and cost benefit analysis.Optimizing membrane contactors for dehumidification is the most well-studied to date with many papers and reviews reporting the effect of operating parameters on the system performance,multi-stage as compared to single-stage dehumidification,flow distribution and membrane deformation in different types of membrane modules,as well as delving into various performance indicators to measure effectiveness,and energy and cost savings [15,16,186].In this respect,studies on optimizing the dehumidification process of separative and adsorptive membranes are significantly lacking and stronger efforts in this area of research will help strengthen the competitive position of these two types of membranes for dehumidification in their targeted applications.

6.Conclusions

In this review,we have put together a comparison of three types of membrane dehumidification,namely,membrane contactors,separative membranes and adsorptive membranes.The fundamentals of each type of membrane dehumidification are explored with discussion focusing on the difference in their transport mechanisms,types of materials used,and their dehumidification performances.Membrane contactors,being the most established process among the three,present a strong promise with diverse applications that can couple dehumidification to air cooling and energy exchange in different system configurations.Separative membranes employs a completely different water vapour transport and may be rationally designed to improve both performance indicators as shown by cited references.Recent research in new polymeric materials seem exciting with desirable intrinsic micro-porosity and hydrolytic stability but their advantages in membrane dehumidification needs further validation.Adsorptive membranes are the least explored amongst the three,where porous nanofibrous membranes allows for extremely high air flux through this type of membrane and moisture capture is usually achieved by water adsorption through either the super absorbent polymers or porous materials as adsorbents.Above all,the possibility of a combination of the benefits of membrane contactors,separative membranes and adsorptive materials to instigate attractive air dehumidification is imaginably probable to have control over humidity and temperature of output air at minimal energetic cost.With impending commercialization in mind,cheap and readily available materials,safe to use and recoverable desiccants,straightforward and scalable fabrication must all be premeditated and designed emphatically for its eventual application.Membrane regeneration should also consider employing free or renewable energy as an essential criterion to minimize energy input.It is only through collectively efforts as such that membrane processes can continue to innovate and push the boundaries of the dehumidification technology.

CRediT authorship contribution statement

Huaixun Lim:Writing–original draft.Kunli Goh:Writing–review &editing.Miao Tian:.Rong Wang:Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by Singapore Membrane Technology Centre(SMTC),Interdisciplinary Graduate Programme,Nanyang Environment and Water Research institute and Nanyang Technological university for this research.