Hongliang Qian,Hongzhou Tian,Guoqiang Yang,Gaodong Yang,Lei Li,Feng Zhang,*,Zheng Zhou,Weihua Huang,Yufu Chen,Zhibing Zhang,*

1 School of Chemistry &Chemical Engineering,Nanjing University,Nanjing 210046,China

2 Department of Pharmaceutical Engineering,China Pharmaceutical University,Nanjing 211198,China

3 Nanjing Institute of Microinterfacial Reaction Engineering,Nanjing 210008,China

Keywords:Hydrogenation and oxidation Microinterface intensification Mass transfer coefficient Structure-effect regulation Online measurement

ABSTRACT Hydrogenations and air oxidations usually have low apparent reaction rate,generally controlled by mass transfer rate,and widely exist in the modern chemical manufacturing process.The key to increase the mass transfer rate is the reduction of the liquid film resistance 1/kLa.In this work,the original concept of microinterface intensification for mass transfer and then for these reactions has been proposed.We derived the regulation model and set up the mathematical calculation method of micron-scale gas-liquid interface structure on mass transfer and reaction,designed the mechanical energy exchange device that can produce gas-liquid microinterface system on a large scale,and established the OMIS system which is able on line to measure the diameter and distribution of millions of microbubbles,interface area a and mass transfer film thickness δM,as well as developed a series of microinterface intensified reactor systems (MIRs) for the applications of hydrogenation and air oxidation processes.It is believed that this research will provide an up-to-date development for the intensification of hydrogenation and air oxidation reactions.

1.Introduction

Modern chemical manufacture process such as oil refining,petrochemicals,fine chemicals,pharmaceuticals,material synthesis,wastewater treatment,etc.involves a large number of reactions of hydrogenation and air oxidation,in which the interphase mass transfer of oxygen and hydrogen as the gas phase to the liquid phase is often the limiting step[1].Therefore,mass transfer intensification of these reaction processes will be greatly beneficial to these processes on energy saving,emission reduction,and intrinsic safety,which will bring significant environmental and economic benefits [2,3].

In these reaction process,the traditional reaction rate refers to the apparent(or macroscopic)reaction rate,rather than the intrinsic reaction rate [4].The apparent reaction rate of hydrogenations and air oxidations is usually much lower than that of the intrinsic,which means that the intrinsic reaction rate is faster and the interfacial mass transfer rate is slower.The fundamental reason is that the gas-liquid interfacial area in the traditional bubble reactors is too small to seriously restrict the interphase molecular transfer[5].For example,the area between the gas and liquid phases is usually 20-300 m2·m-3under normal pressure for the traditional bubbling air oxidation reactor,and in most cases less than 100 m2·m-3since the bubble diameter is generally between 5-30 mm [6-8].Therefore,methods such as increasing the gas circulation or adding reactor internals to enhance mass transfer and improve the mass transfer rate are usually employed in the industrial practice,but their effect is very limited[9,10].For this reason,researchers have focused on the study of new types of reactors such as microchannel(microfluidics) reactors,hyper-gravity reactors,jet reactors,and microwave reactors,trying to find new modes of interfacial mass transfer and reaction intensification,and have achieved gratifying results in the past decades [11].

For a long time,with regard to the development and design mode of the hydrogenation and air oxidation,one is based on the microscopic diffusion mass transfer (especially eddy diffusion)[12,13] and reaction characteristics on the molecular scale,and the other is based on the macroscale research results of bubbling,stirring and mixing,particle dispersion and agglomeration,flow pattern and particle distribution,equipment structure and structure-effect relationship on the millimeter -centimeter scale[14-18].Both of the models neglect the understanding of the mass transfer and reaction behaviors that occur on the mesoscale between the microscale and the macroscale,so it is difficult to know the controllable influence of the critical parameters on the performance of reactors at the mesoscale.As a result,there are thousands of hydrogenation and oxidation reactors in the industry that are still working under high energy consumption,high mass consumption and high-risk state.

The original work of microinterface intensified reactor/reaction(MIR)and the concept of microinterfacial mass transfer intensification has been proposed since 2001 and 2017 by our team,respectively[19-24],which is to enhance the mass transfer and reaction behaviors occurring on the mesoscale in these reactors.It is different from the above-mentioned gas-liquid mass transfer intensification technologies in at least three points: Firstly,it can still form a complete microinterface system in the reactor under the condition of high gas-liquid ratio,eg.more than 10,and the gasliquid mass transfer interfacial area increases by an order of magnitude,and then the mass transfer rate and reaction efficiency are significantly improved;Secondly,to realize the mass transfer intensification of the existing reactors,it is only necessary to modify the existing various gas-liquid,gas-liquid-solidetc.reactors,without the need to manufacture a new reactor,which can minimize the cost of retrofit,while it is impossible to achieve similar results for microchannel (microfluidic) reactors,hyper-gravity reactors,microwave reactors;Thirdly,it is especially suitable for high-capacity reaction devices,such as one hundred thousand tons to one million tons per year.

Different from the traditional reaction intensification technologies that mainly increases the interfacial areaato a certain extentviaimproving stirring,MIR technology not only increases the interfacial areaatens of hundreds of times,but also greatly improves the liquid film mass transfer coefficientkL.It should be noted that the size and its distribution of microbubbles in the MIR system can be accurately measured based on online measuring and imaging system (OMIS) established by our team.In addition,we derived the structure-regulation model of the liquid film mass transfer coefficientkLthough the mass transfer film thickness (δM) of the microbubbles.In the present paper,we focus on the measurement and characterization of the average diameter (d32),liquid film thickness (δL) and mass transfer film thickness (δM) of the microbubbles.The structure-effect regulation models and some cases of applications in hydrogenation and air oxidation processes were introduced as well.

2.Definition of MIR System

The microinterface assembly (MIA) as the core unit of MIR is a well-designed and elaborately manufactured mechanical structure invented by the authors that allows gas flow or/and liquid flow into the reactors nano-microparticles to form a microinterface reaction system.The advantage and mechanism of MIA,the definition of MIR system were introduced in our previous report [19-24].Briefly,the MIR system refers to a system dominated by hundreds of millions of microparticles with an average diameter of 1 μm ≤de＜1000 μm.The above-mentioned‘‘dominated”indicates that the total number of nano-microparticles accounts for more than 99% of the total number of particles and the sum of the surface area of the particles accounts for more than 99% of the total interfacial area of the reaction system.The morphology comparison of microinterface system in MIR and the macrointerface system in the conventional bubbling reactor (CBR) is shown in Fig.1.

3.Microinterface Intensification Philosophy

Taking a chemical reaction with an first-order intrinsic reaction as an example[25],the apparent reaction rate expression is shown in Eq.(1),and it can be seen that the apparent reaction rate is affected by mass transfer and intrinsic reaction.

Eq.(1) shows that the intrinsic reaction rate at this time is determined once the reaction conditions are determined.Then the apparent reaction rate of component A is mainly affected by three mass transfer resistance terms: the gas film mass transfer resistance 1/HAkGa,the liquid film mass transfer resistance 1/kLaand the liquid film mass transfer resistance on the catalyst surface 1/ksas.According to mathematical analysis,the apparent reaction rate will be infinitely close to its intrinsic reaction rate,indicating that all three mass transfer resistances have been eliminated,as long as the first three terms in the right denominatorbecome infinitely small or even zero.Unfortunately,it is so difficult to be achieved on engineering.What we can do is to reduce the mass transfer resistance as much as possible.

For hydrogenation and air oxidation processes,previous studies have shown that the gas film mass transfer resistance is much smaller than the liquid film mass transfer resistance (1-2 orders of magnitude) [26,27].The specific surface area ɑsof the catalyst for certain reaction conditions can be regarded as a fixed value.Therefore,the key problem to intensify the reaction process is how to decrease the liquid film resistance 1/kLa.The philosophy of MIR is to make the interfacial areaatens of hundreds of times larger than that of CBR in the same operational conditions even at lower temperature and pressure as well as gas/liquid ratio,by breaking bubbles and/or droplets with a diameter of centimeter/millimeter in the reactors into particles with a diameter of microns and nanometers.Thus it also naturally results in that the mass transfer film thickness δMmuch thinner and the liquid film mass transfer coefficientkLconsiderably larger than that of CBR,thus achieveing both particularly significant intensifications of the interfacial mass transfer and the reaction [19-24,28].

The structures of the reactor and MIA,the physicochemical properties of the gas and the liquid as reactants,and the operating conditions have an extremely complex effects onkLɑ,and there are many studies on the correlation between them[9,29-46].ButkLais a macroscopic parameter,which cannot reflect the detailed information of gas-liquid mass transfer interface and its two sides.Therefore,more information can be obtained only by separately studying the two parameterskLandainkLa.Namely,the liquid film mass transfer coefficientkLand the gas-liquid interfacial areaaare crucial for the precise design and regulation of the reactor[47-49].

3.1.Determination of a

The formula for calculating the interfacial area in the gas-liquid reaction system is:

The gas holdup can be calculated by the following formula:

Fig.1.Comparison of the morphology of microinterface system in MIR and the macrointerface system in the convectional bubbling reactor(CBR)via our online measurement method (OMIS) [24]: (a) microinterface system,(b) macrointerface system.

where Δpis the pressure difference measured during the experiment,Pa;Δhis the height difference between the pressure measurement points,m;and ρLis the density of liquid,kg·m-3.

The Sauter average diameterd32of the microbubbles in the MIRs is defined by Eq.(4):

Therefore,how to determinate the value ofd32is the key to statistically calculate the interfacial areaa.The measurement of microbubbles and the determination ofd32are described in the section of 4.

3.2.Determination of kL

Danckwertset al.[50] pointed out that the predicted results of the physicochemical properties of the system by Whitman laminar film theory,Higbie surface renewal theory and Danckwerts stochastic surface renewal theory are very similar for the gas absorption process.Then it should be reasonable to use one of the above three classical theories to establish the mathematical expression of the liquid film mass transfer coefficient of the microparticles in the MIR system.

The liquid film mass transfer coefficientkLdetermined by Whitman’s theory can be calculated using Eq.(5).

whereDLis diffusion coefficient of gas molecules in the liquid phase,m2·s-1,which can be calculated by Wilke’s formula.

where Φ is the association factor of liquid components,to be taken as 1 here;MLis the molar mass of liquid components,g·mol-1;VbAis the molecular volume of gas at normal boiling point,cm3·mol-1.

It is worth noting that δMvaries with the diameters of the microbubbles,which is different from the macrobubbles.If the relationship between δMand their average diameterd32can be established,the mathematical model of the liquid film mass transfer coefficientkLwill be obtained.

Therefore,the key to determine the interfacial areaaand the liquid film mass transfer coefficientkLis to measure the average sizesd32(obtained by size and size distribution of microbubbles)and mass transfer film thickness δMof microbubbles in the MIR system.

4.Measurement and Characterization

The measurement and characterization of the characteristics(particle diameter,distribution,number,shape,etc.) of the MIR system are the basis for the determination of the interfacial areaaand liquid film mass transfer coefficientkL,as well as the design and structure-effect regulation of the MIR system[21].As far as an actual MIR is concerned,its particle diameter may range multiorders of magnitude (from nano to millimeter),and the number of the microparticles may be hundreds of millions or even billions per cubic meter with dynamic changes because of the reaction and the movement.Therefore,there rarely is mature measurement and characterization methods to be employed due to the high complexity of spatio-temporal dynamics and vast quantity.

4.1.Overview of measurement methods for microparticles

To accurately measure and characterize the parameters of bubbles and droplets,researchers have made unremitting efforts to precisely determine the particle characteristics of the multiphase system using a variety of the most advanced instruments and measurement methods worldwide over the past three decades.There are many techniques for measuring the overall average parameters,as well as instruments and equipment for measuring the local parameters of the system.According to whether these instruments are in contact with the object under test,these techniques can be classified into two modes: invasive and non-invasive [51].

Invasive techniques generally use various probes as measuring tools [52-54],whose principle is that the continuous phase and the dispersed phase in multiphase fluid systems have different responses to sound,light,electricity and heat signals,so that the probe can obtain the parameters in the system by the signal transformation of the response difference.At present,optical fiber probe,conductivity probe and ultrasonic probe are the main methods.Among the non-invasive technique with good applicability,the techniques used for global parameter measurement mainly include pressure sensing method[23],bed slump method,acoustic wave technique[54],and the techniques used for local parameter measurement mainly include high-speed photography[24],laser doppler velocimetry (LDA),particle imaging velocimetry (PIV)[55],computer-assisted tomography(CT)[56],capacitance tomography (ECT) [57].

Unfortunately,the above exiting techniques are not suitable for the nano-microparticles in the MIR system.Therefore,developing the measuring and characterizing technique is one of the core important jobs for the study of MIR system.

Fig.2.Schematic diagram of microinterfacial intensification philosophy.

4.2.Online measurement method of microparticles

In view of the vast number of microbubbles and catalyst particles in the MIR system,each individual particle is nano/micron in size and spherical in shape,the deformation is negligible.For the measurement and characterization of such a system,an OMIS was set up consisting of a MIR system,a high-speed camera with a set of macro lens,a set of high-brightness light sources,a locating and adjusting system,and a computer with imaging processing software,as shown in Fig.3(a).

By the measurement of OMIS,the particle morphology,size,movement,collision,coalescence,crushing and other characteristics of the MIR system can be quickly obtained,as shown Fig.3(b).Then,with the particle image recognition software (PIRS)developed by our team,the detailed information such as particle size,distribution,gas holdup,motion trajectory,gas-liquid interfacial area,etc.can be analyzed.

4.3.Determination of d32

Based on the images obtained from OMIS,PIRS can automatically analysis microparticle images to realize automatic identification and statistical calculation of microparticles,as shown in Fig.4(a) and (b).Thend32can be calculated according to Eq.(3).

4.4.Determination of δL, δM and kL

As shown in Fig.4(c),the thickness of the liquid film of the microbubbles δLcan be measured by the OMIS system.Previous studies have shown that there is a linear relationship between the bubble diameterdand the liquid film thickness δLfor different microbubble systems.By fitting the experimental data,the following approximate relationship exists between δLof the microbubbles and their average diameter:

Based on the experimental data and the formula in the section of 3.1-3.2,kLcan be determined.δMcan be calculated by Eq.(8).

In-depth analysis of Fig.4 and other images of microbubbles show that there is an approximate relationship between δMand δLfor the microinterface systems as follows:

It indicates that the relationship between δMandd32can be obtained as follows:

5.The Structure-effect Regulation Models of MIR System

Based on the above measurement and the parameter determination,the mass transfer structure-effect regulation models of the MIR system were established,including four equations ofd32,?G,aandkL.

5.1.Structure-effect regulation model of d32

Experimental and theoretical studies [58-70] have shown that the size of particles is log-normal distribution and generally exists in the gas-liquid system,and it is no exception for the MIR system.The microbubbles in the MIR system can be considered as ideal spheres according to Young-Laplace equation and the particle size conforms to a single continuous log-normal distribution.The bubble scale model of the MIR system was obtained through the mathematical analysis of the particle size probability density function,as shown in Eq.(12) [19,71].

Eq.(12)shows thatd32is only related todmaxanddmin,and they can be determined by Eq.(13) and Eq.(14),respectively.

where ε is the energy dissipation rate of the MIA of the MIR and it was derived by our previous study shown in Eq.(15) [72].

Fig.3.(a) OMIS system established by our team;(b) The particle morphology in the MIR taken by OMIS system.

Fig.4.(a) Particle swarm scale identification by PIRS;(b) The microbubbles distribution of (a);(c) and (d) Magnified the real-time image of microinterface system.

whereS0andS1are the cross-sectional area of the reactor and the bubble breaker,respectively,m2;ρLis the density of liquid,kg·m-3;H0is the initial liquid level height in the reactor,m;V0is the initial volume flow,m3·s-1;Pmis the mixing pressure,Pa;QL0is the initial liquid circulation volume flow of the reactor,m3·s-1.

Eq.(15) represents the mathematical relationship between the reactor design parameters and the rate of energy dissipation for bubble collapse.Eqs.(13)-(15) are the structure-effect regulation model ofd32in the MIR system.

5.2.Structure-effect regulation model of ?G

Based on the hydrodynamic behavior of the MIR system,the general expression of the gas holdup ?Gof the MIR system can be derived.

where vGis apparent gas velocity,m·s-1,vLis rising speed of the bubble,m·s-1,v0can be calculated by Eq.(17),which is determined by the physicochemical properties of the system and the values ofd32.where ρLis density of liquid,kg·m-3,ρGis density of gas,kg·m-3.Thus,Eqs.(16),(17) and (12) are the structure-effect regulation model of the gas holdup ?Gin the MIR system.

Fig.5.Operation results of a pilot-scale residue oil hydrogenation on MIR.Material: vacuum residue (VR),ρL=1010 kg·m-3 (20 °C),kinematic viscosity ν=1072.33 mm2·s-1,bp ≥525°C(more than 80%(mass)of the VR),carbon-supported iron catalyst:6 μm average diameter,operating temperature:450-455°C,operating pressure: 4-12 MPa, QG/QL (normal state)=(1500-2000):1,LHSV: 0.5 h-1.

5.3.Structure-effect regulation model of a

Based on Eq.(2),combined with the above-mentioned structure-effect regulation models ofd32and ?G,the structureeffect regulation model of the gas-liquid interfacial areaain the MIR system can be given as Eq.(18) [73].

where v32is the average rising velocity of microbubbles in the MIR system,and its theoretical model can be derived as Eq.(19).

5.4.Structure-effect regulation model of kL

The liquid film thickness δLon the surface of the microbubble was measured through the OMIS [74].Based on Eqs.(6),(11) and(12),the structure-effect regulation model ofkLcan also be established,shown in Eq.(20).

6.Application Cases

The above research shows that the MIR system not only improves the interfacial areaa,but also increases the liquid film mass transfer coefficientkLdue to the reduction of mass transfer film thickness δM.Therefore,the mass transfer resistance of the liquid film 1/kLais substantially reduced by dual factorskLanda.

On the basis of above research,a series of hydrogenation and oxidation industrial reactors were designed and applied to practical processes according to the philosophy shown in Fig.2.Some typical cases are presented as follows.

6.1.Residue oil hydrogenation

The process of residue oil hydrogenation in slurry bed reactor has been studied for more than 100 years.Many typical technologies have been developed,such as VCC [75],HDH/HDHPLUS[76],(CAT/HC)3[77],and SOC [78],UniflexTM[79],ENI [80],etc.However,only a few of them have been implemented actually on a large scale in industry.The actual effect was far from the above-published data in the examples that have been implemented.

High pressure,high energy consumption,high production cost,low liquid hour space velocity(LHSV),and low conversion are their common characteristics.The operating pressure of this process is generally 16-22 MPa in terms of different catalysts.The mostreported operating pressure in slurry bed reactors is 22 MPa and the LHSV is about 0.5 h-1for iron catalysts.The operating pressure disclosed by a few companies is 16-17 MPa for nickel/molybdenum catalysts,but it must be operated at a very low LHSV of 0.08-0.2 h-1.

The MIR employed in a pilot-scale residue oil hydrogenation and the operation results were shown in Fig.5.

It shows that the residue oil hydrogenation of MIR displays a higher efficiency,higher conversion rate,lower pressure,meaning that MIR can make this process much energy-saving,economic and safe.

6.2.Air oxidation of m-xylene to synthesize m-toluic acid

M-toluic acid(MTA),a useful organic intermediate,can produce high-efficiency mosquito repellent and is a good developer for color films and artificial preparation of synthetic flavors.It is generally produced by an oxidation ofm-xylene and air in the presence of organic cobalt as a catalyst.Since the low solubility of oxygen inm-xylene,the mass transfer of the oxygen from the air bubble to the liquid phase in the reactor will play a key role.

Due to the low efficiency of the CBR,excess air and pressure must be exerted on the oxidation reactor to improve the gas-liquid mass transfer efficiency,thus increasing the reaction rate.However,this will inevitably lead to an excess energy dissipation and the toxic m-xylene outflow along with the tail gas at the top of the reactor.Therefore,it is crucial to find a new way to reduce the operation pressure and the air supply to minimize the mxylene escaping and energy consumption.

The MIR was employed instead of CBR to intensify the mass transfer efficiency.The main technical parameters obtained were listed in Table 1,showing that the air oxidationviathe MIR is substantially improved than that of CBR.

Table 1 Comparison of MIR and CBR to produce MTA

Table 2 Comparison of M-WAO with WAO for the wet air oxidation of high-salt wastewater

6.3.Wet air oxidation of high-salt industrial wastewater

Wet air oxidation(WAO)uses CBR reactor and air as an oxidant under high temperature and high-pressure to oxidize the organic matter into CO2and H2O,or partially oxidized salts into the fully oxidized salts in the industrial wastewater so that the left chemical matters can easily be biochemically degraded.The operating pressure for the reactor of WAO is usually 8-10 MPa,and the temperature is between 210-280 °C in terms of the degradation requirement.Therefore,high equipment investment and high security risks are encountered for this technology.

The M-WAO process using MIR technology was designed and applied in treating the wastewater from a glyphosate and glyphosate production units and accomplished in August 2019 in southwest China.The capacity of this oxidation for the wastewater is 1.2 Mt·a-1with the salt content of 25.2% (mass),7902 mg·L-1of ammonia nitrogen,26700 mg·L-1of total COD,and 5028 mg·L-1of formaldehyde.The results of M-WAO compared with those of WAO were shown in Table 2,indicating that there are dramatic improvements in the reaction temperature,pressure,reaction rate and compressor power consumption,etc.by using MIR technology.

7.Conclusions and Prospects

This study indicates that MIR technology may bring us a new way to solve those universal problems caused by low-efficiency mass transfer in modern chemical manufacture processes,which are of high operating pressure,high temperature,high unit investment,high production cost and low reaction efficiency of the hydrogenations and air oxidations.

The application cases of residue oil hydrogenation and air oxidation ofm-xylene and high-salt wastewater displayed that the intensification of mass transfer is very effective for the reaction process controlled by mass transfer.

For the MIR system composed of the nano-micro fluidics (bubbles and liquid drops),it may be different not just from those of the macrointerface systems in CBR on physical level,but also on chemical level,which requires in-depth and more extensive investigations.It is suggested to pay more attentions to theoretical and experimental exploration,especially around the measurement of nano-microbubbles,nano-microdroplets and nano-micro fluidics,as well as innovation of MIR to generate nano-micro fluidics or nano-microinterface system in large-scale.It is believed that this technology will provide a strong technical support for green,low-carbon and sustainable development in the field of chemical industry,and deepen the understanding of nano-micro fluidics(nano-microinterface) and its physicochemical phenomenon from mesoscope.

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

The authors are grateful to the financial support of National Natural Science Foundation of China (No.91634104,21776122 and 22178391),National Key Research&Development Program of China(No.2018YFB0604605),Jiangsu Science and Technology Plan Project (No.BM2018007).

The authors are deeply grateful to Professor Yi Chen,Professor Xianghong Cao,for their precious suggestions.

Nomenclature

agas-liquid interfacial area,m2·m-3

CBaverage concentration of solvent inside the catalyst particles,mol·L-1

DLdiffusion coefficient of gas molecules in the liquid phase,m2·s-1

dmaxmaximum diameter of the bubbles,m

dminminimum diameter of the bubbles,m

d32Sauter mean diameter of the bubbles,m

fScatalyst loading rate,m3·m-3

ggravitational acceleration,m·s-2

HAHenry’s constant,Pa·m3·mol-1

H0initial liquid level height in the reactor,m

kAfirst order intrinsic reaction rate constant based on the reaction rate

kGgas film mass transfer coefficient,m3·m-2·s-1

kLliquid film mass transfer coefficient,m3·m-2·s-1

kSliquid-solid mass transfer coefficient,m3·m-2·s-1

PGpartial pressure of component A in the bubble

Pmmixing pressure,Pa

QL0initial liquid circulation volume flow of the reactor,m3·s-1

S0cross-sectional area of the reactor,m2

S1cross-sectional area of the bubble breaker,m2

V0initial volume flow,m3·s-1

vLrising speed of the bubble,m·s-1

vGapparent gas velocity,m·s-1

v0rising speed of the bubble in an infinite stationary liquid phase

v32mean rising speed of bubbles,m·s-1

δLliquid film thickness of the bubble,m

δMmass transfer film thickness of the bubble,m

ε energy dissipation rate,W·kg-1

λ friction coefficient

ρ density,kg·m-3

ρLdensity of liquid,kg·m-3

ρGdensity of gas,kg·m-3

?Ggas holdup

χAintrinsic reaction effective factor