Xuehua Ruan ,Wenbo Huo ,Jiaming Wang ,Minggang Guo ,Wenji Zheng ,Yun Zou ,Aibin Huang,Jianxiang Shou,Gaohong He,*

1 State Key Laboratory of Fine Chemicals,Engineering Research Center for VOC Control &Reclamation,School of Chemical Engineering at Panjin,Dalian University of Technology,Panjin 124221,China

2 SINOPEC Zhenhai Refining and Chemical Company,Ningbo 315207,China

3 Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology,Guangxi University,Nanning 530004,China

Keywords:Hydrogen production Separation Membranes Process framework H2-selective membrane CO2-selective membrane

ABSTRACT Coal-based H2 generation has abruptly increased in recent years.The PSA-VPSA-SC process is the matured and standard framework for H2 purification and CO2 capture in many existing plants,including normal and vacuum pressure swing adsorption units in series (PSA-VPSA),and shallow condensation unit (SC).However,this standard process is frequently subjected to low H2 recovery ratio and high purification cost.In this work,H2-selective and CO2-selective membrane units, i.e.,HM and CO2M,are attempted to support the standard process and ameliorate constraints.In the beginning,HM unit is arranged after VPSA to enhance H2 recovery from the decarbonized stream, i.e.,the PSA-VPSA-SC/HM process.As a result,H2 recovery ratio can be enhanced significantly from 83%to 98%.In the following,VPSA is replaced with CO2M unit to reduce investment and operation cost, i.e.,the PSA-CO2M-SC/HM process.Accordingly,the specific purification cost is diminished from 33.46 to 32.02 USD·(103 m3 H2)-1,saved by 4.3%,meanwhile the construction cost is falling back and just a little higher than that for the standard process.In the end,another CO2M unit is launched before PSA,i.e.,the CO2M-PSA-CO2M-SC/HM process,which could unbundle CO2 enrichment partially from H2 purification,and then save more investment and operation cost.In comparison with the standard process,this ultimate retrofitted process can be superior in all the three crucial indices,i.e.,recovery ratio,investment,and specific purification cost.On the whole,coal-based H2 generation can be ameliorated significantly through high efficient H2-selective and CO2-selective membrane units.

1.Introduction

In recent years,energy consumption based on fossil fuels has been substituted by either nuclear or renewable energy resources on a large scale [1–3],even though fossil fuels,i.e.,coal,oil,and natural gas,are the remaining dominative role in energy markets.It is reported that coal accounts for about 25% of the total consumption in most of these years [4,5].Hereinto,coal utilization for H2generation is an abruptly increasing field,owing to its substantial demands from petrochemical plants,new energy vehicles,and many other markets [6–9].According to the data from SINOPEC Group,H2index has been higher than 400 m3·t-1crude oil for common refining plants(m3is the abridged expression of standard cubic meter at 101.325 K and 0°C).According to the assessment data from China Energy Group,several millions of H2-powered vehicles would be launched into the transportation market gradually before 2050,and the potential H2consumption would be higher than 1200 m3·a-1for one H2-powered automobile on average [10–12].

Fig.1.Schematic diagram of H2 production based on coal gasification and steam reforming.

The integrated process with steam reforming after coal gasification is the mainstream route for industrial H2production.The schematic block diagram for classical coal-based H2generation has been shown in Fig.1.The overall reaction system is summarized in Eq.(1).It is clear that CO,CH4and CO2are the main coexisting components in reforming effluent [13–15].Besides,a little O2is remaining after reaction,a little N2is brought in by the oxygen stream,and trace H2S might be yielded through sulfate hydrogenation [16,17].The steam reforming process can be scheduled concretely with one-stage or two-stage water–gas shift reaction,and the CO conversion degree is usually between 93% and 98% [18–20].The high conversion cases based on the two-stage shift reaction,although it is relatively easy to conduct H2purification with absorption and methanation,are subjected to the much high conversion cost relevant to high steam–gas ratio and large catalyst loading [21].In contrast,those low conversion cases based on the one-stage shift reaction,owing to their relaxant reaction condition and economical operation,are more favorable currently for H2generation [22].However,the low conversion cases are restricted by both their low H2recovery and complex purification process.In some mature plants,both normal and vacuum pressure swing adsorption units (PSAVPSA) are installed in series for the purification [23–25],even though the recovery ratio is normally remaining to be not more than 85%.

With the intent to reinforce H2generation under lowconversion mode,i.e.,effectively increase the recovery degree,several multi-technique integration separation frameworks were developed in this research.In details,gas membrane permeation(both H2-and CO2-selective modules,HM and CO2M)and shallow condensation (SC) would be launched together with PSA in these separation frameworks.Furthermore,CO2would be liquefied as the byproduct,from the viewpoint of carbon capture,utilization and storage [26–29].The approximated investment together with specific cost for separation would be employed for economical assessment and analysis.During the assessment,the standard PSA-VPSA purification process was referred as the benchmark case.

2.Basic Simulation and Assessment Conditions

The reforming effluent,according to the operation data from SINOPEC Group,is supposed to be H2(58.5%(vol)),CO2(32.8%(vol)),CO (4.3%(vol)),CH4(3.0%(vol)),N2(0.9%(vol)),O2(0.2%(vol))and H2O(0.3%(vol)),with temperature at 40°C and pressureat 2.80 MPa(gage),respectively.In China,the newly built H2generation plants are generally larger than 150,000 m3·h-1,and then coal-based reforming effluent matching with this scale is estimated to be at least 300,000 m3·h-1[30].During the work for separation framework design,the feedstock is defined with these data.In addition,the price for the reforming effluent is estimated to be 0.07 USD·m-3roughly.

During process design and simulation,H2-selective and CO2-selective membranes are supposed to be the Prism-II?hollow fiber module and the PolarisTMspiral wound module,respectively.The Prism-II?membrane is fabricated by Air Products and Chemical Inc.,in which the selective layer is made from glassy polyimide[31–33].The PolarisTMmembrane is manufactured by Membrane Technology Research,Inc.,with selective layer made from rubbery polar polymer [34–36].Their detailed gas permeation properties are listed in Table 1.Hereinto,Prism-II?modules’ parameters are derived from industrial installations (mixed gas at 2.70 MPa,H2content equal to 65.0%(vol)) for hydrogen recovery from refinery gases,and PolarisTMmodules’parameters are collected from the literatures about MTR’s field tests (mixed gas at 1.24 MPa,CO2content equal to 16.5%(vol)) at the National Carbon Capture Center in USA [35,36].The reliable sale prices for Prism-II?and PolarisTMmodules are about 150 US and 240 USD per square meters,respectively.

Table 1Gas permeation rates in Prism-II? and PolarisTM membranes for process simulation

Table 2Economical assessment results of the standard PSA-VPSA-SC process for H2 purification

The above-mentioned selection is in the view of industrial membrane applications.It should be emphasized that many novel membranes with high H2/CO2selectivity are more promising in the future for ameliorating coal-based H2generation.For example,2-D continuous ZIF-8 membranes can separate H2and CO2with selectivity up to 9.9 in our previous research [37],Al2O3/SAPO-34 zeolite composite membranes behaved excellent with selectivity up to 20 in Yu and his co-workers’ research[38],Zr-doped silica membranes are excellent with selectivity close to 23 from Song and his co-workers’ report [39],and amine-modified Mg-MOF-74/CPO-27-Mg membranes increased the selectivity to 28 according to the research from Huang’s group [40].Once the bottlenecks,e.g.,large-scale fabrication and stress-induced module failure,are solved,it is highly attractive to use these new inorganic membranes for coal-based H2generation plants.

Propane refrigeration system is utilized to support condensation in separation frameworks.The refrigeration temperature could be arranged between 0 and -40°C,which is appointed to be 5°C lower than condensation value,considering the minimum temperature difference.The coefficient of performance (COP),according to the industrial data from Bingshan Group working on cooling business in China,is expressed by Eq.(2).With the refrigeration temperature(TR)and the cooling duty(CD,kW)for condensation,the engine power (EP) can be calculated to evaluate investment and running cost.According to engineering experience,the construction cost for propane refrigeration system is supposed to be 9.4×104US dollars per 100 kW engine power.In addition,the recycling water (RW,t·h-1) for refrigeration can be calculated from the cooling duty by Eq.(3).

Fig.2.Sketched structure and crucial formulas for adsorption simulation.

Pressure swing adsorption is the major operation for H2purification.Its towers are packed with activated carbon for CO2,Cu(I)-modified activated carbon for CO,amorphous silica for H2O,and ZSM-5 for CH4,O2and N2,etc.[41,42].Hereinto,the adsorbent loading maximum in amount is activated carbon,which accounts for almost one-third of PSA construction cost.In this instance,its loading mass would be calculated to roughly evaluate the investment.According to the data in literature,its CO2adsorption capacity is around 3.8 mmol·g-1[43,44].According to engineering experience,its adsorption saturation degree during actual operation is assigned to be 70%,and the unit price is suggested to be 2000 US dollars per ton activated carbon [45].

Vacuum pressure swing adsorption is applied for CO2purification.Its towers are mainly packed with activated carbon.The adsorbent would account for one-half of the total construction cost for VPSA,excluding the cost for vacuum equipment.The other details would be assigned identical to the parameters for PSA simulation and calculation.

In addition to separation units,compressors and vacuum pumps are essential for stream transfer and compression.Their adiabatic efficiencies are assigned to be 85%and 70%,respectively,and their construction costs are supposed to be 7.2×104USD per 100 kW engine power.

During economical analysis,the running payout would be simplified to the sum of depreciation expense and operation cost[31].The total construction cost is depreciated by the most-often-used straight-line method,in which the capital investment is divided directly by assets’service life.The lifetimes of Prism-II?and PolarisTMmembrane modules,adsorbents,and the other operation units are empirically specified as 10 years,5 years,10 years,and 15 years,respectively.The annual running time is planned as 8400 h.The operation cost is approximately estimated with the consumption of main utilities,i.e.,electricity (6 kV),recycling water (32°C),and steam (1.0 MPa),and their market prices are 0.13 USD·(kW·h)-1,0.10 USD·t-1and 29.5 USD·t-1,respectively[14].Their demands in the frameworks would be estimated with process simulator UniSim?Design.

According to the basic process framework,there are three main products,i.e.,H2,fuel gas,and CO2liquid.Referring to market information,H2price is estimated to be 0.22 USD·m-3,fuel gas is given to be 0.04 USD·m-3,and CO2liquid is given to be 28.5 USD·t-1,respectively.

3.Modeling and Process Simulation

Before economical evaluation and analysis,the classical and the retrofitted process frameworks were simulated and optimized with the commercial process simulator UniSim?Design.The main separation units include PSA,VPSA,HM,CO2M and SC units,while the associated units include compressors,vacuum pumps,heat exchangers,filters and refrigeration equipment.Most operation units,except adsorption and membrane separation units,have technical simulation modules in the software.Accordingly,the custom-designed models for adsorption and membrane units are given in the following meticulously.

3.1.Modeling for PSA and VPSA units

Referring to the cases in petrochemical industries,a five-bed,11-step PSA system is utilized to separate H2with high purity from the reforming effluent,and a three-bed,nine-step VPSA system is utilized to concentrate CO2from PSA tail gas.The Newton-based solving approach for multiple bed systems,raised by Jiang and his co-workers [46,47],is utilized to make PSA simulation.The multi-component material balance in adsorbent bed is given with the plug flow assumption,while the equilibrium isotherm is assumed to obey the dual-site Langmuir form that has been widely adopted for industrial cases.Gas adsorption rate is governed with the linear driving force model.In addition,heat transfer is supposed to be in equilibrium between adsorbent bed and fluid,while the heat loss across the column wall is ignored.The sketched structure and the governing equations supporting the model for adsorption simulation have been exhibited in Fig.2.More details about the model for multiple-bed PSA simulation can be found in the literatures [46,47].

Fig.3.Sketched structure and crucial formulas for membrane simulation.

3.2.Modeling for membrane units

Hollow fiber membrane modules,owing to the huge separation burden,are employed,and they would be operated under countercurrent mode.The finite element method is applied to make precise simulation for membrane units.The module is divided into 200 hypothetical micro-membrane cells in series in the feed flowing direction,which is nominated as the tank-in-series model[48–50].In this instance,the maximum concentration change in each small cell is less than 0.005,and mass permeation can be precisely estimated with the algebraic average-driving-force model.

For pressure-driven membrane units,there is a porous substrate on the permeated side with the thickness larger than 100 μm to support the dense selective layer.In this instance,gas permeation in each micro-membrane cell could be considered a cross-flow pattern without effect from the bulk permeate stream.The sketched structure and the governing equations supporting the model for membrane simulation have been shown in Fig.3.More details about the micro-cell model can be found in our previous research [51].

3.3.Constraint for process optimization

To fairly make the economical comparison,all process frameworks are analyzed under their optimum behavior.The profit is commonly recognized to be the ultimate economic index for process optimization.In this work,subject to the lacking in some economic parameters,especially the administrative and labor expense,the approximate economic index,i.e.,the gross profit(GP),is used as the criterion [52].It is briefly obtained with the total output value (TOV,USD·h-1),the total input value (TIV),and the total running payout(TRP),while the calculation is exhibited with algebraic expressions Eqs.(4)–(7).For all separation process frameworks,their optimum behavior could be regarded as the operation status with the maximum gross profit.Accordingly,membrane units can be optimized by adjusting parameters to find the optimum behavior of the entire process.

For the total output value,H2,fuel gas (FG) and CO2liquid are taken into account.For the total input value,the estimated value of the reforming effluent (RE) is considered.For the total running payout TRP,the depreciation expense (DE) and the operation cost(OC) are summarized together.

Fig.4.Schematic diagram of the standard PSA-VPSA-SC process for H2 purification.

4.Classical PSA-VPSA-SC Process Framework (CASE-0)

The standard PSA-VPSA-SC purification process is given in Fig.4.The reforming effluent is firstly entering PSA unit,in which H2could be purified to 99.9%(vol) at 2.65 MPa,and the tail gas is released at atmospheric pressure with H2concentration lower than 20.0%(vol).PSA tail gas is then compressed to 0.65 MPa before entering VPSA unit,in which the decarbonized stream is discharged directly with CO2content lower than 10.0 %volat 0.55 MPa and utilized as fuel,and CO2is desorbed with content up to 98.0%(vol) at vacuum condition.The crude CO2is compressed by vacuum pump and compressor installed in sequence.The condensation condition is optimized with pressure at 4.40 MPa and temperature at 5°C and -20°C by stage.The above-mentioned parameters are referred to the industrial plant,and the gross profit is about 13,750 USD·h-1.Again,the operation parameters in Fig.4 have been optimized to make the maximum gross profit.

According to the foundation provided above,the construction costs,depreciation expenses and operation costs are approximately evaluated and listed in Table 2.The summed construction cost for all the major units is assessed to be 39.448×106USD,the annual depreciation expense is about 3.170×106USD,while the annual operation cost for the public utilities,i.e.,31.5 MW electric work and 4020 t·h-1recycling water,is summed to about 37.771×106USD.With this standard separation process,H2output is about 145,700 m3·h-1,its recovery ratio is around 83%,and the specific H2purification cost is about 33.46 USD·(103m3product)-1.

5.Several Retrofitted Separation Frameworks

Based on the classical process,some retrofitted separation frameworks with membrane units to enhance H2purification and/or CO2enrichment are conceived in this research.

5.1.PSA-VPSA-SC/HM process framework (CASE-1)

In the standard PSA-VPSA-SC purification framework,the decarbonized stream from VPSA is abundant in H2,with the content up to 50.0%(vol).However,this valuable stream is usually wasted to fire furnace.It is evident that H2-selective membrane unit can be used to enhance H2recovery.

The retrofitted PSA-VPSA-SC/HM separation framework is roughly exhibited in Fig.5.The decarbonized stream from VPSA is pressurized from 0.55 MPa (gage) to 1.85 MPa (gage),and then entering H2-selective membrane unit with temperature equal to 80°C.During membrane separation,H2is concentrated to be higher than 90.0%(vol) in the permeation stream at 0.25 MPa(gage),and the residual content in the final tail gas as fuel can be decreased to about 10.0%(vol).In the end,the crude H2is pressurized to 2.80 MPa and then entering PSA unit.It should be mentioned that these operation parameters in Fig.5 and utilized above have been optimized to maximize the gross profit.As a result,the PSA-VPSA-SC/HM can achieve the maximum gross profit up to 17,670 USD·h-1.

For the PSA-VPSA-SC/HM process,the approximate construction costs,depreciation expenses and operation costs are evaluated and listed in Table 3.The summed construction cost for all these major units is assessed to be 45.577×106USD,the annual depreciation expense is amounted to be 3.648×106USD,while the annual operation cost for the public utilities,i.e.,37.1 MW electric work,4590 t·h-1recycling water,and 1.70 t·h-1steam,is about 44.716×106USD.With this retrofitted PSA-VPSA-SC/HM process framework,H2output is strikingly increased to about 172,200 m3·h-1,the recovery ratio is about 98%,while the specific H2purification cost is slightly declined to about 33.43 USD·(103m3product)-1.It is obvious that H2-selective membrane unit adding after VPSA is applicable to enhance H2generation and increase economic returns.

Table 3Economical assessment results of the PSA-VPSA-SC/HM process for H2 purification

Fig.5.Schematic diagram of the PSA-VPSA-SC/HM process for H2 purification.

5.2.PSA-CO2M-SC/HM process framework (CASE-2)

In the standard PSA-VPSA-SC framework,VPSA to concentrate CO2is subjected to the highly energy-intensive pressurization operationviaboth vacuum pump and compressor.In this instance,the CO2-selective membrane unit could be attempted to replace VPSA unit.Consequently,we can create the retrofitted framework nominated as PSA-CO2M-SC/HM.

The retrofitted PSA-CO2M-SC/HM process framework is shown in Fig.6.The tail gas from PSA is compressed to 0.97 MPa and afterwards entering CO2-selective membrane unit,in which the decarbonized stream is discharged directly with CO2content close to 10.0%(vol) at 0.95 MPa and utilized as H2resource for H2-selective membrane unit,meanwhile CO2is concentrated in the permeation stream with the concentration close to 90.0%(vol)and the pressure slightly higher than atmospheric condition.The other unit operations would be running in accordance with the normal PSA-VPSA-SC and the PSA-VPSA-SC/HM process frameworks.It should be unfolded that these operation parameters in Fig.6 and utilized above have been optimized and make the maximum gross profit up to 17,840 USD·h-1for the PSA-CO2M-SC/HM framework.

For the PSA-CO2M-SC/HM system,the approximate construction costs,depreciation expenses and operation costs are evaluated and summarized in Table 4.The total construction cost for all the major units is assessed to be about 40.748×106USD,dramatically lower than the investment for PSA-VPSA-SC/HM process.The annual depreciation expense is about 3.489×106USD,and the annual operation cost for the major public utilities,i.e.,35.3 MW electric work,4430 t·h-1recycling water,and 1.76 t·h-1steam,is summed to 42.726×106USD.Both are a little lower than the homologous economical indexes of PSA-VPSA-SC/HM process.

Table 4Economical assessment results of the PSA-CO2M-SC/HM process for H2 purification

With the retrofitted PSA-CO2M-SC/HM separation process framework,H2generation capacity is about 171,800 m3·h-1,and the recovery ratio is slightly lower than 98%.Both are comparable to the PSA-VPSA-SC/HM process.In addition,the specific H2purification cost is further decreased to 32.02 USD·(103m3product)-1.On the whole,the PSA-CO2M-SC/HM separation process,owing to CO2M unit’s lower investment and higher efficiency,is highly competitive for H2purification in coal-based H2Generation plants.

5.3.CO2M-PSA-CO2M-SC/HM process framework (CASE-3)

In the coal-based reforming effluent,CO2concentration is higher than 30.0%(vol).It is the main component adsorbed in PSA unit.On the other side,the packing mode with manifold adsorbents in the PSA towers to remove all impurities is adverse to CO2enrichment.To concentrate CO2to be more attentive,it is attempted to launch the CO2-selective membrane unit before PSA unit in the framework.This retrofitting design is expected to simultaneously reduce PSA operation load and energy consumption for CO2enrichment.

The retrofitted CO2M-PSA-CO2M-SC/HM process framework is roughly exhibited in Fig.7.In this system,the reforming effluent is firstly entering the CO2-selective membrane unit for more attentive CO2separation.CO2is enriched to be higher than 80.0 vol% in the permeation stream at atmospheric pressure,while H2is concentrated slightly in the residual stream before entering PSA unit.The other units would be running in accordance with the PSACO2M-SC/HM framework.In comparison to the direct treatment with PSA unit,CO2concentration after CO2M enrichment can be increased by about 20.0%(vol).It is apparent that this CO2-enriched stream can bridge over the separation units in the following,which can make savings in both investment and running payout.It should be expressly illuminated that the operation parameters in Fig.7 and used above have been optimized for the CO2M-PSA-CO2M-SC/HM process framework and the maximum gross profit is about 17,920 USD·h-1.

Fig.6.Schematic diagram of the PSA-CO2M-SC/HM process for H2 purification.

To demonstrate CO2M-PSA-CO2M-SC/HM framework’s advantage,the approximate construction costs,depreciation expenses and operation costs have been evaluated and summarized in Table 5.The total construction cost for all the major units is assessed to be about 38.960×106USD.In this respect,the benefit from embedding CO2-selective membrane unit before PSA unit is very significant.As a result,the CO2M-PSA-CO2M-SC/HM separation system can be superior to all the other processes in investment.In addition,the annual depreciation expense is reduced to about 3.299×106USD,and the annual operation cost for major public utilities,i.e.,35.0 MW electric work,4450 t·h-1recycling water,and 1.90 t·h-1steam,is summed to be about 42.551×106USD.Both are lower than PSA-CO2M-SC/HM system’s homologous economical indexes.

Table 5Economical assessment results of the CO2M-PSA-CO2M-SC/HM process for H2 purification

With the retrofitted CO2M-PSA-CO2M-SC/HM separation process framework,H2generation capacity is around 171,900 m3·h-1,and the recovery ratio is close to 98%.Both are comparable to the PSA-VPSA-SC/HM or the PSA-CO2M-SC/HM framework.Furthermore,the specific cost for H2purification is reduced to 31.74 USD·(103m3product)-1.Overall,the CO2MPSA-CO2M-SC/HM separation process,owing to CO2M unit’s superiority in investment and efficiency,especially the unique selectivity for CO2permeation,should be highly competitive in the largescale coal-based H2generation plants for H2purification.

6.Comprehensive Economical Comparison

According to the process details and the economic data described above,it is obvious that the retrofitting attempts for separation frameworks,either adding membrane unit directly or replacing VPSA with membrane unit,are effectual to change equipment investment,running payout,as well as H2recovery performance.To thoroughly dissect the effects from both HM and CO2M membrane units,an intensive comparison is conducted in the following.

6.1.Comparison in detailed construction costs

The unfolded construction costs,i.e.,classified as adsorption units (PSA),membrane units (mol·L-1),pressurization units (K+VP),and shallow condensation unit(SC),are illustrated and compared in Fig.8.In all these separation processes,the dominant construction cost parts are accounted by the pressurization units and the adsorption units,and thus they would be considered emphatically in the analyses for economic feasibility.

In the PSA-VPSA-SC/HM process,i.e.,CASE-1,H2-selective membrane unit and its auxiliary units (HM-1,K-3,K-4) are utilized to enhance H2recovery.The feedstock to PSA is increased by about 10% in this design.It is apparent that both the adsorption units and the pressurization units are enlarged.The construction cost for PSA and VPSA units is increased by 4.2%.Owing to high H2content and low CO2content in HM permeation stream,the increment is behind the increase in feedstock.In contrast,the construction cost for pressurization units is increased by 19.3%,much higher than the increase in feedstock,since two additional compressors are launched for the new recycle.All in all,the total construction cost is increased by 15.5%.Even though,it is worthy to make the retrofit,because H2recovery is increased from 83% to 98%significantly.

The next attempt,i.e.,the PSA-CO2M-SC/HM process (CASE-2),is replacing VPSA unit with CO2-selective membrane unit.For an identical CO2capture task,CO2M unit is much cheaper than VPSA unit.Correspondingly,the summed construction cost for adsorption and membrane units is diminished abruptly from 18.282×106to 14.457×106USD.Besides,the CO2-enriched gas is discharged at -0.08 MPa in VPSA unit and at 0.02 MPa in CO2M unit.As a result,the overall pressurization scale can be decreased by about 8.9%,and the construction cost is decreased from 24.665×106to 22.476×106USD.From the viewpoint about saving equipment investment,it is highly applicable to substitute VPSA unit with CO2M unit for CO2enrichment.

The third design is the CO2M-PSA-CO2M-SC/HM (CASE-3)framework.To unbundle CO2enrichment partially from H2purification,another CO2M unit is installed before PSA unit.In this instance,PSA unit could be downsized evidently,and the estimated construction cost is fallen from 10.221×106to 8.503×106USD.It is noteworthy that the increase in construction cost is little for membrane units.The major reason is that the task for CO2enrichment is allocated rather than repeated by the fore-and the post-CO2M units.The fore-CO2M unit can concentrate CO2to about 82.3%(vol),which can be delivered to shallow condensation without enrichment once again by the post-CO2M unit.Furthermore,this retrofitting can diminish the compression scale for PSA tail gas by a certain extent;therefore the construction cost for pressurization units is reduced from 22.476×106to 22.168×106USD.After adding the fore-CO2M unit to optimize PSA unit,the total construction cost is reduced from 40.748×106to 38.960×106USD,even below that for the standard PSA-VPSA-SC process.

Fig.7.Schematic diagram of the CO2M-PSA-CO2M-SC/HM process for H2 purification.

Fig.8.Detailed comparison around main construction costs for H2 purification frameworks.

Fig.9.Detailed comparison around specific H2 purification costs for various frameworks.

Fig.10.Comparison in overall performance for various H2 purification frameworks.

6.2.Comparison in detailed specific costs

In general,the running payout includes depreciation and operation costs.For all H2purification processes in this research,the latter would occupy more than 90%,especially the cost for electric power consumption,which is dominant with the percentage higher than 80%.In this instance,the costs for electric power consumption would be compared emphatically.

The specific cost for H2purification,as shown in Fig.9,is dissected into six items.Hereinto,E(K-1),E(VP-1+K-2),E(K-3+K-4),and E(SC-1)are the detailed electric power costs.E(K-1)is relating to K-1 for PSA tail gas compression;E(VP-1+K-2)is consumed by VP-1 and K-2 for the pressurization of CO2-enriched stream;E(K-3+K-4)is utilized to support HM-1 unit and enhance H2recovery;E(SC-1) is consumed by SC-1 for CO2liquefaction.

For the PSA-VPSA-SC/HM process,HM-1,K-3,and K-4,are used to enhance recovery.In this instance,another specific cost item,i.e.,E(K-3+K-4),is attached by them.Meanwhile,H2yield is strikingly enhanced by 18.1%.It has resulted that the two main specific cost items,i.e.,E(K-1) and E(VP-1+K-2),are reduced by about 13.5%simultaneously.Their decrease has deviation from the increase in H2yield,because the desorbed gas quantities from PSA and VPSA units are increased by the recycle from HM-1 unit.On the whole,the retrofitted PSA-VPSA-SC/HM process can only slightly decrease the specific cost for H2purification.

Based on the PSA-VPSA-SC/HM process,CO2M unit is used to substitute VPSA unit,i.e.,the PSA-CO2M-SC/HM process.These two processes are almost the same in H2yield.In this instance,the change in specific cost items is attributed to pressure optimization around CO2M unit.On the one hand,PSA tail gas pressure after compression with K-1 is adjusted from 0.65 to 0.97 MPa,and then E(K-1)item is increased by 21.4%,the increment is about 1.667 USD·(103m3H2)-1.On the other hand,the CO2-enriched stream pressure is properly adjusted from -0.08 MPa (VPSA unit)to 0.02 MPa (CO2M unit),and E(VP-1+K-2) item is decreased by 21.9%,the decline is around 3.118 USD·(103m3H2)-1.On the whole,the PSA-CO2M-SC/HM process could significantly decrease the specific purification cost to 32.02 USD·(103m3H2)-1.

For the CO2M-PSA-CO2M-SC/HM process,the CO2M unit is added before PSA to unbundle CO2enrichment partially from H2purification.As a result,PSA tail gas is reduced by 16.8%,and the decline in E(K-1) item is 1.604 USD·(103m3H2)-1.Meanwhile,CO2-enriched stream is increased by 7.1%,and the increment in E(VP-1+K-2)item is 0.844 USD·(103m3H2)-1.Again,the increment in E(K-3+K-4) item is about 0.415 USD·(103m3H2)-1.On the whole,the specific purification cost is decreased to 31.74 USD·(103m3H2)-1with the retrofitted CO2M-PSA-CO2M-SC/HM process.

6.3.Comparison in overall performance

To make a final comparison for all frameworks,the crude construction cost,the specific purification cost,and H2production scale are normalized and summarized in Fig.10.

For the PSA-VPSA-SC/HM process,the specific H2purification cost is almost identical to that achieved by PSA-VPSA-SC system.Its main advantage is the increase in H2output.On the one hand,the increase in H2output(18.2%)is higher than the increase in construction cost (15.5%).On the other hand,the specific cost consumed by coal gasification and steam reforming could be decreased abruptly.The saving ratio,even though the decrease in fuel gas back to furnace is taken into account,is higher than 15.0%.Furthermore,the PSA-VPSA-SC/HM process can be carried out based on the existing PSA-VPSA-SC system.From the viewpoint to construct plant,it is clear that the PSA-VPSA-SC/HM process is superior to the other retrofitting cases in this work.

The PSA-CO2M-SC/HM process can be superior to the standard process in both H2output and specific purification cost.The data reveals that H2output is increased by 17.9% and the specific cost for H2purification is diminished by about 4.3%.In addition,the construction cost is only a little higher than the standard PSAVPSA-SC system.

The CO2M-PSA-CO2M-SC/HM process,except the complex degree,should be the optimum cases in this work.Both its construction cost and specific purification cost are the fewest,assessed to be 38.960×106USD and 31.74 USD·(103m3H2)-1,respectively.Besides,its reliable H2output is increased to 171,900 m3·h-1,only slightly behind the PSA-VPSA-SC/HM process.

7.Conclusions

To enhance recovery ratio and save purification cost for coalbased H2generation,three innovative multi-technique separation frameworks based on the standard PSA-VPSA-SC process are proposed in this work.Two kinds of membrane units,i.e.,the H2-selective Prism-II?modules and the CO2-selective PolarisTMmembrane modules are employed to support PSA units.

At first,the PSA-VPSA-SC/HM process is designed,in which Prism-II?modules are added to concentrate H2from VPSA’s decarbonized stream.This concept could be carried out based on the existing PSA-VPSA-SC system.In comparison to the standard process,this retrofitted separation framework could enhance H2recovery ratio abruptly from 83% to 98%.In addition,the specific cost for H2purification is almost the same,while its increase in construction cost is exactly lower than the increment in H2recovery ratio.

Subsequently,the separation framework is further retrofitted with PolarisTMmodules to replace VPSA unit,i.e.,the PSA-CO2MSC/HM process.This novel framework can achieve H2recovery ratio slightly lower than 98%.In comparison to the standard system,the specific purification cost is diminished from 33.46 to 32.02 USD·(103m3H2)-1,saved by 4.3%.Furthermore,the construction cost is falling back and just a little higher than that for the standard process.

In the end,another CO2M unit is added before PSA to unbundle CO2enrichment partially from H2purification,i.e.,the CO2M-PSACO2M-SC/HM framework.Except the complex degree,this retrofitted separation process is superior in all the three critical indices.In details,its H2recovery is very close to 98%,its specific purification cost is decreased to 31.74 USD·(103m3H2)-1,moreover,its crude construction cost is decreased to 38.960×106USD,lower than that for the standard PSA-VPSA-SC process.

On the whole,the retrofitted H2purification frameworks with highly efficient H2-selective and CO2-selective membrane units should be effectual to ameliorate coal-based H2generation.

Acknowledgements

The authors acknowledge the financial supports from the National Natural Science Foundation of China (Grant No.21978033,U1663223 &21978035),China Postdoctoral Science Foundation (2019M650055),Liaoning Province Funds(XLYC1907063),the Changjiang Scholars Program (T2012049),the Fundamental Research Funds for the Central Universities(DUT19TD33),and the Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing &Process Intensification Technology (2018K009).