diban2014

Improved Performance of a PBM Reactor for Simultaneous CO2Capture and DME SynthesisNazely Diban,*, Ane M. Urtiaga, Inmaculada Ortiz, Javier Erena, Javier Bilbao,

and Andres T. Aguayo

Department of Chemical and Biomolecular Engineering, University of Cantabria, Av. de los Castros s/n, 39005 Santander, SpainDepartment of Chemical Engineering, University of Basque Country, Apdo. 644, E-48080 Bilbao, Spain

ABSTRACT: The use of nonideal zeolite membranes for the in situ H2O removal in a packed-bed membrane reactor (PBMR)during the synthesis of dimethyl ether (DME) allows the recovery of CO2 but unexpectedly reduces DME yield by 50% incomparison to a packed-bed reactor (PBR) as previously reported [Diban et al. Chem. Eng. J. 2013, 234, 140]. Due to theadvantageous performance of PBMR, the present work aims to the theoretical analysis and optimization of the workingconditions and system conguration that enhance both DME yield and CO2 recovery. Here, the previously developedmathematical model able to predict the mass transport rate of all the components present in the reactive system through zeolitemembranes has been modied and accounts for the sweep gas recirculation. The inuence of the sweep gas ow-rate in the range0.061.80 molCOxh1 (laboratory scale) and sweep gas recirculation factor (0 < < 1) has been analyzed. Sweep gas ow-rates>0.18 molCOxh

1 favored CO2 conversion but only partial recirculation of the sweep gas promoted DME yields beyond thoseobtained in a PBR due to the synergism between eective H2O removal and MeOH retention in the feed side. Althoughenergetically challenging, these results show promising prospects to apply the existing zeolite membranes for the chemicaltransformation of CO2 into DME on a large scale.

1. INTRODUCTIONThe concerns on the global warming are stressing the interestof the scientic community for CO2 capture and sequestra-tion13 and the chemical valorization of CO2 into the synthesisof valuable products, e.g., formaldehyde, acetic acid, propylene,methanol (MeOH), or hydrocarbons, have been explored inthe literature.47 The synthesis of dimethyl ether (DME) is acatalytic process with promising prospects for CO2 valorizationon a large scale.6 In addition to being a propellant and coolant,DME has a broad range of applications as an alternative cleanfuel for diesel engines, a source of H2 for fuel cells and a keyintermediate for producing automobile fuels and raw materials,e.g., olens and BTX aromatics.810

The catalytic synthesis of DME is industrially performed intwo steps; in a rst packed-bed reactor (PBR), syngas istransformed into MeOH using a metallic catalyst and in asubsequent PBR, MeOH is converted into DME on an acidiccatalyst. The set of chemical reactions that take place isdescribed below.In the rst PBR:

+ hydrogenation of CO CO 2H CH OH2 3 (1)

+ +water gas shift (WGS) reaction H O CO H CO2 2 2 (2)

+ +hydrogenation of CO CO 3H CH OH H O2 2 2 3 2 (3)

+ + + +n n n n

formulation of C C paraffins (HC) (nondesired byproducts)

CO (2 1)H C H H O, (1 10)n n

1 10

2 2 2 2 c (4)

In the second PBR:

+dehydration of MeOH to DME 2CH OH CH OCH H O3 3 3 2(5)

The use of a bifunctional catalyst with metallic and acidicfunctions allows the synthesis of DME in a single step PBRwith the benet of switching the equilibrium of the synthesis ofMeOH (eq 1) toward the dehydration of MeOH into DME(eq 5) and consequently increasing the conversion of CO andCO2 even working at higher temperatures and lowerpressures.11 This bifunctional catalyst promotes cofeeding ofCO2 with the syngas.The conversion of the reactions of CO2 hydrogenation (eq

3) and MeOH dehydration (eq 5) are limited by the presenceof H2O in the reaction site, as previously seen during thesynthesis of DME using feed mixtures of CO2+H2 and CO+CO2+H2.

1214 Hence, the in situ H2O removal from thereaction site with a H2O permeable membrane would enhanceCO2 conversion and DME yield. This strategy has beenexperimentally tested for the FischerTropsch (FT) reactionwith zeolite membranes (ZSM5, Mordenite (MOR) andSilicalite-1 (SIL)), providing successful results in terms ofproduct yields (Espinoza et al., 1999 and 2000; Rohde et al.,2005, 2006 and 2008; Schaub et al., 2008).1520 Iliuta et al.21

have theoretically explored the eciency of a packed-bedmembrane reactor (PBMR) conguration in the synthesis ofDME considering an ideal H2O selective membrane underdierent permeance properties. Among the dierent membranematerials tested in the literature,22 only zeolite membraneswould withstand the high demanding operating conditions(225325 C, 1040 bar) required during DME synthesis.

Received: September 16, 2014Revised: November 12, 2014Accepted: November 24, 2014Published: November 24, 2014

Article

pubs.acs.org/IECR

2014 American Chemical Society 19479 dx.doi.org/10.1021/ie503663h | Ind. Eng. Chem. Res. 2014, 53, 1947919487

Particularly, the zeolite membranes applied by Espinoza etal.15,16 under FT conditions for in situ H2O removal wereZSM5, MOR and SIL and showed improved reactionperformance. Therefore, we theoretically evaluated in aprevious study23 the potential application of these zeolitemembranes in a catalytic PBMR for DME synthesis with CO2cofeeding using a bifunctional catalyst CuO-ZnO-Al2O3/-Al2O3. Interestingly, we observed that the DME yield wasimportantly reduced when introducing these zeolite mem-branes to remove H2O from the feed stream. This low DMEyield was caused by the low selectivity of the zeolite membranesthat allowed not only the permeation of H2O but also ofreactants and intermediate products, mainly the intermediateMeOH that could not be further converted into DME.According to these results, it seems that the selectivity of the

membrane had a strong inuence on the viability of using aPBMR to capture CO2 and to eciently valorise it into DMEunder the simulated conditions. These results led to the logicalconclusion that research eorts should be directed toward theimprovement of the membrane H2O selectivity and to reducethe mass transport of MeOH through the membrane. Thiswould be a challenging defy for zeolite membrane researchers.Attending to the selectivity of the zeolite membranes evaluatedby Diban et al.,23 it is clear that their hydrophilic characteristicswere determinant in the transport mechanism of thecomponents (MeOH selectivity was similar to that of H2Oand higher than the selectivity of smaller molecules such as H2,CO and CO2, see also selectivity values in Table 1). Thesynthesis of a zeolite membrane with ideal H2O selectivity overMeOH selectivity would require that the mass transport of themolecules through the membrane proceeds according to asieving mechanism, thus favoring the permeation of the

smallest molecules (H2 and H2O) over the rest of moleculespresent in the system. This type of zeolite membrane wasalready reported as hydroxy sodalite (H-SOD)19,24 but itpresented low hydrothermal stability and thus it wasinapplicable in the present system.Meanwhile this ideal and stable zeolite membrane is

developed, further assessment of the system aroused thefollowing question: Could the DME yield be enhanced by usingan operational approach, that is, the operational variables andthe system conguration, with the hydrothermally stable zeolitemembranes currently available? The present work aims at goingdeeper into the evaluation of the present PBMR process andthe knowledge of the inuence of the ow rate andrecirculation of the sweep gas stream on the processperformance. The hypotheses to be demonstrated is howthese working conditions would minimize the driving force ofthe mass transport of the reactants and intermediate productsfrom the feed toward the sweep gas side. Additionally, this workintends to evaluate future experimental research priorities basedon the application of theoretical modeling tools for thedevelopment of an ecient PBMR aimed at the chemicalvalorization of CO2 into DME.

2. MATHEMATICAL MODEL MODIFICATIONS

2.1. PBMR Working at Variable Gas Velocities. Amathematical model describing the mass transport through ahydrophilic membrane of the components involved in thecatalytic synthesis of DME from a feed mixture of CO/CO2/H2in a PBMR with in situ H2O removal was developed in ourprevious work.23 Briey, the mass transport conservationequations for component i along the PBMR, with the axial

Table 1. Characteristic Dimensions of the Membrane Reactor, Catalyst Properties and Operational Conditions in the Feed andSweep Gas Streams Employed in the Simulations and Mass Transport Properties of Zeolite (ZSM5, MOR and SIL) Membranes(250300 C)a

parameter value

eective membrane area, Am (m2) 6.13 103

internal membrane support diameter (m) 10 103

external membrane support diameter (m) 13 103

selective layer thickness (m) 100eective length of the xed-bed (m) 0.015voidage of the xed-bed () 0.5catalyst density (kg/m3) 2000catalyst mass (kg) 2.4 103

catalyst mass dilution () 1/7breactants ratio in the feed and sweep gas streams (H2/COx)

c 3/1bCOx molar ow rate in the feed stream, FCOx,0

F (molCOxh1) 0.06

dCOx molar ow rate in the sweep gas stream, FCOx,FSG (molCOxh1) 0.061.8

feed and sweep gas stream pressure, PF and PS (bar) 40reactor temperature, T (C) 275operation time (h) 30

H2O permeance, H O27 (mols1m2Pa1) 6.8 108

H2O/comp. i selectivity SH2O/H2 SH2O/COe SH2O/CO2 SH2O/MeOH SH2O/DME

f SH2O/HC

real zeolite membrane 49 19.6 17.7 2.8 43.2 57.3ideal zeolite membrane 0.5

aDetermination of H2, CO, CO2, H2O, DME and HC properties by Espinoza et al.15,16 in multicomponent mixtures and MeOH properties by Piera

et al.25 in binary H2O/MeOH mixtures.bValues at the entrance of the PBMR. These variables change with the position. cCOx refers to the total

amount of CO and CO2 being the CO/CO2 molar ratio of 50%.dInitial values of molar ow rate in the sweep stream at the entrance of the PBMR

FCOx,FSG = FCOx,=Tao,t=0S . eValues only available at 350 C. fValue of permselectivity of C8 hydrocarbons. It was considered to be similar to DME

properties.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie503663h | Ind. Eng. Chem. Res. 2014, 53, 194791948719480

position expressed in terms of space time, , are (I) in the feedside

+ + = F j AF

Wr F i

d ( )d

( ) ( ) 0 ;i i iF

mCO ,0F

catCO ,0Fx

x

(6)

(II) in the permeate side

= F j AF

Wi

d ( )d

( ) 0 ;i i

S

mCO ,0F

cat

x

(7)

And the mass transport ux of the component i through themembrane can be expressed as

= j p p i( ) ( ( ) ( )) ;i i i iF S7 (8)

There, the simulations were conducted considering thedimensions of a laboratory scale PBR (Table 1) that had beenpreviously employed to determine experimentally the equili-brium thermodynamics and kinetics of the DME synthesis witha bifunctional CuO-ZnO-Al2O3/-Al2O3 catalyst.

26,27 It must benoted that the CO2 hydrogenation in eq 3 (section 1) is linearlydependent on eqs 1 and 2 and thus, it has not been included inthe mathematical modeling of the process thermodynamics andkinetics. The former reaction conditions together with theboundary conditions have been used in the present work (forfurther details, please see Diban et al.23). The mathematicalmodel in Diban et al.23 considered that the feed and sweep gasstreams, circulating counter-currently through the PBMR,owed with constant feed and sweep gas ow velocities andunder plug ow conditions, the stream pressures were xed atthe entrance of the PBMR and suered a pressure drop withthe axial position according to the consumption of reactantsand formation of products and mass transport through themembrane. However, industrially the pressure of the gasstreams is usually xed at the PBR exit while the velocity of thegas stream is widely aected within the PBR axial position, inparticular when there is an important change in the totalnumber of moles during the reaction progress. It must be notedthat considering the particle size (>150 m) and the appliedgas ow rate, the pressure drop along the reactor is negligibleand thus, constant pressure through the PBMR length can beassumed for both the feed and sweep gas streams. This wasconrmed by simulations using COMSOL software. Therefore,in the present work, the simulation conditions were adapted todescribe a PBR with variable fed gas ow rate by changing thecomplementary equations in the mathematical model devel-oped by Diban et al.23 as follows

=

Q FR TP

( ) ( )i

iF F

F(9)

=

Q FR TP

( ) ( )i

iS S

S(10)

2.2. PBMR with Sweep Gas Recirculation. Figure 1shows a ow diagram of the catalytic PBMR with in situ H2Oremoval for DME synthesis with CO2 cofeeding where thesweep gas could be recirculated to the PBMR at will. The feedand sweep streams circulated counter-currently and the feedstream owed in single-pass mode, similarly as in section 2.1.For the recirculation of the sweep gas, additional elements wereincluded in the process diagram (Figure 1) to prevent H2Oaccumulation in the sweep gas. Those elements aimed at the

partial condensation of H2O (separation unit), partial vent andrefreshment of the sweep stream (mixer unit) and the elementsrequired to restore the sweep gas temperature and pressureconditions necessary in the catalytic PBMR (compressor andheater). It must be noted that the unit for H2O and MeOHremoval has been simplied to allow only H2O condensation inorder to focus on the eect of the working conguration on thePBMR performance.For the simulations, the mathematical model reported in

section 2.1 was used. Additionally, the mathematical descriptionof condenser and mixer units has been incorporated. Regardingthe condenser, 95% of H2O condensation was considered(none of the other components were condensed), and thus,only 5% of H2O remains in the dry sweep stream, Fi,dry

S , to berecirculated, which is mathematically described as follows

= =F F tfor H O: 0.052 H O,dryS H O, 0S2 2 (11)

=

=F F

t i H O

for the rest of the components:

,

i i,dryS

, 0S

2 (12)

with FH2O,dryS , and Fi,dry

S corresponding respectively to the H2Oand component i molar ow rates in the recycled sweep streamcoming from the partially dehydrated sweep gas stream. Thesubscript = 0 indicates the axial position at the entrance of thefeed stream to the PBMR that is the position where the sweepstream exits the PMBR.The mass balance in the mixer unit (Figure 1) is

= + F F F i t(1 ) ,i i i,recirculatedS ,dryS ,FSG (13)where is the recirculation factor of the sweep gas streamdened as the fraction of the molar ow rate of the sweep gasstream that is recycled to the PBMR to the total molar ow rateof the sweep gas stream. took values ranging from 0 for norecirculation to 1 for total recirculation. Fi,FSG is the molar owrate of each component i in the fresh sweep gas (FSG) stream.The FSG stream was formed only by H2/CO/CO2 and at t = 0Fi,recircualtedS = Fi,FSG. Attending to Figure 1, it can be seen that

Fi,recircualtedS = Fi,=tao

S , which is the boundary condition for the

Figure 1. Diagram of a PBMR system with partial sweep streamrecirculation.



sweep gas stream at the entrance of the catalytic PBMR and atany time t (being =Tao also the exit position of the feedstream). The boundary condition at the entrance of thecatalytic PBMR for the feed stream is

= =F F i t,i i, 0F ,0F (14)

The set of equations in this section (eqs 1114) were addedto the mathematical model equations described in section 2.1and in Diban et al.23 and were implemented in the simulationsoftware Aspen Custom Modeler v2004.1 (Aspen Technology,Inc., Cambridge, Massachusetts U.S.A.).The characteristic dimensions of the PBMR and operational

conditions employed in the simulations are summarized inTable 1. The total pressure of the sweep gas, PS, was the sameas in the feed side, PF, to keep partial pressures of reactants(H2/CO/CO2) similar in the feed and sweep gas sides, thusminimizing the driving force of the mass transport of thereactants and the consequent reactant losses to the sweep gasside. The operational conditions of temperature, pressure andreactants molar ratio in the feed stream were selected in orderto give the highest DME yields in a PBR with the bifunctionalcatalyst CuO-ZnO-Al2O3/-Al2O3 according to previousresults.26,27 The operational variables studied were (i) themolar ow rate of the FSG stream in terms of COx (CO +CO2) composition,FCOx,FSG, and (ii) the recirculation factor, ,of the sweep gas stream leaving the PBMR.2.3. Membrane Characteristics. As previously indicated,

in the present application zeolite membranes were selectedbecause they could withstand with the demanding experimentalconditions used in the catalytic PBMR in the presentapplication (temperature of 275 C and pressure 40 bar).Particularly, the zeolite materials ZSM5, MOR and SIL weretested by Espinoza et al.15,16 for in situ H2O removal in aPBMR conguration for FT reactions that employed opera-tional conditions very similar to those used in a catalytic PBRfor DME synthesis. The inuence of the mass transportproperties of these zeolite membranes on the processperformance of the present system was evaluated theoreticallyin Diban et al.23 Due to the poor fabrication reproducibility ofthe zeolite membranes,28 it was reported that these membranespresented a wide range of values of H2O permeance, H O27 , andH2O/component i selectivity, SH2O/i. Diban et al.

23observedthat the use of real zeolite membranes with low H O27 and highSH2O/i led to important CO2 conversions in systems operatingin once-through mode and constant feed and sweep gasvelocities despite the yield of DME was reduced almost 50% incomparison to that attained in a PBR. Therefore, the zeolitemembranes with low H O27 and high SH2O/i characteristics (seevalues in Table 1) were selected in the present work to addressthe inuence of the recirculation of the sweep gas stream on theprocess performance. It must be noted that the permeancevalue used in Table 1 for DME was actually for C8hydrocarbons,15,16 and thus, the results herein presented areconsidered a rst approximation. Due to the higher DMEpolarity, water solubility, etc. in comparison to thoseparameters for octane (C8), it is expected that DME permeancewould be higher than octane permeance. Finally, the simulationof the PBMR performance with an ideal zeolite membraneallowing only H2 and H2O permeance (size exclusionmechanism hypothesis) with the same H2O permeance as the

real zeolite membrane previously considered (see character-istics in Table1) was also conducted.2.4. Denition of the Process Performance Parame-

ters. To evaluate the eectiveness of the recirculation of thesweep gas stream in the catalytic PBMR on the processperformance, the conversion of CO2 (XCO2) and the yields ofthe main product DME (YDME), the intermediate MeOH andthe byproduct HC (YMeOH and YHC, respectively) were denedsimilarly to Rhode et al.19 in eqs 15 and 16. This denitionaccounted both for component losses to the permeate sideand/or cofeeding to the feed side.

=| |

|

= =

=

F F j A

F Fii i i

i i

F0

FTao m

F0 tmb, (15)

= | +

|

=

=

n F n j A

F FYi

i iF

cF

Tao c m

CO 0 tmb,CO

i i

x x (16)

=

Figure 3a depicts the change of H2O partial pressure in thefeed and sweep gas sides within PBMR position, , at sweep gasow rates of 0.06 and 0.60 molCOxh

1. This plot shows that atFCOx,FSG = 0.06 molCOxh

1 the sweep gas stream is rapidlysaturated with H2O at the entrance of the PBMR ( = 40 gcathmolCOx

1) and the partial pressure of H2O in the feed side,PH2OF ,

remains very similar to that in a PBR, indicating that H2O is noteectively removed from the feed side. According to eq 2, thehigh H2O pressure displaced the WGS reaction toward theformation of CO2 and caused that the partial pressure of CO2in the feed side, pCO2

F , was always above 5 bar (the initial value

in the entrance of the PBMR) in Figure 3b. When FCOx,FSGincreased, the H2O partial pressure dierence between the feedand sweep sides, that is the driving force of the H2O masstransport through the membrane, also increased. Therefore,FCOx,FSG values higher than 0.06 molCOxh

1 reduced signi-

cantly the presence of H2O in the feed stream. To keep PCO2F

below 5 bar in Figure 3b and thus to achieve positive XCO2, it

was found in Figure 3a that the maximum PH2OF allowable in the

feed side is 0.58 bar.Regarding the product yield, it is observed in Figure 2a,b that

YDME decreased when FCOx,FSG increased in the range 0.061.80molCOxh

1, contrary to YMeOH and YHC trends. Particularly, atlow FCOx,FSG, the partial pressure of MeOH in the feed side,

pMeOHF , rose up to about 1.3 bar and decreased as FCOx,FSGincreased, as illustrated in Figure 4. The higher the values ofpMeOHF , the higher is the dehydration of MeOH into DME in eq5. As it can be seen in Figure 4, pMeOH

F reaches the highest value

Figure 2. Inuence of the sweep gas ow rate in the PBMR on theprocess performance (CO2 conversion and products yields (DME,MeOH and HC)). Comparison between the PBMR (solid color lines)and a PBR (black dotted lines).

Figure 3. Inuence of the sweep gas ow rate, FCOx,FSG (0.06 and 0.60

molh1), on the partial pressure of the H2O in the feed (solid colorlines) and sweep gas (dotted color lines) sides and of the CO2 in thefeed side with the position, , in the PBR (black line) and PBMR(color lines) at the time on stream of 30 h.

Figure 4. Inuence of the sweep gas ow rate, FCOx,FSG (0.06 and 0.60

molh1), on the partial pressure of the MeOH in the feed (solid lines)and sweep gas (dotted lines) sides with the position in the PBMR, , atthe time on stream of 30 h.



at FCOx,FSG = 0.06 molCOxh1 because the dierence between

the feed and sweep MeOH pressures is the lowest at this sweepgas ow rate and increases when increasing FCOx,FSG, similar to

what happened to the H2O partial pressures in Figure 3a. Thelow selectivity of the zeolite membrane to MeOH withSH2O/MeOH values around 2 (see Table 1) caused a highpermeation of MeOH from the feed toward the sweep side. Tofavor higher YMeOH values, MeOH should be retained in thefeed side as much as possible and consequently, low values ofFCOx,FSG would be recommended. However, at low FCOx,FSG,

XCO2 are very low or even negative. The solution to this low

XCO2 values comes from using FCOx,FSG > 0.06 molCOxh1. To

enhance DME yields by minimizing MeOH losses from thefeed side when using FCOx,FSG > 0.06 molCOxh

1, the

recirculation of the sweep stream according to the systemexplained in section 2.2 is evaluated in section 3.2.3.2. Inuence of the Sweep Gas Stream Recirculation.

Figure 5 shows the eect of the molar ow rate of the FSGstream in the mixer unit, FCOx,FSG, between 0.18 and 0.60

molCOxh1, and the recirculation factor, , between 0 and 1, on

the average XCO2, YDME, YMeOH and YHC values after 30 h of time

on stream. The increase of the sweep gas molar ow rate

allowed higher XCO2 values, as it was already observed in once-through mode operations, thus favoring the CO2 capture in theprocess. Albeit XCO2 remains almost independent of at

FCOx,FSG = 0.18 molCOxh1, at FCOx,FSG, =0.60 molCOxh

1, XCO2decreased 27% when increasing from 0 to almost 1. Theeciency of the H2O removal at high FCOx,FSG is much more

pronounced than at low FCOx,FSG; therefore, the eect of therecirculation of sweep gas may aect more signicantly theXCO2 results at high FCOx,FSG, values.YDME values in Figure 5b increased always at increasing

while YMeOH decreased (Figure 5c). By increasing , the MeOHconcentration in the sweep gas stream increased reducing thedriving force for the MeOH mass transfer across the membraneas expected. MeOH was then retained in the feed stream andcould be transformed into DME. This eect is better illustratedin Figure 6. In this gure, the values of pMeOH

F and pMeOHS along

the position of the PBMR, , are depicted for dierentrecirculation factors, , for FCOx,FSG of 0.18 molCOxh

1. It isclearly observed that the increase in the values of from 0 upto 0.975, leads to a noticeable increase in the values of pMeOH

F

and pMeOHS in the PBMR. Furthermore, the driving force for the

mass transfer of MeOH, that is the dierence between thevalues of pMeOH

F and pMeOHS , is signicantly reduced by

Figure 5. Inuence of the recirculation factor, , and sweep gas molar ow rate in terms of COx composition, FCOx,FSG (0.18 and 0.60 molh1), on

(a) the CO2 conversion (XCO2), (b) DME yield (YDME), (c) MeOH yield (YMeOH) and (d) HC yield (YHC). The feed gas molar ow rate FCOx,0F was

0.06 molh1.



increasing . The lower driving force reduced considerably thepermeation of MeOH from the feed phase toward the sweepside at higher values of and it is negligible in the case of =0.975. It is worthy to note that at = 0.975, pMeOH

F in thePBMR reaches values similar to those found in a PBR.The formation of the undesired paran byproducts has to be

taken into account seriously during the analysis of the processperformance. Figure 5d shows the high inuence that the sweepgas ow rate exerted on YHC. The change in FCOx,FSG, from 0.18to 0.60 molCOxh

1 at = 0.975, almost doubled the value ofYHC (from approximately 8 to 15%) but the YDME change wasless noticeable (from 27 to 31%). The formation of HCsprovokes deactivation of the catalyst by coke deposition on theactive sites of the metallic function,29 limits the formation ofMeOH and hinders the selective recovery of DME from the gasstream. Therefore, the formation of HCs should be minimized.In Figure 7, a comparison of the values of XCO2, YDME and

DME between a catalytic PBR and a PBMR conguration underdierent working conditions is presented. As it has beenpreviously indicated in section 3.1, a PBR under the operationalconditions of reactants composition, pressure and temperatureemployed in the present work does not allow the conversion ofCO2 into products; instead, CO2 was formed (see the negativevalues of XCO2). The use of a PBMR conguration at FCOx,FSG =

0.18 molCOxh1 gave positive XCO2 values around 25%. At this

sweep ow rate, the YDME in a PBMR increased from almost18% by working without recirculation of the sweep stream ( =0) to 26% using a recirculation factor = 0.9 and were alwayshigher than in a PBR (16%). The improvement in the YDMEwas caused, as it was previously explained, by the increase of theMeOH concentration in the recycled sweep stream thatreduced the driving force of MeOH transfer from the feedside to the sweep side of the PBMR. This benetted theconversion of MeOH into DME and thus YMeOH dropped fromapproximately 29 to 11% and the DME selectivity DMEincreased from 33 to 55%, similar to the SDME in a PBR. It isimportant to remark the high increase of YHC, which rose from

3.9% in a PBR to 8.2% in a PBMR working at 0.18 molCOxh1.

This was attributed to the reduction of the H2O content in thefeed side. The H2O removal improved simultaneously the CO2conversion and the HC formation. This was in agreement withthe ndings by Sierra et al.10 that observed that cofeeding H2Oin the feed stream at 0.2 H2O/syngas molar ratio led to lowerdeactivation by coke deposition due to a lower HC formationin comparison with systems without H2O cofeeding. Despitethe high values of YHC, the DME in a PBMR with sweep gasrecirculation are still comparable to those in a PBR. However,as it was previously explained, a high formation of paranbyproducts has to be avoided because not only it deactivatesthe catalyst but also adds diculty to the ecient separation ofthe desired DME from the gas streams exiting the PBMR.Therefore, paran formation must be reduced.Finally, the performance of a PBMR working with an ideal

zeolite membrane at a FCOx,FSG = 0.18 molCOxh1 and in once-

through mode operation, is presented in Figure 7. It can beseen that the PBMR at high recirculation factors simulate theconditions to approach the performance results to that of aPBMR that uses an ideal zeolite membrane. The quest for theideal zeolite membrane for selective H2O removal seems veryunrealistic as the hydrophilic character of the zeolitemembranes applicable in the present system also favor theMeOH permeance in addition to that of H2O. At the sight ofthese results, it seems that PBMRs working with low sweep gasmolar ow rates (FCOx,FSG, of 0.18 molCOxh

1) and reasonablerecirculation factors ( = 0.9) resulted in a compromise amongthe results of XCO2, YDME and DME. Furthermore, theseconditions reduce the consumption of reactants in the sweepgas stream, soften the recirculation settings and thereforereduce the costs of this stage. However, a detailed evaluation ofthe costs and technical feasibility of this conguration must befurther addressed.In dierent simulations, independently of the operation

conditions (pressure, ow rate, recirculation and ideal/realmembrane selectivity) the values of YDME and YMeOH reachedasymptotic values of 30 and 10%, respectively (data notshown). However, XCO2 could increase up to values >80% at thecost of the raise of YHC. Further enhancements of the processperformance (high XCO2 and YDME values and low YMeOH and

Figure 6. Inuence of the recirculation factor, , on the partialpressures of MeOH in the feed, pMeOH

F (solid line), and sweep streams,PMeOHS (dotted line), with the position in the PBMR, , at the time on

stream of 30 h. The FSG molar ow rate in terms of COx composition,FCOx,FSG was 0.18 molh

1 and the feed gas molar ow rate FCOx,0F was

0.06 molh1. The entrance of the sweep gas stream was on = 40 gcathmolCOx

1. The eect of on PMeOHF was evaluated in comparison to

the PBR.

Figure 7. Results of the simulation of CO2 conversion DME yield andDME selectivity under PBR and PBMR with a real zeolite membrane(i) without recirculation ( = 0), (ii) with high recirculation factor (= 0.9) and (iii) PBMR with an ideal zeolite membrane withoutrecirculation ( = 0), at a sweep gas molar ow rate FCOx,FSG = 0.18

molh1. The feed gas molar ow rate FCOx,0F was 0.06 molh1.



YHC) will inescapably pass through the improvement of thecharacteristics of the bifunctional CuO-ZnO-Al2O3/-Al2O3catalyst employed in the present system.

4. CONCLUSIONS

The present work aims at demonstrating the CO2 capture andrecovery in the DME synthesis performed in a packed bedcatalytic membrane reactor (PBMR) that incorporates a zeolitemembrane for in situ H2O removal. While in the traditionalpacked bed reactor (PBR) CO2 was formed, the zeolitemembrane of the PBMR allowed in situ H2O removal from thefeed side and therefore the water gas shift (WGS) reaction wasdisplaced toward CO2 conversion. However, the low selectivityof the zeolite membranes toward H2O permeation could causean important reduction of the DME yield in comparison to thatobtained in a PBR depending on the working conditions.In this work, it is concluded that a wise conception of the

operational conditions such as the control of the sweep gas owrates and the recirculation of the sweep gas stream couldovercome the practical limitations on building an ecientPBMR for CO2 transformation into DME, given the dicultyto produce a hydrophilic zeolite membrane with selectivecharacteristics similar to those of an ideal membrane andhydrothermally stable. The PBMR conguration proposed inthe present work gives the following outstanding advantagesthat make the PBMR conguration to surpass the PBRperformance: (1) Increases the conversion of CO2 up to 85%when high sweep gas molar ow rates (FCOx,FSG > 0.60 molCOxh1) are employed. (2) The recirculation of the sweep gasstream reduces the loss of MeOH across the membrane towardthe sweep gas side due to a reduction of the MeOH drivingforce, and thus the yield of DME obtained in the PBMR canreach values around 30% at high recirculation factors. Thevalues of YDME in the PBMR were always higher than thoseachieved in a PBR (16.3%) and similar DME (55%). (3) Therecirculation of the sweep gas stream allows for savings in theconsumption of reactants (H2/CO/CO2).In summary, the strategic design of the operational mode in a

PBMR would favor the presence of the same components inthe feed and sweep gas side, leading to a minimization of thedriving force of the mass transport through the membrane andthus, would allow a higher conversion of reactants (mainly H2and CO2) and intermediate products (MeOH) toward theaimed DME in the feed side of the PBMR.A careful evaluation of the technical diculty of the

recirculation and the high costs associated with the energy ofcooling, heating and compressing the sweep gas stream must bedone. Anyway, although challenging, the quest for a stable andH2O selective membrane still remains the preferable option.Additionally, a yield of the byproduct parans (HCs) higher

than 4% is not acceptable in this system. The formation of HCspromotes the catalyst deactivation by coke deposition andhinders the MeOH formation. Attending to the results of thesimulations in the present work, further improvements of thecatalyst selectivity to reduce the formation of HCs becomes aresearch priority to improve the process eciency andtherefore, this study is currently in progress.

AUTHOR INFORMATIONCorresponding Author*N. Diban. E-mail: [email protected].

NotesThe authors declare no competing nancial interest.

ACKNOWLEDGMENTSFinancial support by postdoctoral grant JCI-2011-10994 fromthe Spanish Ministry of Science and Innovation and by projectsCTQ2008-00690, CTQ2010-19188 and ENE2010-15585 iskindly acknowledged.

NOMENCLATUREAm = eective membrane area (m

2)F = molar ow rate (mol h1)Ftmb = transmembrane ux in eqs 12 and 13 (mol m

2 h1)j = membrane partial ux dependent on time and space time(module position) (mol m2 h1)nc = number of carbon atoms in each componentp = partial pressure (bar)P = total pressure (bar)Q = volumetric total ow rate (m3 h1)r = reaction rate (mol.h1.(g of catalyst)1)R = ideal gas constant (bar m3 mol1 K1)S = membrane permselectivity (dimensionless)t = time (h)T = temperature (K)

Greek Letters = recirculation factorDME = selectivity of the process to the formation of DME = space time ((g of catalyst)hmolCOx

1)Y = products yield (%)X = reactants conversion (%)

Other Symbols7 = membrane permeance (units in the model, molbar1m2h1; units in the text, molPa1m2s1)

SubscriptsCO = carbon monoxideCO2 = carbon dioxideCOx = sum of CO and CO2 reactantsDME = dimethyl etherdry = fraction of the sweep gas after partial H2Ocondensation in Figure 1FSG = fresh sweep gas (new sweep gas introduced in themixer unit in Figure 1)HC = hydrocarbons (parans)H2 = hydrogenH2O = wateri = component (reactant or product)MeOH = methanolrecirculated = stream leaving the mixer unit in Figure 1 thatenters the PBMRTao = feed stream exit position0 = initial

SuperscriptsF = feed sideS = sweep gas or permeate side

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