Synthesis of Dimethyl Ether from CO Hydrogenation: a Thermodynamic Analysis of the Influence of Wate

JumuliddrheaGa ChemitnyJournal of Natural Gas Chemistry 14(2005)47- 53SCIENCE PRESSSynthesis of Dimethyl Ether from CO Hydrogenation:a Thermodynamic Analysis of the Influenceof Water Gas Shift. ReactionGuangxin Jial2,Yisheng Tan'，Yizhuo Hanl*1. State Key Laboratory of Coal Conuersion, Institute of Coal Chemistry, Chinese Acaderny of Sciences, Taiyuan 030001, China2. Graduate School of the Chinese Academy of Sciences, Beiing 100039, China[Manuscript received November 20, 2004; revised March 09, 2005]Abstract: Three reactions involved in dimethyl ether (DME) synthesis from CO hydrogenation: methanolsynthesis reaction (MSR), methanol dehydration reaction (MDR) and water gas shift reaction (WGSR)are studied by thermodynamic calculation. For demonstrating this process in detail, three models, MSR,MSR+MDR, MSR +MDR+ WGSR, are used. Their basic characteristics can be obtained by varying widelythe ratios of H2 to CO in the feed (no CO2). Through thermodynamic analysis a chermical synergic effectobviously exists in the second and third models. By comparison between two models it is found thatWGSR plays a special role in dimethyl ether synthesis. It is possible for the two models to shift one tothe other by regulating CO2 concentration in feed. For Model 2, the selectivity for DME in oxygenates(DME+ methanol) does not change with the ratio of H2 to CO.Key words: dimethyl ether, thermodynamic, syngas, synergic effect, water gas shift reaction1. Introductiontheoretical support to kinetic study. However, most ofresearches for DME synthesis focus on the technolog-Dimethyl ether has been known as an ultra-cleanical conditions, kinetics of chemical reaction [4,5], thefuel used for diesel engine, household, power genera-stability of catalyst [6- 8] and scale-up of the processtion and other purposes. Nowadays, DME is produced[9-11]. Some thermodynamic analysis to this processmainly from synthesis gas through methanol synthesismostly investigated the effects of temperature, pres-and methanol dehydration known as the two-step pro-sure and H2:CO ratio in syngas [12,13] but rarely an-cess [1- 3]. When the two reactions take place withinalyzed the role of water gas shift reaction in DMEone reactor using composite catalyst, namely the one-synthesis process.step process, the equilibrium conversion of syngas im-In this paper, three models, MSR, MSR + MDRproves significantly due to the synergic effect betweenand MSR+MDR+ WGSR, are. used for demonstrat-reactions. Because most of composite catalysts haveing the characteristics of DME synthesis process. Thealso shift activity, water gas shift reaction would pro-influence of a wider range of H2:CO ratio in the feedceed simultaneously along with methanol synthesisvarying from 9.0 to 0.25 is studied by thermodynamicand methanol dehydration. The thermodynamic cal-calculati中国煤化工among the mod-culation for the dimethyl ether synthesis process isels, it is|Y片CNMH GIS chemical syn-useful for understanding the basic characteristics ofergic effect mn dME syntnesis process and WGSR candimethyl ether synthesis process, and it can provide aincrease the overall syngas conversion via synergic* Corresponding author. Tel: (0351 )4049747; Fax: (0351)4044287; E-mail: hanyz@sxicc.ac.cn.This work is supported by the National High Technology Research and Development Prograrm of China (Grant: 2002AA529070).48Guangxin Jia et al./ Journal of Natural Gas Chemistry Vol. 14 No.1 2005effect, but the effect of its product CO2 is unexpected.is permissible thermodynamically because of the pres-In the present paper, the role of WGSR is investigatedence of CO and H2O. Whether WGSR proceeds or notspecially based on the thermodynamic analysis of thedepends on the catalyst used.dimethyl ether synthesis in order to explore the possi-It should be stressed that there is a great con-bility of alviating the negative effect of this process.troversy on the catalytic mechanism of methanol for-mation in Model 3 because MSR and WGSR coex-2. Theoryist in the same systern. Some authors insist thatmethanol is derived from CO hydrogenation [4] while2.1. Foundation of reaction modelsome think that methanol is produced by CO2 hy-drogenation [17], and others believe both CO hydro-For the convenience of investigating the thermo-genation and CO2 hydrogenation contribute to thedynamic characteristics of dimethyl ether synthesismethanol formation at the same time [18]. Neverthe-process, three models are defined as follows,less, the number of independent reactions of methanolModel 1: single reversible reaction system includingsynthesis process in which CO2 or H2O participatesMSRis only two by phase law. Therefore, by selecting anytwo of three reactions, CO hydrogenation, CO2 hydro-C0+2H2台CH3OHgenation and water gas shift reaction as independentreactions, one can obtain the same effluent composi-Model 2: successive reversible reaction system con-tion thermodynamically. In this paper, it is assumedsisting of MSR and MDRthat CO hydrogenation reaction and water gas shiftreaction take place in parallel and all CO2 producedCO+2H2白→CH3OHis contributed by the water gas shift reaction.2CH3OH = = CH3OCH3+H2O2.2. Calculation processModel 3: complex multiple reaction system consistingof MSR, MDR and WGSRIn this paper, the calculation process of Model 3 isselected as an example and the others can be obtainedCO+2H2 = = CHgOHanalogically.2CH3OH≈CH3OCH3+H2OWhile DME synthesis process reaches equilib-CO+H20卡= CO2+H2rium, three reactions are in equilibrium simultane-ously:It should be pointed out that each model has itsown range of application. For Model 1, the CO hydro-K1 =fCH3OH k。 _ fDME:fH20K3=fo2fH2genation process is assumned as the unique approach offcof落。fCHsOH .fcofH2Omethanol formation because the reactants merely con-sist of CO and H2. For Model 2, the presence of COThe f; is the fugacity of component i, given byin feed and H2O in the effluent makes it possible con-the combination of SRK equation and correspondingduct WGSR. Although many papers have referred tothermodynamic relationship formula [19].this model, they failed to point out the reason for the .Kfj is the equilibrium constant of reaction j, ob-absence of WGSR [14,15]. The WGSR needs two pre-tained by Equilibrium Calculator, a non-commercialconditions to undergo, the presence of WGSR catalystsoftware programmed by Dr. Yamazaki. Dimethyland the possibility in thermodynamics. If Model 2ether, methanol and carbon dioxide are consideredemploys a kind of catalyst that has no activity, WGSRas three independent components, the other compo-will not take place. This is very similar to the casenents in effuent can be obtained by balancing theof Fischer -Tropsch reaction using cobalt catalyst thatequations of C. H and O. The Gauss-Newton arith-catalyzes only the CO hydrogenation to hydrocarbonsme中国煤化Ie non-linear equationbut dose not convert the water to hydrogen. When|YHC N M H G of three independentthe iron- based catalyst is used for Fischer Tropsch re-components ranging trom U to 1 are those given ran-action, WGSR will proceed and the main by-productdomly by rand function in MATLAB. After the rightwill no longer be water but CO2 [16]. As for the possi-solutions of equation-group are obtained, CO conver-bility on the ground of thermodynamics, this reactionsion, selectivity and yield of products and equilibriumJournal of Natural Gas Chemistry Vol. 14 No.1 200549components are calculated by the following formulas:To investigate the CO conversion process, theinfluences of CO concentration in feed on the yieldsCo。F.oo-P.yCoof DME, methanol, CO2 and DME+MeOH in threeFo.9Comodels are shown in Figure 2. It is found that with2F . YDMESpME =F.yOO-F.yCothe increase in CO concentration in feed the summa-tive equilibrium yield of methanol and DME in threeF.yiSi =Fo.yCo- F.ycomodels decreases while the equilibrium yield of CO2increases and then starts to decreases after reaching(i=MeOH, CO2)to its maximum.Yd; = Cco.S;1.0(i=DME, MeOH, CO2)0.9(2)Yd(DME+MeOH) = Cco . SDME + Coo . SMeOH-←-Model I MeOH0.8 E-o-Model 2 DME+ -Model2 MeOH0.7o -Model 3 DME3. Results and discussion- Model3 MeOH0.60 Model 3 CO2All calculations in the present paper are per-formed on condition of 5 MPa, 260 °C.0.43.1. Process characteristics of three models0.2 .This section demonstrates the effect of CO con-0.1centration in feed (no CO2) on the three models.Figure 1 shows the effect of CO concentration in0.2 0.3 0.4 0.5 0.6 0.7 0.8feed on CO conversion. It is seen that with increas-CO concentration in feeing CO concentration, the equilibrium conversions ofCO in both Model 1 and Model 2 decrease while that1.in Model 3 maintains a high level first and then has0.(b)a sharp decrease. It is found that Model 3 obtains一+ Model 1 MeOH，0.-- Model 2 DME+McOHa much higher CO equilibrium conversion than the+ - Model 3 DME+MeOHother two when the H2:CO ratio is 1:1.0.6E-0- -Model !- -0-Model30.4F0.803年8 0.7220.5 F0.20.30.5).7).8CO concentration in feed宫03年Figure2. Efect of CO concentration in feed on营equilibrium yields of (a) dimethyl ether,0.2 Emethanol and CO2 (b) DME +MeOH inthree modelsECondition中国煤化工ed0.2 0.3 0.4 0.5 0.6 0.7 0.HCNMHGCo concentration in feedThe calculation results for the effluent equilib-Figure 1. Effect of CO concentration in feed onrium compositions of Model 2 and Model 3 are shownequillbrium conversion of Co in threerespectively in Figure 3 and Figure 4. Differed frommodelsConditions: 260 "C, 5.0 MPa, no CO2 in feedFigure 1 and Figure 2 the distance between two adja-50Guangxin Jia et al./ Journal of Natural Gas Chemistry Vol. 14 No.1 2005cent lines in the two figures stands for the fraction ofsynergic effect in DME synthesis process, and manyone component. From Figure 3, the fraction of DMEprevious papers have emphasized this effect [20,21].reaches its maximum in the equilibrium compositionHowever, the higher yields of the target products arewhen CO concentration in feed is 0.33 in Model 2,more important than CO conversion for DME produc-while it appears at CO concentration in feed of 0.50tion. From Figure 2(b), if methanol+ dimethyl ether isin Model 3 as illustrated in Figure 4.thought as the target product [22], Model 2 gives thehigher total yield of methanol and DME than Model3 and Model 1 in H2-rich environment (CO concen-HOtration is less than 0.30). If DME is considered as the.8 ttarget product, Model 3 gives the highest equilibriumyield of DME in C0-rich environment (CO concen-tration is larger than 0.4). This result suggests that复0.6-Hthe syngas produced by Texaco, Shell and BGG-Lurgigasification plants is fit to the direct dimethyl etherproduction using Model 3 because of their higher CO0.4-/cconcentration, ranging from 0.50 to 0.70 [23].As for the reversible reaction, the driving force ofMDR depends only on the high methanol concentra-0.2-DMEtion or low water concentration in the system. ForModel 2, because water derived from MDR cannot beMeOHremoved from the system, the selectivity for dimethyl0.20.6).81.ether in oxygenates (DME and Methanol) is constantco concentration in feedFigure3. Effect of CO concentration in feed onat 0.7978 in the whole range of H2:CO ratio basedequilibrium composition of Model 2on thermodynamic calculation. In order to increaseConditions: 260 °C, 5.0 MPa, no CO2 in feedthe selectivity for DME in the oxygenates, it is essen-tial to remove water from the reaction system throughWGSR as described in Model 3. However, it is foundfrom Figure 2(a) and Figure 2(b) that the addition ofWGSR brings the opposite effects to the synergy in0.8一diffrent regions. In the H2-rich region, the limiting/coreactant is CO and the presence of WGSR depletesthis limiting reactant, therefore, hurting the synergy.In contrast, in the CO-rich region, the limiting re-actant is H2 and WGSR replenishes this limiting re具0.4-actant, therefore, enhancing the synergy. Especiallywhen CO concentration in feed is higher than 0.50,the methanol concentration in the effluent is closeDME .to zero and CO2 concentration in the effluent approaches dimethyl ether concentration thermodynam-0.0bouu1ically.0.0.2 0.3 0.4).5.60.7From the above analysis, some problems absent inCO concentration in feedthe two-step DME synthesis process are brought forthFigure 4. Effect of CO concentration in feed onwith the use of the chemical synergic effect. There-equilibrium composition of Model 3fore, the analysis of the shortcomings and applicableranges tn Mndel 2 and Mndel 3 is necessary for theBased on the integration of five figures, the pro-con中国煤化工hyl ether. The firstcess characteristics of three models are generalized asprolYHC N M H Ge catalyst applicabil-follows. From Figure 1, Model 2 and Model 3 (es-ty. It has been reported that the catalysts preparedpecially Modet 3) give the higher equilibrium con-from Cu-rare earth alloys are very susceptible to deac-versions of CO than Model 1. This result becomes tivation by CO2, O2, and to a lesser extent, H2O [24-the primary proof of the presence of the chemical26]. This information suggests that the catalysts usedJournal of Natural Gas Chemistry Vol. 14 No. 1 200551in dimethyl ether process should have good water-1resistant and carbon dioxide-resistant properties. Thsecond problem is the carbon utilization. The typical--Case2case is Model 3 has the highest equilibrium conver-0.8sion of CO while the carbon utilization of Model 3 isonly 66.7%. Large amount of CO2 is produced withthe formation of dimethyl ether at a higher selectivityand the disposal of so much CO2 will be more com-plex and costly as compared to the two-step process.The third problem is CO2 cannot be separated easily0.4from the effluent because the liquefied dimethyl etherhas very good CO2 solubility [27].昏0.23.2. The role of CO2 in the process of dimethylether synthesis process.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8As far as the role of CO2 in dimethyl ether syn-CO2 concentration in feedthesis process (for the case of Model 3 in this section)Figure 5. Effect of CO2 concentrations in feed onis concerned, many researchers think that the addi-equilibrium conversion of Co as threemethods of CO2 addingtion of small amount of CO2 to syngas can keep theConditions: 260 °C, 5.0 MPaactivity of Cu-based methanol synthesis catalyst [28].Herein, the role of CO2 in the dimethyl ether synthesis0.process is investigated in a wide range by thermody-namics.- + Case1 DMECase 1 MeOHThere are many methods of adding CO2 to theCase 2 DMECase 2 MeOHfresh syngas. Two typical ways are selected for inves-Case 3 DMEtigating the effect of CO2 on dimnethyl ether synthesis--Case3MeOHprocess. One is adding CO2 directly at a constant ra-; 0.4tio of CO to CO2, namely, the diluting-type addition(Case 1). The other is regulating the ratio of CO2i0.3to CO at a constant concentration of H2, namely, thereplacing-type addtion(Case 2 and Case 3).? 0.2Case1:娟=v&o v8o2 =1-2唱Case2:唱=0.50 /(2o + v2o2)=1,30.4 0.5.6 0.7Case3:唱, =0.66 92/(v8o + v8o2)=2CO2 concentation in feedFigure 6. Effect of CO2 concentrations in feed onequllibrium yields of dimethyl ether andThe influences of CO2 in the three cases aremethanol as three methods of CO2 addingdemonstrated in Figure 5 and Figure 6. For Case1, it is found that with increasing CO2 concentrationin the feed, both the equilibrium conversion of COIt is apparent that the H2 concentration plays anand the equilbrium yield of DME decrease while theimportant role in the process of adding CO2. This canequilbrium yield of methanol increases slightly. CO2be seen from the three points a, b and c shown in Fig-addition has dual effects on the reaction system, oneure 5.中国煤化工d is fixed at 0.10,is depression of the WGSR, which may cause higherH2 condCNMHGthreepointsarewater fraction, and another is dilution of the reactant0.45, 0.5U and 0.67 respectively as calculated results.gas, resulting in a lower CO equilibrium conversionIt is found that CO conversion increases with increas-and a slow overall reaction rate due to lower partialing H2 concentration in the feed. Taking together thepressure of CO+H2 [27].six points shown in Figure 6 which are labelled as.52Guangxin Jia et al./ Journal of Natural Gas Chemistry Vol. 14 No.1 2005a',b',c',a", b" and c", the results indicate that COwhen the CO2 input is more than or equal to the CO2conversion, DME yield and MeOH yield grow with in-output. Therefore, the purpose of improving carboncreasing H2 concentration in the feed. Therefore, theutilization by the recycle of CO2 will not be practi-H2-rich envirnment is favorable for the process withcal and excess CO2 in the effluent must be separatedCO2 recycle.from the system by specific technology [27].In the process of dimethyl ether synthesis, the di-rection and extent of WGSR studied by thermody-3.3. Thermodynamic understanding of WGSRnamics can be judged by the ratio of CO2 input toin DME synthesis processCO2 output. The relationship between CO2 concen-It is emphasized that the present investigation istration in the feed and the ratio of CO2 input to CO2based on the ideal state that three reactions attain theoutput is shown in Figure 7. From this figure, it isthermodynamic equilibrium at the same time. How-seen that the ratios are reduced sharply at first andever, in practical reaction system, it is hardly realizedthen gradually become less than 1 in three cases withthat three reactions take place at comparable reactionincreasing of CO2 concentration in the feed. Whenrates under the same reaction conditions due to thethe ratio equals 1 (CO2 output equals CO2 input),fact that optimal reaction conditions for each reac-WGSR cannot take place according to the thermo-tion are different. The present thermodynamic studydynamics, and the dimethyl ether synthesis processwill help one exactly understand in a wider regiongoes along as Model 2. When the ratio is less than1, WGSR can take place in the opposite directionhow the practical reaction is controlled by the kinet-and CO2 becomes the reactant while CO becomes theics and thus take corresponding measures to deal withproduct, this state belongs to the process of CO2 hy-this controlling step.Previously reported work indicates that WGSRdrogenation as reported by Meshicheryakov et al. [12]can rapidly reach its equilibrium, and MSR kineticallyand Shen et al. [13]. When the ratio is more thanor equal to 1, CO2 can be considered as an inert gascontrols the overall reaction rate in the CO-rich regionlike nitrogen and it can only be accumulated but notwhile MDR does so in the H2-rich region [21]. As acollective result, the maximum methanol equilibriumconverted to DME or methanol.yield can be reached between 1:1 and 2:1 of H2:COratio. It indicates that the practical DME synthesisreaction can fall between Model 2 and Model 3 de-pending on where the feed gas is generated. For nat-ural gas derived syngas that contains more hydrogen,the DME production should be operated at Model 2or close to Model 2, because the stoichiometric ratioof H2 to CO for Model 2 is 2 and the by-product wateris hydrogen containing. As for the coal derived syn-gas with high CO concentration, Model 3 will be moreCase 2applicable for it has the highest synergetic effect and戛七Case3)Case !best utilization of H2 and CO rich syngas althoughthe CO2 emission is unavoidable. By regulating CO2in the feed, one can make the system take a properposition between Model 2 and Model 3 and thus suitCO2 concentration of in feethe production process to the feedstock.Figure 7. Relationship between CO2 concentrationin feed and the ratio of CO2 output to4. ConclusionsCO2 inputConditions: 260 °C, 5.0 MPa中国煤化工，both Model 2 andIt is suggested from the above analysis thatMHC N M H Gf methanol synthesisWGSR can be weakened, stopped and even reversedreaction imposed by thermodynamics, resulting in sig-as long as enough CO2 is added to the fresh syngas.nificant synergic effect. The equilibrium selectivity ofIt also means that the process of adding or recyclingDME in the oxygenates for Model 2 does not changeCO2 is .the process of shifting Model 3 to Model 2with the change of H2:CO ratio. Model 3 can obtainJournal of Natural Gas Chemistry Vol. 14 No.1 200553much higher equilibrium conversion of CO and equi-[5] Ng K L, Chadwick D, Toseland B A. Chem Eng Sci,librium yield of DME than the other two in CO-rich1999, 54: 3587environment especially in the range from 0.50 to 0.66,[6] Peng X D, Parris Gene E, Toseland B A. USP 5 753716, 1998while Model 2 can get a higher yield of DME+MeOH[7] Tatsuya T, Ken-ichi Y, Tormoyuki I et al. Appl Catalthan others. By investigating the effect of CO2 onA, 2000, 191: 201dimethyl ether synthesis process, it is found that CO2[8] Ge Q J, Huang Y M, Qiu F Y. React Kinet Catal ltt,can be considered as an inert gas and it can only be1998, 63(1): 137accumulated but not converted to DME or methanol.[9] DOE Topical Report. Liquid Phase Dimethyl EtherTherefore, by altering CO2 concentration in the feed,Demonstration in the LaPorte Alternative Fuels Dethe dimethyl ether synthesis process can be shiftedvelopment Unit. Cooperative Agreerment No. DE-from Model 3 to Model 2 to make the best use of theFC22-92PC90543 January, 2001feed gas. To depress the CO2 emission and improve [10] Yotaro O, Norio I, Takashi 0 et al. NKK Technicalthe carbon utilization efficiency, the methanol cata-Revivew, 2001, 85: 23lyst without WGSR activity should be employed.[11] Voss B, Joensen F, Hansen J B. WOP 9 623755, 1996[12] Meshcheryakov V D. 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