Model development of a membrane gas permeation unit for the separation of Hydrogen and Carbon Dioxide

Transcription

1 Model development of a membrane gas permeaton unt for the separaton of Hydrogen and Carbon Doxde D. F. Rodrgues Department of Chemcal Engneerng, Techncal Unversty of Venna, Austra and Department of Chemcal and Bologcal Engneerng, Insttuto Superor Técnco, Portugal In ths wor were developed three dfferent membrane gas permeaton unt models n Aspen Custom Modeller. These models were based n the soluton-dffuson mechansm and descrbe a sngle membrane module. Two of the three models bult, consdered a dscretzaton of the membrane s module whereas the other model consdered the membrane as a bloc. The models results were compared aganst expermental data measured from the separaton of carbon doxde and methane for valdaton. The model that presented tself to be the most sutable one was used on senstvty analyss calculatons for the separaton of hydrogen and carbon doxde wth a reverse-selectve PDMS membrane. The models that provded the best results n valdaton were the dscretzed ones. Keywords: Membranes, gas permeaton, Aspen Plus, Aspen Custom modeller, bo-hydrogen, hydrogen and carbon doxde separaton. Introducton Hydrogen s abundant n the unverse and has hgher energy content per weght unt than any other avalable fossl fuel. Thus, the demand for ths energy resource and ts consumpton are trends that n the past years have been ncreasng. Accordngly, the employment of renewable sources rather than fossl fuels n the producton of Hydrogen s an mportant step n the process of achevng a sustanable Hydrogen economy n the future. Large-scale Hydrogen producton generally occurs va steam methane reformng followed by the water-gas shft reacton, partal oxdaton or by the Kvaerner process. Hydrogen can also be produced by the electrolyss of water but, to date, only for a small percentage. Other alternatve ways to produce hydrogen are va bologcal processes such as extractng the gas from bomass through dar fermentaton and/or photo-fermentaton by bactera or the producton of hydrogen by algae. An up-andcomng way for the producton of Hydrogen from bomass n a non-thermal way s a twostage boprocess. Hyvoluton-project presents a bo-process consstng of a thermophlc fermentaton step followed by a photoheterotrophc fermentaton, both producng Hydrogen, Carbon Doxde and ntermedates. A great deal of research at the moment s concerned wth the selecton of mcroorgansms, optmsaton of yeld and rate of Hydrogen producton as well as reactor desgn. In order to obtan pure Hydrogen, Carbon Doxde has to be separated from the product gas. Due to the pressure level of the produced gas, vacuum swng adsorpton was ntally selected. However, durng project wor t turned out that for proper operaton of the thermophlc fermenter Hydrogen s partal pressure had to be reduced. Ths changed sgnfcantly the gas composton up to hgher percentage values of Carbon Doxde. Addtonally, the gas-upgradng system had to cope wth changng gas-flow rates snce the second fermentaton step (photo-heterotrophc fermenter) only produces Hydrogen durng day but not durng nght. Under these condtons, adsorpton operates wth hgh energy demand

2 and hgh Hydrogen losses. In addton to amne-scrubbng, whch needs a hgh heat demand for regeneraton of the absorptonlqud, membrane processes seem to be a good alternatve for upgradng the raw product gas from Hyvoluton-process. Process smulaton s used to select process routes by comparng the performance of dfferent unt operatons and desgn optons for the necessary process steps. It wll help durng ntegraton and optmsaton of the selected process route and provde necessary data for process engneerng and cost estmaton. Approach and worflow The goal of ths wor was to develop a membrane gas permeaton unt for the separaton of Hydrogen and Carbon Doxde. In order to acheve that goal t was necessary, frst of all, to defne whch transport mechansm should be modelled and whch type of membrane system should be used. Snce there were stll no real systems for the separaton of the bo-hydrogen because the process s stll n project phase, the man concept was to fnd a way to buld a general model that could be mproved later on wth the development of the project. Therefore a sngle module usng the soluton-dffuson as a transport mechansm was developed whch ddn t consder the effects from temperature gradents, pressure drop or membrane s geometry. However, three dfferent ways of descrbng the membrane module were consdered, gvng orgn to three dfferent models based on dfferent ways to descrbe the flow, wth or wthout dscretzaton of the membrane module. After ths step came the selecton of the software. The premse was that the models should run n AP and wth that n mnd some optons le developng the model wth the AP user models n connecton wth Excel or Fortran were studed. In the end the models were developed n ACM not only because t was already beng used n the project but also because t presented tself to have the versatlty needed. Snce that there was no expermental data avalable for the Hydrogen separaton, the frst models were buld for the Methane and Carbon Doxde system. After valdaton and testng for ths gaseous par and, assumng that the successful output for ths mxture could be extrapolated to the H 2 /CO 2 par, further calculaton were made. Bacground In the soluton-dffuson model, permeants dssolve n the membrane materal and then dffuse through the membrane along a concentraton gradent. The separaton occurs due to the dfference n rates of dffuson that each permeant has through the membrane s materal as well as the solublty of each permeant n the membrane s materal. The frst assumpton regardng the transport through membranes s that the fluds on both sdes of the membrane are n equlbrum wth the membrane materal at the nterface. Ths means that there s a contnuous gradent n chemcal potental from one sde to the other sde of the membrane. It s mplct that the rates of absorpton and desorpton at the nterface are much hgher than the rate of dffuson through the membrane. The pressure appled across a dense membrane s also consdered to be constant at the hgh pressure value. In gas permeaton, a gas mxture at a pressure p 0 s appled to the sde of the membrane, whle the gas at a lower pressure p s removed from the downstream sde of the membrane.

3 J J P po p (1) o Q p p (2) P Q (3) Model Development Table 2. Software Versons Table 1. Computer characterstcs Mcrosoft Wndows XP OS Professonal Verson 2002 Servce Pac 3 CPU Intel Pentum GHz RAM 512 MB Software Net Vsual Studo Compaq FORTRAN Aspen Custom Modeler (ACM) Verson MS VISUAL STUDIO.NET 2003 Compaq Vsual Fortran Professonal Edton Aspen Plus Model 1 The frst model s a smple model consderng the mass balance n one bloc wthout any dscretzaton as t s shown n Fgure 1. The nputs gven to the model are temperature, pressure and composton as well as, pressure, membrane area and permeances. The assumptons made n ths model are that the total pressures n the, and are constant; the transport mechansm s the Soluton-Dffuson mechansm; permeablty and permeance are ndependent of pressure; the partal pressure of the component n the s the partal pressure before the membrane; the partal pressure of the component n the s the partal pressure after the membrane; perfect mxng n the sde s assumed; there s no pressure drop from to ; temperature and volume are constant. The nputs gven to the model are

4 temperature, pressure and composton as well as, pressure, membrane area and permeances Fgure 1. Schematc drawng of Model 1 F F F (4) F y, F y, F y, (5) J F A y membrane, (6) y, y, J Q P y, y, ln y, P (7) Model 2 In ths model the membrane module was broen down nto cells of equal length and each cell behaves le the smple model. Hence, for each cell the assumptons made for Model 1 apply. The number of cells s defned n the code and t s possble to change. The nputs gven to the model are temperature, pressure and composton as well as, pressure, membrane area and permeances. The condtons for module =0 were also defned. The condtons of =0 are the s condtons and for the they equal zero. Fgure 2. Schematc drawng of Model 2

5 F F 1 1 F F (8) F 1 y 1, F 1 y 1, F y, F y, (9) J F y, (10) A y, y ln y 1, J Q y 1,, y, P P (11) Model 3 In ths model the dscretzaton tools of ACM were used to create yet another dscretzed model. Usng ths dscretzaton method on whch the partton of the membrane s mplemented by ACM, the user only has to wrte n the code the dfferental equatons of the model. It s also possble to change the node spacng so that certan parts of the membrane are dvded nto smaller parts than others,.e. the dscretzaton method, wthout changng the code of the model. In ths model s case, snce t s a smple model, such feature was not analyzed. However, the constructon of a model on whch the dscretzaton s not mplemented by the user s useful to confrm the results obtan wth Model 2. The nputs gven to the model are temperature, pressure and composton as well as pressure, membrane area and the constant parameter. The condtons for module when x=0 were also defned n the code. The condtons of x=0 are the s condtons and for the they equal zero. Fgure 3. Schematc drawng of Model 3 d R R R P yx Rx Q yx p yx P p d (12) d P R R P P y P Q y p y p d x x x x (13) d H dx (14) d y dx P R R P P x Px y x p y x p (15) H Q (16)

6 Results The expermental data used to compare the model s results wth a real system are taen from the lterature and the module used to obtan the expermental data was a hollow fbre module. For each model, three sets of expermental data are compared. Frstly, flow vs. flow, flow vs. flow and fnally composton vs. flow. Snce the membrane from the expermental set-up was a reverse-selectve membrane the composton n the, n terms of methane, concentraton s analyzed. In order to calculate the permeances, the flow through the membrane of the expermental setup of each pure component separately was studed. The pressure of the was ncreased and the pressure of the and rate were measured. The permeances calculated can be found n Table 4 and as well as the membrane s characterstcs. The results for the dscretzed models were obtaned usng 100 modules and 100 nodes of Models 2 and 3, respectvely. Concernng the flow the model that has the closest results to the expermental data s Model 3, followed by Model 2 whch presents smlar results. The same happens wth the results for the flow. Model 3 presents the most approxmate results to the expermental data, followed by Model 2, but n general, all 3 models gve almost smlar results. As far as the concentraton of methane n the s concerned, both dscretzed model present the same results and follow the same trend as the expermental results. Only Model 1 gves a dfferent trend. Table 3. Membrane propertes and component permeances Number of fbers 800 Length (m) 0,38 D (mm) 0,40 At (m²) 0,38 Q(CH 4 ) (mol/h.bar.m²) 2,00x10-4 Q(CO 2 ) (mol/h.bar.m²) 8,27x10-3 Fgure 4. Expermental set-up of the membrane used n the valdaton

7 3,50E-03 0,01 0,009 3,00E-03 0,008 0,007 Permeate Flow (mol/h) 2,50E-03 2,00E-03 1,50E-03 Model 3 Model 2 Model 1 Expermental Retentate Flow (mol/h) 0,006 0,005 0,004 0,003 0,002 0,001 Model 3 Model 2 Model 1 Expermental 1,00E-03 0,002 0,004 0,006 0,008 0,01 0,012 0, ,002 0,004 0,006 0,008 0,01 0,012 0,014 Feed Flow (mol/h) Feed Flow (mol/h) Fgure 5. Comparson between models for the results of flow vs. flow Fgure 6. Comparson between models for the results of flow vs. flow 0,96 0,94 0,92 0,9 Model 3 Model 2 Model 1 Expermental Molar Fracton CH4 0,88 0,86 0,84 0,82 0,8 0,002 0,004 0,006 0,008 0,01 0,012 0,014 Feed Flow (mol/h) Fgure 7. Comparson between models for the results of methane concentraton vs. flow The mmedate concluson one can tae from the plots above s that dscretzaton of the models fts better to the expermental data than a smple model wthout dscretzaton. Moreover, one can also notce that from Model 1 to Model 3 the error n the calculated values for the flow ncrease but the errors on the calculated flow and methane concentraton decrease. The possble reason for the fact that the dscretzed models offer n general better results mght have to do wth convergence of the models. For nstance, when dvdng the membrane nto smaller parts that means the equatons descrbed before are appled to each slce of the membrane tang nto account that, closer to the sde of the membrane the flux to the s hgher due to the bgger dfference n the partal pressures of the components. Ths phenomenon s not accounted for when consderng the membrane as a whole and even the approxmaton of calculatng the logarthmc average of the partal pressure n the sde doesn t offer as good results as the dscretzaton of the module. Model 3 and model 2 presented smlar results n the valdaton wth expermental data and wth the smallest devatons.

8 Senstvty analyss The membrane that was chosen for these senstvty analyss calculatons was a PDMS membrane wth the characterstcs shown n the followng table. The man results analyzed are the carbon doxde removed from the vs. the hydrogen lost to the when the membrane s area ncreases and also the pressure of the. Snce hydrogen s the man product t s mportant to study the better way to retreve the hydrogen present n the usng the smallest area and lower energy consumpton possble. In ths senstvty analyss only calculatons for a sngle module were performed. The reason ths membrane was chosen to perform the senstvty analyss was because t s an example of a polymerc membrane that s reverse selectve at low temperatures and has been used n the separatons of hydrogen and carbon doxde. Snce ths membrane has a good permeablty but a somewhat poor selectvty t s expected that to attan hgh concentratons of hydrogen n the there wll have to be loss to the as well and vce versa. To perform the senstvty analyss model 2 was chosen because the way t was mplemented made t easer to change parameters and t s not necessary to specfy the membrane s length. Table 4. Membrane parameters Permeablty (Barrer) 3200 (CO 2 ) 950 (H 2 ) Thcness 1,3 μm Table 5. Feed composton and operatng condtons 0,9 Feed (mol/h) 23,3 CO 2 mole fracton 0,3 CO2 recovery (fracton) 0,8 0,7 0,6 0,5 0,4 0,3 0,2 H 2 mole fracton 0,7 0, ,1 0,2 0,3 0,4 0,5 0,6 0,7 H2 loss (fracton) Temperature (C) 35 Pressure (bar) 7,7 Fgure 8. CO 2 recovery vs H 2 loss to

9 Conclusons The frst concluson one taes mmedately s that the results of the valdaton can be dvded n two sets, the results of the dscretzed models 2 and 3 and the results of model 1. In both models 2 and 3 the results are very smlar and closer to the expermental data, whch suggests that dvdng the membrane nto smaller parts descrbes the system more accurately. However, there seems to be a tendency of the models methane concentraton to have smaller values than those of the expermental data. Ths suggests features n transport or geometry he/she wants n the system. Another upgrade that can be made s to add to the model the features necessary for dynamc smulaton. Nevertheless, t s crucal that valdaton wth expermental data of the model can be made. Settng up a model for a membrane unt for software le AP should be a compromse between expermental wor and computatonal wor n order not only to troubleshoot but also to mprove the results. Symbols that n the real system there s a phenomenon that blocs the permeaton of carbon doxde or ncreases the permeaton of methane through the membrane. Snce the total flows of the and are almost the same n the models and n the expermental D J Fc s Law Dffuson Coeffcent of component Membrane flux of component Membrane thcness data, one s led to support a combnaton of both hypotheses. Ths addtonal permeaton and blocage mght have to do wth all the factors that were dsregarded such as temperature gradents or geometry. In addton p Partal pressure of o component p Partal pressure of component to ths, t s also necessary to notce that n the expermental setup, the flow was measured wth standard volume unts and that alone ntroduces an error on the readng of the flow and perhaps that s what causes such small P, P Q j Permeabltes of components and j Permeance of component dfferences between the models output for total and flows to expermental data. All n all, both models 2 and 3 seem to be able to predct qute well.e. wth a small error, the outcome of a gas permeaton membrane unt and, for estmaton purposes n a process, these models are sutable to be used. However, the deal case would be that n the future correctons and alteraton to ths model can be made, perhaps even create a sort of a lbrary wthn the model tself that F Membrane Feed Flow F Membrane Permeate Flow F Membrane Retentate Flow y Component molar fracton n, the y Component molar fracton n, the allows the model user to choose whch

10 y Component molar fracton n, the A Membrane s area membrane P Feed total pressure P Permeate total pressure (superscrpt) Membrane module number Constant [3] Wjmans J.G., Baer R.W., (1995), J. Membrane Sc., 107, 1-21 [4] Mulder, M.: Basc Prncples of Membrane Technology, Kluwer Academc Publshers, 2000, Dordrecht. [5] L. Shao, B.T. Low, T.-S. Chung, A.R. Greenberg, Polymerc Membranes for the Hydrogen Economy: Contemporary Approaches, Prospects for the Future, Journal of Membrane Scence (2008) R (superscrpt) P (superscrpt) R(x) P(x) x H A Retentate Permeate Retentate along membrane s length Permeate along membrane s length Membrane s length axs Heght Area [6] S. Adhar and S. Fernando, Hydrogen Membrane Separaton Technques, Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 [7] ASPEN Plus Manual Gettng Started Customzng Unt Operaton Models Verson , AspenTech, 2005 [8] ASPEN Plus Manual User Models Verson , AspenTech, 2005 [9] Aspen Custom Modeler Examples Gude, Verson , AspenTech, 2005 [10] Aspen Custom Modeler Gettng Started Gude, Verson , AspenTech, 2005 Lterature [1] W. Wuovts, A. Fredl, M. Schumacher, M. Modgell, K. Urbanec, M. Ljunggren, G. Zacch, P.A.M. Claassen, 15 th European Bomass Conference & Exhbton, Berln, Germany, 2007 [2] W. Wuovts, Modarres, A. Fredl., Conference PRES08, Prague, Czech.Rep., 2008 [11] Davs R. A., Smple Gas Permeaton and Pervaporaton Membrane Unt Operaton Models for Process Smulators, Chem. Eng. Technol. 25 (2002), 7 [12] Ohlrogge K., Ebert K., Membranen: Grundlagen, Verfahren und ndustrelle Anwendungen, Wley-VCH, 2006 Alessander Maaru, Internal publcaton

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