Energy Procedia 4 (2011) Energy Procedia 00 (2010) GHGT-10

Transcription

1 Enegy Pocedia 4 (011) Enegy Pocedia 00 (010) Enegy Pocedia GGT-10 -Dimensional membane sepaato modelling: mass tansfe by convection and diffusion Juiaan Boon 1*, ui Li, Jan Wilco Dijksta, Johannis A.Z. Pietese ydogen & Clean Fossil Fuels, Enegy eseach Cente of the Nethelands P.O. Box 1, NL1755 ZG Petten, The Nethelands Elsevie use only: Received date hee; evised date hee; accepted date hee Abstact ydogen selective membanes may pesent a technologically and economically efficient method fo the sepaation of fom CO in pe-combustion decabonisation of powe poduction fom fossil fuels. Accuate scale-up and pefomance pediction of membanes stongly depends on adequate epesentation of the pevailing esistances to mass tansfe, especially fo pesent-day high flux membanes. In a seies of expeiments, /N sepaation is measued as a function of feed flow and etentate pessue fo a suppoted palladium membane enclosed by annula channels fo feed/etentante and sweep/pemeate flow. Compaison of model pedictions with measued data eveals that mass tansfe esistances in the gas phase ae significantly educed by a adial velocity component in cases of high tansmembane flux, which can only be adequately descibed by a D model. Fo accuate intepetation of expeiments, scale-up, and design of modules with high flux membanes, D modelling is equied. c Elsevie Published Ltd. by All Elsevie ights eseved Ltd. Open access unde CC BY-NC-ND license. Keywods: membane sepaation; gas pemeation; mass tansfe esistance; D modelling; Pd membane 1. Intoduction In pecombustion decabonisation fo powe poduction, CO is extacted fom the fuel befoe combustion. Initially, fuel is conveted to synthesis gas: mainly CO and. Wate-gas shift (WGS) futhe convets this into a mixtue consisting lagely of CO and. A suitable sepaation technique then yields two steams: a steam of concentated CO and a steam of the latte can be used fo powe geneation without concomitant CO emissions. Seveal techniques ae available fo sepaating fom CO, yet -selective membanes may offe an advantage in tems of economic and enegy efficiency [1, ]. Futhe efficiency benefits may aise if WGS eaction and sepaation ae integated at elevated tempeatues in a sepaation-enhanced WGS eacto [3-5]. Similaly, the fuel efoming eaction, WGS, and sepaation may be integated in a membane efome [6]. In all of these cases minimisation of equipment size and equied membane suface aea is cucial to the economic feasibility and this can be ealised by the use of high pefomance membanes. Theefoe development effots ae diected towads achieving a high hydogen tansmembane flux. Unfotunately, with highe fluxes the esistances to heat and mass tansfe will also become inceasingly impotant. Theefoe, optimal design and opeation of membane units necessitates a fundamental undestanding of the pevailing heat and mass tansfe pocesses [7, 8]. This wok descibes a seies of expeiments of /N sepaation with a suppoted palladium membane enclosed by a double annula geomety. Fo intepetation of the esults and studying the effect of mass tansfe esistance, 1D and D axially symmetic membane sepaato models ae developed. The cucial diffeences between 1D and D modelling will be highlighted. * Coesponding autho. Tel.: ; fax: addess: boon@ecn.nl. doi: /j.egypo

2 700 Juiaan J. Boon et al. / Enegy Pocedia00 4 (010) (011) Expeimental Expeiments on /N sepaation have been pefomed on ECN s Pocess Development Unit (PDU), descibed in moe detail elsewhee [9]. Thee membanes ae used, each consisting of a commecial 14 mm oute diamete α-al O 3 macopoous membane suppot tube, two mesopoous α-al O 3 layes of 45 μm each [10], and a μm Pd laye [11]. Afte sealing [1], the effective length is 44 cm, giving a total suface aea fo thee membanes of m. Membane tubes ae mounted in modules (Figue 1); an inset tube is used fo the sweep gas, ceating a double annulus geomety enclosing the membane. The modules ae placed in an electically heated oven at 400 C, with a maximum tempeatue gadient ove the module length of 5 C. Fo the thee modules combined, feed gases wee pepaed by mass flow contolles, outlet flows and composition wee detemined with a mass flow mete and gas chomatogaph, espectively. The expeimental esults epoted hee wee all obtained with 60.% in N feed gas and pue N sweep gas. The feed flow ate was vaied in the ange of Nl min -1 and the countecuent sweep flow ate was 0 Nl min - 1. Feed-side pessues wee vaied in the ange of MPa, while sweep side pessue was kept at 1.5 MPa. In these conditions, the Reynolds numbe vaies up to 400 and hydodynamics will be lamina at all times. Figue 1. Schematic membane module layout Figue. Membane module annula geomety fo feed/etentate and countecuent sweep/pemeate side ( s = 3 mm, m = 7 mm, f = 13.3 mm) 3. Modelling Modelling is done fo the axisymmetic module geomety depicted in Figue. Both the feed/etentate side and the countecuent sweep/pemeate side of the module have an annula shape. Fo desciption of fluid flow and mass tansfe, D steady state balances ae solved fo both channels in cylindical geomety (, θ, z coodinates). Unde the assumptions of (i) constant density and tempeatue, (ii) no angula gadients ( / θ = 0), (iii) Newtonian fluid of constant viscosity, (iv) ignoing gavity, (v) no adial pessue gadient, and (vi) constant diffusivity, the steady-state equations of continuity and motion, and the hydogen mateial balance educe to [13]: 1 v z v 0 continuity (1) z v z v z p 1 v z v z v v z motion () z z z c c 1 c c v v D mateial balance (3) z z z p 0 no adial pessue gadient (4) z The bounday conditions ae no-slip at the module walls, except fo the membane flux: n n Q p p (5) J mem f,, s

3 Juiaan J. Boon Boon et al. et / Enegy al. / Enegy Pocedia 4 (011) 00 (010) RT v J mem m p and continuity of hydogen flux at the membane suface: c v c D J mem m (6) (7) The value of n=0.7 fo the pessue exponent in the membane flux equation (5) is obtained fom pue expeiments [14], which ae not epoted hee. At the inlets, developed lamina flow velocity pofiles ae assumed[13,.4]. Fou dependent vaiables v, v z, c( ), and p/ z ae detemined by the coupled PDE s (1-4) that ae solved as ODE s in Matlab with built-in solve bvp4c afte discetisation in finite diffeences fo z, using second-ode upwind appoximation fo fist-ode deivatives and a centeed-space appoximation of second-ode deivatives [15]. Afte solving the velocity components and hydogen concentation in two dimensions, the mixing-cup aveage flow and concentation ae obtained by quadatue, to yield obsevable model esults such as the outlet flow and hydogen concentation fo the etentate and pemeate. Typical D model outcome is shown in Figues 3 and 4. The left pat of Figue 3 shows the distibution of hydogen on both sides of the membane, the ight pat shows the calculated gas velocity. Figue 4 shows coss-sectional pofiles at z = m of the hydogen concentation (left) and the adial velocity component (ight). Concentation gadients on both sides of the membane indicate that mass tansfe esistances in the gas phase indeed contibute to the oveall mass tansfe esistance. A adial velocity component is ceated by the tansmembane flux. Figue 3. D model esults fo the concentation (left) and velocity (ight), feed flow 90 Nl min -1 and 3.1 MPa etentate pessue, membane located at = m

4 70 Juiaan J. Boon et al. / Enegy Pocedia00 4 (010) (011) Figue 4. Coss-sectional pofiles at z = m of the concentation (left) and adial velocity component v (ight) fom D model esults, feed flow 90 Nl min -1 and 3.1 MPa etentate pessue Moe simple one-dimensional models can be of geat impotance in design calculations fo membane modules [16, 17]. A 1D plug flow model has been constucted fo compaison, and involves mateial balances fo hydogen on both sides of the membane, combined with the membane pemeation equation (5). Suitable elations fo mass tansfe esistances ( concentation polaisation ) on both sides of the membane elate the flux of hydogen to/fom the membane suface to the concentation diffeence of hydogen and a mass tansfe coefficient accoding to: J k c c (8) m Mass tansfe coefficients (k) ae coelated with the hydaulic diamete (d h ) and molecula diffusivity (D) as dimensionless Shewood numbe: kdh Sh (9) D Combination of equations (8) and (9) allows fo Sh to be obtained fom the D simulation esults. Fo the 1D model, Sh-values ae needed and these can be eadily obtained by using the analogy between heat and mass tansfe [13] and finding liteatue values of Nu fo heat tansfe in lamina flow. These ae available fo developed velocity and concentation pofiles (Nu, Sh ) as a function of the atio of annula adii [18]. Expeimental values fo developing tempeatue (concentation) pofiles ae also available [19] and these have been appoximated as: b v dh d h (10) b b dh Sh a Gz a Re Sc a z D z 0.4 Sh 1.07 Gz Gz 6 feed/etentate (membane at the inne wall) (11) Sh Sh 6.18 Gz Sh 0.88 Gz Gz 51 sweep/pemeate (membane at the oute wall) (1) Sh Sh 5.04 Gz 51 Thus, the cuent wok is based upon thee sepaate models: (i) a full D model, (ii) a simplified 1D model with constant Sh, and (iii) a moe extended 1D model with Sh = Sh(Gz). 4. Results Expeimental esults with thee membane modules fo a feed flow ate of Nl.min -1 and a feed/etentate side pessue of -3.1 MPa ae depicted in Figue 5, togethe with model esults. The pemeance has been detemined as Q = mol m - s -1 Pa -0.7 by fitting of the pessue vaiation data using the D model. With this value of the pemeance, it can be seen that all models can qualitatively descibe the expeimental esults. But fo the highe ecoveies, the 1D models undepedict the ecovey. This is confimed by a paity plot of the pedicted membane flux vesus the expeimentally measued flux as depicted in Figue 6. The 1D models deviate moe than the D model at highe flux, i.e. J mem 0.3 mol m - s -1. It can also be concluded fom the esults that thee is no significant diffeence between the 1D model with constant Sh, and with Sh(Gz) and theefoe that the effect of developing concentation pofiles is sufficiently small to ignoe.

5 Juiaan J. Boon Boon et al. et / Enegy al. / Enegy Pocedia 4 (011) 00 (010) In ode to futhe compae the models, Sh values wee calculated fom the D model as descibed above fo feed flow 90 Nl min -1 and 3.1 MPa etentate pessue, with a measued ecovey of 40% and an aveage membane flux of 0.7 mol m - s -1. An additional computational un was done in which the developed lamina velocity pofile was not distubed by the membane flux, i.e. setting v = 0 in equation (6). Results ae compaed with the liteatue Sh values fom equations (11) and (1) in Figue 7. The Sh values in D simulation ae up to about a facto lage than liteatue Sh values. The diffeence appeas to be due to the adial velocity component ceated by the tansmembane flux: when the adial velocity is set to zeo the diffeence disappeas. Figue 5. ydogen ecovey vesus etentate pessue (fo feed flow 90 Nl min -1 ) and feed flow (fo 3.1 MPa etentate pessue): measued ( ), obtained with the D model ( ), and obtained with the 1D model with constant ( ) and vaying ( ) Sh Figue 6. Pedicted vesus measued values of the length-aveaged membane flux fo the D model ( ), and the 1D model with constant ( ) and vaying ( ) Sh

6 704 Juiaan J. Boon et al. / Enegy Pocedia00 4 (010) (011) Figue 7. Plots of local Sh values fo D simulation ( ), D simulation with v = 0 (--), and fom liteatue ( ) 5. Discussion In the cuent configuation, two annula channels with lamina flow ae sepaated by a -selective membane. The diffeence in concentation on both sides ceates a diving foce fo tanspot fom the feed/etentate to the sweep/pemeate side. The flux of is subject to thee sepaate mass tansfe esistances in seies: diffusion on the feed side, membane pemeation, and diffusion on the sweep side. The diffusion pocesses in the annula channels can be descibed with the D model pesented above, but they can also be appoximated with mass tansfe coefficients and Sh coelations, allowing fo moe simple 1D models. oweve, the 1D models inceasingly deviate fom expeiment at highe membane fluxes, in paticula fo J mem 0.3 mol m - s -1. Compaison of Sh values fo the 1D and the D simulations indeed eveals a lage diffeence in mass tansfe coefficients that oiginates fom the assumptions of Nu coelations in liteatue, in paticula the assumption of developed lamina flow. This assumption ignoes the adial velocity component that is indeed absent in themal conduction poblems, but is ceated hee by pemeating. Radial convection can significantly add to the tanspot to/fom the membane suface by convective tanspot of. In contast, the effect of developing concentation pofile nea the inlet on both sides of the membane may be safely ignoed. Because mass tansfe esistances ae in seies, the effect that the adial velocity component inceases the Sh value is only obseved with sufficiently high membane flux. With the development of high flux membanes, D modelling will become eve moe impotant to allow fo accuate estimation of the esistances to mass tansfe. Fo module design and scale-up calculations, the use of 1D models is esticted to a elatively low tansmembane flux and will othewise pedict too low hydogen flux and accodingly too high a equied membane suface. 6. Conclusion Expeimental esults obtained on /N sepaation expeiments with high flux membanes cannot be descibed accuately with 1D models, including liteatue based Sh coelations fo mass tansfe esistances on both sides of the membane. Simulations with a D model eveal that the high membane flux ceates a adial velocity component that significantly deceases the tanspot esistances to/fom the membane. The distoted velocity pofiles invalidate the Sh coelations that can be deived fom liteatue data on Nu fo heat tansfe in developed lamina flow in identical geomety. The eo that is made in 1D models becomes inceasingly lage fo inceasing membane flux and in this case a D sepaato model is equied fo accuate scale-up and design calculations. Acknowledgement This wok has been pefomed as pat of the Dutch national R&D pogamme fo CO captue, tanspot and stoage CATO-. The authos gatefully acknowledge I. Pete Veensta and Pof. D. Aian Nijmeije of consotium patne Shell Global Solutions, Amstedam (NL) fo fuitful discussions. M. Sande de Munck of ECN ydogen & Clean Fossil Fuels is acknowledged fo pefoming the expeiments.

7 Juiaan J. Boon Boon et al. et / Enegy al. / Enegy Pocedia 4 (011) 00 (010) Nomenclatue c i concentation if i, mol m -3 <c i > mixing-cup aveage concentation of i, mol m -3 d h hydaulic diamete, m -1 D i diffusivity of i in gas mixtue, m s Gz dimensionless Gaetz numbe (equation 10) -1 J mem tansmembane flux, mol m - s k mass tansfe coefficient, m s -1 N nomal gas volume, specified at 73 K and 1 atm p total pessue, Pa p i patial pessue of i, Pa -n Q i pemeance of i, mol m - s -1 Pa adial coodinate, m -1 R gas constant, J mol -1 K Re dimensionless Reynolds numbe (equation 10) f oute adius of feed/etentate side channel, m m membane adius, m s inne adius of sweep/pemeate side channel, m Sc dimensionless Schmidt numbe (equation 10) Sh dimensionless Shewood numbe (equation 9) Sh dimensionless Shewood numbe fo fully developed flow pofile and concentation pofile T tempeatue, K v gas velocity vecto, composed of (v,v z ), m s -1 z axial coodinate, m θ angula coodinate, μ dynamic viscosity, Pa s ρ gas-phase density, kg m -3 List of Refeences [1] Ockwig NW, Nenoff TM. Membanes fo ydogen Sepaation. Chemical Reviews 007; 107(10): [] Lu GQ, niz da Costa JC, Duke M, Giessle S, Socolow R, Williams R, et al. Inoganic membanes fo hydogen poduction and puification: A citical eview and pespective. Jounal of Colloid and Inteface Science 007;314(): [3] Bacht M, Aldeliesten PT, Kloste R, Puschek R, aupt G, Xue E, et al. Wate gas shift membane eacto fo CO contol in IGCC systems: techno-economic feasibility study. Enegy Convesion and Management 1997;38(Supplement 1):S [4] Bedesen R, Jodal K, Bolland O. igh-tempeatue membanes in powe geneation with CO captue. Chemical Engineeing and Pocessing 004;43(9): [5] Kikuchi E, Uemiya S, Sato N, Inoue, Ando, Matsuda T. Membane Reacto Using Micopoous Glasssuppoted Thin Film of Palladium. Application to the Wate Gas Shift Reaction. Chemisty Lettes 1989;18(3): [6] Pietese JAZ, Boon J, Van Delft YC, Dijksta JW, Van den Bink RW. On the potential of nickel catalysts fo steam efoming in membane eactos. Catalysis Today 010; In Pess, Coected Poof. [7] De Falco M, Di Paola L, Maelli L, Nadella P. Simulation of lage-scale membane efomes by a twodimensional model. Chemical Engineeing Jounal 007;18(-3): [8] De Falco M. Pd-based membane steam efomes: A simulation study of eacto pefomance. Intenational Jounal of ydogen Enegy 008;33(1): [9] Dijksta JW. Development of Membane Reacto Technology fo Powe Poduction with Pe-Combustion CO Captue Sep 010; 10 th Intenational Confeence on Geenhouse Gas Contol Technologies, GGT10; 010. [10] Bonekamp BC, van ossen A, Coeia LA, Vente JF, aije WG. Macopoous suppot coatings fo molecula sepaation membanes having a minimum defect density. Jounal of Membane Science 006;78(1-): [11] ou S, Jiang K, Li W, Xu, Yuan L, inventos; A metal palladium composite membane o alloy palladium composite membane and thei pepaation methods.wo [1] Rusting FT, de Jong G, Pex PPAC, Petes JAJ, inventos; Sealing socket and method fo aanging a sealing socket to a tube.wo [13] Bid RB, Stewat WE, Lightfoot EN. Tanspot Phenomena. New Yok etc.: Wiley; 1960.

8 706 Juiaan J. Boon et al. / Enegy Pocedia00 4 (010) (011) [14] Li, Dijksta JW, Pietese JAZ, Boon J, Van den Bink RW, Jansen D. Towads full scale demonstation of hydogen-selective membanes fo CO captue. Jounal of Membane Science 010; In Pess. [15] offman JD. Numeical Methods fo Enginees and Scientists. nd edn. Boca Raton: Taylo & Fancis; 001. [16] ou K, ughes R. The effect of extenal mass tansfe, competitive adsoption and coking on hydogen pemeation though thin Pd/Ag membanes. Jounal of Membane Science 00;06(1-): [17] Koukou MK, Papayannakos N, Makatos NC, Bacht M, Aldeliesten PT. Simulation tools fo the design of industial-scale membane eactos. Chemical Engineeing Reseach & Design 1998;76(8A): [18] Ebadian MA, Dong ZF. Foced convection, intenal flow in ducts. In: Rohsenow WM, atnett JP, Cho YI, editos. andbook of eat Tansfe. 3 edn. New Yok: McGaw-ill; p [19] eaton S, Reynolds WC, Kays WM. eat tansfe in annula passages. Simultaneous development of velocity and tempeatue fields in lamina flow. Intenational Jounal of eat and Mass Tansfe 1964;7(7):