CFD studies of heterocatalytic systems

Transcription

1 CFD stues of heterocatalytc systems György Rá 1, Tamás Varga, Tbor Chován Unversty of Pannona, Department of Process Engneerng, H-800 Veszprém Egyetem u. 10., Hungary tel , e-mal: H-800 Veszprém Egyetem u. 10., Hungary I. INTRODUCTION Nowaays evelopment of flow fel n process equpments s a key ssue n esgn of process unts. Wth a well esgne flow fel the overall process performance an the safety of operaton can be ncrease at the same tme. To support ths step, fferent moelng software packages an flow sheet smulators have been evelope n last ecaes. As computatonal capacty of computers has beng ncrease the role of computatonal flu ynamcs (CFD) n apple moelng techncs an metho has sgnfcantly grown snce the frst computer was swtche on [1]. CFD can be consere as a collecton of numercal solvers for partal fferental equatons. Wth applyng CFD coes more complcate problems n more complex structures can be nvestgate n etals than wth other moelng techncs. In recent years many papers applyng CFD technque to analyze some phenomena or to esgn a reactor have been publshe. The most frequently nvestgate phenomena are the pressure rop, flow profle [,3], heat transfer an mass transfer [4,5,6] for a reactor, the same for a catalytc pellet [8] an ntra-partcle ffuson effects [9] too. These examples show that CFD s capable to smulate transport processes an t can be apple as a part of complex mathematcal moels. Our purpose was to stuy heterocatalytc systems. The moel evelopment process wll be ntrouce n case of an nustral heterocatalytc reactor wth hghly exothermc reactor. COMOL Multphyscs can gve an effectve support snce t can be apple to solve moels whch consst hgher orer partal fferental equatons wth fnte elements metho. In ths artcle COMOL Multphyscs s apple to calculate the evelopment of state varables n a packe be tube reactor. II. CAE TUDY The smplest evelope reactor moels were the one-mensonal quas-sngle phase steay-state an ynamcal moels. It was followe by a two-phase ynamcal moel snce heterocatalytc processes can be more aequately escrbe by applyng separate sol an gas phases. These moels can be apple n the analyss of any possble stuaton n the system, such as reactor runaway. In 1D moels the state-varables are calculate along the catalyst be. However the raal ffuson cannot be consere wthout expanng mensons. D moels take nto conseraton the geometry an the evolve flow fel n the system. nce the geometry of the reactor s symmetrcal some smplfcatons can be apple to ecrease the necessary calculaton cost urng the soluton of moels. The analyss shows the sgnfcant heat sources (hot spots) an the unusual flow fels (back mxng, turbulent waves). These types of results can be sutable n optmzaton of reactor effcency [5,6]. The most complex evelope moel s the D, two-phase moel. In ths moel both sol an gas phases are

2 consere. The etale mathematcal escrpton of the pellet has been performe n COMOL Multphyscs. It can be apple to nvestgate the nfluence of ffuson an the velocty fel on the reactor temperature an the rate of transport processes between the consere phases. Base on the moellng possbltes the pellet moel can be apple to esgn the shape an sze strbuton of catalyst pellets, to get a closer look to specfc locatons of the catalyst be, to analyze the possble evelopment of runaway n a sngle catalyst pellet, an to bul a catalytc be from them. Unfortunately, for solvng of ths type of complex reactor moesl requres horrble harware capacty. III. PELLET MODEL In our prevous works, a quas-sngle phase mathematcal escrpton of the reactor, an D moels, whch take nto conseraton the geometry an the evolve flow fel n the system were ntrouce [4,7,10].The nvestgate, vertcally postone reactor contans a large number of tubes flle wth catalyst. Our concept s the ntegraton of a catalyst pellet moel, mplemente n COMOL Multphyscs nto the catalytc be usng a pellet by pellet bulng technque evelope n MATLAB. The frst step to accomplsh our objectve s the mplementaton of pellet moel n the apple CFD coe. All component mass balances n the gas phase contan terms to calculate convectve an conuctve transport processes an naturally the mass transport process between the sol an gas phase: G G G G ( B c ) ( B c ) x + y = D G c G x G G c + + D A β y G ( c c ) where B G flow rate of gas phase; = {A; B; C}; c G concentraton of th -component n gas phase; x reactor length; D G ffuson coeffcent of th -component n gas phase; A nterface area between the gas an sol phase; β mass transfer coeffcent between the gas an sol phase of th -component; c concentraton of th -component n sol phase. Of course the convectve term n sol phase s mssng: D x c + D y c = A β ( ) r G c c + V ν () where D ffuson coeffcent of th -component n sol phase; V the volume of sol phase; ν the stochometrc coeffcent of th -component; r reacton rate whch s calculate by the followng correlaton: (1) r E RT A = k 0 e (3) where k 0 preexponental factor; E A actvaton energy; R eal gas constant; T temperature of sol phase. Terms n both equatons are the same as those consere n component balances: G G ρ G G G T T + = cp B A α ( ) G G x y T T G G T T λ + (4) x x where ρ G ensty of gas phase; c p G heat capacty of gas phase; T G temperature of gas phase; λ G heat conucton coeffcent n gas phase; α heat transfer coeffcent between the gas an sol phase. An fnally the heat balance of sol phase:

3 T T λ + x y = A α ( ) ( ) G T T + V r H (5) where λ heat conucton coeffcent n sol phase; H r reacton heat. Ths s just a short escrpton of moel equatons to show the structure of pellet moel. All physcal propertes are calculate as the functon of temperature. These equatons have been apple to mplement a pellet moel n COMOL. r IV. CATALYTIC BED MODEL In the followng the pellet moel mplemente n COMOL was use to bul the moel of the catalyst be.. Usng the hgh-level connecton between MATLAB an COMOL, the mplemente pellet moel can be exporte to MATLAB as a functon. The user nterface of MATLAB let you to mofy the pellet moel, an to bul up a structure from them. As Fgure 1 shown, the frst pellet outlet propertes are bounary averages, an these results became the next pellet nlet parameters. Fgure 1. The vsualzaton of the mplemente pellet structure wth average property connecton Then, the average outlet propertes have been replace wth outlet profles. Thus, t has become possble to take nto account the effect of the establshe flow on the next pellet. Fnally, a lnear catalytc be was bult an the frst nvestgatons were conucte on t. Fgure. The vsualzaton of the mplemente pellet structure wth property profle connecton

4 V. REULT AND DICUION For the COMOL Multphyscs pellet moel the value of state varables n front of the reactor were gven as bounary contons n the soluton of the pellet moel. Obtane results of the pellet moel are plotte n Fgure 3. In Fgure 3.a the temperature surface can be seen an as t was expecte the hghly exothermc reacton takng place n catalyst ncreases the temperature of the pellet an t warms up the flowng gas. a) Temperature n catalyst pellet an n surrounngs b) The concentraton of A component n catalyst pellet an n surrounngs c) Concentraton of B component n catalyst pellet an n surrounngs ) Concentraton of C component n catalyst pellet an n surrounngs Fg. 3: Changes n the value of state-varables along the nvestgate part of reactor n steaystate Due to the ffuson n the catalyst pellet the concentraton of reagents ecrease n the recton to the centre of the pellet (see Fgure 3.b an Fgure 3.c); whle the concentraton of the prouct changes n the opposte recton as shown n Fgure 3.. In ths moel all components can be connecte to the same actve stes on the catalyst, however n practse there s a competton between components an the rate an the equlbrum of the asorpton etermne the concentraton n the catalyst. nce only the temperature profle an propertes of nlet an outlet flows are measure urng the reactor operaton, the catalyst pellet moel cannot be valate wthout beng ntegrate nto the reactor smulator.

5 As t can be seen n Fgure 3. the outlet concentraton of the prouct s very low. Ths suggests that the parameters apple to calculate the rate of asorpton must be mofe. tll the characterstc of changes n case of all the calculate state varables are the same as we expecte. Ths pellet moel have been exporte to MATLAB, an use as a functon. The MATLAB software ensures we freeom for ts users, an wth ts bult-n functon t makes possble to mplement a catalytc be pellet by pellet. The effect of the catalyst volume strbuton on the converson was analyse. In ths nvestgaton the frst structute (see Fgure1) was use. The number of pellets was ncrease at near constant catalytc volume. Experments presente n lterature [11] suggest the smaller the pellets are, the better the conversaton/volume nex s. Fgure 4 an 5 show the reagent an Fgure 6 the prouct concentraton along the be. As the number of partcles s ncreasng hgher an hgher converson can be acheve. Due to the exothermc reactons the temperature wll be hgher n the outlet than n the nlet. Fgure 7 show that the temperature has a nearly lnear relatonshp wth the converson. Fgure 4. The concentraton of A component n the catalytc be Fgure 5. Concentraton of B component n the catalytc be Fgure 6. Concentraton of C component n the catalytc be Fgure 7. Temperature n the catalytc be VI. CONCLUION AND FUTURE WORK To montor an operaton of a tube reactor the process operators usually o not have much nformaton, just some temperature measurements along the reactor an composton of nlet an outlet flows. Hence, a reactor smulator can be very useful to follow the operaton an to help process operators, e.g. to avo evelopment of reactor runaways or to optmze reactor

6 operaton. The two mensonal moel of catalyst pellet wth ts close surrounngs was mplemente nto COMOL Multphyscs. Ths pellet moel can be utlse to esgn the shape an sze strbuton of catalyst pellets, to get a closer look to specfc locatons of the catalyst be, an to bul a catalytc be from them. The catalytc be was mplemente pellet by pellet an the result of the frst tests fullflle the expectatons. Our vson s a flexble, pellet by pellet catalytc be moel, whch can be use for the esgn, stuy an evelopment of catalytc pellets an bes wth reuce calculaton cost. VII. ACKNOWLEDGEMENT The fnancal support from the TAMOP /1/ (Élhetıbb környezet, egészségesebb ember - Bonnovácó és zöltechnológák kutatása a Pannon Egyetemen, MK/) project s gratefully acknowlege. VIII. REFERENCE [1] Ranae V. V.: Computatonal flow moellng for chemcal reactor engneerng, Acaemc Press (00) [] A. Jafar, P. Zamankhan,.M. Mousav, K. Petarnen, Moelng an CFD smulaton of flow behavor an spersvtythrough ranomly packe be reactors, Chemcal Engneerng Journal 144 (008) [3] H. P. A. Cals, J. Njenhus, B. C. Pakert, F. M. Dautzenberg, C. M. van en Bleek, CFD moellng an expermental valaton of pressure rop an flow profle n a novel structure catalytc reactor packng, Chemcal Engneerng cence 56 (001) [4] Varga, T., Abony, J., Chován, T., Nagy, L, zefert, F., Rét, J., ProceengsMőszak Kéma Napok 06,, (006) (n Hungary) [5] Tarek M. Moustafa, Mohame Abou-Elreesh an ef-eeen K. Fateen, Moelng, mulaton, an Optmzaton of the Catalytc Reactor for Methanol Oxatve Dehyrogenaton COMOL Conference 007, Boston (007) [6] Maryam Gharan*,1, Ham Mehzaeh Mathematcal Investgaton an CFD mulaton of Monolth Reactors: Catalytc Combuston of Methane, COMOL Conference Hannover (008) [7] Gy. Rá, T. Varga, T. Chován., Mőszak Kéma Napok 09, Proceengs, (009) (n Hungary) [8] Anuraha Nagaraj1 an Patrck L. Mlls, Analyss of Heat, Mass Transport, an Momentum Transport Effects n Complex Catalyst hapes for Gas-Phase Heterogeneous Reactons Usng COMOL Multphyscs, COMOL Conference Boston (008) [9] A. Guaro, M. Casanovas, E. Ramírez, F. Recasens, I. Magańa, D. Martínez, M.A. Larrayoz, CFD moelng on external mass transfer an ntra-partcle ffusonal effectson the supercrtcal hyrogenaton of sunflower ol, Chemcal Engneerng cence 6 (007) [10] Varga T., zefert F., Chován T., Rét J., Mőszak Kéma Napok 07, Proceengs, (007) (n Hungary) [11] J.M. Berty, Experments n Catalytc Reacton Engneerng, tues n urface cence an Catalyss, 14, Elsever,1999