Model-based hybrid reaction-separation process design

Transcription

1 17 th European Symposum on Computer Aded Process Engneerng ESCAPE17 V. Plesu and P.S. Agach (Edtors) 2007 Elsever B.V. All rghts reserved. 1 Model-based hybrd reacton-separaton process desgn Potr Tomasz Mtkowsk, a Gunnar Jonsson, a Rafqul Gan a a CAPEC, Department of Chemcal Engneerng, Techncal Unversty of Denmark, Søltofts Plads, 2800 Kgs. Lyngby, Denmark, rag@kt.dtu.dk Abstract The desgn of hybrd processes takes nto account the performance of each consttuent element and the optmsaton of the desgn must take nto consderaton ther nterdependency. In ths paper, we focus on hybrd process systems consstng of a reactor and a membrane separaton unt whose desgn s usually carred out through tral-and-error approaches nvolvng experments. Through a model-based computer-aded technque t s possble to dentfy relable and feasble desgn alternatves, savng valuable resources. The technque to be presented n ths work conssts of an effcent and systematc approach that generates and test desgn alternatves by avodng duplcaton of work and by effcent decomposton of the problem nto ntegrated subproblems. Keywords Hybrd process, membrane separaton, solvent selecton, model-based desgn 1. Introducton Many chemcal and bochemcal products are obtaned n processes where reactons are equlbrum or knetcally controlled. These types of reactve processes can be usually characterzed by low process yeld or by low selectvty to the desred product, when parallel reactons occur. Irrespectve of the controllng factor of reacton(s), on-ste removal of product(s) enhance the yeld, leadng to reduced processng tmes and may also reduce undesred sde

2 2 P.T. Mtkowsk et al. reacton(s) f they occur. Conventonal separaton processes lke dstllaton can be effcently ntegrated wth reacton as well known reactve dstllaton. However, specfc knock-off crtera wth respect to the reacton phase, catalyst, resdence tme and operatng temperature and pressure need to be satsfed. One avalable opton to remove products, n case when products of the reactons of nterest are heat senstve and separaton technque requres operaton at temperatures below the degradaton temperature (reactants and/or catalyst), s to ntroduce some membrane-based separaton processes. The membrane mparts selectvty to specfc components based on ether the dfference n sze or the chemcal potental of the molecules. Solvents are playng a sgnfcant role n the reacton-separaton systems not only as a reacton medum (e.g. solvent brng reactants together) or as a solublzaton agent but also as a separaton agent. Solvents are wdely use n such processes lke lqud-lqud extracton, extractve dstllaton and crystallzaton. Combnaton of at least two processes s called a hybrd process snce the two processes nfluence the performance of each other and the optmsaton of the desgn must take nto account that nterdependency. Two types of hybrd processes are generally dstngushed n lterature [1]. The frst group conssts of processes whch essentally perform the same functon, for nstant separaton and separaton (S-S). The second group contans off-sprngs of processes whch combne processes orgnally dfferent n nature, for example, combnaton of reactor and separator (R-S). Based on a model-based framework for systematc analyss, t s possble to desgn hybrd process systems to fnd mproved process desgn alternatves n terms of process output parameters such as reacton yeld, selectvty, processng tme and envronmentally frendly solvents. 2. Desgn methodology of hybrd reacton separaton processes 2.1. Desgn algorthm A systematc model-based framework presented n [2] has been extended to analyze not only hybrd reacton-separaton (R-S) but also hybrd separaton -separaton (S-S) systems. The objectve s to dentfy the best possble process set-ups for (R-S) and (S-S) systems wth desred constrants of process parameters lke yeld, reacton tme, selectvty and product purty. The desgn algorthm s depcted n Fgure 1 and conssts of four mans steps where new features are ndcated n bold letters and only these are dscussed below. Step 1 has been expanded to analyze separaton tasks, whch ncludes dentfcaton of azeotrope ponts, phase splt, etc. Step 3 now combnes all collected knowledge from step 1 wth approprate membrane separaton models to generate the feasble hybrd process (desgn) alternatves. If an dentfed

3 Model-based hybrd reacton-separaton process desgn 3 separaton technque requres use of solvents, the solvent selecton method of Gan et. al [3] s used. In the last step (step 4), the generated hybrd process alternatves are tested under dfferent operatonal scenaros. Based on whch feasble alternatves are dentfed Process alternatves The methodology as descrbed above requres the analyss of dfferent process scenaros n order to satsfy the process demands defned n step 2. Dependng on batch or contnues processes, varous process scenaros can be derved. All scenaros need to be nvestgated qualtatvely and quanttatvely n order to explore and propose the best possble process scenaro or at least the feasble operatonal wndow where the optmal desgn may exst. The superstructure of possble alternatves from whch varous process scenaros can be derved s depcted on Fgure 2. Fgure 1 Methodology of desgn/analyze hybrd process system 2.3. Generc model of the hybrd process A generc model gvng the balance equatons ((1)-(2)) for hybrd process are derved based on scheme gven n Fgure 2. The dfferental equatons represent the states of the system at dscrete tme ponts whle the algebrac equatons are the consttutve and control equatons. The process and property sub-models for both processes are not lsted here but can be obtaned from the correspondng author. n ( 1αR ) ( 1βR) ( 2αR) ( 2βR) ( 1αP) = F F t (1) ( P) ( P) ( P) NKR 1β 2α 2β F F F 1β 1 1 kk, r β V β + ν k= 1 H ( F) ( 1αR) ( 1αR) ( 1βR) ( 1βR) ( 2αR) ( 2αR) ( 2βR) ( 2βR) = Fh h h h h t ( 1αP) ( 1αP) ( 1βP) ( 1βP) ( 2αP) ( 2αP) ( 2βP) ( 2βP) NKR F h F h F h F h r 1β H R + k Δ k + Q k= 1 In Equatons (1)-(2): F component molar flow, r reacton rate, t reacton [ ] (2)

4 4 P.T. Mtkowsk et al. tme, V reacton volume, υ,k stochometrc coeffcent, superscrpt: R recycle, P product, α - bottom sub-product, β - top sub-product. For hybrd process desgn, the objectve s to combne Flow out and Recycle terms of (Eq.1) nto a sngle term representng the effluent from the hybrd system. Advantage of such a reformulated model s smplcty to nvestgate the performance of the hybrd system. Moreover, ths reformulaton reduces the number of varables and the degrees of freedom. Usng the generc model and the partcular detals of any problem, the specfc hybrd reactor-membrane process can be generated and tested. Fgure 2 The general scheme of hybrd process 3. Case study Applcaton of the model-based framework s llustrated through an enzymatc esterfcaton reacton of cetyl oleate, a sperm whale ol analogue. Ths ester has mportant applcatons n the cosmetcs, lubrcants, food and pharmaceutcal ndustres. Objectve of ths study s to analyze the esterfcaton process n order to dentfy hybrd process confguratons wth hgher process yelds. Step 1a: Reacton data collecton The esterfcaton of cetyl alcohol wth olec acd s carred out over Novozym 435 (commercally avalable Canada anatarctca mmoblzed lpase on acrylc resn) n the lqud phase. The knetc model publshed by Garca et. al. [4] consders compettve nhbton between substrates and products. However, the overall reacton can be represented as follows: Novozym C H OH C H COOH C H O H O (3) Operatonal wndow for ths reacton wth respect to temperature s between K and 353 K (metng pont temperature of 1-hexadecanol and the temperature of denaturaton of lpase, respectvely). Reacton s lmted by the stablty of the catalyst, whch depends on water actvty and s set to 0.11 based on nformaton obtaned from lterature [5]. Snce reported knetc data [4] were obtaned n solvent free system wth reasonable converson (at atmospherc pressure converson s 0.8), t s consdered that a solvent s not requred. Step 2: Process demands Esterfcaton reacton, whch s knetcally controlled, has a converson around 80mol%. The objectve s to ncrease the process productvty by removng water from the reacton medum. Step 3: Separaton method selecton

5 Model-based hybrd reacton-separaton process desgn 5 Pervaporaton (PV) s selected as the membrane-based separaton technque because of the possblty of ntroducng hydrophlc membranes that would allow only water to permeate. Several authors [6] had reported PV membranes to dehydrate organc mxtures, even wth very small water concentraton. Step 4: Process condtons and feasble desgn Two operatonal alternatves are presented n ths paper: batch and sem-batch hybrd snce small producton rate s consdered. In ths case reactor and PV can be combned together n one unt snce feed to PV unt does not need to be preheated ( TR = K ).All set-ups are nvestgated under assumptons that: reactor s well mxed and actvty of enzyme does not change durng operaton. From the generc hybrd model (Eq. (1)), the problem specfc hybrd process model s generated (Eq.(4)). Wth respect to membrane, water flux depends on molar fracton of water (Eq.(5))[6] and fluxes for all other components present n the system are neglected. NKR dn = JAm + V ν, krk dt (4) k = 1 J w = Px w w (5) Reacton knetcs s descrbed by reversble Mchaels-Menten knetcs. Converson of process s defned as rato of moles of desred product (cetyloleate) to ntal amount of reactant, (X = N Es /N Ac ), and has a lmtng value of The full process model of the hybrd process conssts of 4 ordnary dfferental equatons, 31 algebrac equatons wth 91 varables, plus the equatons for the consttutve model (Mod. UNIFAC (Lyngby)), and t s solved and analyzed through ICAS-MoT[7]. Wth the generated problem specfc hybrd process model, fve scenaros (wth fve dfferent membrane areas) have been nvestgated n terms of process yeld and superorty of the hybrd process over batch reacton. All smulatons have been performed wth the same ntal condtons wth respect to reactor: C Al =1.58 mol/dm 3, C Ac =1.58 mol/dm 3, C Es = 0 mol/dm 3, C W = mol/dm 3 and V = 0.6 dm 3. Performance of the hybrd system s strongly dependent on the membrane area (A m ) and component fluxes (J ). The converson-tme behavour s shown on Fgure 3 whle desgn varables are gven n Table 1. Table 1 Process parameters and process conversons Batch RCPV1 RCPV2 RCPV3 RCPV4 RCPV4 RCPV5 A m [m 2 ] t [mn] X [-] For operatons carred out for 5h, yeld s mproved from 0.84 (batch) to (RCPV3) by removng water from the system usng a reasonable desgn for a PV-unt (A m = m 2 ). However, n 15h wth RCPV3 t s possble to acheve converson close to the lmtng value. What s mportant to observe s that the ncrease of A m from m 2 to m 2 does not gve sgnfcant

6 6 P.T. Mtkowsk et al. mprovement. The feasble membrane whch would meet that desgn s a commercally avalable polyvnyl alcohol membrane (PERVAP1005, GFT). X/(1-X) RCPV5 RCPV4 RCPV3 RCPV2 RCPV1 Batch t [mn] Fgure 3 Comparson of hybrd process systems wth batch n terms of converson 4. Conclusons A model-based computer-aded technque for systematc nvestgaton of hybrd systems has been presented along wth the applcaton of the technque to the study of a enzymatc esterfcaton reacton separaton system. The work-flow and the correspondng data-flow for the desgn methodology and the correspondng computer aded tools needed by the model-based technque have been developed and tested. Problem specfc hybrd process models were generated from the generc model for the llustrated and other case studes. From ths work, t s clear that hybrd processes consstng of a reactor and a membrane unt show advantages wth respect to achevng ncreased product yeld by overcomng lmtatons of knetcally controlled reactons and wthout addtonal process constrants. Acknowledgements P.T.Mtkowsk s pleased to acknowledge fundng to the PRISM the Mare Cure Research Tranng Network, EC s Sxth Framework Program. References 1. F. Lpnzk, R.W. Feld, P-K. Ten, Journal of Membrane Scence, 155, (1999), P.Mtkowsk, G.Jonsson, R.Gan, Comp.-Aded Chem.Eng., 21, (2006), R.Gan, C.Jm ene-gonz alez, D.J.C.Constable, Comp.&Chem. Eng., 29,(2005), T. Garca, A.Coteron, J.Aracl,,Chem. Eng. Scence 55,(2000), P. Adlercreutz, A.M. Lyberg, D.Adlercreutz, J. Lpd Sc. Technol, 105(2003), Z.Koszorz, Z.Zobrowsk, K.Belaf-Bako, R.Krupczka, Desalnaton, 162, (2004), M.Sales-Cruz, R. Gan, 2003, Eds. S.P. Asprey and S. Macchetto, Elsever, Amsterdam