Design of Reactive Distillation with Thermal Coupling for the Synthesis of Biodiesel using Genetic Algorithms

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1 19 th European Symposum on Computer Aded Process Engneerng ESCAPE19 J. Jeowsk and J. Thulle (Edtors) 2009 Elsever B.V./Ltd. All rghts reserved. Desgn of Reactve Dstllaton wth Thermal Couplng for the Synthess of Bodesel usng Genetc Algorthms Erck Yar Mranda-Galndo a, Juan-Gabrel Segova-Hernández a, Salvador Hernandez a, Guadalupe de la Rosa Álvarez a, Clauda Gutérrez-Antono b, Abel Brones-Ramírez c a Unversdad de Guanajuato, Departamento de Ingenería Químca, Nora Alta s/n, Guanajuato, Gto., Méxco, 36050, E-mal:gsegova@qujote.ugto.mx b CIATEQ, A.C., Av. del Retablo 150, Col. Fovssste, 76150, Querétaro, Querétaro, Méxco. c Innovacón Integral de Sstemas S.A. de C.V., Lmas No. 5 Manzana C, Fracconamento Don Manuel, 76114, Querétaro, Querétaro, Méxco. Abstract The esterfcaton of laurc acd and methanol s explored usng a thermally coupled dstllaton sequence wth a sde rectfer and the Petlyuk dstllaton column. The study was conducted usng as a desgn tool a mult objectve genetc algorthm wth restrctons.the product of the esterfcaton can be used as bodesel. It was found that the thermally coupled dstllaton sequence nvolvng a sde rectfer can produce bodesel wth a hgh purty (around 0.999) and also pure water, and the excess of methanol s recovered n a sde rectfer. The results ndcate that the energy consumpton of the complex dstllaton sequence wth a sde rectfer can be reduced sgnfcantly by varyng operatonal condtons. These reductons n energy consumpton can be nterpreted as reductons n carbon doxde emssons. Keywords: reactve dstllaton, bodesel, genetc algorthm, energy consumpton 1. Introducton Due to ncreased energy demand and envronmental concerns worldwde, mportant research s currently underway on bofuels and alternatve energes, e.g., bodesel, bomass, boethanol. In the case of bodesel, t has been reported that ts producton can be compettve wth fossl desel when the prce of crude ol reaches USD 100 per barrel [1]. As a result, mportant process ntensfcaton polces have been taken nto account n the desgn of new processes, due to reducton n ol reserves, and for mnmzaton of carbon doxde emssons and use of alternatve energes. Attenton has been pad to these mportant aspects n the process systems engneerng area of chemcal engneerng. For example, n a chemcal plant, energy consumpton n a separaton process such as dstllaton can be up to 40% of total consumpton. As a result, researchers n the feld of dstllaton are developng new confguratons that can be capable of reducng both energy consumpton and carbon doxde emssons [2]. One alternatve that has been explored n detal s the use of thermally coupled dstllaton sequences (TCDS) that can acheve energy savngs between 30 and 50 percent over conventonal dstllaton sequences for the separaton of some multcomponent mxtures. These energy savngs have been predcted usng steady state smulaton and 549

2 E. Y. Mranda-Galndo et al., mathematcal programmng; also, ther theoretcal control propertes and dynamc behavor have also been determned [3]. Based on these studes, practcal mplementaton of thermally coupled dstllaton sequences has been conducted usng dvdng wall columns. Reactve dstllaton s consdered to be the most representatve ntensfcaton operaton because t combnes reactons and separaton n a sngle process unt. As a result, TCDS optons can be used to carry out reactons of esterfcaton of fatty organc acds, and the produced esters can be used as bodesel. Ths leads to mportant processes to produce bofuels usng complex dstllaton systems that can reduce energy consumpton, captal costs, and carbon doxde emssons. Thus, n ths paper, the producton of bodesel by esterfcaton of methanol and laurc acd s studed usng a thermally coupled dstllaton sequence wth a sde column and the fully thermally coupled dstllaton sequence. We have selected these dstllaton sequences because, for the separaton of ternary mxtures, there are two types of thermally coupled dstllaton sequences: TCDS wth sde columns and the fully thermally coupled dstllaton sequence (Petlyuk dstllaton sequence). The schemes are depcted n Fgure Strategy soluton In order to optmze the thermally coupled reactve sequences we used the multobjectve genetc algorthm wth constrants coupled to Aspen Plus, developed by Gutérrez-Antono and Brones-Ramírez [4]. Ther algorthm manages the constrants usng a multobjectve technque based on the concept of non domnance proposed by Coello-Coello [5]. For the reactve thermally coupled systems the optmzaton problem ncludes as objectves the mnmzaton of the total number of stages, the sze of the reactve secton and the heat duty of the sequence, but t also consders the nterconnecton flows: Mn ( Q, N, N ) = f ( R, N, N, N, N 2, F, N ) subject to y x m m R F, r1 r k k (2) Where R s the reflux rato, N F, s the number of the feed stage and N s the number of stages of the column of the sequence, N r1 and N r2 are the ntal and fnal stages of the reactve secton N R n the column j, y m and x m are vectors of obtaned and requred purtes for the m components, respectvely. F k and N k are the value and locaton of the nterconnecton flow k. In the reactve thermally coupled dstllaton sequences, there are four objectves to mnmze: the number of stages n each column, the sze of the reactve secton and the heat duty of the sequence. For the sequences the objectves are n competton, so they have to be optmzed smultaneously. The manpulated varables nclude reflux rato, total number of stages, value and locaton of the nterconnecton flows, and sze of the reactve secton. For the thermally coupled reactve dstllaton sequences we used 2000 ndvduals and 40 generatons as parameters of the algorthm. These parameters were obtaned through a tunng process, where several runs of the algorthm were performed wth dfferent number of ndvduals and generatons. 550

3 Desgn of Reactve Dstllaton wth Thermal Couplng for the Synthess of Bodesel usng Genetc Algorthms 3. Case study The esterfcaton process can be represented conceptually by equaton 3. Alcohol + Fatty Acd Ester + Water (3) Ths equlbrum reacton can be favored f the products are removed as the reacton proceeds. An addtonal problem may present tself, dependng on the acd and the alcohol used, as bnary or ternary homogeneous azeotropes can be formed n the reactve system. For hghly nondeal systems, heterogeneous azeotropes can be formed. These key factors must be consdered to select the approprate thermodynamc model when the system s studed wth process smulators. For ths class of reactve systems, thermodynamc models such as NRTL, UNIFAC or UNIQUAC can be used to calculate vapor-lqud or vapor-lqud-lqud equlbrums. The systems nclude two feed streams; the frst s laurc acd wth a flow of 100 lbmol/h as saturated lqud at 1.5 bar, and the second s methanol wth a flow of 120 lbmol/h as saturated vapor at 1.5 bar. The reactve system s catalyzed usng sulphurc acd. A mass fracton of was assumed for the purty of the bodesel stream. WATER MEOH LAURIC-A B2 B3 4 5 METHANOL BIODIESE (a) Reactve TCDS wth a sde rectfer. B5 LAURIC-A B METHANOL BIODIESE (b) Reactve Petlyuk column. Fgure 1. Reactve TCDS for the producton of bodesel. 551

4 E. Y. Mranda-Galndo et al., 4. Results The composton profles of the optmzed desgns were analyzed n order to determne bodesel composton. Ths s very mportant because the amount of acd s crtcal n motor vehcles. Fgure 2 presents the composton profles n the lqud phase for the reactve TCDS wth sde rectfer, as a representatve profle of the analyzed reactve systems. In the case of the reactve TCDS opton wth sde rectfer, t s observed that t s possble to obtan almost pure bodesel as the bottom product of the man column (mass fracton equals 0.999). In the dstllate product of ths column, the water produced n the reacton s removed, and the excess of methanol s recovered n the sde rectfer column. Ths methanol, of course, could be returned to the reactve dstllaton column n order to obtan a more effcent reactve dstllaton process. When the composton profles for the Petlyuk dstllaton column are analyzed, a smlar result s obtaned n terms of the purty of the bodesel. Mass fracton n the lqud phase WATER METHANOL LAURIC-A BIODIESE Stage Fgure 2. Composton profles n the lqud phase of the man column of the reactve TCDS wth a sde rectfer. For these complex reactve dstllaton sequences, Pareto front ncludes the complete set of optmal desgns that satsfy the requred purtes: from mnmum reflux rato to mnmum number of stages, and all desgns between them. In ths way, the engneer can establsh the proper tradeoff between energy and equpment accordng to hs partcular needs, both actual and future. In ths study, we choose the optmal values of 1059 and 4083 kw, for the reactve TCDS wth sde rectfer and the reactve Petlyuk column respectvely, snce, for us, they represent a good compromse between the objectves. Regardng envronmental aspects, Kencse and Mzsey [6] have reported that, n fact, gas emssons are drectly lnked to energy consumpton snce, n the chemcal ndustry, the energy requred n dstllaton s obtaned from crude ol. As a result, reductons n energy consumpton can be translated nto reductons n carbon doxde emssons. Ths mportant fact can be observed n Fgure 3. Accordng to Fgure 3, the carbon doxde emsson can be ncremented sgnfcantly when the operatonal condtons are dfferent to those correspondng to the optmum. Ths pont s mportant, because n terms of control and operatonal aspects, t has been reported [7] that the control propertes of 552

5 Desgn of Reactve Dstllaton wth Thermal Couplng for the Synthess of Bodesel usng Genetc Algorthms coupled schemes can be mproved when the operatonal condtons fall outsde the optmum. Ths s mportant because n the selecton of the operatonal condtons, the engneer must take nto account the fact that savngs n carbon doxde emssons can be acheved wth more efforts n the control system. Fnally, regardng recent advances n the use of dvdng wall dstllaton columns, t s possble to propose a sngle dstllaton column usng a dvdng wall and a sde condenser. Addtonally, ths dea leads to reductons n captal costs. The proposed scheme s shown n Fgure 4. Ths complex dstllaton scheme must be subjected to a control study n order to antcpate potental operatonal problems for set pont trackng and load rejecton. Ths topc s currently under study, but t s needed a knetc model to obtan a dynamc model of the reactve system [8]. Fgure 3 Increase n carbon doxde emssons for dfferent operatonal condtons n the thermally coupled dstllaton sequence wth a sde rectfer. Fgure 4. Practcal mplementaton of the reactve TCDS wth a sde rectfer. 553

6 E. Y. Mranda-Galndo et al., 5. Conclusons The esterfcaton of methanol and laurc acd usng sulphurc acd as catalyst was studed n a thermally coupled dstllaton sequence wth a sde column and the Petlyuk dstllaton column usng a mult objectve genetc algorthm wth restrctons. The thermally coupled dstllaton sequence wth a sde rectfer was the best opton n terms of energy consumpton and purty of bodesel n the product. The results for the reactve complex dstllaton sequence wth a sde column showed that energy consumpton can be reduced drastcally dependng on operatonal condtons, and for condtons dfferent than those of the optmal soluton, carbon doxde emssons can ncrease sgnfcantly. Fnally, a practcal mplementaton usng a sngle column wth a dvdng wall s proposed. 6. Acknowledgements We acknowledge the fnancal support provded by Unversdad de Guanajuato, CONACyT and CONCyTEG (Mexco). References [1] M. Frondel and J. Peters, Energy Polcy, 35 (2007) 1675 [2] M. Masca, F. Ferrara, A. Vacca, G. Tola and M. Errco, Appl. Therm. Eng., 27 (2007) 1205 [3] J. C. Cárdenas, S. Hernández, I. R. Gudño-Mares, F. Esparza-Hernández, C. Y. Iranda- Araujo and L. M. Domínguez-Lra, Ind. Eng. Chem. Res., 44 (2005) 391 [4] C. Gutérrez-Antono and A. Brones-Ramírez, Computers and Chemcal Engneerng, In Press (2008). [5] C. A. Coello-Coello, Cvl Engneerng and Envronmental Systems 17, (2000) 319 [6] H. Kencse and P. Mzsey, In proceedngs of 17th European Symposum on Computer Aded Process Engneerng (ESCAPE), Elsever (2007) 883 [7] M. Serra, A. Spuña and L. Pugjaner, Ind. Eng. Chem. Res., 42 (2003) 1773 [8] S. Stengeweg and J. Gmehlng, Ind. Eng. Chem. Res., 42 (2003)