Performance evaluation of bioethanol production through continuous fermentation with a settling unit

Transcription

1 Unversty of Wollongong Research Onlne Faculty of Engneerng and Informaton Scences - Papers: Part A Faculty of Engneerng and Informaton Scences 2013 Performance evaluaton of boethanol producton through contnuous fermentaton wth a settlng unt Mark I. Nelson Unversty of Wollongong, mnelson@uow.edu.au Norhayat Hamzah Unverst Brune Darussalam Publcaton Detals Nelson, M. I. & Hamzah, N. (2013). Performance evaluaton of boethanol producton through contnuous fermentaton wth a settlng unt. Journal of Energy and Power Engneerng, 7 (11), Research Onlne s the open access nsttutonal repostory for the Unversty of Wollongong. For further nformaton contact the UOW Lbrary: research-pubs@uow.edu.au

2 Performance evaluaton of boethanol producton through contnuous fermentaton wth a settlng unt Abstract Ths paper analyses a model for the producton of boethanol that has been calbrated aganst laboratory data by prevous researchers. The authors nvestgate the mprovement n productvty that can be obtaned when a centrfuge s used to recycle cells that would otherwse leave the reactor system n the effcent stream. The authors compare the performance of a double reactor cascade, possble employng a settlng unt, aganst that of a sngle reactor. For the former case, ths paper consders the reactor confguraton n whch the settlng unt recycles from the effluent stream of a reactor back n the nfluent of the same reactor. Keywords boethanol, evaluaton, performance, unt, fermentaton, settlng, contnuous, producton Dscplnes Engneerng Scence and Technology Studes Publcaton Detals Nelson, M. I. & Hamzah, N. (2013). Performance evaluaton of boethanol producton through contnuous fermentaton wth a settlng unt. Journal of Energy and Power Engneerng, 7 (11), Ths journal artcle s avalable at Research Onlne:

3 Journal of Energy and Power Engneerng 7 (2013) D DAVID PUBLISHING Performance Evaluaton of Boethanol Producton through Mark Ian Nelson 1 and Norhayat Hamzah 2 1. School of Mathematcs and Appled Statstcs, Unversty of Wollongong, Wollongong 2522, NSW, Australa 2. Department of Mathematcs, Unversty Brune Darussalam, Bandar Ser Begawan BE 1410, Brune Receved: December 25, 2012 / Accepted: May 23, 2013 / Publshed: November 30, Abstract: Ths paper analyses a model for the producton of boethanol that has been calbrated aganst laboratory data by prevous researchers. The authors nvestgate the mprovement n productvty that can be obtaned when a centrfuge s used to recycle cells that would otherwse leave the reactor system n the effcent stream. The authors compare the performance of a double reactor cascade, possble employng a settlng unt, aganst that of a sngle reactor. For the former case, ths paper consders the reactor confguraton n whch the settlng unt recycles from the effluent stream of a reactor back n the nfluent of the same reactor. Key words: Boethanol, mathematcal modellng, productvty. Nomenclature C The recycle concentraton factor (-) D Dluton rate (h -1 ) F Flow rate nto tank (Lh -1 ) K 1, K 2 Saturaton constants (gl -1 ) m p Mantenance factor of ethanol (h -1 ) m s Mantenance factor of substrate (h -1 ) P Ethanol concentraton (gl -1 ) P c Lmtng ethanol concentraton for vable cells (gl -1 ) P' c Lmtng ethanol concentraton for non-vable cells (gl -1 ) Pr Productvty of ethanol (gl -1 h -1 ) R Recycle rato based on volumetrc flow rates (-) S Substrate concentraton (gl -1 ) S 0 Food substrate concentraton (gl -1 ) t Tme (h -1 ) X d Dead cell concentraton (gl -1 ) X nv Non-vable cell concentraton (gl -1 ) X v Vable cell concentraton (gl -1 ) V Volume of reactor (L) Correspondng author: Mark Ian Nelson, assocate professor, research feld: reacton engneerng. E-mal: nelsonm@member.ams.org. Y ( x p) Yeld coeffcent n converson from bomass to ethanol (-) Y ( x s) Yeld coeffcent n converson from bomass to substrate (-) Greek letters d Growth rate of dead cells (h -1 ) max Maxmum growth rate of vable cells (h -1 ) ' max Maxmum growth rate of non-vable cells (h -1 ) nv Growth rate of non-vable cells (h -1 ) v Growth rate of vable cells (h -1 ) Resdence tme (h) 1. Introducton The nterest n bofuels has ncreased markedly n recent years as they are envronmentally frendly and offer a mechansm to reduce relance on ol. One of the promsng bofuels s ethanol, whch can be derved from renewable sources such as lgnocellulosc waste-materals. Ethanol s a much cleaner fuel than gasolne, reducng CO levels by 25%-30% and dramatcally reducng emssons of hydrocarbons, a major contrbutor to the depleton of the ozone layer. Ethanol blends ncreasngly used worldwde as they

4 2084 Performance Evaluaton of Boethanol Producton through provde hgh octane at low cost and wthout the need for harmful fuel addtves. As long ago as 2002 more than 10% of all gasolne sold n the US contaned ethanol. Recent US legslaton has called for a sx fold ncrease n the use of ethanol to 136 bllon L per year by The model used n ths paper was developed by Jarzebsk [1] to explan oscllatons observed durng the contnuous producton of ethanol usng cultures of saccharomyces cerevsae. Ths extended an earler model proposed by Ghommdh et al. [2] whch accounted for oscllatons observed durng contnuous fermentaton usng Zymononas mobls. Features of the model are descrbed further n Secton 2.1. Jarzebsk estmated bochemcal parameter values for ths model usng laboratory data obtaned from the contnuous fermentaton of sugar-cane molasses at a temperature of 37 C reported by Perego et al. (1985) [3]. The behavour of ths model n a sngle reactor was nvestgated n Ref. [4]. The performance ncrease n yeld of ethanol n a cascade of upto three reactors was nvestgated n Ref. [5]. The mprovement n ethanol productvty that can be acheved by usng a reactor cascade of up to gve reactors was nvestgated n Ref. [6]. Other models for ethanol producton from renewable sources ncludes Refs. [7, 8]. The emphass n ths paper s to compare the productvty that can be obtaned n a sngle reactor aganst that obtaned nboth a sngle reactor and a two-reactor cascade employng a centrfuge to recover cell mass. For the reactor cascade, the authors consder a confguraton n whch the exst stream from a reactor s recycled nto the nfluent stream nto the same reactor. 2. Model Equatons 2.1 Bochemcal Model The authors use the bochemcal mechansm for the producton of ethanol gven n Ref. [1]. The cell populatons are broken nto three groups: vable cells ( X v ), non-vable cells ( X nv ) and dead cells ( X d ). Non-vable cells are non-growng, but retan the ablty to produce ethanol. The bologcal reactons are: S P; Xv 2Xv; Xv X nv; Xv X d ; X nv X d. where, S and P represent the substrate and ethanol respectvely. The reacton rates for these processes, whch are gven n Secton 2.2, nclude both substrate and product nhbton. The second reacton ( X 2X ) denotes vable cell dvson. v v 2.2 Governng Equatons The model equatons for a n reactor cascade wth recycle around each reactor are gven by: 1 1 dxv, V F R 1 Xv, 1 R Xv, V v, nv, d, Xv, dt 1 1, 1 1 nv nv, nv, v, d nv, dxnv, V F R X R X V X X dt 1 1, 1 1 d d, d v, nv, dxd, V F R X R X V X X dt dp v, X v, V FP 1 RV mpx nv, dt Yxp ds v, X v, V FS1 SV m' s X nv, dt Yxs (1) (2) (3) (4) (5)

5 Recycle parameter: Performance Evaluaton of Boethanol Producton through R j Rj Cj 1 (6) Total resdence tme n a cascade of n reactors: nv t (7) F where, denotes the th reactor n the cascade. (By conventon R0 C0 0 ). All parameters are defned n the nomenclature. The value of the reactor cascade parameter ranges 0 R j 1. Note that the mantenance terms (nvolvng m p and m s n Eqs. (4) and (5)) do not appear n Eq. (2) as ths process does not consume or produce non-vable cells. The formulaton of the growth rates ncludes both substrate lmtaton and product nhbton: S P S v1 max 1, K1S Pc K2 S v max 0, v1, S P S d1 max 1 K1S Pc K2 S v max 0, d1, S P S nv1 ' max 1 1 ' c 2, K S P K S vn max max 0, nv1 max 0, v1, S m' s ms 1 S The authors nvestgate the steady-state behavour of the system (1)-(5) and the reactor productvty, Pr, defned by: Pn Pr (8) t as a functon of the total resdence tme and the substrate concentraton n the feed ( S 0 ). For a sngle reactor and a double reactor cascade, the authors have n 1 and n 2, respectvely. 3. Results Fg. 1 shows the productvty of a sngle reactor wthout recycle as a functon of the resdence tme. The sold lnes ndcate a stable steady-state soluton whereas the dotted lnes ndcate an unstable steady-state, Productvty (g/l/h) Resdence tme (h) Fg. 1 Steady-state dagram for the productvty of a sngle tank wth no recycle. The optmal productvty s denoted by the bold crcle. Parameter value: feed substrate concentraton S 0 = 100 gl -1. Reprnted from Ref. [6]. soluton. When the resdence tme s below 4.12 h the washout soluton, where S S0, P 0, X j 0 s stable. The optmal productvty s -1-1 Pr 3.80 gl h whch occurs at a resdence tme 7.47 h. Although the maxmum ethanol concentraton s gven by -1 P gl when h the productvty obtaned at ths resdence tme s only -1 Pr 3.25 gl, about 15% less than the maxmum value. The authors now consder a sngle reactor employng a centrfuge to recycle bomass nto the feed stream. The performance of the centrfuge s charactersed by a sngle number, the recycle parameter, whch takes values rangng from R 1 0 representng operaton wthout a centrfuge, to R 1 1, a perfect centrfuge n whch all the bomass s recycled. Fg. 2 shows how the maxmum productvty of the reactor changed as the recycle parameter s vared. To obtan ths fgure the value of the recycle parameter s fxed. The varaton of the reactor productvty as a functon of the resdence tme s determned and the correspondng maxmum value selected. The performance of the reactor ncreases slowly at frst. When R the maxmum productvty has doubled to -1-1 Pr 7.6 gl h. As the value for the recycle parameter approaches one the lmt asymptotes to nfnty. Ths result s unrealstc and reflects the fact that n ths lmt the steady-state substrate concentraton becomes ndependent of the resdence

6 2086 Performance Evaluaton of Boethanol Producton through 40 4 Maxmum productvty (g/l/h) Maxmum Productvty (g/l/h) Productvty (g/l/h) (a) (b) Recycle parameter R * 1 R Recycle parameter 1 Fg. 2 Maxmum productvty as a functon of the recycle parameter for a sngle reactor. Parameter value: feed substrate concentraton, S 0 = 100 gl -1. tme. Thus the productvty can be made as large as you lke by takng the lmt that the resdence tme approaches zero. Ths unphyscal result ndcates that the bochemcal model s no longer realstc n ths lmt. Fg. 3 shows the productvty as a functon of the resdence tme for both a sngle reactor and a double reactor-cascade (only stable solutons have been plotted). Whereas the washout soluton s stable n a sngle reactor wthout recycle when 0 t (hr) 4.12 n a double reactor cascade wthout recycle t s stable when 0 t (hr) It s evdent that for most values of the resdence tme, the productvty from a sngle reactor outperforms that from a double reactor cascade. And ndeed the optmal productvty n the sngle reactor, Pr 1 = 3.80 gl -1 h -1 at the resdence 7.47 h, s margnally superor to the optmal productvty n the double reactor cascade, Pr 2 = 3.77 gl -1 h -1 at the resdence tme 10.6 h. Thus n the absence of recycle the sngle reactor outperforms the double reactor wth a feed concentraton S 0 = 100 gl -1. Fg. 4 showed the varaton n the maxmum productvty as a functon of the recycle parameter for three reactor confguratons. These are: a sngle reactor wth a centrfuge (lne a); a double-reactor cascade wth a centrfuge operatng around the frst reactor (lne b) and a double-reactor cascade wth a centrfuge operatng around the second reactor (lne c) Total resdence tme (hours) Fg. 3 Steady-state productvty as a functon of the resdence tme for a sngle reactor (a) and a double reactor cascade; (b) In both cases there s no recycle. Parameter value: feed substrate concentraton, S 0 = 100 gl -1. Maxmum Maxmum productvty Productvty (g/l/h) Total resdence tme (h) Recycle parameter R * R Recycle parameter 1 Fg. 4 Maxmum productvty as a functon of the recycle parameter for a sngle reactor (lne a) and a double reactor cascade (b & c). Parameter value: feed substrate concentraton, S 0 = 100 gl -1. Parameter values: (b) R 2 0, R 1 as specfed; (c) R 1 0, R 2 as specfed. It s seen that the reactor cascade wth the centrfuge placed around the frst reactor outperforms the reactor cascade wth the centrfuge placed around the second reactor. However, the sngle reactor wth a centrfuge outperforms both reactor cascade confguratons. Hence, when the nflow substrate concentraton s S 0 = 100 gl -1, the best reactor confguraton s the sngle reactor. In Fg. 5, the feed concentraton has been ncreased to S 0 = 120 gl -1. The authors note two mportant dfferences between ths fgure and Fg. 4. Frstly, t s observed that although the productvty n the cascade (a) (b) (c)

7 Performance Evaluaton of Boethanol Producton through 2087 Productvty Productvty (2) (b) (1) (a) (3) (c) Recycle parameter R * R Fg. 5 Maxmum productvty as a functon of the recycle parameter for a sngle reactor (lne a) and a double reactor cascade (b & c). Parameter value: feed substrate concentraton, S 0 = 120 gl -1. Parameter values: (b) R 2 0, R 1 as specfed; (c) R 1 0, R 2 as specfed. confguraton s maxmsed by placng the centrfuge around the frst reactor the dfference n performance between ths confguraton and the one n whch t s placed around the second reactor s mnmal. Secondly, t s seen that for low values of the recycle parameter the productvty obtaned from a double reactor cascade wth a centrfuge s hgher than that acheved by a sngle reactor wth a centrfuge. However, the latter combnaton s superor for hgher valued of the recycle parameter. The value of the recycle parameter at whch ths transton occurs s approxmately R Conclusons Recycle parameter 1 In ths paper, the authors have nvestgated the productvty of ethanol producton through contnuous fermentaton n a sngle tank and n a cascade of two reactors. In partcular, the authors have nvestgated the ncrease n productvty that s obtaned when the reactor confguraton contans a centrfuge. For the double reactor cascade t has been assumed that the centrfuge recycles bomass from the effluent stream of reactor ( = 1, 2), nto the nfluent stream of reactor. For all three confguratons consdered the effect of the centrfuge can be charactersed by a sngle number, the recycle parameter R. In all cases, the authors treated the total resdence tme as the prmary bfurcaton parameter and determned the value of the total resdence tme whch maxmsed the productvty of the reactor confguraton. It s shown that at feed concentratons S 0 = 100 gl -1 and S 0 = 120 gl -1 that the cascade confguraton wth the centrfuge placed around the frst reactor was superor to that wth the centrfuge paced around the second reactor. However, at the hgher feed concentraton the dfference was slght. At the lower feed concentraton the performance of the sngle reactor confguraton was found to be superor to that of ether cascade. At the hgher feed concentraton the double reactor cascade for low values of the recycle parameter, approxmately for R 0. 3, but at hgher values of the recycle parameter the sngle reactor was agan the best desgn. Thus, the authors conclude that more,.e., an ncreased number of reactors, s not always better when t comes to maxmsng the reactor productvty. The authors are currently extendng the work reported her by consderng hgher feed concentratons. The bochemcal parameters n ths model were estmated by Jarzebsk [1] from expermental data obtaned by Perego et al. [3]. Unless otherwse stated, the parameter values the authors use n ths study are those gven n Ref. [1]: µ max = 0.25 h -1, µ max = 0.21 h -1, P c = 70 gl -1, P c = 130 gl -1, m p = 2.6 h -1, m s = 4.42 h -1, Y (x p) = 0.235, Y (x s) = and K 1 = K 2 = 3 gl -1. Acknowledgments The authors thank Dr. J. Kavanagah (Chemcal Engneerng, Unversty of Sydney) for hs thoughtful nput nto ths work. References [1] A.B. Jarzebsk, Modellng of oscllatory behavour n contnuous ethanol fermentaton, Botechnology Letters 14 (2) (1992) [2] C. Ghommdh, J. Vaja, S. Bolarnwa, J.M. Navarro, Oscllatory behavour of zymomonas n contnuous cultures: A smple stochastc model, Botechnology Letters 2 (9) (1989)

8 2088 Performance Evaluaton of Boethanol Producton through [3] L. Perego, J.M.C.D. Das, L.H. Koshmzu, M.R.D. Cruz, W. Borzan, M.L.R. Varo, Influence of temperature, dluton rate and sugar concentraton on the establshment of steady-state n contnuous ethanol fermentaton of molasses, Bomass 6 (3) (1985) [4] S.D. Watt, H.S. Sdhu, M.I. Nelson, A.K. Ray, Analyss of a model for ethanol producton through contnuous fermentaton, ANZIAM Journal [Onlne], Vol. 49, 2007, cle/vew/322. [5] H.S. Sdhu, J. Kavanagh, S.D. Watt, M.I. Nelson, Performance evaluaton of ethanol producton through contnuous fermentaton, n: Proceedngs of the 36th Australasan Chemcal Engneerng Conference, CHEMECA, Australa, 2008, pp [6] S. Watt, H.S. Sdhu, M.I. Nelson, A.K. Ray, Analyss of a model for ethanol producton through contnuous fermentaton: Ethanol productvty, Internatonal Journal of Chemcal Reactor Engneerng [Onlne]. Publshed Onlne: Vol. 52, 2010, pp. 1-17, [7] P. Garhyan, S.S.E.H. Elnashae, S.M. Al-Haddad, G. Ibrahm, S.S. Elshshn, Exploraton and explotaton of bfurcaton/chaotc behavor of a contnuous fermentor for the producton of ethanol, Chemcal Engneerng Scence 58 (2003) [8] P. Garhyan, S.S.E.H. Elnashae, Bfurcaton analyss of two contnuous membrane fermentor confguratons for producng ethanol, Chemcal Engneerng Scence 59 (2004)