Thermodynamic Analysis for Fischer-Tropsch Synthesis Using Biomass

Transcription

1 A publcaton of CHEMICAL ENGINEERING TRANSACTIONS VOL. 65, 2018 Guest Edtors: Elseo Ranz, Maro Costa Copyrght 2018, AIDIC Servz S.r.l. ISBN ; ISSN The Italan Assocaton of Chemcal Engneerng Onlne at Thermodynamc Analyss for Fscher-Tropsch Synthess Usng Bomass Fernando H. Marques, Regnaldo Gurardello* School of Chemcal Engneergng, Unversty of Campnas (UNICAMP), Campnas-SP, Brazl The use of bomass to produce envronmental cleaner fuels has been ganng more attenton n the past decades due to the soco-economc benefts over the petrochemcal route. For nstance, the converson of synthess gas (H 2 and CO) obtaned from bomass nto fuel usng Fscher-Tropsch (FT) process and catalysts, whch s conventonally based on ron and cobalt. Ths process generates a mxture of lght olefns, wax, gasolne, desel, and alcohols. In fact, operatng condtons (such as temperature, pressure, H 2 /CO rato and catalyst composton) can predct the FT products range. The thermodynamc modelng of ths system s calculated by phase and chemcal equlbrum, applyng global optmzaton technques to mnmze the Gbbs energy of the system, subject to the restrctons of stochometrc balances for each reacton consdered and non-negatvty of the number of moles. In addton, the catalytc effect s taken nto account n the constrants of the model. Thus, the purpose of ths work s the evaluaton of the chemcal equlbrum for the multphase mxture (alkenes, alkanes, alcohols, CO 2, H 2 O, H 2, and CO) n order to provde the phase composton for the FT process. The thermodynamc modelng was developed wth a non-lnear program (NLP) and solved by GAMS (General Algebrac Modelng System) software wth the CONOPT3 solver. Moreover, we analyzed the effect of operatng condtons (temperature, pressure, and H 2 /CO molar rato), consderng an deal system (Raoult s law) n the lqud phase for the thermodynamc characterzaton. 1. Introducton Consume of fuels has been ncreasng; accordng to Al & Dasappa (2016), the energy demand wll ncrease average about 1.6% annually untl These facts lead to a future wth more renewable resources, nstead of fossl energy, due to soco-economc benefts and envronmental mpact. The bomass has been a promsng energy source towards generaton of bofuels lke ethanol and desel. One of these technologes to get fuels from bomass s to convert bomass to synthess gas (H 2 and CO), followed by the converson nto fuels va Fscher-Tropsch Synthess (FTS) (Swan et al. 2011). Fscher-Tropsch Synthess s a catalytc process that produces a large mxture of hydrocarbons (HC), whch ncludes lght olefns, wax, gasolne, desel and alcohols. Ths process s partcularly complex due to dfferent seral reactons that occur, producng alkanes, alkenes, alcohols, and some equlbrum reactons, such as water-gas-shft (WGS), methane reacton and Boudouard reacton (Dry 2002). Fscher-Tropsch products are cleaner than fossl fuels, and qualty of them depend on operatonal condtons n whch the process occurs, on catalyst composton, and the reactor desgn. The practcal condtons that affects the FT process are temperature, pressure and the feed rato of syngas (.e., H 2 /CO rato). The commercal catalysts employed n ths process are based n cobalt and ron; although there s an abundance of research wheren nclude dfferent promoters n the catalyst to ncrease the product selectvty (Everson et al. 1978; Iglesa et al. 1992). The FT reactors can be separated n two classes, correspondng to work condtons, whch are: the Hgh Temperature Fscher-Tropsch (HTFT) and the Low Temperature Fscher- Tropsch (LTFT). The HTFT process nvolves two phases: one gas phase (reactants and products) and one sold phase (catalyst), operatng n a temperature range of 300 to 350 ºC. In the LTFT process there are three phases assocated; these reactors, known as slurry phase bubble column reactor, consst n gas, sold, and a lqud

2 phase where the catalyst partcles are mmersed. These reactors run n a temperature range of 200 to 250 ºC (Espnoza et al. 1999). There are several studes and nvestgatons to comprehend the mechansms and mprovng the FT products selectvty based on knetcs and thermodynamc models. Accordng to Norval (2008), an approach based n the thermodynamc equlbrum can descrbe the FT process performance. Ths method s smpler than knetcs models and can predct the products phase behavour. Besdes, t s possble to evaluate the effect of operatng condtons, such as the temperature, pressure and H2/CO rato (Pacheco & Gurardello 2017). Stenger & Askonas (1986) observed that the thermodynamc product dstrbutons s smlar to the expermental. In a recent research, Fretas & Gurardello (2016) developed a study for FT system combnng Gbbs energy mnmzaton and Soave-Redlch-Kwong equaton of state as optmzaton problem usng lnear programmng; and concluded that hgh pressure and low temperature provded the best condtons for formaton of hgh chan hydrocarbons. Marano & Holder (1997) have formulated a vapor-lqud equlbrum model for FTS whereby the solublty of gas and lght hydrocarbons was calculated usng a correlaton of pseudocomponent, as n-paraffn solvent, n the lqud phase for an ndvdual solute. For these reasons, ths paper amed to evaluate the effect of operatng condtons (temperature, pressure and feed composton) n the FT process, usng a thermodynamc analyss, n a reactor contanng a solvent as the lqud phase n the feed, lke a slurry phase reactor (three-phase). The method employed optmzaton technques, based n the mnmzaton of the Gbbs energy, wth smultaneous phase and chemcal equlbrum. We consdered the possblty of a system that may contan up to four phases: sold, gas, organc lqud and aqueous lqud phases. The thermodynamc analyss allowed to predct the products formed n the chemcal equlbrum for the FTS, and from that, t was possble to defne better operatng condtons to produce hgh weght hydrocarbons (desel and gasolne) and alcohols. 2. Methodology The thermodynamc equlbrum of a multcomponent and multphase system can be calculated va Gbbs energy mnmzaton, by Eq (1). When the system s n equlbrum, the Gbbs energy reaches a mnmum value (Smth & Mssen 1982). G = NC NP n = 1 = 1 μ (1) where and represent the number of moles and the chemcal potental of component n phase, respectvely. NC and NP are the number of compounds and the number of phases, respectvely. The chemcal potental can be calculated by Eq (2). 0 fˆ μ + RT ln μ = 0 f (2) In Eq (2), s the chemcal potental n the reference state, and t s calculated from the formaton propertes under a reference condton, as well as the fugacty n the reference state ( ). R represents the unversal constant of gases, and T s the system temperature. The fugacty of each component ( ) n the gas phase ( ) s obtaned from Eq (3), whle n the lqud phase ( ) t s calculated by Eq (4). f ˆ g g = y φˆ P (3) l sat f ˆ = x γ P (4) where and are the mole fracton of each component n the gas and lqud phases, respectvely; P s the pressure of the system and s the saturaton pressure. In ths paper, we have treated each ndvdual phase as an deal phase ( = 1 and = 1), usng Raoult s law n the lqud phase below the crtcal pont. When the crtcal temperature of one component s lower than the temperature of the system, then ths speces leaves ts vapor behavor, and becomes a gas, there beng no more vapor-lqud equlbrum, but a gas-lqud equlbrum due to the hgh temperature beng over the crtcal temperature of some components. Hence, Eq (4) cannot represent the fugacty n the lqud phase when the temperature of the system s hgher than the crtcal temperature of some component of the system, because there s no more saturated vapor pressure.

3 Thus t s possble to predct the speces dssolved n the lqud phase by usng Eq (5), also known as Henry s law, where s the Henry constant for component. l f ˆ = x H (5) To determne the fugacty n the lqud phase, we use a condtonal expresson between the Raoult s law and Henry s law. The Henry constants for some compounds n the organc lqud phase was calculated from data publshed by Marano & Holder (1997), consderng n-hexadecane as pseudo-solvent. The Henry constants for the compounds dssolved n the aqueous lqud phase were obtaned from NIST Standard Reference Database. In ths paper, we consdered the partal mscblty between the two lqud phases through the Henry s law to represent lqud-lqud equlbrum. The Gbbs energy mnmzaton was modeled as an optmzaton problem, and we consdered the followng constrants for ths model: the non-negatvty of the number of moles Eq (6) and the materal balance represented by stochometrc formulaton Eq (7), nstead of the atom balance. n 0 (6) NP = 1 n = n 0 + NR j= 1 ν ξ j j (7) where s the ntal number of moles of the component, represents the stochometrc coeffcent of the component n reacton, ξ j s the extent of reacton, and NR s the number of ndependent reactons. Other constrants ncluded n ths problem are the nhbton of methane formaton (Fretas & Gurardello 2015) and the lmtaton of CO 2 formaton as catalyst effect of cobalt (van Berge & Everson 1997). We also consdered the possble formaton of one sold phase, composed by coke. We assumed the sold phase as deal. In ths way, the system may present up to four phases: one gas phase, two mmscble or partally mscble lqud phases (organc and aqueous, each one modeled as an deal phase), and one sold phase. The Gbbs energy mnmzaton approach dsregard any knd of flow, snce t s a closed system, or knetc effect. Accordng to Pacheco & Gurardello (2017), wth the thermodynamc analyss, ts possble to predct the chan growth probablty values, ndependent of the mechansm and the catalyst. Ths optmzaton problem, formulated as nonlnear program (NLP), was solved by the software GAMS (General Algebrac Modelng System) usng the CONOPT algorthm. For the thermodynamc analyss of the system n the chemcal equlbrum, we have consdered the possblty of formaton of 20 lnear alkanes (C 1 C 20 ), 7 alpha alkenes (C 2 -C 8 ), 3 alcohols (C 1 -C 3 ), water, sold carbon and carbon doxde; plus the reactants hydrogen and carbon monoxde. These compounds were selected based on the lterature (Dry 2002). From ths model, we evaluated the effect of the temperature ( K), the pressure (5 50 bar) and the H 2 /CO nlet molar rato for 2.0 and 2.3, representng a slurry phase bubble column reactor. 3. Results and dscusson The model gven by Eq (8)-(10) descrbes the generc reactons that occurs durng the FTS to produce paraffns, olefns and alcohols, respectvely; followed by the water-gas-shft reacton Eq (11). From these equatons, the predcton of the best operatng condtons to generate hydrocarbons, wth hgher carbon number, was possble. Under all condtons analyzed n ths work no formaton of sold carbon was observed (8) (9) (10) + + (11) Frst, we have compared the effect of carbon monoxde converson for the temperature range of 400 to 900 K, pressure range of 5 to 50 bar and the H 2 /CO molars rato n the feed equal to 2.0 and 2.3, representng the usually H 2 /CO molar rato appled n the LTFT process (Al & Dasappa 2016). Fgure 1 shows the converson of carbon monoxde as functon of temperature and pressure for the H 2 /CO nlet molars rato of 2.0 and 2.3, respectvely. As can be seen, the ncrease of temperature leads to the decrease of the CO converson, whle the ncrease of pressure causes a rse n the CO converson. However,

4 ths pressure effect s more accentuated for hgh temperatures than low temperatures. Consderng the temperature of 400 K, from the pressure of 5 to 50 bar, there s an ncrease of 1.89%, whle the temperature of 900 K shows an ncrease of 43.65% at the same pressure range. Comparng the CO converson to both H 2 /CO molar rato, we noted that the CO converson s bgger for H 2 /CO molar rato equal to 2.3 than to 2.0. Ths obvously occurs because n the thermodynamc equlbrum, the ncrease of hydrogen nlet causes the elevaton of carbon monoxde consume, promotng hgh CO conversons. Prola et al. (2014) also have reported the molar rato effect at CO converson n ther expermental work usng ron loaded supported catalysts for H 2 /CO molars rato range of 1 to 2. (a) H 2 /CO molar rato = 2.0 (b) H 2 /CO molar rato = 2.3 Fgure 1: The effect of temperature, pressure and H 2 /CO nlet molar rato on the carbon monoxde converson The hydrogen yeld to hydrocarbon for both H 2 /CO nlet molar rato s greater than 57% for all temperature and pressure ranges (Fgure 2). The pressure lkewse does not sgnfcantly affect the hydrogen yeld to hydrocarbon as well as the carbon monoxde converson, as observed above. The H 2 yeld to HC decreases wth the ncrease of H 2 /CO nlet molar rato from 2.0 to 2.3. Pacheco & Gurardello (2017) related the maxmum yeld to HC as 72.4% and 66.1% for a feed molar composton of H 2 /CO of 2.0 and 3.0, respectvely. Whle n ths work, the maxmum hydrogen yeld to hydrocarbon corresponds to 62.3% and 61.3% for H 2 /CO nlet molar rato of 2.0 and 2.3, respectvely. (a) H 2 /CO molar rato = 2.0 (b) H 2 /CO molar rato = 2.3 Fgure 2: The effect of temperature, pressure and H 2 /CO nlet molar rato on the hydrogen yeld to hydrocarbon As seen n the Fgure 2, hgh temperatures exhbt an ncrease n the hydrogen yeld to hydrocarbon. However, ths feature does not lead to a good operatonal condton, because hgh temperatures ncrease the formaton

5 of hydrocarbons wth short chans. Whereas the hgh qualty fuels expected through FTS are hydrocarbons wth long chan, greater than C 5+, n the cuts of gasolne and desel. The Fgure 3 shows how the temperature and the pressure nfluence the dstrbuton of C 5+ Fscher-Tropsch products. As noted before, hgh temperatures have a negatve nfluence on the producton of long chan hydrocarbons durng Fscher-Tropsch process. The maxmum C 5+ molar fracton of FT products (36.6% of the FT products) s acheved at the temperature of 370 K, as shown n the Fgure 3. Whle temperatures above 490 K have a dstrbuton of C 5+ Fscher-Tropsch products lower than 5%. Pressures greater than 25 bar do not affect the product dstrbuton. Also, the product dstrbuton was ftted by the chan growth probablty factor (α) from a log-lnear straght lne (Norval, 2008). Alpha values for H 2 /CO molar rato of 2/1 and 530 K, at pressures of 25 and 40 bar s and 0.082, respectvely. Fgure 3: The effect of temperature and pressure n the dstrbuton of C 5+ Fscher-Tropsch products for H 2 /CO nlet molar rato of 2.0. (The calculatons were appled only at the ponts, and the curves were ftted) (a) (b) Fgure 4: (a) Effect of temperature and pressure on the propanol formaton durng FTS for H 2 /CO nlet molar rato of 2.0. (b) Effect of temperature n the dstrbuton of alcohols durng FTS for pressure of 50 bar and H 2 /CO nlet molar rato of 2.0. (The calculatons were appled only at the ponts, and the curves were ftted) For all condtons analyzed, the majorty of products formed durng Fscher-Tropsch process are alkanes. These alkanes are present n the gas and lqud organc phase. The lqud solvent exhbted a hgh nfluence to C 5+ be n the lqud phase n the equlbrum. But, n hgh temperatures (T > 450 K), only gas phase was formed, even n the presence of solvent. Moreover, we notced the presence of small traces of some hydrocarbons (from ethane to pentane) n the aqueous lqud phase, as result of the pressure ncrease. Usually, alcohols are co-products n the Fscher-Tropsch process, but they have a hgh value. Hence, we also evaluated the nfluence of operatonal condtons n the alcohol producton. The Fgure 4a shows that the

6 pressure has an mportant nfluence n the propanol formaton, such hgh levels of propanol mole fracton are acheved at pressures above 25 bar. Among the three alcohols consdered n ths analyss, the ethanol exhbted the larger mole fracton, over 60% for all temperatures, as can be vewed n the Fgure 4b. Hgh temperatures have a postve mpact on the formaton of methanol, unlke propanol, that shows a negatve effect over hgh temperatures. 4. Conclusons The thermodynamc analyss for Fscher-Tropsch synthess conducted by a condtonal expresson n the lqud phase between the Raoult s law and Henry s law had an accurate effect to represent both lqud phases (organc and aqueous). The temperature range of 370 to 510 K shows the best condtons for producton of paraffns, and co-products, such as alcohols. Moreover, we concluded that a hgher hydrogen yeld to HC at hgh temperatures does not mean a more favourable operatng condton, because the best condtons tested n ths work to acheve a desrable dstrbuton of long chan hydrocarbons are temperatures below 410 K, pressures above 25 bar, and H 2 /CO nlet molar rato of 2.0. Although a real FT system s not at chemcal equlbrum, the Gbbs energy mnmzaton wth the constrants of methane reacton and cobalt catalyst effect, on the WGS reacton, was able to predct the formaton of FT products under dfferent operatng condtons. Acknowledgments The authors would lke to thank the Coordenação de Aperfeçoamento de Pessoal de Nível Superor (CAPES) for all fnancal support. Reference Al, S.S. & Dasappa, S., Bomass to lqud transportaton fuel va Fscher Tropsch synthess - Technology revew and current scenaro. Renewable and Sustanable Energy Revews, 58, pp van Berge, P.J. & Everson, R.C., Cobalt as an alternatve Fscher-Tropsch catalyst to ron for the producton of mddle dstllates. Studes n Surface Scence and Catalyss, 107(15), pp Dry, M.E., The Fscher-Tropsch process: Catalyss Today, 71(3 4), pp Espnoza, R.L. et al., Low temperature Fscher Tropsch synthess from a Sasol perspectve. Appled Catalyss A: General, 186(1 2), pp Everson, R.C., Woodburn, E.T. & Krk, A.R.M., Fscher-Tropsch reacton studes wth supported ruthenum catalysts. Journal of Catalyss, 53, pp Fretas, A.C.D. & Gurardello, R., Predctve Thermodynamc Modellng of Lqud-Lqud-Vapor- Flud (LLVF) Equlbrum n Synthetc Hydrocarbon Synthess from Syngas. Chemcal Engneerng Transactons, 50(3), pp Fretas, A.C.D. & Gurardello, R., Thermodynamc characterzaton of hydrocarbon synthess from syngas usng fscher-tropsch type reacton. Chemcal Engneerng Transactons, 43, pp Iglesa, E., Soled, S.L. & Fato, R.A., Fscher-Tropsch synthess on cobalt and ruthenum. Metal dsperson and support effects on reacton rate and selectvty. Journal of Catalyss, 137(1), pp Marano, J.J. & Holder, G.D., Characterzaton of Fscher-Tropsch lquds for vapor-lqud equlbra calculatons. Flud Phase Equlbra, 138(1 2), pp Natonal Insttute of Standards and Technology, NIST Standard Reference Database 69: NIST Chemstry WebBook. Avalable at: [Accessed October 31, 2017]. Norval, G.W., Notes on the ssues of equlbrum n the Fscher-Tropsch synthess. Canadan Journal of Chemcal Engneerng, 86(6), pp Pacheco, K.A. & Gurardello, R., Analyss and Characterzaton of Fscher-Tropsch Products through Thermodynamc Equlbrum usng Global Optmzaton Technques. Chemcal Engneerng Transactons, 57, pp Prola, C. et al., Bosyngas converson by fscher-tropsch synthess: Expermental results and multscale smulaton of a pbr wth hgh fe loaded supported catalysts. Chemcal Engneerng Transactons, 37, pp Smth, W.R. & Mssen, R.W., Chemcal Reacton Equlbrum Analyss: theory and algorthms, New York: Wley New York. Stenger, H.G. & Askonas, C.F., Thermodynamc product dstrbutons for the Fscher-Tropsch synthess. Industral & Engneerng Chemstry Fundamentals, 25(3), pp