CODEN JYYHE6/ISSN 1001-4160 Computers and Applied Chemistry 2013, V ol.30, No.3, 256-260
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Received: 2012-11-20; Revid: 2013-01-15
Foundation item: Supported by State science and technology support program(No.2006BAE02B02)
新东方戚颖Author information: Li Fang(1975—), Female, doctor, Speciality: Chemical Engineering, E-mail: Tutor: Ying Weiyong(1957—), Male, Professor, Ph.D. Supervisor,E-mail: wying@ecust.edu
Thermodynamic analysis of differentroutes of ethanol synthesis
Li Fang 1,2, Zhang Ke 1, Ma Hongfang 1, Zhang Haitao 1, Ying Weiyong 1* and Fang Dingye 1
(1. Engineering Rearch Center of Larger Scare Reactor Engineering and Technology, Ministry of Education, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China)
(2. School of Biochemical Engineering, Anhui Polytechnic University of Technology and Science, Wuhu 241000, Anhui, China)
Abstract:Thermodynamic analysis of ethanol (EtOH) synthesis using the Gibbs free energy minimization method for three routes(namely, direct synthesis from syngas, methanol homologation and hydrogenation of acetic acid (HAC)) have been performed in the range of (373- 873) K and (0.1-10) MPa in this study. The conversion of key feedstock and product equilibrium composition are obtained as a function of temperature and pressure. For direct synthesis of ethanol, low temperature favors the CO equilibrium conversion. CO conversion would changes from 100% to 1.56% with increasing temperature from 373 K to 873 K at 2 MPa. For the process from methanol homologation, nearly 100% methanol conversion could be gained at all temperatures and pressures. Methanol would be nearly converted into ethanol via homologation reaction at lower temperature (T <473 K) and almost decompod into CO and H 2 at higher temperature(T >673 K). HAC conversion can reach higher value (>70.79%) under the conditions studied for HAC hydrogenation. The maximum et
hanol equilibrium content was 50 %, 50 % and 20 % respectively for the above three routes at T =373 K, P =10 MPa.
Keywords: thermodynamic analysis, ethanol, synthesis from syngas, methanol homologation, hydrogenation of acetic acid
1 Introduction
In recent years, ethanol (EtOH) have spurred increasing interest as a result of limited oil resources, especially to china, a developing country with great demand for energy resource. Ethanol is commercially produced via two routes, namely, fermentation of sugars from corn or sugar cane and hydration of petroleum bad ethylene [1]. Thetwo routes dependent on food or crude oil, bothare unattractive for larger scale production in China. Gasificationroutes to ethanol can be a significant advantage in making the process economically competitive [2-4]. Coal, natural gas or biomass can be transformed into syngas, and then converted into ethanol and higher alcohols. Subramani [5]reported three routes for the process making ethanol from syngas, namely, (A) direct conversion of syngas (B) methanol(MeOH) homologation (C) acetic acid(HAC) hydrogenation (e Fig.1).
Although the alcohol synthes of the schemes from syngas are not carried to equilibrium, reactor
design is bad primarily on reaction rate. However, the choice of operating conditions may still be influenced by equilibrium considerations. Moreover, the equilibrium conversion of a reaction provides a goal by which to measure improvements in a process [6]. Therefore, there is still a need to carry out thermodynamics
Fig.1 Schematic of ethanol synthesis from different three routes.
CHEMCAD is powerful and flexible chemical process simulation software.This program is ud extensively around the world for the design, operation, and maintenance of chemical
process in a wide variety of industries. The aim of this paper is to analyze the equilibrium content of products and conversion of key feed for the above three routes with varying temperature (T ) and pressure (P ) and the CHEMCAD software was ud to the calculation.
2 Methodology
For scheme A and B, methanation reaction is so dominant thermodynamically compared to the other reactions that the methane was the only significant product over the range of conditions studied [7].Therefore, methanation reaction is not considered below.
The reactions considered in scheme A were Eq.(1) and Eq.(2). Eq.(1) is the main reaction of ethanol formation from syngas. Eq.(2) reprents water gas shift reaction (WGS), which is very active for F-T system.
笔友的英文
22522CO 4H C H OH H O ++U (1)
222CO H O CO H ++U (2)
The reactions considered in scheme B wereEq.(2)-Eq.(4). Eq.(3) is the main reaction of ethanol formation from methanol and syngas.
32252CH OH CO 2H C H OH H O +++U (3) 32CH OH CO 2H ++U (4)
The reactions considered in scheme C were Eq.(5) and Eq.(6), which reprent main reaction making ethanol from HAC and esterification reaction formatting ethyl acetate (EtOAc) respectively.
()()()()32252CH COOH g 2H g C H OH g H O g ++R (5)
()()()3323252CH COOH g CH CH OH g CH COOC H g H O(g)++R (6)
2.1 Thermodynamics parameters ∆H R,T , K P ,and ∆G T
The formation of reaction heat ∆ , ) can be calculated from Eq.(7). The reaction equilibrium constant(K P ) can be obtained from Eq.(8).Gibbs free energy ∆ can be expresd as Eq.(9).
T
00
,,298P
R 298
(()()R T R P P H H C
C Δ=Δ+
−∑∑∫ (7)
2013, 30(3)
Li Fang, et al: Thermodynamic analysis of different routes of ethanol synthesis.
257
grow
02982
298
ln d 298R T K
R P G H K T RT −ΔΔ=+∫ (8) 00,000
,011d d ΔΔ−ΔΔΔ=++Δ−∫∫T T
P m T P m T T C G H H G C T T RT RT RT RT R T (9) 2.2 Equilibrium composition
The minimum Gibbs free energy of the system can be
expresd as
()0ln ln ln 0ϕλαΔ++++=∑i f i i k ik k
G RT P RT y RT (10)
The equilibrium composition were calculated using the
Gibbs reactor model entitled in the ChemCad 6.0 software, in which Peng-Robinson equation was chon for the fugacity coefficient calculation.
3 Results and discussion
3.1 Reaction heat, Gibbs freeenergy changes
andequilibrium constant
Reaction heat,Gibbs free energy changes and equilibrium constantof ethanol formation via direct synthesis from syngas and methanol homologation and HAC hydrogenation pathways have been calculated in the temperature range between 373 K and 773 K. The changes in enthalpy for three routes are listed in Table1. It can be en from Table 1, all the reactions are exothermic. Table 2 shows that reaction (1) is unfavorable above 553 K. Reaction (3) is thermodynamic favorable below 723 K, Reaction (5) approaches equilibrium at about 543 K, indicating that this reaction is thermodynamically limited at higher temperature.
Table 1 Reaction heat for three routes.
T /K
大货
Reaction heat /(kJ)
Reaction (1)
Reaction (2) Reaction (3) Reaction (5)
Reaction
(6)
373 -260.67 -40.78 -167.21 -84.35 -15.66 423 -263.59 -40.46 -168.37 -83.36 -15.11 473 -266.15 -40.09 -169.36 -82.23 -14.64 523 -268.38 -39.67 -170.20 -80.86 -14.26 573 -270.32 -39.22 -170.91 -79.01 -13.94 623 -271.98 -38.74 -171.50 -76.22 -13.68 673 -273.40 -38.25 -171.99 -73.17 -13.48 723 -274.62 -37.74 -172.40 -70.13 -13.34 773 -275.64 -37.22 -172.74 -67.10 -13.26
Table 2 Gibbs free energy of reactions.
T /K
Reaction Gibbs free energy changes/(kJ) Reaction
(1)
Reaction (2) Reaction (3) Reaction (5)
Reaction
(6)
fromnowon373 -87.98 -25.49 -79.62 -14.17 -10.95 423 -64.64 -23.46 -67.80 -10.16 -10.35 473 -40.97 -21.47 -55.86 -6.03 -9.82 523 -17.05 -19.53 -43.81 -1.78 -9.33 573 7.069 -17.62 -31.69 2.57 -8.87 623 31.35 -15.76 -19.52 7.10 -8.44
673 55.75 -13.93 -7.31 11.53 -8.03
723 80.25 -12.14 4.94 16.12 -7.63 773 104.82 -10.39 17.22
20.77
-7.24
Equilibrium constant K P for reactions was listed in Table 3. The results show that K P decrea wit
h increa of temperature due to exothermic nature of the reactions. At lower temperature, reaction (1) has a higher value of K P than the other reactions and approaches zero at 773 K. WGS reaction (reaction (2)) keeps higher K P value even at higher temperature. Table 3 Reaction equilibrium constant of reaction.
T /K
Reaction equilibrium constant(K P )
(1) (2) (3) (5)
(6)
373 2.10×1012
3.72×103 1.42×1011 9.71 3
4.17
4239.61×107 7.90×102 2.36×108 3.97 19.01 473 3.35×104 2.35×102 1.48×106 0.33 12.15 523 5.0
5 8.92 2.38×104 4.53×10-2 8.55 573 2.27×10-1 4.04 7.76×102 9.08×10-3 6.44 623 2.35×10-3 2.10 4.34 2.45×10-3 5.11 673 4.71×10-5 1.21 3.69 8.39×10-4 4.20 723 1.59×10-67.54 0.44 3.46×10-4 3.56 773
8.24×10-8
5.04
6.86×10-2 1.65×10-4 3.09
3.2 Equilibrium composition
Equilibrium compositions of three routes were calculated using as parameters temperature 373 K-873 K, and the pressure (0.1-10) MPa. According to literature, the mole ratio of feed was fixed at H 2/CO =2 for scheme A, MeOH/CO/CO 2=1/1/2 for scheme B, H 2/HAC=4/1 for scheme C. 3.2.1 Direct conversion from syngas
EtOH, CO 2, H 2O equilibrium content(mol, %) and CO conversion as a function of temperature and pressurewere depicted in Fig.2(a)-Fig.(d) respectively. EtOH and H 2O equilibrium content monotonically decrea with the increa of temperature. The content of CO 2 increas with tempe
rature until it reaches a maximumvalue of 14% at about 620 K. After maxima, the values show a quicker decrea. As temperature ri from 500 K to 873 K, CO conversion remarkable decreas. 100 % CO conversion can be obrved at lower temperature.
(a) EtOH
(b) H 2O
258Computers and Applied Chemistry / E-mail: jshx@home.ipe.ac 2013,
30(3)
(b) CO
(d) CO2
Fig.2 Plot of thermodynamic equilibrium mole fraction of each
component and CO conversion as a function of pressure and
temperature when route A was employed.
CO2 is formed via shift reaction, who equilibrium is not nsitive to pressure. However, it shows a complicated trend with the increa of pressure. At low temperature, CO2 content decread as pressure increa. At middle range of temperature, it increa initially but decrea afterwards. At high temperature, it increas with the increa of pressure. The phenomena may be owning to the competition between alcohol synthesis reaction (Eq.(1) and water gas shift reaction (Eq.(2)). For H2O equilibrium content, Fig.2(c) show the same trend as EtOH does. Fig.2(d) illustrates that the equilibrium conversion of CO increas a little at lower temperature and remarkably increas when the pressure ris. Most experimental rearch on the formation of ethanol has been conducted in the temperature range of (500-700) K and pressure range of (1-8) MPa[8-10].
3.2.2 Methanol homologation
(a) CO2
(b) Methanol
(c) CO
(d) EtOH
Fig.3 Plot of thermodynamic equilibrium mole fraction of each
component and CO conversion as a function of pressure and
temperature when route B is employed.
The product distribution and CO conversion as a function of temperature and pressurewere shown in Fig.3. It is en that the EtOH content shows to be very nsitive to the changes of temperature. A low temperature can improve the equilibrium composition of EtOH. For instance, at 2MPa, the content of EtOH is approximately decreas from 45.75% to 0.20% when temperature ranges from 423 K to 873 K. It can be obrved from the Fig.3(a) that initially the CO2 content increas very sharply with the increa in temperature but after that (about 500 K) there a slow increa. At low temperature, CO conversion can reach the highest value of 100%. However, the CO conversion decreas greatly with increasing temperature, even to a negative value. This phenomena happens is the result of the decomposition of methanol to CO and H2[5].
Higher content of EtOH, CO2, and H2O were found at higher pressure. Higher pressure also favors the COconversion. The effect of pressure on them prents some difference in detail. For example, at 523 K, the content of CO2
incread 26.4%,
2013, 30(3)Li Fang, et al: Thermodynamic analysis of different routes of ethanol synthesis. 259 when pressure is in the range of (3-6) MPa. In the same range of
pressure, the equilibrium composition of EtOH varies from
38.30 % to 42.55 %, while CO conversion differs from 99.93 %
to 99.97 %. However, at higher temperature, there is no
apparently change of EtOH equilibrium mole fraction when
pressure increas, since at this level the distribution of the
products is mainly determined by water gas shift equilibrium, a
reaction with no volume variation. Also, it can be en from the
Fig.3(a), the undesirable product of CO2 can be lowered under
the conditions of higherpressure and lower temperature. It was
obrved interestingly from Fig.3(b), a complete conversion of
methanol could be got at all temperatures and pressures.
3.2.3 Hydrogenation of HAC to ethanol
Fig.4 illustrated the product equilibrium content and HAC conversion in hydrogenation of HAC to ethanol. The equilibrium content of EtOH and H2O at lower temperature was found to be greater compared to that at a higher temperature and conversion of HAC decread with increa of temperature. HAC conversion can achieve higher level(>70.79 %) under the conditions studied((1-10) MPa, (373-873) K). To get a 20% mole EtOH, reaction should be done under 473 K, suggesting that the reaction (Eq.5) should be carried out at lower temperature. The content of EtOAc increas with an increasing temperature then goes through a maximum.
At lower temperature, pressure shows little effect on the HAC conversion but affects to the HAC conversion in the temperature range between 523 K and 873 K. For the equilibrium composition of products, EtOH and EtOAc are the two products mostly affected by the pressure. Increasing the pressure from 0.1 to 3 MPa results in an increa of EtOH molar fraction at 523 K from 6.6 % to 19 %. Under the same conditions EtOAc content decreas from 7.30 % to 2.40 %. At temperature higher than 623 K the effect of pressure to EtOAc content is weaked.爱在招生部
(a)EtOH
(b) H2O
(c)EtOAc
(d) HAC
Fig.4 Plot of thermodynamic equilibrium mole fraction of each
component and HAC conversion as a function of pressure and
temperature when route C is employed.
4 Conclusions
Three different routes to generate ethanol derived from syngas directly or indirectly have been discusd from the view of thermodynamics. From the thermodynamics analysis, the results showed that any of the three routes can give a EtOH-rich product, and the byproduct CO2 equilibrium content of direct synthesis from syngas and methanol homologation can be reduced to a lower level under lower temperature and pressure. The main byproduct EtOAc of hydrogenation of HAC can be lowered under lower temperature and higher pressure. This work only considered one main reaction
and one or two side reaction, however, the results was still helpful to find out the favorable condition ranges.
Nomenclature
P
C Heat capacity, J·mol-1·K-1
G Gibbs
free
energy,
kJ·mol-1
P
K Chemical equilibrium constant
P Pressure,
MPa
R Gas
constant,
J·mol-1·K-1
T Temperature,
K
Δ
R
H Reaction enthalpies, kJ·mol-1
Δ
f
H0Standard formation enthalpies, kJ·mol-1
y
i
Mole fraction of species i
i
φFugacity coefficient of species i in the gas
mixture
k
λ Lagrange
multipiler
α
ik
Numbers of atoms of the k th element prent in each
molecule of compound
i
260Computers and Applied Chemistry / E-mail: jshx@home.ipe.ac 2013,
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不同路线合成乙醇的热力学分析
李芳1,2，张科1，马宏方1，张海涛1，应卫勇1*，房鼎业1冲账
(1. 华东理工大学大型工业反应器工程教育部工程研究中心，化学工程联合国家重点实验室，上海，200237；
2. 安徽工程大学生物与化学工程学院，安徽，芜湖，241000)
摘要：运用吉布斯自由能最小法对3种不同路线（合成气直接合成、甲醇同系化、醋酸加氢）合成乙醇热力学进行了分析，在温度373 K~873 K，压力0.1 MPa~10 MPa的范围内获得了各路线原料转化率与产物平衡组成随温度与压力变化的关系。对于合成气直接合成乙醇，低温对CO平衡转化率有利，在压力为2 MPa时，温度从373 K升高到873 K，CO平衡转化率从100 %降到1.56 %；在甲醇同系化合成乙醇工艺中，甲醇的转化率在所研究的温度压力范围内都几乎为100 %，低温时(T<473 K)甲醇经同系化反应接近完全转化为乙醇，高温时(T>673 K)甲醇几乎全部分解为CO与H2；醋酸加氢制乙醇工艺中醋酸的转化率均大于70.79 %。对于这3种路线，乙醇的平衡组成在10 MPa、373 K时达到最大，分别为50 %，50 % 与25 %。
关键词：热力学分析；乙醇；合成气合成；甲醇同系化；醋酸加氢
红岩故事中图分类号：TQ021.2
文献标识码：A
文章编号：1001-4160(2013)03-256-260
DOI: 10.11719/com.app.chem20130307
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收稿日期：2012-11-20; 修回日期：2013-01-15
基金项目：国家科技支撑资助计划(No.2006BAE02B02).
作者简介：李芳(1975—)，女，湖北武汉人，博士，主要研究方向为化学工程，E-mail:
联系人：应卫勇(1957—)，男，教授，E-mail:
不同路线合成乙醇的热力学分析
作者：李芳， 张科， 马宏方， 张海涛， 应卫勇， 房鼎业， Li Fang， Zhang Ke， Ma Hongfang， Zhang Haitao， Ying Weiyong， Fang Dingye
作者单位：李芳,Li Fang(华东理工大学大型工业反应器工程教育部工程研究中心，化学工程联合国家重点实验室，上海，200237; 安徽工程大学生物与化学工程学院，安徽，芜湖，241000)， 张科，马宏方，张海涛，应卫勇，房鼎
业,Zhang Ke,Ma Hongfang,Zhang Haitao,Ying Weiyong,Fang Dingye(华东理工大学大型工业反应器工程教
育部工程研究中心，化学工程联合国家重点实验室，上海，200237)
刊名：
计算机与应用化学
英文刊名：Computers and Applied Chemistry
年，卷(期)：2013(3)
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