Thermodynamics and kinetics of the synthesis of

Thermodynamics and kinetics of the synthesis of higher aliphatic alcohols M. Grzesikab, M. Kulawskaa, J. Skrzypeka and M. Witczakb aInstitute bFaculty

of Chemical Engineering, Polish Academy of Sciences, Gliwice, Poland

of Food Technology, Academy of Agriculture, Kraków, Poland

Abstract The thermodynamics of the synthesis of higher aliphatic alcohols is studied in detail. Kinetic model of the synthesis of higher aliphatic alcohols is presented. Experiments were carried out in a high-pressure continuous gradientless stirred tank reactor. Unexpectedly, the reaction rate is independent of partial pressure of carbon monoxide. 1. INTRODUCTION The search for a clean combustion fuel is the most important incentive to improve the synthesis of higher aliphatic alcohols. The mixture of methanol and higher alcohols appears to be a very valuable additive to gasoline as an antiknock agent. They can be a real alternative for MTBE since they are entirely based on natural gas. It is a clean fuel without aromatics, olefins and sulphur. The last review papers on this subject were [1,2]. The stoichiometry of higher alcohol synthesis from syngas is based on the following reaction scheme: m CO + 2mH2

CmH2m+1OH + (m-1)H2O,

m=1,2...

(1)

All the reactions are reversible, exothermic and proceed with volume contraction. The water-gas shift reaction is always present and assumed as attaining to the state of chemical equilibrium at the synthesis conditions: CO + H2O

CO2 + H2 .

(2)

In only a few studies attempts have been made at modelling the kinetics of the overall rate of the synthesis of higher aliphatic alcohols [3,4].

The thermodynamic data concerning the process are scarce and limited in scope. The results cited often refer to the individual reactions only, thus giving an unrealistic general description. The comprehensive work has been published by Mawson et al. [5] . The thermodynamic background of the synthesis has been shown by Xiaoding et al. [6]. Tronconi et al. [7,8] have presented a thermodynamic analysis concerning their experimental results. 2. CHEMICAL EQUILIBRIUM STUDY

Equilibrium conversion degree

A thermodynamic model describing the synthesis of C1-C4 aliphatic alcohols was developed for the system of chemical reactions (1-2). The numerical solution of the nonlinear algebraic equations allowed the estimation of the equilibrium conversion degree of carbon monoxide. Selected results of numerical computations are presented in Fig.1-4 (m=2,3,4): 1.0

Chemical equilibrium Synthesis of higher alcohols XoCO=0.3, XoH2=0.7, p=3 MPa

0.8

Chemical reactions:

0.6

CO --> EtOH CO --> PrOH

0.4

CO --> BuOH reaction (2)

0.2 0.0 550

600

650

700

750

Temperature, K

Fig. 1. Chemical equilibria in synthesis of higher alcohols The effect of the initial mole fraction of CO on the equilibria in the system of (1,2) is highly significant. Pressure has rather a moderate effect on the equilibrium conversion degrees of CO to ethanol and propanol (m=2 and m=3). In the whole range studied the equilibrium conversion degrees increase slightly with increasing pressure and decreasing temperature. Both temperature and pressure have a considerable effect on the equilibrium conversion degree of CO to butanol (m=4). Pressure influences strongly the equilibrium conversion degree in high temperatures as 600-750 K and has rather moderate effect in lower ones. An increase in temperature decreases the equilibrium conversion degree of CO to butanol. The equilibrium conversion degree of (2) increases initially with temperature, passes through a maximum, and then decreases. The position of maximum point depends on both pressure and the initial mole fraction of CO.


1.0


0.8 Chemical reactions:

0.6


0.4


0.2 0.0 550

600

650

700

750

Temperature, K


Fig. 2. Chemical equilibria in synthesis of higher alcohols 1.0


0.8 Chemical reactions:

0.6


0.4


0.2 0.0 550

600

650

700

750

Temperature, K


Fig. 3. Chemical equilibria in synthesis of higher alcohols 1.2 1.0

Chemical reactions: Chemical equilibrium Synthesis of higher alcohols CO --> EtOH XoCO=0,4, XoH2=0,6, p=3 MPa

CO --> PrOH

0.8


0.6 0.4 0.2 0.0 500

550

600

650

700

750

Temperature, K

Fig. 4. Chemical equilibria in synthesis of higher alcohols

3. KINETIC STUDY The catalyst containing mainly CuO and ZnO with Zr, Fe, Mo, Th and Cs oxide addition has been prepared in our laboratory. It is a kind of low - temperature modified methanol synthesis catalyst. Three different methods of catalyst preparation were used; the best results were obtained for the method using citric acid. The catalyst exhibits a remarkable stability during one-year experiments and high selectivity towards alcohols. Experiments were carried out in a high-pressure continuous gradientless stirred tank reactor. This type of reactor allows a direct determination of the reaction rate. The range of experimental parameters used was: P = 4.0-10.0 MPa, T = 553 - 653 K, H2/CO ratio = 0.85 - 3.16, GHSV = 900 - 12 000 h-1. The experimental conditions allowed the process proceeds in the intrinsic kinetic area. At low conversion degrees of carbon monoxide attained, it was far from chemical equilibrium and the reverse reactions were negligible. The liquid product consisted of methanol, ethanol, propanol, C4-C7 aliphatic alcohols and water. Hydrocarbons were practically absent but traces of methane were detected. Unexpectedly, the reaction rate is independent of partial pressure of carbon monoxide. It can be seen that the values of activation energy are typical for catalytic reactions and slightly differ from each other. Using the standard fitting procedures the rate of the reaction was simply described as follows: r k 0 exp( E / RT) pnH 2 , (3) by the mean error not greater than 10%. The detailed values of the parameters are included in Table 1. Table 1 Kinetic parameters in eq. (3) Reaction k0 [mol/g/h/MPan m=1 218.1 m=2 252.3 m=3 84.61

E [cal/mol] 18440 19530 19520

n 2 1.5 1.5

alcohol MeOH EtOH PrOH

4. REFERENCES 1 2 3 4 5 6 7 8

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