(737d) Kinetics of Mixed Alcohol Ethanol/Butanol Esterification of Butyric Acid

The growing need to reduce our
dependence on fossil sources for fuels and chemicals has led to the exploration
of alternate pathways for the use of greener chemicals and their manufacture
from bio based sources. Esters of higher alcohols (alcohols with more than two
carbons) are an industrially important class of compounds and economically
viable processes for making them need to be designed. Blends of esters are
being studied as fuel additives to biofuels for their
high energy density and favorable fuel properties⁽¹⁾.  One route for producing esters that is
currently being extensively studied is the esterification
of carboxylic acids with alcohols using acid catalysts⁽²⁾.
The catalyst used may be homogeneous such as sulfuric acid, or para-toluene sulfonic acid (pTSA). Heterogeneous catalysts used include cationic
exchange resins.

Mixed alcohols are obtained from
the condensation of alcohols to higher alcohols⁽³⁾,
the Fischer-Tropsch synthesis of alcohols from syn gas⁽⁴⁾ and fusel alcohols produced from
ethanol fermentation. The simultaneous esterification
of these alcohol mixtures would be a good use of these mixed alcohol streams
and improve the profitability of these processes. Since these reactions are
equilibrium limited, one option for driving them forward is reactive distillation
where the products are separated from the reaction mixture as and when formed.
The design of a reactive distillation process requires a good understanding of
the kinetics of the reaction system.

It is not known whether the
presence of one alcohol would accelerate or inhibit the rate of reaction of
another, or if the formation of these mixed esters over complicates the
recovery process. In this study, the kinetic behavior of butyric acid
undergoing esterification by ethanol and butanol has been investigated under homogeneous (pTSA) and heterogeneous catalysis (Amberlyst
70). The fermentation of butyric acid from biomass has sparked interest in the
production of many chemicals using butyric acid as a building block. Apart from
their potential as biofuel components, ethyl butyrate
and butyl butyrate serve as food flavoring agents and green solvents⁽⁵⁾.

The kinetics were first studied
separately, and then in alcohol mixtures of varying relative compositions of
ethanol and n-butanol. Reaction equilibrium constants
were first determined experimentally from samples taken after reactions had
reached equilibrium. UNIFAC (UNIversal Functional Activity Coefficient) was used
to calculate the activity coefficient values. Concentration profiles of
activity-based fits and experimental data of mixed alcohol esterification
runs show that the models fit experimental data reasonably well. Activity and
mole fraction based fits were found to be indistinguishable.  An additive combination of individual models
was found to predict the mixed alcohol esterification
system reasonably well. The kinetic model fitted to experimental data is useful
for simulating reactive distillation columns to drive the equilibrium limited
simultaneous esterification reactions forward.

References

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Mbaraka, D. R. Radu, V. S.
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2.         A. Chakrabarti,
M. M. Sharma, Reactive Polymers 20, 1 (1993).

3.         W. Ueda, T. Ohshida,
T. Kuwabara, Y. Morikawa,
Catalysis Letters 12, 97 (1992).

4.         T. J. Mazanec,
Journal of Catalysis 98, 115 (1986).

5.         D. W. Armstrong, H. Yamazaki, Trends in
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