Isoamyl Acetate Esterification: Kinetic and Adsorption Study

ISOAMYL ACETATE ESTERIFICATION: KINETIC AND ADSORPTION STUDY

Yoshinori Casas, Natalia Montoya, Andrés Salazar, Alvaro Orjuela, Gerardo Rodriguez¹

Departamento de Ingeniería Química y Ambiental, Universidad Nacional de Colombia, Bogotá – Colombia.

In Colombia, bioethanol industry has rapidly grown over the last decade as a result of different governmental policies that promote the use of renewable feedstocks to produce biofuels. In fuel ethanol production several by-products are generated, among them fusel oil. It is a mixture of higher alcohols, mainly composed by isoamyl alcohol, which is used as a raw material to produce several value-added products. Currently, fusel oil is burned in furnaces as fuel and used as solvent, representing low benefits for the overall economics of the ethanol industry. Nevertheless, considering that fusel oil contains up to 80% of isoamyl alcohol, the use of this by-product in the production of esters for fragrances and flavoring industries would be an alternative to improve economical benefits.

Since esterification is a reaction limited by chemical equilibrium, high conversions are commonly achieved using a large excess of alcohol. This practice involves bigger equipment sizes and higher energetic requirements in both reaction and separation steps. To solve these problems some hybrid technologies have been used; reactive distillation is one of them. In this case, reaction and distillation processes take place in the same unit. Reagents react at intermediate stages of a column while products are constantly removed; this allows to obtain high conversion and a high purity of the final product. This approach might be useful in the production of isoamyl acetate. 

The more accurate information in terms of kinetic and thermodynamic data, the more reliable is the modeling of the operation. Information such as kinetic expressions, activity coefficients and adsorption behavior on heterogeneous catalysts must be obtained from experimental procedures, in order to get more accurate and reliable estimations used for process design and economics.

Considering that heterogeneous catalysis is frequently used in esterification systems, it is necessary to take into account the adsorption phenomena that take place when reagents interact with the solid phase. Considering the above, kinetics and adsorption constants for esterification of isoamyl alcohol with acetic acid on a ion exchange resin were studied. A Box-Behnken experimental design was performed to carry out the kinetic tests. Batch isothermal reactions were performed at different temperatures (373-393 K), alcohol:acid molar ratios (1:1-1:2) and catalyst loadings (1-2%).  Experimental data were fitted with a non ideal pseudo-homogeneous model and LHHW model using NRTL activity model.

Adsorption constants were determined using two different approaches, Kippling and Pöpken models. The first one joins the composite isotherm model with the law of mass action; while the second one joins the overall mass balance of the binary liquid phase with Langmüir type adsorption based on mass. The difference in the fundamentals of each model leads to very interesting but opposite results, which are studied in this work. Independent binary adsorption experiments were performed. Three non reactive pairs with total solubility were used.