Use of the Langmuir-Hinshelwood-Hougen-Watson equation to describe the ethyl esterification of fatty acids catalyzed by a fermented solid with lipase activity

Abstract

The high cost of lipases is a barrier to producing biodiesel through the enzymatic route. Costs can be reduced by producing the lipases by solid-state fermentation and simply drying the fermented solids to produce the biocatalyst. When dry fermented solids (DFS) are used to catalyze the ethyl esterification of fatty acids, they preferentially sorb hydrophilic components of the reaction medium, thereby complicating description of the reaction kinetics. Here, we use a classical equation from heterogeneous catalysis, the two-site Langmuir-Hinshelwood-Hougen-Watson (LHHW) equation, to describe the ethyl esterification of oleic acid catalyzed by DFS, using Aspen Plus®. The parameters were determined using literature data for the ethyl esterification of bidistilled olein in a closed-loop system with recirculation of reaction medium from a well-mixed reservoir through a packed-bed bioreactor containing DFS. The model was validated with data obtained for ethyl esterification in the same bioreactor, but with the following changes: (1) use of fatty acids from soybean soapstock acid oil as the feedstock and (2) use of bidistilled olein as the feedstock, but with a reservoir with a separation barrier, so that only the organic phase is recirculated through the bed. We conclude that the two-site LHHW equation is suitable for modeling the ethyl esterification of fatty acids in techno-economic analyses of biodiesel production catalyzed by DFS that contain lipases.