Continuous Enzymatic Production of Biodiesel in CSTRs in Series

The demand for biodiesel (BD) is growing as a result of increased focus on reducing greenhouse

gas emissions. Today biodiesel is produced using alkaline catalysis. However, an enzymatic

catalyzed reaction would result in lower costs e.g. fewer process steps, improved glycerol quality,

and allow for 100% feedstock flexibility. To date, soluble liquid lipases have not drawn much

attention in the scientific literature, unlike their immobilized counterparts, because of the

requirements to reusability to make the process cost-effective. However, soluble enzymes are

cheaper, and not inactivated by glycerol and colloids. In this study soluble liquid lipases have been

used to catalyze the transesterification of rapeseed oil with ethanol into fatty acid ethyl esters (BD).

Since BD is produced in huge quantities a continuous production is necessary and a suitable process

layout could include several continuous stirred tank reactors (CSTRs) in series. In order to

determine the optimal configuration of three CSTRs in series, reaction kinetic data has been

collected in batch experiments and based on a Levenspiel plot the reactor volumes have been

calculated. The calculations have been validated experimentally and the steady-state conversions in

the three reactors found to be 61%, 80% and 93% respectively, with a total residence time of 24h. A

way to make the productivity of the enzymes higher is by recirculating the aqueous phase

containing the enzymes, but this is only worthwhile if enzyme activity is retained. It was found that

78% enzyme activity was preserved even after four reuses of the enzymes, proving that

recirculation of the aqueous phase is possible and should be further investigated in an effort to make

the enzymatic biodiesel process profitable.