Biodiesel production via transesterification of different oils with methanol on solid catalyst (heterogeneous catalysis)

The ongoing debate of oil for food or fuel notwithstanding, today’s biodiesel technology leaves much to be desired. It is dominated by batch processes using homogeneous acid / base catalysis.

The batch mode offers simple scale-up of laboratory recipes. It also ensures good flexibility of operation faced to a large chemical variability of raw materials. However, this process also suffers from three serious disadvantages: low productivity, high energy costs, and large amounts of aqueous waste. In fact, the cost of product separation, waste disposal and catalyst neutralisation is the main factor that renders current biodiesel production uneconomical. Interestingly, there is a simple and elegant solution to these problems: Continuous biodiesel manufacturing using heterogeneous catalysis. Esterification and transesterification reactions proceed readily over solid acid/base catalysts. Importantly, such processes have practically no aqueous waste streams, and require no catalyst neutralising. Moreover, they incur lower capital expenses (CapEx) and lower operational expenses (OpEx). Compared to a batch homogeneous process, a continuous process using a solid catalyst can save 40–50% in capital costs and 30–60% in energy. Despite these advantages, biodiesel manufacturing using heterogeneous catalysts remains an emerging technology. The overall transesterification reaction involves three equilibrium reaction steps in which intermediate species are formed. In these steps, the resulting di- and mono-glycerides content is fixed. It cannot be corrected afterwards by any economical separation technique. This is important, as the key biodiesel specifications (in wt%) are: ester content min. 96.5%, mono-glycerides max. 0.8%, di-glycerides max 0.2%, triglyceride max. 0.2%, free glycerine max. 0.02%, and total glycerine max 0.25%.