The Characterization of Bimodal Droplet Size Distributions in the Ultrafiltration of Highly Concentrated Emulsions Applied to the Production of Biodiesel

Abstract

The traditional source of energy, i.e. fossil fuel, will not sufficiently supply our increasing energy demands. This is due to industrialization, population increase and oil price fluctuations. Therefore there is a need to produce renewable energies which not only supply the energy demand that the world is facing but also they are more environmentally friendly in terms of reducing green house gases (GHG) and volatile organic compounds (VOC) emissions. Biodiesel produced from lipid sources is a clean-burning, biodegradable, nontoxic fuel that does not contain sulphur, aromatic hydrocarbons, metals or crude oil residues. Current biodiesel production processes are tedious and involve two to three reaction steps each followed by a separation stage. Process intensification of reaction and separation in a membrane reactor offers several advantages over conventional reactors e.g. using ultralow catalyst concentration, typically 10 to 33 times less than is currently used commercially. This integration leads to the reduction of the environmental footprint of the process while maintaining the quality of the biodiesel produced. However, limiting conditions of operation in such a reactor are not known. These are expected to be dependent on the type of feedstock processed and the operating conditions existing in the reactor. The limiting conditions will affect the quality of the biodiesel produced which might not be in accordance with ASTM and EN standards. This work is aimed at determining the limiting conditions such as the critical flux, based on pressure and composition, and residence time of the transesterification process, for a variety of feedstocks. A non-reactive model system comprising a highly concentrated and unstable oil-in-water emulsion was used to investigate the retention of oil by the membrane in producing biodiesel with a membrane reactor. Critical flux was identified using the relationship between the permeate flux and transmembrane pressure along with the separation efficiency of the membrane. It was shown that separation efficiencies above 99.5% could