Soap Removal from Biodiesel by “Waterless Wash” Methods

Biodiesel is a proven alternative to petroleum diesel fuel. Biodiesel is made using a transesterification reaction. A byproduct of this reaction is the formation of soap. A post-processing washing step is needed to bring the biodiesel product into specification. Washing is typically performed with water, causing many problems. There are several alternative washing methods. These include passing the fuel through a bed of ion exchange resin or mixing magnesium silicate with the fuel then passing the fuel through filtration. The performance and cost of ion exchange resin and magnesium silicate ability to remove soap from biodiesel was tested. Flow rate, methanol concentration and bed height all have an impact on the performance of using ion exchange resin to remove soap from biodiesel. When processing fuel with ion exchange resin a soap-glycerin layer can form on top of the resin bed causing operation problems. Processing fuel with ion exchange resin can increase its acid value. The percentage by weight of magnesium silicate used to remove soap from biodiesel is the primary factor in its performance. Recirculating the biodiesel through a magnesium silicate filter cake improved performance. Comparing the cost of using magnesium silicate versus ion exchange shows that ion exchange resin is more cost effective.