Biodiesel production from waste cooking oil by two-step catalytic conversion

The commercial biodiesel production process is very mature today, but the source of biodiesel is mostly plant oil, which has the drawback of high cost and land competition with food crops. Using waste cooking oil as feedstock for biodiesel production can avoid those problems. However, the transesterification of waste cooking oil involves some challenges. For example, waste cooking oil usually contains a large amount of free fatty acids (FFAs), which could react with base catalyst (such as NaOH) to form soap, resulting in a decrease in biodiesel conversion efficiency. To cope with this, a two-step process, consisting of esterification with acid catalyst and follow-up transesterification with base catalyst was developed. This two-step process could lower the content of FFAs in waste cooking oil in the first step and also improve conversion of transesterification in the second step. Although homogeneous acid catalyst, such as sulfuric acid, could reach a high conversion in a short time, an extra downstream processing is required to remove the acid catalyst (e.g., water rinse). Therefore, we developed a magnetic spinel as acid solid catalyst supporter to replace homogeneous catalyst in order to simplify the overall process. In the first step, esterification of FFAs content in cooking oil was conducted using the self-made solid acid catalyst, which has similar catalytic ability to that of sulfuric acid, and is also much easier for separation. In addition, the residual lipid can be easily transesterified without any pre-T reatment. The self-made spinel-supported catalyst could be regenerated by simple calcination.