Conversion of crude glycerol from the biodiesel industry to value added products

Abstract

Crude glycerol is a major by-product of the biodiesel industries. For every 100 kg of biodiesel produced, approximately 10 kg of the byproduct glycerol is generated. With the large increase in biodiesel production, there is a glut in the glycerol produced. Presently crude glycerol is purified to its purer marketable form, burnt as a fuel or mixed with animal feed. However, none of these options contribute considerable revenues to the concerned biodiesel industry. Additionally, some of these routes are not environmentally friendly. It has thus become imperative to find ways to convert crude glycerol to some value-added products. Bioconversion of crude glycerol to microbial lipids is one possible way to valorize it. However, impurities like methanol, salts and soap present in crude glycerol inhibit the growth of microbes used for such conversions. The research work carried out in this thesis addressed these issues and developed tangible alternatives to overcome these problems. Initially the possible use of a heterogeneous catalyst Calcium oxide (CaO) attached to support alumina (Al2O3) for the production of biodiesel was studied. We found that the use of such a catalyst improves the purity of biodiesel and the glycerol produced. Crude glycerol obtained using such insoluble catalysts contained lower levels of impurities and can be converted relatively easily to other useful products. With CaO anchored on Al2O3 as catalyst, the purity of biodiesel and glycerol were found to be 97.66% and 96.36% respectively. The unanchored heterogeneous catalyst CaO resulted in purities of 96.75% and 92.73% respectively. As the byproduct glycerol containing smaller amount of impurities, the use of anchored heterogeneous catalyst is recommended. The potential use of ash from various sources as a cheap alternative heterogeneous catalyst was also studied. With the use of ash from birch bark and fly ash from wood pellets as catalysts, biodiesel and glycerol with purity in the ranges of 88.06%-99.92% and 78.18%-88.23% respectively were obtained. Since such catalysts are cheap and reusable, their application can reduce expenses and the use of environmentally unsafe compounds. The crude glycerol used in all experiments was obtained from a biodiesel producer in Ontario (Canada). It was found to contain 44.56 wt.% glycerol and many impurities including 13.86 wt.% methanol, 32.97 wt.% soap and 4.38 wt.%. After the characterization of the sample it was converted to microbial lipids using an oleaginous yeast Rhodosporidium toruloides ATCC 10788. When this strain was grown on crude glycerol, double the biomass (21.16 g/L) and triple the lipid concentration (11.27 g/L) was obtained compared to growth on pure glycerol media. The capacity of this strain to grow on crude glycerol with high levels of impurities and produce large amounts of lipids proves its robustness. Investigation of the effect of individual components on the lipid production ability of this strain showed it to be capable of using soap as a sole carbon source. This was also the reason for enhanced lipid production even in the presence of other impurities present in crude glycerol. The lipids obtained were rich in oleic acid (47.16%), a mono-unsaturated fatty acid (MUFA). Feedstock rich in MUFA are considered suitable for biodiesel production. Thus, the process of conversion of crude glycerol to microbial lipids can be integrated to existing biodiesel plants. This will help in the management of crude glycerol produced during biodiesel production, save transportation and disposal costs and contribute to the revenues of such industries.