Reactive coupling for biodiesel production with integrated glycerol valorisation

Abstract

The increasing production of biodiesel globally over the last 20 years has increased the supply of “crude” glycerol. Initially, glycerol was a valuable by-product but is now low value or a waste product, due to mismatch between supply and demand. Valorisation of glycerol is an obvious route to improving the process economics. In this work, we investigated glycerols in situ valorisation by conversion to various oligomers (used in the pharmaceutical, food, and cosmetics industries) and glycerol ethers (used as oxygenated compounds to improve fuel combustion in Diesel engines). This is an example of "reactive coupling", a technique in which the by-product of one reaction is simultaneously converted to an added value product in a second reaction (in a single "pot"), thereby reducing the number of process steps. The main objective for this work is to produce glycerol free biodiesel with reduced methanol usage and fast triglyceride conversion using reactive coupling as a novel technique. This work will for the first time demonstrate the production of biodiesel and glycerol ethers in a single pot. This aimed to reduce glycerol byproduct and methanol recycling. First, the study the convert glycerol in a stainless steel reactor. Reactive coupling was then performed to convert triglyceride with simultaneous conversion of the glycerol to added value products. Sulfuric acid was used as catalyst for the reaction, as it is compatible with all the desired reactions. It is also cheap and can tolerate triglyceride with high FFA levels during biodiesel production. High temperature in transesterification results in fast conversion of triglyceride. The catalyst and temperatures used are suitable for both biodiesel reaction and glycerol etherification. Highest conversion of glycerol achieved was 68%, with over 90% selectivity to diglycerol in 5h. To avoid producing undesired by-products (such as acrolein) and higher oligomers (such as pentaglycerol), the recommended conditions are 3 wt% catalyst concentration, and a temperature of less than 150 oC. Furthermore, a kinetic model was fitted to the experimental data with activation energy of 112 kJmol-1 and pre-exponential factor of 2.18x1011 L.mol-1s-1. The thermodynamic analysis showed the reaction to be endothermic, less disordered, and non-spontaneous with an enthalpy (ΔH) 109 kJmol-1, entropy (ΔS) – 38.1 Jmol-1K-1, and Gibbs free energy (G) 125 kJmol-1 respectively. Reactive coupling achieved complete conversion of triglycerides and 100% FAME yield in 1h. About 60% of the glycerol was converted in parallel, with approximately 90% selectivity to glycerol ether and 10% to diglycerol. A temperature of not more than 150 oC is sufficient for this process with 3 wt% catalyst concentration and molar ratio 4:1 – 6:1. Some of the benefits of the reactively coupled process vs conventional processing are the rapid separation of the biodiesel phase from the glycerol phase, low alcohol to oil ratios, and the production of value-added products from the crude glycerol. The model should make scale-up of this process more predictable and robust. Combined reactive extraction and reactive coupling were also studied, i.e., reactive coupling on the oilseeds, rather than the oil. Over 90% of biodiesel production was achieved and complete conversion of the glycerol to glycerol ether and polyglycerol. However, a substantially higher molar ratio of methanol to oil (400:1) was required, likely to be uneconomic. There were various non-triglyceride products in the extract, which would probably necessitate extra downstream processing. In summary, for the first time, this work demonstrates reactive coupling to produce biodiesel, polyglycerol, and glycerol ether production using sulfuric acid as catalyst. The main advantages of this technique were: i. Reduced glycerol by-product by up to 60%. ii. Reduced methanol usage, from 20:1 to 4:1 – 6:1. This will remove/reduce downstream processing. iii. Rapid conversion of triglyceride. iv. Easy/fast separation of glycerol phase from FAME phase. Furthermore, this study demonstrated proof-of-concept for combined reactive extraction and reactive coupling. Hence, oil in seeds can be converted directly to biodiesel, glycerol, and added-value products. This study’s success shows that the glycerol by-product can be converted to a useful product directly during biodiesel production. Potentially, this will reduce waste generation and diversify the market of biodiesel producers.