Dynamics and control of a biodiesel process by reactive absorption

Integrated biodiesel processes based on reactive separations powered by solid acid/base catalysts are available nowadays, offering significant advantages such as minimal capital investment and operating costs, as well as no catalyst-related waste streams and no soap formation. However, the controllability of the process is just as important as the capital and operating savings. In such processes the small number of degrees of freedom is a drawback which makes it difficult to correctly set the ratio of reactant feeds and consequently to avoid impurities in the products. This work considers the process control of biodiesel production by reactive absorption, the main result being an efficient control structure that ensures the excess of methanol that is necessary for the total conversion of the fatty acids and for prevention of the difficult separations, while maintaining high purity of the water by-product. Rigorous simulations were performed – using Aspen Plus and Aspen Plus Dynamics as efficient computer-aided process engineering tools – for a plant producing 10 ktpy biodiesel from waste vegetable oil with high free fatty acids content, using solid acids as green catalysts. This reactive absorption process eliminates all conventional catalyst-related operations, and efficiently uses the raw materials and the reactor volume in an integrated setup that is well controllable in spite of the reduced number of degrees of freedom.