A Three-Phase Flow in the Chemical Engineering Process—The Modeling of the Digestion in Solid-Liquid System

To simulate the mixing and separation philosophy in the chemical engineering process we need to study a three-phase (solid-liquid-gas) flow problem. Different systems can handle different percentages of solid to liquid. There is roughly an equal split between the ‘wet’ and ‘dry’ technologies in terms of installed capacity. Digestion can take place in one of two different temperature ranges around 35-37oC range) or at the higher temperature of 55oC. The waste is held in the digester for periods varying from 10-20 days, the duration being dictated by differing technologies, external temperature fluctuations, and other variables like the waste composition. The main advantage of operating at the higher temperature is the added benefit of increasing the rate at which pathogensare killed in the process. After the accelerated in-vessel anaerobic biological degradation is complete, the solid residue known, as ‘digestive’ requires further treatment if it is to be used effectively as a compost product. This is known as curing and takes place aerobically. It is also possible at this stage to further enhance the product by post digestion separation so as to remove oversized and unwanted items like glass or pieces of plastic if these were not removed in the source segregation process. The degree of screening required varies according to the intended use of the final product.