Minimization of cost of recycling in chemical processes

This paper considers an important aspect of every chemical process. Most chemical processes have streams being recycled back to different process units. These streams have to meet certain process requirements such as flow rate and composition. Besides these material balance constraints, their relative costs have to be considered. There is a need to determine the global cheapest mixing and recycling scheme for a given process. A generic non-linear program formulation is given and a solution algorithm consisting of two different levels is proposed. The outer level consists of iterations among different variables while the inner level consists of solution of a linear program. In the case of lower number of sources and sinks, this paper introduces application of the simplex algorithm to derive frameworks wherein, the optimal flow rates of the different sources can be obtained as functions of relative costs and compositions. These frameworks make it possible to determine the cheapest recycling scheme for all available streams. Three cases with progressively increasing complexity are considered. These cases cover a large number of potential industrial applications. This methodology is equally applicable to liquid, solid and gaseous streams. It can handle streams coming from unit operations such as crystallization, condensation and adsorption. It requires basic information on flow rate and compositions of all the sources and sinks being considered and the unit costs of the various sources involved. It is easy to apply and is rigorous under the conditions listed for its use. Two different case studies are considered and the application of the proposed solution algorithm is demonstrated.