Minimization of cost of recycling in chemical processes

This paper considers an important aspect of every chemical process. Most ch emical processes have streams being recycled back to different process unit s. These streams have to meet certain process requirements such as flow rat e and composition. Besides these material balance constraints, their relati ve costs have to be considered. There is a need to determine the global che apest mixing and recycling scheme for a given process. A generic non-linear program formulation is given and a solution algorithm consisting of two di fferent levels is proposed. The outer level consists of iterations among di fferent variables while the inner level consists of solution of a linear pr ogram. In the case of lower number of sources and sinks, this paper introdu ces 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 dete rmine 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 eq ually applicable to liquid, solid and gaseous streams. It can handle stream s coming from unit operations such as crystallization, condensation and ads orption. It requires basic information on flow rate and compositions of all the sources and sinks being considered and the unit costs of the various s ources involved. It is easy to apply and is rigorous under the conditions l isted for its use. Two different case studies are considered and the applic ation of the proposed solution algorithm is demonstrated. (C) 2001 Elsevier Science B.V. All rights reserved.