An expert system for solvent-based separation process synthesis
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Expert systems are being used more daily in chemical engineering. This work continues the development of an EXpert system for SEParation flowsheet synthesis named EXSEP. Written in Prolog, it can generate flowsheets for four multicomponent separations: distillation, absorption, stripping and liquid-liquid extraction. For these separations, we describe a large collection of heuristics (or rules) that are used for flowsheet synthesis. EXSEP uses several of these heuristics and the Kremser equation to test the thermodynamic feasibility of separation tasks. EXSEP requires only basic input data such as the expected component flow rates in each product and the component K-values. With those data, EXSEP searches for the sets of the number of theoretical stages, solvent flow rate, and component-recovery ratios that characterize a number of feasible and economical flowsheets. The use of the component assignment matrix (CAM) combined with Prolog list processing makes EXSEP very fast (several seconds) to generate solutions. We test EXSEP with several examples of industrial separation processes and compare the results with the literature. We also compare EXSEP results with rigorous simulations using commercial CAD software (e.g., DESIGN II). In most cases, EXSEP gives very similar and even better flowsheets. However, EXSEP is limited to dilute solvent-based separations and cannot solve problems where the major feed component is also the solvent (e.g. sour-water steam stripper). The development of EXSEP on IBM-PC makes it very "user friendly". In the future, EXSEP should be expanded with additional modules such as extractive and azeotropic distillation, and bulk absorption. It should also include modules for separation method and solvent selections, which are great challenges in flowsheet synthesis.
- Masters Theses