Process design and control for eco-efficiency

Abstract

The main objective of process design is to develop processes or plants that will produce goods of higher value than the raw materials used. Consequently, process design is mostly driven by the economics of production. However, many other factors, such as operability and environmental regulations have to be considered in the design of a process. Due to the dynamic nature of chemical and petrochemical processes, without process control a plant could not be operated safely and at design conditions. Disturbances drive the process in directions that are not the desired/designed operating levels. Process control is the means by which disturbances are rejected. Exergy can be derived from the application of the first and the second laws of thermodynamics to a process. Exergy can be used as a measurement of how valuable the process is, that is, if the exergy values of the products of the process are close to those of the raw materials the process is highly (thermodynamically) efficient. An eco-efficient process can be defined as one that is ecologically friendly and economically viable. This means the process should reduce energy consumption (or exergy destruction), which in tum reduces operating expenses. Exergy along with an integrated framework for process design and process control can be used to develop more efficient processes in a faster and less costly manner than conventional process design. The objectives of this research project were to develop an exergy calculator for chemical process streams and to develop a controllability index based on exergy. These tools will allow the design engineer to gain insight into the eco-efficiency of the process from the early design stages and will reduce the number of design and control structure alternatives that need to be evaluated in detail. The Exergy Calculator was developed and implemented in a commercial process simulator (HYSYS®) and in an open source chemical process simulator (Sim42®). This tool facilitates the generation of an exergy analysis for any process for which a simulation exists. The new Relative Exergy Array (REA) is a controllability index that, when used with the RGA, provides information about the process interactions of a control structure and its relative thermodynamic efficiency.