Work to Chemical Processes: The Relationship between Heat, Temperature, Pressure, and Process Complexity

For a chemical process to be feasible, two levels of energy must be met: the heat and work requirements of the process. Whereas, for most processes, the heat requirement can easily be satisfied, supplying the amount of work needed is a major challenge and is usually the determinant of the process complexity. In some cases, heat, by virtue of its temperature, can satisfy the work requirement for a process; it is the simplest method for supplying work but could result in major irreversibility when applied inappropriately to a process. This article discusses different techniques that can be used to supply work to a process. A graphical approach, namely, the gh diagram, is used to analyze the heat and work requirements of chemical processes and to determine which method of supplying work is suitable for the process to be feasible and reversible. An ammonia process is analyzed as a case study, in which different methods of supplying work are compared and an attempt is made to elucidate the consequences of operating conditions, namely, temperature, on the reversibility and complexity of the process. This is a novel approach to process synthesis that requires knowledge of Δ H and Δ G only to determine the target for a process in terms of heat and work requirements, to determine which method of supplying work is suitable, to provide means of manipulating the process in order to use available technology and get as close to reversibility as possible, and to provide an early understanding of what the process structure would be. It also allows for the assessment of irreversibilities and where they occur within a process; therefore, it can be applied to existing processes to reveal opportunities for improvement and determine how they can be achieved.