Dynamics and control of recycle systems. 2. Comparison of alternative process designs

This paper is the second in a series of papers that explore the challenging problems associated with the dynamics and control of recycle systems. In this paper the authors compare the steady-state economics and the controllability of several alternative process designs. The designs are primarily limited to a reactor/column configuration with recycle, but some consideration is given to several other process configurations (several reactors in series and a tubular reactor). The reactor/stripper configuration is shown to be an economically attractive process configuration even for a simple single irreversible reaction system when fairly high conversions are desired. The optimum steady-state economic design consists of a fairly large reactor and a moderately sized stripping column. As the reactor is made larger, the energy consumption in the column decreases but the number of trays increases. This translates into a taller stripping column (more trays), but one that is smaller in diameter, and into smaller heater exchangers for the reboiler and the condenser. The dynamics of the column stay about the same as its diameter changes, but the dynamics of the reactor slow down as the size of the reactor increases. The gain around the recycle loop increases as the reactor size is described.more » All these competing effects result in a process in which the dynamics of the various alternative designs are quite similar. Temperature control in the reactor is shown to improve as the size of the reactor increases, which is just the opposite of the normal situation when one scales up a reactor. For the smaller reactor process, coupling a reactor, which is stable by itself, with a column, which is stable by itself, is shown to sometimes lead to an unstable coupled system.« less