(408e) Turndown Control Structures for Distillation Columns

Future chemical plants may be required to have much higher flexibility and agility than existing processes in order to be able to handle new hybrid combinations of power and chemical units. An important example is a gasification process producing synthesis gas that can either feed a combustion turbine for the generation of electricity or, during periods of lower power demand, feed a chemical plant. The chemical plant would be designed for the maximum capacity that is associated with periods of minimum electric power demand. But the plant would have to be able to turndown to low throughputs during periods of maximum electric power demand. The 24 hour power demand swings in many locations can be a factor of two or more from day to night.

The separations required in many chemical processes are achieved using distillation columns. If the process must operate over a wide range of throughputs, the columns must also have wide rangeability. There are several low-throughput limitations in distillation columns, usually involving hydraulic constraints. The most common is a low limit on vapor flowrate, below which weeping can adversely affect tray efficiency and separation. The vapor limit depends on tray design, with valve trays being the most rangeable. Even valve trays lose performance below about 50% of design vapor rates. If the plant throughput must be reduced to 25% of design capacity and the column vapor can only be reduced to 50% of design, a control structure that effectively handles this situation is required. The purpose of this paper is to explore three alternative control structures for columns with significant turndown requirements.