A control system for multi-stage flash desalination plants

Abstract

A system of simple analog control loops has been investigated experimentally in a 6-stage flashing test unit, which is aimed to automate the plant operation and to improve the distillation process within any required operating condition. The control circuit includes the control of liquid levels and temperatures in the first and last flashing chamber, adjusting automatically the amount of steam to the brine heater, and the brine flow rates in both the heat rejection and the heat recovery section. Using this system, the load can be changed more easily and quickly than by manual operation, which is an important factor for dual purpose plant coupling. Experiments have been carried out and simulated with a modified. form of a digital computer code published, by ORNL (ref. 3). Calculated and experimental results have been found to agree with one another very closely. The computer program was also applied to a MSF plant design with a higher number of stages. One of the main results is that, by decreasing the load, interstage blow-through can occur in the middle stages of the plant, even if the liquid levels in the first and in the last stage are kept constant. It is shown that the presence of weirs in the flashing chambers reduces this tendency.