Control System

An automatic control system was installed on the pilot plant. The basic system consists of three regulatory loops that stabilise the process and permit manual or one-point composition control. A fourth loop can be closed to provide two-point composition control or alternative one-point configurations.

Unlike the industrial columns that were discussed in Chapters 8 and 10, the pilot plant has only four degrees of freedom since there is no reflux accumulator. The variables pairings are, therefore, somewhat different from a conventional column. Initially, the column pressure was controlled by manipulating the distillate rate, the reboiler level was controlled by manipulating the bottoms draw rate and the reflux rate was flow controlled. However, the equipment is sufficiently flexible to allow the variable pairings to be changed, as required, to investigate and test other combinations.

The configuration of the basic elements of control hardware is indicated in Figure 11.7. The three control loops (i.e. the loops which manipulate the process via the three control valves) which are depicted with solid lines are the basic loops described above. The fourth control uses the voltage regulator as a final control element instead of a control valve. This is feasible as it can receive and interpret a 4-20 mA signal from the controller. This control loop is depicted with dashed lines to indicate that it was initially operated in open-loop (i.e. the voltage is fixed independently of the controllers).

! TEMPERATURE | TRANSMITTER |

PRESSURE TRANSMITTER :

FLOW TRANSMITTER

LEVEL TRANSMITTER

VOLTAGE

! TEMPERATURE | TRANSMITTER |

PRESSURE TRANSMITTER :

FLOW TRANSMITTER

i REGULATOR *

COMPOSITION CONTROLLER

PRESSURE CONTROLLER

FLOW CONTROLLER

LEVEL TRANSMITTER

LEVEL CONTROLLER

TRANSDUCER

OPERATOR INTERFACE

TRANSDUCER

DISTILLATE CONTROL VALVE

COMPRESSED AIR

TRANSDUCER <"

REFLUX CONTROL VALVE

BOTTOMS CONTROL VALVE

Figure 11.7- Control System Block Diagram

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