Brief Introduction to Distillation Control
All distillation columns have to be carefully operated in order to achieve the required production rates and product quality. The 3 main objectives of column control can be stated as:
· To set stable conditions for column operation
· To regulate conditions in the column so that the product(s) always meet the required specifications
· To achieve the above objective most efficiently, e.g. by maximising product yield, minimising energy consumption, etc.
Process variables like temperatures, pressures, flow rates, levels and compositions must be monitored and controlled in all distillation processes. These process variables within a distillation system affect one another, whereby a change in one process variable will result in changes in other process variables. Thus, in column control one should be looking at the whole column and not focusing on any particular sections only.
Each column has a control system that consists of several control loops. The loops adjust process variables as needed to compensate for changes due to disturbances during plant operation.
Each of the process variables has its own control loop,
which typically consists of a sensor and transmitter,
controller and control valve. See the Figure
below. Each control loop keeps track of the associated process variable. An
adjustment is made to a process variable by varying the opening of its control
valve. The stream flow rate is therefore adjusted and a desirable variable is
The sensor measures the process variable from the plant (i.e. plant data) and the transmitter sends the information to the DCS (Distributed Control System) controller located in the control room.
The controller checks if the process variable agrees with the set point. If not, it will send corrective signal to the control valve that will make adjustment in the plant so as to match the process variable to the set point. This goes on continuously, essentially in a loop - hence the term "control loop".
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Distillation Control Philosophy
Some of the general guidelines are noted below:
column pressure normally controlled at a constant value
feed flow rate often set by the level controller on a preceding column
feed flow rate is independently controlled if fed from storage tank or surge tank
feed temperature controlled by a feed preheater. Prior to preheater, feed may be heated by bottom product via feed/bottom exchanger
top temperature usually controlled by varying the reflux
bottom temperature controlled by varying the steam to reboiler
differential pressure control used in packed columns to monitor packing condition, also used in tray columns to indicate foaming
the compositions controlled by regulating the reflux flow and boiled-up (reboiler vapour)
Pressure is often considered the prime distillation control variable, as it affects temperature, condensation, vapourisation, compositions, volatilities and almost any process that takes place inside the column. Column pressure control is frequently integrated with the condenser control system.
Reboilers and condensers are integral part of a distillation system. They regulate the energy inflow and outflow in a distillation column.
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A column is controlled by regulating its material and energy balances. Click here for more information and an example.
The following controls are briefly discussed in this Section:
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