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Reboiler Control

This is required to provide good response to column disturbances, and to protect the column from disturbances occurring in the heating medium. The reboiler boil-up is regulated either: (1) to achieve desired product purity, or (2) to maintain a constant boil-up rate.

In a typical reboiler control (see Figure below), the control valve is located in the reboiler steam inlet line.

Typical reboiler control - steam flow

For inlet steam controlled reboiler, the heat transfer rate is regulated by varying the steam control valve opening, thereby changing the steam condensing pressure and temperature.

When an additional boil-up is required, the valve opens and raises the reboiler pressure, which increase the temperature, and in turn increases the boil-up rate. This scheme has the disadvantage of non-linear relationship between pressure and boil-up, and is affected by fouling in the reboiler.

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An alternative is to control the condensate flow, i.e. by putting the control valve on the condensate line ( see Figure below). The main disadvantage is that this scheme has poorer dynamic response than the previous scheme. Manipulating the inlet valve immediately changes the vapour flow, giving faster dynamic response. On the other hand, the condensate outlet valve has no direct effect on vapour flow. The response time varies with the condensate level in the exchanger.

Alternative reboiler control - condensate side

The other main disadvantage is the sizing of the condensate valve. If condensate cannot be drained in time, vapour flow may be restricted as much of the reboiler remains flooded. On the other hand, too fast of condensate draining (faster than vapour condensation in the reboiler) as result in loss of liquid seal in the reboiler and steam will pass into the plant's condensate recovery system.

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Some reboiler control features the use of condensate pot. This is particularly important in fouling or corrosive services (where leakage is a serious problem). An example is shown in the Figure below.

Reboiler control with condensate pot

In the system shown, by varying the level control set point, the tube surface area in the reboiler that is exposed for vapour condensation can be adjusted, thus changing the available heat transfer area. The heat transfer rate can therefore be adjusted.

This arrangement also automatically minimise the condensation (and therefore, tube wall) temperature. A pressure-balancing line is provided to maintain a steady pressure and level in the condensate pot.

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