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Condenser and Pressure Control

The 3 main methods of pressure and condensation control are:

(1) vapour flow variation,
(2) flooded condenser, and
(3) cooling medium flow variation.

Vapour Flow Variation

The simplest and direct method for column producing a vapour product. The pressure controller regulates the vapour inventory and therefore the column pressure. See the Figure below.

Condener-Pressure control - vapour flow variation

An important consideration here is the proper piping of the vapour line to avoid liquid pockets.

Flooded Condenser

This method is used with total condensers generating liquid product. Part of the condenser surface is flooded with liquid at all times. The flow of condensate from the condenser is controlled by varying the flooded area. Increasing the flooded area (by reducing flow) increases the column pressure (less surface area for condensation).

Flooded condenser pressure control

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Cooling Medium Flow Variation

Pressure can also be controlled by adjusting the flow of coolant to the condenser ( see Figure below). Operation using cooling water can cause fouling problems at low flow condition, when cooling water velocity is low and outlet temperature is high.

Pressure control - CWS flow

For air-fin condensers, the controller varies the fan speed or fan pitch to control pressure (see Figure below). This arrangement is energy-efficient as it minimises fan power consumption, but requires the use of variable-pitch fan or variable speed motor.

Fin-fan pressure control

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Other method: pressure control using inerts (see Figure below).

Pressure control with inerts

When column pressure falls, an inert gas is admitted to raise the column pressure.

Or: split-range pressure control venting excess gas to flare (see Figure below)

Split range pressure control

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In most instances, both vapour and liquid phase are present in the column overhead. The vapour contains components that can condense out but are undesirable in the liquid, i.e. excessive condensation may lead to off-specification liquid product. In addition, it is also undesirable to lose liquid product (through insufficient condensation) to the vapour. It is therefore important to control the rate of condensation to obtain the desired vapour-liquid split.

This is usually done by controlling the temperature of the liquid product just downstream of the condenser. One common scheme used is shown the Figure below.

Offgas pressure control

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