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An Example of Distillation Column Control

A typical distillation column has a combination of different control loops. The control system of a particular column is designed to meet that column's particular process requirements. An example is shown in the Figure below.

There are several control loops associated with the distillation column:

Temperature:

1. Overhead condensation (Fin-fan)
2. Overhead column (Reflux)
3. Feed preheat
4. Column bottom (Reboiler steam)

Pressure:

1. Overhead accumulator (Off gas)

Level:

1. Overhead accumulator (Distillate product)
2. Column bottom (Bottoms product)

Flow:

1. Column feed

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In this distillation column, the material balance (MB) loops consisted of the following:

feed flow control loop (which sets the throughput, i.e. production rate)
bottom level control loop (which controls the column level)
accumulator level control loop (which regulates the product flow by regulating the overhead accumulator level)
off gas pressure control loop (which controls the column pressure)

The energy balance (EB) control loops are the following:

reboiler temperature control loop (which control the column bottom temperature by controlling the steam input to the reboiler)
feed preheater temperature control loop (which controls the feed inlet temperature)
overhead condenser temperature control loop (which regulates amount of cooling in the column)
external reflux temperature control loop (which controls the temperature at the top of the column)

In this example, the main influence on the heat input to the column is the steam flow to the reboiler. Heat also enters the system via the preheater. Heat balance is achieved when the heat input from the reboiler and preheater is removed by the condenser.

(Note that there is also a balance between the energy in the feed stream and product streams, but this balance does not have much effect on the overall energy balance)

In this type of control system, the material balance control loops react to the changes in the column's energy balance.

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For example, a change in the reboiler steam flow will lead to a series of changes in the column; and the column's control system react to this change in order to maintain the material balance and energy balance.

An increased steam flow to the reboiler means an increase in heat input which will result in increased vapourisation in the reboiler and an increased bottom temperature. There will be an increased vapour flow and temperature throughout the column. The liquid level in the bottom of the column decreases as more liquid is being boiled-off, and the bottom product rate decreases. Hence, a change in the EB leads to a change in the MB.

Increased vapour flow to the top will cause a higher temperature at the top of the column, and the temperature (reflux) controller will increase the reflux flow back to the column. Increased reflux flow will condense the additional vapour in the column.

The larger amount of vapour also requires additional cooling in the overhead system and this is handled by the temperature control that increases the fan speed of the overhead condenser. This will increase the heat removal and tends to restore the EB. Increased condensation leads to increased liquid flow into the overhead accumulator (reflux drum). The accumulator level controller responds by increasing the outflow of top product. This increased outflow of materials from the top will offset the decreased in outflow from the bottom, hence the MB is restored.

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Concentrations of the top and bottom product streams are affected as well - higher bottom temperature will results in more heavy components being vapourised from the bottoms product.

This can be illustrated using a multi-component separation of 8 products: C1, C2, C3, C4, C5, C6, C7 and C8+. The main separation is between 2 key components: the light key (C4) and heavy key (C5). This is shown in the Figure below.

If the bottom temperature is too high, more of the heavy key (HK) will be vapourised from the bottom product. The vapour thus had become heavier due to the presence of the HK. The final boiling point (FBP) of the top product will be higher but the initial boiling point (IBP) did not change.

On the other hand, the IBP of the bottoms product will be higher, because the bottoms product has been depleted of the HK and become heavier. The FBP of the bottoms product is not affected by the bottom temperature increase.

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Other possible disturbances

This example illustrated just one of the many disturbances that can upset the smooth operation of a distillation column. Besides the reboiler example, which could be due to controller malfunctioning, other disturbances can also occur. The following list is not exhaustive, but only serves as a reference of what possible events that can disrupt the smooth operation of a plant.

reboiler and other heat exchangers: fouling of heat transfer surfaces, tube leaks, etc
charge heater: loss of fuel gas and/or fuel oil (e.g. due to low fuel gas pressure trip)
overhead condenser: loss of cooling water or loss of power supply (for air-fin coolers)
pumps: overload trip, loss of power, cavitation, etc
control valves failure: e.g. loss of instrument air, jammed valve, faulty positioners, etc.
faulty instruments: wrong signals transmitted, false alarms, etc.
feed changes: feed rate, more lower boiling components, contaminations, etc.
tower internals: e.g. flooding, weeping, channelling, etc.

 

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