Module 3.1: Control of Distillation Columns

Introduction

The aim of this module is to introduce the control of distillation columns. We will start by analysing the degrees of freedom to establish how many and which control parameters it is possible to control and/or manipulate. Then we move on to discuss different ways to control the two most important parameters: composition at the top of the column and the pressure of the column. Finally there are a number of examples showing different control structures.

Degrees of Freedom Analysis

We will use the method developed by Professor Ponton in his Paper

Degrees of Freedom Analysis in Process Control, Chemical Engineering Science, 1994, Volume 49, No. 13, pp 2089 - 2095

to determine the number of control degrees of freedom in a distillation column. There are two equivalent procedures based on the equation -

C.D.F. = Total No. of Streams - No. of Phases Present + 1

All we have to do is count all the streams in the process. Separately count the total number of extra phases i.e. add up all occurrences of phases greater than one in all units. The number of control degrees of freedom is the difference between these two numbers.

Figure 1 below shows this method.

Figure 1 - Degrees of Freedom Analysis of Distillation Column

So the number of degrees of freedom is 5. However, a typical control strategy for such a process would use only 4 of these - feedrate, column pressure, top and bottom composition. This is because the column and condenser are normally maintained at the same pressure.

However, a valve could be placed in the line between. This would actually be undesirable as reducing the condenser pressure will decrease the temperature driving force available from the cooling medium.

Controlling Pressure in Distillation

In a distillation column it is usually necessary to regulate the pressure in some way. Below there are five different methods described for doing this.

One thing to note is that in none of them is a valve simply placed on the vapour line. This would lead to the use of a large expensive control valve. Instead the pressure is controlled indirectly involving the use of the condenser and/or reflux drum.

Vent to Atmosphere

Figure 2 below shows the easiest way to control the pressure in a column operating at atmospheric pressure.

Figure 2 - Vent to Atmosphere

In this case the cooling water flow stays constant and the reflux drum is vented to atmosphere. Thus the reflux drum and hence the top of the column are at atmospheric pressure. The advantage of this scheme is that it requires one less control valve. The disadvantage is that the tops have to be subcooled so that a minimal amount of vapour is lost through the vent. Hence more energy is required from the reboiler when the reflux is added to the top of the column.

Cooling Water

Figure 3 shows the most common method for controlling the pressure - adjustment of the cooling water flow.

Figure 3 - Cooling Water

In this case if the cooling water flow is increased then more vapour is condensed and the vapour pressure is reduced (and vice versa).

Flooded Condenser - 1

Figure 4 shows the classic flooded condenser approach.

Figure 4 - Flooded Condenser - 1

Again in this setup, as with the first example, there is no valve on the cooling water. Instead the valve is in the liquid line between the condenser and reflux drum.

If this valve is closed then the condensed vapour i.e. liquid will build up and flood the condenser. This has the effect of reducing the heat exchange area, thus reducing the amount of vapour being condensed and hence increasing the pressure.

The valve can then be opened, the liquid level will fall, increasing the heat exchange area and hence decreasing the pressure.

Flooded Condenser - 2

Figure 5 shows an alternative arrangement for a flooded condenser.

Figure 5 - Flooded Condenser 2

The first thing to notice about this setup is that the reflux drum and condenser are at the same level. The second important point is that the vapour line, on which there is the control valve, is very small in comparison with the overhead line. If the valve is opened there is a small escape of gas into the reflux drum. This pushes the liquid level down in the drum and up in the condenser, flooding it and reducing the heat exchange area as in the last example.

Therefore to increase the pressure the valve is opened and to decrease the pressure the valve is closed.

Partial Condenser

The final example is the control of a partial condenser.

Figure 6 - Partial Condenser

The above scheme is used if the overhead product is required as a vapour.

Controlling Tops Composition in Distillation

As well as pressure, the other parameter most likely to be controlled is the composition of the tops product. The reason is that the final product will most probably come from the top of the column and it is important to know its composition. Again, as with pressure, there are many different ways of controlling the tops composition. Three methods are described below.

Reflux Rate

In this first example the reflux rate is adjusted to control the composition of the tops product.

Figure 7 - Reflux Rate

As the amount of reflux is changed so the temperature profile in the column changes and hence the composition.

Reflux Ratio

The second example uses the reflux ratio as the control parameter.

Figure 8 - Reflux Ratio

When designing a distillation column it is usually the reflux ratio that is determined. This can be kept constant throughout operation by using two flow indicators and a ratio controller.

Distillate Rate

The third example is for high purity tops. It uses the distillate flowrate to control the distillate composition.

Figure 9 - Distillate Rate

It can be shown that for a high purity column i.e. one with a large reflux, that the composition of the distillate is sensitive to the distillate flow but insensitive to the reflux rate. Therefore for a high purity column the control scheme outlined above is used. It should be noted that tight control on the level in the reflux drum is required using the reflux rate.

Distillation Column Control Examples

The following examples describe alternative control strategies of fairly standard form.

In all cases actual composition controllers are shown. These could of course be replaced by inferential measurement from temperature, with or without cascade of a slower analyser. Unless otherwise stated, it has been assumed that the feed rate to the system is not available as a manipulated variable.

Pressure, Overheads Rate and Composition

This is a fairly standard configuration for a single product column, i.e. when the bottoms streams is a byproduct, recycle or goes to further processing.

Although the overheads composition is regulated by adjusting the steam rate at the base of the column, the response of the column to heat input changes is quite rapid, and so this strategy is acceptable.

Pressure control on condenser cooling water is shown; of course any other pressure control scheme would be acceptable.

Figure 10 - Overheads Rate and Composition

Pressure, Bottoms Rate and Composition

This is the analogous situation to the previous case, in the rather less usual circumstances where a main product is withdrawn from the bottom of the column.

This does not work well, since either the bottom level, as here, or composition, has to be regulated by adjusting the reflux rate. In either case the loop involves a long delay due to the hydraulic lags on each tray.

It is probably marginally better to regulate composition by steam rate since this is a more important quantity than level, although the two loops could be interchanged with the steam adjusting the level, which is quite a good scheme, and the reflux manipulating the bottoms composition, which is very poor. Fotunately this is an unusual requirement, as main products normally come from the top of columns for other reasons.

A standard flooded condenser pressure control system is shown.

Figure 11 - Bottoms Rate and Composition

Pressure, Bottoms Rate and Overhead Composition, With Partial Condenser

This is not a particularly common strategy, but the arrangements for a column with partial condenser are typical. The pressure in such a system is almost always manipulated by a valve on the vapour product line. There is no reflux drum, and reflux rate is often set implicitly by adjusting the cooling load on the condenser.

Figure 12 - Bottoms Rate and Overhead Composition, With Partial Condenser

Pressure, Overhead Rate and Bottoms Composition

This scheme should work satidfactorily as all adjustments are made at the same end of the column as the related measurements. The pressure control scheme is the so-called hot gas bypass. Note that the layout of condenser and reflux drum shown is critical to the operation of this method, which is actually a variation on the flooded condenser approach. The bypass is a very small pipe which bleeds vapour into the reflux drum where it does not immediately condense. The pressure in the system rises as the bypass valve is opened.

Figure 13 - Overhead Rate and Bottoms Composition

Pressure, Bottoms Rate, Overhead Rate and Composition

Since three regulated quantities are specified, the feed to the unit must be available as an adjustment. Apart from this, the arrangements are similar to those of the first example. Level control on the column base is not very satisfactory due to the lags between the feed and the bottom of the column, but any other arrangement would be worse.

Figure 14 - Bottoms Rate, Overhead Rate and Composition

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