Many different operations take place in a chemical plant. The classical approach of Unit Operations might thus be extended to process control, and we could consider in turn the control of heat exchangers, chemical reactors, distillation columns etc.
This turns out not to be a useful approach in most cases. The reason for this is that we are in the end concerned with the control of processes which consist of several operations, and these cannot be considered in isolation. This makes the engineer's task of designing a control system a difficult one, since it is hard to find just where to start!
The starting point we shall choose here is to consider how we regulate each of the basic quantities we may wish to keep constant in a process.
These quantities are
The following sections discuss simple, but real, examples of how feedback control is applied to these basic quantities in a chemical plant. They are primarily examples of control for operability, and most of them will refer to single items of equipment or very simple combinations. A number of safety issues will be identified.
Strategic control for profitability will be dealt with in a later section in the context of control of complete plants and processes.
A number of fundamental concepts will be illustrated in the course of these examples. They are `graded' in the sense that the simplest examples come first; the reader is advised to follow the sequence we have presented. Even apparently trivial examples may be used to introduce important ideas.
The word `control' is sometimes loosely used to mean either regulation or adjustment. We have not actually seen the sentence 'In a control system the controller controls the controlled quantity by controlling a control valve position' However the term is regularly misused in this way. Control should refer only either to the actions of the controller element itself or to the function of the complete system. We are not just being pedantic, it is possible to misunderstand what is happening through misuse of terminology. We may also slip up ourselves, particularly in talking loosely about e.g. 'flow control' when we strictly mean 'flow regulation'.
This completes a control system to regulate the measured quantity, here the flow, by adjustment of the valve position. Compare this with the block diagram which we used earlier to introduce the feedback control system.
Consideration of the properties of flowmeters and valves suggests that we were correct in our first choice. If the valve were upstream of the flowmeter then there are a number of ways in which it might affect the flowmeter calibration.
This is an example of an on-off control algorithm. The heater is either on (full) or off (completely). What will happen if we try to use such an algorithm here, where the objective is to maintain a particular flow?
Clearly, this is unlikely to serve, as rather than maintaining a specified flow the conditions will switch between zero and some maximum value. To achieve a specified steady flow we require something like:
This is a proportional control algorithm; the larger the error in the measured quatity, the larger will be the adjustment. This arrangement should result in the system settling at or near the required flow.
In practice, on-off control is seldom used. Most adjustment elements are valves, or occasionally other mechanical elements. These do not take kindly to being regularly or rapidly swung accross their full range of adjustment; they very quickly wear out or break down.
In most cases, therefore, proportional control or some variant is used. More detailed investigation of control algorithms requires quantitative information about the process. This aspect will be dealt with in a later section.
Inventory may be measured in a number of ways. Mass holdup may sometimes be determined directly, but usually volume is measured. In liquid systems volume is measured by level. In gas or vapour systems pressure is used as a measure of inventory.
Below is a diagram showing the two alternative control systems available for feedback control of the level. Both are equally valid and the decision as to which to use is based on
Raising the pressure of a gas requires energy, and normally this energy is imparted by some mechanical device, such as a compressor. Both the compressor itself, and the energy to drive it, are expensive. To minimise the first cost we try to minimise the number of compressors in a process. Where possible we would use only one, locate it at the front of the process, and perform any subsequent manipulations to obtain the required pressure by downstream valves.
The energy used in compression is expensive, and throttling through a control valve throws this energy away. Therefore in proesses where compressor costs are very significant we may sometimes avoid such valves and manipulate the compressor speed in order to maintain the system at the required pressure. This control system is shown in the diagram below.
To change the temperature of something it is necessary to add or take away energy. This can be achieved in one of two ways.
There are advantages and disadvantages for both methods. With the first there is the problem of transferring heat through the walls of the 'coil'. In the second the energy is absorbed directly but with the additional problem of increased flowrate/volume.
Diagrams of these alternative schemes can be found below.
Simple Composition Control Problem
To illustrate composition control consider the simplest process in which composition can be changed, namely blending. Here two streams of different compositions are mixed together e.g. a concentrate and a diluent as shown in the diagram below.
It is worth mentioning that the composition of a stream is rarely measured directly.
Typical composition analysers include
Features of this type of hardware which make them ineffective for control purposes are
Thus an alternative method has to be sought to control the composition. This could be via the
However, we remember that in a two phase system temperature and pressure are not independent. We can thus change the pressure of a vapour which is in equillibrium with a liquid by changing the temperature of the system. Raising the temperature raises the vapour pressure of the liquid which must equal the equillibrium pressure of the system.
Hence we can manipulate the temperature in the condenser by means of a small valve on the cooling water line, thus changing the pressure in both condenser and column.
