Module 3.2: Extension of Hierarchical Decomposition

Introduction

In this module we aim to introduce an alternative method for the design of control systems for complex processes. In the traditional approach the process engineer is presented with a complete flowsheet and is required to specify the structure of all the loops.

There are two disadvantages to this method.

For these reasons a Hierarchical approach to the design of control systems will be used. In this method controls are placed, as far as possible, on the flowsheet in its simplest form. As the flowsheet is elaborated more loops are added.

This approach has significant advantages over existing practice.

Below is a reference to a paper which has been published on this material.

Ponton, JW and Laing, DM, 1993, A Hierarchical Approach to the Design of Process Control Systems, Chemical Engineering Research and Design, Vol 71, Part A, 181-188.


Outline of Procedure

This hierarchical approach is already well established in the conceptual design of chemical flowsheets. In the Douglas scheme the designer starts with a single block showing only feeds and products and proceeds by sequential elaboration through the following steps: The method used for designing control systems employs a similar set of steps: As with the design outline above, these controls are placed on the flowsheet at different stages in the structure. Advantages of this hierarchical approach are:
  1. At any one time only a subset of the overall problem is considered. This simplifies the problem somewhat and avoids the intellectual overload inherent in the conventional approach.
  2. The more important strategic control loops are emphasised, i.e. those affecting the economic performance of the plant such as product rate, overall conversion and product quality.
  3. Less important decisions are left to the end, i.e. those associated with operability such as regulation of inventory.
  4. By proceeding in parallel with both process and control system design, alternatives for both may be considered at each step.

We will now go through these steps in more depth and discuss what should be considered at each level.


Input-Output Structure

At this stage we can only look at control of the feed and product. However this affects the basic shape of the process and so control decisions at this stage determine the whole strategy. Things to think about at this point are:


Recycle Structure

The next stage in the design of the control system is to elaborate it further to include the recycle structure. In some cases this may be done in more than one step. Things to consider at this stage are:


Functional Subsystems Structure

The next stage is to include sections such as feedsystems, reactors or separation schemes. Important points which can be added to the lists above are: This last point covers systems such as feed/mixing systems or separation schemes. It includes equipment such as flash units or distillation columns. These are discussed in a separate module on control of such units.


Discussion

At each of the above three stages it is important to understand what each stream is there for and to ensure that as much control as possible has been included. Thus the following steps are used:

This is an extremely straightforward procedure. It is easy to see what all streams do when the flowsheet is in a simplified form. On a complete PFS or ELD it will be much harder to identify important streams and their interrelationships. Similarly, the purpose of streams is most clearly established in the designer's mind at the point when he has just identified them in the process design.

The above procedure is best shown as a worked example. There are four of these in a separate section including one which is interactive and so requires the user to think!!

The final stages in the design of a control system are to consider energy integration and inventory control.

For energy integration the general principal is to derive a control system for an integrated energy recovery network from the unintegrated form by replacing valves on utility streams with bypasses. The case of energy integrated distillation can be rather more interesting. For example, in unintegrated distillation systems reboiler heat load is often used as a manipulated variable for bottom product composition regulation. Condenser heat load is used to regulate column pressure. However, what happens when the condenser of the first column forms the reboiler of the second?

We regard the regulation of inventory, i.e. ensuring that all mass balances do in fact balance, as the lowest level of control, to be achieved after all strategic control systems have been specified. It is not usually difficult to place level control loops once the number of alternatives has been reduced.

Consider a process or subsection with n inputs and outputs which contribute to a single total material balance. (n-1) of these may be set by strategic control loops for composition, temperature etc. One remaining stream must be used to ensure material balance via level control in a liquid system or pressure control in a gas system or implicitly by some inherent control mechanism such as the weir on a distillation tray.


Return to Start of Module 3: Control Systems for Complex Processes