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.
- The complexity of the task.
- It is very difficult to choose the control structures for a complete
flowsheet from amongst the large number of alternatives.
- It is made even more difficult in that there is no defined procedure
for tackling this problem.
- It is impossible for any feedback from the control study to be taken
- Decisions about equipment will already have been taken.
- There will be considerable reluctance to change the process
flowsheet to accomodate possible control problems.
This approach has significant advantages over existing practice.
- The task of placing all control loops on a complete and detailed
flowsheet is reduced to more manageable proportions.
- Attention is concentrated on strategically important control loops
rather than those of secondary importance such as level in holding
- Decisions on process and control system structures are taken
together, allowing the latter to influence the former in a manner not
possible with the conventional approach.
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.
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:
- Input-Output structure
- Recycle structure
- Separation sequencing
- Energy integration
As with the design outline above, these controls are placed on the
flowsheet at different stages in the structure.
- Feed and product rate control
- Recycle rates and composition
- Product and intermediate stream composition
- Temperature and energy balance control
- Inventory regulation
Advantages of this hierarchical approach are:
- Input-Output Structure
- Recycle Structure
- Functional Subsystems Structure
- 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.
- 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.
- Less important decisions are left to the end, i.e. those associated
with operability such as regulation of inventory.
- 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.
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:
- Identify likely disturbances
- Ranges and Scaling
- Sensitivities i.e Steady-state gains
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:
- Sensitivities and ranges
- Interactions i.e RGA etc
- Functional controllability
- Multiple forward paths and possible inverse response
- Approximate dynamics
- Time delays
- Approximate time constants
- Simple models
The next stage is to include sections such as feedsystems, reactors or
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
on control of such units.
- Simple dynamic unit models
- Standard control schemes investigated for subsystems
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:
- Consider each stream on the flowsheet at the current level.
- Why is it there?
- What will decide its flowrate?
- Can a complete, partial or tentative control system be added to
manipulate its flow and meet the above objective?
- Draw in as much of the control system as possible.
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
Return to Start of Module 3: Control Systems
for Complex Processes