Cereals processing technology

46 Cereals processing technology
determine optimum control strategies and set points that can be implemented
within existing control systems. They do not, however, take action on the basis
of the optimum solution determined.
On-line Near Infra Red (NIR) apparatus is being used in an open loop system
to control flour ash in flour mills. In this example the measurement device
simply presents real time results of flour ash to the process operator. The
operator closes the control loop by assessing the measured data and acting
appropriately to maximise product yield and minimise product fluctuation. I am
not aware of closed loop control being in place in any operational facilities.
Indeed, the similarities between the closed loop control of flour protein 18 and the
potential for closed loop control of flour ash or starch damage has not been
recognised by many in the flour milling industry. There are mills in operation
today that have the on-line measurement capability and automated roll gap
measurement available, but the link between the two using adaptive control has
not been made.
3.8.3 Application of optimisation techniques
Successful application of optimisation techniques involves a systematic
approach to using any models developed. The first step of the methodology is
concerned with steady state optimisation. This process delivers predictions of
potential benefits and savings. However, plants never actually operate at steady
state, but rather are in a perpetual transient state due to a variety of disturbances.
Nevertheless, the benefits predicted by steady state simulations may be achieved
on average, thereby improving process performance.
Dynamic optimisation must be performed to compensate for process
disturbances and involves reacting to these disturbances in such a manner that
plant productivity is maximised at every instant in time. Dynamic models in the
form of simulations can be used for off-line control strategy development and
on-line dynamic models can be used to help make well-informed responses to
each disturbance.
Reyman (1992) gives an example of the application of an integrated control
system in the design of a new process. The importance of a methodical approach
and especially proper choice of a process model is shown. In summary, the steps
towards integrated control development from beginning to end should consist of
the following:
1. Determination of control performance requirements and collecting process
data.
2. Analysis of process instrumentation.
3. Analysis of process input/output responses.
4. Improvement of the primary control strategy.
18 The protein content of flour is a strictly controlled parameter in flour mills. Most modern mills
employ closed loop control systems to maintain flour protein at predetermined levels.