Page 467 - Design and Operation of Heat Exchangers and their Networks

450 Design and operation of heat exchangers and their networks

responses corresponding to different input disturbances can be given at
the same time, while the traditional open-loop control can only com-
pute the corrective action to one input variable. Therefore, this accu-
rate model predictive feedforward control system can be used for
multistream heat exchanger control, where the traditional open-loop
control is difficult to be implemented.
(2) As previously mentioned, the accurate model predictive control
can choose the optimal control scheme from many choices under dif-
ferent constraints, which is impossible by the traditional open-loop
control.
The implementation of distributed parameter model predictive feedfor-
ward control is a reverse question of dynamic solution obtained. The con-
troller should give the corrective instruction based on the response solution
upon the changes of objective input parameters to adjust the inputs of assis-
tant fluid, namely, the changes of inputs should be given based on the
response solution, but the solution obtaining calculation is to find the output
response based on the information of input changes; the former is much dif-
ficult to be carried out. Thus, two steps are needed in the control execution:
firstly, the solution obtaining calculation is implemented according to the
change of objective input parameter, and then, the adjustment of auxiliary
input is obtained by iterative calculations.
The model predictive feedforward control is a big improvement to tra-
ditional feedback control, but it still needs improvement. Because of the
accuracy of the model adopted and the inertia of the system, it may not give
the complete compensation when disturbances happened, and for no signal
of output is feedback to the controller, the possible error cannot be adjusted.
If the output error is feedback and the controller will act based on the inte-
grated response, the system is a “true” model predictive control according to
the control theory. The model predictive control system can be schemati-
cally shown in Fig. 9.7, where the feedback is included, but it is different
from traditional feedback and feedforward system in that it is not the output,
but the error of output compensation is feedback. Since Richalet et al.
(1978), the predictive control method was developed by many researches
(Clarke et al., 1987a,b; Soeterboek, 1990; Richalet, 1993). Some applica-
tions of model predictive control had been given for heat exchanger control
(Hecker et al., 1997). Several books and thousands of papers about the model
predictive control have been published. Intensive reviews of the develop-
ments in model predictive control and their applications were given by
Garriga and Soroush (2010), Tran et al. (2014), Vazquez et al. (2014),