FC

FT

Steam

R-10A R-10B

T

Bottoms

Figure 7-7.4 Temperature control in a distillation column.

mented. This column uses two reboilers. One of the reboilers, R10B, uses a con-

densing process stream as a heating medium, and the other reboiler, R10A, uses

condensing steam. For ef¬cient energy operation, the operating procedure calls for

using as much of the process stream as possible. This stream must be condensed

anyway, and thus serves as a “free” energy source. Steam ¬‚ow is used to control the

temperature in the column.

After startup of this column, it was noticed that the process stream serving as

heating medium experienced changes in ¬‚ow and in pressure. These changes acted

as disturbances to the column and consequently, the temperature controller needed

to compensate continually for these disturbances. The time constants and dead time

in the column and reboilers complicated the temperature control. After the problem

was studied, it was decided to use feedforward control. A pressure transmitter

and a differential pressure transmitter had been installed in the process stream, and

from them the amount of energy given off by the stream in condensing could be

calculated. Using this information the amount of steam required to maintain the

temperature at set point could also be calculated, and thus corrective action could

be taken before the temperature deviated from the set point. This is a perfect

application of feedforward control.

Speci¬cally, the procedure implemented was as follows. Because the process

stream is pure and saturated, the density r is a function of pressure only. Therefore,

using a thermodynamic correlation, the density of the stream can be obtained:

171

ADDITIONAL DESIGN EXAMPLES

r = f1 (P) (7-7.1)

Using this density and the differential pressure h obtained from the transmitter

DPT, the mass ¬‚ow of the stream can be calculated from the ori¬ce equation:

w = K o hr (7-7.2)

where Ko is the ori¬ce coef¬cient.

Also, knowing the stream pressure and using another thermodynamic relation,

the latent heat of condensation l can be obtained:

l = f2 (P) (7-7.3)

Finally, multiplying the mass ¬‚ow rate times the latent heat, the energy q1 given off

by the process stream in condensing is obtained:

q1 = wl (7-7.4)

Figure 7-7.5 shows the implementation of Eqs. (7-7.1) through (7-7.4) and the

rest of the feedforward scheme. Block PY48A performs Eq. (7-7.1), block PY48B

performs Eq. (7-7.2), block PY48C performs Eq. (7-7.3), and block PY48D performs

Eq. (7-7.4). Therefore, the output of PY48D is q1, the energy given off by the con-

densing process stream.

To complete the control scheme, the output of the temperature controller is

considered to be the total energy required qtotal to maintain the temperature at its

set point. Subtracting q1 from qtotal, the energy required from the steam, qsteam, is

determined:

q steam = q total - q1 (7-7.5)

Finally, dividing qsteam by the latent heat of condensation of the steam, hfg, the

required steam ¬‚ow wsteam is obtained:

q steam

w steam = (7-7.6)

hfg

Block TY51 performs Eqs. (7-7.5) and (7-7.6) and its output is the set point to

the steam ¬‚ow controller FC. The latent heat of condensation of steam, hfg, was

assumed constant in Eq. (7-7.6). If the steam pressure varies, the designer may want

to make hfg a function of this pressure.

Several things must be noted in this feedforward scheme. First, the feedforward

controller is not one equation but several. This controller was obtained using several

process engineering principles. This makes process control fun, interesting, and

challenging. Second, the feedback compensation is an integral part of the control

strategy. This compensation is qtotal or total energy required to maintain tempera-

ture set point. Finally, the control scheme shown in Fig. 7-7.5 does not show dynamic

compensation. This compensation may be installed later if needed.

172 FEEDFORWARD CONTROL

PY 48C

l f2 ( x)

PY 48D PY 48B

r Process stream

w

f1 ( x)

MUL PT

SQRT saturated

PY 48A vapor

q1

DPT

h

SP

qtotal

SUM TC

TY 51

TT

wsteam

FC

FT

Steam

R-10A R-10B

T

Bottoms

Figure 7-7.5 Implementation of feedforward control.

7-8 SUMMARY

In this chapter we have presented in detail the concept, design, and implementation

of feedforward control. The technique has been shown to provide signi¬cant

improvement over the control performance provided by feedback control.

However, undoubtedly the reader has noticed that the design, implementation, and

operation of feedforward control requires a signi¬cant amount of engineering, extra

instrumentation, understanding, and training of the operating personnel. All of this

means that feedforward control is more costly than feedback control and thus must

be justi¬ed. The reader must also understand that feedforward is not the solution

to all the control problems. It is another good “tool” to aid feedback control in some

cases.

It was shown that feedforward control is generally composed of steady-state com-

pensation and dynamic compensation. Not in every case are both compensations

needed. Finally, feedforward control must be accompanied by feedback compensa-

tion. It is actually feedforward/feedback that is implemented.