TEMPERED HEAT TRANSFER

The driving force in any heat exchanger is the temperature difference. The rate of heat transfer can be quickly changed by changing this difference. A tempered

Tempered Heat Transfer 169

system is designed to quickly and accurately control the temperature of an input stream. This system requires two sources of feed. One must be above the desired temperature and the other below it. These are mixed together to obtain the desired temperature. This can be done by using a ratio controller, as is illustrated in Figure 7-6.

Figure 7-5 Material balance control in the direction opposite to flow.

Source: Buckley, P.S.: “Input of Process Control Advances on Process Design,” paper presented at Process Control Workshop in Memphis, Tenn., Feb. 5, 1964.

R-301

A modification of this scheme that can be used when only one stream is available is given in Figure 7-7. It is useful for controlling the input temperature to a reactor. For this system the greater part of a process stream is heated to a temperature above that desired. This is then mixed with a portion (usually around 15%) of the same stream that has not been heated. This system requires a larger heat exchanger than would be required if the whole stream went through the exchanger and the output temperature were controlled by the flow rate of the utility

Q

Figure 7-7 Precise temperature control of the feed to a reactor. CASCADE CONTROL

In one typical situation the temperature of the product stream is controlled by manipulating a valve that regulates the amount of steam entering an exchanger (Fig. 7-8). Should the upstream steam pressure increase, this will increase the flow rate of steam through the control valve. The steam pressure in the exchanger will increase, which in turn will increase the rate of heat transfer to the process stream. The result will be a change in the product temperature due to a change of the steam pressure. The system will eventually return to the desired temperature.

r

I

Cascade and Feedforward Control 171 If very close control is desired, then any disturbance due to steam pressure changes should be minimized. Figure 7-9 shows how this can be done using a cascade control system. In this case, the temperature of the process stream is measured and compared to its desired value, as before. The output of the controller, however, instead of affecting the control valve, regulates the set point of a second controller, the steam-pressure controller. This controller compares the set point determined by the first controller with the pressure downstream of the steam valve. If there are any differences it then adjusts the steam the downstream pressure changes, a correction in the control valve is made immediately, instead of waiting for a product temperature change. Should the output temperature of the process stream rise, this would cause a set point change of the steam-pressure controller, which would cause a decrease in the steam pressure in the heat ex- changer. Cascade control is very useful when the variation in the quality of a utility or other manipulable stream can cause deviations from the desired output.

- - - - 1 I t UTILITY I S T R E A M FEED PRODUCT

Figure 7-9 Cascade temperature control. FEEDFORWARD CONTROL

When close control is desired, usually the variable that is to be closely controlled is monitored and no changes are made until the measurement differs from what is desired. This is feedback control. It obviously is not an ideal system, since the controller can only react to changes. A better system would be one that anticipates a change and takes corrective action that ensures an unvarying output. This is a feedforward control system. This type of control is very advantageous when the input variables have a wide range of variation.

Since it is impractical to measure everything that may affect the output variables, even when feedforward control is used feedback control is also included. Figure 7-10 shows how a feedforward system might be used on a waste neutralizer.” The purpose of the waste neutralizer is to make certain that the streams leaving the plant are neutralized. First, all the streams are combined together and the feed rate and

Neutral

Figure 7-10 A control system for the neutralization of waste by reagent R.

Source: Friedman, P.G., Moore, J.A.: “For Process Control Select the Key Variables,”

Chemical Engineering, June 12, 1972, 90.

acid content are measured before they enter the neutralizer. The amount of reagent necessary to neutralize this feed is determined and is added. A feedback system is also included; it either increases or decreases the calculated amount of base added, depending on the exit value of the acid content

BLENDING

The neutralizer in the previous example might be controlled differently if the main fluctuation in the load occurs in one or two of the streams. Instead of combining all the streams together before they enter the neutralizer, those streams that vary widely might enter an additional holding tank, where they would be neutralized using traditional feedback control. They would then be added to the main neutralizer, which also has a feedback controller. Which system is best can be determined by running an economic analysis (see Chapters 10 and 11).

Both of the control schemes for the neutralizer took measurements on the major varying streams before they were diluted in the large blending tank. This is usually desirable because once the streams are mixed the measurable differences are smaller, and the possibility of noise (the equivalent of static in radio signals) affecting the measurement accuracy is greater.