Control material balance

Source Buckley, P.S. Input of Process Control Advances on Process Design, paper presented at A.I.Ch.E. Process Control Workshop in Memphis, Tenn., Feb. 5, 1964. 

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 

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Bojnowski, J. J., Groghan, R. M., Jr., and Hoffman, R. M., Direct and Indirect Material Balance Control, Chemical Engineering Progress, September 1976. 

Reactor temperature will normally be controlled by regulating the flow of the heating or cooling medium. Pressure is usually held constant. Material balance control will be necessary to maintain the correct flow of reactants to the reactor and the flow of products and unreacted materials from the reactor. A typical reactor control scheme is shown in Figure 5.23 (see p. 235). 

Besides the quality of various streams, their quantity must also be controlled. If the product bins are nearly full the production rate must be slowed down. Later, after a number of shipments to customers have been made, the rate may be increased. This is called material balance control. 

Often the throughputs of the various process steps in a plant are different, even though on paper they were designed to be the same. This could result in an inadequate amount of feed to one unit while for another unit the feed rate is too great to be handled properly. Again, some type of material balance control is necessary. 

Figure 7-2 Material balance control in the direction of flow (Averaging Control).

Figure 7-4 Overall material balance control in direction opposite to flow.

The arguments presented above indicate that the large recycle and coolant flow rates wr and uq are the only manipulated inputs available in the fast time scale, and should be used to control the process temperatures. Likewise, the dynamics of the material-balance variables in the slow time scale are affected only by the small feed and effluent flow rates uq and up, which are thus the manipulated inputs that must be used to tackle control objectives involving the material balance. 6sp, the setpoints of the temperature controllers in the fast time scale, are also available as manipulated inputs in the slow time scale, a choice that leads to cascaded control configurations between the energy- and material-balance controllers. 

Control objectives for a chemical process originate from certain regulation tasks (i.e. product quality control, material balance control, safety, environmental regulations, etc.) and economic objectives (i.e. optimizing the economic performance). Such a classification of control objectives automatically formulates the different design activities for the regulatory and optimizing control structures. 

It should be noted that establishing the product-quality loops first, before the material balance control structure, is a fundamental difference between our plantwide control design procedure and Buckley s procedure. Since product quality considerations have become more important in recent years, this shift in emphasis follows naturally. 

The material balance control structure works opposite to the direction... 

Figure 25.7 Material balance controls for the hydrodealkylation plant.

Remark. A good material balance control system is an imperative requirement for satisfactory operation of a chemical plant. Therefore, if a process design cannot accommodate efficient material balance control, we must change the design of the plant. This is not the case for the assumed design of the hydrodealkylation plant, because the control system of Table 25.3 is acceptable and operationable. 

Long time response (2) long time response and increasing complexity (3) V/F control is not desirable in general (4) column bottom level control by steam flow is desirable. (Comments are due toT. Umeda). b Direct M.B. = direct material balance control Indirect M.B. indirect material balance control V/F = vapor to feed ratio control... 

Changes in the hydrogen composition of the fresh hydrogen stream This disturbance will affect the hydrogen composition of the reactor inlet stream. To eliminate its effect we can use the feedback composition controller, CC1, which was installed for material balance control. It measures the hydrogen recycle composition and adjusts the fresh hydrogen flow rate. 

The material balance control system can be designed using steady-state balances only, while... 

Although a plant is usually designed for a nominal production rate, a design tolerance is always incorporated because the market conditions may require an increase or decrease from the current rate. The control system is then called to ensure a smooth and safe transition from the old to the new production level. This is known as material balance control, because its purpose is to direct the control action in such a way as to make the inflows equal to the outflows and achieve a new steady-state material balance for the plant. Let us now see how to design the material balance control system for the hydrodealkylation plant. 

Changes in the production rate come from the plant s management and are rather infrequent (e.g., once every two weeks, or a month, or longer period). The time required by the plant to reach the new operating level is much shorter than the periods noted above. Consequently, the transient dynamic behavior is very short-lived and not very important. Therefore, we can assume that the plant always operates at steady state. Figure 25.6 demonstrates this pseudo-steady-state behavior. It is clear, then, that steady-state balances are sufficient for material balance control. 

Table 25.2 shows the steady-state material balances for the hydrodealkylation plant. They will guide the design of material balance control system. 

Adjust the benzene product flow rate. This is the overhead product of the benzene distillation column and depends on the value of the feed flow rate. In the next section we will see how to adjust the overhead flow rate in order to maintain the desired benzene purity and fractional recovery by a material balance controller which manipulates the distillate to feed ratio (D/F control). 

We have decomposed the control system into two parts the material balance control and the product quality control. Each has been designed separately, as if they do not interact with each other. 

The material balance control system is called on to compensate for very low frequency disturbances (e.g., plant management decisions for changes in the production rate, once every two weeks, month, or longer). On the other hand, the product quality control system is called on to ensure the operation of the plant against higher-frequency disturbances (once every half a minute, minute, hour, or day). This difference is the basis for the decomposition of the material balance and product quality control systems. It works as follows Figure 25.9 shows the amplitude ratio versus frequency for the material balance and product quality control systems. 

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