Typical Control Schemes

A number of alternative structures are used to control distillation columns. In this section we present some of the most commonly employed strategies and discuss when they are appropriate. The standard termi- 

The simultaneous control of two compositions or temperatures is called dual composition control. This is ideally what we would like to do in a column because it provides the required separation with the minimum energy consumption. However, many distillation columns operate with only one composition controlled, not two. We call this single-end composition control. 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

D-V If the column is operating with a high reflux ratio (RR 4), 

RR-V Reflux ratio is used to control distillate composition and heat input controls bottoms composition. 

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Example 1.3. Our third example illustrates a typical control scheme for an entire simple chemical plant. Figure 1.5 gives a simple schematic sketch of the process configuration and its control system. Two liquid feeds are pumped into a reactor in which they react to form products. The reaction is exothermic, and therefore heat must be removed from the reactor. This is accomplished by adding cooling water to a jacket surrounding the reactor. Reactor elHuent is pumped through a preheater into a distillation column that splits it into two product streams. 

A typical control scheme for a distillation column is shown in Fig. 19. Flow controllers (FCs) regulate the flow rates of the feed and overhead products. Each flow rate is measured by a device such as an orifice plate placed upstream... 

We have presented a very brief discussion of typical control schemes for a number of important unit operations furnaces, compressors, decanters, steam/power processes, liquid-liquid extractors, and evaporators. Each of these units requires the control of certain key variables if it is to do its job. A basic regulatory control system must be in place on each unit. 

The feed split is simply the amount of feed that leaves as distillate versus the amount that leaves as bottoms. The other variable, fractionation, is the amount of separation that occurs per stage. The overall column fractionation depends on the number of stages, the energy input, and the difficulty of separation. A typical control scheme for this column is shown in Figure 8.3. 

Figure 8.5 shows a typical control scheme used for an atmospheric or above-atmospheric tower in a total condensing service with little or no inerts. In this situation the pressure is controlled by regulating the flow of the coolant, which in turn changes the condensing surface temperature and the vapour condensing rate. The pressure response... 

Figure 6.6 Typical block diagram of a W/control scheme with open- or closed-loop control scheme...

The inverter may be a current source inverter, rather than a voltage source inverter (.Section 6.9.4) since it will be the rotor current that is required to be vtiried (equation (1.7)) to control the speed of a wound rotor motor, and this can be independently varied through the control of the rotor current. The speed and torque of the motor can be smoothly and steplessly controlled by this method, without any power loss. Figures 6.47 and 6.48 illustrate a typical slip recovery system and its control scheme, respectively. 

Interlocking and control scheme for a typical air-conditioning plant 13/387... 

Figure 16.10 Typical control and annunciation scheme for an AMF panel...

Figure 25.7 Typical power and control scheme for A/l> starting of a motor with capacitor bank, illustrating capacitor shorting during A...

To illustrate how the control function requires extra capacity of process equipment, let us use a typical fractionation system, as shown in Figure 1. This sample illustrates the point being made rather than recommending any particular fractionation control scheme. The best... 

Some examples of typical (conventional) control systems used for the control of specific process variables and unit operations are given in the next section, and can be used as a guide in preparing preliminary instrumentation and control schemes. 

Shinskey (1984) has shown that there are 120 ways of connecting the five main parts of measured and controlled variables, in single loops. A variety of control schemes has been devised for distillation column control. Some typical schemes are shown in Figures 5.22a, b, c, d, e (see pp. 234, 235) ancillary control loops and instruments are not shown. 

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). 

Figure 5.23. A typical stirred tank reactor control scheme, temperature cascade control, and reagent ...

Total unit heat dnty will typically be in the range of 500-1000 BTU per pound of feed to the unit. This set of process heat requirements establishes the amount of heat that must be supplied by combustion of coke. Because of the process control schemes that are normally employed in FCCUs, the unit operation will automatically adjust itself so that the energy produced via coke combustion equals the heat requirements of the process. If the balance is shifted by changes to the feed quality or operating conditions, shifts in catalyst circulation rate and regenerator temperature will occur until a new equilibrinm set of conditions is established. 

Figure 4.1.3 shows a more concrete scheme of a typical controlled double-jet (CDJ) system, in which the flow rate of the AgN03 solution is previously programed, while the flow rate of the KBr solution is precisely controlled to keep the excess concentration of Br ions constant in response to the silver potential monitored with a silver electrode (7). 

Figure 4 A typical reaction scheme for the synthesis of polylactides. By varying the ratio of the initiator (in this case propylene glycol) to lactide, the molecular weight of the final polymer could be controlled as desired.

FIGURE 19 Typical distillation column control scheme. 

Intramolecular reactions of cyclohexenones follow pathways similar to those of cyclopentenones, both with regard to regio- and stereocontrol. The initially mentioned intramolecular [2 + 2]-photocycloaddition of carvone (1) is a typical example for five-membered ring formation with high diastereofacial control (Scheme 6.1). In this case, the rule of five requires the terminal carbon atom of the intermolecular... 

The equipment used in the unit operations is complex and microprocessor controlled to allow the execution of process recipes. However, advanced control schemes are rarely invoked. The microprocessor adjusts set points according to some sequence of steps defined by the equipment manufacturer or the process operator. Flows, pressures, and temperatures are regulated independently by off-the-shelf proportional-integral-derivative controllers, even though the control loops interact strongly. For example, fluorine concentration, substrate temperature, reactor pressure, and plasma power all influence silicon etch rates and uniformity, but they are typically controlled independently. 

Many effective control schemes have been established over the years for individual chemical units (Shinskey, 1988), For example, a tubular reactor usually requires control of inlet temperature. High-temperature endothermic reactions typically have a control system to adjust the fuel flowrate to a furnace supplying energy to the reactor. Crystallizers require manipulation of refrigeration load to control temperatui e. Oxygen concentration in the stack gas from a furnace is controlled to prevent excess fuel usage. Liquid solvent feed flow to an absorber is controlled as some ratio to the gas feed. We deal with the control of various unit operations in Chaps. 4 through 7. 

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