6 Level Control and Feedforward

Options

6.1 INTRODUCTION

a s indicated in Chapter 1, it is most convenient, when starting the design of controls for a new or modernized plant, first to lay out all of the material- balance controls. These are mostly liquid level controls. It was also indicated that feedforward compensation could be used to supplement feedback composition controls to achieve more constant compositions. In the absence of feedback composition controls-usually because adequate composition measurements are lacking-feedforward compensation is almost mandatory.

In this chapter we will consider only feedforward compensation for production rate changes. These are usually larger and more rapid than composition or thermal changes. The most common compensation technique is that of ratio controls, either open loop via a multiplier or closed loop via a divider. It has been shown 1,2 that for single-loop (no cascade) control, either technique is satisfactory, provided the proper installed valve flow characteristic is used. For cascade loops, however, where the flow-ratio control is the secondary or slave loop, there is a serious variation of control loop gain fiom low to high production r a t a 2 Consequently, as discussed in Chapters 12 and 20, it is usually advisable to use an impulse feedforward technique unless the wild flow rate changes less than 2 : 1. For simplicity, however, we will use ratio controls in most of the illustrations in this chapter.

We will begin with combinations of level control and feedforward com- pensation for applications where material-balance control is in the direction opposite to flow. Then we will consider schemes in which material-balance control is in the direction of flow. Unfavorable schemes-those that are hard to design or to make work-will be pointed out; their use should be avoided unless no suitable option is available.

Only conventional columns with top and bottom drawoffs wdl be considered here. Columns with sidestream drawoffs are discussed in Chapter 7. Note that

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154 Level Control and Feedfmard 0pth.u

if reflux drum level control sets reflux, or if base level sets steam, level control must be as "tight" as possible for best composition control.

6.2 MATERIAL-BALANCE CONTROL IN DIRECTION OPPOSITE TO FLOW

This approach implies that either the distillate or the bottom product is the demand flow and that the column must be operated at a rate to satisfjr that demand. We can now visualize several possible designs.

Bottom-Product Demand: Base-Level Control Via Feed

by feed rate. Then there will be two options for reflux drum level control. Bottom-product demand requires, in most cases, that base level be controlkd

Overhead Level Control Via Top Product There has been considerable controversy in the literature about whether to

have reflux drum level control via top product or via reflux flow. Our own studies f d to show any overwhelming advantages one way or

the other. On the average, however, we find top composition control somewhat more straightforward when reflux is manipulated by a ratio controller or by a direct composition controller as shown by Figure 6.1.

Since the bottom-product flow is the demand flow, steam and reflux are ratioed to it. For all ratio loops, appropriate dynamic feedforward compensators should be provided.

Since there is a dead time involved-the time for liquid to flow from the feed tray to the column base-the base level controller settings should be determined by the method of Chapter 16, Section 6. This section also gives a complete design, including overrides for bottom product and steam flows. The reflux drum level controller settings (if a PI controller is used) should be determined by the method of Chapter 16, Section 2.

Overhead composition may be controlled by trimming the reflux/bottom- product ratio. Base composition may be controlled by trimming the steam/bottom- product ratio.

Reflux Drum Level Control Via Reflux In this case, as shown in Figure 6.2, we have steam/bottom-product and

distiUate/bottom-product ratio controls. In calculating reflux drum level controller settings, whether proportional

only or PI, one should take care to account for reflux subcooling (see Chapter 16, Section 4). Overhead composition may be controlled by trimming the disdlate/bottom-product ratio with perhaps a feedforward compensator connected into the overhead level control loop. Base composition may be controlled by trimming the steam/bottom-product ratio control.

6 2 Ma$eriul-Balana Control an Directwn Opposite to F h 155

DEMAND

FIGURE 6.1 Bottom product demand, overhead level control via top product, base level via feed

156 LeueL Contml and Feedfmard 9th

FIGURE 6.2 Bottom product demand. overhead level control via reflux, base level control via feed

6.3 Material-Balance Contra1 in Direction af Flow 157

Reflux Drum Level Control Via Boilup As shown in Figure 6.3, this case requires reflux/bottom-product and dis-

tillate/bottom-product ratio controls. Determining settings for the reflux drum level controller is, in this case,

difficult unless a large reflux drum holdup is available. Preferably one should make TH 2 5 minutes; level controller tuning will require a dynamic analysis of overall column material balance such as discussed in Chapter 14. If steam flow is metered by an orifice, it should be linearized with a square root extractor.

Base level controller settings may also be determined by the method of Chapter 16, Section 6. Overhead composition control may be accomplished by trimming the top-product/bottom-product ratio. Base composition may be controlled by trimming the reflux/bottom-product ratio. Because of interactions between composition controls, this column control scheme is not desirable, although probably not impossible.

Distillate (Top-Product) Demand: Base Level Control Via Feed

Reflux Drum Level Control Via Reflux This case requires, as shown by Figure 6.4, bottom-product/top-product

and steam/top-product ratio controls. The calculation of reflux dnun level controller settings follows the method

of Chapter 16, Section 4; the calculation of base level controller settings follows that of Chapter 16, Section 6.

Since reflux drum holdups are usually small compared with base holdups, a buffer tank in the top product h e (not in the reflux line) is highly recommended. Top product composition may be controlled by trimming the steam/distillate ratio; bottom composition may be controlled by trimming the bottom-prod- uct/distillate ratio.

Reflux Drum Level Control Via Boilup As shown in Figure 6.5, this scheme requires reflux/distillate and bottom-

product/distillate ratio controls. A buffer tank in the top-product line is recommended.

As indicated earlier, reflux drum level control via boilup is difficult unless a lot of holdup is available. Overhead composition may be controlled by trimming the reflu/distillate ratio; base composition may be controlled by trimming the bottom-product/distillate ratio. Note that if either distillate or bottom product (or side product) is a demand flow, either reflux drum or base level control must manipulate feed rate.

6.3 MATERIAL-BALANCE CONTROL IN DIRECTION OF FLOW

Practically speaking, material-balance control in the direction of flow means that the column must take whatever feed is supplied. This is, however, subject

158 Level Contvol and Feea!jimard Optimu

FIGURE 6.3 Bottom product demand, overhead level control via boil up, base level control via feed

6.3 Matffial-Balunce Conml in Direction of F h 159

FIGURE 6.4 Distillate demand, reflux drum level control via reflux, base level control via feed

160 Level Control and FeerSfmrttard Optimr

FIGURE 6.5 Distillate demand, reflux drum level control via reflux, base level control via feed

6.3 Matetid-BalanCe Control in Direction of Flow 161

to the restriction that column overrides, particularly high base level, can restrict feed flow.

Reflux Drum Level Control Via Distillate; Base Level Control Via Bottom Product (Figure 6.6)

This is one of the most commonly encountered column-control schemes. Both level control systems may be calculated by the method of Chapter 16.

Reflux/feed and steam/feed ratio controls should be provided. Top com- position may be controlled by tnmrmng the rdux/feed ratio; bottom composition by trimming the steam/feed ratio.

Reflux Drum Level Control Via Reflux; Base Level Control Via Bottom Product (Figure 6.7)

This is also a commonly encountered control scheme. Both level controls may be calculated by the method of Chapter 16; care should be taken to include the subcooled reflux effect on overhead level control.

Distillate/feed and steam/feed ratio controls should be provided. Top com- position may be controlled by trimming the distiUate/feed ratio and with perhaps additional feedforward to the reflux drum level controller. Base composition may be controlled by trimming the steam/feed ratio.

In many cases this scheme will require a buffer tank in the top-product line to the next piece of process equipment, unless the top product is going directly to storage.

Reflux Drum Level Control Via Distillate; Base Level Control Via Boilup (Figure 6.8)

Overhead level control may be calculated simply by the method of Chapter 16, Section 3, but base level control by boilup is very difficult. It is normally used only when the average bottom-product flow is very smd. The characteristic time constant ‘TH should be at least 15 minutes and other design factors should be as indicated in Chapter 16, Section 7. In most cases base level control by boilup requires a dynamic analysis, and perhaps supplementary plant tests. If steam flow is measured with an orif?ce, a square root extractor should be used.

Reflux/feed and bottom-product/feed ratio controls should be provided. Top composition may be controlled by trimming the refldfeed ratio while base composition may be controlled by adjusting the bottom-product/feed ratio.

Reflux Drum Level Control Via Distillate; Base Level Control Via Boilup; Reflux Ratioed to Top Product

It is sometimes recommended that reflux be ratioed to distillate to diminish interactions between top and base feedback composition control^.^ If, however, reflux drum level is controlled by manipulating top product, and base level is controlled by manipulating boilup (see Figure 6.9), there will be a dynamics

162 Level Control and Feedjimard Options

FIGURE 6.6 Material balance control in direction of flow, reflux drum level control via distillate, base level control via bottom product

6.3 M a t d - B a l a m Contml in Direction of F h 163

FIGURE 6.7 Material balance control in direction offlow, reflux drum level control via reflux, base level control via bottom product

164 Level Control and Feedfmard OptMns

FIGURE 6.8 Material balance control in direction of flow, reflux drum level control via distillate, base level control via boilup

6.3 Material-Balance Control in Direaim of Flow 165

FIGURE 6.9 Like figure 6.8 but with reflux ratioed to distillate

166 Le91 Control and Feea!jimvard Options

problem since reflux is a fknction of boilup. This is in addition to other problems of controlling base level by boilup (see Chapters 4 and 16).

6.4 UNFAVORABLE CONTROL SCHEMES

Perhaps the most common system that has given trouble is base level control via steam. This is particularly true if a thermosyphon reboiler is employed or if the column has valve trays, or both. It is the result of “inverse response” (see Chapter 13). At low boilup rates, sieve trays give the same trouble. To minimize difficulties the design recommendations of Chapter 16, Section 7, should be followed. One of the authors has shown, in an unpublished study, that an “inverse response compensator” can be designed and implemented on a computer or with some microprocessor controls.

In a recent project we had considerable difficulty with base level control via feed accompanied by steam/feed ratio control. In effect the level controller adjusts both feed and steam. Since bottom product was the demand flow, this was corrected by converting the steaxdfeed ratio control to steamhottom- product ratio control.

High base-level overrides on steam have also given trouble for the reasons mentioned. To a considerable extent, however, these problems can be mitigated by using proportional-reset instead of proportional-only overrides (Chapter 9). A high base-level override on steam should not be used on the same column with either reflux drum level control on reflux or a high reflux drum level override on reflux.

6.5 UNREASONABLE CONTROL SCHEMES

The control schemes in this section have one common fault-the inability to control the overall column material balance. Changes in the demand flow rate are not accompanied by appropriate changes in the other flows.

One of the worst combinations of controls is reflux drum level via reflux and base level via boilup. This configuration produces a positive feedback loop that either will shut the column down or open the steam and reflux valves wide, neither of which is a desirable situation. Operator intervention is eventually required. We have seen it used on columns with very large base holdups-8 to 24 hours.

If the bottom product is the demand stream, then base level control via boilup and reflux drum level control via distillate will not control the column properly. An increase in bottom product will cause a decrease in distillate, with no change in feed rate.

If the distillate is the demand stream, then base level control via bottom product and overhead level control via boilup will not control the column properly. This scheme is analogous to the preceding scheme.

Referemes 167

If distillate, bottom product, or side product is a demand flow, one of the level controllers mwt manipulate feed rate.

REFERENCES 3. Rademaker, O., J. E. Rijnsdorp, and

A. Maarlereld, L?ymma and Gmml $Continuow Distillution Unztr, El- sevier, New York, 1975.

1. Budey, P. S., “Selection of Optimum Final Element Characteristics,” Pro- ceedinfls, Fifth National CHEMPID Symposium, ISA, 1964.

2. Buckley, P. S., “Design of Pneumatic Flow Controls,” Proceedin.s, Texas ACM Symposium, Jan. 1976.