8.minimizing energy requirements

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8Minimizing Energy

Requirements

8.1 INTRODUCTION

t ere are two basic approaches to minimizing &stillation column energy

requirements:

1. Conseraatian-designing and operating a column so that it makes the

specified separation with the least amount of energy per pound of feed.

2. Energy recovq-recovering and reusing the heat in the column productstreams, whether they be liquid or vapor.

The main emphasis of this chapter is on the latter approach, but it is pointless

to uy to recover energy unless we also try to conserve it. Consequently we

will discuss conservation first.

8.2 CONSERVATION

For distillation, conservation means designing and operating a column so

t h a t it makes the specified separation with the least amount of energy per pound

of feed. We have a number of techniques to accomplish this:

1. Automatic control of composition of product streams. Operators commonly

overreflux conventional columns with a single top product and a single bottom

product. Extra heat is used to ensure the meeting or exceeding of specified

product purities.

Geyer and Kline' give, as an example, a 70-tray column separating a mixture

with a relative volatility of 1.4 and with specifications of 98 percent low boilers

overhead and 99.6percent high boilers in the base. If the operator adds enough

boilup and reflux to increase overhead purity to 99 percent and base purity to

99.7 percent, an increase of 8 percent in energy consumption results.

2. Feed provided at the proper feed tray. It can be shown that this results

in a lower energy requirement per pound of feed than would feedmg on any

181



182 Minimtzzw E M - Requirements

other tray. As feed composition or enthalpy deviates &om design values, the

op t i m u feed-tray location also changes.

3. Column operation at min imu pressure.2 Lower pressure usually means

higher relative voIa&ty. Therefore, the necessary separations can be accomplished

with lower boilup/feed and reflux/feed ratios. Condenser capacity may belimited, however, and the column may flood at lower boilup rates than it would

when operating at higher pressures.

4.Use of lowest pressure steam available.' In many plants excess low-

pressure steam is available that otherwise would be vented to the atmosphere.

This steam is usually cheaper than high-pressure steam. Where reboiler AT

might be too small if the steam were throttled, one may use a partially flooded

reboiler (see Chapters 4 nd 15) and throttle condensate. Since low-pressure

steam is seldom available at constant pressure or steam quality, pressure and

temperature compensation of flow measurements is highly desirable if steam isthrottled instead of condensate.

5. Use of steam condensate receivers. In many plants steam traps require

considerable maintenance and have sigmficant leakage.The useof steam condensate

receivers instead of traps reduces maintenance and steam losses.

6. Possible use of mechanical vacuum pumps. For vacuum columns there

is some opinion' that mechanical vacuum pumps offer energy savings over

steam jets. The difference, however, is usually small.

7. Dry distillation. For columns now using live steam, it is sometimeseconomical to switch to steam-heated reboilers.

8. Insulation. Older columns, designed before the energy crunch, can often

benefit from new, increased insulation.

8.3 DESIGN CONSIDERATIONS IN HEAT-RECOVERY SCHEMES

Energy recovery in a distillation column means, practicallyspealung, recoveringor reusing heat contained in the column product streams, whether they are

liquid or vapor. A number of schemes have appeared in the literature. The twochief ones involve (1)"multiple effect" distillation, analogous to multiple effect

evaporation, and (2) vapor recompression. But, regardless of the scheme, there

are five design factors that must be considered:

1. Reserve capacities that may be required:

-Extra heating capacity

-Extra cooling capacity-Extra distillation capacity

These are important for startups and shutdowns, changes in production

rate, changes in feed composition, and changes in product specifications. Generally

speaking, however, auxiliary" reboilers and condensers should be avoided, if

* "Auxiliary"condensers and reboilers are those installed in parallelwith "normal" condensers

and reboilers for startup or peak load purposes.



8.4 Multiple Loads Supplied /y a Sin5h Source 183

at all possible. Their use increases investment, as well as instrumentation and

control complexity.

Some users have had problemswith turning auxiliary condensers and reboilers

on and off,and they prefer not to do it. Instead they always maintain at least

a small load on these heat exchangers. This obviously wastes energy.2. Priorities. If recovered energy is to be distributed to several loads, what

is the order of priorities?

3. Interactions. Elaborate heat-recovery schemes are often highly interactive;

how is this to be dealt with?

4.Overall heat balance. How is this maintained?

5 . Inerts (low boiler) balance. With elaborate heat-recovery schemes, this

is sometimes a problem. Too high a concentration of inerts or low boilers will

blanket process-to-process heat exchangers; too low a concentration will resultin product losses through the vents.

In view of the above, it is apparent that control of columns with heat-recovery

schemes is more difficult than control of conventional columns.

Let us now look at three types of mdtiple-effect distillation.

8.4 MULTIPLE LOADS SUPPLIEDBY

A SINGLE SOURCE

Sometimes, as shown in Figure 8.1, a column that is a very large energy

user becomes the energy source for a number of loads, each of which acts as

a condenser. Two methods have been used to allocate the energy to be recovered

(1) throttling the vapor-heating medium to each condenser, as shown in Figure

8.1, and (2) operating each condenser partly flooded by throttling the condensate.

Some priority scheme must be established for startups and for any other occasion

when vapor supply is temporarily short.

One method of handling the priority problems is to use overrides and tosplit-range the various valves involved, as shown in Figure 8.2. The scheme

shown illustrates the use of pneumatic devices, but the concepts readily may

be implemented with some digital or analog electronic controls. For the six

loads in Figure 8.2, we employ six gain 6 relays. For load 1, which has the

highest priority, the gain 6 relay is calibrated to have an output span of 3-15

psig for an input span of 3-5 psig. Load 2 has the next highest priority, so

its gain 6 relay is calibrated for 5-7 psig input, 3-15 psig output. This continues

until the load with lowest priority, load 6, has a gain 6 relay calibrated for

13-15 psig input, 3-15 psig output. At the design stage of such a system,considerable care must be exerted to obtain suitable value of controller gain

and also proper valve sizing.

Process-to-process heat exchangers are commonly designed for very small

temperature differences, say 8-10°C. If vapor throttling is used, it should be

recognized that the vapor-supply valves will tend to have small pressure drops.

Accordingly, it is advisable to have vapor flow control to each load, with a set



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186 Minimi z i v Energy Requiremoats

point fiom a primary controller. In the absence of vapor flow control, interactions

may be severe, and very close control of supply and load pressures may be

required.

8.5 SINGLE SOURCE, SINGLE LOAD

When there is only one source and one load (see Figure 8.3), control may

be both simpler and more flexible. The column tha t is the source does not

need to be operated at a constant pressure-in the scheme shown, it finds its

own pressure. For the illustrative example, the overhead composition of the

supply column is controlled via reflux; the base composition of the load columnis controlled by boilup in the supply column.

The scheme of Figure 8.3 has an interesting dynamics problem. The controls

must be so designed that changes in vapor flow from the supply column must

reach the condenser-reboiler a t about the same time as feed flow changes from

the supply column. If there is a serious discrepancy, particularly if the second-

column bottom-product flow is smd, base level in the second column may

experience serious upsets.

Another problem associated with this scheme is the selection and sizing of

the feed valve to the first column. This column will run at a low pressure a t

low feed rates and at a higher pressure a t high feed rates. Assuming that the

feed comes from a centrifugal pump, one can see that valve pressure drop will

be very high a t low flow, and low at high flow. The variation in valve pressure

drop with flow will be much greater than that normally encountered in a

pumped system.

In another version a following column in the train supplies heat to a

preceding column as shown in Figure 8.4. In this particular case, the first

column gets only part of its heat from the second column; the remainder comesfrom an auxiliary reboiler. Interactions between the two columns may be severe.

Again, for the cases studied, we have found it advantageous to let pressure

find its own level in the second column, that is, the one supplying heat.

An interesting practical problem here is how to adjust the auxiliary reboiler

on the first column. After examining some complex heat-balance schemes, we

decided that the simplest approach was to use column AI'. Vapor flow to the

first column from the condenser-reboiler will not be constant, but the AI'

control will provide a rapid method of ensuring aonstant boilup. The A P

control, in turn, may have its set point adjusted by a composition controller

for the lower section of the first column.

It should be noted that for the schemes of both Figure 8.3 and Figure 8.4,

maximum column pressure occurs a t maximum feed rate and boilup rate. For

columns we have studied to date, there has been no problem with flooding at

lower rates and pressures.



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8.8 Energy R e m 7 iy VaporRecompresswn 189

8.6 SPLIT FEED COLUMNS

A third arrangement, which is used in some sy~tems,~.~nvolves splitting

feed between two columns that make the same separation (Figure 8.5). The

supply column, however, runs a t a higher pressure than the load column. Thefeed split is controlled to maintain a heat balance.

8.7 COMBINED SENSIBLE AND LATENT HEAT RECOVERY

In addition to the recovery of the latent heat of vapor streams, in manycases it is practical to recover part of the sensible heat in the column bottom

product and steam condensate by exchange with column feed. Such schemes

have been used in the chemical and petroleum industries for years. Since feed

flow is typically set by level controllers or flow-ratio controllers, its flow rate

will not be constant. The feed enthalpy or temperature, therefore, is apt to be

variable. This may make column-composition control difficult unless one employs

either feedforward compensation or a trim heater with control for constant

temperature or enthalpy. (See Chapters 5 and 11.)

8.8 ENERGY RECOVERY BY VAPOR RECOMPRESSION

In the past vapor recompression (“heat pumps”) has often been considered

for distillation of materials boiling a t low temperatures. The incentive in many

instances was to be able to use water-cooled condensers, thus avoiding theexpense of refrigeration. Another factor favoring vapor recompression is a small

temperature difference between the top and bottom of the column.

Today the main interest is in getting the column vapor compressed to the

point where its temperature is high enough to permit using the vapor as a heat

source for the r e b ~ i l e r . ~ . ~n auxiliary, steam-heated reboiler and/or auxiliary

water-cooled condenser may be necessary for startup (see Figure 8.6).A review

of compression equipment and methods of estimating operating costs has been

presented by Beesley and Rhine~mith.~osler7discusses the control of a number

of vapor recompression schemes. N d 9presents investment equations and data.

Fahmi and Mostafa” indicate that the optimum location at which to use the

compressed vapor may not be in a reboiler at the column base, but rather at

an intermediate site.

Other papers on energy integration for distillation columns include those

by OBrien” and by Rathore et al.12



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FIGURE 8.6Heat recovery via vapor recompression



192 Minimking Energy Requirements

1. Geyer, G. R., and P. E. Kline, CEP,

2. Shinskey, F. G., Dtitillation ControL,

3. Tyreus, B. D., and W. L. Luyben,

4. Rush, F. E., CEP, 4-49 (July 1980).

5. Shaner, R. L., CEP, 47-52 (May

1978).6. Bmley, A. H., and R. D. Rhinamith,

CEP, 37-41, (Aug. 1980).7. Mosler, H. A., “Coneol of Sidestream

and Energy Conservation Distdla-tion Towers,” Proceedangs, AIChE

Workshop on Industrial Process

control, Tampa, Fl., 1974.

8. Chiang, T., and W. L. Luyben, “Heat

49-51 (May 1976).

McGraw-Hill, New York, 1977.

CEP, 59-66 (Sept. 1976).

REFERENCES

Integrated Distillation Configura-tions,” paper submitted to IOECProc. Des.Dev. (1983).

9. Null, H. R., “Heat Pumps in Distil-lation,” CEP, 58-64 (July 1976).

10. Fahmi, M. F., and H. A. Mostafa,

“Distillation with Optimum VaporRecompression,” Cbem. Eng., Res,

Des., 391-392 (Nov. 1983).

11. O’Brien, N. G., “Reducing Column

Steam Consumption,” CEP, 65-67(July 1976).

12. Rathore, R. N. S., K. A. Vanwormer,and G. J. Powers, “Synthesis ofDistillation Systems with EnergyIntegration,”AICbEJ., 20(5): 490-

950 (Sept. 1974).

8.minimizing energy requirements

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