8 Minimizing Energy Requirements 8. 1 INTRODUCTION t ere are two basic approaches to minimizing &stillation column energy requirements: 1. Conseraat ian-desi gning and operating a column so that i t 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 product streams, 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 that it makes the spec ifi ed separation with the leas t 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 to p 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 a n exampl e, a 70-tray column separating a mixture with a relative volatility of 1.4 and with specif ications of 98 percent low boilers overhead an d 99.6 percent high boi ler s 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. I t can be shown that this results in a lower energy requirement per pound of feed than would feedmg on a n y 181
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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
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
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