Chemical Engineering October 2020 - 41

Engineering Practice
Evaporators: Energy Conservation
Strategies and Process Control
Several strategies for improving energy consumption and process control in
evaporation processes are presented here
Alan Gabelman
Gabelman Process Solutions, LLC
I
n industrial evaporation, a heat
source (usually condensed
steam) is used to concentrate a
material through the removal of
a liquid solvent. There are several
types of evaporators and a number
of important design principles.
This article describes techniques to
reduce energy usage in industrial
evaporation, and also control methods
for evaporators. For more information
on evaporation fundamentals
and equipment selection, please
see Ref. [1].
Multiple-effect evaporators
To a first approximation, the heat
obtained upon condensation of 1
pound (lb) of steam is enough to
evaporate 1 lb of water, leading to
an expected steam economy of
about 1 lb of water evaporated per
1 lb of steam for an ordinary evaporator.
However, the evaporated
vapor also contains latent heat of
vaporization. Rather than releasing
this heat to cooling water flowing
through the condenser, why not use
it to evaporate more water? Such an
arrangement is shown in Figure 1
Defl ector plate
Steam
Vapor-liquid
separator
(p1, T1)
for a double-effect evaporator. Note
that in Figure 1, the temperature in
the first effect (T1) is greater than that
in the second effect (T2), as required
to provide the driving force for heat
transfer in the second effect, and the
effect pressures also correspond so
that p1 > p2.
The feed is partially concentrated
in the first effect, then the first-effect
product is directed to the second effect
for final concentration. The heat
source for the first effect is steam
from the boiler (or a waste-heat
source), while the second-effect
heat source is the vapor produced in
the first effect. In this double-effect
evaporator, 1 lb of steam results in
approximately 2 lb of evaporated
water, one from each effect.
One can extend the concept and
build an evaporator with N effects,
delivering N lb of evaporated water
per 1 lb of steam. However, there is
an upper practical limit, dictated by
economics. The total available temperature
driving force (∆T), the difference
between the temperature of the
steam to the first effect and the evaporation
temperature of the final effect,
is distributed over all effects. With a
greater number of effects, each effect
receives a smaller portion of the overFirst
effect vapor
Vapor-liquid
separator
(p2, T2)
First effect
concentrate
Second effect
vapor to
condenser
all ∆T, and for a given heat-transfer
area per effect, the evaporation rate
at each effect decreases. Consequently,
more effects are needed to
achieve a targeted evaporation rate,
driving up the required investment.
The optimum number of effects is
reached when the energy savings no
longer supports the increased cost
of capital. In most cases, more than
five or six effects is difficult to justify.
Note that multiple effects do not provide
increased capacity over a single-effect
evaporator, only reduced
energy usage.
By convention, effects are numFeed
First
effect
Steam
condensate
Final concentrate
Condensate
Second effect
FIGURE 1. This double-effect rising-film evaporator uses the latent heat contained in vapor from the first
effect to evaporate additional water in the second effect. More energy is saved by flashing the first-effect
steam condensate into the second-effect heat exchanger
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OCTOBER 2020
bered according to the direction of
the flow of vapor. The first effect is
heated by boiler steam, and effects
2 through N are heated by vapor
from effects 1 through N - 1, respectively.
However, the process liquid
flow is not necessarily sequential in
this manner, as shown in Figure 2 for
a four-effect evaporator. In forward
flow (Figure 2a), feed enters the first
effect, and like the generated vapor
streams, the intermediate product
from each effect flows to the next
one in line. The main advantage is
the successive decrease in pressure
(corresponding to the decrease in
temperature), which either reduces
the size of the required interstage
pumps, or eliminates the need for
them entirely.
In reverse flow (Figure 2b), feed enters
the final effect, and each intermediate
product flows to the preceding
effect, with concentrate removed at
the first effect. The advantage is that
product streams of increasing concentration
are handled at higher temperatures.
This counteracts the high
viscosity often encountered at higher
concentrations, resulting in improved
heat transfer and ease of handling.
The disadvantage is the need for interstage
pumps to overcome the in41

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Chemical Engineering October 2020

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