Chemical Engineering October 2020 - 42

Concentrate
Steam
12 3
Concentrate
Feed
Steam
12 3
(b)
4
Feed
FIGURE 2. By convention, effects of a multiple-effect evaporator are numbered to follow the steam and
vapor flow. Options for process material are as follows: (a) forward flow (in the same direction as vapor
flow); (b) reverse flow (in the opposite direction of vapor flow); and (c) mixed flow (a combination of forward
and reverse flow)
crease in pressure between a given
effect and the previous one.
Mixed flow (Figure 2c) may be a
reasonable compromise. As in forward
flow, feed enters the first effect,
and intermediate product streams
flow sequentially until effect k is
reached. Product emanating from
effect k is directed to the last effect
N, then product from there flows
in reverse, leaving effect (k + 1) as
final concentrate. Mixed flow allows
some interstage pumps to be reduced
in size or eliminated, while still
allowing the viscous concentrate to
be handled at a higher temperature
than with forward flow.
Vapor recompression
Like the use of multiple effects,
vapor recompression is a method
of capturing the energy contained
in the process vapor. This technique
comprises compressing the vapor,
either mechanically or thermally, then
reusing it as heating medium.
Unlike some other evaporator
operations, mechanical vapor recompression
(MVR) does not employ
boiler steam as the heat source
(except for a small amount used at
startup and as makeup). Instead,
the required energy is provided by a
compressor, which raises the pressure
and temperature of the process
vapor high enough for it to serve
as the heat source for additional
evaporation. A schematic diagram
is shown in Figure 3. Note that a
condenser is not needed, because
all of the vapor is recompressed and
reused. Once reused, the vapor is
removed as condensate, to be replaced
by freshly produced and re42
compressed
vapor.
Compressors
can be driven by an electric motor,
steam turbine, gas turbine or internal
combustion engine [2, 3].
Typically, the energy economy of
an evaporator with MVR is equivalent
to 10 to 15 conventional evaporation
effects, although equivalence to
30 effects or more is possible. The
economy is a function of the required
compression ratio, which in turn depends
on the evaporation pressure,
the boiling point elevation (BPE; further
discussed in Ref. [1]), and the
required ∆T. Savings are higher when
the cost of electrical or mechanical
power is low, when low-pressure
steam is not available or when the
cost of providing cooling water is
high. For a reasonable compressor
size and power requirement, usually
the ∆T is kept in the range of 10 to
18°F. This limitation may lead to the
need for a high heat-transfer area,
which in turn may increase the cost of
an MVR evaporator when compared
to a multiple-effect unit
with similar capacity.
However, this is at least
partially mitigated by
the omission of steam
boiler and cooling tower
(or other cooling water
source) capacity, which
are not needed with
MVR [2, 4].
Make-up
steam
MVR is particularly
well suited for film
evaporators, because
the high heat-transfer
coefficients reduce the
required ∆T, and in turn,
the compression ratio.
The
technique
is
Feed
(a)
4
Vapor to
condenser
Steam
12 3
Feed
Vapor to
condenser
(c)
4
Vapor to
condenser
Concentrate
used for high-fouling applications or
if the BPE is high, because the higher
temperature needed to obtain the
necessary ∆T leads to compression
ratios that are not economical. These
cases may be handled by using MVR
in conjunction with a conventional
evaporator. For example, the initial
concentration may be done with a film
evaporator with MVR, then a forcedcirculation
unit may be used for the
final concentration.
Rather than a mechanical compressor,
thermal vapor recompression
(TVR) uses a steam jet ejector
to recompress the process vapor
(Figure 4). Unlike MVR, only part of
the vapor is recompressed, while the
rest is directed to the condenser in
single-effect units, or the second effect
in multiple-effect evaporators.
Because TVR is better suited to vacuum
operation than MVR, the former
is often preferred for heat-sensitive
products that require low temperatures.
Similarly, TVR can add some
steam economy in applications that
cannot use multiple effects because
the temperature in the earlier effects
would be too high. The ratio of motive
steam to entrained vapor depends
on the steam supply and the
evaporation pressures. As a rule of
thumb, a TVR adds the equivalent of
one evaporator effect.
TVR offers simplicity of design
construction,
and
low cost,
low
maintenance, ability to handle large
volumes of vapor and the ability to
handle corrosive gases. The main
disadvantage is lack of operating
flexibility, with a rapid decrease in
efficiency upon deviation from design
conditions. As with MVR, this
Defl ector
plate
Compressor
Vaporliquid
separator
Condensate
Concentrate
Circulation
pump
not
Concentrate
pump
FIGURE
3. This rising-film evaporator uses mechanical vapor recompression,
which typically provides steam economy equivalent to
between 10 and 15 evaporation effects
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