Chemical Engineering October 2020 - 43

Evaporated vapor
to condenser or
second effect
Defl ector
plate
Steam
Steam jet
ejector
Vaporliquid
separator
Feed
Condensate
Concentrate
Circulation
pump
Concentrate
pump
FIGURE
4. Thermal vapor recompression usually increases steam economy by
the equivalent of one evaporation effect
precludes high-fouling applications,
because conditions would change
with the buildup of deposits on the
heat-transfer surface. Installations
that require more flexibility may use
multiple jets in parallel, with valves to
match the number of online jets to
the targeted flowrate.
Control of evaporators
As with most types of process
equipment, there are numerous
ways to control an evaporator, ranging
from simple to sophisticated. An
example of a control strategy that is
intermediate in complexity, applied
to a falling-film evaporator with recirculation,
is shown in Figure 5. The
desired steam flowrate is set by the
operator, and maintained by virtue
of a flow sensor and control valve.
Alternatively, steam pressure can be
controlled instead of flowrate, using
either a pressure sensor and a conPI
Steam
FIC
FS
PI
Cooling
water
return
Heat
exchanger
Air
or
TI
PS PIC
Condenser
TI
inert gas
To vacuum
source
Cooling water
supply
Condensate
T
Steam
condensate
CIC CS
Concentrate
FS
FIC
Circulation
pump
Concentrate
pump
Vaporliquid
separator
LS
LIC
FIC
FS
Feed pump
Condensate
pump
Feed
KEY
CIC: concentration indicatorcontroller
CS:
concentration sensor
FIC: fl ow indicator-controller
FS: fl ow sensor
LIC: level indicator-controller
LS: level sensor
PI: pressure indicator-controller
PS: pressure sensor
T: steam trap
TI: temperature indicator
FIGURE 5. An evaporator can be controlled in a variety of ways. Here, the steam flowrate is set manually,
the feedrate is regulated to maintain the desired level in the vapor-liquid separator, and concentrate
flowrate is varied to maintain the desired concentrate solids concentration
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
OCTOBER 2020
trol valve, or a
simple pressure
regulator with a
spring-loaded
diaphragm.
The latter is the
least expensive,
and provides
adequate control
in many applications.
One
disadvantage of
pressure control,
regardless
of the method,
is the need to
increase the
setpoint over time as the heat exchanger
fouls. This is not necessary
when the steam flow, rather than
the pressure, is controlled, because
the pressure is self-adjusting. That
is, the steam pressure increases
until the ∆T is sufficient to obtain a
heat-transfer rate high enough to
condense the steam entering at the
controlled flowrate.
Feed flowrate is regulated to keep
the vapor-liquid separator level
at
setpoint, using a cascade loop. Concentrate
solids content is maintained
at setpoint by controlling the rate
at which concentrate is removed,
employing a second cascade loop.
Solids content can be inferred from
sensors that measure, for example,
refractive index or density. The former
is obtained from an inline refractometer,
a relatively inexpensive
device. Density can be measured
using a gamma-radiation density
Figure 5, evaporation pressure is regulated
by a controlled bleed of air (or
inert gas). Also, there is no control on
the condenser water, only temperature
indication. While not shown in Figure 5,
some designs include a sensor in the
vapor-liquid separator to detect the
presence of foam at a pre-determined
level, followed by automatic addition of
a chemical antifoamer.
While a single-effect evaporator is
shown in Figure 5, the same control
strategy is applicable to multipleeffect
units. Pressures, temperatures
and solids contents of the intermediate
streams are not controlled, because
the operation is self-regulating,
but a level-control loop at each vaporliquid
separator may be desirable.
Alternatively, separator levels can be
controlled by simple overflow through
a side port, with the connecting pipe
sized to provide sufficient pressure
drop so that the downstream pump
(if used) suction is not starved. ■
Edited by Mary Page Bailey
References
1. Gabelman, A., Evaporators: Design Concepts and Equipment
Selection, Chem. Eng., Jan. 2020, pp. 27-38.
2. Perry, R.H., Green, D.W., Maloney, J.O., eds., " Perry's
Chemical Engineers' Handbook, " 7th ed., McGraw-Hill,
New York, 1997.
3. McCabe, W.L., Smith, J.C., Harriott, P., " Unit Operations
of Chemical Engineering, " 7th ed., McGraw-Hill, New
York, 2005.
4. APV Evaporator Handbook, SPX Flow, Inc., 2009.
Author
Alan Gabelman is president of
Gabelman Process Solutions, LLC
(6548 Meadowbrook Court, West
Chester,
offering
consulting services in process engineering.
years
of experience include numerous
separation processes and other engineering unit operations,
equipment selection, sizing and design, process
simulation, P&ID development, and process economics.
He holds B.S., M.Ch.E. and Ph.D. degrees in chemical
engineering from Cornell University, the University of
Delaware and the University of Cincinnati, respectively. He
is a licensed professional engineer and has served as an
adjunct instructor in chemical engineering at the University
of Cincinnati. Gabelman has edited a book on bioprocess
flavor production, and he has authored several technical
articles and a book chapter.
43
OH 45069; Phone:
513-919-6797; Email: alan.gabelman@gabelmanps.com;
Website:
www.gabelmanps.com),
Gabelman's over 40
meter, which relates the attenuation
of gamma radiation by the process
fluid to its density, and in turn, solids
content. While considerably more
expensive than other methods, this
technique offers high accuracy and
no contact with the process fluid.
For the vacuum evaporator shown in

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