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L I Q U I D RING PUMP APPLICATION I N RECYCLE DRYING
Author : Andrew Kalman Copyright: Date: October 25, 1989
The Upjohn Company, Kal amazoo, Michigan
ABSTRACT
The Upjohn Company has been developing an effective recycle drying system
for batch processes. With the utilization of liquid ring pumps, the system is
more effective, requires less maintenance, and it is easier to comply with GMP.
Attention was given to heat transfer and mass transfer problems. The new design
philosophy is discussed and a detailed design and operating system i s presented.
Liquid Ring PUMP Application in Recycle Drying 2
~
It was reported earlier' that we at the Upjohn Company developed a blower
recycle system for use in drying application. The system was modified several
times to improve its performance. It has been effectively used in drying
operations in conjunction with pressure filters (nutsches) with not quite
pleasing results. We managed to solve the environmental problems associated
with the nutsche drying operation; namely, to eliminate environmental pollution
and we were also able to eliminate operator exposure to solvents. The blower
recycle system works reasonably well in dedicated applications.
The basic flow diagram of the above system is shown in Figure 1. The
drying loop operates as follows: Due to the
adiabatic compression in the blower, the gas heats up. This heated gas stream
goes through a heat exchanger where the gas either gets heated or cooled down,
depending on the desired temperature. In the nutsche the gas stream is
"saturated" by the solvent. The outgoing gas stream from the nutsche passes
through a total condenser before it enters the demister catch tank arrangement.
The demister separates the liquid particles before the gas stream returns to the
suction side of the blower and the cycle is repeated.
The blower compresses the gas.
The pharmaceutical industry places a high demand on its equipment. The
equipment needs to be multipurpose because of product changes. This need is
more pronounced in the batch processing segment of this industry. The high
price of the product forces the industry to maintain low inventory levels which
result in frequent change of products in the same equipment. The characteristics
of the different products can widely deviate from each other.
The temperature-sensitive, sub1 ime or corrosive products can cause serious
probl ems in the recycl e drying appl i cat ion.
Blower Nitrogen Recycle System
;
Heater - Cooler
I t Figure 1
Liquid Ring Pump App7ication in Recycle Drying 4
If the product is temperature sensitive, it can decompose in the blower
(when its temperature is elevated) and can contaminate the system. The
sublimation characteristics of a product can also contaminate the recycle system.
The damage caused by the corrosive properties of different products is self
explanatory.
The blower recycle system has some other shortcomings. It is very noisy,
difficult to clean and oil contamination of the product is possible.
In 1985 we started exploring the possibility of using a liquid ring pump
as a compressor part of the recycle system. Sulzer Burckhardt, which had some
experience in gas compression in this area, joined us in developing a recycle
system using the liquid ring pump. Since Sulzer Burckhardt calls its vacuum
system "Apovac", we retained this name for our recycle system too.
Figure 2 shows the basic arrangement of the Apovac recycle system. The
system works in the following manner: The liquid ring pump acts as the
-3
compressor. The compressed gas passes through the primary heat exchanger which
is located on the top of the liquid ring holding tank. Then the gas, which is
saturated (at the temperature of the primary heat exchanger) with the solvent
used, passes through the secondary heat exchanger. This heat exchanger reduces
the dew point of the gas. In order to ensure that the gas is effectively
separated from liquid entrapment, the gas is led through a demister. Before it
returns to the top of the nutsche it is reheated to the desired temperature.
If we compare the efficiency (liquid removal capability) of the blower
recycle system and the Apovac drying system using a Psychometric chart, one can
easily visualize that there is practically no difference (Figure 3). On
Figure 3, AB represents the effect of the heater, BC and CA represent the effect 3
Brlne
APOVAC Nitrogen Recycle System
Figure 2
Dryi
L .I a 0.03
6 ,0.02
ci, Y 5 0.007 m 9 0.004
Y
).
\
+Ir
cir
ng Cycle on Psychrometic Chart
Saturation
Wet Bulb Temp.
0 10 80
Dry Bulb Temperature (“C) Figure 3
Liquid Ring Pump Application in Recycle Drying 7 ‘3 of the nutsche and the condenser, respectively, on the gas stream in both the
Apovac and blower recycle system.
Then what makes the Apovac system a better one?
.a
The Apovac compressor is a liquid ring pump. Since it i s constantly
flushed with liquid-seal fluid, a pump (as well as other parts of the system)
will not be contaminated with the product to the same extent as the blower
system. Also, since the pump runs at relatively low temperature, it prevents
heat sensitive product from creating tar. Furthermore, the recycle system can
easily be cleaned with the desired solvent between campaigns. Another benefit
is that the ring pump is much less likely to develop mechanical problems and it
has a much lower noise level. Last but not least, the 1 iquid ring pump develops
higher outlet pressure in a single stage and can operate with larger differential
pressure than the blower.
In order to better understand the Apovac recycle system’s operation we are
going to discuss the system in detail, pointing out design parameters and the
principal function of each component.
Although the Apovac recycle drying system is a simple one, building a
successful system requires technical expertise. The system should satisfy two
requirements: Dry the product in an acceptable time frame and provide for easy
cleaning.
Let us start reviewing the drying cycle from the bottom of the nutsche
(Figure 2). The expelled liquid from the product enters the suction side of the
pump. To reduce the contamination of the recycle system by the mother liquor, ~ 1> I.
Liquid Ring Pump Application i n Recycle Drying 8 3 a catch tank is used during the early stage of the drying operation.
catch tank is bypassed by the recirculating gas stream.
Later the
The liquid ring pump, liquid ring holding tank and primary condenser were
all purchased from Sulzer Burckhardt (Figure 4). The pump is a single stage
pump. The shaft of the pump penetrates the pump housing on only one side
requiring a single mechanical seal. The designed outlet pressure is a maximum
of 22 psig with 17 psi differential pressure.
The compressed gas enters the primary condenser, which is mainly to remove
the heat of compression. Since it is located on the top of the liquid ring
holding tank, the tank serves also as a liquid-vapor separator.
During the drying operation this tank acts as the primary catch tank for
the condensed sol vent.
Water is used as cooling media on the primary condenser for several ZI reasons :
1. Two-thirds of the total heat load can be removed here, saving
more valuable 1 ow temperature cool i ng media.
The liquid ring holding tank can contain product. By avoiding
cooling this tank to a too-low temperature, the product most
likely stays in solution. This is important since, as we
mentioned earlier, the ease of cleaning significantly impacts
the usefulness of the system.
Heat insulation of the pump and the entire piping system would
make the maintenance difficult and costly.
2 .
3 .
9
E
0 a > 0 n a
a 0 cn m m
t 3
Q, > - P
Liquid Ring Pump Application in Recycle Drying D
10
The gas leaving the primary condenser enters the secondary condenser
(Figure 5). A single pass shell and tube heat exchanger is used. The cool ing
media temperature is generally -3O’C. The condenser is located in an up-flow
arrangement. We selected this arrangement since we felt that a down-flow
configuration would not provide efficient 1 iquid-vapor separation; furthermore,
it would make the installation more troublesome. Lately, two-pass condensers
are being used to minimize space requirements.
It is very important to have good temperature control on the service side
of this condenser, since different solvents have different freezing points.
Cooling the gas stream below the freezing point of the given solvent will result
in plugging of the heat exchanger. Since this heat exchanger has a very small
heat load, the installation o f a pump in the cooling loop is necessary to create
high velocity and avoid freezing.
3
The gas stream needs to be cooled to the lowest allowable temperature
(by the freezing point) of the solvent. The lower the dew point of the gas, the
lower the final moisture content will be. Other influencing factors which
affect the final moisture content o f the product are: the temperature of the
inlet gas stream (the higher the temperature, the lower the moisture content of
the product will be) and physical property o f the product.
The secondary condenser does not perform as a reflux condenser. Our
measurements showed that practically a11 the liquid which was condensed out from
the gas stream leaves the heat exc’hanger.
Separating this entrained liquid is essential to the success of the drying
operation. If the liquid entrainment is not separated, it is returned to the
Secondary Condenser
C
out In Brine
==1 I I I I I I I I I I I I I I
Figure 5
l i q u i d Ring Pump Application in Recycle Drying 12 ‘3 nutsche, which means that the drying time will increase and the attainable
moisture content is limited by the primary condenser temperature. We found that
demister (Figure 6) is able to separate the liquid particles effectively. We
spent a significant amount of time and effort in determining a properly sized
demister arrangement.
Leaving the demister, the gas steam passes through a steam heater (Figure
7). This heat exchanger is also a single pass shell and tube exchanger. The
temperature of the gas stream is controlled with a conventional control loop.
Leaving the heater, the gas is returned to the top of the nutsche and the
cycle is repeated.
To ensure reliable operation and minimal supervision from the operating
personnel, a certain amount of instrumentation is necessary in addition to the
above mentioned temperature control loops. -a The liquid ring holding tank has high and low level sensors. The
principle operation of the 1 evel sensors is ultrasound. The 1 evel control 1 ers
are connected to pneumatically operated valves. The liquid level is regulated
between the two level sensors.
During the drying cycle the level increases in the tank. When the level
reaches the high level sensor, the drain valve opens and a certain amount of
liquid is discharged. The amount of liquid discharged is determined by a timer
which holds the valve open for a preset time interval. If the high level signal
persists for a certain time interval, the pump will shut down.
If the system is used as a vacuum pump, the liquid level in the seal fluid
holding tank can decrease. When the level reaches the low level sensor, the
air-operated valve on the fill line opens up. A timer keeps the valve open for ,_>
Demister
To Liquid Ring Tank
Figure 6
3
Liquid Ring Pump Application in Recycle Drying 3 15
a predetermined amount of time. If either the high or low level signal is not
responded to for a certain time period, the pump will shut down.
The low level triggered shutdown is necessary since the pump needs a
certain amount of liquid for its operation. The high level initiated shut down
is a safety feature to avoid possible product contamination.
A flow indicator switch is installed in the solvent feed line prior to the
pump. The flow indicator is necessary since a certain amcunt of liquid is
required for optimum operation of the pump. The flow switch is an additional
safety feature used to prevent pump damage if liquid flow is blocked between the
pump and the liquid ring holding tank.
To prevent product damage due to the possible malfunction of the steam
valve at the gas heater, high temperature shutdown is provided.
It is possible that due to the nature of the product to be dried, the
pressure drop through the cake could exceed the maximum allowable 17 p s i
pressure drop of the pump. In this instance, the opening of a by-pass between
the pump inlet and outlet is necessary. We do not feel that automatic
differential pressure regulation is necessary to cope with this limited problem.
It can be seen from the above description of the instrumentation that the
system can shut down for different reasons. For troubleshooting purposes a
small programmable controller i s installed. This shows the operating personnel
the reason for the shutdown. Furthermore, it performs the timing function for
the above controls. We found that due to the simplicity o f the control logic
the use o f programmable contro lers for controlling the different control loops
cannot be justified.
L
Liquid Ring Pump Application in Recycle Drying 16
1 -3 It was mentioned above that the cleanability of the system is a very
We effectively solved this problem. The simp1 icity of important requirement.
the piping system is the primary requirement to accomplish this. The piping
system should not have any solid or liquid trap. To perform the cleaning
operation, the liquid ring holding tank is filled with the solvent selected
for this purpose. The" inlet and outlet sides o f the recycle system are
connected to each other. After the system is pressurized and the pump is
running, solvent i s injected at the top of the secondary heat exchanger. This
solvent will clean the entire loop. The excess solvent is continuously
discharged from the liquid ring holding tank. Cleaning of the system can be
judged from the cleanliness of the solvent in the liquid ring holding tank.
Last but not least the success of the Apovac recycle system depends on the
training of the operating personnel. Naturally, this is not unique to this
system but we repeatedly found that this i s a major contributor.
3
ACKNOWLEDGEMENTS
I wish to express my thanks to H. K. Jaeger and S. I. Amin, Directors of
Pharmaceutical Chemicals Production and Specialty Chemicals Production, The
Upjohn Company, for total support of this project. The dedicated effort of
employees and its management in Units 1380 and 1320 had a major impact on the
success o f the project. The support and cooperation of Sulzer Burckhardt and
Rosenmund, Inc., are highly appreciated.
Thanks to Marilyn Becker for typing and proofreading this manuscript.
_c
Liquid Ring Pump Application i n Recycle Drying '> 17
Andrew Kalman, Dr. Dae Y . Cha, J. Dale Westbrook, James W . Cagle, Jr.
"Recent Developments in Recycle Drying for the Batch Processing Industry"
The Upjohn Company, Kalamazoo, Michigan. AIChE, Miami, Florida, Annual
Meeting (November 2-7, 1986).
11/16/89
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