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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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