684 BIOTECHSOLOGT AND BIOEKGIKEERIKG VOL. X (1968)

Equipment for the Recovery of Precipitates front Tubular Centrifuges

Summary

Equipment has been developed for the recovery of precipitates from rotors of industrial tubular centrifuges. A double piston is described for nonaseptic discharge of precipitates through the outlet holes of the clarifier rotor. For the aseptic resuspension or dissolving of valuable precipitates a closed-circulation system has been developed, which is applied without opening of the clarifier rotor.

INTRODUCTION

Among the machines available for clarification of suspensions on an industrial scale, continuous tubular centrifuges produce the highest centrifugal f0rce.l These centrifuges are recommended for the clarification of suspensions consisting of fine particles and having a low concentration of solids.? Thus bacterial cultures and other bilogical suspensions are processed in the Sharples S o . 6 Presurtite super centrifuge (in the U.S.A. KO. 16), which at 15,000 rpm produces a maximal centrifugal force of 13000g. The rotor of this machine has a volume of G liters and contains a three-wing baffle assembly.2 When the precipitate is the valuable product, one is interested in its complete recovery. KO commercial equipment is available for the recovery of precipitates from the rotor.

Vallet3 ha: developed a system for the aseptic resuspension of precipitates in the fractionation of human plasma. However? this system requires the with- drawal of the three-wing assembly prior to resuspension, whirh is not convenient and may lead to loss of precipitate.

In the present paper we describe two types of equipment for the recovery of precipitates from tubular rotors.

EQUIPMENT AND OPERATION

Double Piston for Nonaseptic Work

Figure 1 shows a double piston made of aluminum. The dimensions of the large piston are fitted to the internal diameter of the rotor, whereas those of the small piston to the outlet neck of the rotor.

Following centrifugation, the cap of the rotor is removed and the three-m-ing assembly is withdrawn. The precipitate adhering to the baffles is recovered by means of a spatula. The open rotor is placed vertically on a wooden board with a central hole, above a stainless steel pail. as shown in Figure 2.

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The double piston is depressed nlanually into the rotor and the precipitate is discharged into the pail. Towards the end, the small piston is unlocked and depressed for the discharge of the precipitate in the rotor neck.

The culture was killed by heating a t 100-llO'C for 10 min, cooled, and centrifuged. From R 150 liter culture 900 g of wet cell mass were collected in the rotor. Using the double piston (Fig. 2), the bacterial mass was completely recovered. -4lthough

Lrsteria momcytogenes w a s grown in a tryptose broth in fe rment~rs .~

SMALL PISTON

\ LARGE PISTON

LOCK OF SMALL h PISTON

. I P .

215

Fig. 1 . Double piston detail. Dimensions in millimeters.

Fig. 2. Diagram of rmmmhmnnmmrrr

work with the double piston. Diniensions in millimeters.

686 BIOTECHKOLOGY AND BIOEXGINEERING VOL. X (1968)

the piston has been very useful in the case of several bacterial species, its suita- bility depends on the physical properties of the precipitate. The cell mass of E . coli It’, grown in a niedium containing casein hydrolyzate and yeast auto- lyzate, could not be discharged with this piston.

Closed Circulation System for Aseptic Work Figure 3 shows schematically the dosed circulation system, which is based on

the spraying of a suitable resuspension medium into the rotor. The rotor is mounted on a rack (not shown) in the vertical and inverted position

(feed port upwards). On the feed port the spraying nozzle (Fig. 4) is mounted. On the outlet neck of the rotor a rubber hose adaptor with a tube elbow is fastened by metal clamps. All rubber hoses are fastened using soft wire. In the aspirator bottle a t least 7 liters resuspension medium, containing a suitable antifoam agent is prepared in advance. The pump should have a high capacity (in our system a centrifugal pump is used: Stuart-Turner Ltd., Henley-on-Thames, England; pump No. 1R; capacity 720 gph a t 5 f t head) in order to produce stong jets on the internal surfaces of the rotor. When the liquid in the aspirator bottle goes down, due to accumulation of liquid in the

Sow the system shown in Figure 3 is assembled.

Cirrulation is continued for about 10 min.

Fig. 3. Diagram of the closed-circulation system: (1) rotor with cap; (2) three-wing asaembly ; (3) spraying nozzle; ( 4 ) T-piece; (5) solenoid valve, nor- mally closed, orifice 5 mm; 16) air filter; (7) rubber hose adaptor; (8) tube elbow; (9) 18-liter aspirator bottle; (10) circulation pump; (11) switchboard.

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Fig. 4. Spraying nozzle detail. Dimensions in millimeters

rotor, the pump is stopped and the solenoid calve iR opened, by moving of a switch. Air enters into the rotor and the suspension is drained into the aspirator bottle (release operation). Circulation and release operations are continued for a duration determined in a preliminary experiment.

The wet mass of L. monocytogenes, prepared as described before, was resus- pended in 8 liters distilled water. In 60 min 900 g wet mass was recovered com- pletely from the rotor. Usually for the resuspension of 1-3 kg wet bacterial m a s , circulation has to be carried out for 60-90 niin.

At the end, by opening of the solenoid value, the whole suspension is collected in the aspirator bottle and the latter is disconnected from the system. In c a e of foaming, additional antifoam may be added into the aspirator bottle during circulation. If aseptic conditions are required, all parts of the system are sterilized and assembled aseptically.

One may centrifuge several hundred liters of culture and using the described system obtain a concentrated suspension of about 10 liters.

This closed system may have similar applications for diesolving of precipitates in biochemical process.

The authors wish to thank Mr. R. Doron for his valuable technical assistance.

References

1. H. Trawinski, Chem. Zng. Tech., 36, 1276 (1964). 2. M. H. Hebb and F. H. Smith, “Centrifugal Separation,” in Encyclopedia of

Chemical Technology, Vol. 3, R. E. Kirk and D. F. Othmer, Eds., Interscience, New York, 1949, p. 501.

688 BIOTECHNOLOGY AND BIOENGINEERING VOL. X (1968)

3. L. Vallet, J . Biochenl. Mierobiol. Tech. Eng., 2, 121 (1960). 4. G. Miller and A. hlizrahi, to be published.

G. MILLER A. MIZRAHI G. SHAWIT

Israel Institute for Biological Research Xess-Ziona, Israel

Received April 2, 1965

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