Introduction

Industrially, centrifuges are used for a variety of purposes related to separation of materials on the basis of density. This separation usually involves separation of insoluble particulates from supernatant liquids, but can also include extraction of dissolved substances from one immiscible liquid to another of different density, separating the mixed liquids centrifugally. The blending of the liquids, transfer of the solute and separation of the immiscible phases are sequentially carried out in the same machine at high speed.

Generally, centrifuges are used throughout many manufacturing industries (Table 1), to separate suspended solids from liquid utilizing the centrifugal acceleration of the suspended particles directed outward from the axis of rotation. This force initiates the particle movement to the centrifuge periphery where it is trapped or contained by the wall of the rotating body. Alternatively, a density difference between two immiscible liquids is exploited to accelerate separation of the liquids (i.e. fat separation in dairies for cream or butter manufacture). A specialized use involves separation of water from fresh-cut vegetables before modified atmosphere packaging.

Much experience and information related to industrial-scale centrifugation exists within companies manufacturing the centrifugal machinery and these sources should not be overlooked when seeking information. Table 2 lists a representative selection of companies involved in the manufacture of centrifuges and their Internet addresses current at the time of writing. The Internet itself should not be overlooked as a source of information: simply typing the word 'centrifuges' in the request space of one search engine provided over 25 000 items for perusal.

Centrifugation is treated as a separation unit operation in chemical engineering and the article in Dahlstrom et al. (1997) by Leung on centrifuges should be consulted for an engineering perspective (see Further Reading). For a comprehensive treatment of industrial centrifugation technology, Leung's book on industrial centrifugation technology should be consulted.

Over the past 10-15 years the growing uses for centrifuges industrially has resulted in a plethora of special centrifuges designed and adapted to particular uses. However, the machines may, in general, be characterized according to the classification of Table 3. Centrifuges fall into two general classifications, termed sedimentation centrifuges and filter centrifuges. In sedimentation centrifuges, solids are transported to the periphery wall of the rotating machine bowl and collected against this surface; liquid is removed from the solids by the close packing of the individual particulates. In filter centrifuges the solids are transported to the surface of a filter element and the solids trapped on this filter, while the liquid drains through the particulates and exits through the filter surface. The mechanism of solids drying is thus quite different between the two types of machine and the types of material each would be expected to treat most efficiently also differ considerably. The other important parameter is whether or not the machines are fed continuously or in batch mode. Generally, batch-mode machines are often considered obsolete for large scale separations, with the important exceptions of the continuing use of batch-mode basket centrifuges for the last recovery stages for white sugar and in the fresh-cut vegetable industry. Other exceptions also exist.

The approximate capabilities of several centrifuge types are indicated in Figure 1. The list is not exhaustive but examples of most types of centrifuge,

Table 1 Industrial use of centrifugal technologies

Food and agri-business Sugar crystal recovery

Dewatering of fresh-cut salad and vegetables

Milk processing, bacterial removal, cream separation

Pulp-free orange juice

Formation of fruit and vegetable juices

Frying oil clean-up

Pharmaceutical/biotechnology Recovery of valuable isolates

Recovery of cells (yeast and bacteria, plant and animal cells) Clarification of fermentation broths

Environmental industries Sewage solids recovery Wastewater treatment

Removal of metal cuttings from industrial cutting lubricants

Chemical industries Black coal separation from slurries Isolation of synthetic products Gas-phase isotope separation

Table 2 Companies manufacturing centrifugal equipment which may be contacted through the Internet3

Alfa Laval Sharpies Barrett Centrifugals Bird Machine

Carr Separations Dorr-Oliver

Eillert Veg. Proc. Mach.b Rousselet Centrifugation Tema Systemsc

Westfalia Separator

http://www.als.thomasregister.com

http://www.barrettinc.com

http://www.bakerhughes.com/bird/

birdhome.htm http: //www.carrsep.com

http://www.dorroliver.

thomasregister.com

http://www.eillert.nl

http: //www.rousselet.com

http://www.tema1-usa.com/

corp.htm

http://www.westfalia.com/ default.htm aThe list is representative, not exhaustive. bFresh cut vegetable processing. cManufactures under licence from Siebtechnik (Germany).

along with an estimate of the range of g forces available from each machine type, is provided for purposes of illustration and estimation of requirements. Basket centrifuges are normally of low speed and provide maximum g forces in the 1500-2000 range.

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