Typical applications in fermentation are thick fermentation broths with high solids concentrations where a relatively drier cake is required. However, protein precipitate cannot be sedimented and animal cell debris, due to their slimy nature, can render scrolling ineffective.

Solids and liquids are discharged continuously in this type of design which can process coarse particles that would blind the discharge system and disks of disk bowl machines. The principle of operation is shown in Fig. 1.

This unit, often referred to as a decanter, is constructed with a conical bowl and an internal rotating scroll conveyor to propel solids or beach them along the inclined wall bowl to then be discharged.

The scroll rotates slower than the bowl at a differential speed of 1/20 to 1/160 of the bowl speed; this differential speed causes translation of the solids along the bowl. Particularly, soft solids can be conveyed with low conveyor differential speeds should higher differential speeds cause resuspensions. Units will have g forces up to 6000 and range in diameter from 6" to 48".(1) The solids discharge outlet is usually smaller than the liquid discharge outlet at the opposite end.

By varying the liquid discharge outlet size the pool level or depth of the pond can be controlled. The lower the level, the greater the length of the dry beach section. These units operate below their critical speeds between fixed bearings attached to a rigid frame. Mechanical seals are available for pressure operation up to 150 psig. Operating temperatures are from -87°C to +260°C

Liquid *

Figure 1. Solid Bowl Decanter.

7.1 Maintenance

Bearings are the primary concern in this design, however, lifetime will depend upon the service and hours of operation. Abrasive materials can cause excessive wear along the feed zone, the conveyor leading to the beach, and the solids-discharge ports. Refacing with replaceable hard surfacing materials such as Hastelloy or tungsten may be required.111

A variation on this design is a screening bowl machine. After the solids have been pulled from the pool of liquid they will pass under a section of a wedge-bar screen to allow for additional dewatering as well as washing the solids more effectively. This design can of course only be used with particles of 80-100 microns or greater as smaller solids will pass into the effluent.

7.2 Typical Problem For Continuous Decanter Centrifuge with


The process has a feed rate of 15 0 gpm of a fermentation broth at 1.5 % solids. We would like to concentrate the feed to 8% solids with no more than 0.2% loss of solids in the effluent. What must the solids phase (underflow) and liquid phase (outflow) throughputs be?

Two mass balance equations must be solved simultaneously.

U+Q = 150 U (.080) + Q (.002) = 150 (.015) U = 150-g

(150-0) (.080) + Q (.002) = 150 (.015) Overflow = 125 gpm Underflow = 25 gpm

U = Underflow Q = Overflow

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