Batch centrifugation

Centrifuges may be grouped into two distinct categories those that utihze the principle of filtration and those that utihze the principle of sedimentation, both enhanced by the use of a centrifugal field. These two categories are analyzed separately below. 

Schematic of the continuous gravity separator separating two immiscible fluids 

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Table 18-15 summarizes the several types of commercial centrifuges, their manner of liquid and solids discharge, their unloading speed, and their relative volumetric capacity. When either the liquid or the solids discharge is not continuous, the operation is said to be cycHc. Cyclic or batch centrifuges are often used in continuous processes by providing appropriate upstream and downstream surge capacity. 

The average prices of the batch centrifuge are shown in Fig. 18-154. All the models include the drive motor and control. In Fig. 18-154, the inverting filter, horizontal peeler, and the advanced vertical peeler are the premium baskets especially used for specialty chemicals and pharmaceuticals. Control versatility with the use of programmable logic control (PLC), automation, and cake-heel removal are the key features which are responsible for the higher price. The underdriven, top-driven, and pendulum baskets are less expensive with fewer features. 

Most batch centrifuges are mounted with their axes vertical and, because of the possibility of uneven loading of the machine, the bowl is normally supported in bearings and the centrifuge itself is supported by resilient mountings. In this way the inevitable out-of-balance forces on the supporting structure are reduced to, typically, 5 per cent or less. 

Batch centrifuges with imperforate bowls are used either for producing an accelerated separation of solid particles in a liquid, or for separating mixtures of two liquids. In the former case, the solids are deposited on the wall of the bowl and the liquid is removed through an overflow or skimming tube. The suspension is continuously fed in until a suitable depth of solids has been built up on the wall this deposit is then removed either by hand or by a mechanical scraper. With the bowl mounted about a horizontal axis, solids are more readily discharged because they can be allowed to fall directly into a chute. 

Based on the differences between the solid and liquid density, batch centrifugation techniques are often used in the discovery phase as a convenient tool to separate the cell from the medium. Highspeed ultracentrifugation instruments have... 

The collection of bacterial cells from nutrient broths is typically done by batch centrifugation. This time-consuming operation can be replaced by membrane filtration. Filtration is faster than centrifugation, and it allows extensive cell washing. 

The liquid products were collected in 4-hour batches, centrifuged, sampled, and analyzed for asphaltene, oil, elementary composition (C, H, N, S), ash, specific gravity, and viscosity. 

This gives, for the supercentrifuge as well as for batch centrifuges ... 

Figure 6.7 A typical batch centrifuge. (This material is used by permission of John Wiley Sons Inc.)...

Figure 11.13 shows schematic diagrams of various types of filtering centrifuges. In Figure 11.13(a) a top suspended batch centrifuge is shown but there are also models that are not suspended but have the drive mechanism beneath the centrifuge. 

A suitable automatic cyclic batch centrifuge for this second stage is shown in Figures 6 and 7. It consists of a perforated rotor or basket that runs continuously at full speed on a horizontal axis. The entire rotating assembly is contained in a casing that may be pressurized to meet system requirements with a... 

The solid bowl batch centrifuge is the simplest type, similar to the tubular bowl machine type but with a smaller bowl length to diameter ratio (less than 0.75). The tubular bowl type is rarely used for solids concentrations above 1% by volume. 

Solids-retaining separators (Fig. 2) are batch centrifuges. They are generally larger in diameter than tubular bowls. Therefore, they are large enough to contain a disk stack but are dependent on at least partial disassembly for manual removal of solids. The larger... 

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