Centrifuges separation duties

This entry has presented a brief description of sedimenting and filtering centrifuges. The types of separation, separation duties, and modes of operation have been discussed. The basic common and distinct mechanical elements have also been dealt with. References are included for those readers seeking a more in-depth historical, theoretical, as well as practical background on the subject. 

Disc stack, decanter and tubular bowl centrifuges are used for a wide range of separation duties in the pharmaceutical and fermentation industries, including liquid-solid, liquid-liquid, liquid-liquid-soUd separations, and liquid-solid extraction. Typical process steps include cell harvesting, concentration and washing, cell debris removal, inclusion body recovery and purification, solvent extraction, dewatering and recovery. 

In their liquid-liquid separation duty, the tubular bowl centrifuges can operate without stopping, but there is always a small amount of solid particulate suspended in the liquids, which is denser than either, and so settles to the bowl wall, where it stays until the centrifuge operation is stopped, and the machine is dismantled for manual cleaning. 

As it exits from the tubes, the liquid and vapor mixture travels into a centrifugal-type vapor separator, and the liquid then flows down the suction line to a heavy-duty pump. 

They are best suited to liquid-liquid separations and clarification duties, but not high solids concentrations due to the limited solids hold-up capacity. They can handle small-density difference separations and best suited to batch processes. They are difficult to clean, and as large-scale laboratory units, best avoided unless another type of centrifuge design will not work. 

Fractional sonication - phase partition. Twenty ml of the lower phase B3 and 10 ml of fresh upper phase was sonicated 4 x 30 s with 2 min resting intervals under continuous cooling using a Vibra-cell ultrasonic processor, Model VC 500, equipped with a 3/4 inch "High Gain" horn. The output was set at 7 with 201 duty pulses. The phases were separated by low speed centrifugation and the upper phase was removed and stored separately. 

A wide variety of screens and meshes are available, ranging from fine photoetched or electroformed perforated screens to the heavy duty wedge wire screens used in centrifuge and high pressure screw press construction. Simple sieves and coarse screens were used as early as the sixteenth century for processing metal ores. Modern woven wire screens are precision made cloths with aperture sizes as small as 20 pm (smaller aperture sizes are supplied by some manufacturers) for industrial separations in filtration, clarification and extraction. Plastic meshes and plastic coated metal meshes are finding an increasing number of applications in separation processes. 

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