Feature Report Filtration Centrifuges: An Overview Peter Schmidt Andritz KMPT GmbH 10,000 Separator Pressure Chamber filter press Belt filter press Centrifuge Driving force of separation pressure (bar) Driving force of separation gravitation (g) The first step in selecting the right centrifuge is understanding what types are available Sedimentation Filtration 30.0 bar 15.0 bar 5.0 bar Atmospheric 3,000 Centrifuge Drum filter Disc filter DCFF Filtration thickener Screen Disc filter Drum filter Pan filter Belt filter Gravity thickener Vacuum 1.0 bar solve the task of solid-liquid (S-L) separation in the chemical process industries (CPI) a wide variety of methods are invariably used. Many of these methods are not mutually exclusive and include a selection of technologies that can be expediently combined to provide an efficient operating system. A good understanding of the options available and details of the individual system modules can enhance the efficiency of the overall concept. For example, mechanical separation using pusher centrifuges has proved to be a very expedient intermediate stage for the dewatering of sodium bicarbonate downstream of the vacuum filter and before calcination. By reducing the residual moisture between filter discharge and centrifuge discharge, some 38% of the required evaporation heat can be saved; and just a fraction of the saved energy is required to operate the centrifuge. This helps to protect the environment and also saves costs. This article presents a basic overview of different types of centrifuges with descriptions of how they operate and where they are applied in the CPI. Cyclone 0.2 bar Figure 1. A classification scheme of the various types of equipment available for solid-liquid separation is shown here Some definitions To begin with, a distinction is made between thermal and mechanical separation of solid products from liquids. Whereas thermal S-L separators can be grouped under the general term it is not so easy to classify the equipment used in the first stage of mechanical S-L separation. For a finer distinction it is necessary to consider the flow directions of the solid and the liquid phases. If these are both in the same direction one talks of filtration; in the case of opposing directions the process is referred to as sedimentation (Figure 1). There is also the special case in which the flow directions of the solid and liquid phases are at right angles to each other, this is referred to as cross flow filtration. In most sedimentation processes, the difference in density between the solid and liquid phase is utilized, but it is also possible to use electric or magnetic fields for separating purposes. To some extent, the natural sedimentation in the gravitational field is used to this end, for example, by gravity thickeners. This natural sedimentation is enhanced by also super- imposing a centrifugal field, as is done by cyclones and centrifuges. Filtration, on the other hand, uses a filter medium that retains the solid phase while allowing the liquid phase to flow through. Another way to distinguish filtration methods is to consider the driving potential that moves the liquid phase, or filtrate, through the filter medium. Separation can take place in the simplest of ways: using a screen in the gravitational field or by imposing a pressure gradient. Such pressure gradients can be generated by applying a vacuum to the filtrate side. Here, however, natural limits are quickly revealed. Far greater potentials are to be found when the pressure gradient is applied to the solids side using overpressure. This brings us to the focus of the article, filtration centrifuges, which are used when a cake-forming filtration is required. Chemical Engineering www.che.com December 2010