Decanter Centrifuges

A schematic diagram of a solid bowl decanter is shown in Figure 2. The machine consists of a horizontally oriented cylindrical bowl with one end tapered to form a cone. Within this cylindrical/conical section is a conveying scroll, with the same profile as the cylindrical/conical bowl. This scroll is rotated at a slightly different speed from the bowl through a gear system or via a separate drive. This arrangement fixes the differential speed, allows adjustment during operation, or accommodates automatic systems. Either method ensures that the scroll turns fast enough to avoid blockage by the solids which accumulate on the scroll faces, while allowing maximum solids retention times for good separations and dry ejecta. An automatic system allows the scroll speed to be adjusted to optimize the differential speeds under operating conditions. The length of the cylindrical and conical sections and the conical angle may be varied, as can the scroll design, to accommodate diverse requirements of feedstock to achieve solids separations. Slurry to be separated is fed continuously through the centre pipe to be distributed evenly near the level of the conical taper and is accelerated to bowl speed. During acceleration, high shear forces are generated and this may result in considerable foam generation in some slurries such as food materials which contain protein or pectin capable of retaining air in suspension.

The depth of the liquid pool rotating against the bowl wall is determined by the positioning of the fluid discharge ports, dams or pick-up tubes. Solids settle through this pool to the bowl wall and are conveyed to the outlet ports at the distal conical end where they are ejected. The solids undergo a drying effect as they are dragged along the bowl wall and elevated to the exit ports as liquid drains back to the pool. Given the decanter configuration, there are four parameters which can be varied: scroll/bowl differential speed, pool depth, bowl rotational speed and feed rate. Fluid or supernatant is conveyed countercurrent to the solids, following the path of the scroll flights, to exit at a discharge port at the end of the cylindrical section. Centrifugal force can vary to over 4000 g depending on application and centrifuge.

The decanter's particular advantage is that it provides continuous separation of a wide variety of feedstocks with a broad range of solids concentrations. It is possible to configure a decanter to process feedstocks containing an aqueous phase, on oil phase and suspended solids, separating the three phases in the same machine. These machines are sometimes termed tricanter (Table 3) because of the three phase separations possible. Thus the decanter can be used to separate solid-liquid two-phase systems or solid-aqueous-oily three-phase systems. Separation

discharging under gravity

Figure 2 Schematic diagram of a decanter (scroll-bowl) centrifuge showing the major parts and indicating mode of operation. Reproduced with permission from Westfalia Separator AG, Oelde, Germany.

discharging under gravity

Figure 2 Schematic diagram of a decanter (scroll-bowl) centrifuge showing the major parts and indicating mode of operation. Reproduced with permission from Westfalia Separator AG, Oelde, Germany.

of two immiscible liquid phases is possible but is normally done in a disc-stack centrifuge specially designed for this purpose (i.e. cream separator). In the solid-liquid mode of operation, the ability to handle high solids content feed streams continuously, effectively and efficiently has made dewatering of municipal and industrial sewage a major use for decanter centrifuges. For similar reasons these machines have been used extensively for dewatering fine coal and for separation of mineral slurries in the mining and mineral-processing industries.

Decanters capable of separating three phases have been used to refine vegetable oils from complex feedstocks such as coconut, producing fat, milk and grated coconut solid fractions, recovery or animal fat from rendering operations and recovery of waste oil in petroleum refining. Watery oil derived from tank bottoms or trapped in containment lagoons which contain suspended solids may have this oil recovered using these machines. In each case value is added through the recovery of the oil phase as a saleable product. More recently, two-phase decanters have been adapted to replace presses in the extraction and further processing of a wide variety of fruit and vegetable juices. Separation in a tricanter of commercially exotic fruits such as sea buckthorn into pulp oil, juice and seed-enriched solid ejecta provides a potential future use of centrifugal technology. New, innovative uses for this versatile machine are still emerging.

The disadvantage of the machine is its inability to clarify liquid streams completely, as some suspended solids remain in the emerging stream. If complete clarity is required, another clarifying method must be used following decanter centrifugation. This may include equipment such as a disc centrifuge (clarifier) or filter system. For example, processes described for the extraction of fruit juices with a decanter replacing the press often have a clarifying disc stack centrifuge in the line following the decanter to provide the final solids removal and provide the brilliant clarity desired in many juice products. Alternatively, the clarifying centrifuge can be operated in such a way as to remove the particles larger than 0.5 |im diameter to provide a stable juice opalescence.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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