Decanter applications

The decanter eentrifuge can be used for most types of liquid/solid separation, and its ability to handle a wide range of feed slurry coricentriition.s adds to its general versatility. 

It can be used for the classification of solids in liquid suspension, where a single cut i.s required between two si es of solid particle (or. less often, between solids of differing density). It is a very good device for this purpose, and its early history included development for the kaolin (china clay) industry. 

The decanter can be used for the clarification of a liquid. It can be operated so as to give a high degree of clarification, although it is not usually used to clarify a slurry that c ontains only a small amount of solids in suspension. 

Where the slurry is a waste needing treatment prior to safe disposal, the decanter again can dewater such slurries to a high level of dryness. 

Finally the decanter can be operated so as to ad as a thickener, producing a clear liquid and a more concentrated slurry — eitlier in a manufacturing process, or in waste treatment. 

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There are various methods of starting a decanter wnth an inherently high inertial load. A variable speed drive might be considered, if changes in machine speed are deemed to be important due to varying process conditions. However, a large majority of decanter applications use a fixed speed main drive motor. Once at speed, a decanter s motor has one of the simplest duties. It is never subjected to cyclic overloads, never subjected l i continuous vibration and never subjected to severe braking, electrically or mechanically. It is seldom stopped and restarted. 

Tn all but the last 10 or 15 years the majority of decanter applications have been limited by their ability to clarify the liquor. Scale up of these centrifuges Would be based on their clariKcation ability. Thus, a lot of the earlier development has been concentrated on enhancing clarification capacity. Bowl speeds have been increased, bowls lengthened and solids discharge areas increased to ensure solids choking does not inhibit clarification volume. Apart from these obvious methods there are some mechanical modifications which can help,... 

In addition to reviewing decanter applications by specific industrial end-use. it is necessary first to recognise that there are ways of looking at decanter applications in a more general sense. Mechanical liquid/solid or liquid/liquid separations serve two broad purposes ... 

Decanter applications certainly obey this differentiation. The number of different utility applications is small, although the main one. the dewatering of waste sludges, is an enormous part of the decanter market. The great variety of decanter uses occurs in the very wide range of process applications. 

For any waste which is produced carrying an oily liquid as well as water, then the oil can be recovered separately from the water and the suspended solids, by the use of the three-phase decanter. This will occur especially in the oil refining and blending processes, but also in several food industry applications. The treatment of refinery slop oils is an important decanter application. 

The beverage industry divides into two main parts — the making of alcoholic and non-alcoholic drinks. For non-alcoholic beverages, the main decanter application is in the separation of fruit and vegetable juices from their crushed pulp. 

Not surprisingly, the main decanter applications lie in the wet polymerisation of monomers to form ... 

In the past, the vast majority of decanter applications were limited by the clarification capacity of the centrifuge. With the development of the decanter, better designs, higher g-levels, and better knowledge of the technology, today many applications are governed by what happens at the other end of the machine. 

In some decanter applications it is required to remove, from the solids, some dissolved impurities in the liquid held in the cake. This is achieved by spraying rinse liquid onto the solids as they are conveyed up the beach. Admitting the rinse too far up the beach can cause problems, by washing the cake back into the pond, or producing too wet a cake. Washing too far down the beach risks poor washing efficiency, when rinse by-passes the cake, by streaming over the surface of the supernatant. 

In decanter applications, the higher molecular weight polymers predominate. Nevertheless, occasionally some lower molecular weight polymers are found very effective when used on some decanter centrifuges. Cross-linked polymers are finding frequent use on decanters, with both municipal and, particularly, industrial effluents. 

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

In some circumstances, separation of solid from a liquid is better achieved by use of a centrifuge than by filtration, and a small, electrically driven centrifuge is a useful piece of equipment for an analytical laboratory. It may be employed for removing the mother liquor from recrystallised salts, for collecting difficultly filterable precipitates, and for the washing of certain precipitates by decantation. It is particularly useful when small quantities of solids are involved centrifuging, followed by decantation and re-centrifuging, avoids transference losses and yields the solid phase in a compact form. Another valuable application is for the separation of two immiscible phases. 

Pharmaceutical Removal of suspended matter is a frequent application for MF. Processes may be either clarification, in which the main product is a clarified liquid, or solids recovery. Separating cells or their fragments from broth is the most common application. Clarification of the broth in preparation for product recovery is the usual objective, but the primary goal may be recovery of cells. Cross-flow microfiltration competes w l with centrifugation, conventional filtration by rotary vacuum filter or filter press and decantation. MF delivers a cleaner permeate, an uncontaminated, concentrated cell product... 

Decanters are normally designed for continuous operation, but the same design principles will apply to batch operated units. A great variety of vessel shapes is used for decanters, but for most applications a cylindrical vessel will be suitable, and will be the cheapest shape. Typical designs are shown in Figures 10.38 and 10.39. The position of the interface can be controlled, with or without the use of instruments, by use of a syphon take-off for the heavy liquid, Figure 10.38. 

The possibility of adjusting solubility properties is of particular importance for liquid-liquid biphasic catalysis. Liquid-liquid catalysis can be realised when the ionic liquid is able to dissolve the catalyst, especially if it displays partial solubility of the substrates and poor solubility of the reaction products. Under these conditions, the product phase, which also contains the unconverted reactants, is removed by simple phase decantation. The ionic liquid containing the catalyst can then be recycled. In such a scenario the ionic catalyst solution may be seen as part of the capital investment for a potential technical process (in an ideal case) or at least as a working solution (only a small amount has to be replaced after a certain time of application). A crucial aspect of this concept is the immobilisation of the transition metal catalyst in the ionic liquid. While most transition metal catalysts easily dissolve in an ionic liquid without any special ligand design, ionic ligand systems have been applied with great success to... 

To assess possible adsorption of antisera to the Ventana dispensers, a Ventana pipeter filled with L26 antisera was stored at 4°C for up to 1 mo. At the end of the storage period, the antisera was decanted, and the pipeter thoroughly rinsed with PBS. The pipeter was then stained to assess antibody adsorption using sequential application of standard immunostaining reagents. After 4 wk of storage, there was no detectable immunoreactivity in the Ventana pipeter. 

Fines removal from the slurry may allow it to be used for anode grade coke rather than fuel grade. Filters have been used successfully in this application and fresh feed or light cycle oil can be used to recycle the fines back to the riser instead of decant oil. 

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