- It includes the following steps.
- Brief Introduction
- Classification of centrifuges
- Selection of centrifuge
- Description of selected centrifuge
- Design consideration
- Brief IntroductionA centrifuge is most often used for the separation of particles from solutions according to their size, shape, density, viscosity of the medium and rotor speed. These machines utilize the natural separation realities present in a high-speed circular G-force environment. Like a high-powered clothes dryer, these exceedingly fast machines spin in order to separate materials from one another. The denser materials separate from the less dense during the centrifugation process. The term “centrifuge” encompasses a wide variety of process equipment used for many different applications in the chemical process industries. Although these units may look different and play key roles in much unrelated processes.
Classification of Centrifuges
Centrifuges are classified according to the mechanism used for solids separation.
- Sedimentation centrifuges
- Filtration centrifuges
In sedimentation centrifuges the separation is dependent on a difference in density between the solid and liquid phases (solid heavier).In filtration centrifuge the separate the phases by filtration. The walls of the centrifuge basket are porous, and the liquid filters through the deposited cake of solids and is removed.
These two categories can be further split into subcategories,
- Tubular bowl High-speed, vertical axis, tubular bowl centrifuges are used for the separation of immiscible liquids, such as water and oil, and for the separation of fine solids. The bowl is driven at speeds of around 15,000 rpm (250 Hz) and the centrifugal force generated exceeds 130,000N.
The conical discs in a disc bowl centrifuge split the liquid flow into a number of very thin layers, which greatly increases the separating efficiency. Disc bowl centrifuges are used for separating liquids and fine solids, and for solids classification.
In this type of machine the solids deposited on the wall of the bowl are removed by a scroll (a helical screw conveyer) which revolves at a slightly different speed from the bowl. Scroll discharge centrifuges can be designed so that solids can be washed and relatively dry solids be discharged.
Solid bowl batch centrifuge
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 per cent by volume. For concentrations between 1 to 15 per cent, any of the other three types can be used. Above 15 per cent, either the scroll discharge type or the batch type may be used, depending on whether continuous or intermittent operation is required.
It is convenient to classify centrifugal filters into two broad classes, depending on how the solids are removed; fixed bed or moving bed.In the fixed-bed type, the cake of solids remains on the walls of the bowl until removed manually, or automatically by means of a knife mechanism. It is essentially cyclic in operation.In the moving-bed type, the mass of solids is moved along the bowl by the action of a scroll (similar to the solid-bowl sedimentation type); or by a ram (pusher type); or by a vibration mechanism; or by the bowl angle. Washing and drying zones can be incorporated into the moving bed type.
Selection of Centrifuge
There are following factors which must be considered in the selection of centrifuge for a process.
- Physical Properties of Materials
The characteristics of the solids and liquids handled in a process will influence centrifuge selection.
Specific Gravities of the Solids and Liquids
If the solids are lighter than the liquid, a decanting centrifuge is not an option. If the specific gravities are very close, but the solids are slightly heavier, a decanting centrifuge may be considered, but only if either the particle size or the centrifugal force improves the settling of the solids.
- Particle Size Coarse solids with particle size greater than 100 um are generally best suited for filtering type centrifuges. Finer solids that measures less than 100 um are best handled in sedimentation centrifuges.
Decanting centrifuge provides the best clarity of all centrifuge types. Filtering centrifuges typically are not used where centrate clarity is the principle process requirement because they use either a filter medium or a screen.
- Process requirements
Continuous centrifuges should be considered when the following criteria are important.
Pressure and Temperature
Continuous centrifuges are used when higher operating pressures are required. Continuous decanters have been used in operations where pressures were as high as 90 psig and temperatures as high as 175 C.Batch centrifuges are restricted to lower pressure applications. There does not appear to be a restriction or preference based on temperature.
The higher the solid flow rate, the greater the tendency to use continuous centrifugation. As a rule of thumb, batch centrifuges are applicable for loadings of up to 1 ton/hr.
Continuous filtering centrifuges, with the possible exception of the screen bowl design, prefer thick feed slurry (typically 50% by weight). There are two reasons for this: continuous filtering centrifuges may be limited by the amount of liquid to be centrifuged. Thus the overall recovery of solids is generally higher with continuous filtering centrifuges when the feed has a high solids concentration. So Scroll Conveyor & Bowl Centrifuge is selected.
Parameters Required For Designing
1: Volumetric Flow Rate
2:Length OF centrifuge
3:Volume OF bowl
5:Relative Centrifugal Force
6:Settling Velocity Of Prticles