Centrifuges

A centrifuge is a piece of equipment, generally driven by a motor that puts an object in rotation around a fixed axis, applying force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to separate substances of greater and lesser density.

The first successful centrifuge was built in 1883 by Carl G. P. de Laval, a Swedish engineer, whose design was used chiefly for cream separators. The ultracentrifuge, devised in the 1920s by the Swedish chemist Theodor Svedberg, found wide application in scientific research.

A mechanical method of separating immiscible liquids or solids from liquids by the application of centrifugal force. This force can be very great, and separations which proceed slowly by gravity can be speeded up enormously in centrifugal equipment.

Centrifugal force is generated inside stationary equipment by introducing a high-velocity fluid stream tangentially into a cylindrical-conical chamber, forming a vortex of considerable intensity.

Much higher centrifugal forces than in stationary equipment are generated in rotating equipment (mechanically driven bowls or baskets, usually of metal, turning inside a stationary casing). Rotating a cylinder at high speed induces a considerable tensile stress in the cylinder wall. This limits the centrifugal force which can be generated in a unit of a given size and material of construction. Very high forces, therefore, can be developed only in very small centrifuges.

There are two major types of centrifuges: sedimenters and filters. A sedimenting centrifuge contains a solid-wall cylinder or cone rotating about a horizontal or vertical axis. An annular layer of liquid, of fixed thickness, is held against the wall by centrifugal force; because this force is so large compared with that of gravity, the liquid surface is essentially parallel with the axis of rotation regardless of the orientation of the unit. Heavy phases “sink” outwardly from the centre, and less dense phases “rise” inwardly. Heavy solid particles collect on the wall and must be periodically or continuously removed.

A filtering centrifuge operates on the same principle as the spinner in a household washing machine. The basket wall is perforated and lined with a filter medium such as a cloth or a fine screen; liquid passes through the wall, impelled by centrifugal force, leaving behind a cake of solids on the filter medium. The filtration rate increases with the centrifugal force and with the permeability of the solid cake. Some compressible solids do not filter well in a centrifuge because the particles deform under centrifugal force and the permeability of the cake is greatly reduced. The amount of liquid adhering to the solids after they have been spun also depends on the centrifugal force applied; in general, it is substantially less than in the cake from other types of filtration devices.

There are many different kinds of centrifuges, including those for very specialised purposes. The decision to use either a batch or a continuous machine depends on several factors. Unlike batch centrifuges, continuous machines are limited to a wash/solids ratio of approximately 10%, with only a short period allocated to the wash zone prior to the finish of the cycle.

If, for example, the material to be processed has low residual impurities and a high washing requirement, the greater flexibility of the batch process allows the necessary adjustments such as extended washes and longer residence times.

Particle size, distribution and shape are also important factors when determining separation capabilities, and whether a batch or continuous centrifuge is the best option. Generally speaking, materials of 45 microns and above that are relatively incompressible are highly suitable for separation by filtration. On the other hand, finer or more compressible materials lend themselves to separation by sedimentation techniques.

Compared with other methods of liquid-solid separation, centrifugal processing provides a number of unique advantages. For example, centrifuges can be installed in a relatively small footprint, have a high washing capability, produce low cake moisture, achieve a high capacity throughput and provide the end user with a totally enclosed, vapour tight processing system.

4. Answer the questions to the text:

1) What is a centrifuge?

2) What was the first successful centrifuge used for?

3) How can separations be speeded up enormously in centrifugal equipment?

4) In what way is centrifugal force is generated?

5) What does rotating a cylinder at high speed induce?

6) What is the design of a sedimenting centrifuge?

7) What is the principle of action of a filtering centrifuge?

8) What are the advantages of batch machines?

9) What factors are important when determining separation capabilities, and whether a batch or continuous centrifuge is the best option?

10) What are the advantages of centrifugal processing compared with other methods of liquid-solid separation?

5. Change the following sentences according to the model:

Model A:	If this mixture is placed in a test tube, it will glow dull red.
B:	If this mixture were placed in a test tube in two hours, it would glow dull red. C: If this mixture had been placed in a test tube two hours ago, it would have glowed dull red.

1. If metals are heated, oxides will be formed.

2. If any liquid is in a vessel, it will take the shape of the vessel containing it.

3. If some air is allowed to mix with hydrogen, the reaction will take place with a characteristic sound.

6. Translate into Ukrainian paying attention to the word “since”:

1. Since diamond is the hardest substance known, it must be polished with diamond dust.

2. Many years have passed since Mendeleyev made his great discovery.

3. This method of mixing has been used since the days of World War II.

4. This process was developed twenty years ago, since then it has been used in many countries.

5. Carbon is one of the most important and interesting elements, since its compounds are widely distributed in nature and play such an important part in daily life.

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Change the following sentences according to the model.	\|	Say if the following statements are true or false. If the statement is false

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