Centrifugal Filtration.pptx

Pharmaceutical Process Chemistry

1. Pharmaceutical Process Chemistry Filtration by Centrifugation S. Maheen Abdul Rahman Master of Pharmacy Pharmaceutical Chemistry

2. Flow of presentation Definitions Process of Centrifugation Theory of Centrifugation Centrifugal Effect Classification of Centrifuges Equipment

3. Definitions Centrifugation: Centrifugation is a Unit operation employed for separating the constituents present in a dispersion with the aid of centrifugal force. Centrifugal force: Centrifugal force is used to provide the driving force for the separation of particles in a liquid. It replaces the gravitational force present in the sedimentation. When it will be use? Centrifugation is particularly useful when separation by normal filtration is difficult. When it will be difficult? For example separating highly viscous mixtures and colloidal dispersion (particle size between 1 nm and 0.1 micrometer ) in which densities of the medium is different.

4. . So, centrifugation provides a convenient method of separating either two immiscible liquids or a solids from a liquid. The equipment used for the separation are known as Centrifuges. Process of Centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The centrifuge consist of essentially a container in which a mixture of solids and liquid is placed and rotated at high speed. The mixture is separated into its constituents parts by the action of centrifugal force based on their densities.  In this process, the denser component of the mixture migrates away from the axis and the lighter component migrates towards the axis. Means a solids retained in a bottom of container and leaving a clear supernatant liquid.

5. Theory of Centrifugation In a colloidal dispersion, the dispersed phase may be either a solid or a liquid. Particle having a size above 5µm sediment at bottom due to gravitational force. In such a cases, separation of solids can be done by simple filtration. If particle size is 5µm or less, they undergo Brownian motion. Hence they do not sediment under gravity. Therefore, a stronger force, centrifugal force is applied in order to separate them. The sedimentation is also depends on the densities of the dispersed phase and dispersion medium. If the difference in the densities of these phases is less, again the separation is difficult So, by applying centrifugal force, it is possible to facilitate the seperation process.

6. Centrifugal Effect The centrifugal operation is described by equations including the gravitational constant. Consider a body of mass m (kg) rotating in a circular path of radius r (m) at a velocity of v (m / s). The force acting on the body is given by, F = centrifugal force, N

7. . The same body is experiencing gravitational force ( newton ). It can be expressed as: Gravitational force, G = mg where g = acceleration due to gravity, m/ s 2 The centrifugal effect is expressed as a ratio of centrifugal force to gravitational force . In other words, centrifugal effect is a force, which is number of times grater than the gravitational force. The centrifugal effect can be expressed as: Centrifugal effect, C = By substituing these,

8. . C = = C = C = 1 C ∞ d & C ∞ n But v=2πrn, where n = speed of rotation (rpm of centrifuge) = = Since 2r=d,

9. Classification of Centrifuges 1. Sedimentation centrifuge : It is a centrifuge that produces sedimentation of solids based on the difference in the densities of two or more phases of the mixture. The efficiency depends on the velocities of rotation. E.g. are Supercentrifuge , Conical disc centrifuge and horizontal continuous centrifuges. 2. Ultracentrifugation : When extremely fine solid matter is to be separated from a liquid for example in a colloidal research, ultracentrifuge is used. In this instrument, a small rotor is operated at a speed exceeding 1,00,000 rpm. So this may used for suspensions containing low concentration of solids. 3. Filtration centrifuge: It is a centrifuge in which solids pass through the porous medium based on the difference in the densities of the solids and liquid phases. In this type the container contains a porous wall through which the liquid may pass and on which the solid phase is retained. E.g. 1. Perforated basket centrifuge (batch type) 2. Semi-continuous centrifuge (short cycle batch centrifuge)

10. Equipment 1.PERFORATED BASKET CENTRIFUGE: Principle: This centrifuge is a filtration centrifuge; the separation is based on the densities difference of solid and liquid phases. The bowel contains a perforated side wall During centrifugation, the liquid phase is passed through the perforated wall, while the solid is retained in the basket. It is removed by cutting the sediment by blade after stopping the centrifuge. CONSTRUCTION: Figure Basket – steel or copper Capacity – 0.085 meter cube and diameter 0.90 meters. Vertical shaft

11. . The diameter of the perforations should be selected based on the size of the crystals. In case, the perforations is bigger then the particles, a filter cloth is employed for the separation Working: Material kept in the basket - Material should be optimum level. 1000 rpm Power (kilowatt) Liquid outlet Power off Solid cake –blade Uses: This type of centrifuges is extensively used for the separating the crystalline drugs (such as aspirin) from the mother liquor. Sugar crystals are separated using this. Merits: It can occupy very little floor space. It can handle the slurries with high proportion of solids and having paste like consistency . The final product has very low moisture content. Demerits: On prolonged operation, the solid may form a hard cake, due to centrifugal force, which is difficult to remove.

12. . Semi-continuous centrifuge or Short cycle automatic batch Centrifuge: The principle and working are same as the Perforated Basket centrifuge ;the only difference is “ it is short cycle automated batch” And the construction is quite different. This containing the vertical basket with horizontal shaft driven by motor. Merits: Short- cycle automatic batch centrifuge is used when solids are drained fast from the bowel! Demerits: During discharge, considerable breakage of particle is possible.

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