Size Separation – Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

 Size Separation

size-separation

Learning objectives  

At the end of this lecture student will be able to: 

  • Explain the various techniques of size separation 
  • Discuss the applications of size separation in pharmaceutical formulations  
  • Explain the Andreasen’s technique of size separation  
  • Recommend the suitability of sedimentation or elutriation method of size separation  
  • Discuss the principle in the size separation process of particles using bag filters  

Introduction  

  • Techniques used to separate one material from the other are called separation  
  • Separations are extremely involved in chemical manufacture  
  • In fact, much processing equipment is devoted to separate one phase or one material from the other  • Size reduction of material never gives particles of same size but particles of varying sizes  
  • Separation technique is used to obtain narrow size ranges  
  • It is an integral part of size reduction process  
  • Assumes greater importance in controlling the size distribution for official specifications  
  • Screening is a method of separating particles according to size alone but particles can be separated into individual sizes  using sieves  

Size separation  

Size separation is an unit operation that involves the separation of a mixture of various sizes of particles into two or  more portions by means of screening surface  

Synonymous terms: Sieving, Sifting, classifying or screening  

Separation is based on physical differences between the particles such as size, shape and density  Classification is applied for the separation of solids from the gases and solids from liquids  

Screening is used for the separation of solids from solids  

Particle size and its importance  

Why measure particle size of pharmaceuticals?  

Final formulation: performance, appearance, stability  

“Processability” of powder (API or excipient)  

Objectives of Size Separation  

To obtain the particles of the same size range or fractions of size ranges  

To obtain granules within narrow size range during granulation to avoid deviation in weight variation in tablets during  punching  

Applications  

1 – Useful in the production of tablets and capsules  

2 – Quality control tool for the analysis of raw materials  

3 – Test the efficiency of a size reduction equipment or process  

4 – Optimizing the processing conditions such as agitation, time of screening, feed rate  

5 – Recovering valuable products or by products  

6 – Prevention of environmental pollution  

Types of Separation  

Diffusional Separation  

  • Diffusional separation is a technique used for the separations of homogeneous mixtures  • This separation involves the transfer of the material between the phases  
  • Following are the well adopted methods for the diffusional separations. (Distillation, Crystallization, Absorption) 

Mechanical Separation  

  • Used for the separation of the heterogeneous mixtures  
  • Based on the physical differences between the particles such as size, shape or density  
  • It can be applied for separating solids from solids, solids from liquids and also solids from gases  • There are further two types of mechanical separations: Classification, – Screening  

General Terminologies  

Over size material  

Material that retain on the screening surface is called over size material.  

Under size material  

Material that passes through the screening surface is called under size material.  

Intermediate material  

When two screens are used for screening, the material that retain on the second screen is called the intermediate  material  

Unsized function  

A single screen can make a single separation into two fractions i.e. under size and over size. Such type of functions is  called the unsized function  

Sized function  

When the material is passed through the series of screens then it is divided into many fractions known as sized function 

Official Grades of Powder as per IP  

S. No.  Grade of Powder  Sieve through which all particles must  pass  Nominal mesh  aperture size  Sieve through which 40% particles must pass  Nominal mesh  aperture size 
Coarse Powder  10  1.7mm  44  355
Moderately Coarse Powder  22  710 µm  60  250
Moderately Fine powder  44  355 µm  85  180
Fine powder  85  180 µm
Very Fine powder  120  125 µm

 

Techniques of Size Separation  

Sieves and sieve shaker  

Sedimentation tanks  

Elutriation tanks  

Cyclone Separator  

Bag Filter  

Air Separator  

Sieves  

  • It is a device consisting of a wire or plastic mesh held in a frame, used for straining solids from liquids, for separating  coarser from finer particles, or for reducing soft solids to a pulp

sieves

Sieving is the most frequently used method for size separation  

Types of Sieves  

Woven wire sieves  

Bolting cloth sieves  

Closely spaced bars  

Punched plates  

Woven wire sieves  

General purpose sieves and widely used in pharmacy practice  Two types;  

– Plain weave  

– Twilled weave  

weave-sieve

Metal wire woven sieves are used for fine siev  

They are included in roller mill, ball mill  

Bolting cloth sieves  

Silk, nylon and cotton are woven from twisted multi-stand fibres nylon cloths are available in different grades  Used for separation of fine powder (Hum-mer screen) 

Bar Screens  

Used for handling large and heavy pieces of materials  

Bars are fixed in parallel position and held by cross bars and spacers (Grizzlies)  

Punched plates (Perforated screens)  

– Used for coarse sieving  

– Metal sheets of varying thickness with perforated holes are used  

– The holes may be round, oval, square or rectangular (hammer mill)  

Standard screens  

  1. Tyler standard sieve series (USA)  
  2. US standard sieve series (USA)  
  3. British standard sieve series (UK)  
  4. IP standard sieve series (India)  
  5. German DIN (Germany & Europe)  
  6. International test sieve series ISO (worldwide)  
  7. I.M.M.S “Institute of Mining and Metallurgy”  
  8. F.S.S “French standard sieves  

Sieve specifications  

Number of sieve  

Indicates the number of meshes per linear length of 25.4mm 

Nominal size of aperture  

Indicates the distance between the two adjacent wires (mm or µm) 

Nominal diameter of the wire  

Wire mesh sieves made from the wire having the specified diameter to give suitable aperture size and strength to avoid  distortion of the sieve  

Approximate percentage sieving area  

It expresses the area of the mesh as a percentage of the total area of the sieve. Generally it is kept within the range of  35 to 40% for the strength of the sieve  

Sieve Analysis  

Sieve analysis is one of the oldest methods of size analysis  

Sieve analysis is accomplished by passing a known weight of sample material successively through finer sieves and  weighing the amount collected on each sieve to determine the percentage weight in each size fraction  

Sieving is carried out with wet or dry materials and the sieves are usually agitated to expose all the particles to the  openings  

Sieve shaker machine  

Principle  

The powdered drug is separated according to its particle size using a number of sieves in a net. These are subjected to  different types of agitation for rapid size separation  

Construction  

Standard sieves of different mesh numbers that are commercially available as per the IP and USP specifications are fixed  in a mechanical shaker in an ascending order

Sieve-shaker-machine

The process of sieving may be divided into two stages.  

-First, the elimination of particles considerably smaller than the screen apertures, which should occur fairly rapidly  

-Second, the separation of the so- called “near-size” particles, which is a gradual process rarely reaching final  completion.  

-The effectiveness of a sieving test depends on the amount of material put on the sieve (the “charge”) and the type of  movement imparted to the sieve.  

Advantages  

– “Simple” and intuitive  

– inexpensive  

– Works well for larger particles  

Disadvantages  

– Can break up weak agglomerates (granulates)  

– Does not give shape information  

– Need substantial amount of material  

– Needs calibration now and then  

– Clogging of aperture if the powder is wet  

Practical Considerations  

  • Type of motion 

Vibratory motion is most efficient followed by side tap motion, bottom pat motion, rotary motion with tap and finally  rotary motion  

  • Weight of the sample  
  • Duration of shaking  

Sedimentation Tank

Sedimentation methods are based on the measurement of the rate of settling of the powder particles uniformly dispersed in a fluid and the principle is well illustrated by the common laboratory method of “beaker decantation”.  

The material under test is uniformly dispersed in low concentration in water contained in a beaker. A wetting agent may need to be added to ensure complete dispersion of the particles.  

A syphon tube is immersed into the water to a depth of ‘h’ below the water level, corresponding to about 90% of the liquid depth ‘L’.  

The terminal velocity ‘v’ is calculated from Stokes’ law for the various sizes of particle in the material, say 35, 25, 15, and  10 μm.

The time required for a 10 μm particle to settle from the water level to the bottom of the syphon tube, distance ‘h’,  is calculated (t = h/v)  

The pulp is gently stirred to disperse the particles through the whole volume of water and then it is allowed to  stand for the calculated time  

The water above the end of the tube is syphoned off and all particles in this water are assumed to be smaller than 10 μm  diameter.  

Andreasen Pipette Technique  

A much quicker and less-tedious method of sedimentation analysis is the Andreasen Pipette Technique  

The method is much quicker than beaker decantation, as samples are taken off successively throughout the test  for increasingly finer particle sizes

Andreasen-Pipette-Technique

Elutriation Tanks  

Elutriation is a process of sizing particles by means of an upward current of fluid, usually water or air.  

Separation of particles in a powder based on the low density of fine particles and high density of the coarse particles  using fluid flow  

The process is the reverse of gravity sedimentation, and Stokes’ law applies.  

All elutriators consist of one or more “sorting columns“ in which the fluid is rising at a constant velocity. 

Elutriation-Tanks

  • In case of sedimentation, particles move in the direction of gravitational force  
  • In case of elutriation, some particles move against the gravitational force  
  • The arrows are vectors showing direction and magnitude of particle movement  
  • In sedimentation, fluid is stationary and particle settle based on the particle velocity  
  • In elutriation, fluid flows in opposite direction of the settling movement  

Sedimentation-and-Elutriation  

Sedimentation                Elutriation  

Uses  

Applicable to insoluble solids such as kaolin or chalk, which are subjected to wet grinding followed by sedimentation or  elutriation  

Advantages  

– It is a continuous process  

– Quick separation of size fractions  

– More compact apparatus compared to sedimentation tank  

– More number of tubes of different area of cross section can be connected  

Disadvantages  

The suspension or solids should be diluted, which may be not desirable in certain cases  

Static tank method  

  • The dry powder or paste made by levigation is placed in the tank  
  • It is mixed with large quantity of water  
  • The solid particles are distributed uniformly in the liquid during stirring  
  • Particles are allowed to settle  
  • Depending on the density, the solid particles may either settle down or remain suspended in water  • Sample are withdrawn at different heights through the outlets  
  • The powders are dried and various size fractions are collected

Static-tank-method

  • Single separation with mobile liquid  
  • Separation as multiple fractions  

Cyclone Separator

Principle 

  • Centrifugal force is used to separate the solids from fluids. The separation depends not only on the particle size  but also on the density of particles.  
  • Depending on the fluid velocity, this can be used to separate all types of particles. Even fine particles can be carried by  the fluid

Cyclone-Separator

Construction  

  • It consists of a short vertical, cylindrical vessel with a conical base  
  • The upper part of the vessel is filled with a tangential inlet  
  • The solid outlet is arranged at the base  
  • Fluid outlet is provided at the center of the top portion, which extends inwardly into the separator • This prevents the air short circuiting directly form the inlet to the outlet of the fluid  

Working  

  • The solids to be separated are suspended in a stream of gas (air)  
  • The above feed is introduced tangentially at a very high velocity, so that energy movement takes place within the  vessel  
  • The centrifugal force and vortexing throws the solids to the walls  
  • As the speed of air diminishes, the particles fall to the conical base and are discharged through the solid outlet  • The fluid can escape from the central outlet at the top  

Uses  

Used to separate the solids from gases  

Used for size separation of solids in liquids  

Used for separating the heavy to coarse fraction from fine dust  

Bag Filter  

Principle  

Size separation of fines or dust from the milled powder is achieved in two steps:  

The milled powder is passed through a bag filter by applying the suction on the opposite side of the feed entry which  facilitates separation  

Pressure is applied in order to shake the bags so that powder adhering to the bag falls off, which is collected from the conical base.

Bag-Filter

Construction  

It consists of a number of bags made of cotton or wool fabric which are suspended in a sheet metal container  A hopper is arranged at the bottom of the filter to receive the feed  

At the top of the metal container, a provision is made for the exhaust  

Adjacent to this, a bell crank lever arrangement is made to bring the filters to normal atmospheric conditions  

Step  Mechanism  Movement of damper  Contact between bags and  suction  Movement of damper  Contact between bags and  atmosphere 
Filtering period  Cam does not press the bell crank lever  Opens  Closes
Shaking period  Cam presses the bell crank lever  Closes  Opens

 

These changes occur at intervals of a few minutes  

Working  

Consists of two steps: 

Firstly, the feed is separated from air by passing it through the cloth bags  

Secondly, the bags are shaken to collect the fines that are adhered to the bags  

These two steps follow in succession and are controlled at different intervals with the help of a bell crank lever  arrangement  

Bell crank lever arrangement  

A shaft with a cam is allowed to rotate at a low speed.  

During rotation, the cam can either press the bell crank lever or does not come into contact.  

Depending on this mechanism, the damper changes its position. The damper is useful mechanism, which allows two  steps to occur  

Filtering period  

  • The exhaust positioned at the top keeps the bags under less pressure than atmospheric pressure 
  • The gas containing fine particles enters the hoper  
  • The gas feed passes through the fabric of bag  
  • During this, the fines are retained in the bags, while the gas reaches the top of the casing 

Shaking period  

  • As vacuum is cut off in the chamber, air from outside enters the casing and passes through the bags  
  • This results in violent shaking of the bags, so that dust and fine particle are displaced from the bags and falls into  the conical base.  
  • It is then removed at intervals  

Uses  

  • Used along with other size separation equipment  

e.g., cyclone separator  

Bag filter is connected to the discharge end of the fluidized energy mill  

Advantages  

  • It is extremely useful for removing fines, which cannot be separated by other methods  
  • It can be used to remove dust (ordinary household vacuum cleaner)  

Disadvantages  

  • It is not a size separation equipment as such  

Air Separator  

Principle  

  • Centrifugal force is used to separate solids rotating disc and blades  
  • Stationary blades are used to improve separation  
  • By controlling these blades and the speed of the rotation, it is possible to vary the size at which separation occurs 

Construction  

  • Consists of a cylindrical vessel with a conical base  
  • The feed inlet is fitted to upper part of the vessel  
  • The rotating disc and rotating blades are attached to the central shaft to produce air environment 
  • At the base of the vessel, two outlets are provided; one for fine particles and the other for heavy particles

Air-Separator

Working  

  • The disc and blades are allowed to rotate by means of a motor. These produce a current of air. 
  • The sample powder is introduced through the feed inlet which falls on the rotating disc  
  • The fine particles picked up and carried into space, where air velocity is sufficiently reduced  
  • The fine particles are dropped and are ultimately collected at the outlet meant for fine particles. 
  • The heavy particles which fall downward are removed at the outlet meant for heavy particles  

Uses  

Often attached to ball mill or hammer mill  

Summary  

  • Size separation is an unit operation that involves the separation of a mixture of various sizes of particles into two or  more portions by means of screening surface  
  • Types of separation includes diffusional and mechanical separation  
  • Sieve number can be defined as the number of meshes present in a linear length of 2.54 cm  
  • Centrifugal force is used to separate the solids from fluids. The separation depends not only on the particle size but  also on the density of particles  
  • Bag filters are used for removing fines, which cannot be separated by other methods  
  • In sedimentation technique, the particles move towards the gravitational force and vice-versa in Elutriation technique

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