dept. of pharmacy ousl - open university of sri lanka school 8 - solid solid... · •a ribbon...

Dept. of Pharmacy

OUSL

OUTLINE

Solid- solid mixing

Mixing mechanisms

Mixing equipment for solids

Mixing of solids and liquid

Factors that affecting selection of suitable mixer

Homogenizing

Solid- solid mixing• There are considerable differences between fluids and

solid mixings.

• Undergo substantial segregation during the routine handling following the mixing.

• Some factors affecting solid- solid mixing are,

– Particle size and particle size distribution -can affect inter particulate movement.

– Particle density, elasticity, surface roughness and shape.

– Efficiency of the mixer.

– Tendency of the materials to aggregate

– Moisture content, surface characteristics and flow characteristics of each component .

Solid- solid mixing contd...

• Generally, materials similar in size, shape, and density are able to form the most uniform mixtures.

• Means of overcoming segregation and poor mixing include:

– Comminution to smaller sizes

– Use of powders with a narrow size distribution

– Use of the same volume-average diameter for all components

– Granulation

– Coating processes

– Controlled continuous mixing

Solid- solid mixing contd...

• Mixers can be classed into two groups with respect to segregation:

– Segregating mixers have mainly diffusive mechanisms, encouraging the movement of individual particles, making segregation more significant. Non-impeller type mixers tend to be of this type.

– Less segregating mixers have mainly convective mixing mechanisms. These are typically impeller types in which blades, screws, ploughs, etc., sweep groups of particles through the mixing zone.

Mixing mechanisms

(1) Convective mixing• Depending on the type of mixer employed

convective mixing can occur by an inversion ofpowder bed by means of blades or paddles, bymeans of revolving screw or by any other methodof moving a relatively large mass of material fromone part of the powder bed to another.

• Mixing groups of particles that are adjacent toeach other are transferred from one location in themass to another.

• This type of mixing predominate in mixers in whichmixing the element moves in a stationarycontainer.

Mixing mechanisms contd…

(2) Shear mixing• As a result of forces within the particulate mass,

slip planes are set up. • Depending on the flow characteristics of powder,

these can occur singly or in such a way as to give rise to laminar flow.

• When shear occurs between regions of different composition and parallel to their interface, it reduces the scale of segregation by thinning the dissimilar layers.

• Shear occurring in a direction normal to the interface of such layers is also effective since it too reduces the scale of segregation.

Mixing mechanisms contd…

(3)Diffusive mixing• Occurs when random motion of particles within a

powder bed causes them to change position relative to one another.

• Such an exchange of positions by single particles results in a reduction of intensity of segregation.

• Occurs at the interfaces of dissimilar regions that are undergoing shear and therefore results from shear mixing.

• It may also be produced by any form of agitation that results in inter particulate motion.

• Predominate in triangular mixers when material lifted pass its angle of repose and stumble.

• Generally mixing containers move or rotate to facilitate mixing.

Mixing equipment for solids

(1) Agitator mixer• The principle of operations is that shear force is applied

to all particles by means of agitating force.

• There are two commonly used agitator types.

– Mechanical agitator - Usually consist of a trough in which one arm rotates .It transmits shear acting on particles. Helical blades are attached to the agitator.

– Air movement agitator - Materials are placed in a vertical cylindrical vessel. Intermittent clusters of air are admitted at the base of the cylinder. The inlets of air are arranged in an angle so that particles could achieve spiral movement.

Mixing equipment for solids contd...

(A) Ribbon/Paddle Blenders

• Consist of a U-shaped horizontal trough and a specially fabricated ribbon agitator.

• A ribbon agitator consists of a set of inner and outer helical agitators.

• The outer ribbon moves materials in one direction and the inner ribbon moves the materials in the opposite direction.

• Particle damage can occur due to the small clearance between the ribbon and the vessel wall and the mixer has a high power requirement.

• Segregation is less of a problem


Paddle or Ribbon blander


(B) Vertical screw mixers

• A rotating screw located in a cylindrical or cone shaped vessel.

• The screw may be fixed centrally or may rotate around the vessel near the wall.

• Quick, efficient and good for mixing a small quantity into a larger one.

• Good for materials prone to segregation .

Vertical screw blender



(2) Tumbler mixer• The principle of operations is that rotation of the vessel imparts

movement to material by tilting the powder where its angle of repose is exceeded and powder starts flowing.

• Rotates on its horizontal axis. • Speed of rotation of the mixer is important as the slow speed gives

rise to slipping/sliding only and the fast speed gives rise to centrifuging only.

Limitations of tumbler mixer• Most of the movement between particles is in one plane only.• No shear forces are introduced along the axis and no ended to end

mixing.• It is advisable to remove the contents in intervals of time and

shake by hands.• Movement of the particles take place only in the upper layer

where slip planes are produced under the gravitational forces.


These mixers are modified to over come some of these weaknesses. This is done by,• Introducing baffles.• Altering the shape of the container to avoid particle movement in one

plane.e.g.: Cone mixer, double cone mixer

• Revolving the container with its axis at an angle to the horizontal plane.e.g.: Ball mill

• Revolving the container at an axis which is at an angle to an axis of symmetry,e.g.: Cube mixer

Uses of tumbler mixer• Mostly used in pharmaceutical industry for mixing powders. Double

cone blender, drum mixer and “v” blender are the most commonly used tumbler blenders.


(A) Double cone blender

• Higher degree of efficiency.

• Used for mixing dry powder and granules homogeneously.

• Only two third of the container is filled to achieve proper mixing.

• Widely used in the pharmaceutical industry to granulate and blend medicated powders.

• The conical shape of both ends enables uniform mixing and easy discharge.


Double cone blender


(B) V blender

• The typical 'V' shaped allows for higher shear of mixing without attrition and particle regeneration, leading to shorter mixing/ blending times.

• Ideal for fragile granules where the percentages of fines needs to be tightly controlled.

• Cylindrical 'V' curves allow easy access to the internal areas for better cleaning.


V -blender


(C) Cage blender and drum mixer

• A functionally similar Tumble Blender to the Double Cone and 'V' Blenders.

• The special degree of mounting of the Cube allows for shorter operational durations as the sides and corners of the cube act as baffles and disturb the internal flow of materials.

• Drum mixer is used when contents in a drum has to be mixed.

• Mostly used for small quantities.


Cage blender

Mixing of solids and liquid

• The flow properties of the mixer containing solid and liquid can change markedly when ratio of 02 phases is changed.

• When the concentration of solid dispersed phase is low the flow properties are Newtonian.

• In that case mixing by an impeller is sufficient.

• When the solid dispersed phase concentration is high non Newtonian flow properties are produced.

• The occurring viscosity is a function of rate of shear.

• Mixing could be done by using an impeller provided that there is sufficient flow patterns in the whole mixing container.

• Turbine mixer could be used for this purpose.

Mixing of solids and liquid contd...

• Where viscosity is very high mixing by impeller is not possible.• For this purpose mixer is needed with mixing element visits all parts of

the mixer containing vessel. • Also thicker paste needs folding, starching and pressing movements for

satisfactory mixing. • In large scale mixing planetary mixer could be used while for small scale

products domestic cake mixer is sufficient. • The paddle is of anchor type and rotates in a cylindrical pan. • The base of the pan is semi spherical. • The paddle rotates on its own axis which moves around in a circle by

eccentrically. • Rotation of the paddles causes all parts of the mixer to be sheared at

intervals. • Very high shear rates are produced as the paddle scrapes the mixer

from the walls of the pan. • This mixer is moved the centre of the pan and simultaneously it is

folded in the mixer that is already present in the pan centre.

Factors that affecting selection of suitable mixer

a) Available floor spaceb) Mechanism of blendingc) Discharge of content - The cone screw blender

and tumble blender give virtually 100% discharge.d) Lumping problems - All blenders can be installed

with lump breakers or choppersf) Heat sensitive products - The blending action of a

ribbon can generate slightly more heat than that of a mixing screw or tumbling blender. In a cone screw blender, heat is easily removed with thermal jacketing.

Factors that affecting selection of suitable mixer contd…

h) Flexibility to operate with smaller batches -cone screw blender or tumble blender can operate efficiently with batches as small as 10% of the rated capacity. The ribbon blender normally requires a minimum of 40-50% capacity.

i) Risk of product contamination - ribbon blender always poses a threat of contamination and new seal designs have reduced the risk dramatically, cone screw blender has less risk

Factors that affecting selection of suitable mixer contd…

j) Cost - the ribbon blender is typically more cost-effective.

k) Blending accuracy - The ribbon blender and cone screw blender both produce a well-blended product.

l) Cleaning between batches - The cone screw blender is easier to clean, especially since you will never have to disassemble a packing gland or seal in the product zone.

Homogenizing

• Homogenize - to make or render homogeneous

• Homogeneous - having the same composition, structure, or character throughout

• The current processes or methods of homogenizing can be broken down into three (3) major categories

– Ultrasonic

– Pressure

– Mechanical

Homogenizing contd…

(i) Ultrasonic Homogenizing /homogenizers

• One widely used method to disrupt cells is ultrasonic disruption.

• These devices work by generating intense sonic pressure waves in a liquid media.

• The pressure waves cause streaming in the liquid and, under the right conditions, rapid formation of micro-bubbles which grow and coalesce until they reach their resonant size, vibrate violently, and eventually collapse. This phenomenon is called cavitation.

• The implosion of the vapor phase bubbles generates a shock wave with sufficient energy to break covalent bonds.

• Shear from the imploding cavitation bubbles as well as from eddying induced by the vibrating sonic transducer disrupt cells.


Ultrasonic Homogenizing contd…

• There are several external variables which must be optimized to achieve efficient cell disruption.

• These variables are as follows:

– Tip amplitude and intensity

– Temperature

– Cell concentration

– Pressure

– Vessel capacity and shape

• Ideally, the temperature of the ultrasonicated fluid should be kept just above its freezing point.

• The ultrasonic disintegrator generates considerable heat during processing and this complicates matters


(ii) Pressure homogenizers

• High-pressure homogenizers have been used to disrupt microbial cells and found to be generally suitable for a variety of bacteria, yeast and mycelia.

• This type of homogenizer works by forcing cell suspensions through a very narrow channel or orifice under pressure.

• Subsequently, and depending on the type of high-pressure homogenizer, they may or may not impinge at high velocity on a hard-impact ring or against another high-velocity stream of cells coming from the opposite direction.


Pressure homogenizers contd…

• The operating parameters which effect the efficiency of high-pressure homogenizers are as follows:– Pressure – Temperature – Number of passes – Valve and impingement design – Flow rate

• High-pressure homogenizers have long been the best available means to mechanically disrupt nonfilamentous microorganisms on a large scale.

• Animal tissue also can be processed but the tissue must be pretreated with a blade blender, rotor-stator homogenizer, or paddle blender.


(iii) Mechanical homogenizers

• Two types,a) Rotor-stator homogenizers

b) Blade type homogenizers.

a) Rotor-stator homogenizers

• Rotor-stator homogenizers (also called colloid mills or Willemshomogenizers) generally outperform cutting blade-type blenders and are well suited for plant and animal tissue.

• Combined with glass beads, this has been successfully used to disrupt microorganisms.

• Cell disruption with the rotor-stator homogenizer involves hydraulic and mechanical shear as well as cavitations.


• The variables to be optimized for maximum efficiency are as follows:– Design and size of rotor-stator (generator)

– Rotor tip speed

– Initial size of sample

– Viscosity of medium

– Time of processing or flow rate

– Volume of medium and concentration of sample

– Shape of vessel and positioning of rotor-stator


b) Blade type homogenizers

• Although less efficient than rotor-stator homogenizers, blade homogenizers (also called blenders) have been used for many years.

• The cutting blades on this class of homogenizer are either bottom or top driven and rotate at speeds of 6,000 to 50,000 rpm.


Blade type homogenizer

dept. of pharmacy ousl - open university of sri lanka school 8 - solid solid... · •a ribbon...

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