PHR311Pharmaceutical Engineering-I

MIXING-IZaki Farhad Habib, MSc

With much appreciated help from Professor Sohel Rana 1

Pharmaceutical MixingMixing is probably the most widely performed unit operation in pharmaceutical manufacturing, in fact, it is difficult to find a product where mixing is not involved in some stage of process.

Mixing may be define as an unit operation in which two or more components in a separate or roughly mixed condition are treated so that each particle lies as nearly as possible in contact with a particle of each of the other components.If this can be achieved it produces a theoretical ideal situation i.e. a perfect mix.

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Objective of mixing

The objective of mixing may be broadly classified as follows:

1. Simple physical mixture:

This may be simply the production of a blend of two or more miscible liquids or two or more uniformly divided solids.

In pharmaceutical practice, the degree of mixing must commonly be of a high order, as many such mixtures are dilutions of a potent substance so consistent dosage must be ensured.

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2. Physical change

Mixing may aim at producing a change that is physical as distinct from chemical, for example the solution of a soluble substance.

In such cases, a lower efficiency of mixing will often be acceptable because the mixing merely accelerates a process that could occur by diffusion, without agitation.

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3. Dispersion This includes the dispersion of two immiscible liquids to form an emulsion or the dispersion of a solid in a liquid to give a suspension or paste. Usually good mixing is required to ensure stability.

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4. Promotion of reaction

Mixing will usually encourage ( and control at the same time ) a chemical reaction, so ensuring uniform products.

E.g. Products or process where accurate adjustment to pH is required and the degree of mixing will depend on the process.

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Types of mixtures

Mixtures may be categorized into three types that differ fundamentally in their behavior:

1.Positive mixtures:Positive mixtures are formed from materials such as gases or miscible liquids which mix spontaneouslyand irreversibly by diffusion without the expenditure of work/energy provided time is unlimited.

In general such materials do not present any problems in mixing.

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2.Negative mixtures

With negative mixtures, the components tend to separate out. If this occurs quickly, then energy must be continuously input to keep the components adequately dispersed.

Suspensions of solids in liquids are examples of negative mixtures that require work for their formulation and the components of which separate unless work is continually expended on them.

Negative mixtures are more difficult to form and a higher degree of mixing efficiency is required.

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3.Neutral mixtures

Neutral mixtures are static in their behavior, the components having no tendency to mix spontaneously nor do they segregate when mixed. Examples: pastes, ointments and mixed powders.

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SOLID-SOLID MIXING

INTRODUCTION:

Solid-solid mixing is a unit operation in particulate processes where consistency and homogeneity of the product is a key requirement.

Mixtures are seen in all different phase of processing, ranging from the mixture of raw materials to the final formulation of product.

The quality of mixing is often key to product performance

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Fundamentals and Importance of solid-

solid mixing:Solid-solid mixing is essential to many industries

including ceramics, chemicals, food, cosmetics, coal, plasticsand pharmaceuticals.

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Mixing Mechanism

It has been generally accepted that solids mixing proceeds by a combination of one or more mechanisms. Some mechanisms are as follows:

Convective Mixing

Shear Mixing

Diffusive Mixing

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particles smaller than 100 m are generally cohesive while larger than that are generally free-flowing

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Selection of machine:

It is very important task for a pharmaceutical engineer to choose the appropriate machine for the best production. The selection of machine may be different depending upon the various aspects.

For example, If the mixing ingredients are in sticky or plastic state,

the ribbon blender machine will be the best solution. While, free flowing powder which requires low shearing

force may be mixed by the double cone blender. It is also suitable for the large volume of production and uniform mixing.

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The zigzag blender, an extension of twin shell blender provides efficient continuous precision blending.

Some machines are capable of mixing in a large range i.e. 50g (laboratory purpose) to 100kg (industrial purpose) e.g. double cone blender.

Twin shell blender is also time saving machine.

In another aspect, among all the machines, double cone machines are easy for cleaning, charging and discharging.

So above all we can say that individually all machines are useful for the mixing of solids but the selection of machine should be dependent upon the procedure, properties of the solids,

economic factors etc.

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THANK YOU

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