Industrial PharmacyLecture 3

Dr. Myasar Alkotaji

Continuous Mixing:The process of continuous mixing produces an

uninterrupted supply of freshly mixed material and is often desirable when very large volumes of material are to be handled. It can be accomplished essentially in two ways:

1-In a tube or pipe through which the material flows & in which there is very little back flow or recirculation.

2- In a chamber in which a considerable amount of holdup & recirculation occur.

To ensure good mixing efficiency, such devices as vanes, baffles, screws, grids or combinations of these are placed in the mixing tube and mixing takes place mainly through bulk transport. In tube the materials flow in one direction. Mixing in such systems requires the careful control of the feed rate of raw materials if a mixture of uniform composition is to be obtained.

The materials to be mixed are continuously charged into the mixer per formulation .

Radial and axial mixing takes place as the material travels from the feeding point to the discharge point .

The time taken by the material to travel from the feeding point to the discharge point is known as the retention time of the

material in the mixer .

Unlike batch mixers where product retention time is carefully controlled, with continuous mixers, material retention time is not uniform and can be directly affected by mixer speed, feed rate, mixer geometry, and the design of mixer internals. Material is continuously discharged at a constant rate which is generally termed as the capacity of the continuous mixer.

This capacity is measured in kg/hr of mixed product .

Advantages of Continuous Mixing

– High Capacity - Compared to batch type mixers, continuous mixers of smaller volumes and power can be used to produce large quantities of uniform mix. Hence for a given capacity they are more compact than batch mixers.

– Lower Mixing Time - The residence time in continuous mixers is lower than in batch mixers.

– Consistent Mixing Performance – With proper feeding arrangements, online instrumentation and operation controls, a consistent mixing performance and uniform product quality can be achieved.

– Suitability for Automatic Control - Operation of continuous mixers can be automated using online monitoring and measuring instruments.

– Minimum Segregation – Continuous mixers can

reduce and control segregation of products as they can be located in proximity of the next processing station.

– Lower Cost of Mixers- Continuous mixers tend to be cheaper than the equivalent batch mixers because they are compact and require less space. However the cost of feeders for metering the product into the mixer, instrumentation and control may result in a higher overall cost of the system.

– Minimum Labor – Since material feeding and discharging processes are automated, minimal labor is required for continuous mixing.

Disadvantages and Limitations of Continuous Mixing– Lack of Flexibility – Continuous mixing systems are designed

for a particular application and cannot be easily tailored to mix different formulations. Even if a new ingredient is to be introduced, it calls for a change in the protocol, and the system has to be recalibrated.

– Component Limitations – When a large number of ingredients are to be added, continuous mixers have limitations with respect to mixing uniformity when compared to batch mixers.

– Higher Overall Maintenance Cost – Continuous mixers heavily depend on feeders, instrumentation and online control systems. Failure, malfunction in any one component can lead to complete stoppage. Hence, overall maintenance costs for continuous mixers are higher compared to batch mixers.

– Calibration and Checking – The feeding devices in a continuous mixing require careful calibration and frequent checking for accuracy.

– Critical Applications – Continuous mixers are not suited for critical applications where product formulations need to be exact. Batch mixers are better suited to processes that require a very tight product formulation, uniform composition.

Continuous mixers are generally dedicated to a single high volume product. Continuous mixers can be designed for capacities as high as 500 Tons/hour. Even though continuous mixing is gaining popularity, selection of continuous mixers is much more complicated than batch mixers.

Mixer Selection Equipment SelectionOne of the first and most often important

considerations in any mixing problem is equipment selection. Factors that must be taken into consideration include:

1- The physical properties of the materials to be mixed such as density, viscosity and miscibility.

2- Economic considerations regarding processing, e.g., time required for mixing and the power expenditure necessary.

3- Cost of equipment and its maintenance.

Monophase Systems• The viscous character and density of the fluid(s) to be mixed

determine to a large extent the type of flow that can be produced and also, therefore, the nature of the mixing mechanisms involved. Fluids of relatively low viscosity are best mixed by methods that generate a high degree of turbulence and at the same time circulate the entire mass of material.

• So these requirements are satisfied by air jets, fluid jets, and the various high-speed impellers.

• Thick creams, ointments, and pastes are of such high viscosity that it is difficult if not impossible to generate turbulence within their bulk and laminar mixing, and molecular diffusion must be relied upon.

• Mixing of such fluids may be done with a turbine of flat blade design.

Polyphase SystemsThe mixing of two immiscible liquids requires the subdivision of

one of the phases into globules, which are then distributed throughout the bulk of the fluid. The process usually occurs by stages during which the large globules are successively broken down into smaller ones by two forces:

1- The interfacial tension of the globules in the surrounding liquids2- Forces of shear within the fluid mass that act to distort and

ultimately disrupt the globules. The relationship between these forces largely determines the

final size distribution in the mixture. Low- viscosity systems, high shear rates are required and are

commonly produced by passing the fluid under high pressure through small orifices or by bringing it into contact with rapidly moving surfaces.

Highly viscous fluids, such as are encountered in the production of ointments, are efficiently dispersed by shearing action of two surfaces in close proximity and moving at different velocities with respect to each other. This is

achieved in paddle mixers .Such mixers are relatively efficient since they not only generate sufficient shear to reduce globule size but if properly constructed, also induce sufficient circulation of material to ensure a uniform dispersion throughout the completed mixture.

The mixing of finely divided solids with a liquid of low viscosity in the production of suspension is often carried out in a single mixing operation, providing that shear forces of sufficient intensity to disrupt aggregates. High-speed turbines, frequently fitted with stators to produce increased shearing action, are often employed.

As the percentage of solids is increased or if highly viscous fluids are employed, the solid liquid system takes on the consistency of a paste or dough. In these cases, the forces required to induce shear are considerable, and equipment used is of heavy design. The choice of a mixer is limited to those that rub the material.

A sigma- blade mixer with overlapping blades is an example of such mixer used for this purpose.

Roller mills consisting of one or more rollers are in common use. Of these, the three-roll type seems to be preferred. A material coming between the rollers is crushed, depending on the gap, and is also sheared by the difference in rates of movement of the two surfaces.