Drying is a mass transfer
process in which water or another solvent is removed by evaporation from a
solid, semi-solid, or liquid. Drying is often a final step in the production or
packaging of pharmaceutical products. For material to be considered as
"dried", the final product must be solid, in the form of a continuous
sheet, long pieces, particles, or powder. The drying process involves a source
of heat and a facility to remove the produced vapors. In the majority of
pharmaceutical intermediates or finished products the solvent removed is water.
Desiccation may be synonymous with drying or considered an extreme form of
drying. In the pharmaceutical industry, final product quality is never be
compromised. The deterioration of the product may be due to microbial
infection, oxidation, thermal decomposition, contamination by metallic
particles, or the presence of an organic solvent. These solvents need to be
removed at any cost.
The materials used in the
construction of dryers should be non-contaminating and should be like polished
stainless steel or enameled iron. Closed dryers are often useful when moisture
removed is organic solvent or their mixture. The oxidative decomposition is
prevented by performing drying in inert gas. Thermal decomposition can be
reduced by drying by vacuum and freeze-drying methods. All these requirements
make dryers for pharmaceuticals use the most expensive.
A variety of drugs are
produced in pharmaceutical companies worldwide in many different forms and for
that dryers need to operate in batch and continuous mode. The manufacturing of
drugs in solid forms such as tablets, capsules, etc. is carried out in three
subsequent stages namely synthesis of intermediate products, final synthesis of
the drug, and manufacture of the dosage form. After each stage, the products
are dried. The selection of a proper dryer for these products depends on the
properties of the materials. Adjustment and control of moisture levels in solid
materials through drying is a critical process in the manufacture of products. The
drying of solid materials is one of the most common and important unit
operations in the pharmaceutical industries, where powders and granules are handled
and manufactured.
The effectiveness of
drying processes has a large impact on product quality and process efficiency.
For example, in the batch processing of pharmaceuticals drying is a key
manufacturing step. The drying process can impact subsequent manufacturing
steps such as tableting or encapsulation and can affect critical quality
attributes of the finished products. Apart from drying of solids for a
subsequent operation, it may be carried out to improve handling characteristics
such as bulk powder filling and powder flow.
Objectives of Drying
The drying unit operation
is used extensively in the pharmaceutical industry, but often a lack of
understanding of the impact of the presence of moisture, environmental
conditions, and drying process parameters on active pharmaceutical ingredients
critical quality attributes can create challenges during product development,
manufacturing, storage, and use. Thus, the following are the objectives of
drying:
- To overcome common challenges in pharmaceutical drying development, including material constraints for scale-up studies and transferring to different equipment types and sizes.
- To understand drying development related to chemical and physical stability, drying kinetics, and powder properties and highlights common development gaps for improving drying development workflows within the industry.
- To encourage further fundamental research and technological advancements for improving the drying process.
- Other objectives are to carry outsize a reduction, to avoid deterioration on storage, to dry the tablet granules to reduce the moisture, to reduce the bulk and weight to lower transportation charges, and for certain preparations such as spray-dried lactose.
- To design and produce a dryer that conserves energy consumption for optimal utilization in terms of acquisition and operating cost and with optimal local content and versatility.
- To understand the impact of factors and establish the product specifications, as well as the nature and limits of residual solvents, in agreement with current regulations.
Applications of Drying
In the manufacturing of
pharmaceuticals the last stage of processing is drying which is carried out for
one or more of the following applications:
- Drying is used to remove excess moisture or other volatiles from coatings and various substrates.
- It is used to reduce and control moisture levels in solid materials in the manufacture of many materials.
- It is most important in the processing of highly thermolabile products which are not stable in liquid form. The lyophilization enables longer shelf life of thermolabile materials and makes them suitable for storage and transport of the product. For example, drying of biological products such as blood plasma, vaccines, enzymes, microbiological cultures, hormones, and antibiotics.
- Drying is used to make the material easy or more suitable for handling and processing. In the manufacturing of bulk drugs or for large-scale production of synthetic drugs, drying is essential to get free-flowing materials. For example, dried aluminum hydroxide, spray-dried lactose, etc.
- It has applications in avoiding or eliminating moisture that initiates corrosion and decreases the product or drug stability. For example, to avoid deterioration or contamination of crude drugs of animal and vegetable origin, synthetic and semi-synthetic drugs.
- It is used to maintain and improve good properties such as flowability, compressibility, etc. of materials. For example, drying fresh plants such as belladonna leaves, nux vomica before subjecting them to size reduction.
- It is used in the production of tablets and granules to improve tablet properties especially, compression of viscous and sticky material.
- Drying is used to improve the solubility of materials by modifying their physical form. For example, milk and coffee extract is dried to convert them into instant soluble powder form.
- Drying is necessary to make material light in weight that helps to reduce the cost of transportation of large volume materials (liquids).
- Drying is used as the final step in evaporation, filtration, and crystallization and to preserve materials from environmental factors.
- Drying is used to maintain and improve the shelf life of thermolabile and hydrolytic substances for a longer period. It is necessary to avoid deterioration of blood products, skin, and tissue that undergo microbial decomposition.
- Drying significantly decreases the rate of chemical reactions as well as chances of microbial attack or enzymatic actions and thus improves stability.
Mechanism of Drying Process
The process of drying
does not mean only removal of the moisture but the physical structure and the
appearance of material have to be preserved. Drying is governed by the
principles of heat and mass transfer. When a moist solid is heated to an
appropriate temperature, moisture vaporizes at or near the solid surface. The
heat required for evaporating moisture from the drying product is supplied by
hot air or gas. Drying involves diffusion in which the transfer of moisture to
the surrounding medium takes place by evaporation from the surface. As some of
the moisture from the surface vaporizes more moisture is transported from the bulk
of the solid to its surface. This movement by diffusion of moisture in a solid
takes place by various mechanisms depending upon the nature and type of the
solid and its state of aggregation. A wide variety of solids are handled for
drying such as crystalline, granular, beads, powders, sheets, slabs,
filter-cakes, etc. The mechanism involved in moisture transport in those solids
is classified as:
- Transport by liquid or vapors diffusion.
- Capillary action.
- Pressure-induced transport.
A specific mechanism that
involves drying a specific solid depends on its nature, pore structure, and rate
of drying. More than one mechanism may come into play and dominate at different
stages of drying of the same material.
There are various common
terms used in designing drying systems. The moisture content of a substance that
exerts as equilibrium vapors pressure less than of the pure liquid at the same
temperature is referred to as bound moisture. The moisture content of the solid
which exerts an equilibrium vapor pressure equal to that of pure liquid at the
given temperature is the unbound moisture.
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