What is drying?


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,s 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 compromised. The deterioration of the product may be due to microbial infection, oxidation, and 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 an 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 at 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. Drying of solid materials is one of the most common and important unit operations in the pharmaceutical industries, where powders and granules 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 attributeThis can create challenges during product development, manufacturing, storage, and use. Thus, following are the objectives of drying :
(i) To overcome common challenges in pharmaceutical drying development, including material constraints for scale-up studies and transferring to different equipment types and sizes.
(ii) 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.
(iii) To encourage further fundamental research and technological advancements for improving the drying process.
(iv) Other objectives are to carry outsize 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.
(v) 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.
(vi) 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.


In the manufacturing of pharmaceuticals, the last stage of processing is drying, which is carried out for one or more of the following applications:

  1. Drying is used to remove excess moisture or other volatiles from coatings and various substrates.
  2. It is used to reduce and control moisture levels in solid materials in the manufacture of many materials.
  3. 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 make 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
  4. 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.
  5. 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.
  6. It is used to maintain and improve good properties such as flowability, compressibility etc. of a materials.
    For example, drying fresh plants such as belladonna leaves, nux vomica before subjecting them to size reduction.
  7. It is used in the production of tablets and granules to improve tablet properties
    especially, compression of viscous and sticky material.
  8. Drying is used to improve solubility of materials by modifying their physical form. For example, milk and coffee extract is dried to convert them into instant soluble power form.
  9. Drying is necessary to make material light in weight that help to reduce the cost of transportation of large volume materials (liquids).
  10. Drying is used as the final step in evaporation, filtration, and crystallization and to preserve materials from environmental factors.
  11. Drying is used to maintain and improve shelf life of thermolabile and hydrolytic substances for longer period of time. It is necessary to avoid deterioration of blood products, skin and tissue that undergo microbial decomposition.
  12. Drying significantly decreases 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 has 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 a gas. Drying involves diffusion in which the transfer of moisture to the surrounding medium takes place by the evaporation from the surface. As some of the moisture from the surface vaporizes more moisture is transported from bulk of the solid to its surface. This movement by diffusion of moisture in a solid takes place by a various mechanisms depending upon the nature and type of the solid and its state of aggregation. 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:

(i) Transport by liquid or vapors diffusion.
(ii) Capillary action, and
(iii) Pressure-induced transport.

A specific mechanism that involves in drying a specific solid depends on its nature, pore structure and the 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 of drying systems. Moisture content of a substance which exerts as equilibrium vapors pressure less than of the pure liquid at the same temperature is referred to as bound moisture. Moisture content of the solid which exerts an equilibrium vapour pressure equal to that of pure liquid at the given temperature is the unbound moisture.

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