Comparison of the drying processes and equipmentEquipmentMode of the heat transferMethod of Material handlingAdvantages and Disadvantages

Desiccation chambers/Tray dryersConvectionStaticAdv: There is no erosion of the solid because it is static and it is economic and small amount of the product can be driedDisadv: Non-homogeneous drying, not appropriate drying method for the thermo-sensitive products and the process is very slow

Drying tunnelsConvectionDynamicsAdv: Compared to the tray dryers, tunnel drying is a continuous process and also a large amount of materials can be driedDisadv: High labor cost for loading and unloading and not suitable for small scale production and also cannot dry if the feed in liquid. Similar to the tray dryers this method is also slow and thermolabile substance cannot be dried

Lyophilisation/freeze dryingAdv: Useful technique for the manufacturing of the sterile drugs and antibodies. And also improves the stability of the drug compounds which are prone to hydraulic degradation and thus increase the shell lifeDisadv: High cost of equipment and high energy consumption

Spray dryingConvectionDynamicAdv: Compared to the slow tray dryers and tunnel drying, spray drying can be used to achieve fast drying by uniform atomization of the feed and by ensuring effective mixing of the droplets with the drying gas.Fast drying and short residence time makes spray drying applicable to dry the heat sensitive materials such as live vaccines and complex proteinsAlso, this method combines crystallization and drying so also applicable to the liquid or slurry feed as compared to tunnel dryingDisadv: Its cost of operation and high volumetric consumption of the inert gas

Fluid BedConvectionDynamicAdv: Good homogeneity, low risk for agglomeration and short drying times.Disadv: Unlike spray drying the fluid bed drying not suitable for pasty or liquid materials and like spray drying it requires high volumetric consumption of the inert gas

Vacuum drying(mixers & Chambers)Convection,Conduction,Radiation

Static& DynamicAdv: fast drying and easy to dry heat sensitice materials and capable of recovering the valuble and useful ingredientsDisadv: Need of a pumping steam vacuum system making the investment in equipment costs and high operating cost and low it has got efficiency problems when it is compared to spray or fluid bed drying in case of the heat sensitive materials

Advantages and Disadvantages of the Fluid bed DryingFluidized-bed drying presents the benefits of high heat and mass transfer coefficients, with leads to the rapid exchange of mass and heat between gas and particles, and hence increased drying rates results in shorter drying times and it eliminates heating of a heat sensitive product. Moreover, control of the process is reliable due to rapid mixing and heat transfer rates are high.Fluidized-bed drying permits large scale continuous operation with easy handling of the feed and product.The dryers do not include any mechanical moving parts and therefore maintenance is not expensive.A major disadvantage of fluidized beds is the potential for particle size reduction due to attrition and collisions between them. In areas of high gas velocities greater rate of attrition occurs accompanied by fracture of particles due to impactThe design of an industrial-scale fluidized bed dryer is a somewhat a challenge as it is based more on empirical knowledge than on fundamental research.

The drying process may greatly influence the quality of the drug substances in several aspects: the polymorphic form of the active ingredient, the structural changes that may have an effect on the activity of the substance, the degradation products formed due to the drying conditions and the presence of residual solvents undesirable or above the permitted limits after drying. For instance, when a drying process is too short it will produce granules with entrapped moisture, but if the process is too long, then the granules become very dry and friable. If granules that dried by the outer layer reach the tablet press, then moisture will escape from the granules during compression and would cause the granules to stick to the tablet press tooling. Important selection of the drying system (static vs agitated/fluidized system), it has been seen that that static drying results in granules possessing higher strength and faster drug dissolution on the other hand it has been seen that fluid bed drying caused less contraction than static bed drying. Choice of drying method also holds its own impacts on the tablet properties, For e.g.: drying in a microwave oven gives rise to more porous granules with a lower fracture strength than drying in a convective dryer. However, in contradiction, it has also been observed that microwave drying may give granules of similar structure as conventional tray drying. In addition, it has been shown that freeze-drying may give extremely porous granules, with a significantly increased deformability and tablet-forming ability, compared with other drying methods. It should also be pointed out in this context that the drying of solids, e.g. the removal of crystal water and spray drying may result in solid-state transformations and in the production of amorphous materials, this is due to stresses occurring during the removal of a liquid by drying may, for instance, produce amorphous materials and influence on the porosity of granules, with subsequent effects on the dosage form.