The process of evaporation is based on several factors and their relationship may be expressed mathematically as: $$ M=\frac{KS(b-b’)}p $$ where, M is mass of vapour formed in unit time (m3/s), K is a constant (m/s), S is surface area of the liquid exposed (m3), b stands for the maximum vapour pressure at the temperature […]
It is convenient to classify evaporators on the basis of heating surface. Horizontal Tube Evaporator Norbert Rilliieux in Louisiana in 1843 for the first time built evaporators which received recognition. It was designed using horizontal tubes. The same was invigorated in 1879 into Wellner-Jelinek type from which evolved the most modern horizontal tube evaporator. Heat […]
Evaporation refers to the change in the phase of a component from liquid to gas. In the pharmaceutical industry, evaporation is chiefly associated with removing water and other solvents in batch operations. The suspended components do not appear in the vapour phase, if they do appear then the same operation is referred to as distillation. […]
Heat can transfer by one or more of the three basic mechanisms viz, conduction, convection and radiation. The Heat Transfer Mechanism of Conduction In conduction, heat transfer occurs by transmission of momentum of individual molecules without the involvement of actual mixing. So, it is limited to solids and ‘static’ fluids that are bound in some […]
Heat is a form of energy associated with random movement of molecules. For our purposes, it can be well understood with the help of an example in which a metal vessel containing liquid is heated, the molecules of the vessel will start vibrating with a higher energy and with the quest of coming to a […]
High-speed mixer/granulators are used extensively for pharmaceutical granulation. They were developed from traditional planetary mixers to speed up the process and to reduce the number of pieces of equipment and separate process steps required. The machines have a stainless steel mixing bowl containing a three-bladed main impeller which revolves in the horizontal plane and a […]
Liquid mixing mechanisms for fluids fall essentially into four categories: bulk transport, turbulent flow, laminar flow, and molecular diffusion. Usually, more than one of these mechanisms is operative in practical mixing situations. Liquid Mixing Mechanisms Bulk Transport The movement of a relatively large portion of the material being mixed from one location in a system […]
In order that powders may be mixed, the powder particles need to move relative to each other. There are three main mechanisms by which powder mixing occurs: namely, convection, shear and diffusion. Mechanisms of Powder Mixing Convective Mixing Convective mixing arises when there is the transfer of relatively large groups of particles from one part […]
Often a mixing process produces a large ‘bulk’ of mixture that is subsequently subdivided into individual dose units (e.g. a tablet, capsule or 5 mL spoonful), and it is important that each dosage unit contains the correct amount/concentration of active component(s). It is the weight/volume of the dosage unit which dictates how closely the mix […]
To discuss the principles of the mixing process, a situation will be considered where there are equal quantities of two powdered components of the same size, shape and density that are required to be mixed, the only difference between them being their colour. This situation will not, of course, occur practically but it will serve […]