Mass Transfer

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Chapter: Pharmaceutical Engineering: Mass Transfer

The following is a brief review of mass transfer to complete the overview of unit processes in pharmacy.

Mass Transfer


The following is a brief review of mass transfer to complete the overview of unit processes in pharmacy. Mass transfer is conceptually and mathematically analogous to heat transfer, as will be seen in the following exposition. Many processes are adopted so that a mixture of materials can be separated into component parts. In some, purely mechanical means are used. Solids may be separated from liquids by the arrest of the former in a bed permeable to the fluid. This process is known as filtration. In other examples, a difference in density of two phases permits separation. This is found in sedimentation and centrifugation. Many other processes, however, operate by a change in the composition of a phase due to the diffusion of one component in another. Such processes are known as diffusional or mass transfer processes. Distillation, dissolution, drying, and crystallization provide examples. In all cases, diffusion is the result of a difference in the concentration of the diffusing substance, this component moving from a region of high concentration to a region of low concentration under the influence of the concentration gradient.

In mass transfer operations, two immiscible phases are normally present, one or both of which are fluid. In general, these phases are in relative motion and the rate at which a component is transferred from one phase to the other is greatly influenced by the bulk movement of the fluids. In most drying pro-cesses, for example, water vapor diffuses from a saturated layer in contact with the drying surface into a turbulent airstream. The boundary layer, as described in chapter 2, consists of a sublayer in which flow is laminar and an outer region in which flow is turbulent. The mechanism of diffusion differs in these regimes. In the laminar layer, movement of water vapor molecules across streamlines can only occur by molecular diffusion. In the turbulent region, the movement of relatively large units of gas, called eddies, from one region to another causes mixing of the components of the gas. This is called eddy diffusion. Eddy diffusion is a more rapid process, and although molecular diffusion is still present, its contribution to the overall movement of material is small. In still air, eddy diffusion is virtually absent and evaporation occurs only by molecular diffusion.

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