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