The ability to analyze, synthesize, and design separation processes is a "distinctive" core capability of ChEs.
Separation is needed to achieve goals of enrichment, concentration, purification, refining, and isolation.
All separations work by exploiting differences between the matter to be separated: size, shape, vapor pressure, solubility, etc. Broadly speaking, separation technology can be divided into two sets:
Mechanical separations depend primarily on differences in particle size, density, velocity, etc. Examples include screening, filtration, sedimentation, centrifugation, decanting, etc. (See MSH6 Chap. 29)
These techniques are good for separating phases (solid from liquid), but not necessarily components within a phase.
Separation of components within a single phase is more difficult -- energy input is required. Mixing is an "irreversibility", so it cannot be spontaneously undone without an energy input. Once the new phase has been created, mass transfer (diffusion) between the phases moves some components relative to others, so that when the phases are separated so are the components. Thus, these are called diffusional separations.
Diffusional separations exploit differences in vapor pressure, solubility, diffusivity, etc. The driving force for these separations is a difference in chemical activity or concentration which leads to migration of components across a phase boundary. The phases are then separated (typically mechanically) to produce products. In most separation processes, it is necessary to create a new phase for transport to occur.
In most industrial applications, the new phase is created by direct energy addition. For instance, in evaporation energy boils off solvent to produce a new vapor phase. Energy addition is characteristic of distillation and crystallization.
A new phase can also be created by adding another component (mass). This characterizes separation techniques such as extraction and absorption.
An efficient separation is thus one that minimizes the energy or mass input required to reach the desired product purities.
In distillation, vaporization separates a liquid mixture into components or groups of components.
In gas absorption or scrubbing, a solute transfers from a carrier gas phase to a liquid solvent phase. The reverse (transfer from liquid to vapor) is called desorption or stripping.
Dehumidification is the removal of a pure liquid from a gas carrier phases by condensation. The reverse (from liquid to vapor) is drying.
Adsorption is the removal of a solute from a fluid by contact with an otherwise inert solid.
When a mixture is treated with a solvent that selectively dissolves certain components, it is called liquid extraction if the initial phase is liquid and leaching if the carrier phase is a solid.
Crystallization and evaporation create a new phase by heat transfer. In these methods, usually only one of the product phases is valuable.
Membrane technologies, including reverse osmosis, ultrafiltration, etc., allow one component to pass through a selective membrane which rejects other components.
SH Tables 1.1-1.4 list many separation types, requirements, etc.
Diffusional separations are governed by transport (mass transfer limits the rate of separation) and by equilibrium (thermodynamic limits on the extent of separation). Earlier courses have provided you with tools for examining thermodynamics and momentum transport, and you are currently studying heat transport. The new material in the present course will begin with the study of component mass transfer.
Essentially all separation problems require you to develop and solve:
References:
R.M. Price
Original: 7/26/2002
Modified: 8/6/2002, 12/31/2003
Copyright 2003 by R.M. Price -- All Rights Reserved