Environmental Engineering Reference
In-Depth Information
of which solutes are being removed from a given fluid by a given sorbent or the amount of
fluid processed in a given time. Hence, the design and analysis of a particular separation
method will be the same regardless of the species and quantities to be separated. Once it
is understood how to evaluate an adsorption column to separate a binary component feed
stream, the same principles can be applied to any binary mixture. An important aspect
of design and analysis is the scale (or size) of a process. Those in which separations
technology is required span several orders of magnitude in terms of their throughput. For
example, industrial separation of radioisotopes occurs at a production rate on the order of
10 6
to 10 3
kg
/
hr, while coal cleaning plants operate at a production rate greater than
10 6
hr. If new design criteria had to be developed for a given separation technique
each time that the scale of the process was changed, the analysis would be of very limited
value and one would have to write a topic for each separation technology to cover all
the potential process sizes. The concept of a unit operation, therefore, allows us to apply
the same design criteria and analysis for a given separation technology, irrespective of the
size. This very important element in evaluation allows one to scale-up or scale-down a
process based upon results obtained on a different-sized piece of equipment. This is the
basis for conducting tests on bench- or pilot-scale equipment and using the results for the
design of the full-scale process. In addition, bench or pilot-plant test results can be used
to determine the effect of a single separation step or other unit operation on an overall
process. The configuration and flow patterns of any single step can affect the entire process
and are usually determined experimentally.
kg
/
2.3
Separation mechanisms
Separation processes rely on various mechanisms, implemented via a unit operation, to
perform the separation. The mechanism is chosen to exploit some property difference
between the components. They fall into two basic categories: the partitioning of the feed
stream between phases; and the relative motion of various chemical species within a single
phase. These two categories are often referred to as equilibrium and mass transfer rate
processes, respectively. Separation processes can often be analyzed with either equilibrium
or mass transfer models. However, one of these two mechanisms will be the limiting, or
controlling, factor in the separation and is, therefore, the design mechanism.
For a separation to occur, there must be a difference in either a chemical or physical
property between the various components of the feed stream. This difference is the driv-
ing force basis for the separation. Some examples of exploitable properties are listed in
Table 2.1. Separation processes generally use one of these differences as their primary
mechanism.
The following factors are important considerations in the choice of a property difference.
(a) Prior experience. The reliability and “comfort” factor go up if there has been prior
positive experience in the use of a certain property difference for certain applications.
(b) The property itself. How simple will it be to implement?
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