Environmental Engineering Reference
In-Depth Information
2.8
Separating agents
Many separation processes are based on the formation of an additional phase which has
a different composition from the feed stream(s). One possible way of forming another
phase is the addition of energy (energy-separating agent) to convert a liquid stream to
a vapor stream. Distillation exploits this idea to separate mixtures of liquids that boil at
different temperatures. Crystallization processes use energy to separate liquid mixtures
with components that solidify at different temperatures. The temperature is lowered until
the species with the higher solidification temperature crystallizes out of solution. Evap-
oration and drying are other processes in which energy addition promotes the separation
by formation of a new phase.
Another large class of separations makes use of a change in solute distribution between
two phases in the presence of mass not originally present in the feed stream. This mass-
separating agent, MSA, is added as another process input to cause a change in solute
distribution. The MSA can alter the original phase equilibrium or facilitate the forma-
tion of a second phase with a concentration of components different from that in the
original phase. One of the components of the original feed solution must have higher
affinity for the MSA than for the original solution. This solute will then preferentially
transfer from the original feed solution to the MSA phase. Once the MSA has been used
to facilitate a separation, it must normally be removed from the products and recov-
ered for recycle in the process. Hence, use of an MSA requires two separation steps:
one to remove a solute from a feed stream; and a second to recover the solute from
the MSA.
General flowsheets for two basic processes using MSAs are shown in Figure 2.6. In
each case, the solute in the feed fluid has a high affinity for the MSA. In Figure 2.6(a), the
MSA is recirculated between two beds, one in which the solute is being sorbed and one in
which the MSA is being regenerated. In this scheme, the MSA is a moving portion of the
system and each vessel serves only one purpose; either the separation or the regeneration
of the MSA. In Figure 2.6(b), the MSA is fixed in each bed. Both beds are capable of
operating in either the sorption or regeneration mode. While one is in the sorption mode,
the other is regenerating the MSA. When the first is saturated, they switch roles. The first
scenario is common in cases in which the MSA is easily transported, as in the case of a
liquid or gas solvent. The second case is more common for solid MSAs that are not easily
transported.
As already stated, MSAs can consist of solids, liquids or gases. Figure 2.7 shows
various combinations of feed phase and MSA phase with examples of various separation
processes involved. In almost all cases, the use of an MSA involves the two steps shown
above.
A separation involving an energy-separating agent (ESA) can involve input and removal
steps, such as in distillation, where there is a reboiler for energy input and a condenser for
energy removal. In other cases, such as evaporation, the vapor can be discharged without
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