Page 47 - Separation process principles 2

12 Chapter 1 Separation Processes

Often, vapor from the top of the column is condensed in a Liquid-liquid extraction, (11) and (12), using one or two
condenser by cooling water or a refrigerant to provide con- solvents, respectively, is widely used when distillation is
tacting liquid, called reflux. Similarly, liquid at the bottom of impractical, especially when the mixture to be separated is
the column passes through a reboiler, where it is heated by temperature-sensitive and/or more than about 100 distilla- i
condensing steam or some other heating medium to provide tion stages would be required. When one solvent is used, it
I
contacting vapor, called boilup. selectively dissolves only one or a fraction of the compo- j
When volatility differences between species to be sepa- nents in the feed mixture. In a two-solvent extraction
rated are so small as to necessitate more than about 100 trays system, each solvent has its own specific selectivity for dis-
in a distillation operation, extractive distillation, Operation solving the components of the feed mixture. Thus, if a feed
(4), is often considered. Here, an MSA, acting as a solvent, mixture consists of species A and B, solvent C might prefer-
is used to increase volatility differences between selected entially dissolve A, while solvent D dissolves B. As with
species of the feed, thereby reducing the number of required extractive distillation, additional separation operations are
trays. Generally, the MSA, which must be completely misci- generally required to recover, for recycling, solvent from
ble with the liquid phase throughout the column, is less streams leaving the extraction operation.
volatile than any of the species in the feed mixture and is in- A variation of liquid-liquid extraction is supercritical-
troduced to a stage near the top of the column. Reflux to the fluid extraction, where the extraction temperature and pres-
top tray is utilized to minimize MSA content in the top prod- sure are slightly above the critical point of the solvent. In this
uct. A subsequent separation operation, usually distillation, region, solute solubility in the supercritical fluid changes
is used to recover the MSA for recycling back to the extrac- drastically with small changes in temperature and pressure.
tive distillation column. Following extraction, the pressure of the extract can be
If condensation of vapor leaving the top of a distillation reduced to release the solvent, which is then recycled. For the
column is not easily accomplished by heat transfer to cool- processing of foodstuffs, the supercritical fluid is an inert sub-
ing water or a refrigerant, a liquid MSA called an absorbent stance such as C02, which will not contaminate the product.
may be introduced to the top tray in place of reflux. The Since many chemicals are processed wet but sold as dry
resulting separation operation is called reboiled absorption, solids, one of the more common manufacturing steps is I
(5). If the feed is all vapor and the stripping section of the drying, (13), which involves removal of a liquid from a solid
column is not needed to achieve the desired separation, the by vaporization of the liquid. Although the only basic re-
operation is referred to as absorption, (6). This operation quirement in drying is that the vapor pressure of the liquid to
may not require an ESA and is frequently conducted at am- be evaporated be higher than its partial pressure in the gas
bient temperature and high pressure. Constituents of the stream, the design and operation of dryers represents a com-
vapor feed dissolve in the absorbent to varying extents de- plex problem in heat transfer, fluid mechanics, and mass
pending on their solubilities. Vaporization of a small fraction transfer. In addition to the effect of such external conditions
of the absorbent also generally occurs. as temperature, humidity, air flow rate, and degree of solid
The inverse of absorption is stripping, Operation (7) in subdivision on drying rate; the effect of internal conditions
Table 1.1. Here, a liquid mixture is separated, generally at el- of liquid and vapor diffusion, capillary flow, equilibrium
evated temperature and ambient pressure, by contacting liq- moisture content, and heat sensitivity in the solid must be
uid feed with a stripping agent. This MSA eliminates the considered. Although drying is a multiphase mass-transfer
need to reboil the liquid at the bottom of the column, which process, equipment-design procedures differ from those of
may be important if the liquid is not thermally stable. If con- any of the other processes discussed in this chapter because
tacting trays are also needed above the feed tray in order to the thermodynamic concepts of equilibrium are difficult to
achieve the desired separation, a rejuxed strippel; (8), may apply to typical drying situations, where the concentration of
be employed. If the bottoms product from a stripper is ther- vapor in the gas is so far from saturation, and concentration
mally stable, it may be reboiled without using an MSA. In gradients in the solid are such that mass-transfer driving
that case, the column is called a reboiled strippel; (9). Addi- forces are undefined. Also, heat transfer rather than mass
tional separation operations are required to recover, for recy- transfer may well be the limiting rate process. Therefore, the
cling, MSAs used in absorption and stripping operations. typical dryer design procedure is for the process engineer to
The formation of minimum-boiling azeotropic mixtures send a few tons of representative, wet sample material for
makes azeotropic distillation, (lo), another useful tool pilot-plant tests by one or two reliable dryer manufacturers
where separation by distillation is not feasible. In the exam- and to purchase the equipment that produces a satisfactorily
ple cited in Table 1.1, the MSA, n-butyl acetate, which forms dried product at the lowest cost. The types of commercial
a heterogeneous (i.e., two liquid phases present), mjnimum- dryers are discussed in detail in Perry's Chemical Engi-
boiling azeotrope with water, is used as an entrainer to facil- neers ' Handbook [5] and Chapter 18.
itate the separation of acetic acid from water. The azeotrope Evaporation, Operation (14) in Table 1.1, is generally
is taken overhead and then condensed and separated into defined as the transfer of volatile components of a liquid into
acetate and water layers. The MSA is recirculated, and the a gas by volatilization caused by heat transfer. Humidifica-
distillate water layer and bottoms acetic acid are removed as tion and evaporation are synonymous in the scientific sense;
products. however, humidiJicatio~l or dehumidijcation implies that