Page 48 - Separation process principles 2

1.3 Separation by Phase Addition or Creation 13

one is intentionally adding vapor to or removing vapor from mechanical devices. A supercritical fluid is sometimes used
a gas, Major applications of evaporation are humidification, as the solvent in leaching.
conditioning of air, cooling of water, and the concentration In adsorptive-bubble separation methods, surface-active
of aqueous solutions. material collects at solution interfaces, establishing a con-
(15),
is
~~~t~llization, carried out in many organic, and centration gradient between a solute in the bulk and in the
almost all inorganic, chemical manufacturing plants where surface layer. If the (very thin) surface layer can be col-
the desired product is a finely divided solid. Since crystal- lected, partial solute removal from the solution will have
lization is essentially a purification step, the conditions in the been achieved. The major application of this phenon~enon is
crystallizer must be such that impurities do not precipitate in ore flotation processes, where solid particles migrate to
with the desired product. In solution crystallization, the mix- and attach themselves to rising gas bubbles and literally float
ture, which includes a solvent, is cooled and/or the solvent is out of the solution. This is essentially a three-phase system.
evaporated to cause crystallization. In melt crystallization, Foam fractionation, (l8), a two-phase adsorptive-bubble
two or more soluble species, in the absence of a solvent, are separation method, is a process where natural or chelate-
separated by partial freezing. A particularly versatile melt induced surface activity causes a solute to migrate to rising
crystallization technique is zone melting or refining, which bubbles and is, thus, removed as a foam. This method is not
relies on selective distribution of impurity solutes between a covered in this book.
liquid and a solid phase to achieve a separation. Many met- Each equipment symbol shown in Table 1.1 corresponds
als are refined by this technique, which, in its simplest form, to the simplest configuration for the operation represented.
involves moving a molten zone slowly through an ingot by More complex versions are possible and frequently desir-
moving the heater or drawing the ingot past the heater. The able. For example, a more complex version of the reboiled
manufacture of single. crystals has been a vital development absorber, Separation Operation (5) in Table 1.1, is shown in
in the semiconductor industry in recent years. Typically sin- Figure 1.8. This reboiled absorber has two feeds, an inter-
gle crystals of very high purity silicon are produced world- cooler, a side stream, and both an interreboiler and a bottoms
wide by the Czochralski technique, wherein a single crystal reboiler. Acceptable design procedures must handle such
is pulled from a melt. Typical crystal dimensions, after shap- complex situations. It is also possible to conduct chemical
ing into a uniform rod with diamond grinding machines, are reactions simultaneously with separation operations in a
150-mm diameter x 1-m long, from which wafers of 675- single column. Siirola [6] describes the evolution of an
micron thickness are sawed. advanced commercial process for producing methyl acetate
Sublimation is the transfer of a substance from the solid by the esterification of methanol and acetic acid. The process
to the gaseous state without formation of an intermediate liq- is conducted in a single column in an integrated process that
uid phase, usually at a relatively high vacuum. Major appli- involves three reaction zones and three separation zones.
cations have been in the removal of a volatile component
from an essentially nonvolatile one. Examples are separation
of sulfur from impurities, purification of benzoic acid, and
freeze-drying of foods. The reverse process, desublimation, 1
(16), is also practiced, for example, in the recovery of Overhead vapor
phthalic anhydride from gaseous reactor effluent. The most k
Absorbent
common application of sublimation in everyday life occurs
in the use of Dry Ice as a refrigerant for storing ice cream,
Intercooler
vegetables, and other perishables. The sublimed gas, unlike El==
water, does not puddle and spoil the frozen materials.
Liquid-solid extraction, often referred to as leaching,
(17), is widely used in the metallurgical, natural product, and
food industries under batch, semicontinuous, or continuous
operating conditions. The major problem in leaching is to
promote diffusion of the solute out of the solid and into the 4
Feed 2
liquid solvent. The most effective way of doing this is to re-
duce the dimensions of the solid to the smallest feasible par- Liquid side stream
ticle size. For large-scale applications, in the metaflurgical
industries in particular, large, open tanks are used in coun- m
tercurrent operation. The major difference between solid- Interreboiler
-
liquid and liquid-liquid systems centers about the difficulty
of transporting the solid, or the solid slurry, from stage to w
stage. For this reason, the solid may be left in the same tank, Bottoms reboiler
with only the liquid transferred from tank to tank. In the ,
pharmaceutical, food, and natural-product industries, Bottoms
countercurrent solid transport is provided by complicated Figure 1.8 Complex reboiled absorber.