Page 19 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN001H-01 May 7, 2001 16:18

16 Absorption (Chemical Engineering)

the contactor. However, the cost of attaining the pressure the main considerations in their design and sizing are
must be considered. Off-gas scrubbers, for example, pro- discussed.
cess large quantities of gas which is then discharged to
the atmosphere; in such a case, the absorber pressure is
1. Packed Columns
set at near atmospheric, and the cost of moving the gas
through the contactor (due to pressure drop) may govern A typical arrangement (Fig. 14) consists of a vertical tower
the decision on operating pressure. containing one or more beds of packings. The descending
4. Determining solvent circulation rate. If the purpose liquid is distributed over the packing layers, forming liquid
of absorption is to generate a solution of a specific con- films that flow along the surfaces of the particles, thus
centration, the circulation rate is a fixed function of this exposing a large surface area for gas–liquid contact. The
concentration. For all other purposes, this circulation rate solute is transferred from the gas to the liquid across this
is determined by optimization. As circulation rate is in- surface. The type and size of packings may be the same
creased, the absorption factor L M /(mG M ) increases, as throughout the column or may differ from bed to bed.
does the distance from the operating line to the equilib- The characteristics considered most desirable for good
rium curve. This leads to a shorter and therefore cheaper packing performance are a high surface area, a uniform
column. On the other hand, the higher the circulation rate, distribution of liquid, and a low resistance to gas flow.
the greater is the cost of separating the solute from the sol- Two types of packings are common: random packings,
vent and the larger is the diameter of the absorber. Many which are discrete pieces of packings randomly dumped
studies have shown that the optimum circulation rate is into the column, and structured packings, which are layers
about 40% greater than the minimum solvent rate. of systematically arranged packings, mostly corrugated
5. Selection of contactor type. Tray and packed sheets or wire mesh. Structured packings provide uniform
columns are most common; other types are generally used channels for gas and liquid flow, a more even distribution,
only for special services. and greater surface area for the same resistance to gas flow.
The main factors favoring packed columns are (1) very In general, they tend to lead a more efficient operation but
corrosive applications, where plastic or ceramic pack- are also more expensive. For absorbers, random packings
ings are favored over trays, which are almost always are more popular, with structured packings being justi-
constructed of metal; (2) low pressure drop requirement, fied only when pressure drop and efficiency demands are
which is easier to achieve with packings than with trays; unusually high.
(3) small-diameter columns, because trays require access Common types of random packings are shown in
for inspection and maintenance; and (4) foaming systems, Fig. 15. The packings shown in Fig. 15a–c have been
which are easier to handle in packed towers. largely superseded by the packings shown in Fig. 15d–h.
The main factors favoring tray columns are (1) pres- Table II shows several common random packings and
ence of solids (packings have a greater tendency to trap compares them on two bases: (1) surface area per unit vol-
solids and to suffer from the resulting blockage and chan- ume, the larger area providing more opportunity for mass
neling), (2) very high or very low liquid rates (trays are transfer, and (2) packing factor, a measure of through-
more suitable to handle these than packings, except for put capacity and pressure drop, the lower is value, the
structured packings, which are also capable of handling higher the capacity and the lower the pressure drop. The
very low liquid rates), (3) slow reaction rate processes table shows that as packing size increases, capacity rises
(trays can provide a greater liquid holdup and therefore while efficiency decreases. The table includes two pack-
more residence time), (4) complexities such as cooling ings fabricated from plastic (usually polypropylene); this
coils or intercoolers, which are easier to incorporate into material of construction is resistant to corrosion and is
tray columns, and (5) column weight (tray columns are light weight. Plastic packing applications range from sul-
generally lighter and easier to support). furic acid absorbers to off-gas scrubbers and stripping
6. The number of theoretical stages, or transfer units, columns.
is calculated using a mathematical model of the type de- Table II shows that, as packing size increases, packing
scribed in Section III. At this stage, it is necessary to allow capacity rises while packing efficiency decreases. It also
for any heat effects; if these are significant, coiling coils shows that both capacity and efficiency are greater for Pall
®
or intercoolers may be required. rings and Intalox saddles than for Raschig rings and Berl
saddles.
The data in Table II are approximate, because the
B. Column Sizing
geometry of each packing varies slightly from one man-
In this section, the main types of absorption equipment ufacturer to another. Usually, the type of data shown in
(packed columns and tray columns) are described, and this table is provided by each manufacturing company for