انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة

AIR POLLUTION 6

of one or more pollutants from a contaminated gas stream by treatment with a liquid. The

necessary condition is the solubility of these pollutants in the absorbing liquid. The rate

of transfer of the soluble constituents from the gas to the liquid phase is determined by

diffusional processes occuring on each side of the gas–liquid interface.

Consider, for example, the process taking place when a mixture of air and sulfur

dioxide is brought into contact with water. The SO2 is soluble in water, and those molecules

that come into contact with the water surface dissolve fairly rapidly. However, the

SO2 molecules are initially dispersed throughout the gas phase, and they can reach the

water surface only by diffusing through the air, which is substantially insoluble in

the water. When the SO2 at the water surface has dissolved, it is distributed throughout

the water phase by a second diffusional process. Consequently, the rate of absorption is

determined by the rates of diffusion in both the gas and liquid phases.

Equilibrium is another extremely important factor to be considered that affects

the operation of absorption systems. The rate at which the pollutant will diffuse into

an absorbent liquid will depend on the departure from equilibrium that is maintained.

The rate at which equilibrium is established is then essentially dependent on the rate

of diffusion of the pollutant through the nonabsorbed gas and through the absorbing

liquid. Equilibrium concepts and relationships are considered in a later section.

The rate at which the pollutant mass is transferred from one phase to another

depends also on a so-called mass transfer, or rate, coefficient which relates the quantity

of mass being transferred with the driving force. As can be expected, this transfer process

ceases upon the attainment of equilibrium.

Gas absorption can also be viewed on a molecular scale as a mass transfer or

diffusional operation characterized by a transfer of one substance through another.

The mass transfer process may be considered the result of a concentration difference

driving force, the diffusing substance moving from a place of relatively high to one of

relatively low concentration. The rate at which this mass is transferred depends to a

great extent on the diffusional characteristics of both the diffusing substance and

the medium.

The principal types of gas absorption equipment may be classified as follows:

1. Packed columns (continuous operation)

2. Plate columns (staged operation)

3. Miscellaneous

Of the three categories, the packed column is by far the most commonly used for the

absorption of gaseous pollutants. The first two types of units are discussed below.

Packed columns are used for the continuous contact between liquid and gas. The

countercurrent packed column (is the most common type of unit encountered

in gaseous pollutant control for the removal of the undesirable gas, vapor, or odor.

This type of column has found widespread application in both the chemical and

pollution control industries. The gas stream containing the pollutant moves upward

through the packed bed against an absorbing or reacting liquid that is injected at the

top of the packing. This results in the highest possible transfer/control efficiency.

Since the pollutant concentration in the gas stream decreases as it rises through the

column, there is constantly fresher liquid available for contact. This provides a

maximum average driving force for the transfer process throughout the packed bed.

Liquid distribution plays an important role in the efficient operation of a packed

column. A good packing from a process viewpoint can be reduced in effectiveness by

poor liquid distribution across the top of its upper surface. Poor distribution reduces

the effective wetted packing area and promotes liquid channeling. The final selection

of the mechanism of distributing the liquid across the packing depends on the size of

the column, type of packing, tendency of packing to divert liquid to column walls,

and materials of construction for distribution. For stacked packing, the liquid

usually has little tendency to cross distribute and thus moves down the column fairly

uniformly in the cross-sectional area that it enters. In the dumped condition, most

flow profiles follow a conical distribution down the column, with the apex of the cone

at the liquid impingement point. For well-distributed liquid flow and reduced channeling

of gas and liquid to produce efficient use of the packed bed, the impingement of the

liquid onto the bed must be as uniform as possible. The liquid coming down through

ABSORBERS

the packing and on the inside wall of the column should be redistributed after a bed

depth of approximately 3 column diameters for Raschig rings and 5–10 column

diameters for other packing (check literature for details of packing). As a guide,

Raschig rings usually have a maximum of 10–15 ft of packing per section, while other

packing can use a maximum of 12–20 ft. As a general rule of thumb, however, the

liquid should be redistributed every 10 ft of packed height. The redistribution brings

the liquid off the wall and outer portions of the column and directs it toward the

center area of the column. As noted earlier, redistribution is seldom necessary for

stacked bed packings, as the liquid flows essentially in vertical streams.

Crossflow packed scrubbers are particularly successful when the process air stream

requires both gas absorption and particulate removal. The crossflow scrubber operates by

allowing the gas to flow horizontally across the scrubber. The scrubbing liquid is introduced

at the top of the scrubber and drains vertically through the packing. Contact

between the gas and liquid occurs at a right angle.

In general, crossflow scrubbers operate at lower pressure drops and liquid recycle

flow rates than do countercurrent packed-bed absorbers. Vendors claim that crossflow

scrubbers operate with a liquid rate and pressure drop approximately 60% less than

a comparable countercurrent packed tower. The crossflow scrubber generally operates

at much higher gas-to-liquid ratios than does the countercurrent packed-bed absorber.

As a result, the liquid stream in the crossflow scrubber is able to “scour” the packing

media more easily than is the countercurrent packed absorber. Thus, there can be a

significant reduction in plugging of the packing. Since the crossflow scrubber reduces

water consumption and the size of the recirculation pump, significant savings in both

operating and capital costs may be realized.

It is also easier to increase the gas flow rate through an existing crossflow absorber

than a countercurrent packed-bed absorber since the crossflow scrubber can operate

over a wider range of gas-to-liquid ratios. Another advantage associated with the crossflow

absorber is that it can be installed horizontally in-line on an existing process. Many

current crossflow scrubbers are using multiple beds with individual scrubbing sections

that can remove a wide variety of pollutants. It is much more difficult and expensive

to provide multiple scrubbing sections in a packed tower. The main advantage that

the packed tower absorber has over the crossflow scrubber is that the packed tower

can achieve very high removal efficiencies for pollutants that are difficult to absorb.

Plate columns may also be employed as absorbers, although they are used only

occasionally for pollution control. These devices are essentially vertical cylinders in

which the liquid and gas are contacted in stepwise fashion on plates or trays in the

manner depicted schematically . The liquid enters at the top and flows downward

via gravity.On theway, it flows across each plate and through a downspout (or downcomer)

to the plate below. The gas passes upward through hole openings in the plate, then

bubbles through the liquid to form a froth, disengages from the froth, and passes on the

next plate above. The overall effect is a multiple countercurrent contacting of gas and

liquid. Each plate of the column is a stage. Since the fluids on the plate are brought into

intimate contact, interphase diffusion occurs, and the fluids are then separated.

The number of theoretical plates is dependent on the difficulty of the separation to be

carried out and is determined solely from material balance and equilibriumconsiderations.

The actual number of plates required for a given separation is greater than the theoretical

number due to plate inefficiency. The diameter of the column, on the other hand, depends

on the quantities of liquid and gas flowing through the column per unit time.

In bubble-cap plates, the vapor moves upward through risers into the bubble cap,

out through the slots as bubbles, and into the surrounding liquid on the plates.

Figure 1 demonstrates the vapor–liquid action for a bubble-cap plate. Although

rarely used today, the bubble-cap plate design is one of the more flexible of plate

designs for high and low vapor and liquid rates. On the average, plates are usually

spaced approximately 24 inches apart.