ABSORPTION AND ADSORPTION

10.1 ABSORPTION AND ADSORPTION

Both absorption and adsorption are diffusional processes employed in the cleanup of effluent gases before the main carrier gas stream is discharged to the atmosphere.

Both of these operations are controlled by thermodynamic equilibrium. In pollution control, the concentrations of gases to be treated are relatively low. Thus, the equipment design is one in which it is reasonable to assume that gas is very dilute.

In absorption it is quite likely that the liquid effluent will be dilute as well. Absorption of the contaminant from the dilute gas results in a chemical solution of the contam-inating molecule. However, adsorption is a surface phenomenon in which the mol-ecules of the contaminant adhere to the surface of the adsorbent.

In diffusional operations where mass is to be transferred from one phase to another, it is necessary to bring the two phases into contact to permit the change toward equilibrium to take place. The transfer may take place with both streams flowing in the same direction, in which case the operation is called concurrent or co-current flow. When the two streams flow in opposite directions, the operation is termed countercurrent flow, an operation carried out with gas entering at the bottom and flowing upward, and the liquid entering at the top and flowing down. This process is illustrated in Figure 10.1. Figure 10.2 shows a combined operation in which the contaminated gas is first cleaned in a countercurrent operation, and then the gas is further treated to remove more of the contaminant in a co-current operation.

TABLE 10.1

VOC Control Technologies

Device

Inlet Conc.

PPMV Efficiency Advantages Disadvantages

Absorption 250 90% Especially good for Limited applicability 1000 95% inorganic acid gasses

5000 98%

Adsorption 200 50% Low capital investment Selective applicability 1000 90–95% Good for solvent recovery Moisture and temperature

constraints

5000 98%

Condensation 500 50% Good for product or Limited applicability

10,000 95% solvent recovery

Thermal incineration

20 95% High destruction

efficiency

No organics can be recovered

100 99% Wide applicability Capital intensive Can recover heat energy

Catalytic incineration

50 90% High destruction

efficiency

No organics can be recovered

100 >95% Can be less expensive Technical limitations than thermal incineration that can poison

Flares >98% High destruction

efficiency

No organics can be recovered Large emissions only 9588ch10 frame Page 110 Wednesday, September 5, 2001 9:51 PM

Countercurrent operation is the most widely used absorption equipment arrange-ment. As the gas flow increases at constant liquid flow, liquid holdup must increase.

The maximum gas flow is limited by the pressure drop and the liquid holdup which FIGURE 10.1 Countercurrent flow.

FIGURE 10.2 Combined countercurrent–co-current operation.

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will build up to flooding. Contact time is controlled by the bed depth and the gas velocity. In countercurrent flow, mass transfer driving force is maximum at the gas entrance and liquid exit. Co-current operation can be carried out at high gas velocities because there is no flooding limit. In fact, liquid holdup decreases as velocity increases. However, the mass transfer driving force is smaller than in countercurrent operation.

Some processes for both absorption and the removal of particulates employ a cross-flow spray chamber operation. Here water is sprayed down on a bed of packing material. The carrier gas, containing pollutant gas or the particulate, flows horizon-tally through the packing, where the spray and packing cause the absorbed gas or particles to be forced down to the bottom of the spray chamber where they can be removed. Figure 10.3 illustrates a cross-flow absorber. The design of cross-flow absorption equipment is more difficult than vertical towers because the area for mass transfer is different for the gas and liquid phases.

Continuous and steady-state operation is usually most economical. However, when smaller quantities of material are processed, it is often more advantageous to charge the entire batch at once. In fact, in many cases this is the only way the process can be done. This is called batch operation and is a transient operation from start-up to shutdown. A batch operation presents a more difficult design problem. Adsorp-tion is a semi-batch operaAdsorp-tion in which the contaminant in the carrier gas adheres FIGURE 10.3 Cross-flow absorber operation.

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to the absorbent until the adsorbent is saturated. The process must then be stopped to regenerate or replace the adsorbent.

Absorption takes place in either a staged or continuous contactor. However, in both cases the flow is continuous. In the ideal equilibrium stage model, two phases are contacted, well mixed, come to equilibrium, and then are separated with no carryover. Real processes are evaluated by expressing an efficiency as a percent of the change that would occur in the ideal stages. Any liquid carryover is removed by mechanical means.

In the continuous absorber, the two immiscible phases are in continuous and tumultuous contact within a vessel which is usually a tall column. A large surface is made available by packing the column with ceramic or metal materials. The packing provides more surface area and a greater degree of turbulence to promote mass transfer. The penalty for using packing is the increased pressure loss in moving the fluids through the column, causing an increased demand for energy. In the usual counter-current flow column, the lighter phase enters the bottom and passes upward.

Transfer of material takes place by molecular and eddy diffusion processes across the interface between the immiscible phases. Contact may be also co-current or cross-flow. Columns for the removal of air contaminants are usually designed for counter-current or cross-flow operation.

10.1.1 FLUID MECHANICS TERMINOLOGY

Defining velocity through a column packed with porous material is difficult. Even if a good measure of porosity has been made, it is not possible to assure that the same porosity will be found the next time a measurement is made after the packing has been changed. Also, during operation the bed may expand or in the case of a two-phase, gas-liquid operation, liquid holdup can occur which varies with the flow.

Therefore, determining the unoccupied tower cross-sectional area is difficult, and it becomes advantageous to base the velocity on the total tower cross-section which is the usual way to calculate tower flow, especially in absorption design.

The conservation of mass principle at steady state is

(10.1) where

m = mass flow rate ρ = mass density A = area

V^— = mean velocity

in compatible units. If G is defined as the mass rate of gas flow, then a superficial mass velocity can be defined as G where

(10.2) m= ρA V

G=G A

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Note that the mean velocity can be calculated from

(10.3) and that the volumetric flow rate Q is given by

(10.4) Thus

(10.5) defines a superficial velocity which is dependent upon the total tower cross-sectional area.

10.1.2 REMOVALOF HAP AND VOC BY ABSORPTION AND ADSORPTION

Absorption is widely used as a product-recovery method in the chemical and petro-leum industry. As an emission control technique, it is more commonly employed for inorganic vapors. Some common absorption processes for inorganic gases are

• Hydrochloric acid vapor in water

• Mercury vapor in brine and hypochlorite solution

• Hydrogen sulfide vapor in sodium carbonate and water

• Hydrofluoric acid vapor in water

• Chlorine gas in alkali solution

In order for absorption to be a suitable process for emission control, there must be a suitable solvent which can readily be treated after it leaves the process. Both vapor–liquid equilibrium data and mass-transfer data must be available or capable of being estimated. Absorption may be most effective when combined with other processes such as adsorption, condensation, and incineration.

Adsorption can be used to treat very dilute mixtures of pollutant and air. Acti-vated carbon is the most widely used adsorbent. Silica gel and alumina are also frequently used adsorbents. Removal efficiencies can be as high as 99%. The max-imum inlet concentration should be about 10,000 ppmv with a usual minmax-imum outlet concentration at 50 ppmv. In some cases it may be advisable to design for minimum outlet concentration of 10 to 20 ppmv. The maximum concentration entering an adsorption bed is limited by the carbon capacity and in some cases by bed safety.

Exothermic reactions can occur when some compounds are mixed in an adsorption bed. Thus, if concentrations are too high, the bed may reach a flammable condition which could lead to an explosion. It is best to keep the entering concentration to less than 25% of the Lower Explosive Limit (LEL). For excessively high concen-trations, condensation or dilution could be used to bring the concentration to a more reasonable lower level.

V=mρA

Q=AV

V=Q A

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Other limitations for adsorption operation are concerned with the molecular mass of the adsorbate. High molecular weight compounds are characterized by low volatility and are strongly adsorbed. Adsorption technology should be limited to compounds whose boiling points are below 400°F or molecular mass is less than about 130. Strongly adsorbed high-molecular-mass compounds are difficult to remove when regenerating the adsorbent. With low molecular mass, compounds below a molecular mass of 45 are not readily adsorbed due to their high volatility.

On the other hand, lower molecular-weight compounds are more readily removed during the regeneration process. Furthermore, gases to be treated may have liquid or solid particles present or have a high humidity. Pretreatment may then be required.

Humidity needs to be reduced below 50% in most cases, or the water will selectively adsorb to such a great extent that the desired adsorbate to be removed will be blocked out. Gases to be treated may also be required to be cooled if the temperature is greater than 120–130°F and the possibility of exothermic reactions exist.

REFERENCE

1. U.S. EPA Handbook: Control Technologies for Hazardous Air Pollutants, EPA/625/6-91/014, Cincinnati, OH, 1991.

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Absorption for HAP