AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 10 doc

Absorption and adsorption are both diffusional sepa-ration processes that can be used to collect hazardous air pollutants.. 10.1 ABSORPTION AND ADSORPTION Both absorption and adsorption

Introduction to Control

of Gaseous Pollutants

Under the auspices of the EPA Center for Environmental Research Information in conjunction with the EPA Control Technology Center, a handbook for design of hazardous air pollution control equipment was published in 1986 A revised version

of the handbook was published in June 1991.1 Current information is furnished through the Clean Air Technology Center (CATC) This center serves as a resource on all areas

of emerging and existing air pollution prevention and control technologies and their cost The information now may be found on the Web at: http//www.usepa.gov/ttn/catc/ There are six main processes by which a gaseous pollutant may be removed from an air stream Table 10.1, taken from the EPA handbook,1 lists those processes with the advantages and disadvantages of using each one The table may be used as

a guide to determine which process may provide the best means of cleaning the air stream Separation processes are used as a means of air pollution control for both particulate matter and gas These processes essentially remove the pollutant from the carrier gas resulting in a cleaned gas stream If the pollutant content of the cleaned stream meets the effluent emission standards, the cleaned stream can be discharged to the atmosphere Absorption and adsorption are both diffusional sepa-ration processes that can be used to collect hazardous air pollutants In the case of absorption, the pollutant is transferred to the solvent which then may need further treatment Recovery of the solvent might be undertaken by distillation or by stripping the absorbed material from the solvent The problem of treating the waste material

in the stream separated from the solvent remains If the pollutant material has a value, adsorption may provide the means for the material to be more readily recov-ered In the case of particulate matter, wet scrubbing collects the particles primarily through the mechanism of inertial impaction Gaseous contaminants such as sulfur oxide, nitrogen oxide, or hydrochloric acid, if present along with the particulates, may be collected simultaneously by absorption

Many organic materials may be removed by condensation, which is essentially

a diffusional operation If a suitable coolant is available and the pollutant concen-tration is high enough, condensation can be very effective in recovering material that may be used again For organic pollutants when the concentration is low or recovering the material is not desired, incineration can be used to convert the pollutant to carbon dioxide and water For large emissions such as would be found

in petroleum refineries the pollutant may be flared

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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.

Absorption 250 90% Especially good for Limited applicability

1000 95% inorganic acid gasses

Adsorption 200 50% Low capital investment Selective applicability

1000 90–95% Good for solvent recovery Moisture and temperature

constraints

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

efficiency

No organics can be recovered Large emissions only

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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.

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 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 F LUID M ECHANICS T ERMINOLOGY

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

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 R EMOVAL OF HAP AND VOC BY A BSORPTION

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

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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

EPA/625/6-91/014, Cincinnati, OH, 1991.