entrance and liquid exit. Co-current operation can be carried out at high gas veloci- ties 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 pack- ing material. The carrier gas, containing pollutant gas or the particulate, flows hori- zontally 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
Gas
Gas Liquid
Gas
Liquid
Liquid
FIGURE 10.2 Combined countercurrent–co-current operation.
Scrubbing water
Packed bed
flowGas
Water outlet
FIGURE 10.3 Cross-flow absorber operation.
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 startup to shutdown. A batch operation presents a more difficult design problem.
Adsorption is a semi-batch operation, in which the contaminant in the carrier gas adheres 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 car- ryover. 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, plastic, or metal materi- als. 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 countercurrent 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 countercurrent 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 pack- ing 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 dif- ficult, 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
m= ρAV (10.1)
where:
m is the mass flow rate ρ is the mass density
V is the mean velocity in compatible units The volumetric flow rate Q is given by
Q AV = (10.2)
Therefore,
V = /Q A (10.3)
If G is defined as the mass rate of gas flow, then a superficial mass velocity can be defined as G, where
G G A = / (10.4)
defines a superficial velocity, which is dependent upon the total tower cross-sectional area.
Note that the mean velocity can be calculated from V m
= A
ρ (10.5)
10.1.2 REMOVALOF HAZARDOUS AIR POLLUTANTSAND VOLATILE ORGANIC COMPOUNDSBY ABSORPTIONAND ADSORPTION
Absorption is widely used as a product-recovery method in the chemical and petroleum industry. As an emission control technique, it is more commonly employed for inor- ganic vapors. Some common absorption processes for inorganic gases are as follows:
• 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. Activated 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 maximum inlet concentration should be about 10,000 ppmv, with a usual minimum outlet concen- tration at 50 ppmv. In some cases, it may be advisable to design for minimum outlet concentration of 10–20 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. For excessively high concentrations, condensation or dilution could be used to bring the concentration to a more reasonable lower level.
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 liq- uid 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°F–130°F and the possibility of exothermic reac- tions exist.
10.2 PROCESS SYNTHESIS TECHNOLOGY FOR THE DESIGN OF