Combustion processes generate various primary and secondary air pollutants such as carbon oxides (mainly CO), nitrogen oxides (NOx), sulphur oxides (SOx), ozone, along with organic acid, inorganic acid, petrochemical oxidant gas and hydrocarbons (HC). Different treatment processes are applied for controlling these and other gaseous emissions. These processes include adsorption and absorption. Selection of appropriate technique depends in part, physical and chemical characteristic of specific gas and the vapour phase compounds present in the gas stream.
For stationary air pollution sources, we can select single or combined air pollution control technique. Variety of devices are used for controlling gaseous pollutant, and choosing most cost effective, most cost efficient unit requires careful attention to the particular operation for which control devices are intended. In order to control the emissions within given standard emissions it is necessary to monitor emissions carefully after selecting best control technique.
PROPERTIES OF GAS STREAM FOR SELECTION OF A CONTROL SYSTEM
The selection and design of a gaseous contaminant control system is done based on some specific information concerning the gas stream to be treated. Following are some factors considered during selection of a process
In adsorption process the contaminant removal is done by passing a stream of effluent gas through a pours solid material (adsorbent) contained in adsorption bed. The surface of porous solid material attracts and holds the gas (the adsorbate) by either by physical or chemical adsorption. The basic difference between physical and chemical adsorption is the manner in which the gas molecule is bonded to the adsorbent.
Table 2.7.1 Difference between physical and chemical adsorption
SALIENT FEATURE OF ADSORPTION PROCESS
(1) Adsorption processes are used extensively on large-scale applications having solvent vapour concentrations in the range of 10 to 10,000 ppm.
(2) Prior to becoming saturated with the solvents, the adsorbent is isolated from the gas stream and treated to drive the solvent compounds out of the solid adsorbent and into a small volume, high concentration gas stream.
(3) The desorbed gas stream is then treated to recover and reuse the solvents.
(4) The adsorbent is cooled (if necessary) and returned to adsorption service.
(5) Because the adsorbent is treated and placed back in service, these adsorption processes are termed regenerative.
(6) Adsorption processes usually operate at efficiencies of 90% to 98% over long time periods.
Table 2.7.2 Physical Properties of Major Type of Adsorbents
STEPS IN ADSORPTION PROCESS
Adsorption occurs in three steps
Step 1: The contaminant diffuses from the bulk gas stream to the external surface of the adsorbent material .
Step 2: The contaminant molecule migrate external surface to the macropores, transitional pores, and micropores within each adsorbent.
Step 3: The contaminant molecule adheres to the surface in the pore. Following figure illustrates this overall diffusion and adsorption process.
Figure 2.7.1: Adsorption steps
Steps 1 and 2 are diffusional processes that occur because of the concentration difference between the bulk gas stream passing through the adsorbent and the gas near the surface of the adsorbent. Step 3 is the actual physical bonding between the molecule and the adsorbent surface. This step occurs more rapidly than steps 1 and 2 .
Three types of equilibrium graphs are used to describe adsorption capacity,
(1) isotherm at constant temperature,
(2) isobar at constant pressure, and
(3) isostere at constant amount of vapour adsorbed.
Isotherm: The isotherm is a plot of the adsorbent capacity versus the partial pressure of the adsorbate at a constant temperature. Adsorbent capacity is usually given as pound of adsorbate per 100 pound of adsorbent. These type of graphs are used to estimate the quantity of adsorption. Isotherms can be concave upward, concave downward, or “S” shaped .
Isostere: The isostere is a plot of the natural log of the pressure versus the reciprocal of absolute temperature (ln(p) vs. 1/T) at a constant amount of vapour adsorbed. Adsorption isostere lines are straight for most adsorbate-adsorbent systems. The isostere is important because the slope of the isostere corresponds to the differential heat of adsorption. The total or integral heat of adsorption is determined by integration over the total quantity of material adsorbed .
Isobar: It is a plot of the amount of vapour adsorbed versus temperature at a constant pressure. Below figure shows an isobar line for the adsorption of benzene vapours on activated carbon.
Figure 2.7.2. Adsorption isobar for benzene adsorption onto carbon.
ADSORBENT REGENERATION METHODS
After a long period of operation and when adsorption bed becomes saturated replacement or regeneration of the adsorbent bed is necessary in order to maintain continuous operation. When the adsorbate concentration is high, and/or the cycle time is short (less than 12 hours), replacement of the adsorbent is not feasible, and in-situ regeneration is required. Regeneration is accomplished by reversing the adsorption process, usually increasing the temperature or decreasing the pressure.
Following four main methods used commercially for regeneration.
Thermal Swing: The bed is heated so that the adsorption capacity is reduced to a lower level. The adsorbate leaves the surface of the carbon and is removed from the vessel by a stream of purge gas. Cooling must be provided before the subsequent adsorption cycle begins. Pressure Swing: The pressure is lowered at a constant temperature to reduce the adsorbent capacity.
Inert Purge Gas Stripping: The stripping action is caused by an inert gas that reduces the partial pressure of the contaminant in the gas phase, reversing the concentration gradient. Molecules migrate from the surface into the gas stream .
Displacement Cycle: The adsorbates are displaced by a preferentially adsorbed material. This method is usually a last resort for situations in which the adsorbate is both valuable and heat sensitive and in which pressure swing regeneration is ineffective.
FACTORS AFFECTING THE PERFORMANCE OF ADSORPTION SYSTEM
Temperature: For physical adsorption processes, the capacity of an adsorbent decreases as the temperature of the system increases. With increase in the temperature, the vapour pressure of the adsorbate increases, raising the energy level of the adsorbed molecules. Adsorbed molecules now have sufficient energy to overcome the van der Waals’ attraction and migrate back to the gas phase. Molecules already in the gas phase tend to stay there due to their high vapour pressure.
Figure 2.7.3. Carbon capacity versus gas stream temperature
Pressure: Adsorption capacity increases with an increase in the partial pressure of the vapour. The partial pressure of a vapour is proportional to the total pressure of the system. Any increase in pressure will increase the adsorption capacity of a system. The increase in capacity occurs because of a decrease in the mean free path of vapour at higher pressures .
Gas velocity: The gas determines the contact or residence time between the contaminant stream and adsorbent. The slower the contaminant stream flows through the adsorbent bed, the greater the probability of a contaminant molecule reaching an available site.
In order to achieve 90% or more capture efficiency, most carbon adsorption systems are designed for a maximum airflow velocity of 100 ft/min (30 m/min) through the adsorber. A lower limit of at least 20 ft/min (6 m/min) is maintained to avoid flow problems such as channeling. Gas velocity through the adsorber is a function of the cross-sectional area of the adsorber for a given volume of contaminant gas.
Humidity: Activated carbon has more affinity towards nonpolar hydrocarbons over polar water vapour. The water vapour molecules in the exhaust stream exhibit strong attractions for each other rather than the adsorbent. At high relative humidity, over 50%, the number of water molecules increases to the extent that they begin to compete with the hydrocarbon molecules for active adsorption sites. This reduces the capacity and the efficiency of the adsorption system .
Bed Depth: Providing a sufficient depth of adsorbent is very important in achieving efficient gas removal due to the rate that VOC compounds are adsorbed in the bed. There are practical minimum and maximum limits to the bed depth.
Figure 2.7.4. Mass transfer zone