The different gaseous control devices are explained as follows:
1. Adsorption:
The forces which hold atoms, molecules or ions together in the solid state exist throughout the body of a solid and at its surface. The forces at the surface may be considered to be “residual” in that they are available for binding other molecules which come in contact or in very close proximity to it. Any gas, vapour or liquid will therefore, adhere to some degree to any solid surface.
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This phenomenon is called adsorption or sorption. The adsorbing solid is called the adsorbent or sorbent and the adsorbed material is the adsorbate or sorbate. Adsorption is useful in air pollution control because it is a means of concentrating gaseous pollutants, thus facilitating their disposal, recovery or conversion to innocuous or valuable products.
The actual adsorption process is classified as either physical adsorption or chemi-sorption. In physical adsorption, the gases molecules adhere to the surface of the solid adsorbent as a result of inter-molecular attractive forces (Vander waals forces) between them. The adsorption process is exothermic and liberates heat depending upon the magnitude of the attractive force.
The advantage of physical adsorption is that the process is reversible. The adsorbed material can be easily removed or desorbed by reducing the pressure or by increasing the temperature without any change in the chemical composition. The amount of gas physically adsorbed decreases rapidly with increasing temperature and is quite small when the temperature is above the critical temperature of the adsorbed component.
Physical adsorption is usually directly proportional to the amount of solid surface available. However these build up is not restricted to a mono molecular layer; a number of layers of molecules can build up on the surface. Another desirable characteristic of physical adsorption is that the rate generally is quite rapid.
Chemisorption, results from chemical interaction between the adsorbate and the adsorbing medium. The bonding force associated is much stronger and the heat liberated during the process is much larger than that in physical adsorption. The energy required for the chemisorbed molecules to react with other molecular species may be considerably less than the energy required when the two species react directly in the gas phase.
This lower energy requirement is one basis of explanation of the catalytic effect of solid surfaces in enhancing the rate of some chemical reactions. Chemisorption is an irreversible process. On desorption the chemical nature undergoes a chemical change. Chemisorption process which is responsible for the catalytic effect is extremely important in number of air pollution control systems.
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The quantity of material that can be adsorbed by a given weight of adsorbent depends on the following factors:
(i) The concentration of the material in the space around the adsorbent,
(ii) The total surface area of the adsorbent
(iii) The temperature
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(iv) The presence of other molecules in the environment which may compete for a place on the adsorbent
(v) The characteristic of the molecules to be adsorbed i.e. weight, electrical polarity, chemical activity, size and shape
(vi) The micro-structure of the adsorbing surface i.e. sizes and shapes of pores and
(vii) The chemical nature of the adsorption surface including electrical polarity and chemical activity.
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Some of the adsorbents commonly used in air pollution control are activated carbon, activated alumina, silica gel and molecular sieves etc. Adsorption techniques are widely used in the field of odour control for removing small quantities of pollutants present in a large volume of air. Adsorption techniques are also used for collecting valuable organic substances that cannot be picked up by scrubbing methods.
Adsorbents:
i. Activated Carbon:
Activated carbon (or activated charcoal) consists of particles of moderately to highly pure carbon with a large surface area per unit weight or volume of solid (of about 1500 m2/gram of adsorbent). It is made by the carbonisation of coal, wood, fruit pits, petroleum residues and coconut or other nut shells.
The extent and micropore structure of the carbon surface are of prime importance. The distribution of pore diameters is also determined both by the nature of the raw material and by the activating process. A typical activated carbon has pore diameters of about 250 micron.
Activated carbon consisting largely of neutral atoms of a single species, presents a surface with a relatively homogeneous distribution of electrical charge. Therefore, activated carbon is effective in adsorbing molecules of organic substances with less selectivity than is exhibited by other more polarsorbents.
Activated carbon is effective in adsorbing organic molecules even from a humid gas stream. The activity is a measure of the maximum amount of a vapour that can be adsorbed by a given weight of carbon under specified conditions of temperature, concentration of the vapour and concentration of other vapours (usually water).
ii. Molecular Sieves:
Molecular sieves have recently been developed for the control of SO2, NOx and Hg emissions. These materials can be tailor-set to adsorb a particular size and type of gas molecules. They can be natural crystalline zeolites or synthetic metal hydrates. The synthetic metal alumina silicates have pore diameters ranging from 3 to 10 Angstrom (10–10m). The usually used metal ions are calcium, sodium, magnesium, potassium or any combination of these.
Molecular sieve pores are custom sized to adsorb only desired molecules. The effect of pore size on separation selectivity is illustrated by some data on molecular sieves compiled by the Union Carbide Corporation. When the nominal diameter is 0.003 micron the molecules adsorbed typically might be H2O and NH3.
By increasing the size to 0.004 μm, larger molecules such as CO2, SO2, H2O, C2H4, C2H6 and C2H5OH are also adsorbed. A further increase to 0.005 (am allows the additional adsorption of natural paraffins to the exclusion of branched and cyclic hydrocarbons. Thus a fairly strong specificity can be built into molecular sieves, which enhances the engineer’s control over the adsorption phenomena.
Molecular sieves contain metallic positive ions. Polar gas molecules are attracted and retained better in sieves.
A new molecular sieve material called silicate has been developed from a polymorph of silica. This sieve will adsorb small non polar organic molecules of the size of benzene and smaller, but will not adsorb the polar water molecules.
Molecular sieve regeneration consists of heating (e.g. by steam), decreasing pressure or stripping with a non-adsorbent phase gas. The adsorbed material in molecular sieves can also be displaced with a purge of adsorbable liquid, frequently water, then dried and rinsed.
iii. Other Adsorbents:
All important sorbents other than carbon are simple or complex oxides. These sorbents show considerably greater selectivity than activated carbon and overwhelmingly greater performance for polar than for non-polar molecules. Due to this, they are more useful than carbon when separations are to be made among different types of pollutants, but much less useful when overall decontamination of an air stream is to be accomplished. They are essentially ineffective for direct decontamination of moist air or gas stream.
Siliceous adsorbents comprise of silica gels, fullers and other siliceous earths etc. These materials are available naturally in a wide range of adsorbent capacities per unit weight or volume of solid and their capacities are of the same order of magnitude as that of the most highly activated carbons.
Silica gels are produced commercially by the reaction of sodium silicate and sulphuric acid. These adsorbents have pore sizes ranging from 20 to 10,000°A and normally break down at temperatures of 250°C and above. Some metallic oxide adsorbents act as desiccants, catalyst carriers or catalysts; they are never used directly for source control of air-borne pollutants by physical adsorption because they are not electrophilic.
Activated alumina (i.e. aluminum oxide) is an example of this type of adsorbent and is popularly used for the adsorption of moisture from gases. It can withstand high temperatures also.
iv. Modified Adsorbents:
For certain gases, it has been observed that the adsorptive affinity has been increased by the use of impregnated activated carbon. The addition of other chemicals to an adsorbent can promote a chemical reaction that can tie up the adsorbent with the additive. This includes chemical adsorption, can greatly increase the rate of adsorption as well as increase the capacity of the system.
If the chemicals are not properly chosen, it can lead to degradation of the adsorbent causing it to be structurally weaker and to wear quicker due to attrition from the gas flow. The additive may react chemically with the adsorbent, especially at higher temperature, which could weaken and ultimately destroy the effectiveness of the adsorbent.
The rate of chemical reaction between the various pollutants adsorbed can be increased by impregnating the adsorbent with a catalyst. The modified adsorbents are often not capable of being regenerated. For example, lead acetate impregnated carbon causes chemical adsorption of hydrogen sulphide from a gas stream. The resulting lead sulphide deposit cannot be recovered without destroying the carbon, so the carbon adsorbent must be discarded when saturated.
Adsorption Equipment:
Adsorbers are the devices that physically contain the adsorbent solid through which the effluent gas passes. Some of these adsorption reactors are Single-bed adsorber and fixed bed unit.
I. Fixed Bed Unit:
In this type the containers are of vertical or horizontal cylindrical shell. Activated carbon, often used as the adsorbent, is arranged as beds or trays in thin layers of 1.5 cm thickness. This saves power because of less resistance to the flow of air. These thin bed adsorbers are most oftenly used in purification of air containing very low concentration of pollutants.
The adsorption is rapid and the contaminants cannot build up on the surface rapidly enough to reduce the collection efficiency of the thin bed adsorber layer. In deep-bed adsorbers, the layers are deeper than 1.5 cm. They occupy the least amount of space and are simpler to fabricate than thin bed adsorbers. Deep-bed adsorbers will be used where the savings on power costs are over ridden by other determining factors.
II. Moving Bed Adsorbers:
In this unit, activated carbons contained in a rotating drum acts as the adsorption bed. The effluent gas contaminant is moved into the rotating drum.
The vapour-laden air enters the ports above the carbon bed, passes through the cylindrical activated carbon bed, enters the space inside of this drum and then finally leaves through the ports at the ends of the drum.
III. Fluidised Adsorber:
It contains a shallow floating bed of adsorbent and when gas passes upward through the bed, it expands and fluidizes the adsorbent. The expanding and fluidizing of the adsorbent provides intimate contact between the contaminated gas and the adsorbent and prevents channeling problems often associated with fixed beds.
Most of the adsorption units are highly efficient until a break point occurs when the adsorbent becomes saturated with adsorbate. At this point, the concentration of pollutants in the exit gas stream begins to rise rapidly and the adsorber must be regenerated or renewed. Depending upon the collected gas desorption, adsorbers can be classified as regenerative or non-regenerative.
The non-regenerative process is more costly because the adsorbent must be disused after exhaustion and replaced with new material. But, non-regenerative systems are economically applicable for control of pollutant sources whose vapours are odourous but present either in low concentrations (i.e. under about 2 ppm) or only at intermittent intervals (e.g. laboratory exhaust systems) or both.
The regenerative systems provide for the on-site periodic recovery of the adsorbent or adsorbate. Such systems may advantageously be used for the removal of vapours from polluting sources in which concentrations are about 1000 ppm (0.1%).
In systems which rely on physical adsorption, regeneration of an adsorbent can be accomplished by use of super-heated steam or circulating hot air. The bed must be cooled before reuse. The system that allows for regeneration usually has more than one adsorption unit in operation so that the flow can be switched on to unsaturated bed while the saturated bed undergoes regeneration.
Applications of Adsorption Equipment:
Recovery of valuable materials from the process industries is economically desirable and adsorption allows for economically feasible recovery when concentrations of organic vapour ate sufficiently high.
The application of equipment include recovery of is propyl alcohol from a citrus-fruit processing plant, recovery of methyl chloroform from a movie-film processing plant, recovery of ethyl alcohol vapours from a whiskey ware house and removal of contaminants from air prior to use in an operating room or an electronics control room.
Organic vapours that can be controlled by various adsorption processes include those discharged by the following industrial processes: dry cleaning, degreasing, paint spraying, tank dipping, solvent extracting and metal foil coating. Emissions from plastics, chemical, pharmaceutical, rubber, linoleum, transparent wrap manufacturing processes and fabric impregnation processes may also be controlled by adsorption.
2. Absorption:
The principle of gas absorption is a gas-liquid contacting process for gas separation that utilises the preferential solubility or chemical reactivity of the pollutant gas in the liquid phase. In the gas absorption technique, effluent gases are passed through absorbers (scrubbers) containing liquid absorbents’ that remove, treat or modify one or more of the offending constituents in the gas stream. Liquid absorbents may utilise either chemical (reactive) or physical (non-reactive) changes to remove pollutants.
Efficiency of absorption depends upon the amount of surface contact between the gas and the liquid (greater the surface, the greater will be the absorption), the time gas is allowed to remain in contact with the liquid, the concentration of the absorbing media, and the speed of reaction between the absorbent and the gas.
Absorption (or scrubbing) has been used in the control of gases such as SOx, NOx, H2S, HCl, Cl2, NH3 and HCs. Removal of HCs by absorption is employed in many industries, notably, asphalt batch plants, coffee roasters, petroleum coker units and varnish and resin cookers. Absorption is also used for recovery of valuable by-products like acetic acid, formic acid, chloroform, amines and ketones etc.
Liquid Absorbents:
Liquid absorbents may be classified as reactive if the absorbent utilises chemical change to remove pollutants. As an example, SO2 may be removed from the gases by injecting water and limestone, which reacts to form calcium hydroxide.
Calcium hydroxide then reacts with sulphur dioxide to form calcium sulphate salt, which can be scrubbed from the gas stream by moving water. If gases are removed by simply dissolving the gas without chemical change, the absorbent is termed as non-reactive absorbent. Water or heavy carbon oil are examples of non-reactive absorbents.
Gas solubility differs from absorbent to absorbent. For example, the solubility of nitrogen, oxygen or carbon dioxide is two to ten times greater in ethanol, acetone or benzene than in water. The absorbents that are chemically similar to the absorbate (solute) generally provide good solubility.
Other than this, absorbates should have a low freezing point and low toxicity and should be non-volatile, non-flammable and chemically stable. Economical operation demands that it should be relatively inexpensive, readily available and non-corrosive, to reduce equipment repair and maintenance costs.
The main absorbents used in the various SO2 absorption processes are aqueous solutions of alkalies (sodium and ammonia) and the alkaline earths (calcium and magnesium). The leading alkali absorbent, sodium has the advantages over ammonia solutions as it is not volatile.
Ammonia solution also is widely used as a scrubber especially in fertiliser industry, as ammonium sulphate, the by-product obtained is more desirable than sodium sulphate. Alkaline earths i.e. magnesium oxide (MgO), calcium oxide (CaO) and calcium carbonate (CaCO3) is also used in many industries.
An absorbent that cannot be regenerated for reuse but instead must be discarded is referred to as a non-regenerative absorbent. Eg – water. Absorbent that can be forced to release the gaseous pollutant that it has captured reversibly by application of heat or steam or by pressure change is referred to as a regenerative absorbent.
A regenerative absorbent may allow reuse of expensive chemicals or catalysts may be necessary to chemically neutralise the pollutant for disposal as a solid or liquid, or may aid in concentration of pollutant for further processing. Eg – carbon tetrachloride, which under pressure combines with chlorine gas and removes it from the effluent gas stream.
Absorption Units (Absorbers):
Absorbers are the devices that physically contain the absorbent liquid through which the effluent gas is passed to provide optimum diffusion of the gas into the solution. Several types of absorbers that are currently used include spray towers, plate or tray towers, packed towers and venturi scrubbers.
3. Combustion:
In the combustion process, organic compounds released from different manufacturing operations are converted to innocuous carbon dioxide and water. The combustion equipment used to control air pollution emissions are designed to push oxidation reactions as close as possible to completion, leaving a minimum of unburned compounds. To obtain complete combustion, a proper proportion of oxygen, temperature, turbulence and time must be provided.
The normal range for the 3Ts is – temperature – 375-825°C; residence time – 0.2-0.6 sec; gas velocity – 4-8 ml/sec. The lower heating values in kJ/kg. mole of some common gaseous fuels are – methane, CH4: 802,000: ethane, C2H6: 1428,000; propane, C3H8: 2044,000:carbon monoxide, CO: 283,000.
The type of combustion process to be used depends upon the type of fuel to be burned and its concentration. Most of the fuels – solid, liquid or gaseous, contain C, H, O and S which produce CO2, CO, SO2, H2O and unburned HCs.
The three methods of combustion commonly used in air pollution control are:
(1) Direct combustion
(2) Thermal combustion and
(3) Catalytic combustion.
(1) Direct-Flame Combustion:
Direct-flame combustion, as the name implies, is a method by which the waste gases are burned directly in a combustor with or without the aid of additional fuel such as natural gas. In some cases the waste gas itself may be a combustible mixture without the addition of air.
In other cases, introducing air and or adding a small amount of supplemental fuel will bring the gaseous mixture to its combustion point. In a well-designed combustor, the combustion can be carried out when the combined mixture has a heating value of 900 kcal/m3 or more. If the heating value is low the gases should be preheated for the combustion process.
Flares:
Flares are usually open ended combustion units maintained at the end of a stack or chimney discharging the waste gas stream. They are equipped with pilots to ensure continuous burning of gases. The combustion process should be designed in such a way that the flame burns at any wind speed, any gas flow rate and fuel compositions.
Most flares are elevated 5-100m above ground level. However, some flares, particularly emergency flares are located at ground level itself but in very well protected area. The height of flares required above the surrounding objects, H is given by –
Direct flame combustion is a relatively safe method of disposal of the large quantities of highly combustible waste gases. Direct flame combustion processes are economical only if the waste gas itself contributes more than 50% of the total heating value required for incineration.
(2) Thermal Combustion:
When the concentration of combustible pollutants is below the lower explosive limit, thermal incinerator or alter burner is one of the choice for combustion. The method is often used when the heating value of the waste gas is in the range of 50-750 kJ/m3.
The waste gas stream is preheated in a heat exchanger and then passed through the combustion zone of a burner supplied with supplemental fuel. As a result, the combustibles in the waste gas stream are brought above their ignition temperatures and burn with the oxygen present in the contaminated stream.
The temperatures of operation depend upon the nature of the pollutants in the waste gas, as given below:
A properly designed and operated incinerator can completely destroy the organic vapour from the exit gases from the coffee roaster and smoke houses. A typical thermal incinerator assembly incorporating a recuperator. The use of a recuperator is to reduce the fuel requirements for thermal incinerator.
In fact, the initial cost of the incinerator may be doubled by the addition of recuperation but the clear gas stream leaving recuperator may still have a high temperature, and hence may be used further as a preheat source for other operations.
(3) Catalytic Combustion:
A catalyst accelerates the rate of a chemical reaction without undergoing a chemical change itself. As a consequence, the residence times required for catalytic units are much less than those required for thermal units. Thermal units require 20 to 50 times as much residence time as a catalytic unit. In catalytic units, the waste gas stream need not be heated to high temperatures as in thermal incineration, because, ignition temperature is lower in catalytic processes.
The catalytic temperature ranges for some pollutants are:
The catalytic oxidation of combustibles proceeds through (i) adsorption of the gas on the active surface (ii) chemical reaction of combustible with the oxygen and (iii) desorption of the reaction products from the surface. Hence the catalytic process is a surface reaction and suitable methods of exposing the maximum surface area are prime design criteria.
Most of, the waste gases containing combustible pollutants from industrial processes are at a fairly low temperature. Therefore, preheating burner is used to bring the waste gas upto that temperature at which the catalyst will be effective. As the efficiency of catalytic combustion is on the order of 95 to 98 per cent, the combustion is almost complete and the effluent gases from the catalytic conversion are CO2, water vapour and nitrogen only.
Many substances have catalytic properties but only a few are used for waste gas treatment. In order to be useful in air pollution control the substance must be relatively inexpensive, long lasting, able to function at the required temperature, and capable of being used in a variety of shapes. Successful catalytic beds have been formed into ribbons, rods, beads, pellets and other shapes. The catalysts used for effective pollution control are the precious metals, primarily platinum and palladium or their alloys.
Catalytic units are widely used for processes involving paint and enamel bakes ovens, varnish kettles etc. The pressure drop through catalytic units is very low. Thus the operating costs are also low with the exception of the cost of maintaining the catalyst. The maintenance cost of the catalyst depends upon the nature of the fume.
The waste gas must be cleaned of particulates before it enters the incinerator. Deposition of particulates on the surface of the catalyst bed decreases the available surface area for catalytic action. This lowers the effectiveness of the bed as well as its life time. The normal operating life of a catalyst without particulate deposition problem may be from 3 to 5 years.
4. Condensation:
Condensation of vapour from the effluent gases as a method of recovery is applicable only if the vapour gas mixture is rich in vapour or saturated with it. In fact, it is most desirable that as much of the material as possible is recovered by condensation by cooling the gases to a temperature which is economically viable.
Condensation of organic material using water at room temperature serves as an effective preliminary removal method prior to treatment by methods such as adsorption or combustion. However, the efficiency of condensation can be increased by employing a refrigerated fluid such as chilled water.
Surface and contact condensers are the two basic types of condensation equipment. In surface condensers, the coolant such as water, refrigerant, chilled water or brine passes through the tubes whereas the vapour is on the shell side.
In this condensation, physical adsorption plays a key role, since contaminants are adsorbed onto a surface as gaseous component condenses. In the shell and tube condenser, as the cooling medium flows through the tubes, the vapour condenses on the surface of the tubes. The condensed vapour collects as a film of liquid and the liquid drains off to storage.
In a contact condenser, the vapour and cooling medium are brought into direct contact. The cooled vapour condenses and the water and condensate mixtures are removed, treated and disposed off. The chief advantages of contact condensers are that they are less expensive and more flexible than surface condenser and they are more efficient in removing organic vapour. The different absorption processes like spray towers, packed towers, venturi and cyclone scrubbers also belong to the family of contact condensers.
The specific application of the process of condensation depends upon the amount and type of coolant used the liquid waste disposal problems and the amount of compound to be recovered. The method of condensation is widely used as an air pollution control device in petroleum refining, petrochemical manufacturing, manufacturing of ammonia and chlorine solutions and miscellaneous processes involving dry cleaning, degreasing and tar dipping.