How to Remove Toxic Gases ? | Pollutants Control | Environmental Engineering

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Gaseous pollutants may be removed from n gaseous stream predominantly by physical or chemical absorption. The pollutant gases are selectively absorbed in a solvent liquid. The solvent liquid so chosen for the purpose must not be costly, operating cost should be minimum and also it should be easily recoverable.

Gas absorption is an operation in which a gas mixture is contacted with a liquid for the purposes of preferentially dissolving one or more components of the gas and to provide a solution of them in the liquid.

For example, the gas from by-product coke ovens is washed with water to remove ammonia and again with an oil to remove benzene and toluene vapours. Objectionable hydrogen sulphide is removed from such a gas or from naturally occurring hydrocarbon gases by washing with various alkaline solutions in which it is absorbed.

Choice of Solvent:

Water is, of course, the cheapest and most plentiful solvent, but the following properties are important considerations:

(a) Gas Solubility:

The gas solubility should be high, thus increasing the rate of absorption and decreasing the quantity of solvent required. Generally solvents of a chemical nature similar to that of the solute to be absorbed will provide good solubility. Thus hydrocarbon oil, and not water, are used to remove benzene from coke oven gas.

A chemical reaction of solvent with solute will frequently result in very high gas solubility, but if the solvent is to be recovered for reuse, the reaction must be reversible. For example, hydrogen sulphide can be removed from gas mixtures using ethanolamine solution since the sulphide is readily absorbed at low temperatures and easily stripped at high temperatures. Caustic soda absorbs hydrogen sulphide excellently but will not release it in a stripping operation.

(b) Volatility:

The solvent should have a low vapour pressure since the gas leaving an absorption operation is ordinarily saturated with the solvent and much may thereby be lost. If necessary, a second, less volatile liquid can be used to recover the evaporated portion of the first.

This is sometimes done, for example, in the case of hydrocarbon absorbers, where relatively volatile solvent oil is used in the principal portion of the absorber because of the superior solubility characteristics and the volatilized solvent is recovered from the gas by a non-volatile oil. Similarly, hydrogen sulphide can be absorbed by a water solution of sodium phenolate, but the, desulphurised gas is further washed with water to recover the evaporated phenol.

(c) Corrosiveness:

The materials of construction required for the equipment should not be unusual or expensive.

(d) Cost:

The solvent should be inexpensive, so that the losses are not costly, and should be readily available.

(e) Viscosity:

Low viscosity is preferred for reasons of rapid absorption rates improved Hooding characteristics in absorption towers, low pressure drops on pumping, and good heat transfer characteristics.

(f) Miscellaneous:

The solvent if possible should be nontoxic, non-flammable, and chemically stable and should have a low freezing point.

The choice among the above mentioned absorbents for a gas also depends upon the concentration of the pollutant and the final product requirement.

Selection of Equipment:

The purpose of the equipment used for the gas-liquid operations is to provide intimate contact of the two fluids in order to permit interphase diffusion of the constituents. The rate of mass transfer is directly dependent upon the interfacial surface exposed between the phases, and the nature and degree of dispersion of one fluid in the other are therefore of prime importance.

The equipment can be broadly classified according to whether its principal action is to disperse the gas or the liquid, although in many devices both phases become dispersed.

(a) Gas Dispersed Devices:

In this group are included those devices, such as sprigged and agitated vessels and the various types of tray lowers, in which the gas phase is dispersed into bubbles or foams. Tray towers are the most important of the group since they produce counter-current, multistage contact, but the simpler vessel contactors have many application.

(b) Liquid Dispersed Devices:

This group includes devices on which the liquid is dispersed into thin films or drops, such as, welled wall towers, sprays and spray lowers, the various packed towers and the like. The packed towers are most important of the group.

Before choosing the type of equipment, the knowledge of the main resistance lo mass transfer in the system being considered is necessary. Gas dispersed type of equipment’s are suitable for the case where major resistance to mass transfer lies in the gas phase and vice- versa.

For various systems, the major resistance to mass-transact may be within one phase only or both the phases may be important. For some of the gas-liquid adsorption systems, the idea of resistance to mass-transfer is given in Table 12.2.

Absorption Processes:

One of the most frequently used techniques for controlling the composition of industrial waste gases before discharge to the atmosphere.

SO₂ is emitted from coal fired power plants (about two thirds of emissions), from industrial fuel combustion, sulphuric acid manufacturing, and smelting of nonferrous metals.

The technical and economic feasibility of SO₂ removal process depends on the type and quantity of effluent gas treatment problems. There are essentially two types of effluent gas treatment problems. The first is the problem of removing SO₂ from power plant flue gases. Power plant flue gases generally contain low concentrations of SO₂ (< 0.5% by volume), but emitted at tremendous volumetric flow rates.

For example, a coal fired power plant burning 2% sulphur coal (by weight) will produce 40,000 kg of SO₂ for every 106 kg coal burned. The second class of SO₂ effluent gas treatment problems comprises those resulting from the need to remove SO₂ from streams containing relatively high concentrations of SO₂ at low flow rates. Streams of this type are typical of those emitted from smelter operations. A smelter emission gas typically contains SO₂ at a concentration of about 10% by volume.

In this article we would largely concentrate on the problem of SO₂ removal from power plant flue gases.

The treatment is based on what is done with SO₂ absorbing or SO₂ reacting medium, and by this means processes are categorized as:

(A) Throw away process or

(B) Regenerative process.

In a throw away process, the sulphur is removed, together with the absorbing or reacting medium, is discarded. But the process is regenerative if the sulphur is recovered in a useable form and the medium is reused.

In the majority of throw away processes and alkaline agent reacts with SO₂ leading to a product that is discarded. Commonly used agents in this type of process are limestone (CaCO₃) and lime (CaO).

In the regenerative alkaline processes, and alkaline agent strips SO₂ from the flue gas stream is combining chemically with the SO₂. In a separate regeneration step, the agent is reconstituted and sulphur is recovered, usually as liquid SO₂ or sulphuric acid. Some of the agents used are MgO, NO₂ SO₃ and metal carbonates.

Throwaway Process:

The most prevalent throw away process involve lime and limestone. Approximately 75% or all installed flue desulphurization systems use a lime or limestone slurry as the scrubbing liquor. It this process SO₂ reacts with the lime or limestone slurry to form a CaSO₃/CaSO₄ sludge that must be disposed of in a pond or landfall. Most wet scrubbing flue gas desulphurisation systems are capable of reducing SO₂ emissions by 90%.

Equipment:

Spray dryers are the units where hot flue gases come in contacted with a line, wet, alkaline spray, which absorbs the SO₂ The temperature of the flue gas (150- 300°C) evaporates the water from the alkaline sprays leaving a dry product that can be collected in a bag-house or electrostatic precipitator. (See Fig. 12.1)

Reactions are as follow: (Calcium Carbonate)

CaCO₃ + SO₂ + H₂O → CaSO₃+ CO₂ + H₂O

(Lime)Ca(OH)₂ + SO₂ → CaSO₃+ H₂O

(A) Advantages:

(a) The use of relatively inexpensive material

(b) The production of an innocuous and more readily disposable solid by product than many other processes.

(B) Disadvantages:

(a) Scaling of the process equipment with inherent high maintenance costs.

(b) Lower reliability because of scaling when compared to other processes.

(c) Because of relative insolubility of lime-stone or lime much more of these chemicals are needed than would be required for complete reaction to calcium sulphite. Therefore a high percentage of unused different lime or limestone are discharged from the process along with the calcium sulphite.

(d) Since different calcium salts have different solubilites (e.g. one will be more soluble in hot water while the other will be less soluble) changes in temperatures are critical for the reduction of scaling, therefore this process requires constant monitoring of the system with the added cost for personnel and equipment.

Sodium Solution Scrubbing (Without Regeneration):

The main problem with lime and limestone scrubbing are scaling and plugging inside the scrubber unit. The Dual alkali system eliminates these problems. A solution of sodium carbonate (Na₂CO₃)/Sodium hydroxide (NaOH) is sprayed in the lower. SO₂ is absorbed and neutralised in the solution, and since both Na₂SO₃ and Na₂SO₄ are soluble in water, no precipitation occurs in the scrubber.

Reactions are as follows:

(Hydroxide) 2NaOH + SO₂ → Na₂SO₃ + H₂O

Na₂SO₃ + 1/2 O₂ → Na₂SO₄

(Carbonate) Na₂CO₃+ H₂O + SO₂ → Na₂SO₃, + H₂O + CO₂

Na₂SO₃+ ½ O₂ → Na₂SO₄

Details of process are given in Fig. 12.2.

(A) Advantages of this process:

(a) Complete reliability in the operating performance.

(b) The SO₂ is converted to a liquid waste which may be more easily disposed or treated than other systems,

(c) This process is capable of removing almost 100% of the SO₂ in the flue gas.

(B) Disadvantages of this process:

(a) This process requires the use of relatively more expensive chemical.

(b) This process can only be used where codes permit disposal of dissolved solids.

(c) Where codes require that a waste should have no C. O.D., a C.E.A. type aeration process must be added to the system to convert the dissolved sulphite to sulphate.

This is a highly reliable process with a relatively low investment in capital requirements. It is ideally suited to older plants and for gases with lower SO₂ concentrations. It is highly adaptable to changes in air pollution codes since capacity can be built in for 50% to almost 100% removal of SO₂ from the gases for less modification needed.

Sodium Solution Scrubbing (With Regeneration):

This process is identical to the sodium solution scrubbing without regeneration except the by-product of SO₂ absorption, sodium sulphite is reacted with either lime or limestone. The sodium sulphite is converted back to its original state with precipitation of the absorbed sulphur in the form of calcium sulphite and the NaOH is regenerated. (Fig. 12.3)

Reactions are as follows:

(Lime) Na₂ SO₃ + Ca (OH)₂ → 2 NaOH + CaSO₃

NaHSO₃ + Ca (OH)₂ → NaOH + CaSO₃+ H₂O

(Lime Stone) 2 NaHSO₃ + CaCO₃ → Na₂SO₃ + CaSO₃ + CO₂ + H₂O

(A) Advantages of this process are:

(i) Reliability, no scaling, good control.

(ii) More attractive cost wise than previous one as the volume and concentration of SO₂ increased in the flue gas.

(c) More easily disposable and innocuous solid which may be simply separated from the regenerated solution.

(d) Less complex than calcium process [A. I].

(B) Disadvantages of this process are:

Relatively more complex process as compared to sodium solution process without regeneration.

Highly reliable and adaptable process. Offers combined advantage of non-regenerative system at a lower cost for chemicals. Disposal of calcium salts creates no water pollution problem.

Magnesium Oxide Scrubbing:

In this process a slurry of MgO is reacted with SO₂ to form insoluble magnesium sulphite of which a bleed in removed to centrifuge where in the other liquor is returned to the scrubbing circuit and the solid cake containing essentially hydrated magnesium sulphite and unreacted MgO is sent to a dryer.

The anhydropus magnesium sulphite and oxide are then sent to a calciner where MgO and SO₂ are liberated. SO₂ is either used to produce sulphuric acid or other sulphur products whereas MgO is returned to the scrubber. (See Fig. 12.4.)

Reactions are follows:

MgO + SO₂+ 6 H₂O → MgSO₃. 6 H₂O

MgSO₃ .6 H₂O – heat → MgSO₃ + 6 H₂O

Mg SO₃ – heat → MgO + SO₂

Advantages of this method are as follows:

(a) No by-products

(b) SO₂ utilized immediately, but needs an acid or similar plant at the power plant site.

Wet Simultaneous No_x/So_x Process (For Removal of Pollutants):

Although wet NO_x removal processes do not as yet complete economically with dry NO_x processes, wet simultaneous NO_x/SO_x processes may be competitive with the sequential installation of dry NO_x control followed by SO₂ control by flue gas desul furization (FGD). The first wet simultaneous NO_x/SO_x systems, called oxidation/absorption/reduction processes, evolved from FGD systems.

Since the NO is relatively insoluble in aqueous solutions, a gas-phase oxidant, such as ozone (O₃) or chlorine dioxide (CO₂) is injected before the scrubber to convert NO to the more soluble NO₂. The absorbent then forms, with SO₂ a sulphite ion that reduces a portion of the absorbed NO_x to N₂.

The remaining NO_x is removed from the waste water as nitrate salts, while the remaining sulphite ions are oxidized to sulphate by air and removed as gypsum. Oxidation/absorption/reduction processes have the potential to remove 90% of both SO_x and NO_x from combustion flue gas.