GASEOUS COMPOUNDS The Control of Organic Compound Emissions

13.3.1.1 Introduction

While process and equipment modification are generally the preferred alternatives for reducing emissions from a plant, some form of control is necessary before emissions are discharged into the environment. Technolo­gies discussed in this section are applicable in preventing emissions from point sources such as process or tank vents. These technologies fall into two main categories:

1. Destruction methods, including incineration and biological gas purification

2. Recovery methods, including adsorption, absorption and condensation

The different technologies can be used separately or combined, such as gas adsorption followed by incineration. Depending on the system used and the organic compound content in the gas stream being treated, the result­ing destruction efficiencies normally range between 90% and 99%.

Thermal and catalytic incinerators, condensers, and adsorbers are the most common methods of abatement used, due to their ability to deal with a wide variety of emissions of organic compounds. The selection between destruction and recovery equipment is normally based on the feasibility of recovery, which relates directly to the cost and the concentration of or­ganic compounds in the gas stream. The selection of a suitable technology depends on environmental and economical aspects, energy demand, and ease of installation as well as considerations of operating and maintenance. The selection criteria may vary with companies or with individual process units; however, the fundamental approach is the same.

The aim of this section is to introduce the fundamentals of incinera­tion, adsorption, absorption, condensation, and biological treatment in order to provide a basic knowledge for the selection of suitable equip­ment. The waste gas characteristics that play a major role in the selection of gas-cleaning equipment are also considered. A detailed presentation of the theory of combustion, adsorption, absorption, condensation, or bio­logical decomposition required for a complete understanding of the sub­ject is not covered in this section (the theory can be found in the handbooks such as Perry’s Chemical Engineers’ Handbook).

Emission abatement methods covered are suitable for the emission control of volatile organic compounds (VOCs). The VOCs include organic compounds existing in the gaseous phase in air at 293.15 K. However, or­ganic compounds, which are not regarded as VOCs, can be treated by the methods covered in this section.

13.3.1.2. Factors Influencing Equipment Selection

General

The first task in selecting an abatement method is the preparation of an emission inventory. The inventory is the basis of planning and the selec­tion of options. By the preparation of an inventory, all emission sources re­quiring treatment can be determined and recorded. The emission inventory

Should cover the entire facility, source by source (Fig. 13.18); the following characteristics must be included in the inventory:

1. Pollutants emitted

2. Individual chemical species within each waste gas stream

3. Composition of waste gas (ranges of each component indicated)

4. Oxygen content of waste gas

5. Operating pressure

6. Operating temperature

7. Emission type (continuous/intermittent)

8. Emission rates (hourly, annual, and average rates and maximum rate at the worst conditions)

9. Condition of source or equipment

10. Availability of suitable control

11. Regulatory considerations, covering national and international standards with proposed future changes and developments

The characteristics of the downstream pollution discharge must be moni­tored (see Fig. 13.18). It is essential that the operation and maintenance of the pollution control equipment be included in a quality audit procedure, assisting in determining the operation efficiency of the equipment and the formation of unwanted and possibly toxic compounds in the pollution control steps. Un­suitable operation of an incinerator may result in partial oxidation and forma­tion of unwanted combustion products or excessive formation of NO.

Effluent Stream Characteristics

Volumetric Flow Rate The equipment size is normally dictated by its ca­pacity and is therefore directly related to investment costs. Incineration systems are capable of handling large amounts of waste gases and are often the most cost- effective method when handling large flows. Adsorption systems can handle large volumes of gases, provided that the gas stream is fairly dilute. Absorption will

Emission 1 — Pressure — Release type — Pollutants — Composition — Flow rate and other stream characteristics — Regulatory aspects

Emission 3 — Pressure — Release type — Pollutants — Composition

Emission 2

— Pressure

— Polluatants

— Composition

— Flow rate and other stream characterisitics

— Regulatory aspects Emission 3b

— Emission data

After pollution

Pollution ‘ control equipment

— Flow rate and other stream characteristics

— Regulatory aspects

— Details of pollution control equipment

Handle large gas volumes, but it is not a cost-effective method when very large col­umns are necessary. Condensation is used mainly for low gas flow rates.:

Concentration and Composition2 The average concentration of organic compounds in a waste gas determines the applicability of the abatement method. Recovery methods usually require high inlet concentrations. They may need a concentrator prior to actual treatment, which increases the investment cost.

Lower and upper explosive limits (LEL, UEL) must be considered in order to avoid dangerous operation in the incinerators. Thermal incinerators are normally designed to operate with concentrations below 25% of the LEL. Typically, the LEL ranges from 2500 to 10 000 ppmv.3

When a mixture of compounds is to be treated, more limitations may be placed upon the selection of a suitable abatement method. There may be sev­eral compounds in the waste gas, some being unsuitable to one method, while others are unsuitable to another method. In such cases, thermal incineration may be the best solution. When recovering mixtures, additional separation equipment may be needed for recycling the reclaimed compounds.

Temperature and Humidity1’2 When adsorption, absorption, or con­densation is employed, the lowest inlet gas temperature is desirable. Adsor­bent and absorbent capacities generally increase with the decreasing gas temperature. High waste-gas temperatures may preclude the use of adsorp­tion or condensation due to the cost of chilling. Thermal and catalytic oxi­dation benefit from a hot effluent gas stream, as that reduces the supplementary fuel requirement. In biological treatment, a wTaste-gas tem­perature of near 37 °C is ideal.

The degree of humidity of the waste gases adds to the heat load on in­cineration options. Condensation of the moist gases produces an aqueous phase, which must be separated to prevent freezing of the condenser. When activated carbon is the adsorbent, 50% relative humidity should not be ex­ceeded. Some zeolites can withstand a relative humidity in the 80-90% range.4 Biological treatment is suitable for humid gases. Relative humidity over 90% is required in the incoming gas stream for compost biofilters.

Particulate Matter, Chlorinated Hydrocarbons, and Toxic Pollutants1’2 Many compounds can cause problems in pollutant-control equipment. Par­ticulate matter, liquids, or solids in the waste stream can plug the adsorber beds, heat-recovery beds in regenerative thermal incinerator systems and biofilters. Conventional filtration systems are used to remove particulate matter before or after the process.

As many emissions involve chlorinated compounds, corrosion is a ma­jor problem in many control methods. The corrosion of columns and sur­face condensers can be prevented or reduced by the correct material selection. However, corrosion remains a constant threat to the interior of incinerators. Additional pollution control equipment such as scrubbers may also be required to remove acidic compounds from treated gases be­fore discharging into the atmosphere.

When toxic or hazardous pollutants are handled, special wasre-disposal and material-handling requirements are essential. Adsorption, absorption, and

Condensation processes all produce some liquid or solid waste. In catalytic in­cinerators the catalyst may become contaminated with toxic or hazardous pol ­lutants and will require replacing. Thermal incineration normally removes all troublesome pollutants.

Removal Efficiency

Emissions can be managed by one of two methods:

1. Total concentration or mass of organics emitted is limited.

2. Resulting ambient concentration of each compound must be below an air quality standard.

Both these methods relate to the required removal efficiencies of the pollution control equipment. All abatement methods achieve high removal efficiencies when used in the correct applications (Fig. 13.19). The highest efficiency (with limitations) is in most cases achieved with incineration. When removal effi­ciencies of 99% or greater are required, incineration is usually recommended.1

Recovery of Pollutants

Recovering and recycling organic compounds make possible some cost savings in the pollution control equipment. Savings may be in raw material costs, which are normally the most significant item of a chemical plant. Sol­vent recovery is best suited for applications dealing with expensive or easily

Thermal incineration 95% 1 * 1	99%
1 > 1	.. W
Catalytic incineration
90%	95%
!—► |—	->				1
Carbon adsorption		50%	95%	99%
	1..	.w	I……. — W	1……	1
	… 1 »		. R — P	1…. *	1
Absorption
		90%	95%	98%
		H*h	—>	1—■>
Condensation
			50%	80% 95%
		H		1 T
Biological treatment
|—— * 95%			. 1
…… 1 ……….. 1……. 1….. _1……. 1 -.1…………….. L…… — I 1 1 1

10 20 50 100 200 300 500 1,000 2,000 3,000 5,000 10,000 20,000 Inlet concentration of total organic compounds (ppmv)

Recoverable compounds. Recovery is economical if the recovered compounds can be reused in a process as a fuel or raw material or sold to another user. Waste minimization practices are important from the environmental regula­tory viewpoint, encouraging the recovery of reusable organic pollutants.

Adsorption and condensation are the normal recovery options. If recy­cling is considered and is economically feasible, consideration of incinerators as destruction devices may be unnecessary. Generally, recovery units like ad­sorbers and absorbers result in higher total capital investment than modular, packaged incinerators.

Location

Many factors must be considered when selecting a location for the control unit, the actual position being partially dictated by the type of unit. Adsorption, absorption, condensation, and biotreatment units can usually be located near the existing process areas. Incinerators and carbon adsorbers cannot always be placed close to hazardous process areas due to the risk of fire and explosion. Po­sitioning the unit remote from the waste gas source increases the cost of ducting and piping, which may be excessive when handling large waste-gas quantities.

Generally, adsorption, absorption, and biofilter units require more space than compact incinerators and condensers. If the plant room is restricted, a local roof-mounted system may be the best alternative. However, roof struc­tural reinforcement may be required even for small and lightweight units. Consideration must be given to the effects of noise and vibration. Small ad­sorption systems, such as adsorption canisters, require an additional central regeneration unit on site, or they must be regenerated or disposed of off site. A central regeneration unit may require long runs of costly ductwork.2

Cost

The economic factors must be considered in every application. It is important to find a technique that will meet both the technical and economical requirements. In short, pollution control costs depend on the system characteristics and the ap­plication. Some cost equations that generalize the economics of the managing sys­tems are available in the literature. Most of these equations give rough estimates and have an accuracy of only about 30% to 50%. For a comprehensive cost com­parison of different units, a detailed cost analysis based on the equipment tender proposals and the special characteristics of the project is necessary.

13.3.1.3 Abatement Methods

Thermal Incineration

General Incineration (oxidation) is the best-known method for the re­moval of gaseous industrial waste. Combustible compounds containing carbon, hydrogen, and oxygen are converted to carbon dioxide and water by the overall exothermic reactions [Eq. (13.72)]. When chlorinated or sulfur-containing compounds are present in the effluent, the products of combustion include HCl/CU or so2/so3.5

CTHvO, + (x + y/4 — z/2) 02 —> x C02 + Y/2 H20 (13.72)

Destruction efficiencies over 99% can be achieved when the inlet concen­tration of organic compounds of the waste gas is over 100 ppmv.6 Thermal

Oxidation units are most commonly used for streams consisting of dilute mix­tures of organic compounds and air7 and when the waste-gas concentration is over 15% of LEL {lower explosion limit). Thermal incineration is also suited if there is large variation in the inlet concentrations or if the waste gas con­tains compounds that could contaminate the catalyst,8 For safety reasons the organic concentration of incoming gas should be limited to less than 25% of LEL.2 If the organics content is over 25% of LEL, the waste gas is diluted prior to incineration by adding air, and the waste-gas composition is continu­ously monitored.9

The advantages of thermal incineration are that it is simple in concept, has a wide application, and results in almost complete destruction of pollut­ants with no liquid or solid residue. Thermal incineration provides an oppor­tunity for heat recovery and has low maintenance requirements and low capital cost. Thermal incineration units for small or moderate exhaust streams are generally compact and light. Such units can be installed on a roof when the plant area is limited.6 The main disadvantage is the auxiliary fuel cost, which is partly offset with an efficient heat-recovery system. The formation of nitric oxides during the combustion processes must be reduced by control of excess air temperature, fuel supply, and combustion air distribution at the burner in­let. The formation of thermal NO increases dramatically above 980 °C. I(I (see Table 13.10)

Types of Thermal Incinerators Three types of thermal oxidation systems are available:

1. Direct-flame incinerator

2. Recuperative incineration unit

3. Regenerative incineration unit

TABLE 13.10 Characteristics of Thermal Incineration Equipment types (a) Direct flame (b) Recuperative (c) Regenerative Capacity range, m s‘1 Over 0.5 Inlet gas concentration, ppmv 100-2500 Removal efficiency, % 95-99 + Recovery No Unsuitable inlet gas characteristics Compounds requiring high destruction temperatures Advantages (a) Can treat dilute streams and complex mixtures of organic compounds. (b) Can treat gases containing particulates, water, or catalyst poisons. (c) Simple operating concept. (d) Wide applicability (e) Compact, light units available. Disadvantages (a) Fuel costs. (b) Further treatment required when Cl — and S-containing gases are treated.

The difference between the units is mainly in their heat-recovery systems. A thermal incineration system generally consists of a refractory-lined chamber. burner(s), a temperature control system, fan(s), and heat-recovery equip­ment. In the incineration process, waste gas is introduced to the combustion chamber, where it is raised to the appropriate combustion temperature by burning an auxiliary fuel such as natural gas or fuel oil. In general, the con­centration of organic compounds in industrial effluents is not high enough to provide self-sustaining combustion. Therefore, additional fuel is required. After heat recovery, the resulting flue gases are exhausted into the atmo­sphere.’*

The direct-flame incinerator is the simplest type of thermal oxida­tion system. It comprises a combustion chamber and supplementary fuel-injection system with no energy-recovery equipment. Direct-flame incineration is suitable only for gases that support combustion without requirements for auxiliary fuel (concentrated streams) or for intermit­tent use.

In the recuperative incinerator (Fig. 13.20), the hot exhaust gases pre­heat the incoming gas in a shell-and-tube heat exchanger (preheater). The heat-recovery efficiency is normally between 50% and 70%. Regenerative oxidation systems offer heat-recovery efficiencies as high as 95% and are used for airstreams with low calorific values and large volumes.8 In the re­generative system (Fig. 13.21), the inlet gas is heated while passing through the hot ceramic bed. After oxidation in the chamber, the hot gases pass through another cold ceramic bed and preheat it to the combustion chamber outlet temperature. The process flows are then reversed, so that the inlet stream can be fed to the hot bed. 5

Catalytic Incineration

In catalytic incineration, organic contaminants are oxidized to carbon diox­ide and water. A catalyst is used to initiate the combustion reaction, which oc­curs at a lower temperature than in thermal incineration. Catalytic incineration uses less fuel than the thermal method. Many commercial systems have removal efficiencies greater than 98%.

Thermal Oxidation

>

Dilution air (optional) Fan Gas to be treated

Preheater > * —I— Air

Fuel

To stack

Optional secondary heat exchanger

FIGURE 13.21 Regenerative thermal incineration unit.

The catalysts used in incinerator systems for gaseous organic compound control are usually precious or base metals or metal salts. The catalysts can be supported on inert materials such as alumina (A1203) or ceramics.9 For the de­struction of organic compound mixtures, a highly active but nonselective cata­lyst is required.11

In catalytic incineration, there are limitations concerning the effluent streams to be treated. Waste gases with organic compound contents higher than 20% of LEL (lower explosion limit) are not suitable, as the heat content released in the oxidation process increases the catalyst bed temperature above 650 °C. This is normally the maximum permissible temperature to which a catalyst bed can be continuously exposed. The problem is solved by dilution; this method increases the furnace volume and hence the investment and oper­ation costs. Concentrations between 2% and 20% of LEL are optimal. The catalytic incinerator is not recommended without prefiltration for waste gases containing particulate matter or liquids which cannot be vaporized. The waste gas must not contain catalyst poisons, such as phosphorus, arsenic, antimony, lead, zinc, mercury, tin, sulfur, or iron oxide.9’12 (see Table 13.11)

The operation of a catalytic incinerator is similar to that of a thermal unit. The preheated waste gas can be further heated in a preheat chamber before passing through the catalyst bed. The most common reactor design for reduc­ing the organics-containing stream with a catalyst is the fixed-bed catalytic in­cinerator with the catalyst monolith (Fig. 13.22).13 Other types are the packed-bed reactor with pelletized catalyst particles on shallow trays and the fluidized-bed reactor. Generally, all types of catalytic incinerators are equipped with recuperative or regenerative heat-recovery equipment. The recuperative exchangers are the most common, giving heat-recovery efficiencies between 50% and 70%.8

F. quipment rypes	Fixed bed
Capacity range, m[5] s’*	0.5-47.2
Inlet gas concentration, ppmv	100-2000
Removal efficiency, %	95-98+
Recovery	No
Unsuitable inlet gas	(a) Particulates and liquids that cannot be vaporized.
Characteristics	Lb) Catalyst poisons
Advantages	(a) Can treat compounds with high destruction temperatures. (b) Low fuel cost.
Disadvantages	(a) Catalyst deactivation. (b) Gas pretrearment usually required. (c) Further treatment if Cl-containing gases are treated.

Adsorption

General Adsorption is a physical process in which organic species are transferred onto the surface of a solid adsorbent. Adsorption is a particularly attractive control method as it can handle large volumes of gases of low pol­lutant concentrations. It is capable of removing contaminants down to very low levels.1 Removal efficiency is typically greater than 95%. The most fre­quently used adsorbent in the organic compound applications is activated car­bon, although zeolites and resins are also used.

Adsorption technologies focus on two applications:

Air Fuel

Dilution air

(optional) Preheatcr

V

Kan

Preheat	Catalyst
Chamber	Bed

Gas to be treated

To stack

Optional secondary heat exchanger

Solvent recovery with adsorption is most feasible when the reusable solvent is valuable and is readily separated from the regeneration agent. When steam-regenerated activated-carbon adsorption is employed, the sol­vent should be immiscible with water. If more than one compound is to be recycled, the compounds should be easily separated or reused as a mix­ture.9 Only very large solvent users can afford the cost of solvent purifica­tion by distillation.1

The advantages include the availability of long-term operating data. In addi­tion, adsorbers can handle varying flow rates or varying concentrations of organic compounds. The main disadvantage of adsorption is the formation of a secondary waste, such as the spent adsorbent, unusable recovered organic compounds, and organics in the waste water if steam is used for regeneration. Secondary waste may require off-site treatment or specialist disposal.12 (see Table 13.12)

Types of Adsorbers Five types of adsorption equipment are used in col­lecting gases containing organic compounds:

1. Regenerable fixed-bed adsorbers

2. Disposable/rechargeable canisters

3. Moving-bed adsorbers

4. Fluidized-bed adsorbers

5. Chromatographic baghouses

Of these, the most commonly used for air pollution control are the fixed-bed and canister units. Fixed beds are also used in solvent-recovery applications. Major process steps include adsorption, regeneration, and further treatment of the desorbed organic compounds. Typically, further treatment includes con­densation and separation.

TABLE 13.12 Characteristics of Adsorption (Fixed-Bed Adsorber) Equipment types Fixed bed Capacity range, nr5 s_l 0.05-125 Inlet gas concentration, ppmv Below 20-5000 Removal efficiency, % Over 95 Over 99.5 with frequent regeneration Recovery Yes Unsuitable inlet gas (a) Complex mixtures Characteristics (b) Particulates (c) Contaminants reacting with adsorbent (d) Compounds difficult to desorb .(e) Compounds with molecular weight of over 1 ■ below 45 G Mol“1 50 G Mol-1 or Advantages (a) Low outlet concentrations possible. (b) Dilute mixtures can be treated. (c) Lots of operating data available. Disadvantages (a) Fjiergy-intensive regeneration. (b) Gas pretreatment usually required.

Fixed-bed adsorbers may be operated in either intermittent or semicon — tinuous mode. A typical removal system is a semicontinuouslv operated dual-bed system; one bed is in adsorption mode while the other is being re­generated (Fig. 13.23).14 The adsorption performance of the bed can be monitored by analyzing the outlet gas. Once organic vapors are detected in the gas stream, the incoming gas stream is routed to the parallel adsorber, and the exhausted bed is regenerated. The adsorption and desorption cycles can also be fixed.

Canister-type adsorbers differ from fixed-bed units in that they are nor­mally limited to the removal of low-volume, intermittent gas streams, such as storage-tank vent gases.3 Process economics usually dictate whether regenera­te or throw-away canisters are appropriate. Each canister unit consists of a vessel, adsorbent, fan (not always necessary), inlet connection and distributor, and an outlet connection for the purified gas. The disadvantage in using canis­ters is that poor operating efficiencies result if the adsorber becomes saturated. Because the adsorber will probably be disposed of, there is a temptation to op­erate it until the adsorber is saturated. Unlike fixed-bed units, the concentra­tion of the outlet gases is not usually monitored.3

Absorption

The removal of one or more components from a gas mixture by absorption is probably the most important and familiar operation in the control of gaseous pollutant emissions. Though most often used for the control of inorganic gases, absorption can also be used for recovery of organic compounds. Absorption in­

Organic compounds FIGURE 13.23 Simplified sketch of adsorption unit.

Volves a mass transfer of one or more components from a gas stream into a non­volatile liquid (solvent, absorbent). The absorption rate is enhanced by using low operating temperatures, high pressures, large mass-transfer surface areas, high liquid-to-gas ratios, and high organic concentration in the gas stream.15

The main advantage of absorption is its applicability to the control of pol­lutant gases present in large concentrations (several percent by volume).1 In these applications, removal efficiencies of 98% or greater can be achieved. The main disadvantage is inflexibility; to achieve the best performance, the gas stream components are fixed once the column is designed, (see Table 13.13)

A typical absorption system (Fig. 13.24) consists of a counterflow absorp­tion column. The regeneration is carried out in a desorption column at a re­duced pressure and increased temperature. Regeneration is achieved by stripping, distillation, or extraction. Packed columns are most commonly used for the absorption of gaseous pollutants and have a high removal efficiency and a low pressure drop.2 The most common absorbents are water, silicon oils, paraffins, high-boiling-point esters, and polyalkylglycolethers.16

Condensation

Condensation can be achieved by either increasing the pressure or re­ducing the temperature of the gas. Generally, condensation systems oper­ate at a constant pressure (at the same pressure as the emission source).12 A removal efficiency of 95% is attainable with high inlet gas concentra­tions. However, the minimum concentration of organic compounds ob­tained in the outlet gas effluent is equal to the saturation concentration at the operation temperature. Chilled water, brine, refrigerant propylene, and liquid nitrogen can be used as coolants.

Condensation is normally used for the recovery of organic compounds from process or tank vent gases or from releases during loading. Condensa­tion is used to recover valuable compounds prior to incineration, or to reduce the organic load entering other control systems, such as adsorbers or absorb­ers.7 Adsorption and absorption processes benefit from low condenser outlet temperatures.

HB’ TABLE 13.13 Characteristics of Absorption

Equipment types	Packed or plate columns
Capacity range, m3 s-1	4.7-52.8
Inlet gas concentration,	Over 800
Ppmv
Removal efficiency, %	Over 90
Recovery	Yes
Unsuitable inlet gas characteristics	(a) Concentration of organics below 300 ppmv (b) Mixtures of organic compounds when recycling is required
Advantages	(a) High organic concentrations can be treated. (b) Moist gases can be treated. (c) Effective when organic compounds are soluble in absorbent.
Disadvantages	(a) More complex than other methods. (b) Inflexible.

Absorber

Gaseous distillate (can be mixed with Regeneration column feed stream)

Absorbent FIGURE 13.24 Absorption unit.

The advantages of indirect condensation are product recovery, modest space requirements, and no additional waste generated if the gas stream is dehumidified prior to the condenser.6 The start-up and shutdown times of the unit are short.17 Condensers can handle nearly any flow stream with adequate concentration of or­ganic compounds (over 3000 ppmv).12 Disadvantages include corrosion or ice for mation if water is allowed to condense in the system. The incoming gas flow rate and its composition must be reasonably constant to avoid changes in the operating temperature.12 One of the biggest problems in surface condensers is the accumu­lation of air or inert gases, which decrease the cooling capacity.18 (see Table 13.14)

TABLE 13.14 Characteristics of Condensation Equipment types Surface condenser Capacity range, m-1 s_l Below 1.4 Inlet gas concentration, ppmv Over 3000 Removal efficiency, % 50-95+ Recovery’ Yes Unsuitable inlet gas (a) Compounds that can polymerize, foul, or solidify in a condenser Characteristics (b) Mixtures when recycling is required Advantages (a) High organic concentrations can be treated. (b) Modest space requirements. (c) Short start-up and shutdown times. Disadvantages (a) Explosion hazard. (b) Corrosion and ice formation. (c) Low operating temperatures normally required.

Surface condensers are typically shell-and-tube heat exchangers. A con­densation unit (Fig. 13.25) generally consists of precondenser(s) for water re­moval, condenser, and demister.17’18 The elimination of an explosion hazard when flammable vapors are condensed requires sophisticated control equip­ment and an inert operating environment of airtight construction.9 Condensa­tion of moisture and possible equipment freezing can be avoided by dessicant that dries the incoming gas stream. Precooling and separation of the organic compounds from water can be omitted by drying.

Biological Gas Purification

In biological gas purification, microorganisms oxidize organic com­pounds to carbon dioxide and water in a moist and oxygen-rich environ­ment. The oxidation of sulfur or nitrogen-containing compounds and chlorinated organic compounds also generates inorganic acids. If unde — gradable compounds are not present, a control efficiency greater than 90% can be achieved with reasonable filter volumes and low investment and op­eration costs. Odors are often completely removed.19

Advantages of the biological gas purification include low operating tem­peratures and the possibility of treating gas streams with low organic loads. Due to the low operating costs, bio-filtration can provide significant advan­tages over other air pollution technologies when the waste gases contain low concentrations of readily biodegradable and water-soluble compounds.1211′ Biotreatment is not recommended for gases containing organic sulfur com­pounds.12 It is essential that dust, oil, and grease are removed from the waste gas stream before they enter the biological treatment zone.20 (see Table 13.15)

Biofilters are used for the control of organic compounds, air toxics, and organic and inorganic odors. Most biofilters are built as open single­bed systems, the most common filter media being compost and soil. H In biofiltration, effluent gases are vented through a biologically active mate­rial, and with sufficient residence time, the contaminants diffuse into a wet, biologically active layer (biofilm) that surrounds the filter particles. Aerobic degradation of pollutants occurs while the microorganisms metab­olize them. The system consists of an effluent gas blower, a gas humidifier,

Equipment types	Compost or soil filters
Capacity range, m’ s-’	0.3-41.7
Inlet gas concentration, pprnv	Below 100
Removal efficiency, %	Over <>0
Recovery	No
Unsuitable inlet gas	(a) Water-insoluble compounds
Characteristics	(b) Very high flow rates (c) High organic concentrations
Advantages	(a) Low organic concentrations can be treated, (bj Simple operating concept.
Disadvantages	(a) Process automation impossible. (b) Sensitivity to pH and moisture in a filter. (c) Excess biomass disposal. (d) Daily manual operation required. (e) Lots of space needed.

And a biofilter including an air-distribution system and filter material (Fig. 1 3.26). For biological purification, bioscrubbers such as fixed-bed bioreac­tors or trickle-bed bioreactors can also be used.

Membranes

Purified gas Filter Material ___

Membrane technology is a new alternative for the treatment of gas-phase organic pollutants. It is commercially viable when combined with other con­trol technologies such as condensation and incineration. Membranes used in the vapor separation processes are selectively permeable to organic com­pounds but relatively impermeable to air and inert gases such as nitrogen and carbon dioxide. Separation occurs as the different compounds transport across the membrane barrier at different rates. The pressure difference be­tween the feed side of the membrane and the permeate side is the driving force of the separation process.-1 The pressure difference across the membrane can be maintained either by compressing the feed stream (Fig. 13.27) or by the Use

Gas to he treated

T Cooling water

CHAPTER 13 GAS-CLEANING TECHNOLOGY Membrane Modules

Collected liquid phase of organic compounds

Compressor

Gas Io Be treated

Collected liquid phase of organic compounds

FIGURE 13.27 Membrane unit.

Of a vacuum pump on the permeate side. Recovery efficiencies of 90-99.99% have been achieved with spiral-wound composite membranes.-2 The primary advantage of the membrane recovery system is that condensation occurs at temperatures above the freezing point of water, resulting in no predrying, mul­tistage cooling, or refrigeration. Another advantage of rhis system is that sec­ondary waste streams are not created.

13.3.1.4 Summary

The technologies used in the control of gaseous organic compound emissions include destruction methods such as thermal and catalytic incin­eration and biological gas treatment and recovery methods such as adsorp­tion, absorption, condensation, and membrane separation. The most common control methods are incineration, adsorption, and condensation, as they deal with a wide variety of emissions of organic compounds. The most common types of control equipment are thermal and fixed-bed cata­lytic incinerators with recuperative heat recovery, fixed-bed adsorbers, and surface condensers. The control efficiencies normally range between 90% and 99%.

In the selection of control equipment, the most important waste-gas characteristics are volumetric flow rate, concentration and composition of organic compounds in the waste-gas, waste-gas temperature and humidity, and the content of particulate matter, chlorinated hydrocarbons, and toxic pollutants. Other factors influencing the equipment selection are the re­quired removal efficiency, recovery requirements, investment and operating costs, ease of installation, and considerations of operation and mainte­nance. The selection of a suitable control method is based on the fundamen­tal selection criteria presented as well as the special characteristics of the project.

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