Control of Volatile Organic Compounds

Dr. Wesam Al Madhoun

Characteristics

• The majority of anthropogenic VOCs released into the atmosphere are from transportation sources and industrial processes utilizing solvents such as surface coating (paints), printing (inks), and petrochemical processing (see Figure 1).

• VOCs are organic compounds that can volatilize and participate in photochemical reactions when the gas stream is released to the ambient air.

• Almost all of the organic compounds used as solvents and as chemical feedstock are VOCs.

• A list of those few organic compounds that are not considered to be VOCs is provided in Table 1.

• Other organic compounds are considered to be VOCs.

Control Techniques

• The dominant source of VOC emissions is the vaporization of organic compounds used in industrial processes.

• A variety of techniques can be used to reduce VOC emissions.

• Using material containing an inherently low quantity of VOC compounds will reduce the release of VOCs.

• Also, the processes can be redesigned to reduce the quantities that are lost as fugitive emissions.

• When these techniques are inapplicable or insufficient, add-on control systems, such as the techniques listed below, can be used:

• Thermal oxidation

• Catalytic oxidation

• Adsorption

• Condensation and refrigeration

• Biological oxidation

Thermal Oxidation

• In a thermal oxidizer, the VOC-laden air stream is heated to gas temperatures several hundred degrees Fahrenheit above the autoignition temperatures of the organic compounds that need to be oxidized.

• Due to these very high temperatures, thermal oxidizers have refractory-lined combustion chambers (also called fume incinerators), which increase their weight and size considerably.

• A sketch of a thermal oxidizer is shown in Figure 1.

• The VOC-laden gas stream is held at this temperature for residence times ranging from a fraction of a second to more than two seconds.

• Temperatures of the exhaust gas from the refractory-lined combustion chambers are often 1,000 to 2,000°F.

• Thermal oxidizers usually provide VOC destruction efficiencies that exceed 95% and often exceed 99%.

• One of the main limitations of thermal oxidizers is the large amount of fuel required to heat the gas stream to the temperature necessary for high-efficiency VOC destruction.

• Heat exchangers are used to recover some of this heat.

• The heat exchanger shown in Figure 1 is sometimes called a recuperative heat exchanger.

• This type of heat exchanger has a heat recovery efficiency ranging from 30 to 60% depending on the size of the unit.

• Some types of thermal oxidizers use large regenerative beds

for heat exchange.

• These beds have heat recovery efficiencies up to 95%.

• Due to the amount of heat that can be recovered and returned to the inlet gas stream,

• these units, termed regenerative thermal oxidizers (RTOs) require less fuel to maintain the combustion chamber at the necessary temperature.

• Thermal oxidizers have the broadest applicability of all the VOC control devices.

• They can be used for almost any VOC compound.

• Thermal oxidizers can also be used for gas streams having VOC concentrations at the very low concentration range of less than 10 ppm up to the very high concentrations approaching 10,000 ppm.

• Thermal oxidizers are rarely used on gas streams having VOC concentrations exceeding approximately 25% of the lower explosive limit (LEL).

• This limit is imposed by safety constraints due to the possibility that a short-term concentration spike would exceed the LEL, and the gas stream would explode.

• The 25% LEL limit depends on the actual gas constituents and usually is in the 10,000 to 20,000 ppm range.

• Thermal oxidizers handling VOC materials that contain chlorine, fluorine, or bromine atoms generate HCl, Cl2, HF, and HBr as additional reaction products during oxidation.

• A gaseous absorber (scrubber) is used as part of the air pollution control system to collect these contaminants prior to gas stream release to the atmosphere.

Catalytic Oxidation

• Catalytic oxidizers operate at substantially lower temperatures than thermal oxidizers.

• Due to the presence of the catalyst, oxidation reactions can be performed at temperatures in the range of 500 to 1000°F.

• Common types of catalysts include noble metals (i.e. platinum and palladium) and ceramic materials.

• VOC destruction by catalytic oxidizers usually exceeds 95% and often exceeds 99%.

• A sketch of a catalytic oxidizer is shown in Figure 2.

• Due to the relatively low gas temperatures in the combustion chamber, there is no need for a refractory lining to protect the oxidizer shell.

• This minimizes the overall weight of catalytic oxidizers and provides an option for mounting the units on roofs close to the point of VOC generation.

• This placement can reduce the overall cost of the system by limiting the distance the VOC-laden stream must be transported in ductwork.

• Catalytic oxidizers are also applicable to a wide range of VOC-laden streams;

• however, they cannot be used on sources that also generate small quantities of catalyst poisons.

• Catalyst poisons are compounds that react chemically in an irreversible manner with the catalyst.

• Common catalyst poisons include phosphorus, tin, and zinc.

• Another potential operating problem associated with catalytic oxidizers is their vulnerability to chemicals and/or particulate matter that masks or fouls the surface of the catalyst.

• (Masking is the reversible reaction of a chemical with the catalyst and fouling is the coating of the catalyst with a deposited material.)

• If the conditions are potentially severe, catalytic units are not installed.

• As with thermal oxidizers, catalytic oxidizers should not exceed 25% of the LEL, a value that is often equivalent to a VOC concentration of 10,000 to 20,000 ppm.

Adsorption

• Adsorption systems beds are generally used in the following two quite different situations:

1- When the VOC-laden gas stream only contains one to three organic solvent compounds, and it is economical to recover and reuse these compounds, or

2- When the VOC-laden gas stream contains a large number of organic compounds at low concentration, and it is necessary to preconcentrate these organics prior to thermal or catalytic oxidation.

• A flowchart for a multi-bed adsorber system used for collection and recovery of organic solvent compounds is shown in Figure 3.

• The VOC-laden gas is often cooled prior to entry into the adsorption system because the effectiveness of adsorption improves at cold temperatures.

• As the gas stream passes through the bed, the organic compounds adsorb weakly onto the surfaces of the activated carbon, zeolite, or organic polymer used as the adsorbent.

• Essentially all of the commercially used adsorbents have a very high surface area per gram of material.

• When the adsorbent is approaching saturation with organic vapor, a bed is isolated from the gas stream and desorbed.

• Low-pressure steam or hot nitrogen gas is often used to remove the weakly adsorbed organics.

• The concentrated stream from the desorption cycle is treated to recover the organic compounds.

• After desorption, the adsorption bed is returned to service, and another bed in the system is isolated and desorbed.

• An adsorption system used for preconcentration is smaller than a system similar to the one in Figure 3 for solvent recovery.

• In preconcentrator systems, the VOC-laden stream passes through a rotary wheel containing zeolite or carbon-based adsorbents.

• Approximately 75-90% of the wheel is in adsorption service while the remaining portion of the adsorbent passes through an area where the organics are desorbed into a very small, moderately hot gas stream.

• The concentrated organic vapors are then transported to a thermal or catalytic oxidizer for destruction.

• The preconcentration step substantially reduces the fuel requirements for the thermal or catalytic oxidizer.

• Adsorption systems (in general) are usually limited to sources generating organic compounds having a molecular weight of more than 50 and less than approximately 200.

• The low molecular weight organics usually do not adsorb sufficiently.

• The high molecular weight compounds adsorb so strongly that is it is difficult to remove these materials from the adsorbent during the desorption cycle.

• These molecular weights are provided as a guideline and the suitability of an adsorption system for a particular situation should be considered on a case-by-case basis.

• Adsorption systems can be used for a wide range of VOC concentrations from less than 10 ppm to approximately 10,000 ppm.

• The upper concentration limit is due to the potential explosion hazards when the total VOC concentration exceeds 25% of the LEL.

• Adsorption systems are not recommended for gas streams that contain particulate matter and/or high moisture concentrations, because the particulate matter and moisture compete with the gaseous pollutants for pore space on the adsorbent material.

• The adsorption removal efficiency usually exceeds 95% and is often in the 98% to 99% range for both solvent recovery and preconcentrator type systems.

• In both types of units, the removal efficiency increases with reduced gas temperatures.

Condensation, Refrigeration, and Cryogenics

• Condensation, refrigeration, and cryogenic systems remove

organic vapor by making them condense on cold surfaces.

• These cold conditions can be created by passing cold water through an indirect heat exchanger, by spraying cold liquid into an open chamber with the gas stream,

- by using a freon-based refrigerant to create very cold coils, or by injecting cryogenic gases such as liquid nitrogen into the gas stream.

• The concentration of VOCs is reduced to the level equivalent to the vapor pressures of the compounds at the operating temperature.

• Condensation and refrigeration systems are usually used on high concentration, low gas flow rate sources.

• Typical applications include gasoline loading terminals and chemical reaction vessels.

• The removal efficiencies attainable with this approach depend strongly on the outlet gas temperature

• For cold-water-based condensation systems, the outlet gas temperature is usually in the 40 to 50°F range,

- and the VOC removal efficiencies are in the 90 to 99% range depending on the vapor pressures of the specific compounds.

• For refrigerant and cryogenic systems, the removal efficiencies can be considerably above 99%,

- due to the extremely low vapor pressures of essentially all VOC compounds at the very low operating temperatures of -70°F to less than -200°F.

• Condensation, refrigeration, and cryogenic systems are usually used on gas streams that contain only VOC compounds.

• High particulate concentrations are rare in the types of applications that can usually apply this type of VOC control system.

• However, if particulate matter is present, it could accumulate on heat exchange surfaces and reduce heat transfer efficiency.

Biological Oxidation

• Biological systems are a relatively new control device in the air pollution control field.

• VOCs can be removed by forcing them to absorb into an aqueous liquid or moist media inoculated with microorganisms that consume the dissolved and/or adsorbed organic compounds.

• The control systems usually consist of an irrigated packed bed that hosts the microorganisms (biofilters).

• A presaturator is often placed ahead of the biological system to increase the gas stream relative humidity to more than 95%.

• The gas stream temperatures are maintained at less than

approximately 105°F to avoid harming the organisms and to

prevent excessive moisture loss from the media.

• Biological oxidation systems are used primarily for very low

concentration VOC-laden streams.

• The VOC inlet concentrations are often less than 500 ppm

and sometimes less than 100 ppm.

• The overall VOC destruction efficiencies are often above 95%.

• Biological oxidation systems are used for a wide variety of organic compounds; however, there are certain materials that are toxic to the organisms.

• In these cases, an alternative type of VOC control system is needed.

General Applicability of VOC Control Systems

• Figures 5 and 6 summarize the general applicability of VOC control systems.

• These two charts apply to gas streams having total VOC concentrations less than approximately 25% of the LEL.

• If the concentrations are above this value, units such as flares (not discussed) are used for control.

• Control system applicability has been divided into two separate groups: low VOC concentration and high VOC concentration.

• There is no generally accepted distinction between these two groups.

• For the purposes of Basic Concepts in Environmental Sciences, total VOC concentrations less than 500 ppm are considered low.

• The low concentration group is further divided into three main categories depending on the number of different VOC compounds in the gas stream and the value of recovering these compounds for re-use.

• If there are a large number of separate VOC compounds, it is usually not economically feasible to recover and reuse the captured organics.

• In this case, thermal or catalytic oxidizers are used to oxidize the VOC compounds.

• Adsorbers can also be used as independent control systems or as preconcentrators for the oxidizers.

• If there are a very limited number of VOC compounds (less than or equal to 3), it is usually possible to use either adsorbers or biological oxidation systems

• It is necessary to confirm that the compounds can be desorbed from regenerative-type adsorbers and that the specific organics are not toxic to the microorganisms in biological oxidation systems.

• Both thermal and catalytic oxidizers can also be used for these types of gas streams.

• If recovery and reuse are necessary, an adsorber system is generally used as the control technique.

• Due to the low VOC concentrations, the cost of organic compound recovery can be quite high.

• The applicability of VOC control systems for high concentration systems also depends, in part, on the number of separate VOC compounds present in the gas stream and the economic incentives for recovery and reuse.

• Thermal oxidizers can be used in all cases in which recovery and reuse are not desired or economically feasible.

• Catalytic oxidizers can be used in these same situations if there are no gas stream components that would poison, mask, or foul the catalyst.

• Adsorbers can also be used for this service as long as there are environmentally acceptable means for disposal of the collected organics.

• If recovery and reuse are desired, either adsorbers or condenser/refrigeration systems can be used.

• These systems are limited to gas streams containing at most three organic compounds due to the costs associated with separating the recovered material into individual components.

• If the process can reuse a multi-component organic stream, both adsorbers and condenser/refrigeration systems can be used without the costs of recovered material purification and reprocessing.

• There are a number of commercial VOC control systems that fall outside the general pattern of applicability indicated in Figures 5 and 6.

• These figures provide a very general indication of the uses and limitations of the five main types of VOC control systems.