4i Control of VOC

8/12/2019 4i Control of VOC

1/43

Control of Volatile Organic Compounds

Dr. Wesam Al Madhoun


2/43

haracteristics 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).
http://www.epa.gov/apti/bces/module6/voc/character/character.htmhttp://www.epa.gov/apti/bces/module6/voc/character/character.htmhttp://www.epa.gov/apti/bces/module6/voc/character/character.htmhttp://www.epa.gov/apti/bces/module6/voc/character/character.htm


3/43

VOCs are organic compounds that can volatilize andparticipate in photochemical reactions when the gas stream is

released to the ambient air.

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

A list of those few organic compounds that are notconsidered

to be VOCs is provided in Table 1.

Other organic compounds are considered to be VOCs.


4/43


5/43

ontrol 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.


6/43

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


7/43

Thermal Oxidation

In a thermal oxidizer, the VOC-laden air stream is heated togas 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.


8/43


9/43

The VOC-laden gas stream is held at this temperature forresidence times ranging from a fraction of a second to more

than two seconds.

Temperatures of the exhaust gas from the refractory-linedcombustion chambers are often 1,000 to 2,000F.

Thermal oxidizers usually provide VOC destructionefficiencies that exceed 95% and often exceed 99%.


10/43

One of the main limitations of thermal oxidizers is the largeamount 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.


11/43

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.


12/43

Thermal oxidizers have the broadest applicability of all theVOC 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).


13/43

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 containchlorine, 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 airpollution control system to collect these contaminants prior to

gas stream release to the atmosphere.


14/43

Catalytic Oxidation

Catalytic oxidizers operate at substantially lowertemperatures than thermal oxidizers.

Due to the presence of the catalyst, oxidation reactions can

be performed at temperatures in the range of 500 to 1000F.

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.


15/43


16/43

Due to the relatively low gas temperatures in the combustionchamber, 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.


17/43

Catalytic oxidizers are also applicable to a wide range ofVOC-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.


18/43

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.


19/43

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 threeorganic solvent compounds, and it is economical to recover

and reuse these compounds, or

2- When the VOC-laden gas stream contains a large number oforganic compounds at low concentration, and it is necessary

to preconcentrate these organics prior to thermal or catalytic

oxidation.


20/43

A flowchart for a multi-bed adsorber system used forcollection and recovery of organic solvent compounds is

shown in Figure 3.


21/43

The VOC-laden gas is often cooled prior to entry into theadsorption system because the effectiveness of adsorption

improves at cold temperatures.

As the gas stream passes through the bed, the organiccompounds 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 avery 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.


22/43

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.


23/43

An adsorption system used for preconcentration is smallerthan a system similar to the one in Figure 3 for solvent

recovery.

In preconcentrator systems, the VOC-laden stream passesthrough a rotary wheel containing zeolite or carbon-based

adsorbents.

Approximately 75-90% of the wheel is in adsorption servicewhile the remaining portion of the adsorbent passes through

an area where the organics are desorbed into a very small,

moderately hot gas stream.


24/43

The concentrated organic vapors are then transported to a

thermal or catalytic oxidizer for destruction.

The preconcentration step substantially reduces the fuelrequirements for the thermal or catalytic oxidizer.


25/43

Adsorption systems (in general) are usually limited to sourcesgenerating 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.


26/43

These molecular weights are provided as a guideline and thesuitability 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.


27/43

Adsorption systems are not recommended for gas streamsthat 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.


28/43

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.


29/43

The concentration of VOCs is reduced to the level equivalentto the vapor pressures of the compounds at the operating

temperature.

Condensation and refrigeration systems are usually used onhigh 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


30/43

For cold-water-based condensation systems, the outlet gastemperature is usually in the 40 to 50F 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 -

70F to less than -200F.


31/43

Condensation, refrigeration, and cryogenic systems areusually used on gas streams that contain only VOC

compounds.

High particulate concentrations are rare in the types ofapplications that can usually apply this type of VOC control

system.

However, if particulate matter is present, it could accumulateon heat exchange surfaces and reduce heat transfer

efficiency.


32/43

Biological Oxidation

Biological systems are a relatively new control device in the airpollution control field.

VOCs can be removed by forcing them to absorb into an

aqueous liquid or moist media inoculated with microorganismsthat consume the dissolved and/or adsorbed organic

compounds.

The control systems usually consist of an irrigated packed bedthat 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%.


33/43

The gas stream temperatures are maintained at less than

approximately 105F 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.


34/43


35/43

General Applicability of VOC Control Systems

Figures 5 and 6 summarize the general applicability of VOCcontrol 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.


36/43


37/43


38/43

If there are a large number of separate VOC compounds, it isusually not economically feasible to recover and reuse the

captured organics.

In this case, thermal or catalytic oxidizers are used to oxidizethe 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


39/43

It is necessary to confirm that the compounds can bedesorbed 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 isgenerally used as the control technique.

Due to the low VOC concentrations, the cost of organic

compound recovery can be quite high.


40/43


41/43

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.


42/43

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

These systems are limited to gas streams containing at most

three organic compounds due to the costs associated withseparating the recovered material into individual components.

If the process can reuse a multi-component organic stream,

both adsorbers and condenser/refrigeration systems can beused without the costs of recovered material purification and

reprocessing.


43/43

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.

Documents

4i Control of VOC