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Module 6: Air Pollutants and Control Techniques - Volatile Organic Compounds - Practice Problems
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
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#1
- Are all organic compounds considered to be VOCs?
- Answer: No.
- Most organic compounds used as solvents and chemical feedstock are VOCs. A list of the few organic compounds that are not considered to be VOCs is provided in Table 1 of the lesson.
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#2
- What characteristics do volatile organic compounds (VOCs) have in common?
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- Answer: VOCs are organic compounds that can volatilize and participate in photochemical reactions once they reach the ambient air.
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#3
- Are all VOCs considered to be toxic?
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- Answer: No.
- All VOCs are not considered to be toxic. Some are inert when they enter the atmosphere. However, many of the air toxic compounds included in Title III of the 1990 Clean Air Act Amendments are VOCs.
Practice Problems
Formation Mechanisms
- Instructions:
- Answer this question on a sheet of paper and check your response against the one provided below.
-
#1
- What is the predominant source of VOC emissions?
-
- Answer: The predominant source of VOC emissions is vaporization of organic compounds used in industrial processes such as solvents and chemical feedstocks. VOC compounds are not formed, but are lost during industrial processes.
Practice Problems
Control Techniques
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
-
#1
- Name the active agent and any other conditions that promote VOC
collection efficiency for each of the following control techniques.
- Thermal oxidation
- Catalytic oxidation
- Adsorption
- Condensation, refrigeration, and cryogenic systems
- Biological oxidation
- Thermal oxidation
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- Answer: i.
- Thermal oxidation must occur at high temperatures of 1000 to 2000°F.
- Answer: ii.
- Catalytic oxidation occurs in the presence of uncontaminated catalyst
composed of noble metals such as platinum and palladium or of ceramic
materials. Also, temperatures should range from 500 to 1000°F.
- Answer: iii.
- Adsorption uses adsorbent beds composed of substances such as activated
carbon, zeolite, or organic polymers. Adsorption improves at cold temperatures.
- Answer: iv.
- Condensation, refrigeration, and cryogenic systems use cold surfaces
to condense organic vapors. These cold conditions can be created by
passing cold water through an indirect heat exchanger, by spraying cold
liquid into an open chamber containing the gas stream, by using a freon-based
refrigerant to create very cold coils, or by injecting cryogenic gases
(e.g. liquid nitrogen) into the gas stream.
- Answer: v.
- Biological oxidation systems use aqueous liquids or moist media inoculated with microorganisms that consume the dissolved or adsorbed organic compound. Other conditions that promote the efficiency of biological oxidation include humidity of more than 95% and temperatures less than approximately 105°F.
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#2
- What is the normal operating temperature range of a thermal oxidizer?
- 500 to 1,000°F
- 1,000 to 1,500°F
- 1,000 to 2,000°F
- 2,000 to 5,000°F
- 500 to 1,000°F
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- Answer: c. 1,000 to 2,000°F
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#3
- What type of VOC control system can be used on essentially all
types of VOC compounds?
- Adsorbers
- Thermal oxidizers
- Catalytic oxidizers
- Biofilters
- Adsorbers
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- Answer: b. Thermal oxidizers
- Thermal oxidizers have the broadest applicability of all VOC control devices. They can be used for almost any VOC compound without risk of damaging the control device as can happen with catalytic oxidizers and biofilters. They can also be used for a very wide range of VOC concentrations, from less than 10 ppm to concentrations approaching 10,000 ppm. Thermal oxidizers do have some limitations. They are rarely used on gas streams having VOC concentrations exceeding approximately 25% of the lower explosive limit. Also, because they destroy VOCs, they offer no mechanism for recovering or reusing the VOC compounds.
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#4
- What is the normal upper VOC concentration limit of thermal oxidizers, catalytic oxidizers and adsorbers?
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- Answer: VOC concentrations should not exceed 25% of the Lower Explosive Limit (LEL), a value that is often equivalent to 10,000 to 20,000 ppm.
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#5
- What is the normal operating temperature range of a catalytic
oxidizer?
- 500 to 1,000°F
- 1,000 to 1,500°F
- 1,000 to 2,000°F
- 2,000 to 5,000°F
- 500 to 1,000°F
-
- Answer: a. 500 to 1,000°F
- The normal operating temperature range of a catalytic oxidizer is 500 to 1,000°F.
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#6
- Do adsorbers work more efficiently when the gas stream is hot or cold?
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- Answer: All adsorption processes work better when the gas temperatures are cold.
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#7
- Condensers, refrigeration systems, and cryogenic systems are generally
used for VOC sources that generate ____________ organic compound concentrations.
- Low
- High
- Any level of
- Low
-
- Answer: b. High
- Condensers, refrigeration systems, and cryogenic systems are generally used for VOC sources that generate high organic compound concentrations.
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#8
- Biofilters are used for VOC sources that generate ____________
organic compound concentrations.
- Low
- High
- Any level of
- Low
-
- Answer: a. Low
- Biofilters are used for VOC sources that generate low organic compound concentrations.
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#9
- What is the purpose of a preconcentrator type adsorption unit?
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- Answer: Preconcentrators increase the organic vapor concentrations to reduce the fuel required for thermally or catalytically oxidizing the contaminants.
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#10
- Assume you are an air pollution control consultant. Name the VOC
control system(s) that you would recommend for each of the following
situations.
- A hot gas stream, moving at a high flow rate, contains more than three VOC compounds as well as some phosphorus-containing compounds. Neither recovery nor reuse of the organic compounds is feasible. The total VOC concentration is approximately 1,000 ppm.
- A hot gas stream, moving at a high flow rate, contains three VOC compounds. There is no indication that the gas contains phosphorus, tin, or zinc. Neither recovery nor reuse of the organic compounds is feasible. The total VOC concentrations are approximately 5,000 ppm. The facility must keep its operating costs to a minimum.
- A gas stream, moving at a low flow rate, contains only three VOCs as pollutants. The VOCs are at a high concentration and have molecular weights less than 50. Recovery of the organic compounds is desired.
- A gas stream, moving at a low flow rate, contains only one pollutant, toluene. The facility would like to recover the toluene. Toluene (C6H5CH) has a molecular weight of 90.
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- Answer: i. Thermal oxidizers
- Thermal and catalytic oxidizers can be good choices for VOC-laden gas streams with high flow rates when recovery and reuse of organic compounds is not desirable or feasible. Catalytic oxidizers in this case would probably not be a good choice because the phosphorus in the gas stream could poison the catalyst. Adsorbers could have been considered in this situation if there had been three or fewer VOC compounds present.
- See Figure 1 (General Applicability of VOC
Control Systems for High Concentration Sources).
- Answer: ii. Catalytic oxidizers
- This situation is similar to (i) above except for the apparent lack of substances that might poison the catalyst and the importance of keeping operating costs to a minimum. Catalytic oxidizers would probably be a better choice than thermal oxidizers here because thermal oxidizers, with their high fuel requirement, can be expensive to operate. Likewise, another possible choice, adsorbers, would require some additional expense in the disposal the desorption air. Also, the gas stream is hot and would have to be cooled for adsorption to be effective.
- See Figure 1 (General Applicability of VOC
Control Systems for High Concentration Sources).
- Answer: iii. Condensation, refrigeration, or a cryogenic system
- Condensation, refrigeration, and cryogenic systems are good choices when the gas flow rate is low, the VOC concentrations are high and limited to three or fewer compounds (unless separation of compounds is not desired). Adsorption systems would not be a good choice here because the molecular weights of the organic compounds do not fall within the prescribed 50 to 200 range.
- See Figure 1 (General Applicability of VOC
Control Systems for High Concentration Sources).
- Answer: iv. Adsorption
- The desire to recover the toluene limits the options to either an
adsorption system or a condensation/refrigeration system. An adsorption
system would work well in this situation because the molecular weight
of toluene falls within the prescribed 50 to 200 range.
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