1. INTRODUCTION

Toxic volatile organic compound (TVOC) abatement has gained recent attention due to a better understanding of the effects of releasing these compounds into the environment. Both the magnitude and hazards of emissions of toxic compounds have led to public health concern and recent government regulations to reduce emissions.

Approximately $19.4 billion will be spent by current sources of releases to meet requirements of Title I and Title III of the 1990 Clean Air Act Amendments (McIlvaine et al., 1992). Of the $19.4 billion direct expenditures for air pollution control, equipment cost is estimated to account for $6.5 billion (McIlvaine et al., 1992). The development of new control equipment is at least in part necessitated by the requirements to remove TVOCs from point source emissions. Because of the size of this investment, there is interest in developing new, efficient means of TVOC control.

In this research, a novel activated carbon fiber cloth (ACFC) adsorption system is designed, developed and characterized for possible use as a method for point source reduction of TVOCs. In the developed system, adsorption by ACFC is followed by electrothermal regeneration resulting in formation of a concentrated organic vapor which is cryogenically condensed from the gas phase. Electrothermal desorption allows for optimizing the desorption time and the concentration profile of the desorbed TVOC to allow minimal use of cryogen. A bench-scale cryogenic condenser is used to assess the fundamental performance of condensing TVOCs at low temperature.

Liquid nitrogen (LN₂) is used as the cryogen. LN₂ can reduce TVOC emissions to parts per billion (ppbv) concentrations due to its high cooling capacity and low temperature. One advantage of the LN₂ system compared to other refrigeration systems is the multiple use of LN₂. The cooling capacity of LN₂ is used to condense TVOCs. Then the evaporated N₂ can be used for other process needs such as inerting and safety blanketing. Another advantage of LN₂ refrigeration over conventional mechanical condensation systems is the absence of moving parts such as compressors. However, LN₂ consumption rates are high for typical industrial effluent concentrations. The effluent TVOC concentrations are typically <1% by volume due to the limitations caused by their lower explosion limits. Ideal LN₂ consumption (minimum mass of LN₂ required per unit mass of TVOC condensed in an isolated thermodynamic system) is proportional to the inverse of the TVOC concentration (Figure 1.1). Results for acetone are reported here too although it is not currently on TVOC.

If TVOC concentration can be increased, then LN₂ consumption can be decreased. This is due to the fact that some portion of the cooling capacity transfers to the non-condensable carrier gas. Therefore, ACFC adsorption technology is used to pre-concentrate the effluent TVOCs for efficient vapor recovery.

Toxic Releases into the Environment

The United States Environmental Protection Agency (USEPA) began compiling a database of toxic releases into the environment in all 50 states and U.S. jurisdictions in 1987. Facilities manufacturing or processing 11,340 kg/yr of toxic materials and/or 4,500 kg/yr of other use toxic materials (e.g., solvents, cleaning materials, etc.) are required to report to the USEPA their toxic releases into the environment. The 1991 Toxic Release Inventory (TRI) included 302 individual toxic chemicals and 20 categories of chemical compounds. Selection of compounds were first compiled by USEPA from lists established by the states of Maryland and New Jersey for state reporting. These compounds have been identified as posing a risk to human health or the environment. The list of 302 toxic chemicals is under continual public, USEPA and U.S. Congressional review for modification. Toxic releases were divided into six different release pathways, the largest of which was direct emissions to the atmosphere. Table 1.1 lists those emission pathways for both the United States and the State of Illinois. Illinois ranked sixth overall in toxic releases into the environment. Emissions to the atmosphere in Illinois accounted for 4.3 H 10⁷ kg of the total 1.1 H 10⁹ kg emitted in the United States (USEPA^b, 1993).

From the list of toxic emissions, USEPA has identified 188 hazardous air pollutants (HAPS; listed in Appendix A) for federal regulation (USEPA, 1990). Many of these HAPs are TVOCs. In general, USEPA categorizes volatile organic compounds (VOCs) as low, moderate and high volatility based on the compounds vapor pressure at 293 K (Table 1.2). Atmospheric emissions of TVOCs appearing on the USEPAs list of 188 HAPs totaled 4.5 H 10⁸ kg in 1991 (USEPA^a, 1993).

Atmospheric releases of the top eight emitted TVOCs account for approximately 95% of the TVOC total emissions HAPS (Figure 1.2). Methanol, toluene, trichloroethane, xylene and methyl ethyl ketone constituted the largest TVOC emissions to the atmosphere in 1991 (USEPA^a, 1993). In general, emissions of VOCs to the atmosphere have declined from 1987 to 1991 by approximately 5% per year (USEPA^a, 1993). This is in large part due to government regulation of VOC emissions (McIlvaine et al., 1992).

Point Source Emission Trends

Atmospheric emissions from point sources can come from a variety of locations within a facility. These emission points include process vents, process vent leaks, transfer operations, storage vessels, waste water treatment and equipment leaks (USEPA^a, 1991). Typically, atmospheric emission inventories at a specific site must include releases from all of these categories and any other pathways that may exist (Ruhl, 1993).

Industrial toxic emissions to the atmosphere are attributed to a number of source types within the United States. Approximately 31% of the total toxic emissions to the atmosphere were emitted from the chemical industry (Figure 1.3). The chemical manufacturing industry emits as much as 150 of the 188 HAPS to the atmosphere (USEPA^a, 1993). Under the USEPA proposed Hazardous Organic National Emissions Standards for Hazardous Air Pollutants (HON) rule, 370 chemical manufacturing facilities and 1,050 chemical manufacturing processes will be required to reduce the TVOC emissions from 5.6 H 10⁸ kg emitted in 1989 to 1.1 H 10 ⁸ kg by 1998 (USEPA^a, 1993).

 

 

Effects of TVOCs on Human Health

Selection of many of the 189 HAPS listed in Section 112 of the 1990 CAAA was based on the compound’s potential to cause increased mortality or serious illness. For example, many of these air toxics are known to be human carcinogens (USEPA^a, 1993). Exposure is also known or suspected to cause many other noncancerous effects (Table 1.3), including poisoning and immunological, neurological, reproductive, developmental, mutagenic and respiratory problems (Burge and Hodgson, 1988). Risk assessment of exposure to air toxics is difficult for cancereous effects and much more difficult for non-cancerous effects due to the many different possible means of exposure pathways (USEPA^a, 1993). USEPA is required under the 1990 CAAA to evaluate residual risk remaining after the implementation of technology-based standards.

Many TVOCs are also known to be tropospheric ozone precursors. Ozone can cause eye, nose, throat and respiratory problems in humans, especially in urban areas (Tolley et al., 1993). This is in part the reason for the mandated reduction of VOCs under Title I in ozone non-attainment areas (USEPA, 1990).

Federal TVOC Regulations

Between 1970 and 1990, the USEPA regulated only eight HAPs (arsenic, asbestos, benzene, beryllium, mercury, radio nuclides, radon-222 and vinyl chloride) under the National Emission Standards for Hazardous Air Pollutants (NESHAPS). During this time, however, the 1977 Clean Air Act Amendments were further changed, and these new amendments were signed into law in 1990. As a result of this 1990 legislation, USEPA is required to establish technology-based guidelines (e.g., RACT and MACT explained below) for the reduction of TVOC emissions to the atmosphere from point sources (USEPA, 1990). Specifically, control-based reduction of VOCs falls under Title I and Title III of the 1990 CAAA. Effectively, VOC emissions in ozone non-attainment areas are governed by Title I, and TVOCs are governed by Title I and Title III.

Under Title I of the 1990 CAAA, USEPA is required to establish reasonable achievable control technology (RACT) standards for point source VOC emissions in ozone-related non-attainment areas. State environmental regulatory agencies are responsible for establishing implementation of RACT standards. RACT is defined as the lowest emission limitation that a particular source is capable of meeting by the application of control technology that is reasonably available considering technological and economic feasibility (USEPA^c, 1991). A compilation of information was developed for state use in determining state specific RACT implementation. For example, the USEPA developed an information base on VOC emissions of 17 categories of industries (USEPA^a, 1993), control technology techniques and cost of implementation of various control technologies (USEPA^c, 1991) have been developed by USEPA.

Title III of the 1990 CAAA requires USEPA to establish maximum achievable control technology (MACT) standards for point source emissions of HAPs. MACT standards are based on the best demonstrated control technology and/or practices and is not dependent on the cost of the technology. These standards apply to all major sources with the potential to emit at least 10 tons per year of any one of the HAPs or at least 25 tons per year of any combination of HAPs. All area sources are also subject to MACT standards. HAP emission standards for point sources based on MACT must be promulgated by November 15, 2000. Furthermore, sources regulated under Title III must meet permit requirements within three years of promulgation. (USEPA^c, 1991)

The USEPA has classified 17 primary categories of industrial sources emitting HAPS (USEPA, 1990) as given in Table 1.4.

These sources fall under new regulations that are the most comprehensive to date for TVOC emission reductions (American Consulting Engineers Council, 1994). McIlvaine (1992) reports that gas phase reductions of air toxics will amount to approximately 70% of the current emission levels. It is likely that new technologies will need to be developed to reach these reduction levels in a cost effective manner. A projected 74% increase in demand for incineration and adsorption systems will result from current facilities attempting to meet the 1990 CAAA VOC and HAP requirements (McIlvaine, 1992).

Point Source Reduction of TVOC Emissions

Point source TVOC emission reduction is generally accomplished by: 1) process modification, 2) feed stream modification, 3) source shutdown and/or 4) utilization of ancillary control devices. While process modification is generally the most economical method of reducing TVOC emissions, further reduction of these emissions below what can be obtained through modification usually requires the addition of control devices along the waste stream (Ruddy and Carroll, 1993).

The seven most widely used control devices that remove VOCs from gas streams are: 1) thermal incinerators, 2) catalytic incinerators, 3) flares, 4) boilers/process heaters, 5) carbon adsorbers, 6) absorbers and 7) condensers (USEPA^c, 1991). These control devices are described below. Selection of an appropriate technology is dependent on the process gas stream characteristics, capital and annual costs and the desired removal efficiency (Table 1.5). Further considerations for TVOC control device selection include: 1) recycling potential, 2) variability of loading, 3) average loading, 4) diversity of TVOCs in the gas stream, 5) lower and upper explosion limits of the TVOCs present in the gas stream, 6) gas stream temperature, 8) fouling problems, 9) locating the control device and 10) required maintenance of the control device (Ruddy and Carroll, 1993).

Thermal oxidation systems include thermal and catalytic incinerators, flares and boiler processes. Thermal incineration systems oxidize the VOCs in the gas stream at temperatures typically between 950 K and 1250 K (Ruddy and Carroll, 1993), producing mostly CO₂ and water vapor. The required temperature is dependent on the composition of the gas stream and the desired removal efficiency. Catalytic incinerators are essentially thermal incinerators with a catalyst present to reduce the operating temperature of the reactor. Typical operating temperatures for catalytic incinerators are 650 K to 750 K.

Flares directly combust the VOC present in the gas stream using an open flame, typically with a secondary fuel such as natural gas (USEPA^b, 1991). This type of VOC abatement technology is often used for gas streams in which the VOC cannot economically be recovered and when process stream characteristics are highly variable. Flares are also used as a safety mechanism in conjunction with other control devices (e.g., a sudden increase in gas flow rate that overloads another control device can be sent to the flare).

The use of boilers to remove TVOCs from gas streams is site specific (USEPA^b, 1991). TVOC destruction is achieved with an on-site boiler that is used for other processes within the facility.

Adsorption systems capture TVOCs at the adsorbents’ internal and external surface areas. Activated carbon is the most widely used adsorbent for VOC removal (USEPA^b, 1991; Noll et al. 1992). Typically the adsorbent is regenerated either thermally at an elevated temperature (typically with steam) or under a vacuum.

Absorbers transfer VOCs from gas streams to a relatively non-volatile liquid (USEPA^b, 1991). Absorption rates are typically dependent on the VOC concentration gradient between the gas phase and liquid phase, the physical properties of the gas/liquid system (e.g., diffusivity, viscosity and density) and the operating conditions of the absorber (e.g., temperature and flow rate). Absorption is typically enhanced by lower operating temperatures, achieving large concentration gradients between the gas and liquid phase and allowing greater interfacial surface area.

Condensers can be used to remove VOCs from gas streams by lowering the temperature of the stream below the saturation temperature of the VOC. The gaseous VOC will change phase from a gas to a liquid at the saturation temperature and concentration, and thus the contaminant would be removed from the gas stream. Recovery of VOCs by condensation should be considered when a relatively pure condensate with a monetary value greater than $0.66/kg can be recovered (Dyer and Muhlholland, 1994).

The applicability of these technologies to a specific gas stream is dependent on the type of VOCs in the gas stream (Table 1.6). Thermal incinerators, catalytic incinerators and condensers can be applied to the widest range of VOC categories.

This manuscript reports on the development of an ACFC adsorption system that has been integrated with electrothermal desorption and LN₂ cryogenic condensation to reduce the TVOC emission levels to the MACT standards and provide for reuse of the TVOCs that are recovered. In the adsorber, TVOCs are selectively separated from the influent gas stream by the attraction force fields generated from micropore walls in the ACFC fibers. ACFC adsorption is followed by electrothermal regeneration, using nitrogen as the carrier gas, resulting in formation of a nonexplosive concentrated organic vapor. Then the concentrated TVOC is efficiently condensed using LN₂ as the required refrigerant. ACFCs higher adsorption capacity and faster adsorption and desorption rates along with higher energy transfer through electrothermal desorption allows for optimizing the desorption time and TVOC concentration profile to allow minimal use of LN₂. LN₂ can reduce TVOC emission concentrations to ppbv levels due to its low temperature and high latent heat of vaporization. This system can enable TVOC sources to meet air quality control regulations while providing a high quality liquid TVOC product for reuse.