Kidd, Donald F. & Nyer, Evan K."Air Treatment for In Situ Technologies" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 6 Air Treatment for In Situ Technologies Donald F. Kidd and Evan K. Nyer CONTENTS Introduction Design Criteria Regulatory Requirements Mass of Contaminants Lifecycle Emission Concentration Citing and Utility Considerations Treatment Technologies Adsorption-Based Treatment Technologies Off-Site Regenerable/Disposable Gas Phase GAC On-Site Regenerable GAC Resin Adsorption Systems Oxidation-Based Technologies Thermal Oxidation/Incineration Catalytic Oxidation Biological Technologies Scrubbers Technology Selection Summary Emission Control Case Study Site Description Remedial Approach Emission Control Design Basis Alternatives Evaluation System Performance References ©2001 CRC Press LLC INTRODUCTION In Chapter 1 we introduced the concept that most in situ treatment processes were simply a switch from water to air as the carrier. This chapter will look at treating the air carrier as it is brought above-ground. Above-ground vapor treatment of emissions from soil vapor extraction, air sparg- ing, and air stripping applications often represents the largest portion of the overall cost of implementing these technologies. Figure 1 represents a pie chart of overall project costs associated with a vapor extraction system which operates for 3 years at a vapor recovery rate of 300 cfm and a declining influent concentration from 2,000 ppm (hydrocarbon vapors) to 5 ppm over the project lifetime. It is assumed that vapors are treated using catalytic oxidation for the first two years, and that granular activated carbon (GAC) is used for treatment during the last year of treat- ment. Figure 1 shows that air emission control and O&M related costs might be over 50 percent of overall project costs. Due to the magnitude of air emission control costs, the design engineer must carefully evaluate and select the most appropriate technology. Control technology selection must consider several criteria that will be introduced in this chapter: • Regulatory requirements • Overall mass of VOCs to be treated • Anticipated decline rate of VOC concentrations over project lifetime (life- cycle design) • Citing considerations • Utility availability Figure 1 Pie chart of overall VES project costs. ©2001 CRC Press LLC • Organic and inorganic composition of process vapor stream • Other project specific considerations The most common air emission control technologies can be classified as adsorp- tive, oxidative, or biological. The adsorption based technologies include off-site regenerable/disposable vapor phase GAC, on-site steam regenerable GAC, and on- site regenerable macroreticular resin systems. Oxidative technologies include cata- lytic oxidation, and thermal oxidation. Biological based systems have gained atten- tion in the last few years and have become commercially available. Less commonly utilized technologies include scrubbing, vapor compression, UV/ozone oxidation, and refrigeration. The most commonly utilized technologies will be introduced in this chapter. DESIGN CRITERIA Regulatory Requirements Vapor emissions from site remediation activities are generally not permanent sources of discharge. The short duration of the emission may exempt its permitting and control in some states. Often however, in cities or states where overall air quality standards are not met (nonattainment areas), or in states with strict emission control standards, permitting and vapor treatment is required. Emission requirements are quite variable within the different states. Emission requirements may be based on total mass emissions per hour or per day. Mass based emission criteria may be for total VOCs and/or may be compound specific. The design engineer must select the air emission control system based upon the most limiting criteria. For example, VES emissions for a gasoline release contain a variety of compounds. If air emission standards require a maximum 15 pounds/day total VOC limit, and an overall benzene limit of one lb/hr, then system design must be based upon the limiting regulatory requirement. In this example, the limiting criterion is likely based on total VOC emissions, since benzene usually constitutes only a small fraction of the total mass of gasoline. Because of the variable composition of gasoline and variations in constituent volatility, vapor samples collected during pilot testing are often used to determine the factors dictating emission control design. Alternately, emission control criteria may simply require that the emission rates not cause an exceedance in ambient air quality or other risk based criteria. This generally requires that the point source of discharge be modeled using air dispersion techniques (Bethea 1978). Several states impose both mass emission and concen- tration criteria. Some states require that all emissions be treated using Best Available Control Technology (BACT) regardless of the magnitude of the emissions (this requirement calls for emission control even if the process stream already complies with mass emission requirements). Therefore, the first thing that the design engineer must do is to acquire the local and state regulations before trying to design a vapor treatment system. Based upon lifecycle design (Chapter 2), emission rates decline during site remediation. Permit preparation should account for this temporal change. ©2001 CRC Press LLC It is plausible to limit the site operation (hours/day or number of extraction wells) to stay within permitting limits (without treatment) until emission rates drop as the cleanup progresses. This approach will likely increase site remediation time frame and the design engineer must conduct a cost-benefit analysis in order to justify merits of this phased start up method. Mass of Contaminants An estimation of the total mass of VOCs that may be recovered by the remedi- ation system is often a requirement prior to determining the appropriate treatment technology. This is particularly true for adsorbent based treatment systems such as carbon or resin-based controls. For example, if 1,000 pounds of gasoline are known to be in the subsurface, and one expects that 65 percent of the mass will be recovered by vapor extraction, 30 percent will be biodegraded, and 5 percent will not be recovered (ratios are based upon empirical projection), an estimate can be formulated for expected adsorbent consumption. Assuming a 7 percent by weight adsorption capacity for GAC, approximately 9,300 lbs of GAC will be required. The cost of other technologies can also be estimated based upon the mass of VOCs and expected flow rates. There are several simple methods to estimate the mass of VOCs in the subsurface. Once this is calculated, an estimate can be made of the amount (percent) that is expected to be extracted for above-ground treatment. Often the final estimate is based on an average of the various estimation methods. An excellent starting point is direct knowledge of the amount of contaminants released. Time is also an impor- tant factor in that spills will weather and naturally degrade (see Chapter 7 on In Situ Bioremediation). Second, the total mass may be estimated by using soil contaminant concentra- tions, groundwater concentrations, soil gas concentrations, and NAPL thickness at the various locations across the site (see Equation 1 in Chapter 3). The use of weighted average methods (concentration and expected flow from each zone) and subdivision of the site into small quadrants (based on the available data) will yield more accurate mass estimates. It should be noted that soil analytical methods often underestimate the amount of adsorbed VOCs due to significant losses during the sampling procedures (USEPA 1991). Use of methanol extraction/preservation methods can often lead to soil con- taminant levels that may be one to two orders of magnitude higher than conventional methods. It should also be noted that core samples represent a small statistical percentage of the sampled media, and therefore, are inherently inaccurate. Third, in instances where limited information is available, gross estimates of the total mass of contamination in the subsurface may be evaluated using partitioning coefficients. For example, if no soil contamination data is available, groundwater data, knowledge of the compound octanol-water partition coefficient, and soil organic content can be used to estimate the amount of VOCs adsorbed to the soil (Equation 3 in Chapter 3). It should be stated that these equations assume equilibrium conditions persist in the subsurface. Nonequilibrium conditions generally dominate ©2001 CRC Press LLC in the subsurface, and partitioning based calculations underestimate the adsorbed mass. This is because a large portion of the mass may be restricted from being in equilibrium with the surrounding soil vapor/groundwater due to nonequilibrium type adsorptive or mass transfer limitations (Brusseau 1991). Lifecycle Emission Concentration Design of cleanup strategies to accommodate the lifecycle of the project has been emphasized several times in this book. This is particularly true for the treatment of vapor emissions from VES, where concentrations may drop four orders of mag- nitude over a project lifetime (Figure 2). Emission control technology selection is more significantly affected by concentration than volume through-put for vapor phase treatment than for liquid phase treatment. For example, an air stripper will generally be chosen for the treatment of 100 ppm or 100 ppb of groundwater contaminated by BTEX compounds. This choice will be made for almost any flow rate. On the other hand, an emission stream of 10,000 ppm vapors from a VES stack (300 cfm), is best treated by a thermal oxidizer. As the concentrations drop to 1,000 ppm, the vapors are best treated by catalytic oxidation. At influent concentrations of 20 ppm, the optimal choice may be GAC. The original design must encompass all of these criteria, not just the initial influent concentration. This dynamic need to modify treatment technologies necessitates foresight from the design engineer for vapor emission system design. The systems must be designed and installed with sufficient flexibility to allow for future modifications. For example, at a site where VES emissions are expected to be above 300 ppm for 6 months and Figure 2 Typical decline curve for VES emissions. ©2001 CRC Press LLC then drop off rapidly (typical small service station, limited spill situation), a catalytic oxidizer may be rented for the first six months and subsequently GAC may be installed at the site. Many localities have recognized the benefits and needs for a dynamic emission control scheme for these typically short duration emission sources. In efforts to streamline the local regulatory approval process, often permits are issued for various locations, allowing one system to be placed at several sites within the agency’s jurisdiction. Such permits allow for more rapid deployment of the appro- priate emission control system, while reducing the burden for detailed evaluation of an often lengthy permit application. When considering the merits of an adaptive emission control scheme, the treatment system citing must accommodate future needs of the contingent system allowing adequate space and utility connections for each component of the treatment process equipment. Typically, the engineer needs to predict the decline curve for the emissions from the air treatment system and subsequently prepare a cost analysis for the various options at varying concentrations. A typical cost analysis table is shown in Table 1. Modeling of the remedial system performance to predict the decline curve may be conducted. In many instances, this modeling is not performed and empirical methods (fitting the concentration decay to an empirical logarithmic decay equation over a time period based upon past experience) are used for its prediction. The use of empirical methods is generally acceptable in the consulting industry for purposes of air emission selection due to the costs of modeling and its inherent uncertainties. For example, it is not critical to know whether a catalytic oxidizer will run for six or seven months before switching to GAC, what is important is the ability to plan for and switch to GAC. Table 1 Cost Analysis Spreadsheet for Vapor Treatment Costs Influent Concentration VOC/day GAC cost/day Catalytic Oxidation cost/day 1 50 ppm 1.47 $59 $136 2 100 ppm 2.93 $117 $128 3 200 ppm 5.86 $234 $120 4 500 ppm 14.65 $586 $112 5 1,000 ppm 29.31 $1,170 $110 6 2,000 ppm 58.62 $2,344 $100 7 3,000 ppm 87.93 $3,516 $100 Cost Assumptions: 1 ppm = 3.26 mg/cu. meter (benzene) 100 cfm operation for 24 hour per day GAC adsorption capacity = 15% by weight Carbon Costs = $6/lb. (new plus regeneration and changeout cost) Catalytic oxidation unit rental is $3,000/month Catalytic oxidation power consumtion is $350/month (at 500 ppm influent); assume costs are slightly higher at lower concentrations; slightly lower at higher concentrations assume costs are slightly higher at lower concentrations; slightly lower at high concentrations. Cost per day of catalytic oxidizer is (3,000+350)/12= 114 ©2001 CRC Press LLC Citing and Utility Considerations There are several citing considerations that need to be evaluated prior to treatment technology selection. Some of these constraints are presented below: • Availability of utilities • Utility cost analysis • Access issues relating to O&M • Aesthetic issues • Proximity to homes and buildings • Winterization • Other site specific considerations The availability of utilities and their ability to accommodate the treatment equip- ment must be carefully evaluated. Most utility evaluations are for thermal and catalytic oxidation. For example, if natural gas is to be selected, it must be available in sufficient pressure to be utilized by the treatment equipment. In residential neigh- borhoods, natural gas lines may not have sufficient pressure for adequate operation of some thermal oxidation units. Even where available, high pressure gas line connections to the oxidizer typically require additional lead time for permitting and installation. Electrical power must be available in the appropriate phase and voltage to power the equipment. At remote sites, where utility availability is limited, propane tanks can be utilized. The design engineer needs to conduct a cost analysis in order to choose the most appropriate power source (natural gas, electrical, propane, oil, etc.) for powering the treatment unit. When available, natural gas tends to be the lowest cost option in many locations. The use of propane, in addition to increased cost per BTU (generally 1.5 to 2 times higher than natural gas), also presents other operational problems such as increased fouling of burner components, as well as logistical problems created from scheduling fuel deliveries. Remedial systems are unplanned installations. Sites and neighborhoods are obvi- ously developed without planning for a potential remedial system installation. This unplanned remedial system, therefore, needs to be located to accommodate several factors that may sometimes be conflicting. It must be located to attain permitting, meet regulatory stack height and air dispersion requirements, fit in with the natural setting, and be accessible for routine operation and maintenance. Concurrently, the system must not be offensive to neighbors and have a stack height that meets local zoning laws. TREATMENT TECHNOLOGIES Adsorption-Based Treatment Technologies Adsorption is a process by which material accumulates on the interface between two phases. In the case of vapor phase adsorption, the accumulation occurs at the air/solid interface. The adsorbing phase is called the adsorbent and the substance ©2001 CRC Press LLC being adsorbed is termed an adsorbate. It is useful to distinguish between physical adsorption, which involves only relatively weak intramolecular bonds, and chemi- sorption, which involves essentially the formation of a chemical bond between the sorbate molecule and the surface of the adsorbent. Physical adsorption requires less heat of activation than chemisorption and tends to be more reversible (easier regeneration). GAC is the most popular vapor phase adsorbent in the site remediation industry. A number of new synthetic resins, however, have shown increased reversibility and have higher adsorption capacities for certain compounds. The most efficient arrangement for conducting adsorption operations is the columnar continuous plug flow configuration known as a fixed bed. In this mode, the reactor consists of a packed bed of adsorbent through which the stream under treatment is passed. As the air stream travels through the bed, adsorption takes place and the effluent is purified (Figure 3). The part of the adsorption bed that displays the gradient of concentration is termed the mass transfer zone (MTZ). The amount of adsorbate within the bed changes with time as more mass is introduced to the adsorbent bed. As the saturated (spent or used) zone of the bed increases, the MTZ travels downward and eventually exits the bed. This gives rise to the typical effluent concentration versus time profile, called the breakthrough curve (Figure 4). The reader is referenced to several text- books for adsorption theory, multicomponent effects, isotherm description, and mod- eling (Noll, Vassilios, and Hou 1992 and Faust and Aly 1987). This basic knowledge of adsorption theory is critical to proper understanding and selection of the various adsorbents. Figure 3 Concentration profile along an adsorbent column. ©2001 CRC Press LLC Off-Site Regenerable/Disposable Gas Phase GAC Gas phase GAC is an excellent adsorbent for many VOCs commonly encountered in vapor extraction, air sparging, vacuum-enhanced recovery, and conventional groundwater extraction. The adsorption capacity of GAC is often quantified as the mass of contaminant that is adsorbed per pound of GAC. This nominal adsorption capacity is a useful guide for pure compound adsorption but can be misleading when complex mixtures of VOCs are treated. Breakthrough, or GAC bed life, is defined when breakthrough occurs for the compound most difficult to adsorb. In instances where multicomponent mixtures are present, the adsorption capacities for each compound are generally lower than for pure compounds. Isotherm data (mg adsor- bate removed per g adsorbent at a constant temperature) and other product specific data are generally available for contaminants of interest from carbon vendors as well as in the published literature. Pilot testing for GAC feasibility is rarely conducted, except in instances where complex mixtures of VOCs are encountered. GAC is generally a good adsorbent for hydrocarbon origin VOCs, and some chlorinated VOCs. GAC has limited adsorption capacity for ketones and generally poor adsorption of volatile alcohols. Table 2 shows typical adsorption removal efficiencies for a variety of VOCs by GAC under constant temperature and moisture conditions (as stated in the table). GAC adsorption capacity is significantly enhanced if the vapor stream’s relative humidity is kept low. The use of water knock outs, demisting, desiccants, and air stream temperature adjustments are therefore common pretreatment steps to enhance GAC performance. Adsorption capacity may be as much as 10 times higher for a low humidity stream than for a humid air stream. This is particularly true for lower Figure 4 Breakthrough curve for a typical adsorber column. [...]... 11-Sep-98 19-Sep-98 2 6- Sep-98 01-Oct-98 21-Oct-98 27-Oct-98 21-Nov-98 19-Dec-98 22-Jan-99 2 6- Feb-99 04-Apr-99 29-Apr-99 11-Jun-99 17-Jul-99 22-Aug-99 Est Catalytic Oxidizer Operating Costs ($/day) $61 .03 $64 .65 $51 .60 $52.33 $61 .75 $59.58 $60 .30 $78.42 $105.09 $90 .60 $72.91 $85.52 $55.81 $107.27 $ 86. 68 $83.50 $111 .62 $101.18 $102.92 Theoretical GAC Consumption Rate ($/day) $183 $138 $110 $98 $1 06 $100 $98... $2,521 $ 360 $ 266 $ 164 $2 16 $60 5 $66 0 $65 ,5 46 $229, 360 $215,9 16 $119, 264 $205 ,61 6 $77,730 $154, 160 $131,092 $238,720 $222,832 $123,528 $211,232 $93, 460 $171,320 $ 262 ,184 $257,440 $2 36, 664 $132,0 56 $222, 464 $124,920 $205 ,64 0 $393,2 76 $2 76, 160 $250,4 96 $140,584 $233 ,69 6 $1 56, 380 $239, 960 O&M Notes: a Capital costs include equipment only b O&M estimated costs provided by vendor in response to In Situ Soil... covering the first year of operation As shown on Figure 16, the daily cost of operation for the catalytic oxidizer is generally consistent, within a narrow range from approximately $50 to $100 per day As expected, operational costs for an oxidizer are not directly dependent on the ©2001 CRC Press LLC Table 5 Case Study—Summary of Daily Operating Costs Date 15-Aug-98 20-Aug-98 29-Aug-98 05-Sep-98 11-Sep-98... chemical waste recycling facility located in Michigan The site occupies an area of approximately eight acres with operation of the facility discontinued in the early 1980s During its operation, storage, handling, and processing of waste solvents and other chemicals resulted in releases of contaminants into the environment Based on initial investigations conducted following discontinuation of facility... significantly increase the O&M complexity of the system The incremental cost of the scrubber maintenance should be factored into a cost analysis when considering competitive alternatives Scrubbers are presented in more detail later in this chapter ©2001 CRC Press LLC Figure 12 Photo of internal combustion engine catalytic oxidizer Biological Technologies Biological treatment of air emissions has gained significant... contaminated steam may undergo water treatment (especially if a groundwater treatment system exists on-site) or may also require off-site disposal If the condensed steam is treated on-site, it should be metered into the groundwater treatment system since it is generally much more contaminated than the groundwater If regenerable systems are used for adsorption of chlorinated VOCs, the vessels should be lined... Priorities List (NPL) in 19 86 The site was subdivided into three distinct units during the investigation As shown on Figure 14, these areas are denoted as: ©2001 CRC Press LLC Figure 14 Case study – site layout • Zone A - solvent loading area • Zone B - solvent processing area • Zone C - soil berm area Consistent between the areas, the primary constituents of concern are VOCs, in particular, tetrachloroethene... activated alumina is placed at the end of the treatment train where vinyl chloride is essentially the only organic compound reaching the oxidizing agent By this configuration, oxidation of other organic compounds is minimized, similarly minimizing the consumption of the oxidizer An additional advantage of the resin system over steam regenerable GAC, is the elimination of steam disposal /treatment Although... been declining for these systems, they are still higher than conventional regenerable GAC systems The average price of the resin is $70/lb in comparison to $1/lb for GAC (if purchased in bulk; $3/lb in canister form) The resin’s superior performance for certain compounds and its minimal disposal and O&M costs, however, may make it more cost effective than regenerable GAC for certain installations In general,... poisoning may occur from entrained water particles that contain chloride (especially in remedial applications involving salt water) or from chlorinated VOCs Metals in entrained water particles may also act as poisons Mercury, arsenic, bismuth, antimony, phosphorous, lead, zinc, and other heavy metals are common poisons Lastly, the presence of high methane levels (either naturally occurring or escaping . Evan K."Air Treatment for In Situ Technologies" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 6 Air Treatment for In Situ Technologies . containing compounds. Halogen poisoning may occur from entrained water particles that contain chloride (especially in remedial applications involving salt water) or from chlorinated VOCs. Metals in. are inherently inaccurate. Third, in instances where limited information is available, gross estimates of the total mass of contamination in the subsurface may be evaluated using partitioning coefficients.