Practical Design Calculations for Groundwater and Soil Remediation - Chapter 7

Kuo, Jeff "VOC-laden air treatment" Practical Design Calculations for Groundwater and Soil Remedition Boca Raton: CRC Press LLC,1999 chapter seven VOC-laden air treatment Remediation of contaminated soil and groundwater often results in trans-ferring organic contaminants into the air phase. Development and imple-mentation of an air emission control strategy should be an integral part of the overall remediation program. Air emission control may affect the cost-effectiveness of a specific remedial alternative. Common sources of VOC-laden off-gas from soil/groundwater remedi-ation activities include soil vapor extraction, air sparging, air stripping, solidification/stabilization, and bioremediation. This chapter illustrates the design calculations for commonly used treatment technologies: activated carbon adsorption, direct incineration, catalytic incineration, IC engines, and biofiltration. VII.1 Activated carbon adsorption Process description Activated carbon adsorption is one of the most commonly used air pollution control processes for reducing VOC emission from soil/groundwater reme-diation. The process is very effective in removing a wide range of VOCs. The most common form of activated carbon for this type of application is granular activated carbon (GAC). Activated carbon has a fixed capacity or a limited number of active adsorption sites. Once the adsorbing contaminants occupy most of the avail-able sites, the adsorption efficiency will drop significantly. If the operation is continued beyond this point, the breakthrough point will be reached and the effluent concentration will increase sharply. Eventually, carbon would be “saturated,” “exhausted,” or “spent” when all sites are occupied. The spent carbon needs to be regenerated or disposed of. Two pretreatment processes are often required to optimize the perfor-mance of GAC systems. The first is cooling, and the other is dehumidifica- ©1999 CRC Press LLC tion. Adsorption of VOCs is generally exothermic, which is favored by lower temperatures. As a rule of thumb, the waste air stream needs to be cooled down below 130°F. Water vapor will compete with VOCs in the waste air stream for available adsorption sites. The relative humidity of the waste air stream generally should be reduced to 50% or less. GAC sizing criteria Various GAC adsorber designs are commercially available. Two of the most common ones are (1) canister systems with off-site regeneration and (2) multiple-bed systems with on-site batch regeneration (while some of the adsorbers are in adsorption cycle, the others are in regeneration cycle). Sizing of the GAC systems depends primarily on the following parameters: 1. Volumetric flow rate of VOC-laden gas stream 2. Concentration or mass loading of VOCs 3. Adsorption capacity of GAC 4. Desired GAC regeneration frequency The flow rate determines the size or cross-sectional area of the GAC bed, the size of the fan and motor, and the duct diameter. The other three, mass loading, GAC adsorption capacity, and regeneration frequency, determine the amount of GAC required for a specific project. Design of vapor-phase activated carbon systems is basically the same as that for liquid-phase acti-vated carbon systems, as described in Section VI.2. VII.1.1 Adsorption isotherm and adsorption capacity The adsorption capacity of GAC depends on the type of GAC and the type of VOC compounds and their concentration, temperature, and presence of other species competing for adsorption. At a given temperature, a relation-ship exists between the mass of the VOC adsorbed per unit mass GAC and the concentration (or partial pressure) of VOC in the waste air stream. For most of the VOCs, the adsorption isotherms can be fitted well by a power curve, also known as the Freundlich isotherms (also see Eq. VI.2.2): q = a(P OC )m [Eq. VII.1.1] where q = equilibrium adsorption capacity, lb VOC/lb GAC, P = partial pressure of VOC in the waste air stream, psi, and a, m = empirical constants. The empirical constants of the Freundlich Isotherms for selected VOCs are listed in Table VII.1.A. It should be noted that the values of these empir-ical constants are for a specific type of GAC only and should not be used outside the specified range. The actual adsorption capacity in the field applications should be lower than the equilibrium adsorption capacity. Normally, design engineers take ©1999 CRC Press LLC Table VII.1.A Empirical Constants for Selected Adsorption Isotherms Compounds Benzene Toluene m-Xylene Phenol Chlorobenzene Cyclohexane Dichloroethane Trichloroethane Vinyl chloride Acrylonitrile Acetone Adsorption Temperature (°F) a m 77 0.597 0.176 77 0.551 0.110 77 0.708 0.113 77 0.527 0.0703 104 0.855 0.153 77 1.05 0.188 100 0.508 0.210 77 0.976 0.281 77 1.06 0.161 100 0.20 0.477 100 0.935 0.424 100 0.412 0.389 Range of PVOC (psi) 0.0001–0.05 0.0001–0.05 0.0001–0.001 0.001–0.05 0.0001–0.03 0.0001–0.01 0.0001–0.05 0.0001–0.04 0.0001–0.04 0.0001–0.05 0.0001–0.05 0.0001–0.05 From U.S. EPA, Control Technologies for Hazardous Air Pollutants, EPA/625/6-91/014, U.S. EPA, Washington, DC, 1991. 25 to 50% of the equilibrium value as the design adsorption capacity as a factor of safety. Therefore, qactual = (50%)(qtheoretical) [Eq. VII.1.2] The maximum amount of contaminants that can be removed or held (Mremoval) by a given amount of GAC can be determined as Mremoval = (qactual)(MGAC ) = (qactual)[(V AC )(rb )] [Eq. VII.1.3] where M is the mass, V is the volume, and r is the bulk density of the GAC, respectively. The following procedure can be used to determine the adsorption capac-ity of a GAC adsorber: Step 1: Determine the theoretical adsorption capacity by using Eq. VII.1.1. Step 2: Determine the actual adsorption capacity by using Eq. VII.1.2. Step 3: Determine the amount of activated carbon in the adsorber. Step 4: Determine the maximum amount of contaminants that can be held by the adsorber using Eq. VII.1.3. Information needed for this calculation • Adsorption isotherm • Contaminant concentration of the influent waste air stream, PVOC ©1999 CRC Press LLC • Volume of the GAC, V • Bulk density of the GAC, rb Example VII.1.1 Determine the capacity of a GAC adsorber The off-gas from a soil venting project is to be treated by GAC adsorbers. The m-xylene concentration in the off-gas is 800 ppmV. The air flow rate out of the extraction blower is 200 cfm, and the temperature of the air is ambient. Two 1000-lb activated carbon adsorbers are proposed. Determine the maxi-mum amount of m-xylene that can be held by each GAC adsorber before regeneration. Use the isotherm data in Table VII.1.A. Solution: a. Convert the xylene concentration from ppmV to psi as PVOC = 800 ppmV = 800 ´ 10–6 atm = 8.0 ´ 10–4 atm = (8.0 ´ 10–4 atm)(14.7 psi/atm) = 0.0118 psi Obtain the empirical constants for the adsorption isotherm from Table VII.1.A and then apply Eq. VII.1.1 to determine the equilibrium ad-sorption capacity as q = a(PVOC)m = (0.527)(0.0118)0.0703 = 0.386 lb/lb b. The actual adsorption capacity can be found by using Eq. VII.1.2 as qactual = (50%)qtheoretical = (50%)(0.386) = 0.193 lb/lb c. Amount of xylene that can be retained by an adsorber before the GAC becomes exhausted = (amount of the GAC)(actual adsorption capac-ity) = (1000 lbs/unit)(0.193 lb xylene/lb GAC) = 193 lb xylene/unit. Discussion 1. The adsorption capacity of vapor-phase GAC is typically in the neigh-borhood of 0.1 lb/lb (or 0.1 kg/kg), which is much higher than the adsorption capacity of liquid-phase GAC, typically in the neighbor-hood of 0.01 lb/lb. 2. Care should be taken to use matching units for P and q in the isotherm equations. 3. The influent contaminant concentration in the air stream, not the effluent concentration, should be used in the isotherm equations to determine the adsorption capacity. 4. There are two sets of empirical constants for m-xylene; one should always check the applicable range for the empirical constants. ©1999 CRC Press LLC ... - tailieumienphi.vn