1、API PUBL*3628D 96 = 0732290 0559379 703 In-Situ Air Sparging API PUBLICATION 1628D FIRST EDITION, JULY 1996 . Environmental Partnership American Petroleum Insti tute API PUBL+L628D 96 = 0732290 0559180 423 s docu- menting performance improvements; and communicating them to the public. The founda- ti
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27、iii API PUBL*Lb28D 96 0732290 0559184 079 CONTENTS SECTION 1-INTRODUCTION 1 1.1 Scope 1 1.1 Techniques 1 SECTION 2-GOVERNING PHENOMENA 1 2.1 In-Situ Air Stripping . 1 2.2 Direct Volatilization 3 2.3 Biodegradation . 3 SECTION 3-APPLICABILITY 3 3.1 3.2 Geological Considerations . 5 Examples of Compou
28、nd Applicability . 3 SECTION this technology has broad appeal due to its pro- jected low capital costs in relation to conventional approaches. The difficulties encountered in modeling and monitoring the multiphase air sparging process (that is, air injection into water saturated conditions) have con
29、tributed to the current uncertainties regarding process(es) responsible for remov- ing petroleum hydrocarbons from the saturated zone. Engi- neering design of these systems is largely dependent on empirical knowledge. It is commonly perceived that the injected air travels up through the saturated zo
30、ne in the form of air bubbles; however, when grain sizes are less than 2 millimeters it is more realistic that the air travels in the form of continu- ous air channels 2. The air flow path will be strongly influenced by the structuring and stratification of the satu- rated zone soils. Significant ch
31、anneling may result from relatively subtle permeability changes, and channeling will increase as the size of the pore throats decrease. Research 3, 41 shows that even minor differences in per- meability due to stratification can impact the sparging effectiveness. It should be noted that in this disc
32、ussion, “air sparging” refers to the injection of air into formations below the water table and should not be confused with processes where air is injected within a well (in-well air sparging) to oxygenate and strip the well water. SECTION 2-GOVERNING PHENOMENA In-situ air sparging is potentially ap
33、plicable when volatile and/or easily aerobically biodegradable compounds are present in water-saturated zones, under relatively permeable conditions. The in-situ air sparging process can be defined as, the injection of compressed air at controlled pressures and volumes into water-saturated soils. Th
34、e phenomena that OCCUT during the operation of air sparging systems include: a. In-situ stripping of dissolved volatile organic compounds (VOCS). b. Volatilization of trapped and adsorbed phase hydrocar- bon compounds present below the water table and in the capillary fringe. c. Aerobic biodegradati
35、on of both dissolved and adsorbed phase hydrocarbon compounds. All three phenomena are dependent on the ability to get air in contact with the soil and groundwater containing petroleum hydrocarbons. 2.1 In-Situ Air Stripping Among the above removal mechanisms, in-situ air strip- ping may be the domi
36、nant process for some dissolved com- pounds. The strippability of any compound is a function of its Henrys Law Constant (estimated for nonpolar substruc- tures, and vapor pressure/solubility). Compounds such as benzene, toluene, xylene, ethylbenzene, trichloroethylene, and tetrachloroethylene are co
37、nsidered to be easily strippa- ble. During air sparging, dissolved compounds that are 1 - API PUBL*Lb28D 9b 0732290 0559387 8BB 2 API PUSUCmON 1628D API PUBL*KLbE!D 96 = 0732290 0559188 714 = IN-SITU AIR SPARGING 3 transferred into the vapor phase and may be capture by a vapor extraction system (VES
38、) once they migrate into the vadose zone. It has been proposed that in-situ air sparging also helps to increase the rate of dissolution of the adsorbed phase com- pounds below the water table. This enhancement dissolu- tion is caused by increased mixing and the higher concentration gradient between
39、the adsorbed and dissolved phases under sparging conditions. 2.2 Direct Volatilization During in-situ air sparging, direct volatilization of the adsorbed and trapped compounds (residual hydrocarbons) is enhanced in the zones where air flow takes place. Direct volatilization of any compound is govern
40、ed by its vapor pressure, and most volatile organic compounds are easily removed through volatilization. In areas where air is brought into contact with significant concentrations of residual VOCs in the saturated zone, direct volatilization into the vapor phase may become the dominant mechanism for
41、 mass removal. 2.3 Biodegradation In most natural situations, aerobic biodegradation of hydrocarbons in the saturated zone is limited by the availability of oxygen. Biodegradability of any com- pound under aerobic conditions is dependent on its chemical structure and environmental parameters such as
42、 pH and temperature. Some VOCs are considered to be easily biodegradable under aerobic conditions (for exam- ple, benzene, toluene, acetone, and so on,) and some are not (for example, trichloroethylene and tetrachloroethyl- ene). Typically the dissolved oxygen (DO) concentration in groundwater is le
43、ss than 4.0 milligrams per liter (mgL), and under anaerobic conditions induced by the natural degrada- tion of petroleum hydrocarbons, is often less than 1 .O ma. DO can be raised to 6 to 10 mg/L by air sparging under equilibrium conditions. This potential increase in the DO levels will contribute t
44、o enhanced rates of aerobic biodegra- dation in the saturated zone. SECTION 3-APPLICABILITY 3.1 Examples of Compound Applicability Based on the previous discussion, Table 1 describes the applicability of a few selected compounds. In practice, the criterion for defining strippability is based on Henr
45、ys Law Constant being greater than 1 x atm- m3/mole. In general, compounds with a vapor pressure greater than 0.5 to 1.0 rnm Hg can be volatilized easily; however, the degree of volatilization is also limited by the flow rate of air in contact with sorbed or dissolved com- pounds. The half lives pre
46、sented in Table 1 are estimates in groundwater under natural conditions without any enhance- ments to improve the rate of degradation. The compounds present in heavier petroleum products such as No. 6 fuel oil will not be amenable to either strip- ping or volatilization (see Figure 2). Hence, the pr
47、imary mode of remediation, if successful, will be due to aerobic biodegradation. Required air injection rates under such conditions will be influenced only by the requirement to introduce sufficient oxygen into the saturated zone. Enhanc- ing DO concentrations in the target area is dependent upon: T
48、able 1-Examples of Compound Applicability for In-Situ Air Sparging 5, 61 Compound Stnppability Volatility Aerobica Biodegradability Benzene Toluene Xylenes Ethylbenzene TCE PCE Gasoline compounds Fuel oil compounds High (H = 5.5 x High (H = 6.6 x High (H = 5.1 x lu3) High (H = 8.7 x High (H = 10.0 x
49、 High (H = 8.3 x High Low High (Vp=95.2) High (Vp= 28.4) High (Vp = 6.6) High (Vp = 60) High (Vp = 14.3) High Very low High (Vp= 9.5) High (1 112 = 240) High (tin = 168) High (fin= 336) Very low (tin = 7,704) Very low (r1/2 = 8.640) High Moderate High (fin= 144) Note: Where: H = Henrys Law Constant (atm-m3/mol). Vp = vapor pressure (mm Hg) at 20C). tia = half life during aerobic biodegradation, in hours. = the estimated half lives could vary depending on site specific environmental conditions. API PUBL*Lh28D 96 = 0732290 0559l189 650 4 API PUBLICATION 1628D I ! 4 K s O
