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Distributor

Packing material

Support plate

Incoming

Effluent

FIGURE 18.10 Schematic diagram of packed tower aerator.

Effluent

To storage tank, distribution

Booster pump system, or additional treatment

FIGURE 18.10 Schematic diagram of packed tower aerator.

Dimensionless units are also used that are valid only for systems that operate at standard pressure (Patm = 1). The actual units are as follows:

where Patm = 1 (standard pressure), Vw = volume of water (m3), and Vc = volume of contaminant (m3).

Typical values of H for gasoline components range between 20 and 500 atm (0.03 to 0.30 in dimensionless units at the standard condition [Patm = 1]).

Henry's law constants for most of the compounds of interest can be found in the literature.54 Figure 18.11 shows Henry's law constants for TCE, EDC and several gasoline compounds.19 These data are derived from water solubility data and the equilibrium vapor pressure of pure liquids at certain temperatures, and may be extrapolated correctly to field design work. Temperature has a major effect on Henry's constant and on stripper performance. Each rise of 10°C in temperature

Concentration in air, |g/L

Concentration in water, |g/L

Ease of stripping

0.01

EDB EDC TEL

Benzene Toluene Xylenes Ethylbenzene

Naphthalene MTBE

EDB = Ethylene dibromide (1,2-dibromoethane)

EDC = Ethylene dichloride (1,2-dichloroethane)

TEL = Tetraethyl lead

MTBE = Methyl tertiary butyl ether

TCE = Trichloroethylene

Dichloroethylene

FIGURE 18.11 A comparison of stripping rates for TCE and gasoline compounds.

may cause an increase of Henry's constant by a factor of about 1.6.55 Consequently, warmer temperatures can achieve higher rates of stripping.

Mass balance and air/water ratio

Contaminant mass transport in an air stripper is schematically shown in Figure 18.12. The removal process can be described mathematically by a mass balance for the contaminant assuming that there is no change in the accumulated contaminant in the stripper under steady-state conditions:

where L — volumetric rate of contaminated groundwater (L3/T), G — volumetric rate of air (L3/T), Xin — influent contaminant concentration in water (M/L3), Xout — effluent contaminant concentration in water (M/L3), Yin — influent contaminant concentration in air (M/L3), and Yout — effluent contaminant concentration in air (M/L3).

For a further application of the mass balance equation to removal processes, four basic assumptions are made:

2. Differential flow holds for air and water.

3. Changes of liquid and air volumes during mass transfer are negligible.

4. Henry's law holds for these conditions.

L = volume of liquid

G = volume of gas

X = concentration in liquid

Y = concentration in gas

Z = depth of packing

FIGURE 18.12 Differential element for an air stripping tower.

L = volume of liquid

G = volume of gas

X = concentration in liquid

Y = concentration in gas

Z = depth of packing

FIGURE 18.12 Differential element for an air stripping tower.

Applying the first assumption, Fin = 0, Equation 18.12 can be rearranged as

Applying Henry's law at the point that air leaves the stripper (i.e., the contaminated water enters stripper) and assuming that equilibrium for mass transfer holds between air and water at that point, Equation 18.9 becomes

Yout HXin

Substituting Equation 18.15 into Equation 18.14 yields

where H = Henry's constant (dimensionless).

Note that (Xin - Xout)/Xin is the removal efficiency of a stripper, denoted as f Then Equation 18.16 becomes

G/L in Equation 18.17a is the theoretical air/water ratio required for the removal efficiency f for a specific contaminant following Henry's law. In this context, the G/L is denoted (G/L)theory, indicating the theoretical air/water ratio. This also means that a minimum amount of air must be brought into contact with the water for a certain length of detention time, the sparging size of the water droplets also affects the mass transfer, as does the air pressure.

Stripping factor

Theoretically, the required air/water ratio for a specific removal can be determined by the mass balance in the stripper:

This air/water ratio is the theoretical or minimum air/water ratio for a given removal. However, in practice, the contaminant mass transfer is a long way from being at ideal equilibrium. A higher air/water ratio, denoted as (G/L)actual, the actual air/water ratio, is required for that removal.

The stripping factor R is used to describe the ratio of the actual operating air/water ratio to the theoretical minimum ratio:

L theory

The air stripping factor is directly related to the air/water ratio, and is in turn related to the gas pressure drop through the stripper.

Gas pressure drop

The gas pressure drop is a function of the gas and liquid flow rates and the size and type of packing. It relates to the overall cost of the air stripper and to its performance. The gas pressure drop through a stripping unit can be determined from the pressure drop curve.

A stripper operating at a high pressure drop will require a smaller volume than a similar stripper at a lower pressure drop. This reduces the capital cost for the tower, but increases the blower cost. Towers designed and built to operate at a low pressure drop have the flexibility to increase the gas flow rate and hence the air/water ratio, should the future influent concentration increase or the effluent limitation decrease. Towers designed for high pressure drops do not have this flexibility in operation and would need to decrease the liquid loading to increase the air/water ratio.

Applicability to air stripping

The removal effectiveness of air stripping depends on the following factors:

1. Physical properties of the contaminants. Physical properties, particularly Henry's constant, determine the ease of air stripping. The higher the value of Henry's constant, the higher is the removal efficiency.

2. Temperature. The temperature of contaminated water significantly influences the removal efficiency because Henry's constant increases with temperature.

3. VOC concentration in water. The higher the concentration of a target component in the contaminated water, the higher is its removal efficiency, because the driving force for the target compound to transfer from the contaminated water to the gas is greater when its concentration is higher than at equilibrium.

4. Air/water ratio. Increasing the air/water ratio will increase the removal efficiency.

5. Packing material. Packing materials are usually designed to be less susceptible to biological and mineral fouling in order to maintain a high surface area and a high void volume, both of which are necessary for maintaining a high operating efficiency.

There are several factors that may limit the use of conventional air stripping for the removal of dissolved gasoline from groundwater:

1. Applicability of air-stripping methods with respect to the type of groundwater contaminants is the most important factor. The major constituents of interest, such as benzene, toluene, xylene, and ethylbenzene, are all fairly volatile and thus are easily removed by this technique. Compounds with low volatility such as 1,2-dichloroethane (DCE) cannot be readily removed.

2. The air pollution impact of the stripping tower is significant, because the air-stripping treatment does not destroy the contaminant; it simply transfers it from the liquid to the gaseous phase. The stripper off-gas, after dilution in the tower, usually mixes with the ambient air in the atmosphere that would further lower the contaminant concentration to values below unsafe levels. Some states have regulated the limit of discharge of volatiles to the atmosphere. In New Jersey, the limit of discharge including benzene is 0.1 lb/h. Off-gas air pollution control is required if a stripper exceeds this limit. Most commonly, GAC adsorption is used to treat the vapor-phase contaminant.56

3. High concentrations of iron and magnesium or suspended solids in the influent will limit the efficiency of air stripping, because iron and manganese facilitate the growth of bacteria on the packing, causing decreased mass transfer rates and higher gas pressure drop (suspended solids can cause a similar problem if they are trapped by the packing).

4. High noise levels associated with tower operation may limit air stripping.

Air stripping processes29,30 and air flotation process57-59 introduced in the 1990s have solved some of the abovementioned problems.

18.8.3.2 Activated Carbon Adsorption

Applications

Many case studies19,60,61 have demonstrated the ability of activated carbon to remove a variety of compounds in gasoline from contaminated water to nondetectable levels (99.99+% removal). GAC is more widely used than powdered activated carbon (PAC). Activated carbon adsorption, in general, is not cost-effective in removal of highly concentrated gasoline compounds in water, where the air-stripping method or biological treatment method may be applied. Thus, GAC is widely used for removing low concentrations of complex pollutants, in particular in polishing effluent or in point-of-entry treatment for drinking water.

The main limitation of GAC in removing gasoline compounds is its cost and the disposal of the generated spent carbon. However, the problem of spent carbon's regeneration has been solved, at least in part.29

The compounds MTBE and disopropyl ether (DIPE) are sometimes found as additives in gasoline. Both have very high carbon usage rates; thus, the costs of removing these compounds are prohibitive, especially if the influent concentrations are substantial. Therefore, the presence or absence of highly soluble compounds such as MTBE or DIPE or other additives may determine the appropriateness of using GAC for a particular gasoline spill.

Petroleum hydrocarbons (benzene, toluene, ethylbenzene, and xylenes—collectively BTEX), particularly benzene, are believed to pose significant health concerns, especially as they are contained in over 99% of all gasoline. However, additives such as MTBE and DIPE, which have high carbon usage rates, are not found in all gasolines and hence pose less significant health concerns. Thus, GAC is generally applicable for the removal of BTEX.

As mentioned, a major potential limitation of GAC use is the disposal of the spent carbon. The spent GAC can be regenerated or disposed of using sanitary landfills or incineration. GAC regeneration is possible and highly feasible by heating the carbon to very high temperatures (e.g., in a kiln) to remove the volatiles and incinerate them. However, on-site regeneration is economical only in very large projects, not in UST sites. Off-site regeneration, on the other hand, may be acceptable at a central regeneration facility. However, U.S. highways authorities consider any carbon with a flash point below 200°F to be hazardous and cannot therefore be transported on the highways. Under RCRA rules, many contaminant-laden carbons are considered hazardous materials, necessitating disposal in a permitted landfill.

Iron and manganese levels in the influent water may also limit the use of GAC. They will precipitate onto the carbon during treatment. If this happens, head losses will increase rapidly, the removal of organics will be hindered, and the carbon filter may eventually get clogged, making it ineffective and increasing cost substantially, or impractical due to space constraints. If these elements are present at concentration levels above 5 mg/L, they must be removed prior to GAC treatment.

Design of GAC systems for groundwater decontamination

An isotherm test can determine whether or not a particular contaminant can be adsorbed effectively by activated carbon. In very dilute solutions, such as contaminated groundwater, a logarithmic isotherm plot usually yields a straight line represented by the Freundlich equation6263:

where X = amount of contaminant adsorbed (M), W = weight of activated carbon (M), k = constant, n = constant, C = unabsorbed concentration of contaminant left in solution (M/L3), and 1/n = represents the slope of the straight-line isotherm. The above equation also indicates the approximate capacity of activated carbon for groundwater decontamination and provides a rough estimate of the activated carbon dosage required.

For the design of a GAC system, the following interrelated parameters should be taken into consideration:

1. Influent flow

2. Carbon contact time

3. Dosage

4. Bed depth

5. Pretreatment requirements

6. Carbon breakthrough characteristics

7. Headloss characteristics

8. On-stream cycle time of carbon (i.e., the time between carbon regenerations)

In general, influent flow and contact time determine the carbon bed depth and size, which in turn determine the breakthrough characteristics of the carbon bed for the influent water, thereby deciding the actual carbon dosage. The carbon dosage determines the volume of influent water that can be treated, which sets the on-stream cycle time of the carbon. These operating variables are related by the following equations:

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