Vl

As seen from equation (7.8) the ratio ammonia / ammonium is disfavored by increased ionic strength, implying that a higher pH is need to obtain the same stripping effect at higher ionic strength.

Table 7.1 gives the activity coefficients for different ionic charges, calculated from the equation (7.5).

TABLE 7.1

Activity coefficient f at different ionic strengths

TABLE 7.1

Activity coefficient f at different ionic strengths

VI

f for

f for

f for

I

1 +VI

Z = 1

Z = 2

Z = 3

0

0

1.00

1.00

1.00

0.001

0.03

0.95

0.82

0.64

0.005

0.07

0.93

0.74

0.51

0.01

0.09

0.90

0.66

0.40

0.02

0.12

0.87

0.57

0.28

0.05

0.18

0.81

0.43

0.15

0.1

0.24

0.76

0.33

0.10

0.2

0.31

0.70

-

-

0.5

0.41

0.62

-

-

I = ionic strength, Z = charge, f = activity coefficient

I = ionic strength, Z = charge, f = activity coefficient

Since calcium hydroxide is the cheapest source of hydroxide ions, it is most often used for adjustment of pH before the stripping process. The addition of calcium hydroxide leads to an increased ionic strength. However, the ionic strength of most waste waters, after addition of sufficient calcium hydroxide to obtain a pH of 10 or above, is only in the order of 0.05-0.1, which implies that the increase of pH needed to obtain the same stripping effect as for distilled water is approximately only 0.1.

7.2 Process Variables

As much as 13 g ammonia gas is soluble at room temperature in 100 ml water. Due to this very high solubility of ammonia in water a large quantity of air is required to transfer ammonia effectively from the water to the air. In principle there are three different configurations of stripping units, as shown in Fig. 7.3; see Montgomery (1985).

Cross Current Flow Chemical
Figure 7.3. Configuration of air stripping units. From left to right: countercurrent, cocurrent and cross flow.

The efficiency of the process depends on:

1. pH, according to the considerations mentioned above. Equations (7.2) and

(7.3) may be applied and in case where the ionic strength is significant, equations (7.7) and (7.8) are used.

2. The temperature. The solubility of ammonia decreases with increasing temperature. The efficiency at three temperatures - 0°C, 20°C and 40°C - is plotted versus the pH in Fig. 7.4 and versus the tower height in Fig. 7.5

3. The quantity of air per m3 of water treated. At least 3000 m3 of air per m3of water are required (see Fig.7.6).

4. The height of the stripping tower. The relationship between the efficiency and the quantity of air is plotted for three heights - Figs. 7.5 and 7.6.

5. The specific surface of the packing (m2/m3). Greater specific surface results in greater efficiency.

Fig. 7.4. Stripping efficiency as function of pH at three different temperatures.
Rate Photosynthesis

Figure 7.5. Effect of water temperature on ammonia stripping. 4 m3 air is used per liter of waste water. The efficiencies are plotted versus the tower height for various temperatures.

Tower depth (m)

Figure 7.5. Effect of water temperature on ammonia stripping. 4 m3 air is used per liter of waste water. The efficiencies are plotted versus the tower height for various temperatures.

Figure 7.7 demonstrates the principle of a stripping tower. The waste water treatment plant at Lake Tahoe, California, includes a stripping process. 10,000 m3 of waste water is treated per 24h at a cost of approximately 8 US cents (1992) per m3. The capital cost is in the order of 20 US cents per m3 (based on 16% depreciation and interest per year of the investment).

The cost of stripping is therefore relatively moderate, but the process has two crucial limitations:

1. It is practically impossible to work at temperatures below 5-7°C. The large quantity of air will cause considerable evaporation, which results in Ihe water in the tower freezing.

2. Deposition of calcium carbonate can reduce the efficiency or even block the tower.

Due to limitation 1) it will be necessary to use warm air for the stripping during winter in temperate climates, or to install the tower indoors. This makes the process too costly for plants in areas with more than 10,000 inhabitants and limits the application for treatment of bigger volumes to tropical or possibly subtropical latitudes.

l600 3200 ¿+800 6^00 8000 96OO

Figure 7.6. Efficiency as function of m3 of air per m3 of water for three different tower heights line = 8 m,_line = 6.7 m, — line = 4 m.

A very important shortcoming of some technological solutions is, that they do not consider a total environmental solution, as they solve one problem but create a new one. The stripping process is a characteristic example, since the

if

y y

/

/

/ /J / ; / / / /

/

/ / / / // // //

ammonia is removed from the waste water but transferred to the atmosphere, unless recovery of ammonia is carried out. In each specific case it is necessary to assess whether the air pollution problem created is greater that the water pollution problem solved. If a significant amount of municipal waste water were be treated by air stripping, the ammonia removed by air would make a crucial contribution to the air pollution problem of nitrogenous compounds on a regional basis.

Aii' out

Aii' out

Fig. 7.7. The principle of a stripping tower.

7.3. Gas Transfer

Both aeration and stripping involve a gas-liquid mass-transfer process in which the driving force is created by a departure from equilibrium. In other words, the driving force in the gas phase is a partial pressure gradient, and is a concentration gradient in the liquid phase.

The transfer of a gas can be treated as a four-step process. The first step of a stripping process involves passage of the dissolved gas from the liquid phase to the gas-liquid interface. The second step is the passage of the gas through a liquid film on the liquid side of the interface The gas must then pass through a gas film on the vapor side of the interface. The gas must in the final step be dispersed throughout the bulk of the gas. General conditions are such that one of the steps is rate-limiting and the overall gas-transfer rate can be calculated on the basis of this step. The remaining steps are most often insignificant in the overall process.

In stagnant conditions diffusion of the gas through the bulk solution is generally the slowest step and an expression for molecular diffusion can be used to predict the transfer rate.

The diffusion can be calculated by means of Fick's Law:

dy where

N = mass transfer per unit time

A = the cross-sectional area across which diffusion occurs dc/dy = the concentration gradient perpendicular to the cross-sectional area, A

D = diffusion coefficient.

If, however, the solution is sufficiently agitated either by natural turbulence or by mechanical mixing, the rate of transfer through the gas-liquid interface becomes the controlling factor. For sparingly soluble gases such as oxygen and carbon dioxide, the resistance of the liquid film controls the rate of gas transfer, while for highly soluble gases such as ammonia, the transfer rate is controlled by the resistance of the gas phase.

Gas solubility

The equilibrium concentration of a gas in contact with a liquid can be calculated by Henry's Law:

where

Ceq = the equilibrium concentration of the gas in solution as molar fraction H = Henry's Constant p = the partial pressure of the gas in the gas phase.

Henry's Constant is roughly proportional to the temperature; i.e., with increased temperature the solubility of a gas decreases. Figure 7.8 gives the relation between solubility of ammonia and the temperature. As can be seen, the solubility changes significantly with the temperature; see also Figs. 7.2, 7.4 and 7.5.

Solubility

0 20 40 60 80 100

Figure 7.8. The solubility of ammonia plotted versus the temperature.

The temperature dependence of Henry's constant may be found by use of one of the following two equations: (Srinath and Loehr 1974 and Montgomery 1985)

Henry's Constant is also influenced by the presence of dissolved solids. The combined effects of dissolved solids and temperature on the solubility of oxygen in water are expressed by the following equation (Gameson and Robertson, 1955):

where

Cds = the concentration of total dissolved solids expressed in g/l T = the absolute temperature expressed in K

It must be emphasized that this equation is developed under the conditions that the pressure is 760 mm Hg and that clean water is in contact with wet air.

In this context it must be stressed that Henry's Law is an ideal law and gives only approximate values. It is preferable to use solubility data if these are available.

Mass transfer

Lewis and Whitman (1924) developed equations for the transfer rate controlled by the gas-film resistance as well as for the transfer rate controlled by the liquid-film resistance:

where

N = mass transfer per unit time

A = area of cross-section

Ceq = concentration at equilibrium (saturation)

p = partial pressure in the gas phase peq = partial pressure at the interface

Kl = liquid-film coefficient defined as Dl/Yl

Kg = gas-film coefficient defined as Dg/Yg

Dl = diffusion coefficient in the liquid

Dg = diffusion coefficient in the gas

Figure 7.9 shows a schematic representation of the liquid-gas mass transfer. The liquid-film-controlled process can be expressed In concentration units by dividing by the volume, V:

KL,a = Kl * (A/V) is termed the overall film coefficient.

The transfer coefficient, Kl, is affected by a number of variables. In general, the liquid-film coefficient increases with increasing temperature according to:

For KL.ain a bubble aeration system, the equation becomes

The presence of surface-active agents in the waste water has a significant effect on Kl and A/V (area to volume ratio). A decrease in surface tension will decrease the size of the bubbles generated, which will increase AA/. In some instances the increase in A/V will exceed the decrease in Kl, with the overall effect that the transfer rate increases. Generally, kl,a decreases with increasing concentration of impurities in water. A coefficient, b, defined as the as the ratio of KL,a for waste water to that for distilled water, is used to account for the influence of the impurities in the waste water on Ki_,a. Figure 7.10 shows a characteristic change in the coefficient b, as a function of BODs of water.

The liquid film resistance is usually not of importance for ammonia stripping. It is therefore possible to relate the transfer process directly to the gas film resistance, which in practice is performed by empirical relations between the resistance coefficients and the tower packing.

Nitrogen Film Coefficient
Figure 7.9. Schematic representation of interfacial mass transfer.

BOD-5 of influent

Figure 7.10. A typical BOD5 / b relationship.

7.4. Design of Stripping Tower

Figure 7.11 shows the application of the mass conservation principle on a countercurrent tower. The tower may be either a packed or a spray tower filled with bubble-cap trays, or of any internal construction to bring about a good gas-liquid contact.

The following relationships are valid (y «1):

where G

Gs : Pt gas stream total moles / h /m2 mole fraction of diffusing solute partial pressure mole ratio of diffusing solute moles/h /m2of non-diffusing, essentially insoluble gas total pressure

Figure 7.11. Principles of mass conservation applied to countercurrent tower.

Similarly, the following equation is valid for the liquid stream (x «1): x

where

L = liquid stream moles/h/m2 x = mole fraction of soluble gas X = mole ratio of soluble gas Ls = moles /h / m2 of non-volatile solvent

Since the solvent gas (air) and solvent liquid (water) are essentially unchanged in quantity as they pass through the tower, it is convenient to express the material balance in terms of these.

The balance in the lower part of the tower (see Fig. 7.11) can be expressed by

This is the equation of a straight line, the so-called operating line, which has a slope of Ls/Gsand passes through (Xi,Yi). The operating line also passes through the point (X2,Y2).

In Fig. 7.12 the operating line is plotted together with the equilibrium solubility curve, which may be found from Henry's law and plotted in terms of the mole ratio.

For a stripping tower, the operating line is always below the equilibrium solubility curve (see Fig. 7.12).

If we consider a packed or spray tower of unit area cross-section, it is convenient to describe the interfacial surface between the gas and liquid as a function of the dispersion of the liquid in the thin film over the packing. The following equation is valid:

where

S = area of the interface expressed as m2/m2 tower cross-section a = m2 interfacial surface/m3 packed volume z

Equilibrium Curve
Figure 7.12. Equilibrium curve (1) and operating line (2) for a stripping process.

The amount of solid in the gas passing the differential section of the tower is G * y mole/h/m2, and the rate of mass transfer to the liquid, d(G * y). This can be related to the mass transfer coefficient as follows:

where Ky = the overall transfer coefficient.

Both G and Y vary from one part of the tower to another, but Gs does not. Therefore, it is more convenient to use Gs in these expressions:

The mass-transfer coefficient for diffusion of one component through a second (the solvent) includes a term involving the average concentration, Ym, of the non-diffusing gas along the path of the diffusion. If the concentration of solute varies considerably from one end of the tower to another, the quantity KG*a(1-y)m will be much more constant than KG*a alone. Therefore, equation (7.25) will be transformed to

0 0

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