1. Advanced Air Pollution Engineering Air Pollution control Devices
Air pollution from industrial operations
Air Pollution control Devices

2. Outline
emphasis will be placed on the design and application of different control; devices , removal of particulate matter,gaseous,dust . Optimum operating conditions for pollutants removal will also be discussed.

3. Introduction 1- ABSORBERS
Gas absorption, as applied to the control of air pollution, is concerned with the removal of one or more pollutants from a contaminated gas stream by treatment with a liquid.
The necessary condition is the solubility of these pollutants in the absorbing liquid.
The rate of transfer of the soluble constituents from the gas to the liquid phase is determined by diffusional processes occurring on each side of the gas–liquid interface.

4. Introduction 1- ABSORBERS
for example, the process taking place when a mixture of air and sulfur dioxide is brought into contact with water. The SO2 is soluble in water, and those molecules that come into contact with the water surface dissolve fairly rapidly.
the SO2 molecules are initially dispersed throughout the gas phase, and they can reach the water surface only by diffusing through the air, which is substantially insoluble in the water. When the SO2 at the water surface has dissolved, it is distributed throughout the water phase by a second diffusional process. Consequently, the rate of absorption is determined by the rates of diffusion in both the gas and liquid phases.

5. Introduction 1- ABSORBERS
Equilibrium is another extremely important factor to be considered that affects the operation of absorption systems.
The rate at which equilibrium is established is then essentially dependent on the rate of diffusion of the pollutant through the non-absorbed gas and through the absorbing liquid.
The rate at which the pollutant mass is transferred from one phase to another depends also on a so-called mass transfer, or rate, coefficient which relates the quantity of mass being transferred with the driving force. As can be expected, this transfer process ceases upon the attainment of equilibrium.

6. Introduction
1- ABSORBERS The principal types of gas absorption equipment may be classified as follows: 1. Packed columns (continuous operation) 2. Plate columns (staged operation)

7. Introduction
Packed columns: are used for the continuous contact between liquid and gas. The countercurrent packed column (Figure 1) is the most common type of unit encountered in gaseous pollutant control for the removal of the undesirable gas, vapor, or odor.

9. Introduction Packed columns:
The gas stream containing the pollutant moves upward through the packed bed against an absorbing or reacting liquid that is injected at the top of the packing. This results in the highest possible transfer/control efficiency.
Since the pollutant concentration in the gas stream decreases as it rises through the column, there is constantly fresher liquid available for contact. This provides a maximum average driving force for the transfer process throughout the packed bed.
Liquid distribution plays an important role in the efficient operation of a packed column. A good packing from a process viewpoint can be reduced in effectiveness by poor liquid distribution across the top of its upper surface.

10. Introduction Packed columns:
Poor distribution reduces the effective wetted packing area and promotes liquid channeling.
The final selection of the mechanism of distributing the liquid across the packing depends on the size of the column, type of packing, tendency of packing to divert liquid to column walls, and materials of construction for distribution.
For stacked packing, the liquid usually has little tendency to cross distribute and thus moves down the column fairly uniformly in the cross-sectional area that it enters.
In the dumped condition, most flow profiles follow a conical distribution down the column, with the apex of the cone at the liquid impingement point.

11. Introduction Packed columns:
For well-distributed liquid flow and reduced channeling of gas and liquid to produce efficient use of the packed bed, the impingement of the liquid onto the bed must be as uniform as possible.
redistribution is seldom necessary for stacked bed packings, as the liquid flows essentially in vertical streams.

12. Introduction Packed columns:
Crossflow packed scrubbers are particularly successful when the process air stream requires both gas absorption and particulate removal.
The crossflow scrubber operates by allowing the gas to flow horizontally across the scrubber. The scrubbing liquid is introduced at the top of the scrubber and drains vertically through the packing. Contact between the gas and liquid occurs at a right angle.
crossflow scrubbers operate at lower pressure drops and liquid recycle flow rates than do countercurrent packed-bed absorbers.

13. Introduction Packed columns:
Crossflow scrubbers operate with a liquid rate and pressure drop approximately 60% less than a comparable countercurrent packed tower.
The crossflow scrubber generally operates at much higher gas-to-liquid ratios than does the countercurrent packed-bed absorber.
the liquid stream in the crossflow scrubber is able to “scour” the packing media more easily than is the countercurrent packed absorber. Thus, there can be a significant reduction in plugging of the packing. Since the crossflow scrubber reduces water consumption and the size of the recirculation pump, significant savings in both operating and capital costs may be realized.

14. Introduction Packed columns:
It is also easier to increase the gas flow rate through an existing crossflow absorber than a countercurrent packed-bed absorber since the crossflow scrubber can operate over a wider range of gas-to-liquid ratios.
Many current crossflow scrubbers are using multiple beds with individual scrubbing sections that can remove a wide variety of pollutants. It is much more difficult and expensive to provide multiple scrubbing sections in a packed tower.
The main advantage that the packed tower absorber has over the crossflow scrubber is that the packed tower can achieve very high removal efficiencies for pollutants that are difficult to absorb.

15. Introduction plate columns:
may also be employed as absorbers, although they are used only occasionally for pollution control.
DESIGN AND PERFORMANCE EQUATIONS
The equilibrium of interest in gas absorption operations is that between a relatively nonvolatile absorbing liquid (solvent) and solute gas (usually the pollutant).
the solute is ordinarily removed from a relatively large amount of a carrier gas that does not dissolve in the absorbing liquid.

16. Introduction
DESIGN AND PERFORMANCE EQUATIONS The equilibrium relationship of importance is a plot (or data) of x, the mole fraction of solute in the liquid, against y* (sometimes denoted simply as y), the mole fraction in the vapor in equilibrium with x. For cases that follow Henry’s law, Henry’s law constant m can be defined by the equation: Y* = y = mx (1) The usual operating data to be determined or estimated for isothermal systems are the liquid rate(s) and the terminal concentrations or mole fractions. An operating line that describes operating conditions in the column is obtained by a mass balance around the column (as shown in Figure 2)

18. Introduction
DESIGN AND PERFORMANCE EQUATIONS Total moles in = total moles out Gm1 + Lm2 = Gm2 + Lm1 (2) The terms G and L represent the molar flow rates of the gas and liquid, respectively. For component A, the mole (or mass) balance becomes: Gm1 yA1 + Lm2 xA2 = Gm2 yA2 + Lm1xA1 (3) Assuming Gm1 =Gm2 and Lm1 =Lm2 (reasonable for most air pollution control application where contaminant concentrations are usually extremely small), Equ. 3 can be re-write as: Gm1 yA1 + Lm2 xA2 = Gm2 yA2 + Lm1xA1 (4)

19. Introduction
DESIGN AND PERFORMANCE EQUATIONS Rearranging the componential mole balance in Equation (4) leads to: This is the equation of a straight line known as the operating line since it describes operating conditions in the column. On x, y coordinates, it has a slope of Lm/Gm and passes through the points (xA1, yA1) and (xA2, yA2) as indicated in Figure 2
(5)

21. Introduction DESIGN AND PERFORMANCE EQUATIONS
With reference to Figure 3, the operating line must pass through point A and must terminate at the ordinate yA1. If such a quantity of liquid is used to determine the operating line AB, the existing liquid will have the composition XA1
If less liquid is used, the exit liquid composition will clearly be greater, as at point C, but since the driving forces (displacement of the operating line from the equilibrium line) for mass transfer are less, the absorption is more difficult. The time of contact between gas and liquid must then be greater, and the absorber must be correspondingly taller.

22. Introduction DESIGN AND PERFORMANCE EQUATIONS
The minimum liquid that can be used corresponds to the operating line AD, which has the greatest slope for any line touching the equilibrium curve and is tangent to the curve at E.
At point E, the diffusional concentration difference driving force is zero; the required contact time for the concentration change desired is infinite and an infinitely tall column is required. This then represents the limiting or minimum liquid-to-gas ratio.
The importance of the minimum liquid-to-gas ratio lies in the fact that column design and operation is frequently specified as some factor of the minimum liquid– gas ratio. For example, a typical situation frequently encountered is that the slope of the actual operating line, (Lm/Gm)act, is 1.5 times the minimum, (Lm/Gm)min:

23. Introduction DESIGN AND PERFORMANCE EQUATIONS
Once all the streams entering and leaving the column and their constituents are identified, flow rates calculated, and operating conditions determined, the physical dimensions of the column can be calculated.
The column must be of sufficient diameter to accommodate the gas and liquid, and of sufficient height to ensure that the required amount of mass is transfered.

24. Introduction DESIGN AND PERFORMANCE EQUATIONS Packed Column
Regarding the diameter, consider a packed column operating at a given liquid rate while the gas rate is gradually increased. After a certain point the gas rate is so high that the resistive force (drag) on the liquid is sufficient to keep the liquid from flowing freely down the column.
Liquid begins to accumulate and tends to block the entire cross section for flow (so-called loading). This, of course, increases both the pressure drop and prevents the packing from mixing the gas and liquid effectively, and ultimately some liquid is even carried back up the column.

25. Introduction DESIGN AND PERFORMANCE EQUATIONS Packed Column
This undesirable condition, known as flooding, occurs fairly abruptly, and the superficial gas velocity (the velocity if the column is empty) at which it occurs is called the flooding velocity.
The calculation of column diameter is based on flooding considerations; the usual operating range is approximately 50–75% of the flooding rate.
One of the more commonly used correlations that has withstood the test of time is pressure drop correlation, as presented in Figure 4.
The procedure to determine the column diameter is as follows:

26. Introduction
DESIGN AND PERFORMANCE EQUATIONS Packed Column 1- Calculate the abscissa, (L/G) (pG/ pL) 2- Proceed to the flooding line and read the ordinate (design parameter). 3- Solve the ordinate term for G at flooding. 4- Calculate the column cross-sectional area S for the fraction f of flooding velocity chosen for operation by the equation: 5- The diameter of the column is then determined by: D=1.13 (s)0.5 (7)

27. 4444

28. Introduction DESIGN AND PERFORMANCE EQUATIONS Packed Column
Note that the proper units as designated in the correlation must be used as the plot is not dimensionless.
The flooding rate should also be evaluated using (total) flows of the phases at the bottom of the column, where they are at their highest value.
The pressure drop may be evaluated directly from Figure 4 using a revised ordinate that contains the actual, not flooding, value of G. The column height is given by
z=NOG HOG (8)

29. Introduction DESIGN AND PERFORMANCE EQUATIONS Packed Column
z=NOG HOG (8)
Where :
NOG: is the number of overall transfer units, dimensionless
HOG: is the height of overall transfer units, ft
Z: is the height of packing, ft

30. Introduction DESIGN AND PERFORMANCE EQUATIONS Packed Column
z=NOG HOG (8)
Where :
NOG: is the number of overall transfer units, dimensionless
HOG: is the height of overall transfer units, ft
Z: is the height of packing, ft

32. Introduction
DESIGN AND PERFORMANCE EQUATIONS Packed Column where the absorption factor is A (not to be confused with area) and is equal to L/mG and m is the slope of the equilibrium curve. The solution to this equation can conveniently be found graphically from Figure 5. Note also that the flow rates L and G are based on moles in Equatio n (9). However, if m = 0, Equation (9) reduces to This equation may be applied if the gas is highly soluble or if the absorbate (pollutant) reacts with the liquid

33. Introduction DESIGN AND PERFORMANCE EQUATIONS Packed Column
Qualitatively, the height of a transfer unit is a measure of the height of a contactor required to effect a standard separation, and it is a function of the gas flow rate, the liquid flow rate, the type of packing, and the chemistry of the system.
the usual practice is to design so that the operational gas rate is approximately 75% of the rate that would cause flooding. If possible, column dimensions should be in readily available sizes (i.e., diameters to the nearest half-foot and heights to the nearest foot). If the column can be purchased “off the shelf” as opposed to being specially made, a substantial savings can be realized.

34. Introduction OPERATION AND MAINTENANCE, AND IMPROVING PERFORMANCE
Packed Column
Normal preventive maintenance requires only periodic checks of the fan, pumps, chemical feed system, piping, duct, and liquid distributor.
removal can be accomplished by recirculating, for a short period of time, a chemical solution into which the solids will dissolve or react.
The normally unirrigated entrainment separator must be periodically flushed with sprays to prevent buildup and eventual plugging.

35. Introduction OPERATION AND MAINTENANCE, AND IMPROVING PERFORMANCE
Fouling
The most common maintenance problem associated with packing media is fouling or plugging. Plugging can result from deposition of undissolved solids in the liquid, deposition of insoluble dust in the inlet air stream, precipitation of dissolved salts in the liquid that exceed their solubility, and precipitation of an insoluble salt that is formed by the reaction of a soluble salt in solution with an absorbed gas.

36. Introduction OPERATION AND MAINTENANCE, AND IMPROVING PERFORMANCE
Fouling
scaling can also be removed by periodic flushing with an acidic solution such as HCl. Scaling can also be reduced by using a packing having a large free (void) volume.
The other plugging problems mentioned can be easily reduced by increasing the liquid flow through the column, adding a strainer in the recycled liquid piping, or increasing the water makeup and overflow rates.

37. Introduction Plastic Packing Advantage
The smooth surface of plastic packings reduces fouling since any solids that may be deposited do not cling, as they would on a rough surface, and are more easily washed away.
Lightweight plastic allows a more open support plate to be used, reducing pressure drop and solids deposition. This lighter weight also reduces the shell cost and is less expensive to install and support.
Plastic packings are easily installed by simply dumping the packing into an empty column. When installing the packing, care should be exercised to prevent bridging of the packing.

38. Introduction Mechanisms of Absorption Two-Film Theory
The two-film theory of absorption is illustrated in Figure 6. All resistance to mass transfer is assumed to be associated with a thin gas film and a thin liquid film immediately adjacent to the gas-liquid interface.
The gaseous contaminant, component A, with mole fraction yA, is transported by turbulent mixing action to the boundary of the gas film.
The contaminant then diffuses through the gas film to the interface where the mole fraction is yAi i. The interface is assumed to be at equilibrium and the mole fraction at the liquid interface is xAi.

39. Introduction Mechanisms of Absorption Two-Film Theory
From the interface, component A then diffuses across the liquid film to the bulk liquid where the mole fraction is xAii. The discontinuity between yAi and xAii is due to the composition difference between the gas and liquid.

40. Introduction Mechanisms of Absorption Two-Film Theory
When the mole fraction of A in the liquid reaches its saturation limit, the rates of mass transfer are equal in both directions. The two phases are in equilibrium and no additional contaminant removal is possible. Accordingly, it is important to design and operate absorbers so that saturation conditions are not reached. There are two ways to achieve this goal.
1- Provide sufficient liquid so that the dissolved contaminants do not reach their solubility limit.
2- Chemically react the dissolved contaminants so that they cannot return to the gas phase.

41. Introduction
Mechanisms of Absorption Solubility The solubility of a gas in a liquid is a function of the temperature and partial pressure of the contaminant in the gas phase. Gas phase total pressure can also influence solubility, but this is not a major variable in absorbers used for air pollution control since they operate near atmospheric pressure. Solubility data for the ammonia- water system are presented in Table 1 as a function of temperature. Ammonia concentration in the gas phase is expressed as partial pressure in units of mm Hg, while liquid phase ammonia concentration is expressed in weight of NH3 per 100 weights of H2O.

43. Introduction
Mechanisms of Absorption The most common method of analyzing solubility data is to use an equilibrium diagram. This is a plot of the mole fraction of solute (contaminant) in the liquid phase, denoted as x, versus the mole fraction of solute in the gas phase, denoted as y. Equilibrium data for the NH3-H2O system given in Table 1 are plotted in Figure 7 at 0oC, 20oC and 40oC. Figure 7 illustrates the temperature dependence of the absorption process. At a constant mole fraction of solute in the gas (y), the mole fraction of NH3 in the liquid (x) increases as the liquid temperature decreases.

45. Introduction
Under certain conditions, the relationship between the gas phase concentration and the liquid phase concentration of the contaminant at equilibrium can be expressed by Henry’s Law. PA=HPXA Where: PA: partial pressure of contaminant in gas phase at equilibrium HP: Henry's Law constant when the gas concentration is expressed in partial pressure XA: mole fraction of contaminant dissolved in the liquid phase at equilibrium

46. by the total pressure, P, of the system
by the total pressure, P, of the system. The left side of the equation becomes the partial pressure divided by the total pressure, which is equal to the mole fraction in the gas phase, yA. The new value of Henry’s constant Hy is simply the old value Hp divided by the total pressure P. It is important to express the contaminant concentrations in mole fraction as indicated below: yA=HyXA Where: yA: mole fraction of the contaminant in the gas phase at equilibrium Hy: Henry’s Law constant when the gas concentration is expressed in mole fraction XA: fraction of contaminant dissolved in the liquid phase at equilibrium

47. Problem 1 Use the NH3-H2O data in Table 1 to show that Henry’s Law is valid at low concentrations of NH3 and calculate Hp and Hy at 20oC in this low concentration range.

51. Liquid-to-Gas Ratios The liquid-to-gas ratio (L/G) is defined as the quantity of liquid fed to the absorber divided by the outlet gas flow rate, often expressed in units of gallons per 1000 ACF. This definition is illustrated in Figure 8.

52. Liquid-to-Gas Ratios The liquid-to-gas ratio (L/G) is defined as the quantity of liquid fed to the absorber divided by the outlet gas flow rate, often expressed in units of gallons per 1000 ACF. This definition is illustrated in Figure 8. The liquid-to-gas ratio is important for two reasons: (1) there must be sufficient liquid to avoid mass transfer equilibrium, and (2) there must be good gas-liquid contact within the absorber. The combinations of factors that influence gas-liquor contact are sometimes referred to as hydraulic factors.

53. Liquid-to-Gas Ratios problem 2
the result of problem 1

54. At the minimum, liquid rate Y1 and X1 will be in equilibrium
At the minimum, liquid rate Y1 and X1 will be in equilibrium. The liquid will be saturated with NH3.