ENVIRONMENTAL ENGINEER’S MATHEMATICS HANDBOOK - CHAPTER 8 ppt

This law states thatfor dilute solutions in which the components do not interact, the resulting partial pressure p of acomponent A in equilibrium with other components in a solution can

Gaseous Emission Control

Be it known to all within the sound of my voice, Whoever shall be found guilty of burning coal Shall suffer the loss of his head.

Figure 8.1)

The applicability of a given technique depends on the properties of the pollutant and thedischarge system In making the difficult and often complex decision of which gaseous air pollutioncontrol to employ, follow the guidelines based on experience and set forth by Buonicore and Davis(1992) in their prestigious engineering text, Air Pollution Engineering Manual Table 8.1 summa-rizes the main techniques and technologies used to control gaseous emissions

In the following, we discuss the air control technologies given in Table 8.1 Much of theinformation contained in this chapter is heavily adapted from Spellman’s The Science of Air (1999)and USEPA-81/12 (1981), Control of Gaseous Emissions, Course 415 The excerpted materials arerearranged and edited to make the materials more accessible for the reader

Absorption (or scrubbing) is a major chemical engineering unit operation that involves bringingcontaminated effluent gas into contact with a liquid absorbent so that one or more constituents ofthe effluent gas are selectively dissolved into a relatively nonvolatile liquid Key terms used whendiscussing the absorption process include:

• Absorbent: the liquid, usually water mixed with neutralizing agents, into which the contaminant

is absorbed

• Solute: the gaseous contaminant being absorbed, such as SO2, H2S, and so forth

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164 ENVIRONMENTAL ENGINEER’S MATHEMATICS HANDBOOK

• Carrier gas: the inert portion of the gas stream, usually flue gas, from which the contaminant is

to be removed

• Interface: the area where the gas phase and the absorbent contact each other

• Solubility: the capability of a gas to be dissolved in a liquid

Absorption units are designed to transfer the pollutant from a gas phase to a liquid phase,accomplished by providing intimate contact between the gas and the liquid, which allows optimumdiffusion of the gas into the solution The actual mechanism of removal of a pollutant from the gasstream takes place in three steps: (1) diffusion of the pollutant gas to the surface of the liquid; (2)transfer across the gas–liquid interface; and (3) diffusion of the dissolved gas away from the interfaceinto the liquid (Davis and Cornwell, 1991)

Figure 8.1 Air pollution control points (From Spellman, F.R., 1999, The Science of Air: Concepts and

Appli-cations Boca Raton, FL: CRC Press.)

Table 8.1 Comparison of Air Control Technologies

Treatment technology Concentration and efficiency Comments

Collection

Source Dispersion

GASEOUS EMISSION CONTROL 165

Several types of scrubbing towers are available, including spray chambers (towers or columns);plate or tray towers; packed towers; and Venturi scrubbers Pollutant gases commonly controlled

by absorption include sulfur dioxide; hydrogen sulfide; hydrogen chloride; ammonia; and oxides

of nitrogen

The two most common absorbent units in use today are the plate and packed tower systems.Plate towers contain perforated horizontal plates or trays designed to provide large liquid–gasinterfacial areas The polluted airstream is usually introduced at one side of the bottom of the tower

or column and rises up through the perforations in each plate; the rising gas prevents the liquidfrom draining through the openings rather than through a downpipe During continuous operation,contact is maintained between air and liquid, allowing gaseous contaminants to be removed, withclean air emerging from the top of the tower

The packed tower scrubbing system (see Figure 8.2) is predominantly used to control gaseouspollutants in industrial applications, where it typically demonstrates a removal efficiency of 90 to95% Usually vertically configured (Figure 8.2), the packed tower is literally packed with devices(see Figure 8.3) of large surface-to-volume ratio and a large void ratio that offer minimum resistance

to gas flow In addition, packing should provide even distribution of both fluid phases; be sturdyenough to support itself in the tower; and be low cost, available, and easily handled (Hesketh, 1991).The flow through a packed tower is typically countercurrent, with gas entering at the bottom

of the tower and liquid entering at the top Liquid flows over the surface of the packing in a thinfilm, affording continuous contact with the gases Though highly efficient for removal of gaseouscontaminants, packed towers may create liquid disposal problems, may become easily cloggedwhen gases with high particulate loads are introduced, and have relatively high maintenance costs

Figure 8.2 Typical countercurrent-flow packed tower (From USEPA, Control Techniques for Gases and

166 ENVIRONMENTAL ENGINEER’S MATHEMATICS HANDBOOK

Solubility is a function of system temperature and, to a lesser extent, system pressure As ature increases, the amount of gas that can be absorbed by a liquid decreases (gases are moresoluble in cold liquids than in hot liquids) Gas phase pressure can also influence solubility; byincreasing the pressure of a system, the amount of gas absorbed generally increases However, this

temper-is not a major variable in absorbers used for air pollution control because they operate at close toatmospheric pressure (USEPA-81/12, 1981)

Under certain conditions, Henry’s law can express the relationship between the gas phase tration and the liquid phase concentration of the contaminant at equilibrium This law states thatfor dilute solutions in which the components do not interact, the resulting partial pressure (p) of acomponent A in equilibrium with other components in a solution can be expressed as

concen-(8.1)

where

p = partial pressure of contaminant in gas phase at equilibrium

H = Henry’s law constant

x A = mole fraction of contaminant or concentration of A in liquid phase

Figure 8.3 Various packing used in packed tower scrubbers (Adapted from Air Pollution Control Equipment,

Part II, American Industrial Hygiene Association, 1968.)

Raschig Ring — most popular type

Berl saddle — efficient but costly

Pall rings — good liquid distribution

Tellerette — very low unit weight

Intalox saddle — efficient but expensive

p = HxA

GASEOUS EMISSION CONTROL 167

Equation 8.1 is the equation of a straight line where the slope (m) is equal to H Henry’s lawcan be used to predict solubility only when the equilibrium line is straight — when the soluteconcentrations are very dilute In air pollution control applications this is usually the case Forexample, an exhaust stream that contains a 1000-ppm SO2 concentration corresponds to a molefraction of SO2 in the gas phase of only 0.001

Another restriction on using Henry’s law is that it does not hold true for gases that react ordissociate upon dissolution If this happens, the gas no longer exists as a simple molecule Forexample, scrubbing HF or HCl gases with water causes both compounds to dissociate in solution

In these cases, the equilibrium lines are curved rather than straight Data on systems that exhibitcurved equilibrium lines must be obtained from experiments

The units of Henry’s law constants are atmosphere per mole fractions The smaller the Henry’slaw constant is, the more soluble the gaseous compound is in the liquid The following examplefrom USEPA-81/12 (1981) illustrates how to develop an equilibrium diagram from solubility data

Step 1 The data must first be converted to mole fraction units The mole fraction in the gas phase y

is obtained by dividing the partial pressure of SO2 by the total pressure of the system For the first entry of the data table:

The mole fraction in the liquid phase x is obtained by dividing the moles of SO2 by the total moles

of liquid:

For the first entry (x) of the data table:

Solubility of SO 2 in Pure Water Equilibrium data Concentration SO 2

2

2 n + moles H O2moles of H O2 = (100 g H O2 = 100 g H 0/18 g2 HH O per mole)2 = 5.55

x = 0.0078/(0.0078 + 5.55)

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168 ENVIRONMENTAL ENGINEER’S MATHEMATICS HANDBOOK

Step 2 The following table is completed The data from this table are plotted in Figure 8.4 Henry’s law applies in the given concentration range, with Henry’s law constant equal to 42.7 mole fraction

SO2 in air per mole fraction SO2 in water.

The simplest way to express the fundamental engineering concept or principle of material or massbalance is to say, “Everything has to go somewhere.” More precisely, the law of conservation ofmass says that when chemical reactions take place, matter is neither created nor destroyed Thisimportant concept allows us to track materials — in this case, pollutants, microorganisms, chem-icals, and other materials — from one place to another

The concept of materials balance plays an important role in environmental treatment ogies, where we assume a balance exists between the material entering and leaving the treatmentprocess: “What comes in must equal what goes out.” The concept is very helpful in evaluatingprocess operations In air pollution control of gas emissions using a typical countercurrent flowabsorber, the solute (contaminant compound) is the material balance Figure 8.5 illustrates a typicalcountercurrent flow absorber in which a material balance is drawn (USEPA-81/12, p 4-14).The following equation can be derived for material balance:

technol-(8.2)

Figure 8.4 SO2 absorption data for Example 8.1 (Adapted from USEPA-81/12, p 4-8, 1981.)

Solubility data for SO 2

GASEOUS EMISSION CONTROL 169

where

Y1 = inlet solute concentration

Y2 = outlet solute concentration

X1 = inlet composition of scrubbing liquid

X2 = outlet composition of scrubbing liquid

L m = liquid flow rate, gram-moles per hour

G m = gas flow rate, gram-moles per hour

Equation 8.2 is the equation of a straight line When this line is plotted on an equilibriumdiagram, it is referred to as an operating line (see Figure 8.5). This line defines operating conditionswithin the absorber, that is, what is going in and what is coming out The slope of the operatingline is the liquid mass flow rate divided by the gas mass flow rate, which is the liquid-to-gas ratio

or (L m/G m) When absorption systems are described or compared, the liquid-to-gas ratio is usedextensively

The following example (using Henry’s law) illustrates how to compute the minimum liquidrate required to achieve desired removal efficiency

Example 8.2

Problem:

Using the data and results from Example 8.1, compute the minimum liquid rate of pure waterrequired to remove 90% of the SO2 from a gas stream of 84.9 m3/min (3000 actual cubic feet perminute [acfm]) containing 3% SO2 by volume The temperature is 293 K and the pressure is 101.3kPa (USEPA-81/12, p 4-20)

Figure 8.5 Operating line for a countercurrent flow absorber (From USEPA-81/12, p 4-17.)

Operating line Driving forces

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170 ENVIRONMENTAL ENGINEER’S MATHEMATICS HANDBOOK

Given:

Inlet gas solute concentration (Y1) = 0.03

Minimum acceptable standards (outlet solute concentration) (Y2) = 0.003

Composition of the liquid into the absorber (X2) = 0

Gas flow rate (Q) = 84.9 m3/min

Outlet liquid concentration (X1) = ?

Liquid flow rate (L) = ?

H = Henry’s Constant

Solution:

Step 1 Sketch and label a drawing of the system (see Figure 8.6).

Step 2 At the minimum liquid rate, Y1 and X1 will be in equilibrium The liquid will be saturated with

GASEOUS EMISSION CONTROL 171

Step 3 The minimum liquid-to-gas ratio is

Step 4 Compute the minimum required liquid flow rate First, convert cubic meters of air to gram-moles:

=( 0 03 0 003− ) / ( 0 000703 0− )

= 38 4 (g mol water)/(g mol air)

At 0 C (273 K) and 101.3 kPa, there are 0.0° 2224 m /g mole of an ideal gas3

172 ENVIRONMENTAL ENGINEER’S MATHEMATICS HANDBOOK

The main parameter that affects the size of a packed tower is the gas velocity at which liquid

droplets become entrained in the existing gas stream Consider a packed tower operating at set gas

and liquid flow rates By decreasing the diameter of the column, the gas flow rate (meters per

second or feet per second) through the column will increase If the gas flow rate through the tower

is gradually increased by using increasingly smaller diameter towers, a point is reached at which

the liquid flowing down over the packing begins to be held in the void spaces between the packing

This gas-to-liquid flow ratio is termed the loading point The pressure drop over the column begins

to build, and the degree of mixing between the phases decreases A further increase in gas velocity

causes the liquid to fill the void spaces in the packing completely The liquid forms a layer over

the top of the packing, and no more liquid can flow down through the tower The pressure drop

increases substantially, and mixing between the phases is minimal

This condition is referred to as flooding, and the gas velocity at which it occurs is the flooding

velocity Using an extremely large diameter tower eliminates this problem However, as the diameter

increases, the cost of the tower increases (USEPA-81/12, p 4-22)

Normal practice is to size a packed column diameter to operate at a certain percent of the

flooding velocity A typical operating range for the gas velocity through the towers is 50 to 75%

of the flooding velocity, assuming that by operating in this range, the gas velocity will also be

below the loading point A common and relatively simple procedure to estimate the flooding velocity

(and thus minimum column diameter) is to use a generalized flooding and pressure drop correlation

One version of the flooding and pressure drop relationship in a packed tower is shown in Figure

8.8 This correlation is based on the physical properties of the gas and liquid streams and tower

packing characteristics We summarize the procedure to determine the tower diameter in the

following set of calculations:

Step 1 Calculate the value of the horizontal axis (abscissa) of Figure 8.8 using Equation 8.3:

Line

57.6

A Minimum operating line

Abscissa = (L/G)(P /P )g 1 0.5

GASEOUS EMISSION CONTROL 173

where

L = mass flow rate of liquid stream

G = mass flow rate of gas stream

P g = gas density

P1 = liquid density

Step 2 From the point on the abscissa (calculated in Equation 8.3), proceed up the graph to the flooding

line and read the ordinate ε

Step 3 Using Equation 8.4, calculate the gas flow rate at flooding and solve G∗

(8.4)

where

G∗ = mass flow rate of gas per unit cross-sectional area at blooding, pounds per second-square feet

ε = ordinate value in Figure 8.8

P g = density of gas stream, pounds per cubic foot

P1 = density of absorbing liquid, pounds per cubic foot

g c = gravitational constant = 9.82 m/sec 2 (32.2 ft/sec 2 )

F = packing factor, dimensionless

φ = ratio of specific gravity of the scrubbing liquid to that of water, dimensionless

µ 1 = viscosity of liquid (for water = 0.8 cP = 0.0008 Pa-sec (use pascal-seconds in this equation)

Step 4 Calculate the actual gas flow rate per unit area as a fraction of the gas flow rate at flooding

0.001

Psi drop, m H2O/ft of packing (in H2O/ft of packing)

Flooding Line 0.0416 (0.5)

0.0208 (0.25) 0.00808 (0.10) 0.00416 (0.05)

0.808 (1.0) 0.125 (1.5)

0.5 (dimensionless)( L) ( )

Example 8.3 Tower Sizing

Problem:

For the scrubber in Example 8.2, determine the tower diameter if the operating liquid rate is1.5 times the minimum The gas velocity should be no greater than 75% of the flooding velocity,and the packing material is 2-in ceramic Intalox™ saddles

Solution:

Step 1 Calculate the value of the abscissa in Figure 8.8 From Example 8.2:

Convert gas molar flow to a mass flow, assuming molecular weight of the gas at 29 kg/mol:

Adjusting the liquid flow to 1.5 times the minimum:

The densities of water and air at 20°C are:

G*operating = ƒG*flooding

Area Total gas flow rate

Gas flow rate per

GASEOUS EMISSION CONTROL 175

Calculate the abscissa using Equation 8.3

Step 2: From Figure 8.8 , determine the flooding line from 1.22 The ordinate ε is 0.019 From Equation

8.4, calculate G ∗:

G* = [(ε)(Pg)(P1)(gc)]/[Fφ(µ1)0.2])0.5 For water, φ = 1.0, and the liquid viscosity is equal to 0.0008 Pa-sec For 2-in Intalox™ saddles, F =

40 ft 2 /ft 3 or 131 m 2 /m 3 :

Step 3 Calculate the actual gas flow rate per unit area:

Step 4 Calculate the tower diameter:

The height of a packed tower refers to the depth of packing material needed to accomplish therequired removal efficiency The more difficult the separations are, the larger the packing heightmust be For example, a much larger packing height is required to remove SO2 than to remove Cl2from an exhaust stream using water as the absorbent because Cl2 is more soluble in water than

SO2 Determining the proper height of packing is important because it affects the rate and efficiency

G*operating = ƒG*flooding = 0.75 × 2.63 = 97 kg/(m -sec)1 2

Tower Area = Gas Flow Rate/Gop

(102.6 kg/min)(min/60 sec)1.97 kg/m -sec2

Tower Diameter = 1.13 A0.5 = 1.05 m or at leeast 1 m (3.5 ft)

Z = HTU × NTU

Z = height of packing

HTU = height of transfer unit

NTU = number of transfer units

The concept of a transfer unit comes from the operation of tray (tray/plate) tower absorbers.Discrete stages (trays or plates) of separation occur in tray/plate tower units These stages can bevisualized as a transfer unit with the number and height of each giving the total tower height.Although packed columns operate as one continuous separation process, in design terminology theprocess is treated as if it were broken into discrete sections (height of a transfer unit) The numberand the height of a transfer unit are based on the gas or the liquid phase Equation 8.9 can bemodified to yield Equation 8.10:

(8.10)

where:

Z = height of packing, meters

NOG = number of transfer units based on overall gas film coefficient

H OG = height of a transfer unit based on overall gas film coefficient, meters

NOL = number of transfer units based on overall liquid film coefficient

HOL = height of a transfer unit based on overall liquid film coefficient, meters

Values for the height of a transfer unit used in designing absorption systems are usually obtainedfrom experimental data To ensure the greatest accuracy, vendors of absorption equipment normallyperform pilot plant studies to determine transfer unit height When no experimental data areavailable, or if only a preliminary estimate of absorber efficiency is needed, generalized correlations

are available to predict the height of a transfer unit The correlations for predicting the H OG or the

H OL are empirical in nature and a function of:

• Type of packing

• Liquid and gas flow rates

• Concentration and solubility of the contaminant

• Liquid properties

• System temperature

These correlations can be found in engineering texts For most applications, the height of a transferunit ranges between 0.305 and 1.22 m (1 and 4 ft) As a rough estimate, 0.6 m (2.0 ft) can be used.The number of transfer units (NTU) can be obtained experimentally or calculated from a variety

of methods When the solute concentration is very low and the equilibrium line is straight, Equation

8.11 can be used to determine the number of transfer units (N OG) based on the gas phase resistance:

(8.11)

where:

NOG = transfer units

Y1 = mole fraction of solute in entering gas

Z = NOGHOG = N HOL OL

Nog

In (Y mX )(Y mX ) 1

mGLm

GASEOUS EMISSION CONTROL 177

m = slope of the equilibrium line

X2 = mole fraction of solute entering the absorber in the liquid

Y2 = mole fraction of solute in exiting gas

Gm = molar flow rate of gas, kilogram-moles per hour

Lm = molar flow rate of liquid, kilogram-moles per hour

Equation 8.11 may be solved directly or graphically by using the Colburn diagram presented in

Figure 8.9 This diagram is a plot of the N OG vs In[Y1 – mX2)/(Y2 – mX2)], reading up the graph

to the line corresponding to (mG m /L m ), and then reading across to obtain the N OG

Equation 8.11 can be further simplified for situations in which a chemical reaction occurs or

if the solute is extremely soluble In these cases, the solute exhibits almost no partial pressure and

therefore the slope of the equilibrium line approaches zero (m = 0) For either of these cases,

Equation 8.11 reduces to Equation 8.12:

(Y1− mX2)/(Y2− mX2)

NOG = In(Y /Y )1 2

units are required to achieve 90% removal of any pollutant Equation 8.12 only applies when theequilibrium line is straight (low concentrations) and the slope approaches zero (very soluble orreactive gases) Example 8.4 illustrates a procedure to calculate the packed tower height.

Example 8.4 Tower (Column) Sizing

Problem:

From pilot plant studies of the absorption system in Example 8.2, the H OG for the SO2-watersystem was determined at 0.829 (2.72 ft) Calculate the total height of packing required to achieve90% removal The following data were taken from the previous examples

Step 1 Compute the N OG from Equation 8.11:

Step 2 Calculate the total packing height:

N

In (Y mX )(Y mX ) 1

mGLm

mGL

(42.7)(3.5)204

GASEOUS EMISSION CONTROL 179

In a plate tower absorber, the scrubbing liquid enters at the top of the tower, passes over the topplate, and then down over each lower plate until it reaches the bottom Absorption occurs as thegas, which enters at the bottom, passes up through the plate and contacts the liquid In a platetower, absorption occurs in a step-by-step or stage process (USEPA-81/12, p 4-32)

The minimum diameter of a single-pass plate tower is determined by using the gas velocitythrough the tower If the gas velocity is too great, liquid droplets are entrained, causing a conditionknown as priming Priming occurs when the gas velocity through the tower is so great that it causesliquid on one plate to foam and rise to the plate above Priming reduces absorber efficiency byinhibiting gas and liquid contact For the purpose of determining tower diameter, priming in a platetower is analogous to the flooding point in a packed tower It determines the minimum acceptablediameter; the actual diameter should be larger The smallest allowable diameter for a plate tower

is expressed by Equation 8.13:

(8.13)

where:

d = plate tower diameter

ψ = empirical correlation, meters0.25(hours)0.25/(kilogram)0.25

Q = volumetric gas flow, cubic meters per hour

Pg = gas density, kilograms per cubic meter

The term ψ is an empirical correlation and a function of the tray spacing and the densities of thegas and liquid streams Values of ψ shown in Table 8.2 are for a tray spacing of 61 cm (24 in.)and a liquid specific gravity of 1.05 If the specific gravity of a liquid varies significantly from1.05, the values for ψ in the table cannot be used

Depending on operating conditions, trays are spaced at a minimum distance between plates toallow the gas and liquid phases to separate before reaching the plate above Trays should be spaced

to allow for easy maintenance and cleaning; they are normally spaced 45 to 70 cm (18 to 28 in.)apart In using the information in Table 8.2 for tray spacing different from 61 cm, a correctionfactor must be used Use Figure 8.10 to determine the correction factor, which is multiplied by theestimated diameter Example 8.5 illustrates how the minimum diameter of a tray tower absorber isestimated

Table 8.2 Empirical Parameters for Equation 8.13

use with Q expressed in cubic feet per minute, and P g expressed in pounds per cubic foot.

Source: Adapted from USEPA, 1972, Wet Scrubber

System Study NTIS Report PB-213016 Research angle Park, NC.

Tri-d = Ψ Ρ[Q( g) ]0.5 0.5

Example 8.5 Plate Tower Diameter

Problem:

For the conditions described in Example 8.2, determine the minimum acceptable diameter ifthe scrubber is a bubble cap tray tower absorber The trays are spaced 0.53 m (21 in.) apart (USEPA-81/12, p 4-34)

Solution:

From Example 8.2 and Example 8.3:

From Table 8.2 for a bubble cap tray:

Before Equation 8.13 can be used, Q must be converted to cubic meters per hour:

Figure 8.10 Tray spacing correction factor (Adapted from USEPA-81/12, p 4-33.)

Tray spacing, inches

0.9

Tray spacing, meters

Gas flow rate = Q = 84.9 m /min3

density=Ρg=1 17 kg/m3

Ψ = 0.0162 m0.25(h)0.50/kg0.25

Q = 84.9(m /min)(60 min/h)3 = 5094 m /h3

GASEOUS EMISSION CONTROL 181

Step 1 Substitute these values into Equation 8.13 for a minimum diameter d:

Correct this diameter for a tray spacing of 0.53 m.

Step 2 From Figure 8.10 , read a correction factor of 1.05 Therefore, the minimum diameter is:

Note: This estimated diameter is a minimum acceptable diameter based on actual conditions In

practice, a larger diameter (based on maintenance and economic considerations) is usually chosen

The several methods used to determine the number of ideal plates or trays required for a givenremoval efficiency can become quite complicated One method used is a graphical technique(USEPA-81/12, p 4-34) The number of ideal plates is obtained by drawing “steps” on an operatingdiagram, a procedure illustrated in Figure 8.11 This method can be rather time consuming, andinaccuracies can result at both ends of the graph

Figure 8.11 Graphical determination of the number of theoretical plates (From USEPA-81/12, p 4-35.)

Equilibrium line

(Note: Lines AB–BC are one theoretical plate

Need a total of 2.3 plates)

G F

D

B A

C

E

X

Equation 8.14 is a simplified method of estimating the number of plates It can only be used

if the equilibrium as well as operating lines for the system are straight, a valid assumption for mostair pollution control systems

(8.14)

where

N p = number of theoretical plates

Y1 = mole fraction of solute in entering gas

X2 = mole fraction of solute entering the tower

Y2 = mole fraction of solute in exiting gas

m = slope of equilibrium line

Gm = molar flow rate of gas, kilogram-moles per hour

L m = molar flow rate of liquid, kilogram-moles per hour

Equation 8.14 is used to predict the number of theoretical plates (N p) required to achieve agiven removal efficiency The operating conditions for a theoretical plate assume that the gas andliquid stream leaving the plate are in equilibrium This ideal condition is never achieved in practice

A larger number of actual trays is required to compensate for this decreased tray efficiency.Three types of efficiencies are used to describe absorption efficiency for a plate tower: (1) anoverall efficiency, which is concerned with the entire column; (2) Murphree efficiency, which isapplicable with a single plate; and (3) local efficiency, which pertains to a specific location on aplate The simplest of the tray efficiency concepts, the overall efficiency, is the ratio of the number

of theoretical plates to the number of actual plates Because overall tray efficiency is an plification of the process, reliable values are difficult to obtain For a rough estimate, overall trayefficiencies for absorbers operating with low-viscosity liquid normally fall in a 65 to 80% range.Example 8.6 shows a procedure to calculate the number of theoretical plates

oversim-Example 8.6

Problem:

Calculate the number of theoretical plates required for the scrubber in Example 8.5, using theconditions in Example 8.4 Estimate the total height of the tower if the trays are spaced at 0.53-mintervals; assume an overall tray efficiency of 70%

Solution:

From Example 8.5 and the previous examples, the following data are obtained:

m = slope of the equilibrium line = 42.7

GASEOUS EMISSION CONTROL 183

Step 2 Assuming that the overall plate efficiency is 70%, the actual number of plates is:

Step 3 The height of the tower is given by:

The top height is the distance (freeboard) over the top plate that allows the gas–vapor mixture to separate This distance is usually the same as the tray spacing:

Note: This height is approximately the same as that predicted for the packed tower in Example

8.4 This is logical because the packed and plate towers are efficient gas absorption devices However, due to the large number of assumptions involved, no generalization should be made.

Adsorption is a mass transfer process that involves passing a stream of effluent gas through thesurface of prepared porous solids (adsorbents) The surfaces of the porous solid substance attractand hold the gas (the adsorbate) by physical or chemical adsorption Adsorption occurs on theinternal surfaces of the particles (USEPA-81/12, p 5-1) In physical adsorption (a readily reversibleprocess), a gas molecule adheres to the surface of the solid because of an imbalance of electrondistribution In chemical adsorption (not readily reversible), once the gas molecule adheres to thesurface, it reacts chemically with it

Several materials possess adsorptive properties These materials include activated carbon; mina; bone char; magnesia; silica gel; molecular sieves; strontium sulfate; and others The mostimportant adsorbent for air pollution control is activated charcoal The surface area of activatedcharcoal will preferentially adsorb hydrocarbon vapors and odorous organic compounds from anairstream

alu-In an adsorption system the collected contaminant remains in the adsorption bed (in contrast

to the absorption system, in which the collected contaminant is continuously removed by flowingliquid) The most common adsorption system is the fixed-bed adsorber, which can be contained in

a vertical or a horizontal cylindrical shell The adsorbent (usually activated carbon) is arranged inbeds or trays in layers about 0.5 in thick Multiple beds may be used; in multiple-bed systems,one or more beds are adsorbing vapors while the other bed is being regenerated

The efficiency of most adsorbers is near 100% at the beginning of the operation and remains highuntil a breakpoint or breakthrough occurs When the adsorbent becomes saturated with adsorbate,contaminant begins to leak out of the bed, signaling that the adsorber should be renewed or regenerated.Although adsorption systems are high-efficiency devices that may allow recovery of product;have excellent control and response to process changes; and have the capability of being operatedunattended, they also have some disadvantages These include the need for expensive extractionschemes if product recovery is required, relatively high capital cost, and gas stream prefilteringneeds (to remove any particulate capable of plugging the adsorbent bed) (Spellman, 1999).

Adsorption occurs in a series of three steps (USEPA-81/12, p 5-2) In the first step, the contaminantdiffuses from the major body of the air stream to the external surface of the adsorbent particle Inthe second step, the contaminant molecule migrates from the relatively small area of the externalsurface (a few square meters per gram) to the pores within each adsorbent particle The bulk ofadsorption occurs in these pores because the majority of available surface area is there (hundreds

of square meters per gram) In the third step, the contaminant molecule adheres to the surface inthe pore Figure 8.12 illustrates this overall diffusion and adsorption process

The adsorption process is classified as physical or chemical The basic difference between physicaland chemical adsorption is the manner in which the gas molecule is bonded to the adsorbent Inphysical adsorption, the gas molecule is bonded to the solid surface by weak forces of intermolecularcohesion The chemical nature of the adsorbed gas remains unchanged Therefore, physical adsorp-tion is a readily reversible process In chemical adsorption, a much stronger bond is formed betweenthe gas molecule and adsorbent; sharing or exchange of electrons takes place Chemical adsorption

is not easily reversible

Figure 8.12 Gas collection by adsorption (Adapted from USEPA-81/12, p 5-2.)

Step 1 Diffusion to adsorbent surface Contaminant molecules

Contaminant molecules

Contaminant molecules

Step 2 Migration into pores of adsorbent

Step 3 Monolayer buildup of adsorbate