518 INTRODUCTIONTO ENVIRONMENTALENGINEERING

Before we leave this example, we should look back and see what we have wrought.
Sincethe absorptiontower neithercreatesnordestroysmatter,the massofNH3 entering
and leaving the column mustbe the same.If we assumeisothermal,steady-statecondi-
tions (that is, gas and liquid rates in and out are equal),we can solve the mass-balance
equation (Equation 6-32) for Xl. After some calculations we find Xl = 0.08734.
This is 90,300 mg/L of NH3. This is a classic example of a multimedia problem. In
solving an air pollution problem, we have createda serious water pollution problem.
Catch-22!

Adsorption. This is a mass-transferprocessin which the gas is bondedto a solid.

It is a surfacephenomenon.The gas (the adsorbate)penetratesinto the pores of the
solid (the adsorbent)but notinto the lattice itself. The bond maybe physical or chem-
ical. Electrostaticforceshold the pollutant gas whenphysical bondingis significant.
Chemical bonding is by reaction with the surface.Pressurevesselshaving a fixed
bed areusedto hold the adsorbent(Figure 6-26). Active carbon(activatedcharcoal),
molecular sieves,silica gel, and activatedalumina are the most commonadsorbents.
Active carbonis manufacturedfrom nut shells (coconutsare great) or coal subjected
to heattreatmentin a reducingatmosphere.Molecular sievesaredehydratedzeolites
(alkali-metal silicates). Sodium silicate is reacted with sulfuric acid to make silica
gel. Activated alumina is a poroushydrated aluminum oxide. The commonproperty
of theseadsorbentsis a large "active" surfacearea per unit volume after treatment.
They are very effective for hydrocarbonpollutants. In addition, they can captureH2S

Pollutant Laden Gas

ent

C
Pollutant

Adsorption Cycle Desorption Cycle

FIGURE 6-26
Adsorption system.
 AIRPOLLUTION 519

and SOl. One special form of molecular sieve can also capture NOz. With the ex-
ception of the active carbons,adsorbentshave the drawbackthat they preferentially
selectwaterbefore any of the pollutants. Thus, water mustbe removed from the gas
before it is treated. All of the adsorbentsare subject to destruction at moderately
high temperatures(150°C for active carbon,600°C for molecular sieves,400°C for
silica gel, and 500°C for activatedalumina). They are very inefficient at thesehigh
temperatures.In fact, their activity is regeneratedat thesetemperatures!
The relation betweenthe amount of pollutant adsorbedand the equilibrium
pressureat constanttem~rature is called an adsorption isotherm.The equationthat
bestdescribesthis relation for gasesis the one derived by Langmuir.54
aC*g
W = (6-48)
l+bC;
where W = amountof gas per unit mass of adsorbent,kg/kg
a,b = constantsdetermined by experiment
c; = equilibrium concentrationof gaseouspollutant, g/m3
In the analysis of experimentaldata, Equation 6-48 is rewritten as follows:

C;
-=
I
-+
b
-C
* (6-49)
W a a g

In this arrangement,a plot of (C;tW) versus C; should yield a straight line with a
slo~ of (bta) and an interceptequal to (Ita).
In contrastto absorptiontowers where the collected pollutant is continuously
removed by flowing liquid, the collected pollutant remains in the adsorptionbed.
Thus, while the bed has sufficient capacity,no pollutants are emitted. At somepoint
in time, the bed will becomesaturatedwith pollutant. As saturationis approached,
pollutant will begin to leak out of the bed. This is called breakthrough.When the
bed capacity is exhausted,the influent and effluent concentrationwill be equal. A
typical breakthroughcurve is shownin Figure 6-27. In orderto allow for continuous
o~ration, two beds are provided (Figure 6-26). While one is collecting pollutant,
the other is being regenerated.The concentratedgas releasedduring regeneration
is usually returned to the processas recoveredproduct. The critical factor in the
o~ration of the bed is the length of time it can operatebefore breakthroughoccurs.
The time to breakthroughmay be calculated from the following:55

Zt -c5
tB = -(6-50)
vI
where Zt = height of bed, m
c5= width of adsorptionzone, m
VI = velocity of adsorptionzone as defined by Equation6-52, m/s

54A. J. Buonicoreand L. Theodore,Industrial Control Equipmentfor GaseousPollutants, Vol. I, Cleve-

land: CRC Press,pp. 149-150, 1975.
ssM. Crawford, Air Pollution Control Theory, New York: McGraw-Hill, p. 516, 1976.
 AIRPOLLUTION 521

Yo = .025

..
"< Y/i = .020

f 015 Ope~ating
=
~. Lme
GO
~
~ .010
II
>- .005 FIGURE 6.28
YB Equilibriumandoperatinglinesfor
adsorptionof benzeneonsilicagel.
.05 .10 .15 .20 .25 XT (Source:JohnH. Seinfeld,Air Pol-
lution, New York: McGraw-Hill,
X = kg Benzene/kg
Silica Gel 1975.Reprintedby pennission.)

The velocity of the adsorption zone may be calculated from the properties of
the system:

vI = (Qg)(l + bC;) (6-52)

apspgAc
where Ps, Pg = density of solid and gas, kg/m3 (Note that Ps is the density of the
absorbent ''as packed.")
Ac = cross-sectional area of bed, m2

Example 6-8. Detennine the breakthroughtime for an adsorptionbed that is 0.50 m

thick and 10 m2 in cross section.The operatingparametersfor the bed are as follows:

Gas flow rate = 1.3 kg/s of air

I Gas temperature = 25°C
Gas pressure = 101.325kPa
Bed density as packed = 420 kg/m3
Inlet pollutant concentration = 0.0020 kg/m3
Langmuir parameters:a = 18; b = 124
Width of adsorptionzone = 0.03 m

Solution. Using Table A-3 in Appendix A and the gas temperatureand pressure,we
interpolate to find Pg = 1.184kg/m3. Then the face velocity of the adsorptionwave is
(1.3 kg/s)[1 + 124(0.0020 kg/m3)]
vI = (18)(420 kg/m3)(1.184 kg/m3)(10 m2)

= 1.8 X 10-5 m/s

The breakthroughtime is calculated directly from Equation 6-50:

tB = 0.50 m10-5
-0.03m/sm
1.8 X

= 2.6 X lW s or 7.2 h l. ~c

"-"",c"",""";c',,," c ;,"',"',
 ,0

522 INTRODUCTIONTO ENVIRONMENTALENGINEERING

!;().
"aY~ ~

Refract
Stee f
~ Refractory t.~
Ring Baffle I:
~

Cf
Inlet for Contaminated
Airstream

Burner
Block
FIGURE 6-29
Directflameincineration.

Combustion. When the contaminant in the gas stream is oxidizable to an inert

gas, combustion is a possible alternative method of control. Typically, CO and
hydrocarbonsfall into this category.Both direct flame combustionby afterburners
(Figure 6-29) and catalytic combustionhave beenused in commercial applications.
Direct flame incineration is the method of choice if two criteria are satisfied.
First, the gas stream must have an energy concentrationgreaterthan 3.7 MJ/m3. At
this energyconcentration,the gasflame will be self-supportingafter ignition. Below
this point, supplementaryfuel is required. The secondrequirementis that none of
the by-products of combustionbe toxic. In somecasesthe combustionby-product
may be more toxic than the original pollutant gas. For example, the combustion
of trichloroethylene producesphosgene,which was used as a poison gas in World
War I. Direct flame incineration has been successfullyapplied to varnish-cooking,
meat-smokehouse,and paint bake-ovenemissions.
Somecatalytic materials enable oxidation to be carried out in gasesthat have
an energy content of less than 3.7 MJ/m3. Conventionally,the catalystis placed in
bedssimilar to adsorptionbeds.Frequently,the active catalystis a platinum or palla-
dium compound.The supportinglattice is usually a ceramic. Aside from expense,a
major drawback of the catalystsis their susceptibilityto poisoningby sulfur and lead
compoundsin trace amounts.Catalytic combustionhas successfullybeenapplied to
printing-press, varnish-cooking,and asphalt-oxidationemissions. :
I
Flue Gas Desulfurization (FGD)
Flue gas desulfurizationsystemsfall into two broad categories:nonregenerativeand
regenerative.Nonregenerativemeansthat the reagentusedto removethe sulfur ox-
ides from the gas streamis usedand discarded.Regenerativemeansthat the reagent
,

r-- .,-~~-"-,~,,, c,!,-c,~~-

 ~
r;~
AIRPOLLUTION 523

is recoveredand reused.In terms of the number and size of systemsinstalled, non-

regenerativesystemsdominate.

Nonregenerative systems. There are nine commercialnonregenerativesystems.57

All have reactionchemistriesbased on lime (CaO), caustic soda(NaOH), soda ash
(Na2C03), or ammonia (NH3)'
The S02 removed in a lime/limestone-basedFGD systemis convertedto sul-
fite. The overall reactionsare generallyrepresentedby:58
S02 + CaC03 -+ CaS03 + CO2 (6-53)
S02 + Ca(OH)2 -+ CaS03 + H2O (6-54)

when using limestone and lime, respectively.Part of the sulfite is oxidized with the
oxygen content in the flue gas to form sulfate:
1
CaS03 + :202 -+ CaS04 (6-55)

Although the overall reactions are simple, the chemistry is quite complex and not
well defined. The choice betweenlime and limestone, the type of limestone, and
method of calcining and slaking can influence the gas-liquid-solid reactions taking
place in the absorber.
The principal types of absorbersused in the wet scrubbing systemsinclude
venturi scrubber/absorbers,static packed scrubbers, moving-bed absorbers,tray
towers, and spray towers.59
Spray dryer-based FGD systems consist of one or more spray dryers and a
particulate collector.60The reagentmaterial is typically a slaked lime slurry or a
slurry of lime and recycled material. Although lime is the most commonreagent,
soda ash has also been used. The reagentis injected in droplet form into the flue
gas in the spraydryer. The reagentdroplets absorbS02 while simultaneouslybeing
dried. Ideally, the slurry or solution dropletsare completelydried before they impact
the wall of the dryer vessel.The flue gas streambecomesmore humidified in the
processof evaporationof the reagentdroplets, but it doesnot becomesaturatedwith
water vapor.This is the single most significant difference betweenspraydryer FGD

\ 57S.B. Hance andJ. L. Kelly, "Status of Flue Gas Desulfurization Systems," PaperNo. 91-157.3, 84th
) Annual Meeting of the Air and Waste ManagementAssociation, 1991.
58H.T. Karlsson and H. S. Rosenberg,"Technical Aspects of Lime/Limestone Scrubbersfor Coal fired
Power Plants, Part I: ProcessChemistry and ScrubberSystems," Journal ofthe Air Pollution Control
Association,vol. 30 (6), pp. 710-714, 1980.
59Black& VeatchConsulting Engineers,Lime FGD SystemsData Book -Second Edition, EPRI Publi-
cation No. CS-2781, 1983.

~;
'
spraydrying. Nonetheless,many authorshave adoptedthe term "spray drying" as synonymouswith dry
scrubbing.-,
L
6OHistorically,from a masstransfer point of view, spraydrying refers to the evaporationof a solventfrom
an atomized spray. Simultaneousdiffusion of a gaseousspeciesinto the evaporating droplet is not true

--'-c--,- -
 524 INTRODUCTIONTO ENVIRONMENTALENGINEERING

and wet scrubberFGD. The humidified gas streamand a significant portion of the
particulate matter (fly ash,FGD reactionproducts,andunreactedreagent)arecarried
by the flue gas to the particulate collector located downstreamof the spray dryer
vessel.61

Control Technologiesfor NitrogenOxides

Almost all nitrogen oxide (NOx) air pollution results from combustionprocesses.
They are produced from the oxidation of nitrogen bound in the fuel, from the reac-
tion of molecular oxygen and nitrogen in the combustionair at temperaturesabove
1,600 K (see Equation 6-12), and from the reaction of nitrogen in the combustion
air with hydrocarbonradicals. Control technologiesfor NOx are grouped into two
categories:those that prevent the formation of NOx during the combustionpro-
cess and those that convert the NOx formed during combustioninto nitrogen and
oxygen.62

Prevention. The processesin this category employ the fact that reduction of the
peak flame temperaturein the combustionzonereducesNOx formation. Nine alter-
natives have beendevelopedto reduceflame temperature:(1) minimizing operating
temperatures,(2) fuel switching, (3) low excessair, (4) flue gasrecirculation, (5) lean
combustion,(6) stagedcombustion,(7) low NOx burners, (8) secondarycombustion,
and (9) water/steaminjection.
Routine burner tune-upsand operationwith combustionzone temperaturesat
minimum values reduce the fuel consumptionand NOx formation. Converting to
a fuel with a lower nitrogen content or one that burns at a lower temperaturewill
reduce NOx formation. For example, petroleum coke has a lower nitrogen content
and bums with a lower flame temperaturethan coal. On the otherhand, natural gas
has no nitrogen content but burns at a relatively high flame temperatureand, thus,
producesmore NOx than coal.
Low excessair and flue gas recirculation work on the principle that reduced
oxygen concentrationslower the peak flame temperatures.In contrast, in lean com-
bustion, additional air is introduced to cool the flame.
In stagedcombustionand low NOx burners,initial combustiontakesplace in a
fuel-rich zone that is followed by the injection of air downstreamof the primary com-
bustion zone. The downstreamcombustionis completed under fuel-lean conditions
at a lower temperature.
Stagedcombustionconsistsof injecting part of the fuel and all of the combus-
tion air into the primary combustionzone.Thermal NOx productionis limited by the
low flame temperaturesthat result from high excessair levels.

61A. L. Cannell and M. L. Meadows,"Effects of RecentOperating Experience on the Design of Spray

DryerFGD Systems," Journal ofthe Air Pollution Control Association,vol. 35 (7), pp. 782-789,1985.
62A. Prasad,"Air Pollution Control Technologies for Nitrogen Oxides," The National Environmental
Journal, May/June,pp. 46-50,1995.

~, oc__c ~---"",c":;j;~_.