Module 5a Incinerators and Adsorbers MCEN 4131/5131 2 Preliminaries 1-minute paper: –things you like about class –helpful suggestions to improve your

Module 5aModule 5a

Incinerators and Adsorbers

MCEN 4131/5131

2

PreliminariesPreliminaries

• 1-minute paper: – things you like about class– helpful suggestions to improve your

learning experience

MCEN 4131/5131

3

Educational ObjectivesEducational Objectives

• Oxidation chemistry of a hydrocarbon in air, including stoichiometry, reaction rates

• The three Ts: temperature, time and turbulence

• Material and enthalpy balance for an incinerator

• Sizing the incinerator: length, volumetric flow rate, diameter

• Adsorption Isotherms• Breakthrough curves• length of adsorption zone

LearningObjectivesfor Today

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Organic CompoundsOrganic Compounds

• Contain carbon (except CO, CO2)• Sources - combustion, unburned fuel,

landfills, chemical manufacturing, bakeries, drycleaners, consumer products, vegetation…

• Nonpolar - charge is evenly distributed around the molecule (methane, benzene)

• Some control technologies require molecules to be absorbed in another liquid

Like Dissolves Like

LearningObjectives

Hydrocarbon oxidationThree T’sIncinerator balancesIncinerator designAdsorption isothermsBreakthrough curvesAdsorption zone

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Thermal OxidationThermal Oxidation

• Organic compounds BURN• Very effective way to get rid of a

pollutant, if take oxidation far enough all you get is CO2 and H2O

• Quiz Question: A major disadvantage of incineration is? (identify one)a. the products of combustion of certain

VOCs are themselves major pollutants b. fugitive vapors from the fuel used for

combustion contribute to water pollutionc. leakage in the compression zone can

occurd. High installation costs

LearningObjectives


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FuelsFuels

• Simplest composition is natural gas (CH4)

• Liquid or solid fuels are complex mixtures of a large number of hydrocarbons

• Composition determined by measuring mass fractions of C, H, S, O, N, and ash

• Heating value is a measure of the heat release during complete combustion

• Ash is noncombustible inorganic (mineral) impurities that remains after combustion

Quiz Question: The net heat of combustion (lower heating value), is the heat that is released when water is in vapor form.True (a) or False (b)

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StoichiometryStoichiometry

• Quiz Question: For the following model of oxidation of a hydrocarbon (HC), what are the values of a and b?

a. a = 0.27, b = 1.45, c = 1, d = 0.9b. a = 3.76, b = 1.45, c = 1, d = 0.9c. a = 3.76, b = 2.9, c = 1, d = 1.8

CH1.8 + (b)O2 + (a)(b)N2 --> (c)CO2 + (d)H2O + (a)(b)N2

LearningObjectives


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The 3 T’sThe 3 T’s

• Keys to getting fuel to combust, turbulence, temperature, time

• Assuming turbulence is adequate, need to figure out how long organic compounds is in “hot zone” and how hot it should be

• Reaction rate constant for the combustion reaction is very temperature dependent

• Time is determined by length of combustor and velocity of gas in combustor

Quiz Question: Temperature, time and turbulence are very important for thermal oxidation. Which set of numbers gives typical values needed for good destruction:

a. 700-1000F, 0.03-0.05 sec, 50-80 fpsb. 1000-2000F, 5-8 sec, 300-400 fpsc. 1200-2000F, 0.3-2 sec, 20-40 fps

LearningObjectives


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How long in the “Hot How long in the “Hot Zone?”Zone?”

• PHC = PHC0 exp (-kt) (Eq 11.19)

• Need to know k and t• t = combustor length/superficial velocity

of gas in the combustor• K = A exp(-E/RT)

Partial pressure of HC at inlet of combustor

LearningObjectives


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ExampleExample• Determine length of combustor required to

reduce benzene concentrations by 99.9% given temperatures of 1000, 1200, and 1400 F. Use superficial gas velocity = 10 m/s

• Efficiency = 0.999 = 1 - exp(-kt) (eq 11.19)• Kt = 6.91• Calculating A using 11.16 to 11.18, A = 7.43 x 1021, E =

95.9 kcal/mol• R = 1.987 cal/mol/K• k @ 1000F = 0.000104 1/s, so t = 18 hours, and length

= 660,000 m• k @ 1200F = 0.14 1/s, so t = 49 sec, and length = 490 m• k @ 1400F = 38.6 1/s, so t = 0.18 sec, and length = 1.8

m

WOW temperature really makes a difference!

LearningObjectives


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LearningObjectives


Isothermal Plug Flow Isothermal Plug Flow Reactor (page 316)Reactor (page 316)

• Imagine incinerator as a one-dimensional flow through a long tube

• Velocity is constant at all radial positions

• No axial dispersion• Material balance for component i

€

QV+ΔVCiV+ΔV = QVCiV + riΔVV

€

dCi

ri

=1

QdV ri = Generation rate of I

Q = volumetric flow rateC denotes concentrationV is volume

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Designing a CombustorDesigning a Combustor

• Need to design for temperature and residence time!

• Mass and enthalpy balance on combustor gives you mass flow rate of fuel gas

• Linear velocity through combustor should be 10-20 fps

• Residence times of 0.4-0.9 seconds sufficient

• Calculate length, volumetric flow rate of exhaust, combustor diameter (eq 11.26-11.28)

LearningObjectives


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Catalytic OxidizersCatalytic Oxidizers

• Used to reduce temperature and space requirements

• Gases are preheated to a lower temperature and passed thru catalyst bed

• Reaction rate depends on mass transfer (diffusion) and rate of chemical oxidation reactions on catalyst

LearningObjectives


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The purpose of flares The purpose of flares is?is?

a. burn off excess VOCs from the incinerator

b. destroy VOCs that are difficult to completely oxidize

c. oxidize emergency releases of VOCs

LearningObjectives


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Adsorption ControlAdsorption Control

• Adsorption control is usually an intermediate step in a larger control scheme that concentrates the pollutants for destruction in a later control step

• Needed because it is cheaper to control a concentrated low flow stream compared to a dilute high flow stream

LearningObjectives


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AdsorptionAdsorption

• Molecules come in contact with a solid surface and stick– Sticks strong enough to be removed

from gas, but not too strong so can be removed from surface

• For air pollution control, adsorption by Van der Wahls forces is most common

• Activate carbon, zeolites, silica

LearningObjectives


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IsothermIsotherm

• Tells us how much material sticks to the adsorbent

• Three kinds: – Linear is used when concentration of

gas is very LOW– Langmuir is determined by assuming

adsorption sites on the surface become unavail. For further adsorption when they are occupied

– Freundlich is the result of fitting experimental data to an exponential type equation

LearningObjectives


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Using an IsothermUsing an Isotherm

• Consider a 10L box containing 10 g of activated carbon. Initial benzene partial pressure in gas phase is 0.0001 psi (~ 20 ug/L). What is equilibrium benzene partial pressure using freundlich isotherm with k = 0.4 g benzene/g carbon/psi, and n = 0.65?

LearningObjectives


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Adsorption BedsAdsorption Beds

• Put adsorbent (activated carbon) in big box (called a bed)

• Force gas thru bed• Contaminant adsorbs • Carbon becomes rapidly saturated

near front of bed so concentration of gas in this area of bed equals concentration of the entering gas

• Beyond saturated zone, concentration drops off rapidly because carbon has not reached capacity

Con

cent

rati

on o

f co

ntam

inan

t in

gas

length

Clean carbonSaturated carbon

Adsorption zone

LearningObjectives


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BreakthroughBreakthrough

• As time goes on, adsorption zone (AZ) moved further down bed

• Eventually outlet concentration will get too high

• BREAKTHROUGH HAPPENS!• Take adsorber off line before breakthrough

occurs and regenerate• Heat it up• Use steam (most commonly used)• Reduce pressure

• Since adsorption zone can be a significant portion of the total length of bed, assume capacity of bed is 25-50% of the theoretical total capacity

LearningObjectives


QUIZ: Increasing the Temperature of a gas stream that is being treated with a fixed bed adsorber does what to the adsorption capacity?

a. Increaseb. decrease

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Adsorption BedsAdsorption Beds

LearningObjectives


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Key ParametersKey Parameters

• Optimum bed velocity 50-100 fpm• QUIZ: Adsorption zone length?

a. 0.5 to 1.5 feetb. 3-6 ftc. 2 inches - 7 inches

• Which of the following statement is not a requirement for the carbon bed design:a. The bed must contain enough adsorbent to

provide reasonable bed cycle timeb. The superficial bed velocity must be high

enough to allow a reasonable pressure dropc. The minimum bed depth must be greater

than the length of one adsorption zone

LearningObjectives


Documents

Module 5a Incinerators and Adsorbers MCEN 4131/5131 2 Preliminaries 1-minute paper: –things you like about class –helpful suggestions to improve your