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Module 5a

Module 5a. Incinerators and Adsorbers. Preliminaries. 1-minute paper: things you like about class helpful suggestions to improve your learning experience. Learning Objectives for Today. Educational Objectives.

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Module 5a

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  1. Module 5a Incinerators and Adsorbers

  2. MCEN 4131/5131 Preliminaries • 1-minute paper: • things you like about class • helpful suggestions to improve your learning experience

  3. MCEN 4131/5131 LearningObjectivesfor Today Educational 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

  4. MCEN 4131/5131 LearningObjectives Organic Compounds Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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

  5. MCEN 4131/5131 LearningObjectives Thermal Oxidation Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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) • the products of combustion of certain VOCs are themselves major pollutants • fugitive vapors from the fuel used for combustion contribute to water pollution • leakage in the compression zone can occur • High installation costs

  6. MCEN 4131/5131 Fuels • 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)

  7. MCEN 4131/5131 LearningObjectives Stoichiometry Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • Quiz Question: For the following model of oxidation of a hydrocarbon (HC), what are the values of a and b? • a = 0.27, b = 1.45, c = 1, d = 0.9 • a = 3.76, b = 1.45, c = 1, d = 0.9 • 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

  8. MCEN 4131/5131 LearningObjectives The 3 T’s Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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 fps • b. 1000-2000F, 5-8 sec, 300-400 fps • c. 1200-2000F, 0.3-2 sec, 20-40 fps

  9. MCEN 4131/5131 LearningObjectives How long in the “Hot Zone?” Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption 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

  10. MCEN 4131/5131 LearningObjectives Example Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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!

  11. MCEN 4131/5131 LearningObjectives Isothermal Plug Flow Reactor (page 316) Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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 DV ri = Generation rate of I Q = volumetric flow rate C denotes concentration V is volume

  12. MCEN 4131/5131 LearningObjectives Designing a Combustor Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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)

  13. MCEN 4131/5131 LearningObjectives Catalytic Oxidizers Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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

  14. MCEN 4131/5131 LearningObjectives The purpose of flares is? Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • burn off excess VOCs from the incinerator • destroy VOCs that are difficult to completely oxidize • oxidize emergency releases of VOCs

  15. MCEN 4131/5131 LearningObjectives Adsorption Control Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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

  16. MCEN 4131/5131 LearningObjectives Adsorption Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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

  17. MCEN 4131/5131 LearningObjectives Isotherm Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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

  18. MCEN 4131/5131 LearningObjectives Using an Isotherm Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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?

  19. MCEN 4131/5131 LearningObjectives Saturated carbon Clean carbon Adsorption zone Concentration of contaminant in gas length Adsorption Beds Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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

  20. MCEN 4131/5131 LearningObjectives Breakthrough Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • 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 • QUIZ: Increasing the Temperature of a gas stream that is being treated with a fixed bed adsorber does what to the adsorption capacity? • Increase • decrease

  21. MCEN 4131/5131 LearningObjectives Adsorption Beds Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone

  22. MCEN 4131/5131 LearningObjectives Key Parameters Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone • Optimum bed velocity 50-100 fpm • QUIZ: Adsorption zone length? • 0.5 to 1.5 feet • 3-6 ft • 2 inches - 7 inches • Which of the following statement is not a requirement for the carbon bed design: • The bed must contain enough adsorbent to provide reasonable bed cycle time • The superficial bed velocity must be high enough to allow a reasonable pressure drop • The minimum bed depth must be greater than the length of one adsorption zone

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