Module 5a Incinerators and Adsorbers
MCEN 4131/ Preliminaries 1-minute paper: –things you like about class –helpful suggestions to improve your learning experience
MCEN 4131/ 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 Learning Objectives for Today
MCEN 4131/ Organic Compounds Contain carbon (except CO, CO 2 ) 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Thermal 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 pollution c. leakage in the compression zone can occur d. High installation costs Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Fuels Simplest composition is natural gas (CH 4 ) 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)
MCEN 4131/ Stoichiometry 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 = 0.9 b. a = 3.76, b = 1.45, c = 1, d = 0.9 c. a = 3.76, b = 2.9, c = 1, d = 1.8 CH (b)O 2 + (a)(b)N 2 --> (c)CO 2 + (d)H 2 O + (a)(b)N 2 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ The 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 F, sec, fps b F, 5-8 sec, fps c F, sec, fps Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ How long in the “Hot Zone?” P HC = P HC0 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Example 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 = = 1 - exp(-kt) (eq 11.19) Kt = 6.91 Calculating A using to 11.18, A = 7.43 x 10 21, E = 95.9 kcal/mol R = cal/mol/K 1000F = /s, so t = 18 hours, and length = 660,000 m 1200F = /s, so t = 49 sec, and length = 490 m 1400F = /s, so t = 0.18 sec, and length = 1.8 m WOW temperature really makes a difference! Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone Isothermal Plug Flow 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 VV r i = Generation rate of I Q = volumetric flow rate C denotes concentration V is volume
MCEN 4131/ Designing 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 fps Residence times of seconds sufficient Calculate length, volumetric flow rate of exhaust, combustor diameter (eq ) Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Catalytic 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ The purpose of flares is? a. burn off excess VOCs from the incinerator b. destroy VOCs that are difficult to completely oxidize c. oxidize emergency releases of VOCs Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Adsorption 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Adsorption 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Isotherm 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Using an Isotherm Consider a 10L box containing 10 g of activated carbon. Initial benzene partial pressure in gas phase is 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? Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Adsorption 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 Concentration of contaminant in gas length Clean carbonSaturated carbon Adsorption zone Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Breakthrough 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 Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone QUIZ: Increasing the Temperature of a gas stream that is being treated with a fixed bed adsorber does what to the adsorption capacity? a. Increase b. decrease
MCEN 4131/ Adsorption Beds Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone
MCEN 4131/ Key Parameters Optimum bed velocity fpm QUIZ: Adsorption zone length? a. 0.5 to 1.5 feet b. 3-6 ft c. 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 time b. The superficial bed velocity must be high enough to allow a reasonable pressure drop c. The minimum bed depth must be greater than the length of one adsorption zone Learning Objectives Hydrocarbon oxidation Three T’s Incinerator balances Incinerator design Adsorption isotherms Breakthrough curves Adsorption zone