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Pitt CHE 0400 - LECTURE NOTES

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Last Class…MTL and Apparent Reaction RatesApparent Activation EnergiesTrue and Apparent Reaction OrderExamplePore Diffusion & Selectivity…MTL & Selectivity – Parallel ReactionsMTL & Selectivity – Series ReactionsSpecial Case: Shape SelectivityExample: Toluene DisproportionationHow can we check for MT limitations?Experimental Test: Internal MTLExperimental Test: External MTLRecapL25L25--11ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringLast Class…ionsconcentratsurfaceexternalatratereactionidealionsconcentratlocalatratereactionreal=ηeffectivenesseffectivenessfactorfactorThiele ModulusThiele Modulus2characteristic reaction ratecharacteristic diffusion rateφ=Let’s have a look at:• diffusion mechanisms• apparent (effective) reaction rate w/ pore diffusion limitationsLet’s have a look at:• diffusion mechanisms• apparent (effective) reaction rate w/ pore diffusion limitations1212ngcat P r Ascatcat P ASkcVnADρτφε−′′+=⋅(generalized form)L25L25--22ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringMTL and Apparent Reaction RatesHow do pore diffusion limitations influence the apparentapparent reaction rate??Pore diffusion will have the strongest influence on reaction for η = 1/φ. (Why?)1121effnn ncateff r As r As r Asncat r AsDAr r kC kC kCVnkCηηφ−′′ ′′ ′′′′== = =+A naïve experimentalist will A naïve experimentalist will measure measure wrongwrongkinetics !!kinetics !!Let’s look at the apparent rate coefficient:Kinetics show Arrhenius dependence on temperature, diffusion much more weaklyT-dependent:reff~(Deffkr)1/2 ~(1)/221ncateff r AscatADk CVn+′′=+k = k0exp(-Er/RT)Deff~ T 3/2or T ½( Deff= Deff,0exp{-ED/RT} )L25L25--33ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringApparent Activation Energies1/T1/Tlnlnrreffeffconc.conc.profiles:profiles:low T ?L25L25--44ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringTrue and Apparent Reaction Orderntruenapp00.5123(1)/2neff AsrC+∝L25L25--55ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringExampleCoal gasification:Coal gasification: production of synthesis gas from coal. C + H2O -> CO + H2Source of synthesis gas, particularly as method for ‘clean coal utilization’. Bernardo & Trimm, Carbon 17 (1979) 115L25L25--66ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringPore Diffusion & Selectivity…Is there an influence of pore diffusion on selectivity?Is there an influence of pore diffusion on selectivity?3 types of selectivities:PP11(I)(I)Selectivity in parallel reactions:Selectivity in parallel reactions:AAPP22(II)(II)Selectivity in series reactions: Selectivity in series reactions: AAPP11PP22(III)(III)Selectivity in independent simultaneous reactions:Selectivity in independent simultaneous reactions:AAPP11BBPP22k1k2k1k2k2k1Since we established that the effectivereaction rate reffis altered by pore diffusion limitations (reff= η.r), and since selectivity is based on relative reaction rates in competitive reaction networks, we should expect that pore diffusion limitations will have a significant impact on selectivity!L25L25--77ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringMTL & Selectivity – Parallel ReactionsReminder:Reminder: differential selectivity S’ = r1/r2Case 1: same order reactions -> dependence on CA:Case 2: different rctn orders (n1≠ n2) -> dependence on CA:Effect of pore diffusion: local reactant concentrations inside catalyst pellet, i.e. at location of reaction Pore diffusion limitations can result in a decrease or increaseof reaction selectivities in parallel reaction systems!Pore diffusion limitations can result in a decrease Pore diffusion limitations can result in a decrease ororincreaseincreaseof reaction selectivities in parallel reaction systems!of reaction selectivities in parallel reaction systems!In practice, this can be used by tailoring the size of the catalIn practice, this can be used by tailoring the size of the catalyst pellets!yst pellets!PP11AAPP22r1r21122()1122nnnAAnAkC kSCkC k−′==-> effect of MTL on S’ : -> MTL effects S’ !How does S’ change?-> higher order reaction by diffusion limitations!L25L25--88ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process EngineeringMTL & Selectivity – Series ReactionsDerivation of this case is beyond the scope of this class. However, the general trend can be rationalized in a straightforward way:• diffusion limitations for feed A should notbe of importance (it’s the onlyonly feed!)• products are also subject to diffusion limitations!• diffusion limitations the residence the residence timetime of the products inside the pellet pores(so-called ‘contact time’‘contact time’)• this increase will alwaysalways favor the final product P2, hence:In this case, one should opt in practice for very small catalyst particles or very large and open pore structures. (Important example: partial oxidation reactions!)AABBCCk1k2For series reactions, MTL always decreaseselectivity towards the desired product P1!For series reactions, MTL always For series reactions, MTL always decreasedecreaseselectivity towards the desired product Pselectivity towards the desired product P11!!L25L25--99ChE 400- Reactive Process EngineeringChEChE400400--Reactive Process EngineeringReactive Process Engineering(I) Reactant selectivity:Due to space restraints, only certain reactants can access thecatalytically active site, other reactants are completely shut out.(Example: branched vs linear hydrocarbons in ZSM-5)(II) Product selectivity:While all reactants can reach the catalytically active site and do react, only certain products can leave the cavity again.(III) Transition state selectivity:No access restriction exists for either reactants or products. However, thetransition state configuration does not ‘fit’ and hence the reaction can not occur. (Example: cis-/trans isomerizations in zeolite Beta)Special Case: Shape SelectivityA special (extreme) case of selectivity change due to transport limitations is the so-called


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