Catalytic Reaction Engineering (CRE)
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Catalytic conversion process Catalytic Reaction Engineering (CRE) Kinetics Catalytic reactions selection and design Minimum cost of overall process • Examples reactor systems Reactants • Description ideal reactors Reactor Energy • batch, CSTR and plug flow • Catalytic kinetics ? Process requirements • Effects of catalyst properties • mass and heat transfer • Labscale reactors - performance testing • purpose • criteria Ammonia oxidation 2NH3 + 2O2 NO + 3H2O •Maximum selectivity •maximum conversion •ease of scale-up •high throughput •low pressure drop •……. Desired products Undesired products Unconverted reactants •Intrinsically safe •WRAP •……. Environment & Safety Pt-Rh gauzes – various structures ‘Bispin’ Ammonia oxidation reactors Economics Installation wire-mesh gauze Pd – gauze (Pt entrapment) ‘Warp knitted’ Fixed bed reactors Aromatization – Amoco Ultrafining Hexane Benzene + 3H2 Spherical reactor Fixed bed Pressure drop Endothermal Hydrotreating adiabatic Methanol synthesis Isothermal reactor (Linde type) FTS – Multi-tubular reactor G L • • • • • • Maximum weight = 900 tonnes Diameter = 6 m Height = 20 m 8000 tubes Reactor productivity = 300 tonnes MD/day Cooling by steam generation: water evaporation Reactors in series adiabatic Fischer-Tropsch synthesis I - Secunda nCO + 2nH2 -(CH2)n- + nH2O Sasol Slurry Phase Distillate (waxes) Bubble column slurry reactor • • • • Fine catalyst particles ~ 50m 2500 bbl/day production Diameter 5m, height 22 m Cooling by steam generation: water evaporation Steam reforming - Catalytic cracking CH4 + H2O 3 H2 + CO CO + H2O H2 + CO2 Heavy feedstock Lighter Products (gasoline) + coke Catalyst deactivation ~50 m particles Batch – CSTR reactor Batch – CSTR reactors 2200 l stirrer motor Hand holes for reactor charging Liquid Gas-liquid Gas-liquid-solid cooling/heating tube/jacket agitators 100 l Cryo-reactor Fermentation - biocatalysis Bioreactors – waste water teatment Beer brewing Aerobic reactors Catalytic process Slow stirring Research facility Diesel - Johnson Matthey CRT Environmental – Automotive TWC gases Monolith wall Pre-oxizider porous support NO +O2 NO2 Wall-flow monolith NO2 + C CO2 + NO Exhaust gas inlet active component Low pressure drop NO, CO, HC removal Pt-Rh/Al2O3 catalyst Solids, gases Separator Bifunctional catalysis Gas distributor Exhaust gas outlet Microreactors Excellent heat removal Higher selectivities CVD reactors - semiconductors Multiwafer reactors Selective oxidation Hot-wall Cold-wall Home appliances Reaction coupling – SMART reactor ABB Ethylbenzene dehydrogenation H 118 kJ/mol Tefal Azura Matsumoto et al., 1993 US 5266543 Elimination heat exchanger, hot piping & steam superheater Higher conversion per pass (80%) Lower energy consumption G packed bed Reactor supermarket methanol (Krishna) G Catalytic reactors L methanol, i-butene, n-butene membrane reactor fluid bed MTBE catalytic distillation reactor Riser circulating fluid bed G multitubular trickle bed slurry reactor L bunker G L reactor Cat G Cat cyclone G Cat stirred tank • What to choose? • How to design? Ideal reactor types Catalytic Reaction Engineering (CRE) Discontinuous Continuous • Examples reactor systems • Description ideal reactors • batch, cstr and plug flow • Effects of catalyst properties • mass and heat transfer Continuous stirred tank reactor (CSTR) • Labscale reactors • purpose • criteria c T • Tutorials – application/illustration Plug flow reactor (PFR) Batch reactor c(z) T(z) c(t) T(t) • Biocatalytic reactor engineering How to describe these? Gas/Liquid/Solid Fixed Trickle-bed Bed Monolith The Chemical Engineer’s tool Reactors Slurry Mechanically agitated Bubble column Input 2 kg/s 3 mol A /s Steady state Production ? Output ?? ?? Accumulation ? Input - Output + Production = Accumulation units: mol/s Steady state = 0 Rate definitions - units Water-tap: Liquid volume in bucket In chemistry usually: mol A / m3 s unsteady state or transient steady state m3reactor ,m3catalyst ??? m3reactor mol / s m 3 reactor rW kg catalyst mol / s kg catalyst rVp m3particle mol / s m3 particle rV t=0 Vliquid tap t Vliquid Vbucket per In mass balance Rate definitions - units RV ,A A rV (CSTR) Molar balance In - Out + Production = Accumulation V rV Vp rVp W rw mol / s m3 particle Continuous (flow) stirred tank reactor Isothermal In mass balance unit: mol A / s mol / s m 3 reactor mol A s mol / s kg catalyst FA0 FA0 FA RW W 0 Introduce X = conversion FA FA0 (1 X A ) FA 0 X A RW W rW,W cA0 cA rate expression stoichiometric coefficient + products - reactants FA W X A FA0 RW ‘space time’ Design equation CSTR Continuous (flow) stirred tank reactor Graphical interpretation (CSTR) Relationship between CA0 and CA?? W X A FA0 RW W X A FA0 RW FA0 FA ‘space time’ A A= -1 rW,W Area CSTR operates here 1 rW Order of reaction? 2 CSTRs in series?? Batch reactor type cA c A0 1 kw 1 0 space time Isothermal Molar balance In - Out + Production = Accumulation d V c A dc V A dt W c A0 FA0 Plug flow reactor (PFR) In - Out + Production = Accumulation dN A 0 0 RW ,A W dt kw 1 0 1 kw 1 0 0 cA zero order? negative order? XA XA cA0 n>0 Molar balance ....... rW k1 cA k1 cA0 1 X A CSTR W 1 XA FA0 rW 1st order reaction FA FA dFA RW ,A dW 0 dt dW FA0 rW FA dc A W RW ,A dt V FA+dFA FA FA 0 (1 X ) dFA FA 0 dX rW,W,V c A c A0 (1 X A ) dX A 1 W RW ,A dt c A0 V B W t V dX A FA0 RW ,A dW ‘batch space-time’ XA cA 0 X dX A RW ,A d W FA0 A 1 W 1 t dX c A0 V R W ,A 0 Design equation Batch reactor cA0 ‘space time’ W X A W 1 dX FA0 RW ,A 0 (integral) Reactor characteristics: CSTR versus PFR Reactor design equations W FA0 Area= B Batch r,V,W PFR CSTR XA W dX t c A 0 V R W ,A 0 similarity ! A dX W c A0 c A 0 R FA0 W ,A 0 1 rW 1 rW PFR CSTR X FA0 r(X,z) FA0 0 Plug flow FA FA Which one is most efficient??? simplicity Similar conditions: • W/F PFR < CSTR positive orders • W/F PFR > CSTR negative orders • CSTR operates at lowest reactant concentrations • PFR at maximum local concentrations RW ,A A rW FA0 V c A0 Series reaction - Profiles most efficient: PFR or CSTR?? A kw1 2s-1 Q Series reaction - max. yields Maximum yields kw2 1.0 P 1s-1 1.00 Plug flow/Batch CSTR 0.80 n<0 n>0 X W c A0 0 c A 0 A RW ,A FA0 CSTR r,W XA XA YQ,max 0.8 0.6 Ci P 0.60 0.4 0.40 CSTR PFR k YQ,max w 2 1 k w1 2 kw 2 k kw 1 kw 2 YQ,max w 2 kw 1 Q 0.2 0.20 A 0.00 0.00 0.50 1.00 0 / kgcat 1.50 2.00 m-3 s-1 Max. yield PFR>CSTR (n>0) 0.0 10-6 10-5 10-4 10-3 10-2 10-1 k2/k1 kw2 /kw1 100 101 102 103 Tutorial 1 A second order reaction A R has been studied in a Berty-reactor, a CSTR suited for the investigation of solid catalysed reactions. The following data are available: V=1l W = 3 g catalyst cA0 = 2.0 mol/l cA = 0.5 mol/l v = 1 l h-1 Tutorial 2 - Batch conversion sucrose At room temperature sucrose can be hydrolysed by the enzyme sucrase: sucrose products Starting with an initial sucrose concentration of 1.0 mmol/l and an enzyme concentration of 0.01 mmol/l the following data have been obtained in a batch reactor. Concentrations have been determined by using polarized light. 1.0 a. Determine the value of the rate constant and give its dimension b. How much catalyst is needed to obtain 80% conversion in a packed bed reactor at a volume flow rate of 1000 l/h and an inlet concentration cA0 = 1 mol/l ? 0.8 Verify that the data can be represented well by a kinetic expression of the Michaelis-Menten type: 0.6 0.4 r k cs cE 0 M cs 0.2 0.0 Determine the parameters k and M 0 2 4 6 t 8 10 12 Ideal reactor types Catalysis Engineering: Questions Do you ever: Discontinuous Continuous • • • Measure and compare activities of catalysts for reactions? Compare catalyst selectivities? For what purpose? How? What does your reactor look like? • • • How do you define your catalyst activity ? Perform kinetic studies? How would you define reaction rate and how to determine it ? • • What do you think plays a role in your measurements? Are you sure you get the information you want? Continuous stirred tank reactor (CSTR) c T Plug flow reactor (PFR) c(z) T(z) Batch reactor c(t) T(t) How to describe these? Phenomena in catalytic reactor Reactor design equations (fluid-solid) Reactor level X Batch r,V,W A W dX B t c A 0 V R W ,A 0 Plug flow 0 A W c A0 dX c A 0 FA0 RW ,A 0 CSTR 0 X W c A0 c A 0 A RW ,A FA0 PLUG FLOW similarity ! MIXING DISPERSION VELOCITY PROFILE X FA0 r(X,z) FA0 FA FA r,W simplicity DIFFUSION REACTION TRANSPORT PHENOMENA Particle level Temperature and concentration profiles within catalyst particle Three-phase catalytic process How would they qualitatively be?? Catalyst Liquid Gas T T c c Exothermal Gas concentration profile Endothermal Rates different from rate at bulk conditions How to handle ? Catalytic reactor design equation External mass transfer - isothermal plug-flow, steady state conversion i r obs stoichiometric coefficient i dX i i rW d W F0 i deactivation function film layer around particle cb real rate r (cs ) rate at c b r (cb ) intrinsic rate cs ‘space time’ ‘catalyst effectiveness’ Use: effective rate effective rate rate at c b ,Tb How to determine cs ? Isothermal - external mass transfer In - Out + Production = Accumulation cb cs mass transfer rate to particle: = reaction rate in particle: mol/s mol/s Isothermal first order - external mass transfer Ap k f cb cs A kv c s p k f c b c s Vp = film layer Vp rv cs rv kv c kf a ' 1 cs cb cb k k f a ' kv 1 v ' kf a cs a' Effective rate: Ap k f cb cs = Vp rv cs cb Ap Vp 1 1 1 c b rvobs k f a' k v rate determined by physical resistance and by chemical resistance or: mass transfer flux (mol/s.m2) rate per particle volume (mol/s.m3p) Limits? rvobs 1 k c e rv ( c b ) kv v b 1 a' k f a ' k f kv Mass transfer control a ' k f kv Kinetic control L/S, G/S, L/L reaction systems Isothermal - internal mass transport Slab-type catalyst Effective diffusivity porous media only fraction open for diffusion combined to ‘tortuosity’ tortuous path longer Flux direction N Deff dc dx Diffusion and reaction Concentration profile Reaction rate profile component gradient in flux direction Profiles? Effectiveness factor Deff D gradient dc/dx direction Isothermal - internal mass transport Slab Mass balance, steady state difusion & reaction 1st order irreversible: De x cosh( ) L c cs cosh( ) Solution: 1.0 0 L d 2c kv c 0 dx 2 c/ci e x e x 2 e x e x cosh( x ) 2 sinh( x ) 1 tanh( x ) cosh( x ) coth( x ) sinh( x ) 4.0 3.0 2.0 sinh' ( x ) cosh( x ) 0.1 0.8 x+dx x Some mathematics - Hyperbolic functions 1.0 tanh cosh' ( x ) sinh( x ) 0.6 1.0 L 0.4 2.0 kv Deff ‘Thiele modulus’ 0.2 0.0 1.0 0.8 0.6 0.4 0.2 x 0 .3 1.0 1.5 2.0 x x 3 0 tanh( x ) 1 Catalyst effectiveness Effectiveness factor- experimental Post et al. AIChE-J 35(1989)1107 Vp rv (c,T )dV observed rate 0 rate at external surface conditions rv (c s ,Ts ) Vp Fischer Tropsch synthesis n CO + m H2 Slab: 0.5 cosh( x ) 1 x 2 2 1 0.0 x/L Limits? 0.0 0.0 sinh( x ) tanh( x ) x 10.0 i sinh cosh i CnH2(m-n) + n H2O tanh 2 1st order irrev. Limits: 1 0 i 1 i 1 Co,Zr/SiO2 catalyst H2/CO=2 21 bar 473-513 K dp= 0.38-2.6 mm spheres rv=kvpH2 (zero order CO) 1 0.1 0.1 1 3 0.1 0.1 1 10 10 Foam structures Classical catalyst particles Monoliths - cell density Generalizations - isothermal - internal L Geometry 200 cpsi 400 cpsi 600 cpsi Kinetics kv Deff i 1 i Vp Ap 1 a' Sphere: 1.80/0.27 mm 1890 m2/m3 = 0.72 1.27/0.16 mm 2740 m2/m3 = 0.76 1.04/0.11 mm 3440 m2/m3 = 0.8 Use: L kv De R 2 R L 3 1 cylinder slab sphere nth order: LL Cylinder: L L 0 Slab: L??? n21 c i n 1 1 tanh 0.1 0.1 1 10 What’s observed? Controlling regimes extraparticle limitation, first order kinetics ' rvobs ,p a kf cb • Kinetic control rvobs rv (cb ) • Diffusion control (internal) rvobs i rv (c b ) • Mass transfer control (external) rvobs e rv (cb ) a' k f cb rv (c b ) a' 1 L LL R Cylinder: L 2 R Sphere: L 3 Slab: a' 1 dp External mass transfer increases at increasing linear velocity From literature kf u 0.6 0.7 • dependent of u, dp • first order • no activation energy • How to determine in which regime? • What do we observe? What’s observed? intraparticle limitation Post et al AIChE-J 35(1989)1107 Observed temperature behaviour Bernardo & Trimm Carbon 17(1979)115 0.1 kvobs Catalysed steam gasification coke on Ni catalyst Limiting case: ‘Falsified kinetics’ dp/mm 0.38 Ni activation energy: Eatrue/2 C + H2O CO + H2 5 Ea(kJ/mol) 0.01 0 1.4 0.001 1.90 1.95 2.00 robs rchem 2.05 rchem 1 n 1 k v De c s L 2.10 1000/T wide pore silica spheres effect dp reaction order (n+1)/2 • p(H2O)=26 kPa • thermobalance • coked catalyst: Ni/Al2O3 1 r(obs) 2.4 61 1 0.75 164 0.1 0.6 order n 0.01 0.9 1.0 1.1 1.2 1000/T particle size dependent Tutorial 8 1.3 1.4 Summary dependencies rv,obs Isothermal - external mass transfer strong mass transport limitations Catalyst effectiveness: Internal mass transfer: robs rchem e 1 n 1 k v De c i L depends on: 1/L, (n+1)/2 reaction order, Eaapp= ½Eatrue r (c ,T ) Vp c s observed rate v s s rate at bulk fluid conditions rv (c b ,Tb ) Vp c b Observable quantity: Ca n robs a ' k f (c b c s ) (c b c s ) c 1 s a' k f c b a' k f c b cb cb 10 External mass transfer: robs a ' kf cb -1 un cb Lm e 1 Ca depends on: L, flow rate, 1st reaction order, Eaapp= 0 e 1 Ca 0.5 1 n e 1 n= 0.1 Criterion: 2 e 1 0.05 How to check if limitations are present ? Kinetics unknown 0.01 0.001 0.01 Kinetics unknown 5%deviation Ca < 0.05/n Diffusion control? Criteria - experimental verification r 1 Ca effectiveness cannot be calculated v ,obs 1 0.05 Criterion: r v ,chem 0.1 Weisz-Prater: effectiveness cannot be calculated i 2 observed rate L2 robs 'diffusion rate' Decs observed rate External transfer: Ca slab rv ,obs 0.05 a' k f c b n Ap Vp particle properties (nth order) cylinder 1 sphere reaction order a' n21 i Criterion: mass transfer coefficient 0.1 0.1 1 i2 10 i G2 rv ,obs L2 Deff c i n 1 0.15 2 When temperature effects? Effect temperature rise 5%Criterion How much increases rate constant ? E T k (Ti ) T exp a b 1 exp b k (Tb ) RT T b i Tb T Prater numbers k(Ti)/k(Tb) 1.5 External transfer: Tb=500 K 1.4 Ea(kJ/mol): A few degrees already critical ! 120 1.3 80 Internal transfer: 1.2 i s i 2 2 40 1.1 1.0 e e bCa 0.05 0 2 4 6 8 e i 10-104 gas-solid 10-4-0.1 liquid-solid 0.05 i E b a RTb 10 T / K ( H )k f c b hTb De ( H )cs eTs 0-0.3 (exothermal) 10-20 Criterion 0.05 1 Ca 1 e e 1 Ca exp b n 1 External gradient criterion more severe than internal criterion e bCa 0.05 External transfer: Temperature and concentration profiles Temperature gradient in catalyst bed Ea R Tw Largest T-gradient in film layer T T c c Exothermal rt 2 eff ,b Tw r criterion bed T - gradient t criterion film T - gradient particle rp 1 b 1 b 1 8 2 p,eff b,eff 1 dp 0.05 Bi w d t s 2 (1 b ) 1 8 temperature gradient in bed always develops first ! Endothermal Largest c-gradient inside particle H r rv ,obs Summary: T grad bed T grad ext c grad int , T grad int c grad ext Tutorials • Tutorials #6, 7 & 10 Screening Laboratory performance testing catalysts Kinetic studies n tio iz a tim Op Preparation Combinatorial stage Quantification stage Reaction network Kinetics Increasing: • time • money • reality Catalyst testing & Kinetic studies Stability tests Scale-up How to obtain intrinsic performance data? solid catalysts Choose a well-defined reactor Information wanted – Ideal type: CSTR, plug flow,.. – Dimensions: L, dt, dp, shape – Hydrodynamics Intrinsic reaction rate data Not obscured by parasitic phenomena • Flow distribution • Wetting, contact phases reactor characteristics mass and heat transport phenomena Avoid undesired gradients particle – reactor scale – C and T gradients on a particle scale – C and T gradients on the reactor scale user manipulations catalyst misbehaviour deactivation/fouling For • • Comparison activities and selectivities Kinetic modeling Starting point for example How ? Rateobserved 1 0.05 Rateideal Laboratory Reactors PFR CSTR – – – – Proper comparison - Selectivity simple yields conversion data, not rates deactivation noted directly small amounts of catalyst needed catalysts of different activity different product yields kinetic selectivity = 2 A k1 2s-1 k2 Q 1s-1 1.00 – direct rate data from conversions – larger amounts of catalyst and flows needed – deactivation noted directly FBR – non-ideal behaviour – continuous handling of solids possible TGA – limited to weight changes – careful date interpretation needed – often mass-transfer limitations Plug flow/Batch 0.80 Ci P 0.60 0.40 Batch P Q 0.20 A 0.00 0.00 – yields conversion and selectivity data quickly over large range – Easy to change feed – catalyst deactivation hard to detect 0.50 1.00 1.50 2.00 k / s-1 Compare selectivities at similar conversion levels ! Important checks Particle level 5% criteria – ‘Observables’ ‘Ten commandments of catalyst testing’ - Dautzenberg Particle criteria: Bed citeria: External temperature rise Internal mass transfer Carberry Weisz-Prater Temperature rise Flow velocity profile Plug flow - dilution Mears • External (film) gradients Ca – Concentration – Temperature • rv ,obs a ' kf cb 0.05 n E a ( H r ) k f c b rv ,obs 0.05 h Tb R Tb k f a ' c b b e Ca Internal (particle) gradients – Concentration (Weisz-Prater) – Temperature i 2 rv ,obs L2 n 1 0.15 Deff c s 2 Ea Hr Deff cs rv ,obs L2 0.1 p,eff Ts R Ts Deff cs s i i 2 Packed bed reactor - assumptions Diagnostic tests mass transport limitations ideal real life 1. Particle size variation egg-shell catalysts? observed rate particle size 2. Flow rate variation at constant space time! XA,1 XA,2 plug flow isothermal XA,3 X W1 W2 equal res,,T W3 FA0,1 FA0,1 FA0,2 FA0,2 FA0,3 FA0,3 Catalyst bed size Practical catalyst: often dp = ~1 - 3 mm Moreover, velocity profiles Dt dp Lb 8n 1 ln dp Pep 1 x Pep X 0.03 X =0.8 n =1 u dp Dax L > 25-75 mm radial temperature gradient res varies 10 X 33 d p dp T varies Impact on observed conversion levels Temperature rise in catalyst bed Mears: Reaction heat production vs. conduction Wall effect heat transfer vs. conduction H rvobs rt2 1 b 1 b E a RTw er Tw Effective thermal bed conductivity ~ 1 J/s.m2K Dt > 25 –75 mm velocity profile “dispersion” analogous to diffusion Dax “Dispersion coefficient” Dt Dt Criterion: Axial dispersion axial dispersion rp 1 8 r Bi t w 0.05 Activation energy Biot wall number ~ 0.8-10 b = fraction inert diluent Generally most severe temperature criterion ‘large’ reactor needed What to do ? Bed dilution - bypassing ? Catalyst testing - Bed dilution decoupling hydrodynamics and kinetics Berger, Perez et al. App.Catal.227(2002)321 Chem.Eng.Sci. 57(2002)4921 Chem.Eng.J. 90(2002)173 inhomogeneous distribution catalyst by-passing Diluent Bed dilution (e.g. SiC) • Hydrodynamics determined by Do not: • dilute too much • use too high conversion small particles (wetting, velocity) • Longer bed, larger L/dp • Testing of real catalyst particles • Better heat conduction • Larger heat transfer area • Less heat produced per volume Heat transfer area Criterion: b xobs 1 b 2 Real particle Lb/dp>~50 Dt/dp>~10-15 or -5 160 Range I -6 140 Eaapp / kJ mol-1 Practical example Effect of Catalyst/Diluent Distribution in Decomposition of N2O 137 kJ mol-1 120 non-porous non-porous quartz quartz -7 100 -8 -9 -10 Range II 80 60 40 20 -11 0 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 -3 -1 10 T / K -1 Achieve a homogeneous mixture of catalyst and diluent ! Catalyst Diluent 0.05 1 k obs = ln 1 (Wcat /FN2O,0) pN2O,0 1-xN O 2 ln(kobs) inhomogeneous distribution dp Lbed (= deviation rate constant less than 5%) Dt/dp<4 Bed dilution: detrimental? catalyst by-passing? b = fraction inert diluent Range I Range II Heat effects in packed-bed reactor Coated wall reactor T-profiles Poor heat transfer • in bed • to wall h 50 W Heat production/ consumption 2 mK Cooling-heating: • Reaction coupling • Heat exchange • through wall • no wall • Evaporation Improvements: • foams (ceramic, metal) • catalytic coatings h 10 4 Better heat removal W m 2K Exothermal reactions oxidation hydrogenation But: Velocity profile? Concentration gradients? • forced flow radial axial Monoliths, microreactors, kinetic studies G.Kolios et al. Chem.Eng.Sci. 57(2002)1505 TCR, UOP Coated wall – flow patterns Coated wall reactors Porous catalytic walls support washcoat • Monoliths Flow pattern Flow pattern Pe ' Concentration profile Criteria X CWR 0.16 0.23 nPe ' u0 L R DA,rad L mm size • Microreactors 2 Concentration profile X CWR 1.48 1.04 nPe ' 0.05-0.2 mm • Kinetic studies 5-15 mm R.J. Berger & F. Kapteijn Ind. Eng. Chem. Res. 46 (2007) 3863 Ind. Eng. Chem. Res. 46 (2007) 3871 Redlingshöfer et al. Ind.Eng.Chem.Res.41(2002)1445-1453 CSTR – fixed bed Internally mixed Alternative reactors for multiphase kinetics measurements Externally mixed Batch – Liquid phase systems – fixed bed Robinson-Mahoney Berty-type Carberry-type Gas Liquid Recirculation reactor 300 ml (turbine) Flat blade basket SISR Pitched blade basket Alternative reactors for multiphase kinetics measurements Alternative reactors for multiphase kinetics measurements (Semi-) batch – G-L-S systems In/out Bentrod Swinging capillary reactor Capillary Fixed point Heating + = Turbine Reactor Screw Impeller Stirred Reactor F.Kapteijn and J.A.Moulijn, Laboratory testing of solid catalysts in: Handbook of Heterogeneous Catalysis Wiley-VCH Verlag, Weinheim, 2008, p. 2019-2045 S.Tajik et al., Meas.Sci.Technol. 1(1990)815 Monolithic Stirrer Reactor Mn-oxide/Alumina H2O2 decomposition Principles Catalyst Performance Testing Down scale as far as possible – – – – – – – Lower cost equipment Less material consumption Lower utility demands Safer Less labour Less synthesis effort More options to test Do not mimic industrial reactor • Output industrial reactor: $$$$ or €€€€ • Output laboratory reactor: knowledge I.Hoek et al. Chem.Engng.Sci. 59 (2004) 4975-4981 R.K.Edvinsson-Albers et al. AIChE J. 44 (1998) 2459-2464 No Dinky Toy / Matchbox approach! Scaling down steps N. van der Puil N. van der Puil Six-flow equipment Observations • Mainly fixed bed and batch slurry systems applied Sie, AIChE-J. 1996 Manhour per reactor hour • Massive parallelization 2 Pilot plant (non-automated) • Cost reduction 1 Bench scale 0.5 (non-automated) • Used for – Catalyst screening 0.1 – Catalyst performance – Kinetic studies Plug flow - parallelization FEED CONTROL ANALYSIS REACTOR P MFC Diesel soot FTS N2O, NOx HDS MFC VENT MFC MFC MFC SV MFC MFC Bench scale (semi-automated) MFC Microflow (automated) BPC MFC BPC parallellization 0.01 1960 1970 1980 1990 2000 ‘Workhorse’ in catalyst testing Year Pérez et al. Catal. Today 60(2000)93 N2O/NOx decomposition set-up FAo FAo FAo FAo FAo FAo FAo (a) (b) x1 FAo x2 x3 x4 FAo (c) x1 FAo dp1 increasing particle size FAo dp5 x1 x1 GC NDIR GC MS, NOx FAo W2 x2 FAo W3 x3 FAo W4 x4 W5 x5 FAo5 FAo1 increasing flow rate (d) W1 o Wi / FAi constant W5 x1 x5 x5 x x5 x1 x5 • FischerFischer-Tropsch • Soot abatement • CFC, Automotive • SCR FAo W1 x5 x Other systems: FAo dp1 dp5 FA0,1 Particle size FA0,5 Flow rate reference catalyst Commercial developments Commercial developments Nelleke van der Puil, dec. 2008 Nelleke van der Puil, dec. 2008 Kinetics Commercial developments Procuring rate data laborious task conversion vs. space time W/F temperature partial pressures / concentrations Improve speed: • PC controlled equipment • Six-flow set-up (parallel reactors) • Temperature scanning • Sequential experimental design Don’t forget: Nelleke van der Puil, dec. 2008 stable catalyst, blank runs, duplicates, criteria Catalysis Engineering: Questions Chapters 3 and 8 Do you ever: Measure and compare activities of catalysts for reactions? Compare catalyst selectivities? For what purpose? How? What does your reactor look like? How do you define your catalyst activity ? Perform kinetic studies? How would you define reaction rate and how to determine it ? What do you think plays a role in your measurements? Are you sure you get the information you want? Kinetics of catalysed reactions Kinetics of catalysed reactions Structure Batch CSTR Plug flow Kinetics of Catalysed Reactions Catalysis Reactor engineering Ideal reactors Kinetics Reactor theory Experimental aspects – Interpretation – Reactors – Interfering phenomena • Mass transfer • Diffusion • Dispersion • Criteria Problems/questions Transport phenomena Reaction models Kinetics Heat & Mass Behaviour single particle Non-ideal reactors Why Reaction Kinetics Derivation rate expressions Simplifications – Rate determining step – Initial reaction rate Limiting cases – Temperature dependency – Pressure dependency Examples Catalytic Reactor Kinetics of catalysed reactions Kinetics of catalysed reactions Utilization of kinetic data for different chemical industry sectors Utilization of kinetic data in industry Questionnaire 1997 (a) Chemical Companies Bos et al. Appl.Catal. A160 (1997) 185-190 Mechanistic Research 6% Catalyst Development 29% Other 1% Process Development 34% Mechanistic Research 8% Catalyst Development 26% Other 1% Catalyst Development 56% www.eurokin.tudelft.nl Process Development 15% Catalyst Development 15% Process Optimisation 28% Other 0% Batch reactor CA design process start-up and control process development and improvement selection reaction model CA CA r=k r = kcA General relationship r = kcn, n ~2 t r f ( pi ,.......T , NT , k i ,........., K i ,......, K eq ) Often used CA t CA t r=k r = k(cA-cp/Keq) m j Process Development 56% Mechanistic Research 1% Rate data, Examples A P Rate expressions in principle crucial for r kp p – power rate models – models based on elementary processes sNT k 2K AK B2 pA pB2 / K eq r 1 K A pA K B pB 2 • extrapolation more reliable • intellectually process better understood Process Optimisation 37% Kinetics of catalysed reactions Rate expressions n i Process Development 30% (d) Engineering Companies Mechanistic Research 12% Process Optimisation 17% Mechanistic Research 2% Catalyst Development 27% Other 4% (c) Catalyst Companies Process Optimisation 30% Kinetics of catalysed reactions Kinetics of catalysed reactions Process Development 34% Process Optimisation 31% Other 0% – – – – (b) Oil Companies r = kcA t Rate equation?? Kinetics of catalysed reactions t Does power rate equation fit? If so, n = ?? Role of catalyst? Rate expression – Catalysed reaction forward rate backward Success frequency Concentrating reactants adsorption/complexation Providing alternative reaction path catalyst selectivity other activation energy barrier affect rate r k c Aads s B k cCads s C rate constant amount of A adsorbed chance of adjacent B adsorbed But: – other components adsorb, too block ‘active sites’ – fixed number of ‘active sites’ affect rate affect rate Note: • cgas and cads differ • ratios components differ other form rate expression expected Kinetics of catalysed reactions Kinetics of catalysed reactions Simple example: reversible reaction A B Elementary processes Rate expression follows directly from rate equation ‘Elementary processes’ A r r1 r1 A* r 2 r2 B r r1 r1 k1 p A NT * k 1 NT A 3 r3 r r2 r 2 k 2 N T A k 2 N T B B* r r3 r3 k 3 NT B k 3 pB NT * ‘Langmuir adsorption’ Eliminate unknown surface occupancies How many unknowns, when the overall rate is known? Kinetics of catalysed reactions Kinetics of catalysed reactions Elementary processes contd. Algebraic eqs. Quasi-equilibrium / rate determining step 1 * A B Site balance: d A 0 dt d B 0 dt Steady state assumption: r1 r1 r2 r-1 r2 r3 r2 r-2 Rate expression: rate determining r3 r NT k1k 2 k 3 ( pA pB / K eq ) (.....) (......) pA (......) pB r-3 Microkinetics Michaelis-Menten ‘quasi-equilibrium’ r with : K eq K1K 2K 3 r = r2 - r-2 Very simple case, nevertheless quite complex equation Kinetics of catalysed reactions Kinetics of catalysed reactions Rate expression, contd. Rate expression - r.d.s. Substitution: Rate determining step: r r2 r2 k 2 NT K1p A * k 2 NT p B * / K3 r r2 r2 k2 NT A k 2 NT B r k 2 NT K1 * p A k 2 p B / k 2 K1K3 Eliminate unknown occupancies where: Quasi-equilibrium: r1 r1 So: A K1p A * k1 p A NT * k 1 NT A with: K1 k1 k 1 Unknown still * p B B * K3 Kinetics of catalysed reactions p Keq K1K2 K3 B p A eq Kinetics of catalysed reactions Other rate determining steps Rate expression, contd. Adsorption r.d.s Site balance: 1 * A B * 1 K1p A p B / K 3 * k1 NT p A pB / Keq 1 1 1 / K2 pB / K3 1 1 K1p A pB / K3 Surface reaction r.d.s. Finally: r r ‘lumped’ k 2 NT K1 p A p B / Keq 1 K1p A pB / K3 r ‘lumped’ k 2 NT K1 p A p B / Keq 1 K1p A pB / K3 Desorption r.d.s. r k 3 NT K1K2 p A p B / Keq 1 1 K2 K1p A Rule of thumb: Generally surface reaction r.d.s. Kinetics of catalysed reactions Kinetics of catalysed reactions Thermodynamics Initial rate expressions Equilibrium constant Forward rates Product terms negligible Reaction entropy RT ln K eq G (T ) H (T ) T S o o o Adsorption Reaction enthalpy r k 'p0A i Gfo,i (T ) i ln K A Adsorption entropy, <0 (J/mol K) S H R RT o T1 atm-1 Kinetics of catalysed reactions T2 Data sources: Handbooks, API, JANAF Chemsage, HSC, YAW’s Handbook pA Kinetics of catalysed reactions T2 T3 Adsorption enthalpy,<0 (J/mol) r k' T1 T1 r0 Desorption high p low p Adsorption constant o Surface k 'p0A r 1 KA p0A T3 pA T2 T3 pA Langmuir adsorption Multicomponent adsorption / inhibition Uniform surface (no heterogeneity) Discrete number of sites No interaction between adsorbed species Langmuir adsorption Irving Langmuir 1881 - 1957 Nobel Prize 1932 A+* A* 1.0 A 0.8 A K A pA 1 K A pA 100 KA /bar -1 10 0.6 K 1p A 1 K 1p A K i p i 1.0 0.4 0.2 Inhibitors 0.1 0 0 0.2 0.4 0.6 0.8 1.0 pA /bar Kinetics of catalysed reactions Kinetics of catalysed reactions Surface occupancies Langmuir adsorption model Generally used Empty sites Occupied by A * A 1 1 K1p A pB / K3 K1p A 1 K1p A pB / K3 1 KB K3 – – – – Simplification (uniform, no interactions) although nonlinear, mathematically simple simple physical interpretation rather broadly applicable • multicomponent adsorption S o H o • non-uniform surfaces K A exp RT R – ‘compensation effect’ – very weak and strong sites do not contribute much to the rate Occupied by B B Kinetics of catalysed reactions pB / K3 1 K1p A pB / K3 • for microporous media (activated carbons) often not satisfactory Kinetics of catalysed reactions N2O decomposition over ZSM-5 (Co,Cu,Fe) Effect of CO on N2O decomposition Kapteijn et al. J.Catal.167(1997)256-265 1.0 Cu-ZSM-5 (673 K) 2N2 + O2 CO + O* CO2 + * CO + * Kinetic model 1. 2. 0.8 CO* (Cu+) X(N2O) 2 N2 O 0.6 Fe-ZSM-5 (673 K) 0.4 0.2 N2O + * N2 + O* N2O + O* N2 + O2 + * Co-ZSM-5 (693 K) 0.0 0.0 0.5 1.0 1.5 molar CO/N2O ratio CO removes oxygen from surface so ‘enhances’ step 2, oxygen removal Rate expression r k1 NT pN 2O k2 1 k1 no oxygen inhibition 1st order Kinetics of catalysed reactions now observed: rate of step 1 increase: ~2, >3, >100 Kinetics of catalysed reactions Dissociative adsorption Dissociative adsorption O2 + 2* r1 = k1 NT pN2O 2O* Gerhard Ertl Nobel laureate Chemistry 2007 H2 + 2* 2H* K p 1 K p 0.5 H Lower pressures: K p 1 K p H2 H2 H2 0.5 H2 0.5 O O2 O2 O2 0.5 Two adjacent sites needed O2 STM oxygen on Ru Two adjacent sites needed Kinetics of catalysed reactions Kinetics of catalysed reactions 2.0 Initial rates - CO hydrogenation over Rh Koerts, Van Santen et al. Langmuir-Hinshelwood/Hougen-Watson models (LHHW) Irving Langmuir Cyril Norman Hinshelwood (1881 – 1957) Nobel Prize 1932 (1897 – 1967) Nobel Prize 1956 Kinetic model 1. 2. CO + * CO* CO* + * C* + O* For: A+B (r.d.s.) includes NT, k(rds) 800 r 600 Rate Initial rate r0 sNT k 2 CO * C+D pApB-pCpD/Keq ( kinetic factor ) ( driving force ) 400 ( adsorption term ) n 200 r0 sk 2 NT KCO pCO 1 KCO pCO molecular: KApA dissociative: (KApA)0.5 0 2 0.2 Oc cu 0.6 pa nc y( -) = 0, 1, 2... number species in and before r.d.s. 600 0.4 550 500 450 0.8 1.0 400 (K) ture pera Tem Leonor Michaelis Kinetics of catalysed reactions Terminology Heterogeneous catalysis Kinetics Langmuir-Hinshelwood 1916/20 Linearization rate expression Biocatalysis Michaelis-Menten 1913 rmax Rate expression Maud Menten (1879-1916) Kinetics of catalysed reactions (1875-1949) Vmax k NT K A pA r 1 K A pA adsorption constant v kE 0 c A K M c A kE 0 c A v K M c A Hougen-Watson Lineweaver-Burke Catalytic centre ‘active site’ enzyme =Vm 1/v Michaelis constant Linearization 1 KM 1 1 v kE 0 c A kE 0 Intercept = 1/Vm Slope = KM/Vm =-1/KM Turnover number Turnover frequency Number of turnovers Kinetics of catalysed reactions k (s-1) r NTsurface 1/cA (s-1) number molecules converted/number complexes Kinetics of catalysed reactions • Lineweaver-Burke • Hougen-Watson Enzyme Catalysis Terminology Biocatalysis Heterogeneous catalysis Reactants Molecules Biocatalysis Heterogeneous Catalysis 1. Irreversible inhibition 2. Competitive inhibition Substrates 1. Catalyst poisoning (irrev.) 2. Competitive adsorption or inhibition k2 Reactor performance CSTR, autoclave Residence time, space time Chemostat, fermentor Flow rate Dilution rate S + E ES E + P I + E EI 3. Non-competitive inhibition 3. Co-adsorbed intermediates change active sites (‘modifiers’) k2 depends on intermediate concentration Affect activity and selectivity k2’ S + E ES E + P I + E EI Kinetics of catalysed reactions v Kinetics of catalysed reactions What about observed: reaction order activation energy ? Determination: Reaction order - Activation energy slope = -Eaobs/R K A pA 1 ln r ln r r limiting cases? ln pi 1/T ln r ln pi Eaobs ln r R 1 T Kinetics of catalysed reactions r k 2 NT K A p A 1 K A p A K B pB Reaction order ? k 2 NT K A p A 1 K A p A K B pB k 2 NT K A pA 1 K B pB r k 2NT K A pA 1 LHHW models ? r rate expression slope = order ni ni k 2' E0 cS K M' cS Activation energy ? depend strongly on occupancy! vary during reaction Svante Arrhenius (1859 – 1927) Nobel Prize 1903 General: Kinetics of catalysed reactions nA 1 A nB B Try it yourself Eaobs E a 2 1 A H A B H B Tutorial 13 Selective hydrogenation benzaldehyde r Surface reaction r.d.s. 700 Concentration / mol/m3 Limiting cases - forward rates 600 Benzaldehyde Benzyl alcohol r k 2 NT 1. Strong adsorption A 500 k 2 NT K A p A 1 K A p A K B pB 400 A#* k#+ 300 kbarrier k#200 Toluene Ea2 H# Eaobs 100 A* 0 0 5000 10000 15000 Time / s 20000 25000 B* Kinetics of catalysed reactions Kinetics of catalysed reactions Limiting cases - forward rates Surface reaction r.d.s. r k 2 NT K A p A Surface reaction r.d.s. 1 K A p A K B pB 3. Strong adsorption B r k 2 NT K A p A 2. Weak adsorption Limiting cases - forward rates k 2 NT K A p A r 1 K A p A K B pB r k 2 NT K A p A K B pB Eaobs =Ea2+ HA HB A#* A#* Eaobs =Ea2+ HA A(g)+ * +B(g) HA Ea2 A(g)+* HA A* HB HR B(g)+* HB A(g)+B* B* Kinetics of catalysed reactions Kinetics of catalysed reactions A* Ea2 Cracking of n-alkanes over ZSM-5 n-Alkanes cracking J. Wei I&EC Res.33(1994)2467 Energy diagram 200 Initial state Ea2 100 kJ/mol 0 -100 -200 0 negative!? Transition state A+* Eaobs Ea,obs Hads HA 5 Ea2 r0 k 2 K A p A 10 15 20 B*,C* E aobs E a 2 H A Carbon number A* Adsorbed state Kinetics of catalysed reactions Kinetics of catalysed reactions Dual site reaction : A+B C Observed temperature behaviour • T higher coverage lower • Step highest Ea increased most Change in r.d.s. adsorption r.d.s. A + * A* B + * B* A* + B* C* + * C + * C* ln robs r k3 NT A sB k 3 NT C s * desorption r.d.s. 4 unknowns, 4 equations r 1/T Ea,observed depends on T, because of change r.d.s. Kinetics of catalysed reactions (r.d.s.) Kinetics of catalysed reactions k3 s NT K1K 2 pA pB pC / K eq 1 K1pA K 2 pB pC / K 4 2 More than one reactant (no product inhibition) Dual site reaction, contd. e.g. hydrogenation, oxidation • One-site models r r3 r3 s NT k 3 A B k 3 C * r kNT AB dual site reaction r kNT s A B • Two-site Number of neighbouring sites (here: 6) kNT K AK AB pA pB 1 K A pA (1 K AB pB ) single site reaction models r kNT s A B different sites • Number kNT s K A pA K B pB (1 K A pA K B pB )2 kNT s K A pA K B pB (1 K A pA ) (1 K B pB ) of sites conditions dependent NT 0 K A pA 12 NT Kinetics of catalysed reactions 1 K pA 12 A optimal surface concentrations optimal adsorption strengths Kinetics of catalysed reactions Reaction kinetics, summary Further kinetics Langmuir adsorption – uniform sites, no interaction adsorbed species, finite number of sites, multicomponent Rate expressions derivation r series of elementary steps steady state assumption, site balance quasi-equilibrium / rate determining step(s) initial rates (model selection) p le sim – – – – LHHW models – inhibition, variable reaction order, activation energy mechanism Kinetics of catalysed reactions kinetics Microkinetics – Keep all elementary processes • Estimate theoretically pre-exponentials (statistical physics) and activation energies (molecular modeling, DFT) or from experimental work (TPD) • Active site concentration and limited number of constants estimated from experimental rate data Single event modeling – Complex reaction schemes reduced to finite number of single events – Detailed composition feed required – Further as microkinetics Transient operation – Active site concentration and rate constant decoupled Include lateral interactions, surface reconstruction, dependency catalyst properties on exposed environment Kinetics of catalysed reactions AppCatA342(2008)3–28 Concepts of Modern Catalysis and Kinetics. I. Chorkendorff, J.W. Niemantsverdriet 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Hydrodesulphurization kinetics Sie, AIChE-J 42(1996)3498 Example HDS vacuum gasoil 2.2 2.0 1/S-1/S0 (1/wt.%) Examples kinetics Gasoil CoMo-alumina trickle flow L=0.2-0.4 m 1.8 • Apparent second order behaviour r = kcS2 • H2S inhibits strongly 1.6 1.4 1.2 1.0 0.8 0.6 0.4 2.0 0.2 concentration 0.0 1.5 0.0 c conversion 0.1 0.2 0.3 0.4 0.5 1/LHSV (h) 1.0 fast decrease followed by slow decrease 0.5 0.0 0.0 0.2 0.4 0.6 0.8 1.0 bed length Second order silly, what is wrong?? Kinetics of catalysed reactions Kinetics of catalysed reactions Composition oil fractions Vacuum gasoil Sulphur compounds R R S S R Thiophene Benzthiophene S Three lump model: first order reactions Simulated model data: 2nd order k=10 m3/mol.s c0=2 mol/m3 Thioethers R S 0 Dibenzthiophene 5 10 15 20 25 30 Three lump model: 1st orders k1=36.1 s-1 c01=1.23 k2=16.0 s-1 c02=0.59 k3=7.5 s-1 c03=0.18 2.0 sum concentration S Simulated profiles - HDS reactivity lumping 1.5 1 1.0 2 3 0.5 Simulated distillation b.p. 0.0 0.0 S 0.2 0.4 0.6 0.8 1.0 bed length S R Kinetics of catalysed reactions R Substituted dibenzthiophene complex mixtures different reactivities lumping Three lump model adequate Inhibition through LHHW models Kinetics of catalysed reactions Which groups lumped? model studies 0.6 N2O decomposition Effect of CO on N2O decomposition 1.0 Cu-ZSM-5 (673 K) CO + O* CO2 + * CO + * CO* (Cu+) X(N2O) 0.8 0.6 Fe-ZSM-5 (673 K) 0.4 0.2 Co-ZSM-5 (693 K) 0.0 0.0 0.5 1.0 1.5 2.0 molar CO/N2O ratio CO removes oxygen from surface so ‘enhances’ step 2, oxygen removal now observed: rate of step 1 r1 = k1 NT pN2O increase: ~2, >3, >100 Kinetics of catalysed reactions Kapteijn et al. J.Catal.167(1997)256-265 Kinetics of catalysed reactions Effect of CO on N2O decomposition rate without CO Apparent activation energies N2O decomposition CO/ N2O = 2 rate with CO Apparent activation energies (kJ/mol) kN p r 1 T N 2O 1 k1 k 2 ratio = 1 + k1/k2 So k1/k2 = : Kinetics of catalysed reactions 1 Co >2 Cu >100 Fe r k1 NT pN 2O only N2O Co and: O * O* k k 1 2 1 k1 k 2 0.5 >0.7 >0.99 CO/N2O=2 115 Cu 138 187 Fe 165 78 Co, Fe r k1 NT p N 2O Cu r Kinetics of catalysed reactions 110 k1 NT p N 2O k N p 1 T N 2O k 2 KCO pCO KCO pCO 1 k1 E aobs E a 1 E aobs E a1 HCO Apparent activation energies N2O decomposition N2O decomposition over ZSM-5 (Co,Cu,Fe) CO/ N2O = 0 Kapteijn et al. J.Catal. 167(1997)256 1.0 Apparent activation energies (kJ/mol) Co 110 r Fe r k 2 NT p N 2O 833 K Oxygen inhibition model 115 Cu 138 187 Fe 165 78 k1 NT pN 2O k2 Co, Cu 1. 2. 3. E 1 k1 obs a 0.6 793 K Cu-ZSM-5 Fe-ZSM-5 0.4 Co-ZSM-5 N2O + * N2O + O* O2 + * N2 + O* N2 + O2 + * *O2 0.2 733 K 688 773 K 0.0 0 2 4 6 8 10 p(O2) / kPa m ix( E a 1, E a 2 ) Rate expression r E aobs E a 2 Kinetics of catalysed reactions k1 NT p N 2O k 2 K3 pO 2 1 k1 Kinetics of catalysed reactions Catalysed N2O decomposition over oxides N2O decomposition over Mn2O3 Winter, Cimino Rate expressions: Kinetic model strong O2 inhibition 0.5 k obs pN 2O 1 pO 2 K 3 0.5 N2O + * N2O* 2 O* 1. 2. 3. 2 r 2N2 + O2 1st order p N 2O pO Yamashita & Vannice J.Catal.1996 2 N2 O r k obs p N 2O r k obs moderate inhibition r k 2 NT K 1 p N 2O 1 K p 1 N 2O = Explain / derive = Kapteijn et al. Appl.Catal.B: Env. 9 (1996) 25-64 N2O* N2 + O* 2* + O2 Rate expression Also: orders 0.5-1 water inhibition Kinetics of catalysed reactions 743 K 0.8 CO/N2O=2 X(N2O) only N2O Kinetics of catalysed reactions pO 2 K 3 0.5 N2O decomposition over Mn2O3 N2O decomposition over Mn2O3 Yamashita & Vannice J.Catal.1996 Yamashita & Vannice J.Catal.1996 Kinetic model order N2O ~0.78 Oxygen inhibition Values 0.4 r / 10-6 mol.s-1.g-1 0.3 0.2 2.0 4.0 6.0 8.0 k 2 NT K 1 p N 2 O 0 .5 N 2O pO 2 K 3 1 K p 1 10.0 = Thermodynamically consistent = = Explain = Kinetics of catalysed reactions Kinetics of catalysed reactions Effect reaction kinetics - batch operation A+B Kinetic coupling between catalytic cycles irreversible C+D Bifunctional catalysis: Reforming cA=cB KA=KB KD small kK A K B c A c B 1 K A c A K B c B K C cC K D c D 2 Isomerization n-pentane: n-C5 -> i-C5 KA=10 KC=1 Pt-function: n-C5 -> n-C5= surface diffusion 1.0 conversion r S 3 109 J/mol K pO2 / kPa r N2O* N2 + O* 2* + O2 H 3 92 kJ/mol 638 K 623 K 608 K 598 K 0.0 0.0 N2O + * N2O* 2 O* Rate expression Ea2 130 kJ / mol 648 K 0.1 H1 29 kJ/mol S1 38 J/mol K pN2O = 10 kPa Eaobs= 96 kJ/mol 1. 2. 3. Acid function: n-C5= -> i-C5= Pt-function: i-C5= -> i-C5 KA=KC= 1 0.8 low concentration close proximity surface diffusion 0.6 KA=KC= 0.1 0.4 KA=1 KC=100 0.2 0.0 Kinetics of catalysed reactions 0 20 40 60 80 100 120 140 160 180 200 time Coupled catalytic cycles on different sites Strong product inhibition Kinetics of catalysed reactions See tutorial NIOK course December 2009 Tutorial 1 A second order reaction A R has been studied in a Berty-reactor, a CSTR suited for the investigation of solid catalysed reactions. The following data are available: v = 1 l h-1 V=1l W = 3 g catalyst cA0 = 2.0 mol/l cA = 0.5 mol/l a. Determine the value of the rate constant and give its dimension b. How much catalyst is needed to obtain 80% in a packed bed reactor at a volume flow rate of 1000 l/h and an inlet concentration cA0 = 1 mol/l ? Tutorial 2 At room temperature sucrose can be hydrolysed by the enzyme sucrase: sucrose products Starting with an initial sucrose concentration of 1.0 mmol/l and an enzyme concentration of 0.01 mmol/l the following data have been obtained in a batch reactor. Concentrations have been determined by using polarized light. c mmol/l 0.84 0.68 0.53 0.38 t (h) 1 2 3 4 c mmol/l 0.27 0.16 0.09 0.04 t (h) 5 6 7 8 c mmol/l 0.018 0.006 0.0025 t (h) 9 10 11 Verify that the data can be represented well by a kinetic expression of the MichaelisMenten type: r k cS c E 0 cS M with M the Michaelis constant Determine the parameter values in this rate expression. Tutorial 3 a. External mass transfer limitations can be verified by the Carberry number, Ca. 1. How would you calculate Ca 2. What are the limiting values of Ca, and why? 3. Give the physical interpretation of Ca b. Pore diffusion limitations in porous catalysts can be verified by the Thiele modulus . 1. Give for a first order irreversible reaction and dimensions of the parameters 2. What is the physical meaning of 2 ? 3. Give the relation between the catalysts effectiveness and for the limits of approaching 1 and approaching 0. 4. To be able to calculatate the kinetics of the reaction has to be known. If the kinetics are unknown give two ways to be able to check the presence or absence of pore diffusion limitations. 5. What is the effect on the observed reaction rate if one increases the dispersion of the active phase of a catalyst by a factor of two, while one operates in a strongly pore diffusion controlled regime? Motivate your answer. Tutorial 4 For a first order catalysed gas-phase decomposition reaction under chemically controlled conditions the following data have been reported: rv = 10-6 mol s-1 (cm3cat)-1 cA = 10-5 mol cm-3 @ 1 bar, 673 K De = 10-7 m2 s-1 Which maximum particle diameter of a spherical catalyst may still be used without diffusional disguise? Tutorial 5 A conversion rate of 8 mol s-1 is being observed for the isothermal gas phase decomposition of a component A in a catalyst bed of 0.5 m3 with a porosity b =0.4 at 600 K and at pA = 1 bar. The spherical catalyst particles have a diameter of 15 mm. In this case De = 2·10-6 m2 s-1. Are diffusion limitations present? Motivate your answer. Use the correct units. Tutorial 6 The data in the table below have been produced in a Berty reactor, a type of CSTR for heterogeneous catalysts with internal recirculation of the fluid. The isothermal reaction conditions were identical in all runs. What can you tell about transport limitations and catalyst porosity ? Run no. 1 2 3 4 Wcat 1 4 1 4 dp 1 1 2 2 FA0 1 4 1 4 Recycle rate High Very high Very high High rvobs 4 4 3 3 Tutorial 7 A first order catalysed decomposition has been studied in a labscale reactor. Use the data below to answer the following questions. a. Has external mass transfer been interfering ? b. Are diffusional disguises present ? c. Do temperature differences exist over the gasfilm or within the particle? Data: Catalyst dp De e kf h cb rvobs Gasfilm Reaction = = = = = = = 2.4 mm 1.4·10-8 m2 s-1 -1 -1 -1 0.45 J m s K 0.083 m s-1 46 J m-2 s-1 K-1 kJ mol-1 20 mol m-3 (@ 1 bar, 609 K) 27 mol s-1 m-3cat Tutorial 8 The Fischer-Tropsch reaction has been studied by Post et al. (AIChE-J. 35 (1989) 1107) using a wide-pore silica supported cobalt based catalysts (spherical particles). The reaction can be described as a first order 0.1 irreversible reaction in the hydrogen partial pressure. They calculated an observed first dp/mm order rate constant at different temperatures obs and for different particle diameters, as kv 0.38 indicated in the graph. 0.01 2.4 0.001 1.90 1.95 2.00 1000/T 1.4 2.05 2.10 A particle size dependency has been observed and the temperature dependency decreases with increasing particle size. Explain these phenomena and by first deriving an expression for the observed reaction rate under extreme diffusion limitations. Tutorial 9 For the irreversible conversion of a component A into a product the following data are available: 1 g catalyst, kw = 10-3 m3 min gcat , cA0 = 3 mol m-3 and v= 10-3 m3 min-1. Calculate the (averaged) exit conversion for an ideal plug flow reactor for the cases a-c. Do the same for a ten times lower catalyst activity. a. For an undiluted catalyst bed b. For the catalyst homogeneously diluted with the same volume of inert particles c. Same as for b., but now the catalyst and inert particles form two parallel beds in the reactor (see drawing) Tutorial 10 In a thermobalance the catalysed oxidation of four char samples has been studied to investigate the effect of the catalyst precursor (copper salts) on the catalytic activity. a schematic diagram of the thermobalance used is given below, together with the observed reaction rate R (mg C per h and per mg C initially present). a. One observes at a certain temperature for each catalyst a strong increase in reactivity and it becomes nearly constant at even higher temperatures. The authors explain this by a changing mode of catalytic action, ‘from a non-wetting to a wetting mode’. Give your explanation for this constant level. b. Why is this level about the same for all samples ? c. Explain the increase in apparent activation energy with increasing temperature in the intermediate temperature regime. gas flow RT (mg/h mgi) 10 cooling water sample in ceramic cup 1.0 heating coil thermocouple 0.1 1.40 1.45 1.50 1.55 1.60 1000/T (K-1) Thermobalance Tutorial 11 The first order irreversible decomposition of N2O into O2 and N2 has been studied in an internally recirculated reactor (Berty type). Under isothermal and kinetically controlled conditions (700 K) the observed conversion amounts to 0.7. The following additional data are available. Total flow rate 200 ml/min, amount of catalyst 1 gram, stainless steel reactor, internal reactor volume 100 ml, feed concentration N2O 40 * 10-6 mol/l. Furthermore, the reaction is not affected by other components that may be present. Design a packed bed reactor that has to convert 2000 ppm N2O (80 *10-6 mol/l) in a stack gas for 90% and a total flow rate of 24000 Nm3/h, i.e. calculate the weight of catalyst needed and the reactor volume needed for the following situation: similar temperature as the Berty reactor, isothermal operation, catalyst effectiveness 0.8 and 100 kg catalyst fits into 1 m3 reactor volume (monolithic catalyst). All volumetric dimensions given are identical in this problem. Hint: Use the design equations for the reactors. Tutorial 12 Hosten and Froment studied the isomerization of n-pentane to i-pentane in the presence of hydrogen over a bifunctional Pt-Al2O3 catalyst. Globally first a dehydrogenation takes place over the metallic function, followed by an isomerization over the acidic alumina sites and finally a hydrogenation of the i-pentene takes place over Pt. The reaction sequences can be given as: Dehydrogenation 1) A + * 2) A* + * 3) H2* 4) M* Isomerization 5) M + # 6) M# 7) N# Hydrogenation 8) N + * 9) H2 + * 10) N* + H2* 11) B* A* M* + H2* H2 + * M+* M# N# N+# N* H2 * B* + * B+* a. Derive a rate expression for this reaction where step 6. is rate determining. b. The overall reaction rate is found pressure independent. Is that in agreement with your result? Tutorial 13 For the catalytic decomposition of alcohols into alkenes and water the following results have been obtained: Alcohol n-propanol iso-propanol n-butanol-1 High pressure 172 163 184 Ea (kJ/mol) Low pressure 119 109 117 Difference 53 54 67 Under all conditions water is adsorbed much stronger at the catalyst than the other two components. The apparent (observed) activation energy, obtained from an Arrheniusplot of ln(r) versus 1/T , is significantly different for high and low pressure conditions. The backward reaction is negligible in all cases and a single-site kinetic model can be assumed for this reaction. 1. Demonstrate by means of a kinetic analysis what the physical meaning of the constant difference of about 58 kJ/mol is. 2. Is it logical that this difference is about the same for all three alcohols?
