Presentation on theme: "Catalysis for Chemical Engineers"— Presentation transcript:
1 Catalysis for Chemical Engineers A Brief History and Fundamental Catalytic Principles
2 What is Catalysis? The science of catalysts and catalytic processes. A developing science which plays a critically important role in the gas, petroleum, chemical, and emerging energy industries.Combines principles from somewhat diverse disciplines of kinetics, chemistry, materials science, surface science, and chemical engineering.2
3 What is Catalyst?A catalyst is a material that enhances the rate and selectivity of a chemical reactions and in the process is cyclically regenerated.Fe2+ + Ce4+ Fe3+ + Ce (Slow Reaction)2Fe2+ + Mn4+ 2Fe3+ + Mn2+Mn2+ + 2Ce4+ Mn4+ + 2Ce3+Fe2+ + Ce4+ Fe3+ + Ce3+Homogeneous Catalysis(Fast Reaction)CO + H2O CO2 + H2@ low temperature (Slow Reaction)S* + H2O H2 + O-S*O-S* + CO CO2 + S*(Faster Reaction)Heterogeneous CatalysisCO + H2O CO2 + H2
4 What is Catalyst? N2 (Desired Reaction) rD NO rU (Undesired Reaction) NH3rDRate of formation of DSD/U==rURate of formation of URhSD/UPtSD/UFrom
5 How Important Is Catalysis? Fibers, Plastics, Food, Home Products, PharmaceuticalsChemicalsRaw MaterialsHeating, Transportation, PowerFuelsFour of the largest sectors of our world economy (i.e. the petroleum, power, chemicals, and food industries), which account for more than 10 trillion dollars of gross world product, are largely dependent on catalytic processes.
6 Development of Important Industrial Catalytic Processes Mittasch investigated over 2500 catalysts compositions!!!
7 Development of Important Industrial Catalytic Processes It played a vital role as a feedstock for chemicals: 30 million tons per year in 2000
8 Development of Important Industrial Catalytic Processes Production of Liquid Fuels!!!
9 Development of Important Industrial Catalytic Processes NOCOCxHyN2CO2H2OO2
10 How to Define Reaction Rate?? 1dniReaction Rate (r) =i * QdtQ = V, W or S.A. of catalysti = Stoichiometric Coefficienti iMi = 0 involving species Mi(i is negative for reactants and positive for products)e.g. 2NH3 = N2 + 3H22 x (NH3) -1 x (N2) -3 x (H2) =2N + 6H – 2N – 6H = 0ni = # of moles of species Mi
11 Chemical Reactions Four Basic Variables to Control Chemical Reactions: TemperaturePressureConcContact timeRate of Reaction = K(T) x F(Ci)K(T) = A exp(-E/RT) i (Ci)iHIHCClHCHIClEnergy Intensive & damaging to equipment and materials & non-selectiveCHClI
17 Active Catalytic Phases and Reactions They Typically Catalyze
18 Typical Physical Properties of Common Carrier (Supports)
19 Heterogeneous Catalysis A (g) B (g)Support(Al2O3)Active Metals(Pt, Co, MoO2)supportMinimize PMinimize Mass Transport ResistancesMaximize ActivityMinimize Poisoning and Fouling
20 Heterogeneous Catalysis Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on tempSteps 3, 4, & 5 are chemical processes => Large dependences on tempFor Knudsen DiffusionOrder of MagnitudedPhasecm2/s m2/sTemp and Pressure Dependencesd < T21.75T1For Bulk, Molecular or Fick’s DiffusiondFrom Elements of Chemical Reaction Engineering, S. Foglerd >
21 Heterogeneous Catalysis Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on tempSteps 3, 4, & 5 are chemical processes => Large dependences on tempGiven that the rates of the chemical steps are exponentially dependent on temperature and have relatively large activation energies compared to the diffusional process (20~200 kJ/mol Vs. 4-8 kJ/mol), they are generally the slow or rate-limiting processes at low reaction temperatures.As the temperature increases, the rates of chemical steps with higher activation energies increase enormously relative to diffusional processes, and hence the rate limiting process shifts from chemical to diffusional.Kapp(T) = Aapp exp(-Eapp/RT)
22 Film Mass Transfer Effect on Reaction Rate If Boundary Layer is Too Thick,Reaction Rate = Mass Transfer RatekA B-rA = kc (CAb – CAs)where Kc = DAB / As the fluid velocity (U) increases and/or the particle size (Dp) decreases, the boundary layer thickness () decreases and the mass transfer coefficient (Kc) increases
23 Internal Diffusion Effect on Reaction Rate kA B-rA = k η CASWhere η = Effectiveness Factorη = (CA)avg / CAScoshcoshΦpore (1 - x/L)CA=CAScosh( Φpore)LxΦpore (Thiele Modulus) = L (k P / Deff)1/2η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore
24 Internal Diffusion Effect on Reaction Rate While the equations above were derived for the simplified case of first-order reaction and a single pore, they are in general approximately valid for other reaction orders and geometry if L is defined as Vp/Sp (the volume to surface ratio of the catalyst particle). Hence, L = z/2, rc/2 and rs/3, respectively, for a flat plate of thickness z, a cylinder of radius rc, and a sphere of radius rs.
25 Elementary Reaction A + B C It is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equationA + B CIf it is an elementary reaction,ABC-rA = k [A]1 [B]1
26 Elementary ReactionIt is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equationO3 O2 + OIs this an elementary reaction?If it is an elementary reaction,-rO3 = k [O3]1
27 Elementary ReactionIt is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equationO3 O2 + OOn molecular level, what really is really happening is:O2 + O3 O2 +O2 + O-rO3 = k [O3]1 [O2]1We never really know for sure if we have an elementary reaction based on the balanced stoichiometric equation!!!
28 Heterogeneous Catalysis A (g) B (g)Active Metals(Pt, Co, MoO2)supportProposed Reaction Mechanismk1A + S A-Sk-1k2A-S B-Sk-2k3B-S B + Sk-3
29 What If Adsorption Is Rate Limiting Step? Length of Vector Is Proportional to RXN RateDirector of Vector Indicates Direction of RXNAdsorption of ANet RXN of AdsorptionNet RXN of AdsorptionSurface RXN of A to BNet RXN of Surface RXNNet RXN of Surface RXNDesorption of BNet RXN of DesorptionNet RXN of Adsorption= Net RXN of Surface RXN =Net RXN of DesorptionFollowing Approximations Can Be Made:Adsorption of A is almost irreversibleBoth surface rxn and desoprtion steps are almost at equilibrium
30 What If Adsorption Is Rate Limiting Step? k1A + S A-SWhere S is a free surface site and A-S is a chemisorbed complexSince it is an elementary reaction,v = the fractional coverage of vacant site-rA = k1 CA CSv = CS / CtotalHow can we experimentally measure Cs ???Cs = functions of parameters that one can experimentally measure or easily obtain
31 What If Adsorption Is Rate Limiting Step? Since both surface rxn and desorption steps are in near equilibrium,k2A-S B-Srnet = k2 CA-S –k-2 CB-S 0k2 / k-2 = K2 =CB-S / CA-Sk-2k3B-S B + Srnet = k3 CB-S –k-3 CB CS 0k3 / k-3 = K3 =CB CS / CB-Sk-3Both K2 and K3 are equilibrium constants which one can obtain:RT ln K = - GLet us do the site balance,Ctotal = CS + CA-S + CB-S = Const.CtotalK2 = CB-S / CA-SCS =1 + [ (1 + K2) CB / (K2 K3) ]K3 = CB CS / CB-S
32 What If Adsorption Is Rate Limiting Step? From the site balance and quasi-equilibrium approximation,CtotalCS =1 + [ (1 + K2) CB / (K2 K3) ]From the rate limiting step,k1CtotalCAk1CtotalCA-rA = k1 CA CS==1 + [ (1 + K2) CB / (K2 K3) ]1 + K’ CBWhere K’ =(1 + K2) / (K2 K3)If A and B behave according to the ideal gas law,CA = PA / RTCB = PB / RT
33 What If Surface Reaction Is Rate Limiting Step? k1A + S A-Sk-1Rate Limiting Stepk2A-S B-Sk-2k3B-S B + Sk-3k2K1PA-rA =1 + K1 PAFigure 1.16 from Fundamentals of Industrial Catalytic Processes
34 What If Desoprtion Is Rate Limiting Step? k1A + S A-Sk-1k3K1K2PAk2A-S B-S-rA =k-21 + (K1 + K1 K2) PARate Limiting Stepk3B-S B + Sk-3
35 Fundamental Catalytic Phenomena and Principles Chemical Properties(Oxidation State, Acidity, Surface Composition)Physical Properties(Surface Area, Pore Structure, Pore Density)CatalystDesignCatalytic Properties(Activity and Selectivity)