Heterogeneous Catalysis

Transcription

1 Heterogeneous Catalysis Main advantages: Convenient technology Easy catalyst separation Relatively easy catalyst regeneration Less expensive Reactor selection: Needs: safety, environmental issues, possibility of scale-up Wants: maximum conversion, maximum selectivity, high throughput, easy scale-up, minimum overall process costs T

2 Catalyst Effectiveness η i = observed reaction rate rate without internal gradients = r r obs ( c, T ) ( c, T v, chem s s ) n th -order reaction: 1 φ gen = V SA k D eff n + 2 first-order reaction: v 1 n 1 cs η i Choose particle size to stay in this region φ gen = V SA k D v eff φ gen T

3 Catalyst Shapes Sphere Pellet Cylindrical Extrudate Trilobe Hollow Extrudate Ring Wagonwheel Minilith Increasing SA / V Decreasing p Decreasing crushing strength Increasing manufacturing costs T

4 Reactor Choice and Design Number and type of phases (gas, liquid, solid) Mass and heat transport Degree of mixing Contacting pattern (co-current, counter-current, cross-current) Concentration and temperature dependence Main and side reactions Desired T and T profile Residence time distribution (degree of mixing) Heat of reaction Temperature control / heat transfer Catalyst deactivation Process design Rate of addition / removal (years, seconds) Regeneration

5 Fixed-bed reactors with one fluid phase Air / O 2 Burner Simple fixed-bed reactor Fixed-bed reactor with combustion zone Radial-flow reactor Monolith reactor

6 Reactors with moving catalyst (one fluid phase) Feed Catalyst addition Product Catalyst in suspension Gas bubbles Product Fresh catalyst Spent catalyst Product Catalyst regeneration Feed Catalyst removal Feed Catalyst regeneration Moving-bed reactor Fluidized-bed reactor Entrained-flow reactor (riser)

7 Reactors for gas-liquid-solid catalyst reactions Fixed-bed reactors Gas Gas Liquid Liquid Product Product Trickle-flow reactor Monolith reactor

8 Reactors for gas-liquid-solid catalyst reactions Reactors with moving catalyst Product Gas Catalyst addition Liquid Catalyst in suspension Baffle Gas bubble Gas Gas bubbles Catalyst removal Cooling jacket Gas Catalyst in suspension Liquid Liquid Three-phase fluidized-bed (ebullated-bed) reactor Mechanically stirred tank reactor

9 Adiabatic fixed-bed reactor Fluid (gas or liquid) Inert beads Catalyst +simple + high catalyst load + little attrition + little backmixing - long diffusion distances - high pressure drop - difficult to cool/heat

10 Fluidized (ebullated) bed reactor + uniform temperature + high heat transfer rates + catalyst replacement -backmixed - cat. attrition and erosion - difficult to scale-up

11 Entrained-flow reactor (riser) + short residence times + high mass transfer rates + replacing catalyst Regeneration - catalyst attrition - erosion of reactor - catalyst separation

12 Trickle-flow reactor film, pocket, filament + high catalyst load + little back-mixing + catalyst separation + no catalyst attrition dry, unwetted, wetted - high pressure drop - flow maldistributions -co-current - long diffusion distance

13 Applications of Reactors for Gas-phase Reactions Simple fixed-bed reactor reacto: : Monolith reactor Fixed-bed with combustion zone Radial-flow reactor Moving-bed reactor Fluidized-bed reactor Entrained-flow reactor HDS of naphtha Catalytic reforming (semi-regenerative) Steam reforming Water-gas shift Methanation Ammonia synthesis Methanol synthesis Exhaust gas cleaning Autothermal reforming Catalytic reforming Methanol synthesis Catalytic reforming (continuous regenerative) Classical FCC process Modern FCC process

14 Applications of Reactors for Gas-liquid Reactions Trickle-flow reactor Moving-bed reactor Three-phase fluidized- bed reactor HDS of heavy oil fractions Hydrogenation of residues (metal removal) Hydrogenation of residues T

15 Heat Management Feed Feed Feed H / C H / C C C Cold feed Product Product Product Multiple adiabatic reactors with intermediate heating (H) or cooling (C) One vessel with intermediate cooling Quench reactor T

16 Heat Management Feed Flue gas Feed Heat-exchange fluid baffle Product Product Fuel for burners Multi-tubular fixed-bed reactor Tubular reformer T

17 Catalyst Deactivation Sintering: loss of surface area due to high T water accelerates this process Poisoning: by impurities in feed sulfur (catalytic reforming, steam reforming) CO, CO2 (ammonia synthesis) ethyne Fouling coke formation by secondary reactions (condensation) optimum conditions for coke formation: high T, low p T

18 Catalyst Deactivation - Coke Formation Addition of H 2 or H 2 O limits coke formation: C + 2 H 2 CH 4 C + 2 H 2 O CO H 2 Alternative: frequent catalyst regeneration - burning off coke Process Steam reforming Catalytic reforming Semi-regenerative Fully-regenerative Continuous regenerative Measure to limit coke formation Excess steam addition Large H 2 /HC ratio Moderate H 2 /HC ratio Moderate H 2 /HC ratio Deactivation time 2 years years days weeks days weeks Reactor type Tubular fixed-bed reactor Fixed-bed reactor Swing reactor Moving-bed reactor FCC None (regeneration) seconds Entrained-flow reactor T

19 Other issues: Mixing, Safety, Scale-up Mixing of reactants: high degree of back-mixing: advantage: isothermal operation (fluidized( bed) disadvantage: lower conversion and selectivity Consecutive reactions: A P S:» PFR yields maximum selectivity for P» but if reactions are very exothermic, CSTR is desired for even T distribution T

20 Other issues: Mixing, Safety, Scale-up Safety: thermal runaways hot spots in trickle-flow reactors Ease of scale-up: Depends on: hydrodynamics of reactor maximum size of equipment, proven versus new technology, experience of the company with the particular technology fixed-bed reactors much easier to scale up than fluidized-bed reactors T

21 Novel Developments in Reactor Technology Combining reaction and heat transfer Structured catalytic reactors Hybrid systems coupling membranes and reaction coupling reaction and adsorption catalytic distillation T

22 Combining reaction and heat transfer Exothermic reactions: T thermodynamics: low T kinetics: high T T increases along reactor low equilibrium conversion T-profile wrong way around: heating of feed with product other solution:» adiabatic fixed-bed reactor with periodic flow reversal (RPFR)» makes use of heat capacity of catalyst bed

23 Reactor with Periodic Flow Reversal (RPFR) Valve open Valve closed Feed Product Feed Product Inert beads Catalyst T

24 Reactor with Periodic Flow Reversal (RPFR) Temperature Profiles Inerts Catalyst Inerts Start of cycle Temperature End of cycle Direction of flow T

25 Reactor with Periodic Flow Reversal (RPFR) Applications Current applications: Oxidation of SO2 for sulfuric acid production Oxidation of volatile organic compounds (VOC) for purification of industrial exhaust gases NOx reduction by ammonia in industrial exhaust gases Possible future applications: Steam reforming and partial oxidation of methane Production of methanol and ammonia Catalytic dehydrogenation T

26 - Structured Catalytic Reactors Monolithic catalysts or monoliths impermeable or permeable (membrane reactor) arranged catalysts structured packings covered with catalytic material monolithic Monolithic catalyst Channel channel catalytic Catalytic layer monolithic substrate washcoat catalytic species T

27 Monoliths - Advantages & Applications Main advantage: low pressure drop Applications: automotive pollution control incineration of industrial off gases Catalytic combustion of fuels for gas turbines Oxidation of SO2 Oxidation of ammonia Hydrogenation processes, including gas/liquid systems T

28 Three-levels-of-porosity reactors Example: Parallel Passage Reactor Gas Macropores (~10 mm) Gauze Mesopores (~ 1 mm) Micropores (1-10 nm) Catalyst particles T Especially suitable for treating dust-containing gases, e.g. flue gas from power plants

29 Hybrid Systems Coupling of operations, e.g.: reaction and separation several separation processes Multifunctional reactor: yield and selectivity enhanced, e.g., by separating product immediately Separation by: membranes, adsorption, distillation, absorption in a solvent Most important benefits: reduction in capital investment equilibrium-limited reactions go beyond equilibrium T

30 Catalytic Membranes - Ethane Dehydrogenation Pt H 3 C-CH 3 H 2 C=CH 2 + H 2 Compression fitting Sweep gas Stainless-steel casing Feed gas Product gas Alumina tube (membrane + support) Product gas Graphitized string T

31 T Catalytic Membranes - Permeation

32 Catalytic Membranes - Conversion

33 Membrane Reactor - Fat Hydrolysis O C-O-C-R fat + water fatty acid + glycerol O C-O-C-R O C-O-C-R O + 3 H 2 O 3 R-C-OH C-OH + C-OH C-OH Oil/fat circuit Fatty acid Water phase Glycerol Hollow-fiber module Oil phase Fiber wall Lipase enzyme

34 Coupling Reaction and Adsorption gas-solid-solid trickle-flow reactor Methanol Synthesis Fresh adsorbent Energy losses: Condensation (inerts( present lower heat transfer rates); Recycles (compressor, heating) due to limited conversion per pass GSSTFR: Adsorption of formed methanol Conversion beyond equilibrium But: recycle of solids Feed gas Product gas Distributor Catalyst bed Support grid Saturated adsorbent

35 Production of Ethylbenzene and Styrene Ethylbenzene: + CH 2 =CH 2 CH 2 CH 3 H r = kj/mol Cumene: + CH 3 CH=CH 2 CH(CH 3 ) 2 H r = - 99 kj/mol Styrene: CH 2 CH 3 CH=CH 2 + H 2 H r = 125 kj/mol

36 B Ethylbenzene - Serial kinetics C-C C-C C=C C=C C=C EB desired product C-C transalkylation DEB C-C PEB Catalysts AlCl 3 (homogeneous) Zeolite (Heterogeneous) shape selective Benzene Selectivity mol% Ethylbenzene C=C/B < Di-ethylbenzene Ethene/benzene molar ratio (-) C=C /Benzene???

37 Ethylbenzene Production - Processes Monsanto-Lummus (homogeneous) - liquid phase Catalyst: AlCl 3 (+ some HCl) K 10 bar B/C=C: mol/mol Mobil-Badger (heterogeneous) - vapor phase Catalyst: zeolite ZSM-5 (shape selectivity) K bar B/C=C: 8-16 mol/mol Catalytic distillation

38 Sensitive to H 2 O Catalyst separation Corrosion Extended PEB formation Monsanto-Lummus Liquid-phase Process - AlCl 3 Off-gas Fresh benzene Recycle benzene L/L sep. water PEBs recycle NaOH Neutralization 370 K Alkylate to distillation Dry salts AlCl 3 catalyst complex Ethene x x x Flash vessel Ethyl chloride in benzene 440 K 10 bar steam Catalyst recycle Benzene drying Gas scrubber Alkylation reactor Transalkylation reactor Settlers

39 Mobil-Badger Vapour-phase Process - ZSM5 Fresh benzene Ethene Benzene Off-gas Ethylbenzene 670 K 20 bar Residue to fuel DEB/PEB recycle Alkylator Translkylator Benzene column Lights stripper Ethylbenzene column Polyethylbenzene column

40 Ethylbenzene - Process comparison Advantages of vapour-phase process (ZSM-5): No aqueous waste Non-corrosive No catalyst separation / recycle No extreme sensitivity towards H 2 O Only small (or no) transalkylation reactor required

41 Catalytic Distillation in EB Production Vent Ethylbenzene Ethene vapor Reactor Distillation column Unreacted benzene stays in reactor Benzene vaporized by heat of reaction Stripper PEBs Benzene

42 Catalytic Distillation in EB Production Incentive keep the ethene concentration low» limits oligomerization and amount of poly-ethylbenzenes Advantages heat of highly exothermic alkylation reaction» used directly in distillation» reduced energy requirements reduced investment costs reduced operating costs (less benzene recycle)

43 Styrene Production - Thermodynamics CH 2 CH 3 CH=CH 2 + H 2 H r = 125 kj/mol Favorable: High T Low p

44 Styrene Production Ethylbenzene 920 K Ethylbenzene Steam 920 K 920 K 590 K 870 K 870 K Crude styrene Crude styrene Adiabatic temperature drop at 100% ethylbenzene conversion (theoretical) Conventional process Steam/EB = mol/mol Adiabatic T too high Two reactors in series Inefficiency??? Outlet temperature 870 K loss of energy

45 Styrene Production: Reverse-flow reactor Inert beads 640 K Catalyst Ethylbenzene + steam 890 K Crude styrene Valve open Valve closed Direction of flow Steam 640 K Direction of flow on reversal Advantages Less steam used Efficient use of energy

46 Exhaust Gas Composition λ = (air/fuel) actual / (air/fuel) stoich. λ > 1: lean/oxidizing environment λ < 1: rich/reducing environment Concentration (ppm) NO CO O 2 HC Concentration (vol%) λ (-)

47 Exhaust gas control Primary measures: Speed limitations: NO x increases with increasing driving speed Fuel purification: reduces SO 2 emissions Engine modifications, e.g. lean burn Secondary measures: Dual-bed catalyst Oxidation reactor + exhaust gas recirculation Three-way catalyst

48 History of of emission control Year HC, CO Fuel Air Engine Oxidation catalyst (Pt/Pd) Exhaust gas NO x HC, CO Fuel Air Engine Reduction catalyst (Rh) Oxidation catalyst (Pt) Exhaust gas OC NO x, HC, CO s Fuel Air Engine Three-way catalyst (Pt/Rh) Exhaust gas

49 Emission Control Problem: Reduction ofno Oxidation of CO Oxidation of hydrocarbons Three-way Catalysis : CO + NO 1/2 N 2 + CO 2 H r = kj/mol CO + 1/2 O 2 CO 2 H r = -288 kj/mol C m H n + (m + n/2) O 2 m CO 2 + n/2 H 2 O H r < 0

50 Monolith reactor and mounting system Stainless-steel catalyst housing Purified Exhaust gas Exhaust gas from engine Catalyst Wire mesh packing

51 Structured Catalysts- Appearance Monoliths Ceramics: Cordierite, alumina, titania, silica Metal Cross-flow Sulzer packing Ceramic foams

52 Efficiency of of three-way catalyst 100 NO 80 Conversion (%) HC CO λ (-) t

53 Three-way Catalyst (TWC) Performance Advantages Catalyst very effective (Pt and Rh preferred metals) Disadvantages Up to 70% of Pt/Rh produced worldwide used for automotive catalysts Fast growing European market Problems: - reduction of SO 2 to H 2 S - formation of N 2 O at intermediate T (ozone destruction in atmosphere)

54 Three-phase monolith reactor Gas Liquid Taylor flow Liquid slug Monolith Gas slug Product (gas + liquid)

55 Taylor flow in channel Liquid circulation Thin film Good mass transfer liquid - solid gas - solid

56 Comparison of of three-phase reactors Characteristic Particle diameter/wash coat thickness (mm) three-phase fluidized bed trickle flow three-phase monolith Fraction of catalyst (m 3 cat/m 3 r) Liquid hold-up (m 3 l/m 3 r) Pressure drop ( kpa/m) Volumetric mass transfer coefficient Gas-liquid, k l a l (s -1 ) Liquid-solid, k s a s (s -1 )

57 Advantages monoliths in multiphase applications Minimization of diffusion limitations (fixed bed), without attrition or separation problems (fluidized bed) Continuous operation easy Countercurrent contacting of gas and liquid possible Relatively large area for mass and heat transfer (better than in fixed-bed reactors) Low pressure drop Straightforward scale-up

58 HDS in in monolith reactor S-compound + H desulfurized compound + H 2 S % HDS 10 Sulfur removal, % % H 2 S Counter-current % H 2 S Co-current H 2 S in gas, vol% x / L 0

59 Incentives for Countercurrent operation Competitive adsorption of gas phase component gas liquid Concentration Conversion cocurrent countercurrent Flooding! Countercurrent gas-liquid flow! Advantageous when conversion is suppressed by competitive adsorption (HDS, HDN, acylation) limited by thermodynamic equilibrium (esterification)! In reactive distillation, stripping,.. " Not possible in a trickle bed reactor

60 Internally-finned monolith Monolith Liquid film Gas