Presentation on theme: "Gas- Liquid and Gas –Liquid – Solid Reactions Basic Concepts."— Presentation transcript:
Gas- Liquid and Gas –Liquid – Solid Reactions Basic Concepts
Proper Approach to Gas-Liquid Reactions References Mass Transfer theories Gas-liquid reaction regimes Multiphase reactors and selection criterion Film model: Governing equations, problem complexities Examples and Illustrative Results Solution Algorithm (computational concepts)
Theories for Analysis of Transport Effects in Gas-Liquid Reactions Two-film theory 1. W.G. Whitman, Chem. & Met. Eng., 29 147 (1923). 2. W. K. Lewis & W. G. Whitman, Ind. Eng. Chem., 16, 215 (1924). Penetration theory P. V. Danckwerts, Trans. Faraday Soc., 46 300 (1950). P. V. Danckwerts, Trans. Faraday Soc., 47 300 (1951). P. V. Danckwerts, Gas-Liquid Reactions, McGraw-Hill, NY (1970). R. Higbie, Trans. Am. Inst. Chem. Engrs., 31 365 (1935). Surface renewal theory P. V. Danckwerts, Ind. Eng. Chem., 43 1460 (1951). Rigorous multicomponent diffusion theory R. Taylor and R. Krishna, Multicomponent Mass Transfer, Wiley, New York, 1993.
Two-film Theory Assumptions 1. A stagnant layer exists in both the gas and the liquid phases. 2. The stagnant layers or films have negligible capacitance and hence a local steady-state exists. 3. Concentration gradients in the film are one- dimensional. 4. Local equilibrium exists between the the gas and liquid phases as the gas-liquid interface 5. Local concentration gradients beyond the films are absent due to turbulence.
Two-Film Theory Concept W.G. Whitman, Chem. & Met. Eng., 29 147 (1923). Bulk LiquidBulk Gas pApA p Ai C Ai C Ab x = 0 x x + x L Liquid FilmGas Film x = L x = G p Ai = H A C Ai
Two-Film Theory - Single Reaction in the Liquid Film - A (g) +b B (liq) P (liq) Closed form solutions only possible for linear kinetics or when linear approximations are introduced B & P are nonvolatile
Gas-Liquid Reaction Regimes Very Slow Rapid pseudo 1st or mth order InstantaneousFast (m, n) General (m,n) or IntermediateSlow Diffusional Instantaneous & Surface
Characteristic Diffusion & Reaction Times Diffusion time Reaction time Mass transfer time
Reaction-Diffusion Regimes Defined by Characteristic Times Slow reaction regimet D <>t R k L =E A k L 0 >k L 0 –Instantaneous reaction regime:k L = E A k L 0
Comparison Between Theories Film theory: –k L D, - film thickness Penetration theory: –k L D 1/2 Higbie model t * - life of surface liquid element Danckwerts model s - average rate of surface renewal = = =
Gas Absorption Accompanied by Reaction in the Liquid Assume : - 2 nd order rate Hatta Number : Ei Number: Enhancement Factor: S31
Eight (A – H) regimes can be distinguished: A.Instantaneous reaction occurs in the liquid film B.Instantaneous reaction occurs at gas-liquid interface High gas-liquid interfacial area desired Non-isothermal effects likely S34
C.Rapid second order reaction in the film. No unreacted A penetrates into bulk liquid D.Pseudo first order reaction in film; same Ha number range as C. Absorption rate proportional to gas-liquid area. Non-isothermal effects still possible. S35
Maximum temperature difference across film develops at complete mass transfer limitations Temperature difference for liquid film with reaction Trial and error required. Nonisothermality severe for fast reactions. e.g. Chlorination of toluene S38
- Summary - Limiting Reaction-Diffusion Regimes - Summary - Limiting Reaction-Diffusion Regimes Slow reaction kinetic regime Rate proportional to liquid holdup and reaction rate and influenced by the overall concentration driving force Rate independent of k l a B and overall concentration driving force Slow reaction-diffusion regime Rate proportional to k l a B and overall concentration driving force Rate independent of liquid holdup and often of reaction rate Fast reaction regime Rate proportional to a B,square root of reaction rate and driving force to the power (n+1)/2 (nth order reaction) Rate independent of k l and liquid holdup Instantaneous reaction regime Rate proportional to k L and a B Rate independent of liquid holdup, reaction rate and is a week function of the solubility of the gas reactant
Gas Limiting Reactant (Completely Wetted Catalyst) S21 Gas – Liquid Solid Catalyzed Reaction A(g)+B(l)=P(l)
Clearly is determined by transport limitations and by reactor type and flow regime. Improving only improves if we are not already transport limited. Our task in catalytic reactor selection, scale-up and design is to either maximize volumetric productivity, selectivity or product concentration or an objective function of all of the above. The key to our success is the catalyst. For each reactor type considered we can plot feasible operating points on a plot of volumetric productivity versus catalyst concentration. S38
Comparison Between Gas-Solid and Gas-Liquid-Solid Catalytic Converters CategoryGas-Solid CatalyticGas-Liquid-Solid Catalytic Design and engineeringSimpleMore elaborate MaterialOften expensive material can be usedCorrosion problems can be critical CatalystPossible poisoning by non-volatile byproducts Resistance to corrosion is required Thermal controlLow thermal stability and low heat capacity require internal heat exchange or low conversion Better stability and higher heat capacity; partial vaporization is possible; better heat exchange coefficient Reactant recyclingOften importantStoichiometric ratio can generally be achieved; hydrodynamics can require gas recycling SafetyTemperature run-away and ignition can occur. Gas mixture must lie outside the explosive range Better stability Operation within the inflammability or explosion limits sometimes possible Dissipated powerHigher pressure dropLow pressure drop but sometimes stirring is required Reactant preheatingAlways importantLess important or unnecessary Heat recoveryGenerally at a high level but low heat transfer rate At a lower level but high heat transfer rate; high efficiency
Classification of Multiphase Gas-Liquid-Solid Catalyzed Reactors 1. Slurry Reactors Catalyst powder is suspended in the liquid phase to form a slurry. 2. Fixed-Bed Reactors Catalyst pellets are maintained in place as a fixed-bed or packed-bed. K. Ostergaard, Adv. Chem. Engng., Vol. 7 (1968)
Modification of the Classification for Gas-Liquid Soluble Catalyst Reactors 1.Catalyst complex is dissolved in the liquid phase to form a homogeneous phase. 2.Random inert or structured packing, if used, provides interfacial area for gas-liquid contacting.
Multiphase Reactor Types for Chemical, Specialty, and Petroleum Processes S42
Multiphase Reactor Types at a Glance Middleton (1992)
Comparison Between Slurry and Fixed-Bed Gas-Liquid-Solid Catalytic Converters CategorySlurry ReactorsTrickle-Bed Reactors Specific reaction rateHigh or fast reactionsRel. high for slow reactions CatalystHighly activeSupported; high crushing strength, good thermal stability and long working life needed Homogeneous side reactionsPoor selectivityGood selectivity Residence time distributionPerfect mixingPlug flow Pressure dropLow or mediumLow except for small particles Temperature controlIsothermal operationAdiabatic operation Heat recoveryEasyLess easy Catalyst handlingTechnical difficultiesNone Maximum volume50 m3 300 m3 Maximum working pressure100 barhigh pressure possible Process flexibilityBatch or continuousContinuous Investment costsHighLow Operating costsHighLow Reactor design and extrapolation Well knowndifficult