Presentation on theme: "Gas- Liquid and Gas –Liquid –Solid Reactions"— Presentation transcript:
1Gas- Liquid and Gas –Liquid –Solid Reactions Basic Concepts
2Proper Approach to Gas-Liquid Reactions ReferencesMass Transfer theoriesGas-liquid reaction regimesMultiphase reactors and selection criterionFilm model: Governing equations, problem complexitiesExamples and Illustrative ResultsSolution Algorithm (computational concepts)
3Theories for Analysis of Transport Effects in Gas-Liquid Reactions Two-film theory1. W.G. Whitman, Chem. & Met. Eng., (1923).2. W. K. Lewis & W. G. Whitman, Ind. Eng. Chem., 16, 215 (1924).Penetration theoryP. V. Danckwerts, Trans. Faraday Soc., (1950).P. V. Danckwerts, Trans. Faraday Soc., (1951).P. V. Danckwerts, Gas-Liquid Reactions, McGraw-Hill, NY (1970).R. Higbie, Trans. Am. Inst. Chem. Engrs., (1935).Surface renewal theoryP. V. Danckwerts, Ind. Eng. Chem., (1951).Rigorous multicomponent diffusion theoryR. Taylor and R. Krishna, Multicomponent Mass Transfer,Wiley, New York, 1993.
4Two-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 interface5. Local concentration gradients beyond the films are absent due to turbulence.
5Two-Film Theory Concept W. G. Whitman, Chem. & Met. Eng pApAipAi = HA CAi•Bulk GasGas FilmLiquid FilmBulk LiquidCAi•CAbxx + xLx = Gx = 0x = L
6Two-Film Theory - Single Reaction in the Liquid Film - A (g) + b B (liq) P (liq)B & P are nonvolatileClosed form solutions only possible for linear kineticsor when linear approximations are introduced
7Gas-Liquid Reaction Regimes InstantaneousFast (m, n)Rapid pseudo1st or mth orderInstantaneous & SurfaceGeneral (m,n) or IntermediateSlow DiffusionalVery Slow
8Characteristic Diffusion & Reaction Times Diffusion timeReaction timeMass transfer time
9Reaction-Diffusion Regimes Defined by Characteristic Times Slow reaction regime tD<<tR kL=kL0Slow reaction-diffusion regime: tD<<tR<<tMSlow reaction kinetic regime: tD<<tM<<tRFast reaction regime: tD>>tR kL=EA kL0>kL0Instantaneous reaction regime: kL= EA kL0
15Eight (A – H) regimes can be distinguished: A. Instantaneous reaction occurs in the liquid filmB. Instantaneous reaction occurs at gas-liquid interfaceHigh gas-liquid interfacial area desiredNon-isothermal effects likelyS34
16D. Pseudo first order reaction in film; same Ha number range as C. C. Rapid second order reaction in the film. No unreacted A penetrates into bulk liquidD. 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
18Temperature difference for liquid film with reaction Maximum temperature difference across film develops at complete mass transfer limitationsTemperature difference for liquid film with reactionTrial and error required. Nonisothermality severe for fast reactions.e.g. Chlorination of tolueneS38
19- Summary - Limiting Reaction-Diffusion Regimes Slow reaction kinetic regimeRate proportional to liquid holdup and reaction rate and influenced by the overall concentration driving forceRate independent of klaB and overall concentration driving forceSlow reaction-diffusion regimeRate proportional to klaB and overall concentration driving forceRate independent of liquid holdup and often of reaction rateFast reaction regimeRate proportional to aB,square root of reaction rate and driving force to the power (n+1)/2 (nth order reaction)Rate independent of kl and liquid holdupInstantaneous reaction regimeRate proportional to kL and aBRate independent of liquid holdup, reaction rate and is a week function of the solubility of the gas reactant
23Our 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.Clearly is determined by transport limitations and by reactor type and flow regime. Improving only improves if we are not already transport limited.S38
25Comparison Between Gas-Solid and Gas-Liquid-Solid Catalytic Converters CategoryGas-Solid CatalyticGas-Liquid-Solid CatalyticDesign and engineeringSimpleMore elaborateMaterialOften expensive material can be usedCorrosion problems can be criticalCatalystPossible poisoning by non-volatile byproductsResistance to corrosion is requiredThermal controlLow thermal stability and low heat capacity require internal heat exchange or low conversionBetter stability and higher heat capacity; partial vaporization is possible; better heat exchange coefficientReactant recyclingOften importantStoichiometric ratio can generally be achieved; hydrodynamics can require gas recyclingSafetyTemperature run-away and ignition can occur. Gas mixture must lie outside the explosive rangeBetter stabilityOperation within the inflammability or explosion limits sometimes possibleDissipated powerHigher pressure dropLow pressure drop but sometimes stirring is requiredReactant preheatingAlways importantLess important or unnecessaryHeat recoveryGenerally at a high level but low heat transfer rateAt a lower level but high heat transfer rate; high efficiency
27Classification of Multiphase Gas-Liquid-Solid Catalyzed Reactors 1. Slurry ReactorsCatalyst powder is suspended in the liquidphase to form a slurry.2. Fixed-Bed ReactorsCatalyst pellets are maintained in place asa fixed-bed or packed-bed.K. Ostergaard, Adv. Chem. Engng., Vol. 7 (1968)
28Modification of the Classification for Gas-Liquid Soluble Catalyst Reactors 1. Catalyst complex is dissolved in the liquidphase to form a homogeneous phase.2. Random inert or structured packing, if used,provides interfacial area for gas-liquid contacting.
29Multiphase Reactor Types for Chemical, Specialty, and Petroleum Processes
30Multiphase Reactor Types at a Glance Middleton (1992)
32Comparison Between Slurry and Fixed-Bed Gas-Liquid-Solid Catalytic Converters CategorySlurry ReactorsTrickle-Bed ReactorsSpecific reaction rateHigh or fast reactionsRel. high for slow reactionsCatalystHighly activeSupported; high crushing strength, good thermal stability and long working life neededHomogeneous side reactionsPoor selectivityGood selectivityResidence time distributionPerfect mixingPlug flowPressure dropLow or mediumLow except for small particlesTemperature controlIsothermal operationAdiabatic operationHeat recoveryEasyLess easyCatalyst handlingTechnical difficultiesNoneMaximum volume50 m3300 m3Maximum working pressure100 barhigh pressure possibleProcess flexibilityBatch or continuousContinuousInvestment costsHighLowOperating costsReactor design and extrapolationWell knowndifficult