Gas- Liquid and Gas –Liquid –Solid Reactions

1 Gas- Liquid and Gas Liquid Solid ReactionsBasic ConceptsProper Approach to Gas- Liquid ReactionsReferences MassTransfertheories Gas-liquidreactionregimes Multiphasereactorsandselectioncriterion Filmmodel:Governingequations,problemcomp lexities ExamplesandIllustrativeResults SolutionAlgorithm(computationalconcepts) Theories for Analysis of Transport Effects in Gas- Liquid ReactionsTwo-film Whitman, Chem. & Met. Eng., 29147 (1923).2. W. K. Lewis & W. G. Whitman, Ind. Eng. Chem., 16, 215 (1924).Penetration theoryP. V. Danckwerts, Trans. Faraday Soc., 46300 (1950).P. V. Danckwerts, Trans. Faraday Soc., 47300 (1951).P. V. Danckwerts, Gas- Liquid Reactions , McGraw-Hill, NY (1970).R. Higbie, Trans. Am. Inst. Chem. Engrs., 31365 (1935).Surface renewal theoryP.

2 V. Danckwerts, Ind. Eng. Chem., 431460 (1951).Rigorous multicomponent diffusion theoryR. Taylor and R. Krishna, Multicomponent Mass Transfer,Wiley, New York, Theory stagnant layer exists in both the gas and the Liquid stagnant layers or films have negligible capacitance and hence a local steady-state gradients in the film are equilibrium exists between the the gas and Liquid phases as the gas- Liquid concentration gradients beyond the films are absent due to Theory Whitman, Chem. & Met. Eng., 29 147 (1923).Bulk LiquidBulk GaspApAiCAi CAbx = 0xx + x LLiquid FilmGas Filmx = Lx = GpAi= HA CAiTwo-Film Theory-Single Reaction in the Liquid Film -A (g)+b B (liq)P (liq)RA kg-moles Am3 Liquid - s = - kmn CAm CBnClosed form solutions only possible for linear kineticsor when linear approximations are introducedB & P are nonvolatileGas- Liquid Reaction RegimesVery SlowRapid pseudo1st or mth orderInstantaneousFast (m, n)General (m,n) or IntermediateSlow DiffusionalInstantaneous & SurfaceCharacteristic Diffusion & Reaction Times Diffusion time Reaction time Mass transfer time2 DLDtk ERCCtr 1 MLBtka Reaction-Diffusion Regimes Defined by Characteristic Times Slow reaction regimetD<<tRkL=kL0 Slow reaction-diffusion regime.

3 TD<<tR<<tM Slow reaction kinetic regime:tD<<tM<<tR Fast reaction regime:tD>>tRkL=EA kL0>kL0 Instantaneous reaction regime:kL= EA kL0 For reaction of a gas reactant in the Liquid with Liquid reactant with/without assistance of a dissolved catalyst PbgA The rate in the composition region of interest can usually be approximated as nBmAACCksmAmolkR 3 Where BACC, are dissolved A concentration and concentration of Liquid reactant B in the Liquid . Reaction rate constant k is a function of dissolved catalyst concentration when catalyst is involved. For Reactions that are extremely fast compared to rate of mass transfer form gas to Liquid one evaluates the enhancement of the absorption rate due to reaction. LAALLAHpEakRgo For not so fast Reactions the rate is LnBmAAACHpkRg Where effectiveness factor yields the slow down due to transport resistances.

4 S30 Comparison Between Theories Film theory: kL D, -film thickness Penetration theory: kL D1/2 Higbie modelt*-life of surface Liquid elementDanckwerts models -average rate of surface renewal'*ALRDkCC '**2 ALRDkCCt '*ALRkDsCC ===Gas Absorption Accompanied by Reaction in the LiquidAssume:-2ndorder rateHatta Number:Ei Number:Enhancement Factor:HkkKgLL111 S31S32In this notation smAmolkNA2 is the gas to Liquid flux sreactormmolkRRsliquidmmolkaNRALAAA3'3' S33 Eight (A H) regimes can be reaction occurs in the Liquid reaction occurs at gas- Liquid interface High gas- Liquid interfacial area desired Non-isothermal effects second order reaction in the film. No unreacted A penetrates into bulk first order reaction in film; same Ha number range as rate proportional to gas- Liquid area.

5 Non-isothermal effects still temperature difference across film develops at complete mass transfer limitationsTemperature difference for Liquid film with reactionTrial and error required. Nonisothermality severe for fast Chlorination of tolueneS38-Summary -Limiting Reaction-Diffusion RegimesSlow reaction kinetic regime Rate proportional to Liquid holdup and reaction rate and influenced by the overall concentration driving force Rate independent of klaB and overall concentration driving forceSlow reaction-diffusion regime Rate proportional to klaB and overall concentration driving force Rate independent of Liquid holdup and often of reaction rateFast reaction regime Rate proportional to aB,square root of reaction rate and driving force to the power (n+1)/2 (nth order reaction)

6 Rate independent of kland Liquid holdupInstantaneous reaction regime Rate proportional to kLand aB Rate independent of Liquid holdup, reaction rate and is a week function of the solubility of the gas reactantKey Issues Evaluate possible mechanisms and identify reaction pathways, key intermediates and rate parameters Evaluate the reaction regime and transport parameters on the rate and assess best reactor type Assess reactor flow pattern and flow regime on the rate Select best reactor, flow regime and catalyst concentration Approximately for 2nd order reaction PBbgA reactorin fractin volumeliquid local reactorliquidfactort enhancemen film, Liquid and gasfor t coefficien transfer mass volumetric1,Afor constant sHenry' phase gas in theA of pressure partial local reactor of eunit volumper ratereaction local observed1113333 mmEEsakHakAmolkliquidmatmHatmpsmAmolkRCk EakHakHPRLLAAAAAALLAAAAAALgLg S29 Gas- Liquid -Solid Reactions Let us consider: EBAEC atalyst Reaction occurring at the surface of the catalyst A Reactant in the gas phase B Non-volatile reaction in the Liquid phase Number of steps.

7 Transport of A from bulk gas phase to gas- Liquid interface Transport of A from gas- Liquid interface to bulk Liquid Transport of A&B from bulk Liquid to catalyst surface Intraparticle diffusion in the pores Adsorption of the reactants on the catalyst surface Surface reaction to yield product The overall local rate of reaction is given as 12*111 lcpsABkwakakARL S45 Gas Limiting Reactant (Completely Wetted Catalyst) pvBpsBlAgHgBvoAslpagBsBpvAvkakaKHAAkRsre actmmolAAaAHAasreactmmolsreactmmolAkscat mmolAkA 11111:. RATE (APPARENT) OVERALL k:solid- Liquid - K: Liquid -Gas - lumereactor vounit per . RATE TRANSPORT lumereactor vounit per .1 : CATALYST IN RATE olumecatalyst vunit per .: RATE KINETIC3s11333S21 Gas Liquid Solid Catalyzed Reaction A(g)+B(l)=P(l)Clearly is determined by transport limitations and by reactor type and flow only improves if we are not already transport 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.

8 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 aS vm maxvm maxx x maxxmaxvm aSionconcentratcatalyst activity specific3 reactormcatkgxhcatkgPkgSaS38 Chemists or biochemists need to improve Sa and together with engineers work on increasing maxx. Engineers by manipulation of flow patterns affect maxvm . In Kinetically Controlled Regime vm aSx, maxx limited by catalyst and support or matrix loading capacity for cells or enzymes In Transport Limited Regime vm ppaxS, 2/10 p Mass transfer between gas- Liquid , Liquid -solid etc. entirely limit vm and set maxvm . Changes in ,aS do not help; alternating flow regime or contact pattern may help!

9 Important to know the regime of operation S39 Comparison Between Gas-Solid and Gas- Liquid -Solid Catalytic ConvertersCategoryGas-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.

10 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 efficiencyKey Multiphase Reactor Types Mechanically agitated tanks Multistage agitated columns Bubble columns Draft-tube reactors Loop reactorsSoluble catalysts&Powdered catalystsSoluble catalysts&Tableted catalysts Packed columns Trickle-beds Packed bubble columns Ebullated-bed reactorsClassification of Multiphase Gas- Liquid -SolidCatalyzed Reactors1.