反应工程基础程易chpt14fixedbedreactorr2206808519.ppt
Summary of Chapter 131.The concentration profile for a first-order reaction occurring in a spherical catalyst pellet is where 1 is the Thiele modulus.For a first-order reaction2.The internal effectiveness factor and overall effectiveness factor.111AAsinhsinh1sCCe2121DRkSummary(contd)3.For large values of the Thiele modulus for an n order reaction,4.For internal diffusion control,the me reaction order is related to the measured reaction order by The true and apparent activation energies are related by2)/2-(1Ae2/12/1312312nsnnCkDRnn12apparenttrue nnapptrue2EESummary(contd)5.The Weisz-Prater Parameter The Weisz-Prater criterion dictates that:If CWP1,internal diffusion limitations present6.Mears Criteria for Neglecting External Diffusion and Heat Transfer315.0 2gAbbrxnThRRErH15.0AbACknRrcbAse2obsA,21WPCDRrCc4Chapter 14Fixed-Bed ReactorDepartment of Chemical EngineeringTiefeng WContentslIntroduction of fixed-bed reactorlAdiabatic fixed bed reactorlFixed-bed reactor with heating or coolinglModel of the fixed-bed reactor514.1 Types of the fixed bed reactor67Fixed-bed reactors for methanol synthesis8多段冷激式反应器列管式合成反应器径向反应器9Fluidized bed reactor10Selection of the reactor type 11Reactor type preferred as a function of on stream time of the catalyst between two regenerations(van Swaaij et al.,Chem.Eng.J.,2002,90:2545)Characters of the fixed-bed reactorlIn passing through fixed beds,gases approximate plug flow.If efficient contacting in a reactor is of primary importance,then the fixed bed is favored.lEffective temperature control of large fixed beds can be difficult because such systems are characterized by a low heat conductivity.If operations are to be restricted within a narrow temperature range,then the fluidized bed is favored.lFixed beds cannot use very small sizes of catalyst because of plugging and high-pressure drop,whereas fluidized beds are well able to use small-size particles.lIf the catalyst has to be treated(regenerated)frequently because it deactivates rapidly,then the liquid-like fluidized state allows it to be pumped easily from unit to unit.lThere is no catalyst attrition in the fixed-bed reactor.12Temperature profile of the fixed bed reactor13Exothermic reactionHot spot14.2 Pressure Drop in the Packed Bed Reactor lIn liquid phase,effect of pressure drop can be ignored.lIn gas phase,for an ideal gas,the concentration of reacting species i islFor isothermal operation:140000()(1)iAiiiFFv XTPCXP T 001PPXXvCCiiAi0A0A0,=iiFyF whereSee Section 4.5Changes in total number of moles=Mole of A reactedAnalyze the following second order gas phase reaction that occurs isothermally in a PBR2A B+C lMole balance(must use differential form),lRate law,lStoichiometry,lCombine,lNeed to find(P/P0)as a function of W(or V if you have a PFR).150AArdWdXF2AArk C0011AAXPCCXP22222A0A0A00001111kCkCdXXPXPdWFXPXPPressure drop in a packed bedlP/P0=f(Volume,V)or f(catalyst weight,W)lMajority gas phase reactions happened in a packed bed of catalyst particles.Equation used to calculate pressure drop is the Ergun Eqn.:16 GDDgGdzdPPPC75.11150130003075.111501TTPPCFFTTPPGDDgGdzdP TTFFTTPP0000 is the superficial mass velocityGuErgun Equation170000TTFFTTPPdzdPGDDgGPPc75.111501Given 300cczAW1 0/PPy 0012 PAcc002TTFFTTydWdyb=bulk densityc=solid catalyst density =porosity(void fraction)TT0T0Change in total number of moles for complete conversionTotal moles fedfNNNPressure drop in reactorlFor single reaction:lFor isothermal operation,T=T0 with =0;180(1)2dyTXdWyT 002TTFFTTydWdy210)1(WPPy01TTFXF2dydWy 0021ccAPThis Eqn.is for multiple reactions and reaction in a membrane reactor.Reaction with pressure drop191/200(1)PPWAA00(1)PCCXPNo PPNo PPPNo PWXWWWWPACAr 1.0(a)(b)(d)(c)(e)Effect of pressure drop on P(a),CA(b),-rA(c),X(d),and (e)No PPReaction with pressure drop20AB,2nd order isothermal reactionAArdWdXF0Rate law:2AArk CStoichiometry:Gas phase isothermal reaction with=00y210)1)(1(WXCCAA0(1)AAAFCCXy21)1(WyMole balance:Combining:2122A 02A 0(1)(1)k CdXXWdWFSeparating variables:A 022A 0(1)(1)FdXW dWk CXIntegrating with limits X=0 when W=0 and substituting for FA0=CA00 yields:0A 0(1)(1)2XWWk CX21Solving for conversion gives:A 00A 00(1)21(1)2k CWXk CWWSolving for catalyst weight,we have:1/200112/(1)Ak CXXW 2214.3 Adiabatic fixed-bed reactor23Dependence of-rA on T-rAirreversiblereversibleexothermicTToptTeIn a general form,A+BR+Sk1k212111AB101AB222RS202RS()(,)(,)()(,)(,)ERTERTrk T f CCk ef CCrk T f CCk ef CCFor irreversible reaction:T -rA A1A0A2R0A0A(1)()rk Cxk CCxFor first-order reactions:For reversible exothermic reactions:-rA has a maximum,and E-E+With fixed conversion2414.3.1 Equilibrium temperature and optimum temperature21)(EEHrwhen-rA=0,the temperature is the equilibrium temperature Tewhen-rA gets its maximum,the temperature is the optimum,ToptFor each conversion x:e101AB202RS()ln(,)(,)rHTRk f CCkf CC Te is obtained by setting0)(21rrrAopt101 1AB2022RS()ln(,)(,)rHTRk E f CCk E f CC 0)(dTrdATopt is obtained by setting251212eqoptopteqeqoptln11EEEERTTTTTTC
