[學(xué)習(xí)]反應(yīng)工程基礎(chǔ)程易chap11-15復(fù)習(xí)

1、期末復(fù)習(xí),第11-15章,2,Chapter 11Catalysis and Catalytic Reactors,參考教材： H. S. Fogler, Elements of Chemical Reaction Engineering,3,Taking part in the reaction Altering the rates of reactions by promoting a different mechanism f

2、or the reactionReturning to its original form (In practice a catalyst deactivates gradually during use)The use of catalyst DOES NOT vary DG & Keq values of the reaction concerned, it merely change the PACE of the p

3、rocess.,Definition of catalystA catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged.,4,Steps in a catalytic reactions (cont’d),Mass transfer (external diffusion) of reactan

4、ts from the bulk fluid to the external surface of the catalyst pelletDiffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surfaceAdsorption of rea

5、ctant A onto the catalyst surfaceReaction on the surface of the catalystDesorption of the products from the surfaceDiffusion of the products from the interior of the pellet to the pore mouth at the external surfaceMa

6、ss transfer of the products from the external pellet surface to the bulk fluid,,,,,,5,總結(jié)：在推導(dǎo)反應(yīng)機(jī)理時，吸附、表面反應(yīng)、脫附速率均按基元反應(yīng)處理，例如分子吸附、解離吸附、單活性位表面反應(yīng)、雙活性位表面反應(yīng)；達(dá)到穩(wěn)態(tài)時，各個步驟的速率相等；利用活性位衡算，總活性濃度為常數(shù)。,6,Is the surface reaction rate-lim

7、iting?,The rate of surface reaction:,Utilize the adsorption and desorption steps to eliminate CC?S and CB?S:,Site balance:,,,,7,The initial rare is,At low partial pressure of C:,At high partial pressure of C:,Adsorption

8、of Cumene (C) is the rate limiting step,Surface reaction is the rate limiting step,Benzene desorption is the rate-limiting step,n-pentene ? i-penteneSurface reaction is rate limiting step.,9,10,Chapter 12External Dif

9、fusion Effects on Heterogeneous Reactions,11,Molar flux,Two contributions to molar flux:(1) Molecular diffusion flux JA(2) Convective flux resulting from the bulk motion BA,In terms of concentration for constant total

10、concentration:,Mass transfer to a single particle,molar flux to catalyst surface = reaction rate on surface,,Fast Reaction Kinetics,Fast reaction kinetics,to increase kc,Molar balance in a packed-bed reactor,14,In –

11、 Out + Generation = Accumulation,rA"= rate of generation of A per unit catalytic surface area, mol/s?m2ac = external surface area of catalyst per volume of catalytic bed, m2/m3? = void fractiondp = particle

12、 diameter. mAc = cross-sectional area of tube containing the catalyst, m2,15,The molar flow rate of A in the axial direction is,If dispersion is negligible,,To simplify further the case studies, assume constant U,,U is

13、 the superficial molar average velocity through the bed, m/s,Boundary condition (Assuming reaction is at steady state):,Then,Integrating with boundary condition: at z=0, CA=CA0,,Thus,Analytical solution,Mass transfer lim

14、ited CA >> CAs,,Parameter sensitivity,Example 11-4For a mass transfer-limited reaction,,17,X=?,,,,,v0,V0/2,X=0.865,v0,X=?,V0/2,The time needed to fully regenerate a coked catalyst particle by consuming all carbon,

15、 tc, is,Shrinking core model,,For a 1 cm diameter pellet with a 0.04 volume fraction of carbon, the regeneration time is the order of 10 s.,The time necessary for the carbon solid interface to recede inward to a radius R

16、 is,19,Chapter 13Diffusion and Reactions,Effective Diffusivity,20,21,dimensionless form,,22,The Thiele modulus ?n,Internal Effectiveness Factor,23,Actual overall rate of reaction (moles per unit time):,,Internal effecti

17、veness factor for the first order reaction:,24,Rate without any diffusion effects,=1 for no diffusion resistance,This sphere expression is a good approximation for all particle shapes,Characteristic length,? = Effective

18、factor, which accounts for the resistance to pore diffusion,?1 = Thiele modulus, useful for predicting reactor behavior from known kinetic information, thus known k’’,CWP = Weisz modulus, useful for predicting experiment

19、s since it only includes observations,Effective diffusion coefficient in porous solids,with,where,,,,,,,,,CWP >> 1, severe diffusion limitationsCWP << 1, kinetically limited,Falsified kinetics,25,A log-log p

20、lot of the measured rate of reaction -rA, as a function of the gas-phase concentration CAs, (external mass transfer is eliminated, thus CAs=CAb),Overall Effectiveness Factor,When both internal AND external diffusion resi

21、stances are important (i.e., the same order of magnitude), both must be accounted for when quantifying kinetics.It is desired to express the kinetics in terms of the bulk conditions, rather than surface conditions:,Over

22、all Effectiveness Factor,reaction rate(internal & external surfaces),mass transport rate,internal surfaces not all exposed to CAs,,Relation between CAs and CA defined by the ? as:,Using Molar rate of mass transfer =

23、 total rate of reaction,Then,,Overall Effectiveness Factor,Summary of factor relationships:,Rearranging the expression:,Overall Effectiveness Factor (?)This eq. is for first-order reaction,29,Chapter 14Fixed Bed Reacto

24、r,參考教材：O. Levenspiel, Chemical Reaction Engineering 陳甘棠，化學(xué)反應(yīng)工程（第三版),,For reversible exothermic reactions: -rA has a maximum, and E->E+,Adiabatic fixed-bed reactor,30,Dependence of -rA on T,I

25、n a general form,,For irreversible reaction: T? -rA ?,For first-order reactions:,31,Equilibrium temperature Te and optimum temperature Topt,when -rA=0, the temperature is the equilibrium temperature Tewhen -

26、rA gets its maximum, the temperature is the optimum, Topt,For each conversion x:,Te is obtained by setting,Topt is obtained by setting,32,,,,Curves of Te and Topt for reversible exothermic reactions,The relationship betw

27、een the equilibrium and optimum temperatures:,For a specific conversion, the same reaction rate corresponds to two temperatures, one smaller than Topt, and the other larger than Topt,,Optimization of the operations of s

28、taged packed bed,33,,,,,,,,,T0,T1’, x1,x1,T1,x3,To get the minimum, the following conditions should be fulfilled:,T2’, x2,x2,T2,優(yōu)化變量,Ti為i-1級入口溫度,xi為 i 級出口轉(zhuǎn)化率,34,以上條件的物理含義：第 i 級出口反應(yīng)速率與第 i+1級反應(yīng)速率相等第 i 級入口溫度是第 i 級出口轉(zhuǎn)換率的最優(yōu)

29、溫度,Two-stage adiabatic packed bed reactors,35,Optimum two-stage packed bed reactor.,The optimization of operations reduces to minimizing the total amount of catalyst needed to achieve a given conversion.,36,Chapter 15Fl

30、uidized Bed Reactor,參考教材：H. S. Fogler, Elements of Chemical Reaction Engineering 補(bǔ)充讀物,The minimum fluidization velocity,37,At the minimum fluidization velocity (umf), the following condition is satisf

31、ied: the weight of the solids = the drag on the particles by the gasAll parameters at umf are characterized by the subscript “mf,” to denote that this is the value of a particular term when the bed is just begi

32、nning to become fluidized.,,38,The weight of the bed,The pressure drop across the bed,The weight of the bed,The pressure drop across the bed (Ergun Eq.),where the particle sphericity ?s is defined as:,39,Conversion of re

33、actant in BFB is usually poorer than for both plug flow and mixed flow. Why??,The Bubbling Fluidized Bed (BFB),The K-L model for BFB,40,Model and symbols used to describe the K-L bubbling gas fluidized bed.,41,,42,,,,祝大家