[學(xué)習(xí)]反應(yīng)工程基礎(chǔ)程易chpt15-fluidizedbedrea

1、Chapter 15Fluidized Bed Reactor,Department of Chemical EngineeringTiefeng Wangwangtf@tsinghua.edu.cn,15.1 Introduction,AdvantagesExcellent heat transfer performanceOn-line catalyst replacement, reaction-regeneration

2、 coupled operationFine catalyst particles, with a high effectiveness factorDisadvantagesNon-uniform gas distributionIntensive backmixing of the solid particles, and bypassing of the gasAttrition and entrainment of c

3、atalyst particles,2,Characteristics of the fluidized bed reactor,Selection of the reactor type,3,Reactor type preferred as a function of on stream time of the catalyst between two regenerations (van Swaaij et al., Chem.

4、Eng. J., 2002, 90: 25–45),Notation for a bed of suspended solids,4,or U,or ?p,The minimum fluidization velocity,5,At the minimum fluidization velocity (umf), the following condition is satisfied: the weight of t

5、he 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 beginning to become fluidized.,,6

6、,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:,7,For fine particles, the second term can be negl

7、ected when Rep<10,,For large particles, the first term can be neglected when Rep>1000,,When ?s and ?mf are unknown, the following estimation can be used (Wen and Yu, 1966),,8,When Rep<10 (fine particles):,When R

8、ep>1000 (large particles):,Using these estimations, Umf can be calculated by:,The Ergun eq. in dimensionless form (for ?s=1):,Void fraction at the point of minimum fluidization (?mf),9,Broadhurst and Becker (1975),Wen

9、 and Yu (1966),,Terminal velocity of the particle,10,Correlation proposed by Haider and Levenspiel (1989),For spherical particles,For irregularly shaped particles of sphericity ?s,where the particle sphericity ?s is defi

10、ned as:,Force balance on the particle: weight=drag,11,Drag coefficient and particle terminal velocity,G/S contacting regimes,12,Geldart classification of solids in BFB,13,g/cm3,14,Distribution of solids in the various c

11、ontacting regimes.,,,,,Flow regime diagram,15,Industrial applications of the fluidized bed reactors,16,“自由”流化床,提升管催化裂化,C2H4氧氯化反應(yīng)器,17,Conversion of reactant in BFB is usually poorer than for both plug flow and mixed flow.

12、 Why??,15.2 The Bubbling Fluidized Bed (BFB),The K-L model for BFB,18,Model and symbols used to describe the K-L bubbling gas fluidized bed.,Davidson’s bubble model,19,Davidson and Harrison (1963)The rise velocity of th

13、e bubble ub depends only on the bubble size;The gas behavior in the vicinity of the bubble depends only on the relative velocity of rising bubble and of gas rising in the emulsion ue.,ue (or uf) is the gas velocity in t

14、he emulsion.,Davidson’s bubble model,PostulateA gas bubble is solid-free and circular in shape.As a bubble rises, particles move aside, as would an incompressible inviscid fluid of bulk density ?s(1-?mf)The gas flows

15、in the emulsion phase as an incompressible viscous fluid. (This is used to solve the pressure distribution)The size of the cloud and the ratio of cloud to bubble volume are:Flow rate of gas into and out of a bubble

16、,20,uf is the gas velocity in the emulsion,,,The assumptions of K-L model,The bubbles are all of one size.The solids in the emulsion phase flow smoothly downward, essentially in plug flow, with velocity us.The emulsion

17、 phase exists at minimum fluidizing conditions, thus,In the wakes, the concentration of solids is equal to the concentration of solids in the emulsion phase. The average velocities of both solid and gas in the wake

18、are assumed to be the same and equal to the upward velocity of the bubbles,21,where ue is the velocity of the gas in the emulsion, and us is the velocity of the moving solids.,Important hydrodynamic parameter in K-L mode

19、l,To determine the velocity of the bubble through the bed we need to first calculate:Porosity at minimum fluidization, ?mfMinimum fluidization velocity, umfBubble size, dbTo calculate the mass transport coefficient,

20、we must first calculate?mf, umf, dbVelocity of bubble rise, ubTo determine the reaction rate parameters in the bed, we need to first calculateFraction of the total bed occupied by bubbles, ?Fraction of the bed consi

21、sting of wakes, ??Volume of catalyst in the bubbles, clouds, and emulsion, ?b, ?c, and ?e,22,The rising velocity of bubbles in BFB is:,Bubble velocity ub,23,According to Davidson and Harrison, the rising velocity of a s

22、ingle bubble in a gas-liquid flow is:,This correlation is based on the following facts:The larger the value of u0, the higher the bubble rising velocityThe higher the minimum fluidization velocity Umf, the lower the bu

23、bble rising velocity,Bubble size db,24,where nd is the number of perforations, and Ac is the cross sectional area of the bed.,The relationship developed by Werther：,The relationship developed by Mori and Wen:,where Dt is

24、 the bed diameter, db0 is the initial bubble diameter, and dbm is the possible maximum bubble diameter,For porous plates:,For perforated plates:,Fraction of bed in the bubble phase, ?,25,Material balance on the solids,Ma

25、terial balance on the gas flows,? is the fraction of the total bed occupied by the bubbles (not including the wake)? is the volume of wake per volume of bubble.,,,,,Here cloud is included in the emulsion phase?,26,Wake

26、angle θw and wake fraction of 3-d bubbles at ambient conditions; evaluated from x-ray photographs by Rowe and Partridge. Kunii & Levenspiel, Fluidized Engineering, 2nd ed. (Stoneham, MA: Butterworth-Heinemann, 19

27、91).,Mass transfer in fluidized beds,The gas interchange between the cloud and the emulsionThe gas interchange between the bubble and the cloud,27,Mass transfer between the fluidized-bed phases,28,The gas interchange be

28、tween the bubble and the cloud:,A typical value of Kbc is 2 S–1.,擴散量,穿流量,參考陳甘棠，p219,Based on per unit bubble volume,29,The gas interchange between the cloud and the emulsion:,This correlation is obtained based on the Hig

29、bie penetration model for mass transfer. A typical value of Kce is 1 S-1.,Reaction behavior in a fluidized bed,30,For nth order, constant-volume catalytic reaction,The reaction rate in the cloud:,The reaction rate in the

30、 emulsion:,where ke, kc and kb are the specific reaction rates in the emulsion cloud, and bubble, respectively.,The reaction rate in the bubble phase:,31,Balance on bubble phase,Balance on cloud and wake phase:,Mole bala

31、nce on the bubble, the cloud, and the emulsion,,32,Balance on emulsion phase,Assuming that the derivative terms on the left-hand side of the material balances on the cloud and emulsion are negligible in comparison with t

32、he terms on the right-hand side, then,Partitioning of the catalyst,33,kb, kc, and ke are dependent on the partitioning of the catalyst,where kcat is the specific reaction rate per volume of catalyst.,34,The volume of clo

33、ud and wakes per volume of bubble is,The value of ?b ranges between 0.001 and 0.01, with 0.005 being the more typical number.,35,In essence, given umf, ?mf, u0, ?, and the effective bubble size in the bed db, this model

34、tells you all the other properties of the bed-flows, region volumes, interchange rates, and consequently reactor behavior.,Design equation of the fluidized bed,To arrive at our fluidized-bed design equation for a first-o

35、rder reaction, we simply express both the concentration of A in the emulsion, CAe, and in the cloud, CAc, in terms of the bubble concentration.,36,First, we use the emulsion balance to solve for CAe in terms of CAc.,,37,

36、Solving for CAc in terms of CAb gives:,We now use this equation to substitute for CAe in the cloud balance,,38,We now substitute for CAc in the bubble balance:,,39,,40,Expressing CAb as a function of X,,,41,Resistance to