Transient Thermal Response in a Fluidized Bed Reactor. Abstract

Transcription

1 ansient hemal Response in a Fluidized Bed Reacto Ese Camcioglu and Daen E. Daugaad Mechanical Engineeing Depatment he Univesity of exas at San Antonio Session A Abstact he objectives of this investigation ae to detemine the heating time equied fo a ceamic-lined fluidized bed eacto to each a steady state tempeatue when stating at oom tempeatue and to study the unsteady themal eaction of injecting wate into the eacto afte eaching initial steady state conditions. Numeical methods fo this investigation ae veified though expeimental methods pefomed at he Univesity of exas at San Antonio (USA) in the Powe Dynamics Systems Laboatoy (PDL) using the fluidized bed potion of the Biomass Pyolysis System. he fluidized bed eacto was modeled using MALAB softwae and its Patial Diffeential Equation (PDE) tool box. Vaious heate settings wee used in each test anging fom a nominal 500 watts to nea 900 watts. he diamete of the fluid bed is 9 cm with a height of 20 cm containing sand with a nominal paticle diamete of 400 µm. his investigation povides insight into the heating time of the eacto. Also, modeling the eacto and compaing the calculated esults to the expeimental esults aids in the design of fluidized beds fo vaious pocesses, which may use diffeent types of insulations, sand sizes, o fluidizing gases. Compaable esults wee obtained between the numeical model and expeimental studies. Backgound Intoduction his study expeimentally and computationally investigates the tansient heating chaacteistics of the fluidized bed located in the Powe Dynamics System Laboatoy (PDL) at he Univesity of exas at San Antonio (USA). his study was undetaken to gain moe insight into this type of fluidized bed and to undestand the tansient esponse of the fluidized bed while heating and injecting wate. Wate was selected as the fluid to simulate the injection of biomass paticles duing actual pyolysis tests. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

2 Gas Exit Sand Plenum Distibuto Plate Compessed Gas Supply Figue : Main fluidized bed components Fluidized bed eactos ae usually vetical cylindical shells that can be insulated and ae often used fo chemical tansfomation of substances. Fluidized beds can be used to enhance themochemical changes of solid biomass paticles into a mixtue of poducts. In fast pyolysis, the eactions poduce cha, liquid and non-condensable gases that can be sepaated by cyclones and condenses 2. In Figue, the citical components of a fluidized bed eacto ae detailed. Pessuized gas passes though a ound sinteed plate into the fluid bed fom a plenum connected to a gas supply. An inet caie gas is often used fo the application of biomass convesion via pyolysis into bio-oil. he pupose of having a sinteed plate is to supply gas unifomly acoss the bed coss-section at low gas velocities into the sand bed. Note, fluidization is defined as a pocess in which fine solid paticles inteact with flowing gas esulting in the solid paticles behaving as a fluid. In fact, movements of the solid paticles in a fluid bed act vey similaly to a boiling liquid. he impotance of this study hee is still a need fo undestanding the heating chaacteistics of the fluid bed in ode to design commecial size fluid beds to pefom these convesions. Histoically, scaling poblems esult with fluid bed design 4 because of a lack of undestanding of the heating chaacteistics. his investigation claifies the heating chaacteistic of a fluid bed though the use of numeical simulations and expeiments. Also, the numeical simulation can be modified to scale up a fluid bed to pedict the heating chaacteistics of the commecial size fluid beds. he Univesity of exas at San Antonio s (USA) bench type fluidized bed eacto is pimaily utilized in the expeimental wok of this study. his fluidized bed eacto has a total of sixteen themocouple and pessue pots fo system monitoing. his eacto is heated by eight Watlow electical catidge heates. Each heate od has a length of eight inches and a maximum powe capacity of 000 Watts at 240 Volts AC. he heates ae configued vetically aound the sand bed in 45 incements. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

3 Methods Enegy flow in the fluidized bed he enegy equation 5 fo the system descibed by Figue 2 is shown by Equation (), which is applied to the fluidized bed. he left side of the equation epesents the ate of enegy stoage tems fo the sand bed and suounding ceamic namely de sand and deceamic. dt dt & H Ai, out Q & Loss Fee boad E ceamic E sand Figue 2: Enegy flow in the fluidized bed he pupose of having the ceamic mateial at the oute suface of the bed is to keep enegy stoed in the sand bed egion of the fluidized bed eacto. Note, heat is stoed in the ceamic egion fom electical heates and povides tansient heat addition at the suface of the sand bed. desand deceamic + = + W& elect Q& Loss + H& Ai, in H& Ai, out () dt dt he ight side of the equation is epesented by W & elect, the electical powe input to the heates; Q & Loss, the heat loss due to fee convection heat tansfe on the oute suface of the fluidized bed eacto; H & Ai, in, the enthalpy ate of the ai, at oom tempeatue enteing the sand bed and H & Ai, out, the enthalpy ate of ai exiting the fluid bed eacto. ypically inet gases ae used in fluid beds as a caie gas. Howeve, in this investigation, oxygenated ai was used because of accessibility to compessed ai and little possibility of significant eactions such as combustion. Fluidized bed model development H Ai, in W & elect In ode to develop a finite element model fo the fluidized bed, thee ae multiple steps that must be taken into account. hese steps include selecting a coodinate system fo the model, selecting the geomety of the model, deciding on govening equations fo the main calculations, selecting pope bounday conditions, discetization of the main equations and the bounday conditions, detemining the popety constants, and then developing a compute pogam that will calculate Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College &

4 Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College the desied vaiable. he tempeatue pofile is the pimay vaiable sought in this fluidized bed model. Fluid beds usually have a cylindical shape; thus, the heat diffusion equation in cylindical coodinate system is utilized. he heat equation, illustated as Equation (2), suggests the tempeatue () is a function of adial (), cicumfeential (θ), and axial (z) diections and time (t). = t C q z k z k k p ρ θ θ & 2 (2) with = (, θ, z, t) Fou diffeent vaiables, thee of which ae spatial (, θ, and z) and the fouth tempoal (t), make the computation time of the CPU potentially lage 5. he tempeatue in this model is consideed constant in the axial diection. As a esult, the z diection in Equation (2) can be eliminated making the tempeatue pofile a function of the adial (), cicumfeential (θ) and time (t) as shown in the Equation (). = + + t C q k k p ρ θ θ & 2 () =(, θ, t) Geomety of the fluidized bed model Figue shows the two dimensional top view of the modeled fluidized bed consisting of eight electical heate ods located in a cicula ing spaced 45 apat. he inne ceamic (2) and oute ceamic (4) egions ae shown in the Figue 4 noting that they have diffeent mateial popeties, which ae consideed in the model. Figue : op view of fluidized bed model. Sand bed, = 4.5 cm 2. Inne ceamic egion, = 8.4 cm. Heate, = 0.79cm 4. Oute ceamic egion, = 2.5cm 2 4

5 Backgound of the Patial Diffeential Equation (PDE) oolbox he Patial Diffeential Equation (PDE) oolbox is a softwae that woks in conjunction with MALAB. his pogam povides a pepocesso and defines a patial diffeential equation poblem. It also ceates the two dimensional egions, defines bounday conditions and defines patial diffeential equation coefficients 6. In addition, it geneates fee meshes, discetizes the patial diffeential equations, solves the discetized equations numeically and also visualizes the esults in gaphs o by animating the esults. ρ C ( k ) = Q& + h( ext ) (4) t he PDE ool Box can handle paabolic and hypebolic patial diffeential equations as well as eigenvalue poblems. Howeve, in this investigation the heat equation coelates with paabolic diffeential equation type as shown in Equation (4). Mesh view of fluidized bed eacto Y axis (metes) X axis (metes) Figue 4: Mesh view of fluidized bed eacto Figue 4 is the mesh of the fluidized bed eacto obtained with MAHLAB PDE ool Box. In this mesh tiangula type of mesh is used with a chaacteistic dimension of 2 mm. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

6 . Ceate the geomety of the fluidized bed 2. Define the PDE coefficients. Meshing of the fluidized bed as shown in Figue PDE solve. Gaphical output of the fee boad model 0. MALAB finite element model of the fee boad egion 9. Cuve fit the solution and detemine the time dependent bounday condition equation 5. Gaphical output 6. Expot the solution to the MALAB wokspace 8. Wate injection calculations 7. Ai calculations Figue 5: Finite element model development with PDE ool Box and MAHLAB he flowchat shown in Figue 5 illustates the steps that wee taken in the numeical analysis. Expeimental Five themocouples ae located at the bottom, top and at the exit of the bed, as well as at a point at the inne and oute ceamic insulation of the fluidized bed. Eight Watlow Fieod catidge heates having a length of 20 cm, a diamete of.59 cm and (240 Volt, 000 Watts) wee used as a heat souce in the fluidized bed eacto. he voltage was contolled by a powe contolle unit made by Payne Engineeing (model of 8-D-2 0i) 7. he wate flow ate was calibated befoe and measued duing each expeiment. he pocedue to detemine the wate flow ate is to measue the initial mass of wate at time zeo and then measuing the wate emaining afte the elapsed time. he mass flow ate of wate was assumed steady and unifom as it was injected into the fluid bed. est pocedue: he inlet aiflow was set at.87 kg/h into the fluidized bed in each expeiment. he voltage and the cuent of each heate was measued by a voltmete and ammete to detemine the heat addition to the unit at pedetemined setting. he fluidized bed eacto was heated until the bed tempeatue eached steady state. Wate was then injected epesenting the endothemic natue of biomass pyolysis. Also, wate popeties can be obtained easily in most themal science books. he wate flow ate of 0.59 kg/h was calculated as the themal load equivalent to the selected biomass flow ate of 2.0 kg/h. he expeiment continued until the bed eached a seconday steady state tempeatue, and it is defined as / t which is 0.2 C pe minute. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

7 Results and Discussion. Expeimental Results A total of six expeimental uns wee completed to detemine the steady state heating time and the length of the unsteady time while injecting wate into the fluidized bed eacto. hee of the uns wee successful out of the six attempted. he successful uns wee numbeed 2, 5, and 6, howeve fo bevity only Run 6 will be discussed in detail. Note that Runs 2 and 5 yielded simila esults. In Run 6 (Fig. 6), the fluidized bed eached the steady state tempeatue 49 C in about 6.8 hous and the fluidized bed tempeatue eached the seconday steady state in an additional.98 hous and tempeatue deceased to 4 C P=69 Watts Wate injection begins 500 empeatue (Celsius) Inne ceamic tempeatue pofile 2. Fluidized bed tempeatue pofile 00. Fluidized bed tempeatue pofile 4. Oute ceamic tempeatue pofile 5. Ai exit tempeatue pofile ime (Hous) Figue 6: empeatue pofile of the fluidized bed duing Run # 6 Numeical Results All input data utilized epesents Run 6 including the heate powe, which was 79.9 watts pe heate. he suface bounday condition was set as a convective bounday condition with a convection coefficient of 5 W/(m 2 -K). Figue 7 shows the simulation esult of Run 6. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

8 Figue 2: Finite element model (PDE) esults fo Run # 6 Compaison of Expeimental and Numeical Methods he compaisons of heating chaacteistics of the fluidized bed ae shown in able. he numeical esults have a faste heating esponse compaed to the actual expeimental uns. One possible eason fo this esponse is heat loss in the axial diection. able : Numeical and expeimental esults when heating the fluidized bed Numeical esults Expeiment esults empeatue C ime in hous empeatue C ime in hous Run Run he numeical and expeimental esults of wate injection into the fluidized bed ae shown in able 2. he finite element model eached the seconday steady state at a highe tempeatue compaed to actual expeimental uns. able 2: Numeical and expeimental esults of wate injection into the fluidize bed Numeical esults of wate injection Expeimental esults of wate injection empeatue C ime in hous empeatue C ime in hous Run Run he finite element model solution was impoved as shown in Figue 8. In ode to impove the solution, one of the paametes of the model was adjusted. he paamete epesenting the conductivity of sand was selected as.9 W/m-K, which coesponds to the paticle conductivity Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

9 and it was utilized in all pevious esults. he volume aveage bulk sand conductivity. W/m- K was used instead of the sand paticle conductivity. Also, the heat tansfe coefficient of the fluidized sand was adjusted to 50 W/m 2 -K. Figue 8: Compaison plot s of numeical esults vesus Run 6 esults with k coection he tempeatue of the fluidized bed model eached the steady state of 504 C in 6.70 hous, and in Run 6 the tempeatue eached the steady state 490 C in 6.70 hous. he fluidized bed tempeatue eached the seconday steady state tempeatue of 2 C in.8 hous afte injecting wate. he esult was veified with expeimental data which was obtained in Expeiment 6, and this was the tempeatue of C in.8 hous. Summay and Conclusions his investigation studies the heating time equied each a steady state bed tempeatue in a fluidized bed eacto. Also investigated is the seconday steady state esulting fom injecting wate into the fluidized bed. he numeical model of the fluidized bed was successfully completed. In ode to detemine the tansient tempeatue pofile of the fluidized bed eacto, the heat equation in cylindical coodinates was solved. he investigation of wate injection into the fluidized bed also showed that the biomass pyolysis system at he Univesity of exas at San Antonio (USA) should opeate at the mass flow ate of 0.2 to 0. kg/h of wate to avoid the lage tempeatue declines. hese wate flow ates ae equivalent to mass flow ates of 0.68 to.02 kg/h of biomass. Heate powe settings of 80.2 to 89. watts should be utilized to pevent damage of heates, which can occu at heate tempeatues of 020 C. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College

10 he heating time of the fluidized bed eacto was detemined by using numeical simulations and these esults wee veified with expeimental data. he numeical simulations wee impoved by using the bulk conductivity of sand instead paticle conductivity. Refeences. Danny Lathouwes and Josette Bellan, Model of Pyolysis of Biomass in a Fluidized Bed Reacto, NASA ech Bief Vol. 25, No. 6, JPL New echnology Repot NPO-20708, June Daen Daugaad and Robet C. Bown, Enthalpy fo Pyolysis fo Seveal ypes of Biomass, Enegy and Fuels, 7, 2 May 200, pp Daizo Kunii and Octave Levenspiel, Fluidization Engineeing, Buttewoth Heinemann, 2 th -ed, D,Geldat Gas Fluidization echnology, John Wiley & Sons, Inc, st -ed, Yogesh Jaluia and Kenneth E. oance, Computational Heat ansfe, aylo & Fancis, 2 th -ed, he MathWoks, Inc., Patial Diffeential Equation oolbox Use s Guide, he MathWoks, Payne Engineeing, Poduct Bochues of Solid State Powe Contolles, Payne Engineeing. ESER CAMCIOGLU M. Camcioglu is a ecent gaduate of the Mechanical Engineeing Mastes of Science pogam at he Univesity of exas at San Antonio. DAREN E. DAUGAARD D. Daugaad cuently seves as an Assistant Pofesso of Mechanical Engineeing at he Univesity of exas at San Antonio. His eseach inteests include bioenewable enegy and geneal themal systems. Poceeding of the 2006 ASEE Gulf-Southwest Annual Confeence Southen Univesity and A&M College