Numerical Simulation on a Gas Distributor Used In Slurry Column Reactor Yating CAI

Transcription

1 2nd Internatonal Conference on Electrcal, Computer Engneerng and Electroncs (ICECEE 2015) Numercal Smulaton on a Gas Dstrbutor Used In Slurry Column Reactor Yatng CAI College of Chemcal Engneerng, Bejng Unversty of Chemcal Technology, Bejng, , Chna Emal: cayatng_shrley@163.com Keywords: CFD; Gas dstrbutor; Slurry column reactor; Multphase flow; Reflux Abstract. The slurry column reactor has the most promsng prospect n FT synthess. In ths paper, the gas dstrbutor developed for slurry column rector was analyzed to fnd an mproved gas dstrbutor wth small energy consumpton, good dstrbuton performance and the good characterstcs of preventng backflow. A novel gas dstrbutor whch s employed n the slurry column reactor was proposed and multphase flow patterns were numercally studed. The sngle phase flow patterns of the new slurry column gas dstrbutor were smulated and the smaller preventng-backflow ball was found out. When connect the upper and lower two openngs to form one openng, there s lower pressure drop n the gas dstrbutor, unevenness also reduced accordngly. The overall gas holdup was consstent wth the expermental data. The performance of the preventng-backflow ball was also studed n ths paper. Some desgn requrements were put forward n order to stroke the lqud flow back. Introducton As a key equpment and nternals n a slurry bubble column reactor, gas dstrbutor has a great nfluence on the ntal gas dstrbuton and flow pattern and also promotes the stable operaton of the entre bed. Gas dstrbutor s located at the bottom of the slurry bed, whose man functon s supportng the catalyst on the slurry column bed, unformly dstrbutng the gas, and nhbtng the development of aggregatve fludzaton and mantanng stablty of the bed. At present engneers always take the perspectve of unform dstrbuton nto consderaton to desgn gas dstrbutor n slurry column reactor, whle someone are begnnng to consder the deposton of catalysts [1]. But less research focus on the descrpton and analyss of the reflux causng by suddenly cuttng off the gas source. Wth the contnuous development of computer technology, CFD Smulatons become a recognzed and mportant research method [2] [3] [4]. Krshna [5], Dhotre and Smth [6] had already modeled the slurry column reactor by multphase flow model and studed ts amplfcaton character. Delnoj [7] [8] modeled a large reactor by takng bubbles as dscrete phase and slurry as the contnuous phase, and the break and gatherng of bubbles were taken nto account. Ths paper based on numercal smulaton of the three dmensonal model, systematcally analyss the gas flow feld dstrbuton nsde and the nfluence of the bubble for the flow outsde the gas dstrbutor. Varous parameters on the performance of the general rule were obtaned, and provde basc theory and optmzaton deas for desgnng blster dstrbutor. Experment Detals Fg.1. shows the schematc of the expermental apparatus. These experments were carred out n plexglass columns. The dstrbutors were also made of plexglass, whch was convenent for us to montor expermental phenomenon and operatonal stablty clearly. The expermental workng medum was ar and water. The compressed ar was blown nto the gas dstrbutor layer through the buffer tank, the flowmeter and the solenod valve, and then flowed out of the bottom of the column and nto the surroundng envronment. Ar flow rate was measured by the rotameter, pressure drop were measured by U tube. All experments were carred out at ambent temperature and atmospherc pressure. The nlet flow rate of the monomer was 20, 32, 56, 80, 100m 3 /h respectvely. The The authors - Publshed by Atlants Press 638

2 experment was carred out n a column wth a dameter of 400mm. The expermental results obtaned by Bed Expanson Method could be used to evaluate the overall gas holdup and prove the valdty of the numercal model. Numercal Models Fg.1. The schematc of the expermental system The nature of the flud has a great nfluence n turbulent form. Therefore the CFX software ntroduced a number of turbulence models to meet the calculaton of turbulent flow. In ths paper, ths model s k-ε model. The model s most frequently used n the smulaton turbulence model, whch was found a best balance pont n ts accuracy and the mathematcal equatons. ρ Contnuty Equaton: + ( ρu ) = 0 (1) ρu, T Momentum equaton: + ( ρ U U) ( µ eff U) = p + ( µ eff U) + B (2) Where B s total volume force; µ eff s effectve vscosty; p, s correcton pressure. K, ε values can be solved drectly from the turbulent knetc energy and turbulent knetc energy dsspaton equaton, turbulent knetc energy equaton s as follows: ( ρ U ) µ t + ( ρ Uk ) = µ + k + Pk + ρε (3) σ k ( ρ U ) µ t ε + ( ρuε ) = µ + ε + ( Cε1Pk Cε2ρε ) (4) σ ε k C ε 1, C ε 2, σ k, σ ε s constant. For multphase, group balance model s used n ths paper. Group balance model s based on populaton balance equatons. The populaton balance equaton s as follows: n( mt, ) + ( U ( mt, ) n( mt, )) = BB DB + BC DC (5) x Where B B, D B, B C, DC respectvely represent the brth rate due to breakup of larger partcles, the death rate due to breakup nto smaller partcles, the brth rate due to coalescence of smaller partcles, and the death rate due to coalescence wth other partcles. The group balance composed of dfferent sze n the model equaton s as follows: m + 1/2 ( ) (, ) N t = n m t dm (6) m 1/2 639

3 ( r ) + ( rru d ) = S (7) x where N represent the number densty of sze group, ρ, r respectvely represent densty and volume fracton of sze group, S represents the source term. Breakup models use Luo and Svendsen Model and coalescence models use Prnce and Blanch Model [9] [10]. Geometrc Modelng and Mesh Generaton As shown n Fg. 2a, the man features of the gas dstrbutor n slurry column reactor mentoned n ths paper are the structure whch combned bubble cap structure wth preventng-reflux element, FT synthess reacton n a slurry bubble column reactor belongs to the gas feed. The pressure drop s smaller when the nlet uses a smple concal structure. Meanwhle, the preventng-reflux ball s put nto the ntake-tube, when workng properly and the flow s smooth, whle the flow s nterrupted supply, the preventng-backflow ball can prevent lqud backflow nto the ntake-tube. The dstrbutor uses a bubble cap structure wth slts, and ths specal structure on the one hand can expand the dffuson area and refne the bubbles, more mportantly, ths structure makes gas flow form a baffle at the bottom, whch prevents the deposton of catalysts and ensure the securty of the whole bed. In ths paper, ths geometrc model s the same experment sze. We use geometres and dfferent meshng strateges to study the aspects of the paper, because the research prortes and methods are dfferent. In ths artcle, ANSYS Workbench provded by Asa-Pacfc was used to repar models and mesh geometres. Not all models were used n sngle gas phase flow calculatons, only gas dstrbutor and the lower part of the outer cylnder was used to smplfy the calculaton of the flow feld. In the calculaton of the gas-lqud two-phase model, ths paper made full use of the symmetry of the geometry, usng 1/ 8 geometry and structured grd. Ths means not only mproved mesh qualty but also reduced the number of grd. Therefore,ths artcle ultmately determnes the amount of grd was 266,929, and the maxmum mesh sze was 1mm. As shown n Fg. 2b. Fg.2. The structure and mesh of bubble column reactor gas dstrbuton Calculaton Results The unformty of the gas dstrbuton, the resstance of flow and the amount of the energy loss wll gve nformaton for the evaluaton of the gas dstrbutor. The unevenness s used to judge the ablty of dstrbuton of gas dstrbutor, whch s as follows: n 1 1 u u 2 2 M f = [ ( )] (8) n = 1 u s local value,u s mean value. It s evdent that n an deal unform feld M f =0 and the value u 640

4 ncreases wth the non-unformty of dstrbuton. Fg.3. shows that the gases mostly flow from the upper entrance to the sleeve and the non-unformty of gas flow wll exst when t passes through the small ball. It s nterestng to note that there s more gas flow n the upper entrance than n the lower entrance. Part of the gas flow wll go n to the sleeve from the lower entrance oblquely, whch wll generate relatve obstructon owng to the gas flow wth dfferent flow path and hence cause the chaotc swrl flowng out from the gap. The streamlne pcture we modeled s exactly dentty n ths stuaton. Meanwhle, because of the unsteady and asymmetry of the gas flow n the nner tube, the movng ball shows a chaotc moton whch leads to further exacerbaton of the chaotc flow n the nner tube. Fg.3. The velocty vectors and streamlnes of the flow feld From above analyss, we can see the upper and lower openngs cannot be fully utlzed and there are blockng ar flowng n the openng of annulus. Therefore, the man mprovement n ths work s to reduce the nfluence on each mouth for avodng the stagnaton of ar flow. Ths work s manly focused on two mproved structures (Fg.4.), one of whch attempt to reduce the relatvely closed area va the connecton between the upper and lower openngs so that can decrease the pressure drop and blockage effectvely (Fg.4a.).Another mproved structure s utlzng a larger ball because of the rregularty of beatng and unsteady of the bed we found n the experment (Fg.4b.). Fg.4. The grds of optmzed structures Fg.5. The chart of unevenness and pressure drops It s evdent that the mpact of these three dfferent structures on pressure drop s bascally consstent (Fg.5). The combnaton of upper and lower openngs can mprove the gas congeston and slghtly reduce to the pressure drop. We can note that the pressure drop ncrease wth the ball dameter, whch s maybe caused by the dsspaton of energy produced va the larger vortex generated by the larger sphercal. From the perspectve of ar flow dstrbuton, the performance of dstrbuton wth the combnaton of upper and lower openngs was best n the three types, because t can reduce unevenness by the promoton of turbulence and larger sphercal s tend to generate more non-unform dstrbuton. The calculaton method of the gas holdup s the Bed Expanson Method [11]. As shown n Fg.6 641

5 (a), the overall gas holdup of slurry column reactor ncreases wth ncreasng workng condtons, ths trend was consstent wth some experment mentoned by lterature [12]. Meanwhle, the overall gas holdup was good n lne wth the paper's experment. Ths proved that the model coupled PBM wth Grace Drag Model has a better predctablty on bubblng behavor of the slurry column reactor. Fg.6 (b) suggest that most bubbles were large sze accountng for 70% of the total bubbles, small sze bubbles was about 20%, and medum bubbles only accounted for around 10%. It could be nferred from the chart that under the llustrated workng condtons, bubble szes were dfferentated states, and medum sze of ar bubbles n ths system was more dffcult to exst, most bubbles tended to form the larger bubbles. Ths phenomenon not only provded a reference to the further mprovement of gas dstrbutor cap, but also bult the foundaton for calculatng model and selectng partcles sze on further optmzaton. Fg.6. The overall gas holdup and bubble sze dstrbuton under dfferent workng In the actual ndustral producton, the reflux of lqud phase and sold phase catalyst often occur because of the dsrupton or nstablty generated durng the gas supply, and lead to the breakdown of gas dstrbuton nozzle caused by obstructon. Therefore, the research and evaluaton of the ablty to prevent reflux n dstrbutor s necessary. In order to smplfy reflux calculatons, we calculated the lqud reflux process under steady bubblng wth fxed movng part. Meanwhle, the gas flow through the narrow slt between ball and nner wall wth hgh speed durng the bubblng state, whch generate a downward force target to ball. We take ths force as securty margn and calculated free-fall tme based on t. Then we compare t wth reflux tme to evaluate the performance. Fg 7 shows very lttle lqud go nto the funnel area from 0.07s and lqud flow out from entrance at 0.08s. Therefore, the ball secure tme should be less than 0.07s. Accordng to the free-fall, we calculated the movng dstance durng ths perod tme as follows: 1 2 s = gt (9) 2 By calculatng know, the ball should complete the non-return effect after 24.5mm. Snce the dfference between the lqud phase reflux under the gas velocty of 5.46m/s and others s neglgble, the tme taken by lqud phase refluxng from the outer tube to nner tube s very short, about s, whch put forward hgher requrements on the heght of entrance and the weght of ball. However, consderng the very short reflux tme and low lqud flow velocty, the amount of reflux wll not be too much. The ball s relatvely good non-return ablty can meet the demands. Fg.7. Reflux fgure of ntake velocty of 5.46m/s 642

6 Concluson The new type gas dstrbutor used n FT synthess slurry column reactor was numercally studed n the paper. The result showed that the orgnal structure wth smaller sphere was benefcal to the reducton of the pressure drop and unevenness. Mergng the two openngs was helpful to reduce the blockng. The results generated by PBM model and drag force model all can provde excellent condton of bubbles aggregaton and broken, whch was mostly dentfed wth the expermental results and lterature. In the modelng of preventng reflux, the basc condton was acheved and t takes less than about 0.1s for the lqud phase to reflux to the gas regon, and hence put up a further requrement for the desgn of the structure. References [1] Krshna R, Ellenberger J. Gas hold-up n bubble column reactors operatng n the chamber and a bubble column, Chemcal Engneerng Journal. 2007,125(3): [2] Ranade V V, Tayala Y. Modellng of flud dynamcs and mxng n shallow bubble column reactors: nfluence of sparger desgn, Chemcal engneerng Scence.2001,56: [3] Dhotre M T, Ekambara K, Josh J B. CFD smulaton of sparger desgn and heght to dameter rato on gas hold-up profles n bubble column reactors, Expermental Thermal and Flud Scence.2004, 28(5): [4] Dhotre M T, Josh J B. Desgn of a gas dstrbutor: Three-dmensonal CFD smulaton of a coupled system consstng of a gas chamber and a bubble column, Chemcal Engneerng Journal, 2007,125(3): [5] Krshna R, van Baten J M, Urseanu M I. Three-phase euleran smulatons of bubble column reactors operatng n the churn-turbulent regme: a scale up strategy, Chemcal Engneerng Scence, 2000,55(16): [6] Dhotre M T, Smth B L. CFD smulaton of large-scale bubble plumes: Comparsons aganst experments, Chemcal Engneerng Scence, 2007,62(23): [7] Delnoj E. Kupers J A M, van Swaaj W P M. Computatonal flud dynamcs appled to gas-lqud contactors, Chemcal Engneerng Scence, 1997,52(21/22): [8] Delnoj E, Lammers F A, Kupers J A M, et al. Dynamc smulaton of dspersed gas-lqud two-phase flow usng a dscrete bubble model, Chemcal Engneerng Scence, 1997,52(9): [9] Prnce, M. and Blanch, H., Bubble Coalescence and Break-Up n Ar-Sparged Bubble Columns, AIChE Journal 36, [10] Luo, S.M., and Svendsen, H., Theoretcal Model for Drop and Bubble Breakup n Turbulent Dspersons, AIChE Journal,42, [11] Songwe, Jngyng, Yubo. Revew of slurry column reactor, Tanjn Chemcal Industry.2009, 23(1): (n chna) [12] Thorat B N,Shevade A V, Bhlegaonkar K N, Aglawe R H, Veera U P et al., Effect of Sparger Desgn and Heght to Dameter Rato on Fractonal Gas Hold-up n Bubble Columns, Chemcal Engneerng Research and Desgn, 498, 76(7):