Influence of Gas Density on Hydrodynamics in a Bubble Column

Transcription

1 Smulaton and Optmzaton Chna Petroleum Processng and Petrochemcal Technology 01, Vol. 16, No. 1, pp March 30, 01 Influence of Gas Densty on Hydrodynamcs n a Bubble Column Tang Xaojn; Hou Shuand; Zhang Zhanzhu (Research Insttute of Petroleum Processng, SINOPEC, Bejng ) Abstract: Two knds of gases were used to nvestgate the nfluence of gas densty on the hydrodynamc characterstcs n a bubble column. It can be found out that hgher gas densty leads to smaller bubble dameter and the flow patterns are dfferent for the both gases. Energy balance mechansms are analyzed by consderng the gas densty dfference. Models were developed to predct the average bubble dameter wth good accuracy. Key words: bubble column; hydrodynamcs, gas densty 1 Introducton Bubble columns are wdely used n chemcal and petrochemcal ndustres where oxdaton [1], hydrogenaton [], neutralzaton [3] and other reactons [-5] are appled. In bubble columns, gas phase s dspersed nto bubbles and lqud phase s a contnuous phase. Thus, the hydrodynamc characterstcs n a bubble column are sgnfcantly nfluenced by bubbles. Untl now, bubble behavors cannot be descrbed clearly due to the complexty of two-phase flow, and the desgn and scale-up of bubble columns are stll very dffcult. Industral applcatons of bubble columns are manly based on a seres of emprcal plot scale tests. Krshna and Se [6] suggested that the combnaton of large cold model experments and small hot model plot tests mght be helpful to the scale-up of bubble columns. For cold model experments, ar at normal pressure and ambent temperature are usually used as the gas phase. However, n most cases the densty of ar s not as the same as that n real reactor under hgh pressure so the applcablty of these cold model expermental results s lmted. Wlknson, et al. [7] found out that the gas holdup ncreases wth the ncrease of gas densty. From ths pont of vew, nvestgatons on the nfluence of gas densty on hydrodynamcs are very essental for the desgn and scale-up of bubble columns based on the cold model experments. In ths study, the hydrodynamc characterstcs n a bubble column were nvestgated. Two knds of gases were used n the experments and the nfluence of gas densty was analyzed. Expermental The scheme of expermental setup s shown n Fgure 1. The bubble column s made of glass, mm n heght and 7 mm n dameter. As shown n Fgure 1, the lqud phase and gas phase are mxed pror to enterng the bubble column at the bottom. In the column, the mxture of gas and lqud frstly goes upward through a bed of ceramc balls, 53 mm n heght, whch s used as the dstrbutor to generate a unform dstrbuton of the gas phase. In the bubble column, the gas phase s dspersed nto bubbles. The upward movng mxture of gas and lqud leaves the column at the overhead, and then enters a gas-lqud separator. In the separator, the gas phase s separated from the lqud phase. The gas phase leaves the separator at the top and the lqud phase leaves the separator at the bottom. Kerosene was used as the lqud phase. Hydrogen and ntrogen were used as the gas phase, respectvely. The physcal propertes of the expermental materals are lsted n Table 1. The experments were conducted at ambent temperature and atmospherc pressure. The superfcal lqud velocty u L was set at m/s and the superfcal gas velocty u g was set n the range of between m/s. Receved date: ; Accepted date: Correspondng Author: Telephone: , E-mal: tangxj.rpp@snopec.com. 66

2 Tang Xaojn, et al. Influence of Gas Densty on Hydrodynamcs n a Bubble Column 3. Flow pattern Fgure 1 Expermental setup of bubble column 1 Gas-lqud separator; Phase nterface; 3 Bubble; Bubble column; 5 Ceramc ball bed Table 1 Physcal propertes of expermental system Item Densty, kg/m 3 Vscosty, Pa s Surface tenson, N/m Kerosene H N The holdup of the gas phase n the bubble column was measured by volumetrc replacement method. The bubble sze s measured by photographc method. Flow pattern can be used to determne the operatng regme n a bubble column. Usually, there are two flow patterns exstng n the column. One s a homogeneous bubbly flow regme at low levels of gas velocty and the other s a churn-turbulent flow regme [6]. In homogeneous bubbly flow regme, the sze of bubbles s unform, whle the gas holdup s low and the dstance between two bubbles s qute long. In ths sense, t s hard for bubbles to coalesce nto bg bubbles. Whle n the churn-turbulent flow regme, the gas velocty and the gas holdup are hgh. It s easy for bubbles to coalesce nto bg bubbles and the bubble sze dstrbuton s wde. The movement of bg bubbles can lead to a turbulent flow n the column. Fgure 3 shows the flow patterns for the both gases under the same superfcal gas velocty. For hydrogen, the bubble sze dstrbuton s much wder than that for ntrogen. It s easy to qualtatvely conclude that the flow pattern of hydrogen s n the churn-turbulent flow regme whle that of ntrogen n the homogeneous bubbly flow regme. 3 Results and Dscussons 3.1 Gas holdup The relatonshp between superfcal gas velocty (u g ) and gas holdup φ s shown n Fgure. It can be dentfed that φ ncreases lnearly wth the ncrease of u g for each knd of gas. When u g s relatvely low (< m/s), φ of hydrogen s hgher than that of ntrogen. When u g s hgh enough (> m/s), φ of ntrogen s hgher than that of hydrogen. Fgure Relatonshp between u g and φ Fgure 3 Flow patterns n bubble column (at u g = m/s) A quanttatve method can be used to determned the flow pattern n a bubble column by the relatonshp between slp velocty and gas holdup [8-9]. The slp velocty V s s defned by Eq. 1. When V s ncreases wth the ncrease of φ, the flow pattern n the column s n the churn-turbulent flow regme. When V s decreases wth the ncrease of φ, the flow pattern n the column s n the homogeneous bubbly flow regme. V s ug ul = (1) ϕ 1 ϕ Fgure shows the relatonshp between V s and φ. It can be found out from Fgures 3 and that n the whole ex- 67

3 Chna Petroleum Processng and Petrochemcal Technology 01,16(1):66-70 permental range, the flow pattern of hydrogen s exactly n the churn-turbulent flow regme and that of ntrogen n the homogeneous bubbly flow regme. Fgure 5 Relatonshp between d 3 and u g Fgure Relatonshp between V s and φ 3.3 Average bubble dameter In a bubble column, the average bubble dameter s usually calculated by Eq. as the Sauter mean dameter d 3. d 0 3 = = = 0 n d n d 3 () The relatonshp between d 3 and u g s shown n Fgure 5. The value of d 3 ncreases wth the ncrease of u g for both gases. Under the same gas velocty, the value of d 3 of hydrogen s hgher than that of ntrogen. Accordng to the theory of sotropc turbulence, the average bubble dameter s determned by the balance of mechancal energy and surface energy n a bubble column. Moreover, the average bubble dameter s proportonal to the characterstc turbulent length. The process of gas beng dspersed nto bubbles s also a process for energy converson from mechancal energy to surface energy. The parameter energy dsspaton rate ε can be used to measure the level of mechancal energy nput and ε can be calculated by Eq. 3, n whch ρ g s the densty of gas phase, ρ L s the densty of lqud phase. It can be found from Eq. 3 that under the same physcal propertes ε s the only functon (u g /φ) that s assumed to be much less than 1. Fgure 6 shows the relatonshp between d 3 and u g /φ whch can be used to measure the level of ε. 1 ρ ε = 1 l ug g (3) ϕ ρg It can be found from Fgure 6 that the value of d 3 ncreases wth the ncrease of ε for hydrogen whle the value of d 3 decreases wth the ncrease of ε for ntrogen. Because the relatonshp for the case of hydrogen s opposte to that for the case of ntrogen, there mght be dfferent energy converson mechansms relatng to both gases. For ntrogen, t s assumed that there s a balance between knetc energy and surface energy. In ths sense, the Weber number (We) n the column s constant as shown n Eq.. In Eq., σ means the surface tenson, u s the characterstc turbulent velocty as shown n Eq. 5. By combnng Equatons and 5, d 3 can be obtaned from Eq. 6. By fttng the expermental data, the parameter K n Eq. 6 s determned to be Fgure 7 shows the comparson between the expermental and calculated values of d 3. It can be found out that Eq. 6 could predct the value of d 3 of ntrogen wth good accuracy. ρlu d3 We = = constant () σ 1 3 u = ( ε d3) / (5) σ 1 ρ l d3 = K 1 ug g (6) ρl ϕ ρg As already mentoned above, the energy converson mechansm for hydrogen s dfferent from that for ntrogen. Consderng that the densty of hydrogen s only one tenth of that of ntrogen, the energy nput level s much lower than that of ntrogen. Thus, hydrogen can hardly form lttle bubble system whch needs more energy n the column and the bubbles are relatvely larger than the case of ntrogen. In ths way, t s assumed that there s a balance between potental energy and surface energy. The Bond number (Bo) n the column s constant as shown n Eq. 7. In Eq. 7, η s the characterstc turbulent length. By combnng Eqs. 7 and 8, Eq. 9 s obtaned to calculate 68

4 Tang Xaojn, et al. Influence of Gas Densty on Hydrodynamcs n a Bubble Column Fgure 6 Relatonshp between d 3 and (u g /φ) Fgure 7 Comparson between expermental and calculated values of d 3 (of ntrogen) exp; cal the value of d 3. By fttng the expermental data, the parameter C n Eq. 9 s derved to be Fgure 8 shows the comparson between the expermental and calculated values of d 3. It can be found out that Eq. 9 could well predct the tendency on the value of d 3 of hydrogen. d ρlgd3 η Bo = = constant (7) σ 3 µ η = 3 L k 3 ρlε 1 1 σ u = C 1 g 3 3 ϕ ρ gg µ L 3. Influence of gas densty on hydrodynamcs From the dscusson n the prevous parts, we can learn that gas densty has an mportant nfluence on the hydrodynamc characterstcs n the bubble column. Hgher densty leads to smaller bubble dameter and homogeneous bubbly flow regme. For most bubble columns, the energy nput s manly dependent on the gas phase, and so the gas 1 (8) (9) Fgure 8 Comparson between expermental and calculated values of d 3 (of hydrogen) exp; cal densty determnes the level of mechancal energy nput by the gas phase to some extent. From ths pont of vew, the energy balance mechansm n the column s determned by the gas densty and also the flow pattern s determned. In ndustral bubble columns, a hgh densty of gas phase resulted from the hgh operatng pressure leads to small bubbles and possbly homogeneous bubbly flow regme. Actually, t s expected that the transfer processes (mass or heat) should be enhanced to create a churn-turbulent flow regme. Thus, hgh superfcal gas velocty should be provded to rase the gas holdup, promote bubble coalescence, produce larger bubbles, and form the churn-turbulent flow regme. Conclusons Hydrogen and ntrogen were used to nvestgate the hydrodynamcs n a bubble column. Based on the expermental data, the nfluence of gas densty on the hydrodynamc characterstcs was analyzed. Hgher gas densty leads to smaller bubble dameter and the flow pattern s n the homogeneous bubbly regme. Energy balance mechansms are dfferent due to the densty dfference between hydrogen and ntrogen. Models were developed to predct the average bubble dameter based on the energy balance mechansms. Acknowledgment: Ths work was fnancally supported by the Natonal Key Basc Research Development Program 973 Project (01CB806) of Chna. References [1] Lucas M S, Peres J A, Puma G L. Treatment of wnery wastewater by ozone-based advanced oxdaton processes 69

5 Chna Petroleum Processng and Petrochemcal Technology 01,16(1):66-70 (O 3, O 3 /UV and O 3 /UV/H O ) n a plot-scale bubble column reactor and process economcs[j]. Separaton and Purfcaton Technology, 010, 7(3): 35-1 [] Herrmann U, Emg G. Lqud phase hydrogenaton of malec anhydrde to 1,-butanedol n a packed bubble column reactor [J]. Ind Eng Chem Res, 1998, 37(3): [3] Tokumura M, Baba M, Znad H T, et al. Neutralzaton of the acdfed seawater effluent from the flue gas desulfurzaton process: φ expermental nvestgaton, dynamc modelng, and smulaton[j]. Ind Eng Chem Res, 006, 5(18): [] Deckwer W D, Alper E. Katalytsche suspensons reacktoren[j]. Chem Eng Tech, 1980, 5(3): [5] Shah Y T, Kelkar B G, Godbole S P, et al. Desgn parameter estmaton for bubble column reactors[j]. AIChE J, 198, 8(3): [6] Krshna R, Se S T. Desgn and scale-up of the Fscher- Tropsch bubble column slurry reactor[j]. Fuel Processng Tech, 000, 6(1/3): [7] Wlknson P M, Spek A P, Derendonck L L. Desgn parameters estmaton for scale-up of hgh-pressure bubble columns[j]. AIChE Journal, 199, 38(): 5-55 [8] Tang X, Luo G, Wang J. Mechansm analyss on the twophase flow characterstcs n coalescence-dsperson pulsedseve-plate extracton columns[j]. Ind Eng Chem Res, 008, 7(3): [9] Smonnet M, Gentrc C, Olmos E, et al. Expermental determnaton of the drag coeffcent n a swarm of bubbles[j]. Chem Eng Sc, 007, 6(3): The Project Development and Applcaton of FCC Addtve for Maxmzaton of Propylene and Isobutylene Passed SINOPEC s Apprasal The research project Development and applcaton of FCC addtve for maxmzaton of propylene and sobutylene jontly undertaken by RIPP, the Balng Petrochemcal Company (BPC) and the SINOPEC Catalyst Branch Company has on January 15, 01 passed the techncal apprasal sponsored by the Scence and Technology Dvson of SINOPEC. The techncal personnel engagng n ths project based on the research on the reacton chemstry for ncreasng the precursors of propylene and sobutylene has formed a techncal dea on catalysts for maxmzng propylene and so-butylene. The ZSP- zeolte wth the MFI structure featurng hgh utlzaton of actve stes and the HSB-l zeolte wth the REA structure characterstc of hgh sobutylene selectvty and hgh hydrothermal stablty were developed, and the yeld of propylene and sobutylene can be selectvely boosted by adjustng the rato of the above-mentoned two types of zeoltes. In the meantme, a metallc alumnum phosphate matrx materal that can enhance the n-butene somerzaton ablty has also been developed. The above-mentoned nnovatve technology has gven rse to the addtve FLOS capable of ncreasng the yelds of propylene and sobutylene. The producton of the addtve FLOS features a ratonal producton flow dagram that can mantan a stable product qualty wthout causng any envronmental problems. The commercal test for applcaton of these addtves n the 1.05 Mt/a FCC (MIP-CGP) unt whch runs on a mxed feedstock composed of mostly paraffnc atmospherc resduum at BPC has revealed that the unt has been operatng smoothly wth these addtves. When the amount of the addtve FLOS accounted for 6% of catalyst nventory, the LPG yeld ncreased by.68 percentage ponts, among whch the propylene yeld experenced an 1.01% ncrease and the sobutylene yeld a 0.5% ncrease. Furthermore, the olefn content n gasolne was reduced coupled wth an ncrease n octane ratng of gasolne along wth a defnte reducton of desel yeld, resultng n a yearly ncremental economc benefts amountng to mllon RMB to the producton enterprse. 70