Temperature distribution and control in liquefied petroleum gas fluidized beds

Journal of University of Science and Technology BeijingVolume ll, Number 3, June 2004, Page 202MetallurgyTemperature distribution and control in liquefied petroleum gas fluidized bedsLi Wang", Ping Wu), Yanping Zhang, Jing Yang", and Lige Tong")1)Mechanical Engineering School, University of Science and Technology Beijing. Beijing 100083, China2)Applied Science School, University of Science and Technology Beijing, Beijing 100083, ChinaReceived 2003-06-15)Abstract: Temperature distribution and control have been investigated in a liquefied petroleum gas(LPG) fluidized bed with hollowcorundum spheres(Al,O, )of 0.867-1.212 mm in diameter at moderately high temperatures(800-1100C). Experiments were carriedout for the air consumption coefficient a in the range of 0.3 to 1.0 and the fluidization number n in the range of 1.3 to 3.0. Particleproperties, initial bed height, a and N all affect temperature distribution in the bed. Bed temperature can be adjusted about 200oC bycombined the adjusting of a and N.Key words: liquefied petroleum gas; fluidized bed; temperature distribution[This work was financially supported by the Key Project Foundation for Science and Technology Research by the Education Ministryof China (No 00020).Fluidized-bed technology can be widely used with in a LPG fluidized bedliquefied petroleum gas(PG)in a wide range of ap-plications, e. g. co-firing, gas re-burn and direct com-I Experimentalbustion in heating fluidized beds in some industrialFigure 1 shows the schematic of the experimentprocesses. In fluidized beds, high-combustion effisystem. The gas fluidized bed furnace is a rectangularciency can also be achieved at a remarkably low temperature(about 1000C)as compared with conventional devices(over 1200C), and the fluidized bedsare capable of meeting all environmental requirementsComputer20Spare information is available in the literature con1516cerning the combustion mode and regime of fluidiza17Mtion in which combustion takes place in pilot or industrial fluidized-bed reactors at moderately highemperatures (1000C)[2] and no such informationabout lpg fluidized beds in the literature. mixingLPG and air is difficult because of the high calorificFigure 1 Schematic of the high-temperature LPG fludized bed test facility: 1--cooling sleeve; 2gas pipevalue of LPG(the volume ratio of air to LPg is about3-lifter; 4-fluidized bed; 5--thermocouple; 6-water-20 to 25), therefore complete combustion is hardlygas separator; 7-silver ball; 8, 9-rotameter; 10-ieved. In the present work, temperature distribution valve; 11-gas chamber; 12--furnacein a pilot LPG fluidized bed and some influence facpressure gauge; 14liquefied petroleurbottletors have been studied extensively and the combustioncompressor; 16--electricmode will be reported in another article. Due to safetydistributor; 18--nozzle: 19-air chamber.concerns and also limited industrial applications of one with a 0.3 mx0.3 m cross-section at the bottom ofpremixing combustion, emi-premixing combustion is the furnace heav凵中国煤化 The inner wallonly considered in this investigation. The aim of this of the furnaceCNMHG with a taperwork is to assess the feasibility of temperature control of 5. Gas anwere luurouuceu into the furnaceCorrespondingauthor:LiWang,E-mail:liwang@meustb.edu.cnL. Wang et al., Temperature distribution and control in liquefied petroleum gas fluidized bedsthrough a semi-premixing metal nozzle distributor [3] kinds of hollow corundum spheres(Al2O3 of 0.867-and the gas used was LPG. Temperature distribution 1. 212 mm in diameter and 386.5-870.0 kg/min pack-in the bed was monitored by shielded nichromealu- ed density were used. Table 1 lists the particles'meanmel-nickel thermocouples fixed on a lifter, which diameters, packed densities and operating conditionscould move vertically or horizontally in the furnace. 5Table 1 Particles and operating conditionsd, /mm p, /kg.mUmf /msHo/mm1.102870.00.3091780.4632.311.194386.50.1932.530.9532.240.6252.223122.152.530.9532.891.212532.00.252460660.4020.98007.00.1961.0001.830.3000.37514551800.1821452 Results and discussionoccurs near the distributor plate. Figure 2 shows typical temperature distributions in the vertical direction2.1 Temperature distributionin the bed under different air consumption coefficientsThe temperature distribution in the bed and the bed and fluidization numbers for No 8 particles. As showntemperature level under these operating conditions in figure 2, the temperature varies very fast near thelisted in table I were measured. The measurement re- distributor. and in a distance about 20-30 mm abovesults show that the temperature difference in the hori- the distributor the temperature rises from tens of dezontal direction in the fluidized bed is small for all op- grees centigrade to about 1000oC(the main bed tem-erating conditions. At about 1000C, the maximum perature). Thislled a temperature variationtemperature difference is less than 5.C, even in a dizone. Beyond中国煤化工 Most dtance about 20 mm above the distributor plate In the not varyCNMHGvertical direction, the variation in temperature mainly204J. Univ. Sci. Technol. Beijing, Vol.IL, No. 3, Jun 2004Under the operating conditions listed in table 1, the which absorb heat, therefore the bed temperature low.height of the temperature variation zone is less than er(a)nt analyses show that the combust1050in the furnace is still processing, meaning uneven in1000zone in which combustion is taking place or not com-950pleted) is larger than the temperature variation zoneThe temperature distribution can be leveled quicklya=0.494,N=1.79through particle cycle movements in the furnace.850-a=0.646,N=1.80▲a=0.931,N=1.78Therefore the temperature variation zone can be th80011ner than the combustion zone. The thin temperature020406080100120140160variation zone is of importance to effectively reduceH/mmthe height and dynamic loss of gas fluidized beds, esially for shallow ones1050110010001050950■900950850a0.375020406080100120140160N=145N=l.61H/ mm0204060801001201401601050HFigure 2 Typical temperature distributions in the vertical1000direction in the lpg fluidized bed under different air con.9502.2 Air consumption coefficient900a=0.300,N=1.47a=0.375,N=1.45In practice applications, it is usually hoped to adjust850=0.500.N=1.45the bed temperature in some ranges. There are some800factors affecting the temperature distribution and the02040608010012014016emperature level in a gas fluidized bed, such as initiFigure 3 Effect of air consumption coefficient on bed tem-distribution:(a) No 0 particles;(b)No 4 particlesnumber, air consumption coefficient and so on. The (c)No8 particles.effects of these factors were investigated separately.The effect of air consumption coefficient on bedtemperature is shown in figure 3. The initial bed1000height is 16and wvalue. As shown in figure 3, with a(a< 1) increasingwhile the other parameters remain the same levelUn=0.309ms,N-1.78-1.80among them, N varies slightly), the bed temperatureP=0252ms,N1.341414-Um=0182ms,N=1.45-147level increases and the temperature profile becomemore even. Figure 4 shows the relationships between0.40.61.0a and main bed temperatureFigure 4 Relationships between air consumption coeffi-When a varies in the range of 0.3-1.0, thecient and main bed temperature.bed temperature varies about 40-110C For the largerminimum fluidization velocity Umf, a has greater ef-According to the results shown above, in a situationwith no strictdieting a is onefect on the main bed temperatureof effective waFor small a, superfluous hydrocarbon decomposes ture.Also, fror中国煤化工 bed tempeCNMH G same valueand carbon precipitates at high temperatures 14,5 while a is changing, so the gas fluidized bed can stillL. Wang et aL., Temperature distribution and control in liquefied petroleum gas fluidized beds20operate at designed conditions (e. g. the optimum heatThe sufficiently cycling and mixing of particles maketransfer operating conditions). This is of great use in the temperature field in the bed more even. Besidepractice applicationsfor larger N, heavier particles(the minimum fluidiza2.3 Fluidization numbertion velocity usually has a small distribution range because of particle screen width) at the bottom of theAs shown in figure 5, with N increasing while the bed can be fluidized sufficiently, and the exchangether parameters remained in constant the main bed between heavier particles at the bottom with the partitemperature increases and the temperature distribution cles in the main bed is modified. This is helpful inbecomes more even. For the same kind particles, with heat transfering down and enlarging the work zoneN increasing, the bed temperature along the vertical1100direction becomes more even and the temperaturevariation zone becomes narrower From figure 6 it can■一U0.182ms,a0.494.0.500·U=0.193m/s,a=0.9530.962be seen that the effect of m on the main bed tempera1060ture increases with the increasing of Umt105010201000·10001.21.41.61.82.02.2242.6283.C900a=0.494,N=1.29Figure 6 Relationships between fluidization number anda=0.500,N1,45the main bed temperature.850▲ax=0.494,N=1.61fi3 and 5. the change in bed tem20406080100120140160perature just by adjusting N is relatively limited sinceH/mma large change in main bed temperature needs a largeadjusting in N. However, if N is changed too much,1050the operating condition will deviate from the optimumoperating condition too much. Besides, N is limited bv1000Umf and particle sedimentation rate. So in practice, the950adjusting of a should be combined with the adjustinga=0.962,N=2.27of N to change the main bed temperature. In the pre900pa-0.960,N-2.57a=0.953,N=2.97sent work, for one kind particles, the temperaturerange for the combined adjusting is about 200C800204060801001201401602.4 Initial bed heightH/ mmThe initial bed height has some effect on the bedtemperature. a deeper or shallower bed has a lower1050bed temperature. For some medium bed height, the1000bed temperature level reaches the maximum valueFigure 7 shows the influence of initial bed height onN=1.38the bed temperature. As Ho=250 mm, the temperature900N=1.59level reaches the maximum value, Ho=160 mm nextN=1.89and Ho=312 mm the lowest but the differences arenot very largea shallow bed, some LPG escapes020406080100120140160the bed before it burns completely. The hot flue gasHtravels a short distance before leaving the bed. Therefore the heat contained in the whole bed is relativelyFigure 5 Effect of fluidization number on the bed tem- less and the temperature level is lower. For a large Hoperature distribution: (a) No8 particles;(b) No l particles(c)No 4 particles.bed, the influences of the temperature variation zoneand the combustion zone are relatively weak. But forFor the same change in N, the change in gas supply the same heat supply, the heat loss through the wallis larger in a fluidized bed with larger Umf, so N has increases with-s is the mainmore effect on the main bed temperature with larger cause for a de中国煤化工er bed temUmf. For the same kind of particles, the larger the N, perature. WherCNMHGin a certainthe more intensive the cycle of the particles in the bed. range, in which the influences of the temperature206J Univ. Sci. Technol. Beijing, VoL. IL, No. 3, Jun 2004variation zone and the combustion zone are relatively ber separately or together, and the adjusting range issmall while the heat loss through the bed wall is not about 200oC.significant, the bed will possess the maximum tem-perature level(3)The air consumption coefficient can be adjustedeffectively in the range of 0.3 to 1.0, and the fluidiza-tion number 1.5 to 3. 0 in a Lpg fluidized bed10501000NomenclatureNo I particles950a=0.463,N=224-2.31d900- H=160 mmH: height from above the distributor, mm850Ho: initial800020406080100120140160180N: fluidization numbeFigure 7 Effect of initial bed height on bed temperature.Thed: main bed temperature, C,2.5 Minimum fluidization velocityT: temperature,CThe main bed temperature of a gas-fluidized bed is Umf minimum fluidization velocity, m/s,mainly decided by gas supply. For a certain a and N, a air consumption coefficientthe larger the Umf, the higher the bed temperatureFor a certain kind of particles, the supply flux of pp: packed density of solid particle, kg/m 3.gas and air affects the fluidization status in the bed diReferencesrectly. For a lower flux(N>1), particles will bebustion in a catalytic turbulent fluidized bed [J],Chemspouted out. Therefore to a certain kind of particles,Eng,Sci,49(1994),No.24A,p4269the flux can only be adjusted in a certain range, so [2 R. Sotudeh-Gharebaagh, J Chaouki, and R Legrosdoes the bed temperature. To obtain a further higher orral gas combustion in a turbulent fluidized bed oflower bed temperature, the particles must be replacedparticles [JI, Chem. Eng. Sci., 54(1999), p. 2029with ones with higher or lower Umt3 L. Wang, P. Wu, and X. Z. Ni, et al., Structure and characteristics of a backfire proof distributor [J], Powder Tech3 Conclusionsnology, 140(200[4] H.G. Chen and K.C. Xie, Cracking of liquefied petroleum(I)Even temperature distribution was obtained in aH/Ar thermal plasma [J].J Sichuan Univ(Eng. SciEd. )(in Chinese), 34(2002), No5, P. 72LPG fluidized bed with semi-premixing gas and air[5] Z.C. Gao, Mechanism of hydrocarbon combustion reacsupply. The height of the temperature variation zone istion [J]. J Liaoning Univ(Natural Sci. Ed. )(in Chinese),less than 20-30 mm29(2002),No.3,p.266(2) The bed temperature can be adjusted by adjusting air consumption coefficient and fluidization num中国煤化工CNMHG