Solids circulation in a recirculating fluidized bed under sticky conditions

Transcription

1 Indian Journal of Engineering Vol. 3, June 1996,pp & Materials Sienes Solids irulation in a reirulating fluidized bed under stiky onditions Bahu J Alappat- & v CRam:'" "Centre for Environmental Siene and Engineering. "Department of Chemial Engineering Indian Institute of Tehnology, Mumhai -WO 07(,. India Reeived July 199:;; revised Il) February Il)l)(, Experiments have been arried out on a semiirular transparent reirulating fluidized hd (RCFB) old model to study the irulation under stiky onditions and to foresee the likely problems of the atual ininerator operations. It is observed that the irulation rate dereased with inrease in stikiness. Auxiliary air supplied as downomer aeration and/or side push below the draft tube bottom failitated the solids movement and inreased the irulation rates. Therefore, it is possible to operate the unit at higher stikiness with proper auxiliary air supply. The problems enountered during the operation of the RCFB under stiky onditions and the orresponding alterations in the design of an ininerator are also disussed. Reirulating fluidized bed (RCFB) is a spouted or spout-fluid bed with a entral riser tube (draft tube) to ontain the spout. These are in use for a variety of operations and proesses due to the possibility of aurate ontrol of solids irulation tate, gas/solids residene time and flexibility in design and operations!". Its use for ininerating distillery spent wash has been disussed". The use of a RCFB old model has also been reported to study the operations of a RCFB ininerator and to selet the best ombination of various design parameters using dry sand and ambient air". But, in an ininerator where spent wash is burnt, sand is sprayed with spent wash and the partiles beome stiky hanging the flow harateristis even as suh and on the evaporation of the moisture. Therefore, it is neessary to arry out experiments under these onditions to foresee the diffiulties in the operation of the atual ininerator and to deide about the auxiliary air supply. The present study inorporates this stikiness in the RCFB old model operations using water mixed sand and humidified air. In few runs, glyerol mixed sand was used along with ambient air. Operations under stiky onditions threw light on the solids movement, auxiliary air requirements, free board dimensions and solids feeding arrangements of the atual ininerator. Experimental Proedure Experiments were arried out on a transparent semiirular RCFB old model shown in Fig. 1. Author to whom orrespondene should be addressed. IS sand of average partile size 1540,urn and speifi gravity 2624 kg/rn' was used as the bed material. Solids inventory was 2.0 kg for the runs with moisture ontent-learane (between. draft tube bottom and perforated plate) ombinations, respetively, as 1.17% and m, 1.56% and 0.04 m and 2.34% and m, while 2.4 kg for the rest of the runs. Sand was mixed with varying perent of water and the operations were with humidified air to avoid drying of sand. The speifi gravity, bulk density and voidage in dense bed onditions of the moistened sand are given in Fig. 2. Humidifiation of the ambient air was aomplished in the humidifying unit by ounter urrent ontat of air with water spray. Saturation was heked by measuring dry bulb and wet bulb temperatures of the air stream. Solids irulation rates were measured using the butterfly valve arrangernent". The learane (x) between the draft Jet diameter Draft tube diameter Area ratio of downomer tube Table I - Experimental and draft Clearane between perforated plate and draft tube hottom (x) Solids inventory Range of moisture in sand Range of glyerol in sand Range of draft tube superfiial air veloity ( V) details (U)4m 0.04 m 3.9ti 0.022,0.04,0.052, and 0.09 m 2.0 and 2.4 kg % % m/s

2 92 INDIAN J. ENG. MATER. SCI., JUNE 19% Fig. I-Experimental set-up (RCFB old model) [I - Air ompressor, 2- Rotameter, - Dry bull) thermometer, 4- Wet bulb thermometer, 5-Air humidifying unit, 6-Water pump, 7-Water tank, R-Drain, 9-Downomer fluidizing air, IO-Side push air, ll-central air jet, 12-Perforated plate, 13-C1earane x, 14-Downomer, 15-Draft tube, 16-Free board (semiirular), 17-Free board (irular) and IS-Free board (enlarged)]..nlrol i-i rfololion. o.oo1meo.0087m (for downom.r o olion).g '" o 4trMl "1r-"'I!I---==9i=:d6=J ) 0 00: l.4----'6--7e-1:-':0--,:-:':2--'./. '110 I.r I gly.rol onl.nt in sand Fig. 2-Effet of water/glyerol ontent on voidage, bulk density and speifi gravity [Water-sand mixtures: I-Speifi gravity, 2-Bulk density, 3- Voidage; Glyerol-sand mixtures: 4-Speifi gravity, 5-Bulk density, 6-Voidage] tube bottom and perforated plate was varied using spaer setions of different dimensions. Details of the experimental parameters are given in Table 1. Experiments were arried out without CLEARANCE!>ECTION WI1H!>IDE JET!> Fig. 3-Auxiliary PERFORATED PlATE air supply arrangements. any auxiliary air supply to find out irulation rates at various moisture ontents for different learanes, maximum possible moisture ontent (beyond whih irulation does not take plae) for eah learane and minimum draft tube superfiial air veloity (V n ) required to maintain solids irulation in the system. Auxiliary air supply was used only inthe experiments with learane x= m. There wert. two arrangements to supply auxiliary air, viz., downomer aeration through the perforated plate and push from sides at the middle level of the

3 AI.APPAT 8:. RANE: RECIRCULATING FLUIDIZED BED , , 0.20 o 0; g ::> 0.15 'v '0 '" ; '" '20.15 > : 0.10 v '0 '" '-- -'- -'- ---"'-- -'- -'- _ '/, Wot.r ontent in sond o l o.s 1 0 I.!> 2 0 Fig. 4-Effetof moisture ontent on solids irulation rate [1-x=O.022 rn, V= rn/s; 2-x=O.022 rn, V=13.94 m/s; 3-x=(l.()4 m, V= m/s; 4-x=O.04 m, V= rn/s; 5-x=().052 m, V= m/s; 6-x=O.052 rn, V= rn/s: 7-x=O.On m, V= m/s; 8-x=O.On m, V= m/s: 9-x=O.09 m, V= m/s; lo-x=o.09 m, V= rn/s] learane setion (Fig. 3). With this learane, few experiments were arried out with glyerol mixed sand and ambient air. Solids inventory was 2.0 kg for the runs with glyerol ontents' 1.56 and 2.34%, while 2.4 kg for the rest of the runs. Results and Disussion Studies with water-sand mixtures-fig. 4 shows that, the effet of varying veloity V for any learane and moisture ontent is same. With the inrease in veloity, irulation rate inreases for the range of veloity tested here. This is due to the inrease in the entrainment apaity of the air stream with inrease in its veloity. For any x, at onstant V, irulation rate dereased with inrease in the moisture ontent of the sand. This is due to the derease in free' flowing nature of the solids with inrease in moisture ontent. The ohesion between the partiles will inrease with inrease in moisture ontent. Inrease in moisture ontent beyond ertain limit (whih varies with the learane) stops the solids irulation. This limit for x= m was 1.17%, for x= 0.04, and 0.09 m was 1.56% and for x= m it was 2.34%. For dry sand and ambient air, at onstant veloity and solids inventory, it was. the learane 0.04 m that gave higher irulation rates than the other learanes tested". In the range of learane to 0.04 m, solids irulation rate inreased as the resistane to the solids Fig. ::;- Effet of glyerol ontent on solids irulation rate for learane x=o.052 m [1- V= m/s: 2- V= mis, 3- V= Vn i.e. the minimum draft tube air veloity required for stable irulation]. flow dereases with the inrease in learane, whereas, in the range 0.04 to 0'{)9 m irulation rate dereased as -the air bypassing from the jet to the downomer side inreased. This, not only offers a resistane to the solids flow, but also dereases the draft tube air veloity. In the present study with moistened sand and humidified air, it was the learane of m that gave higher irulation rates than the other learanes tested. The shifting of this learane (from 0.04 to m) that gives higher irulation rates. is beause of the inreased resistane to the solids flow aross the learane due to the presene of water whih improves ohesion. Clearane x= m was hosen for the experiments with auxiliary air supply. With auxiliary air supply it was possible to operate the unit at relatively higher moisture ontents. With x= m, the unit worked pretty well even at 9.38% moisture ontent, whereas without auxiliary air this limit was 2.34%. The effet of auxiliary air was to inrease the solids irulation rate as it failitates solids movement from the downomer bottom to the draft tube entrene. Studies with glyerol-sand mixtures-fig. 5 shows the variation of solids irulation rates under no auxiliary air supply ondition, for learane x= m at different V and glyerol ontents in the sand. The trend of variation is similar to that of the urves for water mixed sand. With auxiliary air supply, the maximum possible glyerol ontent (beyond whih irulation does not take plae) ould be inreased from 2.34 to

4 94 INDIAN J. ENG. MATER. set, JUNE E i v 0 "' > 0 14.E.'t.... a. :J '" n 2 12 "0 U E i o I. Water /glyerol ontent in s.and Fig. o-effet of moisture/glyerol ontent Oil minimum draft tube air veloity required for stable irulation [Water-sand mixture: l-x=o.022 rn, 2-x=O.04 m. 3-x=O.OS2 m. 4- x=o.072 m..:'i-x=().09 rn; Glyerol-sand mixture: 6- x=o.052 rn] 0.25%. Inrease in the auxiliary air supply within limits dereased the jet air requirements onsiderably. For a glyerol ontent of 1.56%, solids irulation was possible only with veloities above m/s when auxiliary air was absent But with an auxiliary air supply of m 3 /s, the unit worked even for V= 7.3 m/s. Use of auxiliary air inreased the fluidity of the partiles in the downomer, thereby, requiring less veloity in the draft tube. The irulation rates were ompared for the three ways of auxiliary air supply, viz., downomer aeration, side push at the learane setion and both together. For the same quantity of total auxiliary air, it was found that the ombined method gave higher irulations, followed by the side push and then the down omer aeration. This is beause, the already fluidized partiles in the downomer with aeration, are pushed by the side jets into the draft tube. With V= mis, for a fixed quantity of auxiliary air of mvs, irulation rates for different ways of auxiliary air supply were kg/s for downomer aeration, kg/s for side push and kg/s for both together. Fig. 6 shows the minimum draft tube air veloity (V n ) required for maintaining solids irulation for different learanes and moisture/glyerol ontents. When tht; moisture/glyerol ontent was inreased, Vn inreased for all the learanes tried. Similarly, for any moisture/glyerol ontent, V" inreased with learane x. This is beause, as x inreases, jet air bypass to the downomer side inreases. So omparatively higher veloities are required for the jet to penetrate through the learane. Diffiulties enountered during the operations under stiky onditions and alterations of the design of the atual ininerator+ The flow of solids from the hutterfly valve to the downomer through the inlined pipe. (0.054 m i.d.) was not smooth. At higher moisture/glyerol ontents, this pipe was even logged stopping the irulation. But this problem will not our on the atual ininerator as there will not be any butterfly valve fixed to the downomer side. Butterfly valve was fixed on the old model unit for the purpose of measuring solids irulation rate 7. Partiles were found stiking to the free board at high moisture/glyerol ontents, finally forming a bed above the draft tube. To avoid this problem in the atual ininerator, the free board area may be enlarged immediately above the draft tube top. As the moisture/glyerol ontent of the sand was inreased, the solids flow from the downomer bottom to the draft tube through the learane x was found to be more and more diffiult. Auxiliary air supply (both down omer aeration and side push) may be used to assist the flow of solids in the atual ininerator. It was diffiult to feed the stiky sand to the downomer of the unit as the feeding line logged easily. It is not advisable to feed the sand diretly from the top of the unit as the draft tube will also be filled with sand making the start up diffiult. Also, this may ause bed formation at the free board above the draft tube. However, in the atual ininerator it will not take plae as only dry sand will be fed to the downomer through the feeding line. Waste will be sprayed into the unit and the feeding line ross-setional area may he inreased for the atual unit. Wire mesh overing the perforated plate was getting sealed easily beause the dust in the sand beame stiky with the addition of water. In the old model studies, it was taken out frequently and leaned by srathing with nail and blowing ompressed air. In the atual unit, it an be leaned by blowing hot air when the waste is not sprayed into the unit. Partiles were found stiking to the downomer/draft tube walls, sometimes forming a bed like arh logging the entire setion. Tapping on the unit was required to break it At high moisture/ glyerol ontents, the operation ould be started only after tapping on the wall of the unit. In the atual ininerator, these problems an be mini-

5 !\L!\PP!\T & R!\NE: RECIRCULATING FLUIDIZED BED 95 mized by the auxiliary air supply and by maintaining the proper sand to waste ratio. When the auxiliary air was introdued as the side jet push, the hanes of the downomer slugging were onsiderably higher than in the ase where it was supplied as downomer aeration through the perforations on the plate at the bottom of the learane. Therefore, the downomer aeration is preferable in view of the quality of irulation. Conlusions Cold model experiments an provide muh insight about the operation of the atual unit and the diffiulties that may arise. Inrease in the stikiness of sand dereases the solids irulation rate. With a learane of m, the unit ould funtion at relatively higher moisture ontent (2.34%) than with the other learanes tested when the auxiliary air supply was absent. But, it was learane m that gave larger irulation rates than the others. Auxiliary air supply enhaned the irulation rate and made the operation at very high moisture/glyerol ontents possible. With auxiliary air, the unit worked even at 9.38% moisture ontent for the learane m. However, maximum possible glyerol ontent was 6.25%. The minimum draft tube air veloities required to maintain the irulation, inreased with both learane x and stikiness. Referenes I Yang W C, Margaritis P J & Keairns D L, AIChF SYl1lf! Ser, 74 (1978) Judd M R & Rudolf V. in Pro of 5th Eng Foundation Conferene on Fluidization. edited by Ostergaard K & Sorenson A (Engineering Foundation, New York), Khoe G K & Brakel J Y, Can J Chern Eng, 61 (1983) Deamps F, Dumong G & Goossens G. Powder Tehnol. 5 ( ) Alappat B J & Rane V C, lilt J Energy Res, 19 (1995) Alappat B J & Rane Y C. Int J Energy Res. (in press). 7 Alappat B J & Rane V C, Can J Chem Eng, 73 (1995) 248.