Indian Journal of Engineering & Materials SciencesVol. 3, June 1996,pp.91-95

Solids circulation in a recirculating fluidized bed under sticky conditions

Bahu J Alappat- & v CRam:'""Centre for Environmental Science and Engineering. "Department of Chemical Engineering

Indian Institute of Technology, Mumhai -WO 07(,. India

Received ~ July 199:;; revised Il) February Il)l)(,

Experiments have been carried out on a semicircular transparent recirculating fluidized hcd(RCFB) cold model to study the circulation under sticky conditions and to foresee the likely problemsof the actual incinerator operations. It is observed that the circulation rate decreased with increase instickiness. Auxiliary air supplied as downcomer aeration and/or side push below the draft tube bottomfacilitated the solids movement and increased the circulation rates. Therefore, it is possible to operatethe unit at higher stickiness with proper auxiliary air supply. The problems encountered during the op-eration of the RCFB under sticky conditions and the corresponding alterations in the design of an in-cinerator are also discussed.

Recirculating fluidized bed (RCFB) is a spoutedor spout-fluid bed with a central riser tube (drafttube) to contain the spout. These are in use for avariety of operations and processes due to thepossibility of accurate control of solids circulationtate, gas/solids residence time and flexibility indesign and operations!". Its use for incineratingdistillery spent wash has been discussed". The useof a RCFB cold model has also been reported tostudy the operations of a RCFB incinerator andto select the best combination of various designparameters using dry sand and ambient air". But,in an incinerator where spent wash is burnt, sandis sprayed with spent wash and the particles be-come sticky changing the flow characteristics evenas such and on the evaporation of the moisture.Therefore, it is necessary to carry out experi-ments under these conditions to foresee the diffi-culties in the operation of the actual incineratorand to decide about the auxiliary air supply.

The present study incorporates this stickiness inthe RCFB cold model operations using watermixed sand and humidified air. In few runs, gly-cerol mixed sand was used along with ambient air.Operations under sticky conditions threw light onthe solids movement, auxiliary air requirements,free board dimensions and solids feeding arrange-ments of the actual incinerator.

Experimental ProcedureExperiments were carried out on a transparent

semicircular RCFB cold model shown in Fig. 1.

•Author to whom correspondence should be addressed.

IS sand of average particle size 1540 ,urn and spe-cific gravity 2624 kg/rn' was used as the bed ma-terial. Solids inventory was 2.0 kg for the runswith moisture content-clearance (between. drafttube bottom and perforated plate) combinations,respectively, as 1.17% and 0.022 m, 1.56% and0.04 m and 2.34% and 0.052 m, while 2.4 kg forthe rest of the runs. Sand was mixed with varyingpercent of water and the operations were with hu-midified air to avoid drying of sand. The specificgravity, bulk density and voidage in dense bedconditions of the moistened sand are given inFig. 2. Humidification of the ambient air was ac-complished in the humidifying unit by countercurrent contact of air with water spray. Saturationwas checked by measuring dry bulb and wet bulbtemperatures of the air stream. Solids circulationrates were measured using the butterfly valve ar-rangernent". The clearance (x) between the draft

Table I - Experimental details

Jet diameter

Draft tube diameter

Area ratio of downcomer and drafttube

Clearance between perforated plateand draft tube hottom (x)

Solids inventory

Range of moisture in sand

Range of glycerol in sand

Range of draft tube superficial airvelocity ( V)

(U)4m

0.04 m

3.9ti

0.022,0.04,0.052,0.072 and 0.09 m

2.0 and 2.4 kg

0-9.38%

0-6.25%

11.34-15.28 m/s

92 INDIAN J. ENG. MATER. SCI., JUNE 19%

3 4

3 4

18

16

17

152~

14

1312

117

8

8

Fig. I-Experimental set-up (RCFB cold model) [I - Air compressor, 2- Rotameter, ~- Dry bull) thermometer, 4- Wet bulbthermometer, 5-Air humidifying unit, 6-Water pump, 7-Water tank, R-Drain, 9-Downcomer fluidizing air, IO-Side pushair, ll-Central air jet, 12-Perforated plate, 13-C1earance x, 14-Downcomer, 15-Draft tube, 16-Free board (semicircular),

17-Free board (circular) and IS-Free board (enlarged)].

••'".g o·4trMl..- ..•.....•...-"1r-"'I!I---==9i=:d6=J~ )

0·00:---72

--...l.4----'6~--7e-~1:-':0--,:-:':2--'

./. '110 I.r I glyc.rol conl.nt in sand

Fig. 2-Effect of water/glycerol content on voidage, bulkdensity and specific gravity [Water-sand mixtures: I-Specificgravity, 2-Bulk density, 3- Voidage; Glycerol-sand mixtures:

4-Specific gravity, 5-Bulk density, 6-Voidage]

tube bottom and perforated plate was varied us-ing spacer sections of different dimensions. De-tails of the experimental parameters are given inTable 1. Experiments were carried out without

c.nlrol i-I

~rfOlOlion. o.OO1m~eo.0087m(for downcom.r o••olion)

CLEARANCE !>ECTIONWI1H !>IDE JET!>

PERFORATED PlATE

Fig. 3-Auxiliary air supply arrangements.

any auxiliary air supply to find out circulationrates at various moisture contents for differentclearances, maximum possible moisture content(beyond which circulation does not take place) foreach clearance and minimum draft tube superfi-cial air velocity (Vn) required to maintain solidscirculation in the system.

Auxiliary air supply was used only in the exper-iments with clearance x= 0.052 m. There wert.two arrangements to supply auxiliary air, viz.,downcomer aeration through the perforated plateand push from sides at the middle level of the

AI.APPAT 8:. RANE: RECIRCULATING FLUIDIZED BED 93

co0;g::>~0.15'v

'0

'" 0·10

0.05'--__-'- -'- __---"'--__-'- -'- _0·0 1·6 2·0

'/, Wot.r content in sond

Fig. 4-Effectof moisture content on solids circulation rate[1-x=O.022 rn, V= 14.79 rn/s; 2-x=O.022 rn, V=13.94m/s; 3-x=(l.()4 m, V= 14.79 m/s; 4-x=O.04 m, V= 13.94rn/s; 5-x=().052 m, V= 14.79 m/s; 6-x=O.052 rn,V= 13.94 rn/s: 7-x=O.On m, V= 14.79 m/s; 8-x=O.Onm, V= 13.94 m/s: 9-x=O.09 m, V= 14.79 m/s; lO-x=O.09

m, V= 13.94 rn/s]

clearance section (Fig. 3). With this clearance, fewexperiments were carried out with glycerol mixedsand and ambient air. Solids inventory was 2.0 kgfor the runs with glycerol contents' 1.56 and2.34%, while 2.4 kg for the rest of the runs.

Results and DiscussionStudies with water-sand mixtures-Fig. 4 shows

that, the effect of varying velocity V for any clear-ance and moisture content is same. With the in-crease in velocity, circulation rate increases forthe range of velocity tested here. This is due tothe increase in the entrainment capacity of the airstream with increase in its velocity. For any x, atconstant V, circulation rate decreased with in-crease in the moisture content of the sand. This isdue to the decrease in free' flowing nature of thesolids with increase in moisture content. Thecohesion between the particles will increase withincrease in moisture content. Increase in moisturecontent beyond certain limit (which varies withthe clearance) stops the solids circulation. This li-mit for x= 0.022 m was 1.17%, for x= 0.04,0.072 and 0.09 m was 1.56% and for x= 0.052 mit was 2.34%. For dry sand and ambient air, atconstant velocity and solids inventory, it was. theclearance 0.04 m that gave higher circulationrates than the other clearances tested". In therange of clearance 0.022 to 0.04 m, solids circula-tion rate increased as the resistance to the solids

0.25,..--------------------------,

~'"....;'20.15

0.20

c£>:§~0.10v

~'0'"0.05

o~----~-----L----~----~--~0·0 o.s 1·0 I.!> 2·0

Fig. ::;- Effect of glycerol content on solids circulation ratefor clearance x=O.052 m [1- V= 15.03 m/s: 2- V= 14.24 •mis, 3- V= Vn i.e. the minimum draft tube air velocity re-

quired for stable circulation].

flow decreases with the increase in clearance,whereas, in the range 0.04 to 0'{)9 m circulationrate decreased as -the air bypassing from the jet tothe downcomer side increased. This, not only of-fers a resistance to the solids flow, but also dec-reases the draft tube air velocity. In the presentstudy with moistened sand and humidified air, itwas the clearance of 0.072 m that gave higher cir-culation rates than the other clearances tested.The shifting of this clearance (from 0.04 to 0.072m) that gives higher circulation rates. is because ofthe increased resistance to the solids flow acrossthe clearance due to the presence of water whichimproves cohesion.

Clearance x= 0.052 m was chosen for the ex-periments with auxiliary air supply. With auxiliaryair supply it was possible to operate the unit atrelatively higher moisture contents. Withx= 0.052 m, the unit worked pretty well even at9.38% moisture content, whereas without auxil-iary air this limit was 2.34%. The effect of auxil-iary air was to increase the solids circulation rateas it facilitates solids movement from the down-comer bottom to the draft tube entrence.

Studies with glycerol-sand mixtures-Fig. 5shows the variation of solids circulation rates un-der no auxiliary air supply condition, for clear-ance x= 0.052 m at different V and glycerol con-tents in the sand. The trend of variation is similarto that of the curves for water mixed sand. Withauxiliary air supply, the maximum possible glycer-ol content (beyond which circulation does nottake place) could be increased from 2.34 to

94

16

•Eiv0"'>0 14

.E.'t..a.:J

'"..n 122

"0UE~ci 10

00

INDIAN J. ENG. MATER. set, JUNE 1996

• 3

6

o

04 0.8 1.61.2 2·0

-I. Water /glycerol content in s.and

Fig. o-Effect of moisture/glycerol content Oil minimum drafttube air velocity required for stable circulation [Water-sandmixture: l-x=O.022 rn, 2-x=O.04 m. 3-x=O.OS2 m. 4-x=O.072 m . .:'i-x=().09 rn; Glycerol-sand mixture: 6-

x=O.052 rn]

0.25%. Increase in the auxiliary air supply withinlimits decreased the jet air requirements consider-ably. For a glycerol content of 1.56%, solids cir-culation was possible only with velocities above13.48 m/s when auxiliary air was absent But withan auxiliary air supply of 0.0028 m3/s, the unitworked even for V= 7.3 m/s. Use of auxiliary airincreased the fluidity of the particles in the down-comer, thereby, requiring less velocity in the drafttube.

The circulation rates were compared for thethree ways of auxiliary air supply, viz., downcom-er aeration, side push at the clearance section andboth together. For the same quantity of total aux-iliary air, it was found that the combined methodgave higher circulations, followed by the sidepush and then the down comer aeration. This isbecause, the already fluidized particles in thedowncomer with aeration, are pushed by the sidejets into the draft tube. With V= 12.43 mis, for afixed quantity of auxiliary air of 0.0028 mvs, cir-culation rates for different ways of auxiliary airsupply were 0.103 kg/s for downcomer aeration,0.145 kg/s for side push and 0.156 kg/s for bothtogether.

Fig. 6 shows the minimum draft tube air veloc-ity (Vn) required for maintaining solids circulationfor different clearances and moisture/glycerolcontents. When tht; moisture/glycerol content wasincreased, Vn increased for all the clearancestried. Similarly, for any moisture/glycerol content,V" increased with clearance x. This is because, as

x increases, jet air bypass to the downcomer sideincreases. So comparatively higher velocities arerequired for the jet to penetrate through the clear-ance.

Difficulties encountered during the operationsunder sticky conditions and alterations of the de-sign of the actual incinerator+ The flow of solidsfrom the hutterfly valve to the downcomerthrough the inclined pipe. (0.054 m i.d.) was notsmooth. At higher moisture/glycerol contents, thispipe was even clogged stopping the circulation.But this problem will not occur on the actual in-cinerator as there will not be any butterfly valvefixed to the downcomer side. Butterfly valve wasfixed on the cold model unit for the purpose ofmeasuring solids circulation rate 7.

Particles were found sticking to the free boardat high moisture/glycerol contents, finally forminga bed above the draft tube. To avoid this problemin the actual incinerator, the free board area maybe enlarged immediately above the draft tube top.

As the moisture/glycerol content of the sandwas increased, the solids flow from the downcom-er bottom to the draft tube through the clearancex was found to be more and more difficult. Auxil-iary air supply (both down comer aeration andside push) may be used to assist the flow of solidsin the actual incinerator.

It was difficult to feed the sticky sand to thedowncomer of the unit as the feeding line cloggedeasily. It is not advisable to feed the sand directlyfrom the top of the unit as the draft tube will alsobe filled with sand making the start up difficult.Also, this may cause bed formation at the freeboard above the draft tube. However, in the actu-al incinerator it will not take place as only drysand will be fed to the downcomer through thefeeding line. Waste will be sprayed into the unitand the feeding line cross-sectional area may heincreased for the actual unit.

Wire mesh covering the perforated plate wasgetting sealed easily because the dust in the sandbecame sticky with the addition of water. In thecold model studies, it was taken out frequentlyand cleaned by scratching with nail and blowingcompressed air. In the actual unit, it can becleaned by blowing hot air when the waste is notsprayed into the unit.

Particles were found sticking to the downcom-er/draft tube walls, sometimes forming a bed likearch clogging the entire section. Tapping on theunit was required to break it At high moisture/glycerol contents, the operation could be startedonly after tapping on the wall of the unit. In theactual incinerator, these problems can be mini-

!\L!\PP!\T & R!\NE: RECIRCULATING FLUIDIZED BED 95

mized by the auxiliary air supply and by maintain-ing the proper sand to waste ratio.

When the auxiliary air was introduced as theside jet push, the chances of the downcomerslugging were considerably higher than in the casewhere it was supplied as downcomer aerationthrough the perforations on the plate at the bot-tom of the clearance. Therefore, the downcomeraeration is preferable in view of the quality of cir-culation.

ConclusionsCold model experiments can provide much in-

sight about the operation of the actual unit andthe difficulties that may arise. Increase in thestickiness of sand decreases the solids circulationrate. With a clearance of 0.052 m, the unit couldfunction at relatively higher moisture content(2.34%) than with the other clearances testedwhen the auxiliary air supply was absent. But, itwas clearance 0.072 m that gave larger circulation

rates than the others. Auxiliary air supply en-hanced the circulation rate and made the opera-tion at very high moisture/glycerol contents possi-ble. With auxiliary air, the unit worked even at9.38% moisture content for the clearance 0.052m. However, maximum possible glycerol contentwas 6.25%. The minimum draft tube air velocitiesrequired to maintain the circulation, increasedwith both clearance x and stickiness.

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