Analysis Loop Seal Operation

AN ANALYSIS OF LOOP SEAL OPERATIONS IN A

CIRCULATING FLUIDIZED BED

P BASU and L CHENG

Mechanical Engineering Department Dalhousie University Nova Scotia CanadaInstitute for Thermal Power Engineering Zhejiang University PR China

The operation of a loop seal in a circulating macruidized bed is studied on the basis ofpressure balance of the circulation loop The sharp-crested theory of free surface macrowis applied to analyze the solids macrow rate through a loop seal Factors which inmacruence

the solids macrow rate through the loop seal include loop seal air velocity initial bed inventorystandpipe size loop seal slit size and particle size The solids macrow could occur only betweentwo limiting values of each of those parameters The analysis also presented pressuredistributions along the loop for different circulation rates Results from above theoreticalanalyses were compared with experimental results A good agreement between the resultsconregrmed the validity of the present analysis

Keywords loop seal solids recycle device circulating macruidized bed

INTRODUCTION

A typical circulating macruidized bed (CFB) system comprisesa fast macruidized bed (riser) a gas-solid separator a stand-pipe (dipleg) and a solids recycle system Solids particlesmove around these components in sequence and the solidsrecycle system is a key component of the CFB loop A non-mechanical valve is commonly used in this system becauseit is robust inexpensive and simple in construction Seriouserosion of moving parts high solids macrow rate require-ment and high operating temperature preclude the use ofmechanical valves

Solids in a non-mechanical valve are moved by air orgas In certain sections of the valve the solids exhibit aliquid-like behaviour when aeration air is added The solidsmacrow through the valve when the drag of the aeration airexceeds the resistance holding the solids together

Several types of non-mechanical solid recycle valvesare used in a CFB system They are L-valve V-valveJ-valve seal pot and loop seal (macruo seal) Some data areavailable on L-valve12 and V-valve3 but information onloop seal is very limited Some experimental investiga-tions45 were reported but a comprehensive analysis of theperformance of the loop seal is not available in publishedliterature The present paper presents an analysis of theperformance of the loop seal using free surface theory andpressure balance equations

Based on their experiments on a 200 mm acute 200mm crosssection loop seal Luo et al4 presented empirical equationsto calculate the standpipe height and solids macrow rate Theaccuracy and general applicability of their equations havenot been verireged

Horio6 discussed the total pressure balance around aCFB loop taking the loop seal as a simple oriregce or valveThe pressure drop across the seal was obtained by anempirical equation None of the models consider theinmacruence of solids inventory on solids recycle rates

The present work analyses the loop seal taking intoaccount all relevant factors including loop seal air velocityinitial bed inventory standpipe size loop seal slit size andparticle size

THEORETICAL ANALYSIS

Figure 1 shows a CFB loop with a loop seal where solidsare in a fast macruidized condition in the riser Particles enterthe cyclone after they exit from the riser and particlesseparated in the cyclone accumulate in the standpipe Thestandpipe drops the solids into the loop seal which is splitinto two sections supply and recycle chambers (Figure 1)These two sections are connected by a rectangular open-ing called slit Both chambers can be macruidized from thebottom The recycle chamber has an overmacrow weir whichconnects it to a discharge pipe leading the solids to the riser

Solids collected in the standpipe drop into the supplychamber Aeration given at the bottom of the loop sealhelps this solids move through the slit into the macruidizedrecycle chamber The macruidized solids spill over the weirinto the recycle pipe which leads the solids into the riserThus the solids move around the CFB loop without amechanical pump The pressure difference between thestandpipe and the riser drives the solids through the systemSo under a steady state there would be a pressure equilib-rium around the loop

Pressure Balance in a Circulation Loop

For a pressure balance the algebraic sum of pressure dropacross each section of the circulation loop should be equalto zero

Pa Pb Pb Pc Pc Pd Pd Pe

Pe Pf Pf Pg Pg Pa 0 1

991

0263plusmn876200$1000+000q Institution of Chemical Engineers

Trans IChemE Vol 78 Part A October 2000

Point D in Figure 1 is the solids surface in the standpipeThe friction on the discharge pipe (G-A) can be neglectedas it is rarely full Also the section of the cyclone (C-D)offers very low resistance So if any resistance at the exitof the riser is neglected Pb Pc 0 Pc Pd 0 andPg Pa 0 can be written The above pressure balanceequation now simplireges as

Pa Pb Pd Pe Pe Pf Pf Pg 0

2

Pressure Drop in the Riser

The pressure drop along the riser height is determinedby its axial voidage proregle Two main methods are usedto estimate the axial voidage proregle The axial voidageproregle may be divided into two parts a denser section anda leaner section The transition point between these twomay be estimated for riser reactors at a given set of operat-ing and geometric conditions7 In a CFB boiler this pointcoincides with the secondary air injection point8 So theaxial voidage proregle in the leaner section may be calcula-ted using the entrainment model of Kunii and Levenspiel9The voidage decay constant of the leaner section andvoidage of the lower dense bed were based on experimentalresults9

Thus the bed is divided into lower dense region and

upper dilute region along the bed height The total pressuredrop across the riser A-B also includes the frictional lossincluding acceleration loss D Pab Then the total pressuredrop Pa Pb becomes

Pa Pb 1 laquo den ghden 1 laquo dil ghdil D Pab 3

Where laquo den and laquo dil are voidages in lower dense region andupper dilute region respectively hden is dense regionheight in bed hdil is the dilute region height which isriser height less the dense region height Kunii andLevenspiel10 compared voidages in the lower denseregions of a macruidized bed (Table 1) A typical value forthe fast macruidized bed is chosen in the present modelcalculation

Bed internals may affect the axial voidage proregle Thereis also an effect of the bed exit11 However if one assumesthat the furnace is so tall that all particles are completely

992 BASU and CHENG


Figure 1 Loop seal solids recycle system

Table 1 Comparison of voidage inthe lower dense region of a macruidized

bed

Fluidized bed VoidageBubbling bed 045plusmn060Turbulent bed 060plusmn078Fast macruidization 078plusmn084

dispersed near its exit and there is no remacruxing then thesolids macrux leaving the bed may be given by the followingequation

Gs 1 laquo dil raquosAb 4

Where laquo dil is the voidage in the upper region up is theabsolute velocity of solids particle As the voidage is veryhigh up can be approximated by the equation12

up ug ut 5

Where ug is the superregcial gas velocity in the bed ut is theparticle terminal velocity

Pressure Drop in the Standpipe

Particles move down slowly as a moving packed bedmacrow in the standpipe Assume the voidage to have identi-cal values in the standpipe and in the supply chamberSo the pressure drop per unit length is obtained by a slightmodiregcation of the Ergun equation13

Pe Pd

Ls

150 1 laquo s3

laquo 3s

mg D u

w sdp2

175 1 laquo s3

laquo 3s

raquog D u 2

w sdp

6

Where laquo s is the solids voidage in the standpipe D u is therelative velocity of gas with respect to the solids

D u uo us 7

The superregcial rise velocity of gas up the standpipe uo issmall This macrow rate is a fraction d of the air macrow of theloop seal Q Here d is taken as 0095 from Cheng andBasu14

Pressure Drop Across the Slit

Solids from the supply chamber macrow to the recyclechamber through an opening (slit) at the bottom of thedivision wall between these two chambers The pressuredrop across this opening can be calculated by the follow-ing equation suggested by Cheng and Basu14 It is basedon the experimental data of Jin et al15 and Kuramotoet al16

Pe Pf 066As

Asc

12

Gs 8

where As and Asc are cross section areas of the slit and thesupply chamber respectively

Pressure Drop in the Recycle Chamber

Solids in the recycle chamber are macruidized by the airentering from its bottom The bed level must riseabove the weir (Figure 1) in order to overmacrow into therecycle pipe Considering the bed to expand to a heightD hr over the recycle chamber weir height hr (Figure1)

Pf Pg 1 laquo r hr D hr raquos 9

For regrst approximation a simplireged expression for bed

expansion derived by King17 is used

laquo r

ur 1

ur 210

where ur is the macruidizing velocity in the recycle chamber

Solids Flow Rate at the Weir of the Recycle Chamber

Once the recycle chamber is macruidized the solids in itmacrow like a liquid The macruidized bed expands above theheight of the weir (Figure 1) and the solids macrow into theriser through the inclined recycle pipe The inclined pipeis generally not reglled with solids So the overmacrow rateof solids is a direct function of excess bed height abovethe weir which increases with the macruidizing velocityThis situation is similar to that of free surface macrow of aliquid over a sharp crested weir So from sharp-crestedtheory18 the volume macrow Qs of macruidized solids can bewritten as

Qs cg12 D h32W 11

where c is an experimentally derived constant D hr is thebed level above the weir W is the width of the weir and gis the acceleration due to gravity

Thus the solids macrow rate Gs can be calculated by

Gs 1 laquo r raquosQs c 1 laquo r raquosg12 D h32

r v 12

Assuming that the total amount of solids and their sizedistribution do not change during operations the sum ofsolids in each component of the CFB loop system shouldbe equal to the original solids inventory M0 Thus a materialbalance gives us the following equation

1 laquo den raquos Abhden 1 laquo dil raquos Abhdil

1 laquo s raquos Asp Ls lsc 1 laquo s raquos Asclsc

1 laquo r raquos Arc hr D hr M0 13

where Ab Asp Asc and Arc are areas of the riser standpipesupply chamber and recycle chamber respectively Neglect-ing the frictional loss and substituting equations (3) (6)(8) and (9) into (2) relations among different parametersare established

The above model can be used for both understandingthe operating behaviour and the design of a loop seal Forexample the solids circulation rate and gas velocity in theriser may be specireged as input parameters and then com-pute the aeration rate required in the loop seal To validatethe above model it would be used regrstly to interpret theobserved behaviour of the loop seal then the resultscompared

EXPERIMENTS

The tests were carried out in a 152mm diameter circu-lating macruidized bed with a 100 mm width rectangular loopseal The system is shown in Figure 1 Three size sands480 mm 355 mm and 250 mm were used in the tests andtwo techniques were used to measure the solids macrow ratesExperimental details are reported in Basu et al5 so they arenot reported here

993AN ANALYSIS OF LOOP SEAL OPERATIONS IN A CIRCULATING FLUIDIZED BED


RESULTS AND DISCUSSION

The loop seal operates steadily when the pressure balanceequation (1) is satisreged but its operation becomes irrationalor it stops entirely when the pressure balance is lost A verycommon failure occurs when the pressure drop across thestandpipe falls below that required to drive the solidsthrough the loop seal and into the riser Under this conditionthe aeration air takes a short cut from the loop seal to thecyclone directly through the standpipe

When the riser operates at a high gas velocity the solidstransfer rate from the riser to the cyclone and standpipeincreases If there is no corresponding increase in the loopseal aeration the solids return rate will be less than thatleaving the riser So there would be an imbalance in solidsmacrow rate breaking down the loop system pressure balanceSimilarly the balance breaks down with a combination oflow riser gas velocity and high loop seal aeration air rateUnder this condition the solids height in the standpipe dropslowering to an extent that the loop seal air takes the lowresistance path through the cyclone resulting in a drop incyclone separation efregciency This adversely affects theoperation of a CFB

In the present model the calculation was terminatedeither when an imbalance in the loop pressure or air by-passing in the standpipe occurred Air by-passing happenswhen the relative velocity of gas and solids is higher thanthe minimum macruidizing velocity of the solids Under thiscondition further increases in relative velocity do notincrease the resistance of the standpipe So the pressuredrop in the standpipe reaches its peak making the solidsrecycle unstable

In all calculations it is found that the loop systemoperated only within a certain range of loop seal aerationair rate for a given riser gas velocity When the riser gasvelocity varies the operating range of the loop seal aera-tion also changes Besides this the solids inventory alsoaffects the operating range of the loop seal Reasons forthese behaviour are explained in subsequent sections

Solids Flow Rate

Figure 2 shows the relationship between the solids macrowrate and the loop seal aeration velocity at different riser gasvelocities At a regxed riser gas velocity an increase in the

loop seal aeration rate results in an increased solids macrowthrough the loop seal This can be explained by equation(12) which shows that the solids macrow rate increases whenD hr increases Equation (10) shows that higher aerationrate or higher velocity expands the bed in the recyclechamber and increases the D hr

However the pressure seal of solids in the standpipeis broken if the loop aeration is increased continuouslywithout a corresponding increase in the riser gas velocityThis deregnes the maximum operable velocity of the loopseal Similarly if the loop seal velocity is reduced muchthe pressure drop across the standpipe solids falls below thatrequired to drive the solids This deregnes the lower limit ofloop seal aeration

The lines in Figure 2 give the predicted operating rangeof the aeration velocity Within this operating range thesolids macrow rate increases with loop seal air velocity Thesolids macrow also increases as the riser gas velocity increasesHowever at a higher riser gas velocity the operating rangeof the loop seal aeration decreases Thus the control rangeof the solids macrow rate reduces at a higher riser velocityA limited change in the aeration rate gives a very largechange in the macrow rate and regnally there is a breakdownin the macrow

Since the solids carrying capability of the riser increasesat higher riser velocities the solids level in the standpipeincreases This exerts a higher hydrostatic pressure onthe loop seal moving more solids from the standpipe Sothe solids recycle rate increases even when the loop sealaeration rate is unchanged Figure 3 shows how the solidsmacrow rate increases with the riser gas velocity at a regxedloop seal aeration rate

A high riser velocity results in high solids macrow fromthe riser to the standpipe This results in a higher relativevelocity between gas and the solids at a given supplychamber aeration This gives a higher pressure drop acrossthe standpipe Higher resistance in the standpipe wouldcause less air to macrow through the supply chamber to main-tain the pressure balance This would increase the air macrowthrough the recycle chamber which in turn increases therecycle rate

The solids circulation rate predicted from the presentmodels were plotted on Figure 2 along with experimentaldata The latter was shown by points while the predictionsare shown by lines It shows a good agreement between themodel and experiments

994 BASU and CHENG


Figure 2 A comparison of theoretical and experimental variation of thesolids macrow rate with loop seal air velocity (points are experimental data atdifferent loop seal slit height)

Figure 3 Solids macrow rate increases with the superregcial velocity at a regxedloop seal air rate

Solids Height in Standpipe

Figure 4 shows that the height of solids in the standpipedecreases with increasing loop seal aeration rate As theloop seal aeration air increases more solids would macrowout of the standpipe giving a higher circulation rate Thiswould in turn make the riser denser Thus there is anincrease of solids inventory in the riser at the cost of that inthe standpipe This causes the solids level in the standpipeto drop There is however a minimum solids height in thestandpipe for a given solids inventory in the system FromFigure 5 it is found that the pressure drop per unit heightPe Pd Ls increases with the gas-solid relative velocity

in the standpipe So if the pressure across the riser (Pa Pb)increases for any reason a larger fraction of the aera-tion moves to the standpipe to increase the standpipe head(Pe Pd) However it can increase only up to a certainmaximum valve Thus if Ls drops too far the resultant(Pe Pd) will not be able to balance the increased pressuredrop (Pa Pb) in the riser So a solids height lower thanthe minimum will result in an air by-pass through thestandpipe It should be noted that a higher height of solidsin the standpipe does not mean that a higher pressure isproduced The pressure drop in the standpipe is determinednot only by the solids height but also by the relative velocitybetween solids and gas (Figure 5) If the loop seal aerationrate is too small the pressure produced by the solids in thestandpipe will not be enough to balance the loop systempressure and this would stop solids recycle rate (Figure 4)

This aeration rate as explained earlier deregnes the loweroperating limit of the loop seal

Pressure Drops and Pressure Balance

Pressure drops across different sections of the CFB loopvary with changes in the loop seal aeration rate As dis-cussed above the solids macrow rate increases with the loopseal aeration rate and so does with the solids inventory inthe riser Thus the pressure drop in the riser increases withthe increment in the loop seal aeration rate The riserpressure drop also increases with the riser gas velocity at agiven aeration rate

The pressure drop across the standpipe is a combinedresult of the solids height and the relative velocity betweensolids and gas in it (Figure 5) With increasing aerationrates the relative velocity increases but the solids heightdecreases in the standpipe (Figure 4) This explains whythe rate of increase of pressure drop drops off at higheraeration rates

The pressure drop across the opening between the twochambers of the loop seal increases with a rise in the loopseal aeration rate The slit is like an oriregce through whichgas-solids macrow At higher aeration rates there is a highermacrow through the slit which in turn causes higher pres-sure drops The equation used to calculate the slit pressuredrop was taken from the experimental data of Jin15 andKuramoto16

The voidage of the recycle chamber increases as the loopseal aeration rate increases Thus the pressure drop in therecycle chamber decreases with the loop seal aeration rateThe bed expansion equation (10) gives a steady declinein bed density except at very high superregcial gas velocities

Figure 6 shows the pressure drops along the whole loopand their change at different solids macrow rates This reggureexplains the pressure balance and shows how it is main-tained at varying solids macrow rates Similar pressure distri-butions around the CFB loop were found in the experimentsof Basu et al5

Effect of Solids Inventory

Li and Kwauk19 used the results of Weinstein et al20

to suggest that axial voidage proregles are affected by thesolids inventory in the system This implies that the systemsolids inventory has an effect on the pressure distribution



Figure 4 Solids height in standpipe decreases with increasing loop seal airvelocity

Figure 5 Variation of pressure drop across the unit height of the standpipewith increasing gas solids slip velocity in the standpipe

Figure 6 Pressure proregle along the circulation loop at different solidscirculation rate around the CFB loop

around the loop Similar results were obtained by the presentwork Figure 7 shows that at a regxed aeration rate thesolids macrow rate changes with the system solids inventoryAt a given loop seal aeration rate the solids macrow rateincreases as the solids inventory increases And the opera-tional range of the loop seal aeration rate also increaseswith the solids inventory

Effect of Standpipe Size

Inmacruence of standpipe diameter on the solids macrow rateis shown in Figure 8 It shows that a smaller size standpipecan handle a given solids macrow rate at a smaller loop sealaeration rate However this decreases the solids storagecapacity in a smaller standpipe There would be a largechange in the solids height in the standpipe for a rela-tively small change in the solids macrow rate This makes thesystem less stable

Effect of Loop Seal Slit Height

The slit height does not play an important role as long asit does not provide signiregcant resistance (Figure 9) Thesolids macrow rates do not change much at different slitheights This is a result of smaller values of the linearvelocity of solids through the slit (0005 006 m s 1 inFigure 9) which offers a resistance small compared to thatin other components in the loop

Effect of Particle Size

Solids particle size has an effect on the solids macrow rate(Figure 10) Smaller size particle has higher solids macrowrate at a regxed loop seal aeration rate At a given riser gasvelocity smaller size of solids has lower umf and there-fore higher relative velocity (ug umf ) This gives largerpressure drops across the standpipe which result in highersolids macrow rate through the loop seal

Effect of Adjusting Parameters

Finally the adjusting parameters in the model are dis-cussed There are two main adjusting parameters in themodel calculation One is the fraction of the loop seal airentering the supply chamber of the loop seal and this para-meter can be obtained from experiments14 It was taken asa constant in the present model and it might change with thepressure distribution in the loop seal A lower value giveslower solids macrow rate at a given loop seal aeration ratebut the operating range of the loop seal air rate increases(Figure 11)

Another parameter is the constant c of the sharp-crestedtheory in the equation (11) As the pressure drop acrossthis section is not large compared to that in other placesthe inmacruence of c can be neglected

996 BASU and CHENG


Figure 7 Solids macrow rate increases with loop seal air velocity at differentsolids inventories (ug= 3 msplusmn1)

Figure 8 Solids macrow rate increases with loop seal air velocity for differentstandpipe sizes (ug= 3 msplusmn1)

Figure 9 Solids macrow rate varies with the loop seal air rate at different slitheights (ug= 3 msplusmn1)

Figure 10 The variation of solids macrow rate with loop seal air velocity fordifferent particle sizes (ug= 3 msplusmn1)

CONCLUSION

A simple model of the operation of the loop seal incirculating macruidized beds is developed on the basis ofpressure balance It showed that the sharp-crested theory canbe applied to estimate solids macrow rate through the loop seal

The loop system operated only within a certain rangeof loop seal aeration rate for a given riser gas velocity Ata given riser gas velocity the system can not be madeto operate well at any solids recycle rate even throughadjustments of the loop seal aeration When the riser gasvelocity varies the operating range of loop seal aerationrate also changes

Other observations made are

1) The solids macrow rate increases with the loop seal airvelocity

2) The solids inventory in the system has an effect on thesolids macrow rate At a given loop seal aeration ratethe solids macrow rate increases as the solids inventoryincreases

3) The solids macrow rate decreases as the standpipe sizeincreases at a certain loop seal air rate

4) The slit size of the loop seal does not have a major effecton the solids macrow rate if the slit is adequately wide

5) At a given loop seal aeration rate smaller particles willhave a higher solids macrow rate

NOMENCLATURE

Ab cross-section area of the riser m2

Arc area of the loop seal recycle chamber m2

As area of slit between two chambers in the loop seal m2

Asc cross-section area of the supply chamber m2

Asp cross-section area of the standpipe m2

c constant in equations (11) and (12)dp solids particle diameter m

ds standpipe diameter mg acceleration due to gravity m s 2

Gs solids macrow rate kgs 1

hg loop-seal gap height mhden dense region height in the riser mhdil dilute region height in the riser mhr recycle chamber height mD hr suspended solids height above the weir mLs height of solids above the air distributor of supply chamber mlsc height of supply chamber mM0 solids inventory of the system kgPi pressure at Point I i a b c d e f g PaD Pi j friction loss including acceleration loss in IJ section ij ab bc cd

de ef fg ga Pa

Q air macrow of the loop seal m3 s 1

Qs solids volume macrow m3 s 1

u0 velocity of gas in the standpipe m s 1

ug velocity of gas in the riser m s 1

umf minimum macruidizing velocity m s 1

up absolute velocity of solids particle m s 1

ur macruidizing velocity in the recycle chamber m s 1

us superregcial velocity of solids in the standpipe m s 1

usg actual gas velocity in the standpipe msplusmn1

ut particle terminal velocity m s 1

D u superregcial gas velocity in the standpipe m s 1

W weir width mF s sphericity of a particled average fraction of the overall air entered the supply chamber of

loop seallaquo den voidage in lower dense region of the riserlaquo dil voidage in upper dilute region of the riserlaquo r voidage in the recycle chamberlaquo s voidage in the standpipemg viscosity of gas kg ms 1

raquog density of gas kg m 3

raquos density of solids kg m 3

REFERENCES

1 Knowlton T M 1988 Non mechanical solids feed and recycledevices for circulating macruidized beds Circulating Fluidized BedTechnologyII Basu P and Large J F (eds) (Pergamon Press Oxford)pp 31plusmn41

2 Yang W C 1993 L-valve equations Powder Technology 7749plusmn54

3 Chong Y O OrsquoDea D P Leung L S and Nicklin D J 1988Design of standpipe and non-mechanical V valve for a circulatingmacruidized bed Circulating Fluidized Bed Technology II Basu P andLarge J F (eds) Pergamon Press Oxford) pp 493plusmn500

4 Luo Z Y Ni M J Zhou J H Cheng L M Chang Z J andCen K F 1989 Solids recycle systems for circulating macruidizedbeds Proceedings of the 1989 International Conference on Fluid-ized Bed Combustion Manaker A M (ed) (ASME New York)pp 557plusmn562

5 Basu P Luo Z Y Boyd M Cheng L M and Cen K F 1999 Anexperimental investigation into a loop seal in a circulating macruidizedbed 6th Inter Conf on Circulating Fluidized Beds August 22plusmn27Wurzburg Germany pp 805plusmn810

6 Horio M 1997 Circulating macruidized beds Grace J R Avidan A Aand Knowlton T M (eds) Chapter 2 Hydrodynamics (Chapman ampHall London) pp 61plusmn65

7 Li J H Tung Y K and Kwauk M 1988 Axial voidage proreglesof fast macruidized beds in different operation regions Circulating Fluid-ized Bed Technology II Basu P and Large J F (eds) (PergamonPress Oxford) pp 193plusmn203

8 Basu P and Fraser S A 1991 Circulating macruidized bed boilersETHdesign and operations Chapter 2 Hydrodynamics (Butterworth-Heinemann Boston) pp 43plusmn44

9 Kunii D and Levenspiel O 1990 Flow modeling of fast macruidizedbeds Circulating Fluidized Bed Technology III Basu P Horio Mand Hasatani M (eds) (Pergamon Press Oxford) pp 91plusmn98

10 Kunii D and Levenspiel O 1991 Fluidization Engineering 2ndedn Chapter 3 Fluidization and Mapping of Regimes pp 80plusmn82Chapter 8 High-velocity Fluidization p 200 Chapter 15 CirculationSystems pp 374plusmn375 (Butterworth-Heinemann Boston)

11 Werther J 1993 Fluid mechanics of large-scale CFB units Circu-lating Fluidized Bed Technology IV Avidan A A (ed) (AIChE NewYork) pp 1plusmn14

12 Rhodes M J 1990 Principles of Powder Technology M J Rhodes(ed) Chapter 7 Pneumatic Conveying (John Wiley amp Sons NewYork) pp 148

13 Ergun S 1952 Fluid macrow through packed columns Chemical EngProgress 48 (2) 89plusmn94

14 Cheng L M Basu P 1999 Effect of pressure on loop seal opera-tion for a pressurized circulating macruidized bed Powder TechnologyVol 103 203plusmn211

15 Jin Y Wang Z W Zhu J X and Yu Z Q 1985 A study onparticle macrow between macruidized beds Fluidization rsquo85 Science andTechnology Kwauk M and Kunii D (eds) (Science Press BeijingChina Elsevier Amsterdam) pp 172plusmn185



Figure 11 Solids macrow rate increases with the loop seal air velocity atdifferent air fractions (ug= 3 msplusmn1)

16 Kuramoto K Kunii D and Furusawa T 1986 Flow of densemacruidized particles through an opening in a circulation system PowderTechnology 47 141plusmn149

17 King D F 1989 Estimation of dense bed voidage in fast andslow macruidized beds of FCC catalyst Fluidization VI Grace J RShemilt L W and BergougnouM A (eds) (EngineeringFoundationUnited Engineering Trustees Inc New York) pp 1plusmn8

18 Whites F M 1994 Fluidized Mechanics 3rd ed (McGraw-Hill IncNew York) pp 622plusmn623

19 Li J H and Kwauk M 1994 Particle-macruid two-phase Flow theEnergy-minimization Multi-scale Method (Metallurgical IndustryPress Beijing) pp 140plusmn148

20 Weinstein W Graff R A Meller M Shao M J 1983 The effectof the imposed pressure drop across a fast macruidized bed Fluidization

Kunii D and Toei R (eds) (Engineering Foundation UnitedEngineering Trustees Inc New York) pp 299plusmn306

ADDRESS

Correspondence concerning this paper should be addressed to DrP Basu Department of Mechanical Engineering Dalhousie UniversityHalifax Nova Scotia Canada B3J 2X4 E-mail PrabirBasuDalCo

The manuscript was communicated via our International Editor forCanada Professor Philippe Tanguy It was received 19 March 1999 andaccepted for publication after revision 6 September 2000

998 BASU and CHENG


Point D in Figure 1 is the solids surface in the standpipeThe friction on the discharge pipe (G-A) can be neglectedas it is rarely full Also the section of the cyclone (C-D)offers very low resistance So if any resistance at the exitof the riser is neglected Pb Pc 0 Pc Pd 0 andPg Pa 0 can be written The above pressure balanceequation now simplireges as

Pa Pb Pd Pe Pe Pf Pf Pg 0

2

Pressure Drop in the Riser

The pressure drop along the riser height is determinedby its axial voidage proregle Two main methods are usedto estimate the axial voidage proregle The axial voidageproregle may be divided into two parts a denser section anda leaner section The transition point between these twomay be estimated for riser reactors at a given set of operat-ing and geometric conditions7 In a CFB boiler this pointcoincides with the secondary air injection point8 So theaxial voidage proregle in the leaner section may be calcula-ted using the entrainment model of Kunii and Levenspiel9The voidage decay constant of the leaner section andvoidage of the lower dense bed were based on experimentalresults9

Thus the bed is divided into lower dense region and

upper dilute region along the bed height The total pressuredrop across the riser A-B also includes the frictional lossincluding acceleration loss D Pab Then the total pressuredrop Pa Pb becomes

Pa Pb 1 laquo den ghden 1 laquo dil ghdil D Pab 3

Where laquo den and laquo dil are voidages in lower dense region andupper dilute region respectively hden is dense regionheight in bed hdil is the dilute region height which isriser height less the dense region height Kunii andLevenspiel10 compared voidages in the lower denseregions of a macruidized bed (Table 1) A typical value forthe fast macruidized bed is chosen in the present modelcalculation

Bed internals may affect the axial voidage proregle Thereis also an effect of the bed exit11 However if one assumesthat the furnace is so tall that all particles are completely

992 BASU and CHENG


Figure 1 Loop seal solids recycle system

Table 1 Comparison of voidage inthe lower dense region of a macruidized

bed

Fluidized bed VoidageBubbling bed 045plusmn060Turbulent bed 060plusmn078Fast macruidization 078plusmn084




up ug ut 5




Pe Pd

Ls

150 1 laquo s3

laquo 3s

mg D u

w sdp2

175 1 laquo s3

laquo 3s

raquog D u 2

w sdp

6


D u uo us 7




Pe Pf 066As

Asc

12

Gs 8







laquo r

ur 1

ur 210




Qs cg12 D h32W 11




r v 12







EXPERIMENTS









Solids Flow Rate








994 BASU and CHENG
































996 BASU and CHENG






CONCLUSION









NOMENCLATURE










de ef fg ga Pa
















REFERENCES

























ADDRESS



998 BASU and CHENG





up ug ut 5




Pe Pd

Ls

150 1 laquo s3

laquo 3s

mg D u

w sdp2

175 1 laquo s3

laquo 3s

raquog D u 2

w sdp

6


D u uo us 7




Pe Pf 066As

Asc

12

Gs 8







laquo r

ur 1

ur 210




Qs cg12 D h32W 11




r v 12







EXPERIMENTS









Solids Flow Rate








994 BASU and CHENG
































996 BASU and CHENG






CONCLUSION









NOMENCLATURE










de ef fg ga Pa
















REFERENCES

























ADDRESS



998 BASU and CHENG







Solids Flow Rate








994 BASU and CHENG
































996 BASU and CHENG






CONCLUSION









NOMENCLATURE










de ef fg ga Pa
















REFERENCES

























ADDRESS



998 BASU and CHENG






























996 BASU and CHENG






CONCLUSION









NOMENCLATURE










de ef fg ga Pa
















REFERENCES

























ADDRESS



998 BASU and CHENG












996 BASU and CHENG






CONCLUSION









NOMENCLATURE










de ef fg ga Pa
















REFERENCES

























ADDRESS



998 BASU and CHENG


CONCLUSION









NOMENCLATURE










de ef fg ga Pa
















REFERENCES

























ADDRESS



998 BASU and CHENG








ADDRESS



998 BASU and CHENG


Documents

Analysis Loop Seal Operation