Yang Fluidization, Solids Handling, and Processing

	Gas Distributor and Plenum Design 235
Pbed =	pressure drop across the dense bed, Pa
Pgrid =	pressure drop across the grid, Pa
Ph =	pressure drop across the grid hole, Pa

φ= rate of formation of 0-dp micron fines, kg/min

REFERENCES

Blake, T. R., Webb, H., and Sunderland, P. B., Chem. Eng. Sci., 45:365 (1990) Chen, L., and Weinstein, H., AIChE J., 39(12):1901 (1993)

Hiby, J. W., Chem.-Ing.-Techn., 36:228 (1964)

Karri, S. B. R., PSRI Research Report No. 60 (1990)

Karri, S. B. R., Grid Design Chapter, PSRI Design Manual (1991)

Knowlton, T. M., and Hirsan, I., Fluidization, (J. Grace, and J. Matsen,eds.), p. 315, Plenum Press (1980)

Massimilla, L., Fluidization, (Davidson, et al., eds.), p. 133, Academic Press (1985) Merry, J. M. D., Trans. Instn. Chem. Engrs., 49:189 (1971)

Mori, S., and Moriyama, A., Inst. Chem. Eng., 18:245 (1978)

Roach, P. T., Fluid Dyn. Res., 11:197 (1993)

Shakhova, N. A., Inzh. Fiz. Zh., 14(1):61 (1968) Siegel, R., AIChE J., 22:590 (1976)

Sishtla, C., Findlay, J., Chan, I., and Knowlton, T. M., Fluidization VI, (J. R. Grace, L. W. Shemilt, M. A. Bergougnou, eds.), p. 581, Engineering Foundation , p. 581 (1989)

Whitehead, A. B., in: Fluidization, (J. F. Davidson, and D. Harrison, eds.), p. 781, Academic Press (1971)

Yang W. C., and Keairns, D. L., Ind. Eng. Chem. Fundam., 18:317 (1979)

Yates, J. G., Bejcek, V., and Cheesman, D. J., Fluidization V, (K. Ostergaard, and A. Sorensen, eds.), p. 79, Engineering Foundation (1986)

Zenz, F. A., and Othmer, D. F., Fluidization and Fluid-Particle Systems, p. 171, Reinhold Pub. Co. (1960)

Zenz, F. A., Inst. Chem. Eng. Symp., 30:136 (1968)

Zenz, F. A., and Kelleher, E. G., J. of Powder and Bulk Solids Tech., 4:13 (1980)

Zuiderweg, Proc. Int. Symp. On Fluidization, (A. A. H. Drinkenburg, ed.), p. 739, Netherlands University Press (1967)

5

Engineering and

Applications of

Recirculating and Jetting

Fluidized Beds

Wen-Ching Yang

1.0INTRODUCTION

In a conventional fluidized bed, fluid under pressure is passed through a bed of solids via a distributor plate. At a fluid velocity beyond the minimum fluidization or minimum bubbling velocity, visible bubbles appear. The fluid thus passes through the bed in two phases, the bubble and the emulsion phases. The bubble-induced solids mixing and circulation provide the liquid-like behavior of a bed of otherwise immobile solids. The liquid-like behavior of a fluidized bed allows continuous feeding and withdrawal of bed material. The vigorous mixing of solids in the bed gives rise to a uniform bed temperature even for a highly exothermic or endothermic reaction. This leads to easier control and operation. The advantages of a fluidized bed, compared to other modes of contacting such as a packed bed, are numerous and they are described in details in standard textbooks on fluidization. The fluidized beds are widely employed in various industries for both physical and chemical operations.

236

Recirculating and Jetting Fluidized Beds 237

The conventional fluidized beds also possess some serious deficiencies. The bubbles which are responsible for many benefits of a fluidized bed represent the fluid bypassing and reduction of fluid-solids contacting. The rapid mixing of solids in the bed leads to nonuniform solids residence time distribution in the bed. The rigorous solids mixing in the bed leads to attrition of bed material and increases the bed material loss from entrainment. Thus for many industrial applications, the conventional fluidized beds have been modified to overcome those disadvantages. Those modifications, in many ways, alter substantially the operational characteristics of the fluidized beds, and also change the design and engineering of the beds. It is the intent of this chapter to document two of the non-conventional fluidized beds in details: recirculating fluidized beds with a draft tube and the jetting fluidized beds.

2.0RECIRCULATING FLUIDIZED BEDS WITH A DRAFT TUBE

The recirculating fluidized bed with a draft tube concept is briefly illustrated in Fig. 1. In application as a coal devolatilizer, dry coal is introduced into the devolatilizer below the bottom of the draft tube through a coal feeding tube concentric with the draft tube gas supply. The coal feed and recycled char at up to 100 times the coal feed rate are mixed inside the draft tube and carried upward pneumatically in dilute phase at velocities greater than 4.6 m/s. The solids disengage in a fluidized bed above the top of the draft tube and then descend in an annular downcomer surrounding the draft tube as a packed bed at close to minimum fluidization velocity. Gas is introduced at the base of the downcomer at a rate permitting the downward flow of the solids. The recirculating solids effectively prevent agglomeration of the caking coal as it devolatilizes and passes through the plastic stage. Many other applications have also been reported and they will be discussed in Sec. 2.4, “Industrial Applications.”

This concept was first called a recirculating fluidized bed by Yang and Kearins (1974). Several other names have also been used to describe the same concept: the fluid-lift solids recirculator (Buchanan and Wilson, 1965), the spouted fluid bed with a draft tube (Yang and Keairns, 1983; Hadzismajlovic et al., 1992), the internally circulating fluidized bed

(Milne et al., 1992; Lee and Kim, 1992); or simply a circulating fluidized

Recirculating and Jetting Fluidized Beds 239

lating fluidized bed with a draft tube. The downcomer region can be separately aerated. The gas distribution between the draft tube and the downcomer can be adjusted by changing the design parameters at the draft tube inlet. Because the draft tube velocity and the downcomer aeration can be individually adjusted, the solid circulation rate in the bed can be easily controlled. The fact that the solid circulation rate depends primarily on the entrainment rate at the draft tube inlet rather than along the surface of the entire spout, allows easier manipulation through adjustment of design and operational parameters to control the residence time and cycle time distribution. Stable operation over a wide range of operating conditions, a solids circulation rate up to 100 metric tons per hour, and a solids loading of 50 (weight of solids/weight of air) in the draft tube have recently been reported by Hadzismajlovic et al. (1992) in a 95.3 cm diameter bed with a 25 cm diameter draft tube using 3.6 mm polyethylene particles.

Operating conditions for a recirculating fluidized bed can be flexible as well. The bed height can be lower than the draft tube top or just cover the draft tube top so that a spout can penetrate the bed as in a spouted bed. The bed height can also be substantially higher than the draft tube top so that a separate fluidized bed exists above the draft tube. Rather than operating the draft tube as a dilute-phase pneumatic transport tube, one can fluidized the solids inside the draft tube at lower velocities to induce the necessary recirculation of the solids. Several studies were conducted in this fashion (Ishida and Shirai, 1975; LaNauze, 1976; LaNauze and Davidson, 1976). The draft tube wall can also be solid or porous, although most of the studies in the literature employ a solid-wall draft tube. Claflin and Fane (1983) reported that a porous draft tube was suitable for applications in thermal disinfestation of wheat where control of particle movement and good gas/ solid contacting could be accomplished at a modest pressure drop. The concept can also be employed as liquid-solids and liquid-gas-solids contacting devices (Oguchi and Kubo, 1973).

The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keairns (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below.

Recirculating and Jetting Fluidized Beds 241

The pressure balance for the dense phase in the downcomer in the circulating fluidized system shown in Fig. 2 can be expressed as:

Eq. (1)	P	= ρ	b	gH	mf	(1 − ε	bd	) −	τ d Sd
	1−4								Ad

A similar expression can be written for the pressure balance in the draft tube as:

Eq. (2)

− P

= ρ

b

gH

mf

(1 − ε

br

) +

τ r Sr

2−3

Ar

Combining Eqs. (1) and (2), we have

Eq. (3)

ρ b gH mf (εbr

− εbd ) =

τ d Sd

+

τr Sr

Ar

Ad

It was experimentally confirmed, using capacitance probes, that the solids flow down the downcomer at close to minimum fluidization, thus ε bd = 0.

The bubble voidage in the draft tube, ε br , was calculated on the basis of the velocity of a rising gas slug relative to its surrounding solids. The total gas superficial velocity in the draft tube, Ufr , can be derived to be

	Eq. (4)	U fr = U slugε br + Umf	+	Vsrε mf
				1 − ε mf

The slug velocity, Uslug , is defined as the rising velocity of the slug relative to the particle velocity at its nose and can be expressed as

	Eq. (5)		U slug = vp		+ 0.35 gD , and
	Eq. (6)		v p = (U fr		− U mf ) + Vsr
	Substituting Eq. (5) into (4), we have
	Eq. (7)	εbr	=	(U fr − Umf ) −Vsrεmf /(1 − εmf )
				(U fr − U mf ) + Vsr + 0.35 gD

The flow rate of particles in the downcomer and the draft tube are related by a mass balance as follows:

242 Fluidization, Solids Handling, and Processing

Eq. (8)

Vsr ρ s Ar = Vsd ρ s Ad = Wsd Ad = Wsr Ar

By solving Eqs. (4) and (7) simultaneously, the mass flux can be calculated provided the wall shear stress is known as a function of particle superficial volume flow rate. Botterill and Bessant (1973) have proposed several relationships for shear stress, however, these are not general. LaNauze (1976) also proposed a method to measure this shear stress experimentally.

A similar application of the concept as a slugging lifter of solids was studied by Singh (1978) based on the two-phase theory of fluidization and the properties of slugs.

2.2Draft Tube Operated As A Pneumatic Transport Tube

Most of the applications for the recirculating fluidized bed with a draft tube operate the draft tube as a dilute phase pneumatic transport tube. Hence we will discussed this system in more details.

Downcomer and Draft Tube Pressure Drop. Typical experimental pressure drops across the downcomer, P1-4, and the draft tube, P2-3 , show that they are essentially similar. Successful design of a recirculating fluidized bed with a draft tube requires development of mathematical models for both downcomer and draft tube.

Downcomer Pressure Drop. When the downcomer is less than minimally fluidized, the pressure drop can be estimated with a modified Ergun equation substituting gas-solid slip velocities for gas velocities (Yoon and Kunii, 1970), as shown in Eq. (9).

Eq. (9)

L

é

μ (U gd + U pd )(1 - ε d )2

ρ f (U gd + U pd )2 (1 - ε d )ù

DP

=

ê150

+ 1.75

ú

1− 4

g

ê

d

2φ 2ε

2

d

φ

ε

ú

p

p

d

c ë

s

d

s

û

When the downcomer is fluidized, the downcomer pressure drop can be calculated as in an ordinary fluidized bed as:

Eq. (10)

P1− 4 = L(1 − ε d ) ρs

Recirculating and Jetting Fluidized Beds 243

The voidage in the downcomer, ε d , can be assumed to be the same as the voidage at minimum fluidization, ε mf . The voidage at minimum fluidization can be determined in a separate fluidized bed. The agreement between the calculated and the experimental values is usually better than ±10% (Yang and Keairns, 1978a). When the downcomer is not minimally fluidized, the bed voidage depends on the amount of aeration and solid velocity. Use of the voidage at minimum fluidization is only a first approximation.

Draft Tube Pressure Drop. The pressure drop across the draft tube, P2-3, is usually similar to that across the downcomer, P1-4, in magnitude.

Thus, for a practical design basis, the total pressure drop across the draft tube and across the downcomer can be assumed to be equal. In most operating conditions, the pressure drop at the bottom section of the draft tube has a steep pressure gradient due primarily to acceleration of the solid particles from essentially zero vertical velocity. The acceleration term is especially significant when the solid circulation rate is high or when the draft tube is short.

The pressure drop inside the draft tube is more complicated because it involves acceleration of solid particles from essentially zero vertical velocity. However, the model for calculating the pressure drop in vertical pneumatic conveying lines suggested by Yang (1977) can be applied. The acceleration length can be calculated from numerical integration of the following equation.

Up r 2

U prdU pr

Eq. (11)

DL = òUp r1

3

C

DS

ε − 4.7

ρ f (U gr -U pr )2

- (g +

f pU pr

2

)

r

- ρ f )d p

c

4

(ρs

2D

The solid friction factor, fp , can be evaluated with the equation proposed by Yang (1978).

(1- ε

r

)

é

(Re ) ù−0.979

Eq. (12)

f

p

= 0.0126

ê(1- ε

r

)

t

ú

ε

(Re )

3

ê

ú

r

ë

p û

The lower limit of integration, Upr1 , is derived from

Eq. (13)

Wsr = U pr ρs (1 − ε r )

with ε r = 0.5, and the upper limit, Upr2 , by the following equation:

244 Fluidization, Solids Handling, and Processing

							æ	2	ö
Eq. (14)	U	pr	=U	gr	-U	t	ç1 +	f pU pr	÷ ´ε		4.7
							ç	2 gc D	÷	r
							è		ø

The total pressure drop in the acceleration region can be expressed as

Eq. (15)

L

L 2 f g ρ f Ugr

2

DP2-3

= ò0ρs (1 -ε r )dL + ò0

dL

gc D

L f

p

ρ

(1- ε

r

)U

2

é

ρ

(1- ε

r

)U

2

ù

+

ò0

s

pr

dL + ê

s

pr

ú

2gc D

ê

gc

ú

ë

ûat×L

If the draft tube height is less than the acceleration length, the integration of Eq. (15) is carried out through the whole length of the draft tube. If the draft tube height is larger than the acceleration length, the integration of Eq. (15) is carried out for the total acceleration length, and the extra pressure drop for the rest of the draft tube can then be included to give the total pressure drop in the draft tube. The suggested equations have been applied to actual experimental data satisfactorily (Yang and Keairns, 1976a).

Gas Bypassing Phenomenon. Because of different design and operating parameters, the distribution of the total flow between the draft tube side and downcomer side can be very different. A summary of the design parameters that will affect the gas bypassing is presented graphically in Fig. 3 for the flat distributor plate design. Similar parameters apply for the conical distributor plate. The important design parameters that will affect gas bypassing are the area ratio between the downcomer and the draft tube, [(Dc2 - D2)/D2], the diameter ratio between the draft tube and the draft tube gas supply or the diameter of solid feeding tube, (D/dD and D/ds), the distance between the distributor plate and the draft tube inlet, L, the area ratio of the draft tube gas supply and the concentric solids feeder, [(dD2 - ds2)/ds2], and the design of the downcomer gas supply nozzle. In addition to the design parameter, the operating parameters will also affect gas bypassing. The relative strength of the concentric jets of the draft tube gas supply and the solids feeder determines the half angle of the combined jet, and the jet velocity determines the jet penetration. The jet velocity of the downcomer gas supply nozzles is also important if the jets are horizontal and directed toward the draft tube.