8/10/2019 FCC Slip Velocity Characteristics in the Riser of Circulating Fluidised Bed

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Chem. Eng. Technol.

20

(1997) 4 91 -494

49 1

Slip Velocity Characteristics in the Riser of Circulating Fluidised Bed

N.

Bala Subramanian and

C

Srinivasa Kannan*

Experiments were carried out in a conventional circulating fluidised bed to measure the

axial pressure profile and total pressure drop, which covered a wide range of operating con-

ditions. Materials belonging to the Geldart

A fine material) as well as the Geldart

B

coarse

material) categories have been used in the present work. Slip velocity is determined from

the total pressure drop and noticed that the slip velocity is much higher than the free fall

velocity of single particle for Geldart

A

type material, while it is approximately equal to

the free fall velocity of single particle for the Geldart B type materials.

A

model is developed for slip velocity taking into account all the hindrance effects: particle-

particle, particle-wall, and particle agglomeration. Predictions

of

the present model are

validated with the data due to present study and the data reported in the literature.

1

Introduction

Circulating fluidised beds are used for a number of pro-

cesses, mainly gas-solid reactions such as calcination of alu-

minum hydroxide to high grade alumina, ore reduction, and

waste incineration. In recent years, circulating fluidised

beds are increasingly used in catalytic reactions like FCC

and the Fischer Tropsch Synthesis.

The behavior of circulating fluidised bed differs from a

conventional fluidised bed, because of absence of bubbles

and entrained flow of solids. A circulating fluidised bed

operates at much higher gas velocities than those used in

conventional fluidised bed and lower than those used in

pneumatic conveying. Extensive work has been reported on

circulating fluidised beds with respect to regime classifica-

tion, flow structure, residence time distribution, and heat

transfer. The following summary can be made from a criti-

cal review of earlier investigations:

Variation of pressure drop with the solids circulation

rate is a sigmoid curve. The pressure drop is low and in-

creases slowly at low solids rate. On the other hand, if

the pressure drop is high, it approaches an asymptotic

value at high solids rates. Pressure drop increases sharp-

ly with solids circulation between these two boundaries.

The region where low pressure drop is observed refers to

pneumatic conveying. The conventional fluidised bed

regime refers to the region of high pressure drop. The

fast fluidised bed regime occurs between these bound-

aries and the pressure drop or) bed density depends not

only on solids rate but also on the gas velocity [l 1.

In a fast fluidised bed, solids move in entrained flow.

Pressure drop along the riser length is divided into three

zones. A dense bed near the solids feed point, a dilute

* N . Bala Subramanian, Chemical Engineering Department, Indian

Institute of Technology, Madras-600036, India, Dr. C. Srinivasa

Kannan (author to whom correspondence should be addressed),

Chemical Engineering D ivision, Central L eather Research Institute,

Madras-600029, India.

bed near the riser exit, and a transition zone between the

two. Inflection or transition point from dense bed to

dilute bed is marked where the second differential coef-

ficient of axial pressure profile curve is equal to zero

[6 8).The pressure drop in dense as well as dilute beds

is significantly influenced by the flow rate of the phases

and particle characteristics.

Slip velocity is higher than free fall velocity of single

particles and it varies from the bottom to the top of the

riser

[9-111.

Flow is assumed to be core-annulus

[12, 131.

Though extensive work is reported on the basic aspects of

circulating fluidised bed, the work on slip velocity is very

limited. Knowledge of slip velocity is important as it direct-

ly influences the solids concentration and mean residence

time of solids. The present paper studies the slip velocity be-

havior in circulating fluidised bed and covers a wide varia-

tion of material characteristics Tab. 1).

2 Experimental

The schematic diagram of the experimental setup is shown

in Fig. 1; it consists of a riser

5 )

with a provision for con-

tinuously feeding the solids at a controlled rate from the

hopper

(7).

A

gas solid separator

6 )

and a bag filter were

provided at the top of the riser for separating solids and

gases. The movement of solids is cocurrent upward with air

introduced at the bottom of the column. Air for fluidisation

Table

1.

Material characteristics.

Sand 412 2650 668 1

Sand 117 2650 529

Resin 530 1480 7941

Resin

385

1480

3044

Silica gel 384 676

1378

FCC 81 900 17.2

WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1997

0930-75 16/97/0709-0491 17.50+ .50/0

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Chem. Eng. Technol. 20 1997) 491-494

i i

I I I

I ~ I

493

1 I l l l l

Uslip/Uo n Eq. (3) represents the ratio of true fall velocity

of particle in a medium of finite population to free

fall

velocity of particle in an infinite medium; E accounts for the

buoyancy effect. The term [ 1 + 1

E) ~]

n the denomina-

tor represents the hindrance effect due to the presence

of

other particles. The exponential term accounts for the mo-

mentum transfer correction. Since the momentum transfer

effect and hindrance effect are shown in terms of solid con-

centration, Eq. 3) may be modified as

4)

where n is 1 for creeping flow and

2/3

for intermediate

range of Reynolds number [

1

Re

lo3] and nearly four for fine material [Ar