Page 228 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:46 PM Page 224
224 3. Heterogeneous Processes and Reactor Analysis

through an intermediate phase called “cloud” and cannot be o at least in most of v ed, erlook
the fluidized beds.
Fluid–solid mass and heat transfer in particulate fluidization
In fluidized beds, the gas bubbles in combination with the high heat-transfer area per
unit mass of catalyst, lead to mixing in the emulsion phase, and in turn to temperature
uniformity throughout the reactor (Smith, 1981). Considering that the same large area is
,
v also aailable to mass transfer one can easily understand why the temperature and con-
centration gradients between the fluid and particle surface are usually negligible. It should
be noted that the mass and heat transfer coeficients in fluidized beds are less than those f
ix
in fed beds, where large differences in temperature and concentration between the
er
we
fluid and the solids exist. Ho the area is much higher in fluidized beds as a result
,
v
of the use of smaller particles, which leads to higher products of transfer coef icient f
and area. Note that the mass transfer is a function of the transfer coeficient and the f
available surface.
The mass and heat transfer coeficient between the gas or liquid phase and the solid
f
v phase can be ealuated using the Chu–Kaiil–W 1981): etterath correlation (Smith,
 d G p  0.44
j j or 1.77   (3.539)
D H

(1 ) f 
This equation is valid within the limits
dG p
30 5000

(1 )
f
and
k  a  
 23
j f m (3.540)
D    
G a    D 
t f
h a   c  p
 23
j f m (3.541)
H    
cG a    f 
p t
where:
f the void fraction of the fluidized bed
,
G u the fluid mass superficial v kg/m , elocity 2 s
s
a t the total mass transfer area
a m the efe mass transfer area v fecti
D f the solute difvity in the fluid phase fusi
h f the heat transfer coef W/m icient, f 2 K
c p the specific heat capacity of the fluid, J/kg K
the thermal conductivity of the fluid, W/m K.
f
v At temperatures aboe 400 °C and for large particles, heat transfer by radiation should be
also taken into account.