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148 3. Heterogeneous Processes and Reactor Analysis

where: h the total fluid holdup based on the void volume of the bed.
v,t
”
This fraction could be viewed as a “fed-bed wetting ef If y icienc . f h and
ix
e,t
h 1, the bed is completely filled with fluid (fully wetted). The term h is analogous to
v,t v,t
f the catalyst wetting eficiency f for trickle beds (Section 3.7.3). Ho this equality is v er we,
w
valid solely in fed beds where a single fluid flows through. The actie volume of the v
ix
solid, which is occupied by the fluid, amounts to the fed-bed volume occupied by the ix
fluid minus the volume occupied by fluid:
(3.306)
h e,t h e,t h e,t V ( ) h e,t ) V
V R h V e,t R V R R (1 R
and the mass of the solid in contact with fluid is

h h
e,t (1 ) V e,t V (3.307)
p R bR

Then, in the material balance, the following corrections should be made:
h e,t (3.308)

h h
1 e,t (1 ) e,t h h h (3.309)

e,t v,t e,t

h vt , b (3.310)
b

3.6.3 Hydraulics

Nonideal flow in fixed beds
olv
The classic analysis of reactors ines two idealized flow patterns— plug flow and
v
v mixed flow. Though real reactors neer fully follow these flow patterns, in many cases, a
number of designs approximate these ideals with negligible error. However, deviation from
ypically, ideality can be considerable. Tin a reaction v we can hae se essel, eral immedi- v v
ate cases closer to plug or mixed flow. Of course, nonideal flow concerns all types of reac-
ed beds,
tors used in heterogeneous processes, i.e. f fluidized beds, continuous-flo w tank
ix
we
v
reactors, and batch reactors. Ho we will focus on fed beds and batch reactors,
,
er
ix
which are the common cases.
The study of nonideal flow and liquid holdup can be done by residence time distribution
(RTD) experiments (tracing techniques) or by use of correlations deried from literature. v
During this step, physical mechanisms that are sensitie to size are in v estigated separately v
rambouze,
from chemical (kinetic or equilibrium) studies (T 1990). Here, the fed bed is ix