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Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 93
3.3 T Reactors o-Phase Agitated w 93

where:
Q z the volumetric flow rate in the reactor at length z
x the conersion of the limiting reactant at the same length v
the expansion f actor .
R
In f the space-time as defined aboe is the superficial space time, as it is based
v
ed-beds,
ix
on the total volume of the bed. The real space-time for a bed of bed v oidage is
V
R (3.100)
Q
i
ix Another important feature related to mean residence time in fed-beds is the fluid holdup
based on the empty bed v olume h e,t . If the fluid occupies the whole empty bed
,
volume, then h e,t . Ho this is not the case when h e,t i.e. when there is a
v
,
er
we
bypass of the fluid from some regions in the bed, most commonly in the upper section of the
bed in a downflow operation, and the fluid is a liquid. In this case, the real residence time is
hV e,t R
t (3.101)
m
Q
i
The actual residence time of a reactor is measured by employing residence time distrib u-
tion (RTD) experiments utilizing tracing techniques. Furthermore, seeral correlation v
forms estimating the fluid holdup can be found in the related literature.
Space velocity in flow r s eactor
The number of reactor volumes of feed at specified conditions, which can be treated in a
unit time is called “space v and is (Le 1972) enspiel, v
elocity”
1 Q
s i (3.102)
V
R
This parameter is frequently used in ion-exchange and adsorption operations in fixed-beds
and it is frequently called “relati olumetric flo v e v w rate”:

Q QV
s i rel R Q rel (3.103)
V V
R R
The most common unit of Q rel is bed volumes per hour (BV/h). Space velocity is also used
ix in catalytic reactors, especially in three-phase fed-beds, and is referred to as liquid hourly
space velocity (LHSV) for the liquid phase, and gas hourly space v elocity (GHSV) for gas
phase. As mentioned abo space-time and space velocity are measured under the
e,
v
er
,
entrance conditions . Ho for space v other conditions are frequently used
v
,
elocity
we
(Fogler, 1999). F the LHSV is measured at 60 to 75 °F and GHSV at standard or e xample, ,
temperature and pressure.