Mass Transfer Effects: Immobilized and Heterogeneous Reaction Systems - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 867

Therefore, increasing agitation or flow in the reactor leads to a significant increase in the
effective reaction rate.

Example 17-2. Maneuvering a Space Satellite. Hydrazine has been studied extensively for use
in monopropellant thrusters for space flights of long duration. Thrusters are used for altitude
control of communication satellites. Here, the decomposition of hydrazine over a packed bed
of alumina-supported iridium catalyst is of interest (Smith O.I. and Solomon W.C. Ind. Eng.
Chem. Fund . 21: 374, 1982). In a proposed study, a 5% hydrazine in 95% helium mixture is to
be passed over a packed bed of cylindrical particles 2.54 mm in diameter and 5.08 mm in
length at a gas-phase superficial velocity of 10 m/s and a temperature of 750 K. The kine-
matic viscosity of helium at this temperature is 4.5

10 4 m 2 /s. The hydrazine decomposi-
tion reaction is believed to be external mass transfer limited under these conditions. If the
packed bed is 0.0508 m in length, what conversion can be expected? The bed porosity is
35% and the diffusivity of hydrazine in helium is 6.9

10 5 m 2 /s at 298 K. Assume

isothermal operation.
Solution. Hydrazine (A) decomposition reaction is represented by

H 2
which is highly exothermic and thus suitable as rock fuel. The reaction is carried out isother-
mally in a packed bed reactor. As a first approximation, we assume that the reactor can be
characterized as a plug flow reactor or PFR (i.e. neglect the back-mixing caused by dispersion
in the bed). A schematic of the reactor is shown in Fig. E17-2.1 below,
Mole balance of A (hydrazine) in the differential volume (between z and z

N H N

2 þ 2 4 /

2

þ d z ) leads to

A c UC A j Z A c UC A j zþ d z þ r A A c d z ¼ 0 (E17-2.1)

where A c is the cross-sectional area of the bed and U is the superficial flow velocity. Dividing
Eqn (17-2.1) by A c d z and letting d z

0 and assuming U and A c are constant, we obtain

/

d C A
d z þ r A ¼ 0

U (E17-2.2)

The reaction occurs only on the catalyst surface with catalyst not moving,

ðr A ÞA c d z ¼ k c ðC A C AS Þa c A c d z (E17-2.3)

where C AS is the concentration of A on the catalyst surface at axial location z and a c is the
specific surface area of the particles in the bed,

z = 0 z = L

z z+dz

U f Ae

U ,

C A0 , C B0

C Ae , C Be

FIGURE E17-2.1 A schematic of a PFR.