CHEMICAL REACTION ENGINEERING// The catalyst is available as 0.2 cm radius spher
ID: 473434 • Letter: C
Question
CHEMICAL REACTION ENGINEERING// The catalyst is available as 0.2 cm radius spherical pellets that have density of 2 g/cm3
catalyzes the reaction A -> B at 250 C.
a) You fill a 5 L PFR with catalyst pellets, which pack in the tube with a void fraction of 25%.
If you flow 0.1 mol/s of pure A at 2 atm through the reactor, what conversion of A do you
expect to get? (k’=0.8 cm3/ g-cat.s)
b) You find that the actual conversion is only 75% of what you expected, and you conclude
that internal diffusion is limiting the reaction. What is the effectiveness factor, , of the
catalyst particles under these conditions?
c) What would you suggest to make the reaction the surface reaction-limited rather than
diffusion-limited?
Explanation / Answer
Mass of catalyst = volume x density = (1-0.25) x 5 L x 2 g/cm3 = 7500 g
From units of k this is second order reaction,
Q = nRT/P = 0.2 mol/s x 0.082 x 523.15 K / 2 atm == 4.289 L/s
CA0 = 0.2 mol/s / 4.289 L/s == 0.0466 mol/L
For PFR;
(1/CA - 1/0.0466) = 0.8 cm3/ g-cat.s x ( 7500 g / 4289 cm3/s )
1/CA =22.86
CA = 0.0437 M
Conversion = 6%
Effextiveness factor = rate with diffuion / rate without diffuion = 0.75 x rate / rate == 0.75
Thiele modulus = sqrt ( k/d) x L
Increase or higher the value of k will make reaction the surface reaction-limited rather than
diffusion-limited?