## Professional Reference Shelf

#### Example CD12-4: Catalytic Oxidation of Ammonia

 Massimilla and Johnstone 25 studied the catalytic oxidation of ammonia in a fluidized-bed reactor. Under their experimental conditions, the reaction was first order, dependent only on the ammonia concentration, and without a significant change in volumetric flow rate. In one of their runs, 4 kg of catalyst was used with a gas flow rate of 818 cm 3 /s at reaction conditions. A conversion of 22% of the entering ammonia was obtained. Predict this conversion using the Kunii-Levenspiel model. Other data: Solution A. Mechanical characteristics of bed Step 1. Gravitation term, : Step 2. Porosity of bed at minimum fluidization: (CD12-29) Step 3. Gas velocity at minimum fluidization:
 (CD12-25)
 Step 4. Entering gas velocity: Step 5. Is u 0 within a reasonable operating range? Check u t .
 (CD12-33)
 Are the N Re in the proper range for use of Equations (CD12-25) and (CD12-33)? Thus u 0 is 5.4 times, and well below u t . Step 6. Bubble sizes, d b 0 , d bm , and d b :
 (CD12-38)(CD12-39)

Step 7. Bubble sizes, d b 0 , d bm , and d b : The unexpanded bed height is 38.9 cm. The expanded bed height will probably be 40 to 50% greater, say about 60 cm. We will therefore assume that the average bubble size will be taken as the one calculated for h /2 = 30 cm.
Step 8. Average bubble diameter:

Step 9. Rise velocity of single bubble:

(CD12-35)

Step 10. Rise velocity of a bubble when many bubbles are present:

(CD12-36)

From Figure CD12-6 for glass spheres with d p = 0.105 mm,= 0.4.

Step 11: Fraction of bed in bubble phase:

(CD12-46)

Step 12. Bed height:

 Good guess ofh = 60 cm
Since the estimated bed height of 60 cm is sufficiently close to the calculated value of 63.2 cm, one can proceed further in the calculations without making a new estimate of h.

B. Mass transfer and reaction parameters

Step 1. Bubble-cloud mass transfer coefficient:

 Order ofmagnitude parameters

(CD12-53)

Step 2. Cloud-emulsion mass-transfer coefficient.,

(CD12-55)

Step 3. Volume of catalysts in the bubble per volume of bubble:

 b = 0.01 (assumed)

Step 4. Volume of catalyst in clouds and wakes/ cm 3 of bubbles:

(CD12-63)

Step 5. Volume of catalyst in emulsion/cm 3 of bubbles:

(CD12-64)

rearranging Equation (CD12-75).

Step 6. Calculate K R and X from Equation (CD12-27):

where

(CD12-72)

Solving for X gives

This is close to the observed value of 22% conversion.