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:  
161 image  
       
    Solution  
       
    A. Mechanical characteristics of bed  
       
    Step 1. Gravitation term, :  
       
   

image 12eq162.gif

 
       
    Step 2. Porosity of bed at minimum fluidization:  
       

image 12eq164.gif

163 image



(CD12-29)
     
    Step 3. Gas velocity at minimum fluidization:  
       
image 12eq166.gif   image 12eq165.gif

(CD12-25)
       
    Step 4. Entering gas velocity:  
       
image 12eq167.gif   image 12eq168.gif  
       
    Step 5. Is u 0 within a reasonable operating range? Check u t .  
       






image 12eq169.gif 		 

image 12eq170.gif


(CD12-33)
       
    Are the N Re in the proper range for use of Equations (CD12-25) and (CD12-33)?  
       

imag e12eq171.gif

 
       
    Thus u 0 is 5.4 times, and well below u t .  
       
    Step 6. Bubble sizes, d b 0 , d bm , and d b :  
     










image 12eq172.gif

 

image 12eq173.gif


(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:  
       





image 12eq175.gif

 

image 12eq174.gif

 
       
    Step 9. Rise velocity of single bubble:  
       
   

image 12eq176.gif

(CD12-35)
       
    Step 10. Rise velocity of a bubble when many bubbles are present:  
       




image 12eq177.gif

 

(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:  
       

image 12eq180.gif

 


(CD12-46)
       
    Step 12. Bed height:  
       
   

image 12eq181.gif

       

Good guess of
h = 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 of
magnitude parameters






183

 



(CD12-53)
       
    Step 2. Cloud-emulsion mass-transfer coefficient.,  
       

 

image 12eq184.gif

(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:  
       







image 12eq186a.gif

 

imag e12eq186.gif


(CD12-63)
       
       
    Step 5. Volume of catalyst in emulsion/cm 3 of bubbles:  
       

image 12eq188.gif

 

image 12eq187.gif


(CD12-64)
       
    rearranging Equation (CD12-75).
       
    Step 6. Calculate K R and X from Equation (CD12-27):  
       
   

image 12eq189.gif

 
       
    where  
       




 

image 12eq190.gif



(CD12-72)
       
    Solving for X gives  
       
   

image 12eq192.gif

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