4 replies to this topic

[Faizan Ahmed]

Hello, Hope you are all in best of health & spirit, I am a newly appointed process engineer on Ammonia Unit. I need to calculate the performance of the Ammonia Converter with S-200 Basket arrangement. Can i please get a method for evaluation. Thanks  

[sgkim]

Hi Faizan, 

If you are really interested in "the performance" of the ammonia converter, define the term first from your own serious contemplation.  Then evaluate your own plant and compare the results with the others.  If you ever encounter any trouble in your processing, ask some help here in CHeresourses.  Lots of professionals would be eager to help you.  What's your idea on the term "performance" ? 

 

~Stefano

[Faizan Ahmed]

The term ''Performance'' is being referred to as efficiency of Ammonia Converter . We have an S-200 Basket of Haldor Topsoe Make . I was aiming towards calculating the conversion per pass of the converter . Can you please suggest me a method for it.? Thanks in advance 

[ADITYA.DUBEY]

Dear Faizan

 

I have worked with KBR technology ammonia reactor. And I will let you know the process to calculate the performance of the converter.

 

There are two types of performance for the reactor.

 

1) Per pass conversion

2) Per basket conversion

 

Usually in convertors there are multiple baskets (In our case it was 3 baskets) which are in series and Syn gas (N2+H2) enters into this in series(Axial flow) as well as parallel(radial flow) manner to attain the maximum conversion and also to control the bed temperature as the reaction is highly exothermic.

 

It is difficult to calculate per basket conversion as it needs flow & composition details at every streams. Though you can measure the conversion with respect to rise in temperature across each beds.(For each % conversion, temp rise is ~ 40 Deg C .)

 

To calculate per pass conversion you just need the total inlet flow and composition which I assume will be N2,H2 and inerts.

You also need outlet flow and composition which will be N2,H2,NH3 and inerts.

 

By simple stoichiometric balance, you can easily calculate per pass conversion.

It usually comes ~17% in the best cases.

 

I will be happy to further clear any doubts if you provide more data.

 

P.S.- I have worked in Ammonia industry some 5 years before and what I remember ,I have passed it here. Please verify the information as I might miss some information.

 

 

Regards

Aditya

[sgkim]

Ahmed and Aditya, 

 

Conversion would be the most important factor in the ammonia converter considering the factors such as reaction rate, heat of reaction, and also chemical equilibrium.  Assuming that selectivity of the reaction to NH3 be 1.00 and that no vent loss of gas involved, I have tried to find out how to calculate 'the conversion'. 

 

(1)  Measuring flow rate at an elevated temperature is almost impossible. But the conversion per pass or overall can be calculated easily from any single component composition data at inlet and outlet:

 

   mole fractions of component i  at inlet "1"and outlet "2"....xi1    comp. 1=N1, comp. 2=H2, comp. 3=NH3, ...   x11+x21+x31+... =1 at inlet,   x12+x22+x32+....=1 at outlet

   molar flow for comp. i  . ..mi1, at inlet,   mi2, at oulet    then   total flow at inlet "1",  M1= m11+m21+m31+...,   -ditto- at outlet "2",  M2= m12+m22+m32+...     

          

   Let the conversion of N2 be "X",  then molar flows of N2 shall be  x11*M1, and x11*M1(1-X) at inlet and outlet respectively  

   The molar flows of H2 will be (x21*M1), and (x21*M1-3*x11*M1*X) at inlet and outlet respectively since 3 moles of H2 will be consumed to react with 1 mole of N2.

   Likewise the molar flows of NH3 will be (x31*M1), and (x31*M1+2*x11*M1*X) at inlet and outlet respectively since 2 moles of NH3 will be produced from 1 mole of N2.

   Then total number of moles M2 at outlet will become M1 + (-1" x11*M1*X - 3*x11*M1*X + 2x11*M1*X) or  M2 = (M1-2*x11*M1*X).   

   

   While, the outlet mole fraction of N2 at outlet:  x12 = m12/M2 = (x11*M1-x11*M1*X) / (M1-2*x11*M1*X) = (x11-x11*X)/(1-2*x11*X) .......(1),  

   then X cal be calculated  from inlet and outlet N2 composition data only:                   X =  (x11-x12)/x11/(1-2*x12) ....(2a)

   Or, in the similar fashion,  from H2 and NH3 concentration :                                       X =   (x21-x22)/x11/(3-2*x22)....(2b)

                                                                                                                                        X =   (x32-x31)/x11/{2*(1-x32)...(2c)    

 

(2) When it comes to the performance of the catalytic reactor, the following as well as the 'conversion' shall be evaluated together.

 

   Operating temperature and pressure - the lower the better assuming the same evaluation bases,

   Space velocity - the higher the better - ditto- ,

   Catalyst life and/or regeneration cycle time - the longer the better - ditto -,

   Maximum allowable content of inerts and poisonous material - the higher the better - ditto - . 

  

Hope the above helps, 

Stefano G. Kim

Edited by sgkim, 05 January 2017 - 06:23 AM.