9 replies to this topic

[vinnay1999]

Hi

 

Need help with this problem 

 

 

Methanol is synthesized from carbon monoxide and hydrogen in a catalytic reactor. The fresh feed to the process contains 32.0 mol% CO, 64.0 mol% H2 and 4.00 mol% N2. This stream is mixed with a recycle stream in a ratio of 10.00 mol recycle/1 mol fresh feed to produce the feed to the reactor, which contains 14.0 mol% N2- The reactor effluent goes to a condenser from which two streams emerge: a liquid product stream containing essentially all of the methanol formed in the reactor, and a gas stream containing all of the CO, H2, and N2 leaving the reactor.

 

The gas stream is split into two fractions; one is removed from the process as a purge stream, and the other is the recycle stream that combines with the fresh feed to the reactor.

 

For a methanol production rate of 100.0 mol/h, calculate the fresh feed rate (mol/h), the molar flow rate and composition of the purge gas, and the overall and single-pass conversions.

[breizh]

Hi,

Show us your work and your difficulties, we may be able to support.

A process flow diagram associated with the Xcel sheet is welcome.

BTW, we don't do homework.

Breizh  

[vinnay1999]

Hi

 

Here is an excel sheet I made with the data given (attached) . I could calculate some fields given (highlighted in green ) . I could not arrive the value for the purge stream .  May be it is going into a circular reference . What did i miss ? Can't figure out yet 

 

 

 

Vinay Kumar 

Attached Files

•  PROBLEM EXCEL.xlsx   15.57KB   11 downloads

[breizh]

Hi,

Review your mass balance:  Feed =Purge + Product    Mass In=Mass out

 

N2 in feed is transferred into purge, no accumulation.

CH3OH is fully condensed, not the other gas (H2, CO, N2)

 

 

 

Good luck

Breizh  

[vinnay1999]

Hi

 

Thanks for the reply . I incorporated your input but the overall mass isn't balancing (some variation ) . Don't know where it went wrong . My excel sheet is attached 

 

My procedure 

1. The recycle N2 mole percent (15%)  is arrived by N2 balance across fresh  feed (32 %), reactor feed (14 %) and the recycle streams (using the ratio 10:1 recycle to fresh feed ) 

 

2. The fresh feed flow rate is  arrived based on the stoichiometric equation  100 moles methanol requires 100 moles of  CO  , so fresh feed would be 100/32*100 - (given 32 mol % CO in feed). To this we add the purge quantity which will be calculated eventually .

 

3. The reactor inlet molar flow rate is the sum of the recycle  and fresh feed . Also the molar percentage of  nitrogen in reactor feed is 14 %. Remaining 86 % is CO and H2 . Now here I observe that these components are in the molar percentage ratio of 1 : 2 (as they also react in the same ratio)  .So i divided 86 into 1: 2 which gave 28.67 and 57.33 

4 . The reactor product has  15 mol % of N2 as we work backwards from recycle stream (as the same gases are recycled after condensing and purging) . Condensing only strips methanol and purge has all the unreacted nitrogen  (which we put into the system as fresh feed ) plus the proportionate CO & H2. 
 

5. The reactor product CO and H2 (unreacted ) is nothing but the reactor feed quantities - (CO and H2 in methanol (100 and 200 moles ) )



 

Regards 

 

P Vinay Kumar 

Attached Files

•  PROBLEM.xlsx   15.74KB   11 downloads

Edited by vinnay1999, 20 October 2024 - 04:29 AM.

[breizh]

Hi,

 

 

Regarding your process, there appears to be an issue. The product exits the reactor in a gaseous state and then passes through a condenser where the methanol is condensed.

 

this means you need to review block 4, this will change the quantity and composition.

Same for block 5 and 6 ,3,2 which are the consequence.

 

Notes:
 

Your excel sheet is difficult to read btw.

 

Better to have a separate drawing where you identify the streams and balances on another part of the sheet with

composition of all streams, the moles balance and the mass balance. 

Same for energy balance (when applicable).

 

This is the way I was taught 45 years ago.

The solver function is a good tool to support this kind of work.

 

Hope I put you in the right direction. I used Solver to support the calculation

 

Good luck

Breizh

Attached Files

•  vianney Methanol.xlsx   17.28KB   10 downloads

[vinnay1999]

Hi 

 

I was going through your excel calculation. I see that there is methanol in the purge . But the methanol is totally separated in the condenser ???

[breizh]

Hi,

How do you know that all Methanol is fully condensed? Not clear to me.

Regarding Methanol in the purge, you may add a constrain in the solver to get 0.

Note: These are the results of a calculation and optimization.

BTW I'm not very comfortable with the results because the composition in the purge is not the same that the off gas or recycling.

Breizh

[vinnay1999]

It  is stated in the question that all the methanol formed in the reactor is completely condensed . 

[vinnay1999]

Hi Breizh

 

 

Thanks  for your time and valuable inputs/insights . I have revised the calculation based on your inputs and some additional working  . I think it is ok now (attached excel sheet). 

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Regards

 

Vinay Kumar 

 

 

Attached Files

•  RECYCLE AND PURGE.xlsx   100.85KB   9 downloads