5 replies to this topic

[trevortnt]

Hi everyone, I'm a general Chemical Engineering student and I will be resitting an exam in Process Engineerng soon. I'm having problems with a specific question type, involving the Single Pass Conversion formula SPC and Overall Conversion formula. I seem to be completely stomped. Can anyone give me any hints or tips as to how to go about this question.

7. The following flow chart (attached to this thread) describes a continuous steady state process for the catalytic conversion of A to B. The single-pass conversion of the reactor is 20%, and the separator can recover 95% of B from the reactor’s output stream into the product stream. Flow chart is attached.

(a) If the overall conversion is required to be 90%, calculate the component flow rates of A and B in each stream. [10]


any help appreciated.
thanks

Attached Files

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[latexman]

christo7,

It makes the math easier to me to use a basis of 100 moles. 90% overall conversion means the product contains 90 moles of B and 10 moles of A. The separator recovers 95% of B. 90/0.95 = 94.74 moles of B feeding the separator. 94.74 - 90 = 4.74 moles of B recycled to the reactor. SPC = 20%. 90/0.2 = 450 total moles of A fed to reactor. 100 moles of A is fresh feed (overall material balance 100 = 90 + 10). So, 450 - 100 = 350 moles of A recycled to reactor. 450 x 0.8 = 360 moles of A feeding the separator.

[trevortnt]

christo7,

It makes the math easier to me to use a basis of 100 moles. 90% overall conversion means the product contains 90 moles of B and 10 moles of A. The separator recovers 95% of B. 90/0.95 = 94.74 moles of B feeding the separator. 94.74 - 90 = 4.74 moles of B recycled to the reactor. SPC = 20%. 90/0.2 = 450 total moles of A fed to reactor. 100 moles of A is fresh feed (overall material balance 100 = 90 + 10). So, 450 - 100 = 350 moles of A recycled to reactor. 450 x 0.8 = 360 moles of A feeding the separator.

Thanks so much for your reply Latexman, true lifesaver! Wish I could have had an explanation, let alone one as clear and as concise as this. Just one query about the SPC formula. Is the formula like this:
SPC= (starting reactant in input - reactant in output product)/ reactant in input

which reactant in input is the denominator of the formula reffering too? Is it the one being fed to the mixer or the one being fed to the reactor? Thanks so much!

Edited by christo7, 20 August 2009 - 12:44 PM.

[latexman]

The formula I used in my head for SPC was simply (moles A reacted)/(moles A fed), but your equation is the same thing. I think of conversion more clearly in terms of reactants, not products, of course, they are tied together by their stoichiometry. Conversion applies at the reactor, not the mixer.

Edited by latexman, 20 August 2009 - 01:49 PM.

[trevortnt]

I'm getting it! Thanks, so if the second part of the question is:
( If the single-pass conversion was reduced to 5%, what would be the new component flow rates of A and B in each stream? Comment briefly on the
change from (a).

would it now become (90/0.05) for SPC, and now the reactor feed is 1800A, output reactor feed is 1710A which is also in the recycle stream.

thanks so much for the help.

[latexman]

Yes, but don't forget about the 10A that goes out with the product. That will make the recycle 1700A, right?