CHEG Sample Exam Question

[dresherm]

Greetings,

I'm trying to confirm my train of thought is correct for a problem that I've come across. The problem is:

MethylMercaptan (CH3SH) is produced from stoichiometric amounts of H2S and methanol (CH3OH) by reaction:

CH3OH + H2S -> CH3SH + H2O

Molecular Weights:

CH3OH 32.0

H2S 34.1

CH3SH 48.1

H2O 18

The H2S selectivity (percentage of the total conversion to the desired product) is 90%. The other products are formed from the reaction of methanol and H2S. Of the feed to the reactor (fresh feed pluc recycle), 85% reacts on each pass through the reactor. The unreacted feed containing stoichiometric amounts of H2S and methanol is recycled.

The fresh feed (lb mol/hr) of H2S required to produce 1,000 lb/hr of mercaptan is most nearly.

A. 19

B. 21

C. 23

D. 27

My approach:

Lb mols/hr of CH3SH out of seperator

1,000 lb/hr ( 1 lb mol/48.1 lbs) = 20.79 lb mols/hr CH3SH

The way I read it, based on the amount entered into the reactor, 85% actually reacts and 90% of that is converted. So

Lb mols/hr of H2S

20.79 lb mols/hr (1/0.85)(1/0.9) = 27.17 lb mols/hr H2S

The book does not take into account the 85% reacting, so their answer is 20.79/0.9 = 23.1. My question is, is my logic sounds and I should ignore the book?

Thanks a bunch for the help!

 

[snickerd3]

who wrote the question? It is a practice exam question or something else. I ask because it could influence my response.

 

[snickerd3]

FWIW, I got the 23 when I worked the problem.

 

[dresherm]

who wrote the question? It is a practice exam question or something else. I ask because it could influence my response.

Thanks for the reply back Snick . NCEES wrote it.

 

[snickerd3]

The reason I asked the author of the question is that the actual exam questions are FULL of excess information about the situation that have no bearing on the answer they are looking for and really boils down something really simple.

The problem doesn't say after how long the reactor was operating to determine the value so I assumed it was the first hr of the cycle which would have no recycled feed stream. So there is 90% conversion of the H2S.

 

[dresherm]

FWIW, I got the 23 when I worked the problem.

Can you give me an idea as to why to ignore the 85%?

 

[dresherm]

The reason I asked the author of the question is that the actual exam questions are FULL of excess information about the situation that have no bearing on the answer they are looking for and really boils down something really simple.
The problem doesn't say after how long the reactor was operating to determine the value so I assumed it was the first hr of the cycle which would have no recycled feed stream. So there is 90% conversion of the H2S.

I completely understand what you mean about excess information.

I'm still lost as to why to ignore the 85%. The problems says the recycle stream contains stoichiometric amounts of H2S and of CH3OH, which means after it mixes with the feed stream the fractions of each component are the same and just the flow to the reactor increases. Taking this into consideration should mean that the reaction should still only reach 85% completion.

 

[snickerd3]

the way i read the problem as you typed it

the 90% conversion relates to the pure feed stream

the 85% would related to the combined stream of pure + recycle

Without a time given I assumed start up and the recycle part of the stream would not exist only leavinf the pure feed. hence the entire set of info on recycle is superfulous info you can ignore. and it is a quick and simple calc.

it seems deceptive, but things like this happen a lot on that test.

 

[snickerd3]

maybe check the ncees site for erratta to be sure since we both read the problem differently

I didn't buy the NCEES sample tests when I was preparing for the test.

 

[CbusPaul]

The unreacted feed is recycled back so it can be ignored in the overall count. Eventually you'd have 100% reaction of your feed because all of the unreacted continually gets recycled. Is this why you can ignore the 85%?

 

[dresherm]

Ahhhhh, I see now. Combing what both of you said drove it home for me. Thanks a bunch!!!

 

[hombre]

There are very few questions in this official practice exam that have taken me 6 minutes or less. I am hoping they have cherry picked some of the harder questions from past tests, or it could be that I am just in trouble. Anyone else having trouble with it?

 

[dresherm]

There are very few questions in this official practice exam that have taken me 6 minutes or less. I am hoping they have cherry picked some of the harder questions from past tests, or it could be that I am just in trouble. Anyone else having trouble with it?

It has taken me a few min more on some of the problems than others. I did read somewhere tho, that it would be helpful if you did a quick look thru of the test before starting and rate the problems based on difficulty. By doing this I plan on finishing the easier problems in under 6 min leaving extra time to try and solve the more difficult ones. Good luck, I know I'll need it.

 

[hombre]

There are very few questions in this official practice exam that have taken me 6 minutes or less. I am hoping they have cherry picked some of the harder questions from past tests, or it could be that I am just in trouble. Anyone else having trouble with it?

It has taken me a few min more on some of the problems than others. I did read somewhere tho, that it would be helpful if you did a quick look thru of the test before starting and rate the problems based on difficulty. By doing this I plan on finishing the easier problems in under 6 min leaving extra time to try and solve the more difficult ones. Good luck, I know I'll need it.

I did the test again today and was much more comfortable, but my score still wasn't stellar (especially if you factor that I had some of the conceptual questions memorized). Oh well, we'll see what happens. Good luck!

 

[m_mamdouh94]

I don't understand the solution made by NCEES, can anyone help ?

1) the reference manual the selectivity is "defined as the molar ratio of the desired product to undesired product", while here they treat it as the conversion factor ?

2) I don't understand as well, why in the solution they assumed that the selectivity is for the methanol and then they used the stoichiometric amount to find the H2S while in the problem they mentioned that it is the "H2S SELECTIVITY"

can anyone explain it to me please !!!!