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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: spunkee on February 25, 2005, 11:29:21 PM

Title: steamhydrocarbon reforming process
Post by: spunkee on February 25, 2005, 11:29:21 PM
The name is just scary enough... but here is my problem:

"How many kg of methane would be needed to produce 1 kg of hydrogen using the steamhydrocarbon reforming process? (Assume each step is 100% efficient.)"

The steamhydrocarbon reforming process is a three step reaction:

1. First Step At 1100 degrees C and a Ni catalyst:

H2O (g) + CH4 (g) --> CO (g) + 3H2 (g)

2. Second Step At 400 degrees C and metal oxide catalyst:

CO (g) + H2O (l) --> CO2 (g) + H2 (g)

3. No special temps or catalyst for this one:

CO2 (g) + 2OH[-] (aq) --> CO3[2-] (aq) + H2O (l)

Now, I am really lost on this one. Is this a molality problem? And if so, do I use all three reactions, just the first two, or just the first one. I tried finding a similar example of a problem like this in my book but it has been unsuccessful. The section that talks about this in my book simply gives the three reactions without explaining how to do a problem using them. All help is very much appreciated.  :animatedwink:
Title: Re:steamhydrocarbon reforming process
Post by: Mitch on February 25, 2005, 11:59:53 PM
Since your only interested in Hydrogen as a product, you tell us which equations you should use. ;)
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 26, 2005, 12:10:03 AM
hmm, ok. I guess I am using more than one step because the wording in the question says "Assume each step is 100% efficient." This was what was confusing me initially but since both the first and second reactions produce hydrogen as a product, I am guessing I use the first two reactions?? (I'm sorry I am no good at chemistry, please don't laugh)

The tricky thing is, I just don't recall ever using two equations to solve a problem like this before and I can't find an example like it in my book.
Title: Re:steamhydrocarbon reforming process
Post by: Mitch on February 26, 2005, 12:15:55 AM
You will only need equation 1 and 2. First calculate how many moles of hydrogen is 1 kg of hydrogen. In chemistry it is a lot easier to deal with things in moles not killograms.

Quote
I'm sorry I am no good at chemistry, please don't laugh
We're here to help not to laugh.  :D
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 26, 2005, 12:24:05 AM
ok there are 500 mol of H2 in one kilogram of H2
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 26, 2005, 12:25:49 AM
Quote from: Mitch on February 26, 2005, 12:15:55 AM
We're here to help not to laugh.  :D

and it is MUCH appreciated. I try so hard to get chemistry and it just doesn't come easy for me most of the time :-[ so you are a blessing.

Ok, I think I know what to do. Use mole ratios to get the mole of methane and then convert that to kg? I know the first mole ratio is going to be 1 to 3 but I am unsure about the other mole ratio.
Title: Re:steamhydrocarbon reforming process
Post by: dexangeles on February 26, 2005, 04:43:49 AM
hey Mitch,

why are both equations 1 and 2 needed?
i guess i've forgotten my gen chem  ???
Title: Re:steamhydrocarbon reforming process
Post by: Mitch on February 26, 2005, 04:48:28 AM
Because the 2nd equation also produces H2 and it is made by one of the products of equation 1.
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 26, 2005, 05:26:38 AM
do you happen to know what the second mole ratio will be by any chance?  :penguinsmilie:
Title: Re:steamhydrocarbon reforming process
Post by: Mitch on February 26, 2005, 02:20:31 PM
You to tell me how many moles of Hydrogen is in 1 kg of H2.
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 26, 2005, 10:23:58 PM
Quote from: spunkee on February 26, 2005, 12:24:05 AM
ok there are 500 mol of H2 in one kilogram of H2

I thought I figured that out already.. but maybe it is wrong  :-\? This is what I did to get it:
(1 kg H2) * (1000 g H2/1 kg H2) * (1 mol H2/2 g H2)= 500 mol/kg

OK, I think I figured this out... (as long as 1 kg of H2 has 500 moles, that is)
I took the 500 mol H2/kg and multiplied it by the mole ratios. 1 mole of H2 per 1 mole of CO and then 1 mol of CO per 1 mol of CH4, giving me 500 mol of CH4. Next I took 500 mol of CH4 and converted it to kg which gave me 8 kg CH4, and hopefully that is it...  :happy:
 :happy:
Title: Re:steamhydrocarbon reforming process
Post by: Mitch on February 26, 2005, 11:16:52 PM
Sorry you did do it right. I just missed that first post.

Now, I will do something very uncommon and give you a big chunck of the puzzle. This is just because it would be really hard to pepper you with questions, so that you could see it. I think in this case it works out better if I tell you the mole-to-mole relationship and you try to figure out why it is.

For every mole of CH4 you will produce 4 moles of H2.
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 27, 2005, 07:43:49 PM
hmm, I think that I MIGHT know this. I would say the mole ratio is 4 moles of Hydrogen for every one mole of methane because the initial reactant in equation 1 is one mole of CH4. This one mole yields 3 moles of H2 in the products of the first reaction and 1 mole of H2 in the product of the second reaction. If you add the total nuimber of moles of H2 yielded from both reactions, you get 4 moles, thus 1 mole of CH4 yields 4 moles TOTAL of H2. Hopefully, I'm right??  ???
Title: Re:steamhydrocarbon reforming process
Post by: Mitch on February 27, 2005, 08:05:30 PM
Very close, but methane isn't in the second equation so how could it make H2?

What methane does make is 1 mol of CO, and that 1 mol of CO can go on in equation 2 to make 1 mol of H2.
Title: Re:steamhydrocarbon reforming process
Post by: Donaldson Tan on February 27, 2005, 08:21:05 PM
alternatively, given 100% conversion, you can try deriving the overall equation for this system, ie.

H2O +  CH4 + 2OH--> CO32- + 4H2
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 27, 2005, 08:32:59 PM
Ok, thanks  :D Looks like I'm ready to go then. Just need to convert using my mole ratio and then change moles of methane into kg of methane.

Thanks alot! All your help on this problem (and the RV=nRT one from the other day) is soo very greatly appreciated. You all are like the chem gods  :king: This place is great, keep doing what you're doing   :thumbup:
Title: Re:steamhydrocarbon reforming process
Post by: dexangeles on February 27, 2005, 09:28:21 PM
I can't believe I forgot to add all three equations :Flush:
I think it's cause I'm reading physics this weekend :bangin:
Title: Re:steamhydrocarbon reforming process
Post by: spunkee on February 28, 2005, 12:14:17 AM
I know.. after geodome posted, I felt like.. DUH?! I didn't remember using mole ratios from more than one equation... maybe because I should've REALIZED I needed to add them.. :doh: Grr.. Chem is a constant war... and I always seem to lose lol

Physics? :o Sure spells F.U.N. to me...  :-X