Friday, December 24, 2010

For Atrios


Anonymous said...

Merry Christmas Prof Rabett

Little Mouse

Anonymous said...

Merry Christmas!

John said...

OK, this Philadelphia complaint is that NJ drivers can't drive or park.

The NYC complaint is different: One New York City cab driver complained, "it's not the traffic. It's the clueless drivers from Jersey. Ya know what they do?? when the light turns red, [voice rising in fury] they STOP !!

EliRabett said...

As Eli once said to a friend from the midwest who was driving in DC, "Don't be nice, it only confuses them"

mmm onion soup said...

Until this point I have been completely dismissive of the recent arctic sea ice extent dip as just yet another noisy blip. But looking at JAXA today the dip seems to be almost at a point worth thinking about.

David B. Benson said...

To make the chemical reaction

C + 2H2 --> CH4

go, I suppose I have to add some energy?

Mark said...

David, the reaction is actually exothermic (-74.8 kJ/mol), and the Gibbs energy change of the reactio is -12.1 kJ/mol, so it would occur spontaneously under standard conditions. However, at 298 K the reaction rate is very slow because of a high activation energy. Thus, you are correct, you need to add energy to make the reaction go.

Mark said...

Slight unit mistake in my previous post. The Gibbs energy change of the reaction is -12.1 kcal/mol, which is about -48 kJ/mol. (Darn organic chemists!)

Anonymous said...

Sea level rise?

EliRabett said...

Yes, to add to what Mark said, you have to break the bonds between the H atoms first to initiate the reaction which accounts for the activation energy. What you have written is the overall reaction, which is composed of a long chain of reactions, best done on catalysts that atomize the H atoms first (at high temperature and pressure) on the surface of the catalyst, so it is not simple to write what happens. Also, it depends on what form the C atoms are in, graphite, diamond, atoms, etc.

David B. Benson said...

Mark & EliRabett --- The H2 comes from
2H2O + electricity --> 2H2 + O2
unless you know of a way to obtain just 4H- ions from water.
The carbon is 2--10 micron particles initially at 700 K and electrically neutral.

With that setup can I avoid adding energy whilst obtaining the methane?

[Just thought experiments, I'm not planning on building a demonstrator.]

EliRabett said...

First law of thermodynamics:

You can't win.

With the snark out of the way, there is, obviously an accurate way of doing the calculation for the total heat (aka enthalpy) needed to carry out the reactions at constant pressure, using measured heats of reaction. Of course, you also need to account for the heat needed to make your form of carbon.

Let us summarize,

2 H2O --> 2 H2 + O2
C (10 u) ---> C (graphite)
C(graphite) + 2 H2 --> CH4

We don't know the enthalpy of reaction for the second reaction, but it will be something, so that is some additional energy we will need.

To do this we need heats of formation, which is the amount of heat needed to create a molecule from what is called the elemental form, the form that you find most common under normal conditions, 1 atm, room temperature or 298K. For hydrogen, oxygen and carbon these are H2, O2, and graphite. We take their heats of formation as zero, ie you don't have to add any heat to get these.

Note that you can have negative heats of formation for molecules that are created when you react the elemental forms and get heat from the reaction as well as positive heats of formation for molecules where have to provide heat to create them from the elemental forms

The heat of reaction ΔH (some of the super and subscripts are not allowed by Blogger) is then the sum of the heats of reaction of the products less the sum of the heats of reaction of the reactants

ΔHo = ΣnΔHf(products) - ΣmΔHf(reactants)

where n and m are the coefficients for each molecule in the reaction

We also need to specify whether the molecule is in gas, liquid or solid phase. It takes different amounts of heat to form, for example, water in each phase. Using a table of heats of formation: (everything in kJ/mol, that's kilojoules per mole)

2 H2O (l) --> 2 H2(g) + O2 (g)

ΔHo = 2ΔHf(H2) + Delta;Hf(O2)- 2ΔHf(H2O)

ΔHo = 2 x 0 + 0 - 2x-285.8 = 571.6 kJ/mol

and for

C(graphite) + 2 H2(g) --> CH4(g)

ΔHo = ΔHf(CH4) - Delta;Hf(C)- 2ΔHf(H2)

ΔHo = -74.8 + 0 + 2x0 = -74.8 kJ/mol

So the net for your process is 571.6 - 74.8 = 496.8 kJ/mol

This is the BEST you can't do, because there will be inefficiencies in the process, and does not account for the activation energy issue (see above).

In short, people have been trying to improve on this for years. The most efficient way of doing so is the water shift reaction, which essentially gasifies coal using superheated steam, e.g.

2C(coal) + O2(g) --> CO(g) + heat

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

both of these processes are exothermic, and you use the heat to heat the steam as well as using a simple catalyst.

You can throw in the somewhat endothermic water gas reaction which is going to happen to some extent anyhow

C(coal) + H2O(g) --> CO(g) + H2(g)

This was/is used to produce town gas aka coal gas, the stuff we used in ovens stoves before the 60s. Inevitably you wind up with some CO which accounts for the old movies bit where people kill themselves by sticking their heads in the oven and turning the gas on. You can't do this with natural gas, so then you kill yourself by sticking your head in the oven and turning the gas on and when nothing happens after an hour or so, other than you getting light headed, you excerpt your head from the oven and light up a ciggie, killing yourself in the explosion.

John Farley said...

Eli, thanks for the thermochemistry lesson.

Anonymous said...

John Farley said...
Eli, thanks for the thermo chemistry lesson.

15/10/10 11:40 AM

Anonymous said...
Eli...? "Why would Eli want to comment outside his expertise?" Is a_ray_in_dilbert_space, right?

Just askin...

15/10/10 11:40 AM

And so it goes;
We who await with baited breath...
The One who thumped his foot, at us all.

Mark said...

What Eli said, plus:

Second Law of Thermodynamics: you can't break even, either.

Anonymous said...

Mark, that is what Jesus says as well?:o)

David B. Benson said...

Eli --- Thank you, that was helpful.
What I was thinking about was what to do with the carbon from
when there is also a supply of otherwise unused electricity. Of course with carbon monoxide there is also FT to consider...

David B. Benson said...

Here is a better way to make syngas:

EliRabett said...

David, basically the same as this, High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria

the issues are what is the best catalyst, and having found it, is it economic. The energy trade off is the energy cost of the solar concentrator and the catalytic system. The energy to run the process is free. The problem in figuring all this out is that the journals publish chemistry, and clearly all these systems work at some level, but which is cost effective? Eli's friend who will remain nameless gets enthusiastic about this sort of stuff, but doesn't do the costing.

David B. Benson said...

Eli --- Yes, it is now a engineering issue, well motivated by the thought of $5/gallon gasoline by the end of this year.

Anonymous said...

$5 per gallon!

I wish!

UK price is currently hovering around £6 per (UK) gallon, i.e. about $9 per (UK) gallon. :-(

Cymraeg llygoden

David B. Benson said...

Cymraeg llygoden --- Rotate Britian by 90 degrees and the whole thing fits comfortably in the boundaries of this not very large state, the state of Washington.

EliRabett said...

And it's an imperial gallon! (still??)

Anonymous said...

Hello, David B. Benson said...
Here is a better way to make syngas(sic): ...

I think we all know that there are many, very smart, well educated people who comment on Eli's soap box in this veil. The question that bugs us 'the unwashed' is; that you all seem so worried about the little things like CO2 & now a new singas because the world is going to have to pay $6 a gallon-mark. When we the unwashed, are all pissed off because they(Who the Hell are they...)have devalued the dollar by eighty percent in just 6 years. How come the big pressing stuff does not bother you people? Who's watching out for 'the unwashed', do you think? If it is because you might loss your job, don't worry the hole of warren world is facing the same dilemma and they know there is nothin they can do... Don't worry... you know it is all about: 1 & O.

EliRabett said...

Both a good and a bad question. First, this is really not the place to discuss politics and economics except perhaps as it applies to the climate disruption issue. Second, our contamination of the atmosphere with carbon seriously threatens all those things you care about, just not right now, but to deal with the threat to your kids, you have to take action right now.

BTW, Eli knows the dollar has not been devalued by eighty percent in 6 years. Whoever told you that was lying. What you may be talking about is the cost of oil, but the cost of oil is only a small part of the cost of living which has not changed much in the last eight years, because other things have remained stable.

Anonymous said...

UK was easier to spell than impereal after the half a bottle of Châteauneuf-du-Pape I'd imbibed. :*)

Yes, it's still imperial units. Though I've largely retreated from Empire to SI.

Cymraeg llygoden

Anonymous said...

Thank you for your honest answer Eli, No one told me the 'eighty percent' number it is of my own:) making. We have yet to run the six years by my count. I started my count in mid 2007. I know I am a millionair, & I owe me.)