SFRPG

The forum for Science Fiction Role Playing Game inspiration and information! So Say We All!
It is currently Mon Nov 20, 2017 5:44 am

All times are UTC




Post new topic  Reply to topic  [ 13 posts ]  Go to page 1 2 Next
Author Message
 Post subject: Fuel Purification
PostPosted: Wed Feb 01, 2017 3:19 pm 
Offline
User avatar

Joined: Fri Dec 11, 2009 2:26 am
Posts: 238
Location: The DED Zone
This is a rough draft of a treatise on fuel purification plants. Any and all feedback would be appreciated. And if someone spots a flaw in my logic or the science my rusty brain is calling upon, please correct me.

Fuel Purification in Traveller

The other day, a player in one of my campaigns asked me how long it would take to refuel using a world's ocean—the system was devoid of gas giants—and then purify it. I tried to find an answer in the various editions of Traveller that have been published (I consulted CT, MT, TNE, T4, & T20) but came up empty. I thought about giving the standard six hours reply, but it didn't sit right with me. Although my chemical engineering career was rather limited, there's enough there that I decided to explore this question a bit more in depth.

I started with the traditional gas giant skimming operation that's been a staple of player activity since the beginning of the game. The technique, duration, and depth vary among the editions, but the objective is the same: You head into the gas giant to skim hydrogen. It's full of "impurities" and must be refined, otherwise you risk drive damage or misjumps. No one wants that. But what are these impurities that are getting filtered out?

A look at the gas giants in our solar system indicate that besides hydrogen (in the form of H2), there's also helium, methane, ethane, water, ammonia, hydrogen sulfide, ammonium sulfide, carbon dioxide, carbon monoxide, a few heavier hydrocarbons, and various noble gasses.

Intuitively, it makes sense. And since all of these compounds are in a gaseous state, it's easy enough to wave one's hand and say the filters in the fuel purification plant can handle them.

But when a starship is trying to extract hydrogen from water, it's not nearly the straightforward process as obtaining hydrogen from a gas giant. First and foremost, the hydrogen is tied to oxygen in a very strong bond. How strong is this bond? It's so strong that when industry needs hydrogen, it looks elsewhere, namely natural gas (methane).

In chemistry, there's a property known as the heat of formation. Without making your head explode, the heat of formation is the energy change that occurs when compounds are formed. Sometimes heat is released (explosion!); sometimes it's absorbed. The heat of formation for liquid water is -285.8 kJ/mol. The negative sign means that heat is released when hydrogen and oxygen combine to make water. But when you reverse the reaction, the negative sign becomes a positive one. That means if you want to break up a water molecule, you're going to need to supply a lot of energy.

Fortunately, starship crews have plenty of energy available to them thanks to that fusion power plant down in engineering. But before I get into that, I'd like to point out a better source of hydrogen: methane.

Methane (CH4) has a heat of formation of -74.9 kJ/mol. That's just slightly more than 1/4 of the energy requirement (26.2%) to break apart a water molecule. Also, pound for pound (kilogram for kilogram) methane (molecular weight of 16) provides twice as much hydrogen as water (molecular weight of 18).

In fact, the most common alkanes (hydrocarbons that end in -ane) have lower heats of formation and/or supply more hydrogen per unit volume than water. Yet another reason why current industrial practices utilize natural gas rather than water to obtain their hydrogen.

Back to water as a hydrogen source. Another glaring problem with water is it's dirty. Water is typically referred to the universal solvent because nearly everything has a solubility factor in water. We're not just talking dirt either. On a planet with life, you're going to have all sorts of icky microbes to deal with. Before you can even think about liberating the hydrogen from water you've got to filter all that junk out. Anyone who's ever worked in a water treatment plant, whether municipal or corporate, knows how arduous a process this is. There are all sorts of physical and chemical treatments than one can perform, and they're far more intensive than filtering gasses.

Here's a more common example: swimming pools. People or pool vacuums skim the bulky stuff or let their filter basket collect it. The basket needs to be emptied periodically or you lose water pressure. You kill the microbes with chlorine and filter out the tiny detritus and particles suspended in solution with a filtration system that uses diatomaceous earth. But now you've got chlorine in there, which you absolutely do not want in your fusion power plant.

After filtering the water, it's time to split that molecule. There's a process known as electrolysis, whereupon an electric current is passed through the liquid, splitting the molecule. Hydrogen is drawn to the cathode, while oxygen is drawn to the anode. To make the hydrogen extraction process more efficient, a membrane (think of it as a really tight filter) is placed before the cathode so that only hydrogen passes through.

It not a terribly energy efficient process, otherwise hydrogen cars would be everywhere by now. Since energy is money, researchers are trying to drive it using solar or wind. Unfortunately, the watts/sq meter ratios for these two technologies aren't all that good. Fortunately, this isn't a problem in Traveller as our intrepid characters have fusion power plants at their disposal to provide plenty of cheap energy.

There are two methods under study right now to improve the process that I think are applicable to Traveller: high-temperature electrolysis and high-pressure electrolysis.

In the former, water is vaporized, and the ensuing steam heated further to several hundred degrees (peak efficiency appears to be at 850 C, but lower temperatures are good enough) whereupon an electric current is applied to the steam to break the bond. From there, hydrogen can be separated from oxygen via the method used to purify the hydrogen from a gas giant.

With high-pressure electrolysis, water is pumped at high pressure into a tank where it is subjected to an electric current. The freed hydrogen passes through a membrane attempting to reach the cathode. The advantage here is that the hydrogen gas is already compressed and the operating temperature is only 70 C.

Think that's time consuming? Well consider harvesting hydrogen from some icy moon or comet in the Oort Cloud. At least you can pump water. Ice needs to be dug up. Anyone who's dealt with iced over gutters in winter knows how difficult that's going to be. I read somewhere that ice in the outer solar system becomes so hard due to the extremely cold temperatures that it's virtually rock.

Players will have to use a laser drill, or maybe even their weapons, to blast off chunks off ice. I don't foresee a pick axe being all that effective. Afterwards, the ice will need to be melted before being subjected to the purification process that liquid water goes through (No rocks please!). Energy isn't a problem, but it takes time to melt all that ice. After that the meltwater will need to be treated just like above.

A better option this far out from the warmth of stars is our good friend methane. Methane is a liquid below 112 K (-161.5 C) and a solid at 91 K (-182.5 C). In between, you might find methane slush. If you've followed the reports of the Huygens probe at Titan or Far Horizons at Pluto, you know that it's a completely different game out there. Liquid methane can be pumped, and slush is easier to transfer than super chilled water ice. And methane vaporizes at such a low temperature, that unless you apply pressure to it, it will do so shortly after it's pumped into the holding tank inside the ship.

=============================

From here I plan on offering rules that can be applied to the game based on the edition one is using. But before I do that, I want to make sure the science and processes are sound.

_________________
\_/
DED


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Wed Feb 08, 2017 5:38 am 
Offline

Joined: Thu Sep 18, 2008 12:41 pm
Posts: 409
Location: Northern California
Once you have the huge Traveller handwave of Fusion, All The Fusion Ever, Fusion Everywhere, I don't think cracking water ice for hydrogen is much of a problem. Mining ice isn't an issue when you can literally bring the heat of stars to bear on the issue and its available in a form you could fit in the (volume of a) modern automobile. Where you and I might bring a portable gasoline generator to help an issue, in the Traveller universe, you can get a Protium-Protium cycle fusion reactor for perhaps 10 times the volume of a modest gasoline generator except it'd power a small town. This granularity of fusion in many editions of Traveller would make many of our problems, which are typically constrained by the bottleneck of the power we can bring to bear on an issue, trivial.

I think that other chemical compounds are more efficient for hydrogen transport, but water is common in areas where humans hang out, so it's probably the compound of choice for fuel systems. Beyond that, you just apply the 500-pound gorilla of Fusion and brute force the problem.

The (over) plentiful fusion in the Traveller universe and a lack of appreciation of its profound impacts by its original authors I think has always been one of the shortcomings of Traveller as a 'hard' sci-fi universe. I'd think your research would be better off in a milieu like 2300 where fusion isn't quite so common and the playerbase are more interested in 'how would this really work.'


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Fri Feb 10, 2017 8:01 pm 
Offline
User avatar

Joined: Fri Dec 11, 2009 2:26 am
Posts: 238
Location: The DED Zone
But what about time?

I've already conceded that the energy requirements are naught with fusion at the players' disposal, but there still is the matter of time to contend with. There's already several hours listed (the actual number of which varies per edition) for skimming or pumping water. And there's time listed to purify said gaseous mixture and sometimes even crack the water molecule. But the FPP is basically treated as magic black box that ignores the fact that the time to extract hydrogen from water is going to take far more time than the process of filtering the junk gasses from the hydrogen.

And the more I thought about it, the more I came to believe that the current workflow to purify the fuel is wrong. If you fill your tanks with material for purification, how do you then process it? If you pump it back into the tank that you're drawing from, then it's dirty again. In the case of water, the volume is all wrong. Ignoring the dissolved solids, after purging the oxygen you're going to need more water.

A better method would be to have the skimmers (or pumps in the case of water) run directly into the FPP. This way the fuel tanks are only filled with refined fuel.

_________________
\_/
DED


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Sat Feb 11, 2017 6:47 am 
Offline
User avatar

Joined: Sun Jul 06, 2008 1:04 pm
Posts: 1038
Location: the Retirement Coast, NSW, Australia
What source did you have in mind for all this methane? Natural gas? That would involve prospecting and drilling, which are bound to be much slower than dropping the end of a hose into a body of water.

How much would water have to be purified before it was electrolysed? Most impurities won't be converted to gas by running a current through the water, so you can just flush them away. Any chlorine and fluorine produced are going to appear at the cathode with the waste oxygen.

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts to this board are copyright under the Berne Convention. They may be quoted on the board with appropriate attribution. They may not be reproduced beyond the board except with explicit permission from me.


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Sun Feb 12, 2017 7:09 pm 
Offline
User avatar

Joined: Wed May 18, 2011 7:01 pm
Posts: 335
Location: Bristol
Can you do the first stage of purification by freezing? After all, most of those impurities will have different freezing points and space is very, very cold, so you don't even need to build your own freezing equipment.


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Sun Feb 12, 2017 10:37 pm 
Offline
User avatar

Joined: Fri Dec 11, 2009 2:26 am
Posts: 238
Location: The DED Zone
Agemegos wrote:
What source did you have in mind for all this methane? Natural gas? That would involve prospecting and drilling, which are bound to be much slower than dropping the end of a hose into a body of water.


That's certainly true for planets that lie close to the parent star(s), but I was thinking about outer system bodies where methane is freely available in the atmosphere, or even liquid or slush form.

And on many worlds (depending on the edition), siphoning water is illegal so...

Agemegos wrote:
How much would water have to be purified before it was electrolysed?


As pure as possible. I don't have an exact percentage right now. Efficiency suffers as the concentration of dissolved solids increases. Electrolysis experiments were carried out with distilled water. But I guess efficiency doesn't matter when you've got megawatts of nuclear fusion at your disposal.

Agemegos wrote:
Most impurities won't be converted to gas by running a current through the water, so you can just flush them away. Any chlorine and fluorine produced are going to appear at the cathode with the waste oxygen.


Flushing things away requires more water. And impurities in the water are going to lead to side reactions. Sure, nothing but hydrogen is going to get through a polymer membrane, but how long until the anode is gunked up with so much crap that it stops working? I suppose that isn't a big deal either as cleaning it just becomes another maintenance item for the crew.

_________________
\_/
DED


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Sun Feb 12, 2017 10:52 pm 
Offline
User avatar

Joined: Fri Dec 11, 2009 2:26 am
Posts: 238
Location: The DED Zone
strontygirl wrote:
Can you do the first stage of purification by freezing? After all, most of those impurities will have different freezing points and space is very, very cold, so you don't even need to build your own freezing equipment.


If you're talking a gaseous mixture, sure. Nothing is going to freeze or liquefy lower than hydrogen. Although, I would think that forcing gaseous mixtures through molecular filters of decreasing size would be quicker than freezing.

If you're talking water, then no. Dissolved solids are still solid, they've just had their crystalline bonds broken, with the individual molecules suspended in solution. You need either either chemicals to react with the dissolved solids (like chelating agents), filters, and/or distillation (boiling the water and re-condensing it) to purify water.

_________________
\_/
DED


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Mon Feb 13, 2017 2:47 am 
Offline

Joined: Thu Sep 18, 2008 12:41 pm
Posts: 409
Location: Northern California
strontygirl wrote:
Can you do the first stage of purification by freezing? After all, most of those impurities will have different freezing points and space is very, very cold, so you don't even need to build your own freezing equipment.


Space isn't really cold, though. It's the largest vacuum mug, ever. You'd be better off powering refrigeration using your fusion reactor.


Top
   
 Post subject: Re: Fuel Purification
PostPosted: Tue Feb 14, 2017 7:16 am 
Offline
User avatar

Joined: Sun Jul 06, 2008 1:04 pm
Posts: 1038
Location: the Retirement Coast, NSW, Australia
DED wrote:
Agemegos wrote:
What source did you have in mind for all this methane? Natural gas? That would involve prospecting and drilling, which are bound to be much slower than dropping the end of a hose into a body of water.


That's certainly true for planets that lie close to the parent star(s), but I was thinking about outer system bodies where methane is freely available in the atmosphere, or even liquid or slush form.


Then the comparatively large amount of hydrogen per tonne of feedstock isn't really relevant, because this isn't something that you would choose over electrolysing water when you actually had a choice. This is something that you can't do on a habitable planet, it's what you do when you are someplace like Titan where the lakes are full of methane and water is a rock-forming mineral.

Quote:
And on many worlds (depending on the edition), siphoning water is illegal so...


It is? Why?

Quote:
Agemegos wrote:
How much would water have to be purified before it was electrolysed?


As pure as possible. I don't have an exact percentage right now. Efficiency suffers as the concentration of dissolved solids increases. Electrolysis experiments were carried out with distilled water.


Are you quite sure about that? I thought that adding an electrolyte dramatically increased the speed of electrolysis. Of course you need one in which the anion has a lower standard electrode potential than hydroxyl (otherwise it gets oxidised instead of the hydroxy and you electrolyte gets used up) and the cation has a higher standard electrode potentila than hydrogen (otherwise it gets reduced instead of hydrogen being produced).

Quote:
Agemegos wrote:
Most impurities won't be converted to gas by running a current through the water, so you can just flush them away. Any chlorine and fluorine produced are going to appear at the cathode with the waste oxygen.


Flushing things away requires more water.


Not a problem. Water is plentiful and cheap. We use it for flushing toilets.

Quote:
And impurities in the water are going to lead to side reactions. Sure, nothing but hydrogen is going to get through a polymer membrane, but how long until the anode is gunked up with so much crap that it stops working? I suppose that isn't a big deal either as cleaning it just becomes another maintenance item for the crew.

In a reverse-osmosis desalination plant that stuff just gets flushed out with the waste water stream.

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts to this board are copyright under the Berne Convention. They may be quoted on the board with appropriate attribution. They may not be reproduced beyond the board except with explicit permission from me.


Last edited by Agemegos on Tue Feb 14, 2017 9:50 pm, edited 1 time in total.

Top
   
 Post subject: Re: Fuel Purification
PostPosted: Tue Feb 14, 2017 7:29 am 
Offline
User avatar

Joined: Sun Jul 06, 2008 1:04 pm
Posts: 1038
Location: the Retirement Coast, NSW, Australia
DED wrote:
Nothing is going to freeze or liquefy lower than hydrogen.


Except helium. But anyway, I think strontygirl was suggesting freezing the water, straining out pure-water ice crystals, and then electrolysing that.

Quote:
Although, I would think that forcing gaseous mixtures through molecular filters of decreasing size would be quicker than freezing.


That's gaseous diffusion. It's used for separating isotopes in uranium enrichment, where it requires huge plants and hogs power like nobody's business. I don't think it is quick, either.

Of course it'll be more efficient and quicker when you are separating molecules with a larger ratio of molecular weights than that between U-235 hexafluoride and U-238 hexafluoride (which is only 1.0086).

Quote:
If you're talking water, then no. Dissolved solids are still solid,


No they aren't.

Quote:
they've just had their crystalline bonds broken, with the individual molecules suspended in solution.


You're confusing the dissolution of ionic substances with the dissolution of molecular substances.

Quote:
You need either either chemicals to react with the dissolved solids (like chelating agents), filters, and/or distillation (boiling the water and re-condensing it) to purify water.


Another alternative is reverse osmosis, which is what is used in desalination plants.

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts to this board are copyright under the Berne Convention. They may be quoted on the board with appropriate attribution. They may not be reproduced beyond the board except with explicit permission from me.


Last edited by Agemegos on Tue Feb 14, 2017 9:52 pm, edited 1 time in total.

Top
   
Display posts from previous:  Sort by  
Post new topic  Reply to topic  [ 13 posts ]  Go to page 1 2 Next

All times are UTC


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Limited