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.