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Mars Atmosphere Hydrocarbon and Olefin Synthesis System

Propellant production on Mars (in situ propellant production, ISPP) benefits from the easy availability of carbon in the form of atmospheric carbon dioxide. However, the arid nature of the Martian surface means that there is a low availability of hydrogen. Thus, most hydrocarbon or oxygenated hydrocarbon based ISPP plants require liquid hydrogen feedstock imported from Earth. For example, the current Sabatier/Electrolysis (S/E) ISPP scheme combines revaporized liquid hydrogen and Martian carbon dioxide in a catalytic reactor to create methane fuel and water with a fuel H/C ratio of four. The water is condensed and electrolyzed to produce oxidizer (half as much oxidizer as ideal methane stoichiometry requires) and hydrogen for recycle to the reactor. The imported liquid hydrogen is only a small portion of the mass of the total fuel/oxidizer mix (8% for the S/E system), but it has other problems. Since it is an extreme cryogen, it requires extensive insulation, which adds to launch mass. Despite the insulation, a significant fraction of the hydrogen launch mass could be expected to boil off before processing begins on the Martian surface. Finally, the low density of liquid hydrogen (approximately 70 kg/m3) means that the storage tanks take up a large volume. If sufficient hydrogen for the ISPP requirements can be obtained on Mars in an energy efficient manner, this will inevitably be preferred to liquid hydrogen importation. However, the methods for collecting hydrogen on Mars are energy intensive relative to the ISPP processes which use the hydrogen to make hydrocarbon fuels and/or will require substantial industrial infrastructure. Thus, it is clear that, all else being equal, an ISPP technology that uses less hydrogen will be greatly preferable to one which uses more hydrogen.

The Mars Aromatic Hydrocarbon and Olefin Synthesis System (MAHOSS) is a method for producing storable low hydrogen/carbon ratio fuel and oxygen on the surface of Mars with 98% of the required raw material mass derived from the Martian atmosphere. In the MAHOSS system, a reverse water gas shift (RWGS) reactor is integrated with a catalytic fuel production reactor. The reactors combine imported hydrogen with Martian atmospheric CO2 to produce aromatic or olefin fuels and water, with the latter product subsequently electrolyzed to produce oxygen and return hydrogen feedstock to the system. In this system, approximately 40 kg of fuel/oxygen bipropellant are produced for every kilogram of hydrogen imported to Mars, an attractively high leverage ratio. Another significant advantage of the MAHOSS system is its low power consumption, about half the power of the S/E Mars ISPP system.

Product properties

The MAHOSS project demonstrated production of four primary hydrocarbon products: ethylene, propylene, benzene, and toluene. These chemicals are four of the most common commodity hydrocarbons produced today, with a total annual market of more than 50 million tonnes. On Earth, the principal feedstocks for making ethylene and propylene are natural gas or catalytic cracker offgas. The principal feedstocks for benzene and toluene are heavy crude oils.

Table 1: Low H/C ratio fuel properties

Fuel Formula Melting Temp

(K)
Boiling Temp

(K)
Critical Temp

(K)
Heat of Formation

(kcal/mole @ 298K)
Ethylene C2H4 104 169 283 12.496
Propylene C3H6 88 225 365 4.879
Benzene C6H6 278 361 562 19.820
Toluene C7H8 178 384 594 11.950

Note that all of these are endothermic molecules, which is expected given the large amount of carbon relative to hydrogen. They are also all stored relatively easily at Mars ambient temperatures and about one bar of pressure. Ethylene will require refrigeration and/or additional pressurization. Benzene will require some heating at night to maintain it as a liquid.

As fuels, olefins and aromatics burn considerably hotter than fuels such as methane or hydrogen because of the extra carbon. However, their performance is reasonably high. Ethylene ideally gets about 380 seconds specific impulse burning with oxygen, while benzene and toluene are some of the primary ingredients in gasoline and kerosene type fuels, and should be able to achieve on order of 360 seconds specific impulse.

Although all the compounds listed in Table 1 are usable as high quality fuels, their potential as feedstock to plastic production processes should be briefly mentioned. Ethylene has been called the key to the plastics industry. In terrestrial applications, it is the precursor for many different varieties and grades of polyethylene, which is the most widely utilized plastic in the world. Propylene is the precursor for polypropylene, which is the second largest volume commodity terrestrial plastic at this time and is widely used for textiles. Toluene is not directly used as a plastic monomer, but is usually converted to benzene and/or directly combined with ethylene or propylene. With various further processing steps, these chemicals end up in products as diverse as polystyrene, styrene-butadiene rubber, Nylon, polyurethane, epoxies, aspirin, and explosives. While these applications will be irrelevant to robotic and early human missions, they are potentially very valuable when it is time to build a permanent outpost on Mars.

Production of Aromatics and Olefins on Mars

Aromatics and olefins can be produced on Mars by running two reactions in series. The Reverse Water Gas Shift (RWGS) is conducted in the first reactor. The primary products of the RWGS system are water and carbon monoxide. The water is condensed and electrolyzed to produce the oxygen portion of the propellant mixture. The carbon monoxide from the RWGS reactor is mixed with hydrogen to form a mixture called synthesis gas (syngas). In the chemical industry, syngas is a widely marketed and very valuable commodity (terrestrially, it is formed via steam reforming of coal or methane rather than with a RWGS). The syngas is piped off to a second reactor, where it reacts catalytically to produce hydrocarbon products.

The Reverse Water Gas Shift

The reverse water gas shift (RWGS) reaction has been known to chemistry since the mid 1800's. While it has been discussed as a potential technique for Mars propellant manufacture in the literature, until the initiation of the 1997 Phase I MMISPP program by Pioneer Astronautics, there had been no experimental work done to demonstrate its viability for such application. The RWGS reaction is given by equation (1).

CO2 + H2 → CO + ΔH2O H = +9 kcal/mole (1)

This reaction is mildly endothermic and occurs rapidly in the presence of an iron-chrome or copper-on-alumina catalyst at temperatures of 300 C or greater. Unfortunately, at 400 C the equilibrium constant Kp driving it to the right is only about 0.1, and even at much higher temperatures Kp remains of order unity. There is thus a significant problem in driving the RWGS reaction to completion.

However, assuming that reaction (1) can be completely driven to water and CO products, an "infinite leverage oxygen machine" can be created by simply tying reaction (1) in tandem with the water electrolysis reaction. That is, the CO produced by reaction (1) is removed from the RWGS while the water is electrolyzed to produce oxygen (the net product), and hydrogen which can be recycled to reduce more CO2. Since all the hydrogen, barring leakage, is recycled, this process can go on forever, allowing the system to produce as much oxygen as desired. The only imported reagent needed is a small amount of water, which is endlessly recycled.

The RWGS/electrolysis oxygen machine has many advantages over the alternative zirconia/electrolysis system which is sometimes identified for the same purpose. In contrast to the zirconia system, which is composed of thousands of small brittle ceramic tubes manifolded together, the RWGS reactor itself is just a simple steel pipe filled with catalyst, much like a Sabatier reactor, except that the catalyst is different. A similar condenser and identical water electrolysis system to that used in the well demonstrated Sabatier/Electrolysis is also employed. Because the RWGS reaction is only mildly endothermic (9 kcal/mole for RWGS compared to 57 kcal/mole for water electrolysis), system power requirements are dominated by the water electrolysis step, for which the available technology is highly efficient. Moreover, since the thermal power required by the RWGS is less than that produced by a fuel production reactor and their operating temperatures are comparable, such catalytic fuel-making reactors can be used to provide some or all of the heat required to drive the RWGS reactor.

The trick, however, is to find a practical way to drive the RWGS reaction to completion. Pioneer Astronautics demonstrated a way to accomplish this in 1997 during the MMISPP Phase I SBIR. Briefly, in the Pioneer Astronautics Phase I MMISPP device, effluent gas from the RWGS is run through a 5 C water condenser to remove the water vapor, and then the remaining gas is sent to a membrane separator device. The membrane separator returns the hydrogen and CO2 components of the effluent gas to the reactor while the CO is either vented as waste or sent on to be used by the fuel reactor. In operation of this device, simultaneous conversions of over 98% of hydrogen and CO2 were accomplished, or 100% of either when run as the lean reactant. While some hydrogen is lost from the system during recycle, propellant leverages (the ratio of CO/O2 produced to the hydrogen feedstock) as high as 288 were produced. As a result, the practicality of driving the RWGS to completion can now be considered demonstrated.

The discussion so far has shown how a RWGS reactor can be used as the sole component in a loop with an electrolyzer as an "infinite-leverage oxygen machine" on Mars. In addition, a RWGS reactor operating without an electrolyzer can be used to turn imported hydrogen into water on Mars (for crew consumables) with a mass leverage ratio of 9/1. However the RWGS reactor opens up additional remarkable possibilities.

For example, we can operate the RWGS reactor with an excess of hydrogen, but without recycling all the waste hydrogen effluent. As a simplified case, assume that the H2/CO2 molar input ratio is 3/1, and that the CO2 conversion rate is close to 100%. Thus, we have 3 units of H2 and 1 unit of CO2 going into the reactor, 1 unit of H2O collected in the condenser, and 1 unit of CO and 2 units of H2 leaving the reactor. The water is electrolyzed to produce product 0.5 units of O2 for the propellant tanks and one H2 unit for recycle into the RWGS. More importantly, the CO and H2 effluent from the process forms a high quality syngas mixture that can be fed as input into a hydrocarbon fuel production reactor.

Use of RWGS Reactor Outlet to Produce Fuels

Thus, there is no doubt that a RWGS system can produce a feed consisting of carbon monoxide and hydrogen for the MAHOSS fuel production unit. The next step in the MAHOSS is to produce higher hydrocarbons from the syngas, which is done via the Fischer-Tropsch (F-T) synthesis reactions, shown as reactions (2) - (6):

(2n + 1) H2 + n CO → CnH(2n + 2) + n H2O (2)

2n H2 + n CO → CnH2n + n H2O (3)

(n + 1) H2 + 2n CO → CnH(2n + 2) + n CO2 (4)

n H2 + 2n CO → CnH2n + n CO2 (5)

2n H2 + n CO → CnH(2n + 1)OH + (n - 1) H2O (6)

In these reactions, n is any integer. Reactions (2) and (4) produce paraffinic hydrocarbons and are the dominant reactions for most F-T catalysts. Reactions (3) and (5) produce olefins and reaction (6) produces alcohols, although these reactions usually are not as prevalent as those that produce the paraffins. Note that for any n larger than 4, the production of even paraffinic hydrocarbon fuels already has a H/C ratio of less than 2.5, so this reaction by itself is approaching the desired H/C ratio for an ISPP plant. Note also that these reactions are all exothermic with heats of reaction on the order of 30 to 40 kcal/mole.

The F-T reaction was discovered in 1923 in Germany but, even today, has not been fully characterized. The reaction generally uses an iron-based catalyst, which may be mixed with some cobalt, copper, or manganese for hydrogen rich feeds. Actual activity of the catalyst depends on the presence not only of metallic iron, but also of iron oxide and iron carbide. The reaction proceeds at moderate temperatures of about 200 - 280 degrees C and moderate pressures anywhere from 1 to 25 bars. It produces a hugely varying range of product compositions and weights depending on the exact formulation of the catalyst used. The predominate products in most Fischer-Tropsch reactors are middle weight straight chain paraffinic hydrocarbons from ethane (C2H6) up through decane (C10H22), although specific types of catalysts will produce considerable amounts of unsaturated hydrocarbons or alcohols via reactions (3), (5), or (6). In general, the lighter fractions, such as ethane and propane, dominate in molar terms, but in terms of mass fraction, it is not unusual for the heavier hydrocarbons to form the majority of product. In the Phase I MAHOSS program, Pioneer used iron Fischer-Tropsch catalysts doped with potassium to produce high olefin yields, primarily via reaction (5).

Production of Benzene and Toluene

Aromatic components such as benzene and toluene can be produced by subjecting F-T reaction products, particularly olefins, to catalytic reforming using a shape specific zeolite. The zeolite most often mentioned for olefin aromatization in the literature is ZSM-5, a silica/alumina zeolite developed by Mobil in the 1960s. The zeolite pores are just the proper size to piece together multiple olefin molecules via a reaction that looks something like (7):

3 C2H4 → C6H6 + 3 H2 ΔH = -17.7 kcal (7)

Note that if one of the ethylene molecules is a propylene molecule, you will get toluene; if two molecules are propylene, you will get xylene. Since aromatization occurs directly to the F-T products, there are two possibilities for a reactor to create these aromatics. First, the reactor could be a separate vessel immediately following the F-T reactor. Second, you could put a mixture of both catalysts (Fischer-Tropsch and ZSM-5) in the same reactor. Various articles in the literature reported success with the second technique (particularly Caesar, et. al., 1979), so Pioneer pursued this option during the Phase I portion of the MAHOSS program and achieved excellent production of aromatic liquids.

Product Recovery and Separation

The aromatic and heavy olefin products from the reactors can be easily separated in a condenser, since they are liquid at reactor pressure and ice water temperatures, while CO, CO2, hydrogen, and other components are gases at these conditions. A second membrane, similar to the one used in the RWGS system can be used to separate F-T reactants from CO2 effluent; the CO2y and remaining hydrogen can be sent back to the RWGS while the unreacted CO and uncondensed hydrocarbon products can be returned to the fuel production reactor. This process configuration was demonstrated during the Phase I portion of the MAHOSS project.