Journal of Undergraduate Research
Volume 3, Issue 1 - September 2001

Sizing of a Combined Sabatier Reaction and Water Electrolysis Plant for Use in In-Situ Resource Utilization on Mars

Gilbert Canton

 

 

ABSTRACT


A mission to Mars will happen in our lifetimes. NASA's next big project is sending men to Mars, but to accomplish this, research must be carried out and new technologies developed. It has been determined that one element of the Mars mission that will be essential to its success is the In-Situ production of water, oxygen, and fuel for humans and robots. This paper examines the feasibility of one method, the Sabatier/Electrolysis water, oxygen and methane fuel plant, proposed to meet this need. This paper determines the relationship between the necessary mass of fuel to be produced and the mass of the major components of the Sabatier/Electrolysis plant. A non-linear relationship exists between the mass of the Sabatier reactor and the mass of fuel produced, and a linear relationship exists between the condenser mass and mass of fuel to be produced.

 

INTRODUCTION


A mission to Mars, people have dreamed of it, made movies about it, but now it is the next big step for the NASA space program; it is a reality. As with any form of space travel a trip to Mars will require detailed planning and development of technology. This paper focuses on the development of the Sabatier/Electrolysis fuel production unit of the In-Situ Resource Utilization (ISRU) plant. The ISRU plant is designed to make use of the resources available on the surface of Mars, most notably the readily available atmospheric carbon dioxide (CO2). Because the ISRU plant will use the resources on Mars less raw materials will have to be imported from Earth; this significantly decreases the launch mass of the Mars shuttle.

 

The objective of this paper is to study the Sabatier/Electrolysis system to determine how it will have to sized and if it is feasible for use in the upcoming NASA mission to Mars.

 

The NASA Mission

 

The objective of the NASA mission to Mars is to set up a station from which research on Mars can be carried out. The mission will be carried out in two major stages, each stage having a specified objective. These stages are described below.

 

Stage 1. This is a cargo mission originally scheduled for 2011, but now pushed back indefinitely. The following sequence of events will be carried out in this stage.

 

1. Earth launch
2. Arrival and landing of Martian surface habitats
3. All systems including ISRU and human habitat to be used in Stage 2 are set up
4. Sample gathering and research with possible location change
5. Return to earth

 

Stage 2. This is a manned mission in which humans will journey to and land on Mars to carry out research. The following sequence of events will be carried out in this stage.

 

1. Earth launch
2. Arrival and landing on Martian surface
3. Scientists carry out research
4. Return to earth

 

The Sabatier/Electrolysis Plant

 

The ISRU plant will have many functions on the Martian surface; one function is to supply water, oxygen, and fuel for either human or robot use. One method of producing the water, oxygen, and fuel required is the use of the Sabatier/Electrolysis chemical plant shown in Figure 1.

 

Figure 1. Simplified model of A Sabatier/Electrolysis Unit used to Produce O2 and CH4

 

Figure 1. Simplified model of A Sabatier/Electrolysis Unit used to Produce O2 and CH4

 

The Sabatier reaction (given below) will be carried out in this plant.

 

CO2 + 4H2 = CH4 + 2H2O

 

*It is assumed that this reaction goes to completion (all reactants are used).

 

The carbon dioxide used in the reaction will be obtained from the Martian atmosphere, but the hydrogen used will be imported from Earth. The reaction runs best at a pressure of one atmosphere and at a temperature of 573 K (1), so a compressor will be needed to increase the Martian atmospheric pressure. The reaction is exothermic and once started will maintain itself at about 573 K, but an electrical heater will be required for the start up procedure.

 

The methane and the water vapor produced by the reaction are passed through a condenser to separate the water from the methane. From here the methane is put into cryogenic storage and the water goes either for direct use by humans or to a water electrolysis unit. The water electrolysis unit separates the water into hydrogen and oxygen. The oxygen is put into cryogenic storage to be used later as fuel or by humans to breathe; while the hydrogen is returned to the Sabatier reactor to be reused. Reusing the hydrogen helps to decrease the amount that needs to imported from Earth.

 

The Sabatier Reactor. The Sabatier reactor is a small lightweight steel cylinder that has a mixing chamber and a chamber filled with a nickel catalyst. The size of the catalyst-filled chamber will depend on the amount of fuel to be produced and the number of days that the fuel must be produced in. The Sabatier reactor specifications are calculated.

 

The Condenser. The condenser's purpose is to remove the water vapor from the products of the Sabatier reaction. The condenser is a simple pipe with outlets on the bottom to collect water; natural convection on the surface of the pipe is enough to carry out the necessary heat exchange. The amount of heat exchange surface necessary is determined by the amount of fuel to be produces and the time given to produce the fuel. The condenser specifications are calculated.

 

The Electrolysis Unit. The electrolysis unit separates water into hydrogen and oxygen. Electrolysis technology is well developed and units that could be used for a Mars mission already exist; they are made by companies like Hamilton Standard. The electrolysis specifications are not calculated in this paper. Instead it is assumed that the weight of the unit is 15 kilograms; this is more than an existing Hamilton Standard unit (the extra weight is added to account for adjustments that might be made for the specific mission).

 

The Cryogenic Storage. Cryogenic storage is not addressed in this paper, but it is an important part of the ISRU system.

 

ANALYSIS


There are two major portions of the analysis of the Sabatier/Electrolysis system. The first is the analysis of the Sabatier reactor, and the second is the analysis of the condenser. The electrolysis unit will be selected from units already built by Hamilton Standard.

 

The analysis of the Sabatier reactor consists of finding the required specifications of the reactor; the calculations are carried out using the results of Reference 1. Reference 1 is a report describing the kinetics of the Sabatier reaction. From this reference the rate of reaction is described by Equation 1.

 

Equation: The rate of reaction

 

Also a sizing formula can be derived from the results of Reference 1; this is Equation 2.

 

Equation: Sizing formula

 

Once the needed volume of the reactor is calculated then the most efficient cylindrical reactor specifications can be calculated. Equation 3 is the radius formula and Equation 4 is the length formula.

 

Equations: Radius and length formulas

 

The thickness of the reaction chamber can be calculated using pressure formulas, but at the low pressures used the reactor, walls can be thinner than is practical. To solve this problem the walls were given a set thickness of two millimeters.

 

The condenser was sized using basic heat transfer techniques. Average temperatures and values for chemical properties were used. The exit temperature for the methane was specified and the overall heat transfer was calculated using Equation 5.

 

Equation: Exit temperature for the methane was specified and the overall heat transfer

 

Once the total heat transfer has been found the length of the heat exchanger can be found using Equation 6.

 

Equation: Length of the heat exchanger

 

The heat exchanger is also given a set wall thickness of two millimeters. With this information the mass of the heat exchanger can be computed.

 

RESULTS AND DISCUSSION


The relationships between the mass of fuel to be produced and the mass of the Sabatier reactor and the heat exchanger were studied. It was found that in both cases the mass of the components increased as the mass of fuel increased over a set period of time (365 days). Figure 1.2 shows the relationship between the mass of the Sabatier reactor and the mass of fuel to be produced over a 365 day period with the plant operating 12 hours per day.

 

The plot in Figure 1.2 shows that the relationship between the Sabatier reactor mass and the mass of fuel to be produce is:

 

Ms = 0.008Mfo.4556

Ms = mass of Sabatier reactor

Mf = mass of fuel produced

 

Figure 2 shows that the Sabatier reactor is a light weight component; it will also help designers concerned with weight/performance considerations choose performance criteria for the entire system.

 

Figure 2. Plot Showing the Relationship Between the Mass of Sabatier Reactor and Mass of Fuel Produced in 365 Days

Figure 2. Plot Showing the Relationship Between the Mass of Sabatier Reactor and Mass of Fuel Produced in 365 Days

 

The relationship between mass of fuel to be produced and the mass of the heat exchanger is plotted in Figure 3. This relationship is given by Equation 8; it is a linear relationship.

 

Mhe = 0.0036Mf - 0.0916

Mhe = mass of heat exchanger

 

Figure 3. Plot Showing the Relationship Between the Mass of Heat Exchanger and Mass of Fuel Produced in 365 Days

Figure 3. Plot Showing the Relationship Between the Mass of Heat Exchanger and Mass of Fuel Produced in 365 Days

 

In both cases the mass of fuel produced ranges from 100 Kg to 1000 Kg and is produced in a period of 365 days with the plant working for 12 hours on each day. If more than 1000 Kg needs to be produced in this time period it is recommended that more than one Sabatier reactor be used since this will limit the mass flow rate passing through one reactor, and allow the rate of reaction to keep up with the mass flow rate.

 

The mass of the components described here do not make up the entire mass of the Sabatier/Electrolysis unit. The masses of connecting piping, compressors, and other less central heat exchangers will need to be considered. These components have not been considered here because the ISRU plant is so versatile that it is difficult to determine how the Sabatier/Electrolysis unit will be laid out. Heat exchange sources and compressors for the Sabatier/Electrolysis unit may be coupled with heat exchange sources and compressor from other parts of the ISRU, so it is not yet necessary to design independent parts. The parts that are considered in this report are essential to the Sabatier/Electrolysis process.

 

CONCLUSION


Relationships between masses of the components of a Sabatier/Electrolysis unit for use in a Martian ISRU plant were studied. It was found that the relationship between the Sabatier reactor mass and the mass of fuel to be produced is defined by Equation 7; this is a "power" relationship. The relationship between the heat exchanger mass and the fuel mass is a linear one defined in Equation 8. Both relationships are proportional; that is as the mass of fuel to be produced increases so does the mass of the components. The full mass of the Sabatier/Electrolysis unit can only be calculated once the design of the ISRU is developed.

 

 

REFERENCES

  1. Naumov VA, Krylov OV, Gavrilov LI. Kinetics of the Sabatier Reaction on a Nickel Catalyst in a Flow Through System. Journal Title: Kinetics and Catalysts, Volume 20, Pages 740 - 744. 1979.

  2. Salerno LJ, Kittel P. Cryogenics and the human exploration of Mars. NASA Ames Research Center, Moffet Field, California. 1999.

  3. Incropera PI, DeWitt DP. Fundamentals of Heat and Mass Transfer, Fourth Edition. John Wiley and Sons Inc. Press. 1996.

  4. James MG, Timoshenko SP. Mechanics of Materials, Fourth Edition. PWS Publishing Company. 1997.

  5. Floudas CA, Pardalos PM, et al. Handbook of Test Problems in Local and Global Optimization. Kluwer Academic Publishers. 1999.

  6. Hamilton Sundstrand Website - http://www.hamilton-standard.com/ - 2000

  7. Advanced Propulsion Concepts Website, Jet Propulsion Laboratory, California Institute of Technology - http://sec353.jpl.nasa.gov/apc/Introduction/00.html - 2000

 

APPENDIX

 

Formulas for Constants

 

k1 = 1.02*109 exp(-20500/RT)

 

C1 = 7.64*10-2 exp(5200/RT)

 

C2 = 2.02*104 exp(-10500/RT)

 

 

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