Crushing Martian Regolith Simulant for the Extraction of Water

MAE 435 – Project Design & Management II

Mid-Term Report

Team Members:

Nicholas Sestito

Christopher Graham

Project Advisor:

Dr. Robert Ash

Contents

Table of Contents

List of Figures

Abstract

Introduction

Methods

Input Sizing Justification

Output sizing justification

The lumped system Calculation

Preliminary Regolith Crushing Method Research

Future Works

Reference Page

Appendices

Gantt Chart

Budget

List of Figures

Figure 1: Water Extraction Schematic

Figure 2: Cd Vs Log(CdRe^2)

Figure 3: Re Vs Log(CdRe^2)

Figure 4: Pipe Length Vs Grain Diameter

Abstract

With increasing interest and research being done towards the exploration and colonization of the planet Mars, one of the initial demands that following entail whether water can be efficiently produced on the Martian surface. Recent Rovers sent to the surface of Mars have collected a sufficient amount of data regarding water being present in Martian regolith in large enough quantities that theorized excavation and water extraction methods have been researched. Furthermore, research and testing has been performed regarding the mechanical properties of a Martian Simulant known as JSC Mars-1A. These tests assured that the excavation and removal of cubic liters of frozen Martian regolith/ice mix can be performed. This work is an attempt to develop a crushing process with justification of an optimal outlet regolith size to be heats and extracted of water. Through a Lumped system analysis, it is found that a practical heating application can be developed for output regolith grain diameters from one to five millimeters.

Introduction

When planning for Mars colonies, and the potential for the colonists to return to earth, fuel is a problem. If a shuttle were to leave earth carrying enough fuel both to get to mars and come back, the amount of fuel needed would either be prohibitively expensive, or would weigh enough to make escape from earth’s orbit impossible. In this vein, there are plans of producing fuel on mars for the trip back. One method is the Sabatier reaction, which can take hydrogen and CO2 from Mars atmosphere and produce methane and water, followed by electrolysis, which turns water into hydrogen and oxygen[1].

Since CO2 is plentiful in the Martian atmosphere, the only materials needed to start producing fuel for the return trip is hydrogen. Hydrogen is the smallest element, and is the lightest to carry into space, however, containing it over long periods of time is problematic, as it has to be kept cool in order to be contained, and is small enough to escape through most container materials. The surface of Mars is covered in a fine soil like material known as regolith, which shields ice from heat and prevents it from sublimating [2]. If harvested, electrolysis can turn the water from the regolith into hydrogen and oxygen, which can bypass the need to haul hydrogen in from earth in the first place [1].

The Mars rover landings conducted research on the surface of Mars. The Phoenix Lander in particular gathered data in regard to regolith properties, including soil concentration and density. The phoenix lander also observed surface ice, which when exposed, sublimated within a few days[3].

With results from Mars rovers, NASA was able to design and oversee the creation a batch of simulant Mars regolith, called JSC Mars-1, which was believed to accurately resemble the properties of Mars regolith. The company that NASA worked with later made a second batch of simulant, JSC Mars-1A, which was made the same way from volcanic soil from a volcano in Hawaii. JSC Mars-1A has similar chemical composition to Mars regolith, based on Martian rover testing, but has differing density, which is believed to be a result of the presence of organic material and water. To make it more accurate, the simulant is baked at high temperature to remove water and burn off organic compounds[2].

Mined regolith will be processed in a pressurized environment, so that heating the regolith in an oven will allow the ice to melt and collect as water rather than sublimating. By crushing the regolith into smaller sized particles, the surface area of the regolith increases, which reduces the energy needed to extract the water.

The purpose of this project is to design a rock crushingprocess that will operate in Martian temperatures and can crush the required amount of regolith each sol. A prototype of the crushing process will be 3D printed, built and tested with similar required production rates to prove its functionality.

Methods

Input Sizing Justification

The Geometry of the regolith/permafrost input material is one of the initial criteria in developing a rock crushing system. The ODU spring 2015 RASC-AL team had determined a practical regolith excavation method that involves excavation by means of a regolith/ice saw. The ice saw would be used to cut 10 cm deep grooves on the Martian surface, resulting in a grid of 10cm by 10cm squares. This procedure would leave cubic liter cantilever beams on the surface, ready to be broken off and extracted [4]. Furthermore, an excavation area requirement was specified to be 2m by 2m. This geometry of liter sized cubes is used as basis for designing a crushing method for the extraction of water from Martian regolith.

Output sizing justification

In order to determine an optimal regolith grain size, produced by a one or two-stage crushing system determined later, the efficiency in which water can be extracted from an individual regolith grain must be examined. Furthermore, a conservative regolith-water content must be set to determine roughly howmuch excavated regolith is needed to be collected and processed in order to collect a sufficient amount of water each sol. Considering estimates done in the Northern Martian hemisphere, the subsurface percent of water content (by mass) ranges between 44% and 11% [5]. From the previous semester (Spring 2014) group’s publication work it was found that by designating 15% by mass water content in the excavated regolith, less than 0.5 m3/sol of regolithwould need to be collected to yield 100 kg/sol of water[4]. This required volume corresponds to 500 harvested cubic liters of regolith per sol. With a designated water content and an established excavation rate, a thermal model can be developed to determine a range of acceptable output sizes and their heating requirements to extract and collect usable water.

The heating process that is used to determine a grain sized output range has been explored using a hot CO2 oven application seen in the Figure 1 below.

Figure 1: Water Extraction Schematic

Initially, the system is required to be pressurized to 100 kPa in order to raise the boiling point of water above the triple point, as well as to aid in lowing the required heating time by increasing the density of the carbon dioxide. As depicted in the figure, the regolith grains are processed through the crushing system, in an environment where any exposed water on the surface cannot rapidly vaporize or sublime, then transported by a conveyor system towards a hot CO2 pipe. The conveyor then drops the grains down the hot CO2 pipe where forced convection from both the grains terminal velocity and a CO2 blower drives the phase change of water. Finally, the steam is routed to a heat exchanger where it is condensed and stored for use. Some of the main components of the system that are either to be determined or required include the length of pipe the grains fall through, the speed of the blower, the fluid properties of hot CO2, and both the inlet and outlet temperatures of the regolith grains. The method that has been selected to determine such values is a lumped system heat transfer approach. The main assumption of this approach entails that once the center of the regolith grain has reached a specified temperature above the boiling point, in the set environment, the water content in the grain will be vaporized and separated.

The lumped system Calculation

Trials of the calculation were done for grain diameters varying from 1 millimeter to 5 millimeters of regolith permafrost grain size. A heat transfer lumped system calculation, provided the system meets the criteria for one, is capable of determining the time in which a body will uniformly change from an initial to a final temperature. In this case, the initial and final temperature of the regolith grain can be specified. A below average Martian surface temperature of 200 K and a temperature above the boiling point of water of water at 375 K were used as the initial and final temperature of the grain, respectively [6]. Then, the temperature of the hot CO2 was specified while at 600 K. The CO2 is assumed to be heated using the rejected heat of the RAPID-L reactor, with a radiator outlet of 800 K [7].

Working towards a forced convection heat transfer coefficient, the Nusselt number needed to be determined, therefore, the fluid properties of the hot CO2were required. The dynamic viscosity of the surroundings and the grain’ssurface were found using Sutherland’s formula for pure carbon dioxide:

The remaining fluid properties of CO2, including thermal conductivity, specific heat and density were determined using the National Institute of Standards and Technology (NIST) - thermophysical properties of fluids calculator. Furthermore, the Prandtl was determined using dynamic viscosity, thermal conductivity and specific heat of CO2.

Another necessity for determining the Nusselt number was the Reynolds number of the sphere falling in the hot pipe. While the assumption of the grain initially falling at its terminal velocity, according to the gravity on Mars, the coefficient of drag is needed. Since the Reynolds number depended on the terminal velocity over the grain, the terminal velocity depended on the coefficient of drag, and the coefficient of drag depended on the Reynolds number, an interpolative method was invoked using a modification of the terminal velocity equation shown below, as well as a coefficient of drag vs Reynolds number plot for a smooth sphere.

Five points were selected on a drag coefficient vs Reynolds number curve where the Reynolds number ranged from 1 to 1000. The values of CdRe2 were calculated and their logarithm of base ten was taken, considering the plot was a logarithm base ten plot. From there, two individual plots of drag coefficient and Reynolds number versus log10(CdRe2) were created. The most similar curve-fit function was employed using the Excel plotting tool and is shown below. An equation for Reynolds number as a function of log10(CdRe2) and an equation for Drag coefficient as a function of log10(CdRe2) are shown in the lower right portion of each plot. These equations are useful provided the Reynolds number of the grain lies below 1000. However, if the Reynolds number of the grain lies between 1000 and 5x105, meaning the log10(CdRe2) > 5.6, then the drag coefficient is set constant at 0.4. This condition was made by inspection of the coefficient of drag vs Reynolds number plot, where the Drag coefficient remains relatively constant at 0.4.

Figure 2: Cd Vs Log(CdRe^2)

Figure 3: Re Vs Log(CdRe^2)

The curve-fit functions allowed for calculations of the Reynolds number and the coefficient of drag, then lead to the terminal velocity for each grain trial between 1mm and 5mm. The Nusselt number was then determined and, in turn, the Heat transfer coefficient due to forced convection for each grain diameter trial was determined.

Finally the lumped system relationship could be utilized to find the time required for each grain to individually heat to the final temperature of 375 K.

Once the time of each grain was determined, the length of the hot CO2 oven pipe could be justified. Multiplying the terminal velocity with the required heating time, corresponding to the correct grain size, a length of pipe is determined. However, for grain diameters of 3mm and above the length of pipe ranged between 8 and 25 meters. Therefore, the a blower velocity was added to each grain size set to maintain roughly 2m/s less than the terminal velocity. The blower elongated the time required to heat the grain, since the overall velocity over the grain decreased, however, the overall pipe length became significantly shorter and more reasonable.

Figure 4: Pipe Length Vs Grain Diameter

Preliminary Regolith Crushing Method Research

In researching the crusher design schemes and standards for rock crusher design, several factors had to be considered. The most basic factor is the reduction ratio, which is the comparison of the longest dimension of the starting rock compared with the product of the crusher. Jaw crushers and cone crushers had ratios of 6 to 1and 8 to 1, respectively, while some other crushers’ utilized 2.5 to 1 ratios (a double roller crusher), or a 20 to1 ratio (the hammer mill model or the double impeller impact breaker crusher). The greater the reduction ratio, the smaller the output could be, and the more reasonable the ideal size for the thermal modeling could be. [8]

A second concern is how complex a crusher is and how reliable it is at getting rocks to the desired size. Reliability addresses factors like dealing with harder materials that get lumped in the mix, while complexity is less due to design challenge, and more due to the concern of anything space or mars related. Specifically, these crushers are going to be operating near daily on mars to extract water and prepare fuel for the colonies return trip, and some of their time operating is going to be without human operators to fix things. Excessive complexity means more moving parts, which means repairs will require more types of parts and can make automated repairs more complicated. In this regard, we found that the crusher models of the double impeller impact crusher and the hammer mill, while having incredible reduction ratios, were far more complex than we were after, and inversely, the double roller model had a very undesirable reduction ratio.

In the mining industry, it is common for jaw and cone crushers to be used in tandem as stage 1 and 2 crushers. [8] The crushing method of the jaw crusher is better for handling raw mining feed, while the method of the cone crusher is good and pounding small rocks down to more consistent rocks of an even smaller size (the motion of the jaw crusher as the plate moves outward produces a noticeable variance). Early thermal model work suggested that an ideal size for the regolith output would be around 2 millimeters as its longest dimension, which is the maximum reduction ratio of a jaw and cone crusher system. For both jaw and cone crushers, the output size is the maximum width of the jaw or cone crusher during the phase of operation in which the plates are furthest apart. This simplifies some aspects of scaling the machine components down to our non-standard dimensions , but does put pressure on our calculations to make sure of part functionality.

Alternatively, a jaw crusher- ball mill grinder system is used in the mining industry already to reduce ores to a powder for the production of compounds like granite. In this system, a jaw crusher gets the ore down to under half an inch, where it’s fed into the grinder. A ball mill is a spinning drum with steel balls inside of it, as the mill rotates, the balls rise and impact the drum, crushing the material with each impact. A mesh screen determines the output dimension, with the width of the mesh gaps determining what can get through. US 10 mesh has 2 millimeter gaps, while US 18 mesh has 1 millimeter gaps, the US mesh rating is the number of mesh gaps in a 1 inch long length of mesh. In designing the crusher, concern will revolve around the size of the drum/number of balls needed, the operation time needed to grind a day’s regolith into powder, and any friction heat causing the ice in the regolith to evaporate/sublimate, or melt and thoroughly jam the machine with mud like gunk. In this situation, a considerable amount of friction heat will be generated by all the regolith powder/dust rolling around, as well as from the motion of the steel balls.[9]

Future Works

With the thermal model just recently developed, both and input regolith geometry and an optimal output regolith size range has been determined. This allows for the specific crushing process to be narrowed down, a final determination of the stages the regolith will be processed through, and the overall design of the crushing system.

More detail will be added to the thermal model, including a regolith grain processing rate, that will be determined be the crushing system capabilities. This processing rate will allow for a pipe diameter estimation, as well as a volumetric flow rate for the hot CO2. Furthermore, more detail will be added to the heat exchange between the RAPID-L rejected waste heat and the hot CO2 oven.