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PRODUCING LIQUID-SOLID MIXTURES

OF HYDROGEN USING AN AUGER

R. 0. Voth

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80303

February 1978 .

Prepared for:

. - National Aeronautics and Space Administration

. . Kennedy Space Center, Florida 32899

2Qg (Contract No. CC-62530A)

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OCT S 1981

PRODUCING LIQUID-SOLID MIXTURES

OF HYDROGEN USING AN AUGER

R. 0. Voth

Cryogenics Division

Institute for Basic Standards,

National Bureau of Standards

Boulder, Colorado 80303

February 1978

Prepared for:

Wallace H. Boggs

National Aeronautics and Space Administration

Kennedy Space Center, Florida 32899

(Contract No. CC-62530A)

'

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director !
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1
CONTENTS

Page

1.0 INTRODUCTION 1 2.0 DESCRIPTION OF THE AUGER 2 3.0 RESULTS OF TESTS PRODUCING SLUSH HYDROGEN WITH THE AUGER 5 4.0 ENERGY REQUIRED TO PRODUCE SLUSH HYDROGEN 10

5.0 CONCLUSIONS ' 14

6.0 REFERENCES 16

iii
PRODUCING LIQUID-SOLID MIXTURES OF HYDROGEN USING AN AUGER

R. 0. Voth

An auger rotating inside a brass tube refrigerated with liquid helium

was used to produce liquid-solid (slush) mixtures of hydrogen. The auger

produced small particles of solid hydrogen so that the resulting mixture

could be transferred and stored. The auger could produce slush continuously

in an appropriate system; it could produce slush at pressures higher than the

triple point pressure of the hydrogen,- and the energy required to produce the

slush was less than the energy required to produce slush hydrogen using the

freeze-thaw process.

Key Words: Cryogenic; hydrogen; production efficiency; scraping auger;

slush; slush production.

iv
1.0 INTRODUCTION

Liquid-solid mixtures (slushes) of hydrogen exhibit a higher density and

heat capacity than the normal boiling point liquid. These characteristics

are advantageous in space applications where longer storage times and higher

storage densities are important. But to effectively use slush hydrogen, we must be able to produce it economically, to store and transfer it successfully and to measure stored mass and mass flow rates. This paper reports

test results of an auger system used to produce slush hydrogen.

During the late 1960 's and early 1970 's [1-9] slush hydrogen production,

transfer, storage and measurement technologies were investigated. Although

other production methods were tried, the freeze- thaw method was the most

thoroughly investigated and it was used to produce test quantities of slush

hydrogen. The freeze-thaw method of producing slush uses the latent heat of

vaporization to freeze the cryogen. By removing vapor at the optimum rate

from a container of triple point liquid, a porous solid layer is formed on

the liquid surface. Stopping the vapor removal and mixing this solid layer

into the remaining liquid produces a liquid-solid mixture of the cryogen. To

obtain high solid mass fractions, the pumping-mixing or freeze-thaw cycle

must be repeated numerous times. The freeze-thaw method produces small solid

particles so that the resulting mixture can be transferred and stored in

conventional cryogenic systems.

The freeze-thaw production method is technically feasible and fully

developed, however, it has disadvantages. These disadvantages include:

1) The freeze-thaw process is a batch process.

2) The freeze-thaw process operates at the triple point pressure of

the cryogen. For hydrogen this pressure is 0.0695 atmospheres

absolute, a pressure that draws air through inadvertant leaks in

the system. The air can become a safety problem in a hydrogen

system.

3) The freeze-thaw process requires either costly equipment to recover

the generated triple point vapor or the loss of about 16 percent of

the normal boiling point liquid hydrogen if the vapor is simply discarded.

4) The freeze-thaw production process becomes even more difficult to

apply to oxygen (another cryogen used in the space program) because

the triple point pressure for oxygen is 0.0015 atmospheres; this

pressure is nearly 50 times less than the triple point pressure of

hydrogen

.
In this study an auger that scraped frozen solid from the inside of a refrigerated brass tube was used to produce slush hydrogen. The auger system

can continuously produce slush hydrogen and since the auger system can be

immersed in liquid, slush can be produced at pressures above triple point

pressure. The increased pressure can be produced pneumatically or by generating a temperature stratification near the surface of the liquid hydrogen.

The auger system produced particles in the size realm of the particles produced by the freeze-thaw method so that the auger produced slush can be

readily transferred and stored.

The surface freezing occurring in the freeze-thaw production process is

thermodynamically reversible. In contrast, the freezing process in the auger

is irreversible since a temperature difference must exist between the re- frigerant and the freezing cryogen, and work energy added to scrape the solid

cryogen out of the brass tube must be removed by the refrigerant. In spite

of these irreversibilities, the auger system required less energy to produce

a quantity of slush hydrogen than a practical freeze-thaw system. The temperature difference requires refrigeration temperatures below the triple

point temperature of hydrogen so a gaseous or liquid helium refrigerator is

required for the auger. In the experiment described here, liquid helium was

used as the refrigerant.

The auger production method was initially based on the technology of the

commercial shaved water-ice maker. However, during this investigation two

patents describing an auger for making slush hydrogen were discovered [10,11].

The description in the patents confirmed the general conclusions of this

investigation. ,

2.0 DESCRIPTION OF THE AUGER

A cross section of the auger and heat exchanger assembly used to produce

slush hydrogen is shown on figure 1. A photograph of the partially disassembled unit is shown on figure 2. The 400 series stainless steel auger

with an outside diameter of 4.676 cm (1.841 in) is supported within a close

fitting brass tube by a ball bearing at each end. The radial clearance

between the auger and brass tube is 0.0178 cm (0.007 in) at ambient temperature. Because of the difference in thermal expansion between the stainless

steel auger and the brass tube, the radial clearance decreases to 0.0127 cm

(0.005 in) at the operating temperature. Since solid hydrogen has a low

thermal conductivity, this annular clearance should be small to produce a high heat transfer rate.

A ribbon packed, vacuum insulated heat exchanger surrounds the brass

tube. The ribbon packing increases the refrigerant heat transfer area while

the vacuum insulation eliminates heat transfer to the exterior surface.

Vacuum insulated refrigerant lines are provided at the top and bottom of the

heat exchanger.

2
All dimensions in centimeters

except as indicated

Figure 1. Cross section of auger and heat exchanger assembly.

3

The auger assembly was suspended in the vertical position from a top

plate by a stainless steel tube. Located concentrically within the support

tube is the stainless steel torque tube used to rotate the auger. A rotating

shaft seal using a magnetic fluid for a no-leak seal connected the torque

tube to the ambient temperature drive mechanism. The drive motor is an air

motor with numerous gear speed reducers. By changing the number of gear

reducers and air supply pressure to the motor, the auger rotational speed

could be varied from about 1 rad/s to 10 rad/s.

During operation the auger assembly is immersed in the liquid to be

t frozen and liquid is drawn into the holes hear the top of the brass tube and

frozen hydrogen particles are discharged through holes near the bottom. In some of the tests with hydrogen, the bottom support bearing was removed to

allow the solid particles to discharge from the bottom of the brass tube.

Solid particle size was not affected by this alteration, but auger torque

with no freezing (tare torque) was reduced significantly.

The instrumentation used with the auger system measured the refrigerant

inlet and outlet temperatures, the torque and rotational speed of the auger

drive, and the refrigerant flow rate. Refrigerant pressure and slush con- tainer pressure were also displayed on convenient gauges. Clear glass dewars

were used to contain the auger system so that the slush production could be

viewed and photographed.

3.0 RESULTS OF TESTS PRODUCING SLUSH HYDROGEN WITH THE AUGER

Primarily, the auger tests were conducted to determine if the auger

could produce sufficiently fine frozen particles so that the resulting slush

hydrogen could be stored and transferred easily. As a secondary result the

auger power requirements at various rotational speeds versus refrigeration

supplied to the auger were measured to obtain an efficiency figure that could

be compared to the efficiency of the freeze-thaw production method. Also,

the slush was held in the settled condition for approximately 3 hours to

determine if particle size changed with age.

Figure 3 shows freshly produced solid particles floating past a 2 mm

square grid (the grid and shovel used to move the slush is shown in figure

4). The fresh slush particles are random in size varying from approximately

0.1 mm to 8 mm in the largest dimension. The particle size for freeze-thaw

produced slush has been extensively measured [9] and the sizes of both fresh

and aged particles varied from 0.5 to 10 mm with 2 mm being the most common

size. The particles produced by the auger appear smaller; however, a much

more extensive measuring program than conducted during these tests is necessary to determine the actual size distribution. Aged auger produced slush

particles, figure 5, appear to have about the same size distribution as the

fresh particles although they are now more rounded and have a clearer texture.

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One would expect the particles produced by the auger to be curved to the same radius as the auger and not be the fine particles shown in figures 3 and

5. Actually the first particles produced from the auger are curved and quite

large. But as the solid production rate increases (due to cooldown of the

liquid hydrogen to triple point temperature) the curved needle shaped particles disappeared and the small particles shown in figure 3 were produced.

This freezing phenomena is not understood, but the particles produced by the

auger do become acceptably small during steady state operation.

The power required to rotate the auger is based on the torque and

rotational speed measurements. During early tests when the bottom auger

support bearing was in place, the rotational power increased with auger

operating time. This increasing power with time was a result of clear solid

hydrogen closing the solid discharge holes at the bottom of the auger assembly. As the hole size decreased the power required to extrude the solid

increased. This problem was solved by removing the bottom bearing thereby

increasing the solid discharge area. Without the bearing the solid could fall

from the bottom of the auger assembly and the discharge holes were no longer

necessary. Removing the bottom bearing also decreased the no freezing torque

requirements (tare torque) . Reducing the tare torque increased the amount of

refrigeration available to freeze the hydrogen. The tare power (P^) at 1.57

rad/s rotational speed decreased from 0.565 W with the bottom bearing in place

to 0.282 W without the bearing. Tare torque is constant so that the tare

power is directly proportional to rotational speed.

Scraping torque (the torque required by the auger to scrape off the

solid hydrogen) is also nearly constant for a constant refrigeration rate to

the auger. The constant torque is apparently due to a scraping force that

does not depend on the thickness of the solid layer being removed. The

constant torque measurement must be qualified however because theoretically

the scraping power is now directly proportional to rotational speed and the

scraping power approaches zero as the rotational speed approaches zero. In

actual tests the torque would remain constant as the auger rotational speed was reduced until the auger would suddenly stop (freeze in place) with the

torque increasing to a very high value.

The scraping torque does increase, however, as the refrigeration to the

auger increases. This increase in scraping torque reflects the increased

area over which solid hydrogen is formed. Since liquid helium is the refrigerant, heat is rapidly removed at the refrigerant inlet near the bottom of

the auger because of a high helium boiling heat transfer coefficient and a relatively large temperature difference. Above the liquid helium level in

the refrigerant heat exchanger, gas velocities are quite low so that the

heat transfer coefficient is low and the temperature difference is also

decreasing. Thus, at low refrigeration levels the liquid helium level in the

9
heat exchanger is low and the area where the majority of the solid hydrogen

is formed is low. As the refrigeration level is increased due to increased

liquid helium supply flow, the liquid helium level in the heat exchanger

rises increasing the area where frozen hydrogen accumulates. The increased

area of freezing hydrogen requires an increased scraping torque.

Figure 6 shows the scraping power as a function of the refrigeration

supplied to the auger assembly. The points shown are the experimental points

taken at 1.57 rad/s or experimental points adjusted by direct proportion from

a slower or faster rotational speed to 1.57 rad/s. The straight line is a hand fit of the data and is expressed by

- 0.015 (P^), (1)

where P^ is the scraping power at a rotational speed of 1.57 rad/s in Watts,

and P is the refrigeration supplied to the auger assembly in Watts.

The total power required to rotate the auger is the sum of the tare

power and the scraping power. Since this power is proportional to the

rotational speed this power can be expressed as

P^ = R/1.57 (P^ + P^) (2)

where

P = total auger power requirement, W

3. R = auger rotational speed, Rad/s

P = auger scraping power at a rotational speed of 1.57 rad/s

(equation 1 ) , W

and P^ = auger tare power at a rotational speed of 1.57 rad/s, W.

By substituting P^ from (1) and the tare power with the bottom bearing re- moved (0.282 W) while using 1.57 rad/s for R results in

P = 0.015 P^ + 0.282 (3)

4 . 0 ENERGY REQUIRED TO PRODUCE SLUSH HYDROGEN

Producing slush appears to be a reasonable way of increasing the density

and heat capacity of hydrogen, however, the cost of producing the slush must

be acceptable. When large quantities of slush are produced, the cost of the

energy required to produce the slush becomes significant. The production

energy depends on the thermodynamic reversible energy requirements to produce

slush and the energy required to overcome irrversibilities in a practical

system. In this section the energy required to produce slush with a 0.5

solid mass fraction is determined for four practical slush producing systems.

Three of the systems use the freeze-thaw production method and the fourth

uses an auger and helium refrigerator to produce slush from normal boiling

point liquid hydrogen.

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11
The thermodynamic reversible energy required to produce slush is equal

to the thermodynamic availability of the slush. Using normal hydrogen at a temperature of 300 K and a pressure of one atmosphere as the base fluid,

the reversible energy required to produce normal boiling point parahydrogen

is 3971.4 W-hr/kg, and the reversible energy required to produce slush with

a solid mass fraction of 0.5 is 4372.8 W-hr/kg. The energy required by

practical systems is higher because of the component inefficiencies. The

calculated energies for the four cases are based on liquefier and refrigerator efficiencies of 40 percent of Carnot. The vacuum pumps required by

the freeze-thaw production method are assumed to have an efficiency of 50

percent of isothermal. Heat leak into the containers and transfer lines

would also increase the required energy but these increases have not been

included because they are difficult to calculate without a firm system

definition and because, to at least the first approximations, the heat leak

would be nearly equal for all of the systems studied. Because heat leak is

not included, the calculated energies for the four cases will be lower than

those of an actual system, but, the calculations do allow a comparison

between the various slush production systems.

Besides the higher energy due to inefficient liquefiers and refrigerators in the system, the auger introduces additional irreversibilities

because of the rotational power added to the slush generator, and the

temperature difference between the refrigerant and freezing hydrogen. The

ratio of total supplied refrigeration to the refrigeration available for

freezing hydrogen in the auger system varies from 1.15 at low refrigeration

rates to 1.015 for very high refrigeration rates (eq (3)). A ratio of 1.05

will be used to determine the refrigeration required to produce slush with

a solid fraction of 0.5. The input energy to the refrigerator connected to

the auger depends on the refrigeration temperature; the lower the temperature the higher the input energy. For the auger system, we assumed the

lowest refrigeration temperature to be 10 K and since the refrigerator is

also cooling the normal boiling liquid to triple point liquid, the highest

refrigeration temperature is assumed to be 19.76 K. These refrigeration

temperatures are within the capabilities of a closed cycle helium refrigerator . While the power required to produce slush hydrogen using the auger is

relatively easy to calculate, the power required to produce slush hydrogen

using the freeze-thaw production method is very system dependent. For

instance, the simplest freeze-thaw method vents the hydrogen gas after

warming it to ambient temperature and compressing it to ambient pressure.

Much more complex freeze-thaw systems can be visualized. The most complex

system uses the refrigeration available in the vapor to precool a reliquefier. In table 1 five cases used to calculate power requirements are defined.

12
Table 1. Description of cases. CASE 1 Hydrogen liquefier produces sufficient NBP liquid so one kilogram

remains after the slush hydrogen is produced by the freeze-thaw

method. The vapor removed during slush production is vented.

CASE 2 Hydrogen liquefier produces the NBP liquid. Slush is produced by

the freeze-thaw method. Vapor removed during production is warmed,

pumped to ambient pressure and then reliquefied.

CASE 3 Hydrogen liquefier produces the NBP liquid. Slush is produced by

the freeze-thaw method. Vapor removed during production enters a liquefier as a cold gas and is reliquefied.

CASE 4 Hydrogen liquefier produces the NBP liquid. Slush is produced using

a helium refrigerator and the auger. Refrigerator temperatures are between 19.76 and 13.303 K to produce triple point liquid and between 13.7 and 10 K to produce the slush.

CASE 5 Hydrogen liquefier produces the NBP liquid.

Liquefier efficiency = refrigerator efficiency = 40 percent of

Carnot for all cases. Vacuum pump efficiency = 50 percent of isothermal.

13
and table 2 shows the calculated energy required to produce one kilogram of

slush hydrogen with a solid fraction of 0.5 for four cases compared to the

energy required to produce normal boiling point liquid. The first three

cases employ the freeze-thaw production method, the fourth case employs the

auger production method, and the fifth case shows the power necessary to produce NBP liquid. Producing NBP liquid (Case 5) requires the least amount of

energy but for only 11.7 percent more energy slush hydrogen with a 0.5 solid

fraction can be produced using the auger. Case 1, the simplest freeze-thaw

production system, requires the most energy and loses 0.19 kilogram of gaseous

feed hydrogen for every kilogram of slush produced. The most energy economical

freeze-thaw production system (Case 3) requires 516.4 W-hr/kg more energy than

required by the auger method of producing slush. Case 3 would require a significant investment in slush generators and transfer equipment to achieve

the energy economics while Case 1 and Case 4 would require the least capital

for auxiliary equipment. , - . -

5.0 CONCLUSIONS

The auger successfully produces slush hydrogen with sufficiently small

particles, so that the slush can be stored and transferred in cryogenic

systems. Aging characteristics of the auger formed slush appear to be

similar to the aging characteristics discovered for freeze-thaw produced

slush. The energy required to produce 0.5 solid mass fraction slush is only

1.12 X the energy required to produce normal boiling point liquid hydrogen.

This increase in energy input increases the density of hydrogen by 15.5

percent and the heat capacity by 18.3 percent when compared to normal

boiling point liquid. Even though the auger system has some irreversibilities associated with the freezing and scraping processes, the overall

evergy required to produce slush using the auger and a helium cycle refrigerator is less than the energy required by the freeze-thaw production

method

.

The auger method of producing slush eliminates many of the problems

associated with the freeze-thaw production method. The auger can produce

slush continuously versus the batch freeze-thaw process, and it can produce

slush at raised pressures eliminating the air-intrusion safety problems

associated with the production of slush hydrogen in the freeze-thaw production process.

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6.0 REFERENCES

[l] Mann, D. B., Ludtke, P. R. , Sindt, C. F. , and Chelton, D. B. , Advances in

Cryogenic Engineering 207 (Plenum Press, New York, 1966) .

[2] Elrod, C. W. , Technical Report AFALL-TR-65-63 , Air Force Systems Command, Wright-Patterson Air Force Base, Ohio (1965)

.

[3] Neindorf, L. R. and Noichl, O, J., Final Report on Air Force Contract No.

AF 33 (615) -1357 , Union Carbide Corp., Linde Division, Technical Report

AFAPL-TR-64-151, Defense Documentation Center, AD No. 457753 (Alexandria,

Virginia, 1965).

[4] Sindt, C. F. and Ludtke, P. R. , Advances in Cryogenic Engineering 1_5 (Plenum Press, New York, 1970)

.

[5] Sindt, C. F., Ludtke, P. R. , and Daney, D. E. , Nat. Bur. Stand. (U.S.)

Technical Note 377 (1969).

[e] Daney, D. E., Cryogenics 7, 280 (1967).

[7] Mann, D. B., Sindt, C. F., Ludtke, P. R. and Chelton, D. B. , Proceedings

of the Conference on Long-Term Cryo-Propellant Storage in Space, NASA,

Huntsville, Alabama, p. 243 (1966).

[8] Daney, D. E. , Ludtke, P. R. , Chelton, D. B. and Sindt, C. F. , Nat. Bur. Stand.

Stand. (U.S.) Technical Note 364 (1968).

[9] Sindt, C. F., Cryogenics 1_0, 372-380 (1970).

[10] Hemstreet, R. A., Apparatus for making hydrogen slush using nitrogen and

helium refrigerants, Patent No. 3,521,457 (1970).

[11] Huibers, D. Th. A., et al., Apparatus for making hydrogen slush using

helium refrigerant, Patent No. 3,521,458 (1970).

16
NBS-114A (REV. 7-73)

U.S. uth"!. ui- i-uMM. 1 1, PUIiLlC.Al ION OK Rr, FORI NO.

'''''°^h''e''eT ''^'^

1 NBSIR 78-875

Z. Oov c Accession

No.

3. Recipient's Accession No.

4. TITLIi AND SU n 1 1 1 l^r.

PRODUCING LIOUID-SOLID MIXTURES OF HYDROGEN USING AN AUGER

5. Publication Date February 19 78

6« Performing Organization Code

275.05

7. AUTHOR(S)

R. 0. Voth

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2750452

11. Contract/Grant No. CC-62530A

1 2* Spon sor ing Ofg an iz at ion N ame an d Comple te Add re s s (S tteet, dty, Stste , ZIP

)

National Aeronautics and Space Administration

Kennedy Space Center, Florida 32899

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

An auger rotating inside a brass tube refrigerated with liquid helium was used

to produce liquid-solid (slush) mixtures of hydrogen. The auger produced small

particles of solid hydrogen so that the resulting mixture could be transferred and

stored. The auger could produce slush continuously in an appropriate system; it

could produce slush at pressures higher than the triple point pressure of the hydrogen; and the energy required to produce the slush was less than the energy required

to produce slush hydrogen using the freeze-thaw process.

J 17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Cryogenic; hydrogen; production efficiency; scraping auger; slush; slush production.

18. AVAILABILITY [X§ Unlimited

1 ! For Official Distribution. Do Not Release to NTIS

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Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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21. NO. OF PAGES

21

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UNCLASSIFIED

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