THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING Department of Mechanical Engineering FINAL REPORT LIQUID-VAPOR EQUILIBRIJM IN THE SYSTEM4 HYDROGEN-HELIUM R.E. Sonntag R.W. Crain, Jr. W.B. Streett G.J. Van Wylen ORA PROJECT 05664 under contract with NATIONAL AERONAUTICS AND SPACE ADMINISTRATION GEORGE Co MARSHALL SPACE FLIGHT CENTER CONTRACT NO. NAS 8-5256 HUNTSVILLE, ALABAMA administered through: OFFICE OF RESEARCH ADMINISTRATION ANN ARBOR JUNE 1964

PREFACE This report constitutes the final report on Contract No. 8-5256, atio.t Aeronautics and Space Administration, George C. Marshall Space Fli.ght Centser, Huntsville, Alabama. The first phase of the investigation, equilibrium in the system normal hydrogen-helium was conducted as a doctoral thesis by Mr. William B. Streett. A complete description of the apparatus and results obtained have been reported previously, Report No. 05664-1-T. That report may be consulted for details not included here. The second phase of the investigation, equilibrium in the system 20.4~K equilibrium hydrogen-helium was conducted by Mr. Richard W. Crain, Jr., and Mr Sudhir Savkar, who also assisted in the first study. Both phases of the investigation were carried out under the immediate supervision of Professor Richard E. Sonntag. The advice and assistance of Professors J*J. Martin, H. Merte, JrE. E.F. Westrum, and G.E. Smith, is acknowledged with gratitude. Sincere thanks are extended to Mro Frank Be Drogosz and his staff in the Department of Chemical and Metallurgical Engineering Instrumental Analysis Laboratory for their work in the mass spectrometer analyses of the samples. The assistance of Mr. Jack Brigham and the members of the staff of the Automotive Engineering Laboratory is also appreciated. Special thanks are extended to the Linde Company for their many contributiors, which included the construction of the cryostat, supply of the liquid hydrogen, and advice on safety precautions in handling liquid hydrogen. The contribution of the high-purity analyzed helium by the U.S. Bureau of Mines Helium Research Center, Amarillo, Texas, is also gratefully acknowledged. June, 1964 G.J. Van Wylen

TABLE OF CONTENTS I. INTRODUCTION Page A. General 1 B. Present Investigation 2 II. EXPERIMENTAL APPARATUS AND INSTRUMENTATION A. General 4 B. Cryostat 4 C. Vapor Recirculation System 7 D. Vapor-Pressure Temperature Control System 10 E. Liquid and Vapor Sampling Systems 12 F. Instrumentation and Accuracy 14 G. Modifications for Equilibrium Hydrogen Studies 15 III. OPERATIONAL PROCEDURES A. Preparation of Equipment 19 B. Run Procedures 20 C. Operational Difficulties 23 IV. RESULTS IN THE SYSTEM NORMAL HYDROGEN-HELIUM AND COMPARISONS WITH PREVIOUS WORK A. Presentation of Results 27 B. Discussion 37 C. Comparison with Previous Work 39 D. Thermodynamic Treatment 46 V. RESULTS IN THE SYSTEM EQUILIBRIUM HYDROGEN-HELIUM AND COMPARISONS WITH THE SYSTEM NORMAL HYDROGEN-HELIUM A. Presentation of Results 48 B. Discussion 57 C. Comparisons with the System Normal Hydrogen-Helium 58

LIST OF FIGURES Page 1. Schematic diagram of experimental equipment 5 2. Section through cryostat showing equilibrium cell 6 3. Schematic diagram of vapor recirculating system 8 4. Equilibrium cell 9 5. Schematic diagram of vapor pressure control system 11 6. Schematic diagram of sampling system 13 7. Catalytic reactor 16 8. Schematic diagram of thermal conductivity analyzing system 18 9. Isothermal pressure-composition diagram 31 10. Isothermal pressure-composition diagram showing liquid region on expanded scale 32 11. Isobaric temperature-coaposition diagram 33 12. Isobaric temperature-composition diagram showing liquid region on expanded scale 34 13. Pressure-temperature diagram 35 14. Equilibrium K values 36 15. Comparison with previous work, liquid data at 20.4"K 40 16. Comparison with previous work, vapor data at 20.40K 41 17. Comparison with previous work, liquid data at 17.4, 21.8, and 26.0SK 42 18. Comparison with previous work, vapor data at 17.4, 21.8, and 26.00K 43 19. Isothermal pressure-composition diagram 51 20. Isothermal pressure-composition diagram showing liquid region on expanded scale 52 21. Isobaric temperature-composition diagram 53 22. Isobaric temperature-composition diagram showing liquid region on expanded scale 54 23. Pressure-temperature diagram 55

-224. Equilibrium K values 56 25. Comparison of the two systems at 20.4~K 59 26. Comparison of the two systems at 23.0~K 60 27. Comparison of the two systems at 26.0~K 61 28. Comparison of the two systems at 29.0~K 62 29. Comparison of the two systems at 31.0~K 63 30. Comparison of the two systems at 31.5~K 64 - TABIES - I. Normal Hydrogen-Helium Summary of Experimental Data 28 II. Equilibrium Hydrogen-Helium Summary of Experimental Data 49

I. INTRODUCTION A. GENERAL The present investigation was undertaken to determine phase compositions in the liquid-vapor system hydrogen-helium in the range 15.5-32.50K at pressures from hydrogen saturation to 500 psi. Knowledge of the solubility of helium in liquid hydrogen is of particular importance, inasmuch as liquid hydrogen is commonly transferred by pressurization with helium gas. Such information is also of interest in bubble-chamber studies, where it may be possible to utilize hydrogen-helium mixtures to investigate the interaction of high-energy particles with helium nuclei. The hydrogen-helium system is also of interest from a theoretical standpoint, since the two components are the elements with the two lowest critical temperatures, both of which exhibit large quantum effects, and further because of the different ortho-para modifications of hydrogen. Prior to the present investigation, two studies have been made on the liquid-vapor system hydrogen-helium, both covering only a limited experimental range. Both of these investigations apparently used normal hydrogen (75% ortho, 25% para, the room temperature equilibrium composition) since no mention was made of conversion. The first of these is a Ph.D. thesis done by S. R. Smithl at Ohio State University in 1952. Smith reported experimental values of vapor and liquid phase compositions at three temperatures 17.40, 20.39, and 21.80~K, and at pressures up to 850 psia. The experimental apparatus and procedures used by Smith and the results of his work are discussed elsewhere in this paper. More recently Roellig and Giese reported nine isolated pressuretemperature points in the hydrogen-helium system covering a temperature range of 16.3-26.8~K and pressures up to 12.7 atmospheres,

-2Comparison of the data from these two sources is difficult due to the small number of points reported by Roellig and Giese; however a cursory comparison seems to indicate poor agreement, especially in the liquid phase compositions where the reported helium concentrations differ by as much as a factor of ten. Roellig and Giese did not measure temperatures directly, but estimated them by finding the partial pressure of hydrogen in the vapor phase (from the mass spectrometer analysis of the vapor samples), and then taking the temperature from a vapor pressure curve for pure hydrogen, after first calculating the depression of the vapor pressure due to helium in the liquid, using Raoult's law. The assumption of ideal behavior in the liquid solution and in the vapor phase hardly seems justified; moreover the temperature arrived at in this manner is dependent upon the helium content of the liquid, which is perhaps the least reliable portion of the experimental data. Eckert and Prausnitz3 criticized the temperature calculations of Roellig and Giese and concluded that even at the relatively low pressures involved, the temperatures reported were in error by as much as 2.70K. In addition to the two reports of experimental work on hydrogen-helium vapor-liquid equilibrium, two additional reports4'5 have been published which deal specifically with this system, although they contain no original experimental data. The first of these is a bibliography of references on hydrogen and helium and their mixtures with other substances. The second is a theoretical development of a thermodynamic consistency test applied to the data of Smith, and is discussed elsewhere in this work. B. PRESENT INVESTIGATION The principal investigation undertaken on this project deals with the system normal hydrogen-helium, and was the subject of the doctoral thesis by

-36 78 W. B. Streett6. This work has been reported previously in considerable detail7'8 and will be merely reviewed in the present report. Upon completion of that phase of the investigation, it was felt necessary to study this system using 20.4~K equilibrium hydrogen (0.21% ortho, 99.79% para) instead of normal hydrogen, in order to determine the nature and extent of any differences between the two systems. It was not deemed necessary to make as thorough a coverage as in the first phase of the work, since the prime concern here is with differences between the two systems. Consequently, while eleven isotherms were investigated for normal-hydrogen (15.5, 17.07, 20.4, 23.0, 26.0,29.0, 30.6, 31.0, 31.5, 31.9, 32.50K), only six were repeated for equilibrium hydrogen (20,4, 23.0, 26.0, 29.0, 31.0, 31.50K) at pressures from hydrogen saturation to 500 psia.

-4II. EXPERIMENTAL APPARATUS AND INSTRUMENTATION A, GENERAL The experimental apparatus and instrumentation used in the present investigation have been discussed in considerable detail in reference 7, "LiquidVapor Equilibrium in the System Normal Hydrogen-Helium", W. B. Streett, R. E. Sonntag, and G. J. Van Wylen, report 05664-1-T, September, 1963. This equipment will be reviewed here, along with a description of modifications and additions made for studying the system equilibrium hydrogen-helium. Reference 7 should be consulted for additional details regarding the basic equipment. As seen in the schematic diagram Figure 1, the experimental apparatus can be divided into four basic areas: (1) cryostat; (2) vapor recirculation system; (3) vapor pressure temperature control system; (4) liquid and vapor phase sampling; each will be discussed separately below. B. CRYOSTAT A cross-sectional view of the cryostat is shown in Figure 2. It was necessary to design the cryostat of stainless steel, since vapor pressure control was used for the liquid hydrogen bath, with cryostat pressure approaching 200 psi. The inner and outer vessels are made of schedule five type 304 SS pipe (0,108 inch wall) with standard butt-weld pipe caps for the end pieces. The inner vessel is constructed of 6 inch pipe, and has a 3 inch pipe neck as indicated in the diagram, in order to reduce heat flow into the cryostat. The cryostat capacity is approximately 15 liters. The flange was machined from 3/4 inch stainless steel, reduced to 3/8 inch at the outer perimeter to reduce heat conduction to the inner vessel. The top plate is machined from a 1/2 inch stainless plate, and sealed with a teflon-coated stainless steel O-ring.

VENT I'MAGNETIC PUMP rMANOMETER VENT 4 TSUPPLY VAPOT VU / Jb I SadVAPOR RECIRCULATING SYSTEM SO3~~~LENa~VOIOLUM I *MERCURY WATER IIiTC( MAY - M~n~R CHARGING PRESSURE REGULATOR I i 110 VOLTS CAL LARY VENT VAC I "" VCOLS ~VAPO~tR WCRYOTAT AMNSURE CON SYSTEM g. 1. Schematic diagram perental equipment. RPTURWEDISC ~ PRESSURE REGULATOR H 1- -I He EQULIBRIM 1'~- M M M —^r 111 —8-VENT CRYOSTAT SAMPUNO SYSTEM Fig. l. Schematic diagram of experimental equipment.

-6TO VAPORLIUID CIRCULATIONSAMPLE LINE LOOVAPOR { __ SAMPLE LINE COUNTERFLOW HEAT EXCHANGER LINDE SUPERINSULATION VAC BELLOWS TYPE VACUUM VALVE STIRRING MECHANISM — H2EQUILIBRIUM CELL - Fig. 2. Section through cryostat shaing equilibrium cell.

-7Insulation of the cryostat is achieved by 0.8 inch of Linde SI-4 multi-layer insulation, and a liquid nitrogen,reservoir, as shown in Figure 2. The cryostat was also covered with fiberglass and styrofoam insulation to further reduce the heat load. The cryostat is provided with a vertical-acting solenoid operated stirring device actuated by a mechanical-electrical timing circuit. In the operation of the cryostat for investigation of the system normal hydrogen-helium, it was found that the bath liquid hydrogen was boiled away at the rate of about 1 liter in 6 hours. In the later experiments with equilibrium hydrogen, this rate was increased considerably when hydrogen was being converted in the loop, because of the large heat release associated with ortho-para conversion. 3. VAPOR RECIRCULATION SYSTEM The vapor recirculation portion of the experimental apparatus is shown in detail in Figure 3, and includes the equilibrium cell, counterflow heat exchanger volume regulator, vapor sample trap, magnetic pump, pressure gage, flow meter and associated valves and tubing. The total volume of the loop including the equilibrium cell is approximately 180 cc, and was kept to a minimum so that the vapor can be recirculated in a small amount of time. The equilibrium cell, shown in Figure 4, is constructed of free-machining copper, and has an internal volume of about 55 cc. A small piece of copper wool was placed at the top of the cell, to prevent liquid droplets from being carried out with the circulating vapor, and copper wool at the bottom was used to help break up vapor bubbles entering the cell. The 1/8 inch copper vapor line entering the bottom of the cell is loosely coiled several times around the cell to assure that vapor entering the bottom is cooled to the equilibrium

-8Magnetic Pump. I jI ~ Floating Piston Volume Regulator To He ^~ —..L-^-~~~~~ To 16"dia. 0 - harging ~~~~Precoo r~ng ~ P~~ System co"i H l ^ |'==; l (g' PHeise gage Counterflowl Vapor Sample Trap Heat Exchanger ~ ~ Flow _ qu ilibf Final Cooling I ~Coil Equilibrium i Cell Fig. 3. Schematic diagram of vapor recirculating system.

-9Liquid Sample Line 1/16" O.D. x 0.004" I. D. Stainless Steel Capillary Vapor Outlet Line Tubing 1/8" O.D. Copper Tubing tl ^ 1* Silver Soldered Joints ^ _ / 3/16" O.D. Copper Sleeve 1/8"0.D. Stainless Sleeve- Platinum Resistance 1V4" Thermometer —-- _ [:t I.'L"- I ~Copper Wool Resistance Thermometer I Well 7/32" Dia. T=' - \~~ W13/8" 23/8 6 6/41 63/4" i W — |-Copper Wool 3/16"0.D. Copper — Silver Solder Sleeve Vapor Inlet Line 1/8" O.D. Copper Tubing Fig. 4. Equilibrium cell.

-10temperature. The inlet and outlet vapor lines are wound together to form a helical counterflow heat exchanger in the cyrostat neck, as indicated in Figure 2 in order to precool the inlet vapor and reduce the cryostat heat load. The floating-piston volume regulator was placed in the recirculation loop to permit the adjustments in loop volume and consequently in equilibrium pressure. The vapor sample trap consists of two coils of 1/4 inch copper tubing, in the center of which is a sampling valve. The valves at the ends of the coils serve to isolate a portion of equilibrium vapor in the trap before the sampling is performed. The double-acting magnetic recirculating pump and its associated mechanicalelectrical pulsing circuit were constructed for this investigation. It is necessary to have a non-contaminating source of pressure to circulate the vapor at a variable, low rate of flow. D. VAPOR-PRESSURE TEMPERATURE CONTROL SYSTEM Bath temperature control is achieved by an automatic vapor-pressure control system, shown in Figure 5. The mercury U-tube switch S is constructed from a 1 inch thick Plexiglas block, and has platinum wire contacts. Operation of the control system is based upon comparison of the bath pressure with that in a reference volume V. Bypass valve C is manually adjusted to give rough control of the bath pressure at the desired value, after which valve A is closed. Fine control is then achieved automatically by the solenoid valve K, actuated by the mercury U-tube switch. The large bypass D is used in conjunction with the vacuum pump when operating the system at less than ambient pressure. Any tendency for long-term drift of the system can be compensated by making fine adjustments of the quantity of gas in the reference volume through valves G and

-11Manometer 0-200 psi Vent To Charging Heise gage System G oo i Solenoid Stirrer 200 Watt Heoter Ho A t len Temp Sensing /' - IElement Hg T ps ThermoI Ster Bath ll I I i 6v. I iWat.er Bath I sI 11 1v. n! I I. I i l.: I I I Temperature Relay — |' — - -- Controller Cryot IlOv. 110 v. *E F Vent Vac Fig. 5. Schematic diagram of vapor pressure control system.

-12H. Using this system, it is possible to control the cryostat pressure to within 1 mm mercury for long periods of time, which corresponds to a control in temperature of about 0.0050K over most of the temperature range of the investigation. E. LIQUID AND VAPOR SAMPLING SYSTEMS A schematic representation of the liquid and vapor portions of the apparatus, an extremely important part of the experimental equipment, is given in Figure 6. The vapor sample trap has been described previously. A length of stainless capillary tubing connects valve A with the sample bank, in order to control sample flow rate during high pressure runs. Check valve C opens to the vent line at 1.5 psi gage, thereby preventing pressure build-up in the glass sample bottles. Liquid phase sampling is achieved through a 0.004 inch ID stainless capillary located in the equilibrium cell as shown in Figure 4. Extreme care must be exercised in sampling the liquid, inasmuch as the sample vaporizes in the tube as it warms to ambient temperature, and fractionation with subsequent change in composition must be avoided in withdrawing the sample. The first length of liquid sampling line is about 72 inches long, at which point two flow-regulating needle valves J and K are installed. Two bypass loops, each 72 inch lengths of capillary, are installed beyond the control valves, and provide a close control over the sample flow rates. Thus, the liquid can be withdrawn very slowly from the cell over a wide range of cell pressure. The flow meter is used in -onjunction with the liquid sampling system so that the line can be purged by discarding a controlled initial quantity of sample at the time of taking the data. A check valve is also used in this system to prevent excessive pressure in the glass sample bottles..

-13Swagelok Bulkhead Adapter \^ ^^- Hex Nut Tygon Plastic Tubin i ushi ~'~ _,,Bushing ^, %. 1/8"O.D. Copper Vapor Sample Trap X Tubing / (Coiled 1/4"0.D. Copper Solder Tubing) 12-30 Taper Female Glass Joint Front Panel D E Detail of Metal To Glass Seal 1/16" O.D. x 0.004" I.D. Stainless Steel Capillary _I-. - -_.. — Tubing J K L M rF PSI I I -,.i I P Check Gage C Equilibrim I M I Valve -\Zizlr^ JR VT u fv 9W Metal To Glass Seal (See Detail Above) Standard Taper Glass Joint Sample Bottle With Vacuum Stopcock Vac. Fig. 6. Schematic diagram of sampling system.

-14F. INSTRUMENTATION AND ACCURACY The equilibrium temperature is measured using a Leeds and Northrup type 8164 platinum resistance thermometer mounted in a hole in the equilibrium cell block, as shown in Figure 4. Measurements are made by a Leeds and Northrup #8069 type G-2 Mueller bridge, with a null-detecting type 2284-d reflecting galvanometer hung in a Julius suspension. Temperature changes of 0.002 K can be detected with this equipment in the range of the present study, although the temperature scale is accurate to only 0.01 K. Gold-cobalt vs. copper differential thermocouples are used to determine the temperature distribution throughout the cryostat. Five thermocouples were used in the study of the normal hydrogen-helium system; an additional thermocouple was added in the subsequent study using equilibrium hydrogen in order to be able to check the catalytic reactor temperature. The thermocouples are constructed of 30 gage wire, and are all referenced to the platinum resistance thermometer location. Thermocouple EMF's are measured by a Leeds and Northrup type K-3 potentiometer with a type 9834 electronic D-C null detector. Accuracy of these differential measurements is of the order of 0.02 K at the highest temperature and 0.04~K at the lowest temperature of this study. Equilibrium pressure is measured in the vapor recirculation loop on a 0-500 psi, 16 inch, calibrated Heise gage, shown in Figures 1 and 3. The gage is accurate to 0.1% full scale, or 0.5 psi for this work. Vapor pressure of the hydrogen cryostat is measured, for the purpose of rough initial settings, using a 0-200 psi, 16 inch Heise gage for pressures above atmospheric, and by a 0-30 inch mercury manometer for pressures below atmospheric. All auxilliary pressure measurements,are made with standard bourdon tube gages.

-15Samples taken of the liquid and vapor phases were analyzed in the Instrumental Analysis Laboratory of the Department of Chemical and Metallurgical Engineering of the University of Michigan. Analyses were performed using a Consolidated model 21-103 mass spectrometer. The limits of accuracy of this instrument are as follows: Mole % of Max. mole Max, X component % deviation deviation 100.0 + 1.0 + 1.0 10.0 + 0.10 + 1.0 1.0 + 0.05 + 5.0 0.1 + 0.01 + 10,0 G. MODIFICATIONS FOR EQUILIBRIUM HYDROGEN STUDIES Only minor modifications and additions to the equipment have been made for studying the system equilibrium hydrogen-helium. A small catalytic reactor, shown in Figure 7, was designed, and is constructed of copper. Since ortho-para conversion is a highly exothermic reaction, the bar was milled to the crosssection shown to provide maximum surface for heat transfer to the bath hydrogen. The four-parallel-tube construction was used in order to minimize pressure drop in the loop. The catalyst used for conversion in this work is hydrous ferric oxide, ground to about 30 mesh size. The reactor is installed inside the cryostat in the vapor recirculation line immediately above the equilibrium cell. The recirculation line is loosely coiled several times at both the reactor inlet and outlet, to assure temperature equilibrium of the circulating vapor with the bath liquid hydrogen.

-16a III B B A A Fig. 7 Catalytic reactor

-17Before the experimental studies were begun, the catalyst was activated by immersing the reactor in a bath of boiling turpentine (159~C) for a period of four hours while purging with helium gas. It was necessary thereafter to prevent contact of the catalyst with air, so the recirculation system was always filled with helium when not in use, Hydrogen analyses for ortho-para content are performed using a Gow-Mac model 9285 thermal conductivity cell with a power supply-control unit model 405C1. The schematic diagram of the analyzing system is shown in Figure 8. Provision was made to enable drawing a sample from either the vapor recirculating system or from the liquid sampling capillary. The pressure is closely regulated using a Matheson Series 70 low pressure regulator set at 10 inches of water gage. The flow rate through the cell is held at 50 cc/min. STP. The thermal conductivity of the sample is compared to that of a reference stream. Provision was made to pass either room-temperature hydrogen (normal H2) or samples of cryostat boil-off hydrogen (20.4~ equilibrium H2) through the reference cell. Output from the analyzer is read on a Beckman model 93500 potentiometric recorder.

-180 -L LIQUID SAMPLING EXHAUST MICROMETER REGULATING VALVES VAPOR SAMPLING 0 [ FLOWMETERS FLOWMETERS / ICRYOSTAT BOIL-OFF (EQUILIBRIUM H2) LOW PRESSURE CAPILLARY REGULATORS ~ COIL T/C CELL CEL —--- NORMAL H2 Fig. 8 Schematic diagram of thermal conductivity analyzing system

-19III. OPERATIONAL PROCEDURES A. PREPARATION OF EQUIPMENT The first step in the preparation of the equipment is the precooling of the cryostat and equilibrium cell with liquid nitrogen. This is desirable as it avoids the use of large quantities of liquid hydrogen for cooling. The equilibrium cell and recirculation loop are first evacuated and purged several times with helium to insure that no air remains to freeze in the capillary lines upon cooling. Liquid nitrogen is then introduced into the inside of the cryostat and into the nitrogen reservoir at the top. The rapid vaporization of the nitrogen as it first enters serves to purge much of the oxygen from the system. Because of the danger of explosion resulting from mixtures of air and hydrogen, extreme care must be taken to remove all traces of air from the system before hydrogen is introduced. Pure helium or nitrogen gas is allowed to flow continuously through the vent system for approximately 20 minutes before the hydrogen is introduced. The liquid hydrogen storage dewar is brought into the test cell and the transfer line connected to the dewar and cryostat. The cryostat, vapor pressure control system and transfer line are then alternately evacuated and purged with helium gas. Liquid hydrogen transfer to the cryostat is achieved by the hydrogen dewar pressure (10 - 12 psig) at a rate of several liters per minute. The cryostat liquid level is checked with a small carbon resistor probe, and the temperatures indicated by the resistance thermometer and thermocouples are observed. The cryostat was normally filled to a depth of about 20 inches (about 10 liters) for runs involving normal hydrogen, and to about 24 inches for those for equilibrium hydrogen, since the latter boiled away more bath liquid.

-20B. RUN PROCEDURES Operation of the vapor-pressure temperature control system has been described previously. To obtain temperatures below the normal boiling point of hydrogen (20.30K) a vacuum is pumped over the bath as indicated before. For temperatures above the normal boiling point, the vent lines are closed and the cryostat pressure allowed to build up to the desired value. It has been found that this procedure results in a stratification of the bath liquid, with persistance of large temperature gradients. Thus, while a layer of liquid hydrogen near the liquid surface reaches the saturation temperature corresponding to the set pressure quite rapidly, the bulk of the liquid warms only very slowly. When the system was first placed into operation, it was found that in several hours the liquid bulk temperature would rise only a single degree. The cryostat stirring mechanism did not provide sufficient liquid circulation to overcome these gradients, and frequently jammed and did not function properly. This problem of very slow bath temperature rise was eliminated in the normal -H2 study by inserting a length of tubing into the cryostat and pumping a small quantity of warm hydrogen gas through the liquid, thereby achieving a rapid rise to the desired value. It was unfortunately not possible to use this procedure for those runs with equilibrium -H2 when the hydrogen supply was drawn from cryostat boil-off vapor, at least until a sufficient quantity of boil-off had first been collected. Once the desired bath pressure is reached, it is held automatically by the vapor-pressure control system. It is frequently necessary to adjust very slightly the quantity of gas in the reference volume, so as to obtain exactly the temperature of the bath, once the temperature has stabilized the bath becomes uniform in temperature throughout the liquid, with the gas space at the top of the cryostat being somewhat warmer. Charging the loop with hydrogen-helium mixtures is somewhat different for the two systems studied. In the normal hydrogen-helium system, the supply of hydrogen used in Matheson ultra-pure grade having total impurities of less than 10 ppm.

-21Since this is stored at ambient temperature, it exists as normal -H2 (75% ortho, 25% para). The gas is first changed into a 1500 cc charging cylinder to a predetermined pressure. The mass of hydrogen necessary to fill the equilibrium cell approximately half full of liquid at the given temperature has first been calculated, as has the correlation between mass and pressure in the charging cylinder. The hydrogen is then slowly admitted to the loop until the desired amount has been charged, after which the loop is pressurized with helium gas until the desired system pressure is reached. The supply of helium used in this investigation is special analyzed helium donated by the U.S. Bureau of Mines Helium Res_-zrch Center, Amarillo, Texas, and has total impurities less than 12 ppm. In the investigation of the system equilibrium hydrogen-helium, the supply of equilibrium hydrogen was prepared by one of two alternate methods. In early runs, the hydrogen was charged in the same manner as for the previous work, in which case it was necessary to convert the hydrogen using the catalytic reactor. Using this procedure, the hydrogen charged into the loop is circulated by the magnetic pump, passing continuously through the reactor, which is located in the cryostat. Samples are periodically withdrawn and passed through the thermal conductivity analyzet system to be checked for ortho-para content. It was necessary to circulate the hydrogen for a period of several hours to achieve complete conversion to 20.40 equilibrium composition, and this required time gradually increased because of the decrease in catalyst effectiveness. This resulted in the boil-off of large quantities of liquid hydrogen from the bath, as a result of the normal heat load and, in addition, the heat release of ortho-para conversion. Part of the experimental data presented in this report were run using hydrogen converted in this manner. It ultimately proved to be more convenient, however, tD use cryostat boil-off vapor as the source of equilibrium hydrogen. Samples of boil-off gas analyzed by mass spectrometry indicated a few hundredths of one per cent helium (perfectly acceptable in this study), and a trace amount of neon. The problem of using cryostat boil-off is that its pressure is not

-22sufficient to charge the 1500 cc charging cylinder to the desired amount. Since a non-contaminating hydrogen compressor was not available, it was therefore necessary to charge this supply of equilibrium hydrogen into the loop in small low-pressure increments. This process is very time-consuming, but was ultimately felt to be more practical than converting room-temperature hydrogen. Once the desired quantity of equilibrium hydrogen has been charged into the system by either of the described procedures, helium is added until the desired system pressure is reached, as in the earlier study. Repeated experimental runs made using the alternate sources of equilibrium hydrogen showed no differences in composition of the two phases, and it was concluded that both methods are acceptable. In each study, numerous runs were made to determine the approximate time required for the hydrogen-helium mixture to reach equilibrium with continuous vapor recirculation. This was tested by holding pressure and temperature constant and taking samples of each phase at different time intervals. Analyses of these samples indicated that composition remained constant after about 10-15 minutes. Thereafter, samples were taken after about 20 to 30 minutes, and in some instances even longer, to insure that equilibrium was reached. Sampling of the liquid and vapor phases was discussed in the previous section. Extensive tests were run early in this study to determine the reproducibility of liquid sample compositions over a range of sample flow rates and capillary line length and purge time. It was found that by flowing the sample for 15-20 seconds before admitting the sample to the bottle, the results were very consistent and reproducible. It was also found that if the sample flow rate was not too high, there was no pressure drop in the loop during sampling, which indicates that equilibrium between the phases in the cell has not been disturbed by the sampling process. There were in general no difficulties

-23encountered with vapor phase sampling during the course of this investigation. C. OPERATIONAL DIFFICULTIES In general the apparatus functioned satisfactorily, and no serious difficulties were encountered during the majority of this investigation. The principal problem in the first phase of the work was that of raising the bath temperature in a reasonable period of time to study those isotherms above 20.4 K. This was resolved by pumping hydrogen gas through the liquid as described previously. There were difficulties with the bath stirring mechanism throughout the first phase of the study, as the stirrer rod frequently jammed on a guide ring in the cryostat, rendering the stirring device inoperable. This was not felt to be of importance, as temperature gradients were non-existant in the liquid once the temperature had stabilized. This was repaired, however, and the stirrer functioned properly during the second part of the research. The differential thermocouple located at the bottom of the equilibrium cell apparently shorted early in the investigation and was later replaced, although this difficulty recurred. This also presented no problems, since there was another thermocouple nearby in the liquid that consistently indicated no temperature gradient with respect to the resistance thermometer reference location. One phenomenon, not associated with equipment failure, was encountered during the runs at 15.5 and 17.07~K in the system normal hydrogen-helium. At each of these temperatures, there is a pressure in the range studied at which the density of the vapor phase (principally helium) becomes equal to that of the liquid phase (principally hydrogen). At pressures above such a value, the vapor phase is more dense than the liquid, such that the mixture is apparently inverted in the equilibrium cell with the liquid floating on top of the vapor. Smith observed this phenomenon in his studies of the system and concluded that it occurred at pressures slightly above 500 psia in the temperature range in which he was working (17.4 - 21.8~K).

-24Before runs were made at the lower temperatures in the present work, calculations were made to estimate the pressure at which this inversion should occur, using data on hydrogen and helium densities from Chelton and Mann.9 These calculations indicated that at temperatures around 15.0 this inversion would take place at a pressure of about 350 psia, and at higher pressures with increasing temperatures. When the run was made at 17.07, the apparatus functioned normally for pressures up to 400 psia; however when an attempt was made to set the pressure at 500 psia there were two indications that the inversion pressure had been exceeded. First, the pressure in the circulation loop began to oscillate, with a period of several minutes, between about 480 psia and 505 psia; these oscillations leveled off to about 6 lb after 30 min. Secondly, there was a sharp increase in the rate of boil-off of the liquid hydrogen in the cryostat, requiring a readjustment in the valve settings in the vapor; pressure.eotrol system. Neither of these phenomena had been observed previously. Assuming that the inversion had taken place, the probable explanations are as follows. The pumping action of the magnetic pump draws liquid up the outlet line of the equilibrium cell, and it vaporizes as it reaches a warmer portion of the equipment. This causes a gradual pressure rise, which continues until the evaporated liquid has traversed the circulation loop and re-entered the equilibrium cell where it condenses and (presumably) returns to the liquid by floating up through the vapor at the bottom. The increase in boil-off of the bath liquid can be explained as a result of the additional heat supplied by the condensation of the circulating mixture as it re-enters the cryostat. During the 15.50~ run, data were taken in the normal manner at 300 psia, but the phenomena described abn attempt was made to set the pressure at 400 psia.

-25At the two points mentioned above, where the phases were inverted, samples were taken in the usual manner, but the compositions obtained from their analyses were entirely inconsistent with the other results, and the data were discarded. No further attempts were made to take data at pressures above the inversion point, since the inaccessible region included only a small portion of the total area of interest. These isotherms were not run for the system equilibrium hydrogen-helium, since comparisons with normal hydrogen-heilum results at the higher temperatures indicated that differences between the two systems should be extremely small in this region. Other difficulties, mostly of a minor nature, occurred when taking data on the equilibrium hydrogen-helium system. The brass fittings on the mercury U-tube vapor-pressure control failed as a result of corrosion by the mercury and were replaced by stainless steel fittings. The D-C motor operating the magnetic pump switching system also failed and had to be replaced. There were numerous problems with small vacuum leaks in the equipment, such that several fittings and valveshad to be replaced as well. Several times these small leaks apparently developed in the recirculation loop as the loop was being evacuated prior to charging with hydrogen for an experimental run. Consequently, minute quantities of air entered the loop and partially poisoned the catalyst, thereby reducing its conversion effectiveness. More seriously, the air froze solid as it reached the portion of the loop inside the cryostat, so that operation of the equipment would have to be cancelled and the cryostat warmed to ambient temperature. The difficulties described above can all be classed as minor operational nuisances. There was, however, one problem in running the equilibrium hydrogenhelium data that is of a more severe nature. In running the higher temperature

-26isotherms, 31.0 and 31.5 K in particular, it required long periods of time to reach the equilibrium temperature and charge the cell with equilibrium -H2 by either of the methods discussed previously. These times were much longer than for the investigation with normal hydrogen. Consequently, by the time the system was prepared for the collection of data, the bath liquid hydrogen level had frequently fallen to the point where it was possible to take only one or two data points. Refilling the cryostat must be done at atmospheric pressure (20.3K) with subsequent long waiting periods for the bath to again rise to the desired to the desired temperature. When these problems were compounded by those of the vacuum leaks and also the inherent difficulty of obtaining a point inside the samll two-phase loop at the high isotherms, the results were often unsatisfactory. From the results, it will be seen that the data at 31.0~K turned out extremely well, in spite of these problems. The results for the vapor phase at 31.5 K, on the other hand, are uite disappointing, and cannot be considered of the same quality as the results obtained in the rest of the present study.

-27IV. RESULTS IN THE SYSTEM NORMAL HYDROGEN-HELIUM AND COMPARISONS WITH PREVIOUS WORK. A. PRESENTATION OF RESULTS The experimental results for the system normal hydrogen - helium have been presented in the previous report, reference 7, by W.B. Streett, et. al. They are also included here for convenience and for the purpose of comparison with the system equilibrium hydrogen - helium. The results of the experiment are summarized in Table I and Figs. 9-14. In general, points were taken along each isotherm at pressure intervals of 25 psi up to 200 psia, and at the higher pressures 250, 300, 400, and 500 psia. The three points run at 30.600 were taken in an attempt to locate the critical point for that isotherm. Figure 9 is an isothermal pressure-composition diagram, representing the projection on the P-x coordinate plane of curves cut in the P-T-x surface by planes of constant temperature. A portion of this diagram, on which most of the liquid lines cross each other, is shown on an expanded scale in Fig. 10. Figure 11 is a plot of the data on a temperature-composition diagram. These curves are projections on the T-x coordinate plane of curves cut in the P-T-x surface by planes of constant pressure. Because the data were taken at the same pressures at each temperature, very little interpolation of the P-x isotherms was required to plot the curves in Fig. 11. Figure 12 shows the liquid region of the temperature-composition diagram o; an: expanded scale. Figure 13 shows a cross-plot of the data from Figs. 9 and 11 on a pressuretemperature diagram. These curves are lines cut in the P-T-x surface by planes of constant composition. Figure 14 is a plot of equilibrium constants K vs. pressure. The constant

-28TABLE I NORMAL HYDROGEN-HELIUM SUMMARY OF EXPERIMENTAL DATA Temperature Pressure __ Liquid ____Vapor ('K)..(psia) Mole, Hee Mole H2 Mole He Molle% H2 49.5 -. — 93.78 6.22 75.0 0.29 99.71 95.51 4.49 100.0 0.46 99.154 96.31 3.69 125.0 0.51 99.49 96.77 3.23 15.50 150.0 0.55 99.45 97.02 2.98 175.0 o.64 99.36 97.14 2.86 200.0 0.71 99.29 97.29 2.71 250.0 0.82 99.18 97.41 2.59 300.0 0.87 99.13 97.48 2.52 56.5 0.36 99.64 89.67 10.33 75.0 o.43 99.57 91.86 8.14 100.0 0.57 99.43 93.30 6.70 125.0 0.72 99.28 94.12 5.88 150.5 0.80 99.20 94.64 5.36 17.07 175.0 0.89 99.11 95.03 4.97 200.0 1.00 99.00 95.15 4.85 250.0 1.14 98.86 95.56 4.44 300.0 1.31 98.69 95.82 4.18 400.0 1.51 98.49 95.90 4.10 35.0 o.34 99.66 53.60 46.40 50.0 0.41 99.59 65.45 34.55 75.0 0.61 99.39 75.40 24.60 100.0 o.84 99.16 80.30 19.70 125.0 1.05 98.95 83.20 16.80 20.40 150.0 1.30 98.70 84.80 15.20 175.0 1.54 98.46 86.03 13.97 200.0 1.67 98.33 87.13 12.87 250.0 2.05 97.95 88.41 11.59 300.0 2.36 97.64 88.97 11.03 400.0 2.96 97.04 90.08 9.92 500.0 3.39 96.61 90.35 9.65 50.0 o.38 99.62 37.00 63.oo 75.0 0.75 99.25 55.44 46.56 23.00 100.0 1.15 98.85 62.90 37.10 125.0 1.36 98.64 67.87 32.13 150.0o 1.72 98.28 71.75 28.25 175.0 2.15 97.85 74.35 25.65

-29TABLE I (Continued) Temperature Pressure Liquid ____ Vapor (OK) (psia) Mole % He Mole % a2 Mole % He Mole % 11 200.0 2.39 97.61 76.30 23.70 250.0 2.91 97.09 78.78 21.22 23.00 300.0 5.55 96.45 80.55 19.45 400.0 4.58 95.42 82.34 17.66 500.0 5.46 94.54 83.30 16.70 81.0 0.67 99.33 22.60 77.40 100.0 0.97 99.03 35.50 66.50 125.0 1.43 98.57 43.37 56.63 150.0 1.92 98.08 49.77 50.23 26.00 200.0 2.86 97.14 56.90 43.10 250.0 3.64 96.36 62.67 37.35 300.0 4.76 95.24 65.65 34.35 4o0.0 6.64 95.56 68.58.1.42 500.0 8.48 91.52 70.02 29.98 127.0 0.91 99.09 14.03 85.97 151.0 1.50 98.50 21.92 78.08 175.0 2.20 97.80 27.75 72.25 200.0 5.07 96.95 32.93 67.07 29.00 250.0 4.59 95.41 39.54 60.46 300.0 6.21 9379 43.70 56.0 400.o 9.63 90.37 47.95 52.05 500.0 13.35 86.65 49.20 50.8o 300.0 7.29 92.71 30.32 69.68 30.60 400.0 13.24 86.26 32.60 67.40 450.0 17.95 82.05 30.36 69.64 176.5 1.49 98.51 11.28 88.72 199.0 2.51 97.49 15.59 84.41 250.0 4.78 95. 2 22.58 77.42 300.0 7.49 92.51 26.57 73.43 31.00 325.0 8.73 91.27 27.55 72.45 550.0 11.05 88.97 27.85 72.15 375.0 12.93 87.07 27.59 72.41 400.0 15.64 84.36 25.78 74.22 413.0 20.87 79.13 21.85 78.15

..30TABLE I (Concluded) Temperature Pressure LLquid Vapor (~K) (psia) Mole % He Mole % H2 Mole l He Mole % H2 177.0 1.27 98.73 7.67 92.33 200.0 2.50 97.70 11.94 88.06 225.0 3.61 96.39 15.64 84.56 250.0 4.83 95.17 18035 81.65 31.50 275.0 6.26 93.74 20.30 79.70 300.0 8.18 91.82 21.37 78.63 525.0 10.47 89.53 21.49 78.51 351.0 16.59 83.41 17.58 82.42 174.5 0.81 99.19 4o00 96.oo 200.0 2.07 9793 8..31 91.69 220.0 3.17 96.83 11.75 88.25 258.5 4.29 95.71 13.86 86.14 250.5 4.99 95.01 14.82 85.18 51.90 261.5 -- --- 15.62 84.38 275.5 6.87 93.13 16,19 83.81 289.5 8.71 91.29 16.55 83.65 300.0 10.i0 89.90 15.88 84.12 308.5 12.02 87.98. i88.5 0.98 99.02 3.12 96.88 200.0 1.68 98.32 4.8o 95.20 32.50 220.0 3.10 96.90 7.45 92.55 242.0 5.09 94.91 250.0 5.99 94.01 258.0 6.75 93.25

''ca~lc~ CUfVE CRITICA ON v ) O 0) CY~ CY CY 500~~~ w 300/ IY 0 200 100 000 10~~~3 50~6 MoLE %~HLV 710 8 9~ Bstherival. Urssre.,om01to

-32250 ----—' —i -i —-----—, (00 " 0 15.50/ /.// /W~ v 20.40~ 7 26.00" ^\25I //I 2900~. \^/ A31.00e 5C0 - 31.50~ vf/y~ /~rB 31.90~ 0 / 0g O 32.50o o I 2 3 MOLE ~% He IN1 LIOUID Fig. 10. Isothermal pressure-composition diagram showing liquid region.on expanded scale. (I,~~~~~~~~~~~~~~~V2.0 MOLE % HeIN LIQUI ft ef t %'as" &-,koveo&... o.i 26. 00. ftIt a

34 32 30 28 26 2 24....0 aJ 22 l 20 6. 18 o 60 MOLE % HELIUM Fig. 11. Isobaric temperature-composition diagram.'Fig. 11. Isobaric temperatu're-co osition diagram'.

-3434 32 30 26- 0 oo/' 24 Fig. 12. I / daPressure- PSIA i 0egion oo 50 w *, ~ 100 0e 150 e 200 o 250 20 e300 A 400 500 18 16 141I 0 1 2 5 4 5 6 7 8 9 10 11 12 13 14 MOLE % HELIUM IN LIQUID Fig. 12. Isobaric temperature-conrposition diagram shoving liquid region on expanded scale.

-35500 Composition-Mole %H 450- 1.0 o 5.0 A 10.0 0 15.0 400- V 20.0 -- -- DEW POINT CURVE 350- - BUBBLE POINT CURVE - CRITICAL CURVE x CRITICAL POINT 300 ~ 2508 /',~/ T Pure HI2'25 / \I Criticol Point ~ 200 ~ Nool),_c^^00 ^^ — 0~ (Normol Ht) 50 F..I 1I 3 Prsuetm —etI diagnI _.. I. I, 26 27 28 29 30 31 32 33 TEMPERATURE ("K) Fig. 13. Pressure-temperature diagram.

-361000.0 Io 10.0 ni~~~ ~~i lV^~ ~ \ Helium CU)-00 17-07 o _ o_2.e H3ydrogen D-1.0S l0 r. <s^<-^-''-' —< —"^ i I V0.10 — 0 ~ IS,51S000 0.0l,-o -200 300 b20.40050 Pressure (PSIA) Fig. 14. Equilibrium K values.

-37K is defined as the ratio of the mole fraction of a component in the vapor to its mole fraction in the liquid. B. DISCUSSION The behavior of this system, as shown by the curves in Figs. 9-13!,' similar to that of other systems involving components whose critical points are far apart. Similar behavior is exhibited, for example, by the systems 12 nitrogen-helium, 10, 11 and nitrogen-hydrogen. In the P-x diagram of Fig. 9 and the T-x diagram of Fig. 11, each curve is divided into two branches, representing the equilibrium liquid and vapor compositions. At fixed pressure and temperature (represented by a horizontal line in either diagram) the two co-existing phases lie at the ends of a horizontal line between the two curves. The region between the curves represents mixtures consisting of two phases, while points outside are in a single phase region. The two branches of each equilibrium curve on these diagrams merge at a point at which the two phases become indistinguishable. These points are called critical points and their locus on the P-T-x surface is called the critical curve. It projects as a smooth curve on each of the three coordinate planes of the P-T-x diagram. In general, the critical curve will appear on the pressure-temperature plot as an envelope (sometimes called the critical envelope) which is tangent to each of the curves of constant composition; thus critical points will always appear as maxima or minima on the curves in the P-x and T-x diagrams. A further discussion of this subject may be found in the book by Ruhemann.2 Each of the closed loops in Fig. 9 includes a region of retrograde condensation. This is that portion of the curve which lies to the right of a vertical line through the critical point of the loop. For example, if a low pressure gas mixture containing 25% helium is compressed isothermally

-38at 31.0, condensation will occur first at a pressure of about 275 psia. Condensation will continue upon further compression up to about 380 psia; however at higher pressures the liquid will begin to evaporate and will disappear completely around 405 psia, leaving a single vapor phase as in the beginning. In Fig. 10 the scatter in the liquid region of low helium content is greatly magnified. In general, the deviation of points from a smooth curve does not exceed 0.1 mole percent, which for most points does not exceed the limits of accuracy of the mass spectrometer in this region. In Figs. 11 and 12, the data exhibit reverse-order solubility in the liquid region. This is defined as an increase in the solubility of a constituent with increasing temperature, at constant pressure, and is characterized by a positive slope of the liquid lines on the T-x diagram. The area on the P-x diagram of Fig. 9 in which the liquid lines cross represents a region in which a minimum occurs in the bubble point curves in the P-T diagram. Thus a minimum would be expected in the bubble point curves for mixtures with up to 4% helium. (The minimum in the 1% line in Fig. 13 is out of the range of the diagram to the left). The critical curve shown in Figs. 9, 11 and 13 has been estimated from a study of the trend of the data on the P-x, T-x, and P-T plots. Because of the flatness of the P-T-x surface in the vicinity of the critical curve, it is difficult to locate critical points accurately. No visual observation of phenomena in the critical region was possible in this experiment, due to the nature of the apparatus. Several runs were made at the higher temperatures in an attempt to complete those loops which close at pressures below 500 psia. With an apparatus of the type used in this work, the problem is somewhat difficult. At

-39a given temperature, say 31.50, pure hydrogen is charged into the cell and a predetermined amount of liquid is condensed. At this point the contents of the cell are represented by the point in Fig. 9 at which the 31.5~ loop touches the left edge of the diagram. Helium is added to the loop and the pressure rises, and a curve of total composition vs. pressure for the mixture extends into the loop. With further addition of helium, the pressure and total helium content increase; however the path followed by the line representing total composition is not accurately known, aiLd a twophase mixture will exist only as long as the curve remains inside the loop. It may happen that it passes out of the loop and into a single phase region before the critical pressure is reached. This would mean either that all of the liquid had evaporated (exit on the vapor side), or that the cell became swamped with liquid (exit on the liquid side). During the run at 32.50 the cell apparently became swamped with liquid at a pressure above 220 psia, since the samples taken at higher pressure showed the same composition for liquid and vapor, although the compositions fell along a smooth curve with the liquid data at lower pressures. The same phenomenon was observed at 31.9~ and 308.5 psia. In general there is reasonable agreement between the critical curves obtained on each of the three diagrams (P-x, T-x, and P-T). The critical points indicated on these diagrams are estimated to be accurate to within + 10 psi, + 0.2~K, or + 0.2 mole percent. C. COMPARISON WITH PREVIOUS WORK The work of Smithl and of Roellig and Giese2 on the vapor-liquid equilibrium of the hydrogen-helium system has been mentioned previously. A comparison of their results with the present work is shown in Figs. 15-18.

-40500 450 400 A A 350 - Present Work, 20.4~ K A A Smith,20.4~ K v Roellig 8 Giese, 20.3~K |I " H 20.6~K A cA' 300- /~ 250200 150100A 50 0 I 2 3 MOLE % HELIUM IN LIQUID Fig. 15. Comparison with previous work, liquid data at 20.4*K.

600A 500400 en * Present Work, 204~ K Iw~~&A Smitn, 20.4~ K u 300 - Roellig 8 Giese, 20.3~K D * i, 20.6~K r m" i ] " 20.9~K cc 0. 200 - A 100 -I 50 60 7 80 90 100 MOLE /% HELIUM IN VAPOR Fig. 16. Comparison with previous work, vapor data at 20.4~K.

-42500 —, 450400- 0 00, / 350 / 300 a. Lo C,) n^ 250- f, f0 u / X 200 + 150 o 100 A0 Present Work, 17.4~ K (Interpolated) "A ftO m 0 " " 21.80 K Z^6 y + " " 26.0~ K (Experimentol) /A Smith, 17.4~K ~+^~.~//^ " 21.8 K. 50 Ad 6 01 Roellig & Giese, 25.6~K = e " " 25.90K v " v " 26.0 K 0 I 2 3 MOLE %/ HELIUM IN LIQUID Fig. 17.. Comparison with previous work, liquid data at 17.4, 21.8, and 26.0~K.

200 /+.60C 250 600 - 150- 2+0 6100- 100- A 30 40 50 60 70 MOLE % He IN VAPOR | A 0 400 0 0 Present Work, 17.4 K (Interpolated) / J 300 0 2.8~K a 4.+ 3 26.00 K (Experimentol) 200- o H U 2560K c. 25i9K I o A W - - 26.00~K / - 100, 7 2 -0 —— O-""""^ 0 A ^S>x 50 60+.70 K80 p90 1i00 MOLE % HELIUM IN VAPOR Fig. 18. Comparison with previous work, vapor data at 17.4, 21.8, and 26.0~K.

-44Of the three isotherm studied by Smith, only that at 20.40 was duplicated in the present work. A comparison with Smith's results at this temperature is shown in Figs. 15 and 16. The agreement is quite good. Smith's work shows somewhat more scatter, especially in the vapor phase compositions. To obtain a comparison with Smith's work at 17.4~ and 218~0 data at these temperatures were obtained for the present work by graphical interpolation of the temperature-composition diagrams of Figs. 11 and 12. A plot of this data on a large scale was used for this interpolation. The comparison is shown in Figs. 17 and 18. The agreement is not as good as that at 20.40. A study of the data indicates that smooth curves through Smith's data agree reasonably well with isotherms of the present work which are about 0.3 to 0.4 K lower than the temperatures reported by Smith. This discrepancy may be due to the method of temperature measurement used by Smith. He reported temperatures measured with a single copper-constantan thermocouple referenced in an ice bath. The sensitivity of copper-constantan thermocouples at liquid hydrogen temperatures is about 5/uv/degree., and the total EMF (reference junction at 0~C) is over 6000/.v. An error of 1 v would introduce a temperature error of about 0.2 K. Accurate temperature measurement would require very precise calibration of the thermocouple and very accurate measurement of the EMF. To determine the temperature to within + 0.010, as reported by Smith, the EMF would have to be measured to within + 0.05~/Lv, or + 0.008% of the total. Even then, the results would be questionable. Scott13 states that the best copper-constantan thermocouples are rarely free of stray EMF s due to inhomogeneities to better than 1 or 2 AIv in 5000.

-45Comparison with the data of Roellig and Giese cannot be made directly. They reported nine separate points, giving only the temperature, partial pressure of helium in the vapor, and mole percent helium in the liquid. Total pressure and vapor phase composition can be determined for each point by reversing the calculations they used to obtain the values reported. To make the comparison given here, the values of temperature calculated by Eckert and Prausnitz3 have been used with the data of Roellig and Giese. Roellig and Giese measured temperatures indirectly, using experimentally determined liquid and vapor phase compositions and assuming Raoult's law valid for the liquid and ideal gas behavior for the vapor. They obtained a value for the vapor pressure of hydrogen and took the temperature from the hydrogen vapor pressure curve. Eckert and Prausnitz observed that the assumption of Raoult's law and ideal vapor leads to significant errors in the temperature. Using a rigorous thermodynamic equation in place of the ideal relation, they recalculated the temperatures, assuming only that liquid hydrogen is incompressible and that the Lewis-Randall rule can be used to determine the fugacity of hydrogen in the liquid. The resulting temperatures differ from those of Roellig and Giese by as much as 2.7 K. While these temperatures are probably more accurate than those arrived at by Roellig and Giese, they will still reflect any errors in the determination of the phase compositions. The liquid phase compositions for the two temperatures 20.6 and 20.9~ (calculated by Eckert and Prausnitz) are plotted in Fig. 15. The lack of agreement is apparent. The three liquid points at 26.0, 25.6, and 25.90 are plotted in Fig. 17, and agree reasonably well with the 26.00 isotherm of this work. The vapor phase compositions for the above five points, along with that of the 17.20 points are shown in Figs. 16 and 18. They show excellent agreement

-46with the present work. Roellig and Giese concluded from their limited data that the solubility of helium in the liquid increases with a decrease in temperaturejust the opposite of the behavior observed in this work. D. THERMODYNAMIC TREATMENT The original work of Smithl on the hydrogen-helium system includes a review of some thermodynamic relations used to calculate the fugacities of components in vapor and liquid mixtures. Smith applied several of these methods to the calculation of the fuigacities of each component in the vapor, and compared the results to ideal behavior described by Henryts law and Raoultts law. His calculations are somewhat difficult to follow. He first calculated the helium vapor phase fugacity by the Lewis-Randall rule; and then calculated the fugacity of the mixture, using a two constant virial equation of state. He used these results to calculate the vapor phase fugacity of hydrogen, using an equation relating component fugacities to mole fractions and total fugacity. He did not take the obvious step of calculating the fugacity of hydrogen in the liquid and comparing it to the calculated vapor phase fugacity. Brazinsky and Gottfried have pointed out that Smith's vapor phase hydrogen fugacities are in poor agreement with the liquid phase fugacities calculated from the Lewis-Randall rule. They concluded that the disagreement was due to incorrect values of the second virial coefficient for hydrogenhelium mixtures used by Smith. They recalculated the hydrogen vapor phase fugacities, using better values for the virial constants, and obtained reasonable agreement between these values and the liquid fugacities calculated from the Lewis-Randall rule. Eckert and Prausnitz have pointed out however

-47that en o this is a very insensitive test, as five - or ten-fld changes in the helium content of the liquid will not affect the results.

-48V. RESULTS IN THE SYSTEM EQUILIBRIUM HYDROGEN - HELIM AND COMPARISONS WITH THE SYSTEM NORMAL HYDROGEN - HELIUM. A. PRESENTATION OF RESULTS Experimental results of the investigation conducted on the system equilibrium hydrogen-helium are presented in the same manner as for those of the first system studied. Since these results were collected for the purpose of comparison with those found using normal hydrogen, it was not considered necessary to take as many data points as in that study. Six isotherms are reported here, covering the range 20.40 to 31.50~K at pressures from hydrogen saturation to 500 psia. Early results indicated that temperatures below 20o40~ would not be of interest, in that differences between the two systems would be negligibly small. The data are summarized in Table II and in Figures 19-24. The points were in general taken at the same values of pressure and temperature as run for the previous system, for convenience in comparing the results. Figure 19 is an isothermal pressure - composition diagram representing the entire range of data for this system, while Figure 20 is the expanded- scale liquid portion of this diagram. Figure 20 shows the solubility of helium in liquid equilibrium hydrogen, the data of particular interest to the transfer of hydrogen by helium pressurization. Figure 21 shows the data on temperature - composition coordinates, the expanded - scale liquid region of which is given in Figure 22. Figure 23 represents a cross - plot of the data of 19 and 21 on pressure - temperature coordinates, showing lines of constant composition. Finally, Figure 24 is a plot of the equilibrium constant K versus pressure, K being defined as the ratio of vapor phase mole fraction of a component to its liquid phase mole fraction.

-49TABLE II EQUILIBRIUM HYDROGEN-HELIUM SUMMARY OF EXPERIMENTAL DATA Temperature Pressure Liquid Vapor (~K) (psia) Mole % He Mole %H2 Mole % He Mole% H2 34.7 0.23 99.77 49.40 50.60 50.5 0.58 99.42 62.79 37.21 75.0 0.60 99.40 72.80 27.20 100.0 0.87 99.13 78.21 21.79 125.0 1.02 98.98 80.83 19..7 150.0 1.17 98.83 83.00 17.00 175.0 1.55 98.45 84.71 15.29 20.40 200.0 1.77 98.23 85.79 14.21 250.0 1.92 98.08 2.04 97.96 86.64 13.36 300.0 2.25 97.75 86.80 12.20 400.0 2.66 97.34 88.72 11.28 2.94 97.06 88.56 11.44 500.0 3.08 96.92 3.16 96.84 85.99 14.01 64.5 0.54 99.46 45.60 54.40 75.0 0.65 99.35 52.20 47.80 101.0 1.05 98.95 61.70 38.30 125.0 1.38 98.62 67.00 33.00 150.5 1.75 98.25 70.90 29.10 23.00 175.0 2.27 97.73 73.50 26.50 200.0 2.40 97.60 75.35 24.65 250.0 3.00 97.00 77.90 22.10 300.0 3.65 96.35 79.50 20.50 400.0 4.73 95.27 79.10 20.90 500.0 5.62 94.38 82.50 17.50 100.0 0.79 99.21 29.66 70.34 150.0 2.00 98.00 45.57 54.43 20Q.0 3.03 96.97 53.55 46.45 250.0 3.67 96.33 58.77 41.23 26.00 3.90 96.10 59.52 40.48 300.0 5.08 94.92 63.00 37.00 400.0 5.95 94.05 66.51 33.49 6.37 93.63 65.05 34.95 500,0 7.82 92.18 67.15 32.85 8.10 91.90 68.,04 31.96

-50oTABLE II (Continued) Temperature Pressure Liquid Vapor (~K) (psia) Mole % He Mole % H2 Mole % He Mole % H2 142,0 1.19 98.81 17.60 82.40 174.5 2.22 97.78 26.15 73.85 200.0 3.05 96.95 31.15 68.85 29.00 251.5 4.78 95.22 37.90 62.10 300.5 6.33 93.67 41.90 58.10 400.5 10.06 89.94 46.00 54.00 501.0.. 47.00 53.00 200.0 2.17 97.83 13.17 86.83 248.5 4.40 95.60 20.10 79.90 250.0 4.55 95.45. 300.0 7.52 92.48 23.14 76.86 31.00 324.0 9.08 90.92. 332.0 ---- 23.68 76'.32 350.0 -- --- 23.94 76.06 366.5 12.55 87.45 23.53 76.47 390.5 18.12 81.88 18.44 81.56 176.5 1.07 98.93 6.34 93.66 203.0 2.39 97.61 10.99 89.01 225. 0 --- -- 14.02 85.98 31.50 250.0 4.55 95.45 15.68 84.32 275.0 -- --- 16.97 83.03 300.0 8.08 91.92 17.13 82.87 315 0 9.28 90.72 18.70 81.30 332.0 14.90 85.10 16.57 83.43

0 0 it 0 00 500 - 0 ~.~~~~~~~~~~~~~~~~~~~( CM) 0 00 400 00 ( r^ / (^^oo^, r ~ 0 I ~0 10 20 30 40 ~5060 70 80 90 0 0 C)- 0 ~~oo'd rci O oci w (L ~ ~ ~ ML PER CET EL VC 4~~~~~~~~~~~~Fg 19\ 400 0 l 0 CL I 300 L&J D a: cn w cr CL 200 100 0 10 20 30 40 50 60 70 80 90 100 MOLE PER CENT HELIUM Fig. 19 Iaothurml pressuzw-capoition diagram

PRESSURE, PSIA 0C)~- &0 r'7O o o 01 0 en 0s0 0 0 rr- XTt~~~~~~Fig. 20X II Isothermal pressure-caosition diagram showing liquid region on epnded scale Lrrl C c_ Fig. 20 Isothermal pressur-coaposition diagram showing liquid region on expended scale

32 cr~~~~~~~~c 30 11:D 26 2 q 202 0 18 16 L-I - 0 10 20 30 40 50 60 70 80 90 100 MOLE PER CENT HELIUM I3OBARIC 72 1PrRATrV(- nOITTOt. DIAGRiA

-54~3 ~~ ~ ~~2 -—, —- ---— T —-— )- ---- 0 30' \ i, - | 28 M j 26 - 0 0 - HC 20 0 C. 20 0 2 4 6 8 10 MOLE PER CENT HELIUM IN LIQUID MOLE PER CENT HELIUM IN LIQUID~~~~~~~~~~~~~~~~~~~~~~~~~~~r

-55500 _-. DEW POINT CURVE_. ____ BUBBLE POINT CURVE.~~450C~ \ X CRITICAL POINT 450 400 \ N. \ ^ 350 \\ - al 300 a_ I I - c250 - — ) ^UJ20 LINES OF CONSTANT f ] W 200 - COMPOSITION (/ & 4- 150 50 gUI BPURE H2VAPOR PRESSURE 50 ^ ^ (EQUILIBRIUM Hip Q ___.i —1J__,,,,,,1___. __ 26 27 28 29 30 31 32 33 34 35 TEMPERATURE -OK

EQUILIBRIUM CONSTANT K = Y/x 0b b b 0 x jC.).4 0/ (7) ()0C) o 9 0 0 0 0 o 0 0- o., ~, - 0.. 0 mn Fig. 24 Equilibrium K values SI: ~ ~ ~ ~ ~ ~........'~1...... 0~~~~~~~~~~~~~~~7 r!-IFgC 2 r QnlbisK vl~

-57B. DISClUSSION The results are, as expected, very similar to those for normal hydrogenhelium., there, being only minor differences i.rn the behavior of the two systemso The data are in general self-consistent, and show lifttle scatter: IAndiv'idual points that are not consistent with the results in general are mostly vapor p:hase samples. The vapor samples at 20.40~K, 500 psia and at 23.o00~K 400 psia are apparently l.w by a.pproximately 3 an.d 2 per cent:, respectively~ o Me vapor phase curve at 26.00 K seems to be self-consiste-t on a P-x diagramf as ahswn'a shfhwitge 19. However, the T-x plot of Figure 21 and also a comparison with normal thydrogenhelium results at this temperature indicate that the values are somewbat low at pressures below 250 psiao The liquid-phase sample at 29.00 K, 500 psia, had to be discarded. The liquid level in the cell had apparently fallen below the level of the sample probe when this point was taken, such that a sample of vapor instead of liquid composition was withdrawno The vapor phase sample at that point is consistent with the remainder of the data at 29.000K. The vapor phase compositions for the 31.50~K isotherm at pressures above 250 psia are questionable, with appreciable scatter being evident, although the liquid phase results in this region seem acceptable. There was considerable difficulty in obtaining points for this isotherm, as was discussed in detail in Section ITT of this report. From the expanded-scale liquid phase composition plo s, Figures 20 and 22. 9it is seen that throughout the experim.ental range of this inivestigation. the liquid compositions are very consistent and show almost no scatter. This is also evident from the plot of equilibrium constants in Figure 24, especially for the helium9 where the values are large, because of fte small clncentration of helium in the liquid. Any scatter in liquid composition values would be greatly magnified on

-58this plot. Difficulties encountered in e:perimental phase equilibrium ivest'igatic-s are commonaly associa ed wit-h -the liquid phase,, particularly in sai qgling. No such problems are evident from the results of this study. Instead what 8.catter is apparent is concerned primarily with vapor phase.rsults at the points mentioned above. The cause of these minor inconsistencies i.s not knowrs<, and these problems were not evident in the study of the systemn nior-al hydro ge- - helium to It is felt that since these inconsistent values occurred at only isolated points, an'for oniy one phase, the cause must be associated with a poor sampling tecenilque at t.ose points. C. COMPARISONS WITH THE SYSTEM NORlML FYDROGEN - HELIT.-T Comparison of the experimental results in the system equilibrium hydrogen - helium wi-th those for normal hydrogen - helium is best made in graphical formT The two systems are shown in Figures 25-30, isothermal P-x diagrams for the six isotherms studied using equilibrium hydrogen. It is seen that with the exception of thfose few experhmental. points discussed in the previous section, the differences betwgeen the systems are consistent: and increase with increasing temperature. If the systems be.ave i deally then Raoult's Law would describe the liqu-d phase and the.deal gas law the apor phase. Differences between the two systems could tle.n be explained and calculated entirely in terms of the difference in vapor pxressure between normal nhydrogen and equilibrium hydrogen. At any temperature, equilibrium hydrogen has a somewihat higher vapor pressure. Therefore, it would be expected that -the isotherms for the equilibrium hydrogen - helium system should be shifted slightly from those for normal hydrogen - helium in the direction of Lncreasing temperature. Th e two syst:ems do not betave ideally, so that differences i:z -V-T be-avior or te two.afisties

500 - 400 - 0 / TEMPERATURE - 20.40 ~K |/ 0- EQUIL. H2 I A 300 *A- NORM. H2 cr-, 200 a10P - 100 - LIQUID PR VAPOR 0 2 3 50 60 70 80 90 100 MOLE PER CENT HELIUM Fig. 25 Comparison of the two systems at 20.4~K

fs..rW. r i ^ t. t._ yGet, si~ / I ~/ I/ f ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I i/..n.'/~~~~~~~~~~* LflPERtk -, 230 "' a t wf'I~~/ / / |/ 0 Q.... <s i/.;RI. H / / "~~~~~ *,.; 4I' /?~ ~ i ^.^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1"> ~..:.f..,oW-V..G,...:,& I I i /.^ ^ i0 | ff- -^ -" I /1UvD-.^^^:^ VAPOR r/0 -. 2... -. 0....b 6 70 80 90 MOLE:,~ ~' PER.a. CENT HELIt at 23M,Lat''^i~t,4 p9'.,, {l} ~ otz~siiM,,-, i: "9:':'{X { 3 >':1.'1.':....,.i:2,':.',,:'S'~'r_,...... i *'!7._ O ~, i" it ~t

500 400 0 ^ f 04 TEMPERATURE -26.00 *K -EQUIL. H2 / 0 /300 A-NORM. H2 1 ^ 300 /0 2 / LL 200- ~~~a~~~~~~~~~~~. / LIQUID VAPOR 0 t 0 2 46 8 * 30 40 50 60 70 MOLE PER CENT HELIUM F. 27 Comparison of the t7o systems at 26.O'4

500 1 I T 400 / /TEMPERATURE - 29.00 K / -EQUIL. H // I< | ~~ / h~A-NORM. H2 / a. 300- w a: (I / i' s a, 1 200- VAP LIQUID 100/ 0 2 4 6 8 10 12 v 20 30 40 50 MOLE PER CENT HELIUM Fig. 28 Comparison of the tio systems at 29.0 K

-63500 I....l 400- - q300 A C\ o L~~~~~I~~~~~ I llI ~ 0 cr 200 CEN HE L U, / *4 TEMPERATURE - 31.00 ~K o-EQUIL. H. 100 A-NORM. H2 0 5 10 15 20 25 30 MOLE PER CENT HELIUM

-64 — 4001 LO ~ L LIQUID 1 /^Y ^6d ^~~~ VAPOR 3 200 o A-NORME I Ha U 4o 100 - o 0 CO MOLE PER CENT HELIUM

-65of hydrogen in both the liquid and vapor phases will also c...r.bute differences in the behavior of th e two systems. Differences in P-V-T behavior between normal and equilibrium hydrogen are rot iarge, h:owever. Consaequent7.y, the vapor pressure differenice must. be te principal factor azcounting fr di fference beween the two systems The qualitative difference between the two systems discussed above is, in fact the result observed for thie vapor phase at the various temPeratures. The differences between iso-ther-ns on P-x coordinates become larger as temperature >c.Creases, and do so in a regular manner, with the exception of the 26.00~PK isothermi, for which the vapor phase results are apparently low. This isotherm sho ov in Figure 279 consequently indicates a wider separation than does the 29I00r.~ iso-erm plotted in Figure 28. It was previously oncluded from the T-x diagram of Figure 21 that the 26.000 values were somewhat too smallo At 31.00, Figure 29, separatin between the two vapor-phase curves becomes significant, and is even larger at 31,50&, as shown by Figure 30. At temperatires below 20.40~ it wuld be expected that vapor phase differences between kthe two systems would become naegligibyi small. From the qualitative analysis given above, it is to be expected that the liquid phase compositions are also slightly different for the to systems investigated, with isothermrs for equilibrium hydrogen g - helifum Cn a P-x plot being shifted somewhat in the direction of higher temperature. This presum:ption is believed to be correct, although the resul~ts in the * ange 20.40 - 29.3C'K: FK igues 25 - 28, do not indicate any diffeernces between the two systems. I mrust be ecocluded that in this temperature range an.y differences in liquid compositio's for te two systems are smaller thaan the scatter in tee e:xpetirmental data, Thus. only a single curve representin.tg the liquid data has been drawn in Figre 25 — 28..t the 31,00~ isotherm, however, the data indicate a measurable dAifference in liqiXd pase heliung

-66concentration. This resultt shown in Figure 29, is as expected, with the curve for equilibrium hydrogen - helium being shifted slightly in the direction of higher tempera'ture, crossing the curve for normal hydrogen - helium at about 300 psla. The same trend is also indicated at 31.50 as shown in Figure 30. At the higher isotherms the magnitude of the difference in vapor pressure between equilibrium hydrogen and normalhydrogen becomes significant, wnich must be the primary factor contributing to the measurable liquid - phase differences between the two systems. From the results obtained, it can be concluded that liquid phase compositions in the system equilibrium hydrogen - helium at temperatures below 20.400K are, at least within the limits of experimental accuracy, the same as for the system normal hydrogen - helium. The phase inversion phenomenon observed in this low temperature region for normal hydrogen - helium would be expected to occur for equilibrium hydrogen - helium at approximately the same pressure at each temperature, because of only small density differences between the two varieties of liquid hydrogen. While no liquid composition differences were observed ir the temperature range below 29 K, it may be possible that such differences would be found at higher pressures than those studied in the present investigation. It is not possible at the present time to make quantitative predictions of such values, or for that matter quantitative analytical comparisons of thbe two systems in the range studied. Such calculations require P-V-T. data for the liquid-phase solution, which are not presently available.

-67B I B L I O G R A P H Y 1. Smith, S.R.; Ph.D. Thesis, Ohio State University (1952). 2. Roellig, L.O. and C. Giese; J. Chem. Phys., 37, 114 (1962). 3. Eckert, C.A. and J.M. Prausnitz; J. Chem. Phys., 39, 246 (1963). 4. Drayer, D.E. and T.M. Flynn; NBS Tech. Note 109 (1961). 5. Brazinsky, I. and B.S. Gottfried; NASA Tech. Note D-1403 (1962). 6. Streett, W.B.; Ph.D. Thesis, University of Michigan (1963)/ 7. Streett, W.B., R.E. Sonntag and G.J. Van Wylen; Report 0-5664-1-T, NASA Contract NAS 8-5256, (September 1963). 8. Streett, W.B., R.E. Sonntag and G.J. Van Wylen; J. Chem. Phys. 40, 1390 (1964). 9. Chelton, D.B. and D.B. Mann; Cryogenic Data Book, University of California Radiation Laboratory (1956). 10. Buzyna, G., R.A. Macriss and R.T. Ellington; Chem. E. rog. Symp. Ser. 59,-101 (1963). 11. De Vaney, W.E., B.J. Dalton and J.C. Meeks; J. Chem. & Eng. Data, 8, 473 (1963). 12. Ruhemann, M.; The Separation of Gases, 2nd Edition, Clarendon Press, Oxford (1949). 13. Scott, R.B.; Cryogenic Engineering, D. Van Nostrand Co. Inc., Princeton (1959).