Liquefaction of “Permanent” GasesHydrogen as an exampleSee Flynn Ch. 3 and 6Liquefaction of Hydrogen Heat of Normal-to-Para conversion TS diagram for a pure substance Milestones in hydrogen liquefaction Liquid hydrogen production in the last 40 years Hydrogen liquefaction plants in North America Economics of liquefaction Four things that can be done to a gas Gas Liquefaction cycle temperature/Entropy diagram Liquefier block diagram Linde cycle Temperature/Entropy diagram Inversion curve for various gases Linde Cycle with pre-cooling Claude Cycle  Ideal Liquefaction and other cycles  Ortho-Para conversion Mechanics Areas of possible improvement Compressing hydrogen• Liquid hydrogen has the highest storage density of any methodLiquid Hydrogen• But it also requires an insulated storage container and energy-intensive liquefaction process• Liquefaction is done by cooling a gas to form a liquid. • Liquefaction processes use a combination of compressors, heat exchangers, expansion engines, and throttle valves to achieve the desired coolingTwo forms of hydrogen moleculeE=171 K<300 K Normal H2( 300 K and 1 atmosphere) is 75% ortho3 quantum states25% para1 quantum stateLiquid H2(20.4 K and 1 atm.) is almost 100% para (Tboil<<171 K)Heat of conversion o p = 0.15 kWh/kgHeat of liquefaction = 0.12 kWh/kg}more energy to convert than liquefyFigure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 128• Hydrogen molecules exist in two forms, Para and Ortho, depending on the electron configurations• At hydrogen’s boiling point of 20 K(-423°F), the equilibrium concentration is almost all Para-hydrogen• But at room temperature or higher the equilibrium concentration is 25% Para-hydrogen and 75% Ortho-hydrogen• Uncatalyzed conversion from Ortho to Para-hydrogen proceeds very slowly• Ortho to Para-hydrogen conversion releases a significant amount of heat (527 kJ/kg [227 Btu/lb])Ortho-Para conversionPercent para H2vs. TemperatureWe will find that continuous conversion during liquefaction is most efficient, but capital intensivePercent ParaFigure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 129Heat of Normal-to-Para conversionRemember, normal hydrogen is at 300 K (RT). Clearly we need to concern ourselves with catalyzing the ortho-para conversionFigure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 137J/g• Activated charcoal is used most commonly, but ferric oxide is also an inexpensive alternative • The heat released in the conversion is usually removed by cooling the reaction with liquid nitrogen, then liquid hydrogen. • Liquid nitrogen is used first because it requires less energy to liquefy than hydrogen, achieving an equilibrium concentration of roughly 60% Para-hydrogenOrtho-Para conversion Mechanics• If Ortho-hydrogen remains after liquefaction, heat of transformation described previously will slowly be released as the conversion proceeds• This results in the evaporation of as much as 50% of the liquid hydrogen over about 10 days• Long-term storage of hydrogen requires that the hydrogen be converted from its Ortho form to its Para form to minimize boil-off losses• This can be accomplished using a number of catalysts including activated carbon, platinized asbestos, ferric oxide, rare earth metals, uranium compounds, chromic oxide, and some nickel compoundsOrtho-Para conversion notes1/28/2011 10Ortho-para conversion--A. Is a linear process in timeB. Obeys a power law in timeC. Is an exponential process in timeD. Is a logrithmic process in timeE. Obeys a dual power law in timeLiquefaction uses stages of compression with T constant and expansion with constant S. This diagram makes it easy to determine operating parameters and efficiencies. More complicated diagrams have lines of constant Enthalpy (H), which may be used when Joule-Thomson expansion produces cooling at very low T.Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (2005), p. 82TS diagram for a pure substanceLine1/28/2011 12What is the Critical Point of a Fluid?A. Point at which a gas liquefiesB. Point at which a liquid solidifiesC. Point where solid, liquid, and gas phases coexistD. Point above which gas and liquid can not be distinguishedE. Point of no returnFigure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (2005), p. 82A Thermodynamic CycleLine1/28/2011 14Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (2005), p. 82TS diagram for airLine1/28/2011 16T-S Chart forPara HydrogenLow TTE1/28/2011 17T-S Chart for Helium-Low TTEMilestones in hydrogen liquefactionTable adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 127Liquid hydrogen production in the last 40 yearsPage adapted from Air ProductsHydrogen liquefaction plants in North AmericaFigure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 127Economics of liquefactionPage adapted from Air Products“Four things that can be done to a gas”according to Willie Gully (my former post doc)Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 274• The simplest liquefaction process is the Linde or Joule-Thompson expansion cycleLiquefaction Process• Some of the steps in the process are – Gas is compressed at ambient pressure– Cooled in a heat exchanger– Passed through a throttle valve - isenthalpic Joule-Thompson expansion – producing some liquid– Liquid is removed and the cool gas is returned to the compressor via the heat exchanger of step #2Liquefier block diagramLinde or Joule-Thomson (J-T) CyclerSimplest-no LT moving partsHydrogen requires pre-coolingAdapted from Helium Cryogenics, Van Sciver, Plenum (1986) p. 289Isobaric coolingJ-T expansionvalveNon-liquefied return gasMake-up gasLiquid outRT isothermal compression ideal W=T S TemperatureEntropyreal compressionIsobaric heating in counter-flow heat exchangerIsobaric cooling in counter-flow heat exchangerIdeal isenthalpic expansionReal isenthalpic expansionUnliquefied gas returningto compressorFraction of gasliquefied}Linde cycleTemperature/Entropy diagram• The Linde cycle works for gases, such as nitrogen, that cool upon expansion at room temperature.Hydrogen requires more…• But Hydrogen warms upon expansion at room temperature• In order for hydrogen gas