Improved Models for Thermodynamic Properties of Industrially Important Fluids

NIST work on the properties of hydrogen has been incorporated into two new standards that will support the development of hydrogen as a fuel in vehicle applications. A new, simplified equation for the density of hydrogen gas was developed for the draft SAE (Society of Automotive Engineers) procedure J2572. The NIST REFPROP equations for the thermodynamic and transport properties of hydrogen form the basis for the new ASTM International Standard D7265-06. We have also begun an experimental program, which will generate the data necessary to develop improved models for hydrogen and hydrogen-containing mixtures. Context and Approach: One of the challenges in the use of hydrogen in vehicle applications is the seemingly trivial matter of measuring fuel consumption. Consumers and industry are accustomed to the high accuracy that is easily achievable by the volumetric metering of gasoline and other liquid fuels. But hydrogen, used either as a compressed gas or a cryogenic liquid, presents significant metering challenges. Vehicle manufacturers have proposed a number of different protocols and algorithms for their hydrogen-fueled vehicles, and the EPA approached NIST to help provide a baseline for an appropriate standard. To evaluate the consumption of gaseous hydrogen fuel in motor vehicles, determination of the temperature and pressure before and after usage within a storage tank of known, and essentially fixed, volume is one of three methods recognized in the SAE draft procedure J2572 “Recommended Practice for Measuring the Fuel Consumption and Range of Fuel Cell Powered Electric and Hybrid Electric Vehicles Using Compressed Gaseous Hydrogen.” Compared to the alternative methods of continuously metering the flow of hydrogen to the fuel cell or weighing the fuel tank before and after a test, the use of pressure, volume, and temperature measurements has the potential to be the most robust and economical method in terms of instrumentation costs and ongoing personnel test resources, as well as for measurement precision, repeatability, accuracy, and lab-to-lab reproducibility. The EPA National Vehicle Fuel and Emission Laboratory (NVFEL) is currently evaluating this method for fuel cell (FC) and internal combustion (IC) engines. Major Accomplishment(s): In support of SAE J2572, we developed a new equation for hydrogen gas densities using a truncated virial-type equation for the density of hydrogen in terms of pressure and temperature. The new equation reproduces the available experimental data to within 0.2 % (combined uncertainty with a coverage factor of two) and is recommended for use over the temperature range 220 to 400 K at pressures up to 45 MPa. This equation will standardize calculations of hydrogen fuel consumption in vehicles, and represents an initial facet of the NIST effort to help build the infrastructure for a hydrogen-based economy. In a related effort, the hydrogen property values from the NIST REFPROP database form the basis for the new ASTM International standard D7265-06 “Standard Specification for Hydrogen Thermophysical Property Tables.” This standard specification was written by NIST staff who also led the specification through the approval process within ASTM Committee D03 on Gaseous Fuels. The SAE and ASTM documents (as well as the hydrogen properties in the current version of REFPROP) are based on an existing NIST Standard Reference Data formulation dating from 1982. This formulation is “old-fashioned” by current standards and must certainly be improved, but the immediate needs of industry demanded that we provide interim data. In anticipation of the development of more modern equations of state and property formulations, we have collaborated with researchers from the University of Idaho to survey the available thermophysical properties data for hydrogen over the entire fluid surface (not just the compressed gas). This survey will help to identify regions where additional data may be needed to support new applications. We also initiated in FY’06 what is expected to be an ongoing experimental effort on hydrogen systems with measurements of the heat capacity and thermal conductivity of a methane plus hydrogen blend. The measurements extended from 140 to 350 K with pressures up to 20 MPa. This blend is used as a fuel, but the primary motivation was to provide data for developing models of hydrogen-containing mixtures. Impact: The correlations developed this year will satisfy immediate needs of our customers, and will be disseminated through standard reference databases, such as NIST REFPROP. Before this NIST effort, there was no generally accepted industry-wide method of calculation for hydrogen-fueled vehicles. Future Plans: A significant initiative on “codes and standards for the hydrogen economy” is being considered for funding in the FY’07 NIST budget. Under this initiative, we would significantly expand our experimental efforts on hydrogen and hydrogen-containing mixtures, which would be important in the production of hydrogen (e.g. process streams in coal gasification) or as fuels in their own right (e.g. hydrogenmethane blends). We will develop modern equations of state and other property formulations for hydrogen. We will work to satisfy the immediate data needs of industry, delivering interim data and models when necessary, while working towards the broader, long-term goal of developing comprehensive, high-accuracy property models for hydrogen and hydrogen-containing systems. Project Team: E.W. Lemmon, M.L. Huber, D.G. Friend, D.G. Archer, M.O. McLinden, R. Perkins, J. Magee, (Physical and Chemical Properties Div.); R.T Jacobsen and J.W. Leachman (University of Idaho); C. Paulina (EPA) Publications D7265-06 Standard Specification for Hydrogen Thermophysical Property Tables, ASTM International, 2006. E. W. Lemmon, M.L. Huber, D.G. Friend, and C. Paulina, “Standardized Equation for Hydrogen Gas Densities for Fuel Consumption”, Paper 2006-01-0434, Proceedings SAE World Conference, April 3-6, 2006 Detroit, MI R.T Jacobsen, J.W. Leachman, S.G. Penoncello, and E.W. Lemmon, ”Current Status of Thermodynamic Properties of Hydrogen”, Int. J. Thermophysics, submitted Aug 2006. J.W. Leachman, R.T Jacobsen, S.G. Penoncello, and M.L. Huber, “Current Status of Transport Properties of Hydrogen”, Int. J. Thermophysics, submitted Aug 2006. Quote to highlight in a separate box (assuming the same format as FY’05 TARs): “Before this NIST effort, there was no industry-wide method of calculating the fuel consumption of hydrogen-fueled vehicles.” Figure 1. “Hydrogen fuel-cell powered vehicle undergoing tests of fuel economy ” Bringing a Greater in vivo Relevance to in vitro Measurements Contacts: DG. Archer (838), F.P. Schwarz (831) Biosciences; systems biology Context and Approach: Thermodynamic measurements of biochemical reactions are typically conducted in very dilute solutions, but this does not reflect actual in vivo conditions where other, nonreacting molecules are present. There are in vivo system effects, which influence the thermodynamic properties of biochemical reactions, but which are not quantified at all in most in vitro biochemical studies of thermodynamics and kinetics. Providing a framework that allows prediction of the system effects on biochemical reactions is the goal of this project. We have developed new facilities in biological calorimetry to study such “macromolecular crowding phenomena.” In FY ’06 we set up a new laboratory and carried out initial measurements on the denaturation of lysozyme protein. Addressing these needs is well matched to NIST’s expertise in both biochemical measurements and solution physics. Major Accomplishment(s): NIST has begun a program to facilitate the use of existing thermodynamic data for prediction of in vivo processes. Thermodynamic measurements of biochemical reactions are conducted in vitro, in very dilute solutions. Actual biochemical reactions occurring in physiological milieu find themselves in the presence of a very high volume percentage of biomolecules that are non-specific to the reaction being studied, a situation very unlike the in vitro conditions of the thermodynamic measurements. Awareness is growing that these additional molecules affect quite significantly the biochemical reactions. The presence of these additional molecules is theoretically predicted to influence the reactions by pushing them towards a state with smaller molecular sizes. In other words, a chemical reaction that reduces significantly the size of the product molecules, compared to the reactants, should be pushed further towards completion by the presence of a large volume fraction of non-reacting biomolecules than would be the same reaction in a very dilute solution, as in the in vitro measurement. Similarly, reactions that increase the molecular sizes of products over that of reactants should be inhibited by the presence of the additional, non-reacting biomolecules. These general effects are referred to as “macromolecular crowding.” Systematic studies are essential to improve and test theories of the effects of “crowding” on biochemical reactions and to facilitate the use of the large archive of biochemical data. The Physical and Chemical Properties Division has developed new facilities in biological calorimetry and has initiated experimental studies on macromolecular crowding phenomena. The Division has obtained a biological differential scanning calorimeter and an isothermal titration calorimeter, and has placed those instruments into operation. They are now being used to answer fundamental questions about macromolecular crowding. Calorimetry is an essential tool in separating entropic effects from specific energetic (enthalpic) effects. Calorimetry can usually provide more accurately the entropy and enthalpy of a reaction than is obtained by other means. As such, calorimetry can uncover smaller specific enthalpies of interaction – that invalidate the application of theory – than can other measures of biochemical reactions. There are several open questions regarding the measurements of “crowding” in the first place. 0 5 10 15 20 25 30 35 70 72 74 76 78 80 D en at ur at io n Te m pe ra tu re / °C Volume fraction / (g/l) Measured Theory Hen egg white lysozyme in aqueous polyethylene