Munzky
Munzky
pubs.acs.org/jced
■   INTRODUCTION
Recent interest in reducing petroleum usage, lowering green-
                                                                                        critical point that contribute to a weak divergence of the
                                                                                        viscosity at the critical point.6
house gas emissions, improving air quality, and developing a                               The identification of these distinct contributions to the
more diverse energy infrastructure has led to renewed interest                          viscosity is useful because it is possible, to some extent, to treat
in hydrogen as a fuel and as an energy carrier. To advance new                          ηο(T), η1(T), and Δηc(ρ,T) theoretically. Unlike the critical
hydrogen technologies, it is important to have accurate                                 contribution to the thermal conductivity that impacts a
thermophysical property information. Deficiencies identified1                             relatively large region of temperatures and densities around
in currently available formulations for the viscosity surface2,3                        the critical point, the critical contribution to the viscosity is
present an opportunity to provide an improved correlation. In                           limited to a very small region in close proximity of the critical
addition, a powerful new technique, symbolic regression,4 has                           point.6 For several fluids where there are sufficient data in the
become available to identify underlying trends in experimental                          critical region, it has been shown7,8 that the ratio of Δηc/η is
data. The purpose of this work is to demonstrate the use of                             greater than 0.01 only within 1 % of the critical temperature. In
symbolic regression to develop correlations of material                                 this work, we will omit the critical contribution and consider
properties. In particular, we developed an improved, wide-                              only data outside of 1 % of the critical temperature and not
ranging correlation for the viscosity of hydrogen.                                      near the critical pressure. Kinetic theory can be used to
   The viscosity η(ρ,T) of a fluid can be expressed as the sum of
                                                                                        calculate the zero-density viscosity.9 In particular, advances in
independent contributions, as5
                                                                                        theory have led to accurate calculations of the dilute-gas
    η(ρ , T ) = ηo(T ) + Δηexcess(T , ρ) + Δηc (ρ , T )                     (1)
                                                                                        viscosity that can be used to supplement experimental values.
                                                                                        Mehl et al.10 presented quantum mechanical ab initio
                                                                                        calculations of the dilute-gas viscosity and thermal conductivity
    Δηexcess(ρ , T ) = η1(T )ρ + Δηh(ρ , T )                                (2)         of normal hydrogen and parahydrogen. Rainwater and Friend11
                                                                                        presented a theory for the calculation of η1(T). However, there
Here, the first term, ηο(T) = η(0,T), is the contribution to the                         is almost no theoretical guidance concerning the contribution,
viscosity in the limit of zero density, where only two-body                             Δηh(ρ,T), so that its evaluation is based entirely on
molecular interactions occur. The term Δηexcess (T,ρ) accounts
                                                                                        experimentally obtained data. The goal of this work is to
for the increase in viscosity above the zero-density value at
elevated density, and it can be expressed in terms of an initial-                       apply a symbolic-regression methodology to this term to
density viscosity coefficient η1(T) and a term Δηh(ρ,T)                                   identify a functional form that best represents the experimental
representing the contribution of all other higher-order effects                          data.
to the viscosity of the fluid at elevated densities including many-
body collisions, molecular-velocity correlations, and collisional                       Received: November 30, 2012
transfer. The term, Δηc(ρ,T), the critical enhancement, arises                          Accepted: March 7, 2013
from the long-range fluctuations that occur in a fluid near its                           Published: March 27, 2013
                              This article not subject to U.S. Copyright.
                              Published 2013 by the American Chemical             969                      dx.doi.org/10.1021/je301273j | J. Chem. Eng. Data 2013, 58, 969−979
                              Society
Journal of Chemical & Engineering Data                                                                                                  Article
Table 1. continued
          first author            year              method            purity/%       est. unc./%     no. pts.           T range/K                 p range/MPa
  Golubev87                       1970       CAP                       na            na                 58           298 to 523                 0.1 to 81.6
  Kestin88                        1971       OD                        99.9995       0.1                 3           295 to 308                 0.1
  Carey89                         1974       ACST                      na            0.1                18           291 to 299                 0.15 to 11
  Chuang90                        1976       CAP                       99.999        0.5                37           173 to 273                 0.4 to 50.6
  Clifford21                      1981       OD                        99.9995       0.2                 2           298 to 308                 0.1
  Lukin91                         1983       CAP                       na            0.3                23           76.5 to 293.2              0.1
  Nabizadeh12                     1999       OD                        na            0.5 to 1           76           295.6 to 399.2             0.1 to 5.8
  Maltsev13                       2004       CAP                       99.9          3                   3           500 to 1100                0.3
  May14                           2007       2CAP                      99.9999       0.08               32           213.6 to 394.2             0 to 0.11
  Mehl10                          2010       ab initio calculations    na            0.08 to 10         20           20 to 2000                 0
  Hurly15                         2011       GRN                       99.9999       0.5               111           225 to 400                 0.3 to 3.4
  Yusibani16                      2011       CAP                       99.999        2                  17           294 to 400                 4.6 to 99.3
a
  Abbreviations: est. unc., estimated uncertainty; no. pts., number of points; na, not available; 2CAP, twin capillary; ACST, acoustic resonance; CAP,
capillary; CD, constant deflection; FB, falling body; GRN, Greenspan viscometer; HOP, Hoppler viscometer; OIL, oil drop; OD, oscillating disc;
SWP, swinging plate; TOROC, torsional oscillating cylinder; TORC, torsional crystal; TRANSP, transpiration. Values in bold type are considered
primary data. bUncertainty ascribed by Assael et al.20 cIsotherms (32 K) close to critical excluded from primary data set. dOnly points above 150 K
considered in primary data set. eParahydrogen; data adjusted as described in text.
■    EXPERIMENTAL DATA
Previously, Leachman et al.1 reviewed the experimental
                                                                               Table 2. Coefficients of Equation 4
                                                                                             i                                            ai
viscosity data for normal and parahydrogen current to 2006.                                  0                                      2.09630·10−1
                                                                                             1                                     −4.55274·10−1
                                                                                             2                                      1.43602·10−1
                                                                                             3                                     −3.35325·10−2
                                                                                             4                                      2.76981·10−3
                                                                               Figure 3. Deviations of eqs 3 and 4 from the primary data and selected
                                                                               secondary data for dilute gas.
Table 5. continued
        first author           no. pts.          T range/K              p range/MPa           est. unc./%            AAD                 bias              RMS
    Golubev87                    58           298 to 523              0.1 to 81.6             na                        0.7                0.3               0.8
    Kestin88                     3            295 to 308              0.1                     0.1                       0.1              −0.1                0.1
    Carey89                      18           291 to 299              0.15 to 11              0.1                       0.5                0.1               0.5
    Chuang90                     37           173 to 273              0.4 to 50.6             0.5                       0.7                0.7               0.8
    Clifford21                   2            298 to 308              0.1                     0.2                       0.2              −0.2                0.0
    Lukin91                      23           76.5 to 293.2           0.1                     0.3                       1.4              −1.4                1.0
    Nabizadeh12                  76           295.6 to 399.2          0.1 to 5.8              0.5 to 1                  0.9               -0.9               0.7
    Maltsev13                    3            500 to 1100             0.3                     3                         1.4              −1.4                0.3
    May14                        32           213.6 to 394.2          0 to 0.11               0.08                      0.1                0.0               0.1
    Mehl10                       20           20 to 2000              0                       0.08 to 10                0.5               -0.5               0.7
    Hurly15                      111          225 to 400              0.3 to 3.4              0.5                       0.3               -0.2               0.3
    Yusibani16                   17           294 to 400              4.6 to 99.3             2                         1.6                0.3               1.7
a
  Abbreviations: est. unc., estimated uncertainty; no. pts., number of points; na, not available; AAD, absolute average percent deviation; bias, average
percent deviation; RMS, root mean square. Values in bold type are considered primary data. Double asterisk (∗∗) = parahydrogen.
then fit the primary experimental data for the dilute gas10,14,23           where T*, σ are as defined earlier and the coefficients are
with the ODRPACK orthogonal distance regression package93                  presented in Table 3.
and weighting factors of the square of the inverse of the                     Little theoretical guidance exists for the determination of the
experimental uncertainty for all points except the low-                    term Δηh(ρ,T) in eq 2, so we explore the use of symbolic
temperature (T < 100 K) values of Mehl et al.10 that were                  regression to determine both the coefficients and the functional
given increased weights in order to have the final representation           form of this contribution.
be within 1 %. In addition, we constrained the fit to agree with               Symbolic Regression. We employed the technique of
the recommended zero-density value of Berg and Moldover25                  symbolic regression (SR)4,100,101 in our effort to find a new
at 25 °C, 8.8997 μPa·s. The final values of the coefficients are              viscosity correlation for normal hydrogen. Shokir and
given in Table 2. Deviations of the primary data and the                   Dmour102 demonstrated the use of a form of symbolic
correlation given by eq 3 and 4 are shown in Figure 3. Also                regression to obtain a correlation for the viscosity of pure
shown are deviations from secondary data sets measured after               hydrocarbons and hydrocarbon gas mixtures. Symbolic
1950. The dilute-gas expression given by eq 3 and eq 4 with the            regression is a specific application of genetic programming
coefficients in Table 2 represents the experimental data sets of             (GP) that allows one to explore arbitrary functional forms in
                                                                     975                        dx.doi.org/10.1021/je301273j | J. Chem. Eng. Data 2013, 58, 969−979
Journal of Chemical & Engineering Data                                                                                                                  Article
order to fit data. These functional forms are constructed by                  behavior and the relative simplicity of numerous models.
using a set of operators, parameters, and variables as building              Finally, the constant elements of the model were reoptimized
blocks for functions of arbitrary complexity. Using a classic GP             using a differential evolution nonlinear fitting algorithm to
algorithm for this function search is very slow and, even for                produce the final functional form:
relatively simple data fitting problems, requires many days of                                                             ⎡
computing on current computing hardware. An improved GP                          η(T , ρ) = η0(T ) + η1(T )ρ + c1ρr2 × exp⎢c 2Tr + c3/Tr
algorithm based on multiobjective optimization and preferential                                                           ⎢⎣
evolution of models near the Pareto front has recently been
developed103 that greatly improves the efficiency of the search                                     c4ρr 2         ⎤
and brings it into the realm of current, desktop computer                                   +           + c6ρr 6 ⎥
                                                                                                c5 + Tr          ⎥⎦                                           (9)
technology. This method addresses many of the numerous
problems associated with SR including the issues of runaway                  In this equation η0(T) is obtained from eqs 3 and 4, η1(T,ρ)
complexity, evolutionary lock-in, and slow model development.                from eqs 5 to 7, the scaled temperature is Tr = T/Tc and the
There are two available software realizations of SR104,105 that              scaled density ρr = ρ/ρsc where the temperature is expressed in
use similar overall architecture, but vary in the details of the             Kelvin, densities in kg·m−3, and the viscosity η is expressed in
algorithm implementation. (Certain trade names and company                   μPa·s. The quantity ρsc is a compressed-state density used for
names are mentioned to specify adequately the materials used.                scaling that the symbolic regression procedure identified as 90.5
In no case does such identification imply endorsement by                      kg·m−3.
NIST, nor does it imply that the materials are the best.)                       Comparison with Experimental Data and Previous
However, both packages produce a similar result after                        Correlations. Comparisons with the experimental data are
optimization, which is a set of functions that are ranked by a               presented in Table 5, which gives the number of data points,
combination of complexity and quality of data fit. We have                    the original authors’ estimated uncertainty of the data, average
found that using both packages allows us to increase the                     percent deviation, average absolute percent deviation, and a
diversity of models suggested by SR and, therefore, gives us a               root-mean-square error of each data source. We define the
better pool of results from which to choose. Our experience                  percent deviation as P = 100 × (ηexp − ηcalc)/ηexp, where ηexp is
indicates that some criteria necessary for a good viscosity data             the experimental value of the viscosity and ηcalc is the value
correlation are very difficult to automatically enforce in SR and              calculated from the present correlation. The average absolute
must be enforced during the postoptimization model selection                 percent deviation (AAD) is found with the expression AAD =
process. These criteria include reasonable extrapolation                     (∑|P|)/n, where the summation is over all n points; the average
behavior and exclusion of functions that produce nonphysical                 percent deviation (bias) is AVG = (∑P)/n, and the we use
results such as negative viscosities or infinite viscosities.                 RMS = ([n∑P2 − (∑P)2]/n2)1/2. Table 5 summarizes the
   Our specific choice of model search for the viscosity of                   performance of the new model. Figures 5 and 6 show
hydrogen was the following. We initialized SR optimization                   comparisons with the primary data for normal hydrogen over
with the set of operators {+,-,*,/,Exp,∧} and the operands                   the temperature range 200 K to 400 K at pressures to 200 MPa;
{constant,T,ρ} to simultaneously optimize function complexity                the agreement is to within about 4 %. For the region between
and mean absolute error. Initial weights based on the estimated              200 K and 400 K at pressures up to atmospheric, the present
uncertainty of the data were also utilized. The form optimized               correlation reproduces the high-accuracy data of May et al.14 to
was                                                                          within 0.1 %. In Figures 7 and 8 we compare the performance
   f (T , ρ) = η(T , ρ) − η0(T ) − η1(T )ρ                                   of the correlation given by eqs 2 and 3, eqs 5 to 7, and eq 9
                                                                (8)          against that of the correlation of Vargaftik et al.,3 and that of the
where η0 is obtained from eqs 3 and 4 and η1 is from eqs 5 to 7.             McCarty model, as implemented in the NIST Standard
Several hundred individual evolutions that each typically                    Reference Database 23 (REFPROP).95 As mentioned earlier,
involved ∼1011 function evaluations and 100 h of c.p.u. core                 the default correlation in v9.0 and earlier of REFPROP is based
time were performed to identify an optimal function that is                  on the unpublished work of McCarty;94 this model relied
both relatively simple and able to fit the data well. Some terms              heavily on the data of Diller.17 The recently developed model
appeared repeatedly; one was a scaled density, in this case                  of Yusibani et al.16 is not included in the comparisons because it
0.011ρ (where ρ has units of kg·m−3), or equivalently ρ/90.5.                is limited to temperatures above 40 K for the dilute gas and
Therefore, we made additional runs where the density and                     above 100 K for pressures up to 220 MPa. As shown in Figures
temperature were both scaled, with Tr = T/Tc and ρr = ρ/ρsc .                7 and 8, the SR correlation performs comparably to the
For temperature, we scaled with the critical temperature 33.145              Vargaftik3 and McCarty94,95 correlations. The largest deviations
K;18 for density we used the scaling factor ρsc = 90.5 kg·m−3.               shown in Figures 7 and 8 for the SR correlation are for
This value is interesting in that it is a very compressed liquid             parahydrogen data of Diller17 as the critical region is
state, close to the value of the density of the liquid at the triple         approached. There are few liquid-phase data available, but
point (77.00 kg·m−3). In 1971, Hildebrand106 modified the                     comparisons with the data of Johns27 and the saturated liquid
earlier work of Batschinski107 and introduced the concept of an              data of Diller17 indicate agreement with the data to within 4 %.
intrinsic volume where the fluidity is zero. Since then, various              The McCarty model was not developed for pressures above
researchers5,108−110 have successfully used variations of this               100 MPa, and this is evident from Figure 8.
concept to model the viscosity of a range of fluids, and it is
interesting that symbolic regression suggests a scaling in terms
of a very compressed-state density. Future work will investigate
                                                                             ■   CONCLUSION
                                                                             A new wide-ranging correlation for the viscosity of normal
if this same type of term arises for other fluids.                            hydrogen was developed by way of a symbolic regression
   The final choice of model function was done in an ad hoc                   methodology. The correlation covers the temperature range
fashion after reviewing the overall fits to data, the extrapolation           from the triple point to 1000 K and pressures up to 200 MPa
                                                                       976                      dx.doi.org/10.1021/je301273j | J. Chem. Eng. Data 2013, 58, 969−979
Journal of Chemical & Engineering Data                                                                                                                         Article
and extrapolates in a physically reasonable manner to 2000 K.                      (15) Hurly, J. J. Acoustic relaxation and virial coefficients of H2 and
The dilute-gas viscosity agrees to within the experimental                        O2, and viscosity of H2. Int. J. Thermophys. 2011, submitted.
uncertainty of the most accurate gas phase data of May et al.14                    (16) Yusibani, E.; Nagahama, Y.; Kohno, M.; Takata, Y.; Woodfield,
over the temperature range 213−394 K at pressures to 0.11                         P. L.; Shinzato, K.; Fujii, M. A capillary tube viscometer designed for
                                                                                  measurements of hydrogen gas viscosity at high pressure and high
MPa, and also reproduces the recommended value of Berg and
                                                                                  temperature. Int. J. Thermophys. 2011, 32 (6), 1111−1124.
Moldover25 at 25 °C and zero density. Outside of that region,                      (17) Diller, D. E. Measurements of the viscosity of parahydrogen. J.
the estimated uncertainty is 4 % for the saturated liquid and                     Chem. Phys. 1965, 42 (6), 2089−100.
supercritical fluid phases, except along the saturated liquid                       (18) Leachman, J. W.; Jacobsen, R. T.; Penoncello, S. G.; Lemmon,
boundary above 31 K and in the near-critical region, where the                    E. W. Fundamental equations of state for parahydrogen, normal
uncertainty is larger. The simplicity of this new correlation                     hydrogen, and orthohydrogen. J. Phys. Chem. Ref. Data 2009, 38 (3),
makes it easy to implement and demonstrates the power of                          721−748.
symbolic regression in finding relatively simple functional forms                   (19) Assael, M. J.; Ramires, M. L. V.; Nieto de Castro, C. A.;
for data correlation.                                                             Wakeham, W. A. BenzeneA further liquid thermal-conductivity
■
                                                                                  standard. J. Phys. Chem. Ref. Data 1990, 19 (1), 113−117.
    AUTHOR INFORMATION                                                             (20) Assael, M. J.; Mixafendi, S.; Wakeham, W. A. The viscosity and
                                                                                  thermal conductivity of normal hydrogen in the limit of zero density. J.
Corresponding Author                                                              Phys. Chem. Ref. Data 1986, 15 (4), 1315−22.
*E-mail: chris.muzny@nist.gov.                                                     (21) Clifford, A. A.; Kestin, J.; Wakeham, W. A. The viscosity of
Notes                                                                             mixtures of hydrogen with three noble gases. Ber. Bunsenges. Phys.
The authors declare no competing financial interest.                               Chem. 1981, 85 (5), 385−8.
■
                                                                                   (22) Michels, A.; Schipper, A. C. J.; Rintoul, W. H. The viscosity of
    ACKNOWLEDGMENTS                                                               hydrogen and deuterium at pressures up to 2000 atm. Physica
                                                                                  (Amsterdam) 1953, 19, 1011−28.
We thank Mark Kotanchek and Katya Vladislavleva of Evolved                         (23) Guevara, F. A.; McInteer, B. B.; Wageman, W. E. High-
Analytics, LLC, for their encouragement and expertise in the                      temperature viscosity ratios for hydrogen, helium, argon, and nitrogen.
use of the symbolic regression method.
■
                                                                                  Phys. Fluids 1969, 12 (12), 2493−505.
                                                                                   (24) Patkowski, K.; Cencek, W.; Jankowski, P.; Szalewicz, K.; Mehl, J.
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