Gas viscosity correlation

Measurement of gas viscosity is very tedious. Obtaining accurate measurements on a routine basis is difficult. Thus, gas viscosity is estimated from correlations using the values of gas specific gravities measured in the differential liberation. Gas viscosity correlations as given in Chapter 6 or Appendix B are used. 

The excess viscosity has been determined for each datum by the use of equation (14.46) and subtracting the dilute-gas value, and the critical enhancement, A c, from the experimental value, T). For this purpose, reported by the experimentalists, rather than the value obtained from equation (14.47), has been preferred. This choice minimizes the influence of systematic errors in the individual measurements and forces the data of each author to a proper asymptotic behavior for the dilute-gas state. Furthermore, the experimental excess viscosity obtained in this fashion is independent of the choice for a dilute-gas viscosity correlation. Unfortunately, the majority of measurements on viscosity of ethane have been performed at pressures above 0.7 MPa and hence only a few authors reported a >7 value. Therefore, in order to estimate the experimental zero-density viscosity of each isotherm, again an iterative procedure had to be used (Hendl et al. 1994). The correction introduced by the extrapolation to zero density is small and in general does not exceed 0.5%. 

Measurement of gas viscosity at reservoir pressure and temperature is a complex procedure, and correlations are often used as an approximation. 

Generalized Correlations for Viscosity. Gas viscosity has also been predicted by corresponding states theory (90) using... 

The correlation of Jossi el al. (see eq. (1.12) in Appendix I) cannot be used for the determination of gas viscosity, as it is valid only for high pressures. Instead, Table 1.2 in Appendix I can be used, where we find that the viscosity at 523 K is 3.16 X10 5 kg/m s. The next parameter required for the gas phase is the diffusion coefficient of the species that... 

Experimental determination of gas viscosity is difficult. Usually, the petroleum engineer must rely on viscosity correlations. We will look at correlations of gas viscosity data which apply to the gases normally encountered in petroleum reservoirs. 

The composition of the reservoir gas as obtained in Example 7-1 is used in correlations previously given to estimate z-factors, gas viscosities, and other properties. 

Gas viscosity generally decreases with increased molecular size. This trend is reversed for liquids, for which the viscosity increases with increasing Nc within homologous series [2]. The latter observation is confirmed for alkanes, alkanethiols, and n-alkyl /3-ethoxypropionates in Tables 6.2.1a-c. Linear correlations between viscosity and Nc have been evaluated, for example, for n-alkyl n-alkoxypropionates [3] and n-alkyl carbonates of methyl and butyl lactates [4,5]. Similarly, correlations... 

In other words, at low density, the viscosity must equal the ideal gas viscosity. The second property of the correlation is that it must have a smooth transition between the two pieces. In order to achieve this, both the value of the function and the first derivative of the two pieces of the correlation must be equal at the point of transition. 

This dispersion number, (D/uL), for fluid flow in a cylinder can be obtained from a chemical engineering correlation by Levenspiel [9] noting that the intensity of dispersion D/udf, (where df is the diameter of the cylinder) is plotted as a function of Reynolds s number Re = up JlXg, pg is the gas density and p.g is the gas viscosity. (Please note that the Reynolds s number of the flow is altered by the presence of particles. Particles increase the gas density and reduce the effective kinematic viscosity. The net result is to accentuate turbulence and... 

If the composition of the gas is known, a more accurate estimate of the gas viscosity can be made by a method proposed by Carr, Kobayashi, and Burrows. The correlating procedure is based on the Law of Corresponding States. Consider the ratio ix/m where p, is the viscosity of the gas at the reduced reservoir temperature and pressure... 

This method of estimating the viscosity of gas mixtures requires a knowledge of the gas composition so that the pseudo-critical pressure and temperature may be computed. If composition data are not available it is still possible to employ this method, with a sacrifice of accuracy however, if the gas gravity is known. In this event the pseudo-criticals may be estimated by means of the specific gravity correlation already discussed in Chapter 2 and shown in Figures 11 and 12. Having obtained the pseudo-reduced temperatures and pressure in this way the estimation of the gas viscosity at reservoir temperature and pressure is carried out in the same maimer as before. 

The values in these tables were generated from the NIST REFPROP software (Lemmon, E. W., McLinden, M. O., and Huber, M. L., NIST Standard Reference Database 23 Reference Fluid Thermodynamic and Transport Properties—REEPROP, National Institute of Standards and Technology, Standard Reference Data Program, Gaithersburg, Md., 2002, Version 7.1). The primary source for the thermodynamic properties is Lemmon, E. W,. and Span, R., Short Fundamental Equations of State for 20 Industrial Fluids, /, Chem. Eng. Data, 51(3) 785-850,2006. The source for viscosity is Huber, M. L., Laesecke, A., and Xiang, H. W, Viscosity Correlations for Minor Constituent Fluids in Natural Gas n-Octane, n-Nonane and n-decane, Fluid Those Equilibria 224 263-270,2004. The source for thermal conductivity is Huber, M. L., and Perkins, R. A., Thermal Conductivity Correlations for Minor Constituent Fluids in Natural Gas n-Octane, n-Nonane and n-Decane, Fluid Phase Equilibria 227 47-55, 2004. 

At low and moderate pressures, the viscosity of a gas is nearly independent of pressure and can be correlated for engineering purposes as a function of temperatnre only. Eqnations have been proposed based on kinetic theory and on corresponding-states principles these are reviewed in The Properties of Gases and Liquids [15], which also inclndes methods for extending the calculations to higher pressures. Most methods contain molecular parameters that may be fitted to data where available. If data are not available, the parameters can be estimated from better-known quantities such as the critical parameters, acentric factor, and dipole moment. The predictive accuracy for gas viscosities is typically within 5%, at least for the sorts of small- and medinm-sized, mostly organic, molecules used to develop the correlations. 

The DIPPR project (5) was selected for viscosity of gas. Data for gas viscosity as a function of temperature were correlated using Equation (1-7). Results are in favorable agreement with data. Errors are about 1-10% or less in most cases. 

Results from the DIPPR project (5) were selected for viscosity of gas. Since the chemical structure of vinylidene chloride is between that of vinyl chloride and trichloroethylene, the values for vinylidene chloride were estimated from values for vinyl chloride and trichloroethylene. In the absence of data, this estimate should be considered a rough approximation. Data for gas viscosity as a function of temperature were correlated using Equation (1-7). Results are in favorable agreement with data. Errors are about 10% or less in most cases. 

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