Estimating the Viscosity of Gases

The following polynomial can be used to estimate the viscosity of gases  

Compound A BxlO CxlO AppI.Temp. Range,°C 

Note that the kinematic viscosity of a fluid is defined as the ratio of its viscosity to the fluid density. The c.g.s. unit of kinematic viscosity is usually called the stoke, and is equal to 1 cmVsec. 

Compound A BxlO2 CxlO4 Appl.Temp. Range,°C 

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The kinetic theory of gases is far more advanced than that of liquids partly because of complex interactions among the molecules of liquids. We may estimate the viscosity of a pure liquid from the following relation based on the Eyring rate theory ... 

A relatively simple procedure for estimating the viscosity of a liquid is to assume the equation presented earlier for gases under pressure. However, when applying the Jossi et al. equation... 

Three empirical techniques are presented by Reid et al. (1987) to estimate the viscosity of a mixture of gases at low pressure. These are Wilke s method (Wilke 1950), the method of Heming Zipperer (1936) and that of Reichenberg (1974,1975,1977). All three methods require values of the pure component viscosities and are fairly accurate for nonpolar molecules, with expected errors of less than 2 to 3%. Reichenberg s method includes the dipole moment and would be expected to be more successful for mixtures involving polar molecules. This claim is supported by comparisons presented by Reid etal. (1987). 

Information is available on the thermal conductivities of gases, liquids, and dense solids [1, Chapter 8 79 84-86]. Moreover, the thermal conductivity of gases may be estimated by procedures analogous to those employed for the estimation of the dilfusivity and the viscosity of gases. 

The book by Reid et al. [9] is an excellent source of information on properties such as thermal conductivities, diffusion coefficients and viscosities of gases and liquids. Not only are there extensive tables of data, but many estimation methods and correlations are critically reviewed. 

Besides these positive effects, a major disadvantage is introduced a liquid barrier to direct access of gaseous H2 to the catalyst particle The rheological properties of the fluid are also deeply modified, because the viscosity of liquids is many orders of magnitude higher than for gases. Finally, properties such as solubility, molecular diffusivity, etc., of H2 in organic mixtures, difficult to measure and even to estimate, have a vital influence on the mass transport phenomena, which can be schematized as follows ... 

Here n is the number of chemical species in a mixture, the mole fraction of species i, p., the viscosity of species i at the system temperature and pressure, and A /, the molecular weight of species i. Mainly, the dependence of viscosities on composition is nonlinear for mixtures of gases. Many additional empirical equations are available for estimating viscosities of gases and gas mixtures at low and high densities (Reid et al., 1987) as well as for liquids, suspensions, and emulsions (Bird et al., 2002). 

For example, the viscosity of low pressure gases can be estimated by the following combining rule ... 

The viscosity of various specific gravity gases as a function of temperature and pressure are shown in Figure 79. These charts enable a reasonably accurate estimate to be made if experimental data on the gas viscosity are not available. These charts were prepared from gas viscosity data obtained by Bicher and Katz on methane, propane, and methane-propane mixtures. A comparison between vis-... 

Transport coefficients are a group of coefficients used to predict the diffusivity, thermal conductivity and viscosity of gases. These in turn define the impact of the momentum, energy and mass of the gas flow. Two theories have been developed to estimate these coefficients. 

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. 

There are a large number of models used for the correlation and/or prediction of the viscosity of liquid hydrocarbons and their mixtures. Since there is no exact statistical mechanical or molecular-level theory for liquid viscosity, all of the models available contain some degree of empiricism. Also, there is considerable variation in the structure of these models in that most have been formulated to address only a speeific viscosity estimation problem. For example, some liquid hydrocarbon viscosity models have been proposed only for predicting the viscosity of an undefined petroleum mixture, and their input parameters have been selected accordingly. There are models that use some experimental viscosity data, while others are completely predictive, at least within a class of substances. Some viscosity models are suitable for incompletely defined petroleum cuts, whereas others can be used only for well-defined hydrocarbons and their mixtures. Further, some models include the effects of pressure and dissolved gases on liquid hydrocarbon viscosity, while others are for use only at atmospheric pressure. 

Several models that include the effect of pressure on viscosity are outlined herein. For applications at high pressures, one may also require estimates of the viscosity of liquid hydrocarbons and their mixtures with dissolved gases (such as with CO2, Nj, H2S, etc.) because, due to the high solubility of such gases in hydrocarbon mixtures at elevated pressures, there is a very large reduction in the mixture viscosity. Indeed, such behavior is part of the basis for enhanced oil recovery by miscible gas injection. Even though the effect of dissolved gases is beyond the scope of this chapter, some comments about this are included due to the importance of this subject. 

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