The Effects of Pressure on Viscosity

The effect of pressure on viscosity is such that as the pressure goes up, the viscosity increases however the amount is quite small for the situations normally encountered in everyday life. A useful rule-of-thumb is to remember that for most single-phase, organic liquids, 

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The classic Barus equation describes the effects of pressure on viscosity 5... 

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. 

The effect of pressure on viscosity is relatively insignificant in most polymer processing operations, where pressures generally do not exceed 35 MPa (5000 psi). It has been found, however, that the effect of pressure on viscosity becomes quite significant at pressures substantially above 35 MPa. In fact, in careful rheological measurements, the effect of pressure on both viscosity and density has to be considered even at pressures around 35 MPa. 

Special rheometers have been constructed to measure the effect of pressure on viscosity. Various workers have presented data on the pressure dependence on viscosity [33-40]. The viscosity as a function of pressure is generally written as ... 

If the capillary rheometer is used to compare different polymers, it is not necessary to go through the various correction procedures. However, if one wants to know the absolute values of the viscosity, it is important to apply the various correction factors. The most important corrections are the correction of the shear rate for non-Newtonian fluid behavior (often referred to as Rabinowitsch correction) and the correction of the shear stress for entrance effects (often referred to as Bagley correction). These are the most common corrections applied to capillary rheometers. Other corrections that are sometimes considered are corrections for viscous heating, corrections for the effect of pressure on viscosity, corrections for compressibility, correction for time effects, etc. If many corrections are applied to the data, the whole measurement and data analysis procedure can become very complex and time consuming. 

Second, a comparatively simple method was developed for arriving at a first approximation to the limiting curve for E.H.L. conditions where the flow In the film may be considered not only Isothermal but even Isovlscous, l.e. In that the effects of pressure on viscosity may be Ignored. The curve thus obtained was Indeed depicted In Peppler s, or say the discusser s, figure. 

Currently, only five thermal conductivity and six viscosity mixed gas data points have been identified in the literature for xenon mole fractions ranging from 0.1 to 0.4 with temperatures greater than 390K (the lowest HeXe temperature in the SNPP), all at atmospheric pressure. Unless further literature review were to provide substantial additional data, it would be necessary to recommend obtaining additional experimental data against which to benchmark the recommended mixture method. In addition, the effects of pressure on viscosity and thermal conductivity and the deviation of density and specific heat from ideal gas behavior need to be assessed for the ranges of Prometheus system temperature and pressure. 

In the future, the possibility of combiiung measurements in the rheometer with RFM simulations will make it possible to define more accurately the effects of pressure on viscosity by means of optimization algorithms that determine the rheological parameters that best fit the simulated results with experimental data. 

The second part of the chapter is devoted to the effect of pressure on heat and mass transfer. After a brief survey on fundamentals the estimation of viscosity, diffusivity in dense gases, thermal conductivity and surface tension is explained. The application of these data to calculate heat transfer in different arrangements and external as well as internal mass transfer coefficients is shown. Problems at the end of the two main parts of this chapter illustrate the numerical application of the formulas and the diagrams. 

For a gas, the effect of pressure on the viscosity depends on the region of P and T of interest relative to the critical point. Near the critical state, the change in viscosity with T at constant pressure can be very large. The correlation of Uyehara and Watson [15] is presented for the reduced viscosity estimated from the corresponding-states method. The critical viscosities of a few gases and liquids are available [15]. These are necessary to calculate the... 

The figure also shows the effect of temperature on viscosity. At low pressures, gas viscosity increases as temperature increases. However, at high pressures gas viscosity decreases as temperature increases. The reciprocal of viscosity is called fluidity. 

MflM fi and co workers 4 have studied the effect of pressure on live liquid viscosity of ethylcnediamine. Figure 33 <> shows ilwsc results. As cun be seen from the graph, viscosity is only slightly affected by pressure At 250 atmospheres, the v i so wit y Is mil) HFf higher than the viscosity at uimosphcnc pressure... 

Mason and co-workers1 have determined the effect of pressure on the liquid viscosity of isopcopylaminc from 0 C to UWTC. These data are plotted in Figure 34 0. 

The same behavior is observed if the pressure is varied. As the pressure is increased, the free volume between the molecules is reduced, slowing down molecular movement. Here, an increase in pressure is equivalent to a decrease in temperature. In the melt state, the viscosity of a polymer increases with pressure. Figure 1.31 [7] is presented to illustrate the effect of pressure on stress relaxation. 

Experimental confirmations of the relative independence of fcG with respect to total pressure have been widely reported. Deviations do occur at extreme conditions. For example, Bretsznajder (Prediction of Transport and Other Physical Properties of Fluids, Pergamon Press, Oxford, 1971, p. 343) discusses the effects of pressure on the DABpr product and presents experimental data on the self-diffusion of C02 which show that the D-p product begins to decrease at a pressure of approximately 8100 kPa (80 atm). For reduced temperatures higher than about 1.5, the deviations are relatively modest for pressures up to the critical pressure. However, deviations are large near the critical point (see also p. 5-52). The effect of pressure on the gas-phase viscosity also is negligible for pressures below about 5060 kPa (50 atm). 

Another fact which points to a relationship between ionic mobility and viscosity is the effect of pressure on electrolytic conductance. Data are not available for infinite dilution, but the results of measurements on a number of electrolytes at a concentration of 0.01 n in water at 20 are shown in Fig. 23 the ordinates give the ratio of the equivalent conductance at a pressure p to that at unit pressure, i.e., Ap/Ai, while the abscissae represent the pressures in kg. per sq. cm. The dotted line... 

The effect of temperature on viscosity may be linked with its effect on other properties, such as density, surface tension, and vapour pressure. 

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