Critical compressibility factor

Auloignition Tempefafure Cmica) Compressibility Factor Critical Pressure ... 

The critical compressibility factor is estimated using the Lee and Kesler equation (1975) ... 

The critical pressure, critical molar volume, and critical temperature are the values of the pressure, molar volume, and thermodynamic temperature at which the densities of coexisting liquid and gaseous phases just become identical. At this critical point, the critical compressibility factor, Z, is ... 

Critical Compressibility Factor The critical compressibility factor of a compound is calculated from the experimental or predicted values of the critical properties by the definition, Eq. (2-21). 

Critical compressibility factors are used as characterization parameters in corresponding states methods (especially those of Lydersen) to predict volumetric and thermal properties. The factor varies from about 0.23 for water to 0.26-0.28 for most hydrocarbons to slightly above 0.30 for light gases. 

For pure organic vapors, the Lydersen et al. corresponding states method is the most accurate technique for predicting compressibility factors and, hence, vapor densities. Critical temperature, critical pressure, and critical compressibility factor defined by Eq. (2-21) are used as input parameters. Figure 2-37 is used to predict the compressibihty factor at = 0.27, and the result is corrected to the Z of the desired fluid using Eq. (2-83). 

No specific mixing rules have been tested for predicting compressibility factors for denned organie mixtures. However, the Lydersen method using pseudocritical properties as defined in Eqs. (2-80), (2-81), and (2-82) in place of true critical properties will give a reasonable estimate of the compressibihty faclor and hence the vapor density. 

If no value of Zra is available or derivable, the critical compressibility factor can be used in Eq. (2-84) as originally proposed by Rackett. Use of Z increases the average error to about 3.0 percent. 

Compressibility factor (Z) for mixtures when using pseudo-critical mixture constants to detennine ... 

W = gas rate, Ib/hr Z = gas compressibility factor Pc = Pcriv critical pressure, psia... 

A chart which correlates experimental P - V - T data for all gases is included as Figure 2.1 and this is known as the generalised compressibility-factor chart.(1) Use is made of reduced coordinates where the reduced temperature Tr, the reduced pressure Pr, and the reduced volume Vr are defined as the ratio of the actual temperature, pressure, and volume of the gas to the corresponding values of these properties at the critical state. It is found that, at a given value of Tr and Pr, nearly all gases have the same molar volume, compressibility factor, and other thermodynamic properties. This empirical relationship applies to within about 2 per cent for most gases the most important exception to the rule is ammonia. 

These equations will be sufficiently accurate up to moderate pressures, in circumstances where the value is not critical. If greater accuracy is needed, the simplest method is to modify equation 8.3 by including the compressibility factor z ... 

Hence there must be one relation involving pc, Tc and Vc which is independent of the parameters a and b. This relation defines the critical compressibility factor Zc ... 

Estimating the critical density IE, ln(Pc7Pc) No consistency test is available for ln(pc7Pc), but for the original Van der Waals equation and the modified VdW equations discussed in this chapter the critical compressibility factors, Zc(VdW) = Pc/(pcRTc), are equal to 3/8 and (a2 - l)/(4a), respectively. In the latter case,... 

Use the critical constants for hydrogen given in Table 5.3 to calculate the parameters a and b for the van der Waals and Redlich-Kwong equations for hydrogen. Use each of these equations to calculate the compressibility factor z for hydrogen as a function of l/Vm at 50 K between 0.1 Mpa and 20 MP, and compare your calculated values with the experimental values of Johnston and White [16]. 

In terms of reduced parameters the critical point occurs at Vf =1 and Tr=l. Thus from eq 4.1-2 we can note that at critical point the value of "compressibility factor , also known as the universal constant, is ... 

The compressibility factor pV/RT at the critical point for many gases and hydrocarbon liquids is shown in Table 4.1-1, p 237 of Ref 4... 

Fig. 3.2 Plot of the compressibility factor as a function of reduced pressure and parameterized by the reduced temperature. The reduced values are normalized by their corresponding values at the critical point. This plot is adapted from one originally prepared by Nelson and Obert [295,332],...

TABLE 2.4 Critical Constants 7C, Pc, VC9 and Compressibility Factor Zc = PCVC/RTC for Selected Gases... 

Table 2.4 displays critical constants Tc, Pc, Vc and critical compressibility factor Zc for a number of common gases. (Accurate determination of the critical point is experimentally challenging, and quoted values are generally uncertain in the final decimal.) One can see from the table that many common gases (including N2, 02, and CH4) are actually supercritical fluids ( permanent gases ) under ambient temperature conditions, incapable of liquefaction by any applied pressure whatsoever. (Aspects of cryogenic gas-liquefaction techniques are discussed in Section 3.6.3.)... 

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