The Gas Gravity Method

Example 4.2 Calculating Hydrate Formation Using Gas Gravity Chart 

Find the pressure at which a gas composed of 92.67 mol% methane, 5.29% ethane, 1.38% propane, 0.182% i-butane, 0.338% n-butane, and 0.14% pentane form hydrates from free water at a temperature of 283.2 K (50°F). 

Similarly Example 4.1 used the gas gravity method to predict hydrate pressure of 1.95 MPa at 278.2 K for a mixture of 95.6% CH4 + 4.4% C3H8 as compared to the experimental value of 1.3 MPa. 

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Section 4.2 deals with the most useful hydrate equilibria—calculations of temperatures and pressures at which hydrates form from gas and free water. In this section, two historical methods, namely, the gas gravity method (Section 4.2.1) and the Kvs, value method (Section 4.2.2), for calculating the pressure-temperature equilibrium of three phases (liquid water-hydrate-vapor, Lw-H-V)1 are discussed. With the gas gravity method in Section 4.2.1.1, a method is given for limits to expansion, as for flow through a valve. In Section 4.2.2 a distribution coefficient (KVSi) method is provided to determine whether a component prefers residing in the hydrate or the vapor phase. These methods provide initial estimates for the calculation and provide a qualitative understanding of the equilibria. A statistical... 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

Both the gas gravity method and the Kysi-value method enable the estimation of three-phase (Lw-H-V) equilibrium between quadruple points Qi and Q2 for mixtures as well as for simple natural gas hydrate formers such as those in Table 4.2. 

In the present work, the gas gravity method [1] is employed to calculate the pressure-temperature hydrate curve for Tehran city natural gas composition. The average values of gas composition given in Table 3 are used. Fig. 3 shows the hydrate curve for the natural gas composition of Tehran city gas network. To predict the hydrate formation... 

The simplest method of determining the temperature and pressure of a gas mixture three-phase (Lw-H-V) conditions is available through the gas gravity charts of Katz (1945). Gas gravity is defined as the molecular mass of the gas divided by that of air. In order to use this chart, reproduced as Figure 4.5 from Figure 1.4,... 

The inaccuracies listed in the previous section for the gas gravity chart are inherent in the expansion charts of Figures 4.7 through 4.9 due to their method of derivation. Accuracy limits to these expansion curves have been determined by Loh et al. (1983) who found, for example, that the allowable 0.6 gravity gas expansion from 339 K and 24 MPa was 2.8 MPa rather than the value of 4.8 MPa, given in Figure 4.7. 

Brown, and the statistical thermodynamic method of van der Waals and Platteeuw (1959a) was substituted for the three-phase hydrate line prediction by the gas gravity chart of Katz. 

To compute u at a given reservoir temperature and pressure the value of the compressibility factor under these conditions must be known. If an experimental value of Z is not available and it is necessary to estimate a value for a reservoir gas one has recourse to the methods described in Chapter 2. If the composition of the gas is known a pseudo-reduced temperature and pressure may be calculated and the compressibility factor obtained from Figure 10. If, on the other hand, the composition is not known but a value of the gas gravity is available, it is still possible to evaluate the pseudo-critical temperature and pressure from Figures 11 and 12. With these pseudo-critieals and the values of the reservoir temperatiue and pressure, the pseudo-reduced temperature and pressure can be computed and the compressibility factor obtained from Figure 10 as before. 

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. 

Fresh or regenerated catalyst is added to the top of the first reactor to maintain a constant quantity of catalyst. Catalyst transport through the reactors and the regenerator is by gravity flow, whereas that from the last reactor to the top of the regenerator and back to the first reactor is by the gas-lift method. Catalyst circulation rate is controlled to prevent any decline in reformate yield or hydrogen production over time onstream. 

The measured diameters of particles shoiild as nearly as possible represent the effective particle size of a dust as it exists in the gas stream. When significant flocculation exists, it is sometimes possible to use measurement methods based on gravity settling. 

Methods of dust removal depend mainly on the particle size of the dust and the temperature and moisture content of the gas. The methods used are broadly divided into dry methods and wet methods. The dry methods involve the use of gravity and baffle chambers, cyclones, filters, and electrostatic precipitators, while the wet methods involve the use of spray towers and venturi scrubbers. In principle, wet cleaning is preferred to dry cleaning because of the excessive wear associated with and the difficulty in handling the fine dusty material removed in the dry methods. The wet methods, however, must be followed by such operations as filtration, drying of filter cakes, and recycling of water. 

The more carbon there is contained in gas the heavier it is, and specific gravity has, therefore, been often resorted to, as an indication of the relative value of different gases, If carbonic acid has been completely separated, and no excess of carbonic oxide is present, tlie specific gravity may furnish an important indication to the manufacturer of tlie value of gas made under similar circumstances but it can never replace an analytic in judging of the value of different gases, as the amount of carbonic oxide will vary with the kind of coal used, and the method of conducting the distillation, the specific gravity of which is very nearly equal to that of olefiant gas. 

Estimation of the Specific Gravity of Gas.—Although the more determination of the specific gravity of gases is of very little nee as a test of their commercial value —unless the gas is to be used for aeronautic purposes —yet, as it is still much employed by gas engineers, and as such an estimation is occasionally useful for controlling the results of chemical analysis, a method by which such a determination may be made is here subjoined. 

Next we will look at two methods of estimating the specific gravity of the reservoir gas from production data. In the first case, the properties and quantities of all surface gas streams are known. In the second case, only the properties of the gas from the primary separator are known. 

In actual applications, the gas flow in a gravity settler is often nonuniform and turbulent the particles are polydispersed and the flow is beyond the Stokes regime. In this case, the particle settling behavior and hence the collection efficiency can be described by using the basic equations introduced in Chapter 5, which need to be solved numerically. One common approach is to use the Eulerian method to represent the gas flow and the Lagrangian method to characterize the particle trajectories. The random variations in the gas velocity due to turbulent fluctuations and the initial entering locations and sizes of the particles can be accounted for by using the Monte Carlo simulation. Examples of this approach were provided by Theodore and Buonicore (1976). 

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