Bubble point pressure determination

For mixtures, the calculation is more complex because it is necessary to determine the bubble point pressure by calculating the partial fugacities of the components in the two phases at equilibrium. 

EXAMPLE 2-3 Determine the critical temperature and critical pressure of a mixture of50.02 mole percent methane and 49.98 mole percent ethane. Also determine the bubble-point pressure and dew-point pressure of this mixture at 20°F. 

We will see in this chapter that field data can also be used to determine some properties of the reservoir fluid—in particular, bubble-point pressure and solution gas-oil ratio at the bubble point. These two properties are insufficient to permit prediction of reservoir behavior, but they can be used to check the results of reservoir fluid studies or correlations. 

The following physical properties can be determined from the results of a black oil reservoir fluid study bubble-point pressure, formation volume factor of oil, solution gas-oil ratio, total formation volume factor, coefficient of isothermal compressibility of oil, and oil viscosity,... 

Fig. 10-2. Determination of bubble-point pressure with data from flash vaporization.

EXAMPLE 10-1 The data from a flash vaporization on a black oil at 220°F are given below, Determine the bubble-point pressure and prepare a table of pressure and relative volume for the reservoir fluid study. 

First, plot pressure against total volume, determine the point at which the two lines cross, and read bubble-point pressure and volume at the bubble point. 

Determine the solution gas-oil ratio and formation volume factor of oil, both at bubble-point pressure, for Good Oil Co. No. 4. Field operation requires that the separator be operated at 200 psig and 75°F. 

Equation 8-19 may be used to compute the density of a liquid above its bubble point. The value of c0 should be determined at the average pressure between the bubble-point pressure and the pressure of interest. 

Often the pressure in a reservoir is higher than the bubble-point pressure of the reservoir liquid. Figure 11-15 may be used to determine the viscosity of the liquid for pressures above the bubble-point pressure.6 Figure 11-15 is entered with bubble-point pressure and with the viscosity at reservoir temperature and bubble-point pressure from Figure 11-14. The isobars are for any pressure greater than bubble point. The viscosity of the liquid above the bubble point is obtained. This viscosity will be greater than the viscosity at the bubble point due to the compression of the liquid caused by the increase in pressure. 

The bubble-point pressure at a given temperature may be determined by selection of a trial value of pressure, from which values of equilibrium ratios are obtained. Then the summation of Equation 12-21 is computed. If the sum is less than 1.0, the calculation is repeated at a lower pressure. If the sum is greater than 1.0, a higher trial value pressure is chosen. 

First, the initial trial value of bubble-point pressure is 247 psia since this is the bubble-point pressure calculated in Example 12-2 under the assumption of ideal-solution behavior. Determine Kj values at 150°F and 247 psia from Appendix A. 

Second, try a new trial value of bubble-point pressure of 220 psia. Determine Kj values at 150T and 220 psia. 

Third, plot trial values of pressure against the resulting summation as in Figure 12-3, and interpolate to determine a third trial value of bubble-point pressure of 237 psia. 

The bubble point test is simple, quick and reliable and is by far the most widely used method of characterizing microfiltration membranes. The membrane is first wetted with a suitable liquid, usually water for hydrophilic membranes and methanol for hydrophobic membranes. The membrane is then placed in a holder with a layer of liquid on the top surface. Air is fed to the bottom of the membrane, and the pressure is slowly increased until the first continuous string of air bubbles at the membrane surface is observed. This pressure is called the bubble point pressure and is a characteristic measure of the diameter of the largest pore in the membrane. Obtaining reliable and consistent results with the bubble point test requires care. It is essential, for example, that the membrane be completely wetted with the test liquid this may be difficult to determine. Because this test is so widely used by microfiltration membrane manufacturers, a great deal of work has been devoted to developing a reliable test procedure to address this and other issues. The use of this test is reviewed in Meltzer s book [3],... 

Although bubble point measurements can be used to determine the pore diameter of membranes using Equation (7.1), the results must be treated with caution. Based on Equation (7.1), a 0.22-pm pore diameter membrane should have a bubble point of about 200 psig. In fact, based on the bacterial challenge test, a 0.22-pm pore diameter membrane has a bubble point pressure of 40-60 psig, depending on the membrane. That is, the bubble point test indicates that the membranes has a pore diameter of about 1 pm. 

The phase of a given mixture is determined by a method similar to the rules for a pure component. At pressures greater than the bubble point pressure, the mixture exists as a liquid. At pressures less than the dew point, the mixture exists as a gas. At a pressure between the bubble and dew points, the mixture is two phase. 

The pressure at which the first vapor forms when a liquid is decompressed at a constant temperature is the bubble-point pressure of the liquid at the given temperature. Equation 6.4-4 can be used to determine such a pressure for an ideal liquid solution at a specific temperature, and the mole fractions in the vapor in equilibrium with the liquid can then be determined as... 

When the wetting fluid is expelled from the largest pore, a bulk gas flow will be detected on the downstream side of the filter system (Fig. 7). The bubble point measurement determines the pore size of the filter membrane, i.e., the larger the pore the lower the bubble point pressure. Therefore, filter manufacturers specify the bubble point limits as the minimum allowable bubble point. During an integrity test, the bubble point test has to exceed the set minimum bubble point. 

The vapor pressures at 40°C are p = 20 kPa, = 313 kPa. Using the van Laar liquid activity equation, calculate the A -valucs of ethanol and benzene at 40°C when the ethanol mole fraction in the liquid is 0.25, and determine the bubble point pressure. Assume ideal gas behavior in the vapor phase. 

Microfiltration membranes are characterized by bubble point and pore size distribution whereas the UF membranes are typically described by their molecular weight cutoff (MWCO) value. The bubble point pressure relates to the largest pore opening in the membrane layer. This is measured with the help of a bubble point apparatus.t Jt l The average pore diameter of a MF membrane is determined by measuring the pressure at which a steady stream of bubbles is observed. For MF membranes, bubble point pressures vary depending on the pore diameter and nature of membrane material (e.g., hydrophobic or hydrophilic). For example, bubble point values for 0.1 to 0.8 pm pore diameter membranes are reported to vary from 1 bar (equals about... 

The solution of the bubble- and dew-point problems using K values will be illustrated with the bubble-point temperature determination Start by making an initial estimate of the bubble-point temperature. Using a chosen 7 -value chart, read for the components of the mixture at the given pressure and the estimated temperature. Calculate the mole fractions of the incipient vapor by using the K values and the given x, y, = Kpci for all /. Sum the y/s and check if the sum is equal to 1,... 

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