Pseudo Critical Conditions NonIdeal Gases Compresibility Factor Surface Production Operations Design of Oil Handling Systems and Facilities From: Gulf Professional Publishing Surface Production Operations Design of Oil Handling Systems and Facilities Educational training article Nonideal Gas Equations of State 1 de 14 From: Gulf Professional Publishing Surface Production Operations Design of Oil Handling Systems and Facilities Educational training article Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor Tabla de contenido Tabla de contenido ......................................................................................................... 2 Introducción ................................................................................................................... 3 1.- Ecuations .................................................................................................................. 4 2.- Compresibilty ........................................................................................................... 7 3.- NG Compresibility Factor ........................................................................................ 9 4.-Reduce Pressure ....................................................................................................... 10 5.- Reduced Properties ................................................................................................. 11 Example 1 ........................................................................................................ 13 Example 2 ........................................................................................................ 14 F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 2 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor Introducción Hydrocarbon streams are mixtures of hydrocarbons that contain various amounts of impurities such as hydrogen sulfide, carbon dioxide, and water. A single-component system composed entirely of a simple molecule, such as methane or propane, behaves in a very predictable, correctable manner. The performance of a single-component system can be accurately correlated in tabular form. For all others, one must use pressure, volume, and temperature (PVT) equations of state or a weighted average (assumes that the contribution of individual molecules is in proportion to their relative quantity in the mixture). The more dissimilar the molecules, the less accurate the prediction becomes. The of lists properties of some hydrocarbon molecules which are important for process calculations. Water in liquid or vapor form is present to some degree in all systems. Liquid water is basically immiscible in hydrocarbons. Since phase behavior calculations are not applicable to water, special procedures must be used. Equations of state use the values of P, V, and T at the critical point. A specific critical point exists for each hydrocarbon component. For a pure component the critical values represent the maximum pressure and temperature at which a two-phase, vapor-liquid system can exist. Above Pc and Tc, only a single phase is possible. For mixtures, pseudocritical values are calculated. These values are not a point on the phase diagram but a correlation value that allows one to perform routine calculations. Any equation correlating P, V, and T is called an “equation of state.” F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 3 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor Nonideal Gas Equations of State The ideal gas equations of state describe most real gases at low pressure but does not yield reasonable results at higher pressures. Many PVT equations have been developed to describe nonideal, real gas behavior. Each is empirical in that it correlates a specific set of data using one, or more, empirical constants. Unfortunately, there is no correlation that is equally good for all gas mixtures. Some of the more common equations of state that attempt to define the relationship between V, T, and P for the real gases follow: 1.- Ecuations Van der Waals F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 4 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor Several modifications to the above equations have been published to improve the validity of the equation. The above equations are the basis of most computer programs. However, the real accuracy may be no better than some simpler methods when the designer considers the quality of the compositional data usually obtained from a drill stem test in the early stages of a project. Fortunately, all the ideal equations of state can be approximated to the compressibility equation of state by multiplying the “RT” part of the equation by Z: F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 5 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor If the gas acted as if it were an ideal gas, then the Z factor would be 1. The typical range of Z = 0_8 to 1.2. Figure 3-2 shows the Z factor as a function of pressure at constant temperature, various temperatures, and pseudo-critical properties. Figures 3-3 through 3-6 are some correlations for the compressibility factor, Z, which have proven useful for natural gas calculations. F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 6 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor 2.- Compresibilty Figure 3-2. Top: Z factor as a function of pressure for a single hydrocarbon component at various temperatures; bottom: relationship for hydrocarbon mixtures is similar but dependent on reduced properties. F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 7 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities FACTOR Z Compresibility Factor F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 8 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor 3.- NG Compresibility Factor F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 9 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor 4.-Reduce Pressure They depend on the reduced pressures and temperatures which are described below. F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 10 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor 5.- Reduced Properties Reduced properties are used to correlate experimental data. Equations (3-39) and (3-40) can be used to calculate the reduced properties of a flow stream. The reduced pressure and temperature values are not truly a pressure and temperature but a ratio. Thus, it is a nondimensional term and does not take the unit “degrees.” Pseudo-critical properties allow one to evaluate gas mixtures. Equations (3-18) and (3-19) can be used to calculate the pseudocritical properties: F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 11 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 12 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor 6.- Pseudo-reduced properties, are the same equations as for the reduced properties, except that pseudo-critical properties have been substituted for actual critical properties. Equations (3-20) and (3-21) are used to calculate the pseudo-reduced properties: Example 1: Calculate the pseudo-critical temperature and pressure for the following natural gas stream composition: F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 13 | 14 Pseudo Critical Conditions NonIdeal Gases Surface Production Operations Design of Oil Handling Systems and Facilities Compresibility Factor Example 2: Calculate the volume of 1 lb mole of the natural gas stream given in the previous example at 120_F and 1500 psia. F r o m : Gulf Professional Publishing: Design of Oil Handling Systems and facilities P á g i n a 14 | 14
