Chao-Seader thermodynamics

Six alternate methods for predicting the thermodynamic properties are included. These are known by the names of the authors of the methods, which are Chao-Seader (2), Grayson-Streed (3), Lee-Erbar-Edmister (4), Soave-Redlich-Kwong (5), Peng-Robinson (6) and Lee-Kesler-Ploecker (7, 12). 

Example 6 ReboUed Stripper Compute stage temperatures and interstage vapor and liquid flow rates and compositions and reboiler heat duty for the reboiled stripper shown in Fig. 13-48. Thermodynamic properties may be estimated by using the Grayson-Streed modification of the Chao-Seader method. 

The case studies were selected to reflect different separation difficulties and different contents of the intermediate component of the quaternary mixtures. The mixtures considered are displayed in Tables 1-2. The mole fraction of 0.05 was shown to be a borderline value for use of side-stream columns for ternary separations (Tedder and Rudd, 1978). The total feed flowrate for all cases was 45.5 kmol/h. Since the feed involves a hydrocarbon mixture, the Chao-Seader correlation was used for the prediction of thermodynamic properties. The design pressure for each sequence was chosen such that all condensers could be operated with cooling water. 

Step 1 The flowsheet is Figure 6.1, and you enter the components and feed conditions in the usual manner. (See other examples in Chapters 2-5 and Appendix C.) The thermodynamics model chosen is the Refinery, Chao-Seader option. The parts specific to the distillation column are the block parameters. Shown in Figure 6.2 is the screen where you select the block parameters. You choose the light and heavy keys (propane and Z-butane), the splits desired (99 and 1 percent), the pressure of the column (138 psia), a total condenser, and the desired number of stages (10). 

Choose the thermodynamic property method as Refinery/Chao-Seader. Choose the pressures of the columns to agree with with the gas plant in Figure 5.4. Each column has 26 stages, and the temperatures shown in Table 6.6 are the result of thermodynamics predicted with Aspen Plus. They are very close to those listed in Figure 5.4. 

A first issue is the thermodynamic modelling. For the high-pressure part including the stabiliser, an equation-of-state model is appropriate, as for example Peng-Robinson or Soave-Redlich-Kwong. A specific model for hydrocarbons, as Chao-Seader or Grayson-Streed, may be used equally for the low-pressure part. 

Use the Naphtali-Sandholm SC method with the Chao-Seader correlation for thermodynamic properties to calculate product compositions, stage temperatures, interstage flow rates and compositions, reboiler duty, and condenser duty for the following distillation specifications. 

A saturated liquid feed at 125 psia contains 200 IbmoIe/hr of 5 mole% /C4, 20mole% nCj, 35mole% iCs, and 40mole% nCs- This feed is to be distilled at 125 psia with a column equipped with a total condenser and partial reboiler. The distillate is to contain 95% of the nC4 in the feed, and the bottoms is to contain 95% of the iCs in the feed. Use the Naphtali-Sandholm SC method, with the Chao-Seader correlations for thermodynamic properties, to determine a suitable design. Twice the minimum number of equilibrium stages, as estimated by the Fenske equation in Chapter 12, should provide a reasonable number of equilibrium stages. 

For the adjusted material balance of Example 15.2, as shown in Fig. 15.9 (but using the Chao-Seader correlation for thermodynamic properties), stream temperatures, enthalpies, and entropies are as follows. 

Thermodynamic properties (Chapters 4, 5) Soave-Redlich-Kwong Chao-Seader/Grayson-Streed van Laar Wilson UNIQUAC UNIFAC... 

Properties. Propylene is an olefin hydrocarbon that is a gas under ambient conditions bnt is normally stored as a liquid under pressin-e. The physical properties of propylene are given in Table 1. Thermodynamic properties are widely reported in the literature. Vapor-liquid equilibria of mixtures of propylene with other hydrocarbons and hydrogen are accurately represented by correlations for hydrocarbon mixtnres, such as the Chao-Seader correlation. 

Repeat Example 8.14, using the NRTL/Ideal gas thermodynamic module, which works poorly on this problem and with the Chao-Seader module and SRK module, which do much better. Compare these results with the calculated and experimental values in Example 8.14. 

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