1200
1400
Max. 1098 Act. ft /sec
Max. 1195 Act. ft /sec
Max. 1043 Act. ft /sec
1200
1000
Max. 884 Act. ft /sec
1000
800
Max. 898 Act. ft /sec
Max. 1052 Act. ft /sec
800
600
600
400
400
SRK Ideal stage GS Ideal stage PR Ideal stage
200
GS actual
200
0
0
0
5
10
15
20
25
30
0
10
20
30
40
Ideal stages
Actual tray numbers
Figure 1 Vapour flow rates to ideal trays – GS, SRK, and PR
Figure 2 Vapour flow rates based on actual trays – GS
The overall or sectional tray efficiency (OTE), defined as the ratio of the number of ideal stages to the actual num - ber of trays, is commonly used in column simulation models using the actual column’s number of trays. This overall efficiency option is available in most commer - cially available programs. However, some simulation pro - grams like ProII Version 10.2 do not seem to directly provide the equivalent OTE feature but offers Murphree and other point efficiency options. Definitions of these options are described in the ProII reference manual,³ which also alerts inconsistent results from using the point efficiency options. Overall or sectional tray efficiency, defined as the ratio of the number of ideal stages to the actual number of trays, is commonly used in column simulation models using the actual column’s number of trays Typical tray efficiency data are available for commonly known columns and are mainly derived from relevant field operating data. Simulation models for these columns can be based on theoretical stages or the actual number of trays. For revamping existing columns, discussed in the next sec - tion, field operating data should be reviewed to finalise the selected property package and verify the estimated tray efficiencies as necessary. Crude column Crude columns are common in oil refining, and the appli - cable property packages or tray efficiencies are relatively well-known. An example of crude column simulation is presented to evaluate the tray loading data from using dif - ferent property packages with or without tray efficiencies. The first example model uses ProII (Version 10.2) to
simulate an existing 41-tray crude column using 29 ideal stages (30 including the condenser). Three different prop - erty packages – Grayson-Streed (GS), Soave-Redlich- Kwong (SRK), and Peng-Robinson (PR) – are individually used in the separate versions of the model. These packages are commonly used for simulating crude columns, and the default option is used on each package. Input specifica - tions, top and bottom pressures, and pump-around duties are the same in the column simulation model using the three packages. Figure 1 shows the actual volumetric flow rates of the vapour to the ideal stages simulated separately using three different packages. As shown, the calculated vapour load - ing rates in ft3/sec from all packages are in good agreement, with less than 1% maximum deviation. Not shown in Figure 1, other calculated results, such as the condenser duties, deviate as much as 6%, with GS resulting in the lowest duty. For this example, the typical 10% exchanger design margin on flow rate and duty appears necessary to cover the duty variation from different property packages. Liquid flow rates from ideal stages calculated with SRK and PR have good agreements with much less than 1% absolute differences, except for a 2.5% difference for the liquid flow rate from the ideal stage above the feed stage. However, relative to SRK or PR, GS results in about 20% maximum difference for liquid stage-to-stage flow rates. This difference becomes more than 10 times higher for liq - uid flow from the stage above the feed stage. Changing the VLE option in the GS package from GS VLE to SRK VLE reduces the difference to about 7% maximum. This shows that the property estimating options available within a package could result in large differences in tray loading data and need to be properly selected. For revamp projects to evaluate and modify existing col - umns, field operating data typically can be made available and may be used for checking the overall tray efficiency estimates in the column simulation model using the actual number of trays. The overall and/or sectional tray efficiency estimates from references can be fine-tuned to improve
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PTQ Q4 2024
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