Aspen Plus tray design

Figure 4.1 (< ) Aspen Plus Tray Sizing, Specifications, (b) Aspen Plus Tray Sizing-, Design parameters, (c) Aspen Plus Tray Sizing-, Results.

Aspen Plus does not support the direct specification of the feed quality. To overcome this, a design specification was implemented the feed temperature is adjusted to provide the required energy for the change in the internal liquid flow according to Ln = Ln+i + q F where F is the feed flow, q the feed quality, and Ln+i and Ln the liquid entering and leaving the tray, respectively. 

Using the flowsheet simulators, design calculations are needed to estimate the reflux ratio and the theoretical tray requirements for the two towers in each of the sequences. In ASPEN PLUS, this is accomplished with the DSTWU subroutine, which is described in the module ASPEN — Separators Distillation FUG Shortcut Design on the multimedia CD-ROM. 

Aspen Plus provides feature called Conceptual Design that offers another approach to the problem (a design approach). In this method, the product specifications are set at both ends of the column, as is a reflux ratio. Then, the program performs tray-to-tray calculations, both up and down the column, creating composition profiles for both the rectifying and stripping sections. If these two composition profile intersect, the reflux ratio selected is above the minimum, and the number of trays in both sections is now known. The method is applicable to ternary systems with a single feed stream. 

It is important to note that all of these methods use only steady-state information, so steady-state process simulators such as Aspen Plus can be easily used to perform the calculations. The methods all require that various variables are held constant, while other variables change. For example, two product compositions can be held constant, or a tray temperature and reflux flow rate may be held constant. The Design Spec/Vary feature in Aspen Plus is used to achieve the fixing of the desired independent variables and the calculation of all the remaining dependent variables. 

This appendix shows how the Aspen Plus simulator can be used to do detailed tray and downcomer design. 

Lab 13. Aspen Plus uses RADFRAC with the Tray Rating option to do detailed heat- and mass transfer design calculations and tray and downcomer design. Start by setting up the problem below with RADFRAC. 

In the previous sections we utilized the kinetic subroutine supplied by Aspen Technology, which uses a kinetic expression for the reaction rate. It is interesting to compare this column design with the results when chemical equilibrium is assumed on the reactive trays. This is done in Aspen Plus by simply specifying that the reaction is Equilibrium on the Edit Reactions window, as shown in the upper picture of Figure 9.15. The lower picture shows that the chemical equilibrium constant is calculated from the Gibbs free energies. 

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