Aspen Plus stripping

The example CO2 capture process, shown in Figure 8 as an Aspen Plus EO model representation, is part of an ammonia plant. Designed to scrub CO2 from ammonia synthesis gas, it includes an absorber and two solution regeneration columns, one stripping the rich, C02 laden solution leaving the absorber to semilean concentration of absorbed CO2, and the other cleaning the solution even further to lean solution... 

The feasibility of the above setup can be evaluated by simulation with Aspen Plus [19]. The RD column is built-up as a reboiled stripper followed by a condenser and a three-phase flash, with organic phase refluxed to column. The result is that only 3 to 5 reactive equilibrium stages are necessary to achieve over 99% conversion. The stripping zone may be limited at 2-3 stages, while the rectification zone has 1-2 stages. 

This unit can simulate any type of separation processes, as distillation, absorption, stripping, or extraction columns, modelled as cascade of counter-current equilibrium stages. The model Radfrac in Aspen Plus is particular powerful. It is first built on the inside-out algorithm that increased dramatically the robustness in simulating distillation-based operations (Boston, 1980). Columns with multiple feeds, side streams products, stage heaters or coolers, can be treated, as illustrated in Fig. 3.13. The following capabilities are generally available ... 

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. 

The first petroleum fractionator simulated is a simple distillation column that removes some of the light material in the crude. Figure 11.16 gives the Aspen Plus flowsheet of this unit. There are two crude feed streams that are combined and heated in a furnace in which the feed is partially vaporized before entering the bottom of the column. There is no reboiler. Live steam is introduced in the bottom of the column to strip out some of the light components in the bottoms stream, which is fed to a pipestill to be considered in Section 11.4. 

First, find the optimum feed plate and the total number of stages that give Xq = 0.80 0.0012 and Xq = 0.02 + 0.0004. Then, use Eq. tl6-21al and the values obtained from Aspen Plus to determine the height of the stripping section. Conpare to Exanple 16-1. 

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