Optimal Conditions for CO2/n-Hexane Flash Separation

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A liquid stream, composed of carbon dioxide () and -hexane (-), has a flow rate of 100 kg/hr and a pressure of 100 bar. This stream is fed to a flash distillation vessel operating at a lower pressure and a user-set value of the temperature . The phi-phi approach is used with the Peng–Robinson equation of state (PR EoS) to compute the mass flow rates of in the exiting vapor stream (), as well as the mass flow rate of -hexane in the exiting liquid stream (). This Demonstration determines the pressure at which a profit function reaches its maximum value (indicated in the plot by a green dot). It is assumed that this objective function (expressed in $/hr) is equal to . The arc-length continuation technique is employed to generate in one single calculation the red curve (i.e. the profit curve versus pressure). The values of the optimal pressure and maximum profit are both reported in green on the plot.

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In addition, the results are compared with those obtained from a sensitivity analysis using the Aspen Plus process simulator [1, 2] (blue squares for the PR EoS property method). Excellent agreement is achieved for . Alternatively, one can adopt a property package other than the PR EoS. Indeed, we have run our Aspen Plus simulation with the UNIFAC property method for activity coefficients and the PR EoS for vapor phase fugacity coefficients (i.e. gamma-phi approach). The corresponding results are shown by the cyan triangles for . It is clear that despite the small quantitative discrepancies between the two property methods, the general qualitative features are similar (i.e. in both cases we see: (1) a sharp rise in the profit function at low pressure and a gradual decrease of this later function at higher pressure; and (2) the existence of an intermediate value of the pressure of the flash vessel where the profit is maximized).

Also, you can vary the value of the flash vessel temperature using the slider and see its effect on the values of the profit and optimal pressure. Clearly, increasing the temperature decreases the profit.

Finally, by selecting the "outlet mass flow rates" option you can display the values of and in kg/hr (shown in blue and red, respectively).

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Contributed by: Housam Binous and Ahmed Bellagi (January 2017)
Open content licensed under CC BY-NC-SA


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References

[1] M. Morales. Aspen Plus V8.0 Tutorial–Sensitivity Analysis [Video]. (Dec 30, 2016) www.youtube.com/watch?v=L8inROJ10Jw.

[2] Aspentech. "Aspen Plus." (Dec 30, 2016) www.aspentech.com/products/engineering/aspen-plus.



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