Effect of Liquid Murphree Efficiency on the Operation of a Deisobutanizer

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A deisobutanizer, operating at a pressure equal to 500 kPa, contains 40 stages, a partial reboiler, and a total condenser. This column separates a mixture of ‐butane and ‐butane. The feed location is stage 20 counting from the top. The feed flow rate is equal to 21.56 kmol/hr and contains 31.45 mole% ‐butane and 68.53 mole% ‐butane.

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The purity of the distillate and bottom streams are set equal to 90 mole% ‐butane and ‐butane, respectively.

The Peng–Robinson equation of state is used to compute the -values and the departure functions for the liquid and vapor phase enthalpies. Expressions of the constant-pressure heat capacities are obtained from a major process simulator, Aspen HYSYS.

The concept of liquid Murphree efficiency is introduced whenever the stages cannot be considered as equilibrium stages. Murphree efficiency [1] is defined by , where and are the mole fractions of the liquid leaving stages and and is the mole fraction of the liquid leaving stage if equilibrium is achieved (i.e., when we get ). The Murphree liquid efficiency gives the value of the ratio of the actual change in liquid divided by the change in liquid for an equilibrium stage. If , the usual equilibrium stage assumption is recovered.

The Demonstration finds the steady-state composition profile as well as the reflux and reboil ratios for user-set values of the liquid Murphree efficiency. Both the reflux, , and reboil, , ratios become larger as the liquid Murphree efficiency decreases because achieving the separation objectives (i.e., product stream purities) without increasing the number of stages is more difficult (i.e., higher and values) when decreases.

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Contributed by: Housam Binous (November 2012)
Open content licensed under CC BY-NC-SA


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References

[1] P. C. Wankat, Separation Process Engineering, 2nd ed., Upper Saddle River, NJ: Prentice Hall, 2007.



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