Operation of a Depropanizer

A depropanizer is a distillation column that is used in the natural gas industry to isolate propane from a mixture containing butane and other heavy components. This Demonstration shows a depropanizer with three components: propane, -butane, and -pentane. The depropanizer has a total condenser, a partial reboiler, 20 equilibrium stages, and operates at 17 bar. The feed stream, a saturated liquid at 101.6 °C, enters the depropanizer at stage 11 at a flow rate of 100 kmole/hr. The composition of the feed is 23.4 mole% propane, 37 mole% -butane, and 39.6 mole% -pentane.
Equilibrium relationships and enthalpy values are calculated using the Soave–Redlich–Kwong equation of state (SRK EOS).
The Demonstration solves the full MESH (mass, equilibrium, summation, and enthalpy) equations and plots the composition and the temperature profiles inside the column. The orange, green, and blue curves represent the liquid-phase mole fractions of propane, -butane, and -pentane, respectively.
Two case studies are shown in the last two snapshots.
Case 1 shows a situation where pure propane is obtained as a distillate, and with no propane leaving at the bottom of the column. This is the most efficient operation of the depropanizer.
Case 2 shows a situation where the reflux and reboil ratios are improperly chosen and -butane exits at the top of the depropanizer along with propane.
The results from both case studies are in perfect agreement with Aspen–HYSYS simulation results, represented by colored circles. (Aspen–HYSYS is a process modeling tool that is widely used in the oil and gas industry.)
Adding ethane and chemical components heavier than -pentane, including a pseudo-component (e.g., ), requires only minor modification of the Mathematica code used in this Demonstration.


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Expressions for ideal-gas constant-pressure heat capacities were obtained from Aspen-HYSYS.
The feed enthalpy was obtained from a separate calculation.
[1] E. J. Henley and J. D. Seader, Equilibrium-Stage Separation Operations in Chemical Engineering, New York: Wiley, 1981.
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