Operation of a Debutanizer

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Natural gas liquids (NGLs) are hydrocarbon components of natural gas, composed exclusively of carbon and hydrogen. The main components of NGLs are methane, ethane, propane, butane, pentane, hexane, and heptane. In a natural gas processing plant, the various NGLs are separated from pure natural gas (methane) using a train of fractionation columns in which the various hydrocarbons are boiled off one by one. The fractionation columns are called the demethanizer, deethanizer, depropanizer, debutanizer, and so on.

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In this Demonstration we simulate a debutanizer that has as its feed stream a mixture of -butane, -pentane, and other higher hydrocarbons. In order to make the calculation tractable, a pseudo-component, called , is introduced that lumps together all chemical components of oil or gas that are heavier than -pentane. The pseudo-component is hypothetically constructed to have a normal boiling point of , with the following critical parameters: and with heat capacity data close to that of -heptane.

The flow sheet for the debutanizer has a total condenser, a partial reboiler, and 30 equilibrium stages, and operates at 13 bar. The feed stream is saturated liquid at 132.3 °C, and enters the debutanizer at stage 16 at a flow rate of 100 kmole/hr. The composition of the feed is mole% -butane, mole% -pentane, and mole% . Equilibrium relationships and enthalpy values are calculated using the Peng–Robinson equation of state (PR 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 -butane, -pentane, and , respectively.

Drag the sliders to explore the operating behavior of the debutanizer. Two case studies are shown in the last two snapshots.

Case 1 shows a situation where pure -butane is obtained as a distillate, and with no -butane leaving at the bottom of the column. This is the most efficient operation of the debutanizer.

Case 2 shows a situation where the reflux and reboil ratios are improperly chosen and -pentane exits at the top of the debutanizer along with -butane.

These calculations illustrate the need to undertake process simulation studies to determine the optimum operating conditions of the debutanizer: small changes in the reflux ratio and reboiler ratio can lead to dramatic changes in the purity of the butane leaving the top of the column.

Finally, the results from both case studies show good agreement with Aspen-HYSYSsimulation results, represented by colored circles. Aspen-HYSYS is a process modeling tool that is widely used in the oil and gas industry.

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Contributed by: Housam Binous, Naim Faqir, and Brian G. Higgins (November 2012)
Open content licensed under CC BY-NC-SA


Snapshots


Details

Expressions for ideal-gas constant-pressure heat capacities were obtained from Aspen-HYSYS.

The pseudo-component critical temperature and pressure, acentric factor, and ideal-gas constant-pressure heat capacities were obtained from Aspen-HYSYS. A normal boiling temperature of was assumed equal to .

The feed enthalpy was obtained from a separate calculation.

Reference

[1] E. J. Henley and J. D. Seader, Equilibrium-Stage Separation Operations in Chemical Engineering, New York: Wiley, 1981.



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