A New Distillation Process for the Upgrading of Acid Gas
Authors: Carpenter Chris 
Published: May 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 183154, “The Potential of a New Distillation Process for the Upgrading of Acid Gas,” by L.A. Pellegrini, G. De Guido, S. Langé, and S. Moioli, Politecnico de Milano, and B. Picutti, P. Vergani, G. Franzoni, and F. Brignoli, Tecnimont, prepared for the 2016 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 7–10 November. The paper has not been peer reviewed.

In this work, the potential of a new low-temperature distillation process for natural-gas sweetening has been investigated. The proposed technology consists of a dual pressure-distillation scheme designed to avoid the formation of a solid phase in all parts of the process while remaining able to fulfill the required natural-gas-purity specifications. The new process offers the competitive advantages of low-temperature technologies while avoiding carbon dioxide (CO2) solidification and solvent addition.

Introduction

The industry must find technologies that allow the profitable exploitation of low-quality and high-CO2-content gas reserves in order to meet increasing demand while still decreasing production costs. Low-temperature processes are preferred to traditional chemical or physical absorption for gas purification when the CO2 concentration in natural-gas streams is high. For this reason, attention on low-temperature processes for CO2 removal has increased in recent years.

In this work, an analysis has been carried out to determine the trade-off between classical methyldiethanolamine (MDEA) gas-sweetening units, still representing the industry benchmark for acid- and sour-gas purification, and a low-temperature distillation process. The MDEA unit typically requires steam at the reboiler of the regeneration column, while the low-temperature distillation processes consume electric energy to drive the refrigeration cycle. In amine regeneration units, the steam consumption is a significant part of the total energy demand of the process and thus of the total operating costs. The complete paper defines a merit-index function to determine the trade-off between the two considered technologies.

The Low-Temperature Process

The new process for the low-temperature purification of natural gas is based on a dual-pressure (approximately 40/50 bar) low-temperature distillation operation, designed to bypass the solid/liquid/vapor (SLV) locus of the methane (CH4) and CO2 binary mixture through a proper thermodynamic pathway. In this work, an optimized process layout has been taken into account.

The gas stream to be purified is fed to the high-pressure (HP) section at its dewpoint; in this first part of the distillation unit, the natural-gas feed is separated into two streams: a bottom one with a high CO2 content and a top product flow rich in CH4. The top product stream from the HP section is split into two streams: the first is heated and expanded to the low-pressure (LP) section; the second one is cooled so that after expansion it is at its bubblepoint at 40 bar. The superheated gas stream is fed at the bottom of the LP section, while the liquid feed enters the column a few theoretical trays above the bottom of the LP section. In the LP distillation section, a top product CH4 gas stream at commercial grade is obtained, while the bottom liquid stream rich in CH4 is pumped back to the HP section to provide the reflux. The condenser is a full-reflux condenser, and the top product stream is obtained as gas. The rationale of the process to avoid solid CO2 formation is the bypassing of the maximum of the SLV locus of the CH4-CO2 system.