Hybrid Solvent Helps Ease Bottlenecking in Natural-Gas Plant
Authors: Wilson Adam 
Published: April 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 188252, “Natural-Gas-Plant Debottlenecking Thanks to Hybrid Solvent,” by Eric Cloarec, Renaud Cadours, and Claire Weiss, Total, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed.

Processing sour natural gas is a challenge. If mercaptans are present in the sour gas, the limited mercaptan-absorption capacity of the well-known alkanolamine solvents can be a problem. A solution is to replace the usual alkanolamine aqueous solvent with a hybrid formulation that allows simultaneous removal of mercaptans and acid gases. A new solvent has been developed by the addition of a physical component into a generic alkanolamine/water solvent. This hybrid solvent can be used without any plant modification.

Introduction

More than 40% of identified gas reserves contain acid gases. Over the years, solvent technologies have been developed, demonstrated, and improved for hydrogen sulfide (H2S) and carbon dioxide (CO2) removal.

Sour-gas processing has recently seen the requirement of more-stringent specifications for total sulfur compounds, particularly mercaptans and carbonyl sulfide. Producing sour-gas fields in an economic way became a challenge. Indeed, the physicochemical properties of mercaptans allow only very limited reaction with amines under operating conditions. Classical aqueous amine technologies generally are not sufficient to reach mercaptan or total-sulfur specifications. Consequently, additional treatment is required to remove these compounds.

The first option to treat a gas containing CO2, H2S, and mercaptans consists of a polishing stage for further mercaptan removal (for example, molecular sieves) downstream from the amine treating process. One drawback of this option is that the gas used for adsorbents regeneration needs to be sweetened, which requires a dedicated unit. Another drawback is that the gas- sweetening unit is generally designed to use a physical solvent, which has high affinity for hydrocarbon.

The second option is most often used when the recovery of natural-gas liquids is considered. Mercaptans and other sulfur compounds concentrated in the liquid-hydrocarbon cuts are removed through a caustic-soda process or by molecular sieves. With the first option, the succession of treatment stages requires an increase in plant footprint, which brings unavoidable extended lifecycle costs. Meanwhile, the drawbacks of these schemes include the disposal of the disulfide oil with the caustic-soda process or the treatment of the gas used for molecular-sieve regeneration.

The last option is the simultaneous removal of mercaptans, carbonyl sulfide, and acid gases in a single unit by use of a mixture of chemical and physical solvents. However, the mercaptan-removal efficiency is correlated with the solvent composition and flow rate.

Design criteria can be for H2S and CO2 removal or for mercaptan elimination. Expenditure optimization will drive the selection of the process scheme: either total mercaptan removal within the gas-sweetening unit or partial mercaptan removal with the hybrid solvent followed by a polishing unit.