Apr-2012

Solutions for the treatment of highly sour gases

Process technologies for the cost-effective treatment of natural gas with high and ultra-high acid gas content

FRANÇOIS LALLEMAND and GAUTHIER PERDU, Prosernat
CRISTINA MARETTO and CLAIRE WEISS, Total E&P
JULIA MAGNE-DRISCH and ANNE-CLAIRE LUCQUIN, IFP Energies Nouvelles

Article Summary

For decades to come, gas will be the energy source of choice to meet increasing energy demands. Those oil and gas operators who have always preferred to produce gas from the technically easiest to develop reservoirs will in the future have to develop fields with a higher acid gas content. Effectively, 40% of the world’s natural or associated gas reserves currently identified as remaining to be produced, representing over 2600 trillion cubic feet (tcf), are sour, with both H2S and CO2 present most of the time. Among these sour reserves, more than 350 tcf contain H2S in excess of 10%, and almost 700 tcf contain over 10% CO2.¹

Removing the undesired acid gases from highly sour gases is a costly operation. The size and cost of the acid gas separation units and the acid gas-handling facilities (transformation of the H2S into sulphur and forming/shipping of the produced sulphur, or acid gas compression/pumping and re- injection facilities) increase with the amount of acid gases to separate. In the meantime, the exported volume of sales gas decreases because of the reduced hydrocarbon content of the inlet raw gas and the increased auto-consumption for gas treatment. As a consequence, the technical cost per volume of exported sales gas is roughly doubled every 20-25% of additional acid gases present in the raw gas (see Table 1).

Very efficient technologies are therefore necessary to separate the large amounts of acid gases from these fields. Solutions implemented by Prosernat using the AdvAmine series of processes, some of which have been developed in collaboration with Total and IFPEn, to optimise amine processes are presented in this article.

Sulphur, the traditional sub- product from the production of gases containing high amounts of H2S, formerly represented a substantial part of the revenues drawn from the production of such natural gases. It is no longer of economic interest to sell sulphur from several areas today, especially those locations far from the sea and the sulphur users. The world sulphur market is globally saturated, as the supply of sulphur, mainly obtained from H2S separated from sour natural gases or sour crude, has exceeded demand, essentially from the fertilizer industry. Even though some companies have developed new sulphur uses such as sulphur asphalt in the 1970s or, more recently, sulphur concrete, most experts consider that this situation is set to continue for several decades, at least in several parts of the world. The production of major oil or gas fields in the Middle East or in the Caspian Sea area could lead to the production of considerable quantities of additional sulphur in an already saturated market, while the storage of extra sulphur in the long term is an issue that will require significant capital costs to be resolved. This is why new production methods must be developed for sour fields, to limit the over-production of sulphur.

Companies willing to produce large gas fields with very high amounts of CO2 have to face a different constraint related to the essential need to reduce atmospheric emissions of greenhouse gases.

With the growing acceptance of the re-injection of separated H2S and CO2 — either for re-utilisation to enhance oil recovery (EOR) or just simple disposal to a depleted reservoir or an aquifer, as a feasible alternative to costly sulphur recovery to a diminishing sulphur market, or to limit atmospheric emissions of greenhouse gases — a number of highly sour gas fields can be reconsidered as exploitable to produce much-needed natural gas.

These new constraints lead to the development of more energy- efficient technologies for acid gas separation, adapted to these new production schemes. With this objective, in addition to the AdvAmine series of gas sweetening processes using amine-based solvents, Total, IFPEn and Prosernat have developed Sprex and SprexCO2 for the production of highly sour gas reserves with acid gas re-injection.

Optimisation of amine processes for highly sour gas treatment
Amine processes (see Figure 1) have been used for many years to remove acid gases from natural and associated gas streams. They are very versatile processes, which can be used to treat all types of sour gas, regardless of the H2S and/or CO2 content, down to the most severe specifications, such as those imposed by gas liquefaction plants.

But the cost of gas sweetening increases with the amount of acid gases to be separated, requiring larger amine solution flow rates and higher energy consumption for amine solution regeneration. On the other hand, amine processes, because of the variety of amines available and the possibility to adapt and improve the process flow schemes, can be efficiently used for almost any type of gas sweetening application.

Amine-based technology can therefore be considered as the workhorse of the sour gas processing industry and, as such, still justifies continuous technological improvements to extend the economical limits of its applications. Issues like treatment costs and energy consumption are being addressed by such developments.

This article presents solutions proposed by Prosernat for the efficient and economic treatment of gases with a high acid gas content, either H2S or CO2. These solutions benefit from the operating experience of Total in its highly sour gas treatment plants and, while some are proposed by other licensors, others have been developed through the joint R&D effort of Total, IFPEn and Prosernat.

Double-split flow process configuration
Among the oldest process configurations, the double-split flow design, sometimes called the split-flow design, has been used as an alternative flowsheet to minimise capital and operating costs in several sour gas processing plants. Generally used with primary or secondary amines, this design allows for very severe treated gas specifications for the same reboiler duty as a conventional process flow scheme, and was documented probably for the first time by Estep, McBride and West in 1962.2 This design also allows for the same treated gas specification as a conventional amine plant scheme, with a much-reduced reboiler duty when treating very sour gases.

A conventional double-split flow design is shown in Figure 2. Part of the amine solution is withdrawn from the regenerator as a side stream, then cooled and pumped into the lower section of the absorber. The amine in the side draw is not fully regenerated and has a higher residual acid gas loading.