Oct-2013

Extending the treatment of highly sour gases: part 1

The removal of acid gas components from highly or super-sour gases requires the optimum choice of process

FRANÇOIS LALLEMAND, GAUTHIER PERDU and LAURENT NORMAND, Prosernat
CLAIRE WEISS, Total EP
JULIA MAGNE-DRISCH and SEBASTIEN GONNARD, IFP Energies nouvelles

Article Summary

For decades to come, gas will be an energy source of choice to meet increasing energy demands. Oil and gas operators have always preferentially produced the gas from those reservoirs that are technically the easiest and the cheapest to develop, but they will have to develop fields with a higher acid gas content in the future. Effectively, over 40% of the world’s conventional gas resources currently identified as remaining reserves 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

The Middle East, the Caspian Sea area and China have gas reserves with a high H2S content, while large amounts of gases with a high CO2 content are encountered in South East Asia and, to some extent, in South America and North Africa.

Removing the undesired acid gases from highly sour gases is a costly operation. The size and cost of the acid gas separation units and of the acid gas-handling facilities (transformation of the H2S into sulphur and forming/shipping of the produced sulphur, or acid gas compression/pumping and reinjection facilities) increase with the amount of acid gases to separate. In the meantime, the volume of sales gas exported decreases because of the reduced hydrocarbon content of the inlet raw gas, and because of 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).

 To ensure economic development of these fields, very efficient technologies are therefore needed to separate large amounts of acid gases at controlled costs. Prosernat implements several solutions within the AdvAmine series of processes, some of them developed in a collaboration between Total and IFP Energies nouvelles/Prosernat. This article presents some optimisations of the process scheme that give rise to new developments at demonstrated and attractive economic duties.

Sulphur, the traditional sub-
product from the treatment of gases containing high amounts of H2S, formerly represented a substantial part of the revenues drawn from the production of such 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. 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 also an issue that requires significant capital to be resolved. This is why new production methods give the green light to develop ultra-rich H2S gas fields in places where economic interest does not rely on the additional 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.

Economics may also be improved by the growing acceptance of the reinjection of separated H2S and CO2, for enhanced oil recovery (EOR). Separated acid gas reinjection into 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, increases the number of highly sour gas fields that 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, IFP Energies nouvelles and Prosernat have developed the Sprex and SprexCO2 processes for the production of highly sour gas reserves with acid gas reinjection. Discussion of these processes will be the subject of the second part of this article, to be published in PTQ at a later date.

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 gases, 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 that can be used 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 actually 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 such as treatment costs and energy consumption are addressed by such developments.

Treatment of such highly sour gas goes far beyond the usual basic design practices employed in the first stage of a process design of an acid gas removal unit. One usual criterion is the need to stick to moderate acid gas loading of the amine solution.

Capital and operational savings cannot be met without attractive solvent loading and deep thermal integration of the rich solvent regeneration section, based on the self-regenerating properties of highly loaded solvents when they are moderately flashed or heated. The base use of highly loaded solvent at high temperatures should nevertheless be coupled with new recommendations for unit design, especially in terms of sizing criteria and metallurgy requirements.