Dec-2013

There are more roads that lead to Rome. Which is the best route for you?

In our modern society the need for energy is still increasing in a rapid pace, with expectations that hydrocarbon fuels production will have to almost double in the next thirty years. However, the easy to process sources are getting more and more depleted, whereas the new to develop fields are getting increasingly more sour and are going to be build on more and more remote locations.

Gerrit Bloemendal
Jacobs Nederland B.V.

Article Summary

On the other hand, the demand for clean technology with limited emission of sulphurous components is getting stronger and stronger, with new and very tight emission limits being forced by international organisations such as the Worldbank.

In this paper several examples will be shown, which combine state of the art acid gas removal techniques with minimal environmental impact. It will be shown that for the selection of the gas treating technology the removal of H2S and CO2 are only marginally determining the selected technology. More often the technology selection is determined by the requirement to remove trace components such as mercaptans and COS.

Once the acid components are removed from the main gas stream the processing of the acid gas is another challenge. The latest developments in acid gas processing with lean acid gasses and tight emission specs will be discussed. Often a treating solutions selected in an early stage might not be the best fit. Proper evaluation and screening of different options and technologies can decide on a case to case base what the overall best solution is.

Finally, the synergies that can be found between acid gas removal and sulphur recovery will be stipulated, looking at options to integrate the two systems such that both capital and operational cost will be minimised.

Examples that will be shown are the gas development in Turkmenistan using SUPERCLAUS® technology, and the latest development in the Middle East using Sulfinol Technology with a hot flash re-contacting step to enhance the Gas Feed to the Sulphur recovery unit.

Proper evaluation in the right stage of the project will decide success for the project or if additional cost need to be made to make up for not well considered decisions. This paper will try to highlight that it is worthwhile to invest money in proper studies in the feasibility stage of the project. This will be earned back many times in the other stages of the project e.g. when the hardware is being purchased.
 
Conventional treating and sulphur recovery
As stated in the introduction, trace components quite often determine the line-up of a gas treating facility. In case the contaminants in the raw gas are only H2S and CO2, a conventional treater can be used, consisting of an absorber, a flash stage to recover the hydrocarbons in the rich solvent, and a regenerator to strip the acid components from the solvent. Depending on the volume of gas to be processed, the number of trains can be multiple. A good example of such a line-up is the new gas processing facility that is currently being built in Turkmenistan. The huge gas volume in combination with the transportation limitation made it necessary to install four parallel gas treating trains. However, thanks to clever logistics and smart engineering by the detailed contractor, the total acid gas of the four gas treating units can be processed in two parallel sulphur plants applying the SUPERCLAUS process. These units are designed for a capacity of approx. 1200 tpd sulphur production per train. The expected acid gas composition varies between 37 and 45% H2S, and consequently between 50 to 58% CO2. With proper preheating a sufficient hot flame temperature is expected to convert most of the heavy hydrocarbons in the SRU.

The picture above gives a good impression of the typical line-up for such a unit. From right to left the thermal stage, the three Claus reactors and the SUPERCLAUS reactor can be distinguished. In the very back, the incinerator and stack can also be seen.
 
Case 1: An LNG plant in the Middle East
For one of the first LNG plants in the Middle East, the same strategy with a conventional treater and a SUPERCLAUS was applied. However, since for LNG production there is a tight spec on CO2 and organic sulphur (mercaptans, COS) in the treated gas the Sufinol-D process was selected for the main treater.

Sulfinol-D is a mixture of sulfolane, di-iso-propanol (DIPA) and water. Due to its chemical composition it is capable of doing a deep CO2, COS and H2S removal (thanks to the amine component) as well as a substantial removal of mercaptans (thanks to the sulfolane). In combination with a molecular sieve unit, routing the regeneration gas of the mol sieves through a special mercaptans absorber, the Sulfinol solvent is capable of doing a one solvent fits all approach delivering on-spec, dehydrated gas to the liquefaction section and routing all sulphur species including the mercaptans to the SRU.

Since in this case the CO2/H2S ratio was fairly high the acid gas to the SRU was of a rather poor quality, with following design composition: The sulphur recovery unit was designed to handle this low H2S containing gas, and considering the high amount of (especially heavy) hydrocarbons, a straight through process had to be selected, with special effort in preheating the air to meet the flame stability/flame temperature criteria.

The design SRU recovery in those days was 96%, which, despite the poor acid gas quality, could be met with a 2 + 1 reactor SUPERCLAUS plant.

Fairly soon after start-up of the complex it became evident that the CO2/H2S ratio in the raw gas was even worse than designed for, as is shown in Table 2.

Once it was recognised the acid gas was even leaner than expected, a project was started to add an acid gas enrichment unit in between the Sulfinol unit and the sulphur plant. The addition of this unit is shown in Figure 2.