Jan-2014

Optimising hydrogen sulphide scavenging

Selecting the right hydrogen sulphide scavenger is important, but appropriate treatment setup and tailored monitoring procedures are key to avoiding off-specs

GIACINTO GRANDE and ALESSANDRA BERRA
Chimec

Article Summary

Scavenging hydrogen sulphide (H2S) from petroleum products may be easy, but doing it efficiently is much more challenging. Too often some important aspects are overlooked, causing extra costs in the best case and off-specs in the worst ones. Through the example of a case history we want here to show the impact that a proper approach in treatment management can have on dosage rates and therefore costs.

H2S risk
Risk is known to be the product of the probability of an event and its consequences, and H2S is a poisonous gas, lethal at certain concentrations, that can be present in petroleum products. This is why it represents a high risk for the petroleum industry. But there is more to this issue.

H2S’s specific features make it worthy of special attention. Being a colourless gas, its presence can be identified only by smell which can be deceptive. H2S can be detected by a characteristic odour of rotten eggs when present in very low concentrations (1.4 µg/m3 or 0.0047 mg/kg), but the senses are soon saturated and deactivated by an increase in concentration, meaning that when the concentration exceeds 100 mg/kg humans are no longer able to smell it. This is perhaps one of the most dangerous aspects of this gas; its presence ceases to be perceptible by the human senses as its concentration becomes more dangerous. And exposure at a concentration equal to a few hundred mg/kg may already be lethal.

H2S is present, in extremely variable amounts, in crude oils. Advanced refining processes generally allow for high, if not complete, removal of H2S from light and middle distillates, with rare but possible exceptions.

The situation is different when it comes to heavy fuel oils. Generally speaking, H2S content in residues is the result of the sum of what remains trapped during the distillation processes, what is formed as a result of cracking phenomena, and what is removed by desulphurisation and/or stripping processes.

The nature of the sulphur compounds present in the crude, and the severity of cracking applied, influence the amount of H2S that can be produced as a result of these phenomena. Conditions sufficient for the formation of H2S, for example, can be reached by the bottom of the vacuum distillation columns.

The nature of residue refining processes is more oriented towards the formation of H2S (cracking) than its removal (desulphurisation) to the point that its content in the liquid phase can reach values even higher than 50 mg/kg even though it is mostly in the range 5-20 mg/kg.

The values mentioned above may seem lower than the safety thresholds for human health but there is an important difference: the phase to which these concentrations refer. When we speak about petroleum products, H2S content is reported as concentration in the liquid phase. From this liquid, H2S is then released into the vapours above the oil, where it becomes dangerous to humans. The amount of H2S developed in the vapour phase depends on several factors, including the characteristics of the oil, conditions of temperature and pressure, and obviously the H2S concentration in the liquid.

For residues and heavy fuel oils, the levels of H2S in the vapours are normally 50-100 times those in the liquid. It is therefore clear why even a few parts per million of H2S in the liquid phase can be so dangerous.

H2S specification
On 1 July 2012, a H2S specification was implemented in a revision of ISO 8217. A limit of 2 mg/kg in the liquid phase was established for all marine diesel and residual grades. This specification set some new operating requirements and new challenges for all companies producing marine fuels.

Refineries have means of controlling H2S content in the residues and fuel oils they produce, although it is important that this does not have negative effects on other aspects of production. But failing to optimise operating conditions can lead to a H2S content as much as an order of magnitude higher than the minimum required.

Achievement of the H2S specification in ISO 8217 therefore starts with the proper management of fuel oil production processes. When this is not possible, or the H2S level is not sufficiently low, the specification has to be met through the implementation of other solutions.

The main and most common among these is the use of specific additives able to remove H2S by a chemical reaction, so-called H2S scavengers. The alternative is the installation of stripping columns for the residue, but this solution is seldom applied due to high installation and operating costs. In some cases, there may be a competitive solution through the use of additives.

Even though some refineries were already dealing with H2S scavenging before 2012, the number of applications sharply increased after the implementation of the new specification, but experience in the proper management of similar processes did not spread with the same speed.
Through the presentation of a case study, this article will make a general discussion of the relevant aspects of the management of H2S scavenger treatment and the impact they can have on the overall results.

A deeper treatment of the issue would require the use of confidential information that cannot be provided in these circumstances, but Chimec is available to provide further clarification of our experiences and technologies.

Case study
Refinery scheme, operating conditions and H2S content are among the fundamental variables to be taken into account for the proper setup of an 
H2S scavenger application. Monitoring, on the other hand, is a key aspect of treatment management and, to achieve real cost optimisation, specific programmes should be set up based on the characteristics of the site.

The best way to show this is through real experience. It is not the specific case that this article wants to focus on, as it may differ from the situation in other refineries, but the basic principles applied. These principles have wider applicability.

The refinery described in this case study produces marine residual fuels through the blending of a vacuum residue and a mix of fluxants. The main source of H2S is the vacuum residue while its content in the lighter components is negligible.