Apr-2014

Risks of accumulated sulphur in sulphur recovery units

While recovering sulphur in an SRU depends on condensation of sulphur, its unintended condensation and accumulation can present problems.

Ellen Ticheler-Tienstra, Anne van Warners, Rien van Grinsven and Sander Kobussen
Jacobs Nederland B.V.

Article Summary

The paper analyses areas where this may be expected and presents case histories of sulphur build-up in SRU equipment, such as waste heat boilers, reheaters, sulphur condensers and coalescers.

The risks involved in accumulation of sulphur are discussed, both for the operation of Claus plants and for the operation of SUPERCLAUS® plants, for which liquid sulphur may pose extra risks. Ways are discussed to prevent accumulation and minimise the risks.This paper focuses on the most common type of SRU configurations, not on sub-dewpoint type SRUs.

Introduction
From time to time operational issues in an SRU are related to accumulation of liquid sulphur. These issues can be experienced in any SRU but operating experience over the past 25 years has shown that SUPERCLAUS and EUROCLAUS® installations are sensitive to liquid sulphur accumulation, as will be discussed below.

In the modified Claus process, sulphur is produced both in the thermal stage and in the catalytic stages. Since the Claus reaction is a chemical equilibrium, the production of sulphur is inhibited if sulphur vapour is already present in the process gas. Therefore a sulphur condenser is generally used to condense and remove the produced sulphur and ensure continued conversion to sulphur in subsequent stage(s). As a general rule the sulphur production is progressing as shown in Table 1. This also shows the relative contribution of each stage to the total sulphur productionThis table is based on a typical refinery feed of 75 vol.% H2S, 5 vol.% NH3, 10 vol.% CO2, 10% H2O, 0.3 vol.% C2H6 and almost complete removal of sulphur vapour from the process stream. The condensers are not removing all of the sulphur, it is sufficient to stay above the sulphur dew point in the following catalytic stage. In the sub-dewpoint processes (CBA, Sulfreen, MCRC) there is some deliberate condensation of sulphur to below the dewpoint to force the Claus equilibrium closer to completion.

Fire risk of accumulated liquid sulphur
Principles of a sulphur fire
The best known risk of accumulated liquid sulphur is that of a sulphur fire. For this to happen, air (or better: oxygen) has to be present in sufficiently high concentrations (>10%).  At lower concentrations of free oxygen in the process gas the sulphur will be oxidised to SO2 thereby generating extra heat of oxidation. A sulphur fire can also lead to corrosion that is higher than normally experienced.

Besides sulphur vapour (from liquid sulphur) and oxygen in sufficiently high concentrations (>10 vol.%) there has to be an ignition source. The following sources have been identified:
•  No additional source, when the temperature in the process at that point is above the auto-ignition temperature of sulphur. The auto ignition temperature most often mentioned is 230°C but values ranging from 190°C to 261°C have been reported, possibly linked to the sulphur particle size.
•  Pyrophoric iron sulphide (FeS), when exposed to air (oxygen) will ignite a sulphur/air mixture
•  Static electricity generated by liquid sulphur agitation. Sulphur, being one of the best electric insulating liquids known, and having a high dielectric constant can easily generate enough static electricity to cause a spark ignition

Oxygen concentration during normal SRU operation
During normal operation of a Claus unit with a good quality main burner hardly any oxygen will slip from the main combustion chamber to the catalytic stages, therefore the process gas does not contain free oxygen and there is no risk of a sulphur fire. During normal operation (the risk of) oxygen ingress arises from air instead of nitrogen purges on the main burner, fuel gas fired direct reheaters (inline burners) and in the SUPERCLAUS process, where air is introduced to selectively oxidise H2S to elemental sulphur. The oxygen concentrations in these cases are limited and will not lead to sulphur fires.

In case of a SUPERCLAUS or EUROCLAUS installation, oxygen concentration is designed to be just enough to convert H2S and keep the catalyst in an oxidised state. In the reactor outlet there should be just 0.5% to 1.0% oxygen present.

Oxygen concentration during abnormal SRU operation
The risk of a sulphur fire increases as soon as higher concentrations of oxygen enter the system. This happens most often when the main burner is in the start-up phase and/or operated with fuel gas, e.g. during heating up, hot standby or during taking the unit out of operation, when the burner is operated not substoichiometrically, but with excess air.

In the first two modes of operation (heating up with sulphur in the unit and hot standby) this is due to wrong setting of the air to fuel gas ratio. This can occur easily, when the main burner is not operated with a fuel gas of constant composition (preferably natural gas), but with a fuel gas of varying composition (and Mol. Weight, which leads to wrong fuel gas flow metering). A fuel gas analyser (e.g. Wobbe index meter) could be installed to compensate for fluctuations in fuel gas composition and air demand.

In the last mode of operation (taking the unit out of operation for maintenance), it is the intention to gradually convert FeS to Fe2O3 in a controlled way by increasing the oxygen slip from the main burner.
 
Oxygen concentration during abnormal SUPERCLAUS operation
In the selective oxidation stage of a SUPERCLAUS plant, higher concentrations of oxygen can be present during heating up or shutting down or during a bypass of the selective oxidation stage.

A bypass of the selective oxidation stage occurs on high temperature in the catalyst bed or on high inlet H2S concentration and protects the selective oxidation stage during upsets, preventing too high temperatures in the reactor.  In such a case the reactor temperature is maintained by purging the reactor with air. This also keeps the catalyst under oxidising conditions and prevents ingress from sulphur containing tail gas via the back-end of the selective oxidation stage.