Mar-2016

Importance of heat maintenance in sulphur recovery units

While sulphur recovery in an SRU depends on the condensation and removal of elemental sulphur, its unintended condensation and accumulation can present major problems.

Forough Fatemi and Marco Van Son
Jacobs Comprimo Sulfur Solutions

Article Summary

The main areas where this is expected to happen in the SRU equipment include the waste heat boilers, reheaters, sulphur condensers and coalescers. Unintentional condensation and accumulation of sulphur in an SRU can lead to problems such as obstruction of process gas flow, blockage by solid sulphur, fire and equipment damage to name a few. The accumulation of liquid sulphur in an SRU can have various reasons such as wrong piping design in terms of sloping and routing, presence of catalyst dust and debris, loss of plant heating, malfunction of tracing or jacketing and inadequate design of the steam heating system. Experience at the sulphur processing facilities and transport lines around the world reveal that the design of the steam heating systems relies considerably on the industry myths instead of engineering design. This paper examines the main causes and subsequent risks of accumulation of liquid sulphur in the SRUs. Several preventive measures are mentioned to minimise the risk of liquid sulphur accumulation. While all these methods are considered equally important, this paper mainly focuses on the shortcomings of the steam heating systems which are the result of reliance on legacy standards instead of rigorous engineering.

Introduction
From time to time operational issues in an SRU are related to the accumulation of liquid sulphur. In the 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 the subsequent stage(s). 

The condensers do not remove all of the sulphur. It is sufficient to condense and remove as much sulphur to stay above the sulphur dew point in the following catalytic stage. In sub-dewpoint processes such as CBA, Sulfreen and MCRC, there is some deliberate condensation of sulphur in the converters to force the Claus equilibrium closer to completion.

If the liquid sulphur is not removed properly from the SRU it can accumulate in the equipment and piping. Accumulation of the liquid sulphur can impede the flow of process gas in the unit. It can also cause sulphur fires and equipment damage in the presence of oxygen (>10 vol%) and a source of ignition. In addition, the accumulated liquid sulphur can freeze if the plant is let to cool down, or can turn into “sulphur concrete” due to poor housekeeping practices. Solid sulphur not only poses the risk of fire, it can also be difficult and time consuming to remove.

For continuous flow and removal of liquid sulphur in an SRU it is essential that all piping and equipment are designed and sloped properly. The impact of increased viscosity at temperatures above 158°C or at lower concentrations of dissolved H2S must be considered. It has to be ensured that catalyst dusts and debris do not remain in the plant after loading. Furthermore, the SRU must be equipped with an adequately designed heating system to keep the liquid sulphur within a specific range of temperature during all operating conditions. In addition, the heating system is expected to be able to melt the sulphur in case of a freeze up. The most common heating systems utilised in the SRUs include electrical or glycol tracing or some type of steam heating system.

Many Sulphur Recovery Units around the world rely on steam heating systems for the purposes mentioned above. The flowing sulphur must be maintained within an approximate temperature range from 120°C at which sulphur freezes to 160°C at which sulphur viscosity starts to increase rapidly due to polymerisation. When troubleshooting the performance and reliability of a steam heating system the focus is typically on the method of heating (jacketed piping, bolt-on jacketing or tube tracing). However, experience shows that most often the problems with the steam heating systems are not related to the type of the system. The majority of malfunctioning steam heating systems have issues with the steam supply to and condensate removal from the system. These inadequacies are the result of certain rules of thumb and legacy standards which are applied in the design of steam heating systems instead of concrete engineering rules. Finally, some of the wrong beliefs which have crept into the design of the steam heating systems and lead to under-performance and increased capital and operational costs are discussed.
 
Risks of accumulated liquid sulphur
Sulphur fire
The best known risk of accumulated liquid sulphur is a sulphur fire which can cause severe damage to the SRU equipment. The condenser tubes and the mist pads are especially vulnerable. Sometimes the fire is limited either by the available sulphur or oxygen. In this case the sulphur fire will have a noticeably higher temperature but perhaps with no damage. Note however, that in every sulphur fire not only SO2 but also SO3 and sulphuric acid will be produced. Sulphuric acid can eventually lead to extensive corrosion damage in lines and equipment.

For a sulphur fire to happen oxygen has to be present in sufficiently high concentrations (>10 vol%). At lower concentrations of free oxygen in the process gas the sulphur will be oxidised to SO2 thereby generating extra heat of oxidation.

Besides sulphur vapour (from liquid sulphur) and oxygen in sufficiently high concentrations (>10 vol%) there has to be an ignition source. The following sources of ignition have been identified:
• When the temperature in the process is above the auto-ignition temperature of sulphur. The auto-ignition temperature is most often mentioned as 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.

The following sources of oxygen are identified in the SRUs:
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 is increased if air is used instead of nitrogen for the main burner purges or if fuel gas fired direct reheaters (inline burners) are used. In the SUPERCLAUS® process, oxygen enters the plant through the oxidation air which is introduced for the selective oxidation of H2S to elemental sulphur. The oxygen concentrations in all of these cases are limited and will not lead to sulphur fires.

In the SUPERCLAUS or EUROCLAUS® installations, oxygen concentration is precisely controlled such that the H2S is converted to elemental sulphur and the catalyst is maintained in an oxidised state. The oxygen concentration in the reactor outlet is controlled at 0.5 vol% during normal operation.

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, for example, during heating up, hot standby or during taking the unit out of operation, when the burner is operated with excess air.