Ashes entering the converter interferes with the formation of SO3. In a sulphur burning plant, sulphur filters usually catch the blame. A “polishing filter” can solve this problem. However, in numerous inspections of sulphur melting plants, Sulphurnet has discovered that it is not the filtration that is causing the problem. Process design and operation procedures are more likely to be the cause of the poor quality of sulphur entering the sulphur burner.
In the sulphur melting and purification area of the plant, there is a sequence of individual process steps, from storage of solid dirty sulphur to the purified liquid sulphur. Faults in one of these steps add up to difficulties in the filtration step. Incorrect designs are another cause of difficulties that leads to poor quality sulphur.
The combination of good equipment and proper process parameters is of utmost importance.
As we examine the process, we start with the storage of solid sulphur. Storage can be in the open air or in a covered area (or under roof storage). It is a matter of investment cost choosing which type. Sulphur in open storage is affected by the environment, such as wind, dust and rain, all of which can cause pollution of the sulphur. Sometimes rain can affect the acidity of sulphur drastically, for example in dry times, the water will evaporate causing the acidity to increase.
Under roof storage, however, offers controlled parameters resulting in a much more uniform quality sulphur.
In the case of open storage, the extra acidity will require lime dosing for neutralization and the acidity will vary due to the influences of the seasons. Lime dosing must be controlled frequently – two times per shift is recommended. Over dosing as well as under dosing will affect the process.
Under dosing lime will result in high acidity, which can damage process equipment downstream, such as heating coils and filters as well as filter leafs. In addition, all steel will be affected by the acidity.
When the pH is low, large quantities of lime have to be added to the process. The large amounts of solids will disturb the melting process as well as the filtration process. Because of the high acidity and neutralization reaction, the residence time in the melter will be longer affecting the plant capacity. The high amounts of solids must be kept in suspension by the agitation. When you have a rectangular in-ground pit, suspension will be difficult and sedimentation in the dead corner zones will occur. Sedimentation of the solids will lead to reduction of the heat transfer and thus the melting capacity. Overflow systems will cause settling even more. Also, in ground pits have to be cleaned manually. For cleaning in ground pits, the sulphur has to be cooled and the remains in the bottom have to be removed by hammer drill. This is not only a time-consuming exercise, but coils and walls can be damaged as well.
An above ground round melter system has a better flow pattern and solids will remain in suspension more easily. The heat transfer coefficient is also significantly better. Above ground melters have drain systems, so the remains in the bottom can be transferred to the dirty sulphur pump pit and transferred to the liquid sulphur filters. Reduced sulphur loss creates smaller waste streams and reduced labour time loss. In addition, since the heat transfer is very efficient, smaller melters can be installed, which require less plot space.
After melting, purification and the pre-coat filter comprise the next step in the process. Design is also important in this step and a separate pre-coat tank is necessary. A separate tank is necessary for the following reasons:
- To obtain the right dosage of pre-coat sulphur mix
- To reduce the pre-coat cycle
- To obtain a good even pre-coat layer
- To obtain a high filtration efficiency
- To have a dedicated pre-coat pump
Also, the pre-coat pit should be filled with clean sulphur, not with the overflow of dirty sulphur from the pump pit. The pump used for pre-coating should be dedicated for the pre-coat cycle only – the flow ware is much higher than the regular dirty sulphur feed.
In the filtration section, the piping has to be set up correctly. Pressure drop and pressure fluctuations can influence the formation of the filter cake. As incorrect set up can damage the filter cake and cause a breakthrough of particles.
The filtration process and cycle length mainly depend on the solid load. In the case of high acidity and high volume of lime addition, the cycle length is shorter. Overloading the filters with a bulk of solids may damage the filter leafs and cause difficulties when cleaning the filters. For that reason, it is important to monitor the filtration cycle and filter cake formation with a pressure differential measurement device. It is important for filter leafs and filter leaf gaskets to be regularly inspected.
When the setup is correct, and all operation procedures are followed, ash levels of 10.15 ppm can be achieved. This is an acceptable level in a sulphur burning plant.
For security reasons, a polishing filter can also be installed; however, one should realize that a polishing filter is there for protection reasons, in case of emergencies, and not for the constant removal of high levels of impurities. A good set up of a polishing filter can reduce the ash levels down to 5-8 ppm.
Good operation procedures and using improved equipment for melting and purification of sulphur will not only reduce pollution of the catalyst bed but will also contribute to a cleaner and safer environment in the sulphur melting section.
If you want to improve your sulphur processing plant or need more information, contact us.
Author: Jan Hermans.
Article published in Sulfuric Acid Today Fall/Winter 2012