Treatment Options for Molten Sulfur Storage and Transfer Vents
Molten sulfur is present in an ever widening presence in many industries. Besides the traditional sources of sulfur, such as refineries and natural gas plants, and the well known users of sulfur, such as sulfuric acid plants and fertilizer production, sulfur has become a common feedstock in more conventional chemical production such as tire and rubber additives, polymer production, and even food products. The high likelihood of the presence of hydrogen sulfide (H2S), because of the source of most molten sulfur (desulfurization of natural gas and crude oil) further complicates the handling, storage, and transfer of molten sulfur, which presents its own challenges due to its unique physical and chemical characteristics in its many forms. Because the transfer and storage of molten sulfur involves emptying and filling tanks, railcars, and pits, air is displaced, carrying with it H2S evolved from the molten sulfur. Hydrogen sulfide, in addition to being a combustible gas, is also a significant safety hazard, because of its toxic nature, and presents dangers to workers in the area. Molten sulfur is produced in ever increasing amounts, because of increased energy demands, higher energy costs making the processing of sour crudes and natural gas more feasible, more stringent emissions regulations, and the requirements of lower sulfur fuels. As a result, safe handling of the product has become even more important.
To sweep or not to sweep…
Controlling the emissions from a sulfur storage system can be done by two primary approaches:
- Sweeping the vapor space. Or
- Sparging gas in the molten sulfur.
One issue that plays the biggest part in how sulfur storage or transfer vents are handled is the gas used to sweep the vapor space of the tanks or transfer vessels. The primary two options are air or nitrogen, and there are a number of factors that will determine which is used.
Air—the sweep gas of choice
For many reasons, most sulfur storage and transfer is swept and vented using air. It is generally the least expensive, the most available, safest, and allows the largest number of treatment options for handling the H2S present in most sulfur vents.
A sulfur storage tank or transfer system is a very dynamic system, with many compounds and conditions interacting to form secondary compounds that can cause safety concerns. Foremost of these is iron sulfide (FeS), which can form from the reaction between hydrogen sulfide and the iron of the storage vessel. Normally, this product forms at the phase separation at the surface of the molten sulfur, where there is the highest level of turbulence and contact. Iron sulfide, however, only forms in appreciable levels under anaerobic (without oxygen) conditions, such as would be found in unvented tanks, or tanks swept with nitrogen. If left in an anaerobic condition, iron sulfide does not pose a safety risk, and actually is a beneficial corrosion barrier on carbon steel surfaces. It is when these tanks are opened to the atmosphere, for maintenance or cleanout, that the problem surfaces. Iron sulfide is a pyrophoric material, meaning it can spontaneously combust in the presence of oxygen. That, and the fact that sulfur is also combustible, can produce catastrophic results if not handled properly.
While the formation of iron sulfide is a primary reason given that air is used for the sweeping of sulfur pits or storage vessels, there is a reason that air would specifically NOT be chosen. As mentioned earlier, hydrogen sulfide is a combustible gas. H2S evolved from molten sulfur can reach concentrations in air above the lower explosive limit (LEL), at which point an explosive gas mixture is present. Because safety considerations normally require that air streams have a combustible gas content less than 25% of the LEL (or up to 50% LEL with safety monitoring and alarms), sufficient volumes of air must be swept through the tank or pit to keep the H2S content low. Air is inexpensive, and in large supply, however the boosting and moving of large volumes of air can require large blowers, motors, and ductwork on large sulfur storage systems. Alternatively, steam eductors can be used, pulling air through into the vapor space, effectively sweeping the vapor space. The downside of steam educting is the handling of the hot wet vent gas, which can cause sour condensation issues, with the corresponding safety and corrosion problems.
Or not to sweep…..
Sparging Air—a more active approach
Often, the goal of molten sulfur venting is more than simply containing the vapor space and hydrogen sulfide that is there by simple equilibrium- there may be a need to actively remove the hydrogen sulfide from the sulfur itself. In these cases, there are a number of approaches that can be used to drive the hydrogen sulfide from the sulfur, which will be discussed in more detail later. One of the first, and most commonly used is simply air sparging. Air is evenly introduced into the molten sulfur through a network of sparge piping at the bottom of the tank or pit. An air blower, sized to sparge the air as well as provide pressure to the final destination of the vent gas is needed. As with simple air sweeping, sufficient air is needed to keep the air well below the LEL, making this approach much more power consuming, and also requiring that the pit or tank be capable of holding some pressure, or requiring a “push-pull” arrangement with a vent blower to provide the boost to the vent treatment system. Both approaches complicate the sulfur storage system, but the approach does provide significant reduction of the hydrogen sulfide in the molten sulfur itself.
While it does have the disadvantages given above, nitrogen is often used as a purge or blanket gas in sulfur pits or storage facilities. Its use is normally dictated by either the desire to eliminate flammability issues in molten sulfur with high levels of hydrogen sulfide, or the need to have minimum vent gas flow, where H2S in the vapor space can exceed the LEL in an air sweep. Another reason for the use of nitrogen is to minimize oxygen in the vent gas, which may be beneficial for certain vent treatment systems. Overall, the use of nitrogen will be done only where its benefits override the safety issues caused by the formation of the pyrophoric iron sulfide.
Overall, the handling options will be determined by the desired goal. If the desired goal is the purification of the molten sulfur, then the options will be quite different than if the goal is simply to safely vent the vapor space and sweep the tank or pit.
Goal-Purified Molten Sulfur
If removing hydrogen sulfide from the molten sulfur is the goal, then the treatment options can be broken into two categories:
- Liquid Phase Treatment
- Air Sparging
- Or third, a combination of the two
Liquid Phase Treatment
Liquid phase treatment entails introducing a liquid additive into the molten sulfur feed which acts as either a reactant to remove the hydrogen sulfide, or as a promoter to drive the H2S from the molten sulfur easier. There are many products on the market that accomplish sulfur purification through this method, many which use patented methods of either injection, dispersion, or agitation to promote the hydrogen sulfide degassing from the molten sulfur. The product, which is some cases contains reacted byproducts, is normally immiscible in the molten sulfur, and is separated from the molten sulfur by skimming or settling. In the case of catalytic or promoting type additives, the product can be recycled and reused. Reactant type products will require the disposal of a spent byproduct or further treatment.
As discussed earlier, sparging air into the molten sulfur pits or storage tanks can accomplish two goals in one system; the removal of hydrogen sulfide from the molten sulfur, and the dilution of the H2S in the vent system. Simple sparging requires a fairly large volume of air to accomplish adequate gas-liquid contact, especially in shallow pits with a large surface area to sparge. The large volumes of air do keep the hydrogen sulfide concentrations low, but tend to make the air handling systems large, and power high for the boosting of the sparge air.
A Combined Approach…
There are systems available that use both of the above approaches to provide a system that takes advantage of the liquid phase additive’s ability to promote the removal of hydrogen sulfide from molten sulfur, with sparge air’s ability to rapidly remove the H2S from the liquid. These approaches will often also use proprietary contact devices or configurations to optimize the contact of the air, the liquid additive, or both, and will also often include recycle of the liquid additive after separation. These systems capitalize on the liquid additive’s ability to rapidly free the H2S from the molten sulfur, and the air’s ability to remove it quickly, reducing the time and volume requirements that either system alone would require. Air flows needed can be greatly reduced, both because of the decreased hold of the molten sulfur on the hydrogen sulfide, and in many cases by the gas-liquid contacting devices or methods used by these system, which promote intimate and turbulent contact of the gas and liquid. Again, as with the strictly liquid approaches, the vent gas streams from these processes will require further treatment for the removal of the hydrogen sulfide.
Goal-Vent Containment and Treatment
As can be seen, even the removal of hydrogen sulfide with the intent to purify the molten sulfur requires the eventual handling of a vent stream containing H2S (and SO2). So, for the vast majority of handling systems, the treatment or handling of a vent stream rich in hydrogen sulfide will be required. These systems can be broken down as:
- Hydrogen Sulfide Destruction/Conversion
- Hydrogen Sulfide Recycle
- Hydrogen Sulfide Removal
Destruction or Conversion Option
This involves either the thermal or catalytic destruction of the hydrogen sulfide present in the vent stream, converting H2S to another sulfur containing gas component. This usually is accomplished by either sending the vent gas stream to an existing flare, incinerator, or using it as combustion air in an existing burner or boiler. All of these options will present an emission source containing sulfur dioxide (SO2), the primary combustion product of hydrogen sulfide. In systems other than flares or tailgas incinerators that are designed for burning hydrogen sulfide, there may be corrosion and material issues in the burners and downstream equipment, as sulfur dioxide in a wet gas stream presents significant corrosion opportunities.
Recycling the Vent
Popular in applications with existing Claus or other large scale sulfur recovery units (SRU), often times the H2S rich vent stream is simply recycled back into the front end of the SRU, where it usually represents a very small increase to the overall hydrogen sulfide load to the unit.
Hydrogen Sulfide Removal
The last option, actual removal of the hydrogen sulfide, can be accomplished by a number of different approaches, with the best option dependent on the actual amount of H2S that is present in the vent stream. The primary options for H2S removal are:
- Non-regenerable Products (“Scavengers”)
- Regenerable Products
Scavenger Vent Treatment Options
In cases where the total hydrogen sulfide present in the vent stream averages less than 200 lbs/day, a non-regenerable scavenger may be the best option. These system, which use either a solid or liquid chemical reactant to remove the H2S and react it to form a different sulfur containing compound, which must be removed and disposed.
Scavenger systems tend to be relatively simple, low capital installations. However, chemical costs are 10–20 times the chemical cost of regenerable processes per unit of sulfur removed. Disposal costs and logistics can be significant, especially at higher loads. For these reasons, the scavenger products are limited to 200 lb/day sulfur loads, unless other circumstances make it economical at higher loads.
There are generally two types of scavenger products available; liquid or solid materials.
Liquid scavengers work by utilizing some contacting device (either a sparged or spray tower, packed bed, venturi contactor, or other gas-liquid contactor) to contact the liquid and the gas stream, where the hydrogen sulfide is absorbed, and reacted to form another sulfur containing compound in the liquid stream. Typically, these products are water-based, which means that they must operate at a temperature below the boiling point of water, which also means that any sulfur vapor that is present in the gas stream will likely condense and be likewise scrubbed from the gas stream, so particular care must be exercised in the selection of non-fouling contactors for the gas scrubbing.
Liquid scavengers are typically at the higher operating cost end of the scavenger cost range, often costing between $5–10/lb H2S removed, limiting them to the less than 100 lbs/day H2S range. This cost does not include the cost of disposal of the liquid waste product, which can as much as double the total treatment costs.
Solid scavenger products generally consist of granular materials on which the hydrogen sulfide either is adsorbed and reacts with a reactant coated on the media, or is simply adsorbed and held in the pore structure of the media. Solid scavengers available include:
- Activated carbon
- Iron sponge
- Iron-oxide based media
- Zinc oxide based media
- Solid oxidizer media
These all use the same basic principle of adsorbing the hydrogen sulfide and either retaining it or reacting it with a compound that is coated or formed on the surface of an inert substrate.
Operating costs for these products are generally lower than liquid-based scavengers by a factor of 2, are still 10 times that of regenerable products. Their drawback is that the changeout is messy and time consuming, and there is a significant volume of spent material to dispose after each changeout.
More recently, regenerable, non-thermal processes for the removal of hydrogen sulfide have taken the forefront of the “medium scale” (200 lb/day to 20 LTPD sulfur removal) world. These processes include:
- Liquid redox processes
- Direct oxidation
- Non-aqueous liquid based processes
All use different methods of removing hydrogen sulfide and converting it to a stable product, all while not having consumed the primary reactant (or by easily regenerating it completely).
These systems all generally produce solid elemental sulfur as their byproducts, which can either be recycled back into the molten sulfur pit, or used as an agricultural product. Because of their higher capital costs and system complexity (compared to scavenger systems), regenerable system typically not been used for sulfur tank vent treatment, but their very low operating costs (typically, $0.15–$0.30/lb H2S removed) make them very attractive for larger applications where their capital costs can quickly be recovered by the very low operating costs.
Increasing regulations on the emissions of hydrogen sulfide and sulfur dioxide, as a combustion product of H2S, is forcing facilities that produce and handle molten sulfur to treat emissions from their sulfur storage and transfer facilities in a way that does not produce SO2 emissions. As the production of molten sulfur worldwide increases, more and more uses of that sulfur product are bringing new handling and storage issues to facilities that did not historically have those issues. Consequently, there are more issues today with the safe handling of sulfur tank and transfer facilities than ever before. Thankfully, there are also more viable options for the treatment of these vent streams, with variations on the sweep gas, type of venting, and treatment or handling of those vent streams, which makes it possible to tailor a solution to a wide range of requirements. It is critical that the user define their requirements, and work closely with a provider that can design and implement a system that meets the economic, safety, regulatory, and process requirements in an integrated package.