The State of Iron-Redox Sulfur Plant Technology
New Developments to a Long-Establishesd Process Technology

By Douglas L. Heguy and Gary J. Nagl
Gas Technology Products
846 Algonquin Road Suite A100
Schaumburg, IL 60173

Key Words

Iron-redox, LO-CAT®, hydrogen sulfide, chelates, chelated iron, sulfur

Abstract

The iron-redox process has enjoyed commercial success for over 25 years, generally in applications requiring sulfur removal capacity below 20 tons per day. Key process benefits include high H2S conversion efficiency, significant turndown flexibility, and ability to treat a wide range of gas compositions, and environmentally innocuous process and products. The process has also been know to consume expensive chemicals, produce “low-value” sulfur, and plug. This paper reviews the status of the technology, explains how the operating issues are being addressed in commercial practice, and provides a glimpse of improvements that are in the final stages of development.

Introduction

The family of liquid redox processes that has been developed since the 1920’s is best represented, currently, by the “iron-redox process” or “chelated-iron” process. This technology has served its clients well for more than 25 years. Units typically achieve 99.9+% H2S removal efficiency, treat a wide variety of gas types over a wide variety of operating conditions, have substantial turndown capability on H2S concentration and gas flow and produce innocuous products and by-products. No wonder more than 200 such units have been licensed around the world – a technical and commercial success by almost any definition!

The iron-redox technology is typically applied to gas streams requiring less than 20 tons per day sulfur removal capacity, unless operating conditions limit use of other sulfur plant technologies, such as Claus. In such cases, Iron-redox may still be the best sulfur removal technology. Highly variable gas and low H 2S concentration are examples. Iron-redox plants as large as 80 tons per day are in commercial use.

Also well known are the operating issues that have been associated with this process, such as high chemical cost, chemical degradation, plugging, foaming, production of “low-value” sulfur, inability to treat high-pressure applications.

How are the leading developers of this technology addressing these operating issues? What improvements are on the horizon? With 25 years of commercial application, is iron-redox technology still competitive in today’s commercial environment?

The answers are clear. The iron-redox process technology has been improved continuously over the last 25+ plus years. Numerous testimonials confirm that the technologies/solutions described in this paper are being successfully applied in commercial applications. As further evidence, there continues to be significant commercial activity for this well-developed, well-proven technology in applications for natural gas and associated gas processing, geothermal plants, refinery fuel gas, municipal odor control, landfill gas, and recently, municipal waste gasification, as well as a host of others. Furthermore, improvements in the final stages of development will benefit the users of this process technology in the near-term future, and ensure its long-term commercial viability.

This paper will discuss the benefits of H2S removal, provide a basic description of iron-redox technology, a description of the solutions being commercially employed to successfully address past and current operating issues, and, finally, explore the innovations that are on the horizon.

Background

Removal of hydrogen sulfide (H2S) from gas streams has been an issue for the energy industry since its inception. Hydrogen sulfide is an extremely toxic, corrosive and odorous gas, causing safety and materials issues in its unaltered form. After burning, the H2S is oxidized to sulfur dioxide (SO2), a major player in acid rain and greenhouse gas emissions for the downwind neighbors. So, while sulfur removal from gas streams has been an issue since the inception of the hydrocarbon-based energy industry, it also continues to get ever-increasing attention as an environmental issue.
Iron is an excellent oxidizing agent for the conversion of H2S to elemental sulfur. However, due to the very low solubility of iron in aqueous solutions, the iron had to be present in the dry state (iron sponge) or in suspensions (the Ferrox process) or compounded with toxic materials such as cyanides. In the 1960’s development work was begun in England to increase the solubility of elemental iron in aqueous solutions. This work led to the introduction of CIP (Chelated Iron Process). However, it wasn’t until the late 1970s that a system of chelates was developed that had sufficient oxidative resistance to be sufficiently stable and be commercially successful. This development work led to the successful commercialization of the iron redox process.

Iron-redox process

In this process, iron, in its ferric state (+3), is held in solution by chelating agents. The intent of the process is to oxidize hydrosulfide (HS-) ions to elemental sulfur by the reduction of the ferric (Fe+3) iron to ferrous (Fe+2) iron, and the subsequent reoxidation of the ferrous ions to ferric ions by contact with air. The chemistry of all chelated iron processes is summarized as follows with (l) and (v) representing the liquid and vapor states, respectively;

Equations 1 and 2 represent the absorption of H2S into the aqueous, chelated iron solution and its subsequent ionization, while equation 3 represents the oxidation of hydrosulfide ions to elemental sulfur and the accompanying reduction of the ferric iron to the ferrous state. Equations 4 and 5 represent the absorption of oxygen into the aqueous solution followed by oxidation of the ferrous iron back to the ferric state.

Equations 3 and 5 are very rapid. Consequently, iron-based systems generally produce relatively small amounts of by-product thiosulfate ions, and, in properly designed units, air streams can actually be processed. However, equations 1 and 4 are relatively slow and are the rate controlling steps in all chelated iron processes.

It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction, the iron cancels out. So why is chelated iron required at all, if it doesn’t take part in the overall reaction? The iron serves two purposes in the process chemistry. First, it serves as an electron donor and acceptor, or in other words, a reagent. Secondly, it serves as a catalyst in accelerating the overall reaction. Because of this dual purpose, the iron is often called a “catalytic reagent”. The chelating agent(s) do not take part at all in the process chemistry. The sole purpose of the chelating agents is to solubilize iron in water, thus making it possible to have a solution of iron.

Iron-based, liquid oxidation has developed into a very versatile processing scheme for treating gas streams containing moderate amounts of H2S. Advantages of these systems include the ability to treat both aerobic and non-aerobic gas streams, removal efficiencies in excess of 99.9%, essentially 100% turndown on H2S concentration and quantity, and the production of innocuous products and by-products.

The two most common processing schemes encountered in iron-based, liquid oxidation systems are illustrated in Fig. 1 and 2. Fig. 1 shows a “conventional” unit, which is employed for processing gas streams, which are either combustible or cannot be contaminated with air such as carbon dioxide, which is being treated for beverage purposes. In this scheme, equations 1 through 3 are performed in the Absorber while equations 4 and 5 are performed in the oxidizer. Fig. 2 illustrates an “autocirculation” unit, which is used for processing acid gas (CO2 and H2S) streams or for other non-combustible streams, which can be contaminated with air. In this scheme, equations 1 through 3 are performed in the “centerwell” which is nothing more than a piece of pipe open on each end. The purpose of the centerwell is to separate the sulfide ions from the air to minimize by-product formation. The volume within the centerwell is essentially the same as the absorber in a conventional unit. The other unique feature of the autocirculation scheme is that no pumps are required to circulate solution between the centerwell (absorber) and the oxidizer. In these units there is a larger volume of air than acid gas; consequently, the aerated density on the outside of the centerwell is less than on the inside resulting in a natural circulation from the oxidizer into the centerwell.

The sulfur product is typically a sulfur “cake”, with entrained water and catalyst solution. The entrained catalyst solution is effectively a “blowdown” stream for the aqueous process. (The catalyst solution is itself non-toxic and non-hazardous.) Depending on the type of sulfur filter used, the sulfur cake can have a sulfur concentration between 30% (bag filter) to 90% (filter press). The vacuum belt filter is quite common, and produces a sulfur cake that is 60-65% sulfur.

Developments in Iron-Redox Technology

Practitioners of iron-redox technology have as many as 25 years of experience designing process solutions for a wide variety of applications. That can be a very good thing, but it also means that a lot that is “known” about iron-redox characteristics is 25 years old. However, this technology has, indeed, continued to evolve to address the issues of high chemical costs, chemical degradation, plugging, foaming, production of “low-value” sulfur, and an “inability” to treat high-pressure gas.

High Chemical Cost

It is not a simple matter of expensive catalysts used in the process. Rather, chemical costs in the iron-redox process are a function of several variables: catalyst concentration, chemical degradation and chemical recycle.

Catalyst Concentration

Chemical concentration is an issue because the sulfur produced from the process is in the form of a sulfur “cake,” which has significant amount of entrained liquid. The filtered sulfur product exiting the process typically runs between 60% and 80% sulfur, but can run as low as 30% sulfur. The remaining amount is a combination of wash water and catalyst solution. Clearly, the more concentrated the catalyst solution, the more chemical is likely to leave the process with the sulfur.

The main licensees of the iron-redox technology practice two very different philosophies regarding catalyst concentration. The difference in catalyst concentration between the two philosophies is on the order of 20-40x, depending on actual configurations and applications. This difference in chemical concentration has aspects of a capital/operating trade-off, as the more concentrated solution clearly has benefits in vessel and pump size (capital cost), at the expense of operating cost (chemicals). However, that analysis very much understates the impact of the catalyst concentration issue.

The scale factor on capital cost is moderated by several factors. 1) Vessel size is moderated by circulation rates, and 2) there are significant elements of the capital cost that are not affected significantly by this relative equipment size, such as license fee, engineering design, project management, start-up, commissioning, and to a lesser degree, installation. So, while there is likely to be a difference in capital cost between the two philosophies, it will be significantly moderated.

Chemical cost difference will be moderated by the effectiveness of the sulfur wash and chemical recycle. The catalyst cost difference won’t be 20-40x, but it is likely to be at least several multiples. Current practice suggests that a chemical cost difference of 2 to 3 times exists between the two approaches.

Looking at catalyst concentration as solely a cost issue overlooks the operating benefits with the more dilute system, namely:

• Higher capacity to solubilize products and by-products, and
• Moderated response to process changes.

Depending on the feed gas composition, the iron-redox catalyst solution will have varying amounts of thiosulfates, carbonates and bicarbonates and oxalates, in addition to the iron and chelates. It is reasonable that the more dilute catalyst solution will have higher capacities to keep these other chemical species in solution, with less likelihood of creating precipitates, and less likelihood of operating problems resulting from precipitation. In fact, it is these solubility limits and the resulting operating problems that often limit the amount of chemical recycle that can be achieved in efforts to reduce chemical loss. (This issue will be covered in more detail in the following two sections on chemical degradation and chemical recycle.)

The larger catalyst volumes and lower chemical concentrations combine to create a “fly wheel” effect, so that changes to the inlet gas conditions result in greatly moderated changes to the sulfur plant operating conditions, and a commensurate reduction in operator attention.

There are iron-redox clients that operate the two different systems side-by-side that can and have testified to the difference in operating cost and operability between the two systems.

Chemical Degradation

The catalyst regeneration process requires oxidation of the iron catalyst. Unfortunately, chelates are oxidized at the same time. This creates two issues: the need to replace the degraded chelates and the existence of the products of the degradation reaction, which tend to be oxalates. The build-up of the oxalates in the system will limit the amount of chemical recycle that can be achieved before precipitation of the oxalates occurs.

However, there has been significant development work to control the rate of chemical degradation.

Although the iron-redox process is about 25 years old, research to understand and reduce chelate degradation has been ongoing for more than 40 years and continues today. The catalyst systems employed by the leading licensors tend to be proprietary, some are patented. For example, one technology supplier has used a patented blend of chelates to improve stability over a broader range of operating conditions, offering better oxidative resistance than can be accomplished by a catalyst system that is based on only one type of chelate.

In addition, there is significant work done on identifying oxygen scavengers. One elegant solution is the controlled production of thiosulfate ions, which act as an oxygen scavenger in the system and effectively reduces chelate degradation rates.

The chemical equation for the production of thiosulfate (potassium thiosulfate, assuming potassium hydroxide is used to maintain pH) is shown in equation 7.

The trick is to produce thiosulfate in the right amount. Over production of thiosulfate will over consume the caustic used to maintain pH. Underproduction will result in excessive chelate degradation. Controlled thiosulfate production can make a significant contribution to lowering chemical cost and improving process performance.

In summary, prospective buyers of iron-redox processes should ask their supplier about their chelate degradation control mechanism. Best practice includes both oxidation resistant chelates and controlled production or introduction of an effective oxygen scavenger.

Chemical Recycle

The ability to recycle chemicals reduces chemical cost. The more concentrated catalyst system requires more chemical recycle to be economic. However, too much recycle leads to a build-up of unwanted oxalates and other chemicals, which can lead to precipitation and operating problems. Thus, there is a limit to how much chemical recycle can close the chemical cost disparity associated with a large difference in catalyst concentration.

Summary of Catalyst Concentration-Related Issues:

The dilute catalyst system offers significant operating cost benefits, as there are fewer chemicals leaving the system and fewer barriers to recycling chemicals. In addition, there are significant operating benefits, as there is a significantly lower amount of oxalates and other salts being recycled back into a catalyst solution that has a higher capacity for the recycled salts. This means that they dilute system is far less likely to incur operating and maintenance problems associated with precipitates. Also, the relatively large volume attributed to the dilute catalyst system creates a “fly wheel” effect that moderates the impact of variations in the inlet gas on the operation of the sulfur unit.

So, while chemical costs can vary significantly, depending on gas to be treated and sulfur wash and recycle designs, chemical costs for many systems employing the dilute catalyst concentration are in the range of $175-$250/ton. Comparable costs for the more concentrated system, in similar applications, tend to be a factor of 2-3 times that cost.

In conclusion, prospective buyers of modern iron-redox systems should insist in good chemical cost guarantees with his unit. The reputable and experienced firms in this area offer good experience, can estimate chemical usage based on feed gas and equipment configuration with reasonable accuracy, and will back their analysis with good guarantees.

Plugging

Plugging problems have been solved by eliminating packed towers, and by incorporating standard piping designs that limit dead spots and areas of restricted flow—not rocket science, just good basic design discipline.

Some iron-redox suppliers insist sulfur is formed in the oxidizer, and the return catalyst stream can run through a filter to prevent sulfur from going into the absorber to prevent plugging. This design will plug. The sulfur reaction is fast: sulfur will form in the absorber, and the packed tower will plug, regardless of the effectiveness of any filter in the catalyst return line. Prospective customers of this technology need to insist that their design be void of any fixed surfaces that can offer sites for sulfur build-up, or alternatively, insist on good sulfur clean-out provisions as part of the design.

Additional steps to prevent plugging include proprietary heat exchanger designs that minimize plugging. Newly designed absorber spargers and improved oxidizer spargers have significantly improved maintenance requirements and decreased plugging in the oxidizer. Oxidizer vessels and/or separator vessels now have cone-shaped bottoms with additional air injection systems to maintain fluidity in the cone.

For absorber designs that include counter-current gas/liquid flow in which the catalyst solution is sprayed into the column, plugging due to sulfur carryover is being controlled with properly designed and positioned spray nozzles and knockout pots.

Foaming

Chelated-iron systems can foam when two conditions are present: during the initial plant start-up and when a large amount of heavy hydrocarbons enter the system.

In the first case, the surface tension properties of the fresh catalyst solution can lead to foaming issues in the first few days of operation. This is an issue only with the initial start-up with fresh solution, and can easily be handled by following the start-up procedures. This will not be an issue with subsequent start-ups with aged solution.

Continuous incursions of small amounts of liquid hydrocarbons are frequently experienced with no adverse effect on the operation of a unit; however, the introduction of large amounts of liquid hydrocarbons can present foaming problems. The good news is that the unit will continue to operate and treat the gas, but the operation will be “messy”. It is unlikely the plant will need to shut down. (This is in contrast to most Claus-type reactor systems with fixed catalyst beds that would likely experience catalyst fouling, and be forced to shut down and replace catalyst following a similar process upset.) Where this is seen to be a possibility, suitably designed knockouts and separators should be incorporated into the gas inlet piping design. However, should foaming occur, “designer” surfactants1 have been developed, which alleviate the foaming symptoms caused by the introduction of large amounts of liquid hydrocarbons.

Production of “low-value” sulfur

In this world of excess sulfur production due to the large amount of by-product sulfur being produced, when was the last time anyone produced “high-value” sulfur?

It is true that typical iron-redox sulfur has entrained water and residual catalyst in sulfur cake form. The sulfur content of the cake can range from 30% sulfur to 90% sulfur depending on the type of sulfur filter incorporated. Though sulfur in this unmelted “cake” form is typically undesirable as a chemical feedstock; it actually has superior properties as a sulfur fertilizer when compared to typical “pure” sulfur produced by more traditional processes.

One California chemical manufacturer typically handles 20,000 tons of iron-redox sulfur per year, and would like more. The fact that iron-redox sulfur was formed in the liquid phase at low temperature means that the sulfur particle is amorphous (softer) than solidified molten sulfur, and has a smaller particle size, for faster reaction in the soil. In addition, the other catalyst elements in the iron-redox solution, and present in the sulfur “cake” (iron, chelates), are micronutrients in their own right and sold as such by several suppliers of agricultural products.

In order to ensure a market for iron-redox-produced sulfur, commercial proposals have been made that include concurrent fertilizer market development activity.

Commercial sulfur purification systems can convert iron-redox sulfur to molten sulfur of 99.9% purity. However, the appearance (color) will still be slightly degraded due to the presence of iron polysulfides. Where development of a fertilizer market is an option, it is preferable to develop that opportunity rather than install equipment to purify the sulfur, as the cost incurred is generally not matched by a corresponding increase in sulfur value in this market of excess sulfur.

Inability to treat high-pressure gas

Operation of aqueous-based liquid redox systems at high pressure has been a problem due to difficulties with keeping the liquid circulation pumps running. Circulation pumps were always specified as ANSI, open-impeller centrifugal pumps. The logic being that closed-impeller pumps would plug with sulfur particles or possibly erode. Consequently, for high head applications in which open impeller pumps would not apply, plunger type pumps were chosen. The plunger pumps had no difficulty supplying the required head, however, seal rings had extremely short lives. To solve this problem, a multi-staged, closed-impeller, centrifugal pump was installed in one high-pressure application with excellent results. The pump has been in continuous operation for approximately 4 years without any signs of plugging or erosion. Since that installation, similar installations at even higher pressure (as high as 1,000 PSI) have had similar success. For all future high-pressure applications, closed-impeller single or multi-stage centrifugal pumps will be specified. Obviously, the original concern about plugging had no basis.

While iron-redox systems can now be designed to direct treat high pressure gas streams, there will still be occasions where it will make sense to direct treat the high pressure gas with a conventional amine unit and treat the resulting acid gas with an iron-redox autocirculation unit (Figure 2). In applications that benefit from the additional absorption capabilities of a conventional amine unit, such as removal of CO2, along with the required infrastructure, utilities, and experience to run a conventional amine unit, the simplicity of the iron-redox autocirculation can be an advantageous process solution. About 30% of the installed iron-redox plants are this configuration.

The Future of Iron-Redox Systems

While many of the operating issues associated with iron-redox systems have been successfully addressed in current designs, significant advances are being made in the state of the technology which will ensure a bright future well into the next century. Development activities are focusing on improved capital cost and design flexibility, focused on development of improved mass transfer devices in the oxidizer, and improved iron-redox sulfur market development.

An improved mass transfer device in the oxidizer is scheduled for commercial demonstration in early 2003. Successful demonstration of this technology will result in reducing the size of the oxidizer by as much as a factor of ten, creating significant cost, space and weight differences relative to current systems and provide significant design flexibility. Also, the ability to modularize portions of the plant should also create economies in the design and installation of the new units.

Finally, expansion of the successful marketing of iron-redox sulfur can pay large dividends to the operator of the iron-redox plant. In addition to the significant agricultural benefits available to the surrounding agriculture industry, the successful market development of this sulfur should reduce sulfur transport and disposal costs, reduce future liability from disposal issues, and improve plant-permitting prospects for new and/or expanded facilities.

Summary

In summary, the iron-redox technology has been continuously improved over 25 years of successful commercial application. Solutions to most of the “operating issues” associated with the technology have been developed and are being successfully practiced in commercial applications. The most robust iron-redox plant operations will incorporate, as part of the design:

• low concentration iron catalyst (around 1,000 PPM)
• catalyst solutions that incorporate “chelate blends” for enhanced stability and oxidative resistance
• avoidance of “packed” columns for gas-liquid contacting
• “slurry-friendly” piping designs.

New developments on the horizon promise improvements in design flexibility and reduced capital cost.

The implication for operators that must remove sulfur from gas streams is that the iron-redox technology offers a unique combination of proven experience and continuous improvement. It is a technology that is commercially and technically attractive, and in which the operator can have a high degree of confidence.