LO-CAT®: A Flexible Hydrogen Sulfide Removal Process

By William Rouleau & John Watson - Merichem Company

 

Introduction

The hydrocarbon engineering industry faces tighter restrictions for hydrogen sulfide (H2S) emissions and expanding requirements for sulfur recovery. Operators place a high value on developing the right strategy for H2S removal and sulfur recovery.

When H2S exceeds certain levels, sulfur abatement systems are often required. These investments can be expensive and complex. Fortunately, the LO-CAT technology offers a well-developed and economic solution that has been in commercial use for almost 35 years. The LO-CAT technology provides operators features, like: 100% turndown in respect to H2S concentration, flow rate and sulfur loading; single stage removal efficiencies in excess of 99.9%; and an ability to process any type of gas stream. These features make the process attractive as a standalone solution for sulfur removal.

Background

Hydrogen sulfide is an extremely toxic, corrosive and odorous gas, causing safety and material of construction issues in its unaltered form. For many years, high levels of H2S in many raw natural gas streams have required processing to reduce the contained acid gases before transport and distribution of the fuel to market.

Increasing concentrations of H2S can have several detrimental effects: 1) onset of odor problems; 2) corrosion of gas production hardware; 3) increasing SOx emissions from flaring or other combustion processes; and 4) possible health consequences for workers. The odor threshold for H2S is extremely low (0.05 to 0.1 ppmv), and levels of H2S above 10 ppmv are considered toxic, exceeding the Threshold Limit Value (TLV). Moreover, levels of H2S above 1000 ppmv (0.1 V%) in a breathing zone can rapidly lead to unconsciousness and death. Sites with high H2S levels pay special attention to design approval worker health and safety controls throughout the plant.

As environmental considerations increase on designers and site operators to further reduce sulfur emissions, selecting the correct processing choice from several options involves an analysis of capital cost minimization, operational flexibility, operating cost, and the total cost of ownership associated with new facilities.

 

Sulfur Recovery via the LO-CAT Technology

The LO-CAT technology was developed to provide an isothermal, low operating cost method for carrying out the modified Claus reaction. The technology has been adopted by a number of industries, including the Oil & Gas, Coal, Power, and Food & Beverage.

The first commercial installation of LO-CAT technology took place in 1980. Starting with oil and gas production (upstream and midstream), and oil refining (downstream), the basic process has been continually improved and modified over time to allow for expanded use into other markets and industrial segments. From petrochemicals to metals (coke oven gas and direct reduced iron off gas), water and waste water treatment (municipal and industrial) to carbon dioxide products (food and beverage), this simple, robust technology has found success in multiple applications. More recently, unconventional resources and alternative energy sources such as shale gas, stranded offshore gas, biogas, and gasification syngas have been added to the portfolio of applications that successfully utilize LO CAT technology for sulfur recovery.

LO-CAT technology contains a proprietary liquid redox catalyst that converts H2S to solid elemental sulfur by carrying out the direct oxidation of H2S as follows:

Direct Oxidation Reaction H2S + 1/2 O2 → H2O + S°

As embodied in LO-CAT technology, the direct oxidation reaction is divided into five sequential steps:

1 – Absorption of H2S H2S (Gas) + H2O (Liquid) ↔ H2S (Aq) + H2O (Aq)

2 – Ionization of H2S H2S (Aq) ↔ H+ + HS–

3 – Sulfide Oxidation HS– + 2Fe+++ ↔ S° + 2Fe++ + H+

4 – Absorption of Oxygen 1/2 O2 (Gas) + H2O (Liquid) ↔ 1/2 O2 (Aq) + H2O (Aq)

5 – Iron Oxidation 1/2 O2 (Aq) + H2O + 2Fe++ → 2 OH– + 2Fe+++

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 (active) iron to the ferrous (inactive) state. Equations 4 and 5 represent the absorption of oxygen (from ambient air) into the aqueous solution followed by oxidation of the ferrous iron back to the ferric state.

Equations 3 and 5 are very rapid. Consequently, LO-CAT systems generally produce relatively small amounts of byproduct thiosulfate ions. However, equations 1 and 4 are relatively slow and are the rate controlling steps in a LO-CAT System. It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction, the iron effectively cancels out. One may wonder why chelated iron is required at all, if it doesn’t take part in the overall reaction. In the LO-CAT process, 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”. Although there are many metals which can perform these functions, iron (Fe) was chosen for LO-CAT technology primarily because it is inexpensive and safe to operate. The chelating agent(s) do not take part in the process chemistry. Their role is simply to hold the iron ions in solution. Neither ferrous (Fe++) nor ferric (Fe+++) ions are very soluble or very stable in aqueous solutions. Iron will ordinarily precipitate at low concentrations as either ferric hydroxide Fe(OH)3 or ferrous sulfide (FeS). The chelating agents are organic compounds that wrap around the iron in a claw-like fashion, preventing the iron ions from forming precipitates. LO-CAT technology uses a proprietary blend of chelating agents to hold the iron in solution over a wide operating range. LO-CAT has developed into a versatile processing scheme for treating gas streams containing any amount of H2S. Advantages of these systems include the ability to treat both aerobic and anaerobic gas streams, H2S removal efficiencies in excess of 99.9%, essentially 100% turndown on H2S concentration and/or gas flow, and the production of only innocuous products and byproducts.

Elemental sulfur recovered by the LO-CAT technology is profoundly different from sulfur produced by other processes. LO-CAT sulfur has a particle size that ranges from 8 to 45 microns, much smaller than sulfur generated by other means. And since this solid sulfur is formed in an aqueous solution, it harbors no trapped H2S vapors that can evolve during subsequent handling. Sulfur recovered by the LO-CAT technology also has a relatively soft texture that can lead to large internal and external surface areas that support microbial action in the soil (1). LO-CAT sulfur is removed from the process as a “filter cake” composed of approximately 65 wt.% sulfur and 35 wt.% diluted LO-CAT solution. The moisture content of the cake favors low dust production during use. Additionally, particle size, surface area and moisture content characteristics exhibited by the produced LO CAT sulfur cake support good reactivity, applicability and safety for sulfur used in agricultural applications (2).

Some licensees report that the LO CAT sulfur cake has been used as a soil amendment in California with great success since the 1990’s. In addition to being a plant nutrient vital to the formation of plant protein, sulfur use as a soil amendment offers improved soil pH control which can enhance the plant uptake of three specific macronutrients: potassium, phosphorus and nitrogen.

 

LO-CAT Processing Scheme

In applying this chemistry to a wide range of gas streams in diverse industrial applications, many different processing schemes have been successfully employed. The two most common processing schemes utilized are the LO-CAT DirectTreat configuration and the LO-CAT AutoCirc configuration(3). Figure 1 shows a “DirectTreat” unit, which is most commonly employed for processing gas streams that are either combustible or cannot be contaminated with air. This processing configuration treats sour gas directly without the requirement of first using a solvent system to separate out the acid gas. Figure 2 illustrates an “AutoCirc” unit, which is used for processing non-combustible streams that can acceptably be contaminated with air. When CO2 removal from natural gas is desired, it is usually preferable to remove the acid gases (CO2 and H2S) using a solvent-based system, and then recover the sulfur from the acid gas using this indirect processing configuration. The direct treatment of natural gas using the LO-CAT technology can achieve reductions in H2S that are comparable to those achieved by solvent-based acid gas removal units, so the choice of processing configuration is normally determined by considering the economics of the complete project processing scheme.

 

 

 LO-CAT DirectTreatFlowDiagram-rev2

Figure 1. LO-CAT DirectTreat Processing Configuration

 

When using the DirectTreat configuration, the entrained liquids are removed prior to introducing the “sour” gas stream to the absorber. In this scheme, equations 1, 2 and 3 occur in the absorber while equation 4 occurs in the oxidizer. Several different absorber types are routinely used on the DirectTreat configuration. Since solid sulfur will form in solution in the absorber, only non-fouling devices may be used. Figure 1 shows active LO-CAT catalyst solution contacting the sour gas in a liquid full absorber. Merichem’s long history of designing and servicing LO-CAT units across a wide range of processing conditions and objectives has resulted in an extensive portfolio of contacting devices that provide robust absorber operation for every application. “Sweet” gas exits the absorber, and passes through a knockout pot with mist eliminator for removal of entrained solution. In most cases, a single absorber device achieves the desired H2S specification for the sweet gas.

The catalyst solution from the absorber flows to the oxidizer for regeneration. In the oxidizer, air is sparged uniformly through the solution, converting the reduced iron back to an active state. The regenerated catalyst solution from the oxidizer is then pumped back to the absorber to complete the cycle.

Sulfur settles in the cone bottom section of the oxidizer where it is concentrated into slurry prior to being pumped to a filtration step. The cone bottom provides volume for sulfur inventory that enables the operator to take the filter system offline for routine maintenance without shutting down the solution side of the LO-CAT unit.

 

 LO-CAT AutoCirc-rev2

 

Figure 2. LO-CAT AutoCirc Processing Configuration

 

When using the AutoCirc configuration to treat acid gases that have been removed from a sour gas by means of a solvent-based process, the only pretreatment of the acid gas required prior to the LO-CAT unit is a knockout pot to remove entrained liquids. Figure 2, shows that the acid gas and regeneration air are both directed to the AutoCirc vessel, and the excess air and “cleansed” CO2 are combined in the single vent from this vessel.

In the AutoCirc configuration, equations 1, 2, 3 and 4 are all performed in the single AutoCirc vessel, which serves as the absorber and oxidizer. The resulting feature of the AutoCirc configuration is the ability to circulate solution between the absorber and the oxidizer without pumps. Similar to the oxidizer vessel in a DirectTreat configuration, the AutoCirc vessel has a cone shaped bottom where sulfur is concentrated before the filtration step. This conical area serves as an operating inventory for sulfur product during filter maintenance outages.

In either scheme, a chemical addition skid and sulfur filter round out the complete LO-CAT unit. The chemical addition skid consists of small tanks and metering pumps that allow controlled addition of make up LO-CAT catalyst. Controlled addition replaces trace quantities of catalyst that are lost with the sulfur cake, as well as losses due to chelate degradation and byproduct salt formation. Many of the Merichem improvements to the LO-CAT technology over the past 35 years have successfully reduced catalyst solution losses and make-up requirements. Other proprietary chemicals are also pumped from this skid to stabilize the process on an as needed basis.

 

LO-CAT in Refinery Applications

Petroleum refineries have undergone extensive modification over the last 25 years to both reduce sulfur emissions to the air from within refinery battery limits, as well as to produce low sulfur transportation fuels. Low sulfur gasoline and diesel has become the norm in much of the world, and refinery focus is now shifting to reducing sulfur in jet fuel and bunker C. Most of the sulfur removed in refineries is first converted to H2S, prior to being converted into solid elemental sulfur.

A number of small refineries currently rely on the LO-CAT technology to cost-effectively remove hydrogen sulfide from process overhead vents and provide clean gas into their facility fuel gas system. The first refinery installation of the LO-CAT technology occurred in 1980 on a hydrotreating application. Subsequent LO-CAT refinery applications have included asphalt production, ethylene production, diesel production, lube oil production, and propylene production. These LO-CAT units operate across a wide range of pressures, removing H2S from various types of acid gas streams (amine, selexol, sour water strippers) and process overhead gas streams (fuel gas). Sulfur removal capacities for the LO-CAT technology in refinery applications have ranged from 0.6 to 16 tons per day.

 

LO-CAT in Gasification Applications for Power and Petrochemicals

The removal of sulfur from syngas must be accomplished to complete the process of converting coal and/or waste products into clean, environmentally friendly fuels and petrochemical feedstock’s. Hydrogen sulfide is also extremely detrimental to downstream synthesis catalysts on two fronts. The first problem is a revenue loss from the reduced conversion efficiency of synthesis catalyst in the presence of H2S, resulting in lower chemicals production. The second issue is the cost (tens to hundreds of millions of dollars) to replace the synthesis catalyst, as continued exposure to sulfur compounds poison the synthesis catalyst and reduce the time between shut downs.

A number of gasification facilities currently rely on the LO-CAT technology to cost-effectively remove hydrogen sulfide and provide clean syngas. The first gasification-type applications to utilize the LO-CAT technology came online during the early 1990s for the treatment of coke oven gas. By the 2000’s, the high cost of waste disposal and supply/demand gaps for petrochemical feedstock encouraged the development of several industrial and municipal waste gasification projects that also utilized the LO-CAT technology. The experience gained on these projects has recently led to the use of LO-CAT technology on coal gasification projects in the United States, China, and South Africa.

In summary, the LO-CAT technology has been adopted to treat syngas in an array of applications (IGCC Electrical Power Production, Synthetic Ammonia Production, Acetic Acid Production, Synthetic Fuel Gas Production, Fischer Tropsch Fuels Production, Methanol Production, Hydrogen Production, and Urea Production),with an equally diverse spectrum of feedstocks (municipal solid waste, industrial solid waste, petroleum coke, coal). As has been the case with the introduction of LO CAT technology into each new market, initial projects started out small with sulfur removal capacities of less than one ton/day; and have quickly grown to removal capacities greater than 25 tons/day (TPD).

 

Conclusion

As a general rule of thumb, the LO-CAT technology’s sulfur removal capacity niche ranges between 0.5 to 25 TPD. When the ratio of CO2 to H2S is greater than 3 and/or the volumetric sour gas flowrate varies on a frequent basis, LO-CAT technology can be a valid option to remove up to 40 tons/day of sulfur from sour gas streams.

With over 200 licenses globally, the LO-CAT technology is in use within several markets and industrial segments LO-CAT systems are extremely versatile and the plants can process any type of gas stream. The process provides licensees the flexibility of maintaining very high H2S removal efficiencies at 2(in excess of 99.9%)at 100% turndown, regardless of the H2S concentration, flowrate and sulfur loading. These capabilities result in a proven technology that meets the sulfur management requirements of the hydrocarbon engineering world.

 

References

1. Neeley, F., Heguy, D., & Karr, J. “Sulfur: At the Crossroads of Energy, the Environment, and Agriculture”. Fertilizer International, May/June 2002.

2. Adapted from: W.F. Bennett (editor), 1993. Nutrient Deficiencies & Toxicities in Crop Plants, APS Press, St. Paul, Minnesota.

3. Rouleau, W., – Iron Age, Hydrocarbon Engineering, February 2013

4. Rouleau, W., – Small Capacity Sulfur Recovery Units for Coal Gasification, Merichem Company, 2011

5. Watson, J., – Sulfur Recovery Process Modified for FPSO Topside Installation, Offshore Technology Conference, 2010

6. Nagl, G., – Flexibility of Liquid Redox Processing in Refinery Sulfur Management, Hydrocarbon Asia, Nov/Dec 2006