The Use of Aqueous Liquid Redox Desulfurization Technology for the Treatment of Sour Associated Gases at Elevated Pressure

Myron Reicher (Merichem)

846 E. Algonquin Road, Suite A100
Schaumburg, IL 60173
Tel: (847) 285-3850
Fax: (847) 285-3888

Bill Niemiec (Merichem)

846 E. Algonquin Road, Suite A100
Schaumburg, IL 60173
Tel: (847) 285-3850
Fax: (847) 285-3888

Dr. Tamas Katona (Mol Hungarian Oil and Gas Company)

Mol Rt. Szeged
P.O. Box 37
Szeged, Hungary

Abstract

The use of aqueous, liquid redox desulfurization technology for the direct treatment of sour associated and process gas streams at elevated operating pressure and relatively high CO2 partial pressure presents unique design and operational challenges. The LO-CAT II® process, an aqueous liquid redox process that utilizes a ferric (Fe+3)/ferrous (Fe+2) amino-carboxylate redox couple, installed at the MOL Gas Plant in Szeged, Hungary has performed to specification. However, to achieve this goal, resolution of technological challenges related to the solution circulation pump; absorber pluggage with associated high system pressure drop; liquid hydrocarbon influx; foaming with associated liquid entrainment; and corrosion were required.

The design and operational history of the facility is presented, and the aforementioned technological challenges and their resolution are discussed.

Background

ARI Technologies, the developer of the LO-CAT® and LO-CAT II processes; and the predecessor of USF/Gas Technology Products, supplied a LO-CAT II desulfurization system to MOL Hungarian Oil and Gas Company for operation at the Szeged Production Unit. Szeged is Located in southern Hungary, approximately 100 miles southeast of Budapest. This unit was placed in service in late 1992, was only the third of the LO-CAT II units produced, and was the first to treat gas at elevated pressure. Furnished at the same time by others were auxiliary operations such as the water removal system downstream of LO-CAT.


1. Reicher, Myron, This email address is being protected from spambots. You need JavaScript enabled to view it. , Manager of Process Technology, Gas Technology Products, 846 E. Algonguin Road, Suite A100, Schaumburg, IL 60173, USA.

2 .Niemiec, Bill, This email address is being protected from spambots. You need JavaScript enabled to view it. , Technical Service Manager, Gas Technology Products, 846 E. Algonguin Road, Suite A100, Schaumburg, IL 60173, USA.

3. Katona, Dr. Tamas, This email address is being protected from spambots. You need JavaScript enabled to view it. , Manager of Gas Processing Plant, Mol Hungarian Oil and Gas Company, Mol Rt. Szeged, P.O. Box 37, Szeged, Hungary.

LO-CAT and LO-CAT II are registered trademarks of Gas Technology Products.

The LO-CAT II unit was designed to treat natural gas, however the actual gas is a blend of recycled process gas and associated gas collected from the oilfields around the plant. Both oil and gas from the oilfields are simultaneously transported to the plant via a mixed-phase transport pipeline, followed by separation in a bank of gas-liquid separators. After the separators, the associated gas is combined with the recycled process gas stream, compressed to approximately 250 psig, and then sent to the LO-CAT II unit.

Figure 1 is the flow diagram of the LO-CAT II unit. As the design team anticipated some liquid entrainment in the sour gas, the first two unit operations are a knockout pot and a coalescing filter. The knockout pot had a liquid surge volume of approximately 35 ft3. After the coalescing filter, the sour gas is routed to the LO-CAT II static mixer absorbers where it is contacted with oxidized LO-CAT catalyst solution. (Two static mixers had been provided with the forethought that they would probably plug with sulfur and would therefore require periodic cleaning). Exiting the static mixer absorber is a two-phase stream consisting of sweet gas and reduced LO-CAT solution, which then enters the absorber separator. In the separator, the gas and liquid are allowed to separate. The treated gas exits the LO-CAT II unit after passing through a mist eliminator. Downstream of the unit, the gas is then cooled to remove water, and then compressed to 925 psig in the third stage compressor.

The reduced solution from the separator passes through a pressure reducing valve and flash drum, then gravity drains to the oxidizer, where the iron is regenerated for re-use in the absorber. A slipstream of this oxidized solution is then sent to the settler where sulfur is allowed to settle and subsequently transferred as slurry to the belt filter. Filtrate from the filter is returned to the oxidizer while sulfur cake is discharged after washing. A small quantity of flash gas is vented to the flare header.

LO-CAT® II Chemistry

The LO-CAT II process utilizes a ferric (Fe+3)/ferrous (Fe+2) amino-carboxylate redox couple as a catalyst to absorb H2S from sour gas and to oxidize it to elemental sulfur in the absorber according to reactions (1) through (3):

H2S(gas) + H2O(liq) H2S(aq) (1)
H2S(aq) H++HS- (2)
HS-+2Fe+3L S0+2Fe+2L+H+ (3)

The ferrous ion that is produced is then recycled to the oxidizer where it is regenerated by oxygen according to reactions (4) and (5):

1/2O2(gas)+H2O(liq) 1/2O2(aq) (4)
1/2O2(aq)+2Fe+2L+H2O 2Fe+3L+2OH- (5)

The overall reaction (6) is therefore:

H2S 1/2O2 S0+H2O (6)

Equation (6) is recognized as the overall Claus reaction, and it can be seen that the role of iron is catalytic since it is not consumed. Chelating agents, represented by "L" in the above equations, while not partaking in any role in the above reactions, serve to maintain the ferric and ferrous ions in solution.

Design Parameters

The LO-CAT II unit was designed as outlined in Table 1.

Gas to be treated   Natural gas  
Maximum anticipated gas flow, SCFM   47,000  
Design gas flow, SCFM   58,000  
Inlet gas pressure, psig   260-275  
Allowable gas pressure drop through LO-CAT, psi   14.5  
Inlet H2S concentration, ppmv   100  
Outlet H2S concentration, ppmv   3  
Inlet CO2 concentration, %   1.2  
Sulfur load, LTPD   1/3  
Make-up alkali   NaOH  

Start-Up Challenges

During the initial start-up and subsequent restarts of the unit, problems arose in several areas including:

  • Liquid entrainment in sweet gas, thus limiting gas flow
  • Foaming and floating sulfur
  • Chronic failure of the catalyst recirculation pump
  • High system pressure drop
  • Poor H2S removal efficiency

To answer these challenges, modifications to equipment, procedures and chemistry were required. A discussion of the challenges and the countermeasures are discussed below.

Discussion

Liquid Entrainment

During the initial start-up, liquid entrainment was seen to be excessive whenever the sour gas flow exceeded 35,000 SCFM. This was attributed, at that time, to the absorber separator vessel being undersized. As a result, not only was it difficult for the gas and liquid to separate, but sulfur particles were also entrained, thus causing the mist eliminator to foul, resulting in further aggravation of the entrainment issue.

To remedy the situation, the separator was resized and a new separator, complete with modified mist eliminator, was retrofitted into the plant. As a result, gas flow up to 58,000 SCFM was achieved without excessive liquid entrainment.

Foaming and Floating Sulfur

Foaming and floating sulfur are typical start-up problems that oftentimes are remedied in short order. However, such was not the case with this unit.

Foaming and sulfur settling problems usually go hand-in-hand and are a result of condensed hydrocarbons and/or an imbalance in the use of surfactant and anti-foam additives. When condensed hydrocarbons are present, sulfur particles become coated. These hydrocarbon-coated particles cannot be wetted and can trap air or gas bubbles, so they float on free liquid surfaces such as in the absorber separator, oxidizer, and settler and therefore cannot be removed from the system via settling. When surfactant is added in an attempt to wet the sulfur, foam can be created. Further, if anti-foam is added to control the resultant foam, this could actually aggravate the foaming situation.

Eventually, it was determined that there were excessive liquid hydrocarbons entering the LO-CAT II unit. At one point, a sample of solution was found to contain solid wax-like material. Additionally, stable emulsions were forming among the aqueous LO-CAT solution, sulfur particles, and hydrocarbon condensate. In retrospect, hydrocarbon induced foaming may have aggravated the entrainment situation in the absorber separator as well.

If the condensate could have been prevented from entering the system, this entire problem would not have existed. However, it was determined that considerable condensate was frequently being introduced from both the process gas and also as a result of operation of the two-phase oil/gas pipeline. Consequently, it was deemed not possible to easily keep the condensate out. Therefore, a multi-faceted approach to the problem was undertaken. To reduce the quantity of condensate and to provide immediate, short-term relief, modifications were made to the sour gas transfer line to allow condensate to be trapped out. Fortunately, the main line was high on the pipe rack and a simple drainage boot could be retrofitted to catch the condensate. In addition, the knockout pot that was furnished with the unit was replaced with a larger one.

At the same time, MOL’s R&D group went about to develop a new additive, which would allow the stable emulsions to be broken and the sulfur to be wetted. Their goal was met by the formulation of what is termed "B-3". B-3 has replaced ARI-600 and anti-foam in this unit, and has allowed the plant to operate in spite of continued hydrocarbon influx.

Catalyst Recirculation Pump

The catalyst recirculation pump in this plant was a six stage progressive cavity pump. This type of pump was selected as it best fit the hydraulic specifications and had operated successfully in many previous units, albeit at lower pressure. However, during the several attempts to start the plant, it became apparent that this pump would not be dependable in the long-term, even though a larger, slower running pump was installed as recommended by the manufacturer. As a result of the failure of the larger pump on Christmas Eve, 1997 and the need to keep the plant running, plant maintenance personnel retrofitted a scavenged multi-stage centrifugal pump. This style pump had never been used in LO-CAT units before due to fear of erosion and inter-stage pluggage. However, there were no options available on Christmas Eve. A year later, this pump was still operational but the capacity of the unit had been compromised, as the volumetric capacity of this pump was less than that required. As a result, in December of 1998, a new pump with the required capacity was installed. Emboldened by the lack of erosion and inter-stage plugging in the multi-stage pump, but not wanting to chance inter-stage plugging in the long-term, MOL selected a Durco, Mark III pump. This pump is a centrifugal pump with an open impeller, and operates at 3160 rpm. So far, the new pump has performed satisfactorily, having operated successfully for 9 months.

High System Pressure Drop

High system pressure drop was caused by absorber fouling and back-pressure in the water removal train downstream of the LO-CAT unit. Absorber pluggage had been expected; therefore the two static mixers were furnished with flushing connections. Unfortunately, the planned cleaning procedure was not effective, possibly due to the stickiness of hydrocarbon-coated sulfur. An alternate cleaning procedure, using steam, was implemented by the operators, but the pressure drop was never reduced to the as-clean condition. In September of 1998, a revised steaming procedure was implemented that allowed recovery to the as-clean condition. However, to implement this procedure, the absorber required isolation by blinds, as the original isolation valves were not rated for steam service. Replacement of these valves is scheduled shortly. When replaced, more frequent steam outs will be possible, thus allowing low absorber pressure drop and increased gas throughput to be maintained.

Modifications to the water separation train downstream of LO-CAT are also planned for the future and will permit a further increase in gas flow.

H2S Removal Efficiency

From the onset, H2S removal efficiency had been less than designed for. This was largely due to the CO2 concentration of the sour gas being higher than the design, thus greatly reducing the solution pH at the discharge of the static mixer (where liquid and gas emerge as a two-phase stream). To remedy this, it was recognized that additional solution buffering (i.e. more bicarbonate ion) was required beyond that designed for. Unfortunately, increasing the buffer when NaOH is used as alkali supply can cause precipitation of NaHCO3. Usually, KOH is used for higher buffering capacity but it is more expensive. Operators at MOL decided to supplement the NaOH feed with NH4OH. This worked insofar as the solution buffer capacity was increased, but there was a continuous NH3 loss from the oxidizer vent. A packed column scrubber was added to the vent, using oxidizer make-up water as the absorbent. This had some beneficial effect, however ammonia losses continued. Further, a review of solution chemistry in September of 1998 revealed that the solubility product of NaHCO3 was being approached. This prompted a switch to KOH and the discontinuance of NH4OH in December of 1998.

Present Situation

At present, the plant has been operating with a gas flow of 35,000 to 58,000 SCFM, averaging about 38,000 SCFM, with a sulfur load of about 0.05 LTPD (due to low H2S concentration in the gas). The improvements that were made in the hydrocarbon collection system, coupled with improvements in operating procedures have greatly reduced the ingress of hydrocarbon condensates into the unit. In turn, this has reduced the difficulties associated with floating sulfur, foaming, and absorber fouling. The development of B-3 has further improved hydrocarbon-induced problems.

The recently replaced recirculation pump, the switch to KOH, and improved absorber cleaning procedures have combined to permit higher gas flow (58,000 SCFM when clean) and greater H2S removal efficiency (averaging 90%).

Table 2 summarizes the present operating conditions of the unit.

Actual gas flow, SCFM   58,000(clean)/35,000(dirty)  
Inlet gas pressure, psig   230-250  
Gas pressure drop, LO-CAT, psi   6(clean)/9(dirty)  
Gas pressure drop, system, psi   10(clean)/22(dirty)  
Inlet H2S concentration, ppmv   40-50  
Outlet H2S concentration, ppmv   4-5  
Inlet CO2 concentration, %   3.25  
Sulfur load, LTPD   0.08  
Make-up alkali   KOH  

Corrosion is not considered to be a problem at this time, but an inspection of the oxidizer and settler is planned during the next turn-around. Replacement of the EPDM sleeves by polyurethane is a possibility at that time as well.

ANTICIPATED FUTURE CONDITIONS

It is anticipated that when final modifications to the absorber isolation valves and water separation train are made, a sustained gas flow of 58,000 SCFM, meeting pipeline H2S specification of less than 4 ppmv will be achieved (static mixer efficiency improves with increased gas flow). Table 3 summarizes the anticipated future conditions.

Gas flow, SCFM   58,000  
Inlet gas pressure, psig   230-250  
Gas pressure drop, LO-CAT, psi   6(clean)/10(dirty)  
Gas pressure drop, system, psi   10(clean)/14.5(dirty)  
Inlet H2S concentration, ppmv   40-50  
Outlet H2S concentration, ppmv   <4  
Inlet CO2 concentration, %   3.25  
Sulfur load, LTPD   0.14  
Make-up alkali   KOH  

CONCLUSION

After a difficult, extended start-up period, modifications to equipment, procedures, and chemistry has permitted the LO-CAT II unit to operate close to expectations in spite of the continuing ingress of hydrocarbon condensate and higher than designed for CO2 partial pressure. It is anticipated that design gas load and efficiency will be achieved shortly.

The experiences gained in this first LO-CAT II elevated-pressure application, a subsequent 4 LTPD unit operating at 500 psig, and a high pressure pilot plant operating at nearly 1000 psig have been invaluable and will serve as a guide to the design and operation of future units of this nature.

As demonstrated at the MOL site, the impediments to the use of aqueous liquid redox technology for direct treat applications have been resolved. This achievement, when combined with over 20 years of experience in other LO-CAT/LO-CAT II applications, makes LO-CAT II the technology-of-choice for the direct treatment of sour gases at elevated pressure.