A Unique LO-CAT® Success Story — Flexible, High Reliability Operation in Natural Gas Processing

By Conard Baker (Anardarko Pinnacle Gas Treating, Inc), Tony Barnette (Merichem)

Introduction

Anadarko Petroleum operates a gas processing plant in East Texas. Since 1997, they have operated two LO-CAT® H2S Removal Systems, maintaining not only extremely high removal efficiencies, but also keeping one of the highest online time record in the area. For this reason, Pinnacle Gas Treating is bringing on more natural gas wells in the area, and needs to expand the H2S removal capacity of their facility. The ultimate goal of their expansion will require the removal of up to 40 LTPD sulfur from the amine acid gas, while maintaining their high turndown capacity that has consistently given them their reputation for reliability. A modular approach of adding multiple expansion LO-CAT® systems, in stages, as needed meets their requirements in the most cost effective way.

Background

In 1996, Western Gas Resources selected the LO-CAT® Process to treat the amine acid gas generated in their new Pinnacle Gas Processing plant Located in Tennessee Colony, TX. Initially, Western Gas purchased a 2 LTPD LO-CAT® liquid redox-based sulfur recovery unit (SRU), which was started up in June of 1997; however, shortly into the design of the first unit, it became apparent that the sulfur capacity of the unit would be exceeded. Consequently, Western Gas purchased a 7 LTPD LO-CAT® unit, which was started up in November of 1997. In 2001, the facility was purchased by Anadarko Petroleum, who currently operates the facility.

The 7 LTPD SRU was originally designed to treat amine acid gas at the processing conditions itemized in Table 1. As indicated, the SRU was designed to remove 99.99+% of the H2S under rather severe turndown conditions of 155:1 for the gas flow and 45:1 for the H2S concentration.

 Case 1Case 2
Type of gas Amine Acid Gas Amine Acid Gas
Flowrate, MMSCFD dry 9.3 0.06
% H2S in Acid Gas 2.0 90.0
Removal Efficiency 99.99+% 99.99+%
Outlet H2S, lbs/hr <2 <2
LTPD Sulfur 7 7

Additionally, while the unit was designed to handle a wide range of flow and H2S concentrations totaling the design 7 LTPD sulfur, it would also be required to maintain operation with a sulfur load turndown that could go from nearly nothing (100 lb/day) up to the design capacity within a short time.

Because the increase in the sulfur removal requirement at the gas plant was expected to continue to rise, and plans were begun to add a 40 LTPD Claus plant to the SRU capacity at the facility. The goal was to use the LO-CAT® Process up to 7 LTPD removal, and then use the Claus above that, with the LO-CAT® Process acting as the tailgas treater to achieve the high removal efficiency required. Because flexibility was key, the operating plan for the facility was:

Sulfur LoadOperation
< 2 LTPD 2 LTPD LO-CAT® Unit
2 LTPD – 7 LTPD 7 LTPD LO-CAT® Unit
> 7 LTPD Claus Plant, with AGE/ Hydrolysis/ LO-CAT® TGU

Let the Expansion Begin...

From 1997 until 2000, the LO-CAT systems were operated exclusively for the H2S removal. In 2000, the Claus system was completed, consisting of:

  • Acid Gas Enrichment (AGE) to concentrate the 2–3% H2S amine acid gas to 25% for minimum Claus feed concentration
  • 40 LTPD Claus SRU.
  • Tailgas Hydrotreatment to convert all sulfur species to H2S.
  • Cooling
  • Tailgas feed to existing 7 LTPD LO-CAT Unit as tailgas treater.

As this system was operated, several operational difficulties were experienced, specifically with the hydrotreater system and the Claus plant. The Claus plant operational difficulties were related to the previously mentioned load swings, which the Claus could not tolerate. After a year of difficult operation, Anadarko decided to stop operation of the AGE/Claus/Hydrotreater train and began using the LO-CAT® systems exclusively for the SRU. This was a problem, as it severely limited their ability to further expand processing natural gas, as they were now limited to 7 LTPD sulfur throughput.

And Stop…

From 2001 to 2005, operation focused on optimizing the existing LO-CAT operations and maintenance to maximize online time and capacity. In 2005, however, Anadarko began discussions with area gas producers, which would potentially push the SRU requirements at Pinnacle beyond the capacity of the existing LO-CAT® units. Over the next year, expansion requirements expanded from 10 LTPD, to 15 LTPD, finally settling on 40 LTPD total SRU capacity. Several options were considered, and dropped, as the total sulfur load increased, including, shortly, revamping and restarting the Claus system (which was made unfeasible as the AGE unit had been dismantled and moved). In 2007, two options remained:

  • Adding additional LO-CAT system(s)
  • Direct Oxidation w/ existing LO-CAT modified and run as TGCU

Selectox was initially considered for the DO portion, as the sour acid gas would still normally have low H2S concentrations (2–4 vol%). Further investigation of potential operating conditions, however, showed that H2S levels could attain 8% for long periods, and periodically see levels as high as 16%. Selectox could not tolerate these excursions, and was pulled from consideration. Pinnacle’s excellent operating experience and online time with the LO-CAT® system, and the ease at which the unit was operated with the highly variable sulfur load made them very receptive to adding modular trains of larger LO-CAT® systems to operate parallel to their existing 7 LTPD unit, which is how the system is proceeding.

With a total sulfur capacity of 40 LTPD still planned, of which the 7 LTPD would be considered part, 3 x 11 LTPD expansion trains were planned, with the first 11 LTPD system ordered in September 2007. This expansion will meet the needs of the plant through new producers being brought online in June 2008, with the LO-CAT expansion planned to be operational by that time. Future addition of gas producers tied to the Pinnacle plant will require additional acid gas treatment capacity, with a second 11 LTPD LO-CAT expansion plant planned for 2009.

The LO-CAT® Process

A schematic flow diagram of a single train of the LO-CAT Process is contained in Figure 1. The acid gas from the amine unit is passed through a Knockout pot to remove any condensable liquids and/or amine carryover. The acid gas then enters the proprietary Autocirculation LO-CAT unit. In the Autocirculation unit, the sour gas is sparged through a proprietary solution of dilute chelated iron within the absorber sections of the vessel. It is within the absorbers that the iron is absorbed (reaction A) and converted to elemental sulfur (reaction B) as follows:

H2S (gas) + H2O H+ + HS– + H2O (A)
HS– + 2Fe+++ H+ + SO +2 Fe++ (B)

The reduced iron solution then flows into oxidizer sections of the vessel where air is sparged through the solution to reoxidize the chelated iron back to the ferric state (Fe+++) in accordance to reaction C.

œ O2 + H2O + 2Fe++ 2OH– + 2 Fe+++ (C)

Through a unique arrangements of weirs and baffles within the proprietary Autocirculation vessel, liquid circulation between the absorber and oxidizer sections is maintain through a series of airlifts, thus circulation pumps are not required. The air and the sweetened acid gas streams are combined in the headspace of the Autocirculation Vessel and directed to either an incinerator or directly to the atmosphere.

The solid, elemental sulfur, which is formed in the process, is removed by directing a slipstream of solution to a Settler Vessel where the slurry concentration is increased to approximately 10 wt.%. The concentrated slurry is then directed to a vacuum belt filter where the sulfur is dewatered into a 60% moist cake.

Summary

Anadarko’s decision to continue to employ the LO-CAT Process as its primary sulfur recovery unit proved to be not only an economical approach, but also a technical success, even at sulfur loads that normally would be associated with a Claus system. The LO-CAT unit allowed them to achieve removal efficiencies of greater than 99.9% while achieving nearly 100% turndown on not only their sulfur recovery, but also their gas flow, while achieving > 99% online availability, making their facility the most reliable in the area.