ATCO Midstream’s Gas Sweetening Experience Using Iron-Redox Technology
ATCO Midstream (ATCO) works with natural gas producers to gather, process, and store their natural gas and extract liquids from it. The ATCO LO-CAT Unit, located in St. Albert, Alberta, Canada at the Carbondale plant, has provided highly reliable service for over 12 years. The unit has continuously been producing sweet gas at or below the minimum local requirement for H2S emissions. ATCO is very pleased with the performance of the LO-CAT unit, and expects many more years of LO-CAT operation at its St. Albert plant.Importance of H2S Removal Technologies
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.
The iron-redox sulfur removal technology has been commercial for 25 years. The technology has been a dominant process for hydrogen sulfide oxidation and removal since its introduction due to its high removal efficiency, low cost of operation, ease of operation and high reliability. Over 200 iron-redox plants have been installed worldwide since its commercial introduction. The large majority of these have been the LO-CAT process. ATCO’s LO-CAT experience shows why this technology has been so successful.Iron-redox “LO-CAT” process
The LO-CAT process is an aqueous, low temperature process that uses a regenerable iron catalyst. The iron catalyst oxidizes the hydrogen sulfide to elemental sulfur. The process is specific to hydrogen sulfide. Other gases, including carbon dioxide and other sulfur compounds pass through the system unreacted.
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 byproduct thiosulfate and sulfate 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 the obvious question is 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. All the chelating agents do is solubilize iron in water, thus making it possible to have a solution of iron.
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 byproduct 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.
Figure 1. Conventional LO-CAT II® Unit
Figure 2. Autocirculation LO-CAT II® Unit
LO-CAT technology can be applied to natural gas processing in both the conventional configuration and the autocirculation configuration combined with an amine unit upstream of the LO-CAT.
The amine/autocirculation LO-CAT combination is advantaged relative to the conventional LO-CAT when 1) the gas treatment pressure is very high, 2) when carbon dioxide needs to be removed from the gas stream, or 3) when the H2S concentration is very low. The amine serves to concentrate the hydrogen sulfide and reduce the size of the LO-CAT (the materials of construction are lower for the amine unit than the LO-CAT, which is typically made of stainless steel.) Reasons 1 and 2 applied in ATCO’s case.
The conventional LO-CAT is advantageous when the carbon dioxide does not have to be removed. In this case, the LO-CAT can remove the hydrogen sulfide with one process instead of two, reducing complexity, maintenance, and operator attention.Unit History
The unit in St. Albert was originally licensed and built for Norcen Energy Resources in 1991. Located just north of Edmonton, Alberta, Canada, the unit treats the acid gas from an amine unit located upstream. Environmental regulations were and still are very strict in Alberta, especially where local residents and ranch animals are located close to sour gas operations. Norcen Energy did an investigation and found that LO-CAT was the technology best suited for the H2S removal for their needs. The St. Albert unit was commissioned in 1991 and has run continuously (albeit for scheduled shutdowns or shutdowns due to the upstream amine unit) for 12+ years. In 1994, ATCO began the process of purchasing the Industrial Gas System division of Norcen Energy and took over the operations at the St. Albert plant in early 1995.
Being in an extreme winter environment, the ATCO LO-CAT unit (along with 3 other licensed LO-CAT II® units in Alberta) was built to handle the cold temperatures of northern Alberta. The Autocirculation LO-CAT system includes a sulfur melter that produces a pure molten sulfur product. The system was designed to operate in a temperature range of 32? C in summer to -40?C in winter, reflecting the extreme weather conditions in this region of Canada. The vessels and pipes are insulated and heat traced, and a solution heater is installed in the autocirculation vessel. The circulation pumps are indoors.
ATCO employs 3 people full-time to operate the Amine/LO-CAT plant, plus 2 maintenance persons with 50% of their time devoted to the plant. The plant is staffed seven days per week from 8am-4:30pm. The plant runs unattended the rest of the day. Manpower requirements for the LO-CAT unit are around 1 _ - 2 man-hours per day. The shift includes pulling a fluid sample every day and analyzing it for specific gravity, sulfur separation, pH, redox, and catalyst concentration and ratio. Other activities include sulfur handling, pump adjustments, and filter cleanings.ATCO Midstream Current Operating Conditions
The plant currently processes 34 MMSCFD at 600 pounds pressure. The H2S concentration is about .07%.
The gas feed to the LO-CAT unit is about 0.7 MMSCFD, 12 pounds pressure, and an H2S concentration of 33,000 ppm. The outlet H2S concentration is below 1 ppm.
In 12 years of operation, the LO-CAT has never been the cause of a production outage. If a minor upset does occur, the operators can accumulate product in the LO-CAT vessel for some time (24 hours maximum) depending on how much H2S is being processed. The ability to accumulate sulfur in the LO-CAT solution for up to 48 hours without shutting down the production plant provides flexibility in performing and scheduling routine maintenance.Turnarounds:
Shutdowns are done each year over a period of 4-6 days. The liquid from the autocirculation vessel is drained after the sulfur is removed. Storage is rented for the LO-CAT solution. The LO-CAT autocirculation vessel is washed with high-pressure water, along with the air and acid gas spargers, and the cooler downstream of the overhead vent gas. The sulfur separator and melter are both cleaned out (with HP water – 20,000 psi water for melter, 10,000 psi water for the LO-CAT vessels) during the turnaround.
The operators indicate that the LO-CAT is a very easy unit to start-up and operate following shutdown. The solution is heated and circulated until the solution reaches operating temperature (40oC), at which time the gas from the amine unit is fed to the LO-CAT unit. Our new designs for a LO-CAT unit allow for 24-36 months without the need for a turnaround for maintenance. However, if there are other units in the plant that require yearly shutdowns, the LO-CAT unit will be shutdown also for inspection.Modifications to the LO-CAT unit
Modifications to the LO-CAT unit have been made in three major areas over the years:
- Odor control
- Automated sparger washing
- Increased agitation in the sulfur settling cone.
Mercaptan Odor Issues Addressed: The unit was originally designed without provision for addressing odors due to mercaptans in the gas. Following start-up, ATCO and GTP together designed a system to remove the odors being emitted from the plant (see picture below). A fin-fan cooler was installed to dry the saturated gas from the oxidizer vessel upstream of a new catalytic oxidizer (incinerator). The moisture removed from the gas stream is returned to the oxidizer vessel to minimize water lost from the system. Installation of the cooler and incinerator have resolved the odor issues.
Figure 4. Odor control and water collection system
Automated sparger cleaning. The original LO-CAT unit had provision for manually opening valves on a header system and water-washing the sparger lines 3 times a day. After the 1994 shutdown, timers were installed to automate the process. The more frequent washes have improved performance.
Increased agitation in the sulfur settling cone. Air injection “rings” are installed in the sulfur settling cone so that “air blasts” can be injected into the cone to agitate the sulfur slurry in the settling cone and prevent plugging. The original valves did not seal properly and air flow was half the design flow. Properly specified values and air flow have solved the plugging issues.
Figure 5. Air injection rings for cone bottom
ATCO is very satisfied with the operation and performance of their LO-CAT unit.
- The unit requires 1.5 to 2 man-hours of operation per day
- The unit is unattended most of the day
- The unit has never experienced an unplanned shutdown in 12 years of operation.
The unit has provided very reliable service for over 12 years while meeting and exceeding the environmental requirements for H2S removal and odor control. The ATCO facility is in good standing with local residents and provincial regulators.
Neil McCagherty, ATCO Midstream Area Supervisor states: 'We have been very happy with the performance and reliability of our LO-CAT unit. It has helped to establish the Carbondale Plant as a well run environmentally friendly plant in the eyes of regulators and our neighbors'.
The operating issues experienced by ATCO are well understood by ATCO and GTP and are reflected in current LO-CAT design. ATCO expects many more years of reliable LO-CAT operation.
Gary Gialet – Plant Supervisor
Curtis Sexsmith – Operator
Rob Girard – Maintenance
Neil McCagherty – Area Superintendent
Neil Carnegie - Maintenance
Spencer Yelland - Operator
Doug Heguy – Marketing Manager – GTP
The State of Liquid-Redox – July 2002
New Developments to a Long-Established Process Technology
By Douglas L. Heguy and Gary J. Nagl