In 2000, Southwest Oil and Gas Company, a Chinese oil and gas development company, began evaluating H2S removal options to treat the sour acid gas stream from a small amine system treating 400,000 m3/day natural gas in the Longchang area of the Sichuan province in China. The process needed to be flexible enough to operate within a 60–110% range of gas flow conditions, and be easy enough to operate largely unattended. The LO-CAT® process was selected, and designed and installed later that year, and was started early in 2001.
Petrochina, through their Southwest Oil and Gas Fields Company, was bringing natural gas wells online in the Longchang area of Sichuan province, southeast of Chengdu. The wells, producing low volumes of natural gas, were also fairly sour and high in CO2. The gas was to be collected at a central processing gas plant, and the decision had to be made whether to process the gas directly with a desulfurization process, or first treat with an amine system to remove the CO2 and H2S from the natural gas to meet pipeline specifications, and then process the resulting acid gas stream, which would contain 23% H2S. To vent it directly would send over 1250 kg/day H2S (1.14 MTPD as sulfur) into the atmosphere. To flare the gas would require the use of treated natural gas as fuel, plus would discharge over 850 TPY SO2 into the air. Hydrogen sulfide and sulfur dioxide are both toxic and corrosive. Technology reviewed included both non-regenerable scavenger products and regenerable systems such as the LO-CAT process. The scavenger products were considered too costly (removal costs range from $3 –$7/lb H2S removed), and required too much material handling and disposal for this large an H2S load. After a detailed review, the LO-CAT process was selected, and a second review was undertaken to determine whether to treat the natural gas directly with the LO-CAT process, or to use an amine plant for bulk CO2 and H2S removal, with the LO-CAT process used on the amine acid gas. The economics favored the second approach, and the project was awarded to Gas Technology Products. The system would include a LO-CAT auto circulation system, sulfur filter and chemical addition equipment, and all auxiliary rotating equipment and controls.Plant Description
Because the plant was located in a somewhat remote area, it was important that the system be able to operate with a minimum of operator attention. Much of the gas plant would be remotely monitored and operated, and it was important that the desulfurization system also fit this plan. For that reason, and to reduce the overall cost of the system, the autocirculation LO-CAT configuration was selected.
The autocirculation system has three basic functions:
- Absorbs H2S from the acid gas and converts it to sulfur.
- Regenerates the iron catalyst using compressed air.
- Sulfur settler
To accomplish these functions, the vessel has three distinct process zones:
- Absorber Section. The acid gas is sparged into the absorber section of the vessel through spargers which line the absorber cross-section of the vessel above the cone. The H2S reacts with the iron catalyst to form sulfur particles. The catalyst circulates through the vessel by the lift generated by the oxidizing air sparging.
- Oxidizer Compartments. The oxidizer section has a separate sparger system that permits uniform air addition over the entire oxidizer cross-section. The sparger is designed to cause a reduction in solution specific gravity which creates an 'air lift' and promotes circulation of the LO-CAT solution in the oxidizer vessel.
- Settler Cone. Sulfur particles settle into the cone, producing sulfur slurry. The slurry is pumped to the belt filter.
The autocirculation LO-CAT design, in use since 1984, continues to be the most reliable, trouble free, and low cost method to treat amine acid gas. With nearly 40 autocirculation systems in operation, out of over 160 licensed LO-CAT systems worldwide, they are the largest single segment of the LO-CAT operations base.LO-CAT Process Description
The LO-CAT process provides an isothermal, low operating cost method for carrying out the modified Claus reaction:
|H2S + 1/2 O2||
|H2O + S0||(1)|
The reaction is carried out in the aqueous phase using chelated iron as the catalytic reagent. Chelating agents are organic compounds which wrap around iron ions in a claw-like fashion to form chemical bonds between two or more non-iron atoms and the iron atom. The system is typically operated in the mildly alkaline pH range to insure good adsorption of the H2S into the catalyst solution. All LO-CAT systems can be thought of as having three distinct process zones; absorber, oxidizer and sulfur handling.
The absorber contactor for this design consists of an autocirculation unit with a sparged absorber. It is in the absorber where the iron catalyst converts the H2S to elemental sulfur. The reaction in the absorber can be described in the following chemical reactions:Absorption of the H2S
|H2S (gas) + H2O||
|H2S (liquid) + H2O||(2)|
Ionization of H2S
|H+ + HS=||(3)|
Oxidation by Ferric Ions (Fe+++)
|HS- + 2Fe+++||
|S° (solid) + 2Fe++ + H-||(4)|
Overall Absorption Reaction
|H2S (gas) + 2Fe+++||
|2H+ + S° + 2Fe++||(5)|
Whereas the treated gas exits the absorber, the reduced iron catalyst solution with the elemental sulfur is sent to the oxidizer where the iron catalyst regenerated and the sulfur settles out in the cone bottom of the oxidizer. The reaction in the oxidizer can be described in the following chemical reactions:Absorption of O2
|1/2 O2 (gas) + H2O||
|1/2 O2 (liquid) + H2O||(6)|
Regeneration of Ferrous Ions (Fe++)
|1/2 O2 (liquid) + H2O + 2Fe++||
|2OH- + 2Fe+++||(7)|
Overall Regeneration Reaction
|1/2 O2 (gas) + H2O + 2Fe++||
|2OH- + 2Fe+++||(8)|
Adding equations (5) and (8) yields the modified Claus reaction shown as equation (1).
The sulfur slurry produced settles into the cone section of the vessel, and is pumped from the cone to a belt filter which will produce a 60% sulfur cake and the LO-CAT solution recovered is sent back to the oxidizer. The LO-CAT unit for this application has been designed to remove over 99.99% of the H2S leaving less than 10 ppmv H2S in the vented acid gas/ air stream. The sulfur cake produced can be used as fertilizer or fungicide. In the event that a suitable use cannot be found, and it must be disposed of into a landfill, it can be disposed of into a non-hazardous landfill.
The ease of operation of the autocirculation configuration makes it ideal for plants with limited operations personnel. The lack of the need for circulation pumps, piping, and external controls greatly reduce the complexity and make the system extremely simple to operate. Most operations can be accomplished from the control room, with the only direct daily operations attendance required is the checking of the chemical addition rates and visually confirming the sulfur storage volumes.
During the initial start-up period, problems were limited to solution foaming, as the operators “lined out” the LO-CAT unit. Since then, the unit has continued to operate well and meet treatment requirements. Because of the remote location, disposal of the sulfur product has been an issue, but GTP continues to investigate options for the reuse of this valuable product, including education of the local governments of the value of the sulfur cake as a fertilizer additive.
Low cost, easy to operate, able to operate unattended, and reliability all describe the LO-CAT system owned and operated by Petrochina’s Southwest Oil and Gas Company. The LO-CAT system has met all those goals, with the simple autocirculation design that continues to be the cornerstone of the mid-range sulfur load natural gas processing market.