FAQ’s About Sulfur Removal and Recovery using the LO-CAT® Hydrogen Sulfide Removal System
Why choose LO-CAT?
LO-CAT sulfur removal systems are known for:
- Highly efficient H2S removal, always greater than 99.9%
- Good economics for mid-range sulfur removal applications
- Easy operation
- Highly flexible
- 100% turndown for most applications
How do I know if I have a good opportunity to apply LO-CAT?
LO-CAT can effectively treat 100% H2S streams, very dilute H2S streams, and everything in between.
The first criterion is generally the amount of sulfur that needs to be removed. The typical economic niche for LO-CAT is more than 200 kg/day and below 20 tonnes per day. Below 200 kg/day chemical scavengers are generally a good economic choice. (GTP has both a solid scavenger system for H2S removal, Sulfur-Rite®, and a liquid scavenger system, The Eliminator™, for H2S removal.) Above 20 tonnes per day, modified Claus units with LO-CAT tail gas units are generally more economic.
In applications where the H2S concentration is below 15% of the acid gas exiting an amine gas sweetening system, Claus operation can be very troublesome. GTP has designed and built LO-CAT units greater than 80 tonnes per day for this type of application.
What drives the cost of a LO-CAT unit?
A LO-CAT unit has three sections:
- Gas treating
- Catalyst regeneration
- Sulfur handling
The cost of the gas treating section is generally a function of gas volume and pressure. The quantity of sulfur drives the cost of the last two sections.
The major operating costs of the LO-CAT unit are chemical costs and electricity. The chemical consumption is directly related to the actual amount of H2S removed from the gas stream. The power cost is largely constant when the unit is running.
How can I get more information on GTP’s sulfur removal products?
Just ask. You can call us at 847-285-3850, or e-mail your request to: firstname.lastname@example.org or email@example.com
How do I ask for a quotation?
Go to our website at www.gtp-merichem.com where you will find a LO-CAT data sheet. For more information contact William Rouleau, GTP’s Manager of Applications Engineering at firstname.lastname@example.org, or call 847-285-3850.
Is LO-CAT environmentally friendly?
The LO-CAT process is very friendly to the environment. The catalyst solution is made up of organic chelates and iron. Both ingredients are supplied as "micro-nutrients" for use in agriculture. The sulfur product itself is used as a soil amendment in agriculture.
The one warning is that impurities from the gas stream can be absorbed by the catalyst and end up in the sulfur product. Therefore, final assessment of the toxicity of the process and the sulfur product should be made for each application, based on the contents of the gas being treated.
Is LO-CAT old technology?
LO-CAT is celebrating its 25th anniversary in 2003. (Hydrocarbon Engineering magazine acknowledged the occasion on the cover of its February 2003 issue). 151 LO-CAT units have been licensed worldwide, so LO-CAT is a well established, commercially proven technology by any standard.
GTP continues to invest in the technology. Recent advances include a patented means for controlled production of a free-radical scavenger in the catalyst system to reduce chemical consumption. GTP is also in the commercial demonstration phase of a new method for catalyst regeneration. LO-CAT just keeps getting better.
While new methods for treating H2S are emerging, LO-CAT continues to be the leading technology in its niche, and continues to be competitive on performance, price, and operating cost.
Iron has long been known to be an effective catalyst for the oxidation of H2S to elemental sulfur. Iron sponge systems have been in service for over 100 years. The "trick" to getting the cost down to treat larger volumes of gas was to make the iron catalyst regenerable, and to put it in solution.
ARI Technologies, now Gas Technology Products, succeeded in developing a stable system of chelates in the early 1970’s that were both stable relative to iron solubility, and oxidation, as oxygen is used to regenerate the iron catalyst. The result was the development of the LO-CAT Hydrogen Sulfide Removal System.
How does LO-CAT work?
The H2S containing gas comes in contact with LO-CAT solution in the "absorber." The
H2S is converted to elemental sulfur and is removed from the absorber with
the "spent" catalyst solution. "Sweet gas" exits the absorber.
The "spent" catalyst goes to the "oxidizer" where 1) air is blown into the catalyst solution to regenerate the iron catalyst; and 2) the elemental sulfur settles in a cone to create concentrated sulfur slurry. The sulfur slurry is pumped to the sulfur filter where sulfur is washed. Most catalyst is recycled back to the LO-CAT unit, while some catalyst and wash water exit LO-CAT as part of the sulfur product.
In this process, iron, in its ferric state (+3), is held in solution by chelating
agents. 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 are summarized
as follows with (l) and (v) representing the liquid and vapor states, respectively;
H2S (v) + H2O (l) H2S (l) (1)
H2S (l) H+ + HS- (2)
HS- + 2Fe+3 S0 + 2Fe+2 + H+ (3)
1/2O2 (air) + H2O (l) 1/2O2 (l) (4)
2Fe+2 + 1/2O2 (l)+ H2O 2Fe+3 + 2OH- (5)
H2S (v) + 1/2O2 (v) S0 + H2O (6)
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.
Is LO-CAT a typical "iron-redox" technology?
LO-CAT is the first developer of "iron-redox" technology. However, lumping different processes together under the "iron-redox" technology acknowledges the similarities of the processes, but ignore key differences, which can have profound differences in operation. The key differences between "iron-redox" technologies are catalyst concentration and chelate stability.
What is the difference between Sulferox and LO-CAT?
First difference is catalyst concentration. Sulferox iron concentration is
usually 2%; LO-CAT is usually about 0.1% to 0.05%. The more dilute catalyst
system of the LO-CAT has benefits in system stability, ease of operation
and catalyst consumption.
LO-CAT also uses a patented chelate system that is more resistant to oxidation; further enhancing system stability and catalyst cost.
The result is that LO-CAT chemical cost is one-half to one-third of a Sulferox unit, and will have a more stable operation. The LO-CAT unit will have higher circulation rates and slightly higher power costs.
What are the materials used in LO-CAT?
The most common material used for LO-CAT systems is 304 stainless steel. (The same chelates that solubilize the iron catalyst will do the same to iron tanks and pumps.) FRP is also commonly used, and under certain conditions, such as a build-up of chloride ions from the feed gas, offers enhanced stability relative to stainless steel.
Can I treat high-pressure gas with LO-CAT?
Yes. Many LO-CAT units are designed to treat the offgas from a traditional amine unit. In this case, the amine unit will sweeten the high-pressure gas. The H2S and a portion of the CO2 will feed a simplified LO-CAT unit where the H2S is converted to sulfur and the CO2 is vented or recycled to pressurize the producing field. This configuration can be a cost effective way to remove the sulfur and adjust the gas composition to meet sales specification.
In cases where CO2 does not need to be removed, the high-pressure gas can be treated directly with a conventional LO-CAT unit. Past challenges of conventional LO-CAT direct treating high pressure gas have been effectively met with the addition of a controlled degassing vessel between the absorber and the oxidizer, and the use of multi-stage centrifugal pumps to circulate the LO-CAT solution.
LO-CAT's are being designed to treat up to 600 p.s.i. gas streams. Treatment of gas streams over 1,000 p.s.i. have been demonstrated in pilot plant tests.
Does LO-CAT remove all sulfur compounds from the gas stream?
No! LO-CAT is effective in treating and removing H2S, only. Some mercaptans will be converted and removed. However, GTP does not make guarantees based on removal of mercaptans. COS and CS2 will exit the LO-CAT unreacted.
We believe about half of the light mercaptans (methyl and ethyl) will exit with the sweet gas. Of the remainder, some will exit through the oxidizer vent, and some will be converted to disulfides and exit with the sulfur.
How are LO-CAT's used in refineries?
LO-CAT applications include treating fuel gas streams, hydrotreater gas, amine acid gas, sour water stripper gas, sulfur pit vent gas, and Claus Tail Gas.
What advantages does LO-CAT have treating Claus Tail Gas?
LO-CAT’s primary advantage is in retrofitting existing Claus units that are being forced to meet tighter emission limits. The LO-CAT unit would be new capacity, so it would not require debottlenecking of the existing Claus unit. In addition, the LO-CAT unit can treat the sour waterstripper gas, improving Claus operation and freeing up existing capacity in the Claus unit, and/or be sized to provide redundant capacity for the Claus unit.
Is LO-CAT suitable for very cold climates?
Certainly. The LO-CAT reaction is exothermic. The blower for the oxidizer air adds about 50-70 degree C to the ambient air temperature, so the air inlet to the oxidizer is seldom a problem. The primary temperature issue occurs when the plant is shut down for an extended period. In such cases, tank insulation and pipe tracing should be used to limit heat loss from the LO-CAT system.
Do I have to buy LO-CAT chemicals for my LO-CAT unit?
Each LO-CAT unit is sold with a commitment to buy LO-CAT chemicals. This allows GTP to guarantee chemical consumption, outlet gas composition, and sulfur removal capacity. It also improves GTP’s ability to provide effective Technical Service.
What price protection do I have if I must buy LO-CAT chemicals?
LO-CAT has two provisions to protect you from unreasonable price increases. The maximum price that GTP can charge for chemicals is determined by a formula based on escalators published by the U. S. government. 80% of the cost of the chemicals is related to the cost of raw materials, and is allowed to increase according to the Producer Price Index for Specially Chemicals. 20% is related to the cost of labor, and is allowed to escalate according to the cost index for labor in the chemical industry, published by the Department of Labor.
The second provision for price protection is the guarantee that you will receive the lowest price available for LO-CAT chemicals, F.O.B. our factory.
What emissions exit the LO-CAT unit?
Usually there are only two streams that leave the LO-CAT, the sulfur product, and the oxidizer vent gas. In most cases, vacuum belt filter or a simple bag filter produces the sulfur cake. The sulfur product that results from those two filters usually eliminates the need to have a separate liquid blowdown stream for the catalyst.
The oxidizer vent stream may be able to be vented direct to the atmosphere. It depends on the composition of the gas being treated in the LO-CAT unit. In some instances, the oxidizer vent gas may contain some low level of hydrocarbon or ammonia, depending on the gas being treated, and may necessitate incineration or oxidation of the vent stream.
Does LO-CAT sulfur smell?
Usually no, but it depends on the gas being treated. In natural gas processing applications, organic sulfur, including mercaptans, can create disulfides that will exit LO-CAT on the sulfur cake. In this case, the sulfur will have a sulfurous odor. If there are not other organic sulfurs in the gas stream, the sulfur will not smell.
In the United States, Hondo Chemical Company of Bakersfield, California is very active collecting LO-CAT sulfur to use in agriculture in the California grape region. There are proposals to expand this business to other parts of the world.
In other cases, the sulfur can usually be disposed of as a non-hazardous waste. However, in some cases, the gas being treated will have a contaminant that is introduced into the LO-CAT system, and may exit the plant in the sulfur cake. In such instances, the sulfur must be disposed of in the same method as the contaminant itself.