The concept is the same for all scavenging systems where natural, landfill, digester, or any sour gas and preferred chemicals are placed in countercurrent flow for maximum contact. Most chemicals (all Triazine based chemicals) are not regenerative in these systems and are sent to a waste tank after being spent. As with many scavenger chemicals in a fixed bed application, the inlet gas must be water saturated to assure chemical stays active.
These waste chemical byproducts are easy to manage since they can be dispersed into produced water storage tanks for storage and disposal along with the produced water. The produced water can be disposed of in any saltwater disposal well with no manifest required.
This system features a low complexity of operation after startup; two injection pumps are used to inject fresh and recycled chemical, and the recycle loop filter is changed every three to four days, depending on the application. A Lead-Lag tower system is also recommended to assure no H2S break through at any point. It can greatly reduce active chemical waste by assuring the first tower’s chemical is completely spent and assure 100% runtime with a fresh vessel at all times.
Advantages of H2S scavenging at the wellhead and/or earliest location in the process:
Improved operations and maintenance personnel safety
Improved preservation and longevity of equipment mechanical integrity
Reduced production and compression equipment cost
Reduced annual emissions tonnage
Reduced environmental and safety risks
Reduced downstream H2S contamination and monitoring expense
Steady flow to sales and no downstream shut-in due to H2S content
Advantage over solid fixed bed type scavengers:
Targeting economic chemical efficiencies. A liquid chemical H2S scavenging system must be used in a resourceful manner to maximize the value of the process. Most triazine-based systems cannot economically treat over 2,000 ppm, or 0.2%, H2S content; however, this is solely dependent on the gas volume being treated. This is the same issue for all scavenger type H2S treaters, regardless of the chemical.
Typical claims for triazine scavenger byproducts / reaction waste products include:
Water-soluble proprietary formulation
Non-aldehyde/non-oxidizing; product has sufficient ethanolamine for complete aldehyde reaction
For natural gas/liquid hydrocarbon/brine
Forms thermally stable water-soluble reaction byproduct
Reaction byproducts are classified as nonhazardous
Contains scale-inhibitor package to prevent formation of mineral scale
Chemical consumption is key to the economics of the scavenging process. Chemical efficiency is dependent on the mass percent of triazine or on generic blends of scavenger chemical. The equation below is commonly used as a basic starting point to balance chemical consumption to set rates:
Due to the designed bubble-up with atomized gas contact, this combined process enables controlled, constant, maximum loading of triazine with recirculation (recycle) from the bottom to the top of the tower, as well as internal dispersion of gas and a highly efficient injection design. This assortment of internals reduces chemical consumption by reaching 80% or more of the triazine reaction or triazine’s full potential in this system.
The CCT scavenger system routes the sour gas stream through a two-stage process. First, the inlet gas is sparged through a pool of liquid triazine, and then the pretreated gas is exposed to a mist of fresh triazine as it rises upward in a countercurrent flow in the upper section of the contactor. The gas is finely dispersed through multiple perforated baffles that increase the gas chemical agitation and gas-chemical contact surface area. A combination of fresh and recycled chemical is sprayed into the top of the vessel through atomizing nozzles to allow the pretreated gas to come into intimate contact with fresher chemical facilitating a second reaction that takes somewhat longer than the relatively instantaneous first reaction in the liquid-filled lower section of the contactor. A MeOH injection is also included to assure the spent chemical does not cake or become too viscous.
As the gas rises through the fresh chemical spray, any residual H2S is polished from the gas to an H2S content of less than 1 ppm. The gas travels up through a mist extractor and exits the top of the contactor. The spent chemical is collected in the bottom of the vessel for disposal into the produced water tanks.
To assure 100% runtime and significantly reduce any potential for H2S breakthrough, a second vessel is sometimes employed downstream of the first in a Lead-Lag operation. Using the two vessel allows maintenance and refilling to be “planned” rather than “reactive” with field downtime, flaring, or other shutdown implications.