Exporting LNG from the terminal on Quintana Island requires more than cooling natural gas to a liquefied state prior to loading it onto tankers. First, the natural gas must be treated to remove mercury, CO2, H2S, water and heavier hydrocarbons that would be detrimental to liquefaction equipment. This process will occur at the mainland Freeport LNG site referred to as the pretreatment facility or PTF.
Following are the primary steps of the pretreatment process that will take place at the PTF once it is in operation.
First, raw natural gas (at 600 psig) is delivered to the PTF via FLNG's existing 42-inch-diameter pipeline into the facility's inlet separators. The separators remove any bulk liquids that may have been entrained in the gas.
Next, the feed gas is sent to the inlet filter coalescers that are designed to protect the downstream mercury guard beds. The beds remove any elemental mercury present in the feed gas by direct contact with sulfur-impregnated activated carbon adsorbent. The removal of mercury is required as it causes corrosion to the aluminum exchangers in the NGL recovery units and the aluminum tubing in the main cryogenic heat exchangers at the liquefaction plant.
The mercury-free feed gas is then passed to the booster compressors. The booster compressors raise the feed gas operating pressure from 550 psig to 904 psig and the gas is cooled by the booster compressor discharge coolers to 109 °F.
The next destination for the feed gas is the acid gas removal unit. The unit is used to remove CO2, hydrogen sulfide (H2S) and some organic sulfur compounds from the feed gas to provide a sweet gas with a CO2 concentration of 50 or less parts per million by volume and trace concentrations of sulfur. Acid gas contaminants are then removed by countercurrent contact of the feed gas with the circulating amine solution in the amine contactor. The contaminants are sent to the regenerative thermal oxidizer for further destruction before being released to the atmosphere.
Next is the dehydration unit that removes water from the treated gas to less than 0.1 parts per million to avoid plugging the downstream cryogenic equipment with ice. Dehydration of the gas occurs in two stages. In the first stage, the gas is cooled in heat exchangers and the condensed water phase is removed. The gas then passes through molecular sieve beds for adsorption of the remaining water vapor.
From the dehydration unit the feed gas moves to the natural gas liquids recovery unit that is designed to remove heavier hydrocarbon components from the gas feed to make it suitable for liquefaction. These components (benzene, toluene, xylenes, pentanes and all heavier hydrocarbons) would freeze in the cryogenic equipment and cause plugging. They are sent to the adjacent Freeport LNG storage and trucking facility via a new 8-inch pipeline.
From the NGL recovery unit the treated feed gas is passed to the residue gas compressors. These compressors will raise the gas pressure from 593 psig to 1,176 psig. The stream is then cooled by the residue gas compressor discharge coolers. The treated gas along with the debutanizer overhead product is sent to the liquefaction plant at the Quintana Island Terminal via the existing pipeline.
The pretreatment facility has a gas turbine generator—a GE Frame 7— that is used to produce power by burning boil-off gas generated in the LNG storage and loading facilities on Quintana Island. A 12-inch pipeline connects the LQF with the PTF for transporting this boil-off gas. Waste heat from the gas turbine generator provides process heating to all three pretreatment trains.
"Due to the tight tolerances that we have in the specifications of the LNG that we will send to our clients, the success of the liquefaction project depends on the success of the very complex processes at the pretreatment facility," says Roberto Ruiperez Vara, Freeport LNG’s Director of LNG Technology, Liquefaction.
"The pretreatment facility is comprised of a set of very complex processes that will ensure the success of the liquefaction project."