Where are molecular sieve used in natural gas liquification?
In an LNG plant, natural gas from the wellhead must be treated before liquefaction can take place. This treatment happens in dedicated units where contaminants like water, mercury and sour components are removed. These treating facilities are an essential part of LNG plants, helping ensure reliable LNG production.
In a typical LNG plant, a split is made between warm and cold units, as these units can be independently located and operated. The cold units are typically operated at cryogenic conditions. In the treating facilities, front-end processing of the raw natural gas occurs to meet the strict feed gas specifications for liquefaction. The order of connection between the different units is determined by the feed gas requirements and the technology selected for each unit.
The feed gas from upstream typically arrives in a slug catcher before the gas is routed, via a knockout vessel, to the first gas treating unit, the acid gas removal unit (AGRU). To ensure problem-free operation of the AGRU, it is essential to have proper liquid knockout and to prevent carryover of liquid hydrocarbons to the AGRU.3 In the AGRU, the CO2 content is reduced to the LNG specification, usually by an amine-based solvent. The CO2 is removed from the gas by contacting with the circulating solvent in a high-pressure absorber. The solvent is then regenerated by stripping out the CO2, using steam at low pressure.
After contacting with the solvent, the treated natural gas is water-saturated. Before liquefaction of the gas can take place, the water content must be reduced below 0.1 ppmv to prevent freeze out in the cryogenic process units.
Deep removal of water is done by molecular sieve adsorbents in the dehydration unit (DHU). Compared with other adsorbents and water-removal technologies, molecular sieves have a high water capacity at low partial water pressures, which ensures that the tight water specification can be met.
Generally, LNG plant designs require that the DHU inventory be changed out once every four years. The changeout of the molecular sieve is usually aligned with other plant maintenance, such that a dedicated shutdown is not required and the molecular sieve changeout does not determine plant availability. Mercury-free feed gas is required to prevent mercury-induced corrosion of the main cryogenic heat exchanger (MCHE), which is made of aluminum. Mercury is removed by a non-regenerable adsorbent.