I would like to share very good article about “How LNG is LIQUEFIED Intro, adsorption cooling then controls” Written by John Lozinski
Gas is delivered to the processing plant by trucks train or sometimes pipeline. The gas must be cleaned removing all impurities and water that would interfere with the freezing process of methane. Water is removed by adsorption so ice will not form in the liquefaction process. Adsorption is the process in which gas liquid or dissolved solids adhere to a surface of the adsorbent.
The next step is liquefaction. There are three types of liquefaction cycles: Mixed Refrigerants, Turbo expansion and Cascade.
LNG liquefaction is basically the same as a modern refrigerator except the temperature required is negative 161 degrees Celsius or minus 258 Fahrenheit. The temperature is reduced by the Joule-Thomson effect to -161C the temperature at which methane liquefies. The process is similar to refrigeration to drop the temperature down to minus 258 degrees F or negative 161 degrees C.
Often an adsorbent is used to remove mercury. Mercury, when present in gas processing facilities, can be a primary cause of corrosion, equipment failure and downstream catalyst deactivation. Mercury has low vapor pressure and low solubility and is liquid at room temperature.
Each gas field has varied levels of mercury parts per billion. The brazed aluminum heat exchangers commonly found in Liquefied Natural Gas plants and petrochemical plants are particularly susceptible to liquid-metal embrittlement caused by mercury. Varied processes are used to remove most of the mercury including carbon and additives because it causes failures of brazed aluminum heat exchangers.
When mercury is present at very low concentrations in relatively large gas streams powdered adsorbents or pellet adsorbents can be used to remove it. The powdered or pellet adsorbents can be injected into the gas stream and, after an appropriate residence time filtered out in a dust collector.
Most systems are designed to remove trace amounts of Mercury from the gas stream. Mercury needs to be removed as over decades it has been the cause of failure of heat exchangers.
Amine absorbers are used to remove acid gas and impurities. Process removes excess heavy gas components like propane, butane and pentanes with amine absorber or acid gas removal. These can be sold. At this point the cooling begins.
Cooling requires an enormous amount of energy. To this end a number of solutions exist. The Methods used are called C3-MR, AP-X, Cascade, DMR and SMR. You can Google how these methods work exactly. Let me say all these methods require large refrigeration Compressors. Many solutions including gas turbines heat exchangers and large diesels are used to drive them. Designs and solutions vary and with modernization come improved solutions.
One method of liquefaction is using heat exchangers between cooling gasses and refrigerants similar to a refrigerator. Most of us have heard of automotive intercoolers for turbo chargers. You might even know there are air to air and liquid to air intercoolers for car turbos.
What LNG does is cooling liquids to methane gas that they want to liquefy. The refrigerant is liquefied by compressing gaseous refrigerant materials like nitrogen, methane, ethane and propane used as coolants. Processes use heat exchanger cool boxes condensing and LNG sub-coolers using nitrogen coolant.
The cascade method uses propane, ethylene and methane as coolants. This is done in a progression and the cycle continues so as to achieve the highest possible efficiency. DMR uses mixed coolant in Shells process while SMR uses only one type of coolant.