LNG is an acronym for Liquefied Natural Gas. All of the processes to produce LNG are based on a common concept cool the natural gas down to a temperature where it is a liquid at ambient pressure, approximately 162. There are different processes in cooling the natural gas, however all of the LNG processes begin in the same way. Raw natural gas passes through the pre-treatment section of the plant, where it is treated to remove carbon dioxide, hydrogen sulphide, mercury, mercaptans and dehydrated to remove water. The treated, dry gas is then precooled to an intermediate temperature, approximately 30~ 40. This will cause the he avier hydrocarbon components, which can freeze at very low temperatures, to condense and separate from the gas. The lean gas, mainly methane and ethane, is then sentto the liquefaction section of the plant, where it is cooled to approximately 162 before sending it to storage.
Three zones could be noted in the process of the gas being liquefied. A pre-cooling zone, followed by a liquefaction zone, and completed by a sub-cooling zone. The three zones have different temperature zone andcooling zone due to different components and pressure of natural gas. The LNG processes are designed trying to closely approach the cooling curve at the different zones/stages of theliquefaction process to achieve high refrigeration efficiency and reduce energy consumption.
For small sized LNG plant, the Cascade Process and Mixed Refrigerant Process are not generally practical as these processes are too complicated. In our design, isentropic gas turbo expander is used to produce cooling. The gas for expansion shall be either the natural gas or nitrogen.
Features of partly liquefied natural gas by using the pressure difference of gas coming from piping manifold |
Vessels with double layers of molecular sieve are utilized to remove the water vapor and CO2 and ensure the plant is running in long time. Pressure difference between the delivery piping and end user could be used to produce LNG economically by the process of turbo expander cooling combined with Joule-Thompson throttle effect. The turbo expander is marched with a booster that can recovery the expansion work so resulting in low energy consumptionand high operating efficiency. As the cold box encloses some high efficient plate-fin heat exchangers that have small volumes, large heat exchange areas,the cold box is compact and has good insulation effect. Also it is easy to be installed on site, once it is erected on site, the onlyreset work is to connect the surrounding piping so the construction expense could be reduce. The production could be adjusted in large scope in thisprocess and the loading could be changed between 70∼120% economically without any supplementary devices. With reference to the features of the raw natural gas and product, the system design hashigh reliability to prevent the probabilities of fire or explosion and ensure the safe running.
|
Features of small sized natural gas liquefier by using turbo expander |
Extremely simple compared with the Cascade Process and Mixed Refrigerant Process. Cooling cycle could be adjusted to match the cooling duty needed by liquefied natural gas, so it is adaptable to different quality of fered gas. The refrigerant is always in gas phase, which eliminates the requirement of refrigerant storage and separation by using Mixed Refrigerant,with the correspondent impact in the safety on the facility, and therefore simplifies the construction of the liquefaction cold box also. No need for premixing of refrigerant. All of the equipments employed in this process are conventional gas processing equipments using no new or unpracticed technology.
|
