Floating liquefied natural gas pretreatment system

#Offshore production facilities use pipelines to transport natural gas extracted from deep sea gas fields or gas-laden oil-rich fields to processing facilities located on the nearest shore. These onshore facilities chill the natural gas, which turns the gas into a liquid and shrinks its volume by about 600 times. This chilling is accomplished by a liquefied natural gas (“LNG”) refrigeration train (or, simply, an LNG train). The LNG produced by the train is then loaded onto ships for transportation to customers. Alternatively, in a very different and new processing approach, the natural gas is processed into LNG at sea on board a ship. Such ships are known as floating liquefied natural gas (“FLNG”) facilities.

#Regardless of whether the gas is being liquefied at an onshore facility or at sea, some pretreatment of the incoming natural gas stream is required to remove contaminants, such as water, carbon dioxide, hydrogen sulfide, and mercury, so that the gas stream being processed by the LNG train is about 99 percent pure gas. This pretreatment is usually accomplished by way of an amine unit, which removes hydrogen sulfide and carbon dioxide, and a dehydration unit, which removes water. The pretreatment system then delivers the substantially pure natural gas stream to the LNG train at a temperature of about 37° C. to 49° C. (100° F. to 120° F.).

#Energy consumption is a key cost driver in all LNG trains. In particular, the natural gas stream must be cooled or refrigerated to about −160° C. (−256° F.) to make LNG. Because the refrigeration process consumes more energy than any other process in the LNG train, energy optimization and conservation are key design considerations. The volumes of gas being treated are typically very large, so even small decreases in the temperature of the incoming gas stream to the LNG train may translate into significant energy and cost savings.

#Preferred embodiments of the invention will now be described in further detail. Other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings (which are not to scale) where:

#FIG. 1 is a general layout of a FLNG facility showing a pretreatment system located upstream of the LNG train, according to an embodiment of the present invention.

#FIG. 2 is a simplified process flow diagram of a preferred embodiment of a pretreatment system, according to an embodiment of the present invention. The pretreatment system purifies and cools the inlet natural gas stream before it enters the LNG train, and is well-suited for use in the FLNG facility of FIG. 1.

#It is to be understood that the invention that is now to be described is not limited in its application to the details of the construction and arrangement of the parts illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. The phraseology and terminology employed herein are for purposes of description and not limitation.

#Elements shown by the drawings are identified by the following numbers: 10 From FLNG production/topside 19 Inlet natural gas stream 20 Mercury removal system 21 Substantially mercury-free natural gas stream 30 Membrane system 31 Cooled CO2-rich permeate stream 33 Cooled CO2-depleted non-permeate stream 40 Heat exchanger 41 First cooled CO2-depleted non-permeate stream 43 Second cooled CO2-depleted non-permeate stream 45 Heated CO2-rich permeate stream 50 Gas sweetening system 51 Sweetened natural gas stream 60 Gas dehydration system 61 Substantially water-free natural gas outlet stream 70 To LNG train 71 Single cooled CO2-depleted non-permeate stream 80 LNG gathering system

#This invention describes a pretreatment system that delivers the incoming gas stream to the LNG train at a lower temperature than conventional systems. As a result, less energy is needed to convert the gas stream to LNG. In addition, the pretreatment system has a smaller footprint than conventional pretreatment systems.