The present invention and its advantages will be better understood by referring to the following detailed description and the attached drawing which is a flow diagram of a process for removing NGL from PLNG in accordance with the practice of this invention. The drawing presents a preferred embodiment of practicing the process of this invention. The drawing isnot intended to exclude from the scope of the invention other embodiments that are the result of normal and expected modifications of this specific embodiment.
Detailed description of the invention
The present invention provides a process for removing NGL products from pressurized liquid natural gas (PLNG) having a temperature above about 112ºC . (170º F.) and a pressure sufficient for theliquid product to be at or below its bubble point. PLNG, the feed stock for the process, can be manufactured from any suitable natural gas source by processes such as those disclosed in U.S. Pat. Nos. 6,023, 942. 6,016,665. 5,950,453 and 5,950,453. The source gas for making PLNG may comprise gas obtained from a crude oil well (associated gas) or from a gas well (non associated gas) . Although thecomposition of natural gas can vary significantly, it will contain methane (C1) as a major component. The PLNG will typically also contain ethane (C2), higher hydrocarbons (C3+), and minor amounts of contaminants such as carbon dioxide, hydrogen sulfide, nitrogen, dirt, iron sulfide, wax, and crude oil. Liquid C2+ hydrocarbons are referred to herein as NGL. The process for removing NGL from thePLNG will now be described with reference to the drawing.
PLNG feed stream 10 shown as being a passed to a pump 11 to increase the pressure of stream 10. The amount of pressurization will depend primarily on the anticipated pressure losses associated the NGL removal process of this invention, the desired pressure of the fractionation column 15 and the desired pressure of the lean PLNG stream 21.In some applications, the PLNG feed stream 10 may be at a sufficiently high pressure to not need pressurization by pump 11. From pump 11, the PLNG stream 12 is passed through heat exchanger 13 wherein stream 12 provides the required duty for cooling of vapor stream 20 by indirect head exchange. In passing through heat exchanger 13, at least a portion of stream 12 is vaporized. The partiallyvaporized stream 14 is a passed to one or more fractionation columns where the PLNG undergoes typical distillation. The Drawing show only one fractionation column 15, which contains trays and/or packing to provide contact between liquids falling downward and vapors rising upward. The lighter, more volatile hydrocarbons leave the upper end of the fractionation column 15 as stream 17. Liquid stream 17will comprise predominantly natural gas liquids (NGL), primarily ethane, propane , butanes, and heavier hydrocarbons. The overhead vapor stream 20 is a lean vapor stream (without significant quantities of C2+) consisting predominantly of methane. Vapor stream 20 is passed through heat exchanger 13 wherein the vapor is reliquefied to produce lean PLNG stream 21.
The liquid stream 17 is warmedin heat exchanger 22 by indirect heat exchange with any suitable heating fluid. Stream 17 is partially vaporized in this head exchanger. The vaporized portion, stream 18, is returned to the fractionation column 15 to supply the necessary stripping vapors and heating duty to effect the desired hydrocarbon separation. The unvaporized portion of stream 17 is removed from exchanger 22 as the recoveredNGL product stream 19. Although not shown in the drawing, the NGL stream 19 may be further processed by well known fractionation processes (such as deethanizer, depropanizer, debutanizer, and butane splitter) to separate the NGL into separate products including ethane,propane,iso-butane, n-butane, and pentane plus.
In some application, it may be desirable to increase the ethane content of the...