Use of liquefied natural gas (LNG) coupled with a cold expander to produce liquid nitrogen

摘要

The present invention relates to a process for the liquefaction of a nitrogen stream produced by separating air components, by using the combination of cryogenic distillation with improved refrigeration. Very cold liquefied natural gas (LNG) is employed as refrigerant, with the LNG currently being revaporized for transportation. Multi-stage component compression is used, with the component feed to each compression stage being precooled using sequential refrigeration from the LNG. Expander means for the coldest air component product stream provides supplemental refrigeration at the cold end beyond that which is available from the refrigerant LNG. In a preferred embodiment, the feed nitrogen stream(s) are compressed to at least 300 psi in a multi-stage compressor with interstage cooling provided by heat exchange against vaporizing LNG; the resulting compressed stream is directed into first and second nitrogen substreams, followed by further cooling of the first substream by heat exchange against vaporizing LNG and then expanding the cooled first substream to produce an expanded nitrogen substream. Condensing of the second compressed substream against both vaporizing LNG and the expanded nitrogen substream is carried out. Reducing the pressure of the condensed second nitrogen substream produces a two phase nitrogen stream. Phase separation yields a recyclable nitrogen vapor stream and a liquid nitrogen stream as product.