The present invention relates to the well known process (hereafter “Process”) for the cryogenic separation of an air feed wherein:
(a) the air feed is compressed, cleaned of impurities that will freeze out at cryogenic temperatures such as water and carbon dioxide, and subsequently fed into an cryogenic air separation unit (hereafter “cryogenic ASU”) comprising a main heat exchanger and a distillation column system which are contained in a large insulated box (generally referred to as the “cold box” in the industry);
(b) the air feed is cooled in the main heat exchanger by indirectly heat exchanging the air feed against at least a portion of the effluent streams from the distillation column system;
(c) the cooled air feed is separated in the distillation column system into effluent streams including a stream enriched in nitrogen, a stream enriched in oxygen and, optionally, respective streams enriched in the remaining components of the air feed including argon, krypton and xenon; and
(d) the distillation column system typically comprises a first column (hereafter “high pressure column” or “HP column”) which separates the air feed into effluent streams including a nitrogen-enriched vapor stream and a crude liquid oxygen stream; and a second column (hereafter, “low pressure column” or “LP column”) which (i) operates at a relatively lower pressure than the HP column, (ii) separates the crude liquid oxygen stream into effluent streams including an oxygen product stream and one or more additional nitrogen-enriched vapor streams and (iii) is thermally linked with the HP column such that at least a portion of the nitrogen-enriched vapor from the HP column is condensed in a reboiler/condenser against boiling oxygen-rich liquid that collects in the bottom (or sump) of the LP column.
More specifically, the present invention relates to the known embodiment of the Process wherein the refrigeration extracted from liquefied natural gas (hereafter “LNG”) is utilized in order to provide the refrigeration necessary when at least a portion of the oxygen product is desired as liquid oxygen. In particular, the refrigeration is extracted from the LNG by indirectly heat exchanging the LNG in a heat exchanger against one or more nitrogen-enriched vapor streams withdrawn from the distillation column in order to liquefy such nitrogen-enriched stream(s). The skilled practitioner will appreciate the contrast between using LNG to liquefy such nitrogen-enriched stream(s) and the more conventional way of providing the refrigeration necessary to make liquid oxygen product. In particular, the more conventional way consists of turbo expanding a working fluid (typically either nitrogen or air).
A key to the present invention is what happens to the nitrogen-enriched stream(s) that are liquefied against the boiling LNG. In particular, whereas the prior art introduces such stream(s) into the distillation column system, the present invention introduces such stream(s) into a heat exchanger (preferably the main heat exchanger) to be indirectly heat exchanged against at least a portion of the air feed to the distillation column system in order to liquefy at least a portion of the air feed to the distillation column system. In other words, whereas the prior art provides the LNG-derived refrigeration directly to the distillation column system, the present invention provides such refrigeration to the air feed. As further discussed herein, this has the advantage of both reducing the vapor feed to the high pressure column (thereby by allowing a smaller HP column at a smaller capital cost) and avoiding a safety hazard when, as per the prior art, the liquefied nitrogen is introduced into the distillation column directly after being indirectly heat exchanged against natural gas. In particular, in the event there is a defect in the heat exchanger used for the natural gas/nitrogen heat exchange such that natural gas leaks into the nitrogen, the leaked natural gas will be introduced directly into the distillation and thus have the potential to form very hazardous mixtures with oxygen.
The above described safety hazard is an important consideration because it leads to some of the unique features found in the below described prior art processes that utilize the refrigeration contained in LNG to aid in liquefaction.
GB patent application 1,376,678 (hereafter “GB '678”) teaches the very basic concept of how LNG refrigeration may be used to liquefy a nitrogen stream. The LNG is first pumped to the desired delivery pressure then directed to a heat exchanger. The warm nitrogen gas is cooled in said heat exchanger then compressed in several stages. After each stage of compression, the now warmer nitrogen is returned to the heat exchanger and cooled again. After the final stage of compression the nitrogen is cooled then reduced in pressure across a valve and liquid is produced. When the stream is reduced in pressure, some vapor is generated which is recycled to the appropriate stage of compression.
GB '678 teaches many important fundamental principles. First, the LNG is not sufficiently cold to liquefy a low-pressure nitrogen gas. In fact, if the LNG were to be vaporized at atmospheric pressure, the boiling temperature would be typically above −260° F., and the nitrogen would need to be compressed to at least 15.5 bara in order to condense. If the LNG vaporization pressure is increased, so too will the required nitrogen pressure be increased. Therefore, multiple stages of nitrogen compression are required, and LNG can be used to provide cooling for the compressor intercooler and aftercooler. Second, because the LNG temperature is relatively warm compared to the normal boiling point of nitrogen (which is approximately −320° F.), flash gas is generated when the liquefied nitrogen is reduced in pressure. This flash gas must be recycled and recompressed.
U.S. Pat. No. 3,886,758 (hereafter “U.S. '758”) discloses a method wherein a nitrogen gas stream is compressed to a pressure of about 15 bara then cooled and condensed by heat exchange against vaporizing LNG. The nitrogen gas stream originates from the top of the lower pressure column of a double-column cycle or from the top of the sole column of a single-column cycle. Some of the condensed liquid nitrogen, which was produced by heat exchange with vaporizing LNG, is returned to the top of the distillation column that produced the gaseous nitrogen. The refrigeration that is supplied by the liquid nitrogen is transformed in the distillation column to produce the oxygen product as a liquid. The portion of condensed liquid nitrogen that is not returned to the distillation column is directed to storage as product liquid nitrogen.
EP 0,304,355 (hereafter “EP '355”) teaches the use of an inert gas recycle such as nitrogen or argon to act as a medium to transfer refrigeration from the LNG to the air separation plant. In this scheme, the high pressure inert gas stream is liquefied against vaporizing LNG then used to cool medium pressure streams from the air separation unit (ASU). One of the ASU streams, after cooling, is cold compressed, liquefied and returned to the ASU as refrigerant. The motivation here is to maintain the streams in the same heat exchanger as the LNG at a higher pressure than the LNG. This is done to assure that LNG cannot leak into the nitrogen streams, i.e. to ensure that methane cannot be transported into the ASU with the liquefied return nitrogen. The authors also assert that the bulk of the refrigeration needed for the ASU is blown as reflux liquid into a rectifying column.
U.S. Pat. Nos. 5,137,558, 5,139,547, and 5,141,543 (hereafter “U.S. '558”, “U.S. '547”, and “U.S. '543” respectively) provide a good survey of the prior art up to 1990. These three documents also teach the state-of-the-art at that time. U.S. '558 teaches cold compression to greater than 21 bara such that the nitrogen pressure exceeds the LNG pressure. U.S. '547 deals with the liquefier portion of the process—key features are cold compression to 24 bara and refrigeration recovery from flash gas. U.S. '543 further teaches to use turbo-expansion in addition to LNG for refrigeration to liquefy nitrogen.
There is little new art in the literature since the early 90's because the majority of applications for recovery of refrigeration from LNG (LNG receiving terminals) were filled and new terminals were not commonly being built. Recently, there has been resurgence in interest in new LNG receiving terminals and therefore the potential to recover refrigeration from LNG.
With respect to the ASU operation, a fundamental teaching of U.S. '758 is illustrated in FIG. 1. The facility includes an LNG-based nitrogen liquefier (2) and a cryogenic ASU (1). In this example, the cryogenic ASU includes a higher pressure column (114), lower pressure column (116), and main exchanger (110). Feed air 100 is compressed in 102 and cleaned of impurities that will freeze out at cryogenic temperatures such as water and carbon dioxide in unit 104 to produce stream 108. Stream 108 is cooled in 110 against returning gaseous product streams, to produce cooled air feed 112. Stream 112 is distilled in the double column system to produce liquid oxygen 158, high pressure nitrogen gas (stream 174) and low pressure nitrogen gas (stream 180). The nitrogen gases 174 and 180 are warmed in the main exchanger 110 to produce streams 176 and 182. Streams 176 and 182 are processed in the LNG-based nitrogen liquefier to create liquefied nitrogen product stream 184 and liquid nitrogen refrigerant stream 186. Liquid nitrogen refrigerant stream 186 is introduced into the distillation columns through valves 136 and 140.
The principle laid-out in FIG. 1 is also taught in JP 2005134036, JP 55-77680 (JP 1978150868), U.S. Pat. Nos. 4,192,662, 4,054,433, as well as the above referenced U.S. '758 and EP '355. There are two disadvantages related to processes based on FIG. 1. Firstly, should there be a leak of hydrocarbon into ASU refrigerant stream 186, that hydrocarbon will concentrate in the bottom of the lower pressure column and in liquid oxygen stream 158. Since build-up of hydrocarbon in oxygen is to be avoided, for reasons of safety, steps must be taken to ensure that such a leak does not occur in the LNG-based nitrogen liquefier. Secondly, since all of the incoming air to the cryogenic ASU (stream 108) is introduced to the higher pressure column as vapor, this requires a bigger diameter (and thus higher capital cost) for the higher pressure column.
It is therefore desired to provide an efficient process that transports refrigeration of the LNG-based nitrogen liquefier to the cryogenic ASU without the disadvantages associated with directly injecting potentially hydrocarbon laden liquid nitrogen to the distillation columns.
As used herein, “LNG-based nitrogen liquefier” shall be defined as a system that uses the refrigeration contained in LNG to convert gaseous nitrogen into liquid nitrogen. Typical of such systems, the nitrogen will be compressed in stages. If the compression is performed with a cold-inlet temperature, the LNG will be used to cool the compressor discharge by indirect heat exchange. Cooling and or liquefaction of the nitrogen will be accomplished, at least in part, by indirect heat exchange with warming or vaporizing LNG. Examples of LNG-Based Nitrogen Liquefiers can be found in the above referenced GB '678, U.S. '558, U.S. '547, and U.S. '543.