Processes for removing light components such as hydrogen or nitrogen from a hydrocarbon gas mixture represent a problem in petroleum processing. Typically, the hydrocarbon gas mixture is a refining off-gas or a natural gas and the presence of the light components in the mixture limit further processing, or prevent sales of the gas mixture without the removal of the light components. Often these light components include hydrogen, nitrogen, carbon dioxide, carbon monoxide and mixtures thereof. A wide variety of hydrocarbon gas mixtures are found in petroleum refineries. Some streams are integral parts of specific processes. Often these streams are recycled from a fractionation column or a separator to a reactor. One such recycle stream may be an impure hydrogen stream which must be purified before returning the stream to the reactor. Other of these streams may be by-product streams of a major hydrocarbon conversion process such as a fluid catalytic cracking unit or an ethylene plant. Processes available for the rejection of the light components from these gas mixtures can employ solvent absorption, cryogenic separation, adsorption over molecular sieve adsorbents, or membrane separation. The choice of a suitable process depends upon many factors, some of which are the product purity that is desired, the gas recovery levels, available pressure losses, pretreatment requirements, off-gas composition, impact of reaction products remaining in the light component stream, and the turndown capability of the selected process.
When natural gas is produced from a gas well or is recovered as associated gas from an oil well, the natural gas contains a number of light components which can reduce its heating value. Typically, a natural gas stream comprises nitrogen, methane, ethane, carbon dioxide, inerts, and C.sub.3.sup.+ hydrocarbons. In order to improve the heat content of the product natural gas, the concentration of nitrogen and other inerts must be reduced. This reduction of inerts is often required to meet the quality specifications of pipeline companies that transport natural gas from a well head or natural gas processing plant to the consumer. Typically the natural gas must meet the following specifications:
______________________________________ Heat Content 900 to 1000 BTU Total Inerts (N.sub.2 + CO.sub.2) 7 mol % maximum Nitrogen 4 mol % maximum ______________________________________
Actual pipeline specifications vary somewhat depending upon the producer's contract for price and quality. In general, a specification for a higher heating value requires a reduced amount of light components such as nitrogen and carbon dioxide. Typically, natural gas at the well head can contain between 3 and 60 mol % nitrogen, between 0.1 and 10 mol % ethane, between 0.1 and 20 mol % C.sub.3.sup.+ hydrocarbons and between 0.1 and 20 mol % CO.sub.2 with the balance being methane. Light components, of which nitrogen is typically the major component, must be removed from natural gas to improve the heat content of the gas and to meet pipeline specification.
Compositions of the raw gas and the amount of impurities that can be tolerated in the product generally determine the selection of the most suitable process for purification.
Cryogenic technology, which consists of several process variations, has traditionally been employed to separate nitrogen from natural gas. The operating principle of the process entails partially or fully liquefying the high nitrogen content feedstream under pressure and at very low temperatures (e.g., as low as -185.degree. C. (-300.degree. F.)). Afterwards, the partially or fully condensed stream is fractionated in one or two columns, which operate in tandem at two different pressures, to separate the feedstream into a rejected nitrogen stream overhead and a high methane content product stream at the bottom. After heat exchanges with the incoming feedstream, the rejected nitrogen stream is used for gas field operation, vented to atmosphere or reused. The bottom nitrogen-depleted product stream is regasified via process heat exchanges, recompressed, and delivered to the battery limits as an upgraded sales gas.
U.S. Pat. No. 4,936,888 relates to the use of cryogenic technology in an integrated dual distillation system for rejecting nitrogen in concentrations as high as 75 mol % or more from gaseous methane in a distillation system employing a high pressure fractionator and a low pressure fractionator. The low pressure fractionator accepts a feed predominantly comprising liquid nitrogen under conditions to produce a high purity nitrogen overhead stream and a high purity methane bottom stream.
U.S. Pat. No. 4,711,651 relates to a process for the separation of a high pressure gas stream such as refining gas in which the starting gas mixture is cooled and separated into a first vapor portion and a first liquid portion which is expanded to an intermediate pressure. The first vapor portion is further cooled and separated into a second vapor portion which may be further processed for ultimate recovery of a methane-rich product gas. Refrigeration is recovered from the mixed intermediate pressure stream.
U.S. Pat. No. 5,051,120 relates to the cryogenic processing of a feed containing nitrogen and methane. The method is used in treating natural gas which is contaminated with nitrogen. An improved stripping column is provided in which the methane product is produced at a higher pressure than would otherwise be possible.
Other process options have used non-cryogenic routes which employ solvent extraction. U.S. Pat. No. 4,832,718 relates to a process for processing a natural gas, a thermal or catalytic cracking gas or refining off-gas to produce a methane-rich product, a nitrogen-rich product, a hydrogen-rich stream or an olefin-rich product therefrom by solvent extraction. The process further relates to extractive flashing and extractive stripping using selective physical solvents. The process employs a solvent to separate N.sub.2 from hydrocarbons by extraction (i.e., absorption) at moderate temperatures (ambient to -30.degree. F.) and under pressure. As an N.sub.2 -rich gas stream and a solvent stream come in intimate contact in a packed column, the solvent dissolves nearly all of the hydrocarbons and a small amount of N.sub.2 in the feed gas stream, leaving behind a rejected gaseous N.sub.2 stream that contains the major fraction of the N.sub.2 previously present in the feed stream and a small amount of unrecoverable hydrocarbons. The dissolved hydrocarbons are subsequently released from the rich solvent by successive pressure reductions. The gases that evolve from the pressure reductions are recompressed and recombined into an upgraded sales gas stream. The regenerated lean solvent stream is recycled to the absorber. After refrigeration recoveries, the rejected N.sub.2 stream is available at a relatively high pressure and can be used for field applications, vented to the atmosphere, or reused.
U.S. Pat. No. 4,623,371 relates to the recovery of hydrocarbons from natural gas containing acidic compounds and from 3-75 mol % nitrogen to provide up to three products: nitrogen-gas product, C.sub.1 -rich gas product, and a C.sub.2.sup.+ liquid product. The process extracts a natural gas stream with a physical solvent to produce a nitrogen stream and a methane-rich solvent stream. The methane-rich solvent stream is flashed to provide a stripped solvent stream and substantially all of the C.sub.1.sup.+ hydrocarbons as a stream of flashed-off-gases. The stripped solvent is recycled to the extraction step.
U.S. Pat. No. 5,047,074 relates to a process for purging nitrogen from natural gas by passing the gas through an absorption column at elevated pressure and contacting the gas with an absorbent consisting primarily of a poly alpha olefin. The non-absorbed gas consisting predominantly of nitrogen is passed out of the absorption column to waste or to recovery. The resulting rich absorbent is desorbed to obtain the desorbed light hydrocarbons and the lean absorbent.
U.S. Pat. 5,019,143 relates to the use of a particular group of solvents including paraffins, naphthenes, C.sub.6 -C.sub.10 aromatic compounds and dialkyl ethers of polyalkylene glycol to contact the gas feed mixture in a demethanizing absorber with a reboiler operating between 50 to 400 psig and a temperature of +10.degree. to -40.degree. F. for the separation of ethylene and lighter hydrocarbons from the feed with a distillation column to regenerate the rich solvent. U.S. Pat. Nos. 4,511,381 and 4,526,594 relate to the separation of natural gas from natural gas liquids with physical solvents. Natural gas liquids include hydrocarbons heavier than methane. The physical solvent used for the absorption step in U.S. Pat. No. 4,511,381 is described in terms of a solvent having a relative volatility of methane over ethane of at least 5.0 and a hydrocarbon loading capacity at least 0.25 standard cubic feet of ethane per gallon of solvent. The rich solvent stream is flashed and the gas fraction is compressed, cooled, and condensed to produce the natural gas liquids. U.S. Pat. No. 4,526,594 relates to the selective extraction of a stream of hydrocarbons that are heavier than methane from a natural gas stream with a physical solvent by selectively rejecting the consecutively lowest molecular weight portion of the extracted stream by the use of a second extraction step with a physical solvent to reject hydrocarbons heavier than methane, flashing the resulting rich solvent to separate the selected heavier hydrocarbons, and subsequently de-ethanizing the selected heavier hydrocarbons.
U.S. Pat. No. 4,883,514 relates to a process for the removal of nitrogen from nitrogen-rich gases which contain more than 3 mol % nitrogen. The nitrogen-rich gas stream is contacted with a lean oil comprised of paraffins, aromatic or cyclohydrocarbons or mixtures thereof having molecular weight between 75 and 250 at temperature no lower than -40.degree. F. to produce a nitrogen stream as an overhead product and a bottoms methane-rich oil stream. The bottoms methane-rich oil stream is flashed to recover a methane-rich overhead gas product and a lean oil rich bottoms stream, and the lean oil stream is recycled to the absorption step. The patent teaches that the contacting or absorption may take place in a methane extraction column which includes a reboiler and is operable in an extractive stripping mode.
It is an object of this invention to provide a process for the rejection of nitrogen from natural gas streams using noncryogenic absorption with a hydrocarbon solvent. It is a further object of this invention to provide a process for the rejection of nitrogen from natural gas which can achieve gas recoveries of greater than 99 mol % and provides lower compression requirements, lower capital investment, and lower solvent losses than previous technology.