JPH0345310B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] This invention relates to the resolution of light hydrocarbon-containing streams using low temperature phase separation and dephlegmation.
The purpose is to More specifically, the invention provides the recovery of C ₃₊ or C ₄₊ hydrocarbons from light gases and streams containing light hydrocarbons using sequential steps of cooling, phase separation, dephlegmation, and rectification. and purification purposes. [Prior Art Background] Splitting various light gas streams containing hydrocarbons, with the purified light gas stream as one product of the splitting and C ₂ , C ₃ , C as the other product of the splitting. It is known in the prior art to obtain heavier hydrocarbon streams such as ₄₊ or mixtures thereof. It is also known to effect this separation using various combinations of cryogenic separation process steps including phase separation, rectification, and sometimes the use of reflux heat exchangers such as dephlegmators.
Using a combination of cryogenic cooling of the feed gas stream followed by phase separation followed by dephlegmation
It is particularly known for splitting a light gas stream and a heavy hydrocarbon stream from a feed. The heavy hydrocarbon stream can be further divided in a separation column. For example, Tomlinson and Cummings Patent Publication Specification GB2146751A, published in the United Kingdom on April 24, 1985, states that LPG or
NGL and sales gas recovery involves cooling and/or refrigeration of the feed in a process stream, followed by phase separation into light gas and heavy liquid streams, followed by dephlegmation of the light gas stream and removal from the column. This is accomplished by column stripping of the heavy liquid stream which is recycled to the feed. C ₂₊ from a feed gas stream containing light gas components, as described in U.S. Pat. No. 4,002,042.
It is also known to perform phase separation and dephlegmation to recover hydrocarbons. In that method, the gas is cooled, phase separated, and the light gases are split using cryocooled dephlegmation from an external power source. From the phase separator the heavy liquid stream is split in a rectification column using a condenser and reboiling. Another prior art teaching of defragmentation, phase separation and fractionation column processing is described in US Pat. No. 4,519,825. Where,
C ₄₊ hydrocarbons are separated from the feed gas, and in particular the dephlegmator is operated by refrigeration cooling produced by expanding the light gas stream through an expander to a lower temperature and pressure. . Other prior art merely generally related to the technique of this invention, in which a similar feed stream is divided into light and heavy components by cryogenic cooling, phase separation, column separation or dephlegmation alone or in subcombination. Technologies include: D.E.T. Matsukenji and S.T.
Donnelly, “Proven Effectiveness of Mixed Refrigerants in Natural Gas Recovery Processes,” Oil & Gas.
Journal March 4, 1985, p. 116; T. R. Tomlinson and Earl Banks, “LPG extraction process saves energy requirements,” Oil and Gas Journal, July.
15, 1985, p. 81; U.S. Patent No. 3929438; U.S. Patent No. 470940; U.S. Patent No. 4272269; U.S. Patent No. 4303420; U.S. Patent No. 4303427; Patent No. 4443238; U.S. Patent No. 4456460; U.S. Patent No.
4461634; U.S. Patent No. 4482369; U.S. Patent No.
No. 4507133 and U.S. Patent No. 4526596. BRIEF SUMMARY OF THE INVENTION The present invention provides a method for converting a feed gas into C ₃₊ and/or
A heavy hydrocarbon stream containing C ₄₊ hydrocarbons and H ₂ , H ₂ ,
A process for the cryogenic separation of CO, _CO2 , methane and/or _C2 hydrocarbons into light gas streams, the process comprising: cooling a feed gas by indirect heat exchange with a process stream to obtain a portion of said gas; initially phase-separating the partially condensed feed gas into a first heavy liquid stream containing heavy hydrocarbons and a first vapor stream containing light gas components; rectifying by dephlegmation to produce a second heavy liquid stream condensed from the first light gas stream and the first heavy liquid stream; cooling the feed gas with one or both of the streams by expanding to a low pressure and by indirect heat exchange sufficient to partially evaporate the streams; introducing separately into a distillation column operated with an overhead condenser and a bottom reboiler and discharging said streams in said distillation column with heavy carbonization containing C ₃₊ and/or C ₄₊ hydrocarbons as bottom product; separating into a hydrogen stream and a second light gas stream;
the first light gas stream and the second light gas stream;
The method includes recovering to produce one or more product light gas streams comprising H2, _N2 , CO, _{CO2 ,} methane and/or _C2 hydrocarbons. Preferably, the invention cools the first vapor stream in a dephlegmator that indirectly heat exchanges with the first light gas stream expanded to a lower temperature and lower pressure. The present invention can indirectly heat exchange the first light gas with the second light gas to reheat it and provide the reflux duty of the overhead condenser. The invention uses at least a portion of the first or second heavy liquid stream after expansion to a lower temperature and lower pressure to
It can provide part of the vapor phase cooling, or part of the reflux capacity of the overhead condenser of the column, or both. In another embodiment, the invention expands a first light gas stream to a lower temperature and pressure in two or more stages through a working load turbine expander with in-process heating in a dephlegmator. Preferably, the invention is directed to a method such that said first heavy liquid stream comprises primarily C4 ₊ hydrocarbons and said second heavy liquid stream mainly comprises _C1 - _C3 hydrocarbons. . Optimally, the invention is directed to a process in which the feed gas contains no more than 10 mole % C ₃₊ hydrocarbons and the recovery of C ₃ in said heavy hydrocarbon stream is at least 90%. DETAILED DISCLOSURE OF THE INVENTION The present invention provides for the production of C ₃₊ and/or
It provides an efficient method for recovering and purifying C4 ₊ hydrocarbons. Such gas streams include natural gas streams containing NGL hydrocarbons, oil refinery offgas containing LPG hydrocarbons, and various petrochemical offgas mixtures. This invention is particularly useful for containing relatively small amounts of C ₃₊ hydrocarbons, e.g. less than 10 mol%.
The feed gas containing C ₃₊ hydrocarbons is split and at the same time such hydrocarbons are separated in relatively high proportions, e.g. 90% or more of the C ₃ component in such feed gas, and C ₄ and heavier It is convenient to recover essentially all of the components.
The feed gas can include any number of light gas components and heavier hydrocarbons. Such light gas components include methane, ethane, and ethylene, along with hydrogen, nitrogen, carbon monoxide, and carbon dioxide. Propane for heavier hydrocarbons;
Higher saturated and unsaturated hydrocarbons such as propylene, butane, butene, isobutane, isobutylene and pentane, hexane, and potentially residual amounts of heavier hydrocarbons may be included. Throughout this specification, for saturated as well as unsaturated hydrocarbons, propane and propylene equivalents at _C3 ,
And butane, isobutane and its butene derivatives are symbolically represented by C ₄ . The "+" symbol is used to indicate the number of carbon atoms in those hydrocarbon molecular weight compounds and heavier compounds as in the prior art. This invention processes dry feed gases containing light gases and heavy gases including _C3 and heavier hydrocarbons, all of which are in the range of -17.8°C to -45.6°C (0 to 50〓). It is carried out by first condensing a C ₄₊ -rich liquid by phase separation. The remaining vapor is further cooled and dephlegmated to recover C ₃ and residual C ₄₊ hydrocarbons.
The light C ₃ -enriched liquid from the dephlegmator and the heavy C ₄₊ -enriched liquid from the first phase separation are separated and one or both are reheated and added to the distillation column in that column's separated feed. It is partially evaporated before being sent as a gas. The column discharges light gases up to C ₂ hydrocarbons as overhead and removes C ₃₊ hydrocarbons as NGL or LPG products. Refrigeration for dephlegmators is primarily provided by one or more stages of expansion means. Preferably a compressor or an expansion turbine coupled to some type of load such as a generator is used. If necessary, light C ₃ − from the dephlegmator or heavy C ₄₊ − from the separator.
Some or all of one or both of the enriched liquids are heat exchanged and partially evaporated in a dephlegmator to provide additional refrigeration prior to further evaporation by initial cooling of the feed gas. I can do it. Refrigeration of the overhead condenser of a distillation column can be accomplished by external refrigeration, such as in conventional mechanical refrigeration systems, or by recovering refrigeration from the light overhead from the dephlegmator and/or distillation column. Alternatively, it may be carried out by partial evaporation of one or both of the light and heavy liquid streams, or by other suitable methods. Power from the work done expansion of the overhead light gas from the dephlegmator can be applied to the turbine expansion by compressing the feed gas to the method or by a generator or other suitable communication means. It can be recovered from the machine. Phase separators, dephlegmators and multi-stage distillation columns in combination with expansion devices for low level cooling power provision of the feed light components and partial evaporation for high level cooling of the separated light and heavy liquid streams. Compared to other methods for recovering and purifying C ₃₊ liquid hydrocarbons from natural gas and from rectification and/or petrochemical off-gas streams,
Significant energy savings can be achieved. The relatively warm temperature of the partial condensation stage minimizes the amount of _C2 and lighter components condensed into the heavy liquid stream. The reflux action of the dephlegmator minimizes the amount of _C2 and lighter components that are condensed into the light liquid stream and reduces the amount of low level cooling required to condense the light liquid stream. As a result, the C ₃₊ component typically ranges from about 3-8 mol % in the feed to about 40-8 mol % in the feed to the distillation column.
Concentrated to 90 mol%. The energy required in the distillation column to remove residual light liquid components from heavy hydrocarbon products is also reduced. The method of this invention, as shown in Table 1 below,
Compared to many processes described in the prior art, it represents a significant improvement in C ₃₊ hydrocarbon recovery efficiency.

[Table] The invention will be described in more detail with reference to the drawings and preferred operating examples of the invention. 90
A feed gas containing % methane, 6% ethane, 2% propane, 1% butane, and the remainder nitrogen, pentane, and hexane is introduced into dryer 12 from line 10 at a pressure of 550 psiu. . There water and condensed components are removed. The dried feed gas then passes through line 14 at 368.83 kilopascals (535 psiu) and 37.78°C (100°) and enters feed cooler 16 which includes a first heat exchanger. The feed gas is then cooled in the process stream. 3654.35 kilopascals (530psiu) −
The supply gas at 34.5℃ (-30〓) is
is introduced into the phase flow and enters the phase separator vessel 20. Therefore
A significant portion of the C4 ₊ hydrocarbons is separated as the bottom liquid phase, while the main amount of the _C3 -containing gaseous components remains in the gas phase. C ₄₊ hydrocarbons are transferred to phase separator tank 20
It remains to the left of the baffle 22 that defines the tank 20 in the lower, mostly liquid-containing region (note that the baffle does not separate the overhead or vapor region of the tank 20). C ₄₊ liquid 24 is discharged in line 26 and passed through expansion valve 27 -
The temperature and pressure are reduced to 39.4℃ (-39〓) and 2551.15 kilopascals (370psiu). The stream is reheated in feed cooler 16 and then becomes stream 28, which is sent to the distillation column as a separate feed at 56° C. (42°) and 2530.47 kilopascals (367 psiu).
0. The vapor phase of the feed to phase separator vessel 20 ascends in line 60 to dephlegmator heat exchanger 62;
The flow and temperature conditions in the dephlegmator 62 are then such that the condensable liquid is condensed in the heat exchange passages of the dephlegmator and begins to move down the passages, resulting in reflux or rectification conditions in the heat exchanger passages. while the non-condensable light gas components, typically hydrocarbons of lower molecular weight than _C3 , are discharged as vapor through line 64 and reheated in a dephlegmator heat exchanger. Thereafter, the light gas stream is passed in line 66 to a turbine expander 68, where the temperature and pressure of this light gas stream is reduced in line 70 to -17°C. This stream is then fed to the refrigeration and cooling section of the dephlegmator heat exchanger before the gas in line 72 is combined with the overhead from distillation column 30 and discharged as a combined gas stream to line 46 to remove the feed. It is reheated by the cooler 16. The gas in line 48 is then compressed by compressor 50 to 2620.1 kilopascals (380 psiu). The compressor is preferably driven by the expander 68, but may be driven directly or by a generator coupled to the expander. The resulting gas stream is then after-cooled in an after-cooling exchanger 52, further compressed through a series of devices shown as box 54, and after-cooled in an after-cooling heat exchanger 56 as a light gas product stream in line 58. from
3585.4 kilopascals (520psiu), 43.33℃ (110〓)
It can be discharged at a temperature of The light gas product has a composition of 93.7% methane, 6.1% ethane and 0.2% propane. The flow in line 70 can be further reduced in temperature and pressure by an alternative flow diagram shown in line 78, in which case the flow is further reheated and added in a turbine expander 82. After being supplied to line 80 for expansion, it can also be reheated by freezing the dephlegmator 62 in line 84. The C ₃ hydrocarbons condensing and descending in the heat exchanger passages of the dephlegmator 62 return through line 60 to the right side of the buffle 22 of the phase separator vessel 20 and enter the phase separator vessel 20 as a liquid discharged through line 74 . Can be collected. This liquid is expanded through expansion valve 73 to 2551.15 kilopascals (370 psiu) and -
The temperature and pressure are reduced to a temperature of 54.4°C (-66〓). This stream is then reheated with the introduced feed gas into the feed cooler 16.
It has a cooling effect. The stream in line 76 at 2530.47 kilopascals (367 psiu) and 5.6° C. (42°) is then introduced as a feed into distillation column 30 through line 28. By separately introducing the C ₃₊ stream from line 76 and the C ₄₊ stream from line 28 into column 30, the rectification process efficiency of column 30 is increased. The light gases contained in the feeds from lines 28 and 76 ascend the column in the vapor phase, while the C ₃₊ and C ₄₊ hydrocarbons in such streams 76 and 28 descend the column. , is discharged from line 32 as an NGL product. A portion of the product is transferred to a reboiler heat exchanger 3
4 and returned as reboiled steam. NGL products contain major amounts of propane and minor amounts of butane, pentane and higher carbon number hydrocarbons, as well as unsaturated or branched derivatives of such compounds. Light gases from distillation column 30 are removed through line 36 and cooled in overhead condenser 38, and reflux is fed to phase separation tank 40. and reflux capacity from line 42 to the column.
return in accordance with duty). The vapor phase from vessel 40 is removed in line 44 and combined with the flow in line 72 previously generated from the dephlegmator. The condensing capacity of the heat exchanger 38
duty) may be obtained from any type of external cooling means, but alternatively, light gas from line 46 may be used which is discharged from line 90 and returned through heat exchanger 38 from line 92 to stream 46. can. The reflux capacity for overhead condenser 38 also allows for at least a portion of the flow to be removed from line 26 as a flow in line 27, passed through line 90, condenser 38, and line 92, and returned to line 26 from line 29. You can get it even if you twist it. . A similar method can be performed by taking the flow in line 74 to line 75, passing through line 90, condenser 38, and line 92, and returning it to line 74 through line 77. Light gas stream 72 from the dephlegmator and light gas stream 44 from the distillation column may also be recovered as separate light gas products. At least a portion of the flow in line 74 may be branched through line 86 and reheated in dephlegmator or reflux heat exchanger 62 before being recovered in line 88 and returned to line 74. The method of this invention is described in British patent application G.
Although similar to various prior art dephlegmator cycles, such as B.2146751A, it has several advantages compared to such methods.
That is, in prior art methods, the liquid condensed in the feed cooler and dephlegmator is combined, pumped to feed gas pressure, reheated without substantial evaporation, and fed to the top of the evaporation column. ing. As a result, additional cooling is required for feed cooling and additional reboiling capacity is required for C ₃₊ rectification in the column. In the method of the invention, two liquids, a feed cooler and a dephlegmator, containing C4 ₊ and _C3 -enriched liquid streams, are separately reduced in pressure and partially evaporated for feed cooling. cooling is efficient and then partial evaporation for cooling power allows them to be fed to the evaporation column under optimal vapor-liquid feed splitting conditions for the evaporation column. This allows the feed to be provided in the most efficient manner for the operation of the column.
Since the lighter _C3 -enriched liquid evaporates at a lower temperature than the heavier _C4 -enriched liquid, the two liquid streams are separately reheated and partially evaporated. If the two liquids were combined prior to partial evaporation, the resulting mixture would have to be at a lower pressure to provide the necessary cooling with these flows.
This reduces the pressure in the distillation column and requires the overhead condenser to be cooled to a lower temperature, reducing the efficiency of the process. Conventional processes also use the distillation column in a stripping mode without a condenser or rectification capability. As a result, C ₃₊ recovery is uneconomically low unless the overhead vapor from the column is recycled to the feed as shown in the previously described method. However, this recycle stream also requires additional refrigeration cooling for the first feed cooler and second dephlegmator heat exchanger. In particular, dephlegmators require additional low-level refrigeration cooling. Recirculation is uneconomical. This is because the final C ₃₊ recovery must be performed in the purification step from a very dilute C ₃₊ stream comparable to the initial recovery step. The process of this invention operates the distillation column in both the stripping and rectifying regions. The overhead condenser of the column allows for high C ₃₊ recovery at relatively warm temperature levels. This is because the amount of light gas components in the column is typically less than 5% of the light content in the feed gas. This provides a much more efficient purification process than stripping with overhead steam recirculation as in the prior art cycles described above. The present invention clearly shows lower energy consumption, or higher product pressure, or higher recovery, or It shows the advantages of combining the advantages of For these reasons, we believe that the method of the present invention provides significant and unexpected advantages over prior art systems. Although preferred embodiments of this invention have been described, the scope of this invention should be determined from the claims.

[Brief explanation of drawings]

The drawings illustrate preferred embodiments of the low-temperature separation method of the present invention, and dashed lines indicate various modified embodiments.

Claims (1)

[Scope of Claims] 1. Feed gas is a heavy hydrocarbon stream containing C ₃₊ and/or C ₄₊ hydrocarbons and H ₂ , N ₂ , CO, methane,
and/or a process for cryogenic separation into a light gas stream comprising C ₂ hydrocarbons, comprising: a cooling a feed gas to effect partial condensation of the gas by indirect heat exchange with a process stream; b a partial condensed feed; The first gas containing heavy hydrocarbons
initially phase separating into a heavy liquid stream and a first vapor stream comprising a light feed gas component; c rectifying the first vapor stream by cryogenic dephlegmation to separate the first light gas stream and the first vapor stream; producing a second heavy liquid stream that is separated from the heavy liquid stream; d expanding the first and second heavy liquid streams to a lower pressure and temperature and a sufficient amount of water to partially evaporate said fluid stream; cooling a feed gas with one or both of said fluid streams by indirect heat exchange; e. separately applying said first and second heavy streams to a distillation column operated with an overhead condenser and a bottom reboiler; f separating said fluid stream in said distillation column into a heavy hydrocarbon stream comprising C ₃₊ and/or C ₄₊ hydrocarbons as bottom product and a second light gas stream; and g said The first light gas stream and the second light gas stream are recovered to produce H ₂ , N ₂ , CO, CO ₂ , methane and/or
or producing one or more product light gas streams comprising _C2 hydrocarbons. 2. The method of claim 1, wherein the first vapor stream is cooled in a dephlegmator by indirect heat exchange with the first light gas stream expanded to a lower temperature and lower pressure. 3. The method of claim 1, wherein said first light gas stream is reheated by indirect heat exchange with said second light gas stream to provide reflux capacity of said overhead condenser. 4. The first after being expanded to a lower temperature and lower pressure.
2. The method of claim 1, wherein at least a portion of the second heavy liquid stream provides at least a portion of the cooling of the first vapor stream within the dephlegmator. 5. The method of claim 1, wherein the first light gas stream is expanded to lower temperatures and pressures in two or more stages. 6. The method of claim 1, wherein one or both of the produced light gas streams is compressed to an increased pressure after cooling of the feed gas of step a). 7 said first heavy liquid stream comprises primarily C ₄₊ hydrocarbons and said second heavy liquid stream comprises primarily C ₁
2. A method according to claim 1, comprising _C3 hydrocarbons from: 8. The process of claim 1, wherein the recovery of _C3 hydrocarbons from the feed gas of the heavy hydrocarbon stream is at least 90%. 9. The process of claim 1, wherein the feed gas contains less than 10 mol% C3 ₊ hydrocarbons and the _C3 recovery in the heavy hydrocarbon stream is at least 90%. 10. The method of claim 1, wherein at least a portion of the first or second heavy liquid stream after expansion to a lower temperature and pressure provides at least a portion of the reflux capacity of the overhead condenser.