Hydrocarbon gas processing

in 1 x 146554 • t

The present invention relates to treating gases contain, containing hydrocarbons and other gases with similar volatility, in order to separate the condensable fractions. In particular the invention relates to treatment of gases such as natural gas, synthesis gas and refined gases for recovering most of the propane and most of the ethane in the gas along with virtually all the heavier hydrocarbon in the gas, and apparatus for this.

Gases containing hydrocarbons and other gases with similar volatility that can be treated according to the invention include natural gas, synthesis gases from the second hydro-carbonaceous materials such as coal, crude oil, gasoline, oil shale, tar sands and lfgnitt. Natural gases are often a predominant amount of methane and ethane (i.e., the combined and C2 ~ fractions form at least 50% off-gas on a molar basis). There may be minor amounts of relatively heavier hydrocarbons such as propane, butanes, pentanes and the like, and CC> 2 and other gases.

A typical composition of natural gas can be treated according to the invention is in approximate mole%: 80% methane, 10% ethane, 5% propane, 0.5% isobutane, 1.57 »n-butane, isopentane 0.257, 0.257 n pentane, 0.5% hexane + balance nitrogen and carbon dioxide. Sulphurous gases are also often in natural gas.

In recent substantial increases "demand for ethane and propane from natural gas has led to demand for methods that provide high recovery rate or yield of these products. Available processes for preparation of these gases is based on cool-Ofingen and freezing of gas, oil absorption,

II

146554 2 absorption with cooled oil and the later cryogenic processes using the principle of gas expansion through a mechanical device for the production of power while simultaneously heat is drawn out of the system. Depending gasskildens pressure, the discovery wealth (the content of ethane and heavier hydrocarbons) and the desired final products, these prior methods or combinations of them is used.

Recovery by cryogenic expansion currently preferred generally for the recovery of ethane as it combines maximum simplicity with easy start-up, flexibility, good efficacy, safety and good reproducibility. U.S. Patent Nos. 3.360.9 ^, 3-292380 and 3,292,381 describe relevant methods.

In the extraction which is based on a typical cryogenic expansion process, an incoming gas stream (hereinafter also called charge or supply) under pressure is cooled by heat exchange with other · inaterialstrømmer in the process and / or external cooling media like. of a compression refrigeration system with propane. When the gas is cooled is condensed portion liquids collected in one or, more separators as high pressure condensate containing most of the desired C2 + -bestanddeler. Høytrykkskondensatet are then expanded to a lower pressure. The vaporization occurring during expansion of the liquid gives one further cooling 'of the remaining portion of the liquid. De.n cooled stream 'form a mixture of vapor and liquid freed of methane, demetani-ed in a demetaniseringstårn. .Demetaniseringstårnet Is a distillation tower where the expansion cooled stream is fractionated separation of residual methane, nitrogen and other volatile gases as top gas from the desired products consisting of ethane, propane and heavier components which arises as bottom fraction.

If incoming charge not fully condensed, and often do not, it becomes gases from this partial condensation expanded to a lower pressure. Chart liquid condensed due to the additional cooling during expansion. The pressure after expansion is usually the same as the operating pressure of the methane-removal column. They produced fluids used words like depth charge to metanfjernings column. Most often remaining ga'sser and overhead from demethanizer column combined as methane residual gas.

ii 3 146554

Under ideal operation of such a separation process, the gases leaving the process containing substantially all the methane contained in the charge and substantially no hydrocarbons equivalent to ethane or heavier components. The bottom fraction from the demethanizer column will contain virtually all the heavier components and substantially no methane.

In practice achieved, however not the ideal situation for the reason that a common meta removal plant is operated largely as a distillation column .. methane product will therefore typically comprise gases leaving the top fractionation stage of the column together with gases which have not undergone any distillation. Significant losses of ethane occurs because the gases exiting from the low temperature separation contains ethane and heavier components som.kunne recycled if these gases may be further cooled elite if they were brought into contact with a. substantial amount of relatively heavy hydrocarbons, eg. and heavier, which could absorb the ethane.

As described in U.S. Patent Nos. * 1.171.96 / 1. .

and the Norwegian application 772057, improved ethane yield by cooling the condensed high pressure fluid before expansion thereof. How cool will reduce the temperature of the flash expanded floating charge carried at metanavkokeren (decoction column) and thus enhances the ethane yield. As further described in said references can be obtained by pre-cool the high pressure chargen- which are liquids reduce the liquid temperature sufficient that it can be used as inbound peak charge on .metanavkokerkolonnen while the expanded steam is fed to metanavkokeren on an insertion level which is located between the top and the column bottom. This variation allows å.utvinne ethane contained in the expanded gas, which would otherwise be lost.

It is clear that the connection of external cooling at this stage of the process is difficult because of the very low temperatures as it operates under. In typical metanavkoker operations keeps the expanded liquid and gas temperatures in the range -81} 0C to -123 ° C. Accordingly, the cooling of the condensed high pressure liquid best achieved by heat exchange with streams derived in the process as described in the above patent.

It will be this understanding that high pressure liquid danner- depth charge -generelt contain volatile gases

II

146554 *> (as methane) and mixtures with lower volatility and cooling the incoming high pressure supply by expansion due to evaporation of part of the volatile gases. In the present invention, the temperature difference by expansion of the high pressure supply is increased by combining this .tilførsel with a stream from the process that has a bubble point lower than the high-pressure liquid supply bubble point at the pressure that the charge is expanded. Prior to expansion is therefore the combined stream is cooled to a temperature lower than the high pressure feed temperature. '

This can occur by cooling the flowing stream of high-pressure charge or process gas (or both) until they are combined, or by cooling the combined stream if this is more convenient. After expansion, the combined stream "now a lower temperature due to higher contained amount of the more volatile components that reduce the combined flow bubble point and evaporates at the lowest pressure and absorb increased amounts of evaporation.

It will be appreciated that in practice the bubble point temperature of the incoming high-pressure fluid be several degrees or higher Jover the real treatment temperature because of the missing '^ complete equilibration during condensation and separation' r - · in board of high pressure liquid and gas. This condition also occurs when the high pressure fluid is cooled as described in said references.

'When bbblepunktstemperaturen significantly exceed the real treatment temperature for incoming high pressure fluid, the temperature drop at the expansion being less than the temperature drop could be achieved by expanding the high pressure liquid by the bubble point.

In the present invention, such liquid charge of high pressure liquid combined with a more volatile process stream as described above and with moderate additional cooling gl an improved operation and yield. This is because mixing the process gas in the high pressure liquid-charge will lead to the absorption of volatile gases until the actual bubble point for the pressure fluid can be reduced to the treatment temperature. Expansion of a liquid with such a reduced bubble point will give colder refrigeration temperatures.

"According to the present invention there is provided a method and apparatus for separating a feed gas under pressure. INVENTION significant special way I '1 5 146554 character appears from the following claims.

For better understanding of the invention with reference to the attached diagrams and charts in which: FIG. 1 is a strømiiingsdiagram of a single stage refrigeration plant for expansion cooling the natural gas in the prior art, with a set of conditions relating to a typical rich gas; Fig. 2 shows a similar flow diagram of a known type with a set of conditions for typical lean natural gas, Figure 3 shows a flow diagram of Norwegian application no. 772057, and illustrates a technique whereby high pressure incoming liquid may be precooled before expansion, Fig. 4 is a diagram showing application of the invention in pre-cooling of incoming charge according to FIG. 3, and FIG. 5 shows part of a flow diagram with use of the present invention by pre-cooling of incoming 'j air charge by flash expansion of part of the charge.

FIGS. 6A and 6B are graphs of the carbon dioxide content versus temperature of an embodiment of the invention compared with the ratio in known embodiments. .

In the following explanation of the attached diagrams, it is associated tables indicating flow rates calculated for representative process conditions. The flow amounts expressed in kg'mol / hour which is rounded to the nearest whole number. The total flow rate fremgår.av tables include all non-hydrocarbon components and is therefore usually larger than the sum of the hydrocarbon component flow rate The indicated temperatures are approximate rounded to the nearest degree.

Referring to FIG. 1 For a more detailed description of a typical common ethane recovery process, where it operated with an incoming gas from which carbon dioxide and sulfur compounds are removed (if the concentration of such compounds in the incoming gas would cause the product did not meet specifications or caused icing equipment), and that dehydrated, and this gas enters the process at 49 ° C and 64 kg / cm absolute as stream 23. the split into two parallel streams and cooled to -7 ° C by heat exchange with cool residue gas at -15 ° C in heat exchanger 10, with the product liquids (stream 26) at 28 ° C in heat exchanger 11 and with metanavkokervæsken at 12 ° C for metanavkoker 12. From these heat exchangers combined streams

II

146554. 6 again and then into the gas cooler, heat exchanger 13, where do combined stream is cooled to -12 ° C with propane refrigerant at -15 ° C. The cooled stream is divided again into two parallel streams and further cooled by heat exchange with cold residue gas (stream 29) holding ~ 77 ° C in heat exchanger lit and with metanavkokervæsker at -62 ° C for methane sideavkoker 15.

These flows are combined again and goes into a high pressure separator 16 by -it3 ° C and 63.3 kg / cm ^ as stream 23a. The condensed liquid (stream 2H) separated and fed to methane-avkokeren 19 through expansion valve 30. There may be used one expansion engine in place of the expansion valve 30 if desired.

The cooled gas from high pressure separator 16 passes through the expansion machine 17 where the gas is work expanded.

2 rows of 63.3 kg / cm to 20.4 kg / cm. Work expansion cools the gas to -87 ° C. Expansion machine 17 is preferably a turbinekspander with compressor 21 mounted on the expansion machine shaft. In certain prior art embodiments, the expansion machine 17 is replaced by a conventional expansion valve.

Liquid condensed during expansion flushed out in the low pressure separator 18. The liquid goes on level control through line 25 ti'1 metanavkoker column 19 at the top and flows from, a pipe-destillasjonsskål (not shown) as the top-CHARGE to the column 19 ·

It is noted that in certain embodiments the low pressure separator 18 is inserted as part of metanavkoker 19 and then take up the top of the column. In this case, 'go out flow from the expansion machine over a pipe distillation tray in the bottom of the separator section at the top of the column.

The liquid then flows from distillation tray (basin) som.topp-charge to metanavkokerseksjonen column.

Liquid charge to metanavkokeren 19 goes down through the column and in contact with gases such as methane is distilled off from the liquid and generates a meta-free liquid product at the bottom. The heat elapsing for preparing distilling-gases coming from the heat exchangers 12 and 15 ·

Gases freed of condensed liquid in metanavkokeren 19 passes through line 27 together with the cold gaseous effluent from separator 18 through line 28. The combined gas stream going then back through line 29 to heat exchanger 14 and 10. In these heat exchangers, the gas flows through η ii 7 146554 - compressor 21 driven by expansion machine 17 and is directly connected to this. The compressor 21 compresses the gas at an initial pressure of approximately 21.4 kg / cm. The gas passes then into the compressor 22 and compressed to outgoing final pressure of 63.3 kg / cm2.

The flow rate of incoming and outgoing liquid components, the recovery rate of outgoing liquid and compression effect of this prior art method shown in FIG. 1 shown in the following table.

TABLE I (FIG. 1)

Flow volumes - kg mol / hour

Strøm'nr. "Methane Ethane Propane Butane +. Total 23 1100 222 163 130 1647 24 795 202 157. 129 1300 25 16 10 5 1 32 26 3-162 157 130 453

extraction

Etan 72,9¾ 244,458 liters / day '·

Propane 96.2 ^ 327,685 liters / day

Kompressj on-horsepower "

Cooling 564 BHK (brensehestekrefter)

Rekompressjon 1967 BHK Total 2531 BHK

In FIG. 2 treated a typical lean natural gas and it is cooled according to a prior art method of the same structure as shown in FIG. 1. Incoming gas stream 33 is cooled to -55 ° C and flows to high pressure separator 16 as stream 33a in which the contained liquid is separated and fed through the level controller through line 34 and expansion valve 30 to metanavkoker 19 in the middle of avkokerkolonnen.

Cold gas from separator 16 passes through the expansion machine 17, where it due to expansion work from 63.3 kg / cm2 to 17.6 kg / cm2 is cooled to -103 ° C. The liquid that condenses during expansion is separated in low pressure separator

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146554 8:18 and fed through the leveling through line 35 to methane-decoction 19 as top-charge column.

Data for this case shown in the following table: TABLE II (Fig. 2)

Flow volumes - kg 'mol / hour

Power Methane Ethane Propane Butane + Total 33 1447 90 36 43 16 ^ ^ 7:03. _ 280 42 25 39 391. · 35 133 35 '11 4186 36 February 71 36 .43 155 O '

extraction

Ethane 79, US 144 817 liters / day '

Propane 98,2¾ 74,557 liters / day

Kompressjonseffekt Cooling '0 BHK

Rekompressjon 2601 BHK Total 2601 '-BHK

By the previous method shown in FIG. 1 and 2 are e.tanutvinningsgraden 73¾ for charge constituting rich gas and 79% lean gas-charge. It is known that a certain improvement of the yield can be achieved by introducing one or more kj'øletrinn followed by one or more separation steps or by changing the temperature of the separator 16 or the pressure in separator 18. The recovery rate of ethane and propane is produced in this way, although they may be improved compared to the cases illustrated in FIG. 1 and FIG. 2, however, is considerably lower than the yields that can be achieved according to the present invention method.

cross illustration process conditions in FIG. 2p changed by reducing column pressure to 15.8 kg / cm. At this lower pressure will ethane and propane dividend will be slightly increased (to 82,96¾ and 98.66%, respectively), however, requires lower operating pressures significantly increase the power requirement of 2983 BHP (brake horsepower).

in 'in 9 146654 • f%

Pig. 3 shows a method, as described in the aforementioned U.S. Patent Nos. 4,171,964 to precooling the incoming high-pressure charge liquid. According to the method of FIG. 3 father be the delkondenserte gas charge 33a at -55 ° C and 63j3 kg / cm in the same way as shown in FIG. 2. Incoming gas were here assumed to be a lean gas composition as stream 33 in FIG. 2. The partially condensed gas 33a enters the high pressure separator 16 where liquid and gas are separated.

By first follower gases 113 leaving the separator 16 flows these gases into an expansion engine 17 in which mechanical energy is drawn from the gas. As gas 2:02 expanded from a pressure of approximately 63> 3 kg / cm to 1736 kg / cm will work expansion cooling the gas to about 113 -103 ° C. The expanded and partially condensed gas 113 is passed as input charge on metanavkokeren 19 where gas proportion rises to form part of metanavkoker-topputtrekket 117. Toppavdrager 117 from metanavkokeren runs at a temperature of -104 ° C together with the gases 116 from flash vaporization described later to form the residue gas 118. The combined cold residue gas stream 118 goes through heat exchanger 119 · The heated residue gas. leaving -87 ° C heat exchanger 119 and fed back to its previously -gående cooling step such. illustrated in FIG. 2, where additional cooling power from the still cold gas used and the gas is compressed in the compressor 21 (see FIG. 2) driven by the expansion engine 17 and compressed further to the line pressure of 63.3 kg / cm at final compressor 22. ·

One looks at the liquid 34 coming from the separator 16 and goes thereafter through heat exchanger 119 in heat exchange with the cold residue gas 118. This provides a precooling of the liquid portion of the partially condensed high pressure charge is in gaseous form.

The subcooled liquid is expanded through an appropriate expansion device eg. an expansion valve 120 to a pressure of approximately 17.6 kg / cm. During expansion a portion of the charge to evaporate and cool the remainder. For the process illustrated in FIG. 3 leaving the expanded stream expansion valve 120 at a temperature of -106 ° C and enters the separator. The liquid portion is separated and fed as stream 115 to distillation column 19 'as top-charge. It may be noted that

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146554. . On September 10 in comparison with FIG. 2 when the expanded fluid through conduit 34 into metanavkokerkolonnen only a temperature of -92 ° C. Because flow 115 in FIG. 3 is substantially cooler, can be used as top-charge on metanavkokerkolonnen for recovering ethane stream 113 · The recovered ethane drained with methane avkokerens bottom product 125. The bottoms 125 is heat exchanged with incoming charge for utilization of the cooling effect as generally shown in FIG. 1 and 2.

In connection with FIG. 3 mentioned that the heat-economic reasons will be provided one or more methane-decoction sidekokere which heat exchange with the cold incoming charge (not shown in FIG. 3) but as generally illustrated in FIGS .. 1 and 2. With respect to the numbers and calculations are made in connection with FIG. 3 and contained in the following table has been assumed two such sidekokere as shown in FIG. 2. These side cookers are of importance to the overall heat economy in the process. Subcooling liquid stream 34 with tpppgassen 118 reduced the available cooling effect from the stream 118. However, the increased load of metanavkokeren 19 with liquid stream 115 which is cooled as shown in FIG. 3 provide additional available cooling ability in sideavkokerne. Thus, the process overall heat balance essentially unchanged.

Incoming and outgoing flow rates, dividends and expansion / compression needs in the process as shown in FIG. 3 shown in the table: TABLE III (FIG. 3)

Flow rate - kg mol / hour

Power no. Methane Ethane Propane Butane + Total '33a 1447 90 36 43 1647 34 280 42 25 39 391 113 1167 48 11 4 1256 115 251 42 25 39 361 116 29 0 0 0 30 118 1445 10 1 0 1483 125 2 80 35 43 164 '

to I

11 in> 40 554 - Dividends

Ethane 89.1 # 164,018 liters / day

Propane 97.7 # 74,215 liters / day

Kompressj onseffekt Cooling 0 BHK

Rekompressjon 2595 BHK Total 2595 BHK

For further comparison with the method of the present invention which follow in the subsequent examples, was calculated with a second basic case according to the flow diagram of FIG. 3 from the same lean gas. By this modified flow plane, the opening gas charge in the process at 49 ° C and 64 kg / cm2 cooled to -56 ° C in pre-cooler (eg. Heat exchangers 10, 11, 12, 14 and 15 in FIG. 2) instead to -55 ° C and the column was run at a slightly lower pressure of 16.9 kg / cm instead of 17.6 kg / cm. The result was somewhat increased yields of ethane and propane and increased horsepower requirements of the process. - A summary of the altered flow conditions and amounts for this alternative because the case shown in Table IV: TABLE IV (Fig. 3)

Flow rate - kg mol / hour

Power no. Methane Ethane Propane Butane + Total Terms 33a 1447 90 36 43 1647 -56 ° C, 63.3 kg / cm2 34 308 44 26 39 424 -56 ° C, 63.3 kg / cm2 34a 308 44 26 39 424 - 103 ° C, 63.3 kg / cm2 34b 308 44 26 39 424 -10'7 ° C, 16.9 kg / cm2 113 1139 46 10 4 1223 -56 ° C, 63.3 kg / cm2 113a 1139 46 10 4 1223 -103 ° C, l6,9 kg / cm2 115 278 44 26 39 292 -107 ° C, l6,9 kg / cm2 118 1446 8 1:00 1 479 -107 ° C, l6,9 kg / cm2 ll8a 1446 8 1:00 1479 -87 ° C, l6,9 kg / cm2

125 1 82 35 43 168 2 ° C

Dividend

Ethane 90.66 # 166,112 liters / day

Propane 89.08 # 74499 liters / day

"I ......... 1 ...... ................ In

146554 .. 12

Kompressjonseffekt Cooling 0 ΒΗΚ in

Rekompressjon 2773 ΒΗΚ |

Total 2773 ΒΗΚ

example 1

The present invention is illustrated by this example to be viewed together with FIG. 4. Fig. L shows a part of a flow diagram in which a lean gas 33a that hold 63.3 kg / cm 2 is cooled to -55 ° C and enters the separator 16.

The gas charge is cooled and delkondenseres by heat exchange with the various process streams (these heat exchangers are not shown), including sideavkokere on metanavkokerkolonnen 19 (sidekokere not shown) and by heat exchange with metanavkoker bottoms and product gas as described in FIG. 2. If necessary, as indicated in FIG. 1 and 2, the outer cooling also be used. Process conditions as described in FIG. il and the flow rates shown in Table V as follows, corresponding to treat lean gas charge having the composition of Table II and FIG. 2.

According to the method shown in FIG. 4 gets it! partially condensed gas 33a containing a liquid portion and a gaseous portion led into the high pressure l6 where the liquid portion. separated. The liquid from separator 16 (stream 34) is combined with a portion! of gas from separator L6 (current I69). The combined stream 'go' then through heat exchanger 154 in -varmeveksling having top of the gas 158 from metanavkokeren give cooling and condensing the combined stream. The cooled stream at -102 ° C is then expanded through an appropriate expansion means such as. an expansion valve 155 to a pressure of approximately 17.6 kg / cm2. During expansion a portion of the guide evaporate and provide cooling of the remaining portion. By the process of FIG. 4 when the expanded stream 157 which leaves the expansion valve 155 at a temperature of -108 ° C and goes to the top of the distillation column 19 as top-charge. |

The remaining vapor from separator 16 (stream 170) is passed into the expansion engine 17 in which mechanical energy is tapped off from this part of the charge. When gas is expanded from pressure o 2 p of 63.3 kg / cm to about 17.6 kg / cirr will work expansion cooling the gas to about -103 ° C. The expanded and partially in '1 13 14.6554,. * Condensed gas 153 is passed to metanavkoker 19 at an intermediate step.

In comparison with the first basic case of FIG.

3 it will be seen that the liquid 115 in FIG. 3 which enters metanavkoker column when a temperature of -106 ° C. To achieve a lower temperature of -107 ° C at the top of the column in the alternative case because it was necessary to reduce column pressure.

The reduced column pressure increased hestekråftbehovet3 but improved yield only small. In FIG. 4 can be obtained as a result of the combination of liquid 34 from separator 16 with a portion of the high pressure charge is 169 before undergoing cooling in heat exchanger 154, obtaining the incoming charge on metanavkokertoppen keeps -108 ° C without lowering metanavkokertrykket.

Incoming and outgoing flow rates, dividends and expansion / compression needs for Example 1 are found in Table V.

TABLE V (FIG. 4)

Flow rate - kg mol / hour

Power no. Methane Ethane Propane Butane + Total 33a 1447 90 36 43 1647 34 280 42 25 39 391 157 444 48 27 4o (/ 567 '158 1445 71 0 1476 159 2 83 35 43 171 169 164 6 2 1 176 170 1003 42 9 3 IO80

Dividend

Ethane 92.2% 169,066 liters / day

Propane 98.3% 74,674 liters / day

Kompressjonseffekt Cooling 0 BHK

Rekompressjon 2692 BHK Total 2692 BHK

II

146554 .. in "

Comparison of ethane and propane yield Table III and V show that no enrichment of liquid from separator 16 is ethane yield 89> 1% and propane yield 97.1% · Enrichment of separator liquids according to Example 1 (see Fig. B) increases ethane and propane yield to respectively 92,2¾ and 98.3? ·

Comparison between Table IV and V show that the improvement in yields in the present process not only was caused by increased power. On the contrary, Table IV that even when the process conditions in the base case was changed so metanavkokeren was operated at lower pressure, which increased horsepower requirements of 2773 BHK, increased one ethane and propane yield only to 90.66? and 98.08 ?, respectively. When benyttet'foreliggende invention as in Example 1 increased the yield of one ethane and propane compared to the alternative basic case, although somewhat lesser effect elapsed.

From a practical point of view and particularly in connection with lean gases, the entire amount of liquid from separator 16 is combined with a portion of the gas from the separator l6. The combined flow will then be cooled and expanded as described. The amount of gas used in the combined power is sufficient to achieve the desired cooling and temperature control for the top-charge to metanavkokeren. The liquids from separator 16 will by addition to the gas entering the top of the column to increase the entering charge surface tension by 'column conditions and thereby reduce the formation of small liquid particles which are difficult to separate from the top gas.

For richer gases and when it comes more liquid from the separator 16 than is spent on maintaining the conditions of column top may be more economical from the practical standpoint to divide the liquid from the separator L6 and expand a part directly to the tower or optionally after some subcooling. This can lead to savings in heat exchange requirement and higher yield.

As indicated in the above references are characterized a number of modified flow forms by subcooling a portion or the entire amount of liquid from separator 16, which the present invention can be used together with. Two or more of these techniques can be used simultaneously. Among these flow diagrams are as follows: *

II

15 146554 - 1. Non-condensed gases from the separator 16 Expandable example. in an expansion engine for the production of a cold delkondensert liquid and gas. The liquid is separated and fed to metanavkokerkolonnen. All or part of the liquid thus separated can be used as a cooling source of the cooling liquid condensate 34 from separator 16. Alternatively, all or a portion of the expanded gas is used. In addition sideavkokere used for cooling of the condensate 34 from the separator 16. According to the present invention can modify the flow diagrams by combining liquid condensate from separator 16 with a portion of the gas from the separator prior to subcooling and flashing of the liquid condensate.

2. Condensate from separator 16 may be fed to the cooling by heat exchange, and then to an expansion valve needle where it is expanded from line pressure (63.3 kg / cm in FIGS. 1-4) to the operating pressure of metanavkokerkolonnen. This will provide a gas-liquid mixture which can be separated in either a low pressure or introduced directly to metanavkokerkolonnen having internal separation means for separating the gas and liquid within the column. Flash expansion provides further cooling of the charge of the column. A portion of the outer cooled liquid obtained in this way acts as a cooling medium to heat exchange with høytrykkskonden-satet from separator 16 and are then transmitted to metanavkokerkolonnen as a second charge of an intermediate stage of the column. In the present invention, such process is improved by enriching the liquid condensates leaving the separator 16 with a portion of the gases from the separator prior to subcooling and flashing of the liquid condensate.

3. The non-condensed gas leaving separator 16 can be expanded in a arbeidsekspander of a high pressure 63.3 kg / cm of FIG. 1-4 and the operating pressure in metanavkokeren, and all the cooled gas-liquid mixture from the expansion can be used for subcooling of the condensate separator 16. The subcooled condensate is then flash expanded and led to metanavkokeren which incoming charge. This embodiment can be improved by enriching the liquid condensate from separator 16 with a portion of the gases leaving the separator before subcooling and flashing the condensate.

II

146554 i6 Instead of or in addition to these methods, external cooling is used if one wants increased yield, however, is one of the advantages of the invention described in said application consists in that when the condensate is subcooled can achieve improved yields without the need for increased efficacy.

Yet another embodiment of the invention shown in the following Example which is related with Fig. 5:

example 2

Fig. 5 shows part of a flow diagram for the recovery of ethane and heavier components from a hydrocarbon mixture containing methane, ethane and heavier hydrocarbons.

As shown in FIG. 5 recognized one delkondensert high pressure gas 174 to separator 16 at -48 ° C and 63.3 kg / cm2. Cooling of the gas-charge to -48 ° C, for example. shown in Figs, 1 and 2 occur by heat exchange with methane residual gas or other process streams flowing from sideavkokere and bottom currents (These heat exchangers are not shown) and, if necessary, with suitable external cooling. The calculations present example is based on based on two methane sideavkokere. Unlike FIG. 1 and 2, however, the present calculations (current temperature, pressure and flow rates) based on the inlet gas has a composition between lean and rich gas respectively used in FIG. l.og 2 and Tables I and II.

,. .As Indicated in FIG. 5 separated liquid and gas from each other in the separator 16. The gas from separator 16 is divided into two parts. The first section 176 passes through expander 17 where it is due to work expansion of 63.3 kg / cm to 20.4 kg / cm cooled 'to about ~ 92 ° C. From expander 17 runs the cooled gas to metanavkoker 19 mid-charge. The second gas 177 is combined with a portion of the supercooled liquid from the heat exchanger 184 when it goes to heat exchanger 185.

The liquid 175 from the separator L6 passes through heat exchanger 184 wherein it is subcooled to -90 ° C by heat exchange with the cold flow of the expansion valve 182. The subcooled liquid is then divided into two parts. The first part 178 passes through the expansion valve 182 where it is expanded and flash vaporized 'by pressure reduction from 63.3 to 17.6 kg / cm. The cold flow from the expansion valve 182 then flows through the heat * 1 17:01 465 541 ί · * exchanger 184 where it is used during the cooling of the liquids from the separator 16. From the heat exchanger 184, the current flows to metanav reboiler 19 as the lowest charge with -§5 ° C.

The remaining liquid portion 179 from heat exchanger 184, still at high pressure, combined with a portion 177 of the gas from the separator 16. The combined stream passes through heat exchanger 185 which is cooled to about -96 ° C with top-installment of the column 18O. At this temperature, the combined stream substantially condensed. The condensed stream enters • expansion valve 184 where it is expanded and flash vaporized? 2 by pressure reduction from 63 kg / cm to 17.6 kg / cm. From expansion valve 183 runs the cold stream on to metanav boiler 19 as top-charge.

The relevant data in the table below: TABLE V (Fig. 5)

Flow rate - kg mol / hour.

Power no. Methane Ethane Propane Butane * Total 174 1304 162 80 54 1647 175 486 109 .66 51 723 176 723 47 12 2 817 177 95 62 1. 107 178 243 54 33 26 361 179 243 '55 33 25362180 1301. 14 1 0 1362 181 3, 148 79 '54' 285

Dividend

Etan 91j47% 300,724 liters / day

Propane 98.38 $ 164,652 liters / day

Power Cooling 287 BHP

Rekompressjon 2176 BHK Total 246-3 BHK

It is noted that in addition to the process described in FIG. 5 to treat the cooled fluid 175

II

146554 L8 from separator 16, other methods in some situations be used with advantage. Such alternative method consists of passing a portion of the cooled liquid 179 directly from the separator through a second expansion valve to metanavkokerkolonne 19 in an intermediate stage.

In another method, the liquid 175 from the separator 16 during cooling of residual gas in lieu of the cooling as shown in FIG. 5. In such an alternative, the high-pressure condensate is cooled in two successive heat exchangers each user tail gas refrigerant. After passage through the first heat exchanger divided the partially cooled high-pressure charge in two parts. The first part is expanded through expansion valve and passed to metanavkokerkolonnen as the focal transmission. The second part of the delkondenserte condensate continues through the second heat exchanger where it is further cooled and combined with gas from separator 16. The combined stream is thereby further cooled and expanded and fed to column 19 as top-charge. Alternatively, gas from separator 16 is added to the second portion of the partially cooled stream prior to entering the second heat exchanger thereby avoiding one subsequent cooling of the combined stream.

By yet another modification of the present invention, the flash expanded stream, such as 186 in FIG. 5, is passed to heat exchange with the work-expanded stream l87 for cooling thereof and heating current I86. If stream 187 thereby cooled sufficiently, it can advantageously be used as the top-charge to metanavkokeren and stream 186 may be used as the intermediate charge on the column side stream 186 which is clearly shown in plan in FIG. 5 is richer in heavier constituents and stream 187 contains more lighter components, ie methane and incondensible gases.

Other alternative methods for forming the cooled fluid 175 described in said patent applications. These alternative processes can be used in various combinations. These different forms can also be used instead of or simultaneously with cooling of residual gas of the enriched stream prior to use as the top-charge at column 19.

These alternative methods are particularly useful when the cooling capacity for peak installment 180 from column

to I

in? ) 46 554 - because of rich charge into the process, is insufficient to cool the whole volume of liquid coming through line 175 to the desired low temperature.

As is known, natural gas contain relatively large amounts of carbon dioxide. Carbon dioxide in metanavkokeren can cause icing of the internals in the column under the applied cooling conditions. Even when the incoming charge gas contains less than 1% carbon dioxide will be fractionated in metanavkokeren and build up to concentrations of up to 5 or 10 # or more. At such high concentrations, the carbon dioxide freeze out depending on temperature, pressure, the carbon dioxide is in liquid or gas phase and liquid phase solubility.

In the present invention it has been found that when gas from the high pressure separator is expanded and passed to metanavkokeren lower than the peak level, the problem of carbon dioxide icing substantially avoided. The gas from the high pressure separator typically contains a large amount of methane relative to ethane and carbon dioxide. When gas is thus fed to an intermediate level of the column will separatorgass from the high pressure separator dilute carbon dioxide concentration and prevent it from increase of sensitivity level.

The advantage of the present invention can be easily seen by setup carbon dioxide concentration and temperature of the various distillation stages in metanavkokeren. To illustrate the preparation of such a diagram can flow process illustrated in FIG. * 1 used for the treatment of a gas charge with the following composition:

Incoming gas composition Methane 93.82

ethane 3.16

propane 1.06

Butane + 0,8o co2 0.59 N2 0.57

The main operating conditions were the following:

In ...... ...... 'In

1 46 554 20

Pressure in high pressure 16 62.9 kg / cm ^

Temperature in high pressure separator 16 -l8 ° C

¾ of charge condensed in separator 16 0,44¾ j% of gas from separator 16 led to expander 17 60¾

Temperature in combined power to

expansion valve 155 -84 ° C

Outgoing temperature from the expansion valve -99 ° C

The column top temperature -98 ° C

Temperature in the gas from the expander 17 -64 ° C

o

Metanavkoker pressure 25.3 kg / cm% ethane yield 87,33¾% propane yield 97.05%

Power requirement: Rekompressjon 7042 BHK

Cooling 0 BHK

; Total 7042 BHK

: Same incoming gas was processed according to FIG. 2. However, to achieve a more efficient utilization of the cooling effect, the cooling heat exchangers prior to high pressure separator homed portion. The main conditions that process I operated with were the following:

Pressure in high pressure 16 62.9 kg / cm2

the temperature in the high pressure separator 16 -57 ° C

% Gas-charge condensed in separator 16 2.95% • Temperature in expanded gases from

expander 17 ~ 93 ° C

Temperature in the expanded fluid from the flash valve 30 -82 ° C

Temperature of metanavkoker-installment -92 ° C

2

Metanavkokertrykk 25.3 kg / cm ¾ ethane yield 60,92¾ l propanutbvtte 90,58¾

Power requirement: Rekompressjon 6777 BHK

Cooling 0 BHK

Total 6777 BHK

One scheduled curves for each of these cases, with regard .on CC ^ concentration as a function of temperature for metanavkokeren, shown in FIG. 6A and 6B. In FIG. are also equilibria liquid-solid and gas-solid. Equilibrium Data entered on

to I

21 146554 A "· _. Figs. 6a and 6b applies to methane karbondioksydsystemet. This data is believed generally representative of methane and ethane systems.

If CO, concentration at a particular temperature in the column is at or higher than the equilibrium line at this temperature, one can assume that icing occurs. For practical purposes use constructor usually a safety margin, i.e. the real concentration should be less than the "icing" -konsent.rasjonen with a suitable safety factor. 'In Fig. 6A rising carbon dioxide concentration in metanavkokeren over limit level. Such gas could not be applied by a conventional process and can not be used without the front treatment for removal of a substantial amount of carbon dioxide. When the expanded gases used as the center-charge of the column according to the present oppfinnelse.vil however cog concentration reduced metanavkokeren to a point well below the "icing" level.

In constructing metanavkokerkolonner for use respectively in the present invention, the constructor routine assure that icing is not occurring in the column.

Even when the gas is fed at an intermediate stage of the column is possible that icing may occur if a process plant for the highest 'possible etanutbytté. Such a program will usually require the coldest possible temperature at the top of the column. This will cause the carbon dioxide moves to the right of the curves in FIG. 6A and 6B. Depending on the approach can result 'be an impermissibly high concentration of carbon dioxide near the top of the column. It may in such cases be necessary to accept a somewhat smaller ethane yield to prevent icing or to - pretreating the incoming gas to reduce carbon dioxide to a point where the concentration does not present problems in metanavkokeren. Alternatively, one can avoid icing in such cases by other modifications of the conditions. Eg. it may be possible to drive the high pressure separator at higher temperature, increasing the relative quantity of gas from the high pressure separator is expanded through expander 17 or expand a portion of the gases from the high pressure separator through an isenthalpic expansion valve.

If such changes can be made within the process heat balance limitations can avoid icing without reduced ethane yield.

j ί i "in 146554 22 1 regarding the above method should be noted that in some cases the charge being introduced at the top of metanavkokeren a fluid that is expanded from a high pressure to metanavkokerens pressure (see eg. Fig. 4 and 5).

In such cases it may be beneficial to selvavkjøle fluid transmission at the column head. This occurs by splitting the liquid stream into the top of the column into two streams either before or after expansion. (Both flows expanded if top the charge to the column split before expansion). One of the two expanded streams is fed directly to the heat exchange with top the charge before expansion.

• in the