RU2283994C2 - Method of treating carbon-containing gas - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: method comprises treating gas in one or several heat exchange stages to produce first raw flow cooled under pressure, expanding the first raw flow to decrease the pressure, supplying raw to the fractionating column, and fractioning cooled and expanded first raw flow at a lower pressure thus extracting relatively light components. The distillation flow is discharged from the fractioning column and heated. The heated distillation flow is returned to the fractioning column.

EFFECT: enhanced quality of fractioning.

24 cl, 9 dwg, 2 tbl

Description

BACKGROUND OF THE INVENTION

The invention relates to a method for separating a gas containing hydrocarbons.

Ethylene, ethane, propylene, propane and / or heavier hydrocarbons can be extracted from various gases, such as natural gas, oil refining gas and synthesis gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sand and brown coal. Natural gas usually contains a significant proportion of methane and ethane, i.e. methane and ethane together make up at least 50 mol%. gas. The gas also contains relatively smaller amounts of heavier hydrocarbons, such as propane, butanes, pentanes and the like, as well as hydrogen, nitrogen, carbon dioxide and other gases.

The present invention generally relates to the recovery of ethylene, ethane, propylene, propane and heavier hydrocarbons from such gas streams. A typical composition of the gas stream to be processed according to this invention will be the following approximate molar composition: 88.41% methane; 6.65% ethane and other C ₂ components; 2.26% propane and other C ₃ components; 0.36% isobutane; 0.45% normal butane; 0.31% pentanes-plus with a balance (up to 100%) of nitrogen and carbon dioxide. Sometimes gases containing sulfur are also present.

Cyclical fluctuations in prices for both natural gas and liquid components of natural gas (LCPG) sometimes reduced the cost increase of ethane, ethylene, propane, propylene and heavier components as liquid products. Competition to obtain processing rights forced the plants to maximize production capacity and the efficiency of extraction of their existing gas processing plants. Currently applicable methods for the separation of these materials include methods based on cooling and cooling the gas, oil absorption and absorption of chilled oil. In addition, cryogenic methods have also become widespread due to the availability of cost-effective equipment that generates energy and simultaneously dissipates and extracts heat from the gas being treated. Depending on the pressure of the gas source, the saturation (content of ethane, ethylene and heavier hydrocarbons) of the gas and the desired final product, each of these methods or their combination can be used.

The cryogenic expansion process is currently generally preferred for recovering the liquid components of natural gas for the reason that it provides maximum simplicity characterized by easy start-up, operational flexibility, good efficiency, safety and high reliability. U.S. Patent Nos. 3292380, 4157904, 4171964, 4185978, 4251249, 4278457, 4519824, 4617039, 4687499, 4689063, 4690702, 4854955, 4869740, 4889545, 5275005, 5555748, 5568737, 5771712, 5869, 569698, 569, 589, 587, 589, 569, 589, 587, 587, 589, 569, 587, 587, 587, 589, 569, 587, 587, 589, 569, 579, 587, 587, 589, 569, 587, 587, 587, 589, 569, 587, 587, 589, 569, 579, 587, 587 and 580, 769, 587, 587, 569, 587, 587, 587, 587, 587 - 597 - - - - - - - - - - - - - US No. 33408 and co-pending application 09/439508 describe relevant methods (although the description in this invention in some cases is based on other process conditions different from those described in the aforementioned US patents and patent applications).

A typical cryogenic expansion recovery method involves cooling a feed gas stream under pressure by heat exchange with other process streams and / or external cooling sources such as a propane compression-cooling system. During gas cooling, the liquid components can condense and collect in one or more separators in the form of high-pressure liquids containing some of the required C ₂ + components. Depending on the saturation of the gas and the amount of liquids formed, high pressure liquids can be expanded to reduced pressure and fractionated. Evaporation occurring during expansion of liquids leads to further cooling of the stream. In some conditions, it may be desirable to pre-cool the high pressure fluids prior to expansion to further lower the temperature obtained by expansion. An expanded stream containing a mixture of liquid and steam is fractionated in a distillation column (demethanizer). In this column, the cooled expansion stream (s) are distilled to separate residual methane, nitrogen and other volatile gases in the form of vapors discharged from the top of the column from the required C ₂ components, C ₃ components and heavier hydrocarbon components in the form of bottoms (discharged from the bottom of the column) liquid products.

If the source gas has not completely condensed (usually it is), then at least part of the steam remaining after partial condensation can be sent through an expander (expansion machine) or device, or through an expansion valve to reduce the pressure at which additional liquids condense as a result of subsequent cooling of the stream. The pressure after expansion is essentially the same as the pressure at which the distillation column operates. The combined vapor-liquid phases resulting from the expansion are fed as feed to the column. In recent years, preferred hydrocarbon separation methods include supplying this expanded vapor-liquid stream to the midpoint of the column; while the upper absorption section provides additional rectification of the vapor phase. However, there are methods according to which this expanded vapor-liquid stream is used as power to the top of the column. Typically, the steam portion of the expanded stream and the steam discharged on top of the demethanizer are combined in the upper separation section of the fractionation column as the residual gas of a methane product. Alternatively, the cooled and expanded stream may be fed to a separator to produce vapor and liquid streams, so that steam is then combined with the overhead of the column and the liquid is fed to the column as a feed to the top of the column.

For methods involving an upper distillation section, it is necessary to provide an irrigation flow for this section. One of the methods for its implementation is to divert the steam distillation stream from the upper section of the demethanizer column, cooling it for partial condensation by heat exchange with other process streams, for example, with a part of the source gas, which is cooled to substantial condensation and then expanded for further cooling. The liquid condensed from the steam distillation stream is then fed to the demethanizer as a top feed.

The purpose of this method is to carry out the separation to obtain a residual effluent gas containing substantially all of the methane of the feed gas, essentially without C ₂ components and heavier hydrocarbon components; and obtaining a lower fraction from a demethanizer containing essentially all C ₂ components and heavier hydrocarbon components essentially without methane or without more volatile components, while fulfilling the production specifications for the maximum allowable carbon dioxide content. The invention provides a new plant or a means of modifying an existing processing plant to accomplish this separation at significantly lower capital costs by reducing the size or eliminating the need for a product processing system to remove carbon dioxide. Alternatively, this invention, either when used in a new plant, or as a modification of an existing processing plant, can be used to extract more than other treatment methods, the amount of C ₂ components and heavier hydrocarbon components in the bottoms liquid product for a given concentration of dioxide carbon in the feed gas.

The present invention discloses an improved method for separating a gas stream containing methane, C ₂ components, C ₃ components and heavier hydrocarbon components into a volatile residual gas fraction containing a major portion of said methane and a relatively less volatile fraction containing a major portion of said C ₂ -components, C ₃ -components and heavier hydrocarbon components, in which

(a) said gas stream is treated in one or more heat transfer stages to obtain at least a first feed stream that is cooled under pressure;

(b) said cooled first feed stream is expanded to a lower pressure and then fed to a fractionation column at an upper feed point; and

(c) said chilled expanded first feed stream is fractionated at said lower pressure, whereby components of said relatively less volatile fraction are recovered;

where the improvement is as follows:

(1) a liquid distillation stream is withdrawn from said fractionation column and heated;

(2) said heated distillation stream is returned to a lower point in said fractionating column, which is separated from said outlet point by at least one theoretical stage; and

(3) the amounts and temperatures of said feed streams supplied to said fractionating column are effective to maintain the top temperature of said fractionating column at a temperature at which the major parts of the components in said relatively less volatile fraction are recovered.

In another aspect, the present invention discloses an improved method for separating a gas stream containing methane, C ₂ components, C ₃ components and heavier hydrocarbon components into a volatile residual gas fraction containing a major portion of said methane and a relatively less volatile fraction containing a basic a portion of said C ₂ components, C ₃ components and heavier hydrocarbon components, in which

(a) said gas stream is treated in one or more heat exchange stages and at least one separation stage to obtain at least a first feed stream that is cooled under pressure to condense essentially the entire stream, and at least a second feed stream which is cooled under pressure;

(b) said substantially condensed first feed stream is expanded to a lower pressure, as a result of which it is further cooled, and then sent to heat exchange with a warmer distillation stream that rises from the fractionation stages of the fractionation column;

(c) said distillation stream is cooled sufficiently by said first stream to partially condense it, after which said partially condensed distillation stream is separated to obtain said volatile residual gas fraction and an irrigation stream; wherein said irrigation stream is then directed to said fractionating column at a top feed point;

(d) said heated first stream is fed to said fractionating column at a first midpoint of the column feed;

(e) said cooled second feed stream is expanded to said lower pressure and then fed to said fractionating column at a second midpoint of column feed; and

(f) said irrigation stream, said heated first feed stream, and said expanded second feed stream are fractionated at said lower pressure, whereby components of said relatively less volatile fraction are recovered;

where the improvement is as follows:

(1) a liquid distillation stream is withdrawn from said fractionation column and heated;

(2) said heated distillation stream is returned to a lower point in said fractionating column, which is separated from said outlet point by at least one theoretical stage; and

(3) the amounts and temperatures of said feed streams supplied to said fractionating column are effective to maintain the top temperature of said fractionating column at a temperature at which the major parts of the components in said relatively less volatile fraction are recovered.

In one embodiment of said methods, said liquid distillation stream is evacuated after being withdrawn from said fractionating column, and said evacuated liquid distillation stream is separated into at least a first part and a second part; said first part is heated and said heated first part is returned to a lower point in said fractionating column, which is separated from said withdrawal point by at least one theoretical stage.

Said liquid or pumped liquid distillation stream or said first part is sent to heat exchange with at least a part of said gas stream or said raw material streams to provide cooling and for heating, thereby, said liquid or pumped liquid distillation stream or said first part.

The amount and temperature of said heated distillation stream or said heated first portion and heating provided for said fractionating column are effective to maintain the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction is reduced.

According to this invention, it was found that the recovery of C ₂ in a volume of more than 66% can be achieved while maintaining the carbon dioxide content in the bottoms liquid product within the specifications and while ensuring substantially complete removal (emission) of methane into the residual gas stream. Although the invention is applicable at lower pressures and at a higher temperature, it is particularly useful when treating feed gases at pressures in the range of 600-1000 psi or higher under conditions where the top temperature of the column should be -120 ° F or lower .

This invention uses a modified reboiler circuit that can be used for any type of LCPG extraction system. In the case of a typical use of a reboiler or auxiliary reboiler in a distillation column, all of the downward flowing liquid stream in the column is diverted from the column and passed through a heat exchanger, then returned to the column at substantially the same point in the column. In this modified reboiler system, a portion of the downstream fluid of the column is diverted from the higher point of the column, i.e. from a point that is separated from the return point by at least one theoretical stage. Even if the flow rate of the liquid may be lower, it is usually much colder and may have advantages in terms of improving recovery or reducing the size of the heat exchanger.

It has been found that by applying the present invention to known methods for recovering LCGS, the recovery of C ₂ components and heavier components is increased by one to two percent. However, the recovery is increased to a much greater extent if you want to reduce the carbon dioxide content in the recovered product of the LCPG. Extraction of ethane in a conventional LCPG recovery unit also provides recovery of at least a portion of the carbon dioxide contained in the feed gas, since carbon dioxide, from the point of view of relative volatility, is between methane and ethane. Therefore, with an increase in ethane recovery, carbon dioxide recovery in the LCPG product also increases. Using the modified reboiler method according to this invention, the authors found that it is possible to significantly increase the ethane recovery in the LCPG product compared to using conventional reboiler or auxiliary reboiler systems when re-evaporation is carried out in the column to maintain the required carbon dioxide content in the LCPG product.

For a better understanding of the present invention, reference is made to the following examples and figures, in which:

Figure 1 - technological scheme of a cryogenic installation for processing natural gas according to the prior art;

FIG. 2 is a flow chart illustrating how the processing unit of FIG. 1 can be modified into a natural gas processing unit according to the present invention; FIG.

FIG. 3 is a flow chart illustrating an alternative modification of the apparatus of FIG. 1 to a natural gas processing apparatus of the present invention; FIG.

FIG. 4 is a flow chart illustrating an alternative modification of the apparatus of FIG. 1 to a natural gas processing apparatus of the present invention; FIG.

5 is a flow chart illustrating how an alternative prior art method can be modified into a natural gas processing unit according to the present invention;

6 is a diagram illustrating a method of a modified reboiler in accordance with this invention for installing processing, and this method contains a thermosiphon system;

7 is a diagram illustrating a method of a modified reboiler in accordance with this invention for installing processing, and this method contains a pumping system;

Fig. 8 is a diagram illustrating a method of a modified reboiler in accordance with the present invention for a processing unit, said method comprising a pumping system;

Fig.9 is a diagram illustrating a method of a modified reboiler in accordance with this invention for installing processing, and this method contains a split-column system.

In the explanation of the figures below, tables are given that show the flow rates calculated for typical process conditions. In these tables, flow rates (lb mol / hour) are rounded for convenience to the nearest whole number. The total flow rates (flow rates) in the tables include all non-hydrocarbon components and therefore, as a rule, they exceed the sum of the flow rates of hydrocarbon components. Temperatures are approximate values rounded to the nearest degree. It should be noted that the technological calculations performed in order to compare the methods illustrated in the figures are based on the assumption that any leakage of heat from the environment to the process or, conversely, does not occur. The quality of commercially available insulating materials makes this assumption very reasonable and those skilled in the art will usually proceed from this assumption.

State of the art

Figure 1 illustrates a process flow diagram of a processing plant for extracting C ₂ + components from natural gas using the prior art according to US Pat. No. 3,293,280. In this model of the method, the source (incoming) gas enters the installation at a temperature of 90∈F and under a pressure of 915 psi as stream 31. If the source gas contains such a concentration of sulfur compounds, due to which product streams will not meet the technical conditions then the sulfur compounds are removed by appropriate pre-treatment of the source gas (not shown). In addition, the feed stream is usually dehydrated to prevent the formation of hydrate (ice) under cryogenic conditions. A solid dehumidifier is usually used for these purposes.

Feed stream 31 is cooled in heat exchanger 10 by heat exchange with cold residual gas at −108∈F (stream 37), demethanizer reboiler fluids at 59∈F (stream 42) and auxiliary demethanizer reboiler liquids at 30∈F (stream 40). It should be noted that in all cases, the heat exchanger 10 represents either several separate heat exchangers, or one multi-pass heat exchanger, or any combination thereof. (The decision to use more than one heat exchanger for the indicated cooling will depend on a number of factors, including, but not limited to, the flow rate of the incoming gas, the size of the heat exchanger, flow temperatures, etc.). It should also be noted that the heat exchanger 10 was intended to use the liquid demethanizer product (stream 43a) to provide cooling of a portion of the source gas, but according to the explanations below, this stream is too warm to be used for these purposes. The cooled stream 31a enters the separator 11 at -30∈F and 905 psi, where the vapor (stream 32) is separated from the condensed liquid (stream 35).

Steam (stream 32) from the separator 11 enters the expander 14, which uses the mechanical energy of this part of the raw material having high pressure. Expander 14 expands the vapor substantially isentropically from a pressure of about 905 psi to an operating pressure (approximately 315 psi) of the demethanizer column 17; wherein the expansion cools the expanded stream 32a to a temperature of about −108∈F. Conventional commercially available expanders utilize about 80-85% of the work theoretically available with perfect isentropic expansion. Recycled work is often used to drive a centrifugal compressor (key 15), which can be used to re-compress the residual gas (stream 37a), for example. The expanded and partially condensed stream 32a is fed to a separation section 17a at the top of the demethanizer column 17. The liquids separated there become the top feed stream of the theoretical stage 1 of the demethanizer section 17b.

The liquid (stream 35) from the separator 11 is subjected to pulsed expansion using a suitable expansion device, such as an expansion valve 16, to the operating pressure of the demethanizer column 17. During expansion, part of the stream evaporates, cooling the total stream. In the method of FIG. 1, the expanded stream 35a exiting the expansion valve 16 reaches a temperature of −73∈F and is supplied to the midpoint of the demethanizer column 17.

The demethanizer 17 is a conventional distillation column with a plurality of vertically spaced plates arranged with one or more layers of packing or with a combination of plates and packing. The demethanizer can consist of two sections according to the common practice of natural gas processing plants. The upper section 17a is a separator where the partially condensed upper feed (feed) is divided into its corresponding steam and liquid parts and where the steam rising from the downstream distillation or demethanization section 17b combines with the steam of the upper feed to form a cold distillation stream 37 of the residual gas, which goes on top of the column. The lower demethanizing section 17b contains trays and / or a nozzle, and this section provides the necessary contact between the downward flowing liquids and the upward flowing vapors. The demethanization column 17 also contains reboilers that heat and vaporize portions of the liquids going down in the column, in order to provide the driving vapors that go up in the column.

In many cases, the temperature of the liquid product leaving the bottom of the column (stream 43) is controlled by providing the desired ratio of methane and ethane in the liquid product. Typically, this ratio of methane to ethane is 0.025: 1 according to the molar composition of the bottoms product. However, in this case, the concentration of carbon dioxide in the liquid product will exceed the specifications of the manufacturer of the installation in the ratio of carbon dioxide to ethane of 0.05: 1 in molar composition, if the demethanizer was adjusted to maintain this methane: ethane ratio. Therefore, in this operation, for this installation design, an additional processing system is required to remove carbon dioxide from hydrocarbons in order to obtain a marketable liquid product. There are many options for the removal of carbon dioxide (treatment of the feed gas, treatment of the entire liquid product, processing of the ethane product after fractionation, etc.), but all these options will increase not only the capital costs of the installation (due to the costs of installing the processing system), but also the operational costs of the installation (due to the consumption of energy and chemicals in the processing system).

One way to keep the ethane product content within the specifications of carbon dioxide is to use a demethanizer to drive carbon dioxide out of the bottoms liquid by introducing additional heat from the reboiler into the column using an auxiliary reboiler and / or lower reboiler parts of the column according to Figure 1. As a result, a liquid product is obtained (stream 43) exiting from the bottom of the column at a temperature of 77∈F, after which it is pumped to about 480 psi (stream 43a) in pump 20. (The outlet pressure of this pump is usually set the final purpose of the liquid product.Normally, the liquid product is stored after it is used for heat transfer, and the pressure at the pump outlet is set to prevent any evaporation of stream 43a when it is heated to ambient temperature). But since the stream 43a is too warm, it cannot be used to cool the source gas in the heat exchanger 10. Accordingly, the wedge gate valve 21a must be closed and the wedge gate valve 21b is open to bypass (bypass) the stream around the heat exchanger 10 and direct it directly to storage (stream 43d).

The residual gas (stream 37) flows countercurrently to the incoming feed gas in the heat exchanger 10, where it is heated to 33∈F (stream 37a). The residual gas is then re-compressed in two stages. The first stage is the compressor 15, which is driven by the expander 14; and the second stage is a compressor 22, which is driven by an additional source of energy supply. Then, stream 37c is cooled to 120∈F by refrigerator 23 and the residual gas product (stream 37d) is supplied to the finished product pipeline at a pressure of 1015 psi, which corresponds to the specifications of the pipeline (usually corresponding to the inlet pressure).

The summary values of the flow rate and power consumption for the method illustrated in FIG. 1 are given in table I.

Table I
Total Consumption (lb mol / hour) Flow Methane total Ethane Propane Bhutans + Carbon dioxide 31 25338
28659 1905 647 320 307 32 24929
27860 1777 534 181 296 35 409
799 128 113 139 eleven 40 fourteen
3517 1900 730 342 531 37 25338
26977 1129 94 6 268 43 0
1682 776 553 314 39 Extract *
Ethane - 40.74%
Propane - 85.47%
Bhutans + - 98.09%
Horsepower
Compression of the remainder - 13296
* (based on unrounded expense values)

The carbon dioxide: ethane ratio in the bottoms liquid product according to the method illustrated in FIG. 1 is 0.05: 1, which corresponds to the specifications of the plant manufacturer. However, it should be noted that the ratio of methane: ethane in the cubic product is 0.000003: 1 in molar composition compared to the permissible ratio of 0.025: 1, which characterizes the degree of excessive distillation necessary to control the carbon dioxide content in the liquid product at the desired level. A study of the extraction levels according to Table 1 shows that the working method according to Figure 1 for reducing the carbon dioxide content in an ethane product results in a significant reduction in the recovery of liquids. When working with a methane: ethane ratio of 0.025: 1 in a cubic product, calculations show that the method of FIG. 1 can provide ethane recovery of 69.64%; recovery of propane - 96.18% and recovery of butanes +, equal to 99.66%. Unfortunately, the resulting carbon dioxide: ethane ratio (0.087: 1) is too high relative to the specifications of the plant manufacturer when the plant is operated in this way. Thus, the requirement to conduct the method according to Figure 1 to reduce the concentration of carbon dioxide in the liquid product leads to a decrease in the extraction of ethane, propane and butanes + by 28, 10 and 1 percent, respectively, for the methods of the prior art.

In the method of FIG. 1, there are two factors that reduce the recovery of liquids from the bottom of the demethanizer column 17 if the column operates to control the carbon dioxide content of the liquid product. Firstly, when the temperature in the lower part of the column 17 of the demethanizer rises to 77∈F due to additional evaporation in the column, then the temperatures at each point of the column increase. This circumstance reduces the amount of cooling that the column fluid streams (streams 40, 42 and 43) can provide for the feed gas in the heat exchanger 10. Therefore, the cooled feed stream (stream 31a) entering the separator 11 is warmer, which in turn leads to to lower ethane retention in the column 17 demethanizer. Secondly, higher temperatures in the lower section of the demethanizer column 17 also result in higher temperatures in the upper section, resulting in less methane liquid entering the lower section of the demethanizer column 19. In the subsequent evaporation of liquid methane using an additional reboiler and a main reboiler attached to the demethanizer column 17, methane vapors help to drive carbon dioxide out of liquids flowing down the column. With a decrease in the amount of methane that drives off carbon dioxide, in the method of FIG. 1, it is necessary to vaporize more ethane in liquids so that it serves as a stripping gas. Since the relative volatilities of carbon dioxide and ethane are very close, ethane vapor is a much less effective distillation agent than methane vapor, and this circumstance reduces the efficiency of distillation in the column and reduces recovery.

Description of the invention

Example

Figure 2 presents the technological scheme of the method according to this invention. The composition of the source gas and the conditions of the method according to Figure 2 are the same as in Figure 1. Accordingly, the method according to FIG. 2 can be compared with the method according to FIG. 1 in order to illustrate the advantages of the present invention.

According to the model of the method in accordance with FIG. 2, the feed gas enters at a temperature of 90∈F and at a pressure of 915 psi as stream 31. The feed stream 31 is cooled in heat exchanger 10 by heat exchange with cold residual gas at a temperature of -130∈ F (stream 37), with the liquid product of the demethanizer at a temperature of 57∈F (stream 43a), with the liquids of the reboiler of the demethanizer at 33∈F (stream 42) and some of the liquids from the upper section of the column 17 of the demethanizer at a temperature of -130∈F (stream 40 ) The cooled stream 31a enters the separator 11 at a temperature of −59∈F and a pressure of 905 psi. inch where steam (stream 32) is separated from the condensed liquid (stream 35).

The condensed liquid (stream 35) from the separator 11 is pulsedly expanded using an appropriate expansion device, such as an expansion valve 16, to the operating pressure (approximately 315 psi) of the demethanizer column 17. During expansion, part of the stream is evaporated, as a result of which the entire stream is cooled. In the method of FIG. 2, the expanded stream 35a exiting the expansion valve 16 reaches a temperature of −114∈F and is supplied to the demethanizer column 17 at the midpoint of the column.

Steam (stream 32) from the separator 11 enters the expander 14, in which mechanical energy is utilized from this part of the high pressure feedstock. The expander 14 expands the vapor substantially isentropically from a pressure of about 905 psi to the operating pressure of the demethanizer column 17, while the expansion work cools the expandable stream 32a to a temperature of about -132∈F. The expanded and partially condensed stream 32a is then fed to the demethanizer column 17 as a feed to the top of the column. A portion of the vapor stream 32a combines with the vapors rising from the upper fractionation stage of the column and forms a distillation stream 37 that is diverted from the upper region of the column.

The liquid portion of stream 32a is used to contact the vapors rising from the lower fractionation stages of the demethanizer column 17 and to disperse the desired C ₂ components and heavier components from the vapors and then separate it into two parts. One part (stream 41) with a content of about 40% of the total liquid is directed to the lower fractionation stages in the demethanizer column 17 for subsequent contacting and distillation of vapors rising upward.

The other part (stream 40), containing the remaining 60% of the liquid, is discharged from the column and sent to the heat exchanger 10, where it provides part of the initial gas cooling during its heating to 30∈F and partial evaporation. The heated stream 40a is then fed to the demethanizer column 17 at the midpoint of the column, separated from the point where stream 40 was diverted from the column by at least one theoretical stage. In this case, the partially vaporized stream 40a arrives at the same point in the column that was used to return the auxiliary reboiler (theoretical stage 11 in the demethanizer column 17) in the method according to FIG. 1, which is equivalent to ten theoretical stages below the liquid flow outlet point in the fractionation system (theoretical stage 1 in the column 17 demethanizer).

The liquid product stream (stream 43) exits from the bottom of the demethanizer column 17 at a temperature of 54∈F. This stream is pumped out to approximately 480 psi (stream 43a) by pump 20 and then sent to a heat exchanger 10, where it is heated to 72∈F, while it provides partial cooling of the source gas described above. The residual gas (stream 37) flows countercurrently to the incoming feed gas in the heat exchanger 10, where it is heated to 58∈F (stream 37a). The residual gas is then re-compressed in two stages by a compressor 15, which is driven by the expander 14, and a compressor 22, which is driven by an additional power source. Then, stream 37c is cooled to 120∈F using a refrigerator 23, and the residual gas product (stream 37d) enters the finished product pipeline at a pressure of 1015 psi.

The combined values of the flow rates and energy consumption in the method according to Figure 2 are given in table II.

Table II
Total flow rates (lb · mol / hour) Flow Methane Ethane Propane Bhutans + Carbon dioxide Total 31 25338 1905 647 320 307 28659 32 22905 1382 331 87 252 25094 35 2433 523 316 233 55 3565 40 1334 565 186 51 70 2208 37 25306 637 27 one 243 26356 43 32 1268 620 319 64 2303 Extract *
Ethane - 66.58%
Propane - 95.91%
Bhutans + - 99.63%
Horsepower
Compression of the remainder - 15400
* (based on unrounded expense values)

In contrast to the prior art method according to FIG. 1, both the carbon dioxide: ethane ratio (0.05: 1) and the methane: ethane ratio (0.025: 1) in the bottoms liquid product can be adjusted according to the specifications required by the customer in the method according to FIG. .2. Comparison of the extraction levels according to Tables I and II shows that the present invention allows to achieve much higher liquid recovery efficiency than in the method according to Figure 1, when it is carried out in such a way as to limit the content of carbon dioxide in its liquid product. A comparison of Tables I and II demonstrates that, in contrast to the prior art, this invention improves ethane recovery from 40.74 to 66.58%; propane recovery - from 85.40 to 95.91% and butane recovery + from 98.09% to 99.63%. A comparison of Tables I and II also demonstrates that higher product yields are not just the result of increased horsepower requirements. (energy). In contrast, when using the present invention, as in this Example, not only the extraction of ethane, propane and butanes + is improved compared to the prior art, but also the efficiency of liquid recovery is increased by 41% (based on the extracted ethane per unit of power pm.). The method of FIG. 2 extracts 0.83 gallons / hour of ethane per unit of horsepower consumed. compared with 0.59 gallons / hour per unit of power in liters. C. in the method according to Figure 1.

A significant advantage in accordance with this invention, illustrated in FIG. 2, is that the modified reboiler circuitry provides colder liquids in the column for use in cooling incoming feed streams. Due to this, the cooling of the incoming gas is enhanced, since in this case the liquid provides not only a much larger amount of work, but also a lower temperature. At the same time, a larger amount of methane is introduced into the demethanizer column 17 than when re-evaporation is carried out in the column to match the carbon dioxide content. (It should be noted that stream 40 in the method of FIG. 2 contains 1334 lb mol / hour of methane, while stream 40 in the method of FIG. 1 contains only 14 lb mol / hour of methane). This additional methane provided according to the method of the present invention in accordance with FIG. 2 facilitates the distillation of carbon dioxide from liquids going down in the stripping column. The amount of carbon dioxide in the LCFG product in the method of FIG. 2 can be controlled by appropriately adjusting the amount of liquid taken to supply the modified reboiler to the system, instead of sending it to the fractionation stage in the upper section of the demethanizer column 17.

Other embodiments of the invention

Figures 3 and 4 illustrate flow charts of alternative methods according to which the method and apparatus of US Pat. It should be noted that in the embodiment of the invention of FIG. 3, the distillation stream (stream 40) used for the modified reboiler circuit is obtained by separating the liquids formed in stream 32a during expansion (stream 34 from separator 19) outside the demethanizer column 17. This can be done by directing the entire expanded stream (stream 32a) from the expander 14 to the separation section at the top of the demethanizer column 17 to separate the liquids, and then by dividing the liquids to obtain an irrigation stream for the column (stream 41) and a distillation stream for modified reboiler circuits (stream 40). FIG. 5 illustrates a flow chart showing how the method and apparatus of US Pat. No. 4,854,955 can be adapted for a natural gas processing unit in accordance with this invention.

6, 7, 8, and 9 illustrate diagrams depicting some of the alternative embodiments of a modified reboiler circuit. 6 illustrates a typical thermosiphon type in which the partial flow of liquid from the fractionation column 50 to the reboiler 57 can be controlled by a valve 58 in the fluid withdrawal line 61. The part of the liquid that is not discharged from the column simply flows from the vertical plate 51 to the distributor 52 for the nozzle (or plates) 53 below. The heated stream in line 61a from reboiler 57 is returned to the fractionation column 50 at a lower point that contains a suitable raw material distribution mechanism, such as a vertical plate 54 or distributor 55, for mixing the heated stream with downstream liquids of the column from the nozzle (or plates) 53 and for feeding the mixture into the nozzle (or plates) 56. FIGS. 7 and 8 show typical adaptations using pumping, according to which the total downward flow of liquid is diverted to the liquid discharge line 61 and pumped to a higher pressure of the pump 60. The flow of pumped fluid in line 61a is then separated by appropriate control valves 58 and 59 to provide the right amount of fluid in line 62 flowing to reboiler 57. The heated stream in line 62a from reboiler 57 is returned to fractionation column 50 to more low point, as described for implementation in accordance with Fig.6. In the embodiment of FIG. 7, a liquid that does not go into the reboiler (in line 63) is returned to the vertical plate 51 from which the liquid was initially diverted; and then it can transfer from the vertical plate 51 to the distributor 52 for the nozzle (or plates) 52 below. In the embodiment of FIG. 8, liquid that does not flow into the reboiler (in line 63) is returned below the vertical tray 51, from which the fluid was originally diverted, directly to the dispenser 52, which delivers the fluid to the nozzle (or trays) 53 below. FIG. 9 illustrates how the pumping system described in FIG. 8 can be implemented in a split column method, such as upper column 65 and lower column 50.

It will be clear to those skilled in the art that this invention realizes some of its advantages by providing a colder flow to the auxiliary reboiler (s) and / or reboiler (s), thereby providing additional cooling to the column power. This additional cooling reduces the technological requirements for a given level of product recovery, or increases the level of product recovery for a given energy consumption, or provides both. It will be clear to those skilled in the art that this invention also takes advantage of the introduction of large amounts of methane to a lower point in the demethanizer to facilitate the removal of carbon dioxide from downstream liquids. If a larger amount of methane is present for liquid distillation, a correspondingly lower amount of ethane will be required for distillation, while ensuring a greater retention of ethane in the bottoms liquid product. Therefore, this invention is generally applicable to any method, depending on cooling any number of feed streams and feeding the resulting feed stream (s) to a distillation column.

In accordance with this invention, the cooling of the feed streams (feed streams) of the demethanizer can be done in several ways. In the method according to FIGS. 2, 3 and 4, cold residual gas (stream 37) and demethanizer liquids (streams 40, 42 and 43) are used only for cooling the gas stream. In the method of FIG. 5, the feed stream 36 is cooled and substantially condensed with cold residual gas (stream 37); the steam discharged from above the distillation column (stream 47) is cooled and partially condensed by the expanded stream 36b; moreover, the expanded separator liquid (stream 35A) and demethanizer liquid (stream 40) are used only for cooling the gas. However, demethanizer fluids can be used to provide some or all of the cooling and to substantially condense the stream 36 of FIG. 5 or to cool and partially condense the stream 47 of FIG. 5 in addition to or instead of cooling the gas stream. In this case, you can use any stream with a temperature below the temperature of the cooled stream of raw materials. For example, you can divert and use for cooling the side stream of steam from the demethanizer. Other potential cooling sources include, but are not limited to, high-pressure super-fast separator liquids (for example, as indicated by the dashed line in FIG. 3), and mechanical refrigeration systems. The choice of cooling source will depend on several factors, including, but not limited to, the composition and condition of the incoming gas, installation size, heat exchanger size, temperature of the potential cooling source, etc. It will also be clear to a person skilled in the art that to ensure the desired source temperature any combination of the above sources and cooling methods can be applied to the flow.

According to the present invention, external cooling may be used to supplement cooling for the inlet gas from other process streams, especially in the case of a source gas more saturated than indicated in the Example. The use and distribution of demethanizer fluids for heat transfer of the method and the specific design of heat exchangers for cooling the source gas (incoming gas) must be determined for each specific application; and the same applies to the selection of process streams for certain heat transfer operations.

The high-pressure fluid of FIG. 5 (stream 35) can be combined with a portion of the steam of the separator (stream 33) entering the heat exchanger 12. Alternatively, this liquid stream (or part thereof) can be expanded using an appropriate expansion device, such as an expansion valve 16, and feed to the lower midpoint of the column in the distillation column (demethanizer column 17 of FIG. 5). The fluid stream can also be used to cool the inlet (source) gas or other heat exchange operations before or after the expansion operation before entering the demethanizer according to FIG. 5.

It should also be noted that the relative amount of raw material in each line of the column's feed flows will depend on several factors, including gas pressure, composition of the feed gas, the amount of heat that can be economically extracted from the feed, and the amount of available horsepower. More feed to the top of the column can increase recovery, while reducing the power taken by the expander, and thereby increase the power requirements for re-compression. An increase in raw materials at the bottom of the column reduces power consumption, but can also reduce product recovery. However, the relative locations of the mid-column feed can vary depending on the inlet composition or other factors, such as the desired levels of recovery and the amount of liquid formed when the incoming gas is cooled. In addition, two or more streams of raw materials, or parts thereof, can be combined depending on the relative temperatures and quantities of the individual streams; and the combined stream is then directed to the midpoint of the column. Figure 2 illustrates a preferred embodiment for the compositions and pressures shown. Although a separate expansion of the flow is illustrated in specific expansion devices, alternative expansion means may also be used, as appropriate. For example, conditions can guarantee expansion work on the substantially condensed portion of the feed stream (stream 36a in FIG. 5).

Fractional columns, illustrated as single columns in FIGS. 2-5, may also consist of two sections (absorption section and stripping section, for example) due to the size of the installation. The decision on the design of the fractionating column: single (as column 17 in FIGS. 2-5) or multi-section will depend on such factors as the size of the installation, the distance to the production equipment, etc.

Although the above description discloses preferred embodiments of the invention, it will be clear to a person skilled in the art that other and further modifications can be made, for example, the invention can be adapted to different conditions, types of raw materials or other requirements without going beyond the concept of the invention defined by the attached claims.

Claims (24)

1. An improved method for separating a gas stream containing methane, C ₂ components, C ₃ components and heavier hydrocarbon components into a volatile residual gas fraction containing a major portion of said methane and a relatively less volatile fraction containing a major portion of said C ₂ -components, C ₃ -components and heavier hydrocarbon components, in which (a) said gas stream is processed in one or more heat transfer stages to obtain at least a first feed stream that is cooled under pressure; (b) said chilled first feed stream is expanded to a lower pressure and then fed to a fractionation column at an upper feed point; and (c) said cooled expanded first feed stream is fractionated at said lower pressure, whereby components of said relatively less volatile fraction are recovered, where the improvement is as follows: (1) a liquid distillation stream is withdrawn from said fractionation column and heated; (2) the specified heated distillation stream is returned to a lower point in the specified fractionation column, which is separated from the specified outlet by at least one theoretical stage; and (3) the amounts and temperatures of said feed streams supplied to said fractionating column are effective to maintain the top temperature of said fractionating column at a temperature at which the major parts of the components in said relatively less volatile fraction are recovered. 2. The improved method according to claim 1, where the specified liquid distillation stream is pumped out after removal from the specified fractionation columns. 3. The improved method according to claim 1, where the specified liquid distillation stream is sent to heat exchange with at least a portion of the specified gas stream or these streams of raw materials to ensure cooling and thereby heating the specified liquid distillation stream. 4. The improved method according to claim 1, where the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 5. The improved method according to claim 2, where (a) said evacuated liquid distillation stream is divided into at least a first part and a second part; (b) said first part is heated and (c) said heated first portion is returned to a lower point in said fractionating column, which is separated from said tap point by at least one theoretical stage. 6. The improved method according to claim 5, where the specified first part is sent to heat exchange with at least a portion of the specified gas stream or these streams of raw materials to provide it with cooling and thereby heating the specified first part. 7. The improved method according to claim 6, where the amount and temperature of said heated first part and heating provided for said fractionating column are effective for maintaining the temperature of the lower part of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 8. The improved method according to claim 5, where the amount and temperature of said heated first part and heating provided for said fractionating column are effective for maintaining the temperature of the lower part of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 9. The improved method according to claim 2, wherein said evacuated liquid distillation stream is sent to heat exchange with at least a portion of said gas stream or said feed streams to provide cooling and thereby heat said evacuated liquid distillation stream. 10. The improved method according to claim 9, where the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 11. The improved method according to claim 2, where the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 12. The improved method according to claim 6, where the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 13. An improved method for separating a gas stream containing methane, C ₂ components, C ₃ components and heavier hydrocarbon components into a volatile residual gas fraction containing a major portion of said methane and a relatively less volatile fraction containing a major portion of said C ₂ -components, C ₃ -components and heavier hydrocarbon components, in which (a) said gas stream is treated in one or more stages of heat transfer and at least one stage of separation to obtain at least a first stream of feed that is cooled under pressure to condense essentially the entire stream and at least least a second stream of raw materials that is cooled under pressure; (b) said substantially condensed first feed stream is expanded to a lower pressure, as a result of which it is further cooled, and then sent to heat exchange with a warmer distillation stream that rises from the fractionation stages of the fractionation column; (c) said distillation stream is cooled sufficiently by said first stream to partially condense it, after which said partially condensed distillation stream is separated to obtain said volatile residual gas fraction and an irrigation stream; wherein said irrigation stream is then directed to said fractionating column at a top feed point; (d) said heated first stream is fed to said fractionating column at a first midpoint of the column feed; (e) said cooled second feed stream is expanded to said lower pressure and then fed to said fractionating column at a second midpoint of column feed; and (f) said irrigation stream, said heated first feed stream, and said expanded second feed stream are fractionated at said lower pressure, whereby components of said relatively less volatile fraction are recovered; where the improvement is as follows: (1) a liquid distillation stream is withdrawn from said fractionation column and heated; (2) the specified heated distillation stream is returned to a lower point in the specified fractionation column, which is separated from the specified outlet by at least one theoretical stage; and (3) the amounts and temperatures of said feed streams supplied to said fractionating column are effective to maintain the top temperature of said fractionating column at a temperature at which the major parts of the components in said relatively less volatile fraction are recovered. 14. The improved method according to item 13, where the specified liquid distillation stream is pumped out after removal from the specified fractionating columns. 15. The improved method according to item 13, where the specified liquid distillation stream is sent to heat exchange with at least a portion of the specified gas stream or these streams of raw materials to provide it with cooling and thereby heating the specified liquid distillation stream. 16. The improved method of claim 13, wherein the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective to maintain the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 17. The improved method according to 14, where (a) said evacuated liquid distillation stream is divided into at least a first part and a second part; (b) said first part is heated, and (c) said heated first portion is returned to a lower point in said fractionating column, which is separated from said tap point by at least one theoretical stage. 18. The improved method according to claim 17, wherein said first part is sent to heat exchange with at least a part of said gas stream or said feed streams to provide it with cooling and thereby heat said first part. 19. The improved method according to claim 18, wherein the amount and temperature of said heated first portion and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 20. The improved method of claim 17, wherein the amount and temperature of said heated first portion and heating provided for said fractionating column are effective to maintain the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 21. The improved method of claim 14, wherein said evacuated liquid distillation stream is sent to heat exchange with at least a portion of said gas stream or said feed streams to provide it with cooling and thereby heat said evacuated liquid distillation stream. 22. The improved method according to item 21, where the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 23. The improved method of claim 14, wherein the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective to maintain the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction. 24. The improved method according to claim 18, wherein the amount and temperature of said heated distillation stream and heating provided for said fractionating column are effective for maintaining the temperature of the bottom of said fractionating column at a temperature at which the amount of carbon dioxide contained in said relatively less volatile fraction.

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