EP0642649B1 - Method of separating higher-boiling hydrocarbons out of a mixture of gases - Google Patents

Abstract

Described is a method of separating higher-boiling hydrocarbons from a gas mixture containing these hydrocarbons plus lower-boiling compounds by fractional distillation. The gas mixture (6) is partially condensed (7) and passed into a separation column (9). A fraction (27) rich in higher-boiling hydrocarbons is drawn off at the bottom of the column (9) and a fraction (10) rich in lower-boiling compounds is drawn off at the head of the column. The head fraction (10) is partially condensed (11). The condensate thus formed is returned to the separation column (9). Both the partial condensation of the gas mixture and that of the head fraction are produced by indirect heat exchange (7, 11) with a refrigerant which is made up of several components and is circulated in an external circuit (18).

Description

The invention relates to a method for separating higher hydrocarbons from a gas mixture containing these and lower-boiling components by rectification, in which the gas mixture is partially condensed and fed to a separation column. a fraction rich in higher hydrocarbons is withdrawn from the bottom thereof and a fraction rich in lower-boiling components is withdrawn from the top. the top fraction partially condensing and the condensate being fed as reflux to the top of the separation column.

Such methods are known from EP-8-0 318 504 and from EP-A-0 153 984. The cold required for the condensation of feed gas and top fraction is made available in the known processes on the one hand by one or more cooling circuits and on the other hand by work-related expansion of feed or residual gas. The refrigeration circuits work at constant evaporation temperature and cause relatively high temperature differences and thus exergy losses when exchanging heat with a condensing feed or top gas mixture. The turbines used to generate peak cold are not suitable for all processes. In particular in the case of temperature fluctuations, for example as a result of non-stationary process conditions, they exhibit high wear. The previously known method therefore does not work completely economically satisfactorily and is only reliable in operation if certain boundary conditions are observed.

Furthermore, from EP-A-0 132 984 and from the article "The liquefaction of natural gas" by W. Förg and V. Etzbach in the Linde reports from technology and science. No. 28 June 1970. Pages 27 to 39 the use of multi-component refrigerants for the condensation of hydrocarbon-containing process streams is known.

The invention is based on the object of specifying a method of the type mentioned at the outset which works more economically and can be used more flexibly with respect to boundary conditions and is also particularly suitable for relatively strongly fluctuating parameters of the gas mixture to be separated.

This will solve this task. that the condensation of the gas mixture and the condensation of the top fraction by indirect heat exchange with a refrigerant be effected. which consists of several components and is conducted in an external circuit and that compressed refrigerant is separated into a gaseous and a liquid fraction within the external refrigeration circuit and the gaseous fraction is cooled in indirect heat exchange with the portion remaining in gaseous form during the condensation of the top fraction and thereby is condensed and then passed for indirect heat exchange with the top fraction.

Such a procedure enables the refrigerant temperature to be adapted to the requirements specified by the composition of feed gas and products. Compared to a refrigerant cascade, for example, it enables both less equipment expenditure and less exergy losses. Peak cold can also be generated with reasonable effort. so that the method according to the invention can dispense with expansion turbines. The disadvantages associated with turbines in terms of flexibility are avoided.

The energetic advantages of the method according to the invention are surprisingly so great that they do not only outweigh the additional costs caused by the multi-component refrigerant circuit. but overall there is a significant increase in the economics of the process. In addition, possible uses of the method are extremely flexible.

The separation column used in the process is usually operated only as a reinforcement column. that is, the partially condensed gas mixture is fed in at the bottom of the column.

When the heat is exchanged with the gaseous top fraction, the refrigerant is preferably not only completely condensed, but additionally supercooled in order to have as high a proportion as possible in the liquid state after its relaxation. The refrigerant that remains liquid after compression is also supercooled as much as possible.

Downstream of the heat exchanger for generating the return flow, the entire refrigerant flow can be combined again. The refrigerant is exchanged with the top fraction after the heat exchange. usually supplemented by the refrigerant fraction that remained liquid after compression, brought into heat exchange with the gas mixture to be separated, and previously, if provided, brought into heat exchange with the intermediate fraction.

The refrigerant remaining in gaseous form after compression is thus used in a particularly advantageous manner to transfer peak cold to the top fraction of the separation column. This further improves the energy balance of the process,

To further improve the rectifying effect of the separation column, it is advantageous if an intermediate fraction is removed from the separation column at a central point, this is at least partially condensed in indirect heat exchange with the refrigerant and is returned to the separation column.

This heat exchange takes place at a temperature which lies between the temperature levels of the condensation of the feed gas mixture and that of the condensation of the top fraction. The corresponding heat exchangers are preferably connected in series on the refrigerant side, so that optimum use is made of the sliding evaporation temperature profile of the multicomponent refrigerant. As a result, the process is particularly economical to operate. Of course, it is also possible and in many cases also advantageous to remove several such intermediate fractions in an analogous manner and to supply them with an indirect heat exchange with the refrigerant.

According to a development of the concept of the invention, the method is carried out with a throughput and / or a composition of the gas mixture to be separated which is varied over time.

Of course, each process is subject to fluctuations in time, for example when starting and stopping a system. However, changes are meant with a much shorter period, generally less than an hour, preferably in the minute range, which, for example, have temperature fluctuations of approximately 3 K / min and / or 10% load change per minute. Such deviations from static behavior can also be predetermined by preceding process steps, for example if the gas mixture to be separated in the present process comes from a periodically operated apparatus, for example switchable reactors. In particular under such conditions, a process with the generation of peak cooling by turbines (e.g. according to EP-B-O 318 504) would lead to very high wear of the turbines and would therefore mean frequent downtimes and high costs for the plant, in particular due to production downtime. The method according to the invention, on the other hand, can cope with such fluctuations because the multi-component refrigerant run used is not subject to such wear fluctuations and can nevertheless provide cold at different temperature levels, similar to the previously known methods.

In the case of such a non-stationary implementation of the method with relatively short periods, conventional control methods often reach their limits because they react too sluggishly. According to a development of the method according to the invention, it is therefore provided that the throughput and / or the composition of the gas mixture to be separated is measured and the throughput of refrigerant in the various condensation stages is set as a function of this measured value.

The necessary adjustments to the refrigeration budget are therefore not made by a regulation, but by a controller. Certain parameters must be included in the calculation of the manipulated variables, which can only be partially determined in advance by theoretical considerations. In addition, empirical values are necessary, which must be determined by the operating personnel when a system is started up for the first time. Since the fluctuations in throughput and / or composition of the gas mixture to be separated are generally periodic, such values can be determined by tests and then predefined. Self-learning systems are also conceivable that optimize such parameters automatically and also during ongoing operation.

In the case of relatively short-term fluctuations in the compositions of the feed, intermediate and product streams, which arise either indirectly through differently high throughputs or directly via corresponding feed gas, a further problem arises in the previously known generic processes. The commonly used aluminum plate heat exchangers usually only withstand the resulting frequent and short-term temperature fluctuations and the mechanical stresses induced thereby for only a very short time. Even wound heat exchangers with aluminum tubes, the structure of which is more suitable for compensating for thermal changes in length, can leak over time.

According to one embodiment of the invention, heat exchangers are therefore preferably used for the indirect heat exchange between the top fraction and the refrigerant, which are made of a material with high long-term stability against mechanical stresses. Stainless steel is preferred. It is expedient to design the heat exchanger in a wound construction, that is to say with tubes arranged in a helical manner on concentric cylinder surfaces.

Similarly, it is advantageous for the indirect heat exchange between the Antell remaining in gaseous form in the condensate fraction and the gaseous fraction of the refrigerant and / or for the indirect heat exchange (7) between the gas mixture (6) and refrigerant to be separated and / or for to use the indirect heat exchange (24) between the intermediate fraction (28) and the refrigerant in each case a heat exchanger which is made of a material with high long-term stability against mechanical stresses.

According to a further embodiment of the invention, a plate heat exchanger, in particular an aluminum plate heat exchanger, can be used for the indirect heat exchange (7 ') between the gas mixture (6) to be separated and the refrigerant.

The invention and further details of the invention will now be explained in more detail with reference to two exemplary embodiments, which are shown in the drawings as process diagrams. They relate to an application of the method according to the invention in which its advantages are particularly effective, namely the reprocessing of a product gas from a C₃ or C₄ dehydrogenation. Such a gas contains, in addition to the higher hydrocarbons, more volatile components, especially hydrogen, but also lower amounts of water, carbon monoxide, carbon dioxide, nitrogen, C₂-hydrocarbons, etc. The process steps of the invention serve to separate the undesired lighter components, which is a prerequisite for the further processing of the C₃ or C₄ components.

In the process of FIG. 1, the dehydrator product gas is introduced via line 1 and is initially subjected to a pretreatment. After cooling with the help of an external refrigeration system in a heat exchanger 2 and a subsequent phase separation in a separator 3, the gaseous Antell is freed of traces of chlorine in an HCl reactor 4 and dried (5). The pre-cleaned gas in position 6 now represents the gas mixture to be separated for the process according to the invention and is also referred to here as feed gas. For example, it contains 30 to 70% more volatile constituents that are to be separated off. (The percentages here and in the following basically refer to the molar proportions.)

The feed gas in line 6 is cooled in heat exchanger 7 and partially (5 to 40%, preferably 10 to 30%) condensed and fed via line 8 into a separation column 9 above the sump. At the bottom of the separation column, the desired higher hydrocarbons are obtained as bottom product, are drawn off via line 27 and heated in heat exchanger 23. Together with the high-boiling components from separator 3 which have already been condensed out during the pretreatment, they are fed via line 32 for further treatment, for example a depropanizer.

Line 10 leads the top fraction of the separation column to a heat exchanger 11 in which the fraction is partially condensed. The two-phase mixture is fed via line 12 into a separator 13 which is integrated in the separation column. However, a phase separation device designed as a separate component could also be used. The liquid from the isolator flows back into the separation column; the remaining gaseous portion of the top fraction is discharged via a residual gas line 14 and warmed to approximately ambient temperature in heat exchanger 15. This gas can be supplied partially or entirely via line 17 to a compressor unit and then to further processing, for example in a pressure swing adsorption. Alternatively or in parallel, residual gas is either removed via line 16 and used, for example, as fuel gas or to regenerate the dryer 5.

The refrigeration required for the condensation of feed gas (heat exchanger 7) and top fraction (heat exchanger 11) is generated by a multi-component refrigerant circuit 18 in which a refrigerant is compressed and partially liquefied in a known manner. The refrigerant contains, for example, C₂H₄, C₂H₆, Iso-C₄H₁₀ and some CH₄. The exact composition is determined depending on the course of the respective evaporation curves. Here, an exact adaptation to the evaporation properties of feed and intermediate product streams with their respective special composition is possible.

Compressed refrigerant is introduced as a two-phase mixture into a refrigerant separator 19. The gaseous portion (line 20) is condensed to recover peak cold in indirect heat exchange 15 with the gaseous portion 14 of the top fraction and supercooled. The temperature of the refrigerant flow should be as low as possible so that all refrigerant remains liquid even in the subsequent expansion in throttle valve 25. As a result, a maximum amount of latent heat can be converted during the subsequent heat exchange 11 with the top fraction 10.

The liquid portion 21 of refrigerant from the refrigerant separator 19 is also supercooled, namely in heat exchanger 22 against low-pressure refrigerant and in heat exchanger 23 against the C₃₊- / C₄₊ product stream 27 from the bottom of the separation column 9 and again against Low pressure refrigerant. A first part of the supercooled liquid is expanded in the throttle valve 26a, combined with the refrigerant portion remaining in gaseous form in the separator 19, warmed in the heat exchangers 24, 7 and 22 and compressed again. A second part is expanded in FIG. 26 b, heated in the lower part of the heat exchanger 23 and then combined upstream of the heat exchanger 7 with the remaining leather pressure refrigerant.

To further improve the energy balance of the method, an intermediate fraction 28 is led out of the separation column 9 in the exemplary embodiment, partially condensed in a heat exchange 24 with refrigerant and fed back into the separation column 9 via line 29. Similarly, several such intermediate fractions can be removed at different points for partial condensation. In individual cases, this must be decided on the basis of the trade-off between higher expenditure on equipment on the one hand and reduced exergy losses on the other.

The heat exchangers required in the exemplary embodiment are preferably implemented as wound apparatuses with stainless steel tubes.

The method works with a control device instead of an otherwise conventional regulating device. For this purpose, the flow of gas mixture to be broken down is measured in line 6 (30). From this measured value, setpoints for the cooling requirement are determined in a control unit 31 with the aid of additional parameters, which were partly calculated theoretically, partly based on experience, and then the flow rate in the refrigerant lines is set. This manipulation takes place by activating the expansion valves 25, 26a, 26b.

The following numerical example relates to the separation of C₄ hydrocarbons from the product gas of a C₄ dehydrogenation. Due to the discontinuous operation of the dehydrogenation reactors, the throughput and composition of the product gas fluctuate with an approximately four-minute period. Two values are given for each size: left for the phase of maximum throughput of gas mixture to be separated (612 mol / s through line 6) and the associated lower relative but higher absolute hydrogen content (approx. 55%, corresponds to 334 mol / s); right for minimum throughput (423 mol / s) and higher relative but lower absolute hydrogen content (about 64%, corresponds to 275 mol / s).

• A Einsatzgas vor der partiellen Kondensation (Leitung 6)
• B Einsatzgas nach der partiellen Kondensation (Leitung 8)
• C Sumpfprodukt (Leitung 27)
• D Kopffraktion vor der partiellen Kondensation (Leitung 10)
• E Kopffraktion nach der partiellen Kondensatlon (Leitung 12)
• F Zwischenfraktion vor der partiellen Kondensation (Leltung 28)
• G Zwischenfraktion nach der partiellen Kondensation (Leitung 29)

The different streams for which data are given in the table are identified by capital letters A to G. They mean in particular:

• A feed gas before partial condensation (line 6)
• B feed gas after partial condensation (line 8)
• C bottom product (line 27)
• D head fraction before partial condensation (line 10)
• E head fraction after the partial condensation (line 12)
• F intermediate fraction before partial condensation (Leltung 28)
• G intermediate fraction after partial condensation (line 29)

Das Schema von Figur 2 zeigt ein weiteres Ausführungsbeispiel des erfindungsgemäßen Verfahrens, das ebenfalls vorzugsweise zur Aufarbeitung eines Produktgases aus einer C₃- oder C₄-Dehydrierung eingesetzt wird. Einander entsprechende Verfahrensschritte und Vorrichtungen tragen in beiden Zeichnungen die gleichen Bezugszeichen.In this special application, the refrigerant has the following molar composition:
CH₄ 2%
C₂H₄ 20%
C₂H₆ 25%
Iso-C₄H₁₀ 53%

The diagram of Figure 2 shows a further embodiment of the method according to the invention, which is also preferably used for working up a product gas from a C₃ or C₄ dehydrogenation. Corresponding method steps and devices have the same reference symbols in both drawings.

Dehydrogenation product gas is introduced via Leltung 1 and subjected to a pretreatment similar to that of Flgur 1 (cooling by means of external cooling in heat exchanger 2, phase separation in isolator 3, chlorine removal in HCl reactor 4, drying 5). The feed gas in line 6 is cooled in heat exchanger 7 'and partially condensed. The two-phase mixture is fed via line 8 above the bottom of the separation column 9. At the bottom of the separation column, the desired higher hydrocarbons are obtained as the bottom product, are withdrawn via line 27 and heated in the heat exchanger 7 '. They are discharged from separator 3 separately from the high-boiling components that have already condensed out during the pretreatment.

Line 10 leads the top fraction of the separation column to a heat exchanger 11, in which the Fractlon is partially condensed. The two-phase mixture is fed via line 12 into a separator 13 arranged in the upper region of the separation column. The gaseous fraction of the top fraction is discharged via a residual gas line 14 and heated to approximately ambient temperature in heat exchanger 15. This gas can be drawn off via line 16 (for example to regenerate the dryer 5) and / or via line 17.

The cold required for the condensation of feed gas (heat exchanger 7 ') and top fraction (heat exchanger 11) is generated by a multi-component refrigerant circuit 18 similar to the method in FIG.

The gaseous portion of the compressed refrigerant (Leltung 20) introduced into the refrigerant separator 19 is condensed and recovered in indirect heat exchange 15 with the portion 14 of the top fraction remaining in gaseous form in order to recover peak cold, and then expanded in throttle valve 25. and brought into indirect heat exchange 11 with the top fraction 10 from the separation column 9.

The liquefied portion 21 of refrigerant from the refrigerant separator 19 is subcooled in the heat exchanger 7 '. The supercooled liquid is expanded in the throttle valve 26, combined with the refrigerant portion remaining in gaseous form in the separator 19, warmed in the heat exchanger 7 'and completely evaporated and then compressed again.

In order to reduce the investment costs of the plant, the intermediate cooling steps shown in FIG. 1 were dispensed with in the process of Flgur 2. In this variant, the heat exchanger 7 'is designed as a plate heat exchanger. It combines the functions of the heat exchangers 7, 22 and 23 of FIG. 1.

The control in the method in FIG. 2 is similar to that described in FIG. 1. For this purpose, measuring devices are provided for the flow of gas mixture (30) to be broken down in line 6 and for the pressure of the refrigerant (33) in line 20. The measured values are converted in a control unit 31 into target values for the cooling requirement. Then the flow in the refrigerant lines (expansion valves 25, 26) is set.

The numerical examples from the table above are also valid for the variant according to Flgur 2. Dispensing with intermediate cooling (heat exchanger 24 from FIG. 1) causes only minor changes in the parameters of the other flows.

Claims (10)

1. Process for separating off higher hydrocarbons from a gas mixture containing these and lower-boiling components by rectifying fractionation, in which the gas mixture (6) is partially condensed (7; 7') and passed to a separation column (9), at the bottom of which a fraction (27) rich in higher hydrocarbons is taken off and at the top of which a fraction (10) rich in lower-boiling components is taken off, the overhead fraction (10) being partially condensed (11) and the condensate being added as reflux to the top of the separation column (9), characterized in that the condensation (7; 7') of the gas mixture (6) and the condensation (11) of the overhead fraction (10) is effected by indirect heat exchange with a refrigerant which comprises a plurality of components and is externally circulated (18) and in that the compressed refrigerant is separated within the external refrigeration circuit (18) into a gaseous (20) and a liquid (21) fraction and in that the gaseous fraction (20) is cooled and thus condensed in indirect heat exchange with the portion (14) which has remained in the gaseous state in the condensation of the overhead fraction and is then passed for the indirect heat exchange (11) with the overhead fraction (10).
2. Process according to Claim 1, characterized in that an intermediate fraction is withdrawn from the separation column (9) at a central point, this intermediate fraction is at least partially condensed in indirect heat exchange with the refrigerant and is passed back (29) to the separation column (9).
3. Process according to Claim 1 or 2, characterized in that the process is carried out with time-variable throughput and/or time-variable composition of the gas mixture (6) to be separated.
4. Process according to Claim 3, characterized in that the throughput and/or the composition of the gas mixture (6) to be separated is measured (30) and the throughput of refrigerant in the various condensation stages (7; 7', 11, 15, 24) is set (25, 26,; 26a, 26b) as a function of this measured value.
5. Process according to one of Claims 1 to 4, characterized in that a heat exchanger is used for the indirect heat exchange (11) between overhead fraction (10) and refrigerant, which heat exchanger is produced from a material having high long-term stability against mechanical stresses.
6. Process according to one of Claims 1 to 5, characterized in that a heat exchanger is used for the indirect heat exchange (15) between the portion (14) which has remained in the gaseous state in the condensation of the overhead fraction and the gaseous fraction (20) of the refrigerant, which heat exchanger is produced from a material having high long-term stability to mechanical stresses.
7. Process according to one of Claims 1 to 6, characterized in that a heat exchanger is used for the indirect heat exchange (7) between the gas mixture (6) to be fractionated and refrigerant, which heat exchanger is produced from a material having high-long term stability to mechanical stresses.
8. Process according to one of Claims 1 to 7, characterized in that a plate heat exchanger is used for the indirect heat exchange (7') between the gas mixture (6) to be fractionated and refrigerant.
9. Process according to Claim 8, characterized in that an aluminium plate heat exchanger is used for the indirect heat exchange (7') between the gas mixture (6) to be fractionated and refrigerant.
10. Process according to one of Claims 2 to 9, characterized in that a heat exchanger is used for the indirect heat exchange (24) between the intermediate fraction (28) and the refrigerant, which heat exchanger is produced from a material having high-long term stability to mechanical stresses.

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