Method for separating of an ethane-rich fraction from natural gas Field of the Invention The invention relates to a method for separating a C ₂₊ rich fraction from a hydrocarbon- rich feed, preferably a natural gas.

Background of the Invention Conventional natural gas mainly consists of hydrocarbons. After methane, ethane usually has the second highest molar concentration. The extraction of technically pure ethane from natural gas is an important technology to supply so-called gas crackers with the feedstock for to provide production of ethylene. The term "technically pure ethane" is to be understood as meaning a fraction high in ethane with an ethane concentration of >95 mol%, preferably >98 mol%. The recovery of a C ₂₊ fraction from natural gas is usually achieved by a combination of partial condensation and rectification using heat exchanger networks. Since a liquid C ₂₊ product is to be separated from the gaseous natural gas, the cooling capacity required for this must be provided by work-performing expansion and, if appropriate, a refrigeration system. The so-called Gas Subcooled Process (GSP) is exemplified, which is described in U.S. Patents 4,157,904 and 4,278,457.

Such separation method performs the intersection between methane and lighter components, such as nitrogen, ethane and heavier components, such as propane and higher hydrocarbons. In other words, the feed gas is decomposed by a demethanizer into a methane-rich light sales gas (gas high in methane) and a C ₂ + rich liquid (liquid C ₂₊ fraction). The further separation of the C ₂₊ fraction in fractions of the desired composition takes place in a chain from the distillation columns connected downstream of the demethanizer.

The above-mentioned refrigeration system is usually carried out as a closed propane or propylene refrigeration system (hereinafter referred to as a C ₃ refrigeration system), in which the refrigerant is condensed under high pressure against its ambient and then evaporated at lower pressure. The evaporation can also take place in several pressure stages, if the refrigerating capacity is required at different temperatures. However, such C ₃ refrigerators have the following disadvantages: - the condensation of the refrigerant requires a lot of energy of the control unit at the circulating compressor in the case of strongly fluctuating ambient temperatures (due to seasonal and/or day/night changes) to ensure the respectively optimal condensation pressure. If the final pressure of the refrigeration compressor is too low, the condensation cannot take place and the refrigeration system will not work. If, on the other hand, the final pressure is too high, energy is unnecessarily consumed and operating expenses are increased. If a C ₃ refrigeration system is used, for example, at a location with a

pronounced continental climate, the optimum final pressure can vary between 5 and 20 bar.

The considerable investment costs of a C ₃ refrigeration system have to be amortized through an increased product yield (C ₂ + fraction).

- The required refrigerant (propane or propylene) must be provided in appropriate purity and quantity, resulting in additional expenses.

The achievable final temperature of a C ₃ refrigeration system is limited to -40 ° C, since lower pressure at the compressor suction side can occur at lower evaporation temperatures, which can lead to a safety-related undesirable entry of oxygen from the air.

Summary of Invention

It is an object of the present invention to provide a method for separating a fraction high in C ₂₊ from a feed high in hydrocarbon, which does not require a C ₃ refrigeration system.

To achieve this object, a method for separating a fraction high in C ₂₊ from a feed high in hydrocarbon is proposed in accordance with claim 1. In contrast to the prior art, the refrigerating capacity required for the separation process is not provided by a closed refrigeration system with phase change but instead by an open refrigeration circuit by work-performing expansion exclusively in the gas phase.

According to the invention there is provided a method for separating a C ₂₊ -rich fraction + from a hydrocarbon rich feed, preferably from natural gas, wherein the method comprises:

a) partially condensing the feed in a first heat exchanger to provide a condensed feed; and

b) separating the condensed feed into a gaseous fraction and a liquid fraction in a separator,

c) separating the gaseous fraction and the liquid fraction in a rectified manner in a rectification column, to form a methane-rich gas fraction and a C ₂₊ -rich liquid fraction

d) splitting the gaseous fraction into a first gaseous fraction which is at least 60 to 90% of total gaseous fraction and a remaining gaseous fraction

e) depressurizing the first gaseous fraction to the pressure of the rectification column before it is fed to the rectification column, and

f) Liquefying the remaining gaseous fraction against the methane-rich gas

fraction and then feeding the liquefied gas fraction to the rectification column as a reflux,

g) wherein in the first heat exchanger the feed is partially condensed against the methane-rich gas fraction thereby producing a heated methane-rich gas fraction ,

h) subsequently compressing the heated methane-rich gas fraction to a pressure of at least 10 bar (preferably at least 20 bar) above the operating pressure of the rectification column, to form a compressed methane-rich gas fraction, i) splitting the compressed methane-rich gas fraction into a first partial flow and a second partial flow

j) expanding the first partial flow of the compressed methane-rich gas fraction means of work-performing, and then feeding the expanded flow to join with the methane-rich gas fraction to form a combined flow

k) subjecting the combined flow to heat exchange with the feed fraction. According to the invention, the gas fraction high in methane obtained during C ₂ + separation is compressed as a sales gas (to be supplied onwards for storage or consumption) to a pressure which is at least 10 bar, preferably at least 20 bar, above the operating pressure of the rectification column. Usually the gas fraction high in methane is compressed to at least 50 bar, preferably more than 60 bar. This means that the pressure of the sales gas is thus clearly above the maximum operating pressure of the rectification column or of the demethanizer, which is approximately 35 bar. The pressure difference between the suction side and the pressure side of the compressor required for the compression of the gas fraction high in methane is utilized according to the invention to operate a work-performing expander in which a partial flow of the previously compressed gas fraction high in methane is expanded. By suitably selecting the expander inlet temperature and the mass flow of this partial flow, the refrigerating capacity and temperature level can be adapted to the respective, possibly fluctuating requirements of the separation process.

Although the process according to the invention can, under certain circumstances, lead to an increase in the power requirement of the system, this process control is advantageous, since all the previously described disadvantages of a C ₃ refrigeration system are avoided due to the absence of a C ₃ refrigeration system:

Since the cold is not produced by phase changes, but by working relaxation, the operation of the refrigeration is largely independent of the ambient temperature. - The investment costs are comparatively low, since the (required) compressor C already exists and only needs to be designed larger. The additional expander X2 is comparatively inexpensive.

Since this is an open circuit with sales gas as a refrigerant, there is no cost involved to provide the refrigerant.

If desired, temperatures of -40 ° C can also be reached at the outlet of expander X2 by adjusting the inlet temperature at the expander. Further advantageous embodiments of the process according to the invention for separating a fraction high in C ₂₊ from a feed high in hydrocarbon, as outlined in the dependent claims.

The mechanical power of at least one expander (used for the work-performing expansion) may be used to drive at least one generator. The power produced in the generator during normal operation may be fed into one or more of the following: an existing external network, an existing process, a system mains supply.

The mechanical power of at least one expander (used for the work-performing expansion) may be used to drive at least one compressor).

The rectification column may be operated at a pressure of between 15 and 35 bar.

In step (h) the methane-rich fraction high may be compressed to a pressure of at least 50 bar. In step (h) the methane-rich fraction high may be compressed to a pressure of at least 60 bar.

In step (a) the feed (to be partially condensed) may have a pressure between 30 and 100 bar.

The method may further include liquefying and supercooling the second partial flow of the compressed methane-rich gas fraction against the gaseous methane-rich gas fraction which has been withdrawn from the rectification column.

The method may further include feeding the liquefied and supercooled stream to the rectification column as additional reflux.

In step (e) the partial stream of the gaseous fraction, which is expanded to the pressure of the rectification column, may be expanded in working condition.

The pressure and thus the temperature level in the rectification column are

advantageously raised as far as possible (>30 bar, preferably >33 bar). The upper limit of the operating pressure of the rectification column is predetermined by the required density difference (>250 kg/m ³ , preferably >280 kg/m ³ ) between the liquid phase and the gas phase on the column bottoms of the rectification column. By raising the operating pressure, the refrigerant capacity of the expander is reduced, in which at least 60 to 90% of the gaseous fraction is depressurized to the pressure of the rectification column. However, this can be compensated for without limitation with regards to power and temperature level by the additional expander which serves as a work-performing expansion of the partial flow of the compressed gas fraction high in methane.

In summary, the elevated operating pressure of the rectification column allows a higher carbon dioxide tolerance of the entire C ₂₊ separation, which is essentially determined by the lowest temperature in the rectification column (high temperature = high C0 ₂ solubility, and vice versa).

Advantageously, a partial stream of the sales gas, which is not expanded to produce work, can be cooled, liquefied, supercooled and subsequently fed to the rectification column as an additional reflux. This procedure improves the C ₂₊ yield of the process according to the invention.

Specific embodiments of the invention will now be described in detail by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows an embodiment of the invention.

Description of Embodiment s

The process according to the invention for separating a C ₂₊ -rich fraction (fraction high in C ₂₊ ) from a feed high in hydrocarbons and further advantageous embodiments thereof are explained in more detail below with reference to the exemplary embodiment shown of Figure 1 .

The apparatus shown in Figure 1 includes: rectification column (or a demethanizer) T; separator D; a first multi-stream heat exchanger E1 ; a second multi-stream heat exchanger E2; compressors C and C; a generator G, an expansion turbine X1 ; a first expansion valve V1 ; a second expansion valve V2; a third expansion valve V3; a first expander E1 ; a second expander E2; and an aftercooler E4 The feed gas 1 is high in hydrocarbon. It is preferably natural gas and which is usually at a pressure of at least 50 bar. The feed gas 1 is partially condensed in the first multi- stream heat exchanger E1 against process streams to be described later. The partially condensed flow is then fed to the separator D via line 1 ', where it is separated into a liquid fraction 4 and a gaseous fraction 3.

The liquid fraction of the feed obtained in the partial condensation is fed to the rectification column (or a demethanizer) T via line 4 and expansion valve V2. A fraction of at least 60% to 90% of the gaseous fraction 2 obtained in the partial condensation of the feed 1 is expanded in the expansion turbine X1 to the pressure of the rectification column T and fed via line 2'.

Advantageously, mechanical power from the expansion turbine X1 of the gaseous fraction 2 is used for generating current in a generator G. Alternatively or additionally, the work-performing expansion X1 of the gaseous fraction 2 can also be used to compress a gas fraction high in methane 8 in a compressor, which is described below.

Although not shown in the figure, it will be appreciate that the power produced in the generator during normal operation is fed for use as required. For example, the power may be fed into one or more of the following: an existing external network, an existing process, a system mains supply.

The remaining gas phase portion 3 withdrawn from the separator D is completely liquefied in the second heat exchanger E2 against a methane-rich gas fraction 7 (gas fraction high in methane)withdrawn from the rectification column ,T to produce a liquefied portion 3'. In the second heat exchanger E2, the methane-rich gas fraction 7 is heated to form the heated methane-rich gas fraction 8. The liquefied portion is fed to the rectification column T as a reflux via line 3' and expansion valve V1. By means of this recycling step, the ethane yield in the rectification column T is substantially increased.

At least one side stream 6 is also withdrawn from the rectification column T at a suitable point, partially evaporated in the multi-stream heat exchanger E1 , and fed again to the rectification column T below its withdrawal point. In practice, the partial evaporation of at least one further side stream can be realized in the multi-flow heat exchanger E1 (not shown in the figure). The rectification column T is usually operated at a pressure of between 15 and 35 bar. A liquid fraction 5 high in C ₂₊ is withdrawn from the bottom of the rectification column T. A partial stream of this liquid fraction 5 is partially evaporated in the reboiler E3 and fed back to the rectification column T via line 5'.

In a further embodiment (not shown) the liquid fraction 5 is fed to a further separation stage, and the resulting liquid phase is further processed in the reboiler and fed back to the rectification column (as described above).

The gas fraction high in methane 7, which is withdrawn at the top of the rectification column T, is heated in the multi-stream heat exchangers E2 and E1 , as explained above, and subsequently compressed by means of two compressors C C to a pressure which is at least 10 bar, preferably at least 20 bar above the operating pressure of the rectification column T. The maximum operating pressure of the rectification column T is generally approximately 35bar. After removal of the condensing heat in the aftercooler E4, a main stream 9 of the compressed gas fraction high in methane (sales gas) is fed for further use.

A partial stream 10 of the compressed gas fraction high in methane which is not required for further use, is fed back. The stream 10 which is fed back is split into a first partial stream 1 1 and a second partial stream 12.

The first partial stream 1 1 is cooled in the heat exchanger E1 . The cooled first partial stream is expanded in the expander X2 and fed to be combined with the methane rich fraction 7 which has been withdrawn from the rectification column to form a combined flow 7'. The combined flow T is then fed to the first heat exchanger E1 and against the feed flow 1.

According to an advantageous embodiment of the method according to the invention, the second partial stream 12 of the compressed gas fraction high in methane is cooled, liquefied, supercooled in the heat exchangers E1 and E2 against the gas fraction high in methane 7, 7" It is then fed to the rectification column T via the expansion valve V3 as a further reflux. This causes the C ₂ + yield to be increased.

As mentioned above, the work-sustained expansion of the gaseous fraction 2 in expander X1 can be used to compress the heated gas fraction 8 high in methane. In other words it can be used to drive at least one of the compressors C, C. As shown in Figure 1 , the supercharging stage compressor C is advantageously designed in such a way that it is driven exclusively by the expander X1 or the power obtained in it. In principle, it is also possible, alternatively or additionally, to operate a post-compressor stage in the same way.

In Figure 1 , the mechanical power from expander X1 is supplied to the generator A. In a further embodiment (not shown) the mechanical power from the expander X2 is additionally supplied to the same generator or is supplied to a second generator.

Alternatively, in a further embodiment (not shown) only the mechanical power from the expander X2 is supplied to a main generator.

If, as described in German patent application 102015009254, the liquid fraction high in C ₂₊ 5 withdrawn from the bottom of the rectification column T is fed to a C ₂ /C ₃ separation (demethanizer), not shown in Figure 1 , the latter can be cooled as described in the aforementioned patent application and be integrated into the multi- current heat exchanger E1 .

While the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.