MEHOD FOR GRADUAL SEALING OF A GAS

Abstract

A method is proposed for compressing a gas in stages in a compressor arrangement (100, 200, 300, 400) having a plurality of compression stages (I-VI) which are connected together sequentially by a main line (1) and in which the gas, guided through the main line (1), is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level, wherein a feedback amount of the gas, guided through the main line (1), is at least temporarily removed from the main line (1) downstream of one of the compression stages (V), is fed to an expansion process, and is fed back into the main line (1) upstream of the same compression stage (V). It is provided that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line (1) is a supercritical pressure level, that the feedback amount is expanded to a subcritical pressure level, that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which it is removed from the main line (1), and that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line (1). The invention also relates to a compressor arrangement (100, 200, 300, 400).

Claims

1. A method for compressing a gas in stages in a compressor arrangement having compression stages (I-VI) which are connected together sequentially by a main line and in which the gas, guided through the main line, is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level, a feedback amount of the gas, guided through the main line, being at least temporarily removed from the main line downstream of one of the compression stages (V), being fed to an expansion process, and being fed back into the main line upstream of the same compression stage (V), characterised in that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line is a supercritical pressure level, in that the feedback amount is expanded to a subcritical pressure level, in that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which it is removed from the main line, and in that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.

2. The method according to claim 1, wherein a further feedback amount of the gas, guided through the main line, is at least temporarily removed from the main line downstream of a further compression stage (VI), is fed to an expansion process, and is fed back into the main line upstream of the same further compression stage (VI), wherein the pressure-side pressure level of the further compression stage (VI) downstream of which the further feedback amount is removed from the main line is a supercritical pressure level, wherein this further feedback amount is expanded to a supercritical pressure level, and wherein the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.

3. The method according to claim 1, wherein an additional feedback amount of the gas, guided through the main line, is at least temporarily removed from the main line downstream of an additional compression stage (III, IV), is fed to an expansion process, and is fed back into the main line upstream of the same additional compression stage (III, IV), wherein the pressure-side pressure level of the additional compression stage (III, IV) downstream of which the further feedback amount is removed from the main line is a subcritical pressure level, wherein this additional feedback amount is expanded to a subcritical pressure level, and wherein the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.

4. The method according to claim 1, wherein a further heat exchanger is used in a return line used to return the feedback amount and/or in the main line.

5. The method according to claim 1, wherein the feedback amount is fed back into the main line upstream of one or more compression stages (I-1V) which are arranged upstream of the compression stage (V) downstream of which the feedback amount is removed from the main line.

6. The method according to claim 1, wherein the feedback amount is controlled based on an attainable or attained suction-side or pressure-side pressure level of one of the compression stages (I-VI).

7. The method according to claim 1, wherein the gas is cooled between the compression stages (I-VI) using cooling water which is maintained within a predetermined temperature range.

8. The method according to claim 1, wherein the gas is ethylene or an ethylene-rich gas, which is provided in particular using a steam cracking method, or wherein the gas is ethane or an ethane-rich gas, or wherein the gas is carbon dioxide or a carbon dioxide-rich gas.

9. The method according to claim 1, wherein a plurality of the compression stages (1-VI) are driven by one or more common shafts, by which the respective compression stages (I-VI) are mechanically coupled.

10. The method according to claim 9, wherein a plurality of the compression stages (I-VI) are respectively driven by a plurality of common shafts.

11. The method according to claim 10, wherein the plurality of common shafts are mechanically coupled by a transmission.

12. A plant which is configured to compress a gas in stages and which comprises a compressor arrangement with compression stages (I-VI) which are connected together sequentially by a main line and in which the gas, guided through the main line, can be respectively compressed from a suction-side pressure level to a pressure-side pressure level and can be heated by this compression from a suction-side temperature level to a pressure-side temperature level, means being provided which are configured to at least temporarily remove a feedback amount of the gas, guided through the main line, from the main line downstream of one of the compression stages (V), to feed it to an expansion process and to feed it back into the main line upstream of the same compression stage (V), characterised in that the plant is configured to be operated such that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line is a supercritical pressure level and in that the feedback amount is expanded to a subcritical pressure level, in that means are provided which are configured to feed the feedback amount to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which the feedback amount is removed from the main line, and in that means are provided which are configured to cool the feedback amount only after it has been expanded and before and/or after it is fed back into the main line.

13. The plant according to claim 12 which is configured to implement a method for compressing a gas in stages in a compressor arrangement having compression stages (I-VI) which are connected together sequentially by a main line and in which the gas, guided through the main line, is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level, a feedback amount of the gas, guided through the main line, being at least temporarily removed from the main line downstream of one of the compression stages (V), being fed to an expansion process, and being fed back into the main line upstream of the same compression stage (V), characterised in that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line is a supercritical pressure level, in that the feedback amount is expanded to a subcritical pressure level, in that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which it is removed from the main line, and in that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 shows a multi-stage compressor arrangement according to an embodiment which is not according to the invention.

[0041] FIG. 2 shows a multi-stage compressor arrangement according to an embodiment which is not according to the invention.

[0042] FIG. 3 shows a multi-stage compressor arrangement according to a particularly preferred embodiment of the invention.

[0043] FIG. 4 shows a multi-stage compressor arrangement according to a particularly preferred embodiment of the invention.

[0044] FIG. 5 shows a multi-stage compressor arrangement according to a particularly preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0045] In the following figures, mutually corresponding elements have been provided with identical reference signs. For the sake of clarity, they are not described in every figure, unless corresponding elements perform a different function and/or are configured in a different manner.

[0046] FIG. 1 shows a multi-stage compressor arrangement according to an embodiment which is not according to the invention and is designated overall by reference sign 500. The compressor arrangement 500 is configured to provide ethylene at a pressure level of approximately 40 bar, i.e. at a subcritical pressure level. As mentioned, the invention is also suitable for compressing other gases such as methane and carbon dioxide. The compressor arrangement 500 comprises a plurality of compression stages which are designated here by Roman numerals I to IV. The compression stages I to IV are connected together by a main line 1. The compression stages I to IV are arranged on a common shaft 8 in the compressor arrangement 500. Ethylene is fed to compression stages I, II and III from ethylene refrigerant circuits at different pressure and temperature levels via corresponding lines 2 to 4. Line 2 conveys ethylene out of a low-pressure refrigerant circuit at approximately 1.05 bar, line 3 conveys ethylene out of a medium-pressure refrigerant circuit at approximately 3 bar and line 4 conveys ethylene out of a high-pressure refrigerant circuit at approximately 8.1 bar.

[0047] In the first compression stage I, the ethylene is compressed from the mentioned 1.05 bar, the suction-side pressure level of the first compression stage I, to a pressure-side pressure level of approximately 3 bar, which is at the same time the suction-side pressure level of the second compression stage II. Compression stage II compresses the ethylene in the main line 1 to a pressure-side pressure level of approximately 8.1 bar, which is at the same time the suction-side pressure level of the third compression stage III. In compression stage III, the ethylene is compressed to a pressure-side pressure level of approximately 22.5 bar, which is at the same time the suction-side pressure level of the fourth compression stage IV. In compression stage IV, the ethylene is compressed to a pressure-side pressure level of approximately 40 bar, at which it can be released as product via a line 5. Via the line 4, further ethylene is fed in, for example from the top of a C2 splitter. Since the compressor arrangement 500 is configured as a combined refrigerant and product compressor, an intermediate extraction line 6 is provided for extracting refrigerant and optionally a return flow to the C2 splitter.

[0048] To dissipate the compression heat due to the compression in compression stages II to IV, respective aftercoolers IIa to IVa are provided in which the ethylene is respectively cooled to approximately 40 C. Since on the suction side of the third compression stage III cold ethylene is also fed in from the high-pressure refrigerant circuit, upon entry into the third compression stage III a mixed temperature of approximately 18 C. is produced. The entry temperature of the ethylene out of the low-pressure refrigerant circuit into the first compression stage I is approximately 57 C. and the entry temperature of the ethylene out of the medium-pressure refrigerant circuit into the second compression stage II is approximately 14 C.

[0049] Via a plurality of return lines 7, feedback amounts can be respectively removed from the main line 1 downstream of compression stages II to IV and can be fed back into the main line 1 upstream of these compression stages. In this respect, the feedback amounts are expanded via valves which are not denoted separately. In a multi-stage compressor arrangement 500 as shown in FIG. 1, the problem of the initially mentioned liquefying effects typically arises to a lesser extent, because a supercritical pressure level is not reached here.

[0050] FIG. 2 shows a compressor arrangement 600 according to a further embodiment which is not according to the invention. The compression stages I to IV and the interconnection thereof has already been described. In the compressor arrangement 600, the compression stages I to IV are arranged on a common shaft 8.

[0051] In the compressor arrangement 600 according to FIG. 2, two further compression stages V and VI are provided. These are arranged on a common shaft 9 in the compressor arrangement 600 and further compress the ethylene, released via the line 5 as product in the compressor arrangement 500 according to FIG. 1 to a supercritical pressure level. The ethylene, compressed to approximately 40.2 bar and cooled to a temperature level of approximately 40 C., is fed to the fifth compression stage V in the compressor arrangement 600. Thus, the suction-side pressure level of this compression stage V is approximately 40.2 bar. In compression stage V, the ethylene is compressed to a pressure-side pressure level of approximately 70.4 bar from this suction-side pressure level. In so doing it heats up, and is cooled in an aftercooler Va to approximately 40 C. Thereafter, the ethylene is fed to a compression stage VI in which it is compressed to a pressure-side pressure level of approximately 125.6 bar. After cooling in an aftercooler VIa to approximately 40 C., the ethylene is released as product at a temperature level of approximately 40 C. and at the mentioned pressure level via a line 5.

[0052] Also provided downstream of compression stages V and VI are return lines 7, by which feedback amounts can be respectively removed from the main line 1 and can be fed back into the main line upstream of the respective compression stages. However, as mentioned, disadvantageous liquefying effects possibly occur during a compression, particularly in compression stage V, during a feedback and an expansion.

[0053] In the compressor arrangement 600 according to FIG. 2, the shafts 8 and 9 can be connected together by a transmission, as also shown in the following FIGS. 3 and 4. Thus, the rotational speed of compression stages I to VI can no longer be controlled independently of the other compression stages. If the suction-side pressure level of compression stage V is now reduced, for example because a smaller amount of ethylene is fed in via line 4, this can only be counteracted by opening the return line 7 downstream of the aftercooler Va. This is not a problem provided that it is ensured by the cooling water temperature in the aftercooler Va that the feedback amount, guided in the return line 7 downstream of the aftercooler Va, is at a sufficiently high temperature, for example approximately 40 C. However, in an extreme case, with colder cooling water, the feedback amount, guided in the return line 7 downstream of the aftercooler Va, can fall to a value of for example 20 C. This temperature is further reduced due to the expansion in the expansion valve. The suction-side temperature level of compression stage V thereby also falls, and thus also the suction-side pressure level. Here again, this can only be counteracted by returning a greater feedback amount which in turn, however, causes the suction-side temperature level of compression stage V to fall further. Ultimately, a very large amount of ethylene is circulated without any benefit. This also affects the downstream compressor stages.

[0054] FIG. 3 schematically shows a compressor arrangement according to an embodiment of the invention which is designated overall by reference sign 100. The compressor arrangement 100 is largely the same as the compressor arrangement 600 according to FIG. 2. However, whereas in the compressor arrangement 600 according to FIG. 2 the return line 7 is arranged downstream of the aftercooler Va, a corresponding return line, designated here by reference sign 10 for the purposes of clarity, according to the embodiment of the compressor arrangement 100 according to the invention which is shown in FIG. 3, branches off from the main line 1 upstream of this aftercooler and directly downstream of compression stage 5.

[0055] This measure can ensure that a feedback amount which is guided through the return line 10 and is branched off from the main line 1 is expanded in an expansion device 11, for example an expansion valve, from a higher temperature level than in the compressor arrangement 600 according to FIG. 2. In this way, no liquefying effects can occur during expansion in the expansion device 11.

[0056] Provided downstream of the expansion device 11 in the return line 10 is a separate cooler 12 which can cool the expanded feedback amount in the return line 10. After cooling, the feedback amount is fed back into the main line 1 out of the return line 10.

[0057] The embodiment of the compressor arrangement 100 according to the invention which is shown in FIG. 3 also differs from the compressor arrangement 600 according to FIG. 2 in that the common shaft 8 interconnects compression stages I to IV and the common shaft 9 interconnects compression stages V and VI. The shafts 8 and 9 are connected together by a transmission 13. A common drive 14, for example a steam turbine, can thus drive the shaft 8 and the shaft 9. The speed of the transmission 13 can be configured to be variable or fixed.

[0058] The disadvantages in terms of control, described with regard to the compressor arrangement 600 according to FIG. 2, are overcome by the embodiment of the compressor arrangement 100 according to the invention which is shown in FIG. 3. Even when the temperature of the cooling water in the aftercooler Va is reduced, it is ensured that the feedback amount, guided in the return line 7 downstream of the aftercooler Va, is not cooled to the great extent mentioned with regard to the compressor arrangement 600 according to FIG. 2. The maximum cooling is restricted by the heat exchanger 12 because subsequently no further expansion takes place. This can prevent an excessive drop in the suction-side temperature level of compression stage V.

[0059] FIG. 4 shows a compressor arrangement according to a further embodiment of the invention which is designated overall by reference sign 200. The compressor arrangement 200 according to FIG. 4 is largely the same as the compressor arrangement 100 according to FIG. 3, although here as well, the return line is configured downstream of compression stage VI, just as the return line downstream of compression stage V. For the sake of clarity, the same reference signs are used and reference is made to the above descriptions. As stated above, in the case of compression stage VI as well, undesirable liquefaction is thus avoided which could occur during abnormal operating states, such as start-up or malfunction.

[0060] FIG. 5 shows a compressor arrangement according to a further embodiment of the invention which is designated overall by reference sign 300. Like FIG. 4, here the return line 10 branches off directly downstream of compression stage VI and delivers a feedback amount of ethylene to an expansion process in an expansion valve 11. However, here, the ethylene is fed back into the main line 1 directly downstream of the expansion in the expansion device 17, more specifically not directly upstream of compression stage VI, but upstream of compression stage V. A further aftercooler 15 is provided downstream of the ethylene feed-in point of the return flow from the return line 16.

[0061] The shafts 8 and 9 of the compressor arrangement 300 are configured separately from one another, separate drives 14 being respectively provided.