DE10108905A1 - Liquefaction of two-component gas mixture comprises separating mixture into high- and low- boiling fractions, with subsequent cooling and mixing stages avoiding boil-off gases - Google Patents

Abstract

A mixture is separated into two fractions with boiling points above and below that of a feedstock, at the same pressure. The higher-boiling fraction is cooled, liquefying it at the final pressure of the process. The lower-boiling fraction is cooled, such that when combined with the higher-boiling fraction, the result is completely liquid. As coolant for the coolant circuit(s), either a side stream of the lower-boiling fraction; or with two coolant circuits; a side stream of the lower-boiling fraction and a side stream of the gas mixture to be liquefied are used. Preferred Features: In addition, the preferably liquefied, higher-boiling fraction and the preferably liquefied lower-boiling fraction are then combined, to exist in the liquid form. Separation of the mixture to be liquefied into lower- and higher-boiling fractions is effected in any separation or fractionation process, preferably with throttling, permeation and/or rectification. Final combination takes place before supply into a storage tank. In the case of supply of fractions to a storage tank first, the preferably liquefied higher-boiling fraction is supplied to it in the supercooled state and the lower-boiling and the higher-boiling fractions are so cooled before supply, that during their combination, no boil-off results. Cooling of the lower-boiling side stream is by adequate compression, followed by expansion, resulting in the liquid phase following combination. The preferably liquefied higher-boiling fraction before combination with the lower-boiling fraction, or before supply into the storage tank, is super-cooled. This takes place in heat exchange with at least one side stream of the lower-boiling fraction. Before mixing or before supply into the storage tank, the preferably liquefied lower-boiling fraction is subcooled with the higher-boiling fraction. Supercooling is sufficient to assure that no gases are evolved on mixing. Additional processes based on the foregoing principles are carried out.

Description

The invention relates to a method for liquefying at least one two-component gas mixture.

There are a variety of processes for the complete liquefaction of multicomponent gas mixtures known. So the liquefaction by the Use of separate refrigerant with a boiling point below that Boiling range of the multicomponent gas mixture to be liquefied is reached become. For example, when liquefying natural gas in the LNG plants and a refrigeration cycle for the liquefaction of the boil-off gas in natural gas storage tanks used with nitrogen as a refrigerant.

The disadvantage of this method is that the losses of the refrigerant have to be replaced continuously with the resulting further Disadvantage that separate storage for the refrigerant or refrigerant or an oversizing of a part of the refrigeration cycle equipment for storage the refrigerant or refrigerant is necessary.

In addition, methods are known in which the use of separate refrigerant circuit is dispensed with. Instead, a partial flow of the liquefying gas mixture used as a refrigerant, creating a partial liquefaction of the gas mixture is made possible. With these procedures the non-liquefied part of the multi-component gas mixture remains in gaseous form and is used for other purposes or it is used, as is the case with the so-called Peakshaving plants are the case again in the (continuing) pipeline returned.

The disadvantage of this procedure is that part of the multi-component Gas mixture remains gaseous and therefore consumed or in one further pipeline must be given. Often, however, is the consumption of the non-liquefied portion of the multi-component gas mixture not possible or uneconomical, or there is no pipeline to return the non-liquefied  Gas mixture available. For example, the consumption of the non-liquefied Gas mixture during the liquefaction of the boil-off gas on LNG tankers uneconomical, since the value of the boil-off gas is higher than that of the diesel or Heavy oil fuel.

In addition, the efficiency of diesel engines for diesel or heavy oil is considerable larger than that of a gas diesel engine or a steam boiler and one Steam turbine when using the boil-off gas as heating gas to generate Steam for the marine propulsion turbine of an LNG tanker. The exclusive Use a gas diesel engine or a steam boiler and one Steam turbine also requires an additional evaporation system in order to to be able to provide the necessary heating gas even if no or there is insufficient boil-off gas.

The object of the present invention is to provide a method for complete Liquefying an at least two-component gas mixture to specify the avoids the aforementioned disadvantages and also energetic and equipment Has advantages.

To achieve this object, a method for liquefying an at least two-component gas mixture is proposed, in which

• a) the gas mixture to be liquefied into a lower boiling fraction, the one Has boiling temperature that is lower than the boiling point of the liquefying gas mixture, and into a higher boiling fraction, which, at each same pressure, has a boiling temperature that is higher than that Boiling point of the gas mixture to be liquefied is separated,
• b) the higher boiling fraction is cooled at least to the extent that it preferably at the pressure below that at the end of the liquefaction process the liquefied gas mixture is present, preferably in the liquid phase,
• c) the lower boiling fraction is cooled to such an extent that after a Merging with the higher boiling fraction in a completely liquefied state Phase is present  
• d) as a refrigerant for the one or more refrigerant circuits Partial stream of the lower boiling fraction or, if at least two Refrigerant circuits are provided, at least a partial flow of the lower boiling fraction and at least a partial stream of the liquefied Gas mixture can be used and
• e) the preferably liquefied, higher-boiling fraction and the preferred liquefied, lower-boiling fraction then combined and the combined fractions are liquefied.

The separation of the gas mixture to be liquefied into a lower boiling Fraction and into a higher boiling fraction can be done by any Separation or fractionation process, for example a throttling, a permeative or rectification procedure.

The process according to the invention for liquefying an at least two-component gas mixture and further refinements thereof, which are the subject matter of the dependent claims, are explained in more detail below with reference to the exemplary embodiments illustrated in FIGS. 1 to 3.

In the following exemplary embodiments or process descriptions each a liquefied natural gas with a composition of approx. 0.75% nitrogen and approx. 99.25% methane in an unpressurized storage tank T1. At a An ambient pressure of approx. 1.05 bar generates a boil-off gas that is suitable for the present process calculations a composition of 15% nitrogen and has 85% methane.

The compressor systems K1 and K2 shown in FIGS . 1 to 3 are each multi-stage compressors which are equipped with intermediate and final coolers; the compressed gas mixture is cooled to 315 ° K by these coolers. It is assumed that sufficient cooling water or air with the corresponding specification (e.g. sea, river or cooling water and / or air with a temperature of ≦ 310 ° K) is available.

In Figs. 1 to 3 are further only the main process piping, which are necessary for understanding the invention is shown; the start-up and commissioning lines, as well as bypass, pressure compensation and other secondary lines are not shown. Likewise, the representation of the mentioned intermediate and final coolers of the compressor systems K1 and K2 is omitted.

Process data relating to FIGS . 1 to 3 are given in table form, each relating to process points 1 to 16 ( FIG. 1), 1 to 18 ( FIG. 2) and 1 (“encased”) shown in FIGS . 1 to 3 to 22 ( Fig. 3) correlate.

In the embodiment of the method according to the invention for liquefying an at least two-component gas mixture shown in FIG. 1, the lower-boiling fraction is used as the refrigerant circuit.

The boil-off gas with the above-mentioned composition of 15% nitrogen and 85% methane is removed via line 1 from the (storage) tank T1, in which the liquefied natural gas with a composition of approx. 0.75% nitrogen and approx. 99 , 25% methane, is fed to the first stage of the compressor system K1 and compressed therein to the conditions according to method point 2 of Table 1 below.

The boil-off gas is then fed via line 2 to the heat exchanger W1 and is cooled and liquefied in it to the conditions according to method point 3 in Table 1. The liquefied boil-off gas is fed to the throttle valve V1 via line 3 , throttled (method point 4 in Table 1) and fed to the separator A1 via line 4 .

At the top of the separator A1, the gaseous and lower-boiling fraction - hereinafter referred to as ns fraction - is drawn off via line 5 ; this has 35% nitrogen and 65% methane. At the bottom of the separator A1, the liquid and higher-boiling fraction - hereinafter referred to as hs fraction - is drawn off via line 16 and fed to the heat exchanger W2. In the heat exchanger W2, this fraction is then subcooled to the conditions according to process point 5 in Table 1.

The subcooling of the liquid hs fraction in the heat exchanger W2 takes place through a subset of the ns fraction, which is expanded in the throttle valve V3 to the conditions according to method point 8 of table 1. The thus supercooled hs fraction is fed to mixer M1 via line 17 and mixed with the fluid of the ns fraction throttled to the conditions according to method point 6 of table 1 in valve V2.

In stable continuous operation, the ns fraction which is fed to the mixer M1 corresponds exactly in terms of its quantity and composition to the gaseous ns fraction which is drawn off at the top of the separator A1. The quantity and composition of the liquid fraction drawn off from the mixer M1 via line 18 thus corresponds to the boil-off gas fed to the 1st stage of the compressor system K1.

The pressure and temperature of the liquid fraction drawn off from the mixer M1 via line 18 are preferably set here in such a way that no additional boil-off gas is produced when this liquid fraction is fed into the tank T1. In addition, it should be achieved that the liquid in the tank T1 is subcooled by the liquid fraction supplied via line 18 , wherein subcooling of the liquid is not absolutely necessary, but rather serves to cool the liquid in the tank T1.

Instead of mixing or combining the ns and the hs fraction in the mixer M1 shown in FIGS . 1 to 3, the two fractions can also be fed separately to the (storage) tank T1 via separate lines. In this case, the liquefied hs fraction is preferably subcooled before being fed into the tank T1 in order to avoid the formation of boil-off gas.

The gaseous ns fraction drawn off at the top of the separator A1 via line 5 is fed to the heat exchanger W1 together with the partial flow of the ns fraction relaxed in the valve V3, which is drawn off from the heat exchanger W2 via line 14 . In this, the combined ns fraction is heated in counterflow to the boil-off gas to be liquefied and the ns fraction to be liquefied, which is fed to the heat exchanger W1 via line 7 , before being fed via line 6 to the compressor system K2 and in this is compressed to the required pressure and the required temperature for entry into the heat exchanger W1 (process point 12 ).

The compressed ns fraction is now cooled in the heat exchanger W1 and a partial stream is liquefied (process point 16 ), which is then drawn off from the heat exchanger W1 via line 12 . A side stream (process point 13 ) is drawn off from the compressed ns fraction in the heat exchanger W1 via line 8 and in the expansion turbines X1 and X2 for the purpose of additional cooling for the cooling and liquefaction of the two aforementioned process streams - the boil-off gas from the Compressor system K1 and the NS fraction from the compressor system K2 - relaxed.

After the expansion turbine X1 a partial stream of expanded in the expansion turbine X1 ns fraction is from the expansion turbine X1 and X2 connecting line 9 drawn off, heated in heat exchanger W1 and fed to the or a higher compressor stage of the compressor system K2 via line 10 degrees. The partial flow of the ns fraction relaxed in the expansion turbine X2 is again admixed via line 11 to the combined ns fraction in line 5 .

Is shown in Figures 1 not shown to 3 a further embodiment of the inventive method in which the of the separation of the recovered to be liquefied gas mixture lower boiling fraction -. Which is withdrawn at the head of the separator A1 - again at least in a further lower boiling fraction and is separated into a further higher-boiling fraction and the lower-boiling fractions thus obtained are connected to form a refrigeration cycle cascade.

Table 1

In the embodiment of the method according to the invention for liquefying an at least two-component gas mixture shown in FIG. 2, the boil-off gas stream to be liquefied is used in an expander refrigeration circuit as a refrigerant circuit, while the ns fraction is used as refrigerant in a refrigerant circuit with a throttle.

The boil-off gas discharged from the tank T1 via line 1 , before mixing with the partial stream of the boil-off gas supplied via line 26 , before the first stage of the compressor system K1, has the properties listed in table 2 under method point 1 . The boil-off gas is compressed in the compressor system K1 to the conditions according to method point 3 of Table 2, whereby it is mixed with its partial streams supplied via lines 25 and 26 ; these partial flows have an identical composition to the boil-off gas.

The boil-off gas compressed in this way is cooled and liquefied in the heat exchanger W1 and then fed to the separator A1 via line 27 and throttle valve V1. In the heat exchanger W1, a side stream is drawn off during the cooling via line 23 and expanded in the expansion turbines X1 and X2 for the purpose of additional cooling for the cooling and liquefaction of the boil-off gas and the ns fraction to be described from the compressor system K2. After the expansion turbine X1, a partial stream of the side stream is withdrawn via line 25 from the line 24 connecting the expansion turbines X1 and X2, fed to the heat exchanger W1 and heated therein, and then - as already mentioned - fed to the or one of the higher compressor stages of the compressor system K1.

At the top of the separator A1, the gaseous, lower-boiling fraction (ns fraction) is drawn off via line 28 ; this has 35% nitrogen and 65% methane. At the bottom of the separator A1, the liquid, higher-boiling fraction (hs fraction) is drawn off via line 35 and fed to the heat exchanger W2. In the heat exchanger W2, this fraction is then subcooled to the conditions according to process point 11 in Table 2.

The subcooling of the liquid hs fraction in the heat exchanger W2 takes place through a subset of the ns fraction, which is expanded in the throttle valve V3 to the conditions according to method point 14 in table 2. The thus supercooled hs fraction is fed via line 36 to mixer M1 and mixed in it with the fluid of the ns fraction throttled to the conditions according to method point 12 of table 2 in valve V2.

The gaseous ns fraction withdrawn via line 28 from the separator A1 is fed to the heat exchanger W1 together with the partial flow of the ns fraction relaxed in the valve V3, which is withdrawn via line 33 from the heat exchanger W2. In this, the combined ns fraction is heated in countercurrent to the boil-off gas to be liquefied and the ns fraction to be liquefied, which is fed to the heat exchanger W2 via line 30 , before being fed via line 29 to the compressor system K2 and in this is compressed to the required pressure and the required temperature for entry into the heat exchanger W1 (process point 17 ), to which it is fed via line 30 . The pressure of the compressed ns fraction should be chosen to be at least high enough to ensure that it is liquefied in the heat exchanger W1.

The ns fraction liquefied in the heat exchanger W1 is withdrawn via line 31 , divided and fed via lines 32 and 34 to the throttle valves V2 and V3. A further throttle valve V4 is provided in the bypass line 38 connecting the lines 31 and 33 and bypassing the heat exchanger W2. This can be used to generate any additional cooling capacity required and to supply it directly to the heat exchanger W1.

Again, the conditions for the liquid fraction obtained at the outlet of the mixer M1 are set in such a way that no additional boil-off gas is produced when this liquid fraction is fed into the tank T1. In addition, it should be achieved that the liquid in the tank T1 is subcooled by the liquid fraction supplied via line 37 .

Table 2

In the embodiment of the method according to the invention for liquefying an at least two-component gas mixture shown in FIG. 3, both the boil-off gas stream to be liquefied and the lower-boiling fraction are used in an expander refrigeration cycle as refrigeration cycle means.

The boil-off gas discharged from the tank T1 via line 41 before the first stage of the compressor system K1 exhibits the properties listed in table 3 under method point 1 . The boil-off gas is compressed in the compressor system K1 to the conditions according to method point 2 of Table 2, whereby it is mixed with the partial stream supplied via line 44 ; this partial stream has an identical composition to the boil-off gas.

After the compressor system K1, the compressed boil-off gas is divided into two partial flows. The first partial flow of the boil-off gas is fed to the heat exchanger W1 via line 42 , while the second partial flow of the boil-off gas is fed to the heat exchanger W3 via line 46 . This non-mandatory division of the boil-off gas flow makes particular sense when so-called circular surface heat exchangers are used as heat exchangers. The advantage of these heat exchangers is that they do not require external insulation even at the lowest temperatures. However, if conventional heat exchangers are used, the division of the compressed boil-off gas stream shown in FIG. 3 is unnecessary.

The partial flow of the boil-off gas supplied to the heat exchanger W1 via line 42 is cooled in the heat exchanger W1 and partially liquefied (method point 6 in Table 3). A side stream of the boil-off gas stream to be liquefied is drawn off via line 43 , expanded in the expansion turbine X1 for the purpose of cooling, for cooling and partial liquefaction of the boil-off gas and then - as already mentioned - the or again via line 44 a higher compressor stage of the compressor system K1 supplied.

The further partial stream of the compressed boil-off gas stream fed to the heat exchanger W3 via line 46 is cooled in the heat exchanger W3 and - after the boil-off gas stream partially liquefied in the heat exchanger W1 is fed in via line 45 - completely liquefied. Subsequently, the entire boil-off gas flow is led or relaxed into the separator A1 via line 47 and throttle valve V1.

At the top of the separator A1, the gaseous, lower-boiling fraction is drawn off via line 48 , while the liquid, higher-boiling fraction is drawn off from the bottom of the separator A1 via line 60 and is fed to the heat exchanger W4. In the heat exchanger W4, the liquid hs fraction is subcooled to the condition according to process point 10 in table 3. As already with the method shown in FIGS. 1 and 2, the liquid hs fraction is subcooled in heat exchange with a partial flow of the ns fraction, which is expanded in the throttle valve V3 to the conditions according to method point 13 in table 3.

The supercooled hs fraction is withdrawn via line 61 from the heat exchanger W4 and fed to the mixer M1. In this, it is mixed with the partial flow of the ns fraction throttled in valve V2. In stable continuous operation, this ns fraction corresponds exactly in terms of its quantity and composition to the ns fraction which is drawn off from the separator A1 via line 48 . The amount and the composition of the liquid stream withdrawn via the line 62 from the mixer M1 thus corresponds to the amount and the composition of the boil-off gas stream present at the entrance of the first stage of the compressor system K1 in stable continuous operation. The conditions for the liquid fraction obtained in the outlet of the mixer M1 are in turn set such that no additional boil-off gas is formed during the supply of this liquid fraction via line 62 into the tank T1.

The gaseous, ns fraction drawn off at the top of the separator A1 via line 48 is further elucidated with the partial flow of the ns fraction relaxed in the throttle valve V3 via line 58 and a further partial flow of the ns fraction will be received, mixed and fed to the heat exchanger W3. In this, the ns fraction is heated in countercurrent to the boil-off gas stream to be liquefied and then fed to the compressor system K2 via line 49 . In this the ns fraction is compressed to the required pressure and the required temperature for entry into the heat exchanger W2 (process point 16 in Table 3).

In the heat exchanger W2, the ns fraction compressed in this way is cooled, partially liquefied and then drawn off from the heat exchanger W2 via line 55 and fed as a side stream to the heat exchanger W3 and completely liquefied in it (method point 22 in Table 3). The completely liquefied ns fraction is then fed via line 56 to the branching point, in which it is divided into lines 57 and 59 and fed to the throttle valves V3 and V2.

A side stream is drawn off from the ns fraction fed to the heat exchanger W2 via line 50 via line 51 and expanded in the expansion turbines X2 and X3 for the purpose of additional cooling. After the expansion turbine X2, a partial stream of the expanded ns fraction is drawn off from the connecting line 53 via line 52 , warmed in the heat exchanger W2 and fed to the or a higher stage of the compressor system K2. The remaining partial stream of the side stream of the ns fraction drawn off via line 51 , which is expanded in the expansion turbine X3, is - as already mentioned - mixed via line 54 of the ns fraction drawn off at the head of the separator A1 via line 48 .

Table 3

It is particularly advantageous in the method according to the invention that equally dimensioned and therefore interchangeable compressors in the Liquefaction process steps and in the refrigeration circuits are used can; if only one reserve compressor is provided, this is almost a 100% reliability. It is also a much smaller one Warehousing and a smaller space requirement, which is particularly due to the the aforementioned liquid gas tankers represents a not insignificant advantage.

In contrast to the method according to the invention, when using separate refrigerant circuit means the compressor in the refrigeration circuit many times over - usually ten times larger than the compressor in Liquefaction process step. This means a doubling of inventory for the two different machines with a significantly lower reliability.

The inventive method is also characterized by simple Process steps as well as by the self-sufficient liquefaction option, so that this procedure in an advantageous manner in a modular system, as they For example, as a mobile natural gas liquefaction plant in small natural gas fields where the investments for a pipeline or an electrical plant are not worthwhile can be realized.

Claims (10)

1. A process for liquefying an at least two-component gas mixture, wherein

• a) the gas mixture to be liquefied into a lower-boiling fraction which has a boiling temperature which is lower than the boiling temperature of the gas mixture to be liquefied, and into a higher-boiling fraction which, at the same pressure, has a boiling temperature which is higher than the boiling temperature of the gas mixture to be liquefied is separated,
• b) the higher-boiling fraction is cooled at least to the extent that it is preferably in the liquid phase, preferably at the pressure at which the liquefied gas mixture is present at the end of the liquefaction process,
• c) the lower-boiling fraction is cooled to such an extent that it is in a completely liquefied phase after being combined with the higher-boiling fraction,
• d) either at least a partial stream of the lower-boiling fraction or, if at least two refrigerant circuits are provided, at least a partial stream of the lower-boiling fraction and at least a partial stream of the gas mixture to be liquefied are used as the refrigerant for the refrigerant circuit (s), and
• e) the preferably liquefied, higher-boiling fraction and the preferably liquefied, lower-boiling fraction are subsequently combined and the combined fractions are in liquefied form.

2. The method according to claim 1, characterized in that the separation of the gas mixture to be liquefied into a lower-boiling fraction and into a higher boiling fraction by any separation or Fractionation process, preferably by means of throttling, one permeative and / or rectification process.   3. The method according to claim 1 or 2, characterized in that the Process step e) before being fed into a (storage) tank or only in the (Storage) tank is realized, whereby in the case of merging the Fractions only in the (storage) tank the preferably liquefied higher boiling fraction preferably the (storage) tank in the supercooled state is fed and the lower boiling and the higher boiling fraction cooled so far that when merging them Fractions no boil-off gas is formed. 4. The method according to any one of claims 1 to 3, characterized in that the partial flow of the lower-boiling fraction intended for the combination compressed to such a high pressure and then to such a low pressure Cooled temperature, preferably liquefied that at the Merging the lower boiling fraction with the higher boiling Fraction, the resulting mixed fraction is in the liquid phase. 5. The method according to any one of claims 1 to 4, characterized in that the preferably liquefied higher-boiling fraction before combining with the lower boiling fraction or before being fed into the (storage) tank is hypothermic. 6. The method according to claim 5, characterized in that the supercooling of the preferably liquefied higher-boiling fraction in heat exchange with at least a partial stream of the lower boiling fraction takes place. 7. The method according to any one of claims 1 to 6, characterized in that the preferably liquefied lower boiling fraction before mixing with the higher boiling fraction or supercooled before being fed into the (storage) tank becomes. 8. The method according to any one of claims 1 to 7, characterized in that the preferably liquefied lower boiling fraction and / or the preferably liquefied higher-boiling fraction can be subcooled at least to the extent that at there is no outgassing when these fractions are mixed.   9. The method according to any one of claims 1 to 8, characterized in that the lower obtained from the separation of the gas mixture to be liquefied boiling fraction in turn at least in another lower boiling Fraction and into another higher boiling fraction and at least part of the fractions obtained to form a refrigeration cycle cascade is interconnected. 10. The method according to claim 9, characterized in that with the gas mixture an additional refrigeration cycle cascade level is formed.