WO2007110045A1

WO2007110045A1 - Method for liquefying natural gas

Info

Publication number: WO2007110045A1
Application number: PCT/DE2007/000510
Authority: WO
Inventors: Siegfried Westmeier
Original assignee: Technikum Corporation
Priority date: 2006-03-22
Filing date: 2007-03-21
Publication date: 2007-10-04
Also published as: DE102006013686B3; EP1999420A1

Abstract

The invention relates to a method for partially liquefying a flow of natural gas that is continuously supplied at a high pressure of 15 to 70 bar. Said method comprises the following steps: a) the natural gas is first subjected to adsorptive cleaning, subdivided into a plurality of partial flows that are adapted to the inlet conditions, and are individually cooled by at least two process-related partial flows and the exhaust gas flow, are then combined in the total flow (9) while the total flow (8) which is at the inlet pressure is partially liquefied, b) the liquefied natural gas is collected in a high-pressure reservoir (10), the gas component is used to cool at least one partial flow of the inlet flow, the gaseous component being heated, expanded in a first expansion device to an intermediate pressure of 6 to 20 bar, again heated up by at least one additional partial inlet flow and expanded in an additional expansion device to a final pressure of 2 to 5 bar while being again cooled off and then used as a partial flow for cooling at least one additional partial flow of the subdivided inlet flow, c) the liquefied natural gas collected in the high-pressure reservoir is reduced to a distribution pressure of the exhaust gas flow of 2 to 5 bar, a part of the liquefied natural gas being evaporated and being added to the gaseous flow of natural gas downstream of the second expansion device, and the remaining liquid component is further expanded to a pressure of 1 to 2.05 bar, only a small part of the natural gas evaporating, is compressed and supplied to the gaseous residual flow of natural gas.

Description

Title: Process for the liquefaction of natural gas

description

The invention relates to a technically simple and safe method for partial liquefaction of natural gas with further use of the exhaust gas stream as a product gas with high flexibility with respect to different natural gas compositions and input parameters.

State of the art

The increased use of natural gas as a fuel or its use in decentralized facilities on the one hand and the already necessary pressure reduction in the transition of high pressure natural gas pipelines to a operated at medium pressure consumer network on the other suggests that this anyway necessary pressure reduction to liquefy a certain proportion of To use pipeline natural gas to transport this portion in liquid form at low cost to each not connected to the natural gas network consumers or built as an island solution natural gas filling station and there again to evaporate under high pressure, taking over from the high-pressure network gas station configuration and maintained without further changes can.

The pressure reduction in the consumer is usually done by simple relaxation. Also known are solutions in which the pressure difference in natural gas relaxation systems is used to generate energy. One of these systems is e.g. operated since 1989 by the Dortmunder Energie- und Wasserversorgung GmbH. The pressure reduction takes place via two expansion turbines. To compensate for the strong cooling during the relaxation, a combined heat and power plant was built to preheat the gas.

A pipeline-free transport of natural gas and its storage in the highest possible density require the liquefaction of natural gas. The volume of natural gas is reduced to about the six hundredth part of its volume at atmospheric pressure and can be transported and stored inexpensively. Processes for liquefaction both at the point of production and in connection with the line-bound transport are known. They have been used by a number of manufacturers of such plants both as on-site facilities near natural gas fields and as peak-shaving facilities to compensate for high variations in the supply of natural gas through pipelines, and have been described in the literature. They can in principle be based on the three basic processes

- mixture cycle process,

- cycle expander process and

- lead back natural gas expander process. All processes can be carried out in one or two stages depending on the boundary conditions. All these methods are initially common that a complete liquefaction of the natural gas is desired. But for this is an outer

Energy supply necessary. In this case, the processes with low operating costs in the

Usually the higher investment costs and such processes with low

Investment costs the higher operating costs.

In the cycle processes, a separate refrigeration circuit is used to generate the for the

Liquefaction necessary cold.

The cycle expander process works with only one refrigerant. In the

Patent DE 698 19 366 T2 serves e.g. Nitrogen as a refrigerant in the separate

Refrigeration circuit.

In the mixture cycle process, however, a refrigerant mixture is used in the refrigeration cycle to a wider temperature range for the gradual cooling of the under

To be able to use pressurized natural gas. In DE 4440 401 A1, a mixture of nitrogen and methane or mixtures thereof is preferably used for this purpose

Nitrogen, methane and C ₂ to C ₅ hydrocarbons proposed.

In the natural gas expander process, the energy inherent in the compressed natural gas is used to generate refrigeration itself. However, this is not enough to achieve complete liquefaction of the natural gas flow without energy supply. For this reason, it is often combined with a cycle expander process or a mixture cycle process to inject external energy through these circuits. This disadvantage of additional energy costs can be particularly effectively avoided if only a subset of liquefied from a high pressure natural gas, the non-liquefied natural gas can be used in the subsequently achieved low pressure with well-known methods for different applications. Since such relaxation processes are necessarily always necessary in the transition from high to medium pressure nets in a conventional use, these technically induced relaxation processes can be used directly for the present task. Such a process can then do without external energy supply.

Thus, several methods are known which use the pressure energy of the system flowing natural gas and operate without external energy supply. In some patent specifications it is stated, without reference to the source, that in older processes a degree of liquefaction of not more than 8-10% of the total gas is possible.

An improvement of the method for liquefying and storing a continuously flowing under pressure gas, in which the liquefaction is carried out solely by means of the resulting in the work-relaxing expansion of the gas cold, requires very high pressures. This requires additional compression and cooling of the natural gas, which leads to additional energy costs. Therefore, further improvements of these processes were sought.

Thus, the patent DD 71567 describes how a continuously flowing gas is divided into two partial streams, of which the first is working expanded and brought into heat exchange with the second partial flow, which is at least partially condensed and expanded by throttling to storage pressure, while the throttle gases are also used for cooling release. It has been described that investment costs, maintenance costs and energy costs are low compared to other methods. However, the yield remains low with a maximum of 12-14% and only becomes higher if additional compression with subsequent precooling to improve the efficiency is added. However, the equipment and the cold losses are high, due to numerous heat exchangers. A further modification of such a method is described in the patent DE 2036 344 A. Here, too, an adiabatic expansion of a natural gas arriving at high pressure onto the consumer-side base load is to serve for cooling in the natural gas liquefaction process. The cited method, however, can not function as shown, because the gas strands I and II mentioned have different pressure levels when they are combined, the stream II has been heated by the stream III and is subsequently brought together with the liquid intake from the storage tank. Incidentally, the description and illustration contain serious discrepancies.

task

The object of the invention is to develop a technically simple and safe method for partial liquefaction of natural gas without restrictions for the further use of the exhaust gas stream as natural gas. The economy, the yield of liquid product, the adaptability to different natural gas parameters and the simplicity of the system over previously known methods to be improved.

This object is achieved by a method according to claim 1, wherein the liquefaction is carried out with simultaneous production of electrical energy solely by the resulting in the work-performing relaxation cold. For this purpose, the entire amount of natural gas to be expanded under high pressure as input stream is first subjected to adsorptive cleaning with removal of CO ₂ , residual moisture and sulfur compounds in order to avoid system malfunctions due to freezing of these components.

The purified natural gas stream is divided for control technical reasons in at least three partial flows according to the invention adapted to the input conditions and then individually cooled by a plurality of process-related partial flows and the exhaust gas flow. Thereafter, the three substreams are recombined at a pressure close to the inlet pressure, with partial liquefaction of the natural gas cooled under pressure. The quantitative distribution to the individual partial flows is carried out under the procedural aspect of a simple plant structure and under thermodynamic aspects. These in turn depend on the natural gas parameters and the composition of the natural gas, as well as the cooling capacity required individually for each partial flow in the heat exchangers and serve to achieve a uniformly low temperature of all three partial flows with the aim of the highest possible rate of liquefaction. There is also a dependency on the relaxation machines used. The liquefied natural gas is collected in a high pressure vessel. The gaseous portion is used in the heat exchanger as a process-related partial flow for cooling the introduced under pressure partial flow of natural gas. It is heated, expanded in a high-pressure expansion turbine with cooling to an average pressure and then used as a partial flow also for cooling the input partial flow in the heat exchanger. He is reheated, further relaxed over a medium-pressure expansion turbine to the desired final pressure under repeated cooling and then combined as a partial flow with other part streams still to be named and used to cool a third part of the input stream in the heat exchanger. The liquid fraction collected in the high pressure vessel is reduced as a partial flow via a throttle valve to the discharge pressure of the Resterdgases, wherein a portion of the liquefied natural gas is evaporated and added to the gaseous natural gas stream after the second expansion turbine again. The remaining liquid fraction is further expanded to normal pressure as a liquid partial stream for product delivery. In this case, only a small part of the natural gas evaporates, is compressed again in the compressor and fed as a partial flow to the gaseous Resterdgasströmen.

Further advantageous embodiments of the invention are claimed in claims 2 to 8. Thus, according to claim 2, the input current is divided into three different natural gas streams as a function of the composition and the inlet pressure and cooled via a respective heat exchanger with three process-related streams that the optimum for the entire process amount of each partial stream depending on their starting temperature after the heat exchangers is set.

Of the three process-related cold partial flows arise in the preferred process variant two, as the gas line of the high-pressure separator before and after the Hochdruckentspannungsmaschine, the third of the proportion plus the gas components from the low-pressure vessel and the vaporized during expansion gas content from liquid product tank.

In the case of high concentrations of nitrogen in the input stream according to claim 7, the liquid product from the high-pressure vessel for further separation of working at exhaust pressure cryogenic column supplied and decomposed there into a nitrogen or inert rich top product and a methane rich liquid bottom product, wherein the liquid product in the liquid product tank is relaxed, the overhead product is first used to cool the partial flow after the first expansion device and then the exhaust gas flow to the second expansion turbine is supplied and the sump heater is operated with a smaller partial flow of natural gas input stream, which is removed before splitting into the partial streams, and after Heating is fed into the high pressure vessel. According to claim 5, at higher concentrations of ethane, propane and higher hydrocarbons in the natural gas between the first and second expansion device to prevent droplet formation in the second expansion device, first a cooling of the gas stream in a heat exchanger with deposition of said hydrocarbons and only then the heating in countercurrent takes place to the incoming natural gas stream, the condensed hydrocarbons are withdrawn separately as a liquid product.

The advantages of the invention result in the simple control technical adaptation of the optimal modes of operation to existing or occurring changes in the input parameters of the input current of the system and in the simple structural design and in comparison to other methods high yield of liquid product with completely energy-autonomous driving and with Possibility of additional production of electrical energy using appropriate expansion machines. It requires only three, substantially equal sized heat exchanger for optimal use of achievable during the expansion cooling capacity, which is reflected in high and almost equal rates of liquefaction even with greater change, for example, the inlet pressure. embodiments

Other objects, advantages, features and possible applications of the invention will be explained in more detail in the following with reference to the drawings, which represent possible advantageous embodiments. Show it

1 shows the basic principle of an arrangement of a method and the necessary devices for natural gas liquefaction of a commercial, very methane-containing natural gas without substantial amounts of nitrogen and with only small amounts of higher hydrocarbons,

Fig. 2 shows the basic principle of an arrangement of a method and the necessary devices for natural gas liquefaction of a present under high inlet pressure nitrogen-rich natural gas stream and

Fig. 3 shows the basic principle of a process arrangement for natural gas liquefaction of a present under low inlet pressure nitrogen-rich natural gas stream.

Embodiment 1

In the method according to FIG. 1, the natural gas is taken from a natural gas line 1 which is under a pressure of 25 bar and initially passed to remove residual moisture and carbon dioxide via an adsorber 2, which is preferably designed as a multi-bed PSA plant, but also out of any other design designed to remove water and carbon dioxide to concentrations below 1 vpm of water and 50 vpm of carbon dioxide. After the adsorber 2 a splitting takes place in the three partial streams 3, 4 and 5, which are cooled separately in the three heat exchangers 6, 7 and 8 by the three process-related partial streams 12, 14 and 26, then summarized again and as a total stream 9 for condensation be passed into the container 10. Here is a partial liquefaction, wherein about one third liquid product is formed, which is relaxed as a partial flow 11 via the throttle valve 17 in the standing under natural gas discharge pressure medium pressure tank 18. At this Pressure reduction evaporates about one-third of the previously liquid product and leaves as a gaseous stream 19 the container to leave with other exhaust gas streams together through the heat exchanger 6 the system. The emerging from the high pressure vessel 10 as a partial stream 12 gas stream is first used via the heat exchanger 7 for cooling the natural gas inlet partial stream 4 and heats up. Thereafter, it is expanded by a high-pressure expansion turbine 13, cools down and serves as a stream 14 in the heat exchanger 8, the cooling of the natural gas inlet partial stream 3, is heated and with renewed cooling over the

Medium-pressure expansion turbine 15 relaxes, becomes the partial flow 16, forms together with the other process-related partial flows 19 and 24, the exhaust gas stream 26 of the system, which cools the natural gas inlet partial flow 5 in the heat exchanger 6. The liquid fraction obtained in the medium-pressure tank 18 is expanded as a partial flow 20 via a throttle valve 21 for practical reasons to a pressure just above 1 bar, collected and stored in the liquid product tank 22 for the dispensing product and can be dispensed as product 25 as needed. During the last release, only a small amount of liquid product evaporates. The resulting gas fraction is compressed via a small compressor 23 or a similar pressure-generating apparatus to the exhaust pressure and passed as a partial stream 24 in the exhaust stream.

The composition of the exhaust gas differs only slightly from the input composition, therefore represents a full natural gas and can be offered in the low-pressure consumer network without loss of quality.

Two variants with the same arrangement at 25 bar inlet pressure and 40 bar inlet pressure were examined in succession, the differences in the chosen procedure and the liquid fraction surprisingly only differing relatively small. In the case of an inlet pressure of 25 bar, a liquid content of about 18.5% was obtained, at 40 bar, it was about 20%, wherein the discharge pressure was 2 bar. Embodiment 2

The method according to FIG. 2 is preferably used where the natural gas, although under high pressure, in the exemplary embodiment with about 40 bar, but has high levels of nitrogen or light inert. The method is used to recover from this natural gas, a share of about 40% methane-liquid of the methane contained in the input stream.

Unlike Embodiment 1, however, the higher hydrocarbons contained therein must be treated separately. The natural gas is taken, for example, from a natural gas line 1 which is under a pressure of 40 bar and initially passed to remove residual moisture and carbon dioxide via an adsorber 2, analogously to Example 1. The further treatment is initially also analogous to Embodiment 1. After the adsorber 2 is a splitting into the three partial streams 3, 4 and 5, which are cooled separately in the three heat exchangers 6, 7 and 8 by three existing process-related partial streams 12, 44 and 26, are then summarized again and passed as total flow 9 for condensation in the high-pressure vessel 10. Here is a partial liquefaction, wherein about 45% of the natural gas is obtained as a liquid product, which is expanded as a partial stream 11 via the throttle valve 17 in the head of the cryogenic column 34 to the natural gas discharge pressure. In the column, the distillative nitrogen-methane separation takes place. The sump heater 32 is effected by means of a branched off after the adsorber 2 from the natural gas stream 1 small partial stream 31, which is then also fed in the high-pressure vessel 10 the total current again. The liquid methane leaves as part of the current 35 the bottom of the column, is divided into the partial streams 33 and 45, wherein the partial flow 33 serves as a heat transfer fluid for the bottom heater 32 and is fed to one of the lower bottoms of the column. The partial flow 45 is expanded via the throttle valve 21 to the discharge pressure of the liquid product and collected in the liquid product tank 22 and discharged as needed as stream 25 for use. In this case, a small part evaporates and is compressed via a small compressor 23 or a similar pressure generating apparatus to the exhaust gas pressure and as a partial stream 24 in the gaseous nitrogen-rich overhead product 36 of the column 34 and thereafter for the purpose of cooling the partial stream 14 via the heat exchanger 28 directed. It is passed together with the partial flow 16 as exhaust gas stream 26 via the heat exchanger 6 and cools the part input stream 5. The emerging from the container 10 as a partial stream 12 gas stream is first used via the heat exchanger 7 for cooling the natural gas inlet partial stream 4 and heats up. Thereafter, it is expanded by a first high-pressure expansion turbine 13, cools down, is cooled in the heat exchanger 28 by about another 10 K, enters the separator 29 for higher hydrocarbons, where in particular the higher hydrocarbons are separated and thus removed from the further cryogenic cycle. The latter are either withdrawn liquid or get as part of stream 43 through the throttle valve 30 in the exhaust stream 26. The majority of the partial stream 14 now serves as a partial flow 44 in the heat exchanger 8, the cooling of the natural gas input partial stream 3, is heated and with renewed cooling via the second medium pressure expansion turbine 15 relaxed, is the partial flow 16, forms together with the other process-related streams 24, 36 and 43, the exhaust stream 26 of the plant, which cools the natural gas inlet partial stream 5 in the heat exchanger 6. However, the composition of the exhaust gas now differs significantly from the input composition and is in its composition only conditionally an energetically usable gas.

The methane content has dropped from 33.5% to 22.5%. The liquid product yield is about 14%. Thus, about 40% of the methane contained in the input stream was recovered in liquid form.

Embodiment 3

The embodiment relates to a method according to FIG. 3. It differs from the method according to FIG. 2 by a lower inlet pressure, which here amounts to 20 bar, with equal amounts of nitrogen or light inert. The method is also used to recover from this natural gas, a share of about 40% methane-liquid of the methane contained. Due to the lower pressure, the previous adsorber 2 is no longer sufficient to remove the carbon dioxide to levels below 50 vpm. This circumstance requires on the one hand another source for the sump heating, which is branched off from the partial flow 12 by means of a partial flow 41, in order to be fed, after passing through the heat exchanger 32, via a throttle valve directly into the upper region of the cryogenic distillation column 34 for reasons of process. To prevent crystallization phenomena in the system, the additional adsorbers 37 and 38 for removing the carbon dioxide still present in the partial stream 9 are necessary in the gaseous partial stream 12 and in the liquid partial stream 11.

The results of the individual process variants are summarized in Table 1 in the form of the respective process-specific substreams with details of mass, pressure, temperature and composition. The model calculations were related in the embodiment 1 to 1000 Nm ³ input gas quantity, in the embodiments 2 and 3 in each case to 1005 Nm ³ input gas quantity.

table

LIST OF REFERENCE NUMBERS

1 input current

2 upstream Adsorberanlage for water vapor and carbon dioxide removal

3, 4, 5 inventive partial flows

6, 7, 8, 28 heat exchangers

9 total gas flow

10 high pressure tanks

11, 20, 35, 43, 45 Process-related LPG partial streams

12, 14, 16, 19, 20, 24, 31, 33, 36, 41, 44 Process-related partial gas streams

13 high pressure expansion turbine

15 medium pressure expansion turbine

17, 21, 30 Throttle valves for liquefied gas streams

18 medium pressure vessels

22 liquid product tank for dispensing product

23 compressors

25 liquid dispensing product

26 exhaust gas stream 7, 39, 40 water vapor and carbon dioxide-containing exhaust gas 9 separator for higher hydrocarbons

32 bottom heating of the distillation column

34 cryogenic distillation column 7, 38 process-related additional adsorber plants for water and

Carbon dioxide removal 2 Throttling valve for gas flow from the bottom heating of the distillation column

Claims

claims

1. A method for partial liquefaction of a high pressure of 15 to 70 bar continuously supplied natural gas stream (1) characterized by the following features:

1.1 the natural gas is first subjected to adsorptive purification in an adsorber plant (2),

1.2. the purified natural gas stream (1) is divided into the partial streams (3, 4, 5 and 31) and these partial streams (3, 4 and 5) are cooled individually by at least two process-related partial streams (12, 14) and an exhaust gas stream (26), thereafter combined again in the total flow (9), with partial liquefaction of the total flow (9) which is under input pressure,

1.3 the liquefied portion of the natural gas is collected in a high-pressure vessel (10), while the gas fraction is used as a partial stream (12) for cooling the partial stream (4), wherein

1.3.1. the partial flow (12) in the heat exchanger (7) is heated by the partial flow (4) and expanded in a first expansion device (13) to an intermediate pressure of 6 to 20 bar as partial flow (14),

1.3.2. this partial stream (14) is reheated by at least one of the partial streams (3, 5) and expanded in a further expansion device (15) to a final pressure of 2 to 5 bar with renewed cooling and then as partial stream (16) together with other partial streams ( 19, 24, 36) is used as exhaust gas flow (26) for cooling one of the partial flows (3, 5) of the split input flow,

1.4. the liquefied natural gas collected in the high-pressure vessel (10) is reduced to the discharge pressure of the exhaust gas flow of 2 to 5 bar, wherein

1.4.1. a portion of the liquefied natural gas (11) is evaporated and is added as a partial stream (19, 36) the gaseous natural gas stream (16) after the second expansion device (15), and

1.4.2. the remaining liquid content! (20, 35) is further expanded to a pressure of 1 to 2.05 bar, wherein

1.4.3. only a small portion of 3 - 5 vol .-% of the liquid fraction (20, 35) again compressed and as a partial stream (24) the gaseous Resterdgasstrom (26) is supplied and

1.4.4. the separated liquid product (25) of the previous liquid fraction (20, 35) is temporarily stored in the liquid product tank (22) until final withdrawal, to be discharged as stream (25) for further use.

2. The method according to claim 1, characterized in that the input stream (1) of the pressurized natural gas is preferably divided into three partial streams (3, 4, 5), of which a partial stream (4) with the gaseous portion (12) of the in the high pressure vessel (10) collected partially liquefied natural gas, the second partial stream (3) with the output stream (14) of the first expansion device 13 and the third partial stream 5 are cooled with the exhaust stream 26.

3. The method according to claim 1 and 2, characterized in that as expansion devices (13, 15) work-performing expansion turbines or machines are used, which serve to drive generators.

4. The method according to claim 1, 2 and 3, characterized in that at higher levels of ethane, propane and higher hydrocarbons in natural gas between the first (13) and second expansion device (15) to prevent droplet formation in the second expansion device (15) first a cooling of the gas stream in a heat exchanger (28) takes place with deposition of said hydrocarbons and only then the heating takes place in countercurrent to the incoming natural gas stream, wherein the condensed hydrocarbons are withdrawn separately as a liquid product.

5. The method according to claim 1 and 2, characterized in that for the improved removal of CO ₂ , residual moisture and sulfur compounds in the case of a low gas inlet pressure, further adsorber in the gas outlet and the liquefied natural gas flow of the high pressure vessel (10) are arranged.

6. The method according to claim 1 and 2, characterized in that at a content of natural gas to nitrogen of more than 60 vol .-%, the liquid product from the Hochdruckabscheider (10) for further separation of a working at exhaust pressure cryogenic column (34) and there in a nitrogen-rich top product (36) depending on the nitrogen concentration of the input stream and a methane-rich bottom product (35) is decomposed, wherein the liquid bottom product (35) is expanded into the liquid product tank (22), the top product (36) first for cooling the partial stream (14 ) after the first expansion device (13) and then the exhaust gas stream after the second expansion device (15) is fed, and a sump heater (32) with a smaller partial flow (31) of the natural gas stream (1), operated prior to the splitting in the partial streams is removed, and after the sump heater (32) is fed into the high-pressure vessel (10).

7. The method according to claim 1, 2, 5 and 6 characterized in that at high content of natural gas to nitrogen or inert at low inlet pressure of less than 25 bar, the partial flow to the sump heater the purified gas stream of the high-pressure tank (10) is removed and thereafter a throttle relaxation (42) is fed to the cryogenic column (34).