DE102011113548A1 - Method for operating liquefying plant for liquefaction of hydrocarbon-rich gas stream, involves controlling flow rate of gas stream supplied to pre-cooling stage, liquefaction stage and under-cooling stage by control unit - Google Patents

Abstract

The method involves pre-cooling the hydrocarbon-rich gas stream (NG) through a refrigerant mixture (20) in a common refrigerant circuit of a liquefaction device (10) in a pre-cooling stage (11). The hydrocarbon-rich gas stream is liquefied in a liquefaction stage (12) and is under-cooled in an under-cooling stage (13). The flow rate of the hydrocarbon-rich gas stream supplied to the pre-cooling stage, the liquefaction stage and the under-cooling stage is controlled by a control unit (36). Independent claims are included for the following: (1) a regulating device for liquefaction device of liquefaction plant; and (2) a liquefaction device.

Description

The invention relates to a method for operating a plant for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, a corresponding control device and a liquefaction plant.

State of the art

Natural gas (NG) is one of the most important sources of energy in the world. About 15% of world energy demand is covered by natural gas. The transport of natural gas is largely via gas lines. In the past two decades, however, liquefied natural gas (LNG) has also gained in importance on the world energy market. In liquid form, natural gas has only 1/600 of its initial volume and enables long-distance economic transport.

For liquefaction, natural gas must be cooled to -160 ° C. Prior to this, impurities such as methane and heavy hydrocarbons, carbon dioxide, nitrogen, water and a number of other undesired components have to be removed, for example adsorbing, absorbing and cryotechnical processes being used.

For liquefaction of natural gas, for example, base load plants with a product capacity of 1,500-5,000 kt LNG per year are used, by means of which the natural gas is purified, liquefied in cryogenic heat exchangers and stored in tanks. The transport takes place z. B. in tankers.

Different methods are used to optimize the operation of corresponding large-scale installations. Known online optimization methods include, for example, the provision of a device in the system to be optimized which loads the system-describing model equations and an associated optimization problem. The results obtained are used to determine the control variables for components of the plant. However, online optimization methods have disadvantages in practice.

Of course, the model equations describing the system do not completely depict the real situation, so that there are sometimes considerable differences between the model and the real system. However, the closer the online optimization process is adapted to the system, the more complex the implementation becomes. A suitably adapted model can not usually be transferred to other plants or only with considerable additional effort. Finally, online optimization methods are complex in their implementation, since in the occurrence of disturbances in each case extensive calculations must be made in order to bring the system back to an optimal operating condition. Target sizes must be adjusted and set to new default values. Also due to these aspects, online optimization methods are often not accepted by the user. The mentioned disadvantages relate in particular, but not exclusively, to so-called model predictive control (MPC) and real-time optimization (RTO).

There is a need for more efficient operation of such facilities.

Disclosure of the invention

Against this background, the invention proposes a method for operating a plant for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, a corresponding control device and a liquefaction plant with the features of the independent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

Advantages of the invention

The proposed method is based on the so-called LIMUM ^® process of the applicant (Linde Multi Stage Mixed Refrigerant), in which usually a coiled heat exchanger (CWHE, Coil-Wound Heat Exchanger) is used, in which the hydrocarbon-rich stream used against different fractions a common refrigerant mixture is pre-cooled in a mixed refrigerant cycle, liquefied and supercooled. Pre-cooling, liquefaction and subcooling are described in the context of this application with regard to corresponding "heat exchanger stages". It should be emphasized that the individual stages in a single wound heat exchanger, for. B. spatially separated, realized and in a common Heat exchanger jacket can be arranged. A coiled heat exchanger is doing, as explained below 2 visible, arranged vertically. At different heights, the individual fractions of the refrigerant mixture are introduced and cooled by Joule Thomson (JT) valves. From top to bottom, fractions with increasing volatility, ie sinking boiling point, are used for this purpose. Accordingly, from bottom to top results in a gradually decreasing temperature, whereby the heat exchanger stages are realized. At the bottom of the heat exchanger, the completely vaporized refrigerant mixture can be removed substantially in gaseous form. Typically used refrigerant mixtures include, for. As methane, ethane, propane, propylene and butane.

The refrigerant mixture cycle has a two-stage compressor unit, in which two fractions are produced from the refrigerant mixture which is successively completely vaporized in the heat exchanger stages and thus present in gaseous form. A first fraction, referred to in this application as a "high-boiling" fraction, comprises the components of the refrigerant mixture having the highest boiling point. These are liquid after the compression and one of these downstream cooling in air or water coolers. A fraction referred to as "second" fraction in the context of this application comprises the (initially) gaseous radical. The production of the fractions takes place as explained in detail below. Typically, a pressure of about 20 and 45 bar is used to produce the fractions, the cooling is carried out to the temperature of the cooling medium used in the air or water coolers.

The further process steps can be specified as follows:
The named high-boiling fraction is fed to the pre-cooling stage at a pressure of about 20 bar and is cooled there against itself and depressurized to the pressure present on the shell side in the heat exchanger of about 4 bar. In this case, a temperature of about -40 ° C is generated, which is ready for pre-cooling of the hydrocarbon-rich stream in the pre-cooling stage.

The second fraction is also fed to the precooling stage where it is cooled and after cooling in a downstream of the pre-cooling, referred to in this application as "Zwischenabscheider" separator at a corresponding temperature of about -30 ° C and a pressure of about 45 bar separated into a liquid and a gaseous fraction. The liquid fraction produced in said intermediate separator is referred to in this application as a "middle boiling" fraction, the gaseous fraction as a "low boiling" fraction.

The medium-boiling fraction is fed to the liquefaction stage at a pressure of about 4 bar, where it is cooled against itself and also cooled to the mantletide pressure in the heat exchanger. As a result, a temperature of about -110 ° C is generated. This is used for liquefaction in the liquefaction stage.

The low-boiling fraction is also fed to the liquefaction stage, where it is cooled, fed to the subcooling stage after cooling at a pressure of about 4 bar, and cooled in the subcooling stage. Here, a temperature of about -160 ° C is generated. This is available for supercooling the hydrocarbon-rich stream in the subcooling stage.

A hydrocarbon-rich stream to be liquefied passes through said stages of the heat exchanger and is thus cooled successively to the stated temperatures of the separator stages. The flow of the hydrocarbon-rich stream, and thus the throughput of the entire plant or the amount of liquefied hydrocarbon-rich stream is adjusted by a restrictor valve.

According to the invention, it has been found that the targeted selection of controlled variables, namely, in the present case, at least a ratio of the flow rates of the high-boiling fraction fed to the precooling stage and the low-boiling fraction of the refrigerant mixture supplied to the subcooling stage, enables a particularly advantageous operation of a corresponding plant.

The invention is based on the "Self Optimizing Control" principle. This means that by suitable control variable selection operation can be carried out near the optimum, without the need to adjust setpoints or manipulated variables by means of complex online optimization to changed disturbances.

The basics are z. B. in the publication "Self-optimizing control: the missing link between steady-state optimization and control", Computers and Chemical Engineering 24 (2000) 569-575 and "Near-Optimal Operation by Self-Optimizing Control: From Process Control to Marathon Running and Business Systems ", Computers and Chemical Engineering 19 (2004) 127-137 by Sigurd Skogestad described and are summarized here for clarity only briefly.

Self-regulating control methods are not equivalent to conventional online optimization techniques.

Optimization methods are generally based on a scalar cost function to be minimized. To achieve a complete optimization, as mentioned, an ideal model of the system to be optimized would be required, all occurring disturbances would have to be measured continuously, and the resulting optimization problem would have to be solved (on line).

As already mentioned, in practice this is unrealistic and therefore not feasible. Therefore, a simpler implementation is desirable which nevertheless gives satisfactory results (with still acceptable losses). Ideally, according to a self-regulatory control method, this includes selecting particular controlled variables and setting fixed setpoints for them, thereby simplifying a complex optimization problem to a feedback problem.

The controlled variables to be selected represent a subset or a combination of the respectively available measurable process variables. An essential step in the implementation of a corresponding method is the selection of suitable controlled variables. These control variables must be insensitive and easily measurable and controllable against disturbances, have a sufficient amplification factor with respect to the respective control values, and must not be closely correlated (in the case of several controlled variables). The selection of suitable or "best" controlled variables (also referred to in the aforementioned publications as "Best Controlled Variables") therefore represents the central challenge in implementing a corresponding method.

This specific control variable selection proved to be difficult in practice for a variety of reasons. On the one hand, it is in no way obvious which and how many degrees of freedom exist in practice for manipulating the mixed refrigerant cycle. Usually show manipulations of levels, eg. B. of separators, no steady state influence on appropriate facilities. However, this is not the case in the above-LIMUM ^® -cycle surprisingly. In the case of LIMUM ^® -Kreislaufs the individual refrigerant mixture fractions between accumulation points in the form of storage containers and separators are moved so that the levels of them have an impact on the composition of the refrigerant mixture and stationary operation. Since the level of a separator results in each case from the levels of the other separator, two levels can be manipulated independently. It could be shown that the respective fill levels act sufficiently independently on the process to control two controlled variables. Another difficulty was the elaborate modeling of the cycle, including the effects of shifting the refrigerant fractions between the separators.

Natural gas liquefaction plants are geared towards two operational objectives. First, as mentioned, a natural gas stream from the respective initial conditions in a liquid, transportable state (-160 ° C at near atmospheric pressure) to convict. In addition, heavy hydrocarbons must be separated. The separation step is optional and usually takes place after the already partially explained pre-cooling of the natural gas stream. Regardless of the presence of disturbances, such as fluctuating ambient temperatures, these operating goals can be achieved by appropriate regulation, in other words, by using a set of controllers, each of which automatically alters a manipulated variable such that the respective (sub) Goal is achieved.

However, because the systems of interest here for natural gas liquefaction have a larger number of manipulated variables than operating objectives, there are additional degrees of freedom, which are then advantageously used to optimize a profit function, i. d. R. a yield or a throughput of a corresponding system, can be used. On-line optimization methods using the previously explained control techniques (RTO, MPC), which could conventionally be used here, are generally not used in natural gas liquefaction plants because of the disadvantages already mentioned in the introduction.

An essential step in the implementation of a method based on self-optimizing control is, as mentioned, in the selection and / or combination of suitable measurable process variables to controlled variables, which by means of the variable control variables via corresponding actuators on their previous specified setpoints. The technical problem of the control variable selection or combination has a high combinatorial diversity and can only be solved with suitable methods including plant-specific simulations and process know-how.

According to the invention can be achieved by the definition of suitable control variables, omitting elaborate on-line optimization steps and procedures, an operation in the vicinity of the respective optimum. The method is simple to implement, adaptable to different systems and at least partially implementable with existing controllers of corresponding systems, for example PID controllers.

As already explained in the beginning, a compressor unit with two compressor stages is used in a corresponding method. Here, the completely evaporated refrigerant mixture from the individual heat exchanger stages is first precompressed in the first compressor stage with lower pressure, cooled and fed to a first separator. A liquid fraction obtained in the first separator is used as the high-boiling fraction. In contrast, a fraction taken off in gaseous form from the first separator is subsequently compressed in the second compressor stage at a higher pressure, cooled and fed to a second separator. A fraction finally taken off in gaseous form from the second separator is used as the "second" fraction explained above.

A liquid fraction taken from the second separator is at least partially recycled to the first separator. The amount of liquid fraction fed back to the first separator over time, ie a flow rate from the second to the first separator, is referred to as a "compressor valve" by means of a flow control valve, hereinafter referred to as "compressor valve", based on a liquid fraction level in the first fractionator second separator set. The first compressor stage typically produces a pressure of about 20 bar, the second compressor stage a pressure of about 45 bar. The compressor valve relaxes the second fraction taken from the liquid fraction accordingly.

Advantageously, a corresponding regulation is made such that the ratio of the flow rates of the precooling high-boiling fraction and supplied to the subcooling low-boiling fraction of the refrigerant mixture as inventively selected control variable (ie as the explained "Best Controlled Variable") by means of an adjustment of the liquid fraction level is regulated in the second separator as a control variable via a position of the compressor valve as an actuator. Corresponding control strategies can be implemented easily and inexpensively in existing systems and can in some cases resort to existing technical facilities there.

In the context of a corresponding process, expansion valves are used in each case for the high-pressure expansion of the high-boiling fraction, the medium-boiling fraction and the low-boiling fraction. The regulation of the ratio of the flow rates of the high-boiling fraction fed to the pre-cooling stage and the low-boiling fraction of the refrigerant mixture fed to the subcooling stage as a controlled variable (ie as the "Best Controlled Variable" discussed above) can be alternative to the strategy explained above, namely the use of the fill level of the second Separator in the compressor unit, also by means of an adjustment of the level of a liquid fraction in the intermediate separator as a control variable via a position of the expansion valve used to relax the medium-boiling fraction carried out as an actuator.

In particular, in the aforementioned setting the level of a liquid fraction in the Zwischenabscheider arise in combination with other measures special advantages. In this case, again in the compressor unit, a ratio of the second separator removed and the first separator again supplied liquid fraction and the second separator in the form of the second fraction removed gaseous fraction kept constant over a position of the flow control valve, so fixed.

Advantageously, the above-described measures are combined with a method in which the control further regulates a ratio of the pressures of the refrigerant mixture before the first stage and after the second stage of the compressor unit as a controlled variable by means of a flow rate of the low-boiling fraction of the refrigerant mixture in the supercooling stage as a manipulated variable. As an actuator, a limiting valve is used, which is assigned to the subcooling stage. The use of the ratio of the pressures of the refrigerant mixture before the first stage and after the second stage of the compressor unit as a controlled variable already provides in itself, d. H. even without the measures described above, considerable benefits. The throughput of natural gas can be increased by up to 6% compared to conventionally operated systems.

A corresponding control method is based on a gain function in the form of a throughput of the hydrocarbon-rich stream in the liquefaction unit. The flow rate of the hydrocarbon-rich stream in the liquefying means may be adjusted by means of a restrictor valve downstream of the subcooling stage based on a temperature of the hydrocarbon-rich stream after the subcooling stage.

A control device according to the invention for a liquefying device for liquefying a hydrocarbon-rich stream is designed to carry out a method as explained above and has appropriately designed control means. This, as well as the inventively provided liquefaction, benefits from the advantages explained above.

Below, the advantages mentioned above and further advantageous effects of the measures according to the invention are summarized again and briefly explained.

The proposed control variable selection allows a nearly maximum throughput of natural gas, without having to redefine control values after disturbances have occurred. This represents a significant advantage because operators of such plants are generally not readily able to select and properly set correct control values and, as noted, liquefaction plants generally do not have dynamic optimization facilities.

Another advantage is that the installation personnel only have to set the setpoint values of the controllers manually once during commissioning or after maintenance of a corresponding system. After this initial setting, optimum operation can be ensured which is relatively insensitive to disturbances. This frees up appropriate resources. Performance tests can be performed easily and without problems.

A significant advantage, which can be realized by the measures according to the invention, is furthermore that the respective control variables can be kept at or near their respective nominal values over a very large disturbance variable range. A corresponding system is therefore very flexible operable. A complex avoidance of interference can be omitted at least in part.

Moreover, the solution according to the invention is particularly suitable for retrofitting appropriate natural gas liquefaction plants, whereby a relatively high and low-cost liquefied gas yield can be realized relatively easily, without having to implement costly online optimization techniques such as MPC or RTO.

In the following, quantitative results are presented which indicate a maximum loss of a corresponding system in the context of a worst-case scenario with regard to a liquefied gas throughput. The worst-case loss represents the maximum deviation between the best achievable result and the result obtained in the case of a disturbance of a certain size. The values obtained using a conventional control strategy (CONV) and the values obtained according to the invention are indicated here. CFRC (Compression and Flow Ratio Control) refers here and below to the first alternative according to the invention, in which the flow rate ratio of the low-boiling to the high-boiling refrigerant mixture fraction is controlled as a controlled variable via the fill level setting in the second separator of the compressor unit as a manipulated variable. CF2RC (Compression, Flow, and Flow Ratio Control) refers here and below to the second alternative according to the invention, in which the flow rate ratio of the low-boiling to the high-boiling mixed refrigerant fraction is set via a filling level setting in the intermediate separator and, in addition, the time taken over the second separator of the compressor unit Fractions, ie the corresponding flow rates, are set constant. In both alternatives, in addition, the pressure ratio of the refrigerant mixture before the first and after the second compressor stage of the compressor unit, as explained above, used as a controlled variable. CONV CFRC CF2RC > 10% 5% 5%

The values for the conventional control strategy (CONV) should be taken into account that they work in normal operation using a suction pressure control. To enable a comparison of the control strategies, this has been deactivated.

The invention is illustrated schematically with reference to an embodiment in the drawing and will be described below with reference to the drawing.

figure description

1 shows a plant for liquefying a hydrocarbon-rich stream and a control device according to the prior art in a schematic representation.

2 shows an installation for liquefying a hydrocarbon-rich stream according to the prior art in an alternative schematic representation.

3 illustrates a method for operating a system according to 1 according to a particularly preferred embodiment of the invention.

4 illustrates a method for operating a system according to 1 according to a further particularly preferred embodiment of the invention.

5 shows performance characteristics of a plant which is operated according to the prior art and according to particularly preferred embodiments of the invention.

In the figures, like elements are indicated by identical reference numerals. For the sake of clarity, a repeated explanation is omitted.

1 shows a plant 100 for liquefying a hydrocarbon-rich stream NG according to the prior art in a schematic representation, which can be operated by means of a method according to a particularly preferred embodiment of the invention or equipped with a control device according to the invention. The plant has a liquefaction facility 10 on. In the plant realized control or regulating mechanisms are here, as well as in the following 3 and 4 , illustrated with dashed lines. In the 1 are temperature sensors with T, flow rate with F, level sensor with L and pressure transducer with P designated. These may be sensors for providing directly measured measured values, but also corresponding soft-sensing devices. The individual control strategies are shown very schematically.

The liquefaction facility 10 has three heat exchanger stages 11 . 12 . 13 on. The heat exchanger stages 11 . 12 . 13 are as pre-cooling stage 11 , Liquefaction stage 12 and subcooling stage 13 for a hydrocarbon-rich stream NG, for example a natural gas stream formed. The hydrocarbon-rich stream NG successively passes through the precooling stage 11 , the liquefaction stage 12 and the subcooling stage 13 , Is there gradually cooled and liquefied at the temperatures already explained above, and leaves as a liquefied hydrocarbon-rich stream LNG the liquefying 10 , The flow rate of the hydrocarbon-rich stream NG, and thus the flow rate of the liquefier 100 or the amount of liquefied hydrocarbon-rich stream LNG produced per unit time, e.g. B. of liquefied natural gas, is a limiter valve 14 according to a corresponding regulation 34 set. As an associated controlled variable, the temperature of the hydrocarbon-rich stream LNG is in front of the restrictor valve 14 used. The restrictor valve 14 is thus an actuator. The regulation 34 Thus, allows a setting of a setpoint temperature of the liquefied hydrocarbon-rich stream LNG by adjusting a flow rate, and thus a residence time of the hydrocarbon-rich stream NG or LNG in the system 100 or the liquefaction device 10 , This setting of the throughput is common to the prior art and the measures proposed according to the invention.

The individual heat exchanger stages 11 . 12 . 13 have for relaxation of these supplied refrigerant mixture fractions each relaxation valves 16 . 17 and 18 in accordance with the prior art in each case via appropriate, associated with these regulations 31 . 32 and 33 be set according to other sizes.

The regulation 31 of the expansion valve 16 the pre-cooling stage 11 operates according to the prior art based on a temperature of the hydrocarbon-rich stream LNG at the exit of the precooling stage 11 (or between pre-cooling stage 11 and liquefaction stage 12 ) and a temperature of the refrigerant mixture leaving the precooling stage 2 , which is usually in gaseous form and from an evaporation of the fractions in the heat exchanger stages 11 . 12 and 13 comes. This allows a sufficient pre-cooling of the hydrocarbon-rich stream LG in the pre-cooling stage 11 be ensured without condensate, ie liquid refrigerant mixture in the below-described compressor unit 40 can reach.

The regulation 32 using the expansion valve 17 the liquefaction stage 12 takes place according to the state of the art in accordance with a level in the below explained intermediate separator 15 , in turn, as a control variable for the control of the temperature of the hydrocarbon-rich stream LNG at the output of the liquefaction stage 12 (or between liquefaction stage 12 and subcooling stage 13 ) is used.

The regulation 33 using the expansion valve 18 the subcooling stage 13 takes place according to the prior art in accordance with a flow rate of the compressor unit leaving high-boiling refrigerant mixture fraction 21 (for a more detailed explanation see below).

In a two-stage compressor unit 40 with a motor M is a in the heat exchanger stages 11 . 12 and 13 successively completely evaporated refrigerant mixture 2 in two compressor stages 41 and 42 compressed and from this a liquid, high-boiling refrigerant mixture fraction 21 and a gaseous intermediate fraction 24 generated. That in the compressor stages 41 and 42 compressed refrigerant mixture, or its corresponding proportions, each arrive in a cooling unit 45 and 46 and subsequently in first and second separators 43 . 44 ,

In a first compressor stage 41 becomes the refrigerant mixture 20 initially pre-compressed. A little later in the first separator 43 present liquid fraction is called schweresiedede mixed refrigerant fraction 21 used. The fraction which is gaseous in the first separator becomes the second compressor stage 42 fed. In the second compressor stage 42 a compression to a higher pressure takes place. The correspondingly compressed proportion of the Kätemittelgemischs 20 separates in the second separator 44 into a liquid and a gaseous fraction. Part of the liquid fraction may be the second separator 44 via a corresponding compressor valve 47 taken and the first separator 43 be fed again. For this purpose, according to the prior art, a level control 37 used. The gaseous fraction from the head of the separator is called an intermediate fraction 24 used. There are also valves 48 and 49 provided, which are equipped with appropriate, not explained adjusting devices. Another separator 19 Make sure the compressor unit 40 no liquid refrigerant mixture is supplied.

2 shows a plant 200 for liquefying a hydrocarbon-rich stream according to the prior art in an alternative schematic representation, which is based on the actual structural realization. In the 2 shown plant has the elements of the system 100 , in the 1 is shown, but it is additionally expressed that the individual heat exchanger stages 11 . 12 . 13 as part of a common liquefaction facility 10 , z. B. a wound heat exchanger are provided. The system is shown in a very schematic manner, but a representation of a number of components has been omitted for the sake of clarity.

3 shows the plant 100 of the 1 , which is operated according to a particularly preferred embodiment of the invention. Operation takes place using a control 36 in which at least a ratio of the flow rates of the precooling stage 11 fed high-boiling fraction 21 and the subcooling stage 13 supplied low-boiling fraction 23 of the refrigerant mixture 20 is used as a controlled variable. As a manipulated variable, as mentioned, a level in the second separator 44 used, via an adjustment of the compressor valve 47 is set. This corresponds to the illustrated first alternative of the invention, also referred to as CFRC.

Another regulation 38 is based on a pressure ratio of the pressures of the refrigerant mixture 20 before the first 41 and after the second 42 Compressor unit compressor stage 40 , Based on the scheme, the flow rate of the light refrigerant mixture fraction 23 in the subcooling unit 13 over valve 18 set.

The influence of the low-boiling refrigerant mixture fraction 23 associated expansion valve 18 and for returning the liquid fraction from the second separator 44 in the first separator 43 the separator unit 40 serving valve 47 takes place according to the in 3 illustrated embodiment differing from the prior art, in 1 is shown.

Furthermore, an adjustment of the high-boiling refrigerant mixture fraction 21 associated single-voltage valve 16 , according to a regulation 31 ' , based on a temperature of the Refrigerant mixture 20 at the exit of the pre-cooling unit 11 , The adjustment of the medium-boiling refrigerant mixture fraction 22 associated expansion valve 17 carried out in accordance with a regulation 32 ' , based on a level in the intermediate separator 15 .

4 shows the plant 100 of the 1 , which is operated according to another particularly preferred embodiment of the invention. Here, as before, as a control variable, the flow rate ratio of the low-boiling 23 to the high-boiling 21 Refrigerant mixture fraction used, but via a level adjustment in the intermediate separator 15 set. This is a rule 35 intended.

In addition, the ratio of the second separator 44 the compressor unit 40 over the time taken fractions, ie the ratio of the corresponding flow rates, set constant. For this purpose, a scheme 36 ' used. So it's the second alternative, also called CF2RC. The adjustment of the high-boiling refrigerant mixture fraction 21 associated expansion valve 16 takes place as related to 3 explains about a regulation 31 ' ,

5 shows performance characteristics of a plant for liquefying a hydrocarbon-rich stream, which is operated according to the prior art and according to preferred embodiments of the invention. In the upper row, in the graphs A1, A2 and A3, the ratios of product flow F _LNG to a nominal product flow F _{LNG, 0} are indicated in percent on the ordinate. The bottom row shows in graphs B1, B2 and B3, respectively, a percentage loss of flow L on the ordinate. With R / O, the (theoretical) optimum values are denoted by a solid line, CONV (vertical crosses), as explained above, denotes values which are obtained by means of a method according to a control strategy according to the prior art. CFRC (triangles) and CF2RC (stars) denote values according to the control strategies of the invention discussed above.

In the first column with the graphs A1 and B1, the respective ordinate values are plotted against different ambient temperatures T _amb (in K) on the abscissa. The second column with the graphs A2 and B2 shows the ordinate values at different setpoint temperatures of the liquefied natural gas T ^SP _LNG (in K) on the abscissa. The third column with the graphs A3 and B3 shows the ordinate values at different values for the compression capacity of the compressor unit (in MW). The abscissa values are each column by column, the ordinate values in each case are applied identically line by line.

As can be seen, the proposed sets of process variables allow for improved operability since, in contrast to the conventional structure (CONV), the results can be maintained at or near the theoretical optimum over a wide range of disturbances.

LIST OF REFERENCE NUMBERS

100

Plant for liquefying a hydrocarbon-rich stream

NG

hydrocarbon-rich stream

LNG

liquefied hydrocarbon-rich stream

T

temperature sensors

F

Flow rate transmitter

L

level sensor

P

thruster

M

engine

10

liquefier

11

precooling

12

liquefaction stage

13

Hypothermia stage

14

Limit valve in the natural gas path

15

intermediate screen

16

Expansion valve pre-cooling stage

17

Expansion valve liquefaction stage

18

Relaxation valve subcooling stage

19

demister

20

Refrigerant mixture

21

high-boiling refrigerant mixture fraction

22

by means of boiling refrigerant mixture fraction

23

low boiling refrigerant mixture fraction

24

intermediate fraction

31, 31 '

Control with control value expansion valve pre-cooling stage

32, 32 '

Control with control value expansion valve condensing stage

33

Control with control value Expansion valve Subcooling stage

34

Control with manipulated variable limiter valve

35

Control flow rate ratio with control variable level separator

36, 36 '

Control flow rate ratio with control value level second separator

37

Control with manipulated variable compressor valve

38

Control pressure ratio

40

compressor unit

41

first compressor stage

42

second compressor stage

43

first separator

44

second separator

45

first cooler

46

second cooler

47

Valve

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• "Self-optimizing control: the missing link between steady-state optimization and control", Computers and Chemical Engineering 24 (2000) 569-575 [0018]
• "Near-Optimal Operation by Self-Optimizing Control: From Process Control to Marathon Running and Business Systems", Computer and Chemical Engineering 19 (2004) 127-137 to Sigurd Skogestad [0018]

Claims (10)

Process for the operation of a plant for liquefying a hydrocarbon-rich stream (NG), in which the hydrocarbon-rich stream (NG) is gradually transformed by heat exchange against fractions (NG) 21 . 22 . 23 ) of a refrigerant mixture ( 20 ) in a common refrigerant mixture cycle of a liquefying device ( 10 ) in a pre-cooling stage ( 11 ) pre-cooled, in a liquefaction stage ( 12 ) and in a subcooling stage ( 13 ) is supercooled, wherein • off in the pre-cooling stage ( 11 ), the liquefaction stage ( 12 ) and the subcooling stage ( 13 ) successively completely evaporated refrigerant mixture ( 20 ) by means of a compressor unit ( 40 ) a liquid high-boiling fraction ( 21 ) and a gaseous second fraction ( 24 ), • the high-boiling fraction ( 21 ) of the pre-cooling stage ( 11 ) and is cooled down there against itself and is released with a cooling power, • the second fraction ( 24 ) also the pre-cooling stage ( 11 ), cooled there and after cooling in one of the pre-cooling stage ( 11 ) downstream intermediate separator ( 15 ) into a liquid medium-boiling fraction ( 22 ) and a gaseous low-boiling fraction ( 23 ), • the medium-boiling fraction ( 22 ) of the liquefaction stage ( 12 ) and is cooled down there against itself and depressurized, and • the low-boiling fraction ( 23 ) also the liquefaction stage ( 12 ), cooled there, after cooling the subcooling stage ( 13 ), and in the subcooling stage ( 13 ) is released cold-tempered; characterized in that the method by means of a scheme ( 35 . 36 ), wherein at least one ratio of the flow rates of the precooling stage ( 11 ) fed high-boiling fraction ( 21 ) and the subcooling stage ( 13 ) supplied low-boiling fraction ( 23 ) of the refrigerant mixture ( 20 ) is used as a controlled variable. Method according to Claim 1, in which a compressor unit ( 40 ) with two compressor stages ( 41 . 42 ), wherein the completely evaporated refrigerant mixture ( 20 ) first in the first compressor stage ( 41 ) precompressed at a lower pressure, cooled and a first separator ( 43 ), a first separator ( 43 ) fraction taken off in gaseous form in the second compressor stage ( 42 ) recompressed at higher pressure, cooled and a second separator ( 44 ), a second separator ( 44 ) taken gas fraction as the second fraction ( 24 ) and the second separator ( 44 ) removed liquid fraction at least partially the first separator ( 43 ), wherein an amount of the again the first separator ( 43 ) supplied by means of a Abscheiderventils ( 47 ) based on a liquid fraction level in the second separator ( 44 ) and in the first separator ( 43 ) obtained liquid fraction as the high-boiling fraction ( 21 ) is used. Method according to Claim 2, in which the control ( 36 ) the ratio of the flow rates of the pre-cooling stage ( 11 ) fed high-boiling fraction ( 21 ) and the subcooling stage ( 13 ) supplied low-boiling fraction ( 23 ) of the refrigerant mixture ( 20 ) as a controlled variable by means of an adjustment of the liquid fraction level in the second separator ( 44 ) as a manipulated variable via a position of the separator valve ( 47 ) as an actuator regulates. Process according to Claim 2, in which the high-boiling fraction ( 21 ), the middle-boiling fraction ( 22 ) and the low-boiling fraction ( 23 ) Expansion valves ( 31 . 32 . 33 ) and the scheme ( 35 ) the ratio of the flow rates of the pre-cooling stage ( 11 ) fed high-boiling fraction ( 21 ) and the subcooling stage ( 13 ) supplied low-boiling fraction ( 23 ) as a controlled variable by means of an adjustment of the level of a liquid fraction in the intermediate separator ( 15 ) as a manipulated variable on a position of the relaxation of the medium-boiling fraction ( 22 ) used relaxation valve ( 17 ) as an actuator regulates. Method according to claim 4, in which additionally a regulation ( 36 ' ) is used, which is a ratio of the second separator ( 44 ) and the first separator ( 43 ) re-supplied liquid fraction and the second separator ( 44 ) in the form of the second fraction ( 21 ) removed gaseous fraction via a position of the Abscheiderventils ( 47 ) keeps constant. Method according to one of the preceding claims, in which additionally a regulation ( 38 ), which is a ratio of the pressures of the refrigerant mixture ( 20 ) before the first stage ( 41 ) and after the second stage ( 42 ) of the compressor unit ( 40 ) as a controlled variable by means of a throughput of the low-boiling fraction ( 23 ) in the subcooling stage ( 13 ) via the expansion valve ( 18 ) as a manipulated variable. Method according to one of the preceding claims, in which a throughput of the hydrocarbon-rich stream (NG) in the liquefying device ( 10 ) by means of one of the subcooling stage ( 13 ) downstream limit valve ( 14 ) based on a temperature of the hydrocarbon-rich stream (NG) after the subcooling stage ( 13 ) is set. Method according to one of the preceding claims, in which a throughput of the hydrocarbon-rich stream (NG) in the liquefying device ( 10 ) is used as a profit function. Regulating device for a liquefaction device ( 10 ) for liquefying a hydrocarbon-rich stream (NG), which is arranged to carry out a method according to any one of the preceding claims, with control means which are adapted to at least a ratio of the flow rates of the precooling stage (NG). 11 ) fed high-boiling fraction ( 21 ) and the subcooling stage ( 13 ) supplied low-boiling fraction ( 23 ) of the refrigerant mixture ( 20 ) as a controlled variable. Liquefaction facility ( 10 ) for liquefying a hydrocarbon-rich stream (NG), which is adapted to carry out a method according to one of claims 1 to 8 and having a control device according to claim 9.

Images