AND CORRESPONDING REFRIGERANT MIXTURE

Technical field of the present invention

The present invention basically relates to methods and apparatus for cascaded cooling of fluid streams. The present invention is particularly related to methods and apparatus used in offshore gas processing plants.

Technological background of the present invention

In the description to follow, several abbreviations will be used having the meanings set forth below:

HC - hydrocarbon; FC - fluorocarbon; HFC - hydrofluorocarbon; HCFC - hydrochlorofluorocarbon.

In addition, the following terms used in the description will have the meanings set forth below with the terms being capitalized when used in the description:

- azeotropic mixture - a mixture of two or more components in a ratio that the proportions of the components can not be changed by distillation; when boiled, the vapour has the same proportions of components as the unboiled mixture;

- zeotropic mixture - a mixture of two or more components that never has the same vapour phase and liquid phase composition at the vapour-liquid equilibrium state; dew point and bubble point curves do not intersect over the entire composition range;

- key component - major component of a mixture, for example component that has the highest molar fraction;

- azeotropic component - any component that forms an azeotropic mixture in the binary system with the key component;

- impurity - any component that does not form an azeotropic mixture in the binary system with the key component;

- azeotropic composition - the composition of an azeotropic mixture at its azeotropic point, i. e. the composition where the azeotropic mixture cannot be separated by distillation anymore;

- non-azeotropic composition - an azeotropic mixture at any composition other than its azeotropic composition;

- refrigerant mixture - a blend of one key component, at least one azeotropic component, and optionally one or more impurity;

- low pressure - the maximum pressure chosen from these three options - a) lowest possible pressure level such that vapour-liquid envelope of the refrigerant mixture barely does not intersect the freezing point line of the refrigerant;

b) 1 bar(a) or atmospheric pressure;

c) the pressure chosen for the evaporator operating at the lowest temperature;

- irregular phase behaviour - local extremal value in boiling temperature curve of a refrigerant mixture at low pressure when plotted against the content of the key component in respect to a constant ratio of all other mixture components amongst themselves (azeotropic components and impurities), for example 100% CO ₂ ; 90% CO2 + 9% C ₂ H ₆ + 1% CH ₄ ; 80% C0 ₂ + 18% C ₂ H ₆ + 2% CH ₄ ; 90% C ₂ H ₆ + 10% CH ₄ ; the ratio of C ₂ H ₆ to CH ₄ is 9:1 in all mixtures.

The use of C0 ₂ (such as the R-744 product available from the Linde Group) as a refrigerant in trans-critical vapour compression cycles is known. However, the lowest achievable temperature when using C0 ₂ as the refrigerant is limited by the triple point of pure CO ₂ of about -56.6°C.

Further, in order to achieve a sufficiently high heat rejection temperature in the aftercooler/condenser of the system, the CO ₂ has to be compressed to pressures significantly about the critical pressure of pure CO ₂ . Use of such pressures can lead to unsafe operating conditions.

The utilization of zeotropic (R-4xx) and azeotropic (R-5xx) mixtures of refrigerants for single stage refrigerators is also known. This includes standardized mixtures having blends containing HCs, HFCs, FCs, and HCFCs. Mixtures containing CO ₂ (such as R-744 noted above) are not described or used as standardized refrigerant blends.

Offshore gas processing plants require special attention be paid to the safety of the process, at least in part because of the usually partially submerged layout of the floating structure. Such a layout increases the risk of combustible compound accumulation in low points of the system, particularly of heavy weight or low volatility components.

This can in turn lead to vapour cloud formation, which should be strictly avoided. Therefore, a choice of either non-combustible or highly volatile (i. e. at atmospheric temperature supercritical) refrigerants and refrigerant blends are required.

WO 01/69149 A1 describes a natural gas liquefaction apparatus wherein a carbon dioxide based pre-cooling circuit is provided in a cascade arrangement with a main cooling circuit. This publication explicitly notes that the CO ₂ refrigerant should contain no more than five percent of other gases and that essentially pure CO ₂ is preferred.

DE 10 2012 017653 A1 (also published as US 2014/00601 1 1 A1 ) describes a process that utilizes a cascaded CO ₂ pre-cooling cycle for the liquefaction of natural gas. The lowest possible temperature for pre-cooling is limited by the triple point of the pure CO ₂ refrigerant. DE 10 2013 01 1 640 A1 (also published as US 2015/0013380 A1 ) describes a process that utilizes a cascaded

CO ₂ pre-cooling cycle for partial condensation of CO ₂ from a sour natural gas. The yield of the phase separator is directly linked to the temperature of the incoming natural gas. The lowest possible temperature for cooling is again limited by the triple point of the pure CO ₂ refrigerant. Current state of the art processes to achieve efficient pre-cooling for gas liquefaction processes use either wide boiling, zeotropic mixtures for the refrigerant (for example Linde Group LIMUM process) or a cascaded system using pure refrigerants (for example ConocoPhillips LNG or AP-C3MR processes).

Also known are mixtures of CO ₂ and saturated or unsaturated C ₂ -HC and C ₂ -FC that form azeotropic mixtures over a wide pressure range. The composition of the azeotropic mixture is strongly dependent on the choice of the C ₂ compound and the respective system pressure.

The general knowledge about the existence of such mixtures and knowledge of the particular phase behaviour ranges from widely (for example CO ₂ +C ₂ H ₆ ) to marginally (for example CO ₂ +C ₂ H ₄ ) to rarely (for example CO2+C2H2 and CO2+C2F4). The quantitative effect of the C2 compound on the freezing point depression of the mixture is not well known.

Propane (R-290) is known to form azeotropic mixtures with some HFCs but such mixtures are not described as useful for standardized refrigerant blends. To the contrary, some zeotropic mixtures containing propane and HFCs are described as being in the class of standardized refrigerant blends.

As noted above, single stage and cascaded pre-cooling cycles using pure CO2 are limited in their lowest temperature by the freezing point (triple point measurement) of CO2 to approximately -56.6°C.

As will be discussed in greater detail below, the addition of conventional low boilers (for example CH4 or N2) to CO2 cause a steep decline in the bubble line with a limited decrease of the freezing line. This approach does not provide a significant reduction of the lowest achievable temperature with a CO2 rich (> 50 mol% CO2) refrigerant.

Further, the addition of conventional high boilers (for example C3H8 or C4H10) to CO2 causes an increasing temperature effect of the bubble line.

There remains a need in the art for improvements to methods and apparatus for cascaded cooling of fluid streams.

Disclosure of the present invention: object, solution, advantages

Starting from the disadvantages and shortcomings as described above as well as taking the prior art as discussed into account, an object of the present invention is to overcome the limitations and problems that earlier methods and apparatus have experienced.

These objects are accomplished by a refrigerant mixture comprising the features of claim 1 as well as by a method comprising the features of claim 1 1. Advantageous embodiments and expedient improvements of the present invention are disclosed in the respective dependent claims.

The present invention basically provides for improved methods and apparatus for cascaded cooling of fluid streams and particularly for cascaded cooling of fluid streams on offshore chemical and gas processing plants.

The improvements of the present invention are accomplished generally by providing a refrigerant mixture having a key component that is carbon dioxide (CO2) and at least one azeotropic component, such as a saturated or unsaturated hydrocarbon (HC) or fluorocarbon (FC) with a carbon chain length of two, and optionally at least one impurity, in particular less than 30 mol% of an impurity, for example less than 15 mol% of an impurity, such as less than 5 mol% of an impurity.

In an advantageous manner, the azeotropic component may be C2H6, C2F6, C2H4, C2F4, C2H2, or C2F2. Expediently, the mixture may comprise

- 58 mol% to 93 mol% C0 ₂ , in particular 80 mol% to 90 mol% C0 ₂ , with the balance being C ₂ H ₆ , or

- 50 mol% to 95 mol% C0 ₂ , in particular 80 mol% to 95 mol% C0 ₂ , with the balance being C ₂ H ₄ . According to a favoured embodiment of the present invention, the refrigerant mixture may have a freezing point below -56.6°C. Preferably, the refrigerant mixture exhibits irregular phase behaviour at low pressure.

The present invention further relates to a method for cooling, liquefying or processing a gas using a refrigerant mixture, wherein the refrigerant mixture comprises CO2 and at least one azeotropic component. In an advantageous manner, the gas may be natural gas, ethylene, CO2, air, pure air components or mixed air components, said air components expediently being nitrogen, oxygen, or argon.

According to a favoured embodiment of the present invention, the azeotropic component may be a saturated or unsaturated hydrocarbon or fluorocarbon having a carbon chain length of two; in particular, the azeotropic component may be C2H6, C2F6, C2H4, C2F4, C2H2, or C2F2.

The method and apparatus of the present invention may be used for cooling, liquefaction or processing of natural gas, ethylene, CO2, air, and pure or mixed air components (for example nitrogen, oxygen, argon). Brief description of the drawing

For a more complete understanding of the present inventive embodiment disclosures and as already discussed above, there are several options to embody as well as to improve the teaching of the present invention in an advantageous manner. To this aim, reference may be made to the claims dependent on claim 1 , on claim 8 and on claim 1 1 ; further improvements, features and advantages of the present invention are explained below in more detail with reference to the following description of preferred embodiments by way of non-limiting example and to the appended drawing figures taken in conjunction with the description of the embodiments, of which:

Fig. 1 shows a comparison of the principal effect of adding high and low boiling impurities as well as azeotropic components to the phase behaviour of the key component if the key component is CO2 near its triple point;

Fig. 2 shows a variety of refrigerant phase behaviours based on the addition of different compounds to a pure key component (preferably CO2), including

case 1 : azeotropic mixtures formed by a key component and a single (= case 1 a) or multiple (= case 1 b) azeotropic components;

case 2: zeotropic mixtures with irregular phase behaviour formed by a key component and multiple azeotropic components;

case 3: zeotropic mixtures with irregular phase behaviour formed by a key component and one or more azeotropic components and one or more low boiling impurities (= case 3a), one or more high boiling impurities (= case 3b) and the combination of at least two low and high boiling impurities

(= case 3c);

Fig. 3 shows an exemplary phase diagram for CO2 (key component) and C2H6 (azeotropic component) according to the present invention; and Fig. 4 shows an exemplary process scheme according to the present invention.

In the appended drawing figures, like equipment is labelled with the same reference numerals throughout the description of Fig. 1 to Fig 4.

Detailed description of the drawings; best way of embodying the present invention

Before explaining the present inventive embodiment in detail, it is to be understood that the embodiment is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawing, since the present invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. As shown in Fig. 1 , the addition of different compounds creates different effects on the phase behaviour of carbon dioxide (CO ₂ ) near its triple point. The addition of high boiling impurities increases the boiling temperature above that of pure CO ₂ and therefore does not provide any benefit to the goal of achieving the lowest temperature for cooling. The addition of low boilers results in an intersection of the freezing lines and therefore also fails to provide benefit to the goal of the present invention. However, addition of azeotropic components according to the present invention provides the desired benefits by allowing adaptation of the vapour-liquid envelope to the freezing line.

In accordance with the present invention, the refrigerant mixture should exhibit irregular phase behaviour at low pressure. This is true for all cases shown in Fig. 2 for any key component in addition with at least one azeotropic component and particularly for CO ₂ as key component.

In accordance with a first embodiment of the present invention, the refrigerant mixture consists of a mixture of the key component and one or more azeotropic components as shown in Fig. 2, case 1 a, case 1 b and case 2.

The refrigerant mixture of the present invention may be an azeotropic mixture with a preferably Non azeotropic composition as shown in Fig. 2, case 1 a and case 1 b.

In a further embodiment of the present invention, the refrigerant mixture consists of a mixture containing a key component, at least one azeotropic component and at least one impurity. For these mixtures, the low pressure boiling point temperature of the refrigerant mixture should be below the pure substance boiling point temperatures of at least two of the mixture components at the same pressure as shown in Fig. 2, case 2, case 3a, case 3b and case 3c. For refrigerant mixtures containing impurities according to the present invention, the sum of the impurities included is below 30 mol%, preferably below 15 mol% and more preferably below 5 mol%. These impurities may be present in the refrigerant mixture naturally (for example originating from impure CO ₂ makeup) or may be added deliberately. It is noted that small amounts of impurity may be added to the mixture to improve the phase behaviour of the mixture to a more favorable shape of the phase envelope. The effect of impurities may be, for example,

- the lowering of the freezing point temperature of the refrigerant mixture by formation of an eutectic solid;

- the simultaneous increasing of the refrigerant mixture dew point temperature and decreasing of the refrigerant mixture boiling point temperature to allow evaporation and condensation over a wider temperature range;

- the decreasing of the refrigerant mixture boiling point temperature to allow at a lower evaporation temperature of the refrigerant mixture;

- the increasing of the refrigerant mixture dew point temperature to allow at a lower condensation temperature of the refrigerant mixture;

- the generation of a refrigerant mixture exhibiting a reduced effect of its composition on the temperature difference between dew and boiling point.

According to the present invention, the preferred key component is CO2 and the azeotropic components are at least one saturated or unsaturated HC or FC compound having a carbon chain length of two. For example, C2H6, C2F6, C2H4, C2F4, C2H2, and C2F2 all form azeotropic mixtures in a binary system with CO2 as the key component.

The refrigerant mixtures according to the present invention all exhibit lower freezing and boiling point temperatures than the freezing and boiling point temperature of pure CO2 at the same pressure, as shown for all cases of Fig. 2.

One embodiment of the present invention is to a refrigerant mixture comprising CO2 as the key component and at least C2H6 as the azeotropic component. The CO2 content for this refrigerant mixture is less than or equal to 95 mol% and greater than the azeotropic composition of the binary system C2H6 CO2 at low pressure. Preferred compositions for this refrigerant mixture have CO2 content between 58 mol% and 93 mol%, and more preferably between 80 mol% and 90 mol%.

As can be seen in Fig. 3, the addition of a relatively small amount of C2H6 to CO2 causes a steep decline in the boiling and freezing point compared to that of pure CO2. As noted a preferred range for the CO2 component is from 58 mol% to 93 mol% and more preferable from 80 mol% to 90 mol%. These ranges provide the highest benefit for counter-current heat exchangers in the bottom stage evaporator.

Another embodiment of the present invention is to a refrigerant mixture comprising CO2 as the key component and at least C2H4 as the azeotropic component. The CO2 content for this refrigerant mixture is less than or equal to 95 mol% and equal to or greater than the azeotropic composition of the binary system C2H4/CO2 at low pressure. Preferred compositions for this refrigerant mixture have CO2 content between 50 mol% and 95 mol% and more preferably between 80 mol% and 95 mol%.

Fig. 4 shown an exemplary process scheme for use of the refrigerant mixtures according to the present invention. A warm fluid WF passes through a cascade of heat exchangers, E01 , E02, E03 and E04, and is cooled in the course to a lower temperature and condensed fully or partially. The outlet temperature of the cold fluid CF is below the triple point temperature of pure CO2.

A narrow boiling refrigerant mixture is compressed to a preferably supercritical pressure (for example 75 bar to 120 bar) in a compressor stage C02 and is cooled subsequently in the after cooler E06 and economizer E07. The refrigerant is expanded to a sub-critical pressure (for example 60 bar to 70 bar) using valve V01 and is fed into phase separator D01.

The formed vapour is recycled to the cold-end inlet of E07 and the formed liquid is supplied to top stage, kettle type evaporator E01 where it is partially evaporated. The formed vapour is recycled to the cold-end inlet of E07. The pressure of the remaining liquid is expanded down further (for example 30 bar to 50 bar) and the refrigerant is supplied to a second, kettle-type evaporator where it is again partially evaporated.

The formed vapour is fed into the first compressor stage. The pressure of the remaining liquid is expanded down further (for example 15 bar to 25 bar) and the refrigerant is supplied to a third, kettle-type evaporator where it is again partially evaporated. The formed vapour is fed into the first compressor stage. The pressure of the remaining liquid is expanded down further (for example 5.5 bar to 10 bar) and the refrigerant is evaporated completely in a counter flow to the pre-cooled fluid stream in E04.

The formed vapour is fed into the first compressor stage. The compressor stage C01 compresses the vapour from E02, E03 and E04 to a subcritical pressure (ca. 60 bar to 70 bar) and the refrigerant passes through the intercooler E05 and is unified with the vapour coming from E07 and fed into the suction line of the second compressor stage C02.

As described above, the cascaded pre-cooling consists of a sequence of flooded and counter current refrigerant evaporators wherein the evaporators are operated at different pressure levels. The refrigerant evaporator stage operating at the lowest temperature (the bottom stage) is preferably a counter current heat exchanger.

Since the above described refrigerant mixture consist preferably of an azeotropic mixture in a non-azeotropic composition or a zeotropic mixture exhibiting irregular phase behaviour, the partial evaporation of the refrigerant mixture in the heat exchangers E01 through E03 will cause a shift in refrigerant composition along the process where the low boiling components of the refrigerant mixture are somewhat enriched in the refrigerant mixture towards the heat exchanger E04. Thus the composition of the refrigerant is not the same in the cascaded evaporator stages.

At least one of the evaporator stages may be operated such that the refrigerant mixture is in an azeotropic composition. If the critical temperature of the refrigerant mixture is below the maximum ambient temperature, then the refrigerant mixture is compressed to a pressure above its critical pressure.

If the critical temperature of the refrigerant mixture is above the maximum ambient temperature, then the refrigerant mixture is compressed to a pressure below its critical pressure. One or more rectification columns or phase separators may be used to deliberately adjust the refrigerant composition in the pre-cooling cycle.

According to the present invention, the cooling cycle may be utilized to provide refrigeration for at least one fluid wherein the cooled fluid may experience full or partial condensation in the course of cooling. One or more of the stages that provide refrigeration to the fluid to be cooled may be operated at a pressure such that the refrigerant mixture is supercritical.

The method and apparatus of the present invention may be used for cooling, liquefaction or processing of natural gas, ethylene, CO ₂ , air, and pure or mixed air components (for example nitrogen, oxygen, argon).

The present invention provides a number of advantages. In particular, it provides an inherently safe refrigeration process, especially for gas processing plants in offshore locations. Further, temperatures below -56.6°C can be reached with a CO ₂ rich refrigerant. This provides energy savings for subsequent liquefaction or partial condensation of the cooled fluid stream.

When using CO ₂ as the key component, the refrigerant mixture behaves almost as a pure refrigerant at high pressures and as a mixed fluid at low pressures. This provides the advantage that simple, robust, flooded evaporators can be used for the top stages, while a substantial increase of cycle efficiency is realized by using counter current heat exchange in the bottom stages.

It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description, and it is intended that such embodiments and variations likewise be included within the scope of the present invention as set out in the appended claims.