CO2 subcooling process using a fatal cryogenic liquid

Abstract

A process for supplying subcooled liquid CO2 to a site comprising a user station of this liquid CO2, from a liquid CO2 storage tank, site where there is a source of liquid nitrogen suitable for supplying gaseous nitrogen to a user station of this gaseous nitrogen, according to which: a first cooling of liquid CO2 taken from said CO2 storage is organized by heat exchange in an exchanger with liquid nitrogen taken from said nitrogen storage, and the CO2 thus subcooled to a first temperature is returned to the CO2 storage, the taking and the exchange being carried out when there is a demand for consumption of gaseous nitrogen by said nitrogen user station;the consumption of CO2 and nitrogen by said user stations may be synchronized or not, and in that when they are not synchronized, when there is a demand for consumption by the subcooled CO2 consuming station, liquid CO2 is taken from said CO2 storage and this taken CO2 is subjected to a second cooling, by heat exchange in said exchanger with liquid nitrogen taken from said nitrogen storage, to lower the temperature of the taken liquid CO2 to a second temperature, lower than the first temperature; then the liquid CO2 thus subcooled at this second temperature is directed to the user station of this CO2.;

Description

Title of the invention: Method for subcooling CO2 by means of a fatal cryogenic liquid

[0001] The present invention relates to the field of supplying liquid or solid CO2 to processes using such a fluid.

[0002] Examples include dry ice production plants, which are also traditionally centers for bottling gas in cylinders or cylinder frames. Other examples include industrial users of liquid CO2 or dry ice, for instance, for cooling food products in bottom-feed injection systems (mixers, meat grinders, etc.) or for workstations performing machining operations (machining, cutting, etc.), but these are just a few examples among a very large number of industrial applications.

[0003] In the case of machining, the cryogen is used not only to cool the area but also for a "lubricating" effect on the cutting tools.

[0004] A cryogenic liquid is commonly understood as a fluid which, at atmospheric pressure, is liquid at a temperature much below 0°C.

[0005] A consumer of such a cryogenic liquid (for example, liquid nitrogen), whatever its type, is traditionally supplied with such a cryogenic liquid from a cryogenic fluid reservoir connected to the equipment consuming this fluid, which reservoir contains, under a storage pressure greater than atmospheric pressure, a cryogenic fluid which is in liquid phase at the bottom of the reservoir and in gaseous phase at the top of the reservoir, this reservoir being adapted to supply the consuming equipment with liquid which is drawn from the bottom of the reservoir, and to be supplied from the outside with fluid.

[0006] In industry, tanks most commonly used are those called "low pressure storage tanks", that is to say, those whose maximum pressure reached at the top of the tank is generally less than about 4 barg, but depending on the intended applications, there are also "medium pressure" storage tanks going up to 15 barg or even "high pressure" storage tanks going up to 30 barg.

[0007] Since the storage pressure of the tank is greater than atmospheric pressure, opening a valve placed on the connecting pipe of the tank to the consuming equipment (for example a machining machine or a mixer in the food industry) causes the liquid to move from its point of draw-off to its point of use, without any means of forced drive and despite pressure losses on the line (valves, elbows etc...).

[0008] To ensure that the cryogenic liquid entrainment is always effective regardless of the liquid level in the tank, the gas pressure at the top of the tank is conventionally regulated so that this pressure remains substantially equal to a predetermined, fixed value, for example in the order of 2 to 4 bar.

[0009] However, the liquid pressure at the bottom of the tank varies according to the liquid level inside the tank, so that as the liquid level drops, the pressure of the liquid being drawn off decreases and tends to approach the gas pressure at the top. For example, in the case of nitrogen, a liquid height of approximately 10 meters implies a pressure differential of about 0.6 bar between the gas pressure at the top and the liquid pressure at the bottom of the tank, at the point of withdrawal. In the case of a regulated pressure of 3 barg at the top of the tank, the pressure at the bottom of the tank, i.e., the pressure of the cryogenic liquid in the piping, will vary from 3.6 barg when the tank is full to 6 barg when the tank is empty.

[0010] This variation in liquid pressure at the point of extraction necessarily leads to a variation in the flow rate of liquid drawn, resulting in operational disturbances for the downstream consuming equipment. A symmetrical effect occurs when the reservoir is refilled with fluid.

[0011] For well-known reasons of better "cryogenic quality" in terms of available cooling capacity, the literature and these industries using cryogens have been interested in ways of supplying these user stations with free-running or substantially free-running liquid or with subcooled liquid, that is to say, liquid at reduced pressure, and at a lower temperature than when it was at a higher pressure.

[0012] Indeed, let's consider the example of machining: the higher the spray pressure in the machining zone, the better the heat transfer coefficients. Now, when a cryogen, for example liquid nitrogen, is sprayed, gas is created (due to its expansion) at the spray nozzle outlet. The quantity of gas generated is directly proportional to the temperature of the liquid nitrogen and its pressure upstream of the nozzle. The advantage of using a subcooled liquid is therefore clear.

[0013] Some work has recommended the use of phase separation means (degassing) on ​​the line connecting the tank to the consuming equipment, for example, see document EP-2 347 855.

[0014] Other solutions have proposed coupling two tanks and using them alternately after filling and depressurizing. The disadvantages of this solution are obviously the very significant handling involved and the need to use two tanks.

[0015] Another solution is to insert a heat exchanger (for example, a plate heat exchanger) just upstream of the point of use: liquid nitrogen to be subcooled (typically initially at 3 bar) circulates in one of the exchanger's channels (main circuit). and a temperature close to -185 °C), while in another channel of the heat exchanger circulates depressurized nitrogen, typically at a pressure close to 1 bar and a low temperature, close to -196 °C. It is the exchange between these two channels, in co-current or counter-current flow, that allows the nitrogen in the main circuit to be subcooled. However, temperature control is difficult to achieve and stabilize here, particularly when the downstream consuming equipment operates intermittently, forcing the heat exchanger to go through heating, cooling, and other cycles.

[0016] The cryogen can also be subcooled in an exchanger by the intervention of mechanical cooling; this is even a solution that has now become classic and widespread for subcooling liquid CO2.

[0017] Document WO2004 / 00 5791, issued on behalf of the Applicant, may also be consulted. It recommends varying the gas pressure at the top of the tank according to the tank's operating status (downstream user's consumption phase, standby phase, or cryogenic liquid supply phase), and specifically recommends, in one of its embodiments, venting the tank during standby periods. In other words, when the tank is not being used for withdrawals and is not expected to be for a significant period, for example, several hours (e.g., overnight), a control unit opens a vent valve at the top of the tank. The gas pressure at the top of the tank then drops from a storage value to a value approximately equal to atmospheric pressure (residual pressure of a few hundred grams).Thus, by lowering the nitrogen storage pressure in this way, the cryogenic fluid will equilibrate at atmospheric pressure, meaning it will partially vaporize until it reaches its equilibrium temperature at atmospheric pressure. It will then be colder than when it was under pressure. The fluid stored during these periods of non-use of the tank therefore has a lower temperature than usual, guaranteeing better cryogenic quality in terms of available cooling capacity. And indeed, a rapid repressurization (using, for example, its own atmospheric heater or other means) allows the subcooled liquid to be used.

[0018] However, this solution is not without drawbacks. This venting process inevitably results in losses, and moreover, the paradox of this procedure lies in the need to repressurize in order to use the nitrogen, thus introducing heat. Experience with this solution has notably demonstrated vaporization of 4 to 9% of the stored volume. Since this vaporization is not utilized, the cost directly impacts the user site. In summary, we can deduce two major drawbacks of this venting solution: 1. the use of non-recoverable nitrogen for repressurization. 2. The entry of a hot gas into the storage for depressurization and the creation of a thermal bridge.

[0019] We also considered supplying the user station, for example machining, directly from a medium or high pressure cryogen storage, but we then observe the creation, at the outlet of the spray nozzle, of a large quantity of gas, which reduces heat exchange.

[0020] We can finally consider supplying the downstream user machine from a low pressure storage and through a pump, but we are then aware of the difficulties related to the handling of such pumps, to which is added the impossibility of supplying several machining stations on the same site at different pressures and at low flow rates.

[0021] If we now return to the field of subcooled CO2, we can consider subcooling of CO2 via a drop in pressure: the sudden expansion of liquid CO2 below 5.18 bar (triple point pressure) causes the formation of dry ice and gas at a temperature of -78.5 °C when the pressure is equal to atmospheric pressure.

[0022] The proportion of snow and gas depends on the initial state of the liquid and is given to us by the curves of the mass proportions of gas of the Mollier diagram.

[0023] But as mentioned above, liquid CO2 is currently commonly cooled by the intervention of mechanical cold in an exchanger (exchange with a refrigerant fluid). - Liquid CO2 at -20°C / 20 bar gives (by mass percentage) 47% solid and 53% gas. - Liquid CO2 at +20°C / 58 bar gives (by mass percentage) 29% solid and 71% gas. - when the temperature of liquid CO2 is decreased by one degree to 19 barg, snow production is increased by 0.36%. - to increase snow production by 1%, 5.7 kJ of cold must be supplied per kg of liquid CO2. - and each kg of snow gained costs 570 kJ or 0.16 kWh.

[0024] In summary, the production of solid CO2 from liquid CO2 at 20 bar is characterized by a maximum yield of 47%. This means that 100 kg of liquid CO2 are transformed into 47 kg of usable solid CO2 (dry ice) but also 53 kg of gas. This gaseous CO2 is therefore lost and released into the atmosphere.

[0025] As we will have understood, increasing this yield would allow us to consume less liquid CO2 and therefore reduce the cost of producing dry ice.

[0026] We can then make the following comparison of the observable gains for the subcooling of CO2, using the following assumptions: 145 euros / tonne of liquid CO2, and electricity at 0.1 euros per kWh.

[0027] In normal use: to produce 470 kg of CO2 snow, 1000 kg of liquid CO2 is required, i.e. a cost of 145 Euros.

[0028] For cooling to -30C, a pressure of 19 or 13 barg (20 or 14 bara) (the gain is identical regardless of the pressure): 145.4 Euros for 506 kg of CO2 snow.

[0029] To produce 470 kg of CO2 snow, it therefore takes 145.4 / 506x470 = 135 Euros (7% savings compared to 145 Euros).

[0030] For cooling to -40C, a pressure of 19 or 9 barg (20 or 10 bara) (the gain is identical regardless of the pressure): 146.3 Euros for 540 kg of CO2 snow.

[0031] To produce 470 kg of CO2 snow, it therefore takes 146.3 / 540x470 = 127 Euros (12% savings compared to 145 Euros).

[0032] For cooling to -50C, a pressure of 19 or 6 barg (20 or 7 bara) (the gain is identical regardless of the pressure): 149 Euros for 580 kg of CO2 snow.

[0033] To produce 470 kg of CO2 snow, it therefore takes 149 / 580x470 = 121 Euros (18% savings compared to 145 Euros).

[0034] As will be seen in more detail below, the present invention aims to improve existing processes for subcooling liquid CO2, and therefore for supplying such subcooled CO2 to a user station, with the objective, on the one hand, of obtaining a cost of subcooled liquid CO2 that is no higher than that currently borne by industrial users, or even lower, but also with a reduced carbon footprint.

[0035] For this purpose, it is proposed here to cool the liquid CO2 not with a mechanical cooling system as currently practiced, but in an exchanger implementing a heat exchange between CO2 and liquid nitrogen which can be described as "fatal".

[0036] For this purpose, the exchange takes place with liquid nitrogen which is also present on the site, because another application of this site requires gaseous nitrogen resulting from the vaporization of this liquid nitrogen (for example to produce gaseous nitrogen for the packaging of food products under modified atmosphere).

[0037] Cooling is advantageously carried out in the immediate vicinity of the liquid CO2 storage tank, and in the vicinity of the liquid nitrogen storage tank.

[0038] CO2 cooling advantageously occurs when the consumption of gaseous nitrogen by the application in question is activated (gaseous nitrogen demand), on the other hand, CO2 cooling is not necessarily synchronized with the need for subcooled CO2 and therefore with the need for cooling.

[0039] In other words, the CO2 is cooled by the vaporization of liquid nitrogen advantageously when there is a demand for gaseous nitrogen consumption at the station consuming this nitrogen, then this undercooled CO2, if it is not immediately called up to the downstream need, is stored in the CO2 storage tank, thereby lowering the pressure of the tank.

[0040] When the consumption of subcooled CO2 then begins, the CO2 is drawn from the already subcooled tank.

[0041] In summary then: - According to the invention, it is necessary that there be a consumption of nitrogen gas on the site in order to be able to subcool the CO2. - On the other hand, the invention can be implemented in the following two scenarios (i.e., pause or not in CO2 consumption): • CO2 and nitrogen consumption are synchronized, and therefore we take advantage of this fact to subcool the CO2 extracted from storage before sending it to the CO2-consuming station; • the site is in nitrogen consumption phase but CO2 consumption is stopped (pause): we therefore take advantage of this state of affairs to cool the mass of CO2 present in the CO2 storage (we generate a cold inertia). - The gaseous nitrogen resulting from the heat exchange between liquid CO2 and liquid nitrogen is sent to the site's demand for gaseous nitrogen but first preferentially passes through an atmospheric evaporator type exchanger, to better guarantee that this nitrogen has completely changed state and is not too cold, which would present the risk of weakening the transport pipes or disrupting the final user process. - when the consumption of subcooled CO2 begins in the user station of this CO2 and the customer also consumes nitrogen, the CO2 is drawn from the tank already subcooled (for example in the vicinity of -30°C) and while one could initially be satisfied with this already very cold state, it is proposed according to the present invention to cool this CO2 taken from even lower, this by carrying out a second cooling, with this same cold fatal nitrogen, preferably passing through the same exchanger, and to reach lower temperatures, for example in the vicinity of -50°C.

[0042] And preference is given to continuing to lower the temperature of this CO2 not in the storage tank but in line with the heat exchanger, to avoid, for safety reasons, dropping the temperature, and therefore the pressure, too low in the storage tank itself. Therefore, it prefers to "complete" the temperature drop just before the final use point of CO2. - The implementation of the heat exchanger according to the invention therefore makes it possible on the one hand to subcool the CO2 in the cryogenic tank in a first operation, and to subcool the CO2 towards the use of this CO2, in a second operation, this with different temperatures.

[0043] The advantages of the present solution can be summarized as follows: - This system allows, in a way, the storage of the cold that is usually lost during the vaporization of nitrogen and then its reuse in the form of cold with a greater production of dry ice. - Without using electrical energy, this system produces more cooling by increasing the efficiency of dry ice production. In other words, the energy content of the molecule increases, this CO2 will deliver more cooling and the snow production will be greater. - Furthermore, the carbon footprint of this technical solution is reduced: the carbon footprint is reduced proportionally to the reduction in CO2 consumption. This means that for every tonne of CO2 whose emission is avoided, the carbon footprint is reduced by one tonne. - The cooler can be installed outside the production workshop and thus will not disrupt production.

[0044]

[0045] The present invention relates to a method for supplying subcooled liquid CO2 to a site comprising at least one user station for this liquid CO2, from a liquid CO2 storage tank, the tank containing, under a storage pressure greater than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the tank and in gaseous phase at the top of the tank, said tank being adapted to supply said user station with liquid CO2 drawn from the bottom of the tank, as well as to be supplied with fluid from the outside, characterized in that: - there is a source of liquid nitrogen available on site capable of supplying gaseous nitrogen to a station using this gaseous nitrogen; - when CO2 and nitrogen consumption are not synchronized, when there is nitrogen consumption but no CO2 consumption, a first cooling of liquid CO2 taken from said CO2 storage is carried out by heat exchange in a heat exchanger with liquid nitrogen taken from said nitrogen storage, and the CO2 thus subcooled to a first temperature is returned to the CO2 storage, the withdrawal and the exchange taking place when there is a demand for nitrogen gas consumption by said nitrogen user station and no CO2 consumption at the same time; - when the consumption of CO2 and nitrogen by the said user stations is synchronized, when there is a demand for consumption by the subcooled CO2 consumer station and a demand for gaseous nitrogen, liquid CO2 is taken from said CO2 storage and this taken CO2 is subjected to a second cooling, by heat exchange in said exchanger with liquid nitrogen taken from said nitrogen storage, to lower the temperature of the taken liquid CO2 to a second temperature, lower than the first temperature, then the liquid CO2 thus subcooled at this second temperature is directed to the liquid CO2 user station.

[0046]

[0047] According to one of the embodiments of the invention, the gaseous nitrogen resulting from the heat exchange between liquid CO2 and liquid nitrogen carried out during the first or second cooling operation is, before being directed to the nitrogen user station of the site, sent into an exchanger, for example of the atmospheric evaporator type, in order to better guarantee that this nitrogen has completely changed state and that it is not too cold, which would present the risk of weakening the transport pipes or of disrupting the process of said nitrogen user station.

[0048] According to one embodiment of the invention, a cryogenic pump is used during a cooling operation in the heat exchanger to circulate the liquid CO2 within the exchanger. Indeed, releasing the subcooled CO2 into the gas phase would risk causing the storage pressure to drop too rapidly.

[0049] Let us explain below why the presence of this pump (or “circulator”) is very important for the proper functioning of the invention.

[0050] It can indeed be considered that in the case where there is consumption of nitrogen but no consumption of CO2, in this case, it is necessary to take CO2 (it is therefore necessary to "pump" CO2) from the tank, circulate it in the exchanger and then return it to the tank, it can therefore be considered here that the pump is used as a "circulator".

[0051] We will recall a notion well known to those skilled in the art concerning the functions of pump and circulator.

[0052] “Pump” and “circulator” are two terms often used in a interchangeable, but there are subtle differences between the two, particularly in the area of ​​energy transfer.

[0053] A "pump" draws fluid from a low-pressure point and pushes it to a high-pressure point. It can create a significant pressure difference and transport the fluid over long distances. Pumps are available in a wide variety of types and sizes, each designed for a specific use.

[0054] A "circulator" is a type of pump designed to circulate a fluid in a closed circuit. It does not generally create as much pressure difference as a pump, but it does maintain constant fluid circulation in the system. Circulators are often used in heating and air conditioning systems.

[0055] Therefore, within the framework of the present invention, the presence of such a circulator, while it may appear non-essential when the exchange takes place at the time of the use of CO2 towards its application (2nd exchange), this presence is highly recommended in the Nol exchange returning the subcooled CO2 to the CO2 storage.

[0056] Indeed, in an energy transfer circuit, a circulator plays a crucial role in optimizing the circulation of the fluid to be cooled and ensuring efficient distribution of cooling: - The circulator creates a driving force that propels the fluid (CO2) through the circuit (heat exchanger), overcoming the hydraulic resistance of the pipes, valves, and other components. Without a circulator, the natural circulation of the fluid would be slow and insufficiently efficient, resulting in uneven distribution of cooling and average or even poor system performance. - However, proper circulation allows for a faster and more uniform transfer of cold between the source (the CO2 storage) and the cooling emitters (the cryogenic nitrogen exchanger). This results in better energy efficiency of the subcooling system, thus reducing energy (electrical) consumption and operating costs. - The circulator ensures that the cold is distributed evenly throughout all parts of the circuit. Furthermore, a circulator can be considered highly advantageous for protecting the heat exchanger against damage caused by excessive subcooling. Indeed, insufficient circulation can lead to an accumulation of excessive cold within the exchanger, which can impair its operation and reduce its lifespan. - Furthermore, in modern transfer systems, the circulator can be controlled by a thermostat or other control system to adjust The circulation speed is adjusted according to the cooling requirements. This optimizes the nitrogen needed to reach the target CO2 temperature.

[0057] In summary, the use of a circulator in such an energy transfer circuit is really very advantageous to guarantee efficient circulation of the CO2 fluid, homogeneous distribution of cold, better energy efficiency and protection of equipment.

[0058] The pump (circulator) is therefore, for example, operated by a temperature sensor indicating the temperature of the liquid in the CO2 storage, for example close to -30°C.

[0059] Consider the case where the temperature of the liquid CO2 required by the user station on the site is close to -50°C.

[0060] To avoid excessively low temperatures in the exchanger (below -50°C), a liquid CO2 flow sensor allows the exchanger to be put into service.

Claims

1.

2. Demands A method for supplying subcooled liquid CO2 to a site comprising at least one user station of this liquid CO2, from a liquid CO2 storage tank, the tank containing, under a storage pressure greater than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the tank and in gaseous phase at the top of the tank, said tank being adapted to supply said station with liquid drawn from the bottom of the tank, as well as to be supplied with fluid from the outside, characterized in that: - there is available within said site a source of liquid nitrogen suitable for supplying gaseous nitrogen to a station using this gaseous nitrogen; - a first cooling of liquid CO2 taken from said CO2 storage is organized by heat exchange in an exchanger with liquid nitrogen taken from said nitrogen storage, and the CO2 thus subcooled to a first temperature is returned to the CO2 storage, the taking and the exchange being carried out when there is a demand for consumption of gaseous nitrogen by said nitrogen user station; - the consumption of CO2 and nitrogen by said user stations may be synchronized or not, and in that when they are not synchronized, when there is a demand for consumption by the subcooled CO2 consuming station, liquid CO2 is taken from said CO2 storage and this taken CO2 is subjected to a second cooling, by heat exchange in said exchanger with liquid nitrogen taken from said nitrogen storage, to lower the temperature of the taken liquid CO2 to a second temperature, lower than the first temperature; - then the liquid CO2, thus subcooled to this second temperature, is directed towards the user station of this CO2. A method according to claim 1, characterized in that the gaseous nitrogen resulting from the heat exchange between liquid CO2 and Liquid nitrogen produced during the first or second cooling operation is, before being directed to the site's gaseous nitrogen user station, sent into an exchanger, for example of the atmospheric evaporator type, in order to better guarantee that this nitrogen has completely changed state and that it is not too cold, which would present the risk of weakening the transport pipes or even disrupting the process of said nitrogen user station.

3. A method according to either of claims 1 or 2, characterized in that during one or each of said cooling operations in said exchanger a circulator-type cryogenic pump is used to circulate the liquid CO2 in the exchanger and, if necessary, return it to the CO2 storage.