Cryogenic extraction of NOX by freezing

Abstract

Disclosed is a method for the gradual cooling of gas mixtures, in particular fumes, in the presence of oxygen or ozone, to progressively lower temperatures up to – 170° C, thereby allowing both the gradual transformation of nitrogen monoxide NO into nitrogen dioxide NO2 and the direct passage of nitrogen dioxide NO2 from the gas phase to the solid phase on cold-generating exchange surfaces.

Description

[0001] Cryogenic extraction of NOx by icing

[0002] Technical field

[0003] The invention relates to the extraction of NOx by cryogenic icing of nitrogen oxides NOx and more particularly NO2.

[0004] State of the art

[0005] The emission into the atmosphere of nitrogen monoxide NO, nitrogen dioxide NO2, nitrous oxide N2O for the three main NO X is mainly associated with combustion and fertilizer spreading.

[0006] Limiting NOx emissions is subject to numerous regulations in most countries around the world. These regulations aim to preserve air quality and limit acid rain. These regulations have led to the development of numerous NOx reduction technologies.

[0007] The dominant technology for flue gas denitrification is the so-called SCR (Selective Catalytic Reduction) technology. As its name suggests, this selective reduction is carried out on a catalyst, at temperatures of around 220°C to 350°C.

[0008] In document CN 1 05629738 (Innor Mongolia Ruite Optimization Science and Technology, 2016), SCR technology is combined with ammonia injection.

[0009] The major difficulty faced by this technology is the poisoning of the catalyst, particularly by sulfur, which is also very often present in the fumes.

[0010] To avoid the cost of a catalyst, an alternative method called SNCR (Selective Non-Catalytic Reduction) was developed, with direct injection of urea or ammonia at high temperature, as described in US 4,208,386 (EPRI, 1980).

[0011] Document CN 1 0258496 (Hangzhou Tianlan Environmental Prot Equipment Co Ltd, 201 2) describes a multi-stage oxidation and absorption, with the use of hydrogen peroxide as the oxygenation medium.

[0012] Document US 2021 / 0220772 (Huaneng Clean Energy Research Institute, 2021) describes the adsorption on activated carbon or molecular sieve at a temperature below ambient temperature of pressurized flue gases.

[0013] Objects of the invention

[0014] The invention aims in particular to provide a method and a device providing a significant improvement over the state of the art, by extracting NOx and mainly NO2 by a purely physical process of direct passage from the gas phase to the solid phase, this type of icing which does not pass through the liquid phase being called antisublimation or desublimation or deposition according to the authors.

[0015] General presentation of the invention

[0016] For these purposes, there is proposed, according to a first aspect, a method for progressively cooling a gas mixture, such as flue gases, for the denitrification of the gas mixture, in the presence of oxygen, or ozone, at progressively lower temperatures, from ambient temperature to - 170°C, allowing both the progressive transformation of nitrogen monoxide NO into nitrogen dioxide NO2 and the direct passage of nitrogen dioxide NO2 from the gas phase to the solid phase on exchange surfaces generating cold, then the recovery of nitrogen dioxide NO2 in the liquid phase by defrosting or in the gas phase by sublimation. Advantageously, in the method, the oxygen is present in a proportion of one mole for two moles of nitrogen oxide NO.

[0017] Advantageously, in the process, ozone is present in a proportion of one mole to one mole of nitrogen oxide NO.

[0018] Advantageously, the gas mixture is cooled and denitrified at atmospheric pressure, to limit the energy consumption associated with pressurizing the gas mixture.

[0019] In some implementations, defrosting is accomplished by melting NO2 at a pressure just above its triple point pressure and at a temperature above its triple point.

[0020] In some implementations, defrosting is achieved by sublimation of NO2 at a pressure lower than its triple point pressure and at a temperature that promotes cold energy recovery. A process for extracting NO is proposed. Xin a gas mixture, such as fumes, advantageously in the presence of oxygen and / or ozone, the method comprising at least three stages of treatment of the gas mixture by progressive cooling, each of the three treatment stages being carried out by an exchanger of a pair of exchangers, a transfer fluid circulating in each exchanger, each exchanger of a pair of exchangers being in frosting mode for cooling the gas mixture while the other exchanger is in defrosting mode of the ice formed during a cooling stage carried out previously, the transfer fluid being a coolant in the exchangers in frosting mode, the transfer fluid being a heat transfer fluid in the exchangers in defrosting mode, the transfer fluid recovering the energy of sublimation and fusion of the ice obtained in the third stage of cooling and denitrification.

[0021] In some implementations, the method comprises controlling the molar concentration of oxygen in the gas mixture, and advantageously maintaining the oxygen concentration at a value of one mole for two moles of nitrogen monoxide.

[0022] In some implementations, the method comprises controlling the molar concentration of ozone in the gas mixture, and advantageously maintaining the ozone concentration at a value of one mole for one mole of nitrogen monoxide.

[0023] In some implementations, the gas mixture to be treated undergoes initial cooling before entering the first pair of exchangers.

[0024] In particular implementations, the first cooling brings the gas mixture to a temperature of around 4°C.

[0025] In particular implementations, the first cooling is carried out by a counter-current water washer. Advantageously, the pH of the water is regulated, preferably around 7.

[0026] In particular implementations, the volume concentration of water in the gas mixture after initial cooling is of the order of 0.8%.

[0027] In certain implementations, the cooled gas mixture leaving an exchanger of the first pair of exchangers is at a temperature of the order of -40°C. Advantageously, the water concentration in the cooled gas mixture leaving an exchanger of the first pair of exchangers is of the order of 160 ppm.

[0028] In certain implementations, the cooled gas mixture leaving an exchanger of a first pair of exchangers enters an exchanger of a second pair of exchangers, to be cooled there to a temperature of the order of -90°C.

[0029] Advantageously, the water concentration in the cooled gas mixture leaving an exchanger of the second pair of exchangers is less than 0.1 ppm(v).

[0030] In certain implementations, the cooled gas mixture leaving an exchanger of the second pair of exchangers enters an exchanger of a third pair of exchangers, to be cooled there to a temperature between approximately -140°C and -170°C.

[0031] A device is also provided for implementing a method as presented above.

[0032] The advantages of the method can be presented as follows.

[0033] In the presence of oxygen, nitric oxide NO is transformed into NO2, with an acceleration of this transformation in the presence of ozone (O3). On the other hand, the reaction is reversible and exothermic and the heat of reaction is ArH=- 1 13.8 kJ / kmol (N IST Refprop Database).

[0034] In the presence of ozone, the oxidation of nitrogen monoxide is written as this reaction being exothermic.

[0035] Cooling fumes containing mainly nitrogen monoxide NO, in the presence of oxygen, promotes the transformation of NO into NO2 by extracting this heat of reaction.

[0036] The process according to the invention uses the progressive cooling of the fumes in the presence of oxygen, to accelerate the formation of NO2 and to frost this NO2 on the exchange surfaces generating the cooling power.

[0037] The table below shows the important consequence of the formation of NO2 from NO in terms of antisublimation temperature. Denitrification by frosting will take place at high temperature, and therefore with greater energy efficiency, especially since the concentrations of NO in the flue gases are generally much higher than those of NO2.

[0038] The triple point temperatures of NO2 and N2O4 are identical (at -11.2°C) and much higher than that of NO (at -162.7°C). The antisublimation of NO2 and N2O4 will occur identically at much higher temperatures than that of NO.

[0039] The 2 NO2 transformation N2O4 is mainly produced at low temperatures. In the remainder of this description, the frost of these two substances NO2 and N2O4 will be called NO2 frost, designating the mixture of these two nitrogen oxides.

[0040] The triple point pressures of the three substances NO, NO2 N2O4 are around 20,000 Pa.

[0041] Typical partial pressures of NO and NO2 in flue gases are in the order of 300 ppm to 600 ppm, or partial pressures of 3000 Pa to 6000 Pa for flue gases at atmospheric pressure.

[0042] These partial pressures are therefore much lower than the triple point pressures of these substances, consequently the antisublimation temperature will be lower than at the triple bridge pressure.

[0043] Vapor-solid equilibria of NO and NO2 below their triple point pressures are not found in scientific publications.

[0044] It is therefore essential to establish a first empirical law for the transformation of NO into NO2 as a function of temperature and in the presence of oxygen, as well as the evolution of the anti-sublimation temperature of NO2 as a function of temperature. The two phenomena are coupled during cooling. The applicant carried out an experimental study on a test loop comprising exchangers at several temperature levels and where concentrations of NO and NO2 were injected into equivalent flue gases with oxygen contents greater than 3% and a water vapor content of 6% at 25°C.

[0045] To study the transformation of NO into NO2, the volume concentrations of NO were set at values ​​6 times higher than the concentrations of NO2, which is typical of concentrations measured in fumes.

[0046] The equivalent flue gas temperatures are measured at the inlets and outlets of the exchangers, as well as the concentrations of oxygen, NO and NO2.

[0047] The presented measurements show the coupling between the decrease in oxygen and the decrease in NO concentrations upon cooling of the gas mixture, which indicates the transformation of NO into NO2.

[0048] At the same time, the measurements show the decrease in NO2 as a function of lowering temperatures, which indicates the icing of NO2 on the progressively colder exchange surfaces.

[0049] The results obtained are presented in relative values ​​in the following table.

[0050] This table shows on the NO column the transformation of 30% of NO into NO2 between 2°C and -40°C, then the very marked decrease from 70% to 19% between -40°C and -90°C, and finally the almost complete transformation at -170°C of NO into NO2.

[0051] We note the oxygen consumption associated with the transformation of NO into NO2. At the same time, 22% of the NO2 is condensed with water at 2°C, then the 50% reduction of NO2 which includes the 30% of NO which has been transformed into NO2.

[0052] The NO2 frozen at -40°C comes from two sources: injected NO2 and NO transformed into NO2, the initial concentrations of NO being 6 times higher, the total denitrification is at this temperature of -40°C is 30% of the initial NO concentration and 50% of the initial NO2 content.

[0053] At -90°C, the reduction of NO2 is 70% including NO2 from the transformation of NO and finally an almost complete elimination at -170°C of both NO2 and NO.

[0054] These results, presented graphically in Figure 1, materialize the two coupled effects of cooling: on the one hand the transformation of NO into NO2 in the presence of oxygen, and on the other hand the icing of NO2 as the temperatures fall to -170°C.

[0055] Other objects and advantages of the invention will appear during the description of embodiments, a description which will be carried out with reference to the appended figures, in which:

[0056] - Figure 1 shows the two curves of decrease in the concentrations of NO and NO2 as a function of temperature and in the presence of oxygen;

[0057] - Figure 2 represents an example of a cryogenic denitrification device for fumes, the exchanger pairs being omitted for clarity;

[0058] - Figure 3 is a representation of a set of pairs of exchangers, in one embodiment, corresponding to the white rectangle of Figure 2.

[0059] Figure 1 associated with the previous table calls for the following comments.

[0060] Cooling in the presence of oxygen from 25°C to -170°C shows that the transformation of NO into NO2 only initiates in the presence of water at a temperature below 0°C.

[0061] At -40°C, the NO concentration is still higher in absolute and relative value than that of NO2, but at -90°C in relative value, the NO concentration is lower than that of NO2, showing the effectiveness of low temperature cooling in the presence of oxygen by the transformation of NO to NO2 and the associated immediate icing of NO2.

[0062] Finally, at -140°C, the residual contents are respectively less than 5% and 2%, which constitutes a remarkable denitrification, and at -170°C the concentrations are at most 1% of the initial concentrations. The temperature level defines the NOx outlet concentration.

[0063] It is concluded from these measurements that if the oxygen content is insufficient, the injection of gaseous oxygen, ozone or air will allow an efficient transformation of NO into NO2 during cooling.

[0064] Note that ozone O3 has a higher oxygenation power and therefore the transformation of NO into NO2 by ozone injection will be faster and at a higher temperature.

[0065] Figure 2 shows an example of a device 1 for cryogenic denitrification by progressive cooling of a gas mixture, here fumes 10.

[0066] In the embodiment shown, the device 1 comprises three pairs of exchangers 301-302, 303-304, 305-306, shown in Figure 3. Each exchanger 301-306 is integrated into an enclosure connected to an inlet pipe for the fumes to be treated and an outlet pipe for the treated fumes, a valve being placed on the inlet pipe and a valve being placed on the outlet pipe, according to the following references:

[0067] In the remainder of this description, the input and output pipes of the enclosures are called branches.

[0068] The exchangers of each pair are cooled alternately: for example when the valve 133 of the branch 131 is open for the enclosure 1300 of the exchanger 301, the valve 132 of the branch 134 is closed for the enclosure 1310 of the exchanger 302 and the fumes to be treated are cooled in the exchanger 301 which is in frosting mode, the valve 137 of the branch 135 being open for the outlet of the cooled fumes, the valve 138 of the branch 136 is closed, the valve 132 of the branch 134 being closed, the exchanger 302 being in defrosting mode.

[0069] The exchangers of each pair are thus defrosted alternately: for example when the exchanger 302 is in frosting mode, the exchanger 301 is in defrosting mode, and the valves 133, 137 are closed, the valve 65 being open for the evacuation of the liquid contained in the enclosure 1300 of the exchanger 301.

[0070] The flow of fumes 10 to be treated is sucked into the device 1, for example by a centrifugal fan 11.

[0071] Advantageously, a gas analyzer 101 measures the concentrations of NOx: NO, NO2, N2O, as well as the oxygen concentration of the fumes 10 to be treated.

[0072] If the oxygen content is insufficient for the transformation of NO into NO2 during cooling, then an automaton 100 receiving the data from the analyzer 101 opens in a regulated manner a valve 51 arranged on a pipe 50, so that a flow of oxygen, ozone or air is introduced into the flow of fumes 10 to be treated, in order to achieve a targeted molar concentration of oxygen or ozone.

[0073] An oxygen content is insufficient when it is below a target molar concentration. An ozone content is insufficient when it is below a target molar concentration.

[0074] Advantageously, the targeted molar concentration of oxygen is one mole for two moles of nitrogen monoxide NO.

[0075] Advantageously, the targeted molar concentration of ozone is one mole for one mole of nitrogen monoxide NO.

[0076] The fumes 10 to be treated are at variable temperatures, for example of the order of 160°C, and contain water vapor.

[0077] The fumes 10 to be treated will advantageously undergo a first cooling, in a washer 12, subsequently called scrubber 12.

[0078] This cooling of the fumes 10 is advantageously carried out on a lining 121 against the current of a flow of cold water coming from a water shower 122.

[0079] This shower 1 22 is supplied by a recirculation pump 1 23. This recirculated water is advantageously cooled by an exchanger 1 25 supplied with iced water by a branch 223, the heated water leaving by a branch 224.

[0080] This cooling of the fumes 1 0 to advantageously around 4°C causes the water vapor in the fumes 1 0 to condense.

[0081] This condensation is acidic, since approximately 20% of the NO2 will be absorbed by the water, thus forming nitric acid.

[0082] In the embodiment shown, the pH of the water is controlled by an electronic pH meter 102, to keep the pH around 7.

[0083] The signal delivered by the pH meter 102 is advantageously sent to the automaton 100, which will both evacuate the acid solution, by opening a valve 61 arranged on a discharge pipe 60 and bring in the same quantity of water at neutral pH by opening a valve 127 arranged on a water pipe 126.

[0084] The quantitative balance between the incoming water and the outgoing acid solution is advantageously ensured by the indications of an electronic level sensor 1 03 which measures the water level in the hold 1 24.

[0085] In the embodiment shown, the level sensor 103 sends its data to the automaton 100, and the automaton 100 regulates an almost constant water level by the regulated openings of the valves 61 and 127, to keep a constant level and at a pH close to 7.

[0086] The acid solution is recovered by a collector 601, a pump 600 sending the acid solution for further treatment.

[0087] The volume concentration of water in the fumes leaving the scrubber 1 2 is preferably 0.8%.

[0088] These fumes return to a common trunk (referenced 10 in figure 2) and will be alternately cooled counter-currently on exchangers 301 or 302, preferably of the finned tube type, installed respectively in enclosures 1300 and 1310.

[0089] In the embodiment shown, the refrigerating power is provided by a refrigerant circulated by a pump 320 on a circuit 300.

[0090] In the embodiment shown, the refrigerant cools three pairs of refrigerant exchangers 305-306, 303-304, 301-302, preferably of the finned tube type. Advantageously, the cooling of the exchangers 301-306 is carried out from the coldest to the hottest, i.e. in order and alternately in pairs: 305 or 306; 303 or 304, 301 or 302.

[0091] The refrigerant recovers cold from the denitrified fumes by an exchanger 307 integrated in an enclosure 160, before returning to be cooled in a very low temperature refrigeration system 200.

[0092] The refrigerant vapor of the system 200 enters through a pipe 201 into a condenser 220 and returns in liquid phase through a pipe 202 to the system 200.

[0093] The condenser 220 is cooled by the iced water coming through a branch 221 and leaving reheated by a branch 222 of this condenser 220. The exchanger 301, integrated in the enclosure 1300, is in cooling and frosting mode, the fumes passing through the branch 131 whose valve 133 is open, while on the branch 134 the valve 132 is closed.

[0094] The cooled, partially dehumidified and denitrified fumes exit the enclosure 1300 via branch 135, the valve 137 of which is open, while the valve 138 of the outlet branch 136 of the enclosure 1310 is closed.

[0095] These exchangers 301, 302, advantageously of the finned tube type, are preferably alternately cooled to -43°C to cool the fumes to -40°C.

[0096] The water vapor and NO2 are frosted, advantageously on the fins. Advantageously, the water concentration will be reduced from a value of 0.8% at the inlet to a value of 160 ppm at the outlet, and the NO2 concentration will be reduced by 50% compared to the initial NO2 concentration to which must be added the 30% of NO transformed into NO2 and frosted concomitantly.

[0097] When the pressure losses between the inlet and the outlet of the exchanger 301 reach a predetermined threshold, the automaton 100 passes the fumes to the exchanger 302.

[0098] The defrosting of the NO2 ice mixed with the water ice deposited on the exchanger 301 will take place in the enclosure 1300, which is isolated from the flow of fumes by closing the valves 133 and 137 for the enclosure 1300.

[0099] Similarly, the defrosting of the NO2 ice mixed with the water ice deposited on the exchanger 302 will take place in the enclosure 131 0, which is isolated from the flow of fumes by closing the valves 132 and 138 for the enclosure 131 0.

[0100] The frosted exchanger 301 (respectively 302) is heated, advantageously to a temperature of the order of 15°C, the water ice and the NO2 ice melt and the nitric acid solution thus formed is evacuated from the enclosure 1300 via a pipe 64 carrying the open valve 65, (respectively the pipe 62 carrying the open valve 63 for the enclosure 1310).

[0101] The acid solution is sucked towards the collector 601 by the pump 600, for further treatment of this acid solution.

[0102] In order to avoid any corrosion, once defrosting has been carried out, the defrosted exchanger 301 or 302 is advantageously showered with water whose pH is close to 7, by the showers 31 1 and 31 2 to rinse the exchangers 301 and 302 respectively.

[0103] The same applies to exchangers 303, 304, through showers 313 and 314.

[0104] This water is evacuated via the same pipes 62 or 64 for enclosures 1300 and 1310, then collector 601, pump 600 ensuring this extraction.

[0105] The fumes leaving enclosure 1300 join the common trunk (referenced 10 in figure 2), to be cooled and denitrified alternately on exchangers 303 or 304, installed respectively in enclosures 1400 and 1410.

[0106] The exchanger 303 is in cooling mode, the fumes enter through the pipe 141 carrying the valve 143 in the open position, the fumes exit through the pipe 145 carrying the valve 147 open.

[0107] At the same time, the exchanger 304, integrated in the enclosure 141 0, is in defrost mode or in standby, and the valve 144 on the inlet pipe 142 and the valve 148 on the outlet pipe 146 are closed. The temperature of this exchanger 303 is advantageously -93°C, to cool fumes to -90°C, which causes the residual water vapor to freeze, which goes from 160 ppm(v) to less than 0.1 ppm(v) and the NO2 concentration is reduced by more than 75% by freezing, and more than 80% of the NO has transformed into NO2, which is also frozen.

[0108] At this temperature level of -90°C, the process makes it possible to achieve a denitrification efficiency close to 80%, taking into account the six times higher ratio of NO to NO2. For the defrosting of water ice and NO2 ice, the process is advantageously identical to that described for the previous exchangers 301, 302.

[0109] Defrosting will be carried out on the enclosure isolated from the flow of fumes by closing the valve 143 on the branch 141 and the valve 147 on the branch 145 for the enclosure 1400, or the closing of the valve 144 on the branch 142 and the valve 148 on the branch 146 for the enclosure 1410. The frosted exchanger 303 (respectively 304) is heated, advantageously to a temperature of the order of 15°C, the water ice and the NO2 ice melt and the nitric acid solution thus formed is evacuated via the pipe 66 carrying the valve 67 in the open position for the enclosure 1410, (respectively the pipe 68 carrying the valve 69 open for the enclosure 1400).

[0110] The acid solution is discharged through the collector 601, due to the operation of the pump 600, for further treatment of the acid solution.

[0111] In order to avoid any corrosion, once defrosting has been carried out, the defrosted exchanger is advantageously showered with water whose pH is close to 7, by showers 313 and 314, to rinse exchangers 303 and 304 respectively.

[0112] This water is discharged via piping 66 for enclosure 1410, or piping 68 for enclosure 1400, as described above.

[0113] The fumes, advantageously cooled to -90°C with a reduced content of 75 to 80% respectively for NO2 and NO, enter the exchanger 305 integrated in the enclosure 1500 which is in cooling and frosting mode.

[0114] The fumes enter via the pipe 151 carrying the valve 153 in the open position, and exit after cryogenic cooling via the pipe 155 carrying the valve 157 in the open position.

[0115] At the same time, the valve 154 on the inlet pipe 152 of the enclosure 306 is closed as well as the valve 158 on the outlet pipe 156 of the enclosure 306.

[0116] The 306 exchanger integrated into a 1510 enclosure is in defrost mode or in standby.

[0117] The cooling temperature of the exchanger 305, respectively 306 is advantageously set between -143°C and -173°C, to cool the fumes from -140°C to -170°C, depending on the desired denitrification efficiency. As indicated previously, at -140°C the NO2 content is reduced by 95% and that of NO by 98%, and at -170°C the NO and NO2 contents are reduced by 99%.

[0118] Injecting a little ozone raises these frosting temperatures for the same denitrification efficiencies.

[0119] Given the absence of water, defrosting can be carried out either in the liquid phase by melting or in the vapor phase by sublimation.

[0120] For liquid phase defrosting, the temperature is advantageously raised to 15°C and the pressure is brought to a value greater than 20,000 Pa.

[0121] The NO2 / NO2O4 mixture is discharged in liquid phase via pipe 72 for enclosure 1500, (respectively pipe 74 for enclosure 1510), valve 73 carried by pipe 72 and valve 75 carried by pipe 74 being open, a pump 70 placed on a collector 71 sucking the liquid NO2 / NO2O4 mixture, for subsequent treatment. It is also possible to defrost the NO2 / NO2O4 mixture by sublimation. In this case, the temperature and pressure are advantageously free variables and it is then possible to recover the NO2 / NO2O4 mixture in gas phase at a temperature advantageously of the order of -100°C, thus limiting the temperature rise of the exchanger during defrosting and thus gaining in energy efficiency.

[0122] In this case, a vacuum pump 80 is put into operation, a valve 77 carried by a pipe 76 being open for the enclosure 1510, (respectively a valve 79 carried by a pipe 78 for the enclosure 1500). The NC^ / NC^C mixture is sucked in gas phase, discharged by the pump 80 into a tank 81, for subsequent treatment, this gas mixture is advantageously extracted via a pipe 82 carrying an open valve 83.

Claims

Claims 1. Process for the progressive cooling of a gas mixture, such as flue gases, for the denitrification of the gas mixture, in the presence of oxygen or ozone, at progressively lower temperatures, from ambient temperature down to -170°C, allowing both the progressive transformation of nitrogen monoxide NO into nitrogen dioxide NO2 and the direct passage of nitrogen dioxide NO2 from the gas phase to the solid phase on exchange surfaces generating cold, then the recovery of nitrogen dioxide NO2 in the liquid phase by defrosting or in the gas phase by sublimation.

2. Method according to claim 1, characterized in that the oxygen is present in the gas mixture at a concentration of one mole for two moles of nitrogen monoxide NO.

3. Method according to claim 1, characterized in that the ozone is present in the gas mixture at a concentration of one mole for one mole of nitrogen monoxide NO.

4. Method according to claim 1, characterized in that the gas mixture is cooled and denitrified at atmospheric pressure.

5. Method according to claim 1, characterized in that the defrosting is carried out by melting the NO2 at a pressure just above the pressure of its triple point and at a temperature above that of its triple point.

6. Method according to claim 1, characterized in that the defrosting is carried out by sublimation of NO2 at a pressure lower than the pressure of its triple point and at a temperature which promotes the recovery of cold energy.

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