AU2010283603A1

AU2010283603A1 - Improved method and system for extracting a substance by means of anti-sublimation and melting

Info

Publication number: AU2010283603A1
Application number: AU2010283603A
Authority: AU
Inventors: Elias Bou Lawz Ksayer; Denis Clodic; Youssef Riachi; Mourad Younes
Original assignee: Association pour la Recherche et le Developpement des Methodes et Processus Industriels
Current assignee: Association pour la Recherche et le Developpement des Methodes et Processus Industriels
Priority date: 2009-08-13
Filing date: 2010-08-09
Publication date: 2012-03-08
Also published as: EP2464436B1; JP2013501609A; CN102481492B; WO2011018577A3; WO2011018577A2; CN102481492A; FR2949072A1; CA2770594A1; FR2949072B1; JP5748755B2; EP2464436A2; US20120174623A1

Abstract

The invention relates to a method and system for extracting a substance contained in a gaseous mixture, the extraction being cyclically carried out in 2N chambers (AA, BB, CC, DD), each being provided with an exchanger (A, B, C, D), N being an integer greater than or equal to three. Each chamber carries out a cycle comprising the following four consecutive steps: an icing step, a defrosting step, a step of recovering the liquid phase and the residual gaseous phase, and a step of reducing the temperature of the exchanger. Further, the sequencing of passing from one step to another in each of the chambers is such that the number of chambers during the icing step, as well as the total number of chambers during the defrosting, recovery, and temperature reduction steps, are both equal to N.

Description

IMPROVED METHOD AND SYSTEM FOR EXTRACTING A SUBSTANCE BY MEANS OF ANTI-SUBLIMATION AND MELTING Background of the invention 5 The invention relates to the field of separating gases and is targeted at optimizing a method of extracting one or more substances included in a gaseous mixture by anti-sublimation then melting. The anti-sublimation process corresponds to the 10 direct passage of one or more gases from the gas phase into the solid phase (the reverse of the sublimation process). More particularly, the invention relates to optimizing the management of the processes of frosting 15 (or anti-sublimation) and defrosting comprising sublimation then melting on a plurality of heat exchangers; implementing said processes requires managing the timing of distinct elementary operations. In order to obtain anti-sublimation of a substance 20 on a heat-exchanger, it is necessary for the substance to be at a partial pressure that is lower than that defined by its triple point, and also for the temperature of the surface of the heat-exchanger to be lower than that of the triple point of the substance. 25 Various gases are capable of undergoing an anti sublimation process; C02 is one example. Document WO 02/060561 in particular describes a freezing method for extracting C02 present in fumes from energy production units, the C02 being frozen then 30 defrosted in succession on two heat-exchangers. However, that device exhibits undesirable effects, such as significant variations in the flow rate of the refrigerant fluids (or coolant and heat-transfer fluids) flowing in the freezing system, as well as undesirable 35 energy losses. Those undesirable effects are magnified when, as is usual, the extraction systems are large in size.

2 As an example, systems for extracting CO 2 from the fumes from electricity production units using coal are very bulky. It should be recalled that a unit producing 800 MW [megawatts] of electricity with a thermal to 5 electrical conversion efficiency of 44% emits approximately 620 t/h [(metric) tonnes/hour] of CO 2 for a fume flow rate of 2 million Nm /h (normal cubic meters per hour]. The flow cross section of the fumes over the CO 2 frosting heat-exchangers can thus be about 1200 m2 10 Extraction systems with dimensions of the same order are found in many industrial processes where gas separation is necessary and where the flow rates lie in the range 100000 m 3 /h [cubic meters per hour] to several million m 3 /h. 15 Thus, it is essential to reduce variations in the flow rates of refrigerant fluids or coolant and heat transfer fluids as far as possible, and also to reduce variations in the flow rates of the gaseous mixtures passing through that type of facility. 20 Subject matter and summary of the invention A main aim of the invention is thus to overcome the problems identified above. More particularly, the present invention proposes a 25 method of extracting a substance included in a gaseous mixture, in an extraction system comprising: - a first supply circuit for supplying a refrigerant fluid for frosting or a coolant fluid; - a second supply circuit for supplying a 30 refrigerant fluid for defrosting or a heat-transfer fluid; - a third supply circuit for supplying a gaseous mixture; and 2N chambers, N being an integer greater than or 35 equal to three; each chamber comprising: 3 a heat-exchanger that can be alternately connected to the first supply circuit via a first pair of valves and to the second supply circuit via a second pair of valves; 5 * a third pair of valves connecting an inlet and an outlet of the chamber to the third supply circuit, the third pair of valves being capable of controlling movement of the gaseous mixture over the heat-exchanger inside the chamber; and 10 e an evacuation valve to evacuate a residual gas phase or a liquid associated with its gas phase that might accumulate in said chamber; the treatment of the gaseous mixture in each of the chambers comprising, in a cyclic manner, the following 15 steps in succession: e a frosting step, carried out at a first heat exchanger temperature at a first chamber pressure, allowing the substance to pass directly from the gaseous state to the solid state, thereby forming a solid deposit 20 of the substance on the heat-exchanger, the first and the third pairs of valves being closed at the end of the frosting step; e a defrosting step, comprising closing the chamber and allowing the solid deposit to pass directly into the 25 gaseous state, then allowing the solid deposit to pass directly into the liquid state when the temperature of the heat-exchanger and the pressure in the chamber higher respectively than the temperature and the pressure of the triple point of the substance, thereby accumulating the 30 liquid phase of the substance in the chamber, the second pair of valves being closed at the end of the defrosting step; * a recovery step, comprising opening the evacuation valve so as both to allow the liquid phase and the 35 residual gas phase that have accumulated in the chamber to be extracted, and also, simultaneously to reduce the 4 pressure in the chamber so that it returns to substantially the first pressure; and * a temperature-reduction step of reducing the temperature of the heat-exchanger, comprising opening the 5 first pair of valves so that the temperature of the heat exchanger returns to substantially the first temperature; the method further comprising a general sequencing step aimed at sequencing the passage from one step to another in each of the chambers such that the number of 10 chambers undergoing the frosting step and also the total number of chambers undergoing the defrosting, recovery, and temperature-reduction steps are both equal to N, wherein, after a first cycle, the following two conditions are satisfied throughout the method: 15 e each of the frosting, defrosting, recovery, and temperature-reduction steps is carried out in at least one of the chambers; and * each of the chambers carries out one of said steps as dictated by the sequencing step. 20 The present invention can be used to extract a substance that is capable of undergoing an anti sublimation process. More generally, the invention is applicable to any substance included in a gaseous mixture and characterized 25 in that its partial pressure and its temperature are respectively lower than the pressure and the temperature of its triple point. As an example, the invention is applicable to: * CO 2 (or water) contained in air, in fumes, or in 30 process gases; " SO 2 contained in fumes or process gases; and " gaseous mercury contained in fumes, etc. On analyzing in detail a frosting and defrosting method such as that described in WO 02/060561, the 35 inventors have succeeded in identifying a cycle comprising four distinct functional steps: 5 e a frosting step, which means that anti-sublimation of one or more substances can be carried out at a first temperature on the external surface of a heat-exchanger; e a defrosting step, which includes a sub-step of 5 sublimation, involving increasing the pressure in a closed volume, followed by a melting sub-step; e a recovery step, comprising recovering CO 2 in the liquid phase and recovering a residual gas phase until it is returned to the initial pressure of the cycle; and 10 finally e a temperature-reduction step of reducing the temperature of the heat-exchanger in order to return it to the first temperature of the cycle. The inventors have established that subdividing an 15 extraction method temporally into four successive functional steps means that the heat-exchange areas employed and the flow rates of the refrigerant fluids associated either with frosting or else with defrosting can be managed more effectively. 20 Furthermore, it has been shown that appropriately selecting the numbers of chambers respectively undertaking each of the four functional steps throughout the method, produces significant advantages. In particular, the invention makes it possible for 25 the three distinct flows to be managed in coordinated and optimized manner, namely: the internal flow of the coolant fluids or refrigerant fluids for frosting; the internal flow of the heat-transfer fluids or the refrigerant fluids for defrosting, these flows passing in 30 alternation through the heat-exchangers in the chambers carrying out the extraction method; and also the external flow of the gases to be separated moving over the external face of the freezing heat-exchangers. More particularly, by means of the invention, the 35 above three flows can be kept constant over time. This time constancy means that the heat-exchangers can be optimized in terms of energy. In addition, the 6 continuity of the flows obtained means that the compositions, in particular the compositions of the mixtures of refrigerant fluids (when they are used), can be optimized. These advantages are explained in 5 particular by the fact that when the flow rates described above vary with time, the temperature differences between the circulating fluids also vary, which generates losses of efficiency in the system. The continuity and constancy of the flow rates 10 employed can also be used to optimize the operation of the compressors: their compression ratios may remain constant because of the constancy of the thermodynamic properties at the inlet to each compressor. A refrigerant fluid (or a mixture of refrigerant 15 fluids) is a fluid that evaporates when it absorbs heat and condenses when it releases heat. In contrast, a coolant fluid only exchanges sensible heat: it heats up when it absorbs heat and it cools down when it releases its heat, solely by means of sensible 20 heat. It should be understood that in the remainder of the description, that for frosting and for defrosting the invention may make use equally well of refrigerant fluids or of coolant and heat-transfer fluids. 25 The inventors have determined the optimized sequencing for the four functional steps of the invention in each of the chambers of an extraction system. More particularly, the inventors have determined the optimized distribution of the number of chambers of an 30 extraction system that need respectively to undertake the steps of frosting, defrosting, recovery, and temperature reduction. As explained above, said optimized sequencing (or optimized distribution) can in particular mean that 35 continuous or near-constant flow rates can be obtained for the refrigerant fluids or for the coolant and heat transfer fluids that flow successively through the heat- 7 exchangers and through the first and second supply circuits up to the compressor intakes. Thus, the invention can be used to significantly reduce the variations in the three distinct types of flow 5 while extracting a substance. In other words, the invention can be used to significantly improve the distribution of the flows employed throughout the method of extracting the desired substance. It should be noted that optimized energy efficiency 10 of the extraction method is only achieved in reality once the flows employed reach a steady state in the system, i.e. after a first cycle has been carried out. It should be noted that the term "gas to be separated" or "gaseous mixture" as used herein means a 15 mixture of gas from which one or more substances are to be extracted by anti-sublimation. It should also be noted that the description pertains to circumstances in which only a single substance is extracted from the gaseous mixture passing 20 through the chambers. However, the skilled person will understand that the present invention is also applicable circumstances in which two or more substances are extracted from the gaseous mixture. The anti-sublimation or frosting process involves 25 the inverse operation to defrosting. However, the function of a heat-exchanger that freezes then defrosts is by its very nature discontinuous. Thus, one aim of the invention is to render the circulation of the three types of refrigerant fluid flows continuous in order to 30 optimize the energy consumption of the functioning of the freezing systems producing said flows. More precisely, the three types of refrigerant fluid flow are employed as follows: o a heat-exchanger carrying out a frosting step has 35 a first flow of a refrigerant fluid for frosting passing through it; 8 e a heat-exchanger carrying out a defrosting step has a flow of refrigerant fluid for defrosting passing through it; e a heat-exchanger carrying out the recovery step 5 does not have any flows of refrigerant fluid passing through it; and * a heat-exchanger carrying out a temperature reduction step has a second flow of refrigerant fluid for frosting passing through it (i.e. a refrigerant fluid for 10 frosting that will reduce the temperature of the chamber), said second flow of refrigerant fluid for frosting generally having settings in terms of pressure that are different from those applied to the first flow of refrigerant fluid for frosting. 15 In order to ensure that these three types of refrigerant flow circulate continuously within the system, it is initially necessary for the steps of the cycle to be carried out on at least three distinct heat exchanger surfaces operating in parallel, each heat 20 exchanger passing or not passing one of the types of refrigerant fluid flow as dictated by the step being carried out (as described above). In addition, it has been shown that the duration of the frosting step is equal to the duration of the other 25 three steps of the method (i.e. the defrosting, recovery, and temperature-reduction steps). Since the steps of recovery and temperature reduction may be shortened and carried out on a single heat-exchanger surface during the defrosting period and since the defrosting step may be 30 equal to half the duration of the frosting step, it has been found that for continuous operation on the surfaces of the heat-exchangers, it is necessary for the number N of heat-exchangers to be not less than 3 (N being an integer). 35 Furthermore, it has been found that when only three heat-exchangers are in use, the second flow of refrigerant fluid for frosting (i.e. that used for 9 reducing the temperature of the heat-exchangers) is discontinuous in the extraction system since it is used only part of the time. When one of the three heat exchangers carries out the recovery and then temperature 5 reduction steps over a given time sequence, the second refrigerant fluid flows only during the temperature reduction step. That is why it is in fact necessary for the system to comprise 2N heat-exchangers so that all three types of refrigerant fluid flow are effectively 10 continuous. Consider, for example, the situation in which the extraction system comprises 6 chambers (i.e. N = 3). In this situation, after a first cycle has been carried out, the following conditions are satisfied throughout the 15 method: " three heat-exchangers carrying out the frosting step; e one heat-exchanger carrying out the defrosting step; 20 * one heat-exchanger carrying out the recovery step; and " one heat-exchanger carrying out the temperature reduction step. Thus, at any time, there are three flows of the 25 first type of refrigerant fluid for frosting, one flow of the second type of refrigerant fluid for frosting (for temperature reduction), and one defrosting flow. Thus, as explained above, the heat-exchangers of the extraction system of the invention carry out the steps of 30 the method in accordance with the cycle described above. In each predetermined time sequence (10 or 15 minutes, for example), the valves of the system of the invention may change configuration so that the chambers move to a new step in the cycle. Despite the cyclic nature of the 35 method, the flows described above remain continuous when the extraction system is viewed as a whole.

10 In a particular implementation, the frosting step also includes a sub-step of evacuating the chamber after closing the first and third pairs of valves. Carrying out this evacuating sub-step advantageously 5 means that undesirable residual gases present in the chamber can be evacuated at the end of the frosting step. Thus, at the end of the defrosting step, a chamber may contain a very pure gas, i.e. a gas with a very high concentration of the substance to be extracted. In this 10 manner, during the recovery step it is possible to recover an extracted substance of very high purity, both in the liquid form and in the form of residual gases. Recovering a very high purity residual gas is advantageous since the gas may be of significant 15 commercial value. As an example, carrying out this evacuation sub-step in the extraction method of the invention may be used to extract CO 2 in a gaseous form of very high purity; this is highly attractive commercially. Further, it has been recognized that after a first 20 cycle has been carried out, the number of chambers carrying out the defrosting step is equal to N', N' being an integer that depends on the value of N. More particularly, N' is such that the ratio N'/N is equal to a value in the range one-third to one-half, said 25 value being a function of the value of N. As an example, it can be shown that the ratio N'/N is equal to the minimum value 1/3 when N = 3. Similarly, it has been shown, for example, that the ratio N'/N is equal to the maximum value M when N = 4. 30 In a particular implementation, the method of the invention is such that after a first cycle, the number of chambers carrying out the defrosting step is equal to N/2 if N is equal to a power of two. In another implementation, the method of the 35 invention is such that after a first cycle, the number of chambers carrying out the defrosting step is equal to (N/2)-1 if N is even and is not equal to a power of two.

11 In another implementation, the method of the invention is such that after a first cycle, the number of chambers carrying out the defrosting step is equal to (N-1)/2 if N is odd. 5 It should be noted that whatever the selected number of chambers carrying out the defrosting step, the number of chambers carrying out the recovery step and the number of chambers carrying out the temperature-reduction step are, by definition, always selected so as to complement 10 the number of chambers carrying out the defrosting step, i.e. so as to ensure the number of chambers carrying out the frosting step remains equal to the total number of chambers carrying out the defrosting, recovery, and temperature-reduction steps. 15 It should also be noted that when the integer N is equal to a power of 2 (N thus being 4 or more), the ratio N'/N is then equal to M and it is possible to obtain a constructal distribution of the three types of flow employed, i.e. dividing them by two. To this end, it 20 should be recalled that the term "constructal" has its origin in the theory by Adrian Bejan. This theory states that the optimized architecture for access to flows in a plurality of branches is obtained when access to the flows of each branch is maximized. Such maximization is 25 possible when each branch is divided by two, since this guarantees the same path length irrespective of the itinerary adopted by each of the flows under consideration. The constructal distribution is all the more 30 advantageous when, as recalled above, the general dimensions of the extraction systems are very large. It should also be noted that when N is equal to a power of 2, it is not essential for the number of heat exchangers carrying out the recovery step and the number 35 of heat-exchangers carrying out the temperature-reduction step to be equal to a power of 2. When certain constraints are imposed, it is possible to select a 12 different distribution for the recovery and temperature reduction steps among the chambers of the extraction system, provided that the total number of chambers carrying out the defrosting, recovery, and temperature 5 reduction steps nevertheless remains equal to N. In a particular implementation of the invention, the duration of the frosting step is substantially equal to the cumulative duration of said defrosting, recovery, and temperature-reduction steps. 10 It is also possible to carry out a preliminary step of determining the duration of the frosting step, this being determined from a measurement of a maximum pressure drop between the inlet and outlet of the chamber during a frosting step. 15 More particularly, the duration of a frosting step of a given extraction system may be determined by experiment by measuring the difference in the pressure of the gaseous mixture between the inlet and the outlet from one chamber, during a frosting step of the extraction 20 method. This pressure drop is explained by the gradual growth of frost on the external surfaces of the heat exchangers during frosting and by the gradual obstruction of the gas lines. 25 A typical value for the pressure drop is in the range +50 Pa [Pascal] to +500 Pa, this threshold constituting the signal for closing, via two valves (or two flaps), the inlet and the outlet of the chamber that has completed its frosting step. 30 In a particular implementation, the various steps of the extraction method are determined by instructions from computer programs. As a consequence, the invention also provides a computer program on an information medium, said program 35 being capable of being used in an extraction system or, more generally, in a computer, said program including 13 instructions adapted to carrying out the steps of an extraction method as described above. Said program may use any programming language and be in the form of source code, object code, or a code 5 intermediate between source code and object code, for example in a partially compiled form, or in any other desirable form. The invention also provides an information medium that can be read by a computer, and including 10 instructions for a computer program as mentioned above. The information medium may be any entity or device that is capable of storing the program. As an example, the medium may comprise storage means, such as a read only memory (ROM), for example a compact disk (CD) ROM, 15 or a microelectronic circuit ROM, or magnetic recording means, for example a floppy disk or a hard disk. Furthermore, the information medium may be a transmissible medium such as an electrical or optical signal that may be routed via an electrical or optical 20 cable, by radio or by other means. The program of the invention may in particular be downloaded from an internet type network. Alternatively, the information medium may be an integrated circuit into which the program has been 25 incorporated, the circuit being adapted to execute or to be used in the execution of the method in question. The present invention also provides a system for extracting a substance included in a gaseous mixture, the extraction system comprising: 30 - a first supply circuit for supplying a refrigerant fluid for frosting or a coolant fluid; - a second supply circuit for supplying a refrigerant fluid for defrosting or a heat-transfer fluid; 35 - a third supply circuit for supplying a gaseous mixture; and 14 . 2N chambers, N being an integer greater than or equal to three; each chamber comprising: * a heat-exchanger that can be connected both to the 5 first supply circuit via a first pair of valves and also to the second supply circuit via a second pair of valves; e a third pair of valves connecting an inlet and an outlet of the chamber to the third supply circuit, the third pair of valves being capable of controlling 10 movement of the gaseous mixture over the heat-exchanger inside the chamber; and o an evacuation valve to evacuate a residual gas phase or a liquid associated with its gas phase that might accumulate in said chamber; 15 the extraction system further comprising control means to control the opening and closing of each of the valves in order to carry out an extraction method comprising, in a cyclic manner, the following steps in succession: 20 * a frosting step, carried out at a first heat exchanger temperature at a first chamber pressure, allowing the substance to pass directly from the gaseous state to the solid state, thereby forming a solid deposit of the substance on the heat-exchanger, the first and the 25 third pairs of valves being closed at the end of the frosting step; * a defrosting step, comprising closing the chamber and allowing the solid deposit to pass directly into the gaseous state then allowing the solid deposit to pass 30 directly into the liquid state when the temperature of the heat-exchanger and the pressure in the chamber become respectively higher than the temperature and the pressure of the triple point of the substance, thereby accumulating the liquid phase of the substance in the 35 chamber, the second pair of valves being closed at the end of the defrosting step; 15 e a recovery step, comprising opening the evacuation valve so as both to allow the liquid phase and the residual gas phase that have accumulated in the chamber to be extracted, and also, simultaneously to reduce the 5 pressure in the chamber so that it returns to substantially the first pressure; and e a temperature-reduction step of reducing the temperature of the heat-exchanger, comprising opening the first pair of valves so that the temperature of the heat 10 exchanger returns to substantially the first temperature; the control means being configured such that each heat-exchanger is connected alternately to the first and second supply circuits; the control means also being configured to sequence 15 passage from one step to another in each of the chambers such that the number of chambers carrying out the frosting step and also the total number of chambers carrying out the defrosting, recovery, and temperature reduction steps are both equal to N; and 20 wherein the control means are also configured such that, after a first cycle, the following two conditions are satisfied throughout the method: * each of the steps of frosting, defrosting, recovery and temperature reduction is carried out in at 25 least one of the chambers; and e each of the chambers carries out one of said steps as dictated by the sequencing step. The extraction system of the invention and the particular embodiments below have the same advantages as 30 those demonstrated relative to the extraction method and its particular implementations. In a particular embodiment, the control means are also configured such that the frosting step includes a sub-step of placing the chamber under vacuum after said 35 closure of the first and third pairs of valves. Further, and in the same manner as for the extraction method, it has been recognized that once a 16 first cycle has been carried out, the number of chambers carrying out the defrosting step in the extraction system is equal to N', where N' is an integer that depends on the value of N. 5 More particularly, N' is such that the ratio N'/N is equal to a value in the range one-third to one-half, said value being a function of the value of N. It has been shown, for example, that the ratio N'/N is equal to the minimum value 1/3 when N = 3. Similarly, 10 it has been shown, for example, that the ratio N'/N is equal to the maximum value M when N = 4. In a particular embodiment, the extraction system of the invention is such that after a first cycle, the number of chambers carrying out the defrosting step is 15 equal to N/2 if N is equal to a power of two. In another embodiment, the extraction system of the invention is such that after a first cycle, the number of chambers carrying out the defrosting step is equal to (N/2)-1 if N is even and is not equal to a power of two. 20 In another embodiment, the extraction system of the invention is such that after a first cycle, the number of chambers carrying out the defrosting step is equal to (N-1)/2 if N is odd. As with the extraction method, the number of 25 chambers carrying out the recover step and the number of chambers carrying out the temperature-reduction step are always selected so as to complement the number of chambers carrying out the defrosting step, i.e. so as to conserve the equality between the number of chambers 30 carrying out the frosting step and the total number of chambers carrying out the defrosting, recovery, and temperature-reduction steps. When the integer N is equal to a power of 2 (N then being 4 or more), the ratio N'/N is equal to % and a 35 constructal distribution of the three types of flow employed may be obtained, i.e. they may be divided by two.

17 In the particular circumstances explained above, it is not essential to adopt a constructal distribution of the steps over the series of the chambers of the extraction system. In particular, the choice depends on 5 the various constraints imposed on the extraction system. However, it is vital that the total number of chambers carrying out the defrosting, recovery, and temperature reduction steps remains equal to N. In a particular embodiment of the invention, the 10 duration of the frosting step is substantially equal to the cumulative duration of said defrosting, recovery, and temperature-reduction steps. Further, the extraction system may include measurement means to carry out the preliminary step of 15 determining the duration of the frosting step, the duration of the frosting step being determined from a measurement of a maximum pressure drop between the inlet and the outlet of the chamber during a frosting step. 20 Brief description of the drawings Further characteristics and advantages of the present invention become apparent from the description below, made with reference to the accompanying drawings, which illustrate an embodiment that is not limiting in 25 any way. In the figures: * Figure 1 represents an example of an extraction system of the invention, in which N = 4; o Figure 2 represents the principal steps in an example of the extraction method of the invention applied 30 to the chamber AA of the extraction system of Figure 1; * Figure 3 represents, in the form of a table, an example of the sequencing of the functional steps of the extraction method of the invention, in which N = 4; e Figure 4 represents, in the form of a table, an 35 example of the sequencing of the functional steps of the extraction method of the invention, in which N = 6; and 18 e Figure 5 represents, in the form of a table, an example of the sequencing of the functional steps of the extraction method of the invention, in which N = 3. 5 Detailed description of the invention The present invention provides a system for extracting a substance present in a mixture of process gas or fumes, by bringing about anti-sublimation of that substance on the solid surface of a plurality of freezing 10 heat-exchangers. To this end, anti-sublimation of the substance to be extracted is carried out at a temperature and at a pressure lower than that defined by the triple point of the substance under consideration. 15 The extraction system of the invention comprises 2N heat-exchangers, N being an integer greater than or equal to three. Figure 1 represents an example of an extraction system of the invention. In this example, the extraction 20 system comprises eight distinct heat-exchangers, such that N is equal to 4. The eight heat-exchangers have been shown in Figure 1 (denoted A, B, C, D, E, F, G, and H). The extraction system of Figure 1 also comprises 25 eight chambers AA, BB, CC, DD, EE, FF, GG, and HH in which the respective freezing heat-exchangers A, B, C, D, E, F, G, and H are disposed. Each freezing heat-exchanger comprises a circuit in which a refrigerant fluid (or a mixture of refrigerant 30 fluids), a coolant fluid or a heat-transfer fluid may circulate. Said circuit typically comprises ribs associated with tubes in which the fluid in question can circulate. Further, the flow rate of the refrigerant or 35 coolant/heat-transfer fluids circulating in the heat exchangers is controlled by a series of valves described in more detail below.

19 By causing a refrigerant or coolant fluid to circulate at a given temperature, it is possible to control the temperature of the external surface of each heat-exchanger. 5 Thus, during the frosting step, it is possible to reduce the temperature of the gaseous mixture circulating over the heat-exchanger of a given chamber by causing a refrigerant fluid to circulate at low temperature and evaporate or by causing a coolant fluid to circulate and 10 heat up. Thus, it is said that a refrigerant or coolant fluid "for frosting" circulates in the heat-exchanger. Similarly, it is possible, during the defrosting step, to increase the temperature of the solid phase deposited on the heat-exchanger of a chamber. To this 15 end, a refrigerant fluid is caused to circulate in a two phase state at a relatively high temperature (10K [Kelvin] to 70K above the triple point temperature of the substance to be extracted) in the heat-exchanger circuit so that said fluid is condensed. Alternatively, a heat 20 transfer fluid may be caused to circulate in the heat exchanger, which cools down. During the defrosting step, it is said that a refrigerant fluid or a heat-transfer fluid "for defrosting" circulates in the heat-exchanger. 25 In the remainder of the description, it should be understood that when refrigerant fluids for frosting and refrigerant fluids for defrosting are used, it is possible to replace them with coolant fluids and heat transfer fluids respectively. 30 In addition, in the remainder of the description attention is given to a single refrigerant fluid for frosting (or coolant fluid) and to a single refrigerant fluid for defrosting (or heat transfer) circulating in an alternating manner in the heat-exchangers. However, it 35 should be understood that it is possible to cause mixtures of refrigerant fluids for frosting (or coolant 20 fluids) or mixtures of refrigerant fluid for defrosting (or heat-transfer fluids) to circulate. Further, the supply circuit 1000 can be used to route the gaseous mixture from which a substance is to be 5 extracted inside the chambers AA, BB, CC, DD, EE, FF, GG, and HH. Access by the gaseous mixture is controlled upstream of the chambers AA, BB, CC, DD, EE, FF, GG, and HH by valves A100, B100, C100, D100, E100, F100, G100, and H100 10 respectively, which are collectively termed the gas valves V100. The gas mixture located inside the chambers AA, BB, CC, DD, EE, FF, GG, and HH may then be extracted. Said extraction is controlled using valves A101, B101, C101, 15 D101, E101, F101, G101, and H101 respectively, which are collectively termed the gas valves V101. The gaseous mixture extracted thereby may then be re-routed via the evacuation circuit 1010 to another chamber carrying out another functional step. 20 Further, evacuation from the chambers AA, BB, CC, DD, EE, FF, GG, and HH of the substance extracted from the gaseous mixture in the chambers and the residual gas phase is controlled by the valves A130, B130, C130, D130, E130, F130, G130, and H130 respectively, which are 25 collectively termed the valves V130. The substance that is extracted thereby, which is in the liquid state, is routed via the recovery circuit 130 to a reservoir R at the end of gas phase recovery. Further, in a preferred implementation, the recovery 30 circuit 130 is also connected to a vacuum pump P via a valve V200. Using said vacuum pump, it is possible to place the chambers of the system under vacuum. The importance of this pump and said vacuum step is described in more detail below. 35 Further, the refrigerant fluid circuit comprises two distinct supply circuits: a frosting circuit in which a refrigerant fluid for frosting (or a coolant fluid) 21 circulates, and a defrosting circuit in which a refrigerant fluid for defrosting (or a heat-transfer fluid) circulates. The frosting circuit comprises the supply circuit 5 1210, the branches 121 and 120 as well as the return circuit 1200. The defrosting circuit comprises the supply circuit 1100, the branches 110 and 111 and the return circuit 1110. 10 More particularly, the supply circuit 1210 supplies the branch 121 with refrigerant fluid for frosting under high pressure. Said refrigerant fluid for frosting is sent to the heat-exchangers carrying out frosting or proceeding to the heat-exchanger temperature-reduction 15 step. Access by the fluid for frosting to the heat exchangers A, B, C, D, E, F, G, and H is respectively controlled by the valves A121, B121, C121, D121, E121, F121, G121, and H121, which are collectively termed the 20 valves V121. Said valves V121 correspond to the expansion valves that can be used to change the refrigerant fluid for frosting from the high to the low pressure of the system. Said expansion valves can thus be used to evaporate the 25 frosting fluid circulating in the heat-exchanger. Alternatively, the valves V121 can be used to heat up the coolant fluid circulating in the heat-exchanger during the frosting step. In this particular configuration, the valves V121 are not expansion valves 30 but are flow rate control valves. In addition, extraction of the evaporated refrigerant fluid for frosting (or heated coolant fluid) originating from the heat-exchangers A, B, C, D, E, F, G, and H is controlled respectively by the valves A120, 35 B120, C120, D120, E120, F120, G120, and H120, which are collectively termed the valves V120.

22 The refrigerant fluid is then returned to a compressor (not shown) via the return branch 120 then the return circuit 1200. Because 2N heat-exchangers (N 3) are provided in 5 the extraction system, the flow rate of the refrigerant fluid is constant over time and has constant thermodynamic properties, which means that the compressor for the system can function under steady state conditions. Such conditions mean that firstly the 10 compressor can function under optimized compression conditions, and secondly that the heat-exchangers of the system can also function under steady state conditions. In addition, when mixtures of refrigerant fluids are used, the composition of said mixtures may then be 15 optimized since the conditions for said fluids are steady. When a coolant fluid is used to reduce the temperature of the heat-exchanger, it is then returned via the same branch 120 to a cooling heat-exchanger, not 20 shown. This heat-exchanger can be used to cool the coolant fluid so that it can be re-introduced into the system. In the same manner, the steady flow rate of the coolant fluid means that the function of the heat exchangers associated with cooling said coolant can be 25 optimized. Further, the circuit 1100 and the branch 110 supply the heat-exchangers with refrigerant fluid in the vapor phase or partial liquid-vapor phase under condensation conditions (i.e. with refrigerant fluid for defrosting). 30 Access by the refrigerant fluid for defrosting to heat-exchangers A, B, C, D, E, F, G, and H is respectively controlled by the valves A110, B110, C110, D110, E110, F110, G110, and H110, which are collectively termed the valves V110. 35 In addition, extracting the refrigerant fluid for defrosting originating from the heat-exchangers A, B, C, D, E, F, G, and H is respectively controlled by the 23 valves Aill, B111, C111, Dill, E111, F111, G111, and H111, which are collectively termed the valves V111. The collected refrigerant fluid for defrosting, the liquid content of which has been increased, is returned 5 via the return branch 111 and the return circuit 1110 to a heat-exchanger or a series of heat-exchangers at a higher temperature. Because the extraction system of the invention is arranged into 2N heat-exchangers (N 3), the refrigerant 10 fluid for defrosting has a constant mass flow rate in the circuits 1100, 110, 111, and 1110. When a heat-transfer fluid is used during the defrosting step, the cooled heat-transfer fluid is returned to a heating heat-exchanger, not shown. Once 15 heated up, said heat-transfer fluid may be re-introduced into the extraction system. In the example described here, the series of valves identified above are configured so as to allow: * alternate circulation of the gaseous mixture in 20 succession in the four heat-exchangers AA, BB, CC, DD, EE, FF, GG, and HH; e circulation of the refrigerant fluid for frosting in the heat-exchangers of the chambers carrying out frosting; 25 e concomitant circulation of refrigerant fluid for frosting in the heat-exchangers of the chamber proceeding into the temperature-reduction step, the duration of said step being a quarter of the duration of the frosting step; 30 e circulation of the refrigerant fluid for defrosting in the heat-exchangers of the chambers during defrosting, the defrosting step duration being half of the duration of the frosting step; and * the absence of circulation, over a quarter of the 35 duration of the frosting step, of refrigerant fluid in the heat-exchangers of the chambers during the course of 24 recovery of the liquid phase and the residual gas phase (the evacuation valve 130 being open). Reference is made to Figure 2 to describe the principal steps of an example of the extraction method of 5 the invention applied to the chamber AA of the extraction system of Figure 1. As explained above, it has been shown that the extraction method hinges on a cycle of four successive functional steps. 10 In the example described here, a preliminary step (EX) is initially carried out before carrying out the various functional steps composing the cycle of the extraction process. This preliminary step means that the duration of the 15 subsequent frosting step can be ascertained. As described above, this duration may be determined from a measurement of the pressure drop between the inlet and the outlet of the chamber AA during frosting. This pressure drop results from the gradual obstruction of the 20 gas lines as well as gradual deposition of a solid layer on the surface of the heat-exchanger. Next, the first functional step of the cycle is carried out: the frosting step (step FS). This step requires that the gaseous mixture is moved 25 in the chamber AA. To this end, the inlet and outlet valves A100 and A101 connecting the chamber AA to the supply circuit 1000 for the gaseous mixture are opened. The gaseous mixture is then cooled during its passage over the external face of the heat-exchanger A. 30 The gaseous mixture is cooled to a first temperature and to a first pressure so that the substance to be extracted solidifies on the heat-exchanger A: this constitutes anti-sublimation of the substance on the heat-exchanger A. 35 It should be noted that the first temperature and the first pressure are respectively lower than the 25 temperature and pressure defined by the triple point of the substance to be extracted. In order to cool the gaseous mixture, the valves A121 and A120 connecting the heat-exchanger to the 5 refrigerant fluid for frosting or coolant fluid circuit are opened. The refrigerant fluid for frosting circulating in the heat-exchanger evaporates, thereby creating the refrigerating power necessary to freeze the substance on 10 the surface of the heat-exchanger A. Alternatively, a coolant fluid circulates in the heat-exchanger and creates the necessary refrigerating power by heating up. In the embodiment described here, a sub-step 15 (denoted VSS) for placing the chamber AA under vacuum at the end of the frosting step is then carried out in order to extract the residual gases present in said chamber. To this end, the following is carried out in particular: e the evacuation valve A130 is opened; 20 * the valves A121, A120, A100, and A101 are closed; and the vacuum pump P is started up. The valve V200 can be used to control the connection of the evacuation circuit 130 with either the vacuum pump P or the reservoir P. When the vacuum sub-step VSS is 25 carried out, said valve V200 connects the vacuum pump P with the evacuation circuit 130. This vacuum sub-step VSS can typically reduce the pressure in the chamber to approximately 50 Pa absolute. The duration of this vacuum step is of the order of 20 30 seconds, for example, but depends on various parameters such as the pumping power of the pump P in particular. The relative vacuum obtained means that the residual gaseous mixture contained in the chamber AA can be evacuated. Said residual gaseous mixture may, for 35 example, be treated once recovered.

26 However, it should be noted that said vacuum sub step VSS is optional. The advantage of said vacuum sub step is described in more detail below. The second functional step in the cycle corresponds 5 to the defrosting step (step DS). In this implementation, the valve A130 is closed at the start of defrosting step DS. However, it should be understood that said closure is only necessary because the valve A130 was open at the end of the frosting step 10 in the example under consideration in order to carry out the vacuum sub-step VSS. In contrast, when the vacuum sub-step VSS is not carried out at the end of the frosting step, the valve A130 remains closed during the whole of the frosting step, and so the valve A130 is 15 already closed when the defrosting step is commenced. In addition, throughout the defrosting step, the valves A100 and A101 of chamber AA are closed, such that said chamber is no longer supplied with gaseous mixture. Thus, the step is carried out in a closed chamber. 20 Valves A121 and A120 are also closed such that the heat-exchanger A is no longer supplied with refrigerant fluid for frosting. Further, the valves Alll and A110, connecting the heat-exchanger A to the refrigerant fluid for defrosting 25 supply circuit, are open in order to reheat the heat exchanger A. The increase in temperature of the solid deposit of the substance present on the heat-exchanger A causes sublimation of the deposit, namely the direct passage of 30 the substance from the solid phase to the gas phase. The process of sublimation in a closed environment also causes an increase in the pressure in the chamber AA that, as explained above, was initially under vacuum at the start of the defrosting step. 35 Sublimation of the solid deposit still present on the heat-exchanger A then causes the pressure in the chamber AA to rise. When the pressure of the triple 27 point of the substance is reached in the chamber, the solid deposit undergoes a melting process such that the substance passes directly from the solid phase to the liquid phase. 5 To this end, the refrigerant fluid for defrosting is used in the heat-exchanger in the two-phase state and at a relatively high temperature, typically 10K to 70K above the temperature of the triple point of the substance. The refrigerant fluid for defrosting condenses, 10 releasing its heat to the heat-exchanger, thus allowing sublimation then melting of the frozen substance on the external surface of the heat-exchanger. Alternatively, a heat-transfer fluid is used at a temperature of 10K to 70K above that of the triple point 15 circulating in the heat-exchanger. On cooling, the heat transfer fluid allows sublimation then melting of the substance frozen on the heat-exchanger. The defrosting step is managed over time via the trio formed by the flow rate/temperature/vapor content of 20 the refrigerant fluid for defrosting circulating in the heat-exchanger A, or via the duo formed by the flow rate/temperature for the incoming heat-transfer fluid. The third functional step corresponds to the recovery step (step RS). 25 More particularly, this step comprises recovery of the liquid phase of the defrosted substance accumulated in the reservoir after the defrosting step. This step also comprises, simultaneously with recovery of the liquid phase, recovery of the residual 30 gas phase in the chamber before returning to the first pressure of the cycle of the extraction method. The valve A130 is open, thus allowing the liquid phase and the residual gas phase to be evacuated. The valve V200 is configured so that the liquid and gas 35 phases that are thus recovered are evacuated to the reservoir R.

28 It should be noted here that recovery of the residual gas phase is carried out until the pressure in the chamber AA returns to the pressure of the circuits 1000 and 1010. 5 Furthermore, during the recovery step, the valves A100, A101, A110, A111, A120, and A121 are closed. As a consequence, the gaseous mixture does not circulate in the chamber AA and no refrigerant fluid circulates in the heat-exchanger A. 10 It should be noted that when no vacuum sub-step has been carried out during the frosting step, residual gases are present in the chamber AA at the start of the defrosting step. These gases mainly include nitrogen and oxygen, for example. However, such gases are not capable 15 of passing into the solid state during the defrosting step like the substance to be extracted (for example C0 2 ). Thus, as indicated above, said vacuum sub-step (VSS) can advantageously be used to evacuate these undesirable residual gases by means of the vacuum pump P. Thus, at 20 the end of the defrosting step, the chamber AA contains a very pure gas, i.e. a gas with a very high concentration of the substance to be extracted. In this manner, during the recovery step RS, it is possible to recover a high purity extracted substance, both in the liquid form and 25 in the form of residual gases. Recovery of a very pure residual gas is advantageous as that gas may have a significant commercial value. As an example, carrying out said vacuum sub-step VSS in the extraction method of the invention means that very pure 30 CO 2 can be extracted in the gaseous form; this is highly attractive commercially. Finally, the fourth step corresponds to a temperature-reduction step of reducing the temperature of the heat-exchanger (step TS). 35 To this end, the valves A120 and A121 are opened in order to cause the refrigerant fluid for frosting to circulate in the heat-exchanger A.

29 When the temperature of the heat-exchanger A has reached a temperature close to the first temperature at the start of the cycle, a fresh cycle of the extraction method may be initiated by configuring the valves afresh 5 to carry out a frosting step (opening the gas valves A100 and A101). The extraction method of the invention also comprises a general sequencing step (not represented in Figure 2) aimed at sequencing the passage from one step 10 to another in each of the chambers. The sequencing is such that the number of chambers that carry out the frosting step, and also the total number of chambers carrying out the defrosting, recovery and temperature-reduction step are both always equal to 15 N. This equality is, however, only reached once the first cycle has been carried out, so that the extraction system is in a steady state. As explained above, this equality means that the energy efficiency of the extraction method can be significantly improved. 20 Further, the sequencing mentioned above is such that after a first cycle, the following conditions are satisfied throughout the extraction method: e each of the steps of frosting, defrosting, recovery, and temperature-reduction is carried out in at 25 least one of the chambers of the system; and * each of the chambers of the system carries out one of the steps as dictated by the cycle of steps and the sequencing described above. It should also be noted that the ratio of the number 30 of chambers each carrying out the four functional steps throughout the extraction method also corresponds to the ratio of durations between each of these steps. Thus, the cumulative duration of the defrosting, recovery, and temperature-reduction steps in an 35 extraction system is always substantially equal to the duration of the frosting step. It should be noted that this equality is also satisfied when the vacuum sub-step 30 VSS described above is carried out at the end of the frosting step. The duration of said sub-step is in fact negligible compared with the duration of the remainder of the frosting step (i.e. the time during which the 5 substance is deposited in the solid form on the heat exchanger). The duration of the frosting step may typically be of the order of 10 to 15 minutes, while the vacuum sub-step lasts approximately 20 seconds, for example. The vacuum sub-step VSS is thus carried out 10 almost instantaneously compared with the frosting step as a whole. Further, once a first cycle has been carried out, the number of chambers carrying out the defrosting step in an extraction system is equal to N', N' being an 15 integer that depends on the value of N. More particularly, N' is such that the ratio N'/N is equal to a value in the range from one third to one half, the value of the ratio being a function of the value of N. 20 This means that irrespective of the number of heat exchangers 2N present in the extraction system (where N is three or more), the duration of the defrosting step is always between a third and half the duration of the frosting step. 25 As an example, it has been shown that the ratio N'/N is equal to the minimum value 1/3 when N = 3. Similarly, it has been shown that, for example, the ratio N/N is equal to the maximum value M when N = 4. Regarding the value of N, three distinct situations 30 are envisaged: e N is equal to a power of 2: in this situation, the extraction method is such that the number of chambers N' carrying out the defrosting step is equal to N/2. This means that the duration of the defrosting step is 35 substantially equal to the duration of the frosting step; e N is even and is not equal to a power of two: in this situation, the extraction method is such that the 31 number of chambers N' carrying out the defrosting step is equal to (N/2)-1; and 0 N is odd: in this situation, the extraction method is such that the number of chambers N' carrying out the 5 defrosting step is equal to (N-1)/2. Further, the number of chambers carrying out the recovery step and the number of chambers carrying out the temperature-reduction step are by definition always selected so as to complement the number of chambers 10 carrying out the defrosting step. This means that throughout the method, the number of chambers carrying out the recovery step and the number of chambers carrying out the temperature-reduction step are fixed so as to conserve the equality between the number 15 of chambers carrying out the frosting step and the total number of chambers carrying out the defrosting, recovery, and temperature-reduction steps. However, it should be noted that the above ratios in terms of the number of chambers each carrying out the 20 functional steps can only be satisfied once a first cycle has been carried out in the extraction system under consideration. In this manner, it is possible to have, at any given instant, several chambers each carrying out a respective one of the four functional steps of the 25 extraction method cycle. Such distributions of the functional step, throughout an extraction method, over the chambers of an extraction system mean that continuous flows of refrigerant fluids for frosting and defrosting (or 30 coolant and heat-transfer fluids) can be obtained over the series of heat-exchangers. This continuity of the flows thus results in a steady state as regards the supply to the heat-exchangers and the compressor or compressors used in the system, 35 which provides great opportunities for energy optimization.

32 Furthermore, the fixed duration for the defrosting step is a function of the flow rate, the initial quantity of vapor and the temperature of the refrigerant fluid for defrosting circulating in the heat-exchanger during the 5 defrosting step or the temperature and flow rate of the heat transfer. Similarly, the fixed duration for the recovery step is also a function of the recovery flow rate of the liquid phase and the residual gas phase, while the fixed 10 duration for the temperature-reduction step is also a function of the flow rate of the refrigerant fluid for frosting (or coolant) for resetting the heat-exchanger to the first temperature of the cycle. With reference to Figure 3, there follows the 15 distribution over eight heat-exchangers (and thus eight chambers) of the principal steps of the extraction method of the invention. More particularly, Figure 3 represents, in the form of a table, the various functional steps carried out in 20 succession and in a cyclic manner for each of the eight heat-exchangers A, B, C, D, E, F, G, and H of the extraction system of Figure 1. In Figure 3, eight time intervals x1 to x8 are represented on the horizontal axis that corresponds to 25 the time axis. Each of the eight heat-exchangers of the extraction system are indicated on the vertical axis. At each time interval x1 to x8 under consideration, the heat-exchangers A to H carry out a particular 30 functional step, which results in a specific configuration of the set of valves of the extraction system as described with reference to Figure 1. During the first time interval xl, the heat exchanger A carries out the frosting step (start of the 35 step), the heat-exchanger B starts the temperature reduction step, the heat-exchanger C starts the recovery step of the liquid phase of the substance to be extracted 33 and the heat-exchanger D carries out the defrosting step (second half of the step). The table below summarizes the opening or closing positions of the set of valves of the extraction system 5 illustrated in Figure 1, for the time interval xl as described in Figure 3. The "open" and "closed" positions of the various valves are indicated by the letters "O" and "C" respectively. 10 34 Phase of heat- Evaporation Condensation Liquid Gas and gas exchangers in freezing valves freezing valves recovery sequence x1 and expanders valves valves Heat-exchanger A A100: 0 A121: 0 A110: C frosting step A130: C A101: 0 A120: 0 A111: C (1/4) Heat-exchanger B - temperature- D100: C D121: 0 D110: C D130: C reduction step D101: C D120: 0 Dlll: C (1/1) Heat-exchanger C - liquid and gas C100: C C121: C C110: C C130: 0 phases recovery C101: C C120: C C111: C step (1/1) Heat-exchanger D D100: C D121: C D110: 0 - defrosting step D130: C D101: C D120: C D111: 0 (2/2) 1 1 1 1 1 Table 1: Configuration of valves of the extraction system of Figure 1 during the interval x1 defined in Figure 4. 5 The four functional steps of the extraction method are carried out in the same manner on each of the eight heat-exchangers of the extraction system. Thus, it is possible, from Table 1, to determine the configuration of the valves of the extraction system 10 during each time interval x1 to x8. It should be noted that the above table does not include the configuration of the valves when the vacuum sub-step VSS is carried out at the end of the frosting step. This optional sub-step has been deliberately omitted since it does not have a 15 significant impact on the general sequencing of the steps.

35 Further, Figure 3 shows, for example, the time offsets between the heat-exchangers A, B, C, D, E, F, G, and H which occur over the eight time intervals x1 to x8. By studying the successive steps carried out in a 5 cyclic manner for each heat-exchanger, it can be seen that for each heat-exchanger, the extraction method is carried out in two major time sequences: the frosting step followed by the other three functional steps, namely the defrosting, recovery, and temperature-reduction 10 steps. It is clearly seen here that throughout the extraction method, the number of chambers (or heat exchangers) carrying out the frosting step is equal to the total number of chambers carrying out the defrosting, 15 recovery, and temperature-reduction steps. Further, in this particular situation, N is an even integer. Thus, throughout the method: " N = 4 chambers during the frosting step; and e N/2 = 2 chambers during the defrosting step. 20 Further, the number of chambers carrying out the recovery step and the number of chambers carrying out the temperature-reduction step are each selected so that the total number of chambers carrying out the defrosting, recovery, and temperature-reduction steps is equal to 4. 25 In this particular situation, the number of chambers carrying out the recovery step and the number of chambers carrying out the temperature-reduction step are necessarily equal to 1. As a consequence, when N = 4, the duration of the 30 defrosting step is equal to half the duration of the frosting step, while the durations of the steps of recovery and of temperature-reduction are each equal to a quarter of the duration of the frosting step. Controlling the set of valves of the extraction 35 system in accordance with Figure 3 and Table 1 means that the various flow rates employed can be stabilized, namely the flow rates of the gaseous mixtures in each of the 36 chambers of the extraction system, the flow rates of the refrigerant fluids for frosting and for defrosting in each of the heat-exchangers, as well as the flow rates of the substance recovered in the liquid phase from the S various chambers. Thus, a constructal distribution of said three flow rates in the extraction system can be obtained, i.e. division of the flow rates by two. However, it should be noted that in the situation in 10 which N is an integer equal to a power of 2 and N is 8 or more, several configurations are possible concerning the number of chambers carrying out the steps of defrosting and temperature reduction (this is also true for other values of N). 15 As explained above, when several configurations are possible, it is not essential to select the configuration corresponding to a constructal sequencing of the functional steps in the chambers of the extraction system. 20 The choice of a configuration from those that are possible may be made in a random manner or as a function of certain constraints of the extraction system. Further, it can be seen that when N = 4, the ratio N'/N is equal to the maximum value M. 25 An example of an extraction method of the invention in the situation in which N = 6 is described below with reference to Figure 4. More particularly, Figure 4 shows, in the form of a table, the various functional steps carried out in 30 succession and in a cyclic manner for each of twelve heat-exchangers (denoted A' to L') of an extraction system. This situation differs from that of Figure 3 in that the integer N is an even number but is not equal to a 35 power of two. In accordance with the invention, the number of chambers (and heat-exchangers) carrying out the frosting 37 step is equal to the total number of chambers carrying out the defrosting, recovery, and temperature-reduction steps. More particularly, in this particular situation, we 5 have throughout the method: * N = 6 chambers during the frosting step; and * (N/2)- 1 = 2 chambers during the defrosting step. Further, in this particular situation, the extraction system is configured so that throughout the 10 method, there are two chambers carrying out the recovery step and two chambers carrying out the temperature reduction step. As a consequence, the durations of the defrosting, recovery, and temperature-reduction steps are each equal 15 to one third of the duration of the frosting step. Figure 5 represents, in the form of a table, an example of the distribution of the functional steps of the extraction method in an extraction system comprising six heat-exchangers. 20 As a consequence, N = 3 and N is an odd integer. More particularly, in this particular situation, we have throughout the method: e N = 3 chambers during the frosting step; and e (N-1)/2 = 1 chamber during the defrosting step. 25 In this particular situation, the extraction system necessarily comprises one chamber carrying out the recovery step and one chamber carrying out the temperature-reduction step, throughout the extraction method. 30 This means that the durations of the defrosting, recovery, and temperature-reduction steps are each equal to one third of the duration of the frosting step. Further, it can be seen that the ratio N'/N in this situation is equal to one third, i.e. the minimum 35 possible value of the ratio N'/N. In general, the number of heat-exchangers 2N (N being 3 or more) comprised in an extraction system of the 38 invention is selected for specific design reasons linked to the dimensions of the facilities. Further, it is possible to envisage programming a controller to control the opening or closing of the 5 various valves of the extraction system of the invention. The controller may be programmed to satisfy the sequencing of the functional steps as described, for example, with reference to Figures 3, 4 and 5. This controller may also control measuring means in 10 order to carry out a preliminary step EX as described with reference to Figure 2.

Claims

1. A method of extracting a substance comprised in a gaseous mixture, in an extraction system comprising: - a first supply circuit for supplying a refrigerant 5 fluid for frosting or a coolant fluid (1210, 121, 120, 1200); - a second supply circuit for supplying a refrigerant fluid for defrosting or a heat-transfer fluid (1100, 110, 111, 1110); 10 . a third supply circuit for supplying a gaseous mixture (1000); and . 2N chambers (AA, BB, CC, DD), N being an integer greater than or equal to three; each chamber AA comprising: 15 * a heat-exchanger (A) that can be alternately connected to said first supply circuit via a first pair of valves (A121, A120), and to said second supply circuit via a second pair of valves (A110, All); e a third pair of valves (A100, A101) connecting an 20 inlet and an outlet of the chamber to the third supply circuit, the third pair of valves being capable of controlling movement of the gaseous mixture over the heat-exchanger inside the chamber; and * an evacuation valve (A130) to evacuate a residual 25 gas phase or a liquid associated with its gas phase that might accumulate in said chamber; the treatment of the gaseous mixture in each of the chambers comprising, in a cyclic manner, the following steps in succession: 30 e a frosting step (FS), carried out at a first heat exchanger temperature at a first chamber pressure, allowing the substance to pass directly from the gaseous state to the solid state, thereby forming a solid deposit of the substance on the heat-exchanger, said first (A121, 35 A120) and the third (A1O, A101) pairs of valves being closed at the end of said frosting step; 40 e a defrosting step (DS), comprising closing the chamber and allowing the solid deposit to pass directly into the gaseous state then allowing the solid deposit to pass directly into the liquid state when the temperature 5 of the heat-exchanger and the pressure in the chamber become respectively higher than the temperature and the pressure of the triple point of the substance, thereby accumulating the liquid phase of the substance in the chamber, the second pair of valves (A110, Alll) being 10 closed at the end of the defrosting step; e a recovery step (RS), comprising opening said evacuation valve (A130) so as both to allow the liquid phase and the residual gas phase that have accumulated in the chamber to be extracted, and also, simultaneously to 15 reduce the pressure in the chamber so that it returns to substantially said first pressure; and e a step (TS) for reducing the temperature of the heat-exchanger, comprising opening said first pair of valves so that the temperature of the heat-exchanger 20 returns to substantially the first temperature; the method further comprising a general sequencing step aimed at sequencing the passage from one step to another in each of the chambers such that the number of chambers undergoing the frosting step and also the total 25 number of chambers undergoing the defrosting, recovery, and temperature-reduction steps are both equal to N, wherein, after a first cycle, the following two conditions are satisfied throughout said method: e each of said frosting, defrosting, recovery, and 30 temperature-reduction steps is carried out in at least one of said chambers; and e each of the chambers carries out one of said steps as dictated by the sequencing step. 35

2. The method as claimed in claim 1, wherein said frosting step comprises a sub-step (VSS) for placing the 41 chamber under vacuum after closing said first (A121, A120) and third (A100, A101) pairs of valves.

3. The method as claimed in claim 1 or claim 2 wherein 5 after a first cycle, the number of chambers carrying out the defrosting step is equal to N/2 if N is equal to a power of two.

4. The method as claimed in claim 1 or claim 2 wherein 10 after a first cycle, the number of chambers carrying out the defrosting step is equal to (N/2)-1 if N is even and is not equal to a power of two.

5. The method as claimed in claim 1 or claim 2, wherein 15 after a first cycle, the number of chambers carrying out the defrosting step is equal to (N-1)/2 if N is odd.

6. The method as claimed in any one of claims 1 to 5, wherein the duration of said frosting step is 20 substantially equal to the cumulative duration of said defrosting, recovery, and temperature-reduction steps.

7. The method as claimed in claim 6, wherein the extraction method further comprises a preliminary step 25 (EX) for determining the duration of the frosting step, the duration of the frosting step being determined from a measurement of the maximum pressure drop between the inlet and the outlet of the chamber during a frosting step. 30

8. A computer program comprising instructions for executing the steps of an extraction method as claimed in any one of claims 1 to 7, when said program is executed by a computer. 35

9. A computer-readable recording support on which a computer program comprising instructions for executing 42 the steps of the extraction method as claimed in any one of claims 1 to 7 is recorded.

10. A system for extracting a substance comprised in a 5 gaseous mixture, the extraction system comprising: - a first supply circuit for supplying a refrigerant fluid for frosting or a coolant fluid (1210, 1221, 120, 1200); - a second supply circuit for supplying a 10 refrigerant fluid for defrosting or a heat-transfer fluid (1100, 110, 111, 1110); - a third supply circuit for supplying a gaseous mixture (1000); and - 2N chambers (AA, BB, CC, DD), N being an integer 15 greater than or equal to three; each chamber AA comprising: * a heat-exchanger (A) that is capable of being connected both to said first supply circuit via a first pair of valves (A121, A120) and also to said second 20 supply circuit via a second pair of valves (A110, All); * a third pair of valves (AlOO, A101) connecting an inlet and an outlet of the chamber to said third supply circuit, said third pair of valves being capable of controlling movement of said gaseous mixture over the 25 heat-exchanger inside the chamber; and * an evacuation valve (A130) to evacuate a residual gas phase or a liquid associated with its gas phase that might accumulate in the chamber; the extraction system further comprising control 30 means to control the opening and closing of each of said valves in order to carry out an extraction method comprising, in a cyclic manner, the following steps in succession: e a frosting step (FS), carried out at a first heat 35 exchanger temperature at a first chamber pressure, allowing the substance to pass directly from the gaseous state to the solid state, thereby forming a solid deposit 43 of the substance on the heat-exchanger, said first (A121, A120) and third (A100, A101) pairs of valves being closed at the end of said frosting step; e a defrosting step (DS), comprising closing the 5 chamber and allowing said solid deposit to pass directly into the gaseous state then allowing said solid deposit to pass directly into the liquid state when the temperature of the heat-exchanger and the pressure in the chamber become respectively higher than the temperature 10 and the pressure of the triple point of the substance, thereby accumulating the liquid phase of the substance in the chamber, said second pair of valves (A110, A111) being closed at the end of said defrosting step; e a recovery step (RS), comprising opening said 15 evacuation valve (A130) so as both to allow said liquid phase and the residual gas phase that have accumulated in the chamber to be extracted, and also, simultaneously to reduce the pressure in the chamber so that it returns to substantially said first pressure; and 20 * a temperature-reduction step of reducing the temperature of the heat-exchanger (TS), comprising opening said first pair of valves so that the temperature of the heat-exchanger returns to substantially said first temperature; 25 said control means being configured such that each heat-exchanger is alternately connected to said first and second supply circuits; said control means also being configured to sequence passage from one step to another in each of the chambers 30 such that the number.of chambers carrying out the frosting step and also the total number of chambers carrying out the defrosting, recovery, and temperature reduction steps are both equal to N; and wherein which the control means are also configured 35 such that, after a first cycle, the following two conditions are satisfied throughout said method: 44 e each of said steps of frosting, defrosting, recovery and temperature reduction is carried out in at least one of said chambers; and e each of said chambers carries out one of said 5 steps as dictated by said sequencing step.

11. A method according to claim 10, wherein said control means are also configured such that said frosting step comprises a sub-step (VSS) for placing the chamber under 10 vacuum after said closure of the first (A121, A120) and third (A100, A101) pairs of valves.

12. An extraction system as claimed in claim 10 or claim 11 wherein, after a first cycle, the number of chambers 15 carrying out the defrosting step is equal to N/2 if N is equal to a power of two.

13. An extraction system as claimed in claim 10 or claim 11 wherein, after a first cycle, the number of chambers 20 carrying out the defrosting step is equal to (N/2)-l if N is even and is not equal to a power of two.

14. An extraction system as claimed in claim 10 or claim 11 wherein, after a first cycle, the number of chambers 25 carrying out the defrosting step is equal to (N-1)/2 if N is odd.

15. An extraction system as claimed in any one of claims 10 to 14, wherein the duration of said frosting step is 30 substantially equal to the cumulative duration of said defrosting, recovery, and temperature-reduction steps.

16. An extraction system as claimed in any one of claims 10 to 15, the extraction method further comprising a 35 preliminary step (EX) for determining the duration of the frosting step, the duration of the frosting step being determined from a measurement of a maximum pressure drop 45 between the inlet and the outlet of the chamber during a frosting step.