POWDER DOSING METHOD

Description

DOMAINE TECHNIQUE

[0001] The invention relates to the field of powder dosing, and specifically to non-flowable powders which can be of any known type, including high density and / or cohesive powders.

[0002] It applies to any industrial process using powders, particularly non-flowable powders. It concerns a dosing process for non-flowable powders and an associated device. ART ANTÉRIEUR

[0003] Traditionally, different methods exist for performing the function of dosing powders, which are presented below according to four concepts.

[0004] First, bucket dosing systems consist of dosing buckets or forms such as rotary valves, powder gates, and others. This type of solution is described, for example, in the article entitled "Continuous Mechanical Handling of Bulk Products," by Claude Saudemont, Techniques de l'ingénieur, Reference AG7511 v1, July 10, 2002. These systems also have several drawbacks. They can result in dosing that is dependent on the granular state of the powders being dosed, particularly the level of agglomeration and apparent density, among other factors. Furthermore, they lead to intermittent dosing.

[0005] Furthermore, screw dosing systems consist of augers, also known as Archimedes screws, which, due to their rotary motion, push a volume of granular medium between each screw pitch. However, these systems have several drawbacks. Firstly, they are only effective for powders with minimal flowability. Secondly, they cause alterations to the granular medium due to impacts at pipe bends. In addition, they can lead to clogging and only allow for intermittent dosing, i.e., at each screw pitch.

[0006] Furthermore, vibrating plate dosing systems consist of moving parts that are most often subjected to oscillating vibration to collect the granular medium as continuously as possible. This type of solution is described, for example, in the thesis "Modeling the dynamic behavior of a vibrating plate: interaction with the granular medium," by Benoît GELY, PhD thesis, Sigma Clermont Auvergne University, September 2017. These systems also have several drawbacks. They can induce segregation and are sensitive to the granular medium.

[0007] Fluidized bed dosing systems are systems in which the powder to be dosed is subjected to a gas flow, forming a gas suspension with the granular medium, which is then withdrawn. These systems also have drawbacks. They can lead to ineffective dosing for non-fluidizable media and can also cause powder segregation. "Fluidizable" refers to a powder belonging to class A or B of the Geldart classification, as described, for example, in the article "Characterization and Analysis of Powders - Physical Properties of Divided Solids," by K. Saleh and P. Guigon, Techniques de l'Ingénieur, March 10, 2009.

[0008] Thus, the four types of solutions to the problem of powder dosing are not entirely satisfactory, or even not at all satisfactory for the dosing of non-flowable powders.

[0009] In particular, when handling powders that are difficult or impossible to flow, most known prior art dosing devices, such as gates, buckets, and augers, induce a stepped distribution curve. These distributions are discontinuous and occur in fits and starts. For a number of applications, such as the dosing of active pharmaceutical ingredients and sensitive reagents like explosives, discontinuous dosing is unacceptable, primarily due to the excessive impact of heterogeneous concentrations in the products being manufactured.

[0010] It is noted that there remains a specific need to perform the function of dosing a non-flowable, or at least slightly flowable, granular medium, with the following requirements: in a rapid, continuous, precise manner in terms of distributed flow rate, even with granular media considered non-flowable or fluidizable; without risk of dispersion of fine particles constituting the granular medium to be dosed; regardless of the flowability or fluidizability of the powder to be dosed; without inducing segregation of the granular medium to be dosed; without compaction of the granular medium.

[0011] Other cryogenic grinding processes and devices are known from French patent applications FR 3 072 307 A1 and FR 3 072 308 A1, but their operating parameters are not satisfactory with regard to the stated requirements. A powder dosing process and a powder dosing device according to the prior art are disclosed in document EP0732643. EXPOSÉ DE L'INVENTION

[0012] Consequently, the invention aims to meet at least partially the needs mentioned above and to remedy the drawbacks related to prior art achievements.

[0013] Specifically, the invention aims to enable the dosing of a granular medium as precisely as if it were a fluid, but without generating liquid effluent requiring treatment and without the subsequent costly and / or time-consuming separation of the powder from the carrier fluid that might be used for this purpose. It also aims to enable the dosing of any type of powder, with particle sizes ranging from a few nanometers to a few centimeters, and with unlimited densities, from very low to very high. Similarly, the invention aims to minimize the risk of introducing impurities after dosing and to allow for the inerting of the granular medium.

[0014] Also, the invention relates, according to one of its aspects, to a method for dosing non-flowable powders, characterized in that it comprises the following steps: a) introduction of a cryogenic fluid, dosing powders and carbon dioxide in solid form into a mixing and suspension system, the average particle size of the carbon dioxide in solid form being between 0.1 and 10 times that of the particle size of the dosing powders, b) mixing and suspension of the powders with the cryogenic fluid and carbon dioxide in solid form, to obtain a cryogenic suspension, the mass proportion of the powders satisfying the following equation (i): 10 % < poudres vol < 80 % , where: [powders] vol is the proportion by mass density of the powders, c) evacuation of the cryogenic suspension, including the withdrawal of the cryogenic suspension, in particular under conditions of temperature greater than or equal to ambient temperature and pressure less than or equal to atmospheric pressure, d) control of the dosing of the powders according to one or more parameters related to step b) of mixing and suspension, e) dosing of the powders.

[0015] The dosing method according to the invention is thus adapted to powders classified as "non-flowable." The concept of "flowability" refers to the property of a granular medium to flow naturally. It can be characterized by several methods. One of these can be based on a Carr index measurement. By definition, this index is determined as the ratio between the apparent volume occupied by a given quantity of powder and the compacted volume of the same quantity of powder, all normalized to the apparent volume. Above a Carr index of 25, the granular medium is conventionally considered to have very low flowability. Below a Carr index of 15, the granular medium is considered to have relatively good flowability. Thus, for the purposes of the invention, non-flowable powders are defined as powders whose Carr index is strictly greater than 15, and preferably greater than or equal to 25.

[0016] Furthermore, the concept of "average diameter" of a granular medium is used when the granular medium in question is not composed of solid particles all of the same size and is not generally strictly spherical. In this case, particle size distribution is a distribution of size, surface area, or even equivalent volume. To this static distribution, it is possible to associate a concept of average dimension, also called "average diameter." Such a concept is described, for example, in the article "Characterization of Particle Size," by John Dodds and Gérard Baluais, *Sciences Géologiques, bulletins et mémoires*, 46-1-4, pages 79-104, 1993.

[0017] The choice of the range of values ​​of the average diameter of the particle size of carbon dioxide in solid form compared to that of the particle size of the powders to be dosed is advantageous in that it results from taking into account a large number of factors and physical phenomena, simultaneously integrating rheological criteria, stability to sedimentation and energy efficiency.

[0018] Regarding rheological aspects, these must be considered, particularly in terms of viscosity and overall flow behavior. Viscosity is correlated with particle size but also with the incorporation rate (the amount of solid matter in a given volume of liquid). While a link between particle size and viscosity may exist, this link is not direct when the suspended solids are not strictly monomodal (a monomodal case only when the powders are considered model powders, which is not the case in real-world industrial applications). However, the actual powders to be transported are not monomodal, and solid carbon dioxide cannot have exactly the same particle size distribution as the powder being transported (in average diameter and / or particle size range).Therefore, it is important to be able to adjust the average diameter of the carbon dioxide particles to obtain a suspension with an acceptable viscosity for circulation through a hydraulic system without expending too much energy. This adjustment cannot be determined. a priori and must be adjusted on a case-by-case basis. Furthermore, the incorporation rate, the polydispersity of the solids, and even their morphology, also influence the overall behavior of the suspension during flow. There is no direct correlation between all these parameters and the suspension's behavior, which can be Newtonian, shear-thickening, shear-thinning, or even thixotropic. Ideally, Newtonian behavior is desired, but some systems cannot easily achieve this.

[0019] Regarding stability against sedimentation, the behavior of suspensions in liquefied gases is not well understood by those skilled in the art. Indeed, this type of suspension does not induce electrical charge interaction between the liquid and the solid (the liquefied gas being entirely anionic and lacking a charge inversion layer as in conventional liquids). In the case of very fine solids (less than a micron, or even a few hundred nanometers), atypical behaviors can occur (impact of Brownian motion, for example). It is also possible that agglomeration of solid matter aggregates may occur, subsequently leading to accelerated sedimentation of the solid phase.If the carbon dioxide used, instead of being very fine, is relatively coarse compared to the transported powder, this can prevent these phenomena, but the mixture will lose its homogeneity, which is not favorable for the intended transport function. It should also be noted that if the average diameter of the solid carbon dioxide is too large, this phase will be affected (for example, by mixing, high pressure losses, etc.) by the narrow passages of the hydraulic circuits used to convey the suspension.

[0020] To account for all these parameters and phenomena, the average particle size of solid carbon dioxide is between 0.1 and 10 times that of the powders to be analyzed. More preferably, to achieve a more optimized compromise, particularly between rheology and stability during settling, the average particle size of solid carbon dioxide can be between 1 and 8 times that of the powders to be analyzed, specifically between 2 and 6 times that of the powders to be analyzed.

[0021] Furthermore, the conditions of equation (i) given above advantageously allow for the obtaining of a stable and pumpable cryogenic suspension. Also advantageously, the cryogenic suspension is stable and pumpable thanks to the conditions set forth in the invention concerning the average particle size of the carbon dioxide in solid form and the mass fraction of the powders.

[0022] It should be noted that by "stable," we mean that a suspension is considered stable when the time required for its complete settling is at least ten times greater than the time required for its transport or transfer. Typically, in the context of this invention, the transport or transfer time for powders may be on the order of a few minutes, while the stability time may be on the order of one hour.

[0023] Advantageously, the presence of carbon dioxide in solid form in the cryogenic suspension can play the role of steric stabilizer of the powders in order to prevent their sedimentation.

[0024] It should be noted that "pumpable" refers to the ability of a formulation to be dispensed using a conventional pumping system, such as a piston or rotor pump. However, it is important to note that a suspension characterized as "pumpable" is not necessarily intended to be pumped but is capable of being pumped when needed. This concept of "pumpable" appears, for example, in the presentation entitled "Formulation, Homogeneity and Pumpability," by François DE LARRARD, BétonlabPro 3, Lesson No. 13, Laboratoire Central des Ponts et Chaussées - Centre de Nantes (LCPC). More intrinsically, a suspension is considered "pumpable" to the extent that the driving force available to conventional pumping systems (particularly piston or rotor pumps) to allow its movement within a given circuit is greater than the braking force induced by the suspension's viscosity. Typically, a suspension with a viscosity of around 100,000 mPa...A suspension with a viscosity of less than 20,000 mPa.s is considered pumpable.

[0025] The process according to the invention may further include one or more of the following characteristics taken individually or in any possible technical combinations.

[0026] Advantageously, the cryogenic fluid is a gas that liquefies at ambient temperature and pressure. It can, in particular, be liquid nitrogen (N₂). However, this choice is not limited. The cryogenic fluid defines the fluid behavior, especially the liquid behavior, of the cryogenic suspension and, if necessary, allows the carbon dioxide (CO₂) to be maintained in solid form.

[0027] Furthermore, solid carbon dioxide, also known as dry ice, can be in the form of granules and / or powders. This dry ice, by virtue of its bulk or occupancy rate within the cryogenic suspension, stabilizes the powders being measured.

[0028] The first step a) may advantageously include the following successive sub-steps: a1) introduction of cryogenic fluid and carbon dioxide in solid form into the mixing and suspension system, then a2) introduction of powders to be dosed into the mixing and suspension system.

[0029] Advantageously, the introduction of the cryogenic fluid and carbon dioxide, preferably carried out simultaneously, precedes the introduction of the powders to be dosed.

[0030] The time between the two substeps a1) and a2) can be very short, in particular on the order of a few seconds, and thus the second substep a2) can be carried out almost immediately after the first substep a1).

[0031] However, in general, the time between the two substeps a1) and a2) can depend on the time required to obtain a homogenized and high-quality suspension of the cryogenic fluid and carbon dioxide. In particular, the second substep a2) can be implemented when the mixing torque of the cryogenic fluid and carbon dioxide mixture is substantially constant, especially with a variability of less than 10%, or even better, 5%.

[0032] The removal of a cryogenic suspension advantageously involves displacing the cryogenic suspension to facilitate its removal. This can include both the withdrawal of the cryogenic suspension and its volatilization. Volatilization can be induced by the temperature and pressure conditions established during removal or achieved through volatilization devices specifically designed for this purpose.

[0033] Furthermore, the carbon dioxide loading rate in solid form can be between 0.1 and 10 times that in powders to be transported.

[0034] Step d) of powder dosing control can allow the acquisition and processing of the measurement of the agitation torque of the cryogenic suspension, to allow one or more control actions on one or more controllable organs.

[0035] Furthermore, the invention also relates, according to another aspect, to a device for dosing non-flowable powders for implementing the non-flowable powder dosing process as defined above, characterized in that it comprises: a powder, solid carbon dioxide and cryogenic fluid supply system, comprising means for controlled introduction of the powders to be dosed and means for controlled introduction of solid carbon dioxide, a system for mixing and suspending the powders, solid carbon dioxide and cryogenic fluid to obtain a cryogenic suspension, a cryogenic suspension evacuation system, including a cryogenic suspension withdrawal device associated with a mass flow meter, a powder dosing control system.

[0036] The mixing and suspension system may further include: a mixing tank; a mixing and stirring device, located inside the mixing tank; a means for measuring the level of the cryogenic suspension formed, at least partly located inside the mixing tank.

[0037] Furthermore, the mixing and suspension system may include an optical monitoring system to control the homogeneity of concentration in the cryogenic suspension. DESCRIPTION DES FIGURES

[0038] The invention will be better understood upon reading the detailed description of the non-limiting examples of its implementation and upon examining the schematic and partial figures, in which: [ Fig. 1 ] represents a simplified logic diagram of the dosing principle with a dosing device for implementing a dosing process according to the invention, [ Fig. 2 ] schematically illustrates an example of a non-flowable powder dosing device for implementing a dosing process according to the invention, [ Fig. 3A ] is a schematic view along a cross-sectional plane along the Z-axis of the figure 2 and the [ Fig. 3B ] is a schematic view to the left of the cross-sectional view of the figure 3A illustrating the principle of volatilization and evaporation of the cryogenic suspension, [ Fig. 4A ], [ Fig. 4B ] And [ Fig. 4C ] illustrate, according to cross-sectional views, possible variants of mixing and stirring devices for the mixing and suspension system of a dosing device for implementing a dosing process according to the invention, [ Fig. 5 ] graphically illustrates the evolution of the stirring torque of the cryogenic suspension as a function of stirring time and three introductions of solid charge, [ Fig. 6 ] represents the evolution of viscosity as a function of shear rate for suspensions of alumina and dry ice in liquid nitrogen, and [ Fig. 7 ] represents the evolution of viscosity as a function of shear rate for different concentrations of dry ice suspensions in liquid nitrogen.

[0039] Throughout these figures, identical references may designate identical or analogous elements.

[0040] Furthermore, the different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible. EXPOSÉ DES MODES DE RÉALISATION

[0041] The cryogenic fluid FC here is liquefied nitrogen (N2) but this choice is not limiting.

[0042] There figure 1 is a simplified logic diagram of the control necessary for the proper conduct of the dosing to specify the sequence of measurements and input and output data necessary for controlling the dosing of a dosing device for the implementation of a dosing process according to the invention.

[0043] There figure 2 represents an example of a device 30 for dosing non-flowable powders P for implementing a dosing process according to the invention. This firstly comprises a feeding system S1 for powders P and cryogenic fluid FC, which includes means for controlled introduction 43a of the powders P to be dosed and, in this example, means for controlled introduction 43b of carbon dioxide in solid form CO2(s).

[0044] Furthermore, the device 30 includes a mixing and suspension system S2 for the powders P, the cryogenic fluid FC, and solid carbon dioxide CO2(s) to obtain a cryogenic suspension SC. It also includes a discharge system S3 for the cryogenic suspension SC, and finally a control system S4 for the dosing of the powders P.

[0045] Furthermore, the cryogenic suspension SC evacuation system S3 includes a device for drawing off and, in this example, volatilizing the cryogenic suspension SC, associated with a mass flow meter 90. Alternatively, volatilization must be able to occur spontaneously without the need for dedicated volatilization means, due to the applied temperature and pressure conditions. In particular, the temperature may be greater than or equal to ambient temperature, and the pressure may be less than or equal to atmospheric pressure.

[0046] The S4 control system is configured to allow the acquisition and processing of the measurement of the stirring torque Co of the cryogenic suspension SC.

[0047] Such a device 30 makes it possible to implement the process according to the invention. Thus, a step a) allows the introduction of the cryogenic fluid FC, the powders P to be dosed and the carbon dioxide in solid form CO2(s) into the mixing and suspension system S2.

[0048] This step a) can be carried out with a simultaneous introduction of the constituents. However, advantageously, step a) comprises two successive sub-steps: a step a1) of introducing the cryogenic fluid FC and carbon dioxide in solid form CO2(s) into the mixing and suspension system S2, then a step a2) of introducing powders P to be dosed into the mixing and suspension system S2.

[0049] Advantageously, the average diameter of the particle size of carbon dioxide in solid form CO2(s) is between 0.1 and 10 times that of the particle size of the powders P to be measured, and the proportion by mass of the powders P satisfying the following equation (i): 10 % < poudres vol < 80 % , in which [powders] vol is the proportion by mass density of the powders P.

[0050] A step b) is then implemented of mixing and suspending the powders P with the cryogenic fluid FC and carbon dioxide in solid form CO2(s), followed by a step c) of evacuation of the cryogenic suspension SC, including the withdrawal of the cryogenic suspension SC, under conditions of temperature greater than or equal to ambient temperature and pressure less than or equal to atmospheric pressure.

[0051] A step d) allows the dosing of powders P to be controlled according to one or more parameters Co linked to step b) of mixing and suspension, and a step e) ensures the dosing of powders P.

[0052] The mixing and suspension system S2 of powders P, cryogenic fluid FC and carbon dioxide in solid form CO2(s) for obtaining a cryogenic suspension SC may in particular include at least part of the elements of the devices described in French patent applications FR 3 042 985 A1 and FR 3 042 986 A1.

[0053] This S2 system includes a mixing tank 41. The mixing tank 41 is thermally insulated to maintain the liquefied gas in the form of liquid nitrogen without excessive volatilization. Ideally, heat losses would be on the order of 2% per day or even less.

[0054] The S2 system also includes a mixing and stirring device 42, located inside the mixing tank 41. This mixing and stirring device 42 may, in particular, be an agitator, for example, of the blade, propeller, turbine, anchor, attrition type, or others, chosen in particular according to the viscosity of the cryogenic suspension SC under consideration. It may be any other profile, possibly assisted by one or more acoustic stirring systems, such as ultrasonic stirring rods, for example. The mixing and stirring device 42 may, for example, be a blade-type agitator as shown in the figure 4A , or even a turbine-type agitator as shown in the figure 4B , or a porous distribution element with an injection of liquefied nitrogen N 2 (I) as shown on the figure 4C .

[0055] The mixing and stirring device 42 is driven in rotation to generate agitation by means of a drive motor 45. This motor 45 incorporates a means for measuring the torque Co of the cryogenic suspension SC in order to determine whether the suspension is homogeneous, the torque Co being then substantially constant with a variability of less than 10%, or even 5%, and the loading rate adapted. This mixing and stirring device 42 can be further supplemented by an ultrasonic stirring system 98, for example a stirring sonotrode, to prevent possible accumulation at the bottom of the tank 41, and / or by the use of an upward flow of chemically inert gas, such as nitrogen, to promote mixing according to the granular medium to be dosed.

[0056] The means for the controlled introduction 43a, 43b of the powders P to be dosed and of carbon dioxide in solid form CO2(s) into the mixing tank 41 include, in particular, a first feed hopper 43a for the introduction of the powders P to be dosed and a second feed hopper 43b for the introduction of carbon dioxide in solid form CO2(s). The controlled introduction is carried out by weighing or dosing. For this purpose, the feed hoppers 43a, 43b are used in conjunction with weighing systems 46a, 46b respectively, corresponding to overhead balances or load cells. It is thus possible to monitor the mass introduced over time.

[0057] Furthermore, the S1 supply system includes a first insulated liquid nitrogen supply tank 71 and a second nitrogen supply tank 72 in the form of compressed gas. In addition, a mass flow meter 74 is present at the mixing tank 41, for example, of the Coriolis or ultrasonic type.

[0058] It should be noted that, depending on the specific characteristics of the granular medium to be transported, namely the cryogenic suspension, particularly its particle size and density, the proportions of powders P, possibly of carbon dioxide in solid form CO2(s), and of liquid nitrogen may vary. However, in order to obtain a cryogenic suspension SC that is stable and pumpable according to the definitions given previously, the mass fraction of powders P satisfies the following equation (i): (i) : 10% < [powders] vol < 80%, where [powders] vol is the mass fraction of powders P.

[0059] On the figure 2 It is also worth noting the presence of a valve 101 for pressurization and allowing the supply of nitrogen gas for pressurizing the tank 41; and the presence of a three-way valve 102 allowing online sampling for particle size analysis.

[0060] In order to obtain a cryogenic SC suspension that is pumpable and stable within the meaning of the invention, the major parameters to be determined and / or monitored are: the loading ratio of the powders P, namely the volume of solid on the total volume of the suspension SC: it will be advantageous to increase this ratio to the highest possible value to optimize the quantity of powders P dosed for a given volume of cryogenic suspension SC displaced; possibly, the loading ratio of carbon dioxide in solid form CO2(s): this ratio is classically a function of the quantity of powders P to be introduced into the cryogenic suspension SC; the density of the powders P to be dosed: in general, the denser the powders P and the larger the particle size, the more viscous the suspensions incorporating significant quantities of carbon dioxide CO2(s) will be sought to limit the risks of settling of the powders P to be dosed within the cryogenic suspension SC;possibly the particle size distribution of carbon dioxide in solid form CO2(s), given in particular by the average diameter of the particle size distribution of carbon dioxide available for formulating the cryogenic suspension SC; the particle size distribution of powders P given in particular by the average diameter of the particle size distribution of the granular medium to be transported.

[0061] Also, advantageously: The particle size of carbon dioxide in solid form CO2(s) is related to that of the powders P to be dosed: the average diameter being approximately between 0.1 and 10 times the particle size of the powders P to be dosed, or even between 1 and 8 times, or even between 2 and 6 times the particle size of the powders P to be dosed; the loading rate of carbon dioxide in solid form CO2(s) is related to that of the powders P to be dosed: more precisely, it is roughly of the same order of magnitude, the value being between approximately 0.1 and 10 times the content of the powders to be dosed; the liquid nitrogen content is limited as much as possible: it is advantageously less than 70% by volume; this liquid nitrogen content must nevertheless allow the suspension to flow and cannot be less than 5% by volume;the particle size of the solid phase, including the powders P to be dosed and here carbon dioxide in solid form CO2(s), is less than 10 times the diameter of the transport pipe, otherwise segregation could occur and cause the integrity of the granular medium to be dosed to be lost.

[0062] Advantageously, the particle size of dry ice can be between 500 and 900 µm.

[0063] Regarding the suspension of powders P in the cryogenic fluid FC, it is necessary to control the homogeneity of the powder P distribution because this corresponds to controlling the dosed mass. Indeed, the uncertainty of the dosed flow rate can be expressed as a function of the uncertainty in the homogeneity, or more precisely in the variance, by equation (ii): dM / M = dQ / Q + dV / V + dt / t Or : M represents the dosed mass, Q represents the mass flow rate, V represents the variance, and t represents the dosing time.

[0064] To ensure good homogeneity, i.e., a low variance V, ideally close to zero, it is necessary to maintain perfectly stirred reactor-type agitation within the cryogenic suspension formation tank 41. In other words, at every point in the volume of tank 41, there is ideally the same concentration of powder P. Thus, at the outlet of tank 41, the concentration of powder P corresponds to the average concentration in tank 41, and the mass of powder P dosed is therefore controlled by controlling the dosing time through equation (ii).

[0065] Furthermore, the S2 system includes a means for measuring the level of the cryogenic suspension SC formed, at least partly located inside the mixing tank 41. More particularly, this measuring means 44 can take the form of a bubbler or an ultrasonic probe.

[0066] Advantageously, the S2 system also includes an optical monitoring system 94 to control the concentration homogeneity in the cryogenic SC suspension. This can consist of a flow meter coupled to a laser diode or a camera with image analysis for identifying the concentration homogeneity of particles online. It is associated with a peristaltic pump 95 that allows the suspension to circulate to the optical monitoring system 94.

[0067] Furthermore, the S3 evacuation system includes an insulated pipe 53 that transports the formulated suspension while minimizing heat loss to the point of arrival. The length of the insulated pipe 53 must be chosen so that the travel time is significantly shorter than the solution's stability time, by at least a factor of 10. The pipe 53 is connected to a mass flow meter 90, such as a Coriolis or ultrasonic type.

[0068] The draw-off and volatilization device 91, 92 of the evacuation system S3 also includes a heat exchanger and evaporator 92 that allows the exchange and evaporation of the liquefied nitrogen phase N₂(I). The cryogenic suspension SC is thus continuously evacuated by pressurization within the processing tank 41 to ensure easy and controlled draw-off. It also includes a porous element, called a poral, 92, which acts as a nitrogen vapor evacuation tube while also allowing the outlet to be inerted or sheathed to prevent the formation of frost plugs at the outlet.

[0069] THE figures 3A et 3B These figures illustrate the principle of volatilization and evaporation of the cryogenic suspension SC. In these figures, the references Sp, A, X, and Sp represent, respectively, the surface area of ​​the cryogenic suspension, the surrounding circulating air, the axis of symmetry, and a heating element. Additionally, a sealing gasket 110 is provided.

[0070] To stabilize the cryogenic SC suspension, it is also possible to electrostatically charge the powders P to be dosed. To do this, the powders P can be pre-subjected to a significant electrical potential difference or can be charged by induced friction in a container whose oscillation will create friction between the wall and the powders, charging them according to the nature of the container wall used.

[0071] Furthermore, to ensure proper mixing by mechanical action, it is necessary to rotate the agitator 42 in such a way as to prevent any powder from remaining in contact with the bottom wall of the tank 41. The minimum rotational speed of the agitator 42 must induce turbulence with a velocity exceeding the separation velocity. The "separation velocity" refers to the velocity of the liquid phase of a suspension at which separation occurs. In summary, separation is a phenomenon that applies to a laminar boundary layer and is often the source of turbulence because it creates potentially unstable zones. During separation, the point of zero velocity that was initially attached to the wall becomes a volumetric boundary layer: this is separation, and a new boundary layer with a return velocity appears on the wall.Mersmann recommends, in the article "Theoretical prediction of the minimum stirrer speed in mechanically agitated suspensions", Mersmann A., Werner F., Maurer S., Bartosch K., Chem. Eng. Process., vol. 37, pp. 503-510, 1998, imposing a rotation speed (N min ) of the stirring wheel 42 at least equal to a value that can be expressed by the following relationship: . N mix = 5 , 1 N p ⋅ D T D 3 / 2 ⋅ d y g p S − p L D 2 p L 1 / 3 in which: N min represents the minimum stirring speed; Np represents the power number (parameter evaluated according to the type of stirring device); DT represents the diameter of the stirring device; D represents the diameter of the stirring tank; dp represents the diameter of the powder particles; g represents the unit of gravity; ρ s represents the density of the powders; and ρ L represents the density of the liquefied gas.

[0072] To ensure proper mixing by fluidization, the liquefied gas phase must have a sufficient surface velocity compared to that of the powders (P). This velocity depends on the particle size distribution of the granular medium to be fluidized and, in particular, on the physicochemical properties of the liquefied gas. More precisely, the minimum surface velocity required must have the minimum fluidization value umF. This velocity is expressed using the following relationship linking the Reynolds number to the Archimedes' principle: Or with : ε mF represents the porosity of the powder bed to be fluidized; and hk and hB are coefficients corresponding to the configuration of the system to be considered.

[0073] Thus, for example, for a powder bed that can be considered spherical: ε mF ≈ 0.4, with hK ≈ 4.2 and hB ≈ 0.3, we have C1 ≈ 25 and C2 ≈ 0.04.

[0074] Furthermore, the suspension can be stabilized by the use of solid carbon dioxide CO2(s) or by the application of electrostatic charges. The powders P are then charged by charge carriers of the same sign, and since the liquefied gas is neutral, the powder grains, naturally approaching each other through sedimentation, will repel each other due to electrostatic repulsion and thus maintain an equilibrium state in the dispersion.

[0075] Electrostatic charging of powders P can also be achieved through triboelectricity or discharge charging. For electrostatic charging by tribology, the powders are placed in a reservoir whose composition is chosen to allow electron transfer between the powders P and the reservoir walls in contact with them. The transfer is possible and more or less easy depending on the charge potential of each of the materials in contact with each other.

[0076] Furthermore, the S4 control system for the evacuation of the cryogenic suspension SC allows, in particular, the evacuation of the cryogenic suspension SC based on at least one parameter linked to the S2 mixing and suspension system, specifically the Co couple. The S4 control system compiles all the measurements taken on the 30 powder dosing device P, and allows for control actions or feedback on controllable components, such as valves, pump, stirring motor, etc.

[0077] Thus, the S4 control system integrates the acquisition and processing of several data points: the measurement of the quantity of dry ice MCO2 and powders Mpoudre, as well as of the cryogenic fluid FC, in order to evaluate the volumetric and / or mass contents of the constituents of the cryogenic suspension SC; the measurement of the level of the mixing tank 41 to avoid its clogging and to evaluate the density of the cryogenic suspension SC formed; the measurement of the stirring torque Co allowing to evaluate the viscosity of the cryogenic suspension SC and allowing to verify that it is homogeneous and sufficiently stirred; the optical measurement for the solid concentration within the tank 41; the pressure measurement to adjust the movement rate of the suspension measured by mass flow meter.

[0078] Furthermore, in order to achieve a homogeneous suspension and allow for precise dosing of the powders to be distributed, three possible strategies can be distinguished.

[0079] First, the preparation of a dilute suspension. This is the preferred method for powders with a small particle size and low density. The volumetric concentration threshold of the powders to be dosed in the cryogenic fluid FC can be within the range of 0 to 10% for this type of suspension. Stabilization of the suspension by electrostatic charging is feasible for these dilute solutions.

[0080] Secondly, the preparation of dense suspensions. This is the preferred method for powders with larger particle sizes and high density. The volumetric concentration threshold for powders to be dosed in the cryogenic fluid FC can be within the range of 20% to 80% for this type of suspension. Stabilizing the suspension by electrostatic stability becomes difficult above a 20% solid incorporation rate. To stabilize the suspension, starting from these thresholds (which are approximate since they depend not only on the concentration but also on the particle size and density of the powders), it is possible to stabilize the suspensions by increasing their viscosity through the introduction of solid carbon dioxide (CO2(s)) by increasing steric hindrance.

[0081] Thirdly, the preparation of intermediate density suspensions. The volumetric concentration threshold for powders to be dosed in the cryogenic fluid FC can be within the range of 10% to 20% for this type of suspension. In this case, a combination of mechanical and / or ultrasonic and / or convective flow agitation methods should be considered.

[0082] It should be noted that the stirring speed must be sufficient to achieve a high level of turbulence. It should also be noted that when the particles are detached from the bottom of the tank 41, the aim is to prevent particle stagnation at the bottom of the tank, and with an even higher level of turbulence, the goal is to obtain a homogeneous distribution of particles throughout the volume of the suspension. It is this homogeneous particle distribution within the suspension that the present invention makes possible.

[0083] Also, for complete suspension, i.e. when no particles remain at the bottom of the agitated tank 41, all particles must be detached and redeposition must be rendered impossible.

[0084] However, to obtain a homogeneous suspension, it is necessary to increase the stirring speed, bearing in mind that the ideal homogeneity of the solid phase within the suspension represents an asymptote. It should be noted that, in practice, a suspension with an Archimedes' number greater than 10 cannot achieve complete homogeneity.

[0085] Generally, there is a trade-off between homogeneity and the energy to be introduced into the suspension in the form of turbulence. The optimal stirring speed corresponding to this trade-off is on the order of 1.8 to 2 times the value of the minimum stirring speed.

[0086] All of these requirements must optimize the homogeneity of the P powders for a given and optimized cryogenic suspension volume, while ensuring the suspension can be dispensed under pressure. This translates into a limited viscosity, on the order of 100,000 mPa·s, and the formulation of a suspension adapted to the discharge piping at the metering unit.

[0087] Regarding the agitation torque parameter Co, the figure 5 The graph graphically represents the value of the stirring torque Co as a function of time t. References A0, A1, A2, and A3 correspond respectively to the torque at no load, a first addition of solid feed, a second addition of solid feed, and a third addition of solid feed. Thus, with each addition of material A1, A2, and A3, the torque Co increases for a given stirring speed. However, after a certain stirring time, the torque Co tends to stabilize, as shown by the plateaus on the graph. figure 5 This then allows for the possible introduction of an additional quantity of P powders into the cryogenic SC suspension if the transport flow rate instruction requires it, for example.

[0088] Through the figures 6 et 7 , we describe the rheological behavior of several SC cryogenic suspensions that can be considered within the framework of the invention.

[0089] Specifically, the figure 6 represents the evolution of the viscosity v, expressed in mPa.s, as a function of the shear rate tc, expressed in s -1< , for suspensions of alumina (Al 2 O 3 ), dry ice in liquid nitrogen.

[0090] There figure 6 illustrates the rheological behavior of three systems: Al 2 O 3 / CO 2: liquid nitrogen suspension containing nearly 10% (volume) of Al 2 O 3 and 48% (volume) of solid CO 2; CO 2: liquid nitrogen suspension containing 48% of solid CO 2; and Al 2 O 3: liquid nitrogen suspension containing 10% of Al 2 O 3.

[0091] This experimental curve indicates that the viscosities of the suspensions considered exhibit manageable rheological properties in terms of circulation and agitation; that is, the viscosity is not too high to be pumpable, and agitation is not too difficult without excessive energy expenditure. Furthermore, these experimental findings by the inventors show that the behavior of the suspensions, within the experimental range considered, can be likened to that of Newtonian fluids.

[0092] There figure 7 also represents the evolution of the viscosity v, expressed in mPa.s, as a function of the shear rate tc, expressed in s -1< , for different concentrations of dry ice suspensions in liquid nitrogen.

[0093] Specifically, the figure 7 illustrates the rheological behavior of five systems: 34%: liquid nitrogen suspension containing approximately 34% (mass) of solid CO2; 48%: liquid nitrogen suspension containing approximately 48% (mass) of solid CO2; 51.8%: liquid nitrogen suspension containing approximately 51.8% (mass) of solid CO2; 64%: liquid nitrogen suspension containing approximately 64% (mass) of solid CO2; 77.6%: liquid nitrogen suspension containing approximately 77.6% (mass) of solid CO2.

[0094] This experimental curve indicates that the viscosities of the suspensions considered exhibit manageable rheological properties in terms of circulation and agitation; namely, a viscosity not too high to allow pumping or agitation without excessive energy expenditure. This aspect is not trivial, particularly given the significant proportion of solids in these suspensions. Furthermore, the experimental data obtained by the inventors show that the behavior of the suspensions, within the experimental range considered, can be likened to Newtonian fluids, thus enabling more precise control and management of the process.

[0095] Consequently, the figures 6 et 7 allow us to illustrate the viscosities of cryogenic suspensions and to show the influence of the dry ice content on the viscosity of the fluid to be transferred.

[0096] Generally, the rheological behavior of cryogenic SC suspensions can be approximated by semi-empirical laws. As an example, the viscosity of the suspension as a function of the loading rate and particle size of the solid constituting the suspension can be expressed below by equation (ii): μ / μ 0 = 1 + 1 2 ⋅ N ⋅ Φ / 1 − Φ / Φ m 2 , Or : µ is the viscosity of the suspension; µ 0 is the viscosity of the liquid phase; [N] is a constant; Φ is the volume of solid in the volume of the suspension; Φ m is the maximum volume of solid in the volume of the suspension.

[0097] Knowing the viscosity of the suspensions, it is then possible to deduce the possible distribution flow rate Qv of the dosing unit as a function of the overpressure to be applied within the tank to ensure a flow rate conforming to the powder dosing setpoint. Equation (iii) can thus be obtained, given below: ΔP = 8 ⋅ μ ⋅ L ⋅ Qv / π ⋅ R 4 , Or : ΔP is the pressure difference between upstream and downstream of the flowmeter; µ is the viscosity of the suspension; Qv is the volumetric flow rate; R is the radius of the fluid transport pipe; L is the length of the transport pipe.

[0098] The invention is of course not limited to the examples of implementation that have just been described. Various modifications can be made to it by a person skilled in the art.

Claims

1. Method for dosing non-flowable powders (P), characterised in that it includes the following steps: a) adding a cryogenic fluid (FC), powders (P) to be dosed and carbon dioxide in solid form (CO2(s)) in a mixing and suspension system (S2), the mean particle diameter of the carbon dioxide in solid form (CO2(s)) being between 0.1 and 10 times that of the particle size of the powders (P) to be dosed, b) mixing and suspending the powders (P) with the cryogenic fluid (FC) and the carbon dioxide in solid form (CO2(s)), to obtain a cryogenic suspension (SC), the density percentage of the powders (P) verifying the following equation (i): 10 % < powders vol < 80 % , where: [powders]vol is the density percentage of the powders (P), c) removing the cryogenic suspension (SC), comprising the extraction of the cryogenic suspension (SC), under conditions of temperature greater than or equal to ambient temperature and pressure less than or equal to atmospheric pressure, d) controlling the dosage of the powders (P) according to one or more parameters (Co) linked with mixing and suspending step b), e) dosing the powders (P).

2. The method according to claim 1, characterised in that the first step a) includes the following successive sub-steps: a1) adding the cryogenic fluid (FC) and carbon dioxide in solid form (CO2(s)) in the mixing and suspension system (S2), then a2) adding the powders (P) to be dosed in the mixing and suspension system (S2).

3. The method according to claim 1 or 2, characterised in that the mean particle diameter of the carbon dioxide in solid form (CO2(s)) is between 1 and 8 times that of the powders (P) to be dosed, in particular between 2 and 6 times the mean particle diameter of the powders (P) to the dosed.

4. The method according to one of the preceding claims, characterised in that the load ratio of carbon dioxide in solid form (CO2(s)) is between 0.1 and 10 times that of the powders (P) to be dosed.

5. The method according to any one of the preceding claims, characterised in that step d) of controlling the dosing of the powders (P) involves the acquisition and processing of the measurement of the stirring torque (Co) of the cryogenic suspension (SC), to allow one or more controlling actions on one or more controllable members.

6. The method according to any one of the preceding claims, characterised in that the cryogenic fluid (FC) is liquid nitrogen.

7. The method according to any one of the preceding claims, characterised in that the carbon dioxide in solid form (CO2(s)) is presented in the form of granules and / or powders.

8. A device (30) for dosing non-flowable powders (P) for the implementation of the method for dosing non-flowable powders (P) according to any one of the preceding claims, characterised in that it includes: - a system (S1) for supplying powders (P), carbon dioxide in solid form (CO2(s)) and cryogenic fluid (FC), including controlled adding means (43a) of the powders (P) to be dosed and controlled adding means (43b) of carbon dioxide in solid form (CO2(s)), - a system (S2) for mixing and suspending the powders (P), the carbon dioxide in solid form (CO2(s)) and the cryogenic fluid (FC) to obtain a cryogenic suspension (SC), - a system (S3) for removing the cryogenic suspension (SC), comprising a device (91, 92) for extracting the cryogenic suspension (SC) associated with a mass flowmeter (90), wherein said extraction device is configured to extract the cryogenic suspension under conditions of temperature greater than or equal to ambient temperature and pressure less than or equal to atmospheric pressure, - a system (S4) for controlling the dosing of the powders (P), configured to on the one hand, control the system (S1) for supplying powders, carbon dioxide in solid form and cryogenic fluid such that the density percentage ([powders]vol) of the powders (P) verifies the following equation (i): 10 % < powders vol < 80 % , and on the other hand, control the dosing of the powders (P) according to one or more parameters (Co) linked with mixing and suspending step b).

9. The device according to claim 8, characterised in that the mixing and suspension system (S2) comprises: - a mixing tank (41), - a mixing and stirring device (42), located inside the mixing tank (41), - a means (44) for measuring the level of the cryogenic suspension (SC) formed, at least in part located inside the mixing tank (41).

10. The device according to claim 8 or 9, characterised in that the mixing and suspension system (S2) comprises an optical monitoring system (94) for checking the concentration homogeneity in the cryogenic suspension (SC).