FR2868337A1 - Device for mixing reactants where the pistons in a number of reactant reservoirs are connected together rigidly to inject reactants into a mixing reservoir - Google Patents

Abstract

A mixing reservoir (30) is connected to several reactant reservoirs (12) by pipes (32). Each reactant reservoir contains a piston (14) sliding between a first and a second position. The pistons are connected together rigidly by a linkage (16) acting with a spring (34) which is compressed when the pistons are in the first position and brings the pistons into their second position as it expands : The residual space defined in the reservoir by the piston has a smaller volume than that of the first position. Each reactant reservoir has an outlet (24) for the reactant opening into the residual space of the second position. The device includes means of controlling the tension of the spring and contains two reactant reservoirs. The two pipes form the bar of a T to connect to the mixing reservoir. The two pipes have an extension with two branches, each of which connect to the mixing reservoir which is placed in the space defined by the four pipe branches. Alternatively, the two pipes form the branches of a Y with a 90 deg. angle to connect to the mixing reservoir. The bar of the T or the branches of the Y or the extending branches are directly connected to a nucleation tube feeding the mixing reservoir. The mixing reservoir is a nucleation tube. The two pipes connecting at one end of the tube are offset on either side of a plane cutting the tube along its length and passing through its axis. The nucleation tube has a diameter of the order of 2 to 4mm and a length of between 5 and 20cm. The device includes means of detecting the duration of passage of the mixture through the nucleation tube, such as two optoelectronic detectors opposite a piston in its two positions. The mixing reservoir is hydraulically connected to a dilution reservoir which includes an agitator. The linkage is a plate with an axle. The device includes feeder pots connected to the reactant reservoirs by pipes. A first and a second lever (26, 28) operate with the pistons to bring them from the second to the first position. A lock holds the pistons in the first position so it can be released by the first lever and another lock holds the spring until it is released by the second lever. Each feeder pot has a lid. The reactant reservoirs have means of controlling their temperature.

Description

2868337 1

  DEVICE FOR MEASURING NUCLEATION KINETICS

PRIMARY

DESCRIPTION TECHNICAL FIELD

  The invention relates to chemical reactions involving a mixture of reagents. In particular, the invention relates to a device for rapid mixing reagents under controlled conditions that allow to study the parameters affecting this step.

  The invention finds particular application in chemical reactions such as precipitation reactions, the first stage of which is nucleation, homogeneous or heterogeneous, the kinetics of which can thus be studied.

  STATE OF THE PRIOR ART Certain chemical reactions, such as precipitation, occur within supersaturated solutions: depending on the conditions, a process called nucleation causes the formation of microscopic nuclei, that is to say the assembly of a very small number of atoms, ions or molecules to form a stable first solid entity. These nuclei serve as initiators for the solutes which aggregate in a second time; we speak of homogeneous nucleation if there is a spontaneous appearance of a nucleus within the very volume of the supersaturated solution, and of heterogeneous nucleation if the appearance of the nucleus is made in contact with a solid interface. In this second stage, there is competition between the formation of new primary nuclei and the growth of already formed nuclei.

  It appears that the nucleation step is essential for the control of certain reactions: an accurate evaluation of the quantity of nuclei formed and / or their nature makes it possible to optimize the parameters for the synthesis of crystals of controlled nature and shape, for example .

  The study of nucleation is however particularly delicate because of its speed and the very small size of the formed nuclei: it is in fact very difficult to dissociate from the subsequent growth. This is why few methods are described in the literature to directly measure nucleation kinetics. The main difficulty of the experimental studies lies in an extremely rapid contact between the solutions, in order to overcome the influence of the mixture.

  In 1969 a method was proposed for studying the primary nucleation of several salts [Nielsen AE: Kristall und Technik 1969, 4 (1); 17-38].

  As shown diagrammatically in FIG. 1, under the effect of manual pressure, pistons 1 inject the precipitation reagents, initially contained in the tanks 2, by channels 3 into a mixer 4 of the Hartridge-Roughton type. The induction time is measured by turbidimetry at a detector 5 in the mixer 4 downstream of its connection with the channels 3. The concentrated reagent mixture is diluted in saturated solutions containing glycerine, gelatin (0.02%) or sodium citrate (0.01 mol.L-1) in order to overcome the phenomena of aggregation and agglomeration, and the number of germs formed is counted under optical microscope. Assuming all the crystals formed during the induction time tind, the nucleation rate RN satisfies the relation RN = N, with N number of crystals tind per unit volume.

  This method remains the basis of many studies of nucleation and induction time. To overcome the drawbacks inherent in the manual operation of the pistons, unreliable in terms of reproducibility, trigger modifications have also been proposed, in particular with the presence of an engine: the energy supply and the bulk are, however, major handicaps. The expansion of a pressurized gas, also cumbersome, has also been tested, although the risk of overpressure and uncontrolled gas expansion can cause significant damage.

  None of the proposed developments provides a satisfactory solution for a precise and practical determination of the nucleation kinetics, especially in the case of hostile environment studies (such as the glove box, etc.).

  SUMMARY OF THE INVENTION In one aspect, the present invention provides a device for measuring homogeneous and heterogeneous primary nucleation kinetics.

  More generally, the invention relates to a device for controlled mixing of solutions and / or reagents. The device according to the invention uses the principle of the mixer described above, with connection of a mixing tank to reagent tanks which are emptied simultaneously by a piston; the mixture of the reagents is however, according to the invention, initiated by the action of a spring which acts directly on the pistons to evacuate the solution present in the reagent reservoirs to the mixing tank. The use of a spring makes it possible to overcome the technical complexity inherent in engines, gas compressions, or similar solutions. Moreover, the force deployed by the spring is uniform and reproducible.

  In an advantageous embodiment, the spring tension can be adjusted so as to modulate the flow rate of the reagents.

  In particular when two reagent reservoirs are present, different configurations of the conduits connecting them to the mixing tank make it possible to ensure homogeneous and rapid mixing.

  According to an advantageous embodiment, the mixing tank is in the form of a nucleation tube. Advantageously, the nucleation tube is of low volume, and then connected to a dilution reservoir in order to generate the nucleation phenomenon only in the tube. In particular, means for measuring the passage time of the mixture in the tube, for example by two optoelectronic detectors, make it possible to determine the nucleation kinetics with optimized results compared to the prior art.

  Preferably, the reagent reservoirs are connected with feed pots. This allows controlled filling of these tanks, especially with regard to the amount of reagent; temperature control means may optionally be designed on the reagent reservoirs. Advantageously, the reagent reservoirs are filled from the feed pots, via levers acting on the pistons. Preferably, the levers are also activated to allow the relaxation of the spring so as to avoid accidents due to involuntary interactions, and to control the starting point of the mixing and / or nucleation reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

  The appended figures will better understand the invention, but are given for information only and are not restrictive.

  Figure 1, already described, shows the block diagram of a device for applying the Nielsen method.

  Figures 2 show a preferred embodiment of a device according to the invention, Figures 2b and 2c showing in top view a detail of the locking system actuated by the lever.

  FIGS. 3a, 3b, 3c, 3d schematically represent a device according to one of the embodiments of the invention during its operation.

  Figures 4a, 4b, 4c, 4d show different types of connection.

  FIGS. 5 and 6 graphically illustrate the results obtained for the determination of nucleation kinetics by means of a device according to the invention.

  DETAILED DESCRIPTION OF THE PARTICULAR EMBODIMENTS The device according to the preferred embodiment makes it possible in particular to implement the Nielsen method for measuring nucleation kinetics, even if it is not limited to this application.

  As seen in Figures 2a and 3, the device for mixing reagents 10 preferably contains two reservoirs 12 of reagent, which can be filled with reagents, for example precipitation. Each reagent reservoir 12 comprises a sliding piston 14 which makes it possible to vary the volume inside the reservoir 12. The pistons 14 are rigidly secured by a connecting member 16, preferably in the form of a plate. Advantageously, the reagent reservoirs 12 are equipped with a double jacket for controlling their temperature, or any other means for establishing and maintaining a predetermined temperature.

  In a first step, optional but preferred, the reservoirs 12 are filled by means of the pistons 14, in order to guarantee the introduction of constant, and preferably equal, volumes into the two reagent reservoirs 12. The reagents to be mixed are therefore initially introduced into two feed pots 18, for example cylindrical, preferably equipped with a cover 20 to prevent accidental projection, for example during a bad configuration of valves. The two pistons 14, secured by the plate 16, are mechanically connected to a shaft 22. An action on the shaft 22 creates a vacuum in the tanks 12 by increasing the volume defined by the piston 14 and the reservoir 12, and allows aspirate the reagents from the jars 18 in the two reagent reservoirs 12 via ducts which open there through an opening 24. The movement of the shaft 22 is preferably initiated by two levers 26, 28 which act on its end, or near it.

  The amount of reagent taken from the feed pots 18 is determined by the stroke of the pistons 14 and fixed for example by an automatic ball release system.

  Once the reagents have been introduced into the reagent reservoirs 12, the levers 26, 28 are in the lower position within the framework of the preferred embodiment shown (left of FIG. 2, FIG. 3b). Advantageously, as explained below, during suction, a residual volume of air is maintained inside the reagent reservoirs 12.

  In the next step, the reactants are mixed in a mixing tank 30, which is connected to the reagent reservoirs 12 via conduits 32. It is preferred that the conduits 32 be hydraulically connected to the tanks 12 by the same opening 24 as the feed pots 18; three-way valves 33 then connect the feed pots 18, the reagent reservoirs 12 and the mixing tank 30.

  Advantageously for kinetic studies, the mixing tank 30 is a nucleation tube.

  In order to ensure a constant force compression allowing the flow of reagents at rapid speed, the mixing is carried out by a rapid, constant, and controlled thrust on the pistons 14. The flow of the reagents in the nucleation tube 30 is thus ensured. , in the context of the invention, by the compression of the pistons 14 under the effect of a spring 34 acting on the connecting member 16. Minimal space, without intervention of any motorized part, the spring allows a good reproducibility and reliability of the force exerted by its relaxation. The spring 34 is compressed when the pistons 14 define in the tanks 12 the volume of reagent required for the reaction; it is able to relax at least until the pistons 14 define a space of minimum volume, that is to say at least less than or equal to the volume of residual air, preferably until the pistons 14 are in contact with the lower portion of the reagent reservoirs 12. Thus, it can be ensured that the total amount of reagent has been introduced into the mixture, which increases the accuracy of the measurements.

  For the operation to be done safely, the two levers 26, 28 do not remain in contact with the pistons 14 and the plate 16: during spring expansion indeed, if the two levers remain in their low configuration ( Figure 3b), they will be violently ejected, posing a danger to personnel and the environment. That is why a clever system provides for the use of the two levers for triggering the spring 34. Thus, a locking latch is activated at the end of the stroke of the pistons 14 during the suction of the reagents from the power pots 18. This latch is released by the first lever 26 (Figure 3c).

  At the time of initiating the mixing reaction and the possible measures, the other lever 28 is used to release the shaft 22, which allows the spring 34 to relax. The two levers are thus disconnected systematically at the start of each experiment. . See also Figures 2b and 2c.

  The spring 34 being released, it pushes the pistons 14 through the plate 16; the reagents are ejected from the reservoirs 12 in the nucleation tube 30: FIG. 3d.

  For kinetic studies, means for measuring the flow time in the nucleation tube 30 are provided. In particular, two optical sensors connected to an electronic system may for example measure the descent time of the pistons 14, which depends both on the tension of the spring 34 and the volumes of the reagents. A stopwatch is triggered for example when passing a screen in front of the first sensor and stops at the end of piston stroke 14, detected by the second sensor.

  When the tanks 12 are empty, a simple stopwatch can certainly not measure the flow time of the air, but, less than 100 ms, the latter does not influence the measured time: the triggering time of the pistons 14 can This optoelectronic system makes it possible to accurately measure very short flow times.

  The tension of the spring 34 can, in the preferred embodiment, be manually adjusted by screwing at least one fine-pitch nut, which makes it possible to control the passage time in the nucleation tube 30.

  In order to accurately estimate nucleation kinetics, it is important that the nucleation reaction can be stopped very rapidly. The recommended method consists in diluting the solution resulting from the mixture. For this purpose, the nucleation tube 30 is connected to a dilution tank 36, for example a beaker. It is advantageous for the beaker 36 to comprise means 38 for agitating the solution, for example four stainless steel anti-vortex blades agitated by a six-bladed turbine. The discharge opening of the nucleation tube 30 is preferably placed in the discharge stream of the turbine so as to ensure an almost instantaneous drop in the saturation of the mixing solution.

  According to a preferred embodiment, the saturation tube 30 is a cylinder of diameter 2 to 4 mm and length 5 to 20 cm, preferably 10 cm; the reagent reservoirs 12 have a volume of 4 to 25 ml: it is therefore possible to repeat the experiments, even with expensive products, in view of the small quantities involved. The spring 34 advantageously develops a force of 1.5 daN per millimeter of depression; it can consist of 9 turns of diameter 5 mm and length of 40 mm (a total force of 60 daN) for a total height of 85 mm.

  Each part of the device 10 can be made of stainless steel or Teflon for the sliding parts, which makes it possible to withstand a corrosive environment, while keeping costs minimal. Advantageously, a protective plexiglass confines the upper part of the device 10 to prevent accidental contact with the spring 34. The device is more compact and simple to implement. In addition, at the level of cleaning and / or repair, it does not require the use of complex and specific tools. To further increase safety and avoid, for example, any break, it is possible to choose the different elements such that the assembly has no sharp angle.

  Furthermore, in the device 10 according to the invention, many parameters are likely to be modified in a controlled manner: temperature of the reagents through the tanks 12 with thermostats, volumes, flow rate via the voltage control spring 34, speed of the mixing action.

  All of these features make this device particularly suitable for studies in hostile environment on toxic products. It is particularly suitable for handling in a glove box, radioactive material for example. Consequently, its field of application is very broad, from the study of classical precipitation systems to systems involving expensive and / or toxic material. In addition, although described in the specific context of the nucleation kinetics study, it is clear that the principle of controlled mixing can be used for other applications.

  In particular, it is possible to determine the optimum conditions of a mixture. Indeed, in the preferred embodiment, the two ducts 32 meet at the level of the tube 30.

  Different configurations are possible, as represented for example in FIGS. 4: the ducts can be joined along a Y, preferably with a right angle between the branches (FIG. 4a) or according to a T (FIG. 4b). It is also possible to provide a two-stream division of the reagent solutions by branching each conduit 32c into two branches, before joining the tube 30c (Figure 4c). Another possibility is an offset arrival of the reagents within the tube 30d by two ducts 32d facing each other, corresponding to the Hartridge-Roughton configuration (FIG. 4d).

  13 Study of Mixing Conditions The hydrodynamic characterization of different modes of connection of the conduits 32 to the nucleation tube 30 for the device 10 concerned both the micromix and the macromixture (ie at the scale of the reactor), the objective being to quantify the contacting times of the reagents at the inlet of the nucleation tube 30.

  The characterization of the micromixture in the nucleation tube was carried out using the chemical iodide / iodate method [Guichardon, INPL thesis 1996], based on a system of two competitive parallel reactions, one instantaneous and the other rapid . The measured segregation indices were then interpreted in terms of micromixing times using the Incorporation Model to allow a quantitative comparison of the different mixing systems.

  Length Flow Xs (x 103) Tube Tube Setup Time Micromixture Index (cm) (mL.s-1) segregation (ms) Figure 4a 10 30 1.1 2.2 Figure 4b 10 30 0, 9 1, 8 Figure 4d 0.6 0.6 1.2 1, 96 0, 2 3, 8 0, 5 Figure 4a 50 1.4 0.1 2.7 0.3 1, 1 0, 1 2, 2 0, 3 1, 8 0, 2 3, 3 0, 4 Figure 4b 10 33 0.9 0.1 1.8 0.2 0, 6 0, 1 1, 1 0, 1 Table I: Comparison of indices and time micromixing according to the configuration, the length of the tube, and the tension of the spring The nucleation tube 30, whatever the configuration chosen and as shown in Table I, is in fact characterized by a very good micromixture since the values of the indices Xs segregation are very low and the micromixing times are of the order of a millisecond. The tests also showed that the micromixing time remained independent of the length of the nucleation tube and decreased with spring tension.

  The state of the overall mixture in the tube was visualized by an instantaneous neutralization reaction between hydrochloric acid and sodium hydroxide in the presence of bromothymol blue. This one, blue in basic medium, becomes yellow in acid medium; this discoloration accounts for the quality of the mixture both microscopically and macroscopically.

  The fraction of segregated volume depends on the reagent contacting configuration, as shown in Table II which reports the different characteristic times of the mixture in the nucleation tube.

  Configuration Fraction of Volume Time Time fading micromixed blot (ms) (ms) Figure 4a 20% 2.9 4.2 Figure 4b 15% 2 2.9 Figures 4c, 4d 6% 0.8 1.2 Table II: Comparison of the different characteristic times of the system for a flow rate of 25 mL.s-1 Whatever the method, the micromixing times obtained are very low, of the order of a few milliseconds. Therefore, the contacting of the reagents at the inlet of the nucleation tube is fast enough to allow the study of the nucleation phenomenon. In all cases, the Hartridge-Roughton configurations (Figure 4d) and two-stream division of the reagent solutions (Figure 4c) provide the best mixing performance. The micromixing time is then of the order of one millisecond, which makes it possible to guarantee good control of the state of mixing in the nucleation tube.

  In fact, to study the nucleation phenomenon, the contacting of the reagents must be extremely rapid so as to guarantee a limitation by the kinetics of the chemical reaction. With a device 10 as described, the reservoirs 12 are connected to a spring system 34 for simultaneously injecting the reagents into a nucleation tube 30 where they are mixed very rapidly. The high supersaturation thus created in the tube 30 triggers the primary nucleation process. The mixture is immediately diluted in a large agitated volume 36, so as to reduce the supersaturation to stop the nucleation. In this dilution volume, the conditions ensure the growth of the seeds to a detectable size, while avoiding the phenomena of subsequent nucleation, breaking and agglomeration. It is then possible, particularly at the end of the experiment, to measure the number of crystals formed in the nucleation tube 30 by any known means from the measurement of the seed size distribution, for example at using a granulometer.

  The mixing being very rapid, the supersaturation in the tube 30 is homogeneous and remains practically constant, since nucleation consumes very little material. Therefore, the nucleation rate does not vary in the device 10. The crystals are formed only in the Vtube volume nucleation tube, during the flow time tec of the Vreac reagent volume.

  During mixing, the tube 30 corresponds to an open reactor, so that the number of seeds is obtained by the following balance sheet: N = RN.Vreac É 2 = RN É Vtube É tec, Oër is the transit time in the nucleation tube 30. Each experiment makes it possible to measure a nucleation rate corresponding to a given saturation.

  Preferably, and for the examples, the device 10 operates with the following parameters (for the characteristics already specified above): Flow rate (Vreac / tec) 30-60 mL. s-1 Rate of passage: 10-19 ms-1 Time of passage (2): 5-10 ms (tube of 10 cm) Examples of use The device 10 according to the preferred embodiment was used to measure kinetics nucleation. The two examples below relate to reactions whose nucleation kinetics verify the law of Volmer and Weber: ## EQU1 ## Example 1: nucleation of uranium oxalate IV nucleation of uranium IV oxalate was carried out by mixing 5 mL of a nitric solution of uranium IV containing hydrazine (U4 + ion stabilizer) and 5 mL of an oxalic acid solution, the dilution made in 500 mL of a mixture of nitric acid, hydrazine and gelatin.

  For homogeneous primary nucleation (supersaturation ratio S> 210), we have the following parameters AO, homo = 41051 m -3. s-1; Ea, homo = 182 kJ. mol-1; BN, homo = 91.

  For the heterogeneous primary nucleation (S <210), the following parameters are obtained: AO, hetero = 3, 1036 m 3 s-1; Ea, hetero = 102 kJ. mol-1 BNr hetero = 32.

  The nucleation kinetics were measured experimentally over a range of nitric acid concentrations between 0.5 and 2 mol.L1, in the presence of 0.1 mol.L-1 hydrazine and at temperatures 20, 30, 40 and 50 C.

  Figure 5 shows the excellent agreement between the experimental data and the calculated theoretical kinetics, the correlation indices being 0, 96 for the homogeneous primary nucleation and 0.98 for the heterogeneous primary nucleation.

  The induction times being of the order of one second, the passage time in the nucleation tube remains well below the time required for the precipitation to be triggered. In addition, the absence of subsequent nucleation, breakage and agglomeration phenomena could be confirmed by granulometric measurements and scanning electron microscope photos.

  EXAMPLE 2 Nucleation of Neodymium Oxalate III The nucleation of neodymium III oxalate was carried out by mixing 5 ml of a solution of neodymium nitrate III and 5 ml of a solution of oxalic acid. made in 1 L of distilled water containing gelatin.

  In aqueous solution, for this homogeneous nucleation, we have: Ao = 3.1039 m 3 s-1; Ea = 84 kJ. mol-1; BN = 171.

  Nucleation kinetics were measured experimentally for supersaturation ratios in the nucleation tube ranging from 140 to 2300 and for temperatures up to 60 C.

  Figure 6 compares the speeds calculated at the speeds measured experimentally. The correlation index is 0.98. As with uranium IV oxalate, the induction time is longer than the passage time in the nucleation tube and the absence of breaking, subsequent nucleation and agglomeration phenomena in the dilution volume has been confirmed by granulometric measurements and scanning electron microscope photos.

Claims (21)

  A device (10) for mixing reagents comprising: - a mixing tank (30); a plurality of reagent reservoirs (12); each reagent reservoir (12) comprising a piston (14) sliding between a first position and a second position in which the residual space defined in the reservoir (12) by the piston (14) is of a smaller volume than that of the first position; each reagent reservoir (12) comprising an opening (24) for evacuating the reagent and opening into the residual space of the second position; - a plurality of conduits (32) hydraulically connecting and through its opening (24) each reagent tank (12) with the mixing tank (30); - A connecting member (16) rigidly connecting all the pistons (14) between them, characterized by a spring (34) cooperating with the connecting member (16), the spring (34) being in a compressed state when the pistons (14) are in the first position and being able to bring the pistons (14) into their second position by its detent.   2. Device according to claim 1 comprising means for adjusting the tension of the spring (34).   3. Device according to one of claims 1 to 2 comprising two reagent reservoirs (12).   The device of claim 3 wherein the two conduits (32) form the bar of a T to connect to the mixing tank (30).   5. Device according to claim 3 or 4 wherein the two ducts (32) have a branch in two branches, each connecting to the mixing tank (30), which is located in the space defined by the four duct branches.   6. Device according to claim 3 wherein the two ducts (32) form the branches of an angle Y 90 to connect to the mixing tank (30).   7. Device according to one of claims 4 to 6 wherein the bar of the T or branch branches or branches of the Y are connected directly to a nucleation tube (30) which serves as a mixing tank.   8. Device according to claim 3 wherein the mixing tank is a nucleation tube (30), the two conduits (32) connecting to one end of the tube offset on either side of a plane intersecting the tube (30) along its length and passing through its axis.   9. Device according to one of claims 1 to 6 wherein the mixing tank is a nucleation tube (30).   10. Device according to one of claims 7 to 9 wherein the nucleation tube (30) has a diameter of the order of 2 to 4 mm and a length of between 5 and 20 cm.   11. Device according to one of claims 7 to 10 comprising means for detecting the duration of passage of a mixture in the nucleation tube (30).   12. Device according to claim 11 wherein the means for detecting the duration of passage are two optoelectronic detectors located opposite a piston (14) at its two positions.   13. Device according to one of claims 1 to 12 wherein the mixing tank (30) is hydraulically connected to a dilution tank (36).   14. Device according to claim 13 wherein the dilution tank (36) comprises stirring means (38).   15. Device according to one of claims 1 to 14 wherein the connecting member is a plate (16) comprising a shaft (22). 16. Device according to one of the   Claims 1 to 15 comprising pots   (18) connected to the reagent reservoirs (12) via conduits.   17. Device according to claim 16 comprising a first and a second lever (26, 28) in operative relation with the pistons (14) for bringing the pistons from the second to the first position.   18. Device according to claim 17 comprising a latch which locks when the pistons (14) reach their first position and which can be unlocked by the first lever (26).   19. Device according to one of claims 16 or 17 comprising means for locking the spring (34) may be released by the second lever (28).   20. Device according to one of claims 16 to 19 wherein each feed pot (18) is provided with a lid (20).   21. Device according to one of the preceding claims wherein the reagent reservoirs (12) comprise means for controlling their temperature.