FR2877333A1 - PROCESS AND DEVICE FOR PREPARING A COMPOSITE-TYPE PROPERGOL PASTE - Google Patents

Abstract

The present invention relates to a process for preparing a propellant paste of composite type as well as a device suitable for carrying out said process. Said method typically comprises a heat exchange on at least a fraction of pulp extracted from the mixer and put into circulation in an external circulation loop.

Description

2877333 1

  The subject of the present invention is a process for preparing a composite propellant paste and a device that is suitable for carrying out said method.

  The method and device are improvements to the existing method and device to date.

  The invention finds particular application in the field of the manufacture of solid propellant loadings for space launcher, strategic and / or tactical.

  A composite type solid propellant comprises a solid polymeric matrix or binder, powdery solid fillers (oxidizing, optionally reducing and / or energetic) and various additives.

  The binder is prepared from a liquid resin, having chemically reactive terminations, capable of being crosslinked by at least one also liquid crosslinking agent. The various ingredients of the propellant and the crosslinking agent are introduced into the liquid resin in an appropriate order. The resin by hardening (by crosslinking) by baking at a temperature compatible with the presence of pyrotechnic materials, coats all the ingredients and in particular the powdery fillers to form a solid body.

  For example, in DAVENAS 'Solid Propellant Technology', Chapters 10 and 11, Ed. Masson 1989, more details are given on the different ingredients used in the composition of composite type propellants.

  The process currently used to manufacture blocks of such propellants is a batch process, called "batch", consisting of preparing a quantity of product in several kneaded and poured in one or more loads.

  In a first step, the various ingredients are introduced into a kneader in a proper order and carefully and long kneaded, under pressure conditions (usually vacuum) and very precise temperature. For the next step, the mixture which is in the form of a highly viscous paste is poured into a mold, with shaping tools. The whole is baked to ensure the hardening of the dough. The mold is in some cases the same envelope of the propellant, envelope that is properly prepared for carrying out the loading.

  2877333 2 For the manufacture of large loads, from a few tens to a few hundred tons, generally used in space (throttles contiguous to the launcher) or strategic, it is necessary to chain several kneaded-mixes in the same envelope. The sequence of kneaded and poured must be fast, precise and rigorous.

  The mixing phase of highly viscous paste is essential for the quality of the finished product. It is a long and expensive phase by the immobilized material and the consumed energy.

  The duration of this mixing phase, which includes imposed temperature variations (reheating and cooling of the dough), is explained by: d'on the one hand, the difficulty of achieving a good mixture, especially in the vertical direction (by radial opposition) of the mixer. The composition contains very different materials: granules, in large quantities and liquids, in small quantities. The resulting paste has a high viscosity; On the other hand, the poor quality of heat exchange: the propellant paste is a thermally insensitive material (heat exchange is slow); - The heat exchange surface, which is that of the bowl of the kneader, is reduced and moreover an insulating solid layer of dough is formed on the wall.

  In addition, the current constraints in the manufacture of energetic materials require the use of more sensitive loads. However, the implementation of these new raw materials must not lead to an increase in the manufacturing cost because of the increased security measures required for their handling.

  Thus, a first problem to be solved is to develop a composite propellant paste mixing process, in which the mixing time is reduced without altering the quality of the finished product.

  Another problem to be solved is to be able to use, if necessary, sensitive charges (for example charges liable to react very violently, to detonate, for low loads (very low drop height (a few cm), low friction)) without substantial modification of the kneading process, and therefore without increasing cost and risk.

  According to the invention, with reference to these two types of technical problems (reduction of mixing time and intervention of sensitive charges), an improvement is proposed to the mixing phase of the process for preparing a composite propellant paste. . Said improvement is based on the implementation of a heat exchange on at least a fraction of the dough (being kneaded) put into external circulation.

  Thus, according to its first object, the invention relates to a process for preparing a propellant paste based on a binder containing solid charges (composite propellant paste). Said method, in a conventional manner, comprises successively the introduction into a kneader of the constituent ingredients of said dough and then kneading, in said kneader, under conditions of adequate temperature and pressure, the dough constituted.

  Typically, during said process of the invention: at least a fraction of the kneaded dough is extracted from said kneader; said at least a fraction of the extracted kneaded pulp is subjected to heat exchange; and said at least one fraction of the kneaded pulp extracted, heated or cooled, is reintroduced into said kneader.

  During the kneading process, a fraction of the kneaded pulp (or even several fractions thereof) is (are) circulated, in a loop, outside the kneader and undergone (ssen) t an exchange at the level of this external circulation loop (of each of these external circulation loops). When several loops exist, for the circulation of different pulp fractions, it is understood that they are "connected in parallel" on the mixer.

  The term "at least one fraction" in the present text means "one or more fractions" but does not cover the one-off extraction of all the pulp, the entire charge of the mixer. Of course, it is not excluded that, by fraction, the entire load of the mixer is concerned with the release.

  According to an advantageous variant of implementation of the process of the invention, a single dough fraction is removed, undergoes heat exchange and returns to the mixer, cooled or reheated.

  The term "being kneaded" means during kneading but does not necessarily mean during kneading, i.e. throughout said kneading (it actually means that the steps of outside circulation and heating and / or or cooling may occur during the entire mixing phase or for only part of it). Indeed, the skilled person already conceives the interest of heating the dough (via the concept of the invention) at the start of mixing, or even during it and to cool said dough (via the same concept of the invention) at the end of said mixing. Thus, the method of the invention can be implemented according to many variants and in particular: with circulation and heating of a fraction of the dough, at startup only; or - with circulation and cooling of a fraction of the dough at the end of kneading only; or - with circulation and heating of a fraction of the dough at the start and with circulation and cooling of a fraction of the dough at the end of mixing ...

  The process of the invention can be used to prepare propellant pastes containing more or less sensitive powdery solid charges. Thus, said process may comprise the introduction into the kneader of solid powder charges not or little sensitive and / or solid powdery solid charges phlegmatized with at least one phlegmatizing fluid.

  The skilled person is aware that if sensitive charges occur in the manufacture of a propellant, they are manipulated, "incorporated" in a fluid "phlegmatizer". Generally, only one fluid of this type occurs. However, it does not exclude the intervention of at least two fluids of this type, mixed.

  For the purposes of the prior art and the present invention, the term "phlegmatizing fluid" means a liquefied gas or, more often, an inert liquid, that is to say not reacting to the previously mentioned solicitations. When a sensitive or very sensitive charge is suspended in this liquid, it reacts much less to the stresses and can be handled with much less severe precautions. This process of phlegmatization is known per se. When the phlegmatizing fluid is a liquid, this liquid must be a non-solvent for the powdery filler so as not to alter the particle size of said filler; it must also be preferentially non-toxic and non-flammable under the conditions of use.

  As an example of a phlegmatizing fluid that can be used, mention may be made of CO2, N 2, acetone, ethyl acetate and hydrofluoroethers.

  The mass ratio between phlegmatizing fluid and sensitive powdery fillers is generally of the order of 75/25 to 25/75.

  Knowing that the propellant paste ready for casting may comprise from 65 to 80% by weight of sensitive powdery fillers, there is therefore in general a very large amount of phlegmatizing liquid to eliminate.

  According to the prior art, such a phlegmatizing fluid is generally removed before the charges are introduced into the mixer. It will be seen later that, in the context of the process of the invention, the phlegmatized charges can be introduced directly into the mixer, the phlegmatizing fluid being eliminated in said mixer.

  In such a context of intervention of pulverulent solid charges sensitive in the development of the propellant, and therefore of phlegmatizing fluid intervention, it is understood the very particular interest of the method of the invention which by optimizing the heating of the pulp makes it possible to optimize the in situ elimination of the phlegmatizing fluid. This is explained in detail below.

  According to a first alternative embodiment of the process of the invention, at least a fraction of the kneaded dough is extracted from the kneader, reheated and re-introduced into said kneader.

  Thus, it is generally possible to accelerate the rise in temperature of the dough inside the kneader, the energy conferred to the fraction (fractions) of heated dough (s) being better valued than if it had been communicated to the total mass of dough within the mixer ...

  When the process of the invention comprises the introduction into the kneader of flegmatized sensitive powdery solid charges using at least one phlegmatizing fluid, this first variant with heating advantageously comprises the implementation of the heating (at least at startup mixing) under temperature conditions where said at least one phlegmatizing fluid is vaporized. These temperature conditions are obviously to be optimized in view of the nature of the phlegmatizing fluid (s) in question and the possible presence of pyrotechnic materials. It is indicated, in no way limiting, that the at least one fraction of kneaded paste extracted is then advantageously heated to a temperature between 50 C and 95 C, very advantageously at a temperature between 55 and 80 C. In the context of the implementation of this first variant of the method with phlegmatizing fluid (s), it is advantageously provided to recover the said fluid (s) vaporized (s).

  According to a second alternative embodiment of the process of the invention, at least a fraction of the kneaded paste is extracted from the kneader, cooled, and then reintroduced into said kneader.

  It is thus possible, at the end of kneading, to accelerate the cooling of the total mass of dough inside the kneader.

  It is indicated, in no way limiting, that the at least one fraction of milled paste extracted can thus be cooled to a temperature of between 55 ° C. and 20 ° C., more generally at a temperature of between 55 ° C. and 40 ° C.

  It has already been stated that the method of the invention can be implemented firstly according to the first variant and in a second step according to the second variant.

  On reading the foregoing, those skilled in the art have already grasped the whole point of the process of the invention. Referring to the technical problems invoked in the introduction of the present description, it allows: E to reduce the duration of the mixing cycle (by ensuring in particular a good vertical mixing), and thus to reduce manufacturing costs and / or investment; 2877333 7 É to introduce sensitive charges, without significant modification of the procedure and especially without additional risk.

  The process of the invention described above with reference to the preparation of propellant paste, of composite type, in general, is particularly suitable for the preparation of explosive composite loadings. As examples of such loadings, those described in EP-A-1 333 015 may be mentioned.

  The method of the invention does not pose any particular difficulty as to its implementation. The parameters to be managed or even optimized are obviously, for an external circulation loop, the flow rate of the pulp concerned and the intensity of the heat exchange to be implemented. It may be mentioned here, incidentally, that the (the) fraction (s) of pulp is (are) advantageously taken (s), extracted (s), the lower part of the mixing tank, or even the bottom of said tank and reintroduced (s) in the upper part of said tank. The reintroduction operation is generally carried out above the dough mass within the mixer.

  According to its second object, the invention relates to a device suitable for implementing the method described above. Said device is of the type of those of the prior art, iii has been modified, to a lesser extent, to include at least one external circulation loop equipped with at least one heat exchanger.

  More specifically, the device of the invention adapted to the preparation of a propellant paste based on a binder containing powdery solid charges (composite propellant paste) comprises a kneader including a kneading tank and means of mixing, means of thermoregulation and depressurization of said vessel and means for introducing the constituent ingredients of the dough, including means for introducing powdery solid charges. In this, said device is of the type of those of the prior art. Characteristically, it comprises in addition at least one external circulation loop equipped with circulation means and at least one heat exchanger. Each loop with its equipment must allow circulation with heat exchange of a fraction of the kneaded dough.

  The means equipping each of the outer circulation loops (equipping the outer circulation loop) are obviously adapted to the nature of the pulp in question.

  The circulation means of the pump type are more particularly adapted to the viscosity of said paste.

  According to an advantageous variant, said means comprise a peristaltic pump.

  According to another advantageous variant, the at least one heat exchanger is a static mixer of the jacketed type with forced circulation of heat transfer fluid against the current. Such a static mixer not only ensures the heating or cooling of the circulating propellant paste, but also makes it possible to improve the homogenization of said paste before it is reintroduced into the mixer tank. This contributes to the good quality of the finished product.

  As regards the means for introducing powdery solid charges into the mixing tank, they can be arranged on the tank, independently of any external circulation loop. They are advantageously arranged to open into an external circulation loop, very advantageously arranged, in such a case, downstream of the heat exchanger (s).

  The means of thermoregulation and depressurization make it possible to control the conditions of temperature and pressure within the kneader. The depressurization means generally provide vacuum, within said mixer. They intervene in fact in the same way as they do in the devices of the prior art but are in addition capable of contributing to the elimination of the phlegmatizing fluid introduced into the mixer (according to the prior art, said fluid was eliminated off the mixer). Said depressurization means is advantageously coupled to a condensing unit. Such a unit intervenes for the recovery of vaporized phlegmatized fluids, possibly introduced with the pulverulent solid charges.

  The outer loop (s) is (are) advantageously arranged (s) to ensure a flow from a lower part of the tank, or even the bottom of said tank, to the top of said tank.

  We have seen that (the) said (s) loop (s) leads (nt) generally in the upper part, above the filling level provided for said vessel.

  It is now proposed to illustrate the invention, in no way limiting, by the accompanying figures and examples below.

  Figure 1 shows schematically an alternative embodiment of a device of the invention.

  FIG. 2 represents the cooling rate of an inert propellant paste prepared according to the method of the invention.

  Said FIG. 2 is to be considered with Comparative Examples 1a and 1b as well as Examples 2a and 2b of the invention, given below.

  In Figure 1, there is shown in 1 a double jacketed type of mixer in which circulates a heat transfer fluid that can be heated or cool at will, such as brine water; at 10, the vessel of said mixer 1; at 12 and 12a, kneading means, more precisely vertical blades, secured to the cover of the kneader 1 and driven by a motor 11; at 14, thermoregulation means able to control the temperature of the tank 10; at 16, depressurization means able to control the pressure (vacuum or low pressure) of said tank 10. To said depressurization means 16, a condensation unit 17 is coupled capable of allowing the recovery of a phlegmatizing fluid, in the hypothesis where such a phlegmatizing fluid intervenes.

  Characteristically, the tank 10 of the kneader 1 is associated with an external circulation loop 20 equipped with circulation means, a pump 22, and a heat exchanger 30. The temperature of the coolant circulating in said heat exchanger 30, is regulated via the thermal group 32.

  The outer circulation loop 20 has at its reintroduction mouth a tube 24 equipped with a grid 26. Such a grid 26 is advantageously used to prevent the entry of foreign bodies and to promote the degassing of the reintroduced paste, by sharing the flow of said dough in thin threads.

  At 18, there is shown a hopper for the introduction of powdery solid charges. Said hopper 18, according to the variant shown, opens into the circulation loop 20, downstream of the heat exchanger 30.

  With reference to this figure: 1, one can quickly describe an implementation of the method of the invention.

  Conventionally, the open mixer 1, the liquid binder, the reducing charges (for example aluminum powder), the various additives except the crosslinking agent and the crosslinking catalyst, and the oxidizing charges are introduced beforehand into the tank 10. for example ammonium perchlorate.

  The kneader 1 is closed and the ingredients are mixed and heated to about 70 ° C. (for this purpose, the heat transfer fluid of the tank is brought to about 80 ° C.); vacuuming (or low pressure) allows the mixture to be degassed. Continuing to knead and heat, the powdery oxidizing charge is introduced via the hopper 18: the heating makes it possible to compensate for the increase in viscosity due to the introduction of the pulverulent fillers. The kneading and homogenization of the dough are then continued (it is generally intended to bring said homogenized dough to a temperature between 20 ° C. and 80 ° C., preferably between 50 ° C. and 65 ° C.). Said paste is then cooled (for this purpose, the heat transfer fluid of the tank is brought to about 30 ° C.), up to the casting temperature, approximately 50 ° C. Then, by the hopper 18, the crosslinking agent and the crosslinking catalyst, before proceeding to casting after a last homogenization sequence.

  The temperature of the tank 10 of the kneader 1 is controlled by the means 14; the pressure by the depressurization system 16.

  According to the invention, at least one of the above heating and cooling steps is improved by extracting a flow of dough from the bottom of the vessel 1, circulating it in the loop 20 with heating or cooling (effective) at the heat exchanger 30 and reintroducing it, warmed or cooled, in the upper part of the tank 1 (above the level of the dough body surface).

  The reheating in question may be a simple reheating or may also help to effectively extract a phlegmatizing fluid introduced with phlegmatized sensitive powdery solid charges.

11 Examples

  Obtaining an Inert Propellant for Cooling Experiments In a vertical stainless steel kneader with a capacity of 5 gallons (about 19 liters), an inert propellant paste, representative of a real propellant, is prepared (to overcome the measurements safety), by mixing at approximately 65 ° C.-70 ° C., for 2 to 3 hours, the following ingredients: - 12% by weight of binder from a hydroxytelechelic terminated polybutadiene (ATOFINA R45HT), - 68% mass of an inert powdery charge (potassium chloride); and - 20% by weight of aluminum powder.

  Comparative Examples 1a and 1b: Cooling According to a Conventional Process The inert propellant paste obtained above is cooled in the mixer by circulation of coolant at 40 ° C. in the jacket of the mixer tank. The tank is under vacuum (about 5 mm Hg), and the speed of rotation of the blades is 5 rpm. The temperature of the dough at the bottom of the tank is measured during the cooling step. For two separate kneaded kneads, the results are shown in Table 1 and shown in Figure 2.

Table 1

  T initial paste T fluid V coolant cooling Ex. Comp. 64 C 40 C 0.75 C / min Comp. lb 70 C 40 C 0.9 C / min As can be seen in Figure 2, the cooling of the propellant paste, from 64 C to 55 C, takes place in 12 min (eg Comp. and the cooling from 70 ° C. to 55 ° C. takes place over 13 to 14 minutes (Ex.

  Examples 2a and 2b of the invention The same propellant paste as that prepared above is cooled according to the method of the invention. The operating conditions of the kneader are identical to those of the two preceding comparative examples. But, for the cooling phase, the propellant paste is taken at the bottom of the tank via a peristaltic pump, passes into a heat exchanger and is reinjected into the mixer tank according to the diagram shown in Figure 1; for these tests, the device comprises a single external circuit.

  The peristaltic pump is a pump type ABAQUE ALO type, whose flow varies between 250 kg / h and 150 kg / h depending on the increasing viscosity of the dough when it cools and its temperature decreases about 70 C at about 45 C.

  The heat exchanger consists of a SULZER static mixer type SMXL 30 mm in diameter and 800 mm in length; it comprises a double wall with circulation of a heat transfer fluid at 30 C for Example 2a and 20 C for Example 2b.

  The temperature of the propellant paste is measured at the bottom of the tank during the cooling step. For two separate kneaded kneads, the results are shown in Table 2 and shown in Figure 2.

Table 2

  T initial paste T fluid V heat exchange heat exchanger Ex. 2a 66 C 30 C 1.75 C / min Ex. 2b 69 C 20 C 3 C / min As can be seen in FIG. 2, the implementation of the method of the invention makes it possible to cool the propellant paste from 66 ° C. to 55 ° C. in about 6 minutes. (Ex 2a), and from 69 C to 55 C in 5 min (Ex 2b). The cooling rate is thus doubled (eg 2a v. Ex .: Comp., 1a), or even tripled (Ex 2b v. Ex Comp 1).

  EXAMPLE 3 Removal of Phlegmatizing Fluid A sensitive filler (hexogen) is phlegmatized by mixing 1.5 kg of said sensitive filler with 4.5 kg of a liquid hydrofluoroether, sold under the name HFE 7100 by the company 3M. The boiling point of HFE 7100 is 61 C at atmospheric pressure; moreover, this product is neither carcinogenic nor mutagenic and is not harmful for reproduction (non-CMR). Finally it is not flammable in these conditions of use.

  In the tank of a small 1 gallon (3.8 liter) vertical mixer, which already contains 1.5 kg of liquid binder, the 6 kg of phlegmatized mixture is gravity fed through a hopper. The water of the double jacket of the tank is kept at 65 C. By the outer loop is taken about 10 kg / h of the mixture and is passed through a SULZER SMXL exchanger 36 mm in diameter and 240 mm in length. double jacket is also heated to 65 C; the hydrofluoroether (HFE) is vaporized and the vapors are taken up in the mixer, then discharged to the trap when the paste is reinjected into the mixer. After 25 minutes, we see that: • the condenser does not deliver: therefore there is no more HFE vapor; The power demanded by the mixer motor increases: the paste becomes more viscous because the liquid HFE is eliminated.

  The removal of the phlegmatizing liquid is therefore rapid using the method of the invention. As an indication, it is estimated that the removal of the same amount of HFE using a conventional mixing process, that is to say without circulation of the propellant paste in an outer loop, would require between 1h and 2h.

Claims (2)

  A process for the preparation of a propellant paste based on a binder containing solid powdery fillers, said process comprising successively the introduction into a kneader (1) of the constituent ingredients of said dough and then kneading, in said kneader ( 1), under conditions of adequate temperature and pressure, constituted paste, characterized in that during said kneading: - at least a fraction of the kneaded paste is extracted from said kneader (1); said at least one fraction of the extracted kneaded pulp is subjected to a heat exchange; and said at least one fraction of kneaded pulp extracted, heated or cooled, is reintroduced into said kneader (1).   2. Method according to revendiication 1, characterized in that it comprises the introduction into said mixer unpowdered solid charges or not sensitive and / or solid powdery solid charges phlegmatized with at least one phlegmatizing fluid.   3. Method according to one of claims 1 or 2, characterized in that said at least one fraction of milled paste extracted is heated.   4. Method according to any one of claims 1 to 3, characterized in that it comprises the introduction into said mixer pulverulent solids solid charges phlegmatized with at least one phlegmatizing fluid and the heating of said minus a fraction of kneaded paste extracted under temperature conditions in which the at least one phlegmatizing fluid is vaporized.   5. Method according to claim 4, characterized in that said at least one fraction of milled paste extracted is heated to a temperature between 50 C and 95 C, preferably between 55 C and 80 C.   6. Method according to one of claims 4 or 5, characterized in that said at least one vaporized phlegmatizing fluid is recovered.   7. Method according to one of claims 1 or 2, characterized in that said at least one fraction of milled paste extracted is cooled.   8. Process according to claim 7, characterized in that said at least one fraction of kneaded paste extracted is cooled to a temperature of between 55 ° C. and 20 ° C., more generally between 55 ° C. and 40 ° C.   9. Device suitable for the preparation of a propellant paste based on a binder containing powdery solid charges, comprising a kneader (1) including a kneading bowl (10) and kneading means (12, 12a) and that means for thermoregulation (14) and depressurization (16) of said tank (10) and means for introducing the constituent ingredients of said paste, including means (18) for introducing solid powdery charges, characterized in that it further comprises at least one external circulation loop (20) equipped with circulation means (22) and at least one heat exchanger (30), intended for circulation with heat exchange of a fraction of the propellant paste kneaded in said tank (1).   10. Device according to claim 9, characterized in that said circulation means (22) comprise a peristaltic pump.   11. Device according to claim 9 or 10, characterized in that said at least one heat exchanger (30) is a static mixer, of the jacketed type with forced circulation of heat transfer fluid against the current.   12. Device according to any one of claims 9 to 11, characterized in that said means (18) for introducing powdery solid charges open into said at least one outer circulation loop (20), advantageously downstream of ( ) heat exchanger (s) (30).   13. Device according to any one of claims 9 to 12, characterized in that a condensing unit (17) is coupled to the depressurizing means (16).