FR2953844A1 - PROCESS FOR PREPARING A HALOGEN POLYMER AND DEVICE FOR IMPLEMENTING SAID METHOD - Google Patents

Abstract

Continuous process for preparing a halogenated polymer comprising a step of polymerizing at least one halogenated monomer in a medium comprising liquid or supercritical carbon dioxide (CO2 L / SC) in at least one reactor under pressure said method comprising at least: - (1) an expansion of the suspension from the reactor; - (2) a separation of the expanded slurry from the reactor into a first stream, containing the solid phase comprising the halogenated polymer and into a second stream, liquid, gaseous or supercritical, which is a mixture of CO2 and unconverted monomer; polymer; and recovering this first and second streams; - (3) reheating, applied to the relaxed suspension, before or after its separation in first and second flow, associated with a recovery of frigories generated by the relaxation; - (4) a recompression of the second stream and its optional cooling, for recycling to the reactor feed, with a supplement of fresh monomer and, optionally, CO2; - (5) a final step of degassing the halogenated polymer, optionally in combination with a continuous technique of shaping the polymer into granules. Device for implementing the method

Description

The present invention relates to a process for the preparation of halogenated polymers and to a device for carrying it out. It is known to prepare halogenated polymers in a medium comprising supercritical carbon dioxide (see for example: "Polymerization of vinyl chloride in supercritical carbon dioxide", Ge Li et al., Polymer Preprints 2001, 42 (2), page 817 ). US 6258910 B1 describes in particular the polymerization of vinyl chloride in supercritical carbon dioxide. In the known processes, high working pressures are generally used to simultaneously provide the temperature and density required for the polymerization reactions and a moderate solids content is provided to ensure the free flow of the polymer suspension at the outlet. of the reactor. Liquid or supercritical carbon dioxide (also referred to more simply as "CO2 L / SC" in the present description) constitutes an advantageous medium for the polymerization of halogenated monomers, in particular vinyl and vinylidene halides. In addition to the fact that CO2 L / SC is easy to access, non-flammable, non-toxic and inexpensive, it is chemically inert during synthesis. CO2 SC also constitutes a good solvent for these monomers and a bad solvent for the synthesized polymers. The synthesized polymer is thus recovered by precipitation in the form of particles, often extremely fine, whose residual monomer is easy to eliminate. In view of the absence of the compounds which are generally present in the aqueous conventional polymerization medium of most of the halogenated monomers (dispersing agents, surfactants, emulsifiers, etc.), the polymer obtained in the CO 2 L / SC is not contaminated with residues of these compounds and is of very high purity. Finally, the use of CO2 L / SC as the main constituent of the polymerization medium avoids the conventional use of large quantities of demineralized water and, consequently, the consumption of large quantities of steam (or other forms of energy) for drying the polymer. However, the use of CO2 L / SC as the main constituent of the polymerization medium of halogenated monomers has a disadvantage related to the considerable amount of energy required to heat the carbon dioxide and the unreacted monomer, relaxed at the outlet of the polymerization reactor, in order to effectively separate the solid polymer. The object of the present invention is to overcome this drawback by proposing, principally, a new continuous process for preparing halogenated polymers in a medium comprising L / SC CO2. Accordingly, the invention relates to a continuous process for preparing a halogenated polymer comprising a step of polymerizing at least one halogenated monomer in a medium comprising carbon dioxide in the liquid or supercritical state (CO2 L / SC) in at least one reactor under pressure, said process being characterized in that it comprises at least: - (1) an expansion of the suspension from the reactor; - (2) a separation of the expanded slurry from the reactor into a first stream, containing the solid phase comprising the halogenated polymer and into a second stream, liquid, gaseous or supercritical, which is a mixture of CO2 and unconverted monomer; polymer; and recovering this first and second streams; - (3) a reheating, applied to the relaxed suspension, before or after its separation in first and second flow; - (4) a recompression of the second stream and its optional cooling, for recycling to the reactor feed, with a supplement of fresh monomer and, optionally, CO2; (5) a final step of degassing the halogenated polymer, optionally in combination with a continuous technique for shaping the polymer into granules; said continuous process being further characterized by the fact that the reheating (3), applied to the relaxed suspension, is associated with a recovery of the frigories generated by the expansion. By "continuous process" is meant, in the present description, a process in which the supply of carbon dioxide, monomers, initiators and optional additives and the withdrawal of the contents of the reactor are carried out continuously. Preferably, the continuous process according to the invention is such that the control of feeds, withdrawal and other polymerization conditions ensures stationary operating conditions in the reactor. In the present description, the terms "monomer", "polymer", "initiator", "additive", "reactor" "relaxation", "recompression", 5 "reheating" "first stream" and "second stream" are used indiscriminately singular and plural. The polymerization step of the process according to the invention is carried out in a medium which is advantageously inert with respect to the halogenated monomers, comprising carbon dioxide in the liquid state (CO2 L) or supercritical state (CO2 SC). In the present description, the term "medium" is intended to mean the contents of the reactor excluding the introduced halogenated monomer and the halogenated polymer formed. The medium in which the polymerization step of the process according to the invention is carried out may comprise other halogenated monomer solvents. These other solvents are generally selected from those which do not generate an excessive rate of chain transfer reactions. Among these solvents, mention may be made of C 2 -C 8 hydrocarbons, C 1 -C 8 alcohols, methylene chloride, toluene, cyclohexane and the like. The polymerization step of the process according to the invention is advantageously carried out in a medium comprising at least 50% by volume, preferably at least 80% by volume and, more particularly, at least 95% by volume of carbon dioxide ( CO2) in the supercritical state. It is particularly preferred to perform the polymerization step of the process according to the invention in a medium consisting of CO2 in the supercritical state. In the present description, the term "supercritical" is understood in its conventional thermodynamic sense. For pure CO2, it refers to conditions above its critical temperature and pressure (31.1 ° C, 7.38 MPa) ("critical point"). If the CO2 is in the supercritical state, its compression will only result in increasing its density without the appearance of a liquid phase.

Advantageously, the temperature in the reactor is at least -60 ° C, preferably at least 0 ° C. Advantageously, the temperature is at most 200 ° C., preferably at most 150 ° C. In the preferred case of using CO2 SC, the temperature in the reactor is preferably at least 32 ° C, preferably at least 40 ° C. In this case again, the temperature is advantageously at most 100 ° C, preferably at most 85 ° C. Advantageously, the pressure in the reactor is at least 0.5 MPa, preferably at least 4 MPa. Advantageously, the pressure is at most 100 MPa, preferably at most 50 MPa. In the preferred case of using CO2 SC, the pressure in the reactor is preferably at least 4.5 MPa, preferably at least 8 MPa. In this case again, the pressure is advantageously at most 40 MPa, preferably at most 25 MPa. Advantageously, the density of the medium in the reactor is at least 500 kg / m3, preferably at least 600 kg / m3. Advantageously, the density of the medium in the reactor is at most 1200 kg / m 3, preferably at most 1000 kg / m 3. By polymerization of at least one halogenated monomer is meant both the homopolymerization of a halogenated monomer and its copolymerization with one or more other ethylenically unsaturated monomers, in order to obtain halogenated polymers.

Halogenated polymer is understood to mean both homopolymers and copolymers of halogenated monomers. Among these, there may be mentioned homopolymers of halogenated monomers such as vinyl chloride and vinylidene chloride; vinyl fluoride and vinylidene fluoride; trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene and hexafluoropropylene; fluoroacrylates; fluorinated vinyl ethers, for example perfluorinated vinyl ethers bearing perfluoroalkyl groups containing from 1 to 6 carbon atoms. Mention may also be made of the copolymers which these halogenated monomers form with each other and the copolymers of one or more of these halogenated monomers with another ethylenically unsaturated monomer such as olefins, for example ethylene or propylene; styrenic derivatives and styrene; halogenated olefins; vinyl ethers; vinyl esters such as, for example, vinyl acetate; acrylic acids, esters, nitriles and amides and methacrylic acids, esters, nitriles and amides.

Preferably, the process according to the invention is characterized in that the halogenated polymer obtained is a polymer containing chlorine. By chlorine-containing polymer is meant both homopolymers and copolymers of monomers containing chlorine. Among these, there may be mentioned homopolymers of chlorinated monomers such as chloroolefins, for example vinyl chloride and vinylidene chloride; chloroacrylates and chlorinated vinyl ethers, for example perchlorinated vinyl ethers 2953844 -5 bearing perchloroalkyl groups containing from 1 to 6 carbon atoms. Mention may also be made of the copolymers that these monomers containing chlorine form with each other, such as, for example, copolymers of vinylidene chloride with another chlorinated monomer as defined above and the copolymers of one or more of the monomers containing chlorine mentioned above with another ethylenically unsaturated monomer such as olefins, for example ethylene, propylene; styrenic derivatives and styrene; halogenated olefins; vinyl ethers; vinyl esters such as, for example, vinyl acetate; acrylic acids, esters, nitriles and amides and methacrylic acids, esters, nitriles and amides. In a particularly preferred manner, the process according to the invention is characterized in that the halogenated polymer obtained is a vinyl chloride polymer. Vinyl chloride polymer is understood to mean homopolymers of vinyl chloride as well as its copolymers with other ethylenically unsaturated monomers, whether they are halogenated (chloroolefins, for example vinylidene chloride, chloroacrylates, vinyl ethers). chlorinated, for example perchlorinated vinyl ethers bearing perchloroalkyl groups containing from 1 to 6 carbon atoms) or not (olefins such as, for example, ethylene, propylene, styrenic derivatives and styrene; vinyl ethers; vinyl esters; such as for example vinyl acetate, acids, esters, nitriles and acrylic amides, acids, esters, nitriles and methacrylic amides). Homopolymers of vinyl chloride and copolymers of vinyl chloride with a comonomer, halogenated or not, preferably containing at least 50%, preferably at least 60%, particularly preferably at least 70% and most preferably at least 85% by weight of monomeric units derived from vinyl chloride are particularly preferred. Homopolymers of vinyl chloride and copolymers of vinyl chloride with vinyl acetate are most preferred. Homopolymers of vinyl chloride are very particularly particularly preferred. The polymerization step of the process according to the invention can be carried out according to any technique compatible with the medium comprising L / SC CO2 in which this step is carried out. Thus, the polymerization of the halogenated monomer can be carried out by the action of radical initiators, ionizing radiation, Friedel-Crafts catalysts, Ziegler-type catalysts and the like. Preferably, the polymerization step is carried out in a radical manner and usually provides for the use of one or more initiators, the nature, number and concentration of which can also be selected as needed. As initiators useful in the context of the present invention, any suitable radical initiator, particularly an organic radical initiator, may be used. The polymerization step is therefore preferably carried out in the presence of an organic radical initiator. The use of a carrier or a solvent may also be considered to promote the dispersion of the initiator in the polymerization medium. As carriers that can be envisaged for this purpose, mention may be made of silicas, silicagels, aluminas and other metal oxides, such as titanium oxide. As a solvent, there may be mentioned for example toluene, alcohols such as methanol, ethanol and isopropanol and hydrocarbons such as pentane, hexane and cyclohexane.

The initiator usable according to the invention may be chosen for example from peroxides, such as dimethyl peroxydicarbonate, diethyl peroxydicarbonate, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, diacetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxyisopropylcarbonate, t-butyl peroxy-n-decanoate, t-butyl peroxyacetate, 1,1,3,3-peroxymethoxyacetate and tetramethylbutyl, t-butyl peroxypivalate, t-amyl peroxypivalate, di-t-butyl peroxide, 2-2-bis (2,2-dimethylpropanolperoxy) -4-methylpentane, diacyl peroxides, such as diacetyl peroxide; di-n-propionyl peroxide, di-isobutyryl peroxide, dibenzoyl peroxide, diisobutanoyl peroxide, dioctanoyl peroxide, dilauroyl peroxide, dicumyl peroxide, di-t-amyl peroxide t-butyl per-2-ethylhexanoate, t-butyl peroxymaleate, cumyl peroxyneodecanoate and t-amyl peroxynodecanoate cumene hydroperoxide, pinane hydroperoxide, p-menthane hydroperoxide. ; nitriles, such as 2,2'-azobis (methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis (2,4-dimethylvaleronitrile), and similar compounds. Preferred initiators are short half-life peroxides, for example diacyl peroxides, such as diacetyl peroxide, di-n-propionyl peroxide and di-isobutyryl peroxide; as well as 2953844 -7 1,1,3,3-tetramethylbutyl peroxymethoxyacetate, 2-bis (2,2-dimethylpropanolperoxy) -4-methylpentane, diisobutanoyl peroxide, cumyl peroxyneodecanoate and t-peroxyneodecanoate. amyl. The initiator concentration for the polymerization step is advantageously between 0.001% and 2% by weight relative to the total weight of the polymerization medium. The polymerization step of the process according to the present invention may be carried out optionally in the presence of one or more surfactants or one or more dispersing agents. Any suitable surfactant or dispersant known to those skilled in the art may be employed. The polymerization stage of the process according to the invention may be carried out optionally in the presence of other additives than the additives mentioned above (surfactants, dispersing agents) making it possible to improve the implementation of the process and / or the characteristics of the process. resulting polymer. Examples of other additives are chain transfer agents, anti-crusting agents, antistatic agents and co-solvents. The polymerization step of the process according to the invention is usually of an exothermic nature and can be carried out in any suitable type of continuous reactor provided that it withstands the high temperatures and pressures necessary for the production. of the process. One or more reactors connected in series can be used. The reactors that may be used may be chosen, for example, from continuous reactors of the mechanical agitation tank type and the tubular reactors. In the case of continuous reactors of the stirred tank type, several of them are advantageously connected in series. In the case of tubular reactors, one advantageously uses one. These reactors may advantageously be equipped with a heating system and / or a cooling system for controlling the temperature. Tubular reactors are preferred for carrying out the process according to the invention and, among these, tubular reactors with a plug-flow type of flow. By tubular reactor is meant a reactor comprising an elongated tube within which the reaction medium circulates, the heat exchange required for thermal control being through the wall of the tube, for example by means of a double envelope inside which circulates, generally against the current, a coolant (water). The elongate tube may consist of a plurality of straight segments interconnected by 180 ° elbows, together forming a compact assembly. The polymerization step of the process according to the invention is advantageously carried out so that the mean residence time of the reaction medium in the reactor is between 5 and 60 minutes, preferably between 10 and 35 minutes. Frequently, 20 to 90% and, more often, 40 to 80% by weight of the monomer are polymerized in the reactor. In other words, frequently 10 to 80%, more often 20 to 60% by weight of monomer is not converted to polymer. This unconverted monomer should preferably be recycled for the economy of the process. The conversion rate of the monomer to polymer is governed in particular by the choice of temperature and pressure and by the choice of the nature and the concentration of the initiator. The reactor is advantageously fed at least with L / SC CO2, monomer and initiator, to which are optionally added one or more additives mentioned above. The respective amounts of CO2 and monomer introduced into the reactor are such that the weight ratio of CO2 to monomer is generally between 0.5 / 1 and 100/1, preferably between 0.6 / 1 and 30/1. The best results are obtained when the ratio by weight [CO2 / monomer] is between 0.6 / 1 and 5/1, especially between 0.8 / let 3 / 1. The order in which the CO 2 SC, the monomer and the initiator are introduced into the reactor is not particularly critical. Generally, the monomer, withdrawn from a storage vessel and then compressed and heated to the pressure and temperature at which the polymerization step is carried out, is mixed with the L / SC CO2, before adding the initiator to the mixture. thus formed. Most often, a predominant proportion, or even almost all of the CO2 L / SC, circulates in a closed loop in the device in which the process according to the invention is carried out and is therefore recycled to the reactor in a mixture with the monomer not converted into polymer (i.e., not polymerized in the reactor). In the process according to the invention, there are therefore two sources of supply of the monomer reactor: the first source comprises the fresh monomer, withdrawn from the storage container and then compressed, as mentioned above; The second source comprises the monomer, not converted into polymer, recycled. This second monomer reactor feed source may itself comprise several streams, preferably two streams. Indeed, the process according to the invention comprises the recompression (4) of the unconverted polymer monomer mixture and CO2 (second stream). This recompression can advantageously be carried out in several stages, preferably in two stages, such as (see below) the expansion (1) of the suspension coming from the reactor and its separation (2). It can then happen that a monomer-rich liquid is condensed at an intermediate pressure, and separated from a CO2-rich vapor phase, and that the resulting streams are then recycled separately to the reactor feed. For carrying out the process according to the invention, it may be advantageous for the feed of the reactor to be recompressed and recycled in the liquid, gaseous or supercritical state, unconverted polymer monomer and CO2 (second stream), or performed at a plurality of points. It has indeed been found that if part of the feed of the recycled mixture reactor is operated in a delayed manner, by injecting said mixture at several distinct points, succeeding each other over the entire length of the reactor, downstream from the point of contact. feeding fresh monomer, this makes it possible to contribute advantageously to the control of the temperature of the polymerization and thus to reduce the amount of heat transfer fluid required for this control. As a corollary and for the same purpose, it is possible to inject into the reactor, in the same stepped manner, the initiator portions necessary for the polymerization. In this case, initiators with a sufficiently short half-life will preferably be chosen so that active initiator residues do not remain at the outlet of the reactor. The organic radical initiator is thus preferably also introduced at a plurality of points to the reactor. The number of points by which the feed of the unconverted polymer monomer and CO 2 and possibly the initiator can optionally be fed is not critical and depends on the geometry of the reactor. Feeding the reactor in a second stream, and preferably also as an initiator, is advantageously carried out at a number of points of between 2 and 20, preferably between 3 and 10 and particularly preferably between 4 and 8. According to the invention the slurry from the reactor, which is generally in the form of polymer particles dispersed in the liquid, gaseous or supercritical mixture of CO 2 and unconverted polymer monomer (dispersant phase), is subjected to relaxation (1) and a separation (2). Optionally, the expansion (1) to which the suspension from the reactor is subjected and the separation (2) of the expanded suspension from the reactor 5 first and second flow can be advantageously carried out in several steps at successively decreasing pressures. Preferably, the detent (1) and the separation (2) can be operated in two steps (With regard to the relaxation (1), these two stages are also called "first detent" and "second detent" in the present description).

This procedure advantageously reduces the amount of energy required for the recompression of the recycle mixture of CO2 and unconverted polymer monomer. The suspension from the reactor can then be brought back first advantageously, by a first expansion, to an intermediate pressure between that prevailing in the reactor and atmospheric pressure.

This intermediate pressure must be sufficiently low so that the density of the liquid, gaseous or supercritical mixture of CO2 and unconverted monomer in polymer decreases substantially, by simple decompression in the supercritical state, or by vaporization. By "substantially reducing the density of the liquid, gaseous or supercritical mixture of CO 2 and unconverted monomer to polymer" is meant, in the present description, a reduction of this density advantageously by at least a factor of 3, preferably at least a factor of 5. This intermediate pressure is advantageously below the critical pressure, preferably between 2 and 20 times lower, more particularly between 5 and 16 times lower and, very particularly preferably, between 7 and 13. less than that prevailing in the reactor. This first expansion of the suspension from the reactor can be carried out in any known device, such as, for example, an automatic valve. In the case of this optional variant, the second expansion of the suspension from the reactor can be carried out preferably on the optionally heated solid phase of this suspension, to bring it from the intermediate pressure, reached after the first expansion, to the atmospheric pressure. The separation (2) of the halogenated polymer contained in the solid phase of the relaxed suspension from the reactor (first flow) can be by any known means for separating a solid and a gas. Preferably, in view of the generally very fine particle size of the polymer obtained according to the process of the invention, filtration means, preferably sleeves, are used to effect this separation. It is preferred to carry out at least a partial separation of the solid phase of the suspension, after each expansion, with concomitant evacuation, for recycling to the reactor, of the dispersing phase comprising CO 2 and the unconverted polymer monomer.

It may be advantageous and even necessary to recycle the monomer, which is not converted into polymer, which would have possibly condensed on the solid part of the suspension (the synthesized polymer) during expansion (1). According to the invention, heating of said suspension is carried out for this purpose, with recovery of the frigories generated by the expansion, which avoids the partial condensation of the CO 2 mixture with the unconverted monomer, which constitutes the dispersing phase of suspension. This reheating advantageously simplifies the subsequent separation of the dispersed solid phase of the suspension, consisting essentially of the synthesized polymer. According to the invention, the reheating (3) is applied to the relaxed suspension, advantageously before or after its separation in first and second streams, and is associated with the recovery of the frigories generated by the expansion. Reheating (3), associated with the recovery of frigories, can be applied to the first stream from the separation of the relaxed suspension, containing the solid phase comprising the halogenated polymer. It can also be applied after each step constituting the trigger (1). Preferably, however, the heating, associated with the recovery of the frigories, is performed after the first expansion of the suspension from the reactor. This reheating may be carried out in any appropriate device for performing it during an operation simultaneously involving the cooling of the mixture, recycled to the reactor, of CO2 and unconverted polymer monomer. This cooling is advantageously achieved through the recovery, preferably simultaneous, frigories generated during the relaxation of the suspension from the reactor. In this case, the appropriate device for reheating is preferably a cross exchanger providing a heat exchange between, on the one hand, the relaxed suspension from the reactor, which passes through it in one direction and, on the other hand, the mixture of CO2 and unconverted monomer into polymer which travels in the other direction. According to a preferred embodiment of the method according to the invention, it is characterized in that the expansion (1) of the suspension from the reactor is operated in several stages and in that the heating (3) and the recovery of frigories associated with it are performed after the first relaxation. The reheating (3) and the recovery of the frigories generated by the trigger are preferably simultaneous. The solid halogenated polymer obtained according to the process of the invention is generally collected in the form of particles of submicron granulometry. It can optionally be supplemented with conventional additives. Among these additives, mention may be made of: conventional stabilizers derived from calcium, tin or zinc, for example; plasticizers, such as adipates and organophosphates, for example; Inorganic and organic pigments; fillers, such as calcium carbonate, talc and dolomite, for example; lubricants, such as organic waxes, alcohols and fatty acids, esters or metal salts for example; Flame retardants, such as metal oxides for example. The resulting solid halogenated polymer is generally degassed by conventional means to remove residual traces of unconverted monomer. In general, the solid halogenated polymer obtained contains, according to the efficiency and the number of the slops of the suspension coming from the reactor, between 0.2% and 5% by weight, preferably between 0.5% and 3% by weight of monomer. not converted. Among the conventional means for removing residual traces of unconverted monomer (also called "degassing" in the present description), mention may be made of placing it under vacuum, preferably under vacuum, and by means of a carrier gas. or an inert vapor.

In the present description, the terms "degassing" and "degassing" are used to define the passage of the unconverted monomer, present in dissolved, absorbed, adsorbed or occluded form in the solid halogenated polymer, to the gaseous state. According to a variant of the process of the invention, it may be advantageous to combine, continuously, the preparation of the halogenated polymer and its degassing and granular forming to obtain a ready-to-use polymer. The granular shaping of the halogenated polymer is generally carried out in an extruder. By extruder is meant any continuous device comprising at least one feed zone and, at its outlet, an evacuation zone preceded by a compression zone, the latter forcing the melt to pass through the zone. discharge. The evacuation zone is advantageously followed by a granulation device, giving the extruded polymer its final shape. Advantageously, it uses known extruders based on the work of a screw or several screws, preferably two screws which, in the latter case, can cooperate in co- or counter-rotating mode (same 5 directions of rotation or opposite direction of rotation). Counter-rotating twin-screw extruders are preferred because they avoid excessive shear forces that can lead to polymer degradation. The extruder used for carrying out the process of the invention according to this variant is arranged in such a way that it advantageously comprises at least one feed zone, a melting or gelling zone, at least one degassing zone. optionally an additive introduction zone and an evacuation zone. The order in which the areas between the supply and the evacuation are sequenced may be different from the one in which they are mentioned above. Each of these zones has a very specific function and is at a very specific temperature. The extruder preferably comprises at least one degassing zone which itself advantageously comprises at least one degassing vent. At least one degassing vent preferably communicates with means for evacuating, such as for example a vacuum pump, to proceed with the removal of the unconverted residual monomer contained in the halogenated polymer. Preferably, the extruder comprises at least two degassing zones, the first of which is maintained at a pressure close to atmospheric pressure, and the second operates at reduced pressure. The second gaseous or supercritical liquid stream, which is a mixture of CO2 and unconverted polymer monomer from separation (2) of the expanded slurry from the reactor, is (step (4)) recompressed, optionally cooled. , then recycled to the reactor feed where the polymerization step of the process according to the invention is carried out. Optionally, the unconverted polymer monomer from degassing of the halogenated polymer may also be recompressed, cooled, and recycled to the reactor. For this purpose, it can be advantageously recycled separately to the reactor or, preferably, combined with the second stream to produce a further mixture of unconverted polymer monomer and CO2. In the case where the degassing technique releases the monomer into several discrete sources (as for example in an extruder with several degassing zones), it is up to those skilled in the art to determine the practical interest of recycling. each source of monomer. The recompression (4) of the mixture of CO2 and unconverted polymer monomer (second stream) can be carried out in several steps. Preferably, in at least one of these steps, the recompression (4) is carried out at a pressure and a temperature suitable for providing condensation in the liquid phase. The pressure applied to the mixture during its recompression during this step is, for example, between atmospheric pressure and 10 MPa, preferably between atmospheric pressure and 5 MPa.

Cooling of the mixture of CO2 and unconverted monomer to polymer (second stream) is optional in that the recompression (4) can be carried out under isothermal conditions (eg in a liquid ring pump) or under conditions involving a release of heat energy (resulting from the transformation of the work performed for example by a conventional compressor). For practical reasons, in particular when the recompression (4) is not performed under isothermal conditions, it is however preferred to cool the second stream. This cooling can take place in any suitable device, such as one or more condensers and / or one or more heat exchangers, but it is preferred to perform it, at least in part, simultaneously with the reheating of the relaxed suspension. of the reactor, thanks to the recovery of the frigories generated during the relaxation of said suspension. In this case, the appropriate device for cooling the second stream is the cross exchanger whose function has been explained above. The mixture of CO2 and unconverted monomer into polymer, as well as optionally unconverted monomer into the polymer from degassing, advantageously compressed to the supercritical state, or compressed and recondensed in the liquid phase, are then recycled to the feed. of the reactor as described above. This recycling may advantageously comprise one or more purification steps. These may be based on any techniques known to those skilled in the art, including partial condensation, settling, adsorption and their combined use. According to another aspect, the invention also relates to a device, comprising at least one pressurized reactor for the polymerization of at least one halogenated monomer in a medium comprising carbon dioxide in the liquid state (CO2 L) or supercritical (CO2 SC). This device is in particular usable for the implementation of the process for the continuous preparation of a halogenated polymer according to the invention. The characteristics and preferences defined above for the method according to the invention also advantageously apply to the device according to the invention.

This device comprises at least, in addition to the pressurized reactor: a fresh halogenated monomer storage vessel; means for compressing the halogenated monomer at the pressure at which the polymerization step is carried out; means for supplying the reactor with unconverted polymer monomer, and a mixture of this unconverted monomer with CO2, compressed in the liquid, gaseous or supercritical state and recycled; preferably these means are arranged to inject these compressed and recycled fluids at the point of supply of fresh monomer but also at several distinct points downstream thereof, succeeding over the entire length of the reactor; Means for relaxing the suspension from the reactor and comprising the polymer synthesized in the liquid, gaseous or supercritical mixture of CO 2 and unconverted polymer monomer; advantageously, these means can be arranged so that the expansion experienced by the suspension from the reactor is operated in several steps at successively decreasing pressures; preferably, this expansion is carried out in two steps: the first step may advantageously consist of a relaxation to an intermediate pressure between that prevailing in the reactor and atmospheric pressure, sufficiently low for the density of the mixture of CO2 and monomer Unconverted polymer substantially decreases by simple decompression in the supercritical state, or by vaporization; the second step may itself be operated in two stages, the first time resulting in a pressure, intermediate between that obtained after the first step and atmospheric pressure, and low enough so that the density of the dispersing phase decreases substantially. Whatever the number of intermediate expansions, the pressure downstream of the last expansion is advantageously between 0.09 MPa and 0.15 MPa. means for heating the suspension coming from the reactor after its expansion, advantageously after the first expansion, associated with means for recovering the frigories generated by the expansion; these means for heating the expanded suspension from the reactor and the means for recovering the frigories generated by the expansion of the suspension are advantageously associated in the form of a cross exchanger ensuring a heat exchange between, a on the other hand, the relaxed suspension, which runs through it in one direction and, on the other hand, the mixture of CO2 and unconverted monomer into the polymer which travels in the other direction; Means for separating the expanded slurry from the reactor into a first stream, containing the solid phase comprising the halogenated polymer, and a second stream, liquid, gaseous or supercritical; means for recovering this first and second stream; means for recompressing the second stream, which is a mixture of CO2 and unconverted monomer into polymer and, optionally, the monomer resulting from the degassing of the polymer, in whole or in part; optional means for cooling the second stream; means for recycling the second stream to the reactor feed, with a supplement of fresh monomer and, optionally, CO2; Means for degassing the halogenated polymer, optionally in combination with continuous means for shaping the polymer into granules; preferably these means take the form of an extruder with two counter-rotating screws; in a particularly preferred manner, the extruder comprises at least one degassing zone which itself comprises at least one degassing vent communicating with a means for evacuating, such as for example a vacuum pump, in order to carry out the degassing vent. removal of the unconverted residual monomer contained in the halogenated polymer. The continuous process of preparation and the device according to the invention advantageously make it possible, more economically than by conventional polymerization processes in aqueous medium, to obtain dry, ready-to-use halogenated polymers of high purity. , in view of the absence of the compounds generally present in the aqueous conventional polymerization medium of most halogenated monomers. A particular embodiment of the method of the invention and a device for its implementation will now be illustrated with reference to the accompanying drawing. This drawing consists of the appended FIG. 1 schematically representing a practical embodiment of the aspects of the invention, without limiting its scope. According to this embodiment, a carbon steel tubular reactor C comprises (not shown in detail or to scale) six 300 m length sections consisting of straight segments connected by 180 ° bends and jacketed 2953844 An envelope D, itself also in several sections, in which circulates a cooling fluid F. This reactor is fed via the monomer pump B which carries it at a pressure of 15 MPa and at a temperature of 50 ° C, fresh vinyl chloride (monomer) from fresh monomer storage tank (liquid) A and liquid vinyl chloride recycled via line R3. This reactor is also fed with a recycled mixture of vinyl chloride in CO2 SC, respectively, for 20% of the volume of this mixture, via line R2 and, for the remaining 80% of volume, via the injectors schematized at R1. In order to convey at a relatively constant temperature and at an appropriate speed the increasing flow of CO2 SC and monomer to be polymerized circulating in the reactor, the segments of the reactor and their lining D are of increasing diameter between the inlet and the outlet of the reactor. A solution of a heavy organic peroxide in an aliphatic monoalcohol is also introduced into the reactor (feedstock not shown) as an initiator, by injection into the reactor at the same points as the mixture of monomer and recycled CO2 via the pipe. R2 and the injectors Ri. The suspension leaving the reactor first undergoes adiabatic expansion at 1.4 MPa per passage through the automatic expansion valve E. To prevent the unconverted polymer monomer from condensing on the solid part of the suspension, the latter is heated in the cross exchanger O traveled in the other direction by the V4 vaporized mixture of unconverted polymer monomer and CO2. The solid part of the suspension thus expanded and heated, consisting essentially of the synthesized polymer, is then separated in the bag filter Fi, the vaporized part, comprising the unconverted polymer monomer and the CO2, being discharged in Vi for recycling. . The solid part of the suspension separated in the bag filter Fi is subjected to a second expansion at 0.1 MPa via the lock valve (upstream-downstream) or lock G and again separated, in the bag filter F2, the generated vapor phase which is evacuated in V2 to be recycled. The solid polymer (Pol) collected in the form of particles of submicron granulometry, optionally supplemented with stabilizers and conventional additives (Add), is introduced via the hopper H into a counter-rotating twin screw extruder I. After melting / plasticizing the in a first zone of the extruder, the unconverted residual monomer contained in the polymer powder (approximately 1% by weight) is removed in a first vent at atmospheric pressure and at 180.degree. under vacuum at about 133 Pa (absolute) and at a temperature of 180 ° C in a second vent. Vacuum is achieved by vacuum pump K. Finally granules (GR) of ready-to-use vinyl chloride polymer are obtained. The unconverted residual monomer degassed at atmospheric pressure is removed in V3 (vapor phase) for recycling.

The unconverted polymer monomer, evacuated to V3 (vapor phase) and the mixture of unconverted monomer and CO2, evacuated in V1 (vapor phase) and V2 (vapor phase) are recompressed at 2.5 MPa via multi-stage compressors. L1 and L2 with cooling via the heat exchanger M. In the vapor / liquid separator or decanter N is the separation between a liquid phase and a vapor phase comprising the vaporized mixture of unconverted monomer and CO2. The liquid phase comprises a two-phase liquid mixture, the upper organic liquid phase (porg), consisting essentially of monomer, is recycled to R3 to the pump B and the lower aqueous phase (() aq) (from the impurities contained in the additives and initiator solutions) is removed from the device. The vapor phase comprising the mixture of unconverted monomer and CO2, recycled to V4, with additional optional fresh CO2 via the tank A2, is condensed via the cross exchanger O by heat exchange with the suspension from the reactor C flowing through the exchanger in the other direction after adiabatic expansion by passing through the automatic expansion valve E (see above). The condensed mixture of unconverted monomer and CO 2 is then purged from the inert gases at P (vapor / liquid separator) and recycled to the reactor at R 1 and R 2 by means of the pump Q, as mentioned above.

Claims (8)

CLAIMS1 - Continuous process for preparing a halogenated polymer comprising a step of polymerizing at least one halogenated monomer in a medium comprising carbon dioxide in the liquid or supercritical state (CO2 L / SC), in at least one reactor under pressure, said process being characterized in that it comprises at least: - (1) an expansion of the suspension from the reactor; - (2) a separation of the expanded slurry from the reactor into a first stream, containing the solid phase comprising the halogenated polymer and into a second stream, liquid, gaseous or supercritical, which is a mixture of CO2 and unconverted monomer; polymer; and recovering this first and second streams; - (3) a reheating, applied to the relaxed suspension, before or after its separation in first and second flow; - (4) a recompression of the second stream and its optional cooling, for recycling to the reactor feed, with a supplement of fresh monomer and, optionally, CO2; (5) a final step of degassing the halogenated polymer, optionally in combination with a continuous technique for shaping the polymer into granules; said method being further characterized by the fact that the reheating (3), applied to the relaxed suspension, is associated with a recovery of the frigories generated by the expansion. 2 - Process according to claim 1, characterized in that the polymerization step is carried out in a medium consisting of carbon dioxide in the supercritical state. 3 - Process according to claim 1 or 2, characterized in that the halogenated polymer is a vinyl chloride polymer. 2953844 - 20 - 4 - Process according to any one of claims 1 to 3, characterized in that the polymerization step is carried out in the presence of an organic radical initiator. 5 - Process according to any one of claims 1 to 4, characterized in that the expansion (1) of the suspension from the reactor is operated in several stages and that the heating (3) and the recovery of the frigories which it is associated with it are carried out after the first relaxation. 6 - Process according to any one of claims 1 to 5, characterized in that the heating (3) and the recovery of frigories generated by the trigger 10 are simultaneous. A device comprising at least one pressurized reactor for the polymerization of at least one halogenated monomer in a medium comprising carbon dioxide in the liquid or supercritical state, said device further comprising at least one container storing fresh halogenated monomer; means for compressing the halogenated monomer at the pressure at which the polymerization step is carried out; means for supplying the reactor with unconverted polymer monomer and a mixture of this unconverted monomer with CO2, 20 tablets in liquid, gaseous or supercritical state and recycled; - Means for relaxing the suspension from the reactor and comprising the polymer synthesized in the liquid mixture, gaseous or supercritical CO2 and unconverted monomer polymer; means for heating the suspension coming from the reactor after its expansion, associated with means for recovering the frigories generated by the expansion; - Means for separating the expanded suspension from the reactor into a first stream, containing the solid phase comprising the halogenated polymer, and a second stream, liquid, gaseous or supercritical; means for recovering this first and second stream; Means for recompressing the second stream, which is a mixture of CO2 and unconverted monomer into polymer and, optionally, the monomer resulting from the degassing of the polymer, in whole or in part; optional means for cooling the second stream; Means for recycling the second stream to the reactor feed, with a supplement of fresh monomer and, optionally, CO2; means for degassing the halogenated polymer, optionally in combination with continuous means for shaping the polymer into granules. 10 8 - Device according to claim 7, characterized in that the means for heating the relaxed suspension from the reactor and the means for recovering the frigories generated by the expansion of the suspension are associated in the form of a cross exchanger ensuring a exchange of heat between, on the one hand, the relaxed suspension, which travels in one direction and, secondly, the mixture of CO2 and unconverted monomer polymer that travels in the other direction.