SOLID PHASE POLYESTER POLYCONDENSATION WITH PROCESS GAS CLEANING The invention relates to a method for manufacturing a high molecular weight polyester from a solidified polyester prepolymer through solid phase polycondensation, wherein the cleavage products from Polycondensation from the solid phase polycondensation reaction are removed from the product through a process gas, and the process gas is subsequently purified to remove the cleavage products by polycondensation and essentially recycle. The invention also relates to a device for manufacturing a high molecular weight polyester with a crystallization unit and a reaction unit. Methods for making high molecular weight polycondensates in a solid phase polycondensation step are known in the art. Usually, here a process gas is at least partially circulated. This process gas must be subjected to at least partial purification here. These purifications are known, for example, from EP 1100611, wherein the purification is carried out through catalytic combustion. The disadvantage found here includes the relatively high combustion temperatures that must be achieved to achieve complete combustion. This is especially problematic when it comes to processing polyesters loaded with foreign substances. Gas purification systems are also known in which a gas cleaner employing an organic fluid as a washing liquid is used, which is subsequently reprocessed and consumed in a preceding step in the polyester manufacturing process (condensation step in liquid phase ). For example, ethylene glycol is used as the washing liquid for the manufacture of polyethylene terephthalate. The disadvantage of this method is that the condensation step in the liquid phase and the solid phase condensation step and the solid phase polycondensation step must be interconnected. The object of this invention is to provide a method for polycondensation in solid phase of polyesters, said method should be able to be implemented with an improved energy efficiency and independently of any potential application for an organic washing liquid. This object is achieved according to claim 1 and according to claim 17. According to the present invention, the process gas is here purified through an aqueous washing liquid. In accordance with the present invention, the process gas is purified using a gas cleaner operated with an aqueous washing liquid. Useful embodiments of the method according to the present invention and the device according to the present invention are described in the dependent claims. Polyester Polyester is a crystallizable thermoplastic polyester such as for example polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polyethylene naphthalate (PEN), which is present either in the form of a homopolymer or in the form of a copolymer, and which is obtained through a polycondensation from its monomers, diol component, and dicarbonic acid component, accompanied by the dissociation of the low molecular weight reaction product. Several components of diols, mainly linear or cyclic, are used. Several mainly aromatic dicarbonic acid components can also be used. The dicarbonic acid can also be replaced by its corresponding dimethyl ester. The polycondensation can be carried out here directly between the monomers or through an intermediate step, which is subsequently replaced by transesterification, wherein the transesterification can be carried out again with the dissociation of the low molecular weight reaction product or through ring-opening polymerization. . The polyester obtained in this way is essentially linear, where a small number of branches can be formed. The polyester can be a new material, a recycled material or a mixture of new material and recycled material. Additives can be incorporated into the polyester. Suitable additives include catalysts, dyes and pigments, UV blockers, processing aids, stabilizers, impact modifiers, chemical and physical types of foaming agents, fillers such as for example nucleating agents, barriers or particles that improve the mechanical properties , reinforcing bodies such as balls or fibers, as well as reactive substances, such as oxygen absorbers, acetaldehyde absorbers or substances that increase molecular weight, and so on. Polyethylene terephthalate Polyethylene terephthalate is a crystallizable thermoplastic polyester obtained in a polycondensation reaction accompanied by the dissociation of low molecular weight reaction products. The polycondensation can be carried out here directly between the monomers, or through an intermediate step which is subsequently replaced by transesterification, where the transesterification can be carried out again with the dissociation of the low molecular weight reaction product. The polyester obtained in this way is essentially linear, in which a small number of branches can be formed. Polyethylene terephthalate is obtained from a monomer, a diol component and a dicarbonic acid component, wherein the diol components consist largely of ethylene glycol (1,2-ethanediol), and the dicarbonic acid components consist of to a large extent of terephthalic acid. Other dicarbonic acid compounds and linear, cyclic or aromatic diols are possible as the comonomers. Typical comonomers include diethylene glycol (DEG), isophthalic acid (IPA) or 1,4-bis-hydroxymethylcyclohexane (CHDM). Polyester prepolymer Polyester prepolymers are polymerized or polycondensed in a prepolymer in a first stage in the liquid phase. The polyester prepolymer melt obtained in this way is usually manufactured in a continuous process. An esterification step is initially carried out there, and then a prepolicondensation step. In the conventional polyester manufacturing process, a polycondensation step follows in the high viscosity reactor (also known as a finisher). (For example, see: Modern Polyesters, Wiley Series in Polymer Science, Edited by John Scheirs, J. Wiley &; Sons Ltd., 2003, Chapter 4.2). The production of polyester can also be carried out in a batch process (see, for example: Modern Polyesters, Wiley Series in Polymer Science, Edited by John Scheirs, J. Wiley &Sons Ltd., 2003, Chapter 4.1) . Alternatively, the polycondensation step in the high speed reactor may be omitted. This provides a low viscosity polyester prepolymer with a level of polymerization (DP) which is clearly above the level of polymerization of the polyester after the subsequent solid phase treatment. The polymerization level of the low viscosity prepolymer is usually less than 60%, in particular less than 50% of the level of prepolymerization of the postcondensate polyester in the solid phase. Another alternative is to use polycondensated polyesters which, in their crude form, are present in the form of bottle waste, or in a homogenized form due to melting. In the same way, a polyester already polycondensed can be brought to an appropriate viscosity level by melting and depolymerization. Granulation The fusion of polyester prepolymer is usually solidified by granulation, even when alternative solidification methods can also be used in order to generate solid prepolymer particles, such as for example sinter wires, cutting film or chilled grinding pieces. During the granulation, the prepolymer melt is transferred to a defined and solidified mold. The prepolymer melt is pressed here through a die with an opening or with several openings, and cut or subjected to dripping. The die openings are usually round but may also have another profile, for example, openings in the form of slots. The cut can be made both directly at the exit of the die and after passing through a treatment passage first. Cooling solidifies the prepolymer melt. This can be done using a liquid cooling medium (for example, water, ethylene glycol) or a gaseous cooling medium (for example, air, nitrogen, steam), or through contact with a cold surface, where it can also conceive a combination of cooling means. The cooling can take place both simultaneously with the molding of the particles either before said molding or after said molding. Given a prepolymer melt from the conventional polyester manufacturing process, granulation processes such as strand granulation, ring-in-water granulation, granulation under immersion or head granulation (also known as hot-face granulation) are normally used. . Given a low viscosity prepolymer melt, granulation processes such as immersion, ring-in-water granulation, low-water granulation or head granulation (also known as hot-face granulation) are commonly used. The average size of the granulate is usually within a range of 0.4 mm to 10 mm, preferably 0.7 mm to 3 mm. The statistical average for the average diameter of the granulate derived from the average of the granulate height, length and width applies as the average granulate size. The granulates can have a defined form of granulate, for example, they can have a cylindrical, spherical, drop-shaped, and ball-shaped form, or they can have a design form, such as, for example, the type proposed in the document. EP 0541 674, or may present a non-uniform grain product form that arises from a grinding process or a breaking process. Solid granules or porous granules can be used, for example, obtained through sintering, foaming and the like. Solid phase polycondensation The term "solid phase polycondensation" is used to summarize all the process steps necessary to manufacture a high molecular weight polyester from solidified prepolycondensed solid phase. They include steps to heat, crystallize, react and cool. Additional processing steps, for example, treatment with additives, may also be included. The procedure steps are carried out in the respective devices suitable for the passage. However, several procedural steps can be implemented in a single device simultaneously or in several stages. In the same way, several devices can be used for a procedure step. The individual devices are interconnected here by descending pipes or transport lines. Transfer channels can be placed between the individual devices, for example, cell wheel transfer channels or alternate interval isolations. In accordance with the present invention, the polyester prepolymer is supplied to the "cold" process, for example, in the present form after storage in a silo, container or small tambo. This means that it is not sent to the process, conserving a significant part of the calorific content coming from a previous procedural step. Accordingly, the polycondensate prepolymer is directed to the process at an ambient temperature of less than 40 ° C, in particular within a range of -20 to 25 ° C. During solid phase polycondensation, the temperature of the product is increased by at least 160 °, in particular by at least 180 °, which raises the temperature of the product to at least 185 ° C, in particular to at least 205 ° C. Since the energy efficiency of a process can be increased with the rise of the yield, yields between 100 and 800 tonnes per day are common, although even higher yields can be found, or lower yields within a range of 10 to 100 tonnes in the case of special applications such as technical fibers or operations recycling. In spite of the energetic advantages that are obtained when all the steps of the procedure are arranged one over the other and when the product is transferred from one procedural step to the next through gravitation, it is particularly beneficial in systems with high performance rates that the polycondensate is raised through a conveyor to a higher level at least once during the process to limit the overall height of the system. Crystallization The level of crystallization of the prepolymer granulates is high in accordance with methods known in the art. The crystallization is usually carried out thermally, which provides a thermally partially crystallized polyester. Solvent-induced crystallization and crystallization through mechanical stretching can also be used, but are less suitable. The crystallization can be carried out in several stages, that is, before, during and after the granulation step. For this purpose, the prepolymer granules must be treated at a suitable crystallization temperature. The crystallization must result in at least one level of crystallization which allows the treatment during subsequent solid phase polycondensation, without conglutination or lumping there. The appropriate temperature range is evident when the half-life period (t? / 2) of crystallization measured in the DSC is recorded as a function of temperature. It is limited up and down by and, the temperature at which the half-life of crystallization reaches approximately 10 times the average half-life of crystallization. Since very short crystallization half-lives (t? / 2) are difficult to determine, t? / 2 = 1 minute is used as the minimum value. In the case of polyethylene terephthalate, the temperature is within a range between 100 and 220 ° C, and a crystallization level of at least 20%, preferably at least 30%, is reached.
According to the present invention, the temperature of the prepolymer granules after the granulation step is below the suitable crystallization temperature. As a result, the prepolymer granulates must be heated. For example, this can be done using a heated wall of the crystallization reactor, integrated units heated in the crystallization reactor, either radiation, or through the injection of a hot process gas. The suitable crystallization time is found from the time required to heat the product to the point of crystallization plus at least the half-life of crystallization at the given temperature, where 2-20 periods of half-life are added with greater preference to the heating time in order to achieve sufficient crystallization. To avoid the conglutination of the prepolymer granules in the crystallization process, they must be kept in motion in relation to each other. For example, this may be achieved using an agitator, a mobile container or by exposure to a fluidizing gas. Particularly suitable crystallization reactors include fixed bed crystallizers or fluidized bed crystallizers, since these crystallizers have no tendency to generate dust.
At the same time that the crystallization level is raised, any residual liquid is also removed from the granulation process. If a process gas is recirculated in the crystallization process, it must receive a sufficient quantity of fresh gas or purified process gas to avoid excessive enrichment of the liquid. The process gases used for the solid phase polycondensation can also be used in the crystallization step where several process gases can also be used in the various process steps. In one embodiment of this invention, the polycondensate is heated prior to crystallization, which is effected using a process gas stream from a step in the later course of the solid phase polycondensation. A stream of process gas from a passage to cool the polycondensate is particularly suitable for this purpose. Especially preferred is a process gas stream coming from a passage that is made in the air. The polycondensate must be heated to a temperature Tv higher than 50 ° C, in particular higher than 75 ° C, where Tv is more preferably within a range between Tg = -30 ° C and Tg = + 30 ° C, in particular from Tg to Tg + 20 ° C, where Tg refers to the glass transition temperature of the polycondensate. The heating can be carried out in any desired reactor. A device in which the product is displaced is preferred, for example, a stirred reactor or a moving reactor, a fixed bed unit or a fluidized bed unit. Especially preferred is a device that can be operated in variable gas production amounts and a mg / mp ratio of > 2 which makes it possible to regulate the outlet temperature of the product from the heating step, still given the fluctuations in the process gas stream temperature, from one step during the subsequent progression of the solid phase polycondensation through the amount of gas flow. The heating is preferably carried out at an average retention time that ranges from a few minutes to a maximum of one hour, but can also occur in a storage tank during a longer retention period, if Tv does not exceed one hour , in particular 20 minutes. The heating of the polycondensate is simultaneously accompanied by a drying of the polyester, if it continues to react with moisture or another volatile substance. This makes it possible to reduce the volatile substances in the subsequent process steps, thereby reducing the level of contamination of the accompanying process gas streams and thereby decreasing the amount of gas that must be purified. In particular, a wet polycondensate is dried due to the granulation. For this purpose, the amount of process gas and the temperature are selected in such a way that the polycondensate is dried to a moisture content between 50 ppm of water and 2000 ppm of water, in particular between 200 ppm of water and 1000 ppm. of water. Solid phase polycondensation reaction The molecular weight of the polyester granules is brought to a higher polymerization level in a solid phase polycondensation reaction with the dissociation of the cleavage products by polycondensation. If a granulate from the prepolymer melt obtained from a conventional manufacturing process is present, the level of polymerization is normally elevated by a value comprised between 10% and 70%, with an increase of at least 0.10 dl / g being preferred. If a granulate from the low viscosity prepolymer melt is present, the polymerization level is raised to about 1.5 times, in particular to at least 2 times the prepolymer. The solid phase polycondensation reaction is carried out in accordance with methods known in the art, occurring initially at a suitable postcondensation temperature at least during the heating step, and during a step in the post-condensation reaction. Essentially continuous processes are used here, for example, the processes that occur in devices such as fixed-bed reactors, solid-air jet reactors or fluidized bed reactors, and in reactors with stirring attachments or moving reactors, such as rotary kilns. The polycondensation reaction in solid phase can be carried out either at normal pressure, at elevated pressure, or in vacuum. In accordance with the present invention, the polycondensation dissociation products from the solid phase polycondensation reaction are removed through a process gas (carrier gas). In continuous processes with process gas, the process gas here flows around the polycondensate to co-current, counter-current, or cross-current. The amount of carrier gas must be sufficient to discharge the reaction products that diffuse into the surfaces of the particles together with the contaminants, such as carbonyl compounds from the manufacturing process or contaminants from the previous use coming from the reaction step. If the heating step is carried out through exposure to a process gas, a high specific gas amount is used (mg / mp = 2 to 20, in particular 5 to 13), as a result of which the temperature of the product is essentially close to the temperature of the gas. If the heating step is carried out by other means of energy supply, for example, through a heated surface, or radiation, it is still useful to pass a process gas through the product or to apply a vacuum. The postcondensation reaction step can be carried out at the specific low gas quantity (mg / mp = 0.1-1.5, in particular 0.3-1.0), as a result of which the temperature of the gas essentially approaches the temperature of the product, making possible Supply the process gas to the process at a temperature that is below the postcondensation temperature. In this case, mp is the sum of the mass of all product streams supplied to the process, while mg is the sum of the mass of all gas streams supplied to the process. The process gases are circulated through a gas compressor, for example, fans, fans or compressors. The process gas can be air or inert gases, such as nitrogen or C02, as well as process gas mixtures. Inert gases should contain more preferably less than 100 ppm, in particular less than 10 ppm of oxygen, where higher amounts of oxygen are conceivable if the treatment temperature during the process is sufficiently low, or if it is possible to remove oxygen from the process, for example, through combustion. The process gases may contain additives that act either reactively on the product to be treated, or are passively deposited in the product to be treated. In accordance with the present invention, the process gas at least partially circulated. In order not to prevent the polycondensation reaction, the process gas is purified to remove the undesired products, in particular cleavage products from the polycondensation reactions. Dissociation products such as water, ethylene glycol, methyldioxolane or aldehydes (eg, acetaldehyde) are here to be reduced to values below 100 ppm, in particular to values below 10 ppm. To achieve a balance, it may be necessary to leave a residual amount of reaction products in the process gas. At the same time, other unwanted products, for example, contaminants transferred from the polyester to the process gas are removed during the purification. In accordance with the present invention, the purification includes the use of gas cleaning systems known in the art, wherein other purification steps, such as filters, anti-fog devices (drop separators), adsorption systems or cold traps , they can be used. The gas cleaning system is operated with an aqueous washing liquid, where other substances can be added to the water, such as, for example, surfactants, neutralizing agents or solvents. The cleaning system can be operated in one or several stages using a shared washing liquid or different washing liquids. The gas can flow here co-current or countercurrent in relation to the washing liquid. In this case, the washing liquid can be used only once in an open system, circulated in a single circuit in a semi-open or closed system, or circulated in several independent or connected circuits. To improve the exchange between gas and washing liquid, a package sufficiently known in the prior art can be used. The washing liquids circulate more preferably at least partially. Other components such as pumps, overflow containers or coolers can be used in the circulation system. A system for separating small droplets (mist), for example, an anti-fog device (droplet separator) can be placed downstream of the cleaning system. In order to further reduce the dew point of the process gas leaving the washing process, a dryer can be used, for example, an adsorption dryer. It is advantageous to cool the process gas to a temperature below 15 ° C, in particular below 10 ° C, before entering an adsorption dryer. This can be achieved either through the use of cooled scrubber liquids or through a separate cooling device, for example, a cold trap. The adsorption dryer adsorbs water and any undissolved volatile organic component from the washing liquid, for example, acetaldehyde or formaldehyde. A parallel configuration of adsorption beds is more preferably selected in such a way that one bed can be regenerated while the other bed is in use. During the regeneration, water and the organic components are desorbed again, for example, by means of a regeneration gas. Regeneration can be done either in an open circulatory system by releasing the regeneration gas or in a closed circulatory system. The use of a combination can also be conceived. If a circulatory system of closed regeneration is used, the desorbed substances must be removed from the circulation. This is done primarily for water through condensation. The organic components either condense to a sufficient level or must be removed separately, for example, through combustion, or else in a combined system, through the release of regeneration gas. To remove the oxygen, a combustion device can be provided, for example, a catalytic combustion device where the oxygen is burned together with a measured, controlled amount of combustible substances. In a special embodiment of this invention, the washing liquid is subsequently used in a process step for preparing polyester used, for example, waste from polyester bottles. The suitable post-condensation temperature is within a range between 185 ° C and 240 ° C, with temperatures between 190 ° C and 220 ° C being preferred. The adequate post-condensation time is within a range of 2 hours and 100 hours, with retention times of 6 hours to 30 hours being preferred for economic reasons. As an option, the crystallization step and the heating step can be carried out simultaneously or at least in the same reactor at a suitable post-condensation temperature, where the reactor used for this purpose can be divided into several process chambers, where Variable process conditions can prevail (for example, temperature and retention time). Specific energy input The specific energy input is the sum of all the thermal and mechanical energy supplied to the process in relation to the quantity of product processed. Limits to the process extend from the entry of the product into the process before the first processing step until the output of the process product after the last processing step, where the product enters the process at room temperature, that is, a maximum of 40 ° C, usually between 0 and 25 ° C. Any step of preheating the product is therefore part of the process. The process includes all the procedural steps in which the product is subjected to treatment. The process also includes all gaseous streams or liquid streams that are used for direct or indirect input of energy to the product. Accordingly, the process includes process steps in which the product is heated, crystallized, mixed, treated with carrier or vacuum gas, cooled, daced, mixed or transported. Accordingly, a process step in which the product is heated by a hot surface includes the energy required to heat the surface. If the surface is heated through a fluid, the energy inputs to stir the fluid and heat the fluid are taken into account. Accordingly, a process step in which the product is treated with radiation, in particular heated, includes the energy required to generate the radiation, for example, microwave radiation or infrared radiation. Accordingly, a process step in which the product is moved includes the energy used to move the product, for example, pulses for agitators, transfer channels or screw conveyors, or pulses for moving a device or treatment, or part of a treatment device, for example, rotary kilns or vibrating screens. Energy inputs for circulation and heating of a fluid used to move the product are also taken into account, for example, process gases used in fixed bed devices or solid bedding devices-air jets or pneumatic conveyors. Accordingly, a process step in which the product is treated through a fluid, for example, a process gas, includes the energy carriers required to circulate and, if necessary, heat the fluid. If the fluid is subjected to circulation or at least partially subjected to circulation, the energy inputs required to clean the fluid are taken into account.Power inputs required to clean the fluid include energy inputs necessary for the circulation of a washing liquid, for regenerating an adsorption device or for operating a combustion unit. Energy inputs to generate and circulate the energy carrier (= utility company) are not taken into account if they are not directly released to the product. The energy carriers include electric current, cooling water, compressed air, heat carriers such as steam, or heat carrier oils or process gas such as nitrogen, C02 or special gas mixtures. The following are not taken into account: • Energy inputs to circulate cooling water or ice water that is used to indirectly cool a process gas stream;
• Energy inputs to generate cooling water or water with ice; • Power inputs to generate compressed air for process control; • Energy inputs to generate nitrogen or other process gases; • Energy inputs to circulate heat carriers used to indirectly heat a process gas stream • Power inputs to generate and distribute electrical current. The energy inputs to prepare wastewater streams or washing liquids are also not taken into account. Method The embodiment according to Figure 1 indicates that an essentially amorphous polycondensate prepolymer is continuously supplied to a heating and crystallization device (1), after which it is transferred to a reaction area (2). The treatment in the crystallization device is carried out using a process gas at least partially circulated, heated through a heater (H). A carrier gas flowing through the polycondensate in the reaction area is supplied to the process gas circulating in the crystallization system as an exchange gas. As a result, a quantity of process gas must be directed away from the crystallization circulation system and subsequently purified. The contaminated process gas is directed through a gas cleaner (3) for purification with water as washing liquid, subsequently drying in a dryer (4) and returned to the reactor without having been significantly heated. In a variant, the process gas from the reactor and a portion of the process gas from the crystallization circulation system are mixed. A part of the mixed gas is purified, dried and returned to the reactor. Another part of the mixed process gas is returned to the crystallization circulation system, either directly or after passage through the cleaner. As an option, additional stages of purification are carried out through the use of cyclones or filters. The washing liquid is cooled and circulated in two closed systems through coolers (Cl, C2). The overflow of contaminated wash liquid can be used in a special embodiment of the present invention in a process step for preparing polycondensate waste, for example, PET bottle waste. If necessary, the pH value of the washing liquid can be adjusted, for example, by neutralization. Other additives, for example, surfactants, can be incorporated in the washing liquid. An inert gas is used as a process gas. The inert gas is added in an amount sufficient to compensate for the losses due to the material feed line in the inert gas area and the material discharge line from the inert gas area, along with any other point of loss. In order to minimize the losses of inert gas, the supply of material and the unloading of material are effected through a transfer channel, in particular a cell wheel transfer channel. As an option, transfer channels can also be placed between devices within the area of inert gas or outside the area of inert gas. To ensure optimal use of space and energy, the crystallization step is also carried out in the reaction step to allow product transfer by gravity. In another option, the polycondensate can be transferred through a conveyor, for example, a pneumatic conveyor. As an option, the reaction step can be followed by a cooling step (5), more preferably in air. The process gas from the cooling step can be used in an optional heating step (6) which is carried out before the crystallization step. Optionally used valves can be used to adjust or regulate the amount of process gas for the heating step. Advantages, characteristics and additional possible applications of the present invention can be obtained from the following description of examples according to the present invention, which do not intend to limit said invention, and are based on the drawing.
EXAMPLE 1 A regranulated polyethylene terephthalate is supplied to a process according to Figure 1 consisting of devices 1, 2, 3, 4, 5, 6, H, Cl and C2 at an inlet temperature of 10 ° C. Yield is 1 ton per hour. In crystallizer 1, the product is heated to a temperature of 192 ° C, and the crystallinity is increased in
3 ÍD "6 • In reactor 2, the intrinsic viscosity is high from 0.72 to 0.84.9 tons per hour of nitrogen continuously heated to
202 ° C circulate in the circulation system for the crystallizer 1. 0.6 tons per hour of purified nitrogen continuously heated to 50 ° C are supplied to the reactor 2. The cooling is effected at 2.5 tons per hour of air.
The nitrogen is purified through a 2-stage adsorption gas dryer cleaner regenerated with heated process gas. 5 tons per hour of washing liquid or 1 ton per hour of washing liquid circulates in the gas cleaner. A respective transfer channel (not shown) is used before crystallizer 1, before reactor 2, and after reactor 2. A total of 44 kWh of energy is supplied for motors and 72 kWh of energy for heaters to the process, which corresponds to an overall consumption of 116 kWh / ton. This includes heat losses from the devices as well as product lines and process gas. All devices are isolated to maintain their surface temperature below 50 ° C. EXAMPLE 2 In this example, the system of Example 1 is expanded in order to include a product conveyor between crystallizer 1 and reactor 2. As a result , an additional 6 kWh is consumed as energy for transport purposes and compensation for heat losses in the transport path, which corresponds to an overall consumption of 122 kWh / ton. EXAMPLE 3 In this example, the system of Example 2 is operated in such a way that the product in the crystallizer 1 is heated to a temperature of 210 ° C, which raises the intrinsic viscosity to more than 0.9. As a result, an additional 13 kWh is consumed as energy for heating purposes, which corresponds to an overall consumption of 135 kWh / ton. EXAMPLE 4 In this example, the system of example 1 is improved to include a heating device 6 before crystallizer 1, through which 100% of 1 air leaves the cooler, thereby increasing the temperature of the product at the entrance of the crystallizer 1 to 60 ° C. This raises the energy for the engines to 47 kWh, and reduces the energy for the heaters to 54 kWh, which corresponds to an overall consumption of 101 kWh / ton.