MX2008010085A - Acrolein preparation method - Google Patents

Abstract

The invention relates to a method for preparing acrolein from propylene, consisting of a first glycerol dehydration step performed in the presence of a gas containing propylene and, more specifically, in the presence of the reaction gas originating from the propylene to acrolein oxidation step. The inventive method enables the use, in part, of arenewable raw material, while increasing acrolein production.

Description

METHOD OF PREPARATION OF ACROLEIN FIELD OF THE INVENTION The present invention relates to a process for the preparation of acrolein from propylene, comprising a step of dehydrating glycerol in the presence of a gas comprising propylene and, more particularly, in the presence of the reaction gas resulting from the oxidation step of the propylene to yield acrolein.

BACKGROUND OF THE INVENTION The most commonly used process for the production of acrolein is based on the catalytic oxidation reaction of propylene gas phase by atmospheric oxygen as described, for example, in the document Techniques de l'ingénieur , I tried Génie des procédés [Techniques for the Engineer, Process Engineering Treaty], J 6 100 1-4. The acrolein obtained in this way can be incorporated, directly, in a two-step process for the manufacture of acrylic acid from propylene in the gas phase or it can be used as a synthetic intermediate. Acrolein is, in particular, a key intermediate in the synthesis of methionine, a synthetic amino acid used as an animal food supplement which emerged as a substitute for fishmeal. Acrolein also has numerous additional applications in the preparation of derivatives, which can be synthesized in the actual production site of acrolein, thus limiting the storage and transportation of this toxic chemical. In a certain number of cases, it may be advantageous to be able to increase the capacities of the acrolein production of existing units. The production of acrolein is highly dependent on the raw material, propylene. Propylene, obtained by steam reforming or catalytic reforming of petroleum fractions, has the disadvantage of contributing to the increase of the greenhouse effect, as a result of its fossil origin. In addition, propylene resources may become limited. In this way, it seems particularly advantageous to be able to increase the productive output of acrolein while reducing dependence on a fossil resource. It has been known for a long time that glycerol can contribute to the production of acrolein. The glycerol results from the methanolysis of vegetable oils, at the same time as the methyl esters, which are used by themselves in particular as fuels in diesel oil and heating oil. This is a natural product who enjoys a "green" aura, is available in a large quantity and can be stored and transported without difficulty. Numerous studies have focused on giving an economic value to glycerol according to its degree of purity, and the dehydration of glycerol for acrolein is one of the routes considered. The reaction involved in the production of acrolein from glycerol is: CH2OH-CHOH-CH2OH «CH2 = CH-CH0 - 2H20 Usually, the hydration reaction is promoted at low temperatures and the dehydration reaction is promoted at high temperatures . To obtain acrolein, it is then necessary to use a satisfactory temperature and / or a partial vacuum to displace the reaction. The reaction can be carried out in the liquid phase or in the gas phase. This type of reaction is known to be catalyzed by acids. Various processes for the synthesis of acrolein from glycerol are described in the prior art; mention may be made, in particular, of documents FR 695931, US 2 558 520, O 99/05085 and US 5 387 720. It has been found that the reaction for the dehydration of glycerol to result in acrolein can be carried out in the presence of a gas comprising propylene. In this way, it is an advantage introduce glycerol in the process for the catalytic oxidation of propylene gas phase, which makes it possible to use a renewable raw material while increasing the production of acrolein. A process like this becomes an advantage, in particular, for the synthesis of methionine, then it can be said that it is "obtained from biomass". This is because methionine, when used to feed animals, is rapidly metabolized and the carbon dioxide gas, which can be rediscovered in the atmosphere, contributes to the increase of the greenhouse effect. If acrolein is obtained partially from a renewable raw material, such as glycerol that originates from vegetable oil, CO2 emissions are no longer fully involved in the process balance while offsetting the carbon dioxide gas used by the biomass. its growth; There is thus a limitation on the increase of the greenhouse effect. A process like this then corresponds to the criteria related to the new concept of "green chemistry" in the more general context of sustainable development.

SUMMARY OF THE INVENTION The subject matter of the present invention is thus a process for the preparation of acrolein by oxidation of propylene comprising a step of dehydration of glycerol in the presence of a gas comprising propylene. The glycerol dehydration reaction can be carried out in the presence of the gas mixture feeding the reactor for the oxidation of propylene, usually composed of propylene, steam, an inert gas, which can be nitrogen or argon, and molecular oxygen or a gas comprising molecular oxygen. According to a preferred embodiment of the invention, the dehydration step of the glycerol is carried out in the presence of the reaction gas resulting from the oxidation step of the propylene to yield acrolein. This reaction gas, as a rule, is composed of a mixture of unreacted gases (unconverted propylene, propane initially present in propylene, inert gas, steam, oxygen, CO, CO2), acrolein produced and various products secondary, such as acrylic acid, acetic acid and other minor compounds. Without the Applicant Company being responsible for any explanation, it is believed that the dehydration step of glycerol makes it possible to cool the reaction gases resulting from the oxidation step of propylene to result in acrolein. This is because, in the reaction for the oxidation of propylene to result in acrolein, the reaction gases leave the reaction region at a high temperature, the reaction for the oxidation of propylene is exothermic. It is necessary to cool these reaction gases to recover the acrolein. In a process for the preparation of acrylic acid of propylene in two stages, it is also necessary to cool the reaction gases resulting from the first stage of oxidation of propylene to result in acrolein before entering the second stage of oxidation of acrolein to result in acrylic acid as the reaction for the oxidation of acrolein to acrylic acid and is carried out at a lower temperature than the reaction for the oxidation of propylene to result in acrolein. In addition, acrolein can, by itself, ignite at high temperatures, resulting in yield losses. This cooling is obtained, generally, by virtue of a heat exchanger placed downstream of the catalytic region. The same effect may, in whole or in part, be obtained by virtue of the use of an endothermic reaction, such as the dehydration of glycerol. In the present invention, the glycerol dehydration reaction exhibits the advantage of resulting in the same main reaction product (acrolein) as the reaction for the oxidation of propylene. In this way, this results in an increase in the productive output of Acrolein while efficiently recovering the heat from the oxidation reaction.

BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will emerge, more clearly, in reading the following description, with reference to the attached figures in which: Figure 1 represents, diagrammatically, a reactor conventional for the oxidation of propylene to result in acrolein, - Figures 2, 3, 4 and 5 represent, in a diagrammatic way, the different configurations of reactors for the oxidation of propylene to result in acrolein corresponding to the modalities of the process in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION In the process of the invention, the dehydration stage of glycerol is carried out in the gas phase in the presence of a catalyst at a temperature ranging from 150 ° C to 500 ° C, preferably between 250 ° C. C and 350 ° C, and at a pressure between 1.02 and 5.1 kg / cm2 (1 to 5 bars). The dehydration stage of glycerol can be carrying out upstream of the reaction for the catalytic oxidation of propylene in the presence of the feed gas comprising propylene or downstream of the reaction for the catalytic oxidation of propylene in the presence of the gas mixture resulting from this reaction. It can be incorporated, directly, to the oxidation reactor or can be carried out in a reactor placed, immediately, upstream or downstream of the reactor for the oxidation of propylene. As the dehydration reaction is slightly endothermic, it is not necessary to have a multitubular bed for this reaction. A conventional fixed bed may be suitable, and also a configuration in modules (sheets or rafts). The modules exhibit the advantage of being easily loaded and unloaded when the catalyst is deactivated. The catalysts which are suitable are homogeneous or multi-phase materials which are insoluble in the reaction medium and which have a Hammett acidity registered as Ho, of less than +2. As indicated in US Pat. No. 5,387,720, which refers to the article written by K. Tanabe et al. in "Studies in Surface Science and Catalysis", vol. 51, 1989, chap 1 and 2, Hammet acidity is determined by the titration of amines using indicators or by adsorption of a base in the phase soda. The catalysts correspond to the acidity criteria Ho, of less than +2, and can be selected from siliceous materials (natural or synthetic) or acid zeolites; inorganic supports such as oxides, coated with mono-, di-, tri-, or poly-acidic inorganic acids; oxides or mixed oxides; or hetero-polyacids. Advantageously, the catalysts are selected from zeolites, Nafion® compounds (based on fluoro polymer sulfonic acid), chlorinated aluminas, phosphotungstic and / or silicotungstic acids and acid salts, and various solids of the type formed from metal oxides, such as tantalum oxide Ta20s, niobium oxide Nb20s, alumina AI2O3, titanium oxide TiO2, zirconia Zr02, tin oxide Sn02, silica SIOO2 or silicoaluminate SIO2 / AI2O3, impregnated with acid functional groups, such as borate BO3, sulfate SO4, tungstate 03, PO4 phosphate, S1O2 silicate or M0O3 molybdate, acid functional groups. In accordance with the literature data, these catalysts always have a Hammett acidity Ho of less than +2. Preferred catalysts are sulphated zirconia, phosphated zirconia, tungstated zirconia, silica zirconia, sulfated titanium, or tin oxides, or silicas or phosphated aluminas. These catalysts all have an acidity Hammett Ho less than +2; the acidity Ho can then vary to a greater degree, below the values that can reach -20 on the reference scale with Hammett indicators. The table shown on page 71 of the publication on acid-based catalysts (C. Marcilly), vol. 1, in Editions Technip (ISBN number 2-7108-0841-2), illustrates examples of solid catalysts in this range of activity. The glycerol is used pure or in the form of a concentrate or diluted aqueous solution. Advantageously, an aqueous glycerol solution with a concentration ranging from 10% to 100% by weight can be used. In the embodiment of the invention wherein the glycerol dehydration step is carried out upstream of the reaction for the catalytic oxidation of propylene, an aqueous glycerol solution can be used, preferably at a concentration that ranges from about 10 to about 10%. % to 50% by weight, more particularly from 15% to 30% by weight. The concentration should not be too high to avoid side reactions, such as the formation of glycerol ethers or reactions between the produced acrolein and glycerol. In addition, the glycerol solution should not be too dilute due to the energy cost resulting from the evaporation of the aqueous glycerol solution. In the embodiment of the invention wherein the stage of glycerol dehydration is carried out in the presence of the reaction gas resulting from the oxidation step of propylene to result in acrolein, pure glycerol or a concentrated aqueous glycerol solution can be used, said reaction gas comprises steam . Preferably, the concentration of the glycerol aqueous solution ranges from 50% to 100%. Glycerol can be injected in the liquid form or in the gaseous form. The injection in liquid form makes it possible to obtain the benefit of the latent heat of vaporization of the glycerol, thus making it possible to cool the gases resulting from the upstream stage of propylene oxidation. In this case, the dehydration catalyst can be preceded by a bed of inert materials on which the glycerol is vaporized. This can be injected in gaseous form at a lower temperature than that of the gases that exist in the oxidation region, which also makes it possible to increase the cooling effect of these gases. Additionally, glycerol can be injected under pressure, so that the reduction in gas pressure allows additional heat consumption. The dehydration reaction of glycerol is carried out in the presence of molecular oxygen, which occurs in the gas mixture that feeds the reactor for the oxidation of propylene or in the gas mixture resulting from the oxidation stage of propylene. Molecular oxygen may be present in the form of air or in the form of a gas mixture comprising molecular oxygen. In accordance with one embodiment of the invention, it is possible to add an additional quantity of molecular oxygen or a gas comprising molecular oxygen for the dehydration step of the glycerol. The amount of oxygen is selected so that it is outside the range of flammability at all points of the plant. The presence of oxygen makes it possible to limit the deactivation of the dehydration catalyst by carbonization. In addition, the addition of oxygen improves the reaction performance for numerous catalyst systems. The reaction for the catalytic oxidation of propylene to yield acrolein is carried out in accordance with conditions known to a person skilled in the art by passing a gas mixture, which may comprise propylene, steam, an inert gas, which it can be nitrogen or argon, and molecular oxygen or a gas comprising molecular oxygen, on a catalyst for the oxidation of propylene. The oxidation reactor is, in general, a multitubular fixed bed reactor. The oxidation reactor may also be a plate exchanger with a modular arrangement of the catalyst, such as described in EP 995 491, EP 1 147 807 or US 2005/0020851. In the case where the catalytic oxidation of propylene is carried out in the presence of a thermal ballast, as described, for example, in EP 293 224 Al, which makes it possible to use a more propylene flow rate high, the gas mixture resulting from the reaction has a higher specific heat Cp. The process according to the invention is, in particular, advantageous in this case to discharge the excess heat transported by the reaction gases. A preferred embodiment of the invention consists of the use of propane as an inert gas as a replacement in all or part for the nitrogen in the air. Propane, by virtue of its higher specific heat, transports more heat to the reactor which makes it possible to carry out, more easily, the reaction for the dehydration of glycerol. The gas resulting from the dehydration step then comprises, as main constituents, steam, propane, acrolein and residual oxygen. After adsorption of acrolein, the propane-rich gases can be recycled. Preferably, the gas is subject to purification treatments to remove impurities that may be harmful to oxidation and dehydration reactions, such as CO and / or CO2, and to limit the concentration of these impurities in the recycling circuit. In this case, it is an advantage, in particular, to control the concentration of argon in the gas circuit taking into account its very low specific heat. Mention may be made, as separation techniques that can be used alone or in combination, of the selective oxidation of CO to CO2, washing with amines, washing with potassium hydroxide, adsorption techniques, membrane separation or cryogenic separation. With reference to Figure 1, in a conventional process for the oxidation of propylene to result in acrolein, a mixture of gas 1 comprising propylene, steam, nitrogen and molecular oxygen is passed in the multitube reactor from top to bottom on a catalyst 2 for the oxidation of propylene. After cooling using a heat exchanger 8, a mixture 3 comprising the unreacted gases, the produced acrolein and by-products are obtained. The liquid coolers circulate in 6 and 7, in such a way that the reaction temperature which can be between 300 ° C and 320 ° C is maintained. The heat exchanger 8 can be placed directly downstream of the catalytic bed, as in Figure 1, or it can be installed following the oxidation reactor. According to a first method of the process according to the invention, illustrated diagrammatically in Figure 2, the heat exchanger 8 is replaced downstream of the catalyst bed 2 for the oxidation of the propylene (in whole or in part) by a glycerol dehydration step consisting of in passing a mixture 4, composed of glycerol in the form of a vaporized aqueous solution and of oxygen, and at the same time as the gas mixture leaving the oxidation region on a catalyst for the dehydration of glycerol. At the outlet, a mixture of arolein is obtained which results both from the reaction for the oxidation of propylene and from the reaction for the dehydration of glycerol, and also the byproducts resulting from these two reactions. According to a second embodiment of the process of the invention, illustrated diagrammatically in Figure 3, the oxidation reactor is fed by the gas mixture 1 from the lower part upwards, the catalyst 5 for the dehydration of glycerol occurs in this configuration in the upper part of the reactor. In this way, the catalyst change is facilitated. The bed of the dehydration catalyst, which can be conventional fixed bed type or in modules (sheets or trays), can be easily removed and replaced. The regeneration of the catalyst can, thus, be carried out easily outside the reactor. According to a third embodiment of the process of the invention, illustrated in Figure 4, the dehydration catalyst 5 is placed, in whole or in part, in the boiler used to cool the heat exchange fluid circulating in 6 and 7. and which removes the heat of the oxidation reaction by producing steam in the boiler. While the endothermic dehydration reaction takes place partly in the boiler, it is possible to remove more heat, which makes it possible to indirectly increase the flow rate of propylene in the oxidation reactor to thereby , simultaneously produce more acrolein by oxidation of propylene and by dehydration of glycerol. In this embodiment, it is preferable for the catalyst to operate in similar temperature ranges. According to a fourth embodiment of the process according to the invention, illustrated in Figure 5, the catalyst 5 for the dehydration of glycerol is placed upstream of catalyst 2 for the oxidation of propylene. It is necessary, in this case, to heat the glycerol solution to a high temperature to vaporize it on the dehydration catalyst and to keep the reaction gases at a sufficiently high temperature before entering the catalytic region for the oxidation of propylene. The dehydration catalyst can be placed in modules or racks in the upper part of the reactor; In this way, it can be changed easily when it is deactivated. The glycerol 4 can also be attached with the gas mixture 1 comprising propylene. The dehydration catalyst can be placed immediately above the catalyst bed for the oxidation of propylene. It is possible to contemplate the use of another endothermic reaction than the dehydration of glycerol to efficiently recover the heat of the oxidation reaction. In particular, the reaction for the oxydehydration of propane-1,3-diol or the dehydration of propane-1-ol or propane-2-ol is also advantageous in certain aspects, in particular, if the bed of the dehydration catalyst is placed upstream of the reactor for the oxidation of propylene to result in acrolein. This is because the dehydration of propane-1,3-diol can result in allyl alcohol which, in turn, can be oxidized on the catalyst for the oxidation of propylene to result in acrolein. Propane-1-ol or propane-2-ol can be dehydrated to result in propylene and can, therefore, be oxidized to result in acrolein on the oxidation catalyst.

However, the following examples illustrate the present invention without limiting the scope thereof.

EXAMPLES In the examples, the products formed, acrolein and acrylic acid are analyzed by chromatography on an EC-1000 capillary column attached to an HP6980 chromatograph equipped with an FID detector. The quantitative analysis is carried out with an external standard.

Example 1: Use is made of a configuration as represented by Figure 5, in which the glycerol is attached with the gas mixture comprising the propylene, and comprises two catalyst beds. A pyrex reactor equipped with a sintered glass is used to retain the catalysts. First of all, a weight of 6,578 g of catalyst for the oxidation of propylene is loaded to yield acrolein with the reference ACF7 (from Nippon Shokubai), diluted with 7 ml of silicon carbide with a particle size of 0.125 mm. Accordingly, various beds of silicon carbide (SiC) are charged, so that they separate the two beds of catalysts and to control, so independent, its temperature: 2 ml with a particle size of 0.125 mm, then 7 ml with a particle size of 0.5 mm, once again 2 ml with a particle size of 0.125 mm and finally 1 ml with a particle size of 0.062 mm. Accordingly, a weight of 1522 g of catalyst for the dehydration of glycerol is loaded with the reference Z1044 (tungsten zirconia of Dailchi Kigenso KK), diluted with 4 ml of silicon carbide with a particle size of 0.062 mm. The reactor is then worked up to the height of the silicon carbide with a particle size of 0.125 mm (2 ml), 0.5 mm and then 1.19 mm. The reactor is, therefore, connected to the test plant. The temperatures of the two layers of the catalyst are regulated independently at 260 ° C for the upper dewatering layer and at 305 ° C for the lower oxidation layer. The reactor is fed through the top with a mixture of propylene gas / oxygen / helium-krypton / water-glycerol. The mixture of helium-krypton gas comprises 4.92% krypton, which acts as an internal standard. The water-glycerol mixture comprises 30% by weight of glycerol. The molar flow rates of each hour (expressed in micromoles per hour) of the constituents of the mixture are as follows: 30089/55584/288393 / water: 53489-glycerol: 4509. These conditions represent a total molar flow rate of C3 compounds (propylene + glycerol) of 34598 micromol / h. The effluents are collected at the outlet of the reactor by a cold trap comprising ice and the acrolein and acrylic acid produced are determined quantitatively by chromatographic analysis. The effluents are collected in the trap for a period of 82 minutes. The non-condensable gases are analyzed during the evaluation. The amount of acrolein produced is 25302 micromol / h and the amount of acrylic acid is 2103 micromol / h.

Example 2 (comparative): Example 1 is repeated; however, the aqueous glycerol solution is replaced with pure water. The molar flow rates in micromol / h of the reactants are then: propylene / oxygen / helium-krypton / water 30089/55584/288393/76666. The effluents are collected in the trap for a period of 88 minutes. The non-condensable gases are analyzed during the evaluation. The The amount of acrolein produced is 20 391 micromol / h and the amount of acrylic acid is 1157 micromol / h.

Example 3 (comparative): Example 2 is repeated; however, in the meanwhile the dehydration catalyst is replaced with silicon carbide. The same feeding conditions are used. The effluents are collected in the trap for a period of 75 minutes. The non-condensable gases are analyzed during the evaluation. The amount of acrolein produced is 20 821 micromol / h and the amount of acrylic acid is 1223 micromol / h.

Example 4: Use is made of a configuration as depicted in Figure 2, in which the glycerol is introduced over a dehydration catalyst at the same time that the gas mixture results from the region for the oxidation of propylene to result in acrolein. A pyrex reactor provided with a sintered glass is used to retain the catalysts. First of all, a weight of 1,538 g of catalyst is loaded of dehydration with the reference Z1044 (Tungstada zirconia of Dailchi Kigenso KK), diluted with 4 ml of silicon carbide with a particle size of 0.062 mm. Accordingly, various silicon carbide beds are charged, so as to separate the two beds of catalysts and to control, independently, their temperature and make possible the injection of an aqueous solution of glycerol or hydrated glycerol between the two beds of catalyst; 4 ml with a particle size of 0.125 mm, then 7 ml with a particle size of 0.5 mm, and once again 2 ml with a particle size of 0.125 mm are charged. Accordingly, a weight of 6,522 g of catalyst is charged for the oxidation of propylene to yield acrolein with reference ACF4 (from Nippon Shokubai), diluted with 7 ml of silicon carbide with a particle size of 0.125 mm. Finally, the reactor is made up to the height of the silicon carbide with a particle size of 0.125 mm (2 ml), 0.5 mm and then 1.19 mm. The reactor is, therefore, connected to the test plant. The temperatures of the two layers of the catalyst are regulated independently at 260 ° C for the lower dewatering layer and at 305 ° C for the upper oxidation layer. The reactor is fed by the part upper with a mixture of propylene gas / oxygen / helium-krypton / water with the following molar flow rates of each hour (expressed in micromoles per hour): 30089/55584/288393/76666. The mixture of helium-krypton gas comprises 4.92% krypton, which acts as an internal standard. A glycerol / water mixture comprising 80% by weight of glycerol is fed between the two catalyst beds with a flow rate of 4530/5794 micromol / h. These conditions represent a total molar flow rate of C3 compounds (propylene + glycerol) of 34619 micromol / h. The effluents are collected at the outlet of the reactor by means of a cold trap comprising ice and the acrolein and acrylic acid produced are determined quantitatively by chromatographic analysis. The effluents are collected in the trap for a period of 84 minutes. The non-condensable gases are analyzed during the evaluation. The amount of acrolein produced is 25 852 micromol / h and the amount of acrylic acid is 1170 micromol / h. The residual propylene is 2895 micromol / h.

Example 5: Example 4 is repeated, however it uses 95% by weight of glycerol solution (glycerol hydrate). The molar flow rates of each hour (in micromoles per hour) of the constituents of the mixture are as follows: propylene / oxygen / helium-krypton / water 30089/55584/288393/76666 for the upper feed and glycerol / water 8220/2205 micromol / h for intermediate feed. These conditions represent a total molar flow rate of C3 compounds (propylene + glycerol) of 38309 micromol / h. The effluents are collected in the trap for a period of 84 minutes. The non-condensable gases are analyzed during the evaluation. The amount of acrolein produced is 28 099 micromol / h and the amount of acrylic acid is 1237 micromol / h. The residual propylene is 2856 micromol / h.

Example 6: (comparative): Example 4 is repeated; however, in the meantime the dehydration catalyst is replaced with silicon carbide and the glycerol solution is not introduced. The effluents are collected in the trap for a period of 73 minutes. The gases do not condensables are analyzed during the evaluation. The amount of acrolein produced is 22 373 micromol / h and the amount of acrylic acid is 1150 micromol / h. The residual propylene is 2933 micromol / h.

Claims (9)

1. NOVELTY OF THE INVENTION
2. Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as property:
3. CLAIMS 1. - A process for the preparation of acrolein by oxidation of propylene, characterized in that it comprises a step of dehydrating glycerol in the presence of a gas comprising propylene. 2. - The process according to claim 1, characterized in that the gas comprising propylene is the reaction gas resulting from the step of oxidation of propylene to result in acrolein. 3. The process according to claim 1, characterized in that the gas comprising propylene is a gas mixture that feeds the reactor for the oxidation of propylene.
4. 4. - The process according to one of claims 1 to 3, characterized in that the dehydration reaction is carried out in the gas phase in the presence of a catalyst.
5. 5. - The process according to one of claims 1 to 4, characterized in that the oxygen Molecular is added for the dehydration stage of glycerol.
6. 6. - The process according to one of claims 1 to 5, characterized in that the glycerol is injected in liquid form or in gaseous form.
7. 7. - The process according to one of claims 1 to 6, characterized in that use is made of pure glycerol or glycerol in the form of a concentrate or diluted aqueous solution.
8. 8. - The process according to one of claims 1 to 7, characterized in that the oxidation of propylene is carried out in the presence of a thermal ballast.
9. 9. - The process according to one of claims 1 to 8, characterized in that the glycerol dehydration step is carried out in part in the boiler which is used to cool the heat exchange fluid circulating in 6 and 7