Process comprising evaporating glycerol

Abstract

Described is a process comprising a step of evaporating glycerol in single chamber evaporator for purifying glycerol and / or for providing gaseous glycerol for chemical conversion into one or more reaction products.

Description

[0001] Process comprising evaporating glycerol

[0002] Described is a process comprising a step of evaporating glycerol in a single chamber evaporator for purifying glycerol and / or for providing gaseous glycerol for chemical conversion into one or more reaction products.

[0003] Glycerol is notoriously difficult to evaporate due to its high boiling point (290°C) around which thermal decomposition of glycerol starts to occur. Additionally, long-term stability of glycerol at elevated temperatures (e.g. > 100°C) is low as well, resulting in accumulation of products of thermal decomposition of glycerol. Standard evaporation units like a saturator (packed bed column) expose glycerol to high temperatures for prolonged times, therefore fostering accumulation of products of thermal decomposition and loss of value product. Moreover, impurities resulting from thermal decomposition of the glycerol may deactivate catalysts used in subsequent chemical conversion of glycerol into value products like hydroxyacetone.

[0004] Due to the thermal instability of glycerol, common purification technologies for glycerol are complex and time consuming. In order to avoid thermal decomposition, distillation is either carried out as vacuum distillation so that the evaporation temperature is reduced, or as thin film distillation which allows for a very short residence time under thermal stress. Both techniques have the disadvantage that the apparatus has moving elements which are elaborate and require significant efforts in maintenance. More specifically, for pharmaceutical glycerol, a three- or four- step flash evaporator in combination with a falling film evaporator is typically used.

[0005] For additional purification, ion-exchange is carried out to achieve desalination. By using specific adsorption resins, this process may also serve to remove non-ionic impurities like colored matter.

[0006] For detailed information on the purification and refining of glycerol, see Ullmann’s Encyclopedia of Industrial Chemistry, Volume 12, Chapter Glycerol.

[0007] BR 9707072A discloses a method of vaporizing a fluid containing vaporizable oxidationsensitive compounds.

[0008] US 6,875,247 B2 discloses methods of separating fluids using capillary forces and wickcontaining, laminated devices that are capable of separating fluids.

[0009] Certain chemical conversions of glycerol into value products like hydroxyacetone require high reaction temperatures e.g. a reaction temperature of > 240°C. For such reactions, evaporated glycerol is typically supplied by means of a carrier gas stream. Evaporation of glycerol into the carrier gas stream is usually carried out in such manner that the gas stream is not saturated with glycerol, in order to avoid undesired condensation in pipings and reactors. The amount of evaporated glycerol in the carrier gas stream may be controlled by adjusting the temperature of the glycerol and the carrier gas flow rate. Thus, the carrier gas stream loaded with glycerol may have a temperature below the desired reaction temperature, so that there is a need of an additional heating unit to obtain the required temperature of the glycerol containing gas stream.

[0010] It is therefore an object of the invention to provide a process and a use of a device allowing a fast and efficient evaporation of glycerol without significant thermal decomposition. More specifically, it is an object of the invention to provide a process allowing for evaporation and chemical conversion of glycerol at pressures and temperatures as high as possible, without significant thermal decomposition of glycerol. It is a further object to provide an efficient process for purifying glycerol. It is a further object to provide a process for chemically converting glycerol into one or more reaction products having a more efficient preparation of a feed stream comprising evaporated glycerol. These and other objects are achieved by means of the process and the use according to the present invention. Said process comprises the step of evaporating glycerol in a single chamber evaporator comprising a vessel and within the vessel a plurality of internals for flow of a heat carrier medium, said vessel forming one single chamber for evaporating glycerol, wherein a) the ratio of the outer surface of the internals for flow of a heat carrier medium to the volume of the single chamber is at least 300 m2 / m3, b) the hydraulic diameter of the single chamber is in the range of from 0.01 m to 1 m, preferably 0.0172 m to 1 m, more preferably of from 0.03 m to 1 m, c) the length of the single chamber is in the range of from 0.2 m to 5 m.

[0011] A detailed description of a suitable single chamber evaporator is provided in WO 201 1 / 089209.

[0012] The vessel may have any circumference, e.g. circular, oval or polygonal, e.g. hexagonal.

[0013] The vessel and the internals for flow of a heat carrier medium may be made of any suitable material which is durable at the involved temperature and inert with respect to glycerol, e.g. glass or suitable types of steel (1.4571 , 1.4462, 1.4539 and 2.4610).

[0014] The hydraulic diameter of the single chamber corresponds to the fourfold of the ratio between the free cross-sectional area (cross-sectional area minus the area filled by internals like the internals for flow of a heat carrier medium and optional further internals like guide plates) and the circumference of the single chamber.

[0015] The volume of the single chamber corresponds to the free space of the single chamber not occupied by any internals.

[0016] The single chamber is permeated by a plurality of internals for flow of a heat carrier medium. The heat carrier medium may be steam having a pressure corresponding to a condensation temperature at least 5 °C above dew point of glycerin, or a heated gas or a heated liquid having an inlet temperature of at least 5 °C above the dew point of glycerol.

[0017] The hydraulic diameter of the internals for flow of a heat carrier medium may be in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm. The shortest distance between the internals for flow of a heat carrier medium may be 3 mm or less, preferably 1 .5 mm or less.

[0018] Preferably the hydraulic diameter of the internals for flow of a heat carrier medium is in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm, and the shortest distance between the internals for flow of a heat carrier medium is 3 mm or less, preferably 1 .5 mm or less.

[0019] Within the single chamber, the maximum distance of a glycerol molecule from the closest wall is merely a small fraction of the hydraulic diameter of the single chamber, typically in the range of from 100 pm to 1500 pm. The single chamber can therefore be considered as a micro- or milli-structured apparatus. A micro- or milli-structured apparatus has the advantages of high heat transfer performance per unit area, compact design and rapid startup.

[0020] The single chamber evaporator may be manufactured by means of any technique known in the art. An especially preferred technique is 3D printing which allows preparation of complex structures in the micro- or millimeter scale in a very efficient manner.

[0021] The small distance between the internals for flow of a heat carrier medium (preferably 3 mm or less, more preferably 1 .5 mm or less) allows for a high ratio of the outer surface of the internals for flow of a heat carrier medium to the volume of the single chamber, preferably in the range of from 300 m2 / m3to 3000 m2 / m3. This high specific surface allows for short residence time of glycerol, which in turn avoids the thermal decomposition of glycerol. Preferably, the residence time is less than 1 minute.

[0022] Within the single chamber, an effervescing layer is formed having gas bubbles of evaporating glycerol effervescing through the liquid glycerol which is in contact with the hot outer surface of the heating tubes. This improves the heat transfer between the heating tubes and the evaporating glycerol, thereby enabling the input of a high amount of energy per unit of volume of the evaporating liquid glycerol, which in turn allows for a reduction of the residence time. Thus in the process according to the invention the residence time during which the glycerol is under thermal stress is significantly shorter than in a prior art process using a falling film evaporator as e.g. disclosed in BR 9707072A, and the process according to the invention provides for a fast evaporation so that formation of thermal decomposition products is significantly reduced.

[0023] In the single chamber evaporator, the internals may be in the form of heating tubes. The heating tubes may be arranged running parallel to one another and to the longitudinal axis of the single chamber evaporator. The heating tubes may have any circumference, e.g. circular, oval or polygonal, e.g. hexagonal (honeycomb structure).

[0024] The single chamber evaporator may be a vertical tube bundle heat exchanger comprising a vessel and within the vessel a bundle of vertically extending heating tubes for flow of a heat carrier medium. Vertical tube bundle heat exchangers as such are known in the art.

[0025] The internal diameter of the heating tubes for flow of a heat carrier medium may be in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm. The shortest distance between the heating tubes for flow of a heat carrier medium may be 3 mm or less, preferably 1 .5 mm or less.

[0026] Preferably the internal diameter of the heating tubes for flow of a heat carrier medium is in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm, and the shortest distance between the heating tubes for flow of a heat carrier medium is 3 mm or less, preferably 1 .5 mm or less.

[0027] The small distance between the heating tubes for flow of a heat carrier medium (preferably 3 mm or less, more preferably 1 .5 mm or less) allows for a high ratio of the outer surface of the heating tubes to the volume of the single chamber, preferably in the range of from 300 m2 / m3to 3000 nT7m3. This high specific surface allows for short residence time of glycerol, which in turn avoids the thermal decomposition of glycerol. Preferably, the residence time is less than 1 minute.

[0028] The single chamber reactor may be operated at atmospheric pressure as well as at a pressure below or above atmospheric pressure. For instance, the single chamber evaporator may be operated in a pressure range of from 1 kPa (0.01 bar abs) to 500 kPa (5 bar abs), preferably 10 kPa (0.1 bar abs) to 300 kPa (3 bar abs), more preferably 80 kPa (0.8 bar abs) to 200 kPa (2 bar abs). In certain specific processes, especially those directed to purifying glycerol, said single chamber evaporator may further comprise an outlet for non-vaporized matter, e.g. polymerized matter, at or near its bottom.

[0029] In certain specific processes, especially those directed to chemically converting glycerol into one or more reaction products, said single chamber evaporator may further comprise means for feeding a carrier gas stream into the single chamber.

[0030] The carrier gas provides additional gas bubbles which further promote the heat transfer between the heating tubes and the evaporating glycerol.

[0031] A specific process described herein further comprises a step of condensing the evaporated glycerol, wherein the condensed glycerol has a higher degree of purity than the glycerol subjected to evaporation. Accordingly, this process comprises distillation of crude glycerol using a single chamber evaporator as defined above. In said process, crude glycerol is distilled, and the distilled glycerol has a higher degree of purity than the crude glycerol before the distillation.

[0032] Typically such process is directed to the purification of glycerol. Said process is an efficient manner for purifying glycerol from a broad range of typical undesired impurities by means of a single operation, namely distillation, without significant thermal decomposition.

[0033] More specifically, the distilled glycerol obtained by said specific process has on or more of a lower ash content, a lower saponification number, a lower water content, a lower content of chlorine, a lower content of nitrogen, a lower color index than the crude glycerol before the distillation.

[0034] Sources of nitrogen and chlorine may be catalysts and acids used in preparing the glycerol, and the natural starting materials (fats of vegetable or animal origin) used for preparing glycerol. The ash residue after incineration at 600 °C may be determined as follows: A platinum crucible is heated at 600 °C inside a muffle furnace, cooled down to room temperature and stored inside a desiccator. The sample is weighed (to the nearest 0.01 mg) into the dry platinum crucible. In the platinum crucible, the sample is first combusted in an open flame using a Bunsen burner until complete carbonization, and then the residues are incinerated at 600°C inside a muffle furnace for at least 30 min. After cooling down to room temperature inside a desiccator, the crucible with the residue is weighed. This is repeated until a constantweight was reached. Multiple determinations with different net weights are conducted.

[0035] The saponification number may be determined according to DIN EN ISO 3657.

[0036] The water content may be determined according to DIN EN ISO 8534.

[0037] The content of chlorine may be determined as follows: An aliquot of ca. 1 to 10 mg of the sample is burned in oxygen having nitrogen as carrier gas. The resulting hydrochloric acid gas is cleaned from by-products of the combustion in concentrated sulfuric acid and then transferred into a coulometric cell for quantitative analysis.

[0038] The content of nitrogen may be determined as follows: An aliquot of ca. 1 to 10 mg of the sample is burned argon / oxygen atmosphere, nitrogen compounds are converted to NO, then oxidized by ozone to activated NO2* the amount of which is determined by means of chemiluminescence detection.

[0039] The color index may be determined according to DIN EN ISO 6271 :2015.

[0040] Most preferably, the condensed glycerol obtained by said specific process has a lower ash content, and a lower saponification number, and a lower water content, and a lower content of chlorine, and a lower content of nitrogen, and a lower color index than the glycerol subjected to evaporation. It is preferred that in said specific process the single chamber evaporator further comprises an outlet for non-vaporized matter, e.g. polymerized matter, at or near its bottom.

[0041] Another specific process described herein further comprises the step of chemically converting the evaporated glycerol into one or more reaction products. Typically such process is directed to producing one or more reaction products (value products) from glycerol, e.g. hydroxyacetone or a product mixture comprising hydroxyacetone.

[0042] In said process, a reactor for chemically converting the evaporated glycerol into one or more reaction products may be arranged downstream of the above-described single chamber evaporator. Said reactor may comprise a suitable catalyst for the chemical conversion. Since in the upstream single chamber evaporator glycerol is evaporated without significant thermal decomposition, deactivation of the catalyst in the downstream reactor by deposition of products or poisoning by impurities resulting from thermal decomposition of the glycerol is significantly suppressed.

[0043] In said specific process, the step of evaporating glycerol may comprise evaporating glycerol in the presence of a carrier gas stream, so that a carrier gas stream either saturated or unsaturated by glycerol is formed. The carrier gas stream carries the evaporated glycerol to the downstream reactor.

[0044] Said process includes an efficient preparation of a feed stream comprising evaporated glycerol without significant loss by thermal decomposition. Due to the short residence time in the single chamber evaporator used in the process as defined above, the gas stream comprising evaporated glycerol and the carrier gas may be heated up to the temperature necessary for the subsequent chemical conversion without significant thermal decomposition of the glycerol.

[0045] Thus, a complete evaporation of the glycerol is possible, resulting in an overheated gas phase (i.e. a gas phase having a temperature above the dew point of the glycerol at the given gas flow rate), preventing formation of droplets and condensate. In this way, it is achieved that the complete amount of supplied glycerol is transferred into the gas phase when supplied to the reactor where the chemical conversion takes place. Furthermore, due to the overheating, the temperature of the gas phase is higher than dew point / condensation point of glycerol in the given gas stream. This reduces the risk of unwanted glycerol condensation in pipings or reactors. Ideally the temperature of the gas phase is at least 5 °C, preferably 10 °C or more above the dew point.

[0046] The carrier gas stream may comprise or consist of hydrogen and nitrogen. In certain cases, said carrier gas stream comprises or consists of hydrogen and nitrogen in a volume ratio in the range of from 1 :99 to 1 :1 , preferably in the range of from 5:95 to 4:6, most preferably in the range of from 1 :9 to 3:7.

[0047] It is preferred that in said specific process the said single chamber evaporator further comprises means for feeding a carrier gas stream into the single chamber.

[0048] In a preferred specific process the evaporated glycerol is chemically converted into hydroxyacetone. In said preferred specific process, the step of evaporating comprises evaporating a mixture of glycerol and 1 ,2-propanediol, preferably a mixture of glycerol and 1 ,2-propanediol in a mass ratio in the range of from 70:30 to 95:5, more preferably in a mass ratio in the range of from 80:20 to 90:10. A corresponding process for preparing a product mixture comprising hydroxyacetone is described in the non-pre-published patent application EP 23203569.1 of the same applicant.

[0049] A specifically preferred process according to the present disclosure comprises the following steps:

[0050] (i) providing a solid catalyst comprising one or more of copper and compounds of copper

[0051] (ii) providing a feed stream comprising glycerol and 1 ,2-propanediol by evaporating liquid glycerol and liquid 1 ,2-propanediol glycerol in a single ch amber evaporator comprising a vessel and within the vessel a plurality of internals for flow of a heat carrier medium, said vessel forming one single chamber for evaporating glycerol, wherein a) the ratio of the outer surface of the internals for flow of a heat carrier medium to the volume of the single chamber is at least 300 m2 / m3, b) the hydraulic diameter of the single chamber is in the range of from 0.01 m to 1 m, preferably 0.0172 m to 1 m, more preferably of from 0.03 m to 1 m, c) the length of the single chamber is in the range of from 0.2 m to 5 m, in the presence of a carrier gas stream comprising hydrogen and nitrogen, and carrying the feed stream comprising evaporated glycerol and evaporated 1 ,2-propane- diol to the solid catalyst by means of the carrier gas stream,

[0052] (iii) chemically converting glycerol into hydroxyacetone at a temperature in the range of from 200 °C to 270 °C in the presence of hydrogen at said solid catalyst.

[0053] In a further aspect, the present invention relates to the use of a single chamber evaporator comprising a vessel and within the vessel a plurality of internals for flow of a heat carrier medium, said vessel forming one single chamber for evaporating glycerol, wherein a) the ratio of the outer surface of the internals for flow of a heat carrier medium to the volume of the single chamber is at least 300 m2 / m3, b) the hydraulic diameter of the single chamber is in the range of from 0.01 m to 1 m, preferably 0.0172 m to 1 m, more preferably of from 0.03 m to 1 m, c) the length of the single chamber is in the range of from 0.2 m to 5 m.

[0054] A detailed description of a suitable single chamber evaporator is provided in WO 201 1 / 089209.

[0055] The vessel may have any circumference, e.g. circular, oval or polygonal, e.g. hexagonal.

[0056] The vessel and the internals for flow of a heat carrier medium may be made of any suitable material which is durable at the involved temperature and inert with respect to glycerol, e.g. glass or suitable types of steel (1.4571 , 1.4462, 1.4539 and 2.4610).

[0057] The hydraulic diameter of the single chamber corresponds to the fourfold of the ratio between the free cross-sectional area (cross-sectional area minus the area filled by internals like the internals for flow of a heat carrier medium and optional further internals like guide plates) and the circumference of the single chamber.

[0058] The volume of the single chamber corresponds to the free space of the single chamber not occupied by any internals.

[0059] The single chamber is permeated by a plurality of internals for flow of a heat carrier medium. The heat carrier medium may be steam having a pressure corresponding to a condensation temperature at least 5°C above the dew temperature of glycerin, or a heated gas or a heated liquid having an inlet temperature of at least 5°C above the dew temperature of glycerol.

[0060] The hydraulic diameter of the internals for flow of a heat carrier medium may be in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm. The shortest distance between the internals for flow of a heat carrier medium may be 3 mm or less, preferably 1 .5 mm or less.

[0061] Preferably the hydraulic diameter of the internals for flow of a heat carrier medium is in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm, and the shortest distance between the internals for flow of a heat carrier medium is 3 mm or less, preferably 1 .5 mm or less.

[0062] Within the single chamber, the maximum distance of a glycerol molecule from the closest wall is merely a small fraction of the hydraulic diameter of the single chamber, typically in the range or from 100 pm to 1500 pm. The single chamber can therefore be considered as a micro- or millistructured apparatus. A micro- or milli-structured apparatus has the advantages of high heat transfer performance per unit area, compact design and rapid startup.

[0063] The single chamber evaporator may be manufactured by means of any technique known in the art. An especially preferred technique is 3D printing which allows preparation of complex structures in the micro- or millimeter scale in a very efficient manner.

[0064] The small distance between the internals for flow of a heat carrier medium (preferably 3 mm or less, more preferably 1 .5 mm or less) allows for a high ratio of the outer surface of the internals for flow of a heat carrier medium to the volume of the single chamber, preferably in the range of from 300 m2 / m3to 3000 m2 / m3. This high specific surface allows for short residence time of glycerol, which in turn avoids the thermal decomposition of glycerol. Preferably, the residence time is less than 1 minute.

[0065] Within the single chamber, an effervescing layer is formed having gas bubbles of evaporating glycerol effervescing through the liquid glycerol which is in contact with the hot outer surface of the heating tubes. This improves the heat transfer between the heating tubes and the evaporating glycerol, thereby enabling the input of a high amount of energy per unit of volume of the evaporating liquid glycerol, which in turn allows for a reduction of the residence time. Thus in the process according to the invention the residence time during which the glycerol is under thermal stress is significantly shorter than in a prior art process using a falling film evaporator as e.g. disclosed in BR 9707072A, and the process according to the invention provides for a fast evaporation so that formation of thermal decomposition products is significantly reduced. In the single chamber evaporator, the internals may be in the form of heating tubes. The heating tubes may be arranged running parallel to one another and to the longitudinal axis of the single chamber evaporator. The heating tubes may have any circumference, e.g. circular, oval or polygonal, e.g. hexagonal (honeycomb structure).

[0066] The single chamber evaporator may be a vertical tube bundle heat exchanger comprising a vessel and within the vessel a bundle of vertically extending heating tubes for flow of a heat carrier medium. Vertical tube bundle heat exchangers as such are known in the art.

[0067] The internal diameter of the heating tubes for flow of a heat carrier medium may be in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm. The shortest distance between the heating tubes for flow of a heat carrier medium may be 3 mm or less, preferably 1 .5 mm or less.

[0068] Preferably the internal diameter of the heating tubes for flow of a heat carrier medium is in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm, and the shortest distance between the heating tubes for flow of a heat carrier medium is 3 mm or less, preferably 1 .5 mm or less.

[0069] The small distance between the heating tubes for flow of a heat carrier medium (preferably 3 mm or less, more preferably 1 .5 mm or less) allows for a high ratio of the outer surface of the heating tubes to the volume of the single chamber, preferably in the range of from 300 m2 / m3to 3000 nT7m3. This high specific surface allows for short residence time of glycerol, which in turn avoids the thermal decomposition of glycerol. Preferably, the residence time is less than 1 minute. The single chamber reactor may be operated at atmospheric pressure as well as at a pressure below or above atmospheric pressure. For instance, the single chamber evaporator may be operated in a pressure range of from 1 kPa (0.01 bar abs) to 500 kPa (5 bar abs), preferably 10 kPa (0.1 bar abs) to 300 kPa (3 bar abs), more preferably 80 kPa (0.8 bar abs) to 200 kPa (2 bar abs).

[0070] In certain specific uses especially those directed to purifying glycerol, said single chamber evaporator may further comprise an outlet for non-vaporized matter, e.g. polymerized matter, at or near its bottom.

[0071] In certain specific uses, especially those directed to chemically converting glycerol into one or more reaction products, said single chamber evaporator may further comprise means for feeding a carrier gas stream at or near the bottom of the single chamber.

[0072] The invention is described in detail hereinafter with reference to drawings. The drawings show:

[0073] Fig. 1 - a single chamber evaporator, in which the reference numerals mean:

[0074] 1 : inlet and outlet of the heat carrier medium

[0075] 2: connecting flange

[0076] 3: seal

[0077] 4: connecting flange

[0078] 5: guide plate (optional)

[0079] 6: shell of the single chamber

[0080] 7: heating tubes for flow of a heat carrier medium

[0081] 8: hexagonal bolt

[0082] 9: hexagonal nut

[0083] 10: inlet for liquid glycerol

[0084] 11 : outlet for glycerol vapor

[0085] Fig. 2 and Fig. 3 - a single chamber evaporator, in which the reference numerals mean:

[0086] 21 : heating tube for flow of a heat carrier medium 22: free space of the single chamber

[0087] 23: guide plates (optional)

[0088] 24: shell of the single chamber

[0089] The invention is further illustrated by the following non-limiting examples. bed column

[0090] The setup is shown in figure 4. A reactor for chemically converting the evaporated glycerol into hydroxyacetone is arranged downstream (not shown) of the packed bed column. The reactor comprises a catalyst comprising one or more of copper and compounds of copper.

[0091] A carrier gas stream consisting of nitrogen and hydrogen (volume ratio 8:2) to be saturated by glycerol was supplied to the bottom of a packed bed column. By means of an external heat exchanger glycerol was heated to a temperature above the dew point of the saturated carrier gas stream. The heated glycerol was supplied to the head of the packed bed column, and allowed to rinse downward through the packed bed, so that the carrier gas stream was saturated with glycerol. The saturated carrier gas stream was fed into the reactor for chemically converting the evaporated glycerol into hydroxyacetone.

[0092] It was possible to obtain a carrier gas stream saturated by glycerol at 220 °C. Further heating of the glycerol to the reaction temperature of 230 °C had to be achieved within the reactor, i.e. in direct contact with the catalyst.

[0093] After a few hours the glycerol exiting at the bottom of the column exhibited an intense dark color, resulting from thermal decomposition of glycerol due to the long residence time in the column. After a few days, the catalytic activity of the catalyst in the downstream reactor creased significantly due to deposition of impurities resulting from thermal decomposition of the glycerol. Example according to the invention using a single chamber evaporator:

[0094] Glycerol was distilled using a single chamber evaporator of the following dimensions: internal shell diameter (internal diameter of single camber) Dsc: 29.7 mm = 0.0297 m hydraulic diameter Dhsc of the single chamber: 17.2 mm = 0.0172 m length Lscof the single chamber: 400 mm = 0.4 m number N of heating tubes: 37 internal diameter di of the heating tubes: 1.77 mm = 0.00177 m external diameter daof the heating tubes: 3.17 mm = 0.00317 m distance of the heating tubes based on the center thereof: 4 mm ratio of the outer surface Aht of the heating tubes to the volume Vscof the single chamber (specific surface): 919 m2 / m3.

[0095] The single chamber evaporator used in the examples is schematically shown in figs. 2 and 3, except for the real number of heating tubes.

[0096] The hydraulic diameter of the single chamber evaporator used in the example is calculated as follows:

[0097] The hydraulic diameter Dhscof the single chamber is defined as the fourfold of the ratio between the free cross-sectional area AfSCand the circumference Pscof the single cham

[0098] As schematically shown in figs. 2 and 3, the single chamber 24 has a circular circumference and the heating tubes 21 have a circular circumference (number of heating tubes 21 in figs 2 and 3 not corresponding to N of the single chamber evaporator used in the example). Therefore AfSCand Psc are as follows: The hydraulic diameter of a heating tube 21 is calculated as follows:

[0099] The hydraulic diameter dhi of a heating tube 21 having a circular circumference is defined as the fourfold of the ratio between the free cross-sectional area Ai and the circumference Pi of the heating tube 21 , i.e.

[0100] In a tube having a circular circumference the hydraulic diameter corresponds to the internal diameter of the heating tube.

[0101] The specific surface Sspec of the heating tubes 21 of the single chamber evaporator used in the example is calculated as follows:

[0102] The specific surface Sspec of the heating tubes of the single chamber evaporator is defined as the ratio of the outer surface Aht of the heating tubes to the volume Vscof the single chamber 24, i.e.

[0103] SsPec=Aht / V SC

[0104] As schematically shown in figs. 2 and 3, the single chamber 24 has a circular circumference and the heating tubes 21 have a circular circumference (number of heating tubes 21 in figs 2 and 3 not corresponding to N of the single chamber evaporator used in the example), and the length laof the heating tubes 21 corresponds to the length Lscof the single chamber 24. Therefore Aht and Vscare as follows:

[0105] Aht corresponds to the total surface of the 37 heating tubes 21 :

[0106] Vsc corresponds to the free space 22 of the single chamber 24 which is the difference between the volume of the single chamber 24 and the total volume occupied by the 37 heating tubes: Samples of distilled glycerol were analyzed for common impurities by the methods described above.

[0107] The result is given in table 1 :

[0108] Table 1

[0109] Thus, the glycerol distilled by means of the process according to the invention has - a lower ash content, and a lower saponification number, and a lower water content, and a lower content of chlorine, and a lower content of nitrogen, and - a lower color index than the crude glycerol before the distillation.

Claims

Claims:1 . A process comprising a step of evaporating glycerol in a single chamber evaporator comprising a vessel and within the vessel a plurality of heating tubes (7, 21) for flow of a heat carrier medium, said vessel forming one single chamber (6, 24) for evaporating glycerol, wherein a) the ratio of the outer surface of the heating tubes (7, 21) for flow of a heat carrier medium to the volume of the single chamber (6, 24) is at least 300 m2 / m3, b) the hydraulic diameter of the single chamber (6, 24) is in the range of from 0.01 m to 1 m, c) the length of the single chamber (6, 24) is in the range of from 0.2 m to 5 m.

2. Process according to claim 1 , wherein the hydraulic diameter of the single chamber (6, 24) is in the range of from 0.0172 m to 1 m, preferably 0.03 m to 1 m.

3. Process according to claim 2, wherein the hydraulic diameter of the heating tubes (7, 21) for flow of a heat carrier medium is in the range of from 0.1 mm to 6 mm, preferably in the range of from 0.1 mm to 3 mm and / or the shortest distance between the heating tubes (7, 21) for flow of a heat carrier medium is from 3 mm or less, preferably 1 .5 mm or less.

4. Process according to claim 1 to 3, wherein the heating tubes (7, 21) run parallel to one another and to the longitudinal axis of the single chamber evaporator.

5. Process according to any preceding claim wherein the single chamber evaporator is a vertical tube bundle heat exchanger comprising a vessel (6, 24) and within the vessel a bundle of vertically extending heating tubes (7, 21) for flow of a heat carrier medium.

6. Process according to any preceding claim, wherein said single chamber evaporator comprises means for feeding a carrier gas stream into the single chamber.

7. Process according to any preceding claim, wherein said single chamber evaporator further comprises an outlet for non-vaporized matter at or near its bottom.

8. Process according to any preceding claim, wherein crude glycerol is distilled, and the distilled glycerol has a higher degree of purity than the crude glycerol before the distillation.

9. Process according to claim 8 wherein the distilled glycerol has on or more of a lower ash content, a lower saponification number, a lower water content, a lower content of chlorine, a lower content of nitrogen, a lower color index than the glycerol subjected to evaporation.

10. Process according to any of claims 1 to 7, further comprising a step of chemically converting the evaporated glycerol into one or more reaction products.11 . Process according to claim 10, wherein the step of evaporating comprises evaporating glycerol in the presence of a carrier gas stream comprising hydrogen and nitrogen, wherein preferably said carrier gas stream comprises hydrogen and nitrogen in a volume ratio in the range of from 1 :99 to 1 :1 .

12. Process according to claim 10 or 1 1 , wherein the evaporated glycerol is chemically converted into hydroxyacetone.

13. Process according to claim 12, wherein the step of evaporating comprises evaporating a mixture of glycerol and 1 ,2-propanediol, preferably a mixture of glycerol and 1 ,2-propanediol in a mass ratio in the range of from 70:30 to 95:5.

14. Use of a single chamber evaporator as defined in any of claims 1 to 7 for evaporating glycerol.

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