Method for recovering di-trimethylolpropane by distillation

Abstract

Distillative process for obtaining ditrimethylolpropane from solutions includes separating ditrimethylolpropane from the solution in a first distillation unit into a first tops fraction comprising low-boiling compounds having a lower boiling point than ditrimethylolpropane and a first bottoms fraction; introducing the first bottoms fraction into a second distillation unit having at least 5 theoretical plates, said unit being configured as a thin-film evaporator with a column attachment and drawing off a second tops fraction comprising intermediate-boiling compounds having a lower boiling point than ditrimethylolpropane as well as withdrawing a second bottoms fraction from the second distillation unit and introducing the second bottoms fraction into a third distillation unit having at least 4 theoretical plates, said unit being configured as a thin-film evaporator with a column attachment, such that ditrimethylolpropane is obtained as a third tops fraction and high boilers are removed as a third bottoms fraction.

Claims

1. Distillative process for obtaining ditrimethylolpropane from solutions comprising ditrimethylolpropane, consisting of: (a) separating the solution comprising ditrimethylolpropane in a first distillation unit into a first tops fraction comprising low-boiling compounds having a lower boiling point than ditrimethylolpropane and a bottoms fraction; (b) introducing the bottoms fraction as obtained after step a) into a second distillation unit having at least 5 theoretical plates, said unit being configured as a thin-film evaporator with a column attachment, and drawing off a second tops fraction comprising intermediate-boiling compounds having a lower boiling point than ditrimethylolpropane, and withdrawing a bottoms fraction; and (c) introducing the bottoms fraction as obtained after step b) into a third distillation unit having at least 4 theoretical plates, said unit being configured as a thin-film evaporator with a column, attachment, in which ditrimethylolpropane is obtained as a third tops fraction and high boilers are removed as a bottoms fraction.

2. Process according to claim 1, characterized in that the second distillation unit has at least 8 theoretical plates.

3. Process according to claim 1, characterized in that, in step b) the column attachment is configured as a column with random packing or column with structured packing.

4. Process according to claim 1, characterized in that the second distillation unit is operated at a temperature of 210 to 280° C. and at a pressure of 2 to 10 hPa.

5. Process according to claim 1, characterized in that the second distillation unit is operated with a residence time of 2 to 60 seconds.

6. Process according to claim 1, characterized in that the third distillation unit has 4 to 20 theoretical plates.

7. Process according to claim 1, characterized in that, in step c) the column attachment is configured as a column with random packing or column with structured packing.

8. Process according to claim 1, characterized in that the third distillation unit is operated at a temperature of 240 to 280° C. and at a pressure of 0.2 to 5 hPa.

9. Process according to claim 1, characterized in that the residence time of the tops fraction in the third distillation unit is 1 to 30 seconds.

10. Process according to claim 1, characterized in that a solution of ditrimethylolpropane in a polar solvent is separated in the first distillation unit.

11. Process according to claim 10, characterized in that the content of the polar solvent in the bottoms fraction of the first distillation unit is up to 5000 ppm by weight, based on the bottoms fraction.

12. Process according to claim 11, characterized in that the content of the polar solvent in the bottoms fraction of the first distillation unit is up to 1000 ppm by weight, based on the bottoms fraction.

13. Process according to claim 10, characterized in that the polar solvent used is a C.sub.1-C.sub.5 aliphatic alcohol, a C.sub.2-C.sub.10 dialkyl ether or water.

14. Process according to claim 3, characterized in that the second distillation unit is operated at a temperature of 210 to 280° C. and at a pressure of 2 to 10 hPa.

15. Process according to claim 3, characterized in that the second distillation unit is operated with a residence time of 2 to 60 seconds.

16. Process according to claim 7, characterized in that the third distillation unit is operated at a temperature of 240 to 280° C. and at a pressure of 0.2 to 5 hPa.

17. Process according to claim 7, characterized in that the residence time of the tops fraction in the third distillation unit is 1 to 30 seconds.

18. Distillative process for obtaining ditrimethylolpropane from solutions comprising ditrimethylolpropane, consisting of: (a) producing the solution comprising ditrimethylolpropane from the residues of trimethyl propane with the addition of water and separating the comprising ditrimethylolpropane in a first distillation unit into a first tops fraction comprising low-boiling compounds having a lower boiling point than ditrimethylolpropane and a bottoms fraction; (b) introducing the bottoms fraction as obtained after step a) into a second distillation unit having at least 5 theoretical plates, said unit being configured as a thin-film evaporator with a column attachment, and drawing off a second tops fraction comprising intermediate-boiling compounds having a lower boiling point than ditrimethylolpropane is, and withdrawing a bottoms fraction; and (c) introducing the bottoms fraction as obtained after step b) into a third distillation unit having at least 4 theoretical plates, said unit being configured as a thin-film evaporator with a column attachment, in which ditrimethylolpropane is obtained as a third tops fraction and high boilers are removed as a bottoms fraction.

19. Distillative process for obtaining ditrimethylolpropane from solutions comprising ditrimethylolpropane, consisting of: (a) treating the solution comprising ditrimethylolpropane with an ion exchanger and separating the solution comprising ditrimethylolpropane in a first distillation unit into a first tops fraction comprising low-boiling compounds having a lower boiling point than ditrimethylolpropane and a bottoms fraction; (b) introducing the bottoms fraction as obtained after step a) into a second distillation unit having at least 5 theoretical plates, said unit being configured as a thin-film evaporator with a column attachment, and drawing off a second tops fraction comprising intermediate-boiling compounds having a lower boiling point than ditrimethylolpropane, and withdrawing a bottoms fraction; and (c) introducing the bottoms fraction as obtained after step b) into a third distillation unit having at least 4 theoretical plates, said unit being configured as a thin-film evaporator with a column, attachment, in which ditrimethylolpropane is obtained as a third tops fraction and high boilers are removed as a bottoms fraction.

20. Process according to claim 19, characterized in that the ion exchanger treatment is effected both with a basic and with an acidic ion exchanger in any sequence.

Description

BRIEF DESCRIPTION OF DRAWING

(1) The invention is described in detail below with reference to FIG. 1 which is a schematic diagram illustrating the process and apparatus of the present invention.

DETAILED DESCRIPTION

(2) Input streams for the distillation process according to the invention are solutions which comprise ditrimethylolpropane and are obtained independently of a particular production process. For example, these solutions are obtained in the preparation of trimethylolpropane, either by the Cannizzaro process using alkali metal or alkaline earth metal compounds or by the hydrogenation process in the presence of catalytic amounts of trialkylamines or by the Cannizzaro process using stoichiometric amounts of trialkylamines. More particularly, they are obtained in the distillative workup of trimethylolpropane as high-boiling residues. Frequently, these high-boiling residues, for splitting of the formals of the formulae I to V, are taken up in water and treated with an acid or hydrogenated in aqueous solution with hydrogen in the presence of an acidic compound and of a metal catalyst.

(3) Solutions suitable for the distillation process according to the invention are likewise obtained in processes in which ditrimethylolpropane is prepared deliberately, for example by reaction of already isolated trimethylolpropane with 2-ethylacrolein and formaldehyde in the presence of a basic catalyst or by reaction of n-butyraldehyde and formaldehyde in the presence of a basic catalyst under such conditions that 2-ethylacrolein is first distilled out of the trimethylolpropane-containing reaction mixture and is added to the reaction mixture again together with further formaldehyde in order to convert trimethylolpropane formed in the first stage to ditrimethylolpropane. Etherification of a hydroxyl group of trimethylolpropane, optionally after introduction of a protecting group for the two other hydroxyl groups, can also give suitable solutions comprising ditrimethylolpropane.

(4) It is likewise possible in the distillation process according to the invention to use ditrimethylolpropane which has already been purified by a prior recrystallization or has been prepurified by treatment with an ion exchanger. Examples of suitable ion exchangers for the ion exchanger treatment are customary commercially available acidic and basic ion exchangers. The solution comprising ditrimethylolpropane can be contacted either only with a basic or acidic ion exchanger, or with any combination.

(5) The process according to the invention is particularly suitable in the distillative workup of solutions comprising ditrimethylolpropane dissolved in a polar solvent, especially in the distillative workup of aqueous solutions or solutions in a lower C.sub.1-C.sub.5 aliphatic alcohol or a C.sub.2-C.sub.10 dialkyl ether, such as methanol, ethanol, propanol or diethyl ether. Such alcoholic and especially aqueous solutions are obtained particularly in the acidic treatment of residues from trimethylolpropane preparation, which can additionally be effected under hydrogen in the presence of a hydrogenation catalyst.

(6) These residues from trimethylolpropane preparation comprise, as main components still incorporated in a physical mixture, trimethylolpropane, generally in a range from 5 to 30% by weight, ditrimethylolpropane, generally in a range from 10 to 35% by weight, and linear bistrimethylolpropane formal (formula II) in a range from 25 to 70% by weight, based on the overall input composition. Further identified products are 2-ethyl-2-methyl-1,3-propanediol and the monocyclic formal of trimethylolpropane (formula I), which is present only in small amounts of typically up to 3% by weight due to its comparatively low boiling point. As well as the methyl (monolinear) formal of trimethylolpropane (formula III), the methyl (bis-linear) formal of trimethylolpropane (formula IV) and the cyclic formal of ditrimethylolpropane (formula V), or the formals of trimethylolpropane released from the methyl ethers in any upstream hydrogenation, such as:
C.sub.2H.sub.5C(CH.sub.2OH).sub.2CH.sub.2OCH.sub.2OH  Formula VI, CAS 482619-74-9
C.sub.2H.sub.5C(CH.sub.2OH)(CH.sub.2OCH.sub.2OH).sub.2  Formula VII

(7) ##STR00003##
a series of further linear or cyclic formals of ditrimethylolpropane are also present, such as:
C.sub.2H.sub.5C(CH.sub.2OH).sub.2CH.sub.2OCH.sub.2C(C.sub.2H.sub.5)(CH.sub.2OH)(CH.sub.2OCH.sub.2OH)  Formula IX, CAS 97416-99-4

(8) ##STR00004##
and the monocyclic formal formed from linear bistrimethylolpropane formal (formula XII)

(9) ##STR00005##

(10) These residues from trimethylolpropane preparation are admixed with a polar solvent, preferably with water, to form a solution. In general, a solution with a content of organic components, neglecting the polar solvent, of to 90% by weight and preferably of 50 to 80% by weight is prepared, based on the overall composition. Smaller contents of organic components are inappropriate due to the high solvent content; in the case of excessively high contents, particularly at room temperature, inhomogeneities in the solution or the precipitation of solids is to be expected. Appropriately, the solution is prepared at a temperature of more than 50° C. A temperature range for the solution, especially for the aqueous solution, of 60° C. to 80° C. should preferably be established.

(11) Optionally, the resulting solution can be treated at elevated temperature and elevated pressure with hydrogen in the presence of a catalyst and of an acidic compound. The hydrogenation in an acidic medium splits the formals present and converts formaldehyde released to methanol. Optionally, the resulting solution can be contacted with an ion exchanger or with a combination of acidic and basic ion exchangers.

(12) Irrespective of the preparation process, the solution comprising ditrimethylolpropane, in the process according to the invention, is worked up in a three-stage distillation unit. The solution comprising ditrimethylolpropane is also understood to mean a solution which comprises already purified ditrimethylolpropane, for example from a recrystallization process, in which the content of impurities having a similar boiling point to ditrimethylolpropane has already been depleted. Examples of such an impurity having a similar boiling point to ditrimethylolpropane are the linear bistrimethylolpropane formal (formula II), the monocyclic formal formed from the linear bistrimethylolpropane formal (formula XII), the monolinear formal of ditrimethylolpropane (formula IX) or the monolinear formal of the cyclic formal of ditrimethylolpropane (formula XI). Suitable solvents for already purified ditrimethylolpropane are water and lower C.sub.1-C.sub.5 aliphatic alcohols such as methanol, ethanol or propanol. Already isolated, solid ditrimethylolpropane, after melting, is also regarded as a solution comprising ditrimethylolpropane in the context of the present invention.

(13) The solution comprising ditrimethylolpropane is worked up in a three-stage distillation unit. First of all, in a first distillation unit, low boilers are removed as the first tops fraction. Suitable distillation units for the low boiler removal are those which are customary, such as a distillation column having, for example, 2 to 40 theoretical plates, with a boiler, a thin-film evaporator, a short-path evaporator or a vaporization vessel, which is typically operated at bottom temperatures of 100 to 180° C. and at standard pressure or appropriately under reduced pressure down to 70 hPa. The feed to the first distillation unit can be introduced at room temperature, but the feed advantageously has a temperature of 50 to 130° C., especially 80 to 120° C. An elevated temperature is required when a ditrimethylolpropane melt is introduced. The introduction of the feed already at an elevated temperature can cause low boilers to be removed to evaporate instantly and be removed via the first tops fraction.

(14) If a solution of ditrimethylolpropane in a polar solvent is used, the first distillation unit serves to remove the polar solvent and further low boilers, for example methanol, 2-methylbutanol, 2-ethyl-2-methyl-1,3-propanediol or volatile formals. The first distillation unit is operated such that the content of the polar solvent in the bottoms fraction comprising ditrimethylolpropane is not more than 5000 ppm by weight, preferably up to 1000 ppm by weight and especially up to 500 ppm by weight, based on the mass of the bottoms fraction. Complying with a maximum limit for the solvent content in the bottoms fraction has an advantageous effect on the subsequent distillative purification. A suitable polar solvent is a lower C.sub.1-C.sub.5 aliphatic alcohol or C.sub.2-C.sub.10 dialkyl ether, such as methanol, ethanol, propanol or diethyl ether, or especially water.

(15) This bottoms fraction from the first distillation unit is removed and introduced to a second distillation unit.

(16) In the second distillation unit, the second tops fraction obtained is an intermediate-boiling product stream in which compounds having a higher boiling point than the low boilers removed in the first tops fraction but a lower boiling point than ditrimethylolpropane are present. If a mixture comprising trimethylolpropane and ditrimethylolpropane is worked up, the second tops fraction comprises essentially trimethylolpropane and additionally also intermediate fractions and residues of the polar solvent and of low boilers. The intermediate-boiling tops fraction also comprises, for example, the monolinear formal of trimethylolpropane (formula VI), the bis-linear formal of trimethylolpropane (formula VII), the monolinear formal of the cyclic formal of trimethylolpropane (formula VIII), and also the monocyclic formal of ditrimethylolpropane (formula V) and the bis-cyclic formal of ditrimethylolpropane (formula X). These oxygen containing compounds can also be embraced by the intermediate fraction I. The second tops fraction can then appropriately be recycled into the purification stage of the overall process for the preparation of trimethylolpropane, advantageously into the purifying distillation to obtain trimethylolpropane.

(17) The second tops fraction is removed in a distillation unit having at least 5 theoretical plates, preferably at least 8 theoretical plates and especially 8 to 20 theoretical plates. The thermal stress in the second distillation unit should likewise be kept at a minimum, and minimum residence times should be employed. The residence time in the second distillation unit, i.e. in the overall distillation apparatus, is generally from 2 to 60 and preferably from 10 to 30 seconds. The plant connection used is a thin-film evaporator with a column attachment having the required minimum number of theoretical plates. Suitable column attachments are conventional columns with random packing or structured packing, preferably columns with structured packing. Such structured packings are commercially available, for example in the form of Montz or Sulzer packings. The thin-film evaporators for use in the process according to the invention are units customary in the art, which are commercially available. Unsuitable are a boiler with column attachment or short-path evaporator, since, in this arrangement, either the residence time in the distillation unit is too high or the separation performance is inadequate. The second distillation unit is operated generally at bottom temperatures of 210 to 280° C. and at a pressure of 2 to 10 hPa. The bottoms fraction from the second distillation unit is subsequently introduced into a third distillation unit.

(18) The third distillation unit can also be regarded as a tailings removal unit and serves to obtain ditrimethylolpropane in sufficient quality. Ditrimethylolpropane is removed as the third tops fraction, and high boilers are removed as the bottoms fraction from the third distillation unit. The high boilers comprise, for example, the monocyclic formal of linear bistrimethylolpropane formal (formula XII) and the cyclic formal of ditrimethylolpropane (formula XI), and also the monolinear formal of ditrimethylolpropane (formula IX). These oxygen containing compounds can also be embraced by the intermediate fraction II. In order to obtain ditrimethylolpropane in sufficient quality, the third distillation unit has to have at least 4 theoretical plates and especially 4 to 20 theoretical plates. In the third distillation column too, the thermal stress should likewise be kept at a minimum, and minimum residence times should be employed. The residence time of the tops fraction in the third distillation unit is generally from 1 to 30 and preferably from 5 to 20 seconds. The plant connection used is a thin-film evaporator with a column attachment having the required minimum number of theoretical plates. Suitable column attachments are conventional columns with random packing or structured packing, preferably columns with structured packing. Such structured packings are commercially available, for example as Montz or Sulzer packings. The thin-film evaporators for use in the process according to the invention are also units customary in the art, which are commercially available. Unsuitable are a boiler with a column attachment or a short-path evaporator, since, in this arrangement, either the residence time of the top product in the distillation unit is too high or the separation performance is inadequate. The third distillation unit is operated generally at bottom temperatures of 240 to 280° C. and at a pressure of 0.2 to 5 hPa. A maximum bottom temperature of 280° C. should not be exceeded in order to avoid excessive decomposition of ditrimethylolpropane.

(19) The ditrimethylolpropane removed via the tops fraction is obtained in a quality sufficient for industrial applications, and it is possible to obtain product of value contents determined by gas chromatography of greater than 98% by weight. The distillation process according to the invention can effectively remove high-boiling impurities having a comparably high boiling point to ditrimethylolpropane, for example linear bistrimethylolpropane formal (formula II) or compounds having boiling points higher than trimethylolpropane but comparable to ditrimethylolpropane, such as the compounds embraced by intermediate fraction I. It is also possible to achieve sulphate ash contents, determined to DIN 51575, modified by addition of sulphuric acid after the burning and before the annealing of the sample, in the distillatively purified ditrimethylolpropane of less than 100 ppm and generally between 15 and 80 ppm.

(20) FIG. 1 shows a block diagram for the distillative treatment of the solution comprising ditrimethylolpropane. The preferably heated solution introduced via line (1) is fed to a first distillation unit (2) in which the first tops fraction comprising the polar solvent and low boilers, for example water or methanol, is removed at the top via line (3). Via the bottom of the first distillation unit (2), the bottoms fraction is removed with line (4). The first distillation unit is a customary column with, for example, 2 to 40 theoretical plates. If an aqueous or alcoholic solution is being worked up, the first distillation unit is operated such that the water or alcohol content in the bottoms fraction removed via line (4) is not more than 5000 ppm by weight based on the mass of the bottoms fraction. The bottoms fraction from the first distillation unit (2) is fed to a second distillation unit (5) which has at least 5 theoretical plates and which is configured as a thin-film evaporator with a column attachment. Via line (6), the second tops fraction is withdrawn, which comprises, for example, trimethylolpropane, intermediate fractions such as the intermediate-boiling compounds embraced by intermediate fraction I, and residues of the polar solvent and of low boilers. If the trimethylolpropane content in the second tops fraction is sufficiently high, this product stream can appropriately be recycled into the process for preparing trimethylolpropane. The bottoms fraction from the second distillation unit (5) is removed via line (7) and fed to the third distillation unit (8). This third distillation unit or else tailings removal unit has at least 4, for example 5, theoretical plates and is configured as a thin-film evaporator with column attachment. Via line (9), ditrimethylolpropane is removed as the tops fraction in high quality, while high boilers are discharged via line (10) from the process.

(21) The process according to the invention permits the distillative workup of solutions comprising ditrimethylolpropane and the effective removal of high-boiling impurities having boiling points comparable to ditrimethylolpropane, such as the compounds embraced by intermediate fractions I and II. Ditrimethylolpropane can generally be obtained with a high product of value content of greater than 98% by weight, determined by gas chromatography. It is also possible to achieve sulphate ash contents, determined to DIN 51575, modified, of less than 100 ppm.

(22) If the solutions comprising ditrimethylolpropane used in the process according to the invention are the residues from trimethylolpropane preparation, it is simultaneously possible to remove trimethylolpropane-enriched product streams and recycle them into the preparation process for trimethylolpropane, such that the plant efficiency and the trimethylolpropane yield can also be improved overall.

(23) The examples which follow describe the process according to the invention in detail. It is of course not restricted to the embodiment described.

Example 1

(24) For the distillative workup, an aqueous solution which contained 40% by weight of water and 60% by weight of organic components was used, and the organic fraction had the following composition (%) determined by gas chromatography:

(25) TABLE-US-00001 Forerun 6.3 Monocyclic formal (I) 0.3 Trimethylolpropane 64.4 Intermediate fraction I 0.6 Ditrimethylolpropane 28.0 Intermediate fraction II 0.1 Linear bistrimethylolpropane formal (II) 0.1 High boilers 0.2

Example 1a(1): Removal of Water and Forerun

(26) In a first distillation, in a 20-tray column with a boiler at a bottom temperature of 96° C. and at a pressure of 73 mbar hPa, water and low boilers were removed as distillate. The resulting distillation bottoms contained about 800 ppm by weight of water and had the following composition (%) determined by gas chromatography:

(27) TABLE-US-00002 Forerun 0.6 Monocyclic formal (I) 0.1 Trimethylolpropane 72.0 Intermediate fraction I 0.8 Ditrimethylolpropane 26.0 Intermediate fraction II 0.1 Linear bistrimethylolpropane formal (II) 0.1 High boilers 0.3

Example 1a(2): Low Boiler Removal, Trimethylolpropane Depletion

(28) The bottom product according to Example 1a(1) was subjected to another distillation in a 20-tray column with a boiler. The pressure applied was 3 hPa at a bottom temperature of 255° C. A reflux ratio of 1:1 was established. The resulting bottom product had the following composition (%) determined by gas chromatography:

(29) TABLE-US-00003 Forerun 0.1 Monocyclic formal (I) 0.0 Trimethylolpropane 2.4 Intermediate fraction I 1.6 Ditrimethylolpropane 93.4 Intermediate fraction II 0.8 Linear bistrimethylolpropane formal (II) 0.0 High boilers 1.7

Example 1b: Removal of Trimethylolpropane-Enriched Product Streams

(30) The bottom product from the first distillation according to Example 1a(1) (70/30 mixture) was used for the second distillation. The second distillation was configured such that intermediate fractions I and II, which have a boiling point comparable to ditrimethylolpropane, were depleted as far as possible in the distillation bottoms. Table 1 compiles different embodiments for the second distillation. The gas chromatography analysis shows the composition (%) of the input and the composition of the distillation bottoms.

(31) TABLE-US-00004 TABLE 1 Removal of trimethylolpropane-enriched product streams from the distillation bottoms according to Example 1a(1) [input: 70/30 mixture]; gas chromatography analysis of the bottom products 1b (3) 1b (1) 1b (2) comparative Thin-film Thin-film Only column Input evaporator with evaporator with with structured (70/30 column having column having packing and mixture) random packing structured packing a boiler Temperatures Top [° C.] 160 163 165 Side [° C.] — 242 190 Casing/bottom [° C.] 270 265 269 Column top [hPa] 5 5 4 Pressure differential [hPa] 29 11 17 Reflux ratio 1/3 none none Top draw [%] 76 75.3 77 Bottom draw [%] 24 24.7 23 Number of plates 11 15 15 Residence time [s] 10-30 10-30 3-5 hours Gas chromatography composition (%): Forerun 0.6 0.1 0.1 0.1 Monocyclic formal (I) 0.1 0.0 0.0 0.0 Trimethylolpropane 72.0 0.1 0.3 0.1 Intermediate fraction I 0.8 1.5 0.4 0.9 Ditrimethylolpropane 26.0 97.3 98.1 97.5 Intermediate fraction II 0.1 0.1 0.1 0.1 Linear bistrimethylol- 0.1 0.1 0.0 0.1 propane formal (II) High boilers 0.3 0.8 1.0 1.2

(32) As Example 1b(2) shows, the use of a column with structured packing, for example a column having a diameter of 40 mm equipped with a Montz packing, allows depletion of the intermediate fraction I in the bottoms. In Example 1b(3), comparatively long residence times have to be employed such that, at the high distillation temperatures, decomposition occurs to form volatile compounds and a comparatively high pressure differential is observed during the distillation compared to Example 1b(2), which works with the same column type. Nevertheless, in this configuration of the second distillation too, a bottom product with a ditrimethylolpropane content of 97.5% is obtained.

(33) The use of a column with structured packing and a higher number of separation stages is, however, preferable over a column with random packing.

(34) For the subsequent distillation experiments 1b(4)-1b(6), the bottom product from the low boiler removal and trimethylolpropane depletion according to Example 1a(2) (93% material) was used. The conditions of the second distillation and the gas chromatography analysis of the bottom product (%) are compiled in Table 2 below.

(35) TABLE-US-00005 TABLE 2 Removal of trimethylolpropane-enriched product streams from the bottom product according to Example 1a(2) (93% material, input), gas chromatography analysis of the bottom products 1b (4) 1b (5) 1b (6) Thin-film comparative comparative Input evaporator with Only column Only short- (93% column with with random pack- path evaporator material) structured packing ing and boiler with no column Temperatures Top [° C.] 145-175 144-215 185-190 Side [° C.] 149-180 148-223 — Casing/bottom [° C.] 260 270 200 Column top [hPa] 1-2 4 3 Pressure differential [hPa] 8 16 — Reflux ratio ½ 2/1 none Top draw [%] 24.3 13.1 50 Bottom draw [%] 75.7 86.9 50 Number of plates 5 15 1 Residence time [s] 10-30 3-5 hours  5-20 Gas chromatography composition (%): Forerun 0.1 0.1 0.1 0.0 Monocyclic formal (I) 0.0 0.0 0.0 0.0 Trimethylolpropane 2.4 0.3 0.1 2.3 Intermediate fraction I 1.6 0.7 0.7 2.3 Ditrimethylolpropane 93.4 97.7 96.7 94.1 Intermediate fraction II 0.8 0.1 0.1 0.2 Linear bistrimethylol- 0.0 0.0 0.0 0.0 propane formal (II) High boilers 1.7 1.1 2.3 1.1

(36) As Comparative Example 1b(6) shows, it is not possible to deplete the intermediate fraction I in the bottom product of the second distillation when working without a column attachment and only with a short-path evaporator. For the depletion of intermediate fraction I, according to Example 1b(4), a column attachment with theoretical plates is needed, even if the distillation input already has a high content of ditrimethylolpropane.

Example 1c: Tailings Removal

(37) The bottom product obtained according to Example 1b(3) was used for the third distillation for tailings removal. The desired ditrimethylolpropane was obtained as the top product in sufficient quality. The distillation conditions and the gas chromatography analysis (%) of the distillate are reported in Table 3.

(38) TABLE-US-00006 TABLE 3 Tailings removal from the bottom product according to Example 1b(3), gas chromatography analysis (%) of the distillate 1c (7) 1c (8) 1c (9) Thin-film comparative Thin-film Input evaporator with Only column evaporator with Example column with with random pack- column with 1b(3) structured packing ing and boiler structured packing Temperatures Top [° C.] 222 235 155 Side [° C.] 230 240 160 Bottom [° C.] 265 290-70  265 Column top [hPa] 3 5 0.3 Pressure differential [hPa] 10 30-35 — Reflux ratio none none none Top draw [%] 90.1 76.2 95.4 Bottom draw [%] 9.9 23.8 4.6 Number of plates 15 15 15 Residence time [s] 5-8 3-5 hours 5-9 Gas chromatography composition (%): Forerun 0.1 0.0 11.5 0.0 Monocyclic formal (I) 0.0 0.0 0.0 0.0 Trimethylolpropane 0.1 0.6 14.8 0.5 Intermediate fraction I 0.9 0.1 2.2 0.1 Ditrimethylolpropane 97.5 98.6 70.7 98.5 Intermediate fraction II 0.1 0.5 0.3 0.7 Linear bistrimethylol- 0.1 0.0 0.1 0.1 propane formal (II) High boilers 1.2 0.2 0.4 0.1 Ash value DIN 51575 <50 ppm — <50 ppm modified Gardner colour number >6 1 1 1 ASTM D1544

(39) As the comparison of Examples 1c(7) and 1c(9) with Comparative Example 1c(8) shows, the use of a thin-film evaporator with a column attachment is required to obtain ditrimethylolpropane as the top product in sufficient quality. A distillation unit composed of boiler with column attachment is unsuitable for tailings removal since, in the case of this arrangement, the high temperatures and long residence times cause increased decomposition, indicated by distinct formation of forerun and decrease in the ditrimethylolpropane content.