CA1069458A

CA1069458A - Process for isolating propylene glycol diesters in the preparation of propylene oxide

Info

Publication number: CA1069458A
Application number: CA251,271A
Authority: CA
Inventors: Gerd Schreyer; Jorg Krekel; Karl-Hermann Reissinger; Rolf Wirthwein
Original assignee: Bayer AG; Deutsche Gold und Silber Scheideanstalt
Current assignee: Bayer AG; Evonik Operations GmbH
Priority date: 1975-04-30
Filing date: 1976-04-28
Publication date: 1980-01-08
Also published as: SU621313A3; NL175909B; RO69788A; FR2309549B1; DE2519291C3; DE2519291B2; NL175909C; GB1491661A; IE43576B1; DE2519291A1; ES447409A1; JPS51133220A; BE841201A; IE43576L; NL7604535A; FR2309549A1; JPS5946230B2; NO761472L

Abstract

PROCESS FOR ISOLATING PROPYLENE GLYCOL DIESTERS
IN THE PREPARATION OF PROPYLENE OXIDE
ABSTRACT OF THE DISCLOSURE
Process for isolating propylene glycol dicarboxylates in the preparation of propylene oxide by reaction of propylene with a solution of percarboxylic acid in an organic solvent, the boiling point of which is lower than that of the carboxylic acid which corresponds to the percarboxylic acid used as the epoxidising agent, and higher than that of propylene oxide, separation, by distillation, of the reaction mixture which essentially contains propylene oxide, the carboxylic acid corresponding to the percarboxylic acid used as the epoxi-dising agent and one or more of the by-products propylene-glycol, propylene glycol monocarboxylate and propylene glycol dicarboxylate as well as propylene and the organic solvent, into a fraction containing propylene oxide and propylene and a fraction containing the carboxylic acid, the by-products mentioned and the organic solvent and further separation of thesefractions into the individual components by distillation.
The fraction containing the carboxylic acid, one ore more of the by-products propylene glycol, propylene glycol monocarboxy-late and propylene glycol dicarboxylate, and the organic solvent is distilled in a column at pressures of 1.5 to 6 bars and with an average residence time of 10 to 90 minutes in the sump.
The organic solvent is removed as the top product and the carboxylic acid and the corresponding propylene glycol di-carboxylate is obtained as the sump product. From the sump product the propylene glycol dicarboxylate is isolated in a manner which is in itself known.

Description

~.~G945~

The present invention relates to a process for iso-lating propylene glycol diesters in the preparation of propy-lene oxide by reaction of propylene with an organic solution o~ a percarboxylic acid. Propylene glycol diesters are used as solven-ts for polymers or as non-toxic i~hibitors for bac-terial growth (US Patent Speci~ica-tion 2,446,505).
It has long been known to epoxidise propylene with the aid o~ a percarbox~lic acid to give propylene oxide (Prileshaye~, Ber. dtsch. chem. Ges. 42, 4811 (1909)).
Do Swern gives a comprehensive more recent review of this reaction in "Organic Peroxides", Wiley Interscience 1971, Volume 2, pages355 to 533, especially pag~ ~75 to 378. The percarboxylic acid is generally employed in the form of an organic solution for the epoxidation of propyle~e. (D. Swern, i 15 loc. cit., page 377, lin~ 28 to 29). Considerable amounts of f by-products, such as propylene glycol, propylene glycol mono-esters and propylene glycol diesters, readily ~orm in this reaction ~or the preparation of propylene oxide ~rom propy-lene. This is the case especially when water, a carboxylic acid (generally the carboxylic acid correspo~ding to the per-carboxylic acid) or mineral acids are present in the organic percarboxyllc acid solution (for example US Patent Specifica-tion 3,350,422, column 2, lin~ 5 to 11; and also DT OS
(German Published 5peci~ication) 1,92~,~92, page 29 lin~ 19

2~ to 25).
i These contamina~s which promote the ~ormation of the by-products mentioned are generally unavoidable in the organic pe~carboxylic acid solutions used for the epoxidation.
Water is always present, for example ~rom the preparation of ~0 the percarboxylic acid~ Thus, for example 9 there is a possibility of isolating peracetic acid or perpropionic acid Le A 16 426 - 2 - ~

-- . . - . : , . .

~o~s~

~s a solution in an inert organic sol~ent, ~or example in ethyl acetate or acetone, from the o~idation of acetaldehyde or propionaldehyde n~tively (German PatentSpec~cation l~a~7 324), various by-products being ~ormed, including water (N.A. Sokolova a~d others, Zh. Fiz. Khim 357 page 850 (1961) a~d Russian Journal of Ph~sical Chemistry, Volume 35 (1961~, pages~l5 to 419, especially page 415, right hand column~
final paragraph).
If the percarboxylic acid is prepared from hydrogen peroxide and the corresponding carboxylic acid, in general percarboxylic acid solutions are obtained which also contain water and frequently also mineral acids.
For example, according to the process of DT-AS (German Published Specification) 1,043,316, a percarboxylic acid solution is obtained which contains the whole of the catalyst required for reaction of the percarboxylic acid with hydrogen peroxide. In detail, the process is to react acetic acid or propionic acid with aqueous hydrogen peroxide in the presence of sulphuric acid in a water-immiscible sol~ent and azeo-tropically to distil of~ the solvent water and part of the water formed during the reaction. In this process the resulting organic solution o~ the percarboxylic acid contains the acid catalyst used in the reaction, for example sulphuric acid (DT-AS (German Published Speci~ication) 1,043,316 Example 1).
Another means of preparing organic solutions of per-carboxylic acids from carboxylic acids and hydrogen peroxide is to react a corresponding carboxylic acid, in the presence of an acid catalyst, with hydrogen pero~ide in aqueous solu-tion and subsequently to extract the reaction mixture9 which is Xormed, with an organic solvent. A process of this type Le A 16 426 - 3 . ~ . . .

45~

is described, for example, in DOS (German Published Speci~ica tion) 2,14-1,156, according to which an organic solution of percarboxylic acids ha~ing 2 to 4 carbon atoms is obtained by reaction of the corresponding carboxylic acid with aqueous hydrogen peroxide and subsequent extraction wi-th a ; hydrocarbon or a chlorinated hydrocarbon. The percarboxylic acid solutions obtained contain 0.8 to 4.1% o~ water (DOS
(German Published Specification~ 2,141,156, Example 2).
It is apparen-t that with such extraction procedures organic solutions of percarboxylic acids are obtained which contain water and, if a soluble acid catalyst is present in the starting solution to be extracted, also an acid catalyst.
Of course, amounts of unreacted carboxylic acid are also always present.
;~ 15 In order, when using this type o~ organic solutions o~ percarboxylic acids ~or the epoxidation of propylene to give propylene oxide, as far as possible to suppress the i formation o~ the by-product~ propylene glycol, propylene glycol monoesters and propylene glycol diesters, which is due to the presence of water, a carboxylic acid and mineral acids, Z it has been attempted, on the one hand, to remove, as com-pletely as possible, the contaminating water or mineral acid which promote the ~ormation o~ these by-products; on the other hand, the process design conditions during the reaction and when working up the reaction mixture have been so selected that, as far as possible, the~e side reactions do not occur.
Azeotropic dehydration has been proposed in order to obtain percarboxylic acid solutions which are dehydrated to the desired degree (DOS (German Published Speci~ication) 2S141,156). However, it must be pointed out that the Le A 16 426 _ 4 , .

694S~3 required degree o~ dehydration of percarboxylic acid solutions cannot be achieved without considerable expenditure. Thus, it is stated, ~or exa~ple~ in DOS (German Published Speci~ica-tion) 1,618,625, page 3, final paragraph, to page 4, first line: "The use of an anhydrous reaction mixture is desired, but the preparation of solutions o~ per~ormic acid having less than 0.3~ of water is neither simple nor ec~nomically tenable. The use of a reaction mixture which contains only a small amo~mt o~ water is pre~erred. Il The formation of the by-prod~cts mentioned can also be suppressed -to a certain degree by excluding water and mineral acid as far as possible in the epoxidation reaction of propylene (DOS ~German Published Speci~ication~ 1,618,625, page 5, lineslO to 14); however9 the side reactions which lead to propylene glycol, propylene glycol monoester and propylene glycol diester cannot be completely stopped.
For example7 according to Example 3 o~ DT-OS (German Pub- ;
lished Speci~ication) 1,618,625, the yield of propylene oxide, relative to percarboxylic acid consumed~ is only 85%. The reason for this is that, on the one hand, the organic per-carboxylic acid solutions can generally not be employed in the absolutely a~hydrous state in the epoxidation and~ on the other hand, even when water and mineral acid are completely excluded durlng the epoxidation, the carboxylic acid, which corresponds to the percarhoxylic acid and which necessarily forms in the course of the reac-tion, can be added onto ' propylene oxide, the oxirane ring being opened, as is shown in ; equation (1), wherein R denotes a hydrocarbon radical.
. .
Le A 16 426 - 5 -6~

~\ H
(1) CH3-CH--CH2 + R-COOH--~ CH3-C-CH2-0_C_R
OH O
H
or 3 ~ ~ 2 o OH
; R-~
o The glycol monoester formed by this opening of the propylene oxide ring can itsel~ reac-t further to give propylene glycol and propylene glycol diester 9 as is shown in equation (2) which follows:

(2) 2 CH~-IH-CH2-o-ll R = CH3-CH~--fH2 OH O OH OH

+ CH3-fH-CH2_0_~_R
l OC R
,. O

As is known, an equilibrium between glycol monoester on the one hand and glycol and glycol diester on the other hand is set up in this reaction (DOS (German Published Specification~ 2,425,844, pages8 and 9, linking paragraph).
I It can be seen ~rom equations ~1) and (2) that a :~' mixture of propyle~e glycol, propylene glycol monoes~er and , propylene glycol diester as by-products must always be expected when propylene oxide is prepared from propylene and percarboxylic acid, even: i~ the reaction is carried out I with very pure percarboxylic; aff,id solutions.
The process design conditions in the reaction also ~' 20 influence the formation o~ the by-products mentioned and there has been no lack of ef~ort so to select the conditions Le A 16 426 ~ 6 -.

~69~5~

when carrying out the process industrially that the formation of by-product is prevented as far as possible.
In this context, it is proposed in British Patent Specification 1,105,261 to use a series of closed reaction loops, in which mixing of reaction product with the starting substances is largely prevented, for carrying out the reaction of a non-aqueous solution of peracetic acid with propylene.
However, even under these conditions 2.5 mol % of propylene glycol monoacetate and a further 20 5 mol % of other higher-boiling by-products are formed (British Patent Specification 1,105,261, page 3, lines63 to 68).
In the process of DT-OS (German Published Specifica-tion) 1,923,39Z it is said that a reduc-tion in the formation of the products from side-reactions is achieved when propy-; 15 lene is introduced in the form of bubbles into -the reaction system (a peracetic acid solution), a reaction system con-sisting of a multiplicity of reaction zones (in practice a multi-stage bu~ble column) being used. Howe~er, in this case also ? the formation of the by products mentioned cannot be completely suppressed (DOS (German Published $pecification) 1,923,392, page 14~ linesl9 and 20)o However, the glycol and glycol ester by-products which have been mentioned not only form during the reaction between propylene and the percarboxylic acid but can also form during workîng up o~ the reaction mixture con-taining the propylene oxide (for example DOS (German Published Specifica-tion) 2,013,877, page 3, lln~ 9 to 16).
In order as far as possible to eliminate this increased formation of by product during working up, an ~0 epoxidation mixture containing propylene oxide, propylene, acetic acid, a solvent and the by-products is fed, for example Le A 16 426 - 7 ~

~ ~ 6~ ~5 ~

according to the process of German Patent Specification 1,802,241, continuously to a first distillation column, in which propylene and propylene oxide are distilled off over the top and then fed to a second distillation for separating propylene oxide and propylene. The solven-t and -the acetic acid can be isolated from the s~mp of the first dis-tillation column by redistilla-tion, by-products ha~ing higher boiling points~ such as propylene glycol, propylene glycol monoacetate and polypropylene glycol acetates 9 which may in part emanate from the epoxidation and in part have been formed during the distillation by side-reactions, being obtained as the residue. According to Example 1 and 3 of German Patent Specification 1,802,241~ 4.4 to 6.2 per cent by weight of such high-boiling es-ter by-products, relative to the propylene oxide charged into the distillation, are obtained (German Patent Speci~ication 1,802,2415 column 3, line~2 and 3; column 4, lines24 to 26, and column 2, lines 28 to 41).
i The problem o~ the ~ormat~on o~ propylene glycol, propylene glycol monoester and propylene glycol diester during the preparation of propyle~e oxide from propylene with the aid of a percarboxylic acid can be summarised by stating that although it is possible, by using an organic solution, of a percarboxylic acid, which is as ~ar as possible absolutely anhydrous and ~ree from mineral acid and by selecting specific process design conditions for -the reaction and for the subsequent working up of the reaction mi~ture containing propylene oxide, to depress the formation of ` the sai~ by-products, it is not possible complet01y to pre-~0 vent this. However, since indus-trial propylene oxide plants as a rule are very large production units, the formation o*
Le A 16 426 - 8 -only a few per cent of a mixture of propylene glycol, propylene glycol mono-ester and propylene glycol diester, for example 1 to 3 mol % of the amount of propylene oxide, in absolute terms can signify a considerable amount9 the working up and further use of which can present difficulties. It is a parti-cular disadvantage that the product obtained is always a mixture of the three by-products mentioned, which are derived from propylene glycol, and not a product which is at least substantially homogeneous.
A process has now been found for isolating propylene glycol dicarboxylates formed in the preparation of propylene oxide. Accord-ing to the invention, there is provided a process for isolating a propylene glycol dicarboxylate rom the reaction mixture produced in the preparation of propylene oxide by reaction of propylene with a solution of a percarboxylic acid in an organic solvent, the boiling point of which is lower than that of the carboxylic acid which corresponds to the percarboxylic acid, and higher than that of propylene oxide, and which mixture comprises propylene oxide, the carboxylic acid corresponding to the percarboxylic acid, one or more of the by-products propylene g}ycol, propylene glycol monocarboxylate and pro~
pylene glycol dicarboxylate, and unreacted propylene and the organic solvent, the said process comprising separating the reaction mixture by dis~illation into a first fraction containing propylene oxide and propylene and a second fraction containing the carboxylic acid, one or more of the by~products and organic solvent, distilling the second fraction in a column at a pressure of 1.5 to 6 bars with an average residence time in the sump sufficient to pro-duce a sump product which comprises the carboxylic acid and the corresponding propylene glycol dicarboxylate, the organic solvent being removed as the top ;~ -product, and separating the propylene glycol dicarboxyla~e from the sump product.

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1~694~
Th~ process according to the invention can be ~mployed in the preparation of propylene oxide by reaction of propylene with solutions of very diverse percarboxylic acids, that is to say for isolating the dica~-boxylates of propylene glycol which correspond ~o the particular different percarboxylic acids. For example, the percarboxylic acids mentioned in D.
Swern "Organic Peroxides" J Volume I, Chapter VI, page 313 et seq. can be employed. Percarboxylic acids with 1 to 4 carbon atoms are particularly suitable. Acids which may be mentioned individually are performic acid>
peracetic acid, perpropionic acid and the perbutyric acids. Peracetic acid, perproyiollic acid and perisobutyric acid are particularly suitable. However, percarboxylic acids which are substituted, for example by chlorine, fluorine or a nitro or alkoxy groupJ are also suitable. Examples which may be mention~
ed individually are monofluoroperacetic acid, trifluoroperacetic acid, 1-fluoroethanepercarboxylic acid, 2-chloroethane-1-percarboxylic acid, 2-fluoropropan0-l-percarboxylic acid, l-fluoropropane-l-percarboxylic acid, 3-fluoropropanepercarboxylic acid and 2-chloropropane-2-percarboxylic acid.
Aromatic per-acids, such as perbenzoic acid, p-nitrobenzoic acid and mono-perphthalic acid can also be used. The use of perpropionic acid is particu-larly preferred.
In principle, all ~he compounds which are essentially inert under the conditions of the reaction and which have a boiling point which is higher than that of propylene oxide ,~ ~

~:3t69458 and lower than that of the carboxylic acid corresponding to the percarboxylic acid used are suitable as solvents for the per-acids. In general, the boiling point of the solvent is above 37C. The upper limit ~or the boiling point oP a suitable solvent is generally at about 220C. However, in individual cases a solvent having an even higher boiling point can also be used. Usually the boiling poin-t of the solvent used for the percarboxylic acid is in the range ~rom about 40 to 150C, for example between 60 and 120C.
Sui-table solvents which may be mentio~ed are: hydro-carbons, such as alkanes with 5 to 10 carbon atoms, cyclo-alkanes, such as cyclohexane, methylcyclohexane or ethyl-cyclohexane, aromati~ hydrocarbons, such as benzene, toluene, xylene, ethylbenzene, chlorobenzene or dichlorobenzene, chlorinated hydrocarbons, such as methylene chloride, chloro-form, 1,2-dichloroethane, 172-dichloropropane or chlorobenzene, esters of carboxylic acids containing 1 to 4 carbon atoms with alcohols containing 1 to 5 carbon atoms, such as eth~l formate 9 propyl ~ormate, methyl acetate, ethyl acetate 7 iso-propyl acetate, butyl acetate, methyl propionate, ethyl j propionate, methyl butyrate, ethyl butyrate or eth~l iso-butyrate. Particularly suitable solvents are chloro~orm, methylene chloride, methyl acetate, ethyl acetate and benzene~
Ben~ene is used preferentially.
Solutions of percarboxylic acid~ in a solvent mixture can also be employed for carrying out the epoxidation reac-tion within the sc~pe of the process accordi~g to the in~en-tion. It is also possible to render solutions of percarboxylic acids in a solvent which is less suitable for the epoxidation of propylene, for example triisooctyl phosphate, accessible to the process by adding a more suitable solvent.
Le A 16 426 945~3 The concentration of the percarboxylic acid in the organic solution can vary within wide limits. The upper limit is given by the explosibility o~ such solutions, which increases with rising concentration of the percarboxylic ; 5 acid. Customary concentrations are, for example, 5 to 50 per cent by weight o~ percarboxylic acid. Appropriately, a stabiliser is added to the percarboxylic acid solution.
Examples of sultable stabilisers are partially esterified phosphorus-containing acids or the salts thereo~ for example Na5(2-e~hylh~yl~(P301~z. The sta biliser i~ frequently present in an amount of 50 to 500 mg/kg of percarboxylic acid solu-tion; the usual amount is 80 to lG0 mg/kg.
The reaction between propylene and the organic per;
carboxylic acid can be carried out according to the cus-tomary methods. The reaction can be carried out in a homo-ge~eous liquid phase. A heterogeneous reaction mixture (for example gaseous/liquid) c~ also be used- The reaction is carried out at normal pressure or at elevated pressures o~
up to, ~or example, 50 bars. A suitable pressure range is - 20 2 to ~0 bars. The reaction temperature is generally 0 to 1~0C, preferably 20 to 100C.
The molar ratlo of propylene to percarboxylic acid can be varied within wlde ranges. It is appropriate to employ propylene in exce~s. For exampleg the reaction is carried out with a propylene excess o~ 0.01 to 8.00 mols, relative to the percarboxylic acid.
All the customary equipment, such as stirred kettles or tube reactors of very diverse dimensions with regard to diameter and length, can be used as the reactor system for the reaction of propylene with the percarboxylic acid. A cascade of very diverse reactor units9 for e~ample o~ kettlesy loop Le A 16 426 - 12 -~6~5~

reactors or reaction loops, with, for example9 2 to lO units, can also be used.
In general the reaction is carried out in such a way that the percarboxylic acid is reacted a3 completely a~ po~-~ib1e.
The composition of the mixture obtained ~rom thereaction can also vary within wide range~. Th~ propylene oxide content is, for e~ample, l to 50 per cent by weight, preferably 3 to 25 per cent by weight. The concentration of carboxylic acid corresponding -to the percarboxylic acid employed can accordingly also be 1 to 50 per cent by weight~
preferably 3 to 30 per cent by weight. As can be ~een from the descriptions of the processes disclo~ed hitherto, the content of the by-products prop~lene glycol, propylene glycol monoester and/or propylene glycol diester can, depending on the purity (water~ mineral acid and carboxylic acid content) ¦ and on the reaction conditions, be up to about 25 mol per cent of the amount o~ propylene oxide ~ormed. Eowever, it is alæo possible, without di~iculty, to obtain an even larger amount of the by-products by optionally adding water or mineral acîd.
However, the conditions o~ the reaction are generally ~o selected that only -the unavoidable amount of these by-products is formed. ~hiæ is frequently 1 to lO, usually about 2 to 5, mol per cent o~ the amount of propylene o~ide formed. ~he amount of solvent is generally 20 to 90 per cent by weight9 but in specific cases amou~ts above or below these i limits can alæo be used. The reaction mixture generally still contains unconverted propylene in accordance with its solubility.
Generally, the reac-tion mixtu~e is worked up by dis-tillation by taking o~f propylene and propylene oxide ~e A 16 426 - 13 -~ 6~ ~ 8 together over the top in a first distillation, the solvent, the carboxylic acid and the by product~ derived from propy-lene glycol being obtained as the sump phase. Another possible procedure is to take part o~ the solvent over the top with the propylene and the propylene oxide. By separating the top product, which essentially contains propy-lene, propylene oxide and solvent if appropriate, propylene oxide is obtained in a known manner, -the other componen-ts being separated off.
The sump product from the first distillation, which contains the carboxylic acid, the solvent and the by-products derived from propylene glycol3 is now again distilied, according to the invention, at a pressure o~ 1.5 to 6 bars and with an average residence time of 10 to 90 minutes in the ; 15 sump, the organic solvent being taken off as the top product and the carboxylic acid and the corresponding propylene glycol ~, dicarboxylate being obtained as the sump product from this second distillation column. The pressure in the distillation column is preferably 2.5 to 4 bars and preferentially 2.8 to ~.2 bars. In general 9 the sump temperature is about 1~0 to 250C, preferably 150 to 220~C and preferentially 160 to 190C. The temperature at the top is likewise determined by the pressure and also by the boiling point o~ the solvent.
The temperature at the top of the column is usually approxi-mately up to 160C, preferably 100 to 140C.
The ~ractionation column is generally designed in such a way that substantially pure solvent is obtained at the top of the distillation column. Small amounts of carboxylic acid, for example less than 1 per cent by weight, are possible~
Appropriately, the conditions are so selected that the top product contains le~s than 0.3 per cent by weight, preferably Le A 16 426 - 14 -~6~4 S 8 less than 0.1 per cent by weight, of carboxylic acid When the distillation is carried out according to the invention virtually only propylene glycol dicarboxylate is formed instead of the propylene glycol/propylene glycol mono-ester/propylene glycol dies-tsr by~product mixture. The water of esterification produced in these reactions is rémoved over the top together with the solvent and condensed. Part of the condensate is ~ed, appropriately a~ter separating of~
the water, as reflux into the column. The refluE ratio is generally 0.2 to 10~ pre~erably 0.3 to 5 and pre~erentially 0.5 to 2Ø
In addition to the propylene glycol diester, the car-boxylic acid is present i~ the sump. In addition, small amounts o~ solvent and of higher-boiling components may be present. The amount of solvent is, ~or example, less tha~
1 per cent by weight, usually less than 0.3 per cent by weight.
In general an average residence time of at least 10 minutes is required to guide the ~ormation o~ by-product as completely as possible in the direction of propylene glycol dicarboxylateO Usually a residence time o~ 90 minutes is suf~icient. In many cases an average residence time o~ 20 to 40 minutes is appropriate.
All the industrial equipment which is customarily used is suitable as the evaporator unit ~or the distillation column. Appropriately, reboilers or circulation re~oilers are used. The known ~ractionation units, such as trayed ; columns or packed columns can be used as the columns.
Particular attention must be paid to the sump material because o~ the corrosive properties o~ the carboxylic acids~
The solutio~ to this problem is known. Suitable materials ~Le A 16 426 - 15 ~

~g~8 are high grade stainless steels which, in addition to iron, essentially also contain chromium and nickel. Examples o~
high grade stainless steels which may be mentioned are a ma-terial with the DIN designation 1,4571, which, in addition to iron, contains 17.5 per cent by weight of chromium, 11.5 per cent by weight o~ nickel, 2.25 per cent by weight o~
molybdenum and also up to 2 per cent by weight of manganese, up to 1 per cent by weight o~ silicon, u~ to 0.1 per cent by weight of carbon and small amounts of titanium, or a material which, in addition to iron, contains 25 per cent by weight o~
chromium, 25 per cent by weight o~ nickel, 2.25 per cent by - weight of molybdenum and up to 2 per cent by weight of man-ganese, up to 1 per cent by weight of silicon, up to o.o6 per cent by weight of carbon and also small amounts of titanium and which is designated, according to DIN, by the number 1,4577. The material~ which is designated according to DIN by the number 2,4812 and which, in addition to nickel, contains 16% by weight o~ molybdenum and 16% by weight of chromiumS or a material which is designated by DIN 1,4439 and which, in addition to iron, contains 16.5 to 18.5 per cent by weight of chromium, 12.5 to 14.5 per cent by weight o~
nickel, 4 to 5 per cent by weight of molybdenum as well as 0.12 to 0.22 per cent by weight o~ nitrogen and up to 0.04 per cent by weight o~ carbon, up to 0.1 per cent by weight o~
silicon, up to 2 per cent by weight o~ manganese, up to 0.03 per cent b~ weight of phosphorus and up to 0.02 per cent by weight of sulphur, is also suitable.
Surprisingly, it is not necessary to add esteri~ica-tion catalysts in the distillation. However, small amou~ts of an esteri~ication catalyst can also be added. Catalysts which ca~ be admixed are, for example, sulphuric acid, phos-Le A 16 426 - 16 ~ 6 phoric acid, polyphosphoric acid or sulphonic acids~
The mixture of carboxylic acid and propylene glycol - diester obtained as the sump product from the distillation column when the process according to the invention is carried out can easily be separated into the individual components.
In general, the carboxylic acid is distilled o~ ~rom the dicarboxylate in a ~urther distillation. The propylene glycol dicarboxylate can then be subjected in a known manner, ; for example by vacuum distillation, to final purification ùntil the desired purity is obtained.
In a particular embodiment o~ the process according t~
the invention, ~or example, a benzene solution containing about 15 to 25 per cent by weight o~ perpropionic acid is employed for epoxidation of the propylene. The water content o~ this solution is 0.1 to 2 per cent by weight. The solu-tion of perpropionic acid in benzene is reacted with propy-lene, using a molar ratio of perpropionic acid to propylene of 1 : 2 to 3, at a temperature of 60 to 80C and at a pres~ure of 6 to 12 bars. 3 stirred kettles arranged in a cascade ~re . ' used, ~or example, as the reaction vessel and the reaction ie carried out in these with a residence time o~ 1.5 to 3 hours9 The conversion of the perpropionic acid is 98 to 100%. The selectivi-ty of the reaction for propylene oxide is 90 to 98%.
About 1 to 6 mol % o~ propylene glycol, propylene glycol mono propionate and propyle~e glycol dipropionate are formed, relative to the amount of propylene oxideO The composition of the reaction mixture is about 8 to 12 per cent by weight of propylene oxide, 1 to 5 per cent by weight o~ propylene, 20 to 35 per cent by weight of propionic acid and 0.15 to 008 ~0 per cent by weight of propylene glycol) propylene glycol mono--propio~ate and propylene glycol dipropionate; the re~ainder Le A 16 426 17 -. . ..

~6~45~i is benzene.
Working up b~ distillation starts with separatio~ into a top product of propylene oxide and propyle~
which contains about 30 to 50% o~ benzene~ and a ~ump product which contains the remainder of the benzene, the propionic acid and the by products derived ~rom propylene glycol. The product withdrawn from the sump o~' this dis tillation column is trans~erred to a second fractionation column, which is provided with a circulation reboiler and a condenser with a phase separator. At a pressure of 2 to 3 bars, benzene is distilled over the top, the water of esteri~ication being separated from the condensate. Part of the upper phase o~ the condensate~ returned to the column as reflux. The re~lux ratio is 0.5 to 2. A solution o~ about 1 to lO per cent by weight of propylene glycol dipropionate in propionic acid is obtained as the sump product.
In a further distillation column, propionic acid i8 distilled o~ over the top at a pressure of lOO to 400 mm Hg.
The propylene glycol dipropionate which is thus obtained as 2U the sump product is isolated in a purity of more than 99~ in a ~inal fractionation column provided with a thin layer evaporator, It can be put directly to ~urther u~e.
The advantage o~ the process according to the inven-tlo~ is that, in the prep~ration of propylene oxide from propylene and a percarboxylic acid, the propylene glycol di~
carboxylate corresponding to the percarboxylic acid is iso lated without any particular additional e~fort because o~
greater uniformity of the ~y-products derived ~rom propylene glycol. The problem o~ the separation and/or suitable ~ur-ther use o~ the by product mixture o~ propylene glycol andpropylene glycol carboxylic acid esters, which is obtained Le A 16 4Z6 - 18 -. . ~ . . .

1~699L5~3 with the processes known hitherto, is thus also eliminated.
Ex mple In a reaction system, 7.4 kg pure hour o~ high puritypropylene (= 175.8 mols/hour) are epoxidised with 68.27 kg per hour of a ben~ene solution o~ perpropionic acid (20.48%
by weight = 155.2 mols/hour)~ which also contains 12.67% by weight of propionic acid, 0.16% by weight o~ hydrogen per oxide, less than 0.1% by weight of water and 250 mg/kg o~ a Na salt of a partially esteri~ied polyphosphoric acid as the stabiliser.
~ he excess propylene, relative to the perpropionic acid feed, is 13.3 mol %. The reaction system co~sists o~
two loop reactors arranged in series and a downstream delay tube. The reaction is carried out at a pressure of 4 bars.
All the propylene is ~ed into the first loop reactor~
The reaction temperature in the ~wo loop reactors is 65C and the avera~e residence time of the reaction mixture is about 45 minutes in each case. In the delay tube, the reaction temperature is 70C and the average residence time o~ the reaction mixture is about 70 minutes. On leaving the second loop reactor, the perpropionic acid is about 9Q% converted and a~ter the delay tube a conversion of 99.8% is achieved.
The reaction mixture then contains on average 1.16% by weight o~ propylene~ 11.8% by weight of propylene oxide, 2~.5% by weigh~ o~ propionic acid and about 0.2% by weight of propylene glycol monopropionate in addition to the solvent benæene.

This reac~ion mixture is directly let down into a distillation column (I)~ in which propylene, the whole of the propylene oxide and about 10% o~ the benzene are separated o~f Le A 16 426 - 19 -1~69~S8 as the distillate. In a distillation column (II), this dis-tillate is separated into its components propylene, propylene oxide (8.91 kg/hour, 99.9% puri-ty = 98.7%, relative to the perpropionic æ~ ~oy~)and benzene as the sump product.
The sump product from column (1) is combined with the sump product from column (II) and the combined products are ~ed into a distillation colum~ (III). The combined product streams contain, o~ average, 30.5~ by weight of propionic acid, 69.1% by weight of benzene, about 0.2% by weight o~
propylene glycol monopropionate as well as small amounts o~ propylene glycol and propylene glycol dipropionate.
Distillation column (III) is a packed column (length =
6 m9 diameter = 150 mm), which is provided with a circulation reboiler, a condenser and a separator for phase separation of the distillate at the top o~ the column. The feed is in the centre of the column. At a pressure of 2.6 bars, a residence time of 80 minutes in the sump of the column, a sump tempera~
ture o~ 180C~ a temperature at the top of the column of 116C
and a re~lux ratio of about 1.0, 45.5 kg per hour of benzene (with 0.11% by weight of propionic acid and 0.09% by weight of water) ars obtained. In the course of 24 hours, 0.5 kg of aqueous phase, which contains about 5% by weight of propionic acid, are obtained in the separator.
The sump product, which contains about 1% by weight o~ propylene glycol dipropionate, is fed to a distillation column (IV) (packed column, length = 4 m, diameter = 150 mm).
At a pressure of 100 mm Hg, a sump temperature of 170C, a temperature at the top of the column of 89C and a reflux ratio of about 0.29 19.8 kg per hour of propionic acid (approximately 99.8% purity~ are distilled off. 0.25 kg per hour of crude propylene glycol dipropionate are withdrawn Le A 16 426 - 20 -~;9458 ~rom the sump of this column~ This product is distilled batchwise at 50 mm Hg in a thin layer evaporator ~itted with a packed column (length = 2 m, diameter _ lOQ mm), 0.22 kg of propylene glycol dipropionate o~ approæimately 98% purity being obtained from 0.25 kg of the crude productO This amount corresponds to 0.75%, relative to the perpropionic acid employed.
. .

Le A 16 426 21 -

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for isolating a propylene glycol dicarboxylate from the reaction mixture produced in the preparation of propylene oxide by reaction of propylene with a solution of a percarboxylic acid in an organic solvent, the boiling point of which is lower than that of the carboxylic acid which corresponds to the percarboxylic acid, and higher than that of propyl-ene oxide, and which mixture comprises propylene oxide, the carboxylic acid corresponding to the percarboxylic acid, one or more of the by-products pro-pylene glycol, propylene glycol monocarboxylate and propylene glycol dicar-boxylate, any unreacted propylene and the organic solvent, the said process comprising separating the reaction mixture by distillation into a first frac-tion containing propylene oxide and propylene and a second fraction containing the carboxylic acid, one or more of the by-products and organic solvent, dis-tilling the second fraction in a column at a pressure of 1.5 to 6 bars with an average residence time in the sump sufficient to produce a sump product which comprises the carboxylic acid and the corresponding propylene glycol dicarboxylate, the organic solvent being removed as the top product, and separating the propylene glycol dicarboxylate from the sump product.

2. A process according to Claim 1, wherein the propylene glycol dicarboxylate is a diester of a carboxylic acid containing from 1 to 4 carbon atoms.

3. A process according to Claim 1 or 2, wherein the percar-boxylic acid is perpropionic acid and the propylene glycol dicarboxylate is propylene glycol dipropionate.

4. A process for isolating propylene glycol dipropionate from the reaction mixture produced in the preparation of propylene oxide by re-action of propylene with a solution of perpropionic acid in an organic solvent, the boiling point of which is lower than that of propionic acid and higher than that of propylene oxide, and which mixture comprises propylene oxide, pro-pionic acid and one or more of the by products propylene glycol, propylene glycol monopropionate and propylene glycol dipropionate, any unreacted pro-pylene and the organic solvent, the said process comprising separating the re-action mixture by distillation into a first fraction containing propylene oxide and propylene and a second fraction containing the propionic acid, one or more of the by-products and the organic solvent, distilling the second fraction in a column at a pressure of 1.5 to 6 bars with an average residence time of 10 to 90 minutes in the sump, sufficient to produce a sump product which comprises propionic acid and propylene glycol dipropionate, the organic solvent being removed as the top product, and separating the propylene glycol dipropionate from the sump product.

5. A process according to Claim 1, 2 or 4, wherein the dis-tillation is carried out at a pressure of from 2.5 to 4 bars and at a sump temperature of 160 to 190°C.

6. A process according to Claim 1, 2 or 4, characterized in that benzene is used as the organic solvent.