Process for the purification of tetrahydrofuran

Tetrahydrofuran is recovered by a two stage distillation procedure from a crude hydrogenation product resulting from vapor phase hydrogenation of diethyl maleate and containing water, ethanol and a minor amount of n-butanol, and possibly also dissolved hydrogen, in addition to butane-1,4-diol, gamma-butyrolactone and "heavies" such as diethyl ethoxysuccinate. In the first distillation stage, conveniently operated substantially at atmospheric pressure, ethanol, water, and tetrahydrofuran are recovered as overhead product, are condensed to separate the condensible components from a hydrogen stream which can be vented, and then redistilled in the presence of a molar excess of a hydroxylic solvent containing at least two hydroxyl groups, such as butane-1,4-diol, in a second distillation zone. Pure tetrahydrofuran is recovered as overhead product from the second distillation zone, while the bottom product therefrom is stripped in a third distillation zone of tetrahydrofuran, ethanol and water which are recycled to the first distillation zone and the stripped bottom product is recycled to the second distillation zone.

This invention relates to a process for the production of tetrahydrofuran. 
More particularly it relates to a continuous process for the separation of 
tetrahydrofuran from a crude reaction product containing, in addition to 
tetrahydrofuran, also minor amounts of ethanol, water, n-butanol and 
significant amounts of high boiling materials including butane-1,4-diol 
and gamma-butyrolactone. 
Production of tetrahydrofuran by hydrogenation of butyl maleate or 
succinate in the presence of a catalyst containing cobalt, molybdenum and 
aluminium oxides is described in SU-A-968034. 
In EP-A-0143634 there is described a process for the production of 
butane-1,4-diol in which vapor phase hydrogenation of an ester of a 
C.sub.4 dicarboxylic acid, for example an ester of maleic acid, such as 
diethyl maleate, is carried out in two reactors in series, using a copper 
chromite hydrogenation catalyst. Further teaching regarding this process 
can be found in WO-A-86/03189. A modified process in which the major 
product is gamma-butyrolactone is described in WO-A-86/07358. 
In such processes the reaction conditions can be varied so as to produce 
different proportions of butane-1,4-diol, of gamma-butyrolactone and of 
tetrahydrofuran, which is a co-product of the process. If desired, a 
dehydration section can be included in the plant as taught, for example, 
by WO-A-86/03189, in order to enhance the yield of tetrahydrofuran. By 
varying the conditions in the hydrogenation reactors and by use, if 
necessary, of the dehydration section, the output from the hydrogenation 
plant can be varied in response to market demand for the three products, 
i.e. butane-1,4-diol, gamma-butyrolactone, and tetrahydrofuran. 
In all cases the hydrogenation product contains, in addition to 
butane-1,4-diol and gamma-butyrolactone, a number of low boiling materials 
including tetrahydrofuran, ethanol, water and traces of n-butanol. 
However, the separation of these low boiling materials with a view to 
recovering a pure tetrahydrofuran product is problematic as various 
components of the mixture form azeotropes one with another. In particular, 
tetrahydrofuran forms azeotropes with water, as well as with ethanol. The 
azeotrope with water has a boiling point at atmospheric pressure of 
64.degree. C., while the azeotrope of tetrahydrofuran and ethanol boils at 
65.5.degree. C. at atmospheric pressure. Hence any attempt to separate the 
low boiling components of the crude reaction product by normal 
distillative methods results in production of an azeotrope, usually a 
tetrahydrofuran/water azeotrope, rather than leading to recovery of pure 
tetrahydrofuran (boiling point 66.degree. C. at 1 bar). 
In US-A-4665205 butane-1,4-diol is contacted with a dehydration catalyst, 
such as 97% aqueous sulphuric acid, to yield a reaction mixture comprising 
an aqueous 80% by weight tetrahydrofuran solution with a water content of 
20% by weight. This is then subjected to extractive distillation using 
butane-1,4-diol as an extraction solvent at a temperature in the range of 
from 40.degree. C. to 200.degree. C. and at a pressure of from 0.1 to 9.8 
bar (0.1 to 10 kg/cm.sup.2). Tetrahydrofuran is recovered as one stream 
from the extractive distillation column, from which is also recovered a 
second stream containing water and butane-1,4-diol. Water is stripped from 
this second stream and the resulting butane-1,4-diol is recycled to the 
dehydration step. 
Extractive distillation with added water is used to separate 
tetrahydrofuran from one or more aliphatic alcohols, such as ethanol, 
according to the teachings of US-A-4175009. Use of ethylene glycol or 
butane-1,4-diol as extractive solvent for the separation by extractive 
distillation of a methanol/tetrahydrofuran mixture is described in 
SU-A-1025709. A dilute solution of an alkali in a polyhydric alcohol, such 
as ethylene glycol or butane-1,4-diol, is used to separate the components 
of a methanol/tetrahydrofuran azeotrope in the process of DD-A-237056. 
In EP-A-0255400 there is described a process for the production of 
substantially pure gamma-butyrolactone from a feed mixture containing a 
major molar amount of gamma-butyrolactone and a minor amount of diethyl 
succinate. Such a feed mixture may result from hydrogenation of diethyl 
maleate according to the teachings of EP-A-0143634, WO-A-86/03189, and/or 
WO-A-86/07358. In EP-A-0255400 which was published on Feb. 3, 1988, there 
is described a process in which the "light ends", i.e. a mixture of 
tetrahydrofuran, ethanol, water, and n-butanol, are stripped off in a 
first distillation column which is operated under vacuum at a pressure of 
0.27 bar with a head temperature of 48.degree. C. The "light ends" which 
are recovered overhead are passed to a second distillation column which is 
operated at 1.2 bar at a head temperature of 58.degree. C. A first 
tetrahydrofuran/water azeotrope is recovered overhead from this second 
distillation column which is passed to a third distillation column 
operated at 7.0 bar with a head temperature of 126.degree. C. Essentially 
pure tetrahydrofuran is recovered as a bottom product from the third 
distillation column, whilst the overhead product therefrom is a second 
tetrahydrofuran/water azeotrope which is markedly richer in water than the 
first tetrahydrofuran/water azeotrope from the second column. This second 
azeotrope is recycled to the second column. 
This method of recovering tetrahydrofuran requires distillation under three 
different pressures, i.e. under vacuum (at 0.27 bar), under slightly 
elevated pressure (at 1.2 bar) and at a moderately elevated pressure (7.0 
bar). Distillation of mixtures containing tetrahydrofuran under vacuum is, 
however, best avoided since there is a risk of formation of explosive 
peroxides by reaction of air sucked into the system, possibly as a result 
of an inadvertent leak, with tetrahydrofuran. Moreover, as the crude 
hydrogenation product contains dissolved hydrogen, there is a risk of 
venting an explosive mixture of air and hydrogen if a vacuum system is 
used. Furthermore, the operation of the third distillation column at 
elevated pressure (at 7 bar) adds to the cost of the plant and of the 
operating costs therefor. 
The present invention seeks to overcome the disadvantages associated with 
the recovery of tetrahydrofuran described in EP-A-0255400. It further 
seeks to provide a process in which a vacuum distillation step, with its 
attendant risk of formation of explosive peroxides, is avoided. Yet again 
it seeks to provide a process for recovery of pure tetrahydrofuran in 
which hydrogen dissolved in the crude hydrogenation product can be safely 
vented. It further seeks to provide a process for recovery of 
tetrahydrofuran from a crude hydrogenation product obtained by 
hydrogenation of a dialkyl maleate, such as diethyl maleate, which can be 
operated at atmospheric pressure. 
According to the present invention there is provided a continuous process 
for the separation of tetrahydrofuran from a crude reaction product 
containing, in addition to tetrahydrofuran, also ethanol, water, and 
n-butanol, and significant amounts of high boiling materials including 
butane-1,4-diol and gamma-butyrolactone, which process comprises: 
(a) supplying the crude reaction product to a first distillation zone; 
(b) recovering a vaporous first overhead product from the first 
distillation zone containing tetrahydrofuran, ethanol and water; 
(c) redistilling the overhead product from the first distillation zone in a 
second distillation zone; 
(d) supplying to an upper part of the second distillation zone a hydroxylic 
solvent containing at least two hydroxyl groups; 
(e) recovering an overhead product from the second distillation zone 
consisting essentially of tetrahydrofuran; 
(f) recovering a bottom product from a lower part of the second 
distillation zone comprising hydroxylic solvent, tetrahydrofuran, ethanol 
and water; 
(g) stripping tetrahydrofuran, ethanol and water from the bottom product of 
step (f) by distillation in a third distillation zone; and 
(h) recycling stripped bottom product of step (g) to the second 
distillation zone. 
The crude reaction product may comprise a crude hydrogenation product 
obtained by vapor phase hydrogenation of an ester of maleic acid, such as 
diethyl maleate, which has been carried out according to the teachings of 
EP-A-0143634, WO-A-86/03189 and/or WO-A-86/07358. Such crude hydrogenation 
products typically contain at least about 10 mol %, up to about 70 mol % 
or more, of ethanol, at least about 5 mol % each, up to about 20 mol % 
each or more, of butane-1,4-diol and gamma-butyrolactone, not more than 
about 10 mol % of tetrahydrofuran, not more than about 15 mol % of water, 
and not more than about 2 mol % of n-butanol. They may further include a 
minor amount of diethyl succinate and minor amounts of other by-products, 
such as diethyl ethoxysuccinate. Typically such minor amounts do not 
exceed about 5 mol % each. 
In a preferred process according to the invention the first distillation 
zone is operated substantially at atmospheric pressure. Preferably the 
second distillation zone is operated substantially at atmospheric 
pressure. 
The overhead product from the first distillation zone is advantageously 
condensed prior to entry to the second distillation zone. In this case the 
condensate resulting from condensation of the overhead product from the 
first distillation zone can be separated from a gas stream containing any 
hydrogen, which was dissolved in the crude reaction product, and the gas 
stream can then be purged from the process. This purge gas can be burnt as 
fuel in the process or passed to a flare stack. In this way formation of 
potentially explosive mixtures of air and hydrogen, such as might be 
formed in a tetrahydrofuran recovery plant of the type described in 
EP-A-0255400 is avoided. 
Hence, the invention further provides, in a preferred aspect thereof, a 
process for the separation of tetrahydrofuran in a distillation plant from 
a crude reaction product containing, in addition to tetrahydrofuran, also 
dissolved hydrogen, ethanol, water, and n-butanol, and significant amounts 
of high boiling materials including butane-1,4-diol and 
gamma-butyrolactone, which process comprises: 
(i) supplying the crude reaction product to a first distillation zone which 
is operated substantially at atmospheric pressure; 
(ii) recovering a vaporous first overhead product from the first 
distillation zone containing hydrogen, tetrahydrofuran, ethanol and water; 
(iii) cooling said vaporous first overhead product to effect separation by 
condensation of a first condensate from a gaseous stream containing 
hydrogen, said first condensate containing tetrahydrofuran, ethanol and 
water; 
(iv) purging the gaseous stream of step (iii) from the distillation plant; 
(v) redistilling the first condensate in a second distillation zone which 
is operated substantially at atmospheric pressure; 
(vi) supplying to an upper part of the second distillation zone a 
hydroxylic solvent containing at least two hydroxyl groups; 
(vii) recovering a second overhead product from the second distillation 
zone consisting essentially of tetrahydrofuran; 
(viii) condensing said second overhead product; 
(ix) recovering a bottom product from a lower part of the second 
distillation zone comprising hydroxylic solvent, tetrahydrofuran, ethanol 
and water; 
(x) stripping tetrahydrofuran, ethanol and water from the bottom product of 
step (ix) by distillation in a third distillation zone; and 
(xi) recycling stripped bottom product of step (x) to the second 
distillation zone. 
As examples of hydroxylic solvents containing at least two hydroxyl groups 
there can be mentioned butane-1,4-diol, glycerol, diethylene glycol, 
butane-1,2-diol, triethylene glycol and the like. Preferably the 
hydroxylic solvent has a boiling point at atmospheric pressure of at least 
about 200.degree. C. and preferably at least about 230.degree. C. 
In the process of the invention the hydroxylic solvent is preferably 
supplied to the second distillation zone in an amount sufficient to 
provide a molar ratio of hydroxylic solvent:tetrahydrofuran in the second 
distillation zone of from about 3:1 to about 10:1. 
The material which is stripped in step (g) from the bottom product from the 
second distillation zone is conveniently returned to the first 
distillation zone. 
The bottom product from the first distillation zone in the process of the 
invention contains, in addition to butane-1,4-diol and 
gamma-butyrolactone, also water, ethanol, n-butanol and any other minor 
constituents present in the crude hydrogenation product, such as diethyl 
succinate and a minor amount of diethyl ethoxysuccinate. This bottom 
product from the first distillation zone is conveniently passed to a 
fourth distillation zone, which can also be operated at atmospheric 
pressure but is preferably operated under vacuum (e.g. at a pressure of 
from about 0.01 bar up to about 0.7 bar) so as to strip any remaining 
"light ends" from the bottom product. From this fourth distillation zone 
is recovered overhead a vaporous mixture containing water, ethanol and 
n-butanol. This mixture can then be redistilled in a further distillation 
column to give, as overhead product, a water/ethanol azeotrope and, as 
bottom product, substantially pure n-butanol. 
In the process of the invention the hydroxylic solvent is circulated in a 
loop between the second and third distillation zones. Provision may be 
made for supplying to this loop any necessary make up hydroxylic solvent 
and for taking a purge therefrom as may be necessary. 
In order that the invention may be clearly understood and readily carried 
into effect a preferred process for the continuous production of 
tetrahydrofuran in accordance with the invention and a plant designed for 
operation thereof will now be described, by way of example only, with 
reference to the accompanying drawing.

It will be understood by those skilled in the art that, as the drawing is 
diagrammatic, further items of equipment such as condensers, heat 
exchangers, reflux drums, column reboilers, pumps, vacuum pumps, 
temperature sensors, pressure sensors, pressure relief valves, control 
valves, flow controllers, level controllers, holding tanks, storage tanks, 
and the like, would additionally be required in a commercial plant. The 
provision of such additional items of equipment forms no part of the 
present invention and is in accordance with conventional chemical 
engineering practice. 
Referring to the drawing, maleic anhydride is supplied in line 1 to an 
esterification plant 2 which is also supplied in line 3 with ethanol. 
Esterification plant 2 produces a stream of acid-free diethyl maleate in 
line 4, part of which is fed by way of lines 5, 6 and 7 to a vapor phase 
catalytic hydrogenation plant 8 which is also fed with hydrogen in line 9. 
In plant 8 the diethyl maleate is hydrogenated in the presence of excess 
gaseous hydrogen by passage, in the vapor phase, over a copper chromite 
catalyst to produce a crude product stream in line 10 that is 
substantially free from diethyl maleate and contains, as products, a 
mixture of butane-1,4-diol, gamma-butyrolactone, and tetrahydrofuran, and, 
as recyclable materials, diethyl succinate, and ethanol, as well as minor 
amounts of by-products, including water, n-butanol, and "heavies" such as 
diethyl ethoxysuccinate. 
Esterification plant 2 may include a non-catalytic monoesterification 
stage, in which maleic anhydride is reacted with excess ethanol to yield 
monoethyl maleate according to the following equation: 
##STR1## 
and one or more catalytic esterification stages, in which the resulting 
monoethyl maleate is further reacted with ethanol to yield diethyl 
maleate, according to the following equation: 
##STR2## 
Although homogeneous liquid phase esterification catalysts, such as 
sulphuric acid, can be used, it is preferred to use in the catalytic 
esterification stage or stages a heterogeneous solid catalyst, such as an 
ion exchange resin containing sulphonic acid groups, for example Amberlyst 
16. (The word "Amberlyst" is a trade mark). This obviates the need to 
neutralise the catalyst as is necessary when using a homogeneous catalyst, 
such as sulphuric acid. Hence the production of significant quantities of 
waste liquors and loss of potential product, in the form of monoethyl 
maleate, therein is avoided by use of a heterogeneous catalyst. Since 
equation (2) is reversible, as much water of esterification as possible 
must be removed if the yield of diethyl maleate is to be maximised. 
In one scheme monoethyl maleate is passed in co-current with excess ethanol 
through a primary esterification reactor containing a charge of a suitable 
ion exchange resin (e.g. Amberlyst 16), the resulting intermediate 
reaction mixture is distilled to remove excess ethanol and water 
therefrom, and then the bottom product containing a mixture of mono- and 
diethyl maleates is fed in countercurrent to dry ethanol through one or 
more further esterification stages, each also containing a charge of a 
resin catalyst (e.g. Amberlyst 16). Further details of such a plant can be 
found in EP-A-0255399. 
Final traces of monoethyl maleate and any other acid material present can 
be removed from the system by a two stage distillation procedure according 
to the teachings of EP-A-0255401, possibly followed by the washing 
procedure taught in GB-A-2193207. In this two stage distillation procedure 
monoethyl maleate is allowed to decompose thermally in the first 
distillation stage to yield ethanol, which is removed overhead, and maleic 
anhydride, which co-distils with product diethyl maleate and is separated 
therefrom in a second distillation stage. Further distillation stages can 
be used to remove the final traces of acid materials therefrom. The 
alternative washing procedure involves washing the ester with an alkaline 
solution of disodium maleate containing an alkali metal hydroxide, 
carbonate, bicarbonate or a mixture thereof, following by distillation, to 
remove traces of water and sodium ions. 
In an alternative esterification process described in WO-A-88/00937, a 
primary esterification reactor is used that contains a charge of Amberlyst 
16 resin, or similar solid catalyst. The resulting mixture of diethyl 
maleate, monoethyl maleate, ethanol and water is distilled to remove 
substantially all the ethanol and water therefrom, and the ester mixture 
(typically containing an approximately 65:35 molar mixture of diethyl and 
monoethyl maleates) is reacted with further ethanol in a continuously 
stirred tank reactor containing also a charge of Amberlyst 16 resin or 
other solid catalyst from which a stream containing an approximately 85:15 
molar mixture of diethyl and monoethyl maleates, water and ethanol is 
recovered. This is then distilled to remove substantially all water and 
ethanol therefrom and the residue is subjected to the procedure of 
EP-A-0255401 and possibly also to the procedure of GB-A-2193207. 
Hydrogenation plant 8 may include a single catalytic zone or may include 
two hydrogenolysis zones operated according to the teachings of 
EP-A-0143634, of WO-A-86/03189 or of WO-A-86/07358. 
The crude hydrogenation product is fed by way of lines 10 and 11 to a first 
distillation column 12 which is operated at a pressure of 1.1 bar and at a 
head temperature of 66.1.degree. C. A mixture of tetrahydrofuran, ethanol 
and water is recovered overhead in line 13, together with any hydrogen 
dissolved in the crude product in line 10. This mixture is condensed by 
cooling in a condenser 13C before being passed in line 14 to a second 
distillation column 15. A vent gas stream consisting mainly of hydrogen is 
taken in line 16 for use as a fuel or for venting to a flare stack. Column 
15 is operated at 1.1 bar and with a head temperature of 68.3.degree. C. A 
stream of butane-1,4-diol is fed to an upper part of second distillation 
column 15 in line 17 at a mass flow rate which is approximately 6 to 7 
times that of the mass flow rate in line 14 so as to give a 
butane-1,4-diol:tetrahydrofuran molar ratio of approximately 4.5:1 in 
second distillation column 15. Essentially pure tetrahydrofuran is 
recovered as an overhead product from second distillation column 15 in 
line 18. The tetrahydrofuran product is condensed in condenser 18C. 
The bottom product in line 19 from second distillation column 15 is a wet 
mixture of tetrahydrofuran, ethanol, and a minor amount of n-butanol, 
dissolved in butane-1,4-diol. This is passed to a third distillation 
column 20 in which tetrahydrofuran, ethanol, and any n-butanol are 
stripped from it and appear overhead in line 21. The mixture in line 21 is 
admixed with recycled material in line 22 and returned to first 
distillation column 12 by way of lines 23 and 11. The stripped 
butane-1,4-diol is recycled to the second distillation column 15 by way of 
lines 24, 25 and 17. A purge stream can be taken in line 26 and any make 
up butane-1,4-diol required to sustain the circulating flow of 
butane-1,4-diol between second and third distillation columns 22 can be 
supplied in line 27 from the downstream part of the plant, which is 
described further below. 
The bottom fraction from first distillation column 12 contains, in addition 
to the high boiling materials present, such as butane-1,4-diol, 
gamma-butyrolactone, diethyl succinate, and a minor amount of diethyl 
ethoxysuccinate and other "heavies", also ethanol, water, and n-butanol, 
but only a trace amount of tetrahydrofuran. This bottom fraction is passed 
in line 28 to a fourth distillation column 29 which is operated at a 
pressure of 0.26 bar. Low boiling materials, i.e. remaining traces of 
tetrahydrofuran, water, ethanol and n-butanol are recovered overhead in 
line 30 at a head temperature of 47.8.degree. C. and are mixed with 
make-up ethanol supplied in line 31. The resulting mixed stream is 
supplied in line 32 to a fifth distillation column 33. Column 33 is 
operated at 2 bar and at a head temperature of 96.7.degree. C. A wet 
ethanol stream is recovered overhead in line 3 for use in the 
esterification plant 2. Esterification plant 2 includes a water recovery 
section (not shown) whereby the water mass balance of the plant can be 
maintained. 
The bottom product from fifth distillation column 33, which is recovered in 
line 34, is substantially pure n-butanol. 
The "heavy ends" fraction in line 35 from fourth distillation column 29 is 
a mixture containing, in addition to butane-1,4-diol and 
gamma-butyrolactone, a minor amount of diethyl succinate, as well as a 
minor amount of "heavies", such as diethyl ethoxysuccinate. This is fed to 
a sixth distillation column 36 which is operated under vacuum at a 
pressure of 0.13 bar with a head temperature of 136.degree. C. The 
overhead product from column 36 is a mixture of diethyl succinate, 
gamma-butyrolactone and a minor amount of butane-1,4-diol; this is passed 
by way of lines 37 and 38 to a seventh distillation column 39 which is 
operated at a pressure of 0.13 bar and at a head temperature of 
approximately 141.degree. C. Column 39 is also supplied by way of line 40, 
at a point above the point of connection of line 38, with diethyl maleate 
from line 4 via line 41. Hence the mixture of gamma-butyrolactone, diethyl 
succinate and butane-1,4-diol in line 38 is distilled in seventh column 39 
in the presence of diethyl maleate. The overhead product in line 42 from 
column 39 is a mixture of diethyl maleate and gamma-butyrolactone. The 
bottom product from column 39 comprises a mixture of diethyl succinate and 
diethyl maleate, and possibly a trace amount of "heavies"; this is taken 
by way of line 43 and admixed with diethyl maleate in line 5 to form the 
stream in line 6. Hence the diethyl succinate and diethyl maleate 
recovered from the bottom of column 39 are recycled to the hydrogenation 
plant 8 by way of lines 6 and 7. 
If desired some of the material in line 37 can be recycled to the 
hydrogenation plant by way of lines 44 and 7. 
As already mentioned, the stream in line 42 is substantially free from 
diethyl succinate and consists predominantly of a mixture of 
gamma-butyrolactone and diethyl maleate. This is passed to an eighth 
distillation column 45 which is operated at a pressure of 0.13 bar and at 
a head temperature of 135.degree. C. A side stream is taken from near the 
top of column 45 in line 46. This stream consists essentially of 
gamma-butyrolactone. A purge stream can be taken in line 47 for recycle of 
any "lights" which reach column 45 to first distillation column 12; this 
purge stream is recycled from line 47 by way of lines 22, 23 and 11. 
The bottom product from column 45 is mainly diethyl maleate but contains 
also a minor amount of gamma-butyrolactone. This is recycled to seventh 
distillation column 39 by way of lines 49 and 40. 
Although columns 39 and 45 could be combined into a single column, it is 
preferred to utilise two columns 39 and 45 so as to reduce the danger of 
carryover of diethyl succinate. Thus, if for any reason, the output from 
esterification plant 2 should be interrupted so that no diethyl maleate is 
temporarily available in line 4 for supply to line 41, then diethyl 
maleate can be recycled between columns 39 and 45 by way of lines 49 and 
40, thus ensuring that diethyl succinate appears in the bottom product in 
line 43 from column 39 and not in the gamma-butyrolactone product in line 
46, until either columns 39 and 45 can be shut down or else the supply of 
diethyl maleate in line 4 can be restored. 
As described above, part of the diethyl maleate in line 4 of the plant of 
the drawing flows to hydrogenation plant 8 in lines 5, 6 and 7, whilst the 
remainder flows in lines 41 and 40 to column 39. If desired, or if more 
convenient or expedient, line 5 can be omitted so that all of the diethyl 
maleate from line 4 passes by way of lines 41 and 40 to column 39 and 
thence by way of lines 43, 6 and 7 to the hydrogenation plant 8. 
Reverting to sixth column 36, the bottom product therefrom in line 50 is a 
mixture of butane-1,4-diol and "heavies". This is distilled in ninth 
distillation column 51 which is operated at a pressure of 0.1 bar and at a 
head temperature of 162.2.degree. C. A stream of substantially pure 
butane-1,4-diol is recovered from near the top of column 51 in line 52. 
Part of this can be used to supply make up butane-1,4-diol to second 
distillation column 15 in line 27, whilst the remainder is passed on as 
product butane-1,4-diol in line 53. A bleed stream may be taken from the 
reflux stream for column 51 in line 54 and recycled to first distillation 
column 12 by way of lines 55, 22, 23 and 11 for the purpose of recycling 
any "lights" which may reach column 51. 
The bottom product from distillation column 51 contains butane-1,4-diol and 
"heavies", such as diethyl ethoxysuccinate. This stream in line 56 is 
passed to a tenth distillation column 57 which is operated at a head 
temperature of 165.degree. C. and at a pressure of 0.1 bar. The overhead 
product in line 58 is combined with overhead product in line 54 and passed 
by way of lines 55, 22, 23 and 11 to first distillation column 12. A 
bottom product stream consisting mainly of diethyl ethoxysuccinate and 
other "heavies" in line 59 can be exported beyond site limits or can be 
used as boiler fuel in the plant. 
In the plant of the drawing distillation columns 39 and 45 are operated 
according to the teachings of EP-A-0255400. Instead of supplying diethyl 
maleate to column 39 in line 40 it is alternatively possible to omit line 
41 and to supply in line 40 to column 39 another ester, such as diethyl 
ethoxysuccinate, as taught by copending U.S. Patent Application No. 
07/223,079, filed July 22, 1988, or di-n-butyl maleate, as taught by 
copending U.S. Patent Application No. 07/222,728, filed July 22, 1988, now 
abandoned. In this case the bottom products from columns 39 and 45 should 
be distilled to separate the more volatile materials present, such as 
gamma-butyrolactone, from the ester supplied by way of line 40. That ester 
can then be recycled to column 39, after addition of any necessary make-up 
ester, while the more volatile products are recycled to hydrogenation 
plant 8 by way of lines 6 and 7. If diethyl ethoxysuccinate is used then, 
as this is a minor by-product of the hydrogenation step effected in 
hydrogenation plant 8, a suitable make-up supply of diethyl 
ethoxysuccinate can be obtained by distillation of the stream in line 52. 
The invention is further illustrated in the following Example. 
EXAMPLE 
Distillation of a crude product obtained by vapor phase hydrogenation of 
diethyl maleate according to the teachings of EP-A-0143634 was effected on 
a pilot plant scale using a 12.2 m (40 feet) distillation column with an 
internal diameter of 0.1 m (4 inches) made of 316 stainless steel packed 
with six beds of Sulzer Mellapak 500Y stainless steel packing. (The word 
"Mellapak" is a trade mark). 
The column was provided with an oil heated reboiler. Each section of the 
column corresponding to a particular bed was lagged and could be 
electrically heated to enable adiabatic conditions to be maintained in the 
column. A liquid distributor/re-distributor and a vapor distributor were 
positioned between each adjacent pair of beds within the column. 
The crude product was supplied continuously to the column above the third 
bed of packing from the bottom at a feed temperature of 70.degree. C. 
using a column top pressure of 1.03 bar (770 mm Hg). The column head 
temperature was 65.8.degree. C. and the reflux ratio was 20:1. 762.39 kg 
of crude product was fed to the distillation column; 50.80 kg of overhead 
product was recovered from the top of the column as a condensate and 
709.48 kg of bottoms product was recovered from the sump of the column, 
corresponding to a weight balance of 99.7% in this first distillation 
step. (The small apparent loss can be attributed to experimental error). 
The mol % composition of the various streams was as set out below in Table 
I. 
TABLE I 
______________________________________ 
Component Feed Overhead Bottoms 
______________________________________ 
Tetrahydrofuran 
4.82 80.64 0.04 
Ethanol 59.08 5.11 63.10 
Water 9.52 14.17 9.10 
-n-butanol 0.17 0 0.19 
gamma-butyrolactone 
10.27 0 11.00 
butane-1,4-diol 
14.71 0 15.07 
diethyl succinate 
1.22 0 1.27 
"unknown(s)" 0.21 0.08 0.23 
______________________________________ 
The overhead product from this first distillation step was re-distilled in 
the same column at 1.03 bar (770 mm Hg), being continuously supplied as 
feed above the first bed of packing from the bottom of the column at 
18.degree. C. Butane-1,4-diol was supplied to the top of the column at a 
feed temperature of 61.degree. C. An overhead vaporous product was 
recovered from the top of the column and condensed. A bottom product was 
taken from the sump of the column. The weight ratio of 
feed:butane-1,4-diol was 0.21:1. The compositions of the various stream in 
mol % were as set out in Table II below. 
TABLE II 
______________________________________ 
Component Butane-1,4-diol 
Overhead Bottoms 
______________________________________ 
Tetrahydrofuran 
0 96.37 5.69 
Ethanol 0 0.21 2.60 
Water 0.48 3.10 2.82 
gamma-butyrolactone 
0 0 0 
Butane-1,4-diol 
99.21 0.21 88.66 
"Unknown(s)" 0.31 0.11 0.23 
______________________________________ 
614.3 kg of butane-1,4-diol was supplied to the column in this second 
distillation step whilst 114.0 kg of feed material was fed to the column 
above the first bed of packing. 65.8 kg of condensate comprising 
essentially pure tetrahydrofuran was recovered overhead from the column, 
as well as 662.5 kg of bottoms product. The mass balance was calculated to 
be 100%. 
The bottoms product from the first distillation step was re-distilled in 
the same column at a pressure of 0.26 bar (197 mm Hg) and at a head 
temperature of 44.degree. C. The feed was continuously supplied, above the 
second bed of packing from the bottom of the column, at a temperature of 
82.2.degree. C. under a reflux ratio of 0.35:1. 705.75 kg of feed yielded 
379.01 kg of overhead condensate and 324.57 kg of bottoms product, 
corresponding to a weight balance of 99.7% (the balance being attributable 
to experimental error). The compositions in mol % of the streams were as 
set out in Table III. 
TABLE III 
______________________________________ 
Component Overhead Bottoms 
______________________________________ 
Tetrahydrofuran 0.02 0.09 
Ethanol 87.17 0.20 
Water 12.55 0.06 
-n-butanol 0.23 Trace 
gamma-butyrolactone 
-- 39.68 
Butane-1,4-diol -- 54.90 
Diethyl succinate -- 4.27 
"Unknown(s)" 0.03 0.80 
______________________________________