PURIFICATION DE FLUX DE SOUS-PRODUITS DANS LA PREPARATION DE POLYESTERS A BASE DE PROPANEDIOL-1,3

BYPRODUCT STREAM PURIFICATION IN THE PREPARATION OF 1,3-PROPANEDIOL-BASED POLYESTERS

Abrégé français

La présente invention concerne un procédé de préparation d'un polyester à base de 1,3-propanediol. Ce procédé comprend la mise en contact d'un flux aqueux, contenant le sous-produit qu'est l'acroléine, avec une base, pendant un temps suffisant pour réduire la teneur en acroléine du flux aqueux. Dans un mode de réalisation spécifique, un procédé de préparation de poly(téréphtalate de propylène) comprend les étapes suivantes consistant à: (a) mettre en contact du 1,3-propanediol et de l'acide téréphtalique à une température élevée pour obtenir un mélange de produits de réaction comprenant un poly(téréphtalate de propylène) et une solution aqueuse comprenant de l'acroléine; (b) extraire du mélange de produits de réaction susmentionné la majeure partie de la solution aqueuse; (c) ajouter une quantité suffisante d'une base à la solution aqueuse pour former une solution basique dont le pH soit supérieur à 7,5 et maintenir cette solution pendant un temps qui permette d'abaisser sa teneur en acroléine; (d) éventuellement diluer la solution basique avec de l'eau, et (e) si on le désire, faire subir à la solution basique diluée un traitement biologique. Le procédé de l'invention donne un flux de sous-produits de polyesters dont les niveaux d'acroléine sont réduits.

Abrégé anglais

A process for preparing a 1,3-propanediol-based polyester which comprises
contacting an aqueous stream containing acrolein byproduct
with a base for a time effective to reduce the acrolein content of the aqueous
stream. In a specific embodiment, a poly(propylene terephthalate)
preparation process comprises the steps of (a) contacting 1,3-propanediol and
terephthalic acid at elevated temperature to produce a reaction
product mixture comprising a poly(propylene terephthalate) and an aqueous
solution comprising acrolein; (b) removing a major portion
of the aqueous solution from said reaction product mixture; (c) adding a
sufficient quantity of a base to the aqueous solution to form a
basic solution having a pH greater than 7.5 and maintening the basic solution
for a time effective to lower the acrolein content thereof
(d) optionally diluting the basic solution with water, and (e) optionally,
biotreating the dilute basic solution. The process of the invention
provides a polyester byproduct stream containing reduced levels of acrolein.

Revendications

-11-
1. A process in which at least one dicarboxylic acid and
1,3-propanediol are contacted at elevated temperature to
produce an aqueous product mixture comprising a
1,3-propanediol-based polyester and an aqueous solution
of acrolein which comprises:
(a) removing a major portion of said aqueous solution
from said aqueous product mixture;
(b) adding to the aqueous solution an amount of a base
effective to form a basic solution having a pH greater
than 7.5; and
(c) maintaining the basic solution for a time effective
to reduce the amount of acrolein therein.

-11-
2. The process of claim 1 in which said basic solution
has a pH greater than 10.
3. The process of claim 1 in which the base is selected
from alkali and alkaline earth metal hydroxides,
carbonates and bicarbonates.
4. The process of claim 3 in which the base is selected
from sodium hydroxide and potassium hydroxide.
5. The process of claim 1 in which the temperature of
the basic solution of step (c) is within the range of 0
to 50 °C.
6. The process of claim 1 in which step (c) is carried
out over a time within the range of 1 to 100 hours.
7. The process of claim 1 which further comprises:
(d) adding an aqueous liquid to the basic solution to
form a dilute solution containing less than 3 ppm
acrolein and
(e) subjecting the dilute basic solution to biological
treatment.

-12-
8. The process of claim 1 in which the dicarboxylic acid
is selected from at least one of terephthalic acid,
isophthalic acid and 2,6-naphthalene dicarboxylic acid.

Description

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BYPRODUCT STREAM PURIFICATION IN THE
PREPARATION OF 1,3-PROPANEDIOL-BASED POLYESTERS
This invention relates to the preparation of
polypropylene terephthalate) ("PPT") and related
copolyesters. In a specific aspect, the invention
relates to the treatment of a byproduct-containing stream
in a PPT preparation process.
The preparation of PPT involves the reaction of
terephthalic acid and excess 1,3-propanediol (PDO) at
about 250 °C under pressure to form an oligomer and
water. The water is then distilled from the PPT. The
major byproducts of the polymerization reaction,
acrolein and allyl alcohol, are contained in solution in
,the distillate.
Although the amount of these byproducts is low, it
would be desirable to further reduce the level of
byproducts in the aqueous distillate especially for
subsequent activated sludge treatment.
It is therefore an object of the invention to provide
a process for preparing a 1,3-propanediol-based poly-
esters~in which the level of acrolein byproduct in the
aqueous stream distilled from the product polyester is
reduced. This object is achieved by base treatment of the
aqueous byproduct stream, optionally followed by
biotreatment thereof.
The present invention therefore provides a process
for preparing a 1,3-propanediol-based polyester,
comprising contacting an aqueous stream containing
acrolein byproduct with a base for a time effective to
reduce the acrolein content of the aqueous stream. In a
specific embodiment, the invention provides in a process

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CA 02213745 1997-08-25
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in which at least one dicarboxylic acid and 1,3-propane-
diol are contacted at elevated temperature to produce a
n
aqueous product mixture comprising a 1,3-propanediol-
based polyester and an aqueous solution of acrolein
,
which comprises:
' (a) removing a major portion of said aqueous solution
from said aqueous product mixture;
(b) adding to the aqueous solution an amount of a bas
e
effective to form a basic solution havin
a
H
g
1G p
greater
than 7.5; and
(c) mai:ltainirg the basic solution for a time effective
to reduce the amount of acrolein therein;
(d) optionally, diluting the basic solution with
water;
and
1S (e) optionally, biotreating the dilute basic sohition.
The process of the i~l:vention proz~-ides a by
rodu
t
p
c
stream contair_ing reduced levels of acrolein.
The invention process involves forming a basic
aqueous solution of the acrolein byproduct of the
3G condensation pclyrnerization of 1,3-propanediol and
at
least one dicarboxylic acid to prepare a 1,3-propane- -
diol-based polyester. In addition to 1,3-propanediol, the
polyester reaction product mixture can contain
one or
more additional diols such as ethylene glycol,
2S 1,~-bu~.anediol, 1,4-cyclohexane dimethanol and
neopentyl
glycol. Suitable dicarboxylic acids include
for
,
example, terephthalic acid, isophthalic acid and
2,6-naphthaler~e dicarboxylic acid. The polyester and
copolyester condensation products of such diol and diacid
3G monomers are referred to herein as 1,3-propanediol-based
polyesters or PDO-based polyesters. "
Water-soluble byproducts are found in the a
queous
stream distilled from the PDO-based polyester reaction
product mixture. The amount of typical byproducts in th
e
35 aqueous distillate will generally be within th
e range of
AMENDED SHED

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about 100 to about 2500 ppm acrolein, 0.1 to 5 weight
percent ally! alcohol and 0.1 to 10 weight percent PDO,
based on the weight of the distillate.
The invention process involves addition of a base to
the aqueous byproduct solution. The base is preferably
an inorganic base, most preferably selected from alkali
and alkaline earth metal hydroxides, including lithium
hydroxide, sodium hydroxide, potassium hydroxide,
magnesium hydroxide and calcium hydroxide, and the
corresponding carbonates and bicarbonates. The preferred
base, because of its cost and effectiveness, is sodium
hydroxide.
The amount of base added to the aqueous byproduct
solution is that which is sufficient to impart a pH (at
25 C) of the aqueous solution of greater than 7.5,
preferably greater than 8, most preferably greater than
10. For the preferred inorganic bases such as alkali and
alkaline earth hydroxides, carbonates and bicarbonates,
the amount added will typically be such that the weight
ratio of base:acrolein is within the range of 0.01 to 20,
preferably 0.1 to 10, most preferably 0.2 to 5. For a
typical concentration of acrolein of up to 2500 ppm, the
preferred amount of base will be up to 2.5o by weight
based on the total weight of the aqueous solution,
although higher levels of base can be used if desired for
faster reaction and if not detrimental to subsequent
biotreatment.
The temperature during the base treatment can affect
the rate of disappearance of the acrolein and will
generally be within the range of 0 to 50 C, preferably
from 10 to 40 C. Temperatures above 50 C, i.e., near
or above the boiling point of acrolein, are effective but
generally to be avoided since this will increase the
evaporation of acrolein. The process is effective at

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temperatures less than 0 °C, but treatment times may be
prolonged.
After the addition of the base, the aqueous stream is
held in a vessel such as a tank or pipe for a length of
time to allow the acrolein to react with the base. The
time will typically be from a few minutes to several days
and will depend on the amount of base, the temperature,
the initial concentration of acrolein and the final
concentration of acrolein desired. Generally, treatment
times will fall within the range of 1 to about 100 hours.
After a sufficient treatment time for reduction of
the concentration of acrolein in the aqueous solution,
the solution may be optionally diluted with additional
aqueous fluid such as a second aqueous byproduct stream.
The treated aqueous solution can optionally be passed to
a biotreatment process. Biotreatment will generally
involve contact with activated sludge in an aeration
basin at a temperature within the range of 1 to 40 °C, a
dissolved oxygen content within the range of 0.5 to
8 mg/1, and a typical dilution ratio of 10-500:1. It is
preferred that treatment with base combined with optional
dilution be carried out so as to reduce the concentration
of acrolein to less than 3 ppm, most preferably to less
than 0.3 ppm, prior to any biotreatment.
Other treatment methods such as reverse osmosis,
ultrafiltration and adsorption may be used in combination
with the invention process.
The following examples will illustrate the invention.
Example 1
Treatment of PPT Byproduct Solution with NaOH
PPT byproduct solution (76.6427 g) was spiked with
0.1247 g acrolein (97a) to bring the acrolein level to
about 1950 ppm by GC analysis. Two samples of this water
in screw cap vials were treated with caustic. In one
vial, 0.0980 g of 1N sodium hydroxide was added to

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10.380 g of the solution. In the other, 0.2341 g of 1N
sodium hydroxide was added to 10.380 g of the solution.
The vials were sealed and allowed to stand at room
temperature. The vials were sampled periodically and
s 5 analyzed by gas chromatography using clean dioxane as a
diluent and tetrahydrofuran as an internal standard. The
results are shown in Table 1.
Example 2
Treatment of PPT Byproduct Solution with NaOH
Treatment of samples of PPT byproduct solution
containing 1000 ppm acrolein was carried out essentially
as in Example 1. Sodium hydroxide was then added to the
solution to bring the concentration of NaOH to 1000 ppm.
The initial pH was 12.5. The sample vials were sealed,
allowed to stand at room temperature and analyzed
periodically for acrolein. Results are shown in Table 1.
Example 3
Treatment of Byproduct Solutions with NaOH
A simulated PPT byproduct mixture was prepared by
mixing together 0.4809 g allyl alcohol, 2.6063 g
1,3-propanediol and 0.0484 g acrolein and diluting with
distilled water to a final solution of 56.2798 g. Two
samples of this solution in screw cap vials were treated
with caustic. In sample C, 0.0964 g 1N NaOH was added to
9.9768 g solution. In sample D, 0.198 g 1N NaOH was added
to 10.2325 g of the solution. The vials were sealed and
allowed to stand at room temperature. Results are shown
in Table 1.

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' TABLE 1
Example Sample NaOH pH Hours Acrolein (ppm)
1 Untreated 0 0 1950
6 2200
76 1460
Treated 380 1.5 380
A
75 60
Treated 880 2 240
B
7 200
2 Untreated 0 5.6 0 1000
5.5 96 430
Treated 1000 12.5 1 13
4 <1
24 <1
12.0 96 <1
3 Untreated 0 890
79 780
Treated 380 1 80
C
Treated 760 1 160
D
76 ND
ND = not detected (estimated 10 ppm)
<
Example 4
Treatment of PPT Byproduct Solutions with NaOH/Ethylene
Glycol
PPT byproduct solution (70.428 g) was spiked with
0.1690 g acrolein to bring the acrolein level to about
2630 ppm by GC analysis and 6.58 g was transferred to a
screw cap vial. To the remaining solution was added
approximately 0.3 g 1N sodium hydroxide solution to give
a final solution of 64.24 g with a pH of about 10 '
(sample E). Ethylene glycol was added to portions of
sample E to make sample F with about l.lo ethylene glycol
and sample G with about 10~ ethylene glycol. The sealed

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samples were allowed to stand at room temperature and
were analyzed by GC. The results are shown in Table 2.
The presence of added ethylene glycol does not appear
to affect the results. The samples treated with sodium
S 5 hydroxide show lower levels of acrolein than the
untreated sample. Note that the concentration of base
was lower and the initial concentration of acrolein
somewhat higher than in Example 1.
TABLE 2
Sample NaOH Hours Acrolein Conc. (ppm)
Untreated 0 1 2630
25 2260
120 1520
Treated 190 2 280
E
7 310
22 260
120 250
Treated 190 3 280
F
Treated 170 3 450
G I
Comparative Example 5
Treatment of PPT Byproduct Solutions with Acid
For comparison, PPT byproduct solution (53.1842 g)
was spiked with 0.090 g acrolein to bring the acrolein
level to about 2610 ppm. Three samples were prepared
from this solution. Sample H containing 10.1576 g
byproduct solution with 0.512 g O.1N hydrochloric acid
(final pH 3). Sample I contained 9.5485 g by product
solution with 0.5212 g O.1N HCl and 0.1063 g ethylene
glycol. Sample J contained 9.4565 g byproduct solution
with 0.4976 g O.1N HC1 and 0.5092 g ethylene glycol. The
sealed vials were allowed to stand at room temperature
and analyzed by GC. Results are shown in Table 3. The
acid treatment, with or without added glycol, had

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relatively little effect on the concentration of
acrolein.
C
TABLE 3
Sample Hours Acrolein Conc. (ppm)
Untreated 0 2610
24 2120
Sample H 1 1970
4 1910
25 1740
Sample I 1 1960
4 1800
25 1700
Sample J 1 1870
5 1730
25 1550
Example 6
Treatment of Byproduct Solution at Zower Temperature
Similar to the process described in Example 3, a
simulated PPT byproduct solution was prepared by mixing
together 0.41 g allyl alcohol, 2.75 g 1,3-propanediol and
0.052 g acrolein and diluting with 51.69 g distilled
water. To test the invention process under low-temper-
ature conditions, the solution was cooled to about 1 °C.
Samples of the solution in screw cap vials were tested
with caustic. In sample K, 0.12 g 1N NaOH was mixed with
10.304 g of solution. In sample L, 0.266 g 1N NaOH was
mixed with 9.760 g of solution. In Sample M, 0.504 g 1N
NaOH was mixed with 9.989 g of solution. The vials were
capped and stored at about 1 °C in a refrigerator.
Results are shown in Table 4.

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TABLE 4
Sample NaOH Acrolein Conc.
(ppm) pH Hours ( m)
Untreated 4.5-5 0 1120
' 25 1110
49 1060
Treated K 460 10.5-11 2 230
24 130
48 110
Treated L 1060 11.5-12 2 260
24 130
48 100
Treated M 1920 12-12.5 2 170
25 130
48 120
Example 7
Effect of Alkali Pretreatment on Byproduct Solution
Biotreatability
Respirometric tests were conducted to compare the
oxygen utilization rates of activated sludge or biomass
under various feed conditions. The oxygen utilization
rate is a direct measurement of the microbial consumption
rate of the waste materials. The activated sludge was
taken from a chemical manufacturing plant where polyester
(PET) based on ethylene glycol is produced. A byproduct
solution from the synthesis of polypropylene
terephthalate (PPT) containing 1000 ppm acrolein was
treated with sodium hydroxide as described above in
Example 2 and used as a feed. The byproduct solution
from the PET plant was used as a standard feed for
comparison. Acrolein was added at various levels to the
PET solution to show whether it had any adverse effect on
oxygen uptake rates. The results of a typical test are
summarized in Table 5. Oxygen uptake rates are in

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millilitres of oxygen at 20 °C and atmospheric (100 kPa)
pressure.
The PPT solution treated with caustic did not affect
the oxygen uptake rate, while 5 to 15 ppm of acrolein
reduced the rate by 29-52~ in 12 hours and 34-65~ in
28 hours. These results indicate that the caustic
treatment removed the inhibitory effect of the acrolein
in the PPT byproduct solution and that the organic
products formed in the caustic treatment were nontoxic to
the biomass.
TABhE 5
Oxygen Uptake Oxygen Uptake
Feed Source at 12 Hoursa at 28 Hours
Endonenous (no feed) 14 26
PET plant byproduct 85 250
stream
PET plant byproduct 82 255
+ caustic PPT
byproduct stream
(3~ of total COD)
PET byproduct stream 60 168
+ 5 ppm acrolein
(0.3~ of total COD)
PET byproduct stream 41 88
+ 15 ppm acrolein
(0.9~ of total COD)
COD = Chemical Oxygen
Demand aMillimetres
02 at 20 C,
1 atm.
i