Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE RECOVERY OF PURIFIED TEREPHTHALIC ACID (PTA)
The present invention relates to a new process for the production and recovery
of
purified terephthalic acid ("PTA"). More particularly, this application
relates to the use of a
s rotary pressure filter to recover crystalline terephthalic acid and a
process to recover the
resulting crystals at atmospheric pressure. The process involves moving the
crystalline
material through at least two valves defining separate zones, wherein each
succeeding zone
is at a pressure slightly less than the previous zone.
Terephthalic acid is used in the production of many different polymers,
including
1 o polyethylene terephthalate (PET). The typical process for PET is the
direct condensation of
terephthalic acid with a polyalcohol. This direct esterification reaction
requires purified
terephthalic acid, for the reaction product to be acceptable.
Terephthalic Acid is produced by direct oxidation of p-xylene and subsequent
crystallization from the mother liquor to recover the Crude Terephthalic Acid
(CTA). This
~5 CTA still contains approximately 0.2-0.4 percent by weight of 4-
carboxybenzaldehyde (4-
CBA) as major impurity. To reduce the content of 4-CBA, the CTA is typically
dissolved in
water and then the resulting solution is treated in a hydrogenation reactor,
to convert the 4-
CBA into p-toluic acid. The solution from the hydrogenation reactor is then
typically
cooled by flash in a battery of crystallizers to precipitate the purified
terephthalic acid (PTA)
2 o as a crystal. The slurry coming from the crystallizers still contains a
significant amount of
p-toluic acid that needs to be separated from PTA, to meet the usual
commercial
specification of no more that 150 ppm.
In order to purify the PTA two subsequent stages of solid separation are most
currently used. The traditional method to separate the PTA from its mother
liquor consists
25 in centrifuging the slurry at a temperature of from 100°C to
170°C and a pressure of from 1
to 7 bar. Under these conditions the majority of the p-toluic acid will remain
in solution,
allowing it to be separated.
The crystals of PTA coming from the centrifuges contain only a small amount of
p-
toluic acid, but do contain residual mother liquor (typically 10-15 percent).
To get rid of
3 o these impurities, the crystals are usually mixed with additional water,
typically in a ratio of
1.1 to 1.5 m3 water/ton PTA, to wash the mother liquor still entrained. This
results in a
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slurry having a 45 ~5percent solids. This slurry is then flashed to
atmospheric pressure and
fed either to a second stage of centrifuges or to Rotary Vacuum Filters (RVF).
The PTA,
containing a residual 10=15 percent of water, is then dried, typically in a
rotary dryer, and
stored. The PTA crystals still contain minor amounts of p-toluic acid (usually
less than 150
ppm) while 4-CBA content is typically lower than 25 ppm.
These traditional methods have high capital costs in that they require a large
scale
pressure centrifuge as well as either a large scale RVF or a second large
scale centrifuge.
This in turn leads to high maintenance costs with accompanying lack of
reliability.
Additionally the centrifuge causes significant crystal breakage such that the
particle size
1 o distribution is greatly enlarged, especially of particle sizes less than
44 microns. This
process also requires a lot of water that needs to be heated up to the process
temperature.
Finally the process results in a product with a relatively large amount of
moisture which
must be removed in the dryer, resulting in high energy consumption.
U.S. Patent 5,175,355 teaches a method of purifying the terephthalic acid
comprising
pressure filtering. This reference teaches introducing an aqueous slurry
(comprising purified
terephthalic acid present as crystals and p-toluic acid present in the aqueous
solution and as
a co-crystallized form) into one or more filter cells. The slurry is filtered
at a system
pressure of from atmosphere to 16 atm. The filter cell with the resulting
filter cake is then
transported into a wash zone where a stream of water heated to 38°C to
205°C is introduced
2 o to the filter cell to form a reservoir of water over the filter cake.
Displacement washing is
then achieved by forcing the water through the cake at a pressure gradient,
which is at least
0.5 atm above the system pressure while maintaining the reservoir. The
displacement
washing is allowed to continue for a sufficient time to remove a desired
amount of
impurities. The filter cell is then transported to a pressure release zone
wherein the system
2s pressure is quickly released to flash evaporate the water remaining in the
filter cake and the
product is recovered. The pressure release zone is then pressurized back up to
the system
pressure so as to be ready to accept additional product. This process
reportedly results in
terephthalic acid that contains less than 200 ppm by weight of p-toluic acid.
While this process satisfactorily produces pure product, it is a time
consuming
3 o process as the pressure release zone repeatedly has to be pressurized back
up to the system
pressure before it can accept more displacement washed material.
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The applicants of the present invention have found that the process can be
improved
by transferring the washed filter cake to a letdown zone (or pressure release
zone) which is
at a pressure less than the pressure of the washing zone. In this way the time
required to
pressure up the letdown zone is dramatically reduced. The applicants of the
present
invention have discovered that the pressure of the letdown zone approaches the
desired
pressure asymptotically. That is to say that the majority of the pressure
differential between
the release pressure and the system pressure is made up in the early stages of
re-
pressurization, whereas the final bit takes a relatively longer period of
time. Thus, by
allowing the letdown zone to be pressurized to a pressure less than the system
pressure,
to much time can be saved. Shortening the cycle time allows the front end of
the production to
be run faster, or alternatively allows smaller rotary pressure filters to be
used without
causing a bottleneck.
Furthermore, it has been discovered that a pressure differential between the
letdown
zone and the washing zone actually facilitates the movement of product, as it
is earned to
1 s some extent by the flow of gas, which occurs when the barrier between the
two zones is
removed. This also has been observed to help keep the filter itself free from
clogging.
Additionally the applicants of the present invention have discovered that it
is
advantageous, for the best mechanical performance of the system mainly to
avoid plugging
of the vent line, to release the pressure more slowly than the flash
evaporation taught by
2 o U.S. Pat. No. 5,175,355.
One aspect of the invention is a process for the preparation of purified
terephthalic
acid which comprises first introducing a liquid slurry containing crystallized
terephthalic
acid with impurities into a high pressure rotary filter, then filtering the
slurry with the high-
pressure rotary filter and collecting at least some of the solid portion. Then
the solid portion
2s is washed with additional amounts of water. The washed solid portion is
then transferred to
a letdown zone, which is at a pressure less than the zone in which the solid
portion was
collected. After the solid portion is transferred into the letdown zone, the
connection
between the letdown zone and the collection zone is sealed, such that a change
of pressure
in the letdown zone will not effect the pressure in the collection zone. The
pressure of the
3 0 letdown zone is then reduced and the solid removed. The letdown zone is
then pressured
back up so that it is ready to accept additional solid from the rotary filter.
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The liquid slurry containing the terephthalic acid containing the impurities
can come
from any terephthalic acid production scheme. These are known in the art and
are of
minimal importance to the invention at hand. The particular high-pressure
rotary filter used
to filter the slurry is similarly not critical to the present invention. Any
filtering system
s capable of operating at a pressure greater than atmospheric pressure may be
used.
Preferably, for standard operating conditions, the filter is capable of
handling the full
throughput of terephthalic acid of the plant, and capable of operating under
pressures of
from 1.0 to 10.0 bar. A suitable filter is the Bird Young Rotary Filter sold
by Baker Process
Inc. The filter used in the apparatus of the present invention was a Bird
Young Rotary Filter
1 o having a filtration area of 1 sq. ft and was able to process from 1 to 5
MT/h of solid PTA.
The filter typically consists of a case, pressurized at process pressure, and
a drum
covered by a filtering device such as a cloth or equivalent filtering device,
pressurized at a
pressure suitably lower than the case.
The drum is ideally divided into three zones:
i s ~ First zone, where the mother liquor is removed.
~ Second zone, where the solid is washed.
~ Third zone, where the excess of washing liquor is removed and the solid is
discharged.
The pressure of the case is preferably in a range of from 3 to 6.5 bar, with
about 4.5
2 o bar being most preferred. The filtering is carried out at a temperature of
from 133°C to
161 °C, with 147°C being most preferred.
The pressure difference between case and drum is in the range of 0.1 to 2.0
bar,
preferably in the range of 0.3 to 0.7 bar, most preferably 0.5 bar.
The mother liquor removed from the rotary pressure filter can be separately
2 s recovered and reused into the production process or sent to waste
treatment facilities, as is
known in the art.
The remaining solid portion is then washed with additional amounts of water.
It has
been discovered that less wash water is required in the present process to
achieve purity
similar to the traditional methods. Thus while any amount of water may be used
in the
3 o washing stage of the present invention, it is preferred that less than 1
cubic meter per ton of
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PTA be used, in order to conserve water and reduce the energy associated to
heating up this
water to process temperature. The water amount is preferably in a range of
from 0.2 to 0.7
m3/MT of PTA, with about 0.5 m3/MT of PTA being most preferred. The washing is
preferably done at the same temperature as the filtering, although this is for
convenience and
is not mandatory. The temperature of the water used to wash the solid material
is in the
range of 50°C to 161°C, preferably in the range of 130°C
to 150°C, most preferably 147°C.
The wash liquor can then be separately collected from the mother liquor and
reused into the
production process or recycled back into the production process, as is known
in the art.
After washing, the solid material is preferably collected in a collection zone
and then
1 o passed into a let down zone, which is at a pressure that is less than the
collection zone. To
maintain the pressure difference the collection zone is preferably separated
from the letdown
zone by a device of some sort, which can readily open and close to seal the
letdown zone
from the collection zone. Thus in operation, when an amount of solid material
had collected
in the collection zone, the device would be opened and the combination of
gravity and the
15 pressure differential would cause some (preferably all) of the solid
material to move to the
letdown zone. The device is not critical to the invention, however hemispheric
valves
(Dome Valves) such as those manufactured by Macawber Engineering Inc., have
been
shown to be effective.
When the device is opened the letdown zone is at a pressure that is somewhat
less
2 o than the collection zone. Preferably this pressure difference is in the
range of 0.01 to 0.3 bar,
preferably in the range of 0.03 to 0.1 bar, most preferably 0.05 bar.
After the solid has moved into the letdown zone the device is closed, sealing
off the
letdown zone from the collection zone and/or rotary pressure filter. At this
time the
pressure is released. While flash evaporation can be used with the invention,
it is preferred
2 s that the pressure be released more slowly. Ideally the pressure is
released over a period
from 0.5 to 10 seconds, more preferably from 4 to 7 seconds, although the
optimal time will
be a factor of the individual machinery and how fast the train is running. The
slow release of
pressure can be achieved by many different methods. One way is just to have a
narrow
restriction in the vent line, which can be controlled. This can be used alone
or in conjunction
3 o with a check valve, which maintains a certain pressure in the line (and
therefore the letdown
zone). Other methods such as bleed valves are well known in the art and can be
used with
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the present invention. A second vent line without any restrictions can also be
used as a
double check to ensure that the letdown zone does not remain pressurized.
Once the letdown zone has been depressurized a second sealing device is opened
and
the solid contents are removed. As in the transition from the collection zone
to the letdown
s zone, this transition is preferably conducted with a slight pressure
differential so that when
the sealing device is opened the contents are carried by gravity as well as
the flow of gasses
into the region of lesser pressure. Preferably this pressure difference is in
the range of 0.01
to 0.3 bar, preferably in the range of 0.03 to 0.1 bar, most preferably 0.05
bar. Again any
device capable of sealing the letdown zone can be used, however the Dome
Valves are
1 o preferred. The solid material can then be passed to a drier by mean of
suitable device, such
as a screw, for further processing as is known in the art.
When the contents have been removed from the letdown zone, the letdown zone is
once again sealed off with the Dome Valve (or other suitable means) and then
the letdown
zone is pressured back up to a pressure suitable to accept more washed
material. Preferably,
15 the time it takes to release the pressure in the letdown zone, remove the
product, and re-
pressurize the letdown zone is calculated to correspond to the time it takes
to fill up the
collection zone, so that as soon as the letdown zone is emptied, sealed and re-
pressurized,
the collection zone is full, so that there is no down time in the cycle. As
stated before, by
not requiring the letdown zone to be at the same pressure as the collection
zone (or rotary
2 o pressure filter case), the letdown zone can be pressurized up to
acceptable limits much more
quickly. Thus the process can begin again almost immediately.
Preferably, a Programmable Logic Controller (PLC) is used to run the sequence
above described, in order to optimize the opening and closing of the various
sealing devices,
venting devices, and pressurization lines.
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EXAMPLES
Example no. 1
Various tests have been performed to investigate the quality results versus
different
operating conditions. This example describes only tests performed keeping
constant the
pressure differential (dp) between the case and the drum to a value of 0.55
bar.
A description of the operating values is reported in table 1.
to A description of the quality results is summarized in table 2.
Table 1
O eratin conditions Pressure filter Two sta es of centrifu
es
Case ressure 4.3 bar 3.8 bar
D casin / drum 0.55 bar _
Washin water 200=500 k TPTA -
Reslu water - 1100 k TPTA
De ressurization ! =10 sec. -
time ( 1 )
Differential pressure0.01=0.05 bar
(2)
Between zones
Table2
Product uali Pressure filter Two sta es of centrifu
es
Other b roducts 90=230 m 120-240 m
P-toluic acid SO-140 m 20-140 m
Humidi 8=10 ercent 10=15 ercent
15
( 1 ) Time of depressurization of the let down chamber.
(2) Differential pressure between the accumulation chamber and let down
chamber and
between the let down chamber and the final discharge line at atmospheric
pressure.
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