U.S. patent application number 17/137268 was filed with the patent office on 2021-12-09 for air-isolated continuous feeding system for synthesizing polylactic acid from lactide and feeding method thereof.
This patent application is currently assigned to Nutrition & Health Research Institute, COFCO Corporation. The applicant listed for this patent is COFCO Biotechnology Co., Ltd, COFCO (jilin) Bio-Chemical Technology Co., Ltd, Jilin COFCO Biomaterial Co.,Ltd, Nutrition & Health Research Institute, COFCO Corporation. Invention is credited to Tai AN, Bo CHEN, Yi LI, Fang TIAN, Yi TONG, Lida WU.
Application Number | 20210379548 17/137268 |
Document ID | / |
Family ID | 1000005983339 |
Filed Date | 2021-12-09 |
United States Patent
Application |
20210379548 |
Kind Code |
A1 |
TONG; Yi ; et al. |
December 9, 2021 |
AIR-ISOLATED CONTINUOUS FEEDING SYSTEM FOR SYNTHESIZING POLYLACTIC
ACID FROM LACTIDE AND FEEDING METHOD THEREOF
Abstract
The present invention relates to the field of high polymer
material manufacturing, and discloses an air-isolated continuous
feeding system for synthesizing polylactic acid from lactide and a
feeding method thereof. The continuous feeding system comprises a
raw material bag/box and a raw material collector for collecting
and outputting lactide, the raw material bag/box is connected with
a shielding gas input pipeline, a discharge pipe is movably
inserted into the raw material bag/box, a cyclone separator is
connected downstream of the discharge pipe, and a solid substance
outlet of the cyclone separator is connected with the raw material
collector. According to the invention, deterioration of the lactide
raw material incurred by moisture absorption and oxidation is
avoided, and the reaction conversion ratio and final product purity
are improved. The continuous feeding system is easy to operate, can
save manpower and material resources, and is applicable to
industrial application.
Inventors: |
TONG; Yi; (Beijing, CN)
; LI; Yi; (Beijing, CN) ; TIAN; Fang;
(Beijing, CN) ; CHEN; Bo; (Beijing, CN) ;
AN; Tai; (Beijing, CN) ; WU; Lida; (Changchun,
CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nutrition & Health Research Institute, COFCO Corporation
COFCO (jilin) Bio-Chemical Technology Co., Ltd
Jilin COFCO Biomaterial Co.,Ltd
COFCO Biotechnology Co., Ltd |
Beijing
Changchun
Changchun
Bengbu |
|
CN
CN
CN
CN |
|
|
Assignee: |
Nutrition & Health Research
Institute, COFCO Corporation
Beijing
CN
COFCO (jilin) Bio-Chemical Technology CO., Ltd
Changchun
CN
Jilin COFCO Biomaterial Co.,Ltd
Changchun
CN
COFCO Biotechnology Co., Ltd
Bengbu
CN
|
Family ID: |
1000005983339 |
Appl. No.: |
17/137268 |
Filed: |
December 29, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 4/007 20130101;
C08G 63/08 20130101 |
International
Class: |
B01J 4/00 20060101
B01J004/00; C08G 63/08 20060101 C08G063/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 8, 2020 |
CN |
202010509990.7 |
Claims
1. An air-isolated continuous feeding system for synthesizing
polylactic acid from lactide, comprising a raw material bag/box and
a raw material collector for collecting and outputting lactide,
wherein the raw material bag/box is connected with an shielding gas
input pipeline A, a discharge pipe is movably inserted into the raw
material bag/box, a cyclone separator is connected downstream of
the discharge pipe, and a solid substance outlet of the cyclone
separator is connected with the raw material collector.
2. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 1, wherein a gaseous
substance outlet of the cyclone separator is connected with a
filter, and a solid substance of the filter is connected with the
raw material collector.
3. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 2, wherein a blower is
arranged at the gaseous substance outlet of the filter, and the
outlet of the blower comprises two branches, wherein one branch is
connected with the shielding gas input pipeline A, and the other
branch is connected with an air separation station B arranged
outside the air-isolated continuous feeding system for synthesizing
polylactic acid from lactide.
4. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 1, wherein a gas distributor
for loosening the solid substance contained in the raw material
collector is provided in the raw material collector, and the gas
distributor is directly connected with the shielding gas input
pipeline A.
5. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 1, wherein an outlet of the
raw material collector is connected with a screw conveyor, and an
outlet of the screw conveyor is connected with a reaction system C
arranged outside the air-isolated continuous feeding system for
synthesizing polylactic acid from lactide.
6. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 2, wherein the shielding gas
input pipeline A is directly connected with the filter.
7. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 1, wherein the shielding gas
is nitrogen, argon or helium.
8. The air-isolated continuous feeding system for synthesizing
polylactic acid from lactide of claim 1, wherein a vibration
squeezing crusher for fully breaking and dispersing the
agglomerated lactide raw material is provided outside the raw
material bag/box.
9. A feeding method of an air-isolated continuous feeding system
for synthesizing polylactic acid from lactide, the air-isolated
continuous feeding system comprising a raw material bag/box and a
raw material collector for collecting and outputting lactide,
wherein the raw material bag/box is connected with a shielding gas
input pipeline A, a discharge pipe is movably inserted into the raw
material bag/box, a cyclone separator is connected downstream of
the discharge pipe, and a solid substance outlet of the cyclone
separator is connected with the raw material collector, the method
comprising the following steps: step 100: introducing dry shielding
gas into an enclosed space containing lactide raw material to
enable the lactide raw material and the dry shielding gas to form a
gas-solid mixture, and pneumatically outputting the lactide raw
material by means of the shielding gas; step 200: performing
gas-solid separation for the outputted gas-solid mixture, and
collecting the separated lactide raw material and outputting it to
an external reaction system; wherein the volume fraction of water
in the dry shielding gas in the step 100 is 15 ppm or lower; and
the volume fraction of oxygen in the dry shielding gas is 50 ppm or
lower.
10. (canceled)
11. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 9,
further comprising the following step before the step 100: when the
continuous feeding system is used for the first time, purging and
replacing the residual air in the entire continuous feeding system
with the dry shielding gas till the volume fraction of the
shielding gas in the entire continuous feeding system exceeds 99%
.
12. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 9,
further comprising the following step before the step 100: fully
squeezing and vibrating the lactide raw material to fully break up
and disperse agglomerated lactide raw material in the raw material
package; wherein the duration of the squeezing and vibrating
treatment of the material is 5-30 min.
13. (canceled)
14. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 12,
wherein the duration of the squeezing and vibrating treatment of
the material is 5-20 min.
15. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 9,
wherein the step 200 further comprises: filtering the gas outputted
after gas-solid separation, and collecting the filtered solid
lactide raw material; returning a part of the shielding gas in the
filtered gas to the enclosed space containing the lactide raw
material in the step 100, so that the shielding gas is recycled and
reused in the air-isolated continuous feeding system for
synthesizing polylactic acid from lactide; and outputting the
remaining part of the shielding gas for separate recovery; wherein
the ratio of the part of shielding gas that is recycled and reused
to the remaining part of shielding gas that is outputted to the air
separation station for recovery is 5:1-1:2.
16. (canceled)
17. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 15,
wherein the ratio of the part of shielding gas that is recycled and
reused to the remaining part of shielding gas that is outputted to
the air separation station for recovery is 4:1-1:1.
18. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 15,
wherein the collected lactide raw material is purged with the
shielding gas periodically; the frequency of the periodical purging
with the shielding gas is once after each batch of feeding is
completed; the batch is feeding of 4-6 sealed packages of lactide
raw material.
19. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 9,
wherein the step 200 further comprises: blowing the collected solid
lactide raw material with the shielding gas for loosening, wherein
the duration of blowing for loosening is 5-30 min; preferably,
wherein the duration of blowing for loosening is 5-20 min.
20. The feeding method of the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide of claim 9,
wherein the environment temperature for the feeding is
20-35.degree. C.
Description
PRIORITY CLAIM & CROSS REFERENCE
[0001] This application claims priority to Chinese Application No.
202010509990.7, filed on Jun. 8, 2020, entitled "Air-Isolated
Continuous Feeding System for Synthesizing Polylactic Acid from
Lactide and Feeding Method Thereof", which is specifically and
entirely incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the technical field of high
polymer material manufacturing, in particular to an air-isolated
continuous feeding system for synthesizing polylactic acid from
lactide and a feeding method thereof.
BACKGROUND
[0003] Polylactic acid (PLA) is an environmentally-friendly high
polymer material with good biocompatibility and biodegradability,
which can be completely degraded into water and carbon dioxide
without pollution. Its raw material is lactic acid, which can be
obtained through fermentation of grain crops and biological fibers.
The development and application of polylactic acid can reduce white
pollution of waste polymer materials to the environment and save
oil resources. As the research on polylactic acid is deepened,
presently, PLA products have been widely applied in medical,
packaging material, textile, daily necessities, and industrial
product domains, etc. At present, PLA with high relative molecular
weight is usually synthesized through a lactide ring-opening
polymerization process, which has good controllability and achieves
stable product quality. Patents related with engineering
application of ring-opening synthesis of polylactic acid from
lactide mostly focus on equipment, such as CN105348499B,
CN101353417B, CN109280156A, CN105694017A and CN110606838A, etc.,
but there are few patents related with air-contactless lactide
feeding system at present.
[0004] In the process of ring-opening synthesis of polylactic acid
from lactide, lactide must be fed into the reactor under a
condition of strict isolation from air and moisture; otherwise
lactide will be degenerated and the reaction conversion ratio and
product purity will be degraded. Furthermore, a large quantity of
degenerated and useless waste material existing in the system not
only causes material waste but also brings great difficulties to
the subsequent monomer separation procedure. Consequently, the
overall operation becomes complex and the production cost is
increased. However, owing to the fact that lactide is packed in
boxes or bags, the manual unloading and feeding operations can't be
performed strictly in an isolated state, and it is inevitable that
some air and moisture enters into the lactide.
[0005] It is urgent task to realize continuous material feeding in
a state of isolation from air and moisture, so as to ensure the
purity of lactide while avoiding raw material loss and reducing
useless waste in the system.
SUMMARY
[0006] To solve the problems of lactide degeneration, low reaction
conversion ratio and low product purity in the existing production
process in the prior art, the present invention provides an
air-isolated continuous feeding system for synthesizing polylactic
acid from lactide and feeding method thereof. The air-isolated
continuous feeding system and feeding method can continuously feed
the raw material in a state of isolation from air and moisture,
employs an shielding gas as propulsion power and a protective
agent, thus avoid deterioration of lactide raw material resulted
from moisture absorption and oxidation, and improve reaction
conversion ratio and final product purity. Specifically, in the
present invention, the reaction conversion ratio is as high as 97%,
and the monomer content in the final product can be reduced to
0.7-2.0.Salinity.. In addition, the continuous feeding system
provided by the present invention is easy to operate and can save
manpower and material resources, and is a continuous feeding system
applicable to industrial application.
[0007] To attain the above object, in a first aspect, the present
invention provides an air-isolated continuous feeding system for
synthesizing polylactic acid from lactide, which comprises a raw
material bag/box and a raw material collector for collecting and
outputting lactide, wherein the raw material bag/box is connected
with an shielding gas input pipeline, a discharge pipe is movably
inserted into the raw material bag/box, a cyclone separator is
connected downstream of the discharge pipe, and a solid substance
outlet of the cyclone separator is connected with the raw material
collector.
[0008] Preferably, a gaseous substance outlet of the cyclone
separator is connected with a filter, and a solid substance of the
filter is connected with the raw material collector.
[0009] Preferably, a blower is arranged at the gaseous substance
outlet of the filter, and the outlet of the blower comprises two
branches, wherein one branch is connected with the shielding gas
input pipeline, and the other branch is connected with an air
separation station arranged outside the air-isolated continuous
feeding system for synthesizing polylactic acid from lactide.
[0010] Preferably, a gas distributor for loosening the solid
substance contained in the raw material collector is provided in
the raw material collector, and the gas distributor is directly
connected with the shielding gas input pipeline.
[0011] Preferably, an outlet of the raw material collector is
connected with a screw conveyor, and an outlet of the screw
conveyor is connected with a reaction system arranged outside the
air-isolated continuous feeding system for synthesizing polylactic
acid from lactide.
[0012] Preferably, the shielding gas input pipeline is directly
connected with the filter.
[0013] Preferably, the shielding gas is nitrogen, argon or
helium.
[0014] Preferably, a vibration squeezing crusher for fully breaking
and dispersing the agglomerated lactide raw material is provided
outside the raw material bag/box.
[0015] In a second aspect, the present invention provides a feeding
method of the air-isolated continuous feeding system for
synthesizing polylactic acid from lactide as described above, which
comprises the following steps:
[0016] step 100: introducing dry shielding gas into an enclosed
space containing lactide raw material to enable the lactide raw
material and the dry shielding gas to form a gas-solid mixture, and
pneumatically outputting the lactide raw material by means of the
shielding gas;
[0017] step 200: performing gas-solid separation for the outputted
gas-solid mixture, and collecting the separated lactide raw
material and outputting it to an external reaction system.
[0018] Preferably, the volume fraction of water in the dry
shielding gas in the step 100 is 15 ppm or lower; and the volume
fraction of oxygen in the dry shielding gas is 50 ppm or lower.
[0019] Preferably, the method further comprises the following step
before the step 100: when the continuous feeding system is used for
the first time, purging and replacing the residual air in the
entire continuous feeding system with the dry shielding gas till
the volume fraction of the shielding gas in the entire continuous
feeding system exceeds 99% .
[0020] Preferably, the method further comprises the following step
before the step 100: fully squeezing and vibrating the lactide raw
material to fully break up and disperse agglomerated lactide raw
material in the raw material package.
[0021] Preferably, the duration of the squeezing and vibrating
treatment of the material is 5-30 min., preferably is 5-20 min.
[0022] Preferably, the step 200 further comprises: filtering the
gas outputted after gas-solid separation, and collecting the
filtered solid lactide raw material; returning a part of the
shielding gas in the filtered gas to the enclosed space containing
the lactide raw material in the step 100, so that the shielding gas
is recycled and reused in the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide; and
outputting the remaining part of the shielding gas for separate
recovery.
[0023] Preferably, the ratio of the part of shielding gas that is
recycled and reused to the remaining part of shielding gas that is
outputted to the air separation station for recovery is 5:1-1:2,
preferably is 4:1-1:1.
[0024] Preferably, the collected lactide raw material is purged
with the shielding gas periodically; the frequency of the
periodical purging with the shielding gas is once after each batch
of feeding is completed; the batch is feeding of 4-6 sealed
packages of lactide raw material.
[0025] Preferably, the step 200 further comprises: blowing the
collected solid lactide raw material with the shielding gas for
loosening, wherein the duration of blowing for loosening is 5-30
min., preferably is 5-20 min.
[0026] Preferably, the environment temperature for the feeding is
20-35.degree. C.
[0027] With the above technical scheme, continuous feeding is
carried out under the condition of isolation from air and moisture,
nitrogen is used as propulsion power and a protective agent, so
that deterioration of the lactide raw material incurred by moisture
absorption and oxidation is avoided, and the reaction conversion
ratio and final product purity are improved. Specifically, the
reaction conversion ratio can be as high as 97%, and the monomer
content in the final product can be reduced to 0.7-2.0.Salinity..
In addition, the continuous feeding system provided by the present
invention is easy to operate and can save manpower and material
resources, and is a continuous feeding system applicable to
industrial application.
BRIEF DESCRIPTION OF DRAWINGS
[0028] FIG. 1 is a schematic diagram of the overall structure of an
embodiment of the air-isolated continuous feeding system for
synthesizing polylactic acid from lactide in the present
invention.
REFERENCE NUMBERS
[0029] 1--raw material bag/box; 2--connecting hose; 3--cyclone
separator; 4--filter; 5--blower; 6--raw material collector; 7--gas
distributor; 8--screw conveyor; 9--vibration squeezing crusher;
A--shielding gas input pipeline; B--air separation station;
C--reaction system.
DETAILED DESCRIPTION
[0030] Hereunder some embodiments of the present invention will be
detailed with reference to the accompanying drawings. It should be
understood that the embodiments described herein are only provided
to describe and explain the present invention rather than
constitute any limitation to the present invention.
[0031] In the present invention, unless otherwise specified, the
terms that denote the orientations are used as follows, for
example: "top", "bottom", "left" and "right" usually refer to
"top", "bottom", "left" and "right" as shown in the accompanying
drawings; "inside" and "outside" usually refer to inside and
outside in relation to the profiles of the components; and "distal"
and "proximal" usually refer to distal and proximal positions with
respect to the outlines of the components.
[0032] As shown in FIG. 1, the present invention provides an
air-isolated continuous feeding system for synthesizing polylactic
acid from lactide, which comprises a raw material bag/box 1 and a
raw material collector 6 for collecting and outputting lactide,
wherein the raw material bag/box 1 is connected with an shielding
gas input pipeline A, a discharge pipe is movably inserted into the
raw material bag/box 1, a cyclone separator 3 is connected
downstream of the discharge pipe, and a solid substance outlet of
the cyclone separator 3 is connected with the raw material
collector 6. To facilitate movement, both the shielding gas input
pipeline A and the discharge pipe may employ a connecting hose 2
respectively. The specific material of the shielding gas input
pipeline A and the discharge pipe may be selected according to the
actual requirement in the actual application. That is to say, in
the present invention, a dry shielding gas is introduced into the
enclosed space of the raw material bag/box 1 containing lactide raw
material through the shielding gas input pipeline A, so that the
lactide raw material and the dry shielding gas form a gas-solid
mixture, the lactide raw material is pneumatically outputted by
means of the shielding gas, the outputted gas-solid mixture is
subjected to gas-solid separation by the cyclone separator 3, and
the separated lactide raw material is collected in the raw material
collector 6. It is be seen from the above description: in the
present invention, the raw material is fed continuously in a state
of isolation from air and moisture, an shielding gas is used as
propulsion power and a protective agent; thus, high reaction
conversion ratio and high final product purity are achieved, the
loss is reduced, manpower and material resources are saved. In
actual applications, nitrogen is usually used as the shielding
gas.
[0033] In order to reduce the loss of the lactide raw material, a
small amount of solid substance mixed in the gaseous substance
separated by the cyclone separator 3 is further separated and
collected. The gaseous substance outlet of the cyclone separator 3
is connected with a filter 4, and the solid substance outlet of the
filter 4 is connected with the raw material collector 6. In this
embodiment, the filter 4 is a bag filter in order to collect the
gaseous substance separated from the cyclone separator 3
conveniently. In the actual application, the shape and structure of
the filter 4 may be selected as required. In order to keep the
lactide raw material isolated from air and moisture all the time,
the shielding gas input pipeline A is directly connected with the
filter 4 to facilitate purging the material in the filter 4.
[0034] A blower 5 is provided at the gaseous substance outlet of
the filter 4 to make the gas used as conveying power circulate in
the air-isolated continuous feeding system for synthesizing
polylactic acid from lactide and improve the fluidity of the gas in
the circulation process. The outlet of the blower 5 comprises two
branches, wherein one branch is connected with the shielding gas
input pipeline A and the other branch is connected to an air
separation station B arranged outside the air-isolated continuous
feeding system for synthesizing polylactic acid from lactide. That
is to say, the gas outputted after gas-solid separation in the
cyclone separator 3 enters the filter 4 again for filtering, and
the filtered solid lactide raw material is collected in the raw
material collector 6; a part of the shielding gas in the filtered
gas is returned to the shielding gas input pipeline A and conveyed
again to the raw material bag/box 1, so that the shielding gas is
recycled and reused in the air-isolated continuous feeding system
for synthesizing polylactic acid from lactide; the remaining part
of the shielding gas is outputted for separate recovery. For
example, in an embodiment of the present invention, the remaining
part of the shielding gas is outputted to the air separation
station B for recovery. The two parts of shielding gas are
allocated at a certain ratio.
[0035] To prevent the lactide raw material collected in the raw
material collector 6 from coalescence and agglomeration, a gas
distributor 7 for loosening the solid substance contained in the
raw material collector 6 is provided in the raw material collector
6, and the gas distributor 7 is directly connected with the
shielding gas input pipeline A, so as to loosen the collected
lactide with the shielding gas.
[0036] To facilitate transfer, an outlet of the raw material
collector 6 is connected with a screw conveyor 8, and an outlet of
the screw conveyor 8 is connected with a reaction system C arranged
outside the air-isolated continuous feeding system for synthesizing
polylactic acid from lactide.
[0037] Usually, the shielding gas may be nitrogen, argon or helium.
For the sake of cost and operation convenience, nitrogen is used in
this embodiment.
[0038] In addition, the structure of the raw material bag/box 1 may
be in different forms according to the actual requirement. For
example, the raw material bag/box 1 may be an enclosed box or bag.
In this embodiment, the raw material bag/box 1 is an enclosed box.
Since the lactide raw material may be packed in boxes or bags, the
nitrogen pipeline and the lactide discharge pipe may be connected
with hoses. In view that it is more convenient to move connecting
hoses, pneumatic conveying of the lactide raw material can be
realized by inserting a connecting hose into the packing box or
packing bag of the lactide raw material, regardless of the
packaging structure. Furthermore, a vibration squeezing crusher 9
may be provided outside the raw material bag/box 1 to prevent the
lactide raw material from being affected by moisture and
agglomerating in the raw material package, which may adversely
affect the material feeding. Before the feeding process is
commenced, the lactide raw material affected by moisture and
agglomerated is intensively squeezed and vibrated to fully break up
the lactide raw material agglomerated in the raw material
package.
[0039] As shown in FIG. 1, in a second aspect, the present
invention provides a feeding method of the air-isolated continuous
feeding system for synthesizing polylactic acid from lactide as
described above, which comprises the following steps:
[0040] step 100: introducing dry shielding gas into an enclosed
space containing lactide raw material to enable the lactide raw
material and the dry shielding gas to form a gas-solid mixture, and
pneumatically outputting the lactide raw material by means of the
shielding gas;
[0041] step 200: performing gas-solid separation for the outputted
gas-solid mixture, collecting the separated lactide raw material
and outputting it to an external reaction system.
[0042] The volume fraction of water in the dry shielding gas in the
step 100 is 15 ppm or lower; and the volume fraction of oxygen in
the dry shielding gas is 50 ppm or lower.
[0043] In order to ensure that the lactide raw material is always
isolated from air and moisture, the method further comprises the
following step before the step 100: when the continuous feeding
system is used for the first time, purging and replacing the
residual air in the entire air-isolated continuous feeding system
for synthesizing polylactic acid from lactide with the dry
shielding gas till the volume fraction of the shielding gas in the
entire continuous feeding system exceeds 99%.
[0044] In order to prevent the raw material package of lactide from
being affected by moisture and caking, which may affect the
material feeding adversely, the method further comprises the
following step before the step 100: fully squeezing and vibrating
the lactide raw material to fully break up and disperse
agglomerated lactide raw material in the raw material package. The
duration of the squeezing and vibrating treatment of the material
is 5-30 min., preferably is 5-20 min.
[0045] In order to save energy and reuse the shielding gas in the
entire feeding system, the shielding gas is circulated in the
air-isolated continuous feeding system for synthesizing polylactic
acid from lactide. Specifically, the step 200 further comprises:
filtering the gas outputted after gas-solid separation, and
collecting the filtered solid lactide raw material; returning a
part of the shielding gas filtered to the enclosed space containing
the lactide raw material in the step 100, so that the shielding gas
is recycled and reused in the air-isolated continuous feeding
system for synthesizing polylactic acid from lactide; outputting
the remaining part of the shielding gas filtered for separate
recovery, for example, in an air separation station. More
specifically, the ratio of the part of shielding gas that is
recycled and reused to the remaining part of shielding gas that is
outputted to the air separation station for recovery is 5:1-1:2,
preferably is 4:1-1:1.
[0046] The collected lactide raw material must be purged with the
shielding gas periodically; the frequency of the periodical purging
with the shielding gas is once after each batch of feeding is
completed; the batch is feeding of 4-6 sealed packages of lactide
raw material.
[0047] In addition, in order to prevent the collected lactide raw
materials from agglomeration, which may affect the transportation
of the solid material, the step 200 further comprises: blowing the
collected solid lactide raw material with the shielding gas for
loosening, wherein the duration of blowing for loosening is 5-30
min., preferably is 5-20 min.
[0048] Under the condition of complete isolation from air and
water, the environment temperature of the air-isolated continuous
feeding system for synthesizing polylactic acid from lactide is
20-35.degree. C.
[0049] It is seen from the above description: the air-isolated
continuous feeding system for synthesizing polylactic acid from
lactide provided by the present invention utilizes an shielding gas
(e.g., nitrogen) as the propulsion power for conveying the lactide
raw material and a protective agent in the conveying process, so as
to isolate air and moisture from the system and ensure the purity
of lactide. Thus, the reaction conversion ratio and the purity of
the final product are improved, raw material loss is avoided,
useless waste generated in the system is reduced, and manual
operations are reduced manpower and material resources are
saved.
[0050] As shown in FIG. 1, the specific working process of the
air-isolated continuous feeding system for synthesizing polylactic
acid from lactide provided by the present invention is as
follows:
[0051] A sealed raw material bag/box 1 filled with lactide raw
material is transported to a raw material processing table and
fixed there. In the embodiment shown in FIG. 1, the vibration
squeezing crusher 9 mainly comprises a vibration mechanism arranged
on the raw material processing table, and the vibration mechanism
usually may be a vibration spring. The lactide raw material
agglomerated in the raw material package is completed broken up and
dispersed by driving the raw material processing table to vibrate
up and down in the vertical direction, or vibrate left and right in
horizontal direction, or vibrate in up, down, left and right
directions in combination. Usually, the vibration squeezing crusher
9 fully squeezes the lactide raw material for 5-30 min., preferably
5-20 min. For example, in this embodiment, the lactide raw material
agglomerated in the raw material package can be completely broken
up and dispersed by squeezing the material for 10 min. The
shielding gas outputted from the shielding gas input pipeline A is
fed to the bottom of the raw material bag/box 1 through the movable
shielding gas input pipe connected with a connecting hose 2 and is
in communication with the interior of the raw material bag/box 1.
In this embodiment, the shielding gas is nitrogen. Moreover, one
end of movable raw material discharge pipe connected with another
connecting hose is also inserted into the raw material bag/box 1
and is in communication with the interior of the raw material
bag/box 1, the other end of the raw material discharge pipe
communicates with the interior of the cyclone separator 3, and the
raw material discharge pipe is provided with a control valve for
controlling the open/close of the material discharge pipe. Usually,
the inputted nitrogen is dry nitrogen, in which the volume fraction
of water is 15 ppm or lower and the volume fraction of oxygen is 50
ppm or lower. In this embodiment, the volume fraction of water is
10 ppm and the volume fraction of oxygen is 40 ppm.
[0052] It should be noted: when the system is started for the first
time, it is necessary to purge and displace the air in the entire
system with fresh and dry nitrogen till the volume fraction of
nitrogen in the system exceeds 99%. Then the nitrogen pipe and
discharge pipe are inserted into the raw material bag/box 1, and at
the same time, the blower 5 is started to pneumatically convey the
lactide raw material.
[0053] The nitrogen feeding valve on the nitrogen pipe is opened,
and the nitrogen drives the lactide to the cyclone separator 3
through the discharge pipe. In the cyclone separator 3, the lactide
is separated from the nitrogen and falls to the bottom of the
cyclone separator 3, while the nitrogen is outputted from the top
of the cyclone separator 3 to the filter 4.
[0054] The lactide raw material falling to the bottom of the
cyclone separator 3 enters the raw material collector 6 through the
bottom discharge port, and then is sent to the reaction system C by
the screw conveyor 8. A gas distributor 7 is arranged at the bottom
of the raw material collector 6. In case the lactide raw material
is jammed, nitrogen may be introduced to loosen up the lactide raw
material, so as to ensure smooth conveying of the raw material.
Each loosening cycle takes 5-30 min., preferably 5-20 min. For
example, in this embodiment, the loosening cycle takes 10 min. In
addition, the lactide raw material collected in the raw material
collector 6 has to be periodically purged with the shielding gas,
and the frequency of the periodical purging with the shielding gas
is once after every batch of feeding is completed; wherein, each
batch consists of feeding of 4-6 sealed packages of lactide raw
material, and the frequency of purging may be selected as required
in the actual operation process. In this embodiment, the lactide
raw material collected in the raw material collector 6 is
periodically purged with nitrogen by means of the gas distributor
7.
[0055] The gas outputted after gas-solid separation is filtered by
the filter 4, and the filtered solid lactide raw material is
collected in the raw material collector 6; a part of the nitrogen
gas filtered is returned to the raw material bag/box 1, so that the
nitrogen gas is recycled and reused in the air-isolated continuous
feeding system for synthesizing polylactic acid from lactide; and
the remaining part of nitrogen gas is outputted to the air
separation station B for recovery. Usually, the allocation ratio of
the part of nitrogen returned to the raw material bag/box 1 to the
remaining part of nitrogen outputted to the air separation station
B is 5:1-1:2, preferably is 4:1-1:1. In this embodiment, the
lactide raw material carried by the nitrogen entering the filter 4
is collected in the filter 4; after the nitrogen is pumped out by
the blower 5, 80% volume fraction of nitrogen is returned to the
nitrogen pipeline at the raw material bag/box 1 to supplement the
fresh nitrogen to pneumatically convey the lactide raw material,
while the remaining 20% volume fraction of nitrogen is sent to the
air separation station B for regeneration. That is to say, in this
embodiment, the allocation ratio of the part of nitrogen returned
to the raw material bag/box 1 to the remaining part of nitrogen
outputted to the air separation station B is 4:1.
[0056] In the above material feeding process, the environment
temperature of the air-isolated continuous feeding system for
synthesizing polylactic acid from lactide is 20-35.degree. C. For
example, in this embodiment, the environment temperature is
25.degree. C.
[0057] It is seen from the above-mentioned working process of the
air-isolated continuous feeding system for synthesizing polylactic
acid from lactide provided by the present invention: in the
embodiment shown in FIG. 1, the entire system only requires
supplementing a small fraction of fresh nitrogen in operation
cycle, while most of the nitrogen can be recycled and reused. Thus,
the energy consumption is reduced. Besides, the entire system is
enclosed and isolated from air, so as to ensure the purity of the
lactide raw material and avoid raw material loss.
[0058] After the lactide is fed by the feeding system, the
following process is applied to synthesize polylactic acid from
lactide, and the entire reaction process of synthesizing polylactic
acid is carried out in high vacuum or under nitrogen shielding,
specifically including: [0059] (a) Under nitrogen shielding,
lactide is fed into a lactide melting tank by means of a screw
feeder for melting, so as to obtain molten lactide, wherein the
melting temperature is 90-110.degree. C. and the reaction time is
1-1.2 h. [0060] (b) In the presence of a catalyst and an initiator,
the molten lactide and a composite stabilizer are kept in a first
polymerization reactor to have a first polymerization reaction, so
as to obtain a first melt, wherein the reaction temperature is
140-160.degree. C., the pressure is 50-53 kPa, and the reaction
time is 3-3.2 h; [0061] (c) The first melt is fed into a second
polymerization reactor to have a second polymerization reaction, so
as to obtain a second melt, wherein the reaction temperature is
170-200.degree. C., the pressure is 6-6.5 MPa, and the reaction
time is 1-1.2 h; [0062] (d) the second melt is fed into a monomer
removal reactor for monomer removal at 210-215.degree. C.
temperature, 1-1.5 kPa pressure and 5-30 rpm stirring speed for
0.5-0.6 h reaction time; [0063] (e) The polylactic acid melt
obtained after the step (d) is treated by water-cooled pelleting,
dehydration, crystallization and drying, and finally polylactic
acid resin is obtained, wherein the pelleting temperature is
210.degree. C.
[0064] The reaction conversion ratio of polylactic acid produced
with the feeding system and the feeding method provided by the
invention through actual industrial production is as high as 97% or
above; the obtained polylactic acid product has good color and
luster, and the weight average molecular weight is 130,000-250,000;
the content of lactide monomer in the obtained polylactic acid is
0.7-2.0.Salinity.; the melt index of the polylactic acid at
190.degree. C. under 2.16 kg load is 6-27 g/10 min. Compared with
the situation of about 95% reaction conversion ratio and
2-5.Salinity. monomer content in the prior art, the present
invention utilizes an shielding gas as propulsion power and a
protective agent so as to isolate air and moisture from the system
and avoid deterioration of the lactide raw material incurred by
moisture absorption and oxidation, and improve the reaction
conversion ratio and the final product purity. Specifically, the
reaction conversion ratio is as high as 97%, and the monomer
content in the final product is reduced to 0.7-2.0.Salinity..
Moreover, the present invention can avoid raw material loss, reduce
useless waste generated in the system, reduce labor and save
manpower and material resources owing to its simple operation, and
is a continuous feeding approach applicable to industrial
application.
[0065] While the present invention is described above in detail in
some preferred embodiments with reference to the accompanying
drawings, the present invention is not limited to those
embodiments. Within the scope of the technical concept of the
present invention, various simple modifications may be made to the
technical scheme of the present invention. For example, the raw
material box may be replaced with a lactide raw material bag or a
closed container in a different shape. To avoid unnecessary
repetition, the possible combinations are not described
specifically in the present invention. However, such simple
variations and combinations shall also be deemed as having been
disclosed and falling in the scope of protection of the present
invention.
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