This invention relates to a method for continuously
removing lower molecular materials from granulated
polyamides.
It is well known that polyamides, particularly those of
the type of poly—e-caprolactam, contain after completion
of polymerization considerable amounts (up to 12% by
weight) of the starting monomers as well as small
amounts of oligomers. The oligomers are troublesome
since the mechanical properties of the products of polymerization, particularly nylon threads and ?bers, are severely impaired. It is therefore common to reduce the
content of lower molecular components by extraction with
solvents, preferably ‘water, to lower concentrations of, for
example, about 0.5% by weight.
It has vbeen the usual practice to perform such extractions discontinuously in large washing boilers or continuously in extraction towers. The discontinuous
extraction processes have the known drawbacks of down time
on opening, closing, and charging the extraction installation, and low efficiency
due to waste of input energy be
cause of periodic reheating for each batch.
In usual continuous extraction processes, the granulate
is introduced into an extraction tube at the top and hot
water enters at the bottom thereof and ?ows upwardly
counter?ow to the polyamide particles. During the down
ward movement of the particles in the Water stream, theextraction of the lower molecular components occurs. At
the bottom of the extraction tube the granulate is discharged, while the water which has become charged with
lower molecular components is drawn o? at the upper
part of the extraction tube. Inside the elongated extraction tube, counterflow currents are set up because of density ?uctuations in the water. The density fluctuations
occur for various reasons. Sometimes the density varies be
cause of excessive heating, for example, along the heated
wall. It also increases because of cooling in the upper
part of the tube upon entry of the considerably colder
polyamide cuttings. These temperature variations result
in corresponding convection currents. Upon charging of
the water with the heavier lower molecular components,
especially monomers, the density increases in the main
flow direction of the water, which is an additional cause
for counter?ow currents. The total result of the counter
flows of current causes the solvent, severely charged with
monomers, to ?ow down from the upper zones of the
extraction tube to the inlet of the fresh water. Because
of these interference currents, the counter?ow principle
of the extraction and thereby the Washing effect are
seriously disturbed so that in the conventional extraction
tubes, to sufficiently free the cuttings of the monomers,
very large amounts of water are necessary. The maximum
monomer concentration in the extraction tubes is about
the middle so that the upper half of the tube remains
virtually unused.
SUMMARY OF THE INVENTION
It is therefore an important object of the invention to
eliminate the disadvantages of prior art methods of extracting lower molecular components from granulated
polyamides.
It is also an object of the invention to provide a method
for extraction which is simple and economical, yet highly
efficient.
Further purposes and objects of the invention will ap
pear as the speci?cation proceeds.
It has been found that the objects of the invention may
be accomplished by localization of secondary counter
flows inside the extraction tube, whereby a more efficient
and more extensive removal of the lower molecular substances from the polyamide cuttings is achieved. This is
accomplished by providing a plurality of installations over
the height of the extraction tube for constricting the flow
cross section of the extraction tube. The ?ow cross sections are chosen so that the upward ?ow of the solvent in
the narrowest tube cross sections is equal to or greater
than the counter?ow of the solvent resulting from temperature variations and charging of the solvent with lower
molecular components and/or forced counter?ow of the
solvents. ‘
These constrictions in the tube increase the ?ow rate
of the solvent ?owing in the tube. Experimentally, it has
been found that the effect of improving the elutriation
manifests itself effectively only if the narrowest of the
flow cross sections are chosen so that the upward flow
of the solvent in the constrictions is equal to or greater
than the counter?ow currents due to convection or density variations upon charging the solvent with lower molecular components. The constrictions in the tube act to block the downwardly directed interference currents of
the solvent above the particular constriction in the tube.
A particular advantage of the invention is that the
counter?ow currents, which are actually desirable for
intensifying the elutriation by providing good intermixing
of granulate and solvent are not prevented, but rather are
only localized. To provide further intensification of elutriation, additional streams created by suitable agitation
or mixing assemblies may be utilized. In this case, the
constrictions in the tube are dimensioned so that the up
ward movement of the solvent at the constrictions is equal
to or greater than the counter?ow of the solvent due to
fluctuations in density and/or the forced counterflow of
the solvent.
The size of constriction in the tube that is required for
the desired blocking effect is readily calculated for each
specific case from the mean flow velocity of the solvent
and the flow brought about through density fluctuations
of the solvent resulting from temperature variations and
charging of the solvent with lower molecular components
and/ or a forced flow. The size of the constriction is limited
upon adversely affecting the downward movement of the
cuttings by reason of excessive solvent acceleration.
The extraction method according to the invention has
many advantages. In comparison with conventional methods, and with equal granulate throughput and equal conventional apparatus dimenisons, to provide equal end
monomer content in the granulate, much smaller amounts
of solvent are required in the invention method, for exam
ple, only about 50% to 70% of the amount normally required. In addition to the saving in solvent, there is also
an improved working of the extraction agent for the recovery of monomers because of the more concentrated
collection thereof in water.
With equal amounts of water and like other conditions,
the final concentration of the polyamide cuttings in caprolactam, using the method of the invention, amounts to,
for example, 0.3% by weight as compared to 1% to 1.5%
by weight in the extraction in conventional apparatus.
Furthermore, with the invention, a smaller extraction unit
accomplishes the same ?nal concentration in caprolactam
as in a conventional extraction tower with the same amount
of solvent. Further, it is important that the extraction
method according to the invention, particularly in the case
of operational disturbances, such as breakdown of the
heating system and pressure loss in the solvent feed, has
a more stable opearting behavior than conventional extraction methods.
BRIEF DESCRIPTION OF THE DRAWINGS
Particular embodiments of the apparatus used in practicing the method of the present invention are illustrated
in the accompanying drawings, wherein:
FIG. 1 is a fragmentary, side elevational view of an
extraction tube made utilizing the method of the invention;
FIG. 2 is a fragmentary, side elevational view of another extraction tube embodiment; and
FIG. 3 is a side eleva
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
FIG. 1 shows an insert 2 being a funnel-shape shell
truncated in the shape of a cone shell which narrows in
the direction of movement of the granulate, which construction is advantageous in the case of a circular cross
section extraction tube l. The narrowing of the flow path
is designated by the numeral 3. In the case of rectangular
tubes, on the other hand, the installations are preferably
in the shape of truncated pyramid shells. The direction of
movement of the granulate is indicated by the arrow drawn
in solid lines, while the arrow shown in broken lines shows
the direction of flow of the solvent.
There are also other forms of construction, such as, for example, simple horizontal plates with corresponding restricted central apertures (not shown).
The restricted portions 3' may also, as shown in FIG.
2, be formed by funnel-shaped contrictions 2' defined in
the outer wall of the extraction tube 1' itself, so that additional constricting inserts are unnecessary. In the funnel
shaped constrictions 2’ the use of the shown opposed double-funnel arrangements, with opposed aperture directions
and meeting at the conical or pyramidal apexes of the funnel portions 2’, are particularly advantageous since neither
the flow of the solid matter nor that of the solvent is disturbed. In order to make a commercial scale extraction apparatus, for example, with funnel-shaped constrictions
readily accessible for repairs and for the purpose of cleaning, short funnels with correspondingly large access openings, about the size of manholes, may be used.
สิ่งประดิษฐ์ที่เกี่ยวข้องกับวิธีการอย่างต่อเนื่อง เอาวัสดุต่ำกว่าโมเลกุลจากแต่ polyamides เป็นที่รู้จักว่า polyamides โดยเฉพาะอย่างยิ่งบรรดา ชนิดของโพลี — อี-caprolactam ประกอบด้วยงาน ของ polymerization มากยอด (ถึง 12% น้ำหนัก) ของ monomers เริ่มต้น และขนาดเล็ก จำนวน oligomers Oligomers กำลังลำบาก เนื่องจากคุณสมบัติทางกลของผลิตภัณฑ์ของ polymerization โดยเฉพาะอย่างยิ่งไนลอนเธรด และ? bers มีความรุนแรง จึงไปลดการ เนื้อหาส่วนประกอบโมเลกุลต่ำโดยการสกัดด้วย หรือสารทำละลาย ควร ' น้ำ การลดความเข้มข้นของ สำหรับ ตัวอย่าง ประมาณ 0.5% โดยน้ำหนัก มันมี vbeen ขนบการสกัดเช่น discontinuously ในหม้อไอน้ำขนาดใหญ่ซักผ้า หรืออย่างต่อเนื่อง ในอาคารแยก การไม่ต่อเนื่อง กระบวนการสกัดมีข้อเสียที่ทราบของการลงเวลา เปิด ปิด และชาร์จไฟติดตั้งสกัด และประสิทธิภาพต่ำ เนื่องจากการเสียของพลังงานที่ป้อนเข้าได้ สาเหตุของ reheating ประจำงวดแต่ละชุด ในกระบวนการสกัดต่อเนื่องปกติ granulate นำเข้าสู่การท่อแยกที่ด้านบน และร้อน ป้อนน้ำที่ด้านล่างดังกล่าว และ? ows upwardly เคาน์เตอร์? อ่าว ให้อนุภาคใยสังเคราะห์ ในระหว่างการลง ผู้ป่วยเคลื่อนไหวของอนุภาคในกระแสน้ำ theextraction ส่วนประกอบโมเลกุลต่ำเกิดขึ้น ที่ the bottom of the extraction tube the granulate is discharged, while the water which has become charged with lower molecular components is drawn o? at the upper part of the extraction tube. Inside the elongated extraction tube, counterflow currents are set up because of density ?uctuations in the water. The density fluctuations occur for various reasons. Sometimes the density varies be cause of excessive heating, for example, along the heated wall. It also increases because of cooling in the upper part of the tube upon entry of the considerably colder polyamide cuttings. These temperature variations result in corresponding convection currents. Upon charging of the water with the heavier lower molecular components, especially monomers, the density increases in the main flow direction of the water, which is an additional cause for counter?ow currents. The total result of the counter flows of current causes the solvent, severely charged with monomers, to ?ow down from the upper zones of the extraction tube to the inlet of the fresh water. Because of these interference currents, the counter?ow principle of the extraction and thereby the Washing effect are seriously disturbed so that in the conventional extraction tubes, to sufficiently free the cuttings of the monomers, very large amounts of water are necessary. The maximum monomer concentration in the extraction tubes is about the middle so that the upper half of the tube remains virtually unused. SUMMARY OF THE INVENTIONIt is therefore an important object of the invention to eliminate the disadvantages of prior art methods of extracting lower molecular components from granulated polyamides. It is also an object of the invention to provide a method for extraction which is simple and economical, yet highly efficient. Further purposes and objects of the invention will ap pear as the speci?cation proceeds. It has been found that the objects of the invention may be accomplished by localization of secondary counter flows inside the extraction tube, whereby a more efficient and more extensive removal of the lower molecular substances from the polyamide cuttings is achieved. This is accomplished by providing a plurality of installations over the height of the extraction tube for constricting the flow cross section of the extraction tube. The ?ow cross sections are chosen so that the upward ?ow of the solvent in the narrowest tube cross sections is equal to or greater than the counter?ow of the solvent resulting from temperature variations and charging of the solvent with lower molecular components and/or forced counter?ow of the solvents. ‘ These constrictions in the tube increase the ?ow rate of the solvent ?owing in the tube. Experimentally, it has been found that the effect of improving the elutriation manifests itself effectively only if the narrowest of the flow cross sections are chosen so that the upward flow of the solvent in the constrictions is equal to or greater than the counter?ow currents due to convection or density variations upon charging the solvent with lower molecular components. The constrictions in the tube act to block the downwardly directed interference currents of the solvent above the particular constriction in the tube. A particular advantage of the invention is that the counter?ow currents, which are actually desirable for intensifying the elutriation by providing good intermixing of granulate and solvent are not prevented, but rather are only localized. To provide further intensification of elutriation, additional streams created by suitable agitation or mixing assemblies may be utilized. In this case, the constrictions in the tube are dimensioned so that the up ward movement of the solvent at the constrictions is equal to or greater than the counter?ow of the solvent due to fluctuations in density and/or the forced counterflow of the solvent. The size of constriction in the tube that is required for the desired blocking effect is readily calculated for each specific case from the mean flow velocity of the solvent and the flow brought about through density fluctuations of the solvent resulting from temperature variations and charging of the solvent with lower molecular components and/ or a forced flow. The size of the constriction is limited upon adversely affecting the downward movement of the cuttings by reason of excessive solvent acceleration. The extraction method according to the invention has many advantages. In comparison with conventional methods, and with equal granulate throughput and equal conventional apparatus dimenisons, to provide equal end monomer content in the granulate, much smaller amounts of solvent are required in the invention method, for exam ple, only about 50% to 70% of the amount normally required. In addition to the saving in solvent, there is also an improved working of the extraction agent for the recovery of monomers because of the more concentrated collection thereof in water. With equal amounts of water and like other conditions, the final concentration of the polyamide cuttings in caprolactam, using the method of the invention, amounts to, for example, 0.3% by weight as compared to 1% to 1.5% by weight in the extraction in conventional apparatus. Furthermore, with the invention, a smaller extraction unit accomplishes the same ?nal concentration in caprolactam as in a conventional extraction tower with the same amount of solvent. Further, it is important that the extraction method according to the invention, particularly in the case of operational disturbances, such as breakdown of the heating system and pressure loss in the solvent feed, has a more stable opearting behavior than conventional extraction methods. BRIEF DESCRIPTION OF THE DRAWINGSParticular embodiments of the apparatus used in practicing the method of the present invention are illustrated in the accompanying drawings, wherein: FIG. 1 is a fragmentary, side elevational view of an extraction tube made utilizing the method of the invention; FIG. 2 is a fragmentary, side elevational view of another extraction tube embodiment; and FIG. 3 is a side eleva DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an insert 2 being a funnel-shape shell truncated in the shape of a cone shell which narrows in the direction of movement of the granulate, which construction is advantageous in the case of a circular cross section extraction tube l. The narrowing of the flow path is designated by the numeral 3. In the case of rectangular tubes, on the other hand, the installations are preferably in the shape of truncated pyramid shells. The direction of movement of the granulate is indicated by the arrow drawn in solid lines, while the arrow shown in broken lines shows the direction of flow of the solvent. There are also other forms of construction, such as, for example, simple horizontal plates with corresponding restricted central apertures (not shown). The restricted portions 3' may also, as shown in FIG. 2, be formed by funnel-shaped contrictions 2' defined in the outer wall of the extraction tube 1' itself, so that additional constricting inserts are unnecessary. In the funnel shaped constrictions 2’ the use of the shown opposed double-funnel arrangements, with opposed aperture directions
and meeting at the conical or pyramidal apexes of the funnel portions 2’, are particularly advantageous since neither
the flow of the solid matter nor that of the solvent is disturbed. In order to make a commercial scale extraction apparatus, for example, with funnel-shaped constrictions
readily accessible for repairs and for the purpose of cleaning, short funnels with correspondingly large access openings, about the size of manholes, may be used.
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