Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR DEPOLYMERIZING POLYCAPROLACTAM
PROCESSING WASTE TO FORM CAPROLACTAM
BACKGROUND OF THE INVENTION
The present invention relates to a process for the
depolymerization of polycaprolactam processing waste to form
caprolactam.
The processing of polycaprolactam into intermediate articles
such as fiber, chip, film, or molded articles results in polycaprolactam
(hereinafter "nylon 6 ") processing waste, i.e., scrap nylon 6
1o polymeric and/or oligomeric materials. Examples of such scrap nylon
6 polymeric and/or oligomeric material are yarn waste, chip waste, or
extruder slag. Examples of scrap nylon 6 oligomeric materials are the
linear and cyclic oligomers of caprolactam. The nylon 6 intermediate
articles are then incorporated or transformed into end use products
i5 such as fabrics, engineered plasics, carpets, and packaging.
The current worldwide production of polycaprolactam is
enormous and this polycaprolactam is then processed into the
intermediate articles. The scrap nylon 6 which results from this
2o polycaprolactam processing into intermediate articles is sizeable.
In order to improve the yield in .the processing of
polycaprolactam, the scrap nylon 6 materials are depolymerized to
caprolactam and the caprolactam is then reused. Recovery of
25 caprolactam from polycaproiactam processing waste scrap, i.e. nylon
6 which is substantially free of non-nylon 6 materials, has been
practiced for at least twenty years. In general, nylon 6 is
depolymerized by heating at elevated temperatures, usually in the
presence of a catalyst and/or steam. See U.S. Patents 4,107,160;
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5,233,037; 5,294,707; 5,359,062; 5,360,905; 5,468,900; German
4,421, 239A 1; Example 5 of European Patent Application 608,454;
and Chem. Ing. Techn. _4~, 1509 (1973). The caprolactam produced
may be removed as a vapor stream as taught by AlIiedSignal's U.S.
Patent 3,182,055. In most of the above processes a catalyst such as
phosphoric acid is used to promote depolymerization of
polycaprolactam. An extensive review of the field has been given by
L. A. Dmitrieva et al, Fibre Chemistry, Vol. 17, No. 4, March 1986,
pp. 229-241. U.S. Patent 5,495,014 teaches the depolymerization of
1o nylon 6 wherein the reaction is in the liquid phase at elevated
temperatures in the presence of a heterogeneous catalyst and in
organic solvent.
U.S. Patent 3,939,153 to Fowler teaches a polycaprolactam
1s depolymerization process wherein superheated steam and melted
scrap nylon 6 are combined in a tubular elongated reactor. The
reference teaches that the average temperature in the reactor is about
343°C to about 677°C, the average residence time for the nylon
melt
in the reactor is from about one to about 40 minutes, and the average
2o residence time for the superheated steam in the reactor is from about
0.01 to about ten seconds. The reference makes no mention of
pressure in the reactor. The steam and nylon melt decomposition
product then pass as a combined stream out of the reactor into a
nylon column wherein the steam and nylon decomposition products
2s pass overhead in vapor phase and the unconverted nylon 6 and
byproduct oligomers are withdrawn from the nylon column and
recycled back to the feed tank. The reference teaches that in a
typical system, about 20°~ of the nylon 6 passing through the reactor
is depolymerized to caprolactam and the remainder is recycled back to
so the feed tank.
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The Fowler polycaprolactam depoiymerization process which
has only a 20% first pass yield is unacceptable in industry.
Additionally, we have found that under the process conditions
employed by Fowler, relatively large amounts of caprolactam cyclic
s dimer and ammonia are produced. Thus, a need exists in the industry
for an improved process for the depolymerization of polycaprolactam
processing waste.
to SUMMARY OF THE INVENTION
The invention provides an improved process for depolymerizing
polycaprolactam waste to form caprolactam. The process comprises
the step of: in the absence of added catalyst, contacting
is poiycaprolactam waste with superheated steam at a temperature of
about 250°C to about 400°C and at a pressure within the range of
about 1.5 atm to about 100 atm and substantially less than the
saturated vapor pressure of water at the temperature wherein a
caprolactam-containing vapor stream is formed.
Optionally, the polycaprolactam waste is contacted for a short
time period with liquid water under elevated temperatures and
pressures for time sufficient to reduce the molecular weight of the
polycaprolactam, prior to contacting with steam as discussed above.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described in more detail below with
3o reference to drawings, wherein FIG. 1 through 3 are graphs illustrating
advantages of the present invention.
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FIG. 1 illustrates at a temperature of 320°C the effect of
pressure on caprolactam production.
FIG. 2 illustrates at a temperature of 320°C the effect of
pressure on caprolactam, ammonia, and cyclic dimer production.
FIG. 3 illustrates at a temperature of 340°C the effect of
pressure on caprolactam, ammonia, and cyclic dimer production.
io
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The term "polycaprolactam waste" as used herein means scrap
nylon 6 polymeric and/or oligomeric material. Examples of scrap
nylon 6 oligomeric materials are linear and cyclic oligomers of
caprolactam. Such scrap nylon 6 polymeric andlor oligomeric material
is generated during the production of intermediate articles such as
fiber, chip, film, or molded articles. The nylon 6 intermediate articles
2o are then incorporated or transformed into end use multi-component
products such as fabrics, engineered plastics, carpets, and packaging.
Examples of such scrap nylon 6 polymeric andlor oligomeric material
are yarn waste, chip waste, or extruder slag. The term
"polycaprolactam waste" excludes the presence of non-
polycaprolactam components in significant amounts. However, it does
not exclude small amounts of adventitious contaminants, such as
environmental dust or humidity, or processing oils and lubricants, or
fiber opacifiers such as titanium dioxide. Such non-polycaprolactam
components will generally be present in no more than about 10% by
3o weight with respect to polycaprolactam.
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The term "fiber" as used herein means an elongated body
wherein the length dimension is much greater than the transverse
dimensions of width and thickness. Accordingly, "fiber" includes, for
example, monofilament, multifilament yarn (continuous or staple),
5 ribbon, strip, staple and other forms of chopped, cut or discontinuous
fiber, and the like having regular or irregular cross-sections. "Fiber"
includes a plurality of any one of the above or a combination of the
above.
1o According to the process of the current invention, caprolactam
is formed by contacting the polycaprotactam waste with superheated
steam at elevated temperatures and superatmospheric pressures and
removing a vapor stream containing caprolactam from the contact
region. The term °superheated steam" as used herein means steam
that is heated to a temperature substantially higher than the
temperature at which condensation to liquid water would takq place
at the pressure used to convey said steam. An important benefit of
the process is that no catalyst is needed for recovering caprolactam
from polycaprolactam waste.
Accordingly, for the present process, no acidic catalyst is
added to the vessel in which the polycaprolactam waste is contacted
with superheated steam. It should be understood, however, that the
waste material feedstock may include minor amounts of materials (for
2s example, contaminants) that incidentally are recognized in the art as
catalysts. However, the subject process does not rely on the
presence or addition of any such catalytic materials in the vessel.
The polycaprolactam waste is preferably fed to the reactor as a
so melt. This feeding may be achieved by using an extruder, gear pump,
or other means known in the art. Some feeding systems, such as
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extruders, allow the development of relatively high pressures in the
melt. This offers the option of contacting the melt with liquid water
at elevated temperatures for a short period of time at little added cost.
This may be achieved, for example, by introducing water under
s pressure in the extruder barrel. The contact time between the melt
and water may be extended by placing a high pressure pipe between
the extruder exit and reactor. In this optional pretreatment step, the
polycaprolactam waste is combined with liquid water and heated at a
sufficient temperature for a time period sufficient to effect ~n initial
1o depolymerization of the polycaprolactam waste. The depolymerization
products formed in this step may include reduced molecular weight
polycaprolactam, caprolactam, caprolactam linear oligomers, and
caprolactam cyclic oligomers. Such contact accelerates caprolactam
production in subsequent process steps as disclosed in AlIiedSignal's
is U.S. Patent 5,457,197 to Sifniades et al.
For the recovery of caprolactam to be economical, it is
2o desirable to utilize as inexpensive equipment and as little steam as
technically feasible. A good index of the economy of the process is
the concentration of caprolactam obtained in the overheads, which
bears an inverse relationship to the amount of steam used.
Concentrations in excess of 15 weight percent may be obtained by
25 appropriate design of the reactor and choice of operating conditions
as described below.
The reaction temperature should be at least about 250°C but
not higher than about 400°C. Generally, the rate of caprolactam
3o formation increases with~increasing temperature. However, the rate of
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side reactions of nylon 6 such as evolution of ammonia also increases
with temperature.
Temperatures of at least about 250°C are preferred because
below 250°C, caprolactam formation may be too slow. Temperatures
no greater than about 400°C are preferred, as above 400°C side
reactions of nylon 6 may become prohibitively fast. A preferred
temperature range is about 280°C to about 350°C, more preferably
a
temperature in the range of about 300°C to about 340°C.
The pressure should be moderately above atmospheric but
higher pressures offer certain advantages as will be explained below.
Other factors, such as the availability and operating cost of high
pressure equipment may influence the choice of pressure.
Regarding the effect of pressure, it has been found that for a
given temperature and steam flow, jncreasing the reactor pressure
generally increases the caprolactam concentration in the overheads up
to an optimal pressure. Further small increases in pressure have little
2o effect on caprolactam concentration. However, a large increase in
pressure beyond the optimat pressure results in decreased
caprolactam concentration. Generally, the higher the operating
temperature, the higher is the optimal pressure at which maximum
caprolactam concentration is obtained. For example, at an hourly
steam flow equal to twice the mass of the polycaprolactam charged,
the optimal pressure is about 17 atm (about 1720 kPa) at about
320°C and about 19 atm (about 1920 kPa) at 340 °C. Optimal
pressure conditions under different operating conditions within the
scope of this invention can be determined by those skilled in the art.
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It will be appreciated that the optimal pressure is well below the
saturated vapor pressure of water at the operating temperature. For
example, the saturated vapor pressure of water is 111 atm at 320°C,
and 144 atm at 340°C. Therefore, it is clear that in the current
process, no liquid aqueous phase is present.
The effect of pressure on caprolactam concentration at
constant steam flow is matched by its effect on the rate of production
of caprolactam. Therefore, operating near the optimal pressure
io minimizes not only steam usage but also reactor volume.
A further benefit of operating close to the optimal pressure is the
suppression of side reactions leading to ammonia formation. We have
found that at a given temperature, ammonia production relative to
caprolactam production is lowest at pressures close to the optimal
pressure for caprolactam production as will be discussed later relative
to Figure 2.
Although not wishing to be bound by any theory, we wish to
2o rationalize our findings by means of the following theory. One useful
outcome of the theory is that it allows the construction of a computer
model that may be used to optimize the process once sufficient data
have been collected to calibrate the model. We believe that as
pressure increases at a given temperature and steam flow, the
amount of water that dissolves in nylon 6 is increased resulting in the
acceleration of depoiymerization reactions. It will be appreciated that
the action of water in the depolymerization of nylon 6 to caprolactam
is catalytic, that is, no net amount of water is consumed in the overall
conversion of nylon 6 to caprolactam. Caprolactam is generally
3o formed by cleavage of caprolactam molecules from the ends of the
nylon 6 chain, in a reversal of the polyaddition reaction which
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constitutes caprolactam polymerization. Water promotes caprolactam
formation by virtue of promoting the cleavage of amide bonds, which
results in the formation of more end groups. Water is consumed only
to the extent that some of the nylon 6 charged is not converted to
s caprolactam. As caprolactam is produced at a faster rate, its partial
pressure in the vapor phase increases. However, the partial pressure
of water also increases, approximately in proportion to the applied
pressure. The caprolactam/water ratio in the overheads is
proportional to the ratio of the corresponding vapor pressures.
io
Therefore, increasing the reactor pressure can result in an
increase or a decrease of caprolactam concentration in the overheads,
depending on whether the caprolactam vapor pressure increases
faster or slower than the water vapor pressure. Evidently, at
1s pressures below the optimal pressure, the caprotactam partial
pressure increases faster than the partial pressure of water as the
reactor pressure is increased. At pressures above the optimal
pressure, the partial pressure of water increases faster than the partial
pressure of caprolactam as the reactor pressure is increased.
A secondary effect of pressure is the suppression of
caprolactam cyclic dimer. The dimer is formed reversibly along with
caprotactam during nylon 6 depolymerization. When the
depolymerization is carried out at atmospheric pressure, relatively
2s large amounts of the dimer are found in the overheads, as much as 3-
4 wt% of the caprolactam. Increasing the pressure decreases the
ratio of dimer to caprolactam in the overheads. Since dimer formation
is reversible, dimer that does not distill over is converted eventually to
caprolactam. Suppressing dimer concentration in the overheads is
3o beneficial not only from the point of view of product yield, but also
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because the dimer, when present at high concentrations, may be
deposited as a solid and clog the transfer lines and the condenser.
In view of these findings, the operating pressure should range
s from about 1.5 atm up to about 100 atm (about 152 kPa to about
10130 kPal. However, the pressure should be substantally less than
the saturation vapor pressure of water under the operating
temperature to ensure that liquid water does not condense in the
reactor. For example, at 300°C, the saturated vapor pressure of
io water is 85 atm. Operation at that temperature should be carried out
at pressures ranging from about 1.5 atm to about 75 atm. For the
preferred temperature range of about 280°C to about 350 ° C, the
preferred pressure range is about 2 atm to about 30 atm (about 203
kPa to about 3940 kPal. For the more preferred temperature range of
~s about 290°C to about 340°C, the preferred pressure range is
about 3
atm to about 15 atm (about 304 kPa to about 1520 kPa). The rate of
steam flow should be sufficient to remove caprotactam from the
reactor, but not so high as to cause undue dilution of caprolactam in
the overheads. Since a high caprolactam concentration in the
overheads is desired, the steam flow should be proportional to the
rate of production of caprolactam, which is generally proportional to
the mass of nylon 6 charged and also increases with temperature.
The contact of the potycaprofactam waste with superheated
steam is effected in a vessel designed to withstand the requisite
temperature and pressure, as well as the corrosiveness of the
reactants. Since no corrosive catalysts, such as acids, are required in
this process, no special alloys are required, and a stainless steel
vessel is adequate.
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Good contact between steam and the polycaproiactam waste is
essential for an effective operation. Such contact may be achieved
by various means known generally in the art. As an example, steam
may be sparged through the material using a multiplicity of inlets, far
s example, using a steam distributor. Improved contact may be
achieved by including mechanical agitation in the reactor, for
example, using a combination of rotating paddles and static fins.
The process of the current invention may be carried out either
io continuously or in batch fashion. In the latter case, the
polycaprolactam waste is charged to the reactor all at once and steam
is sparged continuously until most of the.caprolactam has been
recovered. Generally, in the batch process, caprolactam concentration
in the overheads diminishes as the charge is depleted of nylon 6.
is Said concentration may be maintained at relatively high levels
throughout the process by gradually increasing the temperature
and/or decreasing the steam flow as the run process.
In a continuous process, both the polycaprolactam waste and
2o the steam are fed continuously to the reactor. Caprolactam is
recovered overhead, while a nylon 6 depleted melt is discharged from
the bottoms. This melt is a mixture of low molecular weight
polycaprolactam with polycaprolactam degradation products and with
non-polycaprolactam materials, and their degradation products, that
2s may have been present in the feed stream. The volume of the bottoms
stream will depend on the purity of the potycaprolactam waste and the
extent of caprolactam yield. Generally, yields of caprolactam should
exceed 90%. Therefore, for substantially pure polycaprolactam waste,
the bottoms stream is generally less than 10% of the feed stream.
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To maintain a high caprolactam concentration in the overheads,
it is desirable to run the steam countercurrent to the melt flow. This
can be achieved by using a series of continuous stirred reactors
(CSTRs) in which melt flows from the first reactor to the last while
steam flows in the opposite direction. However, it is also possible to
operate with steam crossflow or crosscurrent flow. In this mode, the
melt flows from the first reactor to the last, whereas fresh steam is
supplied to each reactor. If desired, the steam flow to each reactor
may diminish as the nylon content of the melt diminishes. Although
1o crossflow may generally result in higher overall consumption of steam,
it is simpler to implement and may require lower capital investment.
In a preferred embodiment of the process, the polycaprolactam
waste melt is fed at the top of a continuous flow reactor.
i5 Superheated steam is fed through a distributor at the bottom of the
reactor countercurrent to the flow of the melt. A vapor stream
containing caprolactam is collected at the top of the reactor and nylon
6 depleted melt exits at the bottom. The polycaprolactam waste may
be fed by means of an extruder, gear pump, or other device. The
2o reactor may be divided into several stages by means of baffles.
Means may be provided for mechanical agitation in each stage. Heat
is provided to the reactor mainly by means of the superheated steam.
Additional heat may also be provided through the polycaprolactam
waste feed, especially if an extruder is used, and through the wall of
2s the reactor.
Caprolactam may be separated from other components of the
distillate. The vapors from the reactor overhead may be sent to a
partial condenser to obtain a condensate containing caprolactam.
3o Caprolactam suitable for the production of fiber, film, or engineered
resin may be obtained from this condensate by further purification
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including distillation, crystallization and other conventional techniques
known in the art. For example, the caprolactam purification process
of AlIiedSignal's U.S. Patent 2,813,858; 3,406,176 or 4,767,503 to
Crescentini et al. may be used.
The purified caprolactam may then be used to make
poiycaprolactam using a known process such as disclosed in
AlIiedSignal's U.S. Patent 3,294,756; 3,558,567; or 3,579,483. The
polycaprolactam may then be used in known engineered materials
io such as disclosed in AlIiedSignal's U.S. Patent 4,160,790; 4,902,749;
or 5,162,440; spun into fiber using a known process such as
disclosed in AlIiedSignal's U.S. Patent 3,489,832; 3,517,412; or
3,619,452; or made into film.
The following examples illustrate various preferred
embodiments of the invention.
EXAMPLE 1
Nylon 6 chips, 15 g, having a molecular weight of about
20,000, were charged to a stainless steel cylindrical reactor of 24.5
mm diameter and 300 mm height. The reactor was connected to a
condenser equipped with a back-pressure valve set at 6.4 atm (650
kPa). Superheated steam was blown through the bottom of the
reactor at the rate of 0.4 g per minute while the temperature was
2s maintained at 330 °C. A small stream of nitrogen, 30 mL(STP)/min,
was mixed with the steam to prevent backing of melt into the steam
line. Overhead cuts were collected periodically and analyzed for
caprolactam. After 225 min of operation, the combined overheads
contained 12.0 g caproiactam.
EXAMPLE 2
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Effect of Prehvdrolvsis on the Rate of Depolvmerization
Nylon 6 chips, 15 g, having a molecular weight of about
20,000, and 7 g water were charged to a stainless steel cylindrical
reactor of 24.5 mm diameter and 300 mm height. The reactor was
s seated and the temperature was raised rapidly to 290 °C and held for
min. The molecular weight of the mixture (excluding caprolactam)
was about 1,000 and the amount of caprolactam present was about
20% of the total amount of caprolactam theoretically recoverable
from the amount of nylon 6 charged. The reactor was next cooled to
10 100 °C and connected to a condenser equipped with a back-pressure
valve set at 6.4 atm (650 kPa). Superheated steam was blown
through the bottom of the reactor at the rate of 0.4 g per minute
while the temperature was maintained at 330 °C. A small stream of
nitrogen, 30 mL(STP)/min, was mixed with the steam to prevent
backing of melt into the steam line. Overhead cuts were collected
periodically and analyzed for caprolactam cyclic oligomers of.
caprolactam. After 225 min of operation, the combined overheads
contained 14.6 g caprolactam, 0.098 g cyclic dimer, and 0.0058 g
cyclic trimer.
COMPARATIVE EXAMPLE
Effect of Pressure on the Rate and Selectivity of Deoolvmerization
The procedure of Exampie 2 was repeated, except that the
pressure was maintained at 1 atm (101 kPa). At the end of 225 min,
the combined overheads contained 7.8 g caprolactam, 0.16 g cyclic
dimer, and 0.060 g cyclic trimer.
EXAMPLE 3
Nylon 6 melt was charged via an extruder to a 2 L cytindical
3o stainless steel reactor. The reactor was agitated by means of a down
flow helical agitator that scraped the reactor walls and was equipped
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with two circular windows at mid-height that allowed observation of
the reaction mass. The reactor vent was connected to a condenser
via a back-pressure regulator. Superheated steam was blown
throughout the operation at the rate of 35 to 40 g/min via a sparger
5 at the bottom of the reactor and caprolactam containing vapor was
drawn overhead and condensed. The pressure and temperature in the
reactor were maintained at about 312 °C and 9.2 atm respectively.
During the first 105 min of operation, 1100 g of nylon 6 were
charged to the reactor at the rate of 10 to 12 g/min. At that point,
the mass in the reactor was estimated at about 700 g. Feeding of
nylon 6 was then interrupted for 120 min. The mass remaining in the
reactor at that point was about 150 g. An additional 900 g nylon 6
was added during the next 45 min and the operation continued for an
additional 60 min after termination of the second nylon 6 addition.
i5 The total amount of caprolactam found in the overheads condensate
was 1558 g. The remaining melt that was drained from the reactor
was 440 g and consisted essentially of low molecular weight nylon 6.
The instantaneous rate of caprolactam production was roughly
proportional to the amount of melt held in the reactor. The normalized
2o rate of caprolactam production was about 0:7 to 0.8 g caprolactam
per gram of melt held in the reactor per hour. The overall
concentration of caprolactam was 11 % by weight.
EXAMPLE 4
The apparatus of Example 3 was used. Nylon 6 melt was fed to
the reactor throughout the run at a rate adjusted to maintain the melt
level in the reactor at about 800 g, while superheated steam was
blown at the rate of 4,000 g/h. The temperature and pressure were
held at about 310 °C and 9.2 atm respectively. The run lasted for 6
3o hours. The rate of caprolactam production was 500 to 600 glh, or
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0.62 to 0.75 grams per gram of melt held in the reactor per hour. The
overall concentration of caprolactam was 11 % by weight.
EXAMPLE 5
A tube reactor of 24.5 mm diameter and 1070 mm height was
charged with 100 g nylon 6 and steam was blown through the
bottom. Overheads were collected at time intervals during the
operation and analyzed for caprolactam, caprolactam cyclic dimer,
and ammonia. Runs were carried out at various temperatures,
io pressures, and steam flows. The data were used to construct a
computer mode! of the process in the framework of the theory
presented earlier. The model was then used to draw the curves
shown in Figures 1 through 3. In all Figures, an hourly steam flow
equal to twice the mass of the polycaprolactam charged was
is assumed. It will be understood that slightly different results may be-
obtained under different reactor configurations and steam floyvs, but
we believe that the trends shown in the Figures have general validity.
Figure 1 shows the expected rate of caprolactam production as
2o a function of pressure at 320 °C . The labels on each curve
represent
the pressure in the reactor in atmospheres. Thus, 1 means 1 atm, 2
means 2 atm, and so on. It is evident that increasing the pressure
increases the rata of caprolactam production overhead, but the
greatest impact is at relatively low pressures. For example, the rate
25 increases the most in going from 1 atm to 2 atm, and the least in
going from 5 atm to 6 atm.
Figures 2 and 3 illustrate the effects of pressure and
temperature on the concentration of caprolactam in the overheads at
30 90% caprolactam yield. Since the curves were drawn at constant
steam flow, the concentration of caprolactam is proportional to the
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rate of caprolactam production. It is evident that at relatively low
pressures the rate sharply increases with pressure but the effect
diminishes at higher pressures and a maximum rate is reached at ca
17 atm at 320 °C or at 19 atm at 340 °C. The Figures also show
(right Y-axis) the production of cyclic dimer and ammonia relative to
caprolactam. It is evident that these rates sharply decrease at lower
pressures but the effect diminishes at higher pressures. In the case of
ammonia, a minimum is shown at about the pressure of maximum
rate of caprolactam production. The production of the cyclic dimer of
io caprolactam monotonically decreases with increasing pressure.
Finally, comparison of the two Figures shows that increasing the
temperature increases the rate of caprolactam production but even
more so the rates of ammonia and dimer production.
EXAMPLE 6
For a continuous process, the apparatus comprises at least
three reactors equipped with inlet at the top and outlet at the bottom
for liquid flow, and inlet at the bottom and outlet at the top for vapor
flow. The three reactors are connected in series so that liquid flow
2o runs in one direction while vapor flow runs in the opposite direction.
Each reactor is equipped with a mechanical agitator and baffles that
ensure intimate mixing between liquid and vapor. Polycaprolactam
waste is continuously fed to the first reactor by means of an extruder
and exits from the last. Superheated steam is fed to the last reactor at
a rate approximately 5 times the extrudate flow and exits from the
first reactor. The reactors are held at about 330°C and 15 atm. The
overall residence time of the melt in the reactors is about 4 hours.
The exit vapors are sent to a partial condenser where a condensate
containing about 90% caprolactam is obtained. Fiber grade
so caprotactam may be obtained from this condensate by further
purification including filtration, distillation, crystallization, and other
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conventional techniques known in the art. A portion of the remaining
vapor is purged while the rest is mixed with makeup steam, sent to a
superheater, and recycled through the process.
From the foregoing description, one skilled in the art can easily
ascertain the essential characteristics of this invention, and without
departing from the spirit and scope thereof, can make various
changes and modifications of the invention to adapt it to various
usages and conditions.
