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BACKGROUND OF THE INVENTION
Present invention relates to the polymerization of polyvinyl chloride, more
particularly to suspension polymerization of polyvinyl chloride (PVC), and
still more particularly to post polymerization treatment of the PVC
suspension resin slurry which provides for more efficient removal of
residual vinyl chloride monomer.
Homopolymers and copolymers of polyvinyl chloride have long been one of the
largest and most versatile thermoplastic polymers, the vast majority of
which has traditionally been manufactured by suspension polymerization
techniques. In suspension polymerization, an intimate mixture of at least
one polymerizable monomer and at least one suspending agent are suspended
in aqueous medium and subjected to suitable polymerization conditions of
time, temperature, and pressure. While there are countless literature and
patent teachings of such processes, U.S. Pat. Nos. 2,833,754, 3,026,308,
and 3,054,786 may be mentioned as three typical examples of such
teachings.
In such techniques, the water and suspending agent(s) are first charged to
the reactor, then the vinyl chloride monomer (and any comonomers) and the
catalyst(s) are charged to the reactor, and the reaction is then permitted
to continue until the desired degree of polymerization is achieved,
usually as evidenced by a pressure drop in the reactor. At this point, the
liquid medium is transferred to a blowdown tank or other suitable raw
slurry storage tank (hereinafter "blowdown tank" for convenience), any
gaseous vinyl chloride is collected, and the liquid phase is then
subjected to a series of drying steps in which water is removed and the
polymer is ultimately dried.
The most significant inherent undesirable property of polyvinyl chloride
polymers is their susceptibility to degradation on exposure to heat and/or
light. It is, of course, well known that such degradation, particularly
heat degradation, is a function not only of the time and temperature of
the immediate heat environment to which the PVC is subjected, but also a
function of the "heat history" of the polymer, that is to say the
cumulative effect of all the heat environments to which it has heretofore
been subjected. Thus, where a particular mass of polyvinyl chloride
polymer is subjected to even a brief period at moderately high temperature
(e.g., in excess of about 200.degree. F.) though it may not evidence any
discernable evidence of degradation, it will indeed have suffered adverse
heat stability effects. If such a mass is subsequently subjected to
additional heat conditions (e.g., during extrusion or other end use
forming procedures), the cumulative heat history of the particular resin
mass, as well as the immediate heat conditions to which it is then
subjected, wil determine whether or not objectionable degradation will
occur.
Certain essential process steps, of course, must be carried out in spite of
this potential deleterious effect on the heat history of the polymer mass
being treated. For example, the polymer must be dried, and all free water
must be removed. While the last traces of water will indeed be removed by
heat, the major portion of the water removal will be achieved by
mechanical means, usually centrifugal or other mechanical filtration
procedures.
With the increasing awareness of the potential dangers of vinyl chloride
monomer (VCM), and the recognition that potentially significant
concentrations of residual VCM can be entrapped in the polymer particle,
new procedures have been introduced to promote recovery of unreacted VCM,
and reduction of the residual VCM in the finished polymer particle. A wide
variety of specific procedures have been suggested, and in many cases are
in commercial use, to achieve these results.
In one such procedure, the PVC suspension resin slurry is stripped of
residual vinyl chloride monomer in a colum using a combination of steam
and pressure/vacuum control. The slurry from the blowdown tank is fed to
top of the column, and flows downward over a series of trays to the
bottom, while stream is added near the botom of the column and flows
upward through the slurry, stripping it of VCM. The VCM-water vapor stream
passes through the top of the column, the water is removed by
condensation, and the recovered monomer is returned to the process. The
stripped PVC slurry is fed from the bottom of the column to a cooling
means and then to the drying and resin fin hing systems. See, for example,
U.S. Ser. No. 592,119 filed July 1, 1975 and since abandoned in favor of
C-I-P application Ser. No. 728,521, filed on Oct. 1, 1976.
Like drying of the resin, such a stripping procedure has an adverse effect
on the heat history of the resin treated, but clearly it is now essential.
In order to minimize the deleterious effect on heat history, the slurry
being removed from the stripping column, which is usually at a temperature
of from about 200.degree. to 240.degree. F., (about 93.degree. to
116.degree. C.) was fed to a cooling means where it was cooled as quickly
as possible. One such cooling means is a conventional jacketed tank, using
cool water to quickly lower the temperature of the slurry. On the other
hand, one type of cooling means which has not heretofore been employed for
such applications is a conventional heat exchanger, because of thhe
tendency of the suspension resin particles to settle out, with the
resultant blockage of the cooling means.
DRAWINGS
FIG. 1 is a plan view from the top of a horizontal spiral heat exchanger
suitable for use in the process of the present invention.
FIGS. 2 and 3 are sectional views of the heat exchanger of FIG. 1
illustrating the flow paths through the heat exchanger.
FIG. 4 is a flow diagram illustrating one embodiment of the present
invention.
FIG. 5 is a flow diagram illustrating another embodiment of the present
invention.
SUMMARY OF THE INVENTION
We have now found that a substantial energy saving can be achieved, and/or
the VCM removal can be more effectively carried out, by employing a spiral
heat exchanger, and using the slurry feed from the blowdown tank in place
of water as the cooling medium. Thus, the feed stream from the blowdown
tank to the stripping column acts as a cooling medium for the outlet
stream from the stripping column, while the outlet stream from the
stripping column acts as a preheating medium for the feed stream to the
stripping column.
This, of course, results in a substantial savings in terms of heat energy
(steam) input to the stripping column, and/or higher efficiency in removal
of VCM. In some cases, this could possibly involve subjecting the polymer
to a potentially longer period of exposure at high temperature, and/or a
potentially slower cooling curve for the outlet feed from the stripper.
Very surprisingly, however, it does not appear to have any substantial
additional adverse effect on the heat history of the polymer being treated
(over the above that imparted in normal treatment in the stripping
column). Also, the heat exchanger operates surprisingly effectively in a
slurry-to-slurry configuration, without any significant clogging.
FIGS. 1, 2, and 3 illustrate a spiral heat exchanger suitable for use in
conjunction with the process of the present invention. FIG. 1 is a top
view showing inlet ports 1a and 2a, and outlet ports 1b and 2b. FIGS. 2
and 3 illustrate the flow from paths of the raw PVC resin slurry (from the
blowdown tank) through the cold side of the heat exchanger, and of the
stripped PVC resin slurry through the hot side of the heat exchanger.
FIG. 4 depicts a flow diagram for one embodiment of the present invention.
Raw PVC resin slurry from the blowdown tank is fed (Stream A) to a spiral
heat exchanger; from said heat exchanger, it is fed (Stream B) to the top
of a VCM stripping column. Stripped slurry is removed from the bottom of
VCM stripping column and fed (Stream C) to said heat exchanger, and from
there, fed (Stream D) to a collecting means from which it can then be
dried and finished in the conventional manner.
In the process illustrated by FIG. 4, Stream A from the blowdown tank is at
a temperature of from about 70.degree. to about 115.degree. F. (about
21.degree. to about 46.degree. C.) as it is fed to the heat exchanger, for
example, to inlet port 1a of the heat exchanger illustrated in FIG. 1.
Stream B is at a temperature of from about 150.degree. to about
190.degree. F. (about 66.degree. to about 88.degree. C.) as it leaves the
heat exchanger, e.g., as it exits from outlet port 1b of the heat
exchanger illustrated in FIG. 1.
Stream C is at a temperature of from about 180.degree. to about 230.degree.
F. (about 82.degree. to about 110.degree. C.) as it exits from the
stripping column to be fed to the heat exchanger, e.g., the inlet port 2a
of the heat exchanger illustrated in FIG. 1. Stream D is at a temperature
of from about 90.degree. to about 140.degree. F. (about 32.degree. to
about 60.degree. C.) as it exits from the heat exchanger, e.g., from
outlet port 2b of the heat exchanger illustrated in FIG. 1.
While average feed rates will vary according to the amount of slurry to be
treated, the capacity of the heat exchanger, and the like, it is to be
expected that the stripped slurry, Streams C and D, will be fed through
the heat exchanger (or at least withdrawn from the stripping column) at a
somewhat faster rate than the raw PVC resin slurry feed, Streams A and B,
generally about 5 to about 20 percent faster. Thus, where, for example,
raw slurry is fed to (and through) the heat exchanger the rate of about
107 gallons per minute (gpm), steam condensation will increase slurry
volume during treatment in the stripping column, requiring that about 122
gpm be withdrawn from the stripping column. Employing the process of FIG.
4, to treat a 30 percent solids slurry of PVC suspension resin (Sp. G 1.1)
at the above rates (107/122 gpm), it is possible to effect energy savings
of 2.56.times.10.sup.10 BTU/year.
PREFERRED EMBODIMENT
In the preferred embodiment of the present invention, only a portion of the
stripped slurry exiting from the stripping column is fed to the heat
exchanger, and the cooled slurry exiting from the heat exchanger is
recycled to the bottom portion of the stripping column where it reduces
the temperature of the slurry being withdrawn and allows the largest
portion of this stream to be fed directly to the collecting tank without
further cooling and with a substantially more desirable heat history.
This preferred embodiment is illustrated by the flow diagram of FIG. 5.
Stream A is still fed to the heat exchanger at a temperature of from about
70.degree. to about 115.degree. F., (about 21.degree. to about 46.degree.
C.) preferably 110.degree..+-.5.degree. F., (43.degree..+-.3.degree. C.)
but Stream B will exit from the heat exchanger at a temperature of from
about 150.degree. to about 170.degree. F., (about 66.degree. to about
77.degree. C.) preferably 160.degree..+-.5.degree. F.
(71.degree..+-.3.degree. C.). Stream C exits from th bottom of the
stripping column at a temperature of from about 150.degree. to about
180.degree. F., (about 66.degree. to about 82.degree. C.) and preferably
170.degree..+-.5.degree. F. (77.degree..+-.3.degree. C.). Prior to feed
into the heat exchanger, Stream C is broken into two smaller feed streams,
Streams C' to the heat exchanger, and Stream C" directly to the collecting
tank. Stream D, the cooled stripped stream, exits from the heat exchanger
at a temperature of from about 90.degree. to about 130.degree. F., (about
32.degree. to about 54.degree. C.) preferably 120.degree..+-.5.degree. F.
(49.degree..+-.3.degree. C.). Stream D, however, is not fed to the
collecting tank, as in the process of FIG. 4, but is recycled to the
bottom of the stripping column. It will, of course, be clear that the flow
rate of Stream C" must be greater than that of the raw PVC slurry feed,
Stream A (or Stream B).
It was noted earlier that the slurry reaches a temperature of about
230.degree. F. (110.degree. C.) as it reaches the bottom of the stripping
column. Since the slurry tends to collect as a pool at the bottom of the
column, it tends to be adversely effected on terms of heat history. By
recycling Stream D from the heat exchanger to the bottom of the stripping
column, at a temperature of about 120.degree. F., (49.degree. C.) it
serves to immediately reduce the temperature of the slurry collected in
the pool at the bottom of the stripper to a temperature of about
190.degree. F., (88.degree. C.) well below the critical temperatures, of
about 200.degree. F., (93.degree. C.) where adverse heat history problems
are likely to be encountered. While mechanical mixing can be employed, we
have found it advantageous to locate the inlet means for addition of the
recycled Stream D into the stripping column at a point below the outlet
means for Stream C (or at least below the level of the pool of PVC slurry
collected at the bottom of the column), so that Stream D is added into the
slurry collected in a pool at the bottom of the stripping column, thereby
providing its own agitation to promote mixing and cooling.
EXAMPLE
An aqueous (30% solids) raw slurry of PVC (Sp.G 1.1) at a temperature of
110.degree..+-.5.degree. F. (43.degree..+-.3.degree. C.) was fed at an
average rate of 77 gpm (Stream A of FIG. 5) from a blowdown tank to the
cold inlet port (port 1a of FIG. 1) of a horizontal spiral heat exchanger
(American Heat Reclaiming Corp.; Type 1-H; area 240 sq. ft., minimum
spacing of channels=0.5 inches). The raw slurry exited from outlet port 1b
at a temperature of 160.degree..+-.5.degree. F. (71.degree..+-.3.degree.
C.) and was fed (Stream B of FIG. 5) to the top of a VCM stripping column.
Stripped slurry was removed from the bottom of the stripping column at an
average rate of 130 gpm at a temperature of 195.degree..+-.5.degree. F.
(49.degree..+-.3.degree. C.) and fed to a valve means where it was divided
into two portions Stream C', which was fed to the heat exchanger, and
Stream C", which was fed at an average rate of 84 gpm directly to a
collection means, from which it was dried and finished by conventional
procedures. Stream C' was fed at an average rate of 46 gpm to the hot
inlet port 2a of the heat exchanger. The cooled stripped slurry exited
from the heat exchanger through outlet port 2b at an average rate of 46
gpm and a temperature of 120.degree..+-.5.degree. F. and was recycled
(Stream D) to the bottom of the stripping column where it was allowed to
mix with the stripped slurry pooled at the bottom of the column, cooling
it from a previous average temperature of about 230.degree. F.
(110.degree. C.) (as it left the bottom tray in the stripping column) to
a temperature of about 195.degree. (91.degree. C.) as it collected in a
pool at the bottom of the stripping column preparatory to withdrawal from
the stripping column into Stream C.
It will, of course, be obvious that the foregoing example is presented by
way of illustration and not by way of limitation, and that many changes
can be made without departing from the spirit of the present invention.
Thus, the flow rates of Stream A and B can vary from as low as 36 to as
high as 100 gpm and that of Stream C can vary from 70 to 140 gpm. This
represents the preferred range of feed rates, and even greater variations
of feed rates can, of course, be achieved, though a smaller (or larger)
surface area heat exchanger may be required. Similarly, heat exchangers
having channel width as narrow as 0.1 inches can be employed.
Also, while the present discussion speaks of feeding Stream B to the "top"
of the stripping column, in many cases the level of residual VCM is
relatively low, and Stream B may be fed into the stripping column at a
point only halfway to the top, or even less. Similarly, while we discuss
withdrawing Stream C from the bottom of the column (and/or recycling
Stream D to the bottom of the column), this really denotes the lower area
of the column, which may indeed be somewhat above the actual bottom.
Where desired, additional cooling of Stream C can be achieved, either using
conventional PVC slurry cooling procedures, or using an additional spiral
heat exchanger and a supplementary cooling medium. This would, of course,
more likely be useful with the embodiment illustrated by FIG. 4 than that
illustrated by FIG. 5. Also, while the foregoing discussion has referred
to PVC suspension polymerization, similar procedures might be employed in
conjunction with emulsion polymerization of PVC, though special control
might be required in view of the greater heat sensitivity of PVC paste
resin, and the potential mechanical instability of the latex itself.
In view of the foregoing, it will be clear that a wide variety of changes
can be made without departing from the scope of the invention herein
disclosed, and it is our intention to be limited by the appended claims.
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