Note: Descriptions are shown in the official language in which they were submitted.
2~
A Process for Separating Volatile Components
from Reaction Mixtures Obtained through High~pressure
Polymerization
The invention relates to a process for separating
volatile components from polymer melts which are obtained
from the polymerization of ethylene and the
copolymerization of ethylene with other copolymerizable
compounds under high pressure.
BAC~GROUND OF THE INVENTION
The radical high~pressure polymerization of
ethylene alone or together with comonomers is performed on
a commercial scale in tubular reactors or in autoclaves
with agitation at pressures of 50 to 400 MPa, in
particular 140 to 250 MPa, and temperatures of 100 to
400C.
A diagram of the process is shown in Figure 1.
Fresh ethylene is first fed into low~pressure compressor 8
via line 9, compressed to 10 to 35 MPa and then, together
with the unreacted ethylene from the polymerization stage
and which is recycled, it is raised to the reaction
pressure in a high~pressure compressor 5. With the aid of
suitable pumping equipment, e.g, piston pumps, the
initiators present in solution are mixed into the
compressed ethylene or metered into the reactor directly.
2 20~850
Comonomers such as vinyl esters, olefinically
unsaturated carboxylic acids, or alpha~olefins are also
fed into the high~pressure compressor 5 via feed line 10.
In reactor 1 between 10 and 45 % by weight of the monomers
entering the reactor are reacted in one run. The polymer
is precipitated in high-pressure separator 3 by reducing
the pressure on the mixture to 5 to 50 MPa, preferably 10
to 35 MPa, after optional cooling in cooler 2. During
pressure~relief, the gaseous components released are
returned to the high~pressure compressor 5 via
high~pressure return gas system 4. The polymer, which
contains dissolved unreacted monomers, is
pressure~relieved via a system consisting of low~pressure
separator 6 and low~pressure cooler 7. The low-molecular
weight compounds released are returned via the
low~pressure compressor g to the high~pressure compressor
5. The polymer precipitated in the low-pressure separator
6 contains not only residual amounts of ethylene and
comonomers but also oligomers.
Even when the polymerization products are
separated carefully, it is impossible to avoid volatile
admixtures such as comonomers, their decomposition
products, and other low~molecular weight compounds being
retained in the polymers. During storage and processing
of the polymers, these residues lead to undesirable
emissions and odors; in addition, the flame point of the
polymers is considerably reduced. Therefore, efforts are
made to insure that the products are free of volatile
-- 201 0850
compounds by taldng suitable measures during the plessule-relieving process and by
subjecting the polymer to an additional tre~tm~-nt
Further difficulties are caused by the fact that the ethylene liberated in the
low-ple~ulc sepal~or contains comonomers, oligomers, and other compounds which
5 condense out when the sepal~ed ethylene is recycled, forming deposits in the
colllplcssors and pumps which can lead to clogging.
According to a process described in DD-PS 131 824, issued July 26, 1978,
either the reaction ~ lure is brought into contact intensively with the ethylene
downstream of the polymP.ri7~tion reactor or the polymer melt is brought into contact
10 intensively with the ethylene after separation of the majority of the unreacted gas
llli~lurc in a pressure stage and the llli~lure is then sep~ t-0A in a stabilizing zone.
According to an al)pr~liate embodiment of this process, the reaction l~ lurc is fed
c~unlel-;ullelllly into a widening tube, ethylene also being ~im~llt~n~ously introduced
and the Illi~lurc thus formed is separated in the low-pressure sepal~tor.
Another procedure for high-plcs~ulc polymeri7~tion of ethylene is
disclosed in the DE-AS 21 31 145, issued January 14, 1982. It mixes the reaction
Illi~.lulc coming from the reaction zone with the fresh ethylene feed and the recycled
ethylene coming from the low-pres~.lre sep~tor.
JJ: ~l . 3
A~
``_ 20 1 0850
Both processes lead only to a slight reduction in the concentration of low-
molecular weight ad~ Lures in the polymer. Moreover, additional appa,~tus is
required for their pt;lÇol",al~ce.
It is also known that unreacted gas origin~ting from the low-pres~ure
S sep~lor can be recycled to the high-plt;s~u~ cycle with the aid of an injector (cf. DD-
PS 202 882, issued October 5, 1983). However, this process does not reduce the
concentration of the low-molecular components re~ inillg in the product.
With liquid products, the low-molecular weight compounds can be
sepal~led by gas scrubbing with nitrogen or other inert gaseous substances. However,
10 additional appa-~lus is also required for this process and the sep~r~ted co",~onents
cannot be returned without çl~ning.
BRIEF DESCRIPIION OF THE INVENTION
Therefore, the problem was to develop a process which avoids the
disadvantages described above, guarantees nearly complete removal of the volatile
15 co,.,~nents from the polymer melt, and insures that little or no condensed conll?ollents
are contained in the cycle gas of the low-pressure stage.
JJ: 4
The invention consists in a process for separating
volatile components from polymer melts formed during the
homopolymerization of ethylene or the copolymerization of
ethylene with other copolymerizable compounds at elevated
pressures and elevated temperatures. It is characterized
in that ethylene is passed in countercurrent flow through
the melt at pressures of 5 to 70 MPa and a temperature in
the melt of at least approximately 120C.
Surprisingly, with the new process it is possible
to almost completely remove the volatile components
contained in the polymer melt formed during high-pressure
polymerization. Further cleaning, e.g. in the extruder or
during storage of the product, is not necessary. The
concentration of the low~molecular components in the
recycled ethylene is reduced until condensation products
do not form during the compression of the ethylene and
product losses are avoided.
The claimed procedure can be performed with
different polymerizations and copolymerizations o~
ethylene using the high-pressure process, in both tubular
reactors and autoclaves. It can be used both with solvent
and solvent~free processes, and also with processes for
the preparation of linear low~density polyethylenes
(LLDPE).
8S0
Volatile components contained in the polymer melt
are understood to be low~molecular weight compounds which
are contained in the polymerization products, e.g. as
comonomers, decomposition products of comonomers, and
oligomers. Thus, for example, in the preparation of
copolymers of ethylene and vinyl acetate, acetic acid
formed by the decomposition of vinyl acetate is to be
found in the products. Furthermore, volatile compounds
include solvents which, for example, enter the reaction
mixture with initiators and molecular mass regulators.
Lubricants and other auxiliaries, which are required to
operate the polymerization plant, also are present in the
polymer melt.
According to the claimed process, the polymer melt
is treated at elevated pressure, preferably at 5 to 70 ~Pa
and in particular at 10 to 40 MPa. It is not necessary to
provide a separate reactor for treatment; it can be
successfully performed in the polymer separator of a
high~pressure polymerization plant. It is expedient to
remove most of the unreacted gas mixture by reducing the
pressure to the above~mentioned range and then bringing
the melt into contact with the ethylene. The
high~pressure separator used in the polymerization process
is particularly suitable for this purpose.
~ 2010850
To insure that the volatile components are
separated either completely or nearly completely from the
melt, it is necessary to maintain a temperature of at
least 120C therein. It has proved particularly
valuable to treat the ethylene at temperatures above
approximately 150C. The higher the temperature, the
more completely the volatile substances are separated.
The only upper limit for the treatment temperature is the
temperature at which the melt from the polymerization
enters the chamber where the volatile substances are
separated.
In order to guarantee as high a temperature as
possible in the melt, it is expedient to pass heated
ethylene in countercurrent flow to the melt. It is
important to the claimed procedure that the ethylene and
melt flow in opposing directions when they meet each
other. This insures an extremely intimate mixture of melt
and olefin.
The effectiveness of the ethylene stream can be
further increased by forcefully diverting it from a
straight path in the chamber where the melt is treated.
In this manner, fresh ethylene is passed several times
through the precipitated product. The ethylene is
diverted by means of suitable installations in the
reactor, in the simplest case by plates which are located
on opposite sides thereof and in staggered formation.
- 8- 201~850
The amount of ethylene required for treating the
polymer melt depends on the temperature, the viscosity,
and the comonomer content of the melt. The required
amount of ethylene increases with the viscosity and the
comonomer content and decreases with increasing
temperature. In general, 0.1 to 2 parts by weight of
ethylene are used per one part by weight of melt.
Pure ethylene can be used to separate the volatile
components from the melt. It is compressed in the
low~pressure compressor and fed into the polymer melt in
the high~pressure separator. The ethylene charged with
the volatile components of the melt is passed through the
high~pressure gas system and then, after separation of the
volatile components, is fed via a partial stream to the
inlet side of the high pressure compressor and on into the
polymerization reactor.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, constituting a part
hereof and in which like reference characters indicate
like parts,
Figure l is a flow diagram of a prior art
process;
~ 9 _ 20~C~8S0
Figure 2 is a flow diagram of the process of
the present invention; and
Figure 3 is a schematic view of the high
pressure separator of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
A diagram of the claimed process is shown in
Figure 2. Fresh ethylene from pipe 19 is compressed in
low~pressure compressor 18 to 10 to 35 MPa, heated in
heating device 22 to at least 120C and passed through
line 21 into high~pressure separator 13. There it meets
the polymer flowing in the countercurrent direction.
Charged with volatile, low~molecular weight
components of the polymer melt, the ethylene enters a
high~pressure gas recycling system. The low molecular
components such as solvent, lubrication oil, and oligomers
are separated by means of pressure-relieving device 23 and
line 24. The ethylene, optionally containing comonomers
which can be replenished via line 20, is fed to
high~pressure compressor 15 and then to reactor 11. The
polymer is separated at a pressure of 5 to 70 MPa in
high~pressure separator 13, after it has been cooled in
cooler 12, if necessary. The gaseous parts enter
high~pressure gas recycling system 14 together with the
fresh ethylene passed in the countercurrent direction.
201(~850
--10--
The polymer is pressure~relieved via low-pressure
separator 16 and low-pressure cooler 17; the liberated
low-molecular weight compounds are compressed in the
low~pressure compressor together with fresh ethylene from
line 19.
Figure 3 shows separator 13 with baffles 25 where
the polymer is treated with ethylene. The melt enters
into the separator through line 27 and the ethylene
through line 21. The melt leaves the separator through
line 29 and the ethylene charged with volatile components
from the melt through line 30.
The new process is explained in the following
examples which are intended to be illustrative, not
limitative.
Example 1
A reaction mixture obtained from the
polymerization of 720 parts by weight of ethylene and 280
parts by weight of vinyl acetate, and which has a
percentage of 32.9 % by weight of polymer, is fed into a
high~pressure separator without baffles. The polymer is
precipitated and can be drawn off via a bottom valve.
Countercurrent to the polymer, 250 parts of ethylene are
introduced and withdrawn from the head together with the
low-molecular weight components of the melt. The treated
polymer contains 0.62 % by weight of monomeric vinyl
acetate, and the vinyl acetate concentration of the
low-pressure gas cycle is 12.5 % by volume.
20~0850
Example 2 (Comparison)
The same procedure as in Example 1 is carried out,
except that the ethylene is not fed into the high~pressure
separator but is passed directly to the high-pressure
compressor in known manner. The polymer contains 2 % by
weight of monomeric vinyl acetate, and the vinyl acetate
concentration in the low~pressure gas cycle is more than
40 ~ by volume.
Example 3
A reaction mixture obtained from the
polymerization of 720 parts by weight of ethylene and 2~0
parts by weight of vinyl acetate, and which has a
percentage of 33 % by weight of polymer, is fed into a
high~pressure separator with baffles according to Figure
3. The polymer is precipitated and can be drawn off via
the bottom valve.
Countercurrent to the polymer, 260 parts of
ethylene are introduced and withdrawn from the head
together with low~molecular weight components of the
melt. The treated polymer contains 0.33 % by weight of
monomeric vinyl acetate, and the vinyl acetate
concentration in the low~pressure gas cycle is 4.4 ~ by
volume.
~ _ 20iO850
Example 4 (Comparison)
The procedure of Example 3 is followed except that
the ethylene is not fed into the high-pressure separator,
but is passed directly to the high~pressure compressor in
known manner. The polymer contains 1.1 % by weight of
vinyl acetate, and the vinyl acetate concentration in the
low~pressure gas cycle is 33 % by volume.
The viscosity of the polymers prepared according
to Examples 1 to 4 is the same and is 250 mPa x sec at
140C.
