U.S. patent application number 09/891318 was filed with the patent office on 2002-02-14 for process optimization of peroxide injection during polyethylene production.
This patent application is currently assigned to BASF Aktiengesellschaft. Invention is credited to Deckers, Andreas, Groos, Georg, Hammer, Klaus, Stenger, Michael, Weber, Wilhelm.
Application Number | 20020019502 09/891318 |
Document ID | / |
Family ID | 7646348 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020019502 |
Kind Code |
A1 |
Hammer, Klaus ; et
al. |
February 14, 2002 |
Process optimization of peroxide injection during polyethylene
production
Abstract
In a process for the high-pressure polymerization of ethylene in
a high-pressure autoclave or in a high-pressure tube reactor having
one or more metering points for one or more peroxides, no peroxide
or a reduced amount of peroxide is metered in during the time
interval in which the pressure is lowered by opening a pressure
maintenance valve and the original amount of peroxide per unit time
is metered in again as soon as the pressure after closure of the
pressure maintenance valve has built up again so that it exceeds
90% of the pressure difference between nominal pressure and lowered
pressure.
Inventors: |
Hammer, Klaus; (Mutterstadt,
DE) ; Groos, Georg; (Dannstadt-Schauernheim, DE)
; Stenger, Michael; (Munchen, DE) ; Weber,
Wilhelm; (Neustadt, DE) ; Deckers, Andreas;
(Flomborn, DE) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
BASF Aktiengesellschaft
Ludwigshafen
DE
67056
|
Family ID: |
7646348 |
Appl. No.: |
09/891318 |
Filed: |
June 27, 2001 |
Current U.S.
Class: |
526/61 ; 526/227;
526/352; 526/64 |
Current CPC
Class: |
C08F 10/02 20130101;
C08F 10/02 20130101; C08F 2/002 20130101; C08F 2/01 20130101; C08F
2/00 20130101; C08F 10/02 20130101; C08F 10/02 20130101 |
Class at
Publication: |
526/61 ; 526/64;
526/227; 526/352 |
International
Class: |
C08F 002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 2000 |
DE |
10030306.4 |
Claims
We claim:
1. A process for the high-pressure polymerization of ethylene in a
high-pressure autoclave or in a high-pressure tube reactor having
one or more metering points for one or more peroxides, wherein no
peroxide or a reduced amount of peroxide is metered in during the
time interval in which the pressure is lowered by opening a
pressure maintenance valve and the original amount of peroxide per
unit time is metered in again as soon as the pressure after closure
of the pressure maintenance valve has built up again so that it
exceeds 90% of the pressure difference between nominal pressure and
lowered pressure.
2. A process as claimed in claim 1, wherein the amount of peroxide
metered in during the pressure-reduction procedure is reduced by at
least 15% of the original amount.
3. A process as claimed in claim 1, wherein the amount of peroxide
metered in during the pressure-reduction procedure is reduced by
from 15 to 90% of the original amount.
4. A process as claimed in claim 1, wherein the amount of peroxide
metered in during the pressure-reduction procedure is reduced by
from 30 to 40% of the original amount.
5. A process as claimed in claim 1, wherein the amount of peroxide
metered in during the pressure-reduction procedure is reduced by
from 45 to 55% of the original amount.
6. A process as claimed in claim 1, wherein the amount of peroxide
metered in during the pressure-reduction procedure is reduced by
from 60 to 70% of the original amount.
7. A process as claimed in claim 1, wherein the amount of peroxide
metered in during the pressure-reduction procedure is reduced by
from 70 to 80% of the original amount.
Description
[0001] The present invention relates to a process for preparing
polyethylene, in particular low density polyethylene (LDPE), in
which multiple injection of peroxide takes place. Specifically, the
present invention relates to a process for the high-pressure
polymerization of ethylene in a high-pressure autoclave or in a
high-pressure tube reactor having one or more metering points for
one or more peroxides, wherein no peroxide or a reduced amount of
peroxide is metered in during the time interval in which the
pressure is lowered by opening a pressure maintenance valve and the
original amount of peroxide per unit time is metered in again as
soon as the pressure after closure of the pressure maintenance
valve has built up again so that it exceeds 90% of the pressure
difference between nominal pressure and lowered pressure.
[0002] The synthesis of polyethylene, for example LDPE, is carried
out industrially either in continuous stirring autoclaves or in
tube reactors. In both cases, the process employs different
pressure stages. A low-pressure compressor compresses the fresh gas
(ethene) provided with catalyst to from 150 to 300 bar. It is mixed
there with the unreacted circulating gas of the high-pressure
circuit and brought by means of the high-pressure compressor to
operating pressure (generally from 1 400 to 3 500 bar at from 130
to 270.degree. C.). Both the tube reactor and the stirring
autoclave can be operated with multiple injection of initiator
(peroxide or oxygen). This allows control of the reaction and
correspondingly better heat removal.
[0003] Peroxidic initiators used are, for example:
[0004] didecanoyl peroxide,
2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexa- ne, tert-amyl
peroxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butyl
peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate,
tert-butyl peroxydiethylisobutyrate,
1,4-di(tert-butylperoxycarbo)cyclohexane as an isomer mixture,
tert-butyl perisononanoate, 1,1-di(tert-butylperoxy)-3,3,-
5-trimethylcyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, methyl
isobutyl ketone peroxide, tert-butyl peroxyisopropylcarbonate,
2,2-di-tert-butylperoxy)butane or tert-butylperoxyacetate, also
tert-butyl peroxybenzoate, di-tert-amyl peroxide, dicumyl peroxide,
the isomeric di(tert-butylperoxyisopropyl)benzenes,
2,5-dimethyl-2,5-di-tert-- butylperoxyhexane, tert-butylcumyl
peroxide, 2,5-dimethyl-2,5-di(tert-buty- lperoxy)hex-3-yne,
di-tert-butyl peroxide, 1,3-diisopropyl monohydroperoxide, cumene
hydroperoxide or tert-butyl hydroperoxide.
[0005] In the case of a tube reactor, heat removal is greatly
inhibited by a laminar boundary layer having a high polymer content
at the wall. This is removed, for example, by briefly opening the
pressure maintenance valve at the reactor outlet further at
intervals of from 10 seconds to a few hours and/or varying the hot
water temperature; the boundary layer is then detached and carried
out by the turbulence of the outflowing gas. To handle ethene
safely, it has to be remembered that ethene decomposes with
explosion-like rapidity under certain temperature and pressure
conditions to form carbon, methane and hydrogen. This undesirable
reaction can occur particularly in the case of the high-pressure
polymerization of ethene. The drastic pressure and temperature
increase which follows such an event represents a considerable
hazard potential for the operational safety of the production
plant. Even when the pressure relief devices are actuated in good
time, the decomposition leads, owing to carbon formation, to
considerable production disruption and additional costs for
cleaning the plant and restoring it to an operational state.
Furthermore, the emission of particulate carbon leads to pollution
of the environment.
[0006] The reaction conditions under which decomposition of ethene
is to be expected may be found in the literature (see, for example,
Chem.-Ing.-Tech. 67 (1995), pages 862 to 864).
[0007] It is an object of the invention to provide a process for
preparing ethylene polymers in a tube reactor or autoclaves which,
by avoiding or removing relatively severe deposits, ensures good
heat transport and also drastically reduces the risk of spontaneous
decomposition of ethene. It is a further object of the invention to
increase the conversion by increasing the amount of peroxide
metered in per unit time and unit weight of starting material.
[0008] According to the present invention, the solution to these
objects starts out from a process for the high-pressure
polymerization of ethene in a tube reactor with multiple injection
of peroxide at various peroxide addition points along the main flow
direction of ethene. This amount of peroxide metered in per unit
time will hereinafter also be referred to as the original amount.
The reaction pressure is temporarily lowered (pressure reduction)
at approximately constant time intervals. In the process of the
present invention, the amount of peroxide introduced per unit time
into the tube reactor at the respective peroxide addition point is
reduced during the time interval between the point in time at which
the temporary pressure decrease produced by a pressure-reduction
procedure goes through a pressure minimum and the point in time at
which a pressure corresponding to the pressure minimum of the
respective pressure-reduction procedure plus 90% of the pressure
drop amplitude of the respective pressure-reduction procedure is
reached. This is achieved according to the present invention by
metering in no peroxide or a reduced amount of peroxide during the
time interval in which the pressure is lowered by opening a
pressure maintenance valve and once again metering in the original
amount of peroxide per unit time as soon as the pressure after
closure of the pressure maintenance valve has built up again so
that it exceeds 90% of the pressure difference between nominal
pressure and lowered pressure. The amount of peroxide metered in
during the pressure-reduction procedure is preferably reduced by at
least 15% of the original amount. The amount of peroxide metered in
during the pressure-reduction procedure is particularly preferably
reduced by from 15 to 90%. The amount of peroxide metered in during
the pressure-reduction procedure is very particularly preferably
reduced by from 30 to 40% or by from 45 to 55% or by from 60 to 70%
or by from 70 to 80%. Here, the percentages are in each case by
weight.
[0009] Some of the terms used in the achievement of the object of
the invention will be explained below.
[0010] In the known processes, the reaction pressure is lowered
temporarily at approximately constant time intervals. This is done
by temporarily opening the pressure maintenance valve, as a result
of which the pressure firstly drops and then, after reactivation of
the pressure control circuit, approaches the set value again. Such
a pressure transient has a relative pressure minimum in the plot of
pressure versus time. The pressure drop until the relative pressure
minimum is reached is relatively steep, while the pressure rise
after going through the relative pressure minimum is firstly steep
then increasingly flat. For the purposes of the invention, the time
interval between the point in time at which the temporary pressure
decrease produced by a pressure-reduction procedure goes through a
pressure minimum and the point in time at which a pressure
corresponding to the relative pressure minimum of the respective
pressure-reduction procedure plus 90% of the pressure drop
amplitude of the respective pressure-reduction procedure is reached
is of particular importance. For the purposes of the present
invention, pressure drop amplitude is the difference between the
set value of the pressure control circuit and the relative pressure
minimum of the temporary pressure decrease produced by a
pressure-reduction procedure, i.e. the pressure transient.
[0011] It goes without saying that the pressure is not only a
function of time but also a function of the reactor coordinates.
All statements made with regard to pressure in the process of the
present invention therefore relate to the pressure prevailing in
the reactor tube at the level of the respective peroxide addition
point.
[0012] For the purposes of the present invention, a peroxide
addition point is the reactor coordinates at which peroxide is
introduced into the reactor (in the course of multiple injection of
peroxide). This applies individually to each peroxide addition
point. Since the pressure is also a function of the reactor
coordinates, the lowering of the amount of peroxide metered in per
unit time generally has to be calculated and controlled
individually for each peroxide addition point.
[0013] The percentage decrease in the amount of peroxide metered in
per unit time is based on the amount of peroxide per unit time
metered into the reactor at the respective peroxide addition point
during all other operating phases, in particular when no
pressure-reduction procedure is being carried out.
[0014] It has been observed that the pressure rise to the set value
following the pressure decrease in conventional processes and the
associated slowing of mass flow strongly favor the decomposition of
ethene. It has also been observed that local decomposition
processes generally occur from 5 to 6 seconds after the pressure
minimum occurring as a result of pressure reduction.
[0015] According to the present invention, during the critical time
period during which the pressure rises to the set value
[0016] the polymerization in the vicinity of the addition point is
retarded and
[0017] the local peroxide concentration which has been increased as
a result of the decreased mass flow and the likewise decreased
convection is decreased
[0018] by lowering the amount of peroxide introduced per unit time
into the respective peroxide addition point, preferably by at least
15% of the amount of peroxide metered in per unit time during other
phases of operation.
[0019] This measure increases reactor safety without having to omit
the pressure-reduction procedure. Alternatively, the amount of
peroxide per amount of ethene and thus indirectly the conversion
could be increased in this way while retaining the same level of
reactor safety.
[0020] In a preferred embodiment of the process of the present
invention, the amount of peroxide introduced is decreased by at
least 15%, particularly preferably by from 15 to 90%, very
particularly preferably by from 30 to 40% or by from 45 to 55% or
by from 60 to 70% or by from 70 to 80%.
[0021] The polyethylenes obtainable by means of the process of the
present invention have a density of from 0.900 to 0.955 g/cm.sup.3,
preferably from 0.910 to 0.945 g/cm.sup.3 and particularly
preferably from 0.915 to 0.940 g/cm.sup.3, measured at 23.degree.
C. Polyethylene waxes having a molecular weight M.sub.n of not more
than 20 000 g, preferably not more than 10 000 g and particularly
preferably not more than 7 500 g, can be readily prepared by the
process of the present invention. The molecular weight distribution
is in the range from 2 to 10. The melting points are in the range
from 60 to 125.degree. C., preferably from 80 to 120.degree. C.
However, the process of the present invention is also well suited
to the preparation of relatively high molecular weight LDPE having
a molecular weight M.sub.n of more than 20 000 g, preferably more
than 80 000 g and particularly preferably more than 120 000 g. The
molecular weight distribution of the relatively high molecular
weight LDPE prepared by the process of the present invention is in
the range from 2 to 20.
[0022] The process of the present invention will now be illustrated
with the aid of the drawing below. In the drawing,
[0023] FIG. 1 schematically shows an apparatus for carrying out the
process of the present invention,
[0024] FIG. 2 shows the relationship between reactor pressure (P)
and amount of peroxide (n) fed in per unit time (t), in each case
as a function of time (t).
[0025] In FIG. 1, 1 denotes a high-pressure gas compressor which
compresses ethene, with or without addition of oxygen, to the
operating pressure. Here, the compressor 1 together with the
pressure maintenance valve 2 form a control circuit 4 which is
controlled by a control computer 3. Along the tube reactor, i.e.
along the main flow direction of ethene, there are provided various
peroxide addition points 9, 10, 11 at which metering pumps 6, 7, 8
feed peroxide into the tube reactor. The metering pumps 6, 7, 8
together with the pressure maintenance value 2 form a control
circuit 5 which is likewise controlled from a control computer 3.
In addition, the control computer 3 is connected to the pressure
meter 12 which allows measurement of the pressure as a function of
time. The local pressure at the peroxide addition points 9, 10, 11
is calculated by mathematical modeling of the pressure as a
function of time and taking into account the material properties of
the polymer (density, viscosity) and the amount of peroxide to be
fed in as a function of time is determined therefrom.
[0026] FIG. 2 shows a temporary pressure decrease, namely a
pressure transient, resulting from a pressure-reduction procedure.
The pressure-reduction procedure is commenced at the time t.sub.1
by opening the pressure maintenance valve. The relative pressure
minimum is reached at the time t.sub.2 immediately before
reactivation of the pressure control circuit and the associated at
least partial closure of the pressure maintenance valve. The time
interval between passage through the relative pressure minimum at
the time t.sub.2 and the time t.sub.3 at which a pressure
corresponding to the pressure minimum plus 90% of the pressure drop
amplitude of the respective pressure-reduction procedure (AP) is
reached is denoted by .DELTA.t. During this time interval, the
amount of peroxide fed in per unit time 1 ( n t )
[0027] is reduced by at least 15%.
* * * * *