U.S. patent application number 09/976525 was filed with the patent office on 2003-04-17 for recovery of olefin monomers.
Invention is credited to Golden, Timothy Christopher, Johnson, Charles Henry, Weist, Edward Landis JR..
Application Number | 20030073788 09/976525 |
Document ID | / |
Family ID | 25524181 |
Filed Date | 2003-04-17 |
United States Patent
Application |
20030073788 |
Kind Code |
A1 |
Golden, Timothy Christopher ;
et al. |
April 17, 2003 |
RECOVERY OF OLEFIN MONOMERS
Abstract
In a process for the production of a polyolefin, an olefin
monomer is polymerised said polyolefin and residual monomer is
recovered. A gas stream comprising the monomer and nitrogen is
subjected to a PSA process in which said monomer is adsorbed on a
periodically regenerated silica gel or alumina adsorbent to recover
a purified gas stream containing said olefin and a nitrogen rich
stream containing no less than 99% nitrogen and containing no less
than 50% of the nitrogen content of the gas feed to the PSA
process.
Inventors: |
Golden, Timothy Christopher;
(Allentown, PA) ; Weist, Edward Landis JR.;
(Macungie, PA) ; Johnson, Charles Henry; (Coplay,
PA) |
Correspondence
Address: |
AIR PRODUCTS AND CHEMICALS, INC.
PATENT DEPARTMENT
7201 HAMILTON BOULEVARD
ALLENTOWN
PA
181951501
|
Family ID: |
25524181 |
Appl. No.: |
09/976525 |
Filed: |
October 12, 2001 |
Current U.S.
Class: |
526/72 ; 422/131;
422/132; 528/482 |
Current CPC
Class: |
C08F 6/001 20130101;
C08F 6/005 20130101; C08F 6/001 20130101; B01D 2253/106 20130101;
F25J 3/062 20130101; F25J 2205/60 20130101; F25J 2210/42 20130101;
F25J 2245/02 20130101; B01D 2256/24 20130101; F25J 2215/62
20130101; F25J 2205/40 20130101; F25J 2210/04 20130101; B01D
2253/306 20130101; F25J 3/064 20130101; B01D 2253/104 20130101;
F25J 3/066 20130101; F25J 2270/90 20130101; C08L 23/00 20130101;
C08L 23/00 20130101; B01D 53/047 20130101; B01D 2253/308 20130101;
C08F 6/005 20130101; F25J 2210/12 20130101 |
Class at
Publication: |
526/72 ; 528/482;
422/132; 422/131 |
International
Class: |
C08F 002/00 |
Claims
1. A process for the production of a polyolefin in which an olefin
monomer is polymerised to produce said polyolefin and residual
monomer is recovered, said process comprising subjecting a gas
stream comprising said monomer and nitrogen to a PSA process in
which said monomer is adsorbed on a periodically regenerated solid
adsorbent to recover a purified gas stream containing said olefin
and a nitrogen rich stream containing no less than 99% nitrogen and
containing no less than 50% of the nitrogen content of the gas feed
to the PSA process.
2. A process as claimed in claim 1, wherein the adsorbent is silica
gel.
3. A process as claimed in claim 1, wherein the adsorbent is
alumina having a surface area of no more than 900 m.sup.2/g.
4. A process as claimed in claim 1, wherein the adsorbent is
alumina having a surface area of no more than 800 m.sup.2/g.
5. A process as claimed in claim 1, wherein the adsorbent is
alumina having a surface area of no more than 600 m.sup.2/g.
6. A process as claimed in claim 1, wherein the adsorbent is
alumina having an average pore diameter of at least 1.7 nm.
7. A process as claimed in claim 1, wherein the adsorbent is
alumina having an average pore diameter of at least 2.0 nm.
8. A process as claimed in claim 1, wherein the recovery of
C.sub.2+ hydrocarbons from the monomer and nitrogen containing gas
stream is no less than 99%.
9. A process as claimed in claim 1, wherein at each stage of the
PSA process, breakthrough of C.sub.2+ hydrocarbons from the
adsorbent is avoided.
10. A process as claimed in claim 1, wherein the ratio of ethylene
in the monomer and nitrogen gas stream to the ethylene in the
nitrogen rich stream is at least 500.
11. A process as claimed in claim 1, wherein the ratio of ethane in
the monomer and nitrogen gas stream to the ethane in the nitrogen
rich stream is at least 50.
12. Apparatus for use in the polymerisation of olefin comprising a
polymerisation unit having an inlet for olefin monomer and an
outlet for polymer connected to a polymer degassing unit having an
inlet for nitrogen purge gas and an outlet for a nitrogen/monomer
mixture connected to an olefin recovery unit comprising a
compressor feeding said nitrogen/monomer mixture in a compressed
state to an inlet of a liquid olefin separator having outlets for
liquid olefin and for an olefin depleted nitrogen/olefin mixture,
said outlet for liquid olefin being connected to recycle the olefin
to the polymerisation unit, and a PSA gas separation unit connected
to receive said olefin depleted nitrogen/olefin mixture and to
produce therefrom a purified nitrogen stream containing no less
than 99% nitrogen and a recovered olefin stream, said PSA unit
being connected to return said recovered olefin stream to the inlet
of said liquid olefin separator.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to the recovery of olefin
monomer from a nitrogen purged degassing step in the manufacture of
polyolefin.
[0004] Following the polymerisation of olefin such as ethylene or
propylene to polyethylene or polypropylene, the polymer contains
residual olefin monomer. This is conventionally removed in a
degassing step using nitrogen to strip the monomer from the polymer
material. The nitrogen purge gas, now contaminated with olefin
monomer and other reaction by products or monomer impurities such
as ethane and isobutylene and methane, may be flared off or burned
with fuel.
[0005] U.S. Pat. No. 4,498,910 (Benkmann) discloses a process in
which ethylene is reacted to produce ethylene oxide and a purge gas
is withdrawn that contains residual ethylene together with a large
amount of methane and a smaller proportion of other gases which
include oxygen and `inert gases`, which may be argon and nitrogen.
The ethylene is recovered by pressure swing adsorption (PSA) using
a silica gel adsorbent. The reported recovery of ethylene is about
97%, but the recovery of ethane is only about 62%. There are
significant levels of both ethane and ethylene in the residual gas
from the PSA. Indeed, most of the ethane appears here and not in
the ethylene rich stream.
[0006] U.S. Pat. No. 5,245,099 (Mitariten) discloses the recovery
by PSA over silica gel of ethylene and heavier components from a
hydrocarbon stream which also contains a minor amount of nitrogen.
The light cut which is the residual gas from the PSA contains
percent levels both of ethylene and of ethane and very large
amounts of methane, which like the ethane concentrates there.
[0007] U.S. Pat. No. 4,849,537 (Ramachandran) discloses a process
for making acrylonitrile from propane by dehydrogenation of the
propane to propylene followed by aminoxidation using oxygen
enriched air leading to a product stream containing the
acrylonitrile from which acrylonitrile is removed as a liquid by
quenching. This leaves an off-gas containing propylene which is
recovered and recycled by subjecting the off-gas to PSA over silica
gel. Once again, the residual gas from the PSA, or light stream,
contains percent levels of hydrocarbons, with methane, ethane and
ethylene concentrating there.
[0008] U.S. Pat. No. 4,781,896 (Wilmore) discloses a polyolefin
manufacturing process in which olefin monomer is recovered by
de-gassing polyolefin product and is recycled. The recovery is done
without the supply of a purge gas and it is remarked that the use
of a nitrogen purge stream makes it difficult and expensive to
recover the olefin.
[0009] U.S. Pat. No. 5,769,927 (Gottschlich) recovers olefin from
such a nitrogen purge gas for recycle to the polymerisation by a
process of condensation, flash evaporation and membrane
separation.
[0010] There is a need for an economic method for recovering olefin
monomer from olefin polymer using a nitrogen purge gas and
recovering the nitrogen without significant olefin content whilst
maintaining a sufficient degree of recovery of the nitrogen.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention now provides a process for the
production of a polyolefin in which an olefin monomer is
polymerised to produce said polyolefin and residual monomer is
recovered, said process comprising subjecting a gas stream
comprising said monomer and nitrogen to a PSA process in which said
monomer is adsorbed on a periodically regenerated solid adsorbent
to recover a purified gas stream containing said olefin and a
nitrogen rich stream containing no less than 99% nitrogen and
containing no less than 50% of the nitrogen content of the gas feed
to the PSA process.
[0012] The problems of recovery of the olefin from the nitrogen
remarked on in Wilmore are overcome by appropriate choice of
adsorbent and process conditions and a nitrogen stream is obtained
of sufficient purity for reuse or for use elsewhere in the plant,
for instance for the pneumatic movement of solids. This is in
contrast to the results obtained in other olefin recycling
proposals such as those of Benkmann and Ramachandran. In Benkmann a
significant amount of ethylene and most of the ethane find their
way into the light stream, i.e. the residual gas from the PSA,
which in addition to nitrogen contains substantial amounts of
oxygen. In Ramachandran, the PSA waste stream contains percent
levels of C.sub.2+ hydrocarbons (i.e. ethane, ethylene and heavier
hydrocarbons). Furthermore, the ethane and ethylene, present only
in small amounts in this system, actually concentrate in the light
stream, i.e. the residual gas from the PSA.
[0013] The term "PSA" is used herein to include processes in which
regeneration is accomplished essentially by pressure reduction
rather than by heat addition and specifically includes VSA (vacuum
swing adsorption) processes.
[0014] The adsorbent may be silica gel but is preferably alumina of
suitable pore size and surface area, on the grounds of price and
performance. Preferably then, the adsorbent is alumina having a
surface area of no more than 900 m.sup.2/g, more preferably no more
than 800 m.sup.2/g, most preferably no more than 600 m.sup.2/g.
Where silica gel is used, the surface areas preferred are as given
for alumina.
[0015] On option is to use a layered bed or a pair of beds in
series containing an upstream (with respect to the on-line feed
direction of gas flow) silica gel portion and a down-stream alumina
portion. Adsorption of C.sub.2H.sub.4 is characterised by a higher
rate of mass transfer or alumina. The linear driving force mass
transfer coefficients that match measured breakthrough curves are
0.55 and 0.88 sec.sup.-1 for alumina and silica gel respectively.
Placing the alumina at the product end of the adsorbent bed
sharpness the C.sub.2 mass transfer zone.
[0016] For both silica gel and alumina it is preferred that the
alumina has an average pore diameter of at least 1.7 nm, more
preferably at least 2.0 nm.
[0017] Preferred particle sizes for the adsorbent are 0.25 to 4
mm.
[0018] The recovery of C.sub.2+ hydrocarbons from the monomer and
nitrogen containing gas stream is preferably no less than 95%, more
preferably no less than 99%. The ratio of ethylene in the monomer
and nitrogen gas stream to the ethylene in the nitrogen rich stream
is preferably at least 500 and the ratio of ethane in the monomer
and nitrogen gas stream to the ethane in the nitrogen rich stream
is preferably at least 50.
[0019] Preferably, at each stage of the PSA process, breakthrough
of C2+ hydrocarbons from the adsorbent is avoided.
[0020] The PSA process is preferably operated using four beds,
generally as described in U.S. Pat. No. 3,430,418, but any number
of beds may be employed as known in the art. Feed temperatures are
suitably from 5 to 100.degree. C. at pressures of 1 to 20 bara. The
beds may be regenerated at lower pressures from 0.1 to 2 bara.
[0021] The invention includes apparatus for use in the
polymerisation of olefin comprising a polymerisation unit having an
inlet for olefin monomer and an outlet for polymer connected to a
polymer degassing unit having an inlet for nitrogen purge gas and
an outlet for a nitrogen/monomer mixture connected to an olefin
recovery unit comprising a compressor feeding said nitrogen/monomer
mixture in a compressed state to an inlet of a liquid olefin
separator having outlets for liquid olefin and for an olefin
depleted nitrogen/olefin mixture, said outlet for liquid olefin
being connected to recycle the olefin to the polymerisation unit,
and a PSA gas separation unit connected to receive said olefin
depleted nitrogen/olefin mixture and to produce therefrom a
purified nitrogen stream containing no less than 99% nitrogen and a
recovered olefin stream, said PSA unit being connected to return
said recovered olefin stream to the inlet of said liquid olefin
separator.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0022] The invention will be further described and illustrated with
reference to the accompanying drawing, in which:
[0023] FIG. 1 shows a schematic flow sheet of a monomer recovery
and recycle system according to the invention.
[0024] FIG. 2 is a graph of nitrogen recovery against adsorbent
surface area produced in Example 1.
[0025] FIG. 3 is a graph of nitrogen recovery against adsorbent
average pore size produced in Example 1.
DETAILED DESCRIPTION OF THE INVENTION
[0026] As shown in FIG. 1, an olefin polymerisation reactor 2
receives olefin monomer through an inlet 4 and feeds a degassing
unit 6 purged by nitrogen introduced via a line 8. Vent gas from
the nitrogen purged degassing of the polyolefin is received at an
inlet to a cooler 10, is compressed by a compressor 12, is cooled
in a heat exchanger 14 against liquefied recovered hydrocarbons in
line 16 and is dried in a drier 18. The stream is further cooled in
a heat exchanger 20 against olefin containing gas in a line 22 and
is still further cooled in heat exchanger 24 against refrigerant in
a refrigeration system 26 before passing to a liquid hydrocarbon
separator 28 from which liquid hydrocarbon exits through line 16
and is fed to heat exchanger 14 by a pump 30. Gas separated in the
separator 28 passes via line 22 and heat exchanger 20 to a PSA
separation plant 32 which produces a residual gas stream of
essentially pure nitrogen through line 34 and a recycle stream rich
in recovered olefin and other hydrocarbons through line 36 which is
fed back up stream of the compressor 12.
[0027] The PSA system is of conventional construction and has
multiple beds of adsorbent which are on-line adsorbing hydrocarbons
and are undergoing regeneration in a cyclic manner. A four bed
system is used in the following example, but other configurations
of more or fewer beds may be used as known in the art.
EXAMPLES
Example 1
[0028] A four bed PSA cycle with one pressure equalisation step as
described in U.S. Pat. No. 3,430,418, Example 1, is used. The cycle
consists of the following steps:
1 1 Adsorption 8 minutes 2 Pressure equalisation 1 minute 3
Cocurrent depressurisation 7 minutes 4 Countercurrent
depressurisation 1 minute 5 Purge 7 minutes 6 Repressurisation 8
minutes
[0029] The cocurrent depressurisation effluent gas is sent to the
effluent or light cut stream, rather than to another bed.
Repressurisation is with nitrogen rather than feed gas. The
cocurrent steps(feed, pressure reduction and purge are all stopped
prior to C.sub.2 breakthrough.
[0030] The following parameters are used:
2 1 Feed pressure 16 bara (220 psig) 2 Feed temperature 27.degree.
C. 3 Feed time 160 sec 4 Equalisation and depressurisation 85 sec 5
Purge time 75 sec 6 Purge pressure 1.33 bara (4.8 psig)
[0031] The adsorbents evaluated are as shown in the following
table:
3 TABLE 1 Adsorbent Surface area Average pore diameter activated
alumina 325 m.sup.2/g 4.0 nm silica gel 750 m.sup.2/g 2.0 nm
Activated carbon 1200 m.sup.2/g 1.2 nm
[0032] The molar compositions of the feed, effluent (low cut) and
recycle (heavy cut) steams are as follows:
4 TABLE 2 Component Feed % Effluent % Recycle % Nitrogen 49.5 99.9
27.7 Ethylene 30.3 <0.1 43.3 Ethane 5.4 <0.1 7.7 Isobutane
14.2 <0.1 20.4 Methane 0.5 <0.1 0.4
[0033] The performance of the adsorbents in terms of recovery of
nitrogen in the low cut and ethylene in the heavy cut is as
follows:
5TABLE 3 Adsorbent Surface area (m.sup.2/g) N.sub.2 recovery %
C.sub.2+ recovery % Alumina 325 65.6 99.4 Silica gel 750 61.2 99.6
Activated carbon 1200 4 99.0
[0034] Plots of the nitrogen recovery against adsorbent specific
surface area and pore size are shown in FIGS. 2 and 3
respectively.
[0035] In general, adsorbents with high selectivity are desired for
adsorption processes. The following Table shows the Henry's Law
constants (initial isotherm slope) and Henry's Law ethane/nitrogen
selectivity (ratio of Henry's Law constants) for various adsorbents
at 30.degree. C.
6TABLE 4 (nm) Average pore (mmole/g/atm) (mmole/g/atm) S
C.sub.2H.sub.6/ Adsorbent diameter K N.sub.2 K C.sub.2H.sub.6
N.sub.2 Alumina 4.0 0.019 0.30 15.8 Silica gel 2.0 0.054 0.94 17.4
Activated 1.2 0.44 29.7 67.5 carbon 13X zeolite 1.0 0.20 5.8
29.0
[0036] The results show that adsorbents with average pore diameters
less than 2.0 nm have high selectivity for ethane over nitrogen.
Unexpectedly, these high selectivity adsorbents are not preferred
for nitrogen recovery from C.sub.2+ hydrocarbon containing
streams.
[0037] Whilst the invention has been described with particular
reference to the preferred embodiments thereof, it will be
appreciated that many modifications and variations are possible
within the scope of the invention.
* * * * *