Recovery of unreacted monomers in an olefin polymerization process

A method of recovering and recycling monomers from a gas phase olefins polymerization reactor through the use of the monomer feed stream. A high pressure monomer source is reduced in pressure so as to reduce the temperature of the monomer source. The monomer source is then used to cool and condense monomers contained in various vent gas streams. The condensed monomers are then separated from the gas stream and recycled back to the reactor.

FIELD OF THE INVENTION 
The invention relates to a method for recovering unreacted monomers from 
gas streams from an olefin polymerization process. 
BACKGROUND OF THE INVENTION 
Polymers and copolymers of C.sub.2 -C.sub.10 olefins, particularly 
copolymers of ethylene and higher alpha-olefins are produced in gas phase, 
fluid bed reactors, see for example, Karol, et al. U.S. Pat. No. 
4,302,566. In the fluid bed reactor, the monomer or monomers are provided 
to a fluid bed reactor at pressures slightly above the normal operating 
pressure of the reactor, which is typically around 200-350 psig. The 
monomer or monomers are frequently compressed to a pressure above the 
reaction pressure of the polymerization reactor in order to fed monomer to 
the polymerization reactor. However in some regions of the world, monomers 
are delivered to a polymerization facility at very high pressures, 
sometimes in excess of 1,000 psig. In these instances the monomer feed 
pressure is typically reduced to a pressure 50-100 psi above reaction 
pressure through a series of expansions. Reducing the pressure of the 
monomer feedstream through a series of expansions helps to improve control 
of the monomer feedstream and also helps to prevent pressure surges within 
the reactor. 
After the monomer is fed into the reactor it reacts with a catalyst to form 
a polymer. The polymer is then discharged from the reactor. The resulting 
polymer may also contain gaseous unpolymerized hydrocarbon monomers. An 
inert gas, typically nitrogen, may also be added to the polymer product 
stream to aid in the transfer of the polymer product from the reactor and 
to reduce the risk of explosion if the hydrocarbon monomer content becomes 
excessive in the presence of oxygen. The polymer product is separated from 
the gas stream through the use well known methods and devices in the art, 
see for example U.S. Pat. No. 4,372,758. The gas stream, which still 
contains the various hydrocarbon monomers, can then be cooled to condense 
and remove as much of the monomer gases as possible. For economic 
considerations, it is desirable to recover the monomer gases and recycle 
the monomer gases to the reactor. Furthermore, environmental regulations 
require proper disposal of hydrocarbons, such as incineration or other 
suitable techniques, in order to meet emission standards. 
Cooling water is sometimes employed to recover the monomer gases. However, 
cooling water is ineffective in recovering most of the monomers because 
the monomer gases condense at temperatures lower than the temperature of 
the cooling water. Refrigeration equipment is often necessary to chill 
water, brine or other suitable media used in the condensation and recovery 
of the hydrocarbon monomer gases. However, refrigeration equipment is 
expensive to install, expensive to operate and may be a significant 
operational problem due to extensive maintenance requirements. 
Accordingly, a need exists to provide a reliable, inexpensive method for 
the recovery and recycle of condensable monomer gases from a gas phase 
fluidized bed reactor. Preferably, the cooling source should be readily 
available and inexpensive to operate. 
SUMMARY OF THE INVENTION 
The present invention provides a method which can be used to cool, condense 
and recover monomers so that they may be recycled back to the reactor 
rather than being incinerated. By recovering the monomer gases and 
recycling them back to the reactor, improved polymer product efficiencies 
are achieved. 
The invention utilizes the cooling effect the expansion of the monomer feed 
creates as a means to absorb heat. The expanded monomer feed is then 
introduced into the gas phase fluid bed reactor and reacted using methods 
known in the art. The monomer is expanded prior to being fed to the 
reactor and used in indirect heat exchange to recover monomers contained 
in various gas streams. The use of the expanded monomer feedstream may 
reduce or eliminate the need for refrigeration systems within monomer 
recovery processes. 
The method of the present invention comprises: 
(a) providing a monomer source under pressure; 
(b) reducing the pressure of said monomer source thereby reducing the 
temperature of the monomer source; 
(c) providing a source of condensable gases; 
(d) condensing at least a portion of said condensable gases through 
indirect heat exchange with the reduced pressure monomer; 
(e) separating said condensed liquids from the gas stream; 
(f) recovering said condensed liquids; and 
(g) feeding said monomer source to a gas phase fluid bed olefin reactor. 
The expansion of the monomer stream may cause a portion of the monomer 
stream to liquefy. In a preferred embodiment of the present invention, 
partially liquefied monomer is created by the expansion of the monomer 
feedstream. The partially liquefied monomer feedstream is then 
sufficiently heated through indirect heat exchange with the various 
condensable gas-containing streams so as to vaporize all of the monomer in 
the feedstream. Vaporizing all of the monomer feed stream is preferable, 
inasmuch as it maximizes the cooling effect of the expansion, by utilizing 
the entire heat of vaporization of the liquid, while also allowing the 
monomer feed stream to be directly fed to the gas-phase reactor without 
having to heat the monomer.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention can be used in conjunction with a process for 
polymerization or copolymerization of C.sub.2 -C.sub.10 olefins. In the 
preferred embodiments, the invention is used in the polymerization of 
ethylene and propylene or the copolymerization of ethylene and propylene 
with higher olefins. 
The operation of a prior art gas phase, fluid bed polymerization process is 
presented in FIG. 1. The present invention is schematically presented in 
FIGS. 2a and 2b. 
Referring to FIG. 1, monomer 3 and catalyst 4 are provided to a gas-phase 
fluidized bed reactor 1. A recycle stream 2 is also present to recycle 
unreacted reactor components back to the reactor. Polymer and some reactor 
gases are withdrawn from the reactor through valve 5 and conducted to a 
first product chamber 6, through valve 7 and into a product blow tank 8. 
The reactor contents pass through valve 9 and are conveyed through conduit 
10 to a product separator 11. Within the product separator the solids from 
the reactor, i.e. the polymer, are separated from the reactor gases. The 
polymer is purged with an inert gas, typically nitrogen, fed through line 
12 and fed through valve 13 into a product conveying line 15. A compressor 
14 is employed to convey the polymer to the product handling and storage 
area. Alternatively, the product can be pelletized in a dose-coupled 
pelletizing system. Reactor gases and inert gas, collectively referred to 
herein, as product purge bin gases, are removed from the product separator 
through conduit 16 and are typically sent to a flare to be incinerated or 
to a monomer recovery operation. If the product purge bin gases are 
incinerated a substantial amount of reactor gas, containing various 
amounts of monomers and solvents are lost resulting in efficiency losses. 
Alternatively, a refrigeration system would be necessary to condense and 
separate the monomers contained in the product purge bin gas stream from 
the non-condensable gases. 
Throughout this disclosure reference will be made to product purge bin 
reactor gas and the condensation and recovery of monomer gases contained 
within this stream. While this gas stream is used to illustrate the 
present invention, those with ordinary skill in the art will readily 
appreciate the applicability of the invention to other gas streams which 
contain monomer gases that are to be cooled, condensed and recovered. 
Referring to FIG. 2a, a gas stream 101 containing various monomers, 
typically at a pressure ranging from about 0.5 pounds per square inch 
gauge (psig) to about 20 psig and a temperature of from 50.degree. C. to 
about 110.degree. C., such as the product purge bin gas stream, is cooled 
in a heat exchanger 102 to about -10.degree. C. to partially condense a 
portion of the reactor gas stream. The product purge bin gas stream is 
then directed to a liquid/gas separator 103 to separate the condensed 
liquids such as hexene, hexane, butene, isopentane and the like from the 
remaining non-condensable gases. The liquids are removed and sent to a 
recovered monomer tank 113. The condensed monomers are sent to the reactor 
feed system, or a monomer purification system, if necessary, by pump 114. 
The non-condensable gases are then compressed by a compressor 104 to a 
pressure of from about 30 psig to about 90 psig and a temperature of from 
50.degree. C. to about 200.degree. C. and sent to a second heat exchanger 
105 to be further cooled to a temperature of about 40.degree. C. The 
intermediate pressure gas stream created by the discharge of compressor 
104 is then sent to a second liquid/gas separator 106 in which the 
condensed gases are removed. The non-condensable gases are then fed to a 
second compressor 107, having a temperature of from about 30.degree. C. to 
about 200.degree. C. and a pressure range of from about 120 psig to about 
300 psig before being conveyed into a third heat exchanger 108. Any 
condensed liquids formed in heat exchanger 108 are then removed in a 
liquid/gas separator 109 and sent to a recovered monomer tank 113. Any 
product purge bin gases not condensed in the process are then typically 
sent to a flare 110 or used as a conveying gas for the product blow tanks 
8 (as depicted in FIG. 1). 
Those with ordinary skill in the art will recognize that additional 
compressors, liquid/gas separators, heat exchangers etc. can be added to 
the above method without departing from the spirit of the invention. For 
simplicity, multiple pumps and multiple recovered monomer tanks are not 
depicted in FIG. 2a. Pumps, compressors and additional equipment will be 
necessary depending on the final design which is selected. The selection 
and specification of the type of equipment needed for a particular design 
is within the ability of a person with ordinary skill in the art. 
Still referring to FIG. 2a, the monomer feed stream 112 at a pressure of 
from about 1000 psig to about 1600 psig and temperature of from 0.degree. 
C. to about 25.degree. C. is expanded at the expansion station 115 to 
create the reduced pressure monomer feed stream having a pressure range of 
from 350 psig to 450 psig and a temperature of from about -10.degree. C. 
to about -20.degree. C. The reduced pressure monomer feed stream is 
preferably fed to the various heat exchangers so that the highest pressure 
reactor gases are contacting the coldest monomer feed source. This enables 
the low boiling gases to be indirectly contacted with the coldest monomer 
feed stream. In each successive heat exchanger the monomer feed stream is 
warmer than in the previous heat exchanger. Preferably, any liquefied 
monomer created by the expansion of the monomer feed stream is vaporized 
to recover the heat of vaporization of the monomer and maximize the amount 
of condensable gases recovered by the process. Preferably, the monomer 
feed stream is on the shell side of the heat exchangers. The monomer feed 
stream is then fed to the gas phase fitted bed reactor through conduit 
116. 
A preferred embodiment of the present invention is presented in FIG. 2b. A 
triple pass heat exchanger is employed to cool the product purge bin gases 
201 in a single heat exchanger 203. On the shell side of the exchanger, 
the expanded monomer feed, preferably a partially liquefied monomer gas is 
supplied, while in the tube side of the heat exchanger the reactor gases 
are cooled and condensed. This embodiment of the invention is preferred 
because a single heat exchanger is employed rather than multiple heat 
exchangers. Furthermore in the method of the invention depicted in FIG. 
2b, all of the reactor gases are in indirect heat exchange with the 
partially liquefied monomer feed gas which is the coldest stream created 
by the expansion of the monomer gas. 
Still referring to FIG. 2b, the product purge bin gas stream at a 
temperature of from about 50.degree. C. to about 110.degree. C. and a 
pressure of from 0.5 psig to about 20 psig is provided to an optional 
filter 202 and then to the low pressure tubes of a multipass heat 
exchanger and cools it to a temperature to of from about -5.degree. C. to 
about -10.degree. C. Any condensable gases are then separated from the gas 
stream in the low pressure liquid/gas separator 204. The remaining gas 
stream is compressed by a compressor 205 which provides the product purge 
bin gas stream to the intermediate pressure tubes of the heat exchanger. 
The gases are then cooled to a temperature of from approximately 
-5.degree. C. to about -10.degree. C. Any condensed gases are separated 
from the gas stream in the intermediate pressure separator 206. The 
remaining product purge bin gases, are compressed again by a second 
compressor 207 and sent to the high pressure tubes of the heat exchanger. 
The condensed gases are cooled to a temperature of from -5.degree. C. to 
about -10.degree. C. and then removed from the product purge bin gas 
stream in the high pressure separator 208 and the non-condensable gases 
are then sent to an optional surge tank 209 and then sent to a flare 216. 
Alternatively, the gas stream could be sent to the product handling area 
217 for further use of the gas stream which is not shown for simplicity. 
If further refrigeration is available, the gas stream could then be further 
cooled and compressed so as to recover a higher percentage of any monomer 
remaining in the product purge bin gas stream. The use of the present 
invention in combination with refrigeration system will reduce the heat 
load on the refrigeration system thereby requiring less energy to operate 
the refrigeration system than a refrigeration system not employing the 
method of the recent invention. 
On the shell side of the heat exchanger of FIG. 2b is the monomer source 
210 which has been expanded from about 1200 psig and 18.degree. C. to 
about 430 psig in an expansion station 211 and cooled to a temperature of 
about -12.5.degree. C. After cooling the various gas streams, the monomer 
source is then feed to the gas phase fluid bed reactor through conduit 
215. As in FIG. 2a, a single recovered monomer tank 212 is depicted in 
FIG. 2b. The recovered monomers are then recycled to the reactor through 
pump 213 and conduit 214. For simplicity, other necessary equipment, such 
as pumps, are not shown in FIG. 2b. 
The monomer source can be expanded to as low a pressure as feasible. For 
economic reasons, it is undesirable to have to compress the monomer in 
order to feed it to the reactor. Preferably, a 50 to 100 pounds per 
square inch pressure differential between the expanded monomer gas source 
and the reactor pressure is maintained to feed the monomer to the reactor. 
Most preferably, the pressure of the monomer source is reduced to as low a 
pressure as possible so as to maximize the cooling effect of the expansion 
and to provide as low a temperature monomer feed stream as possible. For 
example, if the monomer supply pressure is about 800 to 1000 psig and the 
reactor pressure is about 300 psig then expansion of the monomer supply 
stream to about 350 to 400 psig would advantageously recover most of the 
cooling effect of the expansion while not requiring further recompression 
of the monomer feed gas to supply it to the reactor. 
The present invention preferably employs a large volume monomer source for 
cooling. In most instances, the monomer is ethylene or propylene, but 
other monomer sources may also be expanded and used as a cooling source 
without departing from the spirit of the invention. 
Depending upon many variables, including but not limited to the discharge 
pressure created by the compressors, the initial pressure of the monomer 
source, the final pressure to which the monomer source is expanded, how 
much of the monomer source is liquefied, and the heat of vaporization of 
the monomer source, will determine how much and what monomers are 
recovered by the method of the invention. The present invention is ideally 
suited for the recovery of the higher boiling components from the product 
purge bin gas stream. These components include hexane, pentane, butene, 
isobutene, isopentane, 4-methyl 1-pentane, octane and the like. The 
ability to condense and recover these monomers is based upon the operating 
temperatures and pressures of the system. Lower boiling components, such 
as ethylene and propylene, may also be condensed and recovered by the 
method of the invention. Condensing the low temperature boiling components 
will generally require that higher pressure systems are employed. 
The Example which follows is presented for the purpose of illustrating the 
invention and are not to be construed as unduly limiting the claims. All 
parts and percentages are by weight unless otherwise specified. 
EXAMPLE 1 
Ethylene was introduced into a system at high pressures, 1200 pounds per 
square inch gauge and was reduced in pressure through a series of valves 
to approximately 430 psig. The result was a mixture of ethylene liquid and 
vapor which is approximately 62% liquid/38% vapor and had a temperature of 
approximately -12.5.degree. C. The liquid/vapor ethylene mixture was fed 
to the shell side of a heat exchanger. 
A monomer-containing vent gas was provided to a triple pass, heat 
exchanger, similar to the one depicted in FIG. 2b. The vent gas was 
initially cooled to a temperature of -5.degree. C. Approximately 850 
pounds of condensate was collected, which was comprised primarily of 
hexene. The remaining gas stream was then compressed to approximately to 
50 psig and then cooled by the intermediate tubes of the heat exchanger to 
approximately -5.degree. C. Approximately 800 pounds of condensate, hexene 
and butene were condensed, separated from the gas stream and sent to a 
recovery tank. The remaining gas was then compressed again to 
approximately 235 psig and then sent to the high pressure tubes of the 
heat exchanger and cooled again. Approximately 200 pounds of condensable 
gases, primarily hexene, hexane, isopentane and/or butene were collected 
and removed from the stream. The remaining non-condensable gases were sent 
to a flare for incineration.