Process and apparatus for recovering diluent, monomer, and comonomer from a polymerization reactor effluent

A process and apparatus are provided for recovering diluent, unreacted monomer, and unreacted comonomer from a polymerization reactor effluent. The comonomer has a boiling point higher than the boiling point of the monomer and the diluent has a boiling point between the boiling points of the monomer and comonomer. The process and apparatus employ at least one flash tank, a first fractionation stage including a first column and operating at a first fractionation pressure, and a second fraction stage including a second column and operating at a higher second fractionation pressure. Comonomer is withdrawn from the first column as a fractionation product, and overhead vapor containing diluent and monomer is substantially condensed to yield a substantially condensed overhead stream. Liquid and vapor from the stream are separated in an accumulator. Vapor and liquid from the accumulator are compressed and pumped, respectively, for delivery to the second fractionation stage at approximately the second fractionation pressure. The pumped liquid is a major portion by weight of the total flow of pumped liquid and compressed vapor to the second fractionation stage. Diluent and monomer are withdrawn from the second column as fractionation products.

BACKGROUND OF THE INVENTION 
The invention relates to the recovery of diluent, unreacted monomer, and 
unreacted comonomer from the polymerization reactor effluent as produced 
by a polymerization reactor in which monomer and comonomer are reacted in 
the diluent. 
The polymer effluent comprises a slurry of polymer solids in a liquid. 
Liquid is typically flashed to a vapor and subjected to fractionation to 
recover diluent, monomer, and comonomer for recycle to the polymerization 
reactor. Continuing concerns in such recovery are power requirements and 
cost. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a process and 
apparatus for recovery of diluent, monomer, and comonomer from a 
polymerization reactor effluent which minimizes power requirements and 
cost. 
The above object is realized by a process for recovering diluent, unreacted 
monomer, and unreacted comonomer from a polymerization reactor effluent 
comprising a slurry of polymer solids in a liquid as produced by the 
polymerization reaction of the monomer and comonomer in the diluent, 
wherein the comonomer has a boiling point higher than the boiling point of 
the monomer and the diluent has a boiling point between the boiling points 
of the monomer and comonomer, the process comprising: passing the effluent 
to a flash means for flashing liquid in the effluent to a vapor; providing 
a first fractionation stage including a first fractionation column and 
operating at a first fractionation pressure; passing vapor from the flash 
means to the first fractionation stage; withdrawing comonomer from the 
first column as a fractionation product; passing overhead vapor containing 
diluent and monomer from the first column to a cooler which substantially 
condenses the overhead vapor to thereby yield a substantially condensed 
overhead stream; passing the substantially condensed overhead stream to an 
accumulator in which liquid and vapor from such stream are allowed to 
separate; providing a second fractionation stage including a second 
fractionation column and operating at a second fractionation pressure 
higher than the first fractionation pressure; passing vapor from the 
accumulator to a compressor to compress such vapor for delivery to the 
second fractionation stage at approximately the second fractionation 
pressure; passing liquid from the accumulator to a pump so as to increase 
the pressure of such liquid for delivery to the second fractionation stage 
at approximately the second fractionation pressure, wherein the liquid as 
pumped to the second fractionation stage is a major portion by weight of 
the total flow of pumped liquid and compressed vapor to the second 
fractionation stage; and withdrawing diluent and monomer from the second 
fractionation column as fractionation products. 
By condensing and pumping mostly liquid (preferably about 75-95% by weight) 
to the second fractionation stage, this minimizes the power requirement 
because pumping liquid to a higher pressure requires less power than 
compression of vapor. Moreover, because of the relatively small flow of 
vapor through the above-mentioned compressor, the compressor employed can 
be a single stage centrifugal compressor which is less expensive (as a 
capital expenditure and operationally) and more reliable than the more 
conventional reciprocating compressor. 
According to another aspect of the invention, there is provided an 
apparatus for recovering diluent, unreacted monomer, and unreacted 
comonomer from a polymerization reactor effluent as described above, 
wherein the diluent, monomer, and comonomer have relative boiling points 
as also described above, the apparatus comprising: a flash means for 
receiving the effluent and flashing liquid in the effluent to vapor; a 
first fractionation stage which includes a first fractionation column and 
which receives vapor from the flash means; a first compressor for 
compressing at least a portion of the vapor passing from the flash means 
to the first fractionation means; means for withdrawing comonomer from the 
first column as a fractionation product; a cooler for receiving and 
substantially condensing overhead vapor containing diluent and monomer 
from the first column to thereby yield a substantially condensed overhead 
stream; an accumulator for receiving the substantially condensed overhead 
stream and in which liquid and vapor from such stream are allowed to 
separate; a second fractionation stage which includes a second 
fractionation column; a second compressor for receiving and compressing 
vapor from the accumulator for delivery to the second fractionation stage; 
a pump for receiving and increasing the pressure of liquid from the 
accumulator for delivery to the second fractionation stage; and means for 
withdrawing diluent and monomer from the second column as fractionation 
products. 
According to a preferred embodiment hereafter described, the flash means 
comprises two flash tanks at different pressures. Vapor from the lower 
pressure flash tank is compressed by the first compressor. Such first 
compressor may be a reciprocating compressor or a screw compressor. The 
second compressor is preferably, as discussed above, a centrifugal 
compressor.

DETAILED DESCRIPTION OF THE INVENTION 
The polymerization reactor effluent, a slurry of polymer solids in a liquid 
with some constituents or portions thereof in a vapor phase as entrained 
in the liquid, is produced by a reactor (not shown) in which the monomer 
and comonomer are reacted in the diluent under suitable polymerization 
conditions. The diluent is maintained in a liquid phase during the 
polymerization reaction, and the polymer solids produced are not soluble 
in the diluent. A suitable catalyst, such as a chromium oxide-containing 
catalyst, can be used in the polymerization reaction. 
According to broad aspects of the invention, the invention is applicable to 
any monomer, comonomer, and diluent having relative boiling points 
previously described. That is, the boiling point of the comonomer is 
higher than the boiling point of the monomer, and the boiling point of the 
diluent is between the boiling points of the monomer and comonomer. It is 
understood that such description and the appended claims assume such 
relative boiling points are at a constant pressure. 
Both the monomer and comonomer are preferably olefins. The monomer is most 
preferably ethylene, and the comonomer is preferably an olefin having 4 to 
10 carbon atoms per molecule, such as butene, hexene, octene, or decene, 
most preferably hexene. Where the monomer is ethylene and the comonomer is 
hexene, the diluent is preferably a paraffin having 3 to 5 carbon atoms 
per molecule, such as propane, n-butane, isobutane, n-pentane, or 
isopentane, most preferably isobutane. 
A preferred embodiment will now be described where the monomer is ethylene, 
the comonomer is hexene, and the diluent is isobutane. In the 
polymerization reaction of the ethylene and hexene, of the combined 
weights of ethylene and hexene, the hexene preferably comprises less than 
10 weight percent, most preferably about 0.25 to 6 weight percent. 
The polymerization reactor effluent contains polymer solids as high as 55 
weight percent (as based on the total weight of the effluent). The 
remainder of the effluent typically includes the following constituents in 
the indicated weight percentages (based on the total weight of the 
remainder): about 90 weight percent isobutane; about 2-8 weight percent 
ethylene; about 1-5 weight percent hexene; and small amounts of heavy 
hydrocarbons (having 7 or more carbon atoms per molecule), hexane, ethane, 
hydrogen, nitrogen, and other impurities and by-products. 
Now referring to the FIGURE, the effluent enters line 10 at a pressure of 
about 350 psia and a temperature of about 100.degree. F., and proceeds to 
flow through line 10 so as to be heated by flashline heater 12 prior to 
its introduction to flash tank 14. The preferred temperature in flash tank 
14 is about 170-190.degree. F. Flash tank 14 is operated at a pressure 
hereafter referred to as the first flash pressure. The first flash 
pressure can be within a broad pressure range of about 110-210 psia, but 
is more preferably about 140-190 psia, and is most preferably about 
150-170 psia. In any event, the first flash pressure is preferably about 
10 psia higher than what is hereafter referred to as the first 
fractionation pressure, or that pressure at which the first fractionation 
stage operates. The first fractionation stage includes fractionation 
column 16. A substantial portion of liquid in the effluent is flashed to 
vapor in flash tank 14, and the small difference between the first flash 
pressure and the first fractionation pressure induces the flow of such 
vapor through line 18 to cyclone collector 20 and also the flow of vapor 
from cyclone collector 20 to column 16 via line 22. 
Cyclone collector 20 removes fine polymer solids, hereafter referred to as 
polymer particles, which are entrained in vapor. The thus removed polymer 
particles are passed through line 24 to flash tank 30. Valves 26 and 28 
along line 24 are operated by a sequencing controller (not shown) to 
alternately open and close such valves. For example, valve 26 can be 
opened for a predetermined period of time while valve 28 remains closed, 
during which time the portion of line 24 between valves 26 and 28 fills 
with polymer particles. Valve 26 is then closed and valve 28 opened to 
allow the passage of polymer particles through valve 28 and line 24 to 
flash tank 30. Such sequence is then repeated similarly. 
Polymer solids and residual liquid (that liquid not vaporized in flash tank 
14) are passed from flash tank 14 to flash tank 30 in a similar manner by 
means of line 31 and associated valve 32, surge vessel 34 which is allowed 
to fill with polymer solids and residual liquid periodically during the 
sequence, and line 36 and associated valve 38. Valves 32 and 38 are 
operated by a sequencing controller which is not shown. 
Flash tank 30 is operated at a second flash pressure substantially lower 
than the first flash pressure, preferably about 20-30 psia. The 
temperature in flash tank 30 is preferably about 155-175.degree. F. If 
present, a substantial portion of residual liquid is flashed to vapor in 
flash tank 30. Polymer solids and any small amounts of liquid not flashed 
to vapor are passed to a nitrogen purge column (not shown) through line 40 
and associated sequencing valves 42 and 44. 
Vapor from flash tank 30 is passed through line 46 to cyclone collector 48, 
in which entrained solid particles are removed and passed to the 
above-mentioned purge column through line 50 and associated sequencing 
valves 52 and 54. Vapor from cyclone collector 48 flows through line 56 to 
cooler 58, and then through line 60 to compressor 62. Compressor 62, 
preferably a reciprocating or screw compressor, compresses the vapor to 
approximately the first fractionation pressure. The thus compressed vapor 
is passed to column 16 via line 64. By cooling and then compressing the 
vapor, rather than compressing followed by cooling, this reduces the power 
required for compression. 
Although the illustrated embodiment uses only cyclone collectors to remove 
polymer particles from vapor in passage from the flash tanks to column 16, 
bag filters and guard filters can also be used if desired. 
The first fractionation pressure, the pressure at which column 16 operates, 
can be in the broad range of about 100-200 psia, but is more preferably 
about 130-180 psia, and most preferably about 140-160 psia, to minimize 
total power requirements. Preferred temperature conditions in column 16 
include an overhead temperature (temperature at the top of the column) of 
about 130-160.degree. F. and a bottoms temperature (temperature at the 
bottom of the column) of about 310-340.degree. F. 
Liquid bottoms product, containing heavy hydrocarbons and some hexene, is 
withdrawn from column 16 through line 66. A first portion of the bottoms 
product passes through flow control valve 68 and then to flare. A second 
portion of the bottoms product is passed from line 66 and through line 70 
to heater (reboiler) 72, and from there through line 74 back to column 16. 
A preferably vaporous sidedraw stream is withdrawn from column 16 through 
line 76. The sidedraw stream typically contains about 90-95 weight percent 
hexene and also contains isobutane and hexane. The sidedraw stream is 
cooled and condensed by cooler 78, and the resulting condensate flows 
through line 79 and flow control valve 80 to storage for recycling to the 
reactor (not shown). 
Overhead vapor from column 16, containing isobutane, ethylene, ethane, 
hydrogen, and nitrogen, passes through line 82 to cooler 84. Cooler 84 
substantially condenses the overhead vapor. The resulting substantially 
condensed overhead stream, at a preferred temperature of about 
90-110.degree. F., flows through line 86 to overhead accumulator 88 in 
which liquid and vapor from such stream are allowed to separate. The 
contents of accumulator 88 are at the first fractionation pressure and at 
the aforementioned temperature of about 90-110.degree. F. 
Vapor from accumulator 88, containing ethylene and isobutane and also 
containing ethane, hydrogen, and nitrogen, is passed through line 90 to 
compressor 92. Compressor 92 is preferably a single stage centrifugal 
compressor such as, for example, the Sundyne Model LMC-311P compressor. 
Compressor 92 compresses the vapor to approximately a second fractionation 
pressure, preferably about 220-250 psia, at which a second fractionation 
stage operates. Such fractionation stage includes a fractionation column 
94 (having a preferred overhead temperature of about 110-120.degree. F. 
and a preferred bottoms temperature of about 180-200.degree. F.) and an 
associated overhead accumulator 96. Vapor as compressed by compressor 92 
flows through line 98 to line 100. 
Liquid from accumulator 88, containing about 95-98 weight percent isobutane 
and about 2-5 weight percent ethylene, passes through line 102 to pump 104 
(preferably centrifugal) which increases the pressure of the liquid to 
preferably about 250-280 psia. Liquid is pumped by pump 104 through line 
106, and a first portion (preferably about 1/3 by weight) flows through 
flow control valve 108 to column 16 as reflux. A significant pressure 
drop, typically about 100 psia, occurs across valve 108. A second portion 
(preferably about 2/3 by weight) of the liquid pumped by pump 104 flows 
from line 106 through line 110 to flow control valve 112, across which the 
liquid undergoes a pressure drop of typically about 30 psia to 
approximately the second fractionation pressure. The liquid as reduced in 
pressure continues to flow through line 110 to line 100. 
Liquid as pumped through line 110 is a major portion by weight of the total 
flow of pumped liquid and compressed vapor flowing through lines 110 and 
98, respectively. This major portion is preferably about 75-95%, and most 
preferably 80-90%. The advantage of this feature has been previously 
discussed. 
Vapor from line 98 and liquid from line 110 flow through line 100 to and 
through cooler 114, and then through line 116 for delivery to accumulator 
96 at approximately the second fractionation pressure. Liquid and vapor 
separate in accumulator 96. Accumulator 96 has a vent column condenser of 
which the upper portion is cooled by a propylene refrigerant to achieve an 
overhead temperature of about -25 to -10.degree. F. At this temperature 
most of the isobutane vapor is condensed, while some ethylene, hydrogen, 
nitrogen, and ethane, as well as a small amount of isobutane, are vented 
through line 118 and flow control valve 121. The thus vented vapor can go 
to flare or to an ethylene plant. 
Liquid from accumulator 96 flows through line 120 to pump 122, and pump 122 
pumps the liquid through line 124 and flow control valve 126 to column 94 
at approximately the second fractionation pressure. Overhead vapor from 
column 94, containing ethylene and isobutane, is passed through line 128 
to line 100. At least a portion of such overhead vapor is condensed by 
cooler 114 and then returned by pump 122 to column 94 as reflux. 
A sidedraw stream of liquid, containing about 95-99 weight percent 
isobutane and about 1-4 weight percent ethylene, is withdrawn from column 
94 through line 130. The sidedraw stream flows to and through a cooler 132 
and then through line 134 and flow control valve 136 before going to 
storage for recycling to the reactor (not shown). 
Liquid bottoms product, comprising substantially pure olefin-free 
isobutane, is withdrawn from column 94 through line 136. A first portion 
continues to flow through line 136, to and through cooler 138, and then 
through line 140 and flow control valve 142 to storage for recycling to 
the reactor (not shown). A second portion passes from line 136 to heater 
144 (reboiler) via line 146, and from there through line 148 back to 
column 94. 
In the above description, very small pressure drops across coolers and very 
slight nonuniformities in pressure in the fractionation columns were not 
considered for simplicity of description. 
An example will now be described to further illustrate the invention. 
In substantial accordance with the preferred embodiment described above 
with reference to the FIGURE, a heat and material balance computer 
simulation was carried out using Aspen Plus.TM. computer software, 
available from Aspen Technology, Inc. of Cambridge, Mass. The first flash 
pressure is about 10 psia above the first fractionation pressure, which is 
varied from 110 psia to 190 psia. The second flash pressure is fixed at 22 
psia. The total vapor flow into fractionation column 16 is 75,030 1 b/hr. 
The second fractionation pressure is fixed at 232.4 psia. In addition, 
several assumptions are made in the simulation: compressors 62 and 92 
operate at 72% polytropic efficiency; pump 104 operates at 65% efficiency; 
and a by-pass or inefficiency exists in the flash process by which higher 
pressure vapor escapes to lower pressure streams and this by-pass rate is 
proportional to the square root of the pressure difference. 
Results of the simulation are presented in the following Example Table. As 
the first fractionation pressure increases, the total work decreases to a 
minimum at 145 psia, and then gradually increases through the highest 
pressure of 190 psia. Total work is defined as the sum of the individual 
requirements of compressor 62, compressor 92, and pump 104 in reaching the 
second fractionation pressure. Vapor flow through line 98 and liquid flow 
through line 110 are also indicated in the Table, as well as the liquid 
flow/total flow. This percentage increases from slightly above 50% at the 
lowest pressure of 110 psia to above 98% at the highest pressure of 190 
psia. The minimum total work is at about 85%. 
______________________________________ 
Example Table 
1st Frac. 
Vapor Flow 
Liq. Flow Liq. Flow/ 
Total 
Pressure Line 98 Line 110 Total Flow 
Work 
(psia) (lb/hr) (lb/hr) (%) (HP) 
______________________________________ 
110 32857 37885 53.6 556.3 
115 27116 43626 61.7 494.2 
120 22790 47952 67.8 448.8 
125 19390 51350 72.6 414.3 
130 16629 54111 76.5 387.4 
135 14326 56413 79.7 366.0 
140 12364 58375 82.5 348.8 
145 10661 60077 84.9 335.8 
150 9162 61575 87.0 337.1 
155 7827 62910 88.9 341.2 
160 6625 64111 90.6 347.7 
165 5536 65200 92.2 356.2 
170 4543 66191 93.6 366.7 
175 3637 67097 94.9 378.9 
180 2811 67922 96.0 392.7 
185 2063 68670 97.1 408.0 
190 1393 69339 98.0 424.8 
______________________________________ 
Duties (not in Table) for heater 72 and cooler 84 increase as the first 
fractionation pressure increases. The heater duty increases from about 
1,650,000 to about 2,700,00 BTU/hr, and the cooler duty increases from 
about 9,500,000 to about 14,400,000 BTU/hr. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. For example, instead of using 
two flash tanks, only a single flash tank could be employed. In such an 
embodiment, the preferred first fractionation pressure would be about 
110-140 psia to minimize total power requirements. It is, therefore, to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described.