Method and apparatus for high solids slurry polymerization

Abstract

An olefin polymerization process wherein monomer, diluent and catalyst are circulated in a continuous loop reactor and product slurry is recovered by means of a continuous product take off. The continuous product allows operating the reaction at significantly higher solids content in the circulating slurry. In a preferred embodiment, the slurry is heated in a flash line heater and passed to a high pressure flash where a majority of the diluent is separated and thereafter condensed by simple heat exchange, without compression, and thereafter recycled. Also an olefin polymerization process operating at higher reactor solids by virtue of more aggressive circulation.

Description

BACKGROUND OF THE INVENTION

[0002] This invention relates to the polymerization of olefin monomers in a liquid diluent.

[0003] Addition polymerizations are frequently carried out in a liquid which is a solvent for the resulting polymer. When high density (linear) ethylene polymers first became commercially available in the 1950's this was the method used. It was soon discovered that a more efficient way to produce such polymers was to carry out the polymerization under slurry conditions. More specifically, the polymerization technique of choice became continuous slurry polymerization in a pipe loop reactor with the product being taken off by means of settling legs which operated on a batch principle to recover product. This technique has enjoyed international success with billions of pounds of ethylene polymers being so produced annually. With this success has come the desirability of building a smaller number of large reactors as opposed to a larger number of small reactors for a given plant capacity.

[0004] Settling legs, however, do present two problems. First, they represent the imposition of a “batch” technique onto a basic continuous process. Each time a settling leg reaches the stage where it “dumps” or “fires” accumulated polymer slurry it causes an interference with the flow of slurry in the loop reactor upstream and the recovery system downstream. Also the valve mechanism essential to periodically seal off the settling legs from the reactor upstream and the recovery system downstream requires frequent maintenance due to the difficulty in maintaining a tight seal with the large diameter valves needed for sealing the legs.

[0005] Secondly, as reactors have gotten larger, logistic problems are presented by the settling legs. If a pipe diameter is doubled the volume of the reactor goes up four-fold. However, because of the valve mechanisms involved, the size of the settling legs cannot easily be increased further. Hence the number of legs required begins to exceed the physical space available.

[0006] In spite of these limitations, settling legs have continued to be employed where olefin polymers are formed as a slurry in a liquid diluent. This is because, unlike bulk slurry polymerizations (i.e. where the monomer is the diluent) where solids concentrations of better than 60 percent are routinely obtained, olefin polymer slurries in a diluent are generally limited to no more than 37 to 40 weight percent solids. Hence settling legs have been believed to be necessary to give a final slurry product at the exit to the settling legs of greater than 37-40 percent. This is because, as the name implies, settling occurs in the legs to thus increase the solids concentration of the slurry finally recovered as product slurry.

[0007] Another factor affecting maximum practical reactor solids is circulation velocity, with a higher velocity for a given reactor diameter allowing for higher solids since a limiting factor in the operation is reactor fouling due to polymer build up in the reactor.

SUMMARY OF THE INVENTION

[0008] It is an object of this invention to produce olefin polymers as a slurry in a liquid diluent utilizing continuous product slurry takeoff;

[0009] It is a further object of this invention to operate a slurry olefin polymerization process in a diluent at a reactor solids concentration high enough to make direct continuous product takeoff commercially viable;

[0010] It is a further object of this invention to operate a slurry olefin polymerization process in a diluent at higher circulation velocities.

[0011] It is yet a further object of this invention to operate a slurry olefin polymerization process in a diluent in a reaction zone of greater than 30,000 gallons; and

[0012] It is still yet a further object of this invention to provide a loop reactor apparatus having a capacity of greater than 30,000 gallons and having a continuous take off means.

[0013] In accordance with one aspect of this invention, an olefin polymerization process is carried out at a higher reactor solids concentration by means of continuous withdrawal of product slurry.

[0014] In accordance with another aspect of this invention, a loop reactor olefin polymerization process is carried out by operating at a higher circulation velocity for a given reactor pipe diameter.

[0015] In accordance with another aspect of this invention, a loop polymerization apparatus is provided having an elongated hollow appendage at a downstream end of one of the longitudinal segments of the loop, the hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the drawings, forming a part hereof,

[0017] FIG. 1 is a schematic perspective view of a loop reactor and polymer recovery system;

[0018] FIG. 2 is cross section along line 2--2 of FIG. 1 showing a continuous take off appendage;

[0019] FIG. 3 is a cross section along line 3--3 of FIG. 2 showing a ram valve arrangement in the continuous take off assembly;

[0020] FIG. 4 is a cross section of a tangential location for the continuous take off assembly;

[0021] FIG. 5 is a side view of an elbow of the loop reactor showing both a settling let and continuous take off assemblies;

[0022] FIG. 6 is a cross section across line 6--6 of FIG. 5 showing the orientation of two of the continuous take off assemblies; FIG. 7 is a side view showing another orientation for the continuous take off assembly;

[0023] FIG. 8 is a cross sectional view of the impeller mechanism;

[0024] FIG. 9 is a schematic view showing another configuration for the loops wherein the upper segments 14a are 180 degree half circles and wherein the vertical segments are at least twice as long as the horizontal segments and

[0025] FIG. 10 is a schematic view showing the longer axis disposed horizontally.

[0026] FIG. 11 is a schematic diagram illustrating a process for separating polymer from diluent in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Surprisingly, it has been found that continuous take off of product slurry in an olefin polymerization reaction carried out in a loop reactor in the presence of an inert diluent allows operation of the reactor at a much higher solids concentration. Commercial production of predominantly ethylene polymers in isobutane diluent has generally been limited to a maximum solids concentration in the reactor of 37-40 weight percent. However, the continuous take off has been found to allow significant increases in solids concentration. Furthermore, the continuous take off itself brings about some additional increase in solids content as compared with the content in the reactor from which it takes off product because of the placement of the continuous take off appendage which selectively removes a slurry from a stratum where the solids are more concentrated. Hence concentrations of greater than 40 weight percent are possible in accordance with this invention.

[0028] Throughout this application, the weight of catalyst is disregarded since the productivity, particularly with chromium oxide on silica, is extremely high.

[0029] Also surprisingly, it has been found that more aggressive circulation (with its attendant higher solids concentration) can be employed. Indeed more aggressive circulation in combination with the continuous take off, solids concentrations of greater than 50 weight percent can be removed from the reactor by the continuous take off. For instance, the continuous take off can easily allow operating at 5-6 percentage points higher; i.e., the reactor can be adjusted to easily raise solids by 10 percent; and the more aggressive circulation can easily add another 7-9 percentage points which puts the reactor above 50 percent. But, because the continuous take off is positioned to take off slurry from a stratum in the stream which has a higher than average concentration of solids, the product actually recovered has about 3 percentage points (or greater) higher concentration than the reactor slurry average. Thus the operation can approach an effective slurry concentration of 55 weight percent or more, i.e. 52 percent average in the reactor and the removal of a component which is actually 55 percent (i.e. 3 percentage points) higher.

[0030] It must be emphasized that in a commercial operation as little as a one percentage point increase in solids concentration is of major significance. Therefore going from 37-40 average percent solids concentration in the reactor to even 41 is important; thus going to greater than 50 is truly remarkable.

[0031] The present invention is applicable to any olefin polymerization in a loop reactor utilizing a diluent so as to produce a product slurry of polymer and diluent. Suitable olefin monomers are 1-olefins having up to 8 carbon atoms per molecule and no branching nearer the double bond than the 4-position. The invention is particularly suitable for the homopolymerization of ethylene and the copolymerization of ethylene and a higher 1-olefin such as butene, 1-pentene, 1-hexene, 1-octene or 1-decene. Especially preferred is ethylene and 0.01 to 10, preferably 0.01 to 5, most preferably 0.1 to 4 weight percent higher olefin based on the total weight of ethylene and comonomer. Alternatively sufficient comonomer can be used to give the above-described amounts of comonomer incorporation in the polymer.

[0032] Suitable diluents (as opposed to solvents or monomers) are well known in the art and include hydrocarbons which are inert and liquid under reaction conditions. Suitable hydrocarbons include isobutane, propane, n-pentane, i-pentane, neopentane and n-hexane, with isobutane being especially preferred.

[0033] Suitable catalysts are well known in the art. Particularly suitable is chromium oxide on a support such as silica as broadly disclosed, for instance, in Hogan and Banks, U.S. Pat. No. 2,285,721 (March 1958), the disclosure of which is hereby incorporated by reference.

[0034] Referring now to the drawings, there is shown in FIG. 1 a loop reactor 10 having vertical segments 12, upper horizontal segments 14 and lower horizontal segments 16. These upper and lower horizontal segments define upper and lower zones of horizontal flow. The reactor is cooled by means of two pipe heat exchangers formed by pipe 12 and jacket 18. Each segment is connected to the next segment by a smooth bend or elbow 20 thus providing a continuous flow path substantially free from internal obstructions. The polymerization mixture is circulated by means of impeller 22 (shown in FIG. 8) driven by motor 24. Monomer, comonomer, if any, and make up diluent are introduced via lines 26 and 28 respectively which can enter the reactor directly at one or a plurality of locations or can combine with condensed diluent recycle line 30 as shown. Catalyst is introduced via catalyst introduction means 32 which provides a zone (location) for catalyst introduction. The elongated hollow appendage for continuously taking off an intermediate product slurry is designated broadly by reference character 34. Continuous take off mechanism 34 is located in or adjacent to a downstream end of one of the lower horizontal reactor loop sections 16 and adjacent or on a connecting elbow 20.

[0035] The continuous take off appendage is shown at the downstream end of a lower horizontal segment of the loop reactor which is the preferred location. The location can be in an area near the last point in the loop where flow turns upward before the catalyst introduction point so as to allow fresh catalyst the maximum possible time in the reactor before it first passes a take off point. However, the continuous take off appendage can be located on any segment or any elbow.

[0036] Also, the segment of the reactor to which the continuous take off appendage is attached can be of larger diameter to slow down the flow and hence further allow stratification of the flow so that the product coming off can have an even greater concentration of solids.

[0037] The continuously withdrawn intermediate product slurry is passed via conduit 36 into a high pressure flash chamber 38. Conduit 36 includes a surrounding conduit 40 which is provided with a heated fluid which provides indirect heating to the slurry material in flash line conduit 36. Vaporized diluent exits the flash chamber 38 via conduit 42 for further processing which includes condensation by simple heat exchange using recycle condenser 50, and return to the system, without the necessity for compression, via recycle diluent line 30. Recycle condenser 50 can utilized any suitable heat exchange fluid known in the art under any conditions known in the art. However preferably a fluid at a temperature that can be economically provided is used. A suitable temperature range for this fluid is 40 degrees F. to 130 degrees F. Polymer particles are withdrawn from high pressure flash chamber 38 via line 44 for further processing using techniques known in the art. Preferably they are passed to low pressure flash chamber 46 and thereafter recovered as polymer product via line 48. Separated diluent passes through compressor 47 to line 42. This high pressure flash design is broadly disclosed in Hanson and Sherk, U.S. Pat. No. 4,424,341 (Jan. 3, 1984), the disclosure of which is hereby incorporated by reference. Surprisingly, it has been found that the continuous take off not only allows for higher solids concentration upstream in the reactor, but also allows better operation of the high pressure flash, thus allowing the majority of the withdrawn diluent to be flashed off and recycled with no compression. Indeed, 70 to 90 percent of the diluent can generally be recovered in this manner. This is because of several factors. First of all, because the flow is continuous instead of intermittent, the flash line heaters work better. Also, the pressure drop after the proportional control valve that regulates the rate of continuous flow out of the reactor has a lower pressure which means when it flashes it drops the temperature lower thus further giving more efficient use of the flash line heaters.

[0038] Referring now to FIG. 2, there is shown elbow 20 with continuous take off mechanism 34 in greater detail. The continuous take off mechanism comprises a take off cylinder 52, a slurry withdrawal line 54, an emergency shut off valve 55, a proportional motor valve 58 to regulate flow and a flush line 60. The reactor is run “liquid” full. Because of dissolved monomer the liquid has slight compressibility, thus allowing pressure control of the liquid full system with a valve. Diluent input is generally held constant, the proportional motor valve 58 being used to control the rate of continuous withdrawal to maintain the total reactor pressure within designated set points.

[0039] Referring now to FIG. 3, which is taken along section line 3-3 of FIG. 2, there is shown the smooth curve or elbow 20 having associated therewith the continuous take off mechanism 34 in greater detail, the elbow 20 thus being an appendage-carrying elbow. As shown, the mechanism comprises a take off cylinder 52 attached, in this instance, at a right angle to a tangent to the outer surface of the elbow. Coming off cylinder 52 is slurry withdrawal line 54. Disposed within the take off cylinder 52 is a ram valve 62 which serves two purposes. First it provides a simple and reliable clean-out mechanism for the take off cylinder if it should ever become fouled with polymer. Second, it can serve as a simple and reliable shut-off valve for the entire continuous take off assembly.

[0040] FIG. 4 shows a preferred attachment orientation for the take off cylinder 52 wherein it is affixed tangentially to the curvature of elbow 20 and at a point just prior to the slurry flow turning upward. This opening is elliptical to the inside surface. Further enlargement could be done to improve solids take off.

[0041] FIG. 5 shows four things. First, it shows an angled orientation of the take off cylinder 52. The take off cylinder is shown at an angle, alpha, to a plane that is (1) perpendicular to the centerline of the horizontal segment and (2) located at the downstream end of the horizontal segment 16. The angle with this plane is taken in the downstream direction from the plane. The apex for the angle is the center point of the elbow radius as shown in FIG. 5. The plane can be described as the horizontal segment cross sectional plane. Here the angle depicted is about 24 degrees. Second, it shows a plurality of continuous take off appendages, 34 and 34a. Third, it shows one appendage, 34 oriented on a vertical center line plane of lower segment 16, and the other, 34a, located at an angle to such a plane as will be shown in more detail in FIG. 6. Finally, it shows the combination of continuous take off appendages 34 and a conventional settling leg 64 for batch removal, if desired.

[0042] As can be seen from the relative sizes, the continuous take off cylinders are much smaller than the conventional settling legs. Yet three 2-inch ID continuous take off appendages can remove as much product slurry as 14 8-inch ID settling legs. This is significant because with current large commercial loop reactors of 15,000-18000 gallon capacity, six eight inch settling legs are required. It is not desirable to increase the size of the settling legs because of the difficulty of making reliable valves for larger diameters. As noted previously, doubling the diameter of the pipe increases the volume four-fold and there simply in not enough room for four times as many settling legs to be easily positioned. Hence the invention makes feasible the operation of larger, more efficient reactors. Reactors of 30,000 gallons or greater are made possible by this invention. Generally the continuous take off cylinders will have a nominal internal diameter within the range of 1 inch to less than 8 inches. Preferably they will be about 2-3 inches internal diameter.

[0043] FIG. 6 is taken along section line 6-6 of FIG. 5 and shows take off cylinder 34a attached at a place that is oriented at an angle, beta, to a vertical plane containing the center line of the reactor. This plane can be referred to as the vertical center plane of the reactor. This angle can be taken from either side of the plane or from both sides if it is not zero. The apex of the angle is located at the reactor center line. The angle is contained in a plane perpendicular to the reactor center line as shown in FIG. 6.

[0044] It is noted that there are three orientation concepts here. First is the attachment orientation, i.e. tangential as in FIG. 4 and perpendicular as in FIG. 2 or 7 or any angle between these two limits of 0 and 90 degrees. Second is the orientation relative to how far up the curve of the elbow the attachment is as represented by angle alpha (FIG. 5). This can be anything from 0 to 60 degrees but is preferably 0 to 40 degrees, more preferably 0 to 20 degrees. Third is the angle, beta, from the center plane of the longitudinal segment (FIG. 6). This angle can be from 0 to 60 degrees, preferably 0 to 45 degrees, more preferably 0-20 degrees.

[0045] FIG. 7 shows an embodiment where the continuous take off cylinder 52 has an attachment orientation of perpendicular, an alpha orientation of 0 (inherent since it is at the end, but still on, the straight section), and a beta orientation of 0, i.e. it is right on the vertical centerline plane of the lower horizontal segment 16.

[0046] FIG. 8 shows in detail the impeller means 22 for continuously moving the slurry along its flow path. As can be seen in this embodiment the impeller is in a slightly enlarged section of pipe which serves as the propulsion zone for the circulating reactants. Preferably the system is operated so as to generate a pressure differential of at least 18 psig preferably at least 20 psig, more preferably at least 22 psig between the upstream and downstream ends of the propulsion zone in a nominal two foot diameter reactor with total flow path length of about 950 feet using isobutane to make predominantly ethylene polymers. As much as 50 psig or more is possible. This can be done by controlling the speed of rotation of the impeller, reducing the clearance between the impeller and the inside wall of the pump housing or by using a more aggressive impeller design as is known in the art. This higher pressure differential can also be produced by the use of at least one additional pump.

[0047] Generally the system is operated so as to generate a pressure differential, expressed as a loss of pressure per unit length of reactor, of at least 0.07, generally 0.07 to 0.15 foot slurry height pressure drop per foot of reactor length for a nominal 24 inch diameter reactor. Preferably, this pressure drop per unit length is 0.09 to 0.11 for a 24 inch diameter reactor. For larger diameters, a higher slurry velocity and a higher pressure drop per unit length of reactor is needed. This assumes the density of the slurry which generally is about 0.5-0.6.

[0048] Referring now to FIG. 9 the upper segments are shown as 180 degree half circles which is the which is the preferred configuration. The vertical segments are at least twice the length, generally about seven to eight times the length of the horizontal segments. For instance, the vertical flow path can be 190-225 feet and the horizontal segments 25-30 feet in flow path length. Any number of loops can be employed in addition to the four depicted here and the eight depicted in FIG. 1, but generally four or six are used. Reference to nominal two foot diameter means an internal diameter of about 21.9 inches. Flow length is generally greater than 500 feet, generally greater than 900 feet, with about 940 to 1,350 feet being quite satisfactory.

[0049] Commercial pumps for utilities such as circulating the reactants in a closed loop reactor are routinely tested by their manufacturers and the necessary pressures to avoid cavitation are easily and routinely determined.

EXAMPLES

[0050] A four vertical leg polymerization reactor using a 26 inch Lawrence Pumps Inc. pump impeller D51795/81-281 in a M51879/FAB casing was used to polymerize ethylene and hexene-1. This pump was compared with a 24 inch pump which gave less aggressive circulation (0.66 ft of pressure drop vs 0.98). This was then compared with the same more aggressive circulation and a continuous take off assembly of the type shown by reference character 34 of FIG. 5. The results are shown below.

DATA TABLE
Description	24 in Pump	26 in Pump	26 in Pump + CTO
Date of Operation	Oct 4-9, 1994	May 24-28, 1995	Nov 15-18, 1996
Avg. Reactor Solids	39	45	53
Concentration, wt %
Polymer Production	40.1	40.7	39.9
Rate, mlbs/hr
Reactor Circulation	430	691	753
Pump Power, kw
Circulation Pump	14.3	22.4	23.7
Pressure Diff, psi
Circulation Pump	61.8	92.5	92.4
Head, ft
Reactor Slurry Flow	39	46	45
Rate, mGPM
Reactor Slurry Density, gm/cc	0.534	0.558	0.592
Reactor Temperature, F.	215.6	218.3	217.0
Ethylene	4.43	3.67	4.9
Concentration, wt %
Hexene-1	0.22	0.17	0.14
Concentration, wt %
Reactor Heat Transfer	270	262	241
Coefficient
Reactor Inside	22.0625	22.0625	22.0625
Diameter, inches
Reactor Volume, gal	18700	18700	18700
Reactor Length, ft	941	941	941
Pressure Drop per Foot	0.066	0.098	0.098
of Reactor, ft/ft

[0051] As noted above, a flash vessel design which may be used in conjunction with the continuous take off techniques discussed herein is disclosed in U.S. Pat. No. 4,424,341 to Hanson and Sherk, which is incorporated by reference. For the convenience of the reader, aspects of the Hanson and Sherk technique applicable to the present continuous take off techniques are reproduced below.

[0052] While the present invention is applicable to any mixture which comprises a slurry of polymer solid and diluent, it is particularly applicable to the slurries resulting from olefin polymerizations. The olefin monomers generally employed in such reactions are 1-olefins having up to 8 carbon atoms per molecule and no branching nearer the double bond than the 4-position. Typical examples include ethylene, propylene, butene-1,1-pentene, and 1,3-butadiene.

[0053] Typical diluents employed in such olefin polymerizations include hydrocarbons having 3 to 12, preferably 3 to 8 carbon atoms per molecule, such as propane, propylene, n-butane, n-pentane, isopentane, n-hexane, toluene, isooctane, isobutane, 1-butene, and the like. In some cases, naphthene hydrocarbons having 5 to 6 carbon atoms in the naphthenic ring are also used. Examples of such naphthenic hydrocarbons include cyclohexane, cyclopentane, methylcyclopentane, ethylcyclohexane, and the like.

[0054] The temperature to which the slurry is heated for vaporization will vary of course depending upon the nature of the diluent, the nature of the polymer, and the temperature of the heat exchange fluid that is used to condense the vaporized diluent. Obviously, the temperature must be raised above the dew point of the diluent at the flashing pressure. Further the temperature should be below that of the melting point of the polymer to preclude accumulation of polymer in the process vessels and to preclude agglomeration of the polymer particles.

[0055] The pressure for the first flash step will likewise vary depending upon the nature of the diluent and the temperature selected. Typically, pressures in the range of about 30 to about 300 psia can be employed, preferably about 150 to 250 psia.

[0056] The heat exchanging fluid used to condense the vapor from the first flash step is, as indicated above, at a temperature in the range of about 40° F. to 130° F. A particularly preferred embodiment uses a heat exchange fluid at a temperature of moderate ambient conditions, for example, temperatures in the range of 60° to 100° F., more preferably 86° to 960 F.

[0057] A further understanding of the present invention will be provided by referring to FIG. 11 which illustrates a system comprising an embodiment of the invention.

[0058] In the embodiment illustrated in FIG. 11, the polymerization is carried out in a loop reactor 110. The polymerization mixture is circulated by agitator 111. Monomer and diluent are introduced through conduits 114 and 116, respectively, connected to conduit 113. Catalyst is added through conduit 117. Normally catalyst is introduced as a suspension in a hydrocarbon diluent.

[0059] Polymer slurry is removed from the loop to a settling leg 118. The slurry passes from settling leg 118 to conduit 119 and into flash chamber 120. Conduit 119 has an indirect heat exchange means such as a flash line heater 121. The flash chamber 120 as illustrated includes in its lower end a gas distribution plate 122. Heated diluent vapor provided via conduit 123 is passed into the flash chamber 120 and through the distributor plate 122 in such a fashion as to cause a fluidized bed of polymer solids to occur in the flash chamber.

[0060] Vaporized diluent exits the flash chamber 120 via conduit 124 through which it is passed into a cyclone 125 which separates entrained polymer particles from the vapor. Polymer particles separated by the cyclone are passed via line 126 to a lower pressure flash chamber 127.

[0061] The polymer particles in the fluidized bed are withdrawn via conduit 128 and also passed into the lower pressure flash chamber 127. In flash chamber 127 substantially all the diluent still associated with the polymer is vaporized and taken overhead via conduit 129 to a second cyclone 130.

[0062] The major portion of the diluent associated with the polymer solids as they leave settling leg 118 will have been taken to cyclone 125 as vapor via conduit 124. The vapor after having a substantial part of any entrained solids removed is passed via line 131 through a filter capable of removing any remaining polymer fines. The vapor stream is then split. One portion is passed via conduit 133 through a heat exchanger 134 wherein the vapor is condensed by indirect heat exchange with a heat exchange fluid. The condensed diluent is then passed to an accumulator 135 via conduit 136. Any uncondensed vapors and gases can be removed overhead from the accumulator 135. A pump 137 is provided for conveying the condensed diluent back to the polymerization zone.

[0063] The other portion of the diluent vapor is passed via line 138 through a blower 139 which forces the vapor into conduit 123 to provide at least part of the diluent vapor needed to provide the fluidized bed in flash chamber 120. The vapor that is passed into conduit 123 is first passed through a heat exchange zone 140 wherein the vapor is heated if desired to provide part or all of the heat needed for heating the polymer slurry provided by conduit 119.

[0064] The polymer solids in the lower pressure flash tank are passed via line 141 to a conventional conveyor dryer 142 from which the polymer can be packaged or otherwise handled while in contact with the atmosphere.

[0065] The vapors exit the secondary cyclone 130 via line 143 to a filter 144 such as a bag filter capable of removing any substantial amounts of polymer fines. The filter vapor is then passed to a compressor 145 and the compressed vapors are passed through conduit 146 to an air-fin cooler 147 wherein a portion of the compressed vapors are condensed. The remaining vapors are passed through conduit 148 to a condenser 149 where most of the remaining vapors are condensed and the condensate is passed through conduit 150 to knockout drum 151 or a fractionator. The condensed diluent can then be removed via conduit 152 and recycled to the polymerization process. Since the major portion of the diluent is recovered from the intermediate pressure flash chamber, the load on compressor 145 is much lower than in prior art techniques of the type illustrated in U.S. Pat. No. 3,152,872.

[0066] It is important to note that there are many variations of the illustrated embodiment which fall within the scope of the present invention. For example, it is within the scope of the present invention to eliminate the flash line heater 121 and to have all the heat supplied by the heated diluent vapor that is used to provide a fluidized bed in flash chamber 120. Further, in some instances, it may be desirable to have the cyclone 125 actually present in the flash chamber rather than being connected to it by a conduit. Still further, it is within the scope of the present invention to eliminate the fluidized bed concept and to supply all the heat needed by other means such as the flash line heater 121. In such a modification, there obviously would no longer be a need for the gas distributor plate 122.

[0067] It is noted that when recycled diluent vapor from the first flash step is used as the fluidizing medium in the first flash step, it can sometimes lead to alterations in the properties of the polymer since it often will contain monomer that could react in the flash step. Under such circumstances, it is thus preferred to use a substantially pure heated diluent as the fluidizing medium or to eliminate the fluidized bed concept and use flash line heaters to provide all the necessary heat.

[0068] In regard to embodiments employing the fluidized bed concept, experiments were conducted to determine the conditions that would be most suitable for producing a fluidized bed of the polymer particles. The particles employed were polyethylene particles having sphericities in the range of about 0.55 to 0.60 as determined by the Ergun equation as disclosed in Zenz, F. and D. Othmer, Fluidization and Fluid-Particle Systems, New York; Reinhold, 1960, p. 75. The Ergun equation is 1$\frac{\Delta P}{L}\mathrm{gc}=\frac{150{\left(1-\mathrm{Em}\right)}^2}{{\mathrm{Em}}^3}\frac{\mu U_O}{{\left(\phi \mathrm{dp}\right)}^2}$

[0069] where:

[0070] ΔP=pressure drop over the bed length.

[0071] L=bed length.

[0072] gc=dimensional constant when units of force such as lbs-force or Kg-force are used.

[0073] Em=porosity of packed bed.

[0074] μ=viscosity of flowing gas.

[0075] Uo=gas superficial velocity (based on bed cross-sectional area).

[0076] φs=sphericity of the particles.

[0077] dp=mean particle diameter for mixture.

[0078] For the polyethylene fluff particles having sphericities in the range of about 0.55 to 0.60, it was determined that good fluidization was obtained with the superficial velocity of the fluidizing gas being in the range of about 0.4 to 0.8 ft/sec. It was further noted that slugging of the bed was a problem when the height of the bed was allowed to be more than about 3 times its diameter. Generally, it would be preferable for the bed height to be no greater than two times its diameter.

[0079] The preferred bed diameter and rate of feeding such a polymer slurry can be calculated by the formula: 2$t=\frac{750\pi D^3}{W}$

[0080] where:

[0081] t=Residence time in minutes necessary for desired level of diluent separation.

[0082] D=Bed diameter, ft.

[0083] W=Fluff feed rate, lb/hr.

[0084] Rate data obtained during measurement of equilibrium isobutane absorption on polymer fluff indicated that 2 to 3 minutes should be adequate for such polyethylene fluff. Thus, for a pilot plant scale process producing 22 pounds per hour of fluff, a bed diameter of at least about 4 inch would be preferred. For a commercial process producing 17,500 pounds of fluff per hour, a bed diameter of at least about 4 feet would be preferred. Residence times greater than 10 minutes generally should not be necessary.

[0085] The following example sets forth typical conditions that can be used in a commercial scale process in employing the present invention.

EXAMPLE

[0086] A typical ethylene homopolymerization process would be the polymerization conducted at a pressure of about 650 psia and a temperature of about 225° F. The settling leg would be operated to accumulate and discharge about 55 weight percent solids. An example of such a process would result in a polymer slurry product containing about 17,500 pounds per hour of polyethylene and about 14,318 pounds per hour of isobutane diluent. This slurry would then be flashed to 180 psia and 180° F. to vaporize the major portion of the diluent. The auxiliary heat necessary to cause the effluent to be at 180° F. after the pressure drop to 180 psia can be supplied by preheating the effluent, by heating recycled fluidizing diluent, or by a combination of the two methods. About 90 percent of the diluent is taken overhead from flash zone 120 at 180 psia. Even assuming that there would be a further pressure drop between flash zone 120 and accumulator 135, the isobutane diluent could readily be condensed against 60° to 80° F. cooling water without compression. The remaining 10 percent of the diluent and the fluff are then passed into a lower pressure flash tank wherein they are exposed to a pressure in the range of about 20 to 30 psia. The diluent vapor from the lower pressure flash tank can then be condensed using compression and cooling. The use of the preliminary higher pressure tank results in a significantly lower compression load than was required in the conventional process in which slurry was immediately flashed to a pressure in the range of 20 to 30 psia.

Claims

1. A method of continuously obtaining polymer product from an olefin polymerization reactor comprising an endless loop of pipe, the method comprising: circulating within the loop a slurry of polymer particles and liquids while maintaining the reactor at a first pressure above 400 psia; continuously conveying an amount of the polymer particles from the reactor first through a discharge means located below a horizontal midline of a cross-section of the reactor pipe and then through a transfer line; and receiving the polymer particles at the inlet of a non-cyclonic primary flash vessel having vertical sidewalls and a conical bottom and maintained at a second pressure less than 25 psia, whereupon the particles settle to the bottom of the flash vessel.

2. The method of claim 1 in which the reactor is a horizontal loop reactor.

3. The method of claim 2 in which the discharge means is located upstream of a reactor-circulating pump in the reactor.

4. The method of claim 1 in which the first pressure is above 600 psia.

5. The method of claim 2 in which the first pressure is from 635 to 675 psia and the liquids include isobutane and ethylene.

6. The method of claim 1 including controlling the flow through the discharge means in response to the pressure of the reactor, and adding fresh olefin feedstock to the reactor at a constant rate.

7. The method of claim 6 in which the reactor is a horizontal loop reactor.

8. The method of claim 6 in which the reactor is a vertical loop reactor.

9. The method of claim 1 in which the particles enter the non-cyclonic primary flash vessel at a tangent to the vertical sidewall in the upper half of the vessel.

10. The method of claim 1 further comprising removing a portion of the polymer particles via an outlet at the bottom of the flash vessel while retaining an amount of particles sufficient to maintain a dynamic seal between the inlet and the outlet of the vessel.

11. The method of claim 1 in which the reactor is a vertical loop reactor.

12. The method of claim 11 in which the particles enter the non-cyclonic primary flash vessel at a tangent to the vertical sidewall in the upper half of the vessel.

13. The method of claim 11 further comprising removing a portion of the polymer particles via an outlet at the bottom of the flash vessel while retaining an amount of particles sufficient to maintain a dynamic seal between the inlet and the outlet of the vessel.

14. The method of claim 13 in which polymer particles are removed while maintaining a level which fills the conical bottom of the vessel.

15. The method of claim 11 including the additional step, before the polymer particles enter the primary flash vessel, of receiving the polymer particles at the inlet of an intermediate non-cyclonic flash vessel with an upper section having vertical sidewalls and the inlet being tangential to the sidewall, a conical bottom with an outlet therein, and operated at a third pressure intermediate between the first and second pressures.

16. The method of claim 15 in which the first pressure is above 600 psia and the third pressure in the intermediate flash vessel is above 180 psia, and the second pressure in the primary flash vessel is below 25 psia.

17. A method of continuously obtaining polymer product from an olefin polymerization reactor comprising an endless loop of pipe, the method comprising: circulating within the loop a slurry of polymer particles and liquids while maintaining the reactor at a first pressure above 400 psia; continuously conveying an amount of the polymer particles from the reactor first through a discharge means located below a horizontal midline of a cross-section of the reactor pipe and then through a transfer line; receiving the polymer particles at the inlet of a non-cyclonic primary flash vessel having vertical sidewalls and a conical bottom and maintained at a second pressure less than 25 psia, whereupon the particles settle to the bottom of the flash vessel; and including the additional step, before the polymer particles enter the primary flash vessel, of receiving the polymer particles at the inlet of an intermediate non-cyclonic flash vessel with an upper section having vertical sidewalls and the inlet being tangential to the sidewall, a conical bottom with an outlet therein, and operated at a third pressure intermediate between the first and second pressures.

18. The method of claim 17 in which the first pressure is above 600 psia and the third pressure in the intermediate flash vessel is above 180 psia, and the second pressure in the primary flash vessel is below 25 psia.

19. A method of continuously obtaining polymer product from an olefin polymerization reactor comprising an endless loop of pipe, the method comprising: circulating within the loop a slurry of polymer particles and liquids while maintaining the reactor at a first pressure above 400 psia; continuously conveying an amount of the polymer particles from the reactor first through a discharge means located below a horizontal midline of a cross-section of the reactor pipe and then through a transfer line; receiving the polymer particles at the inlet of a non-cyclonic primary flash vessel having vertical sidewalls and a conical bottom and maintained at a second pressure less than 25 psia, whereupon the particles settle to the bottom of the flash vessel; removing a portion of the polymer particles via an outlet at the bottom of the flash vessel while retaining an amount of particles sufficient to maintain a dynamic seal between the inlet and the outlet of the vessel and maintaining a level which fills the conical bottom of the vessel.

20. A method of continuously obtaining polymer product from an olefin polymerization reactor comprising an endless loop of pipe, the method comprising: circulating within the loop a slurry of polymer particles and liquids while maintaining the reactor at a first pressure above 400 psia; continuously conveying an amount of the polymer particles from the reactor first through a discharge means located below a horizontal midline of a cross-section of the reactor pipe and then through a transfer line; and receiving the polymer particles at the inlet of a non-cyclonic first flash vessel having vertical sidewalls and a conical bottom and maintained at a second pressure less than 25 psia, whereupon the particles settle to the bottom of the first flash vessel.

21. The method of claim 20 in which the reactor is a horizontal loop reactor.

22. The method of claim 21 in which the discharge means is located upstream of a reactor-circulating pump in the reactor.

23. The method of claim 20 in which the first pressure is above 600 psia.

24. The method of claim 21 in which the first pressure is between 635 to 675 psia and the liquids include isobutane and ethylene.

25. The method of claim 21 in which the first pressure is about 650 psia and the liquids include isobutane and ethylene.

26. The method of claim 20 including controlling the flow through the discharge means in response to the pressure of the reactor, and adding one or more input streams to the reactor at a constant rate.

27. The method of claim 26 in which the reactor is a horizontal loop reactor.

28. The method of claim 26 in which the reactor is a vertical loop reactor.

29. The method of claim 20 in which the particles enter the non-cyclonic first flash vessel at a tangent to the vertical sidewall in the upper half of the vessel.

30. The method of claim 20 further comprising removing a portion of the polymer particles via an outlet at the bottom of the first flash vessel while retaining an amount of particles sufficient to maintain a dynamic seal between the inlet and the outlet of the vessel.

31. The method of claim 20 in which the reactor is a vertical loop reactor.

32. The method of claim 31 in which the particles enter the non-cyclonic first flash vessel at a tangent to the vertical sidewall in the upper half of the vessel.

33. The method of claim 31 further comprising removing a portion of the polymer particles via an outlet at the bottom of the first flash vessel while retaining an amount of particles sufficient to maintain a dynamic seal between the inlet and the outlet of the vessel.

34. The method of claim 33 in which polymer particles are removed while maintaining a level which fills the conical bottom of the vessel.

35. The method of claim 31 including the additional step, before the polymer particles enter the first flash vessel, of receiving the polymer particles at the inlet of a second non-cyclonic flash vessel with an upper section having vertical sidewalls and the inlet being tangential to the sidewall, a conical bottom with an outlet therein, and operated at a third pressure intermediate between the first and second pressures.

36. The method of claim 35 in which the first pressure is above 600 psia and the third pressure in the second flash vessel is above 180 psia, and the second pressure in the first flash vessel is below 25 psia.

37. A method of continuously obtaining polymer product from an olefin polymerization reactor comprising an endless loop of pipe, the method comprising: circulating within the loop a slurry of polymer particles and liquids while maintaining the reactor at a first pressure above 400 psia; continuously conveying an amount of the polymer particles from the reactor first through a discharge means located below a horizontal midline of a cross-section of the reactor pipe and then through a transfer line; receiving the polymer particles at the inlet of a non-cyclonic first flash vessel having vertical sidewalls and a conical bottom and maintained at a second pressure less than 25 psia, whereupon the particles settle to the bottom of the first flash vessel; and including the additional step, before the polymer particles enter the first flash vessel, of receiving the polymer particles at the inlet of a second non-cyclonic flash vessel with an upper section having vertical sidewalls and the inlet being tangential to the sidewall, a conical bottom with an outlet therein, and operated at a third pressure intermediate between the first and second pressures.

38. The method of claim 37 in which the first pressure is above 600 psia and the third pressure in the second flash vessel is above 180 psia, and the second pressure in the first flash vessel is below 25 psia.

39. A method of continuously obtaining polymer product from an olefin polymerization reactor comprising an endless loop of pipe, the method comprising: circulating within the loop a slurry of polymer particles and liquids while maintaining the reactor at a first pressure above 400 psia; continuously conveying an amount of the polymer particles from the reactor first through a discharge means located below a horizontal midline of a cross-section of the reactor pipe and then through a transfer line; receiving the polymer particles at the inlet of a non-cyclonic first flash vessel having vertical sidewalls and a conical bottom and maintained at a second pressure less than 25 psia, whereupon the particles settle to the bottom of the first flash vessel; removing a portion of the polymer particles via an outlet at the bottom of the first flash vessel while retaining an amount of particles sufficient to maintain a dynamic seal between the inlet and the outlet of the vessel and maintaining a level which fills the conical bottom of the vessel.