Anchor agitator for gaseous phase polymerization vessel

An improved anchor agitator is provided for performing uniform and effective stirring of the fluidized bed zone of a polymerization vessel in a gaseous phase polymerization, especially of olefins such as ethylene and propylene. The anchor agitator includes at least two pairs of arms provided on a rotating shaft in mutually crossing positions. A pair of main blades is provided at opposite outward end portions of one pair of arms and at the opposite outward portions of each remaining pair of arms a pair of subsidiary blades is provided. The main blades and subsidiary blades extend substantially parallel along the axis of the rotating shaft and each pair of blades is located substantially equidistant from the axis of the rotating shaft. The pair of main blades is located farther from the axis of the rotating shaft than the subsidiary blades. The main blades and subsidiary blades are uniquely designed to avoid the formation of coherent crusts or lumps near the inner wall of the polymerization vessel and to avoid formation of a vortex formed centrally in the fluidized bed zone of the vessel in the vertical direction of the zone. The unique angular relationships and distance relationships of the main blades and subsidiary blades also avoid the likelihood of vibration due to mechanical imbalance of the agitator while at the same time allowing the agitator to be easily designed and built.

This invention relates to an anchor agitator for a gaseous phase 
polymerization vessel, which is provided in a fluidized bed zone of a 
polymerization vessel in the gaseous phase polymerization of olefins such 
as ethylene and propylene and is suitable for performing uniform and 
effective stirring of the fluidized bed zone. According to the anchor 
agitator of the invention, it is possible to avoid the formation of 
coherent crusts or lumps near the inner wall of the polymerization vessel, 
and the formation of agglomerated masses or lumps of a polymer owing to a 
vortex formed centrally in a fluidized bed zone of the vessel in the 
vertical direction of the zone, i.e. in the direction of its depth. 
Furthermore, there is no likelihood of vibration owing to the mechanical 
imbalance of the agitator, and the agitator can be easily designed and 
built. 
In the gaseous phase polymerization of olefins, it is desired to mix solid 
particles of the resulting pulverulent polymer and catalyst particles 
uniformly and sufficiently with gaseous olefin monomers and, in some 
cases, also with an easily volatile hydrocarbon or a liquid olefin by 
using an agitator which can impart a uniform and effective agitating 
action to the fluidized bed zone of a polymerization vessel. However, no 
industrially satisfactory means for uniform and effective mixing has been 
provided to date. 
In addition to the anchor agitator, a paddle type agitator comprising a 
plurality of agitating blades spaced at suitable intervals along a 
rotating shaft, and a screw agitator for circulating the fluidized bed 
zone in a vertical flow pattern have also been known heretofore for use in 
a gaseous phase polymerization vessel. These conventional agitators, 
however, have some drawbacks when used in a gaseous phase polymerization 
vessel. 
The most common type of the anchor agitator comprises a pair of arms 
secured to a rotating shaft at right angles thereto and agitating blades 
extending substantially parallel along the axis of the rotating shaft and 
provided at opposite outward end portions of the arms located 
substantially equidistant from the shaft. With this type of anchor 
agitator, a powdery polymer undergoes a centrifugal action by the rotation 
of the agitator and gathers on, and adheres to, the inner wall surface of 
the polymerization vessel to form coherent crusts or lumps. In addition, a 
vortex is formed at the central part of the fluidized bed zone in the 
perpendicular direction of the zone, that is, in the direction of its 
depth. A monomer gas introduced from the bottom of the zone flows 
deviatingly into this part to form agglomerated masses or lumps of the 
polymer. Investigations of the present inventors have shown that the 
deviating flow of the monomer gas to the central part of the fluidized bed 
zone cannot be blocked even by providing two additional stirring blades in 
those parts of the arms which are nearer to the rotating shaft. 
The paddle-type agitator, on the other hand, does not cause the trouble of 
vortex formation in a gaseous phase polymerization vessel, but has the 
defect that the flowability of the polymer near the inner wall surface of 
the polymerization vessel is poor, and the polymer strongly tends to 
adhere and agglomerate near the wall surface of the vessel to form 
coherent crusts or lumps. 
With the screw agitator, a deviated flow of the polymer occurs in the 
downward portion of the fluidized bed because the fluidized bed makes a 
circulating motion up and down. Accordingly the powdery polymer changes 
into agglomerated masses or lumps in the above portion of the fluidized 
bed, and the agitator is unsuitable for long-term continuous operation on 
an industrial scale. 
As a modified anchor agitator provided in an attempt to remove the trouble 
of formation of coherent crusts or lumps in a gaseous phase polymerization 
vessel, U.S. Pat. No. 3,545,729 discloses an anchor agitator having two 
arms for use in a stirring vessel, said arms being rotatable about the 
center line of said vessel, wherein the arms are disposed parallel to the 
center line of the vessel at different distances from the center line, the 
outer arm being longer and closer to the bottom of the vessel than the 
inner arm and both arms having a streamlined cross section externally 
defined by arcs of a circle or of a logarithmic spiral. 
Investigations of the present inventors have shown that when the rotating 
speed of the anchor disclosed in the above-cited U.S. patent is increased 
in order to give a sufficient agitating effect, vibration occurs in the 
agitator, presumably because of an imbalance in mechanism owing to the 
different arm lengths and the different arm mounting positions on the 
rotating shaft. Furthermore, it has been found that since the rising part 
of the arms, that is the cross sectional shape of the stirring blades, is 
complex, the agitator is complex and costly to design and build. Another 
defect the inventors have found is that the trouble of vortex formation in 
the anchor agitator cannot virtually be avoided in practice. 
The present inventors have undertaken extensive investigations in order to 
provide an anchor agitator for gaseous phase polymerization which is free 
from the aforesaid defects and can exhibit an exceptionally good agitating 
performance in gaseous phase polymerization. 
These investigations have led to the discovery that an anchor agitator for 
a gaseous phase polymerization vessel, comprising a rotating shaft, at 
least two pairs of arms provided on the rotating shaft in mutually 
crossing positions, and agitating blades provided at opposite outward end 
portions of each pair of arms, said blades extending substantially 
parallel along the axis of the rotating shaft and being located 
substantially equidistant from the axis of the rotating shaft in each pair 
of arms, which anchor agitator have the structural features (I), (II) and 
(III) given below, can achieve a uniform and effective stirring of a 
fluidized bed in a gaseous phase polymerization vessel while preventing 
formation of coherent crusts or lumps, a vortex, and agglomerated masses 
or lumps due to the vortex and the occurrence of vibration. 
(I) Of said at least two pairs of blades, one pair of blades is a pair of 
main blades located farther from the axis of the rotating shaft than the 
remaining at least one pair of blades, and said remaining blades are 
subsidiary blades located nearer to the axis of the rotating shaft. 
(II) Each of the main blades meets the following requirements. 
(II-1) its horizontal section crossing at right angles to the axis of the 
rotating shaft is of a triangular shape, and in rotation, the interior 
angle .alpha. of the vertex A forming the forward end of the triangle in 
the advancing direction is defined by 
10.degree..ltoreq..alpha..ltoreq.45.degree.; 
(II-2) the internal angle .beta. of the vertex B of the triangle which is 
farther from the axis of the rotating shaft of the two vertexes B and C in 
the rearward portion of the triangle in the advancing direction is defined 
by 80.degree..ltoreq..beta..ltoreq.150.degree.; 
(II-3) the shortest distance r.sub.1 between the vertex A and the axis of 
the rotating shaft and the shortest distance r.sub.2 between the vertex B 
and the axis of the rotating shaft satisfy the relation 0.8r.sub.1 
.ltoreq.r.sub.2 &lt;r.sub.1 ; and 
(II-4) the distance l between the vertex A and the vertex C and the 
distance r.sub.1 satisfy the relation 0.1r.sub.1 
.ltoreq.l.ltoreq.0.5r.sub.1 ; 
(III) Each of the subsidiary blades meets the following requirements. 
(III-1) the shortest distance r.sub.3 between the axis of the rotating 
shaft and that site of each subsidiary blade which is farthest from the 
axis of the shaft and the distance r.sub.1 satisfy the relation 0.2r.sub.1 
.ltoreq.r.sub.3 .ltoreq.0.8r.sub.1 ; and 
(III-2) each of the subsidiary blades is located within a range of 
.+-.60.degree. with respect to a straight line perpendicularly crossing 
the straight line which passes through the centers of the triangles of the 
main blades in the horizontal section. 
It is an object of this invention therefore to provide an improved anchor 
agitator for a gaseous phase polymerization vessel. 
The above and other objects and advantages of the invention will become 
apparent from the following description.

Referring to FIGS. 1 and 2, two pairs of arms 2,2' and 3,3' are provided on 
a rotating shaft 1 of the agitator at mutually crossing positions. 
Agitating blades 4,4' and 5,5' are provided at outward end portions of the 
arms 2,2' and 3,3'. The agitating blades extend substantially parallel 
along the axis X of the rotating shaft 1, and in each pair of the arms, 
the blades are located substantially equidistant from the axis X of the 
rotating shaft 1. 
In the anchor agitator of the above structure, one pair of blades 4,4' is a 
pair of main blades which are located farther from the axis X than the 
other pair of blades 5,5', and the other pair of blades 5,5' are 
subsidiary blades [requirement (I)]. If, contrary to the above requirement 
(I), the main blades 4,4' are located near the axis X, and the subsidiary 
blades 5,5' are located farther away, a vortex will be formed at the 
central part of the fluidized bed zone, and agglomerated masses or lumps 
will be formed at this part. 
It is essential that the main blades should meet the requirements (II-1) to 
(II-4) described below. 
(II-1) The horizontal section of each main blade crossing the axis X at 
right angles thereto is substantially of a triangular shape as shown in 
FIG. 2, and in rotation, the interior angle .alpha. of the vertex A 
forming the forward end of the triangle in the advancing direction, i.e. 
the vertex A forming the forward end in the advancing direction of the 
triangle ABC in FIG. 2 when the blade rotates in the direction of arrows 
along the inside wall 6 of the polymerization vessel, is defined by 
10.degree..ltoreq..alpha..ltoreq.45.degree.. If the interior angle .alpha. 
is less than 10.degree., the ability of the blades 4,4' to scrape a 
powdery polymer into the central part of the polymerization vessel is 
reduced. If the angle .alpha. is larger than 45.degree., the powdery 
polymer is pushed toward the inner wall 6 of the polymerization vessel, 
and coherent crusts or lumps of the polymer tend to form. The angle 
.alpha. is therefore set at 10.degree..ltoreq..alpha..ltoreq.45.degree., 
preferably 20.degree..ltoreq..alpha..ltoreq.40.degree.. 
(II-2) The interior angle .beta. of the vertex B which is farther from the 
axis X than the two vertex C in the rearward portion of the triangle in 
the advancing direction is defined by 
80.degree..ltoreq..beta..ltoreq.150.degree.. A a result of designing the 
main blades in this way, a suitable turbuent flow zone having a reduced 
pressure is formed rearwardly of the blades 4,4' (in the vicinity of the 
side BC of the triangle ABC in FIG. 2), which, however, is not the case 
with the agitator disclosed in the above-cited U.S. Pat. No. 3,545,729. As 
a result, in conjunction with the action of the subsidiary blades 5,5', 
the main blades 4,4' serve to eliminate the trouble of vortex formation at 
the central part of the fluidized bed zone which is attributed to the 
deviating flow of gas toward the central portion of the agitator. The 
preferred interior angle .beta. is 
85.degree..ltoreq..beta..ltoreq.135.degree.. 
(II-3) In each of the main blades 4 and 4', the shortest distance r.sub.1 
between the vertex A and the axis X and the shortest distance r.sub.2 
between the vertex B and the axis X satisfy the relation 0.8r.sub.1 
.ltoreq.r.sub.2 .ltoreq.r.sub.1. If r.sub.2 is equal to, or larger than, 
r.sub.1, the powdery polymer tends to be readily pushed densely into the 
space between the inner wall 6 and the blades, and formation of polymer 
coherent crusts or lumps is facilitated. If r.sub.2 is smaller than 0.8 
r.sub.1, the powdery polymer tends to stagnate in the vicinity of the 
inner wall 6, and the reaction conditions at this portion differ from 
those at the central portion of the polymerization vessel. Consequently, 
the quality of the resulting polymer is likely to vary. 
(II-4) In each of the main blades 4 and 4', the distance l between the 
vertex A and the vertex C and the distance r.sub.1 satisfy the relation 
0.1r.sub.1 .ltoreq.l.ltoreq.0.5r.sub.1. If l is smaller than 0.1r.sub.1, 
the ability of the blades 4 and 4' to convey the powdery polymer toward 
the central portion of the fluidized bed zone is reduced, and a vortex is 
likely to form at the central portion of the fluidized bed. Accordingly, 
the main blades should be designed to meet the above conditions. If the 
distance exceeds 0.5 r.sub.1 the space occupied by the blades becomes 
excessive, and the power consumption for operating the agitator increases 
disadvantageously. 
In addition to the above requirements (II-1) to (II-4), of the main blades, 
the subsidiary blades 5,5' must meet the requirement (III) consisting of 
(III-1) and (III-2). 
(III-1) The shortest distance r.sub.3 between the axis X of the rotating 
shaft 1 and that site of each of the subsidiary blades 5 and 5' which is 
farthest from the axis X and the distance r.sub.1 should satisfy the 
relation 0.2r.sub.1 .ltoreq.r.sub.3 .ltoreq.0.8r.sub.1. If r.sub.3 is less 
than 0.2 r.sub.1 or exceeds 0.8 r.sub.1, a vortex is formed in the 
interface at the central portion of the fluidized bed zone, and a deviated 
flow of the gas toward the central portion is generated. 
(II-2) Each of the subsidiary blades 5 and 5' is located within a range of 
.+-.60.degree. with respect to a straight line Y.sub.1 perpendicularly 
crossing the straight line Y.sub.2 which passes through the centers of the 
triangles of the main blades in the horizontal section (the straight line 
Y.sub.2 is the line formed by connecting the center points of the sides AC 
of the main blades 4 and 4'). The range is shown by P and P' in FIG. 2. 
If the positions of the subsidiary blades 5 and 5' come too close to the 
main blades 4 and 4' beyond the above-specified ranges, formation of a 
vortex at the central portion of the fluidized bed zone is further 
facilitated to form large amounts of agglomerated masses or lumps of the 
polymer. 
The lengths of the main blades 4 and 4' and the subsidiary blades 5 and 5' 
can be changed as desired in the anchor agitator of the invention. 
Preferably, the blades are designed such that their lengths correspond to 
about 30 to about 120%, especially about 60 to about 100%, of the height 
of the fluidized bed zone in the gaseous phase polymerization vessel. 
In providing the anchor agitator of the invention in a gaseous phase 
polymerization vessel, they are preferably positioned such that the 
vertexes A and B of the main blades have a clearance of about 0.003 D to 
about 0.2 D, where D is the diameter of the polymerization vessel, from 
the inside surface 6 of the polymerization vessel. 
If desired, two or more pairs of subsidiary blades may be provided as shown 
in FIG. 3. The sectional shape of the subsidiary blades can be changed as 
desired, and may be triangular as shown in FIG. 4. In FIG. 4, the patterns 
of the agitated flows are shown by arrows. 
Below are shown an example in which propylene was polymerized using a 
gaseous phase polymerization vessel having the anchor agitator of the 
invention, and a comparative example in which the polymerization of 
propylene was effected by using a comparative agitator not including the 
subsidiary blades. 
Experimental Apparatus 
An apparatus of the type illustrated in FIG. 5 was used. An anchor agitator 
12 equipped with main blades 13 and subsidiary blades 14 was set in a 
reactor 10. The anchor agitator 12 was secured to a rotating shaft 15 for 
rotation via a reduction gear 17 by a motor 18. To prevent gas leakage, a 
shaft seal 16 was provided in the rotating shaft 15. A gas sparger plate 
19 was provided at a position 400 mm from the bottom of the reactor, and 
the clearance between the gas sparger plate 19 and the lower end portion 
of the blades 13 and 14 was adjusted to 20 mm. 
Specification of the reactor: 
Total height: 1500 mm 
Inside diameter: 300 mm 
Mounting position of the gas sparger plate: 400 mm above the bottom of the 
reactor 
Material: SUS 304 stainless steel 
Specification of the agitator: 
(Main blades) 
Sectional shape: triangle 
Interior angle .alpha.: 30.degree. 
Interior angle .beta.: 90.degree. 
Distance r.sub.1 : 135 mm 
Distance r.sub.2 : 130 mm 
Distance l: 45 mm 
(Subsidiary blades) 
Farthest position (r.sub.3): 70 mm 
Near position: 40 mm 
Width: 20 mm 
With respect to the straight line connecting the centers of the main 
blades: crossing at right angles 
(Lengths of the main and subsidiary blades): 460 mm 
(Diameter of the rotating shaft): 40 mm 
Synthesis of a catalyst: 
A 200 ml flask was charged with 7.2 g of anhydrous MgCl.sub.2, 23 ml of 
decane and 23 ml of 2-ethylhexanol, and they were reacted at 120.degree. 
C. for 2 hours to form a uniform solution. Then, 1.68 ml of ethyl benzoate 
was added. 
To a 400 ml flask was added 200 ml of TiCl.sub.4, and all of the uniform 
solution obtained as above which was cooled at -20.degree. C. was added 
dropwise over 1 hour, and then the mixture was heated to 80.degree. C. It 
was stirred at 80.degree. C. for 2 hours. The solid portion was collected 
by filtration, and suspended in 200 ml of a fresh supply to TiCl.sub.4. 
The mixture was stirred at 90.degree. C. for 2 hours. After the stirring, 
the solid portion was collected by hot filtration, and washed thoroughly 
with hot kerosene and hexane to give a titanium catalyst component which 
contained 4.5% by weight of Ti, 60% by weight of Cl and 18% by weight of 
Mg and had an average particle diameter of 15 microns and a specific 
surface area of 195 m.sup.2 /g. 
Pre-treatment of the catalyst: 
The resulting catalyst slurry was again suspended in hexane so that its 
concentration was 5 mmoles/liter as Ti atom. Triethyl aluminum was added 
so that its concentration reached 15 mmoles/liter. Furthermore, propylene 
was fed in a proportion of 0.5 g per gram of the titanium catalyst 
component and the treatment of the catalyst was performed at 40.degree. C. 
Polymerization: 
In the experimental apparatus shown in FIG. 5, polypropylene powder having 
an average particle diameter of 450 microns was filled to a height of 450 
mm from the gas sparger plate 19. Propylene gas was fed into the apparatus 
through an opening 23, and circulated by operating a circulating blower 
25. 
The titanium catalyst slurry whose concentration was readjusted to 1.0 
mmoles/liter of hexane was fed from an opening 20 at a rate of 1 liter/hr, 
and triethyl aluminum was fed through an opening 21 at a rate of 25 
mmoles/hr. Furthermore, as a third component, methyl p-toluate was fed 
through an opening 22 at a rate of 8.5 mmoles/hr. 
Propylene was fed from the opening 23 at a rate of 5 kg/hr, and hydrogen 
was also fed therefrom at a rate of 0.04 mole per mole of propylene. The 
reaction temperature was set at 80.degree. C. by using a cooler 26. The 
reaction pressure was set at 9.8 kg/cm.sup.2.G. The rotating speed of the 
stirring blades was 150 rpm. The superficial velocity in a column of the 
gases was 10 cm/sec. 
The resulting polymer was withdrawn from an opening 24 so that the amount 
of the propylene polymer in the polymerization vessel was substantially 
constant. There was obtained a propylene polymer having an isotacticity 
index of 94.5%, a melt flow index of 8.5, a bulk density of 400 kg/m.sup.3 
and an average particle diameter of 450 microns. 
When the polymerization was performed continuously for 70 hours, no trouble 
attributed to the formation of polymer masses or the adhesion of the 
polymer to the reactor wall occurred. 
For comparison, the above procedure was repeated except that the anchor 
agitator did not contain the subsidiary blades. 
In 20 hours from the start of the reaction, the driving power for driving 
the motor for the agitator increased to 3 KW from 2 KW. Hence, the 
reaction was stopped and the inside of the apparatus was inspected. A 
polymer mass having a diameter of about 200 mm was seen to form at the 
lower portion of the fluidized bed zone.