Vertical autoclave for bulk polymerization of vinyl chloride based polymers and copolymers

A vertical autoclave suitable for the bulk polymerization of vinyl chloride based polymers and copolymers embodying agitation means comprising at least one agitator (A) in the form of helicoidal ribbon about a rotatable shaft in the upper portion of the autoclave and an agitator (B) having a rotatable shaft passing through the bottom portion of the autoclave with at least one arm located adjacent to and having a profile corresponding to the bottom of the autoclave, and means for independently driving the agitators (A) and/or (B).

This invention relates to a vertical autoclave for bulk preparation of 
vinyl chloride based copolymers and polymers. It also relates to a method 
of preparation, in bulk, of vinyl chloride based polymers and copolymers 
which employs said autoclave. 
It has already been suggested to effect preparation, in bulk, of vinyl 
chloride polymers and copolymers in two stages carried out in different 
vessels by a method consisting of carrying out, in the first stage, a 
limited polymerization, known as prepolymerization, with agitation at high 
turbulence, until a conversion of the monomer or monomers of the order of 
7% to 15% is achieved, then to effect, in a second stage, a final 
polymerization operation under low agitation, the speed of agitation in 
this step however remaining sufficient to maintain the temperature of the 
reaction medium substantially uniform until the end of the polymerization 
or copolymerization reaction. 
This process and variations thereon have been described in French Patents 
and Certificates of Addition Nos. 1,382,072, 84,958, 84,965, 84,966, 
85,672, 89,025 and Nos. 1,436,744, 87,620, 87,623, 87,625, 87,626. 
In a particular embodiment of the process, the final polymerization is 
effected in a vertical autoclave provided with an agitator comprising a 
single agitator element consisting of at least one ribbon wound in 
helicoidal turns, fixed by means of at least one support member, on a 
rotating shaft extending into the autoclave along the axis of symmetry 
thereof and integral, at its lower end, with at least one arm which is 
adjacent and follows the shape of the curved bottom of the autoclave. 
Such agitation apparatus has the following disadvantages, which increase 
with the size of the autoclave; 
The agitator must be constructed in two parts to be assembled inside the 
autoclave if it is desired to use a one-piece autoclave vessel; the use of 
two-piece vessels, in which the vessel parts are held together by a large 
flange or the like, considerably increases the cost. 
Withdrawal of the agitator from a one-piece autoclave cannot be carried out 
without initial cutting. 
The vertical shaft has to be supported in a mounting at its lower end to 
avoid unacceptable stress on the associated mechanical equipment at the 
top of the autoclave. 
The driving equipment has to have a large power transmission capacity 
because the speed of rotation of the ribbon, which determines the 
efficiency of mixing, is equal to that of the arm or arms which are 
integral with the rotating shaft, and the latter consume an increased 
amount of power at certain critical phases of the process, especially at 
the end of the operation of polymerization during degassing of the 
monomeric composition which has not reacted and breaking of the vacuum. 
The design is complicated, in the case of embodiments having facilities for 
internal cooling by circulation of refrigerating fluid. 
The autoclave, according to the present invention, is intended and has for 
its object to avoid or reduce the disadvantages mentioned above. 
According to one aspect of the invention, there is provided a vertical 
autoclave suitable for the bulk preparation of vinyl chloride polymers and 
copolymers provided with agitation means comprising at least one agitator 
(A) formed of a ribbon coiled in helicoidal turns about a rotatable shaft 
passing through the upper part of the autoclave and an agitator (B) 
comprising a rotatable shaft passing through the bottom of the autoclave 
along the vertical axis of symmetry thereof, connected to at least one arm 
adjacent to and having a profile closs to the bottom of the autoclave, the 
agitator or agitators (A) and the agitator (B) being independently driven. 
The agitator (B) generally comprises two arms. The agitators (A) and (B) 
may be hollow and have a heat exchange fluid circulating therein. 
According to one embodiment of the invention, a single agitator (A) and the 
agitator (B) are coaxial. 
According to another embodiment of the invention, agitators (A) are 
equidistant from the central axis of the autoclave and arranged in a 
generally regular manner about said axis. For example, agitators (A) may 
be three in number, equidistant from said axis and arranged at the corners 
of an equilateral triangle. This embodiment is especially favorable in the 
case of an autoclave of large capacity since it avoids the use of a very 
large transmission for driving agitators (A), which requires special 
design and construction.

The embodiments of FIGS. 1 and 3 comprises a single agitator (A). The 
embodiment of FIGS. 2 and 4 has three agitators (A). 
As shown in the drawings, a vertical axis autoclave 1 is surrounded by a 
jacket 2 in which a heat exchange fluid is circulated from an entrance at 
inlet 3 to an exit at outlet 4. The autoclave 1 comprises, at its upper 
part, an inlet 5 through which prepolymer and the necessary other 
reactants and possibly monomers are introduced and an outlet 6 for removal 
of the monomeric composition which has not reacted at the end of the 
operation. At its lower part, autoclave 1 has an outlet 7 for discharge of 
the polymer. The bottom 8 of the autoclave 1 is provided with a shaft 9 to 
which are connected two arms 10 and which is supported by the bearing 11, 
the liquid tightness between the shaft and the bottom of the autoclave 
being assured by the stuffing box 12. 
As shown in the embodiment of FIGS. 1 and 3, the upper part of the 
autoclave 1 is provided with a shaft 13 mounted for rotational movement 
along the central axis of the autoclave and supported by the bearing 14, a 
fluid tight seal between the shaft and the upper part of the autoclave 
being assured by means of a stuffing box 15. A ribbon 16 is wound in 
helicoidal turns on the shaft 13. 
In the embodiment of FIGS. 2 and 4, the upper part of the autoclave 1 is 
traversed by three rotatable shafts 17, 18 and 19 which are equidistantly 
spaced from the central axis of the autoclave, arranged at the apices of 
an equilateral triangle, and supported respectively by the bearings 20, 21 
and 22, the fluid tightness between the rotatable shafts and the upper 
part of the autoclave being assured by means of stuffing boxes 23, 24 and 
25. On the rotatable shafts 17, 18 and 19, ribbons 26, 27 and 28 are 
respectively arranged in helicoidal turns. 
The autoclave, according to the invention, has the following advantages 
when used for the final bulk polymerization of vinyl chloride based 
polymers and copolymers in two stages, carried out in different respective 
vessels. 
The possibility of using the optimum speeds for each of the agitators (A) 
and (B) and of reversing the sense of rotation of one or both to obtain 
optimum stirring. 
The possibility of reducing the speed of agitator (B), when desired, and of 
stopping the or one of the agitators (A) during evacuation of the 
autoclave to reduce the amount of power consumed. 
A less complex arrangement of the cooling circuits in embodiments using 
fluid circulation in the agitators (A) and (B). 
The use of integral components for the agitators, which are relatively 
light and therefore easy to manufacture and to mount without welding at 
the interior of the autoclave. 
The possibility of cooling agitators (A) and (B) to different temperatures. 
Formation of crust is reduced due to more efficient agitation of the 
reaction medium in the lower part of the autoclave. 
The possibility of keeping only one agitator in use in the case of failure 
of power supply, which allows for a reduction in the power of the standby 
electricity provided when the agitators are driven electrically. 
The possibility of cleaning the autoclave by means of a jet of water under 
very high pressure fed from a head, preferably rotating, which is 
introduced in the proximity of agitator (B) to reach the parts where the 
probability of formation of crust is high and where crusts formed are 
especially adherent. This advantage avoids the necessity for manual 
operations inside the autoclave and the risk of accident which may result 
therefrom.