Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a method for removing incrusta-
tions from the portions of reactors which are in contact with the
gaseous phase that occurs in the polymerization of vinyl chloride
in suspension, either alone or in admixture with other monomers.
; ` It is known that one of the major problems in the poly-
~,` merization in suspension of vinyl chloride, either alone or in
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admlxture with other monomers is caused by the incrustations of
~, polymer which are deposited on the reactor walls and which, if
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they are not removed, pollute the product of the subsequent batches.
The removal, either manually or mechanically with water under
; high pressures, requires that the reactor be opened for one hour
or more.
There are a number of patents which have been taken
recently, which claim to have solved the problem of the incrusta-
~^ tions on the walls of the reactor which are in contact with the
liquid phase.
Generally speaking, specific formulations are involved,
; ~ which prevent the deposition and require cleaning operations only
after some ten batches.
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The problem of the incrustations on those parts of the
;j, reactor which are in contact with the gaseous phase is still un-
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tS, solved. The tendency toward exploiting the reactors to the
utmost (high filling coefficients, vigorous stirring, high dis-
tlllations speeds of the residual monomer,and others) has aggra-
vated the phenomenon by encouraging a more intense formation of ~ -
foams and squlrts which deposit the polymer on the reactor ceiling.
After a few batches, the nozzles begin to become clogged, with the
danger that large lumps of polymer are detached and hinder the
;1 discharge, thus compromising the tightness of the bottom valves.
This impairs the correct performance of the loading and unloading
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~1 operations and the safety of the reacitor during polymerization
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becomes questionable.
~ Summing up, the removal of incrustations on the reactor
;~ ceiling with an antisoiling formulation is not the answer.
It has now been found tha-t with the present invention
; it is not necessary to open, at each batch, the reactor or the
`. visual inspection of its celling and for possibly removing -the
''.G~ incrustations.
. Surprisingly and unexpectedly, it has been found that :
the elimina-tion of incrustations can be achieved while ~
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;~ 10 maintaining polymeriza-tion runs of commercial importance by
keeping the ceiling under a continuous Elushing of water under
; a~low pressure when the conversion rate is from 15% to 25% to a
` conversion rate of at least 40%, and then as the reac-tion is
being completed by flushing the ceiling with wa-ter under high
~ pressure.
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The present invention therefore provides a method for
;~ polymerizing vinyl chloride either alone or in admixture with,.,,~ .
other monomers within a reactor, charac-terized in that, during
the reaction, the reactor ceiling is kept under a continuous
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flushing of water under a low pressure when the conversion rate
is from 15% to 25%, and which is continued at leas-t until 40%
conversion is attained and, as the reaction has been completed,
the residual incrustations on the reactor ceiling are removed
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mechanically with water under a high pressure.
, According to a preferred embodiment, the continuous
` flushlng of the reactor ceiling during the reaction is carried
with water under a pressure of at least 15 kilograms/square
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centimeter and such an operation is followed by a washing (for
example during the batch discharge and anyhow when the autoclave -
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is sealed? with water under a pressure of at least 100 kilograms/
~ square centimeter.
- The operation with water under a low pressure is
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`~ attained preferably with rates of flow of not less than 1,000
liters an hour.
Cleaning operations which are started when the conversion
rate is over 25% do not prevent the localization of the incrusta-
` tions in the form of large lumps, and premature operations do not
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afford any advantage inasmuch as the reduced free volume availablewith the usual industrial batches does not permit that the Lequired
rate of flow of water be maintained.
The subsequent operations with water under a high pressure
is made absolutely necessary to prevent the slight residual in-
crustation from growing thicker as the subsequent batches are fed
in: on the other hand, the flushing with water alone, as carried
out during the entire reaction time is capable of permanently
preventing the deposit of the polymer only with batches with a
low filling coefficient, slow stirring and the like, that is, in
polymerization runs which have an industrial importance of little
value.
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The ensuing examples, without limiting the scope of the
invention, demonstrate the necessity of the simultaneous adoption
of the two measures of the invention.
; EXAMPLE 1
A 25-cubic meter autoclave equipped with a stirrer and
a Pfaudler antislosh device, was filled to 70% oE its volume
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with a ratio water to vinyl chloride equal to 1:1 and stirred at
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a speed of 120 rpm.
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The ceiling was flushed during the whole reaction time
at a rate of flow of 1,000 liters an hour of water under a pres-
sure of 20 kilograms/square centimeter.
The final filling was as high as 90%. On completion of
the reaction, the ceiling was perfectly clean and remained clean
during all of the subsequent batchloads.
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. EXAMPLE 2
; The same autoclave as in Example 1 was filled to 90%
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0 with the same ratio of water to an 85/15 mixture of vinyl chloride
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and vinyl acetate. After flushing the ceiling with water under
a pressure of 25 kilograms/square centimeter, when the conversion
rate was from 20% to 40%, there were recovered 3 kilograms of
crusts which were evenly distributed all over the surface.
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In the subsequent batchloads washing with water under a
~; pressure of 300 kilograms/square cm was effected, using orientable
i. 10 nozzles.
.~ The autoclave ceiling was perfectly clean and no problems
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were~ experiencecl as to the unloading and the tightness of the
bottom valve.
, EXAMPLE 3
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In the same autoclave of the previous examples, there
were kept unaltered, relative to Example 2, the filling coefficient
and the ratio of water to vinyl chloride.
Upon flushing of the ceiling with water under a pressure
of 30 kilograrns/sq.cm. when the conversion rate was from 40% to
65%, there were collected 10 kilograms of crusts in the shape
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of coarse lumps.
Washing during subsecluellt batchloads with waker uncler
a pressure oE 350 kilograms/sq.cm. ~7as such so as to remove the -~
soiling integrally but it did considerably extend the unloading
times so that a manual operation was required for removing the
;i, polymer which clogged the bottom valve.
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The above described method can be supplied for autoclaves
of any size used in industry and to all known formulations used
in the industrial practice, including those adopted in patented
anti-soiling procedures.
The polymer1zatlon runs to wh1ch this application applies
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`'. are those of vinyl chloride, either alone or in admixture with
other monomers which contain 20% or less of comonomer; the comono-
' mers are those which are known to those skilled in the art in
': . that they can be copolymerized with vinyl chloride. For example,
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'' and without being limited to them, there are vinylidene chloride,
j'~- vinyl acetate, vinyl butyratej methyl acrylates and metacrylates,
'.. ~ butyl acrylates and metacrylates, isooctyl.a'crylates and metacry-
''~'' lates, diethyl and dipropyl maleates and fumarates, styrene,
~: ethylene, propylene, butylene, vinyl-ethyl ether, aIlyl acetate,
'' 10 diallyl phthalate, diallyl maleate, acrylonitrile, metacrylonitrile, :.
'~ and others.
'.;:` When reducing to practice what has been described herein- ~'
1 above, it is necessary to bear in mind expedients which are aimed .: :
.1at exploiting the effects oE the flushing and washing water without .:.
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prejudicing the correct performance of the reaction. . ' '
; For example, fixed installations jutting towards the `'-~''
;~ ~ interior' of the autoclave could be preferential polnt of polymer .' -'
~1 ; incrustations build-up, that which'considerably reduces the ef- :-'
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~ ficiency of the flushing and washing treatment.
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^~i. 20 Movable installations should thus be preferred, of the
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kind depicted in the appended drawings in which -the sole figure is
' a sectional view of a portion of an autoclave with an embodiment ;`'~
: of the'apparatus of the invention il; its withdrawn or inoperative, '. .. '.
..~ : non-cleaning position. As shown, water (under either low or high
'~ pressure) flows through the hollow shaft 1 and is projected against '
'.~; the autoclave ceiling 2, by means of nozzles 3, which are properly
:: sized and oriented, when the system has been lowered in the interior '':
of the autoclave. ; .
~ ' In the inoperative condition, a sealing packing 4 between .'
; 30 the stem and the stub S and the outline of the.bottom end of the :
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: stub ensures a tight seal without jutting from the autoclave ceiling. .
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In addition, the rotary movement of the stem as indi-
~ cated by the arrow in the drawings, enables the entire circum-
i~ ference of the autoclave to be hit by the washing water.
, The size and the orientation of the nozzles is a function
- of the operation and thus of the properties of the water (for
~; flushing under a pressure equal to or greater than 15 kgs/sq.cm.;
and washing under a pressure equal to or greater than 100 kys.sq.
cm.).
The number and the position of such installat:ions are a
function of the internal arrangement of the autoclave; antislosh
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plates, cooling pipes, stirrer shaft, thermocouples and all the
component parts which could interfere with the water jets.
The installation, irrespective of the type of sealing
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appliances and the way in which the necessary movemen-ts are ob-
tained, that which is a general matter of design, is characterized
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in any case by the fact that water lS caused to flow through a
hollow stem 1, which is axially rotatable, and water emerges throuyln
nozzles 3 which are appropriately oriented and designed.
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