Note: Descriptions are shown in the official language in which they were submitted.
~3~69
This invention is directed to an improved process
for the removal of a volatile material from an aqueous latex.
The art is well known for the removal of a volat~le
material, such as for example, styrene or acrylonitrile, from
an aqueous latex by means of a steam stripping type operation
whereby the aqueous latex flows downward through a column
opersting at sub-atmospheric pressure containing a series of
trays and steam is passed countercurrently upward through the
column passing through apertures in the trays and then through
the aqueous latex on each tray and the volatile material is
removed together with the steam from the top of the column.
Such stripping columns are well known for the high steam
consumption necessary to achieve the low residual monomer levels
necessary in the latex and for the propensity to foul or
plug-up, necessitating extensive cleaning and the concurrent
downtime.
It is an ob~ective of our invention to provide an
improved process for the removal of volatile material from an
aqueous latex, said process comprising passing said latex
downwardly through a column and passing steam upwardly through
said column, said column being equipped with trays having
perforations thereon and a downcomer attached to each tray
whereby said latex flows across each tray and downward from
one tray to the next lower tray by said downcomer and said
steam passes through said trays and the latex thereon by said
perforations, the improvement being that, in combination, at
least the top one-fourth of the trays are equipped with weirs
of at least 15 cm in height and the downcomers from at least
the top tray to the second tray and from the second tray to
the third tray terminate at least 5 cm above the surface of
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the tray to which they are supplying the latex.
According to our invention, there is provided an
improved process for the removal of volatile material from an
aqueous latex which process comprises passing said latex
downwardly through a column and passing steam upwardly through
said column, said column being equipped with from eight to
fourteen trays having perforations thereon and a downcomer
attached to each tray whereby said latex flows across each
tray and from one tray to the next lower tray by said town-
comer and said steam passes through said apertures on said
trays and the latex thereon, said column being operated at a
pressure at the upper end thereof of about 5 to about 20 cm of
mercury, the improvement being that, in combination, at least
the top one-fourth of said trays in said column are equipped
with weirs of at least 15 cm and up to 25 cm height and the
downcomers from at least the top tray to the second tray and
from the second tray to the third tray terminate at a point
at least 5 cm but not more than 15 cm above the surface of
the tray to which they are supplying latex, such that there
i8 at least 7.5 cm but not more than 15 cm between the lowest
point of said downcomer and the uppermost point of the weir
on the tray being supplied by said downcomer.
Aqueous latices which may be used in our invention
are prepared by emulsion-free radical polymerization of C4-C6
conjugated diolefins and vinyl or vinylidene aromatic hydro-
carbon monomers or vinyl or vinylidene nitrile group con-
taining monomers. Suitable vinyl or vinylidene aromatic
hydrocarbon monomers include styrene, alpha-methylstyrene,
vinyl toluene and divinyl benzene. Suitable vinyl or vinyli-
dene nitrile group containing monomers include acrylonitrile,
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methacrylonitrile and ethacrylonitrile. The polymers of
such latices are high molecular weight, solid, normally
rubbery materials. Subsequent to the polymerization in a
reactor chain, the aqueous latex containing unreacted monomers
is transferred optionally to a storage tank and then to one
or more flash tanks which are operated at reduced pressure and
serve to flash off the unreacted volatile C4-C6 conjugated
diolefin, e.g. butadiene or isoprene. The latex is then
transferred to a column for removal of the last traces of the
volatile conjugated diolefin and for removal of the unreacted
vinyl or vinylidene monomers, especially for removal of
unreacted styrene and acrylonitrile and small residual amounts
of butadiene. Because this stripping column i8 operated at a
slightly elevated temperature there is a tendency for the latex
to coagulate and deposit on the surfaces within the column
thereby causing fouling of the column. When the extent of
fouling has reached the point that the latex flow rate or the
stripping efficiency i8 too low, the column has to be taken
out of service for cleaning.
Within the column, w~ich is a vertical elongated
tubular shell, there are from eight to fourteen, preferably
from ten to twelve, substantially horizontal trays spaced
vertically apart and each tray i8 equipped with a weir and a
downcomer. The latex is fed to the top tray by a suitable
distributor system and the stripped latex is removed from
the bottom tray for subsequent passage to the coagulation
facilities or to facilities for storage of the latex. Each
tray contains a number of perforations, usually about 0.4
to 1.3 cm diameter on a 1.5 to 4 cm triangular pitch. The
weir is located at one side of the tray with the downcomer
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from that tray to the next lower tray being attached to the
tray at a point below the weir. Diametrically opposite to
the weir is where the downcomer from the next upper tray
directs the flow of latex onto the tray. Preferably, each
tray is equipped with a weir which is from 15 to 25 cm high.
For a better understanding of the invention,
reference is now made to the drawings.
Figure 1 is a schematic outline of a tray, weir
and downcomer according to the prior art.
Figure 2 is a schematic outline of a tray, weir
and downcomer according to improvements in the prior art.
Figure 3 is a schematic outline of a tray, weir
and downcomer as one embodiment of the present invention.
In Figure 1, 1 represents the tray, the perforations
not being shown. A weir 2 controls the level of liquid on
' the tray. Attached to the tray at a point below the weir is
the downcomer 3 leading to the next lower tray. A foam baffle
4 prevents excessive foam from flowing back into the downcomer
5 from the next upper tray and assists in achieving uniform
distribution of the latex flow to the tray. Sump 6 provides
a limited reservoir for latex flowing down the downcomer from
the next upper tray. The shell of the column is generally
shown at 9. The weir 1 is generally from about 2 to about 8
cm in height. The foam baffle is about 5 to about 8 cm in
vertical height from the tray, although it is slanted at an
angle of about 20 from the vertical. The depth of the sump
6 is shown at 7 and is generally about 20 to about 25 cm. The
distance between the bottom of downcomer 5 and the bottom of
sump 6, shown at 8, is about 15 to about 17 cm. The downcomer
terminates at a point from ~bout 3 to about 10 cm below the
~.3E~
level of the tray.
An improved form of a tray is shown in Figure 2,
in which 21 represents the tray, equipped with a weir 22 and
having a downcomer 23 attached to the tray below the weir.
The downcomer from the next upper tray 24 allows the latex to
flow into the sump 25. The shell of the column is generally
show~ at 28. The weir 22 is at least 15 cm and up to 25 cm
in height thereby providing a greater depth of latex on the
tray. The depth of the sump 25 is shown at 26 and is generally
about 20 to about 25 cm. The distance between the bottom of
the downcomer 24 and the bottom of the sump 25, shown at 27,
is about 15 to about 17 cm. The downcomer terminates at a
point from about 3 to about 10 cm below the level of the tray.
Figure 3 shows one embodiment of the present
invention, the tray 41 being equipped with a weir 42 and an
associated downcomer 43. The downcomer 44 from the next upper
tray provides for the flow of latex onto the tray. The sump
45 is as found in the prior art. The shell of the column is
generally shown at 48. The depth of the sump is shown at 46
and is generally about 20 to about 25 cm. The weir 42 i8 at
least 15 cm and up to 25 cm in height. The distance between
the bottom of downcomer 44 and the bottom of the sump 45,
shown at 47, is from about 30 to about 35 cm and the distance
between the bottom of downcomer 44 and the top of weir 42
(the top of weir 42 being above the bottom of downcomer 44)
is from about 7.5 to about 15 cm.
~he process of the present invention is applicable
to the removal of a range of volatile materials from an
aqueous latex. To illustrate an operation of the present
process, there will now be described the removal of styrene
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1~3~7d~9
ant butadiene from a styrene-butadiene polymer in aqueous
latex. Such description does not limit the scope of the
invention and only serves to illustrate the same.
A styrene-butadiene polymer was prepared by a con-
ventional free radical emulsion polymerization process. The
polymerization was stopped, for quality control reasons, at a
total monomer conversion of about 65 to 67 per cent. The latex
from the reactor thus contained a high level of unreacted
butadiene and unreacted styrene. The latex was fed to the first
of two flash tanks connected in series in which the pressure is
finally reduced to about 20 cm of mercury and a major propor-
tion of the butadiene is vaporized therein and removed.
The latex from which the majority of the unreacted
butadiene had been removed was then fed to the top of a
styrene stripping column. The stripping column comprised a
vertical elongated tubular shell having twelve substantially
horizontal trays spaced vertically apart, each tray having
a plurality of apertures. Each of the trays was equipped with
a weir and a downcomer communicating with said weir in order
that the aqueous latex may flow downwardly from one tray to
the next lower tray. The downcomer was directed to and in
contact with the aqueous latex on the next lower tray so that
upward flow of steam does not pass through the downcomer.
The downcomer was eccentrically located with respect to the
tray to which it was attached and the downcomer on one tray
was located at an essentially diametrically opposite position
to the location of the downcomers of both the next upper tray
and the next lower tray. The top tray of the column was
equipped with a downcomer directed to the second tray from the
top. The outline of the tray may be appreciated by reference
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to Figure 3, the downcomer 44 from the top tray being directed
to the second tray from the top 41 and terminating 10 cm below
the top of weir 42 which was 15 cm high. Sump 45 was 25 cm
deep, the downcomer terminating 30 cm above the bottom of the
sump and therefore 5 cm above the level of tray 41. The second
tray from the top 41 was similarly equipped with a downcomer
directed to the third tray from the top, said downcomer
terminating 5 cm above the level of said third tray. The weir
and sump on the third tray were of the same dimensions as
- 10 those of the second tray from the top. The remaining trays
of the column were as shown in Figure 2, the weir 22 being 15
cm high, the sump 25 being 25 cm deep and the downcomer 24
terminating 16.5 cm above the bottom of the sump and therefore
below the level of the tray. The stripping fluid used was
desuperheated steam at about 1 kg/cm2 gage supplied to the
bottom of the column and passed upwardly through the column
by passing through the apertures in the trays and thereby
stripping the styrene, residual butadiene and any other
volatile hydrocarbon impurities from the latex as it flowed
over the ~rays. The steam and stripped hydrocarbons were
removed from the top of the column and the s~ripped latex was
removed from the bottom of the column. The pressure in the
column was about 20 to about 40 cm of mercury at the lower end
of the column and about 5 to about 20, preferably from about
7 to about 15, cm of mercury at the upper end of the column.
Residual styrene in the stripped latex removed from the bottom
of the column was found to be from about 0.03 to about 0.07
weight per cent based on the latex.
Operating the process in this manner, it was found
that the column could be used for times from 30 per cent to
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50 per cent longer, without having to be shut down because of
fouling, than when the prior art processes were used.
Using the process of the present invention, it was
- found that when fouling of the columns did occur, it was as
balls of agglomerated polymer which collected mainly on the
trays and in the æump. Using the present process, the fouling
on the trays finally becomes sufficient to prevent the flow
of the steam through the perforations of the trays, thereby
necessitating cleaning of the column. It appears that the
time for the build-up of such a state of fouling is much
longer than when using processes of the prior art. Accumula-
tion of fouling material in the sump does not appear to be of
large enough magnitude to cause blockage of the flow of latex
therethrough. In the prior art processes, accumulation of
fouling material in the sump caused about 50~/O of the column
shut downs for cleaning and accumulation of fouling material
on the tray~ accounted for about 50% of the column shut downs
for cleaning.
In a similar manner, a stripping column may be used
in which from three to all twelve trays may be equipped in
the manner described herein and the process may be operated
using such a column for further im~rovement in the operating
time before a shut down becomes necessary due to fouling.
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