Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BRIEF SUMMARY OF THE INVENTION.
This invention relates to latex stripping methods and,
more particularly, to a new and improved method o~ continuous
and efficient stripping of latices which are~sensitive to
excessive heat and agitation.
Heretofore, monomers have usually been extracted from
polymeric latices of low stability by steam sparged, batch-type,
vacuum stri~ping methods., Continuous stripping methods such as
' countercurrent tray columns, continuouS feea and overflow stirred
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tan}c~ (similar physically to batch), and tortuous path dispersed
flow ,~trippers as described by Palmason in U. S. patent No.
3, 469, 617, have been used to remove monomers from latices which
are relatively insensitive to shear or overheating. For continuous
production of foamy, low stability latices, by the batch method,
multiple batch units must be installed, each with a complete
- 3et of controls, to avoid foaming, and close operator attention
.
to each batch unit is required. Coagulation yield losses and
associated cleaning costs are high for many latices.
Continuous tortuous path type strippers, as described,
for example, in Palmason U. S. patent No. 3, 469, 617, have been
employed widely in latex stripping to overcome many of the
above-mentioned problems of batch stripping methods. The
Palmason method, by its very nature, treats a latex under highly
turbulent conditions involving considerable shear and relatively
high inlet steam-latex "mixing" temperatures. While these
conditions enhancP stripping efficiencies, some latices are
too unstable to process by this method. Continuous tower ~ype
strippers are also undesirable for very unstable and foamy
latices due to the cascading actions and the tendency of the
trays to become plugged by coagulum, and because of increasing pressure
and temperature from top to bottom of a column, they tend to cause
dilution of the processed latex.
Accordingly, it is an object of the pxesent invention
to provide a method for the continuous stripping of monomers
from synthetic polymcr latices which are sensitive to agitation
and high temperatures.
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Thus, in accordance with the present invention, a
metl~od is provided for stripping a volatile constituent from a
mixture containing a latex polymer which is sensitive to shear
and high temperature. The method comprises injecting the latex
into a confined stream of steam and passing the steam-latex
mixture concurrently through a tubular contactor having a length-
to-diameter ratio of at least 20:1. The temperature at the point
of injection is maintained between about 115F. and about 18~F.
Substantially all of the heat is supplied in the thbular
contactor by means of the steam-latex mixture passing through
the tubular contactor. The pressure at the point of injection
is maintained between about 1.5 psia and about 7.5 psia. The
steam-latex mixture is passed through at least one bend having
an angle of at least 45 within the contactor and directly into
a liquid-vapor separator. The pressure in the liquid-vapor
~eparator is maintained at at least 0.5 psia lower than the
pressure maintained at the point of injection of the latex and
the steam within the tubular contactor.
In certain embodiments of the invention, a latex to be
stripped is initially injected into and contacted with stripping
steam and then passed in cocurrent flow with the steam through
an unheated tubular contactor section from which the stripped
latex and monomer and/or solvent laden steam enter a cyclonic
vapor-liquid separator under vacuum. The stripped or partially
stripped latex flows out through the separator bottom and is
pumped either to a subsequent stripping stage or a storage tan~
or other process step.
DESCRIPTION OF THE DRAWING
The single figure of the drawing schematically
illustrates a representative apparatus arranged to carry out the
invention.
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1~70~79
DETAILED DESCRIPTION
- In the typical embodiment of the apparatus for carrying
out the invention schematically illustrated in the drawing, a
latex polymer containing a monomer to be stripped is supplied at
a controlled rate by a pump 10 through a temperature control 11 to
an injecting "fid" 12 disposed near one end of a tubular contactor
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1C~7~3~379
13, Steam supplicd through a pressure control 14 and a
ternperature control 15 is applied near the end 16 of the tubular
co~ntactor 13 wh~ch is adjacent to the fid 12.
The fid 12 may be of the type described and illustrated
in Fig. 3 of patent No. 3, 469, 617 and is positioned to provide
an annular space bet~,veen its outer surface and the interior wall
of the contactor 13 through which steam flows. An adjustable
an~ular groove 17 in the fid injects the latex into the annular
6pace, causing it to mix with the steam without producing
significant turbulence. To assure complete stripping without
overheating the latex, the temperature of the steam and latex
~upplied to the contactor are controlled so that the temperature
of the mixture at the point of mixing is at least 115F and no
more than 180F, the particular temperature being dependent on
the characteristics of the latex and the monomer as well as the
structure of the contactor. Below 115 F stripping is not ac-
complished efficiently and above 180 F heat sensitive latices are
degraded.
In order to assure adequate contact between the steam
and the latex without e~ccessive agitation, the tubular contactor
is shaped with two successive 90 bends, designated 18 and 19
in the drawing. The contactor may be of the general type described,
for example, in thc U. S. patent to Morro-v, No. 2, 487, 769, but no
heat is applied to ihe wall o~ the contactor. The tubular con-
2~ tactor may be madc of any materials such as carbon steel, stain-
less stecl, or glass, which has sufficicnt strength and is
resist~nt to corrosit-n by chc~ icals ancl thc total lcngth-to-
diamctcr ratio o the activc part of thc tul)ular contactor
should bc at lcast 20:1 and may be as hi~h aY 200:1, I~ desircd,
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~7~879
thc tubular contactor may have only one bend, and the angle of
each bend may be as small as 45,
Froln the outlet end 20 of the tubular contactor the
mixture of steam, latex polymer, and extracted monomer flows
to a cyclone-type vapor-liquid separator 22 from which the vapor
constituents are removed through a line 23 connected through a
.. . . .
pressure control unit 24. If desired, the exhausted vapor Irom the
line 23 may be passed through a monomer and solvent recovery
arrangement utilizing appropriate condensing and condensate removal
equipment (not shown).
To assure effecient stripping of the monomer in the
contactor 13 without e~;cessive heating or turbulence of the latex,
the pressure control unit 24 maintains the pressure in the
separator 22 at leasl: 0. 5 psi lower and preferable 1 to 7 psi
lower than the pressure at the mixing point adjacent to the fid
12, which is maintained between about 1. 5 psia and about 7. 5 psia.
Pressures at the mixing point below about 1. 5 psia, seriously
limit throughput capac ity and eficiency, while pr e s sure s greate r
than about 7. 5 psia cause degradation of heat sensitive latlces.
For substantially complete and effect1ve stripping
operation of certain latices at high flow rates while maintaining
opcration within the required temperature and pressure ranges,
a two-stage stripping operation is desirable. For this purpose,
the latex drawin from the bottom of the separator 22 through a
line 25 is transn~itted by a pump 26 into a second stage arranged
and operated in the same manner descri~ed in connection with the
first stage. For convcnicnce, the elements of the second stage
corresponding to tho5e of the first stage are identified in the
drawing with primed refcrence numerals,
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.
~ he following typical examples are illustrative of the
advantages achieved by the method of the invention:
Example 1
An acrylic polymer latex containing approximately 26-28%
solids and approximately 4% acrylonitrile monomer was to be
stripped at a feed rate of approximately 24,000 lbs./hr. to a level
of 0.4% acrylonitrile. To illustrate the disadvantages of the
~rior art, a two-stage process arra~ged in the manner shown in
the drawing, but using tortuous path plate type contactors of the
type described in U.S. patent No. 3,469,617! in place of the
tubular contactors 13, and 131. While this arrangement worked
well with respect to acrylonitrile removal, the rate of formation
and accumulation of polymer coagulum in the plate contactors was
excessive. This coagulum buildup caused the contactor pressure
drop to increase, thus incxeasing the temperature which, in turn,
accelerated the coagulum formation. The net result was an
excessive required shutdown and cleaning frequency.
The same acrylic latex polymer was processed in accor-
dance with the present invention utilizing dual parallel 6-inch
~iameter tubular contactors with length to diameter ratio of
approximately 25:1 in the arrangement shown in the drawing. ~his
~ystem operated successfully without the problem of capacity reduc-
tion due to coagulum deposits and the necessity for frequent
cleaning. Typical operating parameters for an embodiment of the
in~ention arranged for production at a commercial rate are given
below:
1st Stage 2nd Stage
~atex feed rate, lbs./hr.24,000 ------
Latex feed wt.% total solids 26 ------
0 Latex feed wt.~ acrylonitrile 4.2 0.8
Steam rate, lbs./hr. 1,690 1,690
Mixing Temperature, F 151 148
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379 l~t. Stage 2nd. Stage
Separator Temperature, F lZ0 120
- Stripped latex wt; % total solid~ _ 26. 4
5tripped latex wt. % acrylonitrile 0.8 0. 4
The above data represents fully acceptable operation and a sub-
stantial improvement over the operation of a system as described
in U, S. patent No. 3, 469, 617.
Example 2
It was desired to strip toluene from a feed emulsion
1~ containing 15% toluene, 30~o rosin, and 55% water. This emulsion
comprises emulsified particles of rosin-in-toluene solution in a
continuous water phase. Rapid removal of toluene leaves a rosin-
in^water emulsion which is very foamy and very sensitive to
temperature and shear coagulation, To illustrate the disadvantages
of the prior art, a run was made using a plate contactor of the
type disclosed in U. S. patent No. 3, 469, 617 in a single stage
process. Data from a 45-minute run are given below:
Start Run End Run
Emulsion feed rate, lbs. hr 81 81
Feed wt. ~o toluene 14, 3 14. 3
Feed wt, % rosin solids 27. 8 27. 8
Stripping steam rate, lbs. /hr. 44 41
Mixing temperature, F 162 172
Separator pressure, mm Hg 116 125
Product wt. % toluene 0. 52 1. 4
Lbs. steam/lb. rosin 1. 9 1, 7
During this short run, coagulum formed and deposited in the plate
contactor at an unacceptably high rate, causing the mixing
pressure and temperature to ri se and requiring shutdown. The
plates were opened and found to be heavily fouled.
In accordance with the present invention, a tubular
contactor having a 1-1/2" diameter and a length to diameter
ratio of ~8:1 and containing two 45 bends was assembled. When
the same emulsion was supplied to the tubular contactor system,
it operated successfuliy for 5 hours with no indication at that
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time of serious coagulum deposits. Typical data taken during
~his two-stage run are given below.
1st Stage 2nd Stage
Emulsion feed rate, lbs./hr. 245 211
Feed wt.~ toluene 14.1 2.2
~eed wt.% rosin solids 26.6 27.8
Stripping steam rate, lbs./hr. 62 51
Mixing temperature, F 135 136
Separator pressure, mm Hg.118 115
Product wt.~ toluene 2.3 0.5
Lbs. steam/lb. rosln 1.9 0.93
m e above data represented fully acceptable operation and a dis-
tinct advantage over the process of patent No~ 3,469,617.
The invention is especially effective for stripping feed
emulsions in which the volatiles (monomers or solvents) to be
stripped are mainly present, either dissolved or dispersed, in
the water phase from which they can be rapidly removed. One such
example is acrylonitrile which is water soluble. In the case of
Example 2, the toluene is not water soluble; but, because of its
high concentration, it acts similarly to a dispersed free solvent
(in the water phase).
Attempts to use the invention to strip monomers or sol-
vents which are closely bound to the latex have not thus far been
successful. ~or example, the stripping of styrene from car-
boxylated styrene-butadiene latices has not been accomplished
apparently because the styrene is contained mainly inside the
polymer particles and can not diffuse readily out to the water
phase under the conditions of the test. The results were
that styrene removal was poor and that severe foaming occurred,
probably because styrene diffused from the polymer and vaporized
as the latex discharged from the contactor into the separator.
From the fore~oing, it will be apparent that the present
invention provides a high capacity continuous stripping method
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which operates without foaming, with low process holdup, good
control, and OGCUpieS reiatively little plant space while'at the
same time overcoming the relatively rapid coagulum plugging and
necessary cleanout experienced by the Palmason method in this
5 ' service.
' Although the invention has been described herein with'
reference to specific embodiments and typical examples, many
mo*ifications and variations therein will readily occur to those
skilled in the art. Accordingly, all such variations and modi-' ~
'fications are included within the intended scope of the invention
as defined by the appended claims.
