Note: Descriptions are shown in the official language in which they were submitted.
~L$~3,~
Background of the Invention
Field of the Invention
This invention relates to organic polymeric
isocyanates and more particularly pertains to an improved
process for purifying and reducing the acidity in methylene-
bridged polyphenylpolyisocyanate mi~tures.
Description of the Prior Art
Organic isocyanates are generally prepared by the
phosgenation of corresponding amine compounds by one of the
many well-known phosgenation processes. These phosgenation
processes usually result in crude isocyanate products that
contain residual acidic materials which adversely affect the
reactivity of the isocyanate. The acidic material contaminants
are generally those which respond as acids in standard
analytical tests known and employed in the art. The acidic
material contaminants are known to include hydrogen chloride
and a variety of unknown by-product materials of which only
some appear to be hydrogen chloride precursors. These
materials also respond as acids in the aforementioned standard
analytical tests.
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There are procedures known for purifying organic
isocyanates to reduce acidity levels and otherwise increase
their reactivity rates. Generally, the most common practice
in the industry is to subject crude isocyanate mixtures to
one or more distillation steps, such as by passing a stream
of the crude reaction mixture of the phosgenation reaction
mentioned hereinabove, which contains the organic isocyanate,
solvent and impurities, through one or more distillation
columns whereby the solvent employed in the phosgenation
lQ reaction and acidic material contaminants are carried overhead,
leaving the organic isocyanate as residue. For example,
United States Patent No. 3,264,336 discloses the employment of
fractional distillation as a method for removing acid
contaminants from organic isocyanates.
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~' . .
,
However, conventional distillation techniques have
left much to be desired in regard to reducing the acidity
levels of organic isocyanates. Apparently, it is difficult
to separate many acidic material contaminants from the
isocyanate material by conventional distillation.
There are several procedures described in the
literature for improving the separatlon of acidic material
and other contaminants from isocyanates by distillation.
For example, a number of methods have been described which
comprise treatment of crude organic isocyanates with metals,
such as copper, silver, nickel, iron, zinc, co~alt, alumunum,
bismuth, and the like, and then distilling the mixtures for
separation. It is disclosed that the metallic compounds
form materials or complexes with acidic material contaminants
of the crude isocyanate which can be readily separated by
distillation, thereby resulting in an isocyanate having
reduced acidity. See U.S. Patent No. 3,155,69~, 3,264,336,
3,373,182 and 3,458,558.
In addition, U.S. Patent No. 3,219,~7~ discloses a
process for purifying organic isocyanates for the reduction
of the hydrolyzable chloride content therein which includes
the steps of subjecting a crude organic isocyanate mixture
that has been previously degassed and subjected to distillation
for sol-vent removal (employed in the aforementioned conventional
phosgenation procedure) to a temperature considerably above
those tem2eratures required and used in conventior.al degassing
procedures for the cleavage of organic carbamyl chlorides
- formed in the phosgenation reaction into organic isocyanate
and hydrogen chloride, for extended periods of time, and
then distilling the mixture to separate the organic isocyanate.
--3--
It is disclosed that the heatin~ prior -to distillation
apparently removes hydrolyzable chloride contaminants or
those responsible for hydrolyzable chloride content which
are no~ removed by simple dis-tillation of the organic
isocyana-te. However, the process disclosed in U.S. Patent
No. 3,219,678 leaves much to be desired from a commercial
operation standpoint inasmuch as i-t would apparently require
the utilization of a plurality of distillation columns
and/or extensive tie-up of plant production equipment which
necessarily reduce the economics of the process.
Furthermore, the above-mentioned procedures have
especially left much to be desired for purifying and reducing
,
` the acidity levels of polymethylene polyphenylpolyisocyanate
', mixtures. Generally, polymethylene polyphenylpolyisocyanate
mixtures are prepared by the well-known procedures of mixing
and reacting phosgene, in the presence of a compatible
~`~ solvent such as monochlorobenzene, with a corresponding
methylene-bridged polyphenyl polyamine mixture prepared by
the condensation reaction of formaldehyde and aniline or a
related polyamine in the presence of a strong mineral acid
or alumina-silica catalyst. Illustrative methods of the
preparation of methylene-bridged polyphenyl polyamines and
corresponding polymethylene polyphenylpolyisocyanates are
described in U.S. Patents No. 2,~83,730; 2,950,263; 3,012,008;
3,344,162; and 3,362,979, to name a few. The primary
disadvantage of employing the above-mentioned processes for
purifying and reducing the acidity of polymethylene poly-
phenylpolyisocyanates is the fac~ that these materials are
heat-sensitive. Exposure to high temperatures for extended
time periods adversely affect the chemical and physical
.
- ~ -a-
:-'
z
properties o polymethylene polyphenylpolyisocyanate mixtures,
such as viscosity, isocyanate equivalent weight, weight
percent free isoc~vanate, and the like. Furthermore, poly-
methylene polyphenylpolyisocyanates prepared by the afore-
S mentioned processes exist as mixtures of methylene diphenyl-
; isocyanate and higher functionality, higher molecular
weight methylene-bridged polyphenylpolyisocyanates which
have variable boiling points.
For these reasons, it has heretofore been a common
practice in the industry to purify and reduce the acidity of
polymethylene polyphenylpolyisocyanate mixtures by a distil-
lation procedure employing a fractional distilla~ion column
having a reboiler consisting of a thin film evaporator
means. More particularly, i.n accordance with conventional
techniques, a crude polymethylene polyphenylpolyisocyanate
mixture, from the phosgenation reaction containing solvent
` and impurities is initially degassed by rapidly heating the
crude mixture to about 70 to about ~0C., under about 60 to
about 90 mm. Hg absolute pressure, to remove unreacted
phosgene and other highly volatile impurities, and then ~-
passed through the fractional distillation column and thin
film evaporator means where the mix~ure is subjected to high
tempera~ure for only a few seconds to reduce affecting the
polymethylene polyphenylpolyisocyanate mixture physical and
2S chemical characteristics. The solvent and impurities are
taken overhead.
Although the distillation procedure described
immediately hereinabove has been found to be effective in
removing the solvent and high volatile impurities from the
polymethylene polyphenylpolyisocyanate mixtures without
-5- -
.
., .
adversely a~fectin~ the physical and chemical properties of
the mixt~re, it has left much to be desired in regard to
reducing the acidity of the isocyanate product.
- Accordingly, it is the primary obiect of the
present invention to provide an improved process for treating
a polymethylene polyphenylpolyisocyanate mixture whereby the
resulting isocyanate has a reduced acid level without adversely
affecting the viscosity, isocyanate equivalent weight,
isocyanate reactivity and like physical and chemical character-
istics.
It is another object of the present invention to
provide an improvement in the process for purifying and
reducing the acidity level of a polymethylene polyphenyl-
polyisocyanate subjected to a distil]ation step for separation
; 15 of the solvent employed in a conventi.onal phosgenation
; reaction and other contaminants.
-` It is yet another object oi the present invention
; to provide an improvement in the process for puriying and
reducing the acidity of polymethylene polyphenylpolyiso~yanate
mixture subjected to distillation procedures which does not
require the employment of an extensive series of distillation
columns and related apparatus and/or extensive tie-up of
-~ plant equipment and related long time requirements.
Other objects and advantages of the present invention
will become readily apparent to those having ordinary skill
in the art from the following description oE the invention
along with the attached drawing.
Summary of the Invention
The present invention is an improvement in the
process for purifying and reducing the acidity level of a
''
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:,'
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rJ~i2
polymethylene polyphenylpolyisocyanate mixture prepared by
the phosyenation of the corresponding methylene-bridged
polyphenylpolyamine mixture wherein the crude phosgenated
reaction product, after being subjected to conventional
degassing procedures, is subjected to at least one distil-
lation step to remove excess solvent and impurities therefrom,
such as by passing a flowing feed stream of the degassed
crude phosyenated mix-ture through a distillation column
having a thin film evaporator means moùnted thérewith for
heating the mixture to from about 190C. to about 250C.,
taking the excess solvent and impurities overhead from the
distillation column, and then allowing the resulting poly-
methylene polyphenylpolyisocyanate product residue from the
distillation column and evaporator means to cool. The
improvement of the invention comprises maintaining the
polymethylene polyphenylpolyisocyanat:e product residue from
the distillation column and evaporator means at a temperature ~;
: of from about 190C. to about 250C. for about 1 to 60
minutes, such as by holding in a temperature controlled : : :
. 20 holding tank, and recirculating continuously a portion of
the product residue being maintained at that temperature to
the distillation column for admixture with the flowing feed
stream of crude polymethylene polyphenylpolyisocyanate
mixture. It has been found that the maintaining and holding
. 25 of the polymethylene polyphenylpolyisocyanate product resid~e
at tem~eratures within the above-mentioned range ror the
described times in combination with the continuous recir-
culation of a portion thereof for admixture with the ~rude
feed stream in the distillation column results in a sub-
: 30 stantial reduction in the acid level of product mixture
., .
--7--
,
3~?~2
without adversely afecting the isocyanate equivalent
weigh~, viscosity and other physical and chemical properties
of the product. Moreover, the reduction of acidity of
polymethylene polyphenylpolyisocyanate mixtures treated by
the process of the invention i5 obtained without the use of
a plurality of distillation steps heretofore usually required,
thereby eliminating the necessity of employing extensive
distillation equipment and/or extended process time require-
ments. The process of the invention is especially effective
for reducing the acidity of methylene-bridged polyphenyl
polyisocyanate mixtures prepared by the aforementioned
b procedures which are known to be extremely heat-sensitive
and thus adversely affected by the extensive distillation
procedures heretoore employed for removing impurities from
crude organic isocyanate products.
Description of the Drawing
The detailed description of our invention r which
` follows herein, will be further illustrated in connection
with the attached drawing, which is a schematic flow sheet
illustrating a preferred embodiment of the invention. In
order to simplify the drawing, conventional details, such as
valvesr pumpsr condensersr reboilers, flow and temperature
control devices, and the like r have not been shown since the
construction, operation and function thereof is known to
- 25 those of ordinary skill in the ar-t.
Detailed Description of the Invention
As hereinbefore mentioned, the process of the
invention is particularly useful for purifying and reducing
the acidity of polymethylene polyphenylpolyisocyanate mixtures.
Any polymethylene polyphenylpolyisocyanate mixture, prepared
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~ 3~l~6~
by any process ~nown iIl the art can be treated by the
inventive process. Inasmuch as polym2thylene polyph~nyl-
polyisocyanate mixtures are well-known, and methods for
their preparation and the methods for the preparation of the
corresponding methylene-bridged polyphenylpolyamine mixtures
employed for their preparation are well-known, as shown by ~-
the above-noted patents, further detailed description thereof
will not be set forth herein.
Thus, for the purposes of brevity only, the term
"polymethylene polyphenylpolyisocyanate mixture" ~Jill here-
after be referred to as the isocyanate mixture or product.
Referring now to the drawing, in accordance with ~;;
the process of the present invention, a crude isocyanate
feed stream, obtained from any of the aforementioned phos-
genation processes of corresponding methylene-bridged poly-
phenyl polyamines and which contain the isocyanate, a
compatible organic liquid solvent, such as monochlorobenzene,
and impurities, is fed rom the phos~enation reactor 10
through line 12 through one or more flashing units 14,14a (in
phantom) wherein the feed stream is heated at a temperature
of from about 70 to about 90C. at about 60 to about 90
` mm. Hg absolute pressure for about 15 minutes to about 2
hours. As the isocyanate feed stream passes -through the -
flashing units 14,14a, unreacted phosgene, some solvent and
some high volatile impurities are taken overhead through
line 16,16a (in phantom). The liquid bottoms from the
flash units 14,14a, i.e. the degassed isocyanate feed stream,
then moves through line 18 and is fed to the fractionation
distillation column 20, preferably at about mid-point of the
column 20. The fractionation column 20 is mounted with a
_g_
~3~2
reboiler means consisting of a thin film evaporator 22. The
thin film evaporator 22 is mo~lnted to the fractionation
column by any conventional means, such as by lines 24 and
26. For example, the isocyanate feed s-tream passes through
the fractionation column 20 and line 24 to the thin film
evaporator 22 wherein it is heated to a temperature of from
about 190-250C. for only a few seconds. The solvent and
other impurities volatilized in the thin film evaporator 22
are then passed through line 26 back to the bottom of the
fractionation column 20, passing countercurrent to the feed
stream, and are taken overhead through line 28 for reuse or
discard. The volatili~ed solvent and impurities are preferably
taken overhead ~rom the thin film evaporator at a temperature
of about 110C. to about 215C. and passed through the
~ractionation column 20 whereby the isocyana-te feed stream
flowing countercurrently therewith is heated so as to provide
a temperature of about 50-80C. at th~ fractionator column
20 mid-point and an overhead temperature oE about 30-50C,
depending upon the particular solvent employed during the
aforementioned phosgenation reaction. In addition, the
fractionation column 20 is preferably operated under a
pressure of from about atmospheric to about 15 mm. Hg absolute
and includes reflux condenser means (not shown) to provide
reflux ratios of from about 0.5:1 to about 5:1 to prevent
excessive loss of the isocyanàte products.
In accordance with the process of the present
inventian, the isocyanate product residue or bottoms Erom
the thin film evaporator 22 is then removed therefxom
through line 30 to a holding tank 32 wherein the residue is
maintained at a temperature of from about 190C. to about
,
j, --10--
3~
250~C., i.e. essen-tially the same temperature as heated to
in the thin film evaporator 22, for a time period of from
about 1 minute to about 60 minu-tes. The temperature can be ~-
maintained in the holding tank 32 by any conventional means,
such as by equipping the holding -tank 32 with appropriate
insulation and/or additional heating means. The desired
holding time can be provided by any conventional means such
as by equipping the holding tank 22 with appropriate product
removal means ~or continuously removing portions of the
heated isocyanate product from the holding tank 32 when it
collects and reaches a certain level. Preferably, the
isocyanate product residue is maintained at a temperature of
from about 210C. to about 240C. for about 1 to about 15
minutes. It has been determined that, as higher temperatures
are employed, lower holding times are required.
A portion of the isocyanate product is removed
; continuously from the holding tank through line 34 and
. .
continuously recirculated back to the fractionation col~nn
20 at its mid-point for intimate admixture with the crude
isocyanate feed stream entering the fractionation column 20
; through line 18 from the flash unit 14,14a. Most unexpectedly,
it has been found that the continuous recirculation of the
isocyanate product residue from the thin film evaporator 22
which has been maintained at the aforesaid temperature for
the aforesaid time at recirculation volume ratios of from
about 1:1 to as high as 3:1, temperature-maintained product
residue:crude flashed feed product, results in drastic
reduction of the acid content of the polymethylene polyphenyl-
; polyisocyanate mixture being treated, as further shown in
the examples set forth hereafter. Preferably, a recirculation
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: ` ~
rate of from about 2 to abou-t 3 volumes recirculated product
per volume of crude feed is employed.
The isocyanate produc-t residue or evaporator
bottoms recirculated through line 34 can be recirculated at
the same temperature maintained in the holding tank 32 if
desired. However, the product residue can be recirculated
at temperatures as low as about 25C., such as by passing
the recirculated residue through a cooling tank 36 (in
phantom1 without adverse results so as to reduce potential
equipment maintenance problems.
The isocyanate residue product is also removed
from the holding tank 32 through line 38, which can include
the above-mentioned product removal means (not shown). The
product removal means can comprise a nozzle having an
` 15 orifice of desired size to provide the desired removal rate
and holding time. It can he readily appreciated that ~he
; volume of product removed through line 38 is preferably
substantially the same as the volume of crude isocyanate
-~ feed fed to the fractionation column 20 through line 18 to
~0 provide processiny equilibrium~ The removed product is then
.: ,
passed through conventional co~ling means ta storage receivers,
both not snown.
The invention will be ~urther illustrated by the
following specific examples which are given by way of
~5 illustration and not as limitations on the scope of this
inventlon.
All of the following examples were carried out in
a fractional distillation column which consisted of a ~-inch
column packed with Intalox Saddles (Norton Company, Akron,
Onio), A reboiler was mounted with the column which consisted
~ R~g;s~Q~e~ ~rade n~4~
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of a thin film evaporator which had a heat transfer surface
of 1.4 sq.ft. The column also had a 3-way valve means ~;
activated by a timer mounted therewith to provide the
desired reflux, along with pressure means. A double-pipe heat
exchanger was attached by appropriate conduits to the head
of the column to condense vapors taken overhead. The crude
isocyanate ~eed mixture to be treated, previously degassed,
was introduced through a conduit connected at the mid-point
o the column. For heat treatment, a small tank was attached
to the bottom o~ the evaporator. The heated isocyanate
product residue from the bottom of the heat exchanger entered
the tank through a dip-tube mounted therein, filled the tank
to a level needed to give the desired holding time, and
overflowed through a no2zle and then through a cooler to
rec~eivers. Holding time in the tank was adjusted by selecting
one o~ multiple nozzles provided on t:he side o~ the tank.
The tank was wrapped with a coil oE tubing through which hot
oil flowed to maintain a constant temperature in the iso-
cyanate product residue, and the entire tank was insulated
` 20 to prevent heat loss. Recirculating means were provided for
recirculating a portion of the product held in the ~olding
tank back to the mid-point of the column for admixture with
; the fresh crude isocyanate feed. The recirculating means
included appropriate conduits and metering devices attached
~ 25 respectively to the holding tank and distillation column at
mid point for removing the isocyanate product residue from
- the holding tank to the distillation column mid-point. The
recirculating means included a cooling tank mounted with the
conduit for reducing the temperature thereof to about 50C.
to reduce metering device maintenance. The thin film
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: .
evapora-tor also had appropriate conduits mounted therewith
for passing the isocyanate product residue direc~ly therefrom
to the cooler and receivers bypassing the holding tank
This bypass means and the recirculating means both haa cut-
off valves for operation when desired.
The various data relating to acid levels in the
following examples were obtained as follows:
Acidity Determination
1.2 to 1.4 g. of the isocyanate mixture to be
analyzed is weighed, to the nearest 0.1 mg., into each of
two 250-ml. beakers. 50 ml. of methanol and then 50 ml. o~
toluene are pipetted into each beaker. A stirring bar is
added to each beaker which are -then placed on a pre-heated
(maximum heat) stirrer hotplate. Thermometers are placed in
each solution and the solutions are heated to 60C. in less
; than 3 minutes. The beakers are then removed from the
hotplate, covered with watch glasses and allowed to stand
for one hour, plus or minus 5 minutes. With a pipette, the
thermometers and the walls of each beaker are washed down
with 10 ml. methanol. Using a pH meter with glass and
calomel electrode, each solution is then titrated with 0.02N
methanolic sodium hydroxide, to pH 7. The acidity, deter-
mined as HCl, is then calculated according to the following
formula:
cid (Basis HCl) = ( 1. of NaOH)(N of NaOH)(3
EXAMPLE I
A crude reaction mixture resulting from the
phosgenation of a polymethylene polyphenylpolyamine mixture
from the condensation reaction of aniline and formaldehyde,
~ ,o~
containillg 20-25 w-t.% monochlorobenzene and 75-80 wt.%
polymethylene polyphenylpolyisocyanate mixture, previously
flashed at a~out 7~C. to about 90C. at about 60 to 90 ~m.
Hg absolute pressure, was fed to the distillation column
described hereinabove which was adjusted to provide holding
of the isocyanate product residue in the holding tank and
recirculating a portion therefrom. The crude flashed mixture
was ~ed to the distillation column at a feed rate o~ 8.3
lbs./hr. The distillation column was operated under 15 mm.
Hg absolute pressure and a reflux ratio of 2/1. The holdin~
~ank was adjusted with appropriate nozzles for a holding
time of 1.5 minutes. The recirculating means was adjusted
.,~ . . .
;` to provide a recirculation rate of 20.7 lbs./hr. Temperatures
were recorded at various points as follows:
Point Temperature Range
.
Column head 27-32C.
` Column mid-point 57-81C.
Vapor from evaporator 126-212C.
; Product from evaporator 200-229C.
Holding tank 216-225C.
A portion of the product was removed from the
holding tank at a rate of 8.3 lbs./hr. and passed through
` the cooler to receivers. A portion of the product was
removed from a receiver and analyzed, the results of which
are set forth in the following Table 1~ -
TABLE 1
Product Analysis
` Acidity, wt.~ as HCl0.054
Isocyanate content, meq/g 7.42
Viscosity, cp. at 25C. 285
MDI content, wt.% 43.5
4,4'-isomer, wt.~ of MDI 75.7
:` :
~15-
E:XA~IPLE I I
A crude reaction mixture o about 50 wt. 6 mono- -
chlorobenzene and 50 wt.~ polymethylene polyphenylpolyiso-
cyanate, obtained ~rom the phosgenation of a polymethylene
polyphenylpolyamine mixture prepared by the condensation
- reaction of aniline and formaldehyde which had been previously
;~ degassed by passing through a fIash unit operated at about
70C. to about 90C. at 60 to 90 mm. Hg absolute pressure,
was fed to the distillation column described hereinabove at
a feed rate of 31.6 lbs./hr. The crude reaction feed mixture
was the same as described in Example I except for the wt.%
monochlorobenzene present. The distillation column apparatus
described hereinabove was ad~usted so as to pass the treated
.
isocyanate product residue from the evaporator directly
lS through the cooler to receivers bypassing the holding tank
` and recirculating means. The distillation column was
operated under 15 mm. Hg absolute pressure and a reflu~ ratio
of 2/1. Temperatures, measured at various points, were as
follows:
Point Temperature Ran~e
Column head 32-47C.
` Column mid-point 53-61C.
Vapor from evaporator 111-215C.
Product from evaporator 230-242C.
` 25 A portion of the isocyanate product residue from a
receiver was analyzed, the results being set forth in the
following Table 2.
TABLE 2
- Product Analysis
Acidity, wt.% as HCl 0.20
Isocyanate content, meq/g 7.60
Viscosity, cp. at 25C. 160
MDI content, wt.% 45.7
4,4'-isomer, wt.% of MDI76.6
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A comparison of the results of Table 2 to those of
Table 1 illustrates the improvement ln reduction o~ acidity
of organic polyisocyanate mixtures trea-ted in accordance
with the process of the invention over a conventional distil-
lation technique.
EXAMPLE III
The crude flashed reaction feed mixture described
in Example II was fed to the distillation column described
hereinabove at a feed rate of 27.4 lbs./hr. The distillation
column was adjusted so that the isocyanate product residue
from the evaporator would be removed to the holding tank
which was adjusted to maintain the temperature of the product
residue at about 172-219C. for about 11 minutes holding
time. The distillation column was operated under 15 mm. Hg
absolute pressure and a reflux ratio of 2/1. Temperatures
were recorded at various points as follows:
Point emperature Range
Column head 31-33C.
Column mid-point 58-76C.
Vapor from evaporator 150-199C.
Product from evaporator
Holding tank 172-219C.
- The product passed from the holding tank at the
- feed rate through a cooler to receivers. A portion of the
treated isocyanate residue product was then analyzed, the
results of which are set forth in the following Table 3.
TABLE 3
Produc~ Analysls
Acidity, wt.% as HCl 0.15
Isocyanate content, meq/g 7.59
Viscosity, cp. at 25C. 174
MDI content, wt.% 47.5
4,4'-isomer, wt.~ of MDI 76.0
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' .
A comparison oE tne results set forth in Table 3
~ith the results of Table 2 shows that holding the mix-ture
at the elevated temperature provides improved acidity
reduction over conventional distillation. However, a
comparisOn of the results of the table with those of Table 1
illustrates the unexpected improvement of acidity reduction
by recirculatin~ a portion of the isocyanate product residue
in accordance with the present invention.
EXAMPLE IV
', 10 A crude reàction mixture resulting from the
phosgenation of a polymethylene polyphenylpolyamine mixture
of the condensation reaction of aniline and formaldehyde,
containing 20-25 wt.% monochlorobenzene and 75-80 wt.~ of
the polymeth,ylene polyphenylpolyisocyanate mixture, which
had been previously flashed by passing through a flash unit
operated at about 70C. to about 90C'. at 60-90 mm. Hg
absolute pressure, was fed to the dic;tillation colu~n
described hereinabove. The distilla~ion column was adiusted
as described in Example I to provide holding of the iso-
cyanate product residue in the holding tank and recirculating
' , a portion therefrom back to the distillation column. The
crude flashed mixture was fed to the distillation column at
a feed rate of 7O0 lbs. per hour and the distillation column
t~as operated under 15 mm. Hg absolute pressure and a reflux
-, 2~ ratio of 2/1. The holding tank was adjusted with appropriate
ozzles for a holding time of 1.5 minutes and the recirculating
~ ~Qans was adjusted to provide a recirculation rate of 23.0
,, lbS./hour~ During operation, temperatures were recorded at
-~ariOus points as follows:
-18-
Poin~ Temperature Range
Column head 23-27C.
Column mid-point 58-77C.
Vapor from evaporator 173-200C.
Product from evaporator 174-190C.
Holding tank 194-209C.
A portion of the product was removed from the
holding -tank at the feed rate of 7.0 lbs.~hour and passed
through the cooler to receivers. Analysis of a portion of
the product removed from a receiver provided the following
results set forth in Table 4:
TABLE 4
Product Analysis
Acidity, wt.~ as HCl 0.068
Isocyanata content, meq/g 7.47
Viscosity, cp. at 25C. 80
MDI content, wt.% 66.2
4,4'-isomer, wt.% of MDI 75.8
A comparison of the result~i of Table 4 to those
set forth in Tables 2 and 3 ùrther illustrate the improved
reduction of acid content in polymèthylene polyphenylpoly~
isocyanate mixtures treated in accordance with the present
invention.
From the foregoing description and examples of
this invention, those of ordinary skill in the art may make
many modifications and variations therefrom without departing `~
from the scope of the inventlon as h~reinafter claimed.
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