Low bromine content glacial acetic acid

Excessive energy consumption of a combination of multi-fractionations and multi-distillations of concentrating aqueous acetic acid product of liquid phase oxidations, especially oxidation of liquid n-butane with oxygen gas while the butane is dissolved in liquid acetic acid containing a catalyst system comprising Co-Br or Co-Mn-Br, is avoided and an otherwise hard to remove bromo-ketone is readily removed by a combination of sequential steps of decompressing the oxidation reaction mixture to remove unreacted butane as well as gaseous products, heat treating the decompressed liquid at a temperature of from 150.degree. C. up to 200.degree. C. for from 15 up to 150 minutes, subjecting the heat treated liquid to fractionation while recycling to the rectification zone thereof an aqueous portion of low boiling impurities as a means for concentrating the acetic acid and thereafter further concentrating the acetic acid produced by continuous fractional crystallization.

FIELD OF INVENTION 
This invention relates to the preparation of glacial acetic acid from 
n-butane and more specifically pertains to the preparation of glacial 
acetic acid from the liquid reaction effluent or its debutanized liquid 
fraction obtained by liquid n-butane oxidation with oxygen gas at a 
temperature of from 120.degree. C. up to 235.degree. C. in the presence of 
an acetic acid solution containing the system of catalysis comprising 
bromide ions in combination with ions of cobalt or cobalt and manganese by 
decreasing the bromine content of the reaction effluent or said liquid 
fraction thereof by a step of continuous fractional crystallization and a 
step of heat treating at a temperature of from 150.degree. C. up to 
200.degree. C. for from 15 minutes up to 150 minutes before or after the 
distillative removal of organic impurities boiling lower than acetic acid 
and separation of acetic acid from organic materials boiling higher than 
acetic acid including cobalt or cobalt and manganese salts of organic 
acids. 
STATE OF THE ART 
Acetic acid at high selectivity can be produced at high conversion of 
n-butane by its oxidation as a liquid with oxygen gas at a temperature in 
the range of from 120.degree. C. up to 235.degree. C. and a gauge pressure 
of from 35 up to 210 kg/cm.sup.2 in the presence of an acetic acid 
solution containing bromide ions in combination with ions of one or more 
transition metals as components of catalysis. According to U.S. Pat. No. 
3,293,292 it is essential to use both cobalt and manganese as the 
transition metal component of said catalysis. 
However, according to the later U.S. Pat. No. 4,111,986 the same high 
conversion of n-butane at high selectivity to acetic acid can be 
accomplished using cobalt as the sole transition metal component of said 
catalysis provided that for each gram mole of n-butane to be so oxidized 
there are employed from 1.0 up to 50 milligram equivalents of cobalt and 
from 2 to 50 milligram equivalents of bromine as components of the needed 
catalysis. 
Said oxidations of n-butane produce acetate esters and ketones boiling at 
temperatures below the boiling point temperature of acetic acid as well as 
the higher carbon atom content aliphatic mono-carboxylic acids propionic 
and butyric acid which have boiling temperatures above the boiling 
temperature of acetic acid, such acetate esters, ketones and higher 
aliphatic acids as products are produced in impurity level amounts and can 
be removed by simple distillation from the debutanized (removal of 
unreacted n-butane) liquid reaction effluent. Such debutanized reaction 
effluent contains mainly acetic acid (65 to 80 weight percent) and water 
(20 to 30 weight percent). However, there is one co-product produced as a 
result of the bromide ion component of catalysis which is difficult to 
remove to the impurity level which can be tolerated in acetic acid used as 
reactant and/or reaction solvent. It is appreciated that for some uses of 
acetic acid (glacial) as reactant and/or reaction solvent, that only 
substantially zero bromine content is acceptable but is not specified in 
commercial specifications. 
Said difficultly removable impurity co-product has now been found to be 
3-bromo-2-butanone. Its boiling temperature and that of acetic acid and of 
acetic acid-water compositions formed during fractionation are so close 
that to effect separation of said bromoketone by fractionation would 
require an inordinately large number of theoretical separation (tray or 
packed) units not acceptable for commercial operation. 
Techniques have been proposed for decreasing the bromine content of acetic 
acid. It is not apparent from the description of such techniques that they 
are directed to decreasing the 3-bromo-2-butanone content of acetic acid 
even though there is mention of converting the bromine in organic 
(coordinate bound) bromides to inorganic (ionic) bromides. 
According to U.S. Pat. No. 3,578,706 bromine is removed from bromine 
contaminated acetic acid by stirring such contaminated acetic acid at 
elevated temperatures (30.degree. to 118.degree. C.) in the presence of a 
finely divided metal having an electrochemical potential between magnesium 
and iron, or the oxides, hydroxides or salts of such metals and then 
subjecting the acetic acid so treated to ion exchange. The treatment with 
the metal converts organic bromides to inorganic bromides. 
It might be thought that the catalytic hydrogenation technique of U.S. Pat. 
No. 2,884,451 for removal of odorous substances and materials of a 
reducing nature from acetic acid obtained by the non-catalytic oxidation 
of C.sub.4 to C.sub.8 paraffinic hydrocarbons might also convert organic 
bromide impurities to easily removable inorganic bromides. However, it has 
been found in our laboratories that such catalytic hydrogenation of the 
liquid phase of the organic bromide contaminated acetic acid does not 
suitably decrease the organic bromide contamination. 
It has also been found in our laboratories that treatment of the organic 
bromide contaminated acetic acid with an alkali metal hydroxide, 
bicarbonate or carbonate and then distilling the treated acid or that 
treatment of the organic bromide contaminated acetic acid with a solid 
absorbant does not suitably decrease the organic bromide contamination. 
Rather it has been found that more severe treatment is necessary. For 
example, the organic bromide contamination can be suitably decreased by 
first contacting a vapor phase of the organic bromide contaminated acetic 
acid with hydrogen and a hydrogeation catalyst (e.g. metallic platinum or 
palladium per se or disposed on the surface of activated carbon) and then 
either (1) contacting the vapors with a bed of solid absorbant (e.g. 
alumina or activated carbon), or (2) condensing the treated acetic acid 
vapors and treating the liquid state of acetic acid with an alkali metal 
hydroxide, carbonate or bicarbonate followed by distillative recovery of 
acetic acid. Such combinations of vapor phase catalytic hydrogenation of 
organic bromide contaminated acetic acid with solids are the subject 
matter of claims in the copending U.S. patent application Ser. No. 
970,226, now U.S. Pat. No. 4,228,307 and Ser. No. 970,222, now U.S. Pat. 
No. 4,227,971, both filed on Dec. 18, 1978. 
It has now been discovered that the 3-bromo-2-butanone contamination of 
glacial acetic acid can be substantially decreased by the use of a heat 
treating step and a cryogenic fractional crystallization step before the 
removal of the last amounts of water from the acetic acid, that is, before 
the last step of forming glacial acetic acid. 
STATEMENT OF THE INVENTION 
The present inventive technique to decrease the 3-bromo-2-butanone 
contamination of a useful acetic acid produced from an acetic acid product 
obtained from the oxidation of liquid n-butane with oxygen gas at a 
temperature of from 120.degree. C. up to 235.degree. C. in the presence of 
an acetic acid solution of a bromine liberating compound and a cobalt salt 
or a cobalt and manganese salt providing bromide ions in combination of 
ions of cobalt or of cobalt and manganese introduces a thermal step of 
converting 3-bromo-2-butanone to 1-butene-3-one and inorganic bromide or 
bromides, and a cryogenic step of concentrating the aqueous acid mixture 
and rejecting 3-bromo-2-butanone into the recovery of acetic acid which 
includes the steps of debutanizing the liquid reaction effluent by 
decreasing its pressure (decompressing) to a gauge pressure of from 22 
down to 0 kg/cm.sup.2, removing by distillation organic compounds (esters 
and ketones) boiling lower than acetic acid and then an acetic acid 
fraction containing from 5% to 10% water and C.sub.1 to C.sub.4 aliphatic 
acid homologues of acetic acid (formic, propionic and butyric acids) 
leaving a residue containing co-products boiling higher than acetic acid 
and catalyst metal salts of organic acids (mainly acetates), and 
dehydrating the aqueous acetic acid fraction. 
The 3-bromo-2-butanone decreasing effects of the thermal conversion step 
and the cryogenic step depend on the co-presence of water with acetic 
acid. The thermal conversion is believed to involve the reaction of water 
with 3-bromo-2-butanone to produce 1-butene-3-one and one or more 
inorganic bromides, probably catalyst metal bromides. The cryogenic step 
is a continuous fractional crystallization which rejects organic 
impurities not by precipitation in a crystalline form, but rather as a 
solute in an acetic acid-water mother liquor of higher water content than 
the acetic acid-water crystalline product frozen out of the feed to the 
cryogenic step. 
Each of said steps can operate effectively on the debutanized liquid 
portion of the liquid reaction effluent. For example, continuous 
fractional crystallization practiced on the liquid debutanized fraction 
containing 66.4 wt.% acetic acid, 24.85 wt.% water and 0.72 wt.% 
3-bromo-2-butanone, on a once through basis, can produce a product 
containing 78.7 wt.% acetic acid, 18.73 wt.% water and 0.06 wt.% 
3-bromo-2-butanone and a waste liquor containing 60.6 wt.% acetic acid, 
30.1 wt.% water and 0.77 weight percent 3-bromo-2-butanone. Thus a product 
containing 3-bromo-2-butanone of only 8.33% of that in the feed (a 91.66% 
decrease) is achieved by the cryogenic process. 
The thermal conversion step is carried out at temperatures of from 
150.degree. C. up to 200.degree. C. for from 15 minutes up to 150 minutes 
while some water is still present for its aforementioned reaction with 
3-bromo-2-butanone to produce 1-butene-3-one and one or more inorganic 
bromides. There is substantial evidence that a substantial proportion of 
the inorganic bromide formed is catalyst metal bromide (e.g., by reaction 
of catalyst metal acetate with HBr) because additionally formed inorganic 
bromide appears in the bottom fraction of distillation as does the 
catalyst metal acetate. 
More specifically, a total liquid effluent from the oxidation of n-butane 
with oxygen gas according to the processes before described has an 
inorganic bromide content of 0.2122 weight percent and a 
3-bromo-2-butanone content of 0.765 weight percent. Maintaining such 
effluent at a temperature of 150.degree. C. for 80 minutes decreased the 
bromoketone content to 0.019 weight percent and increased the inorganic 
bromides to 0.72 weight percent. But maintaining said liquid effluent at a 
temperature of 200.degree. C. for 40 minutes or 80 minutes decreased the 
bromoketone content to 0.016 weight percent and to a not detectable level, 
respectively, while increasing the inorganic bromides to the respective 
levels of 0.68 and 0.736 weight percent. After such heat treatment and 
upon debutanizing the liquid reaction effluents and distilling them to 
recover a product fraction (85 to 90% of the charge to distillation), a 
product fraction containing less than 0.01 weight percent 
3-bromo-2-butanone can be recovered. The foregoing indicates that the 
thermal debromination of 3-bromo-2-butanone is effective even at pressures 
well above atmospheric pressure and prior to debutanization of the liquid 
reaction effluent. 
The best mode presently contemplated for the practice of the present 
invention comprises decompressing the liquid reaction effluent from 56 to 
63 kg/cm.sup.2 gauge pressure and 176.degree.-177.degree. C. down to a 
gauge pressure of from 15 down to 2.5 kg/cm.sup.2 and a temperature of 
from 176.degree.-177.degree. C. while maintaining the remainder of the 
effluent at a temperature between 118.degree. C. and 177.degree. C. which 
causes at least 80 weight percent of the unreacted butane to be removed in 
a mixture which comprises from 14 up to 32 weight percent of the liquid 
reaction effluent before being decompressed. 
The effectiveness of such preferred conditions for decompressing and 
debutanizing the liquid reaction effluent can be demonstrated by data from 
such operations on liquid reaction effluent having the composition shown 
in TABLE I to follow which effluent is prepared by the oxidation of 
n-butane with oxygen gas at a temperature of 182.degree. C., a gauge 
pressure of 63.6 kg.cm.sup.2, a residence time of 41.4 minutes, and a 
molar ratio of cobalt bromide to butane of 0.005:1.0. 
TABLE I 
______________________________________ 
COMPOSITION OF THE LIQUID REACTION 
EFFLUENT FOR BUTANE OXIDATION 
Component Weight % 
______________________________________ 
Butane 9.40 
Acetone 0.123 
Methyl Acetate 1.67 
Ethyl Acetate 1.45 
Methyl Ethyl Ketone 1.70 
s-Butyl Acetate 0.479 
n-Butyl Acetate 0.037 
Propionic Acid 0.890 
Butyric Acid 0.365 
3-Br-2-Butanone 0.726 
Unknowns 1.03 
Water 16.39 
Formic Acid 0.318 
Acetic Acid 61.54 
CO 0.281 
CO.sub.2 3.56 
CH.sub.4 0.016 
C.sub.2 H.sub.6 0.025 
Cobalt 0.25 
______________________________________ 
Portions of such liquid reaction effluent are subjected to decompression to 
gauge pressures of 2.6, 4.24, 6.0, 9.5 and 14.0 kg.cm.sup.2 at 
temperatures which permit removal of at least 80 weight percent of the 
unreacted butane. The weight percent of each of the original components in 
the liquid reaction effluent are shown in TABLES II and III to follow: 
TABLE II 
______________________________________ 
DECOMPRESS LIQUID REACTION EFFLUENT 
EFFECT OF TEMPERATURE AND PRESSURE 
______________________________________ 
Temperature, .degree.C. 
119.4 122.8 124.8 
128.4 
135 138 
Pressure, 
kg/cm.sup.2 2.6 2.6 2.6 2.6 4.24 4.24 
Amount Removed, 
wt. % 14.8 16.8 18.2 21.8 14.7 18.1 
Component Removed 
wt. % 
Acetic Acid 5.2 6.7 7.8 10.8 5.4 8.0 
Water 10.2 12.9 14.8 20.1 10.7 15.4 
Butane 86.6 88.9 90.1 92.6 83.7 88.0 
Methyl Acetate 
30.6 36.0 39.3 47.4 29.4 37.6 
Ethyl Acetate 24.1 29.0 32.0 39.8 23.5 31.1 
Formic Acid 6.4 8.1 9.3 12.8 6.5 9.4 
Propionic Acid 
2.7 3.6 4.2 6.0 3.0 4.6 
Acetone 25.9 30.8 33.8 41.6 24.7 32.3 
MEK 18.1 22.1 24.7 31.5 17.7 24.0 
______________________________________ 
TABLE III 
______________________________________ 
DECOMPRESS LIQUID REACTION EFFLUENT 
EFFECT OF TEMPERATURE AND PRESSURE 
______________________________________ 
Temperature, .degree.C. 
144.7 147 161.8 
176.9 
176.7 
148.9 
Pressure, 
kg/cm.sup.2 6.0 6.0 6.0 9.5 14.0 14.0 
Amount Removed, 
wt. % 14.0 15.3 32.1 26.7 15.0 10.9 
Component Removed 
wt. % 
Acetic Acid 5.1 6.0 20.8 16.0 4.3 6.6 
Water 10.3 12.0 36.3 29.5 8.3 3.0 
Butane 80.4 82.7 92.1 87.1 88.8 82.8 
Methyl Acetate 
26.8 29.9 60.7 50.8 14.1 3.9 
Ethyl Acetate 21.6 24.4 54.2 44.8 14.4 4.6 
Formic Acid 6.1 7.1 23.7 18.2 -- 1.5 
Propionic Acid 
3.0 3.5 13.3 10.6 4.1 1.2 
Acetone 22.4 25.2 54.9 45.0 14.1 4.2 
MEK 16.2 18.5 45.5 36.6 13.1 4.4 
______________________________________ 
The data in TABLE IV illustrates that the reaction effluent from the 
oxidation of liquid n-butane with oxygen gas in liquid acetic acid 
containing cobalt, manganese and bromine as components of catalysis is 
quite similar to the reaction effluent obtained from the same oxidation 
conducted in liquid acetic acid containing cobalt and bromine as 
components of catalysis as illustrated by TABLE I. 
TABLE IV 
______________________________________ 
OXIDATION OF n-BUTANE IN PRESENCE 
OF Co--Mn--Br CATALYST 
______________________________________ 
Conditions: 
Temperature 182.degree. C. 
Pressure 63.6 kg/cm.sup.2 gauge 
Residence time of 51 to 54 min. 
Gram Atom Ratio of Co:Mn of 1:1 
Gram Atom Ratio Br:Co + Mn of 2:1 
Milligram Atom Metal:Gram Mole Butane 50:1.0 
Mole Ratio O.sub.2 to Butane of 78:1.0 
______________________________________ 
Reaction Effluent Composition 
Component Weight % 
______________________________________ 
Butane 0.14 to 0.27 
Acetone N.D. 
Methyl Acetate 0.58 to 0.69 
Ethyl Acetate 1.11 to 1.19 
Butyl Acetates 0.37 to 0.61 
Propionic Acid 3.51 to 5.09 
Butyric Acid 0.52 to 0.69 
3-Br-2-Butanone 0.23 to 0.33 
Unknowns 0.41 to 0.65 
Water 18.0 to 18.8 
Formic Acid 1.16 to 1.34 
Acetic Acid 65.3 to 71.2 
______________________________________ 
"N.D." is none detected. 
The decompression-debutanization of liquid reaction effluent is followed by 
the thermal conversion of 3-bromo-2-butanone to MEK and inorganic bromide 
conducted at a temperature of from 150.degree. C. up to 200.degree. C., 
preferably at a temperature of from 170.degree. C. up to 200.degree. C. 
and at a gauge pressure of from 10 up to 30 kg/cm.sup.2 for from 40 up to 
150 minutes, preferably from 80 to 60 minutes. Under the preferred 
conditions of maintaining the decompressed and debutanized liquid reaction 
mixture at a temperature of from 170.degree. C. up to 200.degree. C. and 
at a gauge pressure of from 10 up to 30 kg/cm.sup.2 for from 80 to 60 
minutes (longer time at lower temperature and shorter time at higher 
temperature), substantially all of the 3-bromo-2-butanone will have been 
converted to MEK and inorganic bromide or bromides. For example, with an 
initial 3-bromo-2-butanone content of 0.35 to 0.80 weight percent in the 
decompressed-debutanized portion of the liquid reaction effluent, the 
preferred heat treatment of said portion converts its 3-bromo-2-butanone 
content to the range of from not detectable to 0.02 weight percent. 
Such preferred heat treatment of the decompressed-debutanized liquid 
reaction mixture provides a feed for fractional distillation from which a 
low boiling organic (butane, acetone, methyl acetate, ethyl acetate, MEK, 
sec. butyl acetate and n-butyl acetate) fraction including acetic acid and 
water with a total bromine content of 3.5 to 4.5 weight percent and 
amounting to 7 to 10 wt.% of the feed can be removed and recycled to the 
n-butane oxidation. An acetic acid fraction amounting to 70 to 78 weight 
percent of the feed and containing 85 to 95 weight percent acetic acid, 
0.5 to 5 weight percent water, 3-bromo-2-butanone of from 0 (not 
detectable) to 0.005 wt.% and a total bromide content of not more than 
0.04 wt.%, and a residue or bottoms fraction amounting to 7 to 10 wt.% of 
the feed and containing from 49 to 85 wt.% acetic acid, from 0 to 0.5 wt.% 
water, from 1.4 to 2.8 wt.% catalyst metals, up to 8 wt.% total bromine, 
and up to 0.01 wt.% 3-bromo-2-butanone. 
The distillation of the heat treated liquid to obtain a feed for the 
cryogenic concentration of acetic acid can be a simple distillation 
suitable to separate the water, acetic acid and associated organic 
materials from catalyst metal and inorganic bromides together with 
materials boiling higher than acetic acid as well as some acetic acid to 
leave a fluid residue fraction. However, it is preferred that the 
distillation be a continuous fractionation which, advantageously, can 
provide as a second (acetic acid) fraction one with a minimum amount of 
water by forming in the rectification zone an azeotropic mixture with the 
acetates and low boiling ketone impurities. The first fraction comprises 
water, some acetic acid and said acetates, and ketones which upon 
condensation form two immiscible liquid phases. The top phase is the 
acetate-ketone or organic phase and, of course, contains a small amount of 
acetic acid and water. The bottom phase is mainly a water phase with from 
5 to 10 wt.% acetic acid. Together those two phases amount to from 5 to 25 
weight percent of the feed to the fractionation. It is preferred to 
conduct the fractionation by charging the feed between the stripping and 
rectification zones, withdrawing a vapor acetic acid product below said 
feed entry, that is, withdrawing the vapor product or second fraction from 
the stripping zone so no catalyst metal or inorganic bromide contaminates 
the second fraction, to discard the organic phase portion of the first 
fraction or recycle it to the oxidation of n-butane, and to use the 
aqueous phase as reflux to the rectification zone. By such recycle of the 
aqueous phase portion of the first fraction there can be withdrawn from 
the stripping zone an acetic acid vapor fraction containing from 0.5 up to 
8 weight percent water which can readily be concentrated to a "glacial" 
product by the cryogenic final step of this invention. 
Suitable for such continuous fractionation is a column having trays or 
packing of 50 to 60% of theoretical separation efficiency. Such a column 
will have as its top rectification zone from 15 to 12 trays or packed 
units and as its stripping from 10 to 20 trays or packed units. The second 
or concentrated acetic acid fraction is withdrawn 10 trays or packed units 
below the feed entrance. Such a continuous fractionation is conducted at 
essentially atmospheric pressure at the top of the column with a bottom or 
reboiler temperature of from 120.degree. C. up to 135.degree. C. using a 
reflux ratio of from 20:1 to 30:1. 
The acetic acid fraction removed from the heat treated 
decompressed-debutanized liquid reaction mixture is subjected to one or 
more sequences of continuous fractional crystallization by the technique 
which cools the fraction to freeze out an acetic acid-water eutectic 
crystalline magma having an acetic acid content higher than the acetic 
acid content of said fraction and leaves an acetic acid mother liquor 
having a water content higher than the water content of said fraction, 
moves said crystalline magma countercurrent to the flow of the mother 
liquor, melts at least a portion of the crystalline magma before its final 
composition is removed from the fractional crystallization system as 
product, and moves the melt liquor also countercurrent to the movement of 
the crystalline magma so that said flowing melt liquor and mother liquor 
wash the oppositely moving crystalline magma and mix to form a single 
waste liquor to be removed from the continuous crystallization system. 
By the use of two or more of such continuous fractional crystallization 
systems in series flow relationship the origin acetic acid fraction can be 
processed to an anhydrous product from which the remaining C.sub.1 to 
C.sub.4 homologues of acetic acid can be separated by distillation. 
The waste liquor from the foregoing continuous fractional crystallization 
system or from the first of two or more such systems is, according to the 
concept of the present invention, returned to the distillation step for 
concentration of its acetic acid content by removal of water. Such return 
can be to the still's boiler or after preheating to below, at or above the 
point of charging to the still column, the feed liquor which is the heat 
treated liquid portion of the butane oxidation effluent after its 
decompression and debutanization. 
The conduct of two or more of the foregoing continuous fractional 
crystallization systems are carried out in the following manner. For two 
series connected systems, the waste liquor withdrawn from the second 
system is added as part of the feed to the first system and the lastly 
washed crystalline magma produced in the first system, and leaving it as a 
melt is fed to the second system. For the conduct of three or more 
systems, the waste liquor of the third system and melt of the lastly 
washed crystalline magma from the first system becomes the total feed to 
the second system, the melt of the lastly washed crystalline magma from 
the second system is the feed to the third system and the melt of the 
lastly washed crystalline magma from the third system becomes the product. 
Three systems for effective continuous fractional crystallization are 
described by Gerard J. Arkenbout in CHEMTECH, vol. 6, September 1976, 
pages 596 to 599. Two of such systems comprise slow crystallization to 
maximize crystal purity and conveniently separable sized crystals followed 
by washing of the crystals formed by a melt of at least an outer portion 
of the last to form crystals in countercurrent flow with respect to 
crystal formation. One system effects such cooling and countercurrent 
washing by chilling the liquid feed in a long horizontal crystallizer 
whose inner surfaces, cooled by indirect heat exchange, are scraped by a 
helical screw end which advances the crystals as they begin to form near 
the feed end through to the discharge end. The resulting suspension of 
crystals in mother liquor discharges into the upper portion of a vertical 
column having a reciprocating piston periodically pushing down from the 
top of the column past the entry of the slurry into the column and forcing 
the slurry downward and then withdrawing toward the top of the column. The 
column also has, at the upper portion thereof a wall filter which extends 
from just below entry of the suspension down to slightly below the 
furthest downward thrust of the piston. The compression of the entering 
suspension by the piston forces mother liquor through the wall filter and 
compacts the crystals against the downwardly moving bed of previously 
compressed crystals. Near the bottom portion of the column a heating zone 
is provided to melt the compacted crystals reaching said heating zone. A 
valved liquid product exit is provided in the bottom of the column. The 
flow of liquid through the valve is adjusted so that the downwardly moving 
bed of compacted crystals forces only a part of the melt of the crystals 
out of the bottom of the column which forces upwardly the remaining 
portion of the melt of the crystals. The upwardly forced portion of the 
melt of crystals flows past the next upward adjacent portion of crystals 
before they move into the melting zone and displaces mother liquor from 
and/or melts the outer surfaces of the next upward adjacent portion of 
crystals, thus forming a new liquid in contact with them of lower impurity 
content which continues upward displacement of mother liquor from and/or 
melting outer layers of crystals contacted. As the bed of compressed 
crystals moves downward in contact with the upwardly moving liquid, new 
crystals form or crystals grow which have a lower impurity content. 
The second system containing the scraped wall surface chilling zone and 
vertical washing column has a long horizontal freezing zone made up of a 
series of chilled, scraped inner surface crystallization zones cooled by 
indirect heat exchange with a cold liquid. Each crystallization zone has 
not only scrapers to remove crystals from the cold inner surfaces but also 
has means for pumping least pure melt in the direction of the mother 
liquor discharge. The feed enters near the center of the last 
crystallization zone and the mother liquor is forced out one end of said 
zone. A temperature gradient is imposed on each of the crystallization 
zones such that a countercurrent flow of melt and crystals is established. 
Crystals formed in the coldest portion of the last crystallization zone 
are forced into the preceding zone and are first partially melted, the 
melt returning to the last crystallization zone and the unmelted crystals 
in contact with melt of purer crystals grow on the chilled surface and are 
forced further in the direction of the washing column. The crystals of 
increasing purity are forced into and downwardly through the column in 
contact with rising melt formed at the bottom of the column in its heated 
portion and rising through the column of the downwardly moving crystals 
and thence into the first section of the series of crystallization zones. 
In this system crystals are grown from a melt as pure as possible rather 
than from the least pure rejected waste. 
In neither of the two foregoing systems does recrystallization contribute 
to product purification. Consequently the separation power of those two 
separation systems is rather limited. 
The third system is a continuous purification accomplished not only by 
crystallization and countercurrent washing of crystals but also by 
repeated continuous recrystallizations accomplished in quite an unusual 
manner. The recrystallizations are not conducted by redissolving each 
crystal crop in an extraneous solvent. Rather the recrystallizations are 
accomplished by several steps of grinding crystals during their travel 
down through the wash column toward its bottom heating zone which melts 
the final crystals. Such grinding steps result in high separation 
efficiency per unit height of the wash column. The comminuting of the 
large crystals results in small particles which are not stable and 
dissolve in the upwardly moving surrounding liquid. The continuous 
comminuting of crystals and the travel of particles by countercurrent flow 
of melt liquor to a new pure liquid phase and recrystallization from such 
purer liquid phase ultimately results in the growth of larger purer 
crystals through the imposition of concentration differences, analogous to 
those of distillation or extraction. 
Such grinding can be accomplished by a plurality of ball mills set at 
various levels in the wash column, for example, steel or ceramic balls on 
perforated trays or sieve discs with vibration of the balls and/or the 
trays or discs. 
Such a system comprises a cooled and scraped surface crystallizer mounted 
in a vertical position at the top of a washing column having a plurality 
of perforated trays or sieve discs (e.g., 5 to 40 per meter of column 
height), a bottom heating zone to melt the last formed crystals washed 
with rising melt, a bottom discharge for liquid purified product, a feed 
inlet below the top, several (e.g., 2 to 4) trays or discs, and an upper 
outlet above the top disc or tray but below the crystallizer for discharge 
of impurity enriched liquor. An example of the number and size of the 
balls for the needed comminuting are 30 balls of 12 mm diameter per 80 mm 
diameter sieve disc having openings of 0.6.times.0.6 mm. 
The temperatures suitable for fractional crystallization of the acetic acid 
distillative fractions, compositions according to this invention, are 
governed by the freezing temperatures of their acetic acid-water contents. 
Such temperatures, for example, are known from tables of the freezing 
temperatures of acetic acid-water compositions, for example, those at 
pages 359 to 360 of volume 4 of the Physico-Chemical Constants of Binary 
Systems in Concentrated Solutions by Jean Timmermans, Interscience 
Publishers, Inc., New York (1960). 
The following is provided to illustrate our presently contemplated best 
mode of conduct of the present invention so that those skilled in this art 
can readily practice our invention. However, as those skilled persons will 
appreciate, equivalent results can be obtained by selecting different 
operating conditions for each step from among the preferred operating 
conditions therefore suited to the needs of such persons as indicated by 
the compositions of each mixture to be processed. Thus the illustrative 
example of least mode of operation is not intended to impose any 
limitation on the conditions for the practice of the present invention, 
for such limitations are only imposed by the terms and conditions set out 
in the appended claims.

EXAMPLE 
The impure acetic acid for use in this example is obtained by the 
continuous oxidation of liquid commercial n-butane (95% n-butane) with 
oxygen gas at a temperature of 193.degree. C. and a gauge pressure of 77.3 
kg/cm.sup.2 in the presence of cobalt added as cobalt acetate 
tetrahydrate, and bromine added as hydrogen bromic acid (48% HBr). The 
continuously removed total liquid reaction effluent is decompressed to a 
gauge pressure of 28 kg/cm.sup.2 and a temperature of 193.degree. C. by 
venting unreacted n-butane together with oxides of carbon, methane, 
ethane, ethylene, propane, and butane. Such vented gases contain 60 to 65 
weight percent n-butane, 30 to 35 weight percent oxides of carbon (mainly 
carbon dioxide and rather small amounts of the other named organic 
compounds). Said decompressed-debutanized liquid, hereafter designated 
"starting material," is heat treated at a temperature of 193.degree. C. 
and 28 kg/cm.sup.2 gauge pressure continuously at a residence time of 
ninety minutes. 
The resulting liquid (hereafter "heat treated") is cooled to 105.degree. C. 
and at that temperature is continuously fed, at the rate of 12.6 grams per 
minute to a distribution tray of a fractionation column comprising an 
upper rectification zone above the distribution tray, and below said tray, 
a stripping zone and a reboiler. The stripping zone is 50.8 mm internal 
diameter and has 10 trays functioning at 55 to 60% separation efficiency 
(i.e., 55 to 60% of a theoretical tray) between the distribution tray and 
the product vapor draw-off and 10 more trays in a 76.2 mm internal 
diameter column. The rectification zone is 50.8 mm in diameter and has 15 
trays of 50 to 55% separation efficiency. The fractionation system is 
operated at one atmosphere (0 kg/cm.sup.2 gauge) pressure, a reboiler 
temperature of 125.degree. C. and a temperature of 105.degree. C. at the 
top of the column. Concentrated acetic acid is withdrawn as a vapor from 
above the tenth tray below the distribution tray. The 105.degree. C. 
temperature vapor at the top of the column is removed, cooled to condense 
the vapors to liquid and the resulting liquid condensate is collected in a 
settling tank from which the organic (top) phase and the aqueous (bottom) 
phase can be separately withdrawn. In this example the aqueous phase is 
used as reflux to the rectification zone and at equilibrium steady flow 
operation the reflux ratio is 20.4:1 with a reflux rate of 17.95 grams per 
minute. 
The organic phase at 0.88 gram per minute (6.98% of the starting material) 
and the withdrawn reboiler liquid at 1.5 grams (11.9%) per minute are 
recycled to the butane oxidation. The concentrated acetic acid vapor 
(98.13 wt.% acetic acid and 0.5 wt.% water) is withdrawn at 8.1 grams 
(64.3% of the starting material) per minute, cooled to condense the vapor 
to liquid as feed for the cryogenic step for further concentrating the 
acetic acid to a water-free product. The aqueous phase (about 17.86 of the 
starting material) not recycled to the fractionation contains 18.8 weight 
percent acetic acid and 58.9 weight percent water. 
TABLE V to follow provides the compositions of the materials acted upon and 
produced from the decompression step through the fractionation. Under the 
heading "Components," "MEK" is used to designate 2-butanone, both for the 
unsubstituted ketone and the 3-bromo-substituted ketone (e.g., 3-Br-MEK); 
and "C.sub.4 " is used to designate a butyl group (e.g., "Sec.-C.sub.4 " 
for the secondary butyl group and "n-C.sub.4 " as the normal butyl group). 
TABLE V 
______________________________________ 
DECOMPRESSION THROUGH FRACTIONATION 
Component, Starting Heat 
Weight Percent Material Treated 
______________________________________ 
Butane 0.02 0.02 
Acetone 0.46 0.46 
Methyl Acetate 1.80 1.80 
Ethyl Acetate 1.16 1.16 
MEK 1.69 2.015 
Sec-C.sub.4 Acetate 
0.29 0.29 
n-C.sub.4 Acetate 
0.05 0.05 
3-Br--MEK 0.69 0 
Formic Acid 0.12 0.12 
Water 23.62 23.62 
Acetic Acid 65.43 65.43 
Propionic Acid 2.38 2.38 
n-Butyric Acid 0.91 0.91 
Unknowns 0.95 0.95 
Ionic Bromine 0.18 0.548 
Cobalt 0.25 0.25 
______________________________________ 
TABLE V 
______________________________________ 
DECOMPRESSION THROUGH FRACTIONATION 
Fractions 
Component, First 
Weight Percent 
Organic H.sub.2 O 
2nd 3rd 
______________________________________ 
Butane 0.46 0 0 0 
Acetone 2.94 1.45 0 0 
Methyl Acetate 
17.66 5.90 0 0 
Ethyl Acetate 15.62 4.15 0 0 
MEK 19.43 6.18 0 0 
Sec-C.sub.4 Acetate 
5.59 0.96 0 0 
n-C.sub.4 Acetate 
0.89 0.17 0 0 
3-Br--MEK 0 0 0 0 
Formic Acid 0.49 0.31 0.29 0.25 
Water 11.77 58.9 0.50 0.39 
Acetic Acid 13.87 18.79 98.13 84.49 
Propionic Acid 
1.13 0.48 0.98 6.07 
n-Butyric Acid 
0.64 0.17 0.09 3.09 
Unknowns 3.21 0.99 0 3.12 
Ionic Bromine 0 0 0 4.48 
Cobalt 0.01 0.01 0 1.40 
______________________________________ 
The foregoing second fraction (condensed concentrated acetic acid vapor 
stream with only 0.5 wt.% water) is charged as feed to a combination of 
indirectly cooled horizontal tubular cyrstallizer closed at one end and at 
the other end joined to the top of a vertical cylindrical column in fluid 
flow relationship. Said column has a closed bottom so that the combination 
of tube and column comprises a closed, fluid retaining system. Said 
horizontal tube having an inner helical ribbon screw driven at one end of 
the helical screw and pivotally supported at each end of the tube, to its 
discharge end and to scrape material frozen to the inner wall of the tube; 
a feed inlet 75 to 85% of the length of the tube away from its closed end 
and a waste liquid outlet near said closed end; and an inlet to the jacket 
around said tube near the closed end thereof and an outlet from said 
jacket near the junction of said tube and column; means for supplying a 
flow of chilled coolant to the inlet of the jacket, withdrawing warmed 
coolant from the outlet of the jacket and extracting heat by ndirect heat 
exchange from the circulating coolant to chill it for its return to the 
jacket's inlet; a crystalline product melter near the bottom closed end of 
the column. The helical ribbon screw-scraper can be driven at a rate of 
from 0.5 up to 2 revolutions per minute. 
Said second fraction is precooled to a temperature of 18.degree. C. and fed 
to the foregoing apparatus chilled by a solution of 50:50 water and 
ethylene glycol cooled by indirect heat exchange with a refrigerant to a 
temperature of -35.degree. C. Said solution enters the jacket of the 
horizontal tubular crystallizer near the closed end of the horizontal 
tube. The ribbon screw-scraper is operated at 0.7 to 0.8 rpm. The heater 
at the bottom of the washing column is operated to provide a melt at a 
temperature between 16.degree. C. and 17.degree. C. The temperature of the 
crystallizer at its feed inlet is between -20.degree. C. and -30.degree. 
C. The melted product contains no water. The waste liquor contains 75 to 
80 weight percent water and is recycled to the feed distribution tray of 
the fractionation column as part of the feed thereto for concentration.