Monochloroacetic acid process

Production of monochloroacetic acid with a minimum of polychlorinated derivatives in a two stage process by reaction of acetic anhydride and hydrogen chloride at a low temperature to produce a solution of acetyl chloride in acetic acid and chlorinating the said solution at a higher temperature which may be effected in a continuous manner.

STATE OF THE ART 
The production of monochloroacetic acid by reaction of elemental chlorine 
and acetic acid is known. For instance, U.S. Pat. No. 2,539,238 describes 
a process in which acetic acid and acetic anhydride are chlorinated under 
various conditions. U.S. Pat. No. 2,688,634 discloses the reaction of 
acetic acid with chlorine in the presence of acetyl chloride or acetic 
anhydride. The processes disclosed are normally batch processes which 
usually entail periodic opening of a reactor and exposing personnel to the 
fumes coming therefrom which are toxic. Also, difficulty is encountered in 
avoiding loss of highly volatile by-products such as acetyl chloride and 
avoiding overchlorination to produce polychloroacetic acid. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a novel process for the 
production of monochloroacetic acid with a minimum of polychlorinated 
impurities. 
It is another object of the invention to provide a two stage process for 
the production of monochloroacetic acid in good yields with a minimum 
amount of acetic anhydride catalyst. 
It is a further object of the invention to provide a continuous process for 
producing monochloroacetic acid without exposing personnel to dangerous 
fumes and with a minimum loss of acetyl chloride and other volatile 
products. 
These and other objects and advantages of the invention will become obvious 
from the following detailed description. 
THE INVENTION 
The novel process of the invention is conducted in two separate and 
distinct stages and in the first stage, a mixture of acetic acid and 
acetic anhydride is contacted with hydrogen chloride, preferably in the 
form of a gas comprising hydrogen chloride which may contain some amount 
of acetyl chloride and chlorine in small amounts. This step achieves a 
reaction of hydrogen chloride with acetic anhydride producing acetyl 
chloride and preferably, the reaction is conducted at a low temperature, 
usually below 60.degree. C. and generally at 35.degree. C. or below, 
preferably at or below 25.degree. C. but not less than the freezing point 
of acetic acid so that the resulting acetyl chloride largely or entirely 
remains dissolved in the acetic acid. The conversion of acetic anhydride 
to acetyl chloride is normally very high and usually approaches 100%. 
However, lower conversion ratios may be tolerated. 
In the second and separate step which is generally remote from the first 
step, the resulting mixture of acetic acid and acetyl chloride is 
subjected to chlorination, usually at a temperature at least 10.degree. C. 
above the temperature of the reaction of Step 1 and generally above 
60.degree. C., and usually at a temperature of 75.degree. to 150.degree. 
C. preferably about 90.degree. to 120.degree. C. The amount of chlorine 
supplied is rarely in excess of that amount which is theoretically 
required to convert the incoming acetic acid to monochloroacetic acid and 
preferably, the amount of chlorine fed to the chlorination zone is below 
about 85% of this amount. To achieve relatively high reaction rates, 
however, at least 65% of such theoretical amount of chlorine should be 
introduced. 
Summarizing, a concentration of acetyl chloride is established in acetic 
acid by reacting hydrogen chloride with acetic anhydride before or at 
least separate from the chlorination step, preferably at a relatively low 
temperature, and this mixture is separately reacted with chlorine 
advantageously at a higher temperature. This process is most effectively 
conducted in a continuous cyclic operation wherein acetyl chloride is 
recycled to minimize losses and the process is broken down into a series 
of individual steps with the respective conditions of each operation being 
adjusted to those which are optimum for that particular step. 
For example, the reaction of chlorine with acetic acid and acetyl chloride 
occurs more readily at an elevated temperature above 60.degree. C. as 
discussed above. This elevated temperature which rarely exceeds 
150.degree. C. can be established by exothermic heat of the chlorination 
reaction with cooling if necessary. On the other hand, if hydrogen 
chloride is contacted with acetic anhydride at an excessive temperature, 
the resulting acetyl chloride is volatilized and may have to be recovered 
by condensation. Also if the temperature is too high, undesirable 
by-products may be formed. Therefore, this reaction is better conducted at 
the lower temperature specified above. Even at such low temperatures, if 
excess hydrogen chloride is used, some acetyl chloride may escape with the 
escaping hydrogen chloride. However, this may be at least partially 
recovered by condensation and/or by scrubbing with cold acetic acid and 
the resulting acetyl chloride-acetic acid solution is forwarded to the 
chlorination reaction.

As shown in the drawing, a mixture containing 7% acetic anhydride and 93% 
of acetic acid (glacial) is fed at room temperature continuously through 
line 12 into the top of a packed glass-lined, polytetrafluoroethylene or 
graphite-lined gas-liquid contact Column C-1 and flows down over the 
packing (or through bubble plates) countercurrent to an upwardly rising 
continuous stream of hydrogen chloride gas which enters the column at the 
bottom thereof from line 14. This gas also may contain acetic acid, acetyl 
chloride and some chlorine. 
The temperature of the column C-1 or at least the top thereof is held low 
enough to minimize the escape of acetyl chloride, for example at about 
35.degree. C. or below, and thus acetic anhydride reacts with the hydrogen 
chloride and forms acetyl chloride thereby producing a liquid mixture of 
acetic acid-acetyl chloride which is essentially water free. 
Since the hydrogen chloride stream fed to line 14 conveniently comes 
originally from the monochloracetic acid reaction, it has the following 
composition in gaseous state: 
Acetic Acid (AA): 2.38% percent by weight; 
Hydrogen Chloride: 84.17% percent by weight; 
Acetyl Chloride (AC): 8.11% percent by weight; 
Chlorine: Balance. 
The height of the column C-1 and the depth of the packing therein is great 
enough to convert all of the acetic anhydride to acetyl chloride. 
The amount of hydrogen chloride passing through the column C-1 is 
considerably in excess of that amount of hydrogen chloride required to 
convert the acetic anhydride to acetyl chloride, generally being 100 to 
1000 mol percent of more in excess of that amount. The off gas from column 
C-1 may contain substantial acetyl chloride which may be scrubbed with 
cold acetic acid and the recovered acetyl chloride-acetic acid solution is 
sent to the chlorination stage as discussed below. 
A liquid mixture of 94.79% of acetic acid and 5.21% of acetyl chloride is 
withdrawn from the lower part of column C-1 through line 16 and is mixed 
with a recycling stream coming from line 50 containing about 99.28% of 
acetic acid and about 0.72% of acetyl chloride to produce a mixture 
containing 93.86% acetic acid and about 6.14% of acetyl chloride. Thus, 
the molecular ratio of acetyl chloride to acetic acid therein is about 
0.05. 
This mixture is fed continuously through line 16 and gaseous elemental 
chlorine is fed through line 20 into the bottom of reactor R-1 and, as 
diagrammatically illustrated, the mixture of gas and liquid reactants and 
reaction products flow concurrently and successively from reactor to 
reactor through a series of reactors R-1, R-2 and R-3. Reactors R-2 and 
R-3 are connected to reactor R-1 and R-2 through exit lines 24 and 26, 
respectively. These lines lead from the top of one reactor to the bottom 
of the next succeeding reactor in the series with chlorine, acetic acid 
and acetyl chloride being maintained in the reaction mixture by thorough 
mixing while the reaction is under way. 
The composition of the respective gas and liquid phases flowing through the 
respective lines are as follows: 
______________________________________ 
Line 24 Line 26 
to Reactor R-2 
to Reactor R-3 
Gas Liquid Gas Liquid 
Phase Phase Phase Phase 
______________________________________ 
Acetic Acid 1.39% 57.82 1.85% 32.85 
(AA) 
Monochloro- 
acetic Acid (MCA) 
trace 39.63 trace 64.13 
Acetyl Chloride 
4.60% 2.55 6.33 1.96 
Chlorine Balance trace Balance trace 
Hydrogen Chloride 
23.92% trace 54.61 trace 
Dichloroacetic Acid 
Nil Nil Nil 1.06 
______________________________________ 
Temperature in the reactors is maintained by cooling the reaction mixture 
to absorb heat of chlorination and to hold the temperature at a suitable 
level of about 90.degree. to 120.degree. C., for example at 110.degree. 
C., and the liquid reactants and reaction mixtures are stirred to avoid 
channeling. The total retention time of the reactants in the reactors is 
about 3 to 8 hours. 
The reactors may be of any convenient construction such as a jacketed 
glass-lined kettle provided with a stirrer. The reactors are largely 
filled with the liquid phase reactants and the chlorine or gas phases are 
intimately mixed with the liquid phases. 
Ultimately, the reaction mixture flows from the top of reactor R-3 through 
line 30 to a phase splitter 32 which separates the liquid phase from the 
gas phase. 
This gas is fed through line 33 to a cooler 34 where it is cooled to about 
35.degree. C. or below and then fed through line 14 to the bottom of 
column C-1. Therefore, this gas constitutes the hydrogen chloride gas 
which was referred to above as flowing counter-currently to an acetic 
acid-acetic anhydride mixture in column C-1. The temperature of the column 
C-1 is held at or below about 35.degree. C. and as stated above acetyl 
chloride is generated therein. 
Gas or vapor escaping from the top of the column C-1 has the following 
compositions: 
Acetic Acid: 2.43 Percent; 
Hydrogen Chloride: 83.88 Percent; 
Acetyl Chloride: 8.25 Percent; 
Chlorine: Balance. 
The gas leaves column C-1 at a temperature of about 35.degree. C. through 
line 36 and is further cooled to about 15.degree. C. by passing through 
cooler 37. After cooling, the cooled gas is sent through line 41 to the 
lower part of column C-2. Condensate of acetic acid and acetyl chloride 
from cooler 37 is transferred by means not shown to one of the reactors 
R-1, R-2 or R-3. 
The liquid phase from phase splitter 32 has the following composition: 
Acetic Acid: 20.12 Percent; 
Monochloroacetic Acid: 74.08 Percent; 
Di Chloroacetic Acid: 4.21 Percent; 
Acetyl Chloride: 1.59 Percent. 
This liquid phase is delivered to the upper portion of packed column C-2 
through line 38 and thus flows counter current to the incoming hydrogen 
chloride delivered through line 41. The temperature of the liquid is at 
reaction temperature of the reactor or, in this example, about 110.degree. 
C. The gas exits from the top of column C-2 at a temperature of 90.degree. 
C. and is cooled by passing through a condenser-cooler 39 to a temperature 
of about 15.degree. C. The liquid escaping from the bottom through line 42 
has a temperature of about 155.degree. C. and this is the crude 
monochloroacetic acid product which is to be purified by crystallization 
or other means to separate monochloroacetic acid from unreacted acetic 
acid and polychloroacetic acids. 
The effect of the counter current gas-liquid contact in column C-2 is to 
strip acetyl chloride from the liquid phase so that the raw product 
contains little or no acetyl chloride. Also, some acetic acid is removed 
from the gas phase by condensor 39 and is recycled to preferably reactor 
R.sub.1 by means not shown. Thus the gas phase escaping through line 44 
from the top of the column has the composition: 
Acetic Acid: Trace; 
Hydrogen Chloride: 89.66; 
Acetyl Chloride: 4.56; 
Chlorine: Balance. 
This gas phase is delivered by line 44 to the bottom of packed column C-3 
where it is counter currently scrubbed with cold liquid glacial acetic 
acid by line 52 to remove acetyl chloride therefrom. The gas leaving the 
top of the column C-3 through line 48 has a temperature of about 
24.degree. C. and has the composition: 
Acetic Acid: Trace; 
Hydrogen Chloride: 94.18 Percent; 
Acetyl Chloride: Nil; 
Chlorine: Balance. 
This gas is led to a suitable hydrogen chloride recovery system. 
The acetic acid liquid collected from the bottom of the column C-3 contains 
about 99.28% of acetic acid and 0.72% of acetyl chloride and is delivered 
through line 50 to be mixed with incoming acetyl chloride and acetic acid 
of line 16 as discussed above. 
Gaseous chlorine fed to the reactor R-1 through line 20 generally is 
proportioned to ensure introduction of a small amount of unreacted 
chlorine in the off gas coming from line 48 and to ensure a good 
selectivity of monochloroacetic acid with low polychloroacetic acids 
production. Preferably, the amount of chlorine is in the range of 65 to 85 
mol percent of the theoretical amount required to react with the acetic 
acid to produce monochloroacetic acid. That is, about 0.65 to 0.85 moles 
of chlorine is fed per mole of acetic acid fed to the Reactor R-1. While 
higher proportions of chlorine, rarely in excess of one mole per mol of 
acetic acid, may be used, this increased proportion tends to increase 
polychloro derivatives produced. 
Acetic anhydride is fed largely to compensate for loss of acetyl chloride, 
some quantity of acetyl chloride usually being in the off gas escaping to 
line 48. This line delivers the gases to a system for hydrogen chloride 
and chlorine absorption and recovery. In any event, the better the 
recovery of acetyl chloride, the lower the amount of acetic anhydride is 
required. 
The acetic acid used is anhydrous or substantially so, rarely containing 
more than five percent by weight of water and generally having a water 
content not over about 2 or 3%, preferably being anhydrous. The acetic 
anhydride added may be partially consumed removing water in the other 
reactants. Accordingly, the hydrogen chloride and the monochloroacetic 
acid produced as well as the acetic acid-acetyl chloride mixture leaving 
column C-1 through line 16 are substantially anhydrous. 
Some variation is possible in the composition of the respective streams 
illustrated in the drawing and as discussed above may be conducted without 
departure from the spirit of the invention. For example, as the 
temperature within column C-1 is raised, some increase may occur in the 
acetyl chloride and acetic acid content of the gas in line 36. In that 
case, greater care is required to scrub these out with cold acetic acid in 
column C-3 and/or to condense acetyl chloride and acetic acid in condenser 
37 and to recycle the condensate to the reactor. 
Also, some acetic acid is stripped along with acetyl chloride from the hot 
liquid phase entering column C-2 by the hydrogen chloride passing upward 
therein. The gas or vapor escaping through line 44 which has a temperature 
of about 90.degree. C. is cooled below 35.degree. C. for example, to 
15.degree. C. or below in condenser 39 and this condenses the acetic acid 
therefrom. The condensed acetic acid is returned to one of the reactors or 
to line 12 and the cooled gas is forwarded to the lower part of column 
C-3. 
The temperature in the reactors R-1, R-2 and R-3 may be higher or lower 
than 110.degree. C., generally above about 75.degree. to 150.degree. C., 
but rarely above 125.degree. C. 
The time of retention of reactants in the reactors should be sufficient to 
achieve substantial conversion (more than 50%) of the acetic acid to 
monochloroacetic acid. The exact length of time depends upon reaction 
temperature but generally is in the range of 1 to 12 hours. 
It will be noted that acetic acid is supplied to the reactor in two streams 
(lines 16 and 50). The amounts thereof in these respective streams are 
proportioned to hold the acetyl chloride largely in solution in column C-1 
and to recover all or at least an optimum amount of acetyl chloride from 
the gas stream passing through column C-3. This can be effective 
accomplished by feeding substantially equal amounts of acetic acid to each 
of the lines 12 and 52. However, it will be understood that these 
proportions may be varied to long as the general objectives are 
accomplished. 
Some chlorine remains unabsorbed in the reactors in most cases. Therefore, 
the hydrogen chloride stream fed to column C-1 usually contains a small 
residue of elemental chlorine. Because of the lower temperature and high 
dilution, also perhaps because of the other conditions of gas-liquid 
contact (rate of gas flow-through, low solubility of chlorine etc.), 
little or no chlorine is absorbed or reacted in column C-1 as a general 
rule, and chlorination of acetic acid in column C-1 is minor if it occurs 
at all. Accordingly, the temperature easily may be held low because 
substantial heat evolution from exothermic chlorination is not 
encountered. Excessive temperature, of course, is objectionable not only 
because acetyl chloride enters the gaseous phase but also because thermal 
disassociation tends to reduce the conversion to acetyl chloride. 
While the flow sheet illustrates a concurrent flow of the reactant of 
chlorine and acetic acid, the process may also be conducted counter 
currently, for example, by feeding chlorine into reactor R-3 and acetic 
acid into reactor R-1 while withdrawing gaseous hydrogen chloride from 
reactor R-1 and liquid reaction product from reactor R-3. 
The various steps in the process may be conducted readily at or near 
atmospheric pressure; for example, 10 to 200 millimeters of mercury above 
atmospheric pressure. If desired, a slight vacuum may be imposed on the 
system or parts thereof such as in line 48 and other gaseous lines in 
order to reduce the risk of leaking fumes into the surrounding atmosphere. 
Of course certain advantages may accrue if certain parts of the system such 
as column C-1, column C-3 or at least one of the reactors are at a 
superatmospheric pressure since vaporization of acetyl chloride may be 
reduced and chlorine absorption can be improved. However, in such a case, 
greater precaution must be taken to provide a tight well-gasketed system 
to avoid fume leakage. 
It will be understood that as the conditions of operation are varied, the 
compositions of the respective gas and liquid phase also change within the 
scope of the invention. 
Pure or substantially pure monochloroacetic acid (MCA) can be recovered 
from the crude product by cooling and crystallization of the 
monochloroacetic acid and draining and washing off the mother liquid. 
One advantage of the process herein contemplated is that it does not 
require the presence of inorganic catalysts or inhibitors such as 
phosphorous trichloride, sulfates, phosphates, nitrates or acetates of 
cobalt, manganese, chromium, nickel, sodium, barium, lithium, or the like 
such as stannous chloride, chromic acetate, manganese acetate, etc. The 
presence of these agents complicate the problems of recovering pure 
monochloroacetic acid from the crude product produced by this process. 
Although the present invention has been described with reference to 
specific details of certain embodiments thereof, it is not intended that 
such details shall be regarded as limitations upon the scope of the 
invention except insofaras included in the accompanying claims.