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Vol. 26, No. 2 (2014)
Study on Gas-Phase Mechanism of Chloroacetic Acid 477
concentration was from 25 to 98 % (mass percentage), sample
weighted 30 g and dropping time was from 1h to 1.5 h. The
average residence time of packing material was from 15 to
25 s. The excessive amount of chlorine, whose airflow was
22mL/min, should flow into the reactor along with the material.
Hot water was passed into the external cavity of the reactor,
which was not only used for heating but also for heat extraction.
The conversion ratio of the sample varied from 40 to 60 %.
Liquid phase bubbling reaction: A certain amount of
acetic acid was filled into the chlorination reactor and the circu-
lating pump (glycol solution as low temperature medium) was
opened after the solution was cooled down to a set temperature.
The quantitative catalyst was put into the chlorination reactor
after it was heated to the specified temperature using a mag-
netic stirring heater. Then the chlorine was passed into and
the timer was started at the same time. Rising steam was cooled
and condensed in the condenser. Part of the condensate was
taken out as samples, while the rest returned to the reactor.
The overflow hydrogen chloride exhaust would be discharged
into the air after it was absorbed by exhaust absorber, cooled
by secondary condenser and absorbed by hydrochloric acid
absorption bottle and alkaline washing bottle respectively.
Chlorine was stopped passing into after the reaction was
finished.
At the same temperature, the value of Y=/Y– decreased with
the acetic acid content increasing in the reactant. However, the
trend slowed when the content of acetic acid was up to 40 %.
0.4
a
0.3
0.2
b
c
0.1
0.0
0
20
40
60
80
100
Acetyl chloride (wt %)
Fig. 3. Gas-liquid reaction for monochloroacetic acid and dichloroacetic
acid; (a) 60 ºC; (b) 80 ºC; (c) 90 ºC
At the same temperature, the yield ratio of dichloroacetic
acid to chloroacetic acid reduced with the acetic acid content
increasing in the reaction system, which showed a fast reaction
rate of acetic acid with chloroacetyl chloride and the reaction
was a fast reaction. The presence of acetic acid not only
reduced the concentration of acetyl chloride in the reaction
system relatively but also reduced the series reactions. This
result was consistent with the previous conclusion.
The temperature of liquid-phase bubbling reaction was
90 ºC and the mass percentage of the added acetyl chloride
was 25 %. Rising steam was cooled and condensed by the
glass spherical condenser. The condensate was separated from
the gas-liquid separator. Hydrogen chloride exhaust was
absorbed and processed. Part of the condensate was taken out
as samples, while the rest returned to the reactor.
When the conversion rate of acetic acid was elevated, the
yield ratio of dichloroacetate increased faster, which was one
of the late term of acetic acid chlorination reaction phenomenon.
When the conversion rate of the acetic acid was higher, the
concentration of acetic acid in the reaction system would be
lower, then the mass transferring would be harder. If chlorine
airflow of reaction medium and late term was consistent,
chlorine would be in excess, which caused more chloroacetyl
chloride further to convert into dichloroacetyl chloride and
the generation rate of dichloroacetic acid would be faster and
faster. Therefore, it is believed that the generation of dichloro-
acetic acid was in line with the series reactions mechanism.
Parallel reactions are trimolecular reaction. According to a
statistical point of view, there is a very slight chance for tri-
molecular to react.
Detection method: In this experiment, chloride solution
was analyzed after esterification13,14 by GC2900 gas chromato-
graph. The chromatography working conditions are as follows:
Column: ϕ mm × 3000 mm stainless steel column; Detector:
Thermal conductivity (TCD); Column temperature: 130 ºC;
Vapourizer temperature: 180 ºC; TCD temperature: 150 ºC;
Carrier gas (H2) flow: 35-40 mL/min; Bridge flow : 80-100
mA.
Qualitative analysis of each main component by means
of the qualitative of standard compound; while quantitative
analysis of each component is calculated by the absolute
retention time and the correction area normalization method.
RESULTS AND DISCUSSION
Gas-liquid mixed reaction of surface:According to Fig. 3,
no matter what ratio (acetyl chloride/acetic acid) of the reaction
materials and no matter how much catalyst acetyl chloride
was put. The dichloroacetic acid and chloroacetic acid yield
molar ratio (Y=/Y–) decreased with the reaction temperature
increasing. When the temperature increased, both the reaction
rate and the conversion rate increased, which showed the
concentration of chloroacetyl chloride in the system increased,
while dichloroacetic acid and chloroacetic acid yield ratio
decreased obviously. Therefore, the activation energy of acetyl
chloride in the chlorination process is higher than that of
chloroacetyl chloride and reaction rate of acetyl chloride with
chlorine increases relatively with the increasing temperature.
Therefore, chlorine airflow should be controlled effec-
tively throughout the process of the chlorination reaction so
that the best reaction effect could be obtained in the case of
the same amount of catalysts.
In late term of the reaction, the amount of acetyl chloride
solution also decreased with the decreasing of the acetic acid
concentration. At the same time, the generation rate of acetyl
chloride slowed, while the dichloroacetic acid increased rapidly.
Sometimes, chloroacetic acid concentration no longer increases
but tends to reduce with the reduction of acetic acid concen-
tration in the final stage of the late term of the reaction. How-
ever, at the same time, the concentration of the dichloroacetic
acid has been increasing.