Chapter 5

5. Process Description

5.1. Photo-Chlorination of toluene

5.1.1. Over View

The chlorination of toluene in the absence of the catalysts that promote nuclear

substitution generally occurs in the side chain. The reaction is promoted by free

radical initiators such as ultraviolet light or peroxides. The chlorination takes place in

a step wise manner and can be controlled to give a good yield of intermediate

products. Small amounts of sequestering agents are sometimes used to remove trace

amounts of heavy metal ions that cause ring chlorination.

CH3 CH2Cl CHCl2 CCL3

Cl2 Cl2 Cl2

k1 k2 k3

IN PRESENCE OF UV LIGHT

Benzyl Chloride is manufactured by thermal or photochemical chlorination of toluene

at 65-1000C (Lin and Krishnamurthy, 1975). At lower temperatures the amount of

ring chlorinated products is increased. The chlorination is usually carried to 50%

toluene conversion in order to minimize the amount of Benzal chloride formed.

Overall yield based on benzyl chloride is more than 90%.Various materials, including

phosphorous pentachloride, have been reported to catalyze the side chain chlorination.

These compounds and others such as amides also reduce the ring chlorination by

complexing metallic impurities.

In commercial practice, chlorination may be carried out either in batches or

continuously. Glass lined or nickel reactors may be used. The crude product is purged

of dissolved hydrogen chloride, neutralized with alkali and distilled. Chlorine

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Chapter 5

efficiency is high; the product distribution depends on the so called degree of

chlorination or the molar feed ratio of chlorine to toluene.

In particular the chlorination of toluene has been studied by several authors: Veyola

(1955) found that the reaction proceeds slowly near room temperature without an

activator, and that side chain and nuclear substitutions take place simultaneously .Side

chain chlorination is facilitated by light under conditions in question. High chlorine

concentrations at 00C or below accelerate the rate of chlorination of toluene

tremendously, but substitution in the nucleus and in the side chain as well as addition

takes place.

In light or darkness a mixture of side chain substituted compounds, benzyl chloride,

benzal chloride, and benzo-trichloride is obtained. The ratio of these compounds

depends above all on the ratio of the chlorine to toluene used, the so called degree of

chlorination or the chlorination level. Veyola chlorinated to level of only 0.25, but

Benoy and De Maeyer (1954), studied the complete product distribution pattern upon

batch chlorination of toluene at low temperatures (150C), under the influence of light

of standard fluorescent lamp. Various such experiments have been carried out at

different temperatures. The product distribution patterns for side chain chlorination of

toluene depending on the degree of chlorination are shown for continuous

chlorination:

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Chapter 5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5

benzyl toluene

benzal

0

20

40

60

80

100

120

140

160

Wt%

Degree of Chlorination

Weight % of chlorinated products as a function of degree of chlorination at 100 degree celcius

benzyl

toluene

benzal

Experiments (Haring and Knol, 1964) have shown that the product distribution pattern

remain unaffected by the type of catalysts such as actinic light, phosphorous

trichloride or peroxides.

Experiments have summarized that liquid phase chlorination of toluene consists of

two simultaneous reaction systems:

1. Side chain chlorination as the main reaction system, in turn consisting of three

pseudo first order consecutive reactions:

2. Substitution of chlorine in the aromatic nucleus, accompanied by side chain

chlorination of the nuclear chlorine compounds, as a simultaneous side

reaction system.

5.1. 2. Side Chain Chlorination Products:

Although the rate of chlorination seems to be markedly accelerated by the use of

catalysts such as actinic light, phosphorous trichloride4 or peroxides5, 6, experimental

determination of the reaction product distributions for continuous or batch

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Chapter 5

chlorination, under various reaction conditions, reveals that the reaction rate constant

ratios and , the so called selectivity parameters, are not affected by the type of

catalyst. The product distribution apparently corresponds to a reaction kinetic

equilibrium condition, fixed by the degree of chlorination and by the selectivity

parameters of the consecutive reactions. Catalysts essentially accelerate the rate of

formation the chlorine radicals and, thus, the velocity with which the fixed

equilibrium condition is reached. The selectivity parameters are functions of the

activation energies required for the formation of compounds in question and thus

functions of temperature: at 1000C the rate constant ratios are about 6, at 400C about

8, which means that the selectivity of the rapid reactions is enhanced by lowering the

temperature.

Since the formation of the compounds chlorinated in the benzene ring is

favored by lower temperatures, this reaction counteracts the increase in selectivity for

benzyl chloride observed at lower temperatures. This accounts for the observations

that the weight percentages of benzyl chloride, benzal chloride at a given chlorination

level are nearly independent of the temperature , between 400C and 1300C.However

the net amount of side chain chlorinated products from a given amount of toluene is

higher at higher temperatures towards 1300C.

5.1. 3. Order of the reaction

All chlorine was consumed immediately and completely in most of the continuous

experiments and in first stages of batch chlorination. This means that the chlorine

input rate was reaction-rate controlling (an overall reaction of zero order in

toluene).Nevertheless there is experimental evidence that the product distribution

pattern of the consecutive reaction products in toluene chlorination is unaffected by

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Chapter 5

the chlorine input rate and was identical with that expected with first order reaction

kinetics. This can be explained by the assumption that the main part of the reaction

will take place in the a narrow reaction zone near the chlorine gas inlet where excess

chlorine is present in solution, making the reaction obey first order kinetics in situ.

Increasing the chlorine input rates will then extend the reaction zone, which can

indeed be observed in the reactor by a broadening of yellow- colored region near the

chlorine feed point.

For the production of benzyl chloride we operate at a degree of chlorination = 0.5 so

that very low quantities of benzal chloride is formed.

5.1. 4. Classical Reactor for the photo-chlorination of Toluene

(Andre and Tournier, 1983)The industrial photo-chlorination of toluene is carried out

in a continuously stirred gas-liquid photo-reactor. Residence time is 2Hr and the

irradiation is performed by using ten 1000W lamps. Working temperature is 1000C

with toluene reflux. The extent of chlorination is fixed at 0.5 and the non transformed

toluene is separated by distillation, cooled and recycled. The reactor operates under

the diffusion regime. The rate of reaction is limited by the rate of chlorine supply and

as a consequence the there is a very low stationary concentration of chlorine in the

liquid phase. The weight fraction product distribution at 1000C, 0.5 degree of

chlorination for continuous photochlorination is toluene 44%, benzyl chloride 47%

and benzal chloride 9%.

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Chapter 5

Industrial reactor for Photo-chlorination of toluene

(Ref: Tournier and Deglise, 1983)

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Chapter 5

5.2. Unit wise process description

1. Toluene is stored in the main storage tank MST1001.From here it is pumped

to the reactor R1001 through a day storage tank DST1001 at 250C.The

recycled toluene from the distillation operation is also introduced in DST1001.

Chlorine is available in chlorine tonners of 900kg each. This is evaporated in

an evaporator FE1001. The expansion operation of chlorine endothermic and

is used for the generation of chilled water. Chlorine gas is sparged at the

bottom of the reactor at 50C. The reactor is a continuous stirred reactor

operated at 1000C and a pressure of 1.3 atmospheres. In the reactor at the

given temperature, pressure and 0.5 degree of chlorination Benzyl chloride,

Benzal chloride and HCl is produced. Chlorine is supplied in

stoichiometrically deficient amount and hence completely utilized.

2. HCl gas produced in the reactor goes to the HCl scrubber arrangement. This

gas is assumed to be saturated with organic vapors. Hence organic vapors

corresponding to their vapor pressure at that temperature go with HCl. This

gaseous stream is passed through the Heat Exchanger HE1001.Here the gases

are cooled from 1000C to 350C by cooling tower water. The condensed

organics are collected in a separate vessel SP1001. The gases are further led to

Heat Exchanger HE1002 where they are further cooled to 150C by using

chilled water. Some more amounts of organics that condense are also directed

to FL1001.The condensed organics in FL1001 are fed back to the reactor

R1001. The HCl gas from HE1002 with little amount of organics is the fed to

the HCl scrubber.

3. HCl scrubber consists of a tail gas absorber PB1001 (packed bed absorber)

and a falling film absorber (FF1001). Scrubbing is done with water. The

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Chapter 5

quantity of water is so decided that the HCl on absorption gives 30% w/w

hydrochloric acid solution which has commercial value. The absorption of

HCl in water is highly exothermic and so the absorbers should have

arrangement for heat removal; hence the choice of Falling Film absorber. The

system is so designed that 90% of the HCl is absorbed in FF1001 and

remaining 10% in PB1001. Water is introduced from the top in the packed bed

absorber. It strips the 10% of the remaining HCl that has escaped form the

falling film absorber. This forms a dilute acid which is led to the falling film

absorber. Here 90 % of the HCl from the gaseous stream from HE1002 is

scrubbed off. One important aspect of the HCl scrubbing is that though the

temperature of that gases in the scrubber are higher than in HE1002,

absorption of HCl results in saturation of the gaseous stream and hence in

condensation of the organic vapors. The 30% w/w Hydrochloric acid from

FF1001 is taken to a gravity decanter FL1002 where phase separation between

the condensed organic phase and aqueous phase is effected. The acid is taken

to DST 1002.The condensed organics are pumped to the liquid organic stream

from the reactor. Since no equipment works at 100% efficiency some little

amount of HCl with marginally low trace amounts of organics is sent to the

flare. Actually two decanters FL1002 and FL1003 are used in parallel, as the

liquid- liquid separation of the organics and the aqueous acid takes time.

Decantation time is typically 1hr.

4. The liquid stream from the reactor at 1000C is fed to the distillation column

D1001. Here the toluene and dissolved HCl corresponding to its saturation

solubility is removed as distillated while the benzyl chloride and Benzal

chloride is removed as bottoms.D1001 is a packed column using 16mm pall

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Chapter 5

rings (ceramic) as packing. The distillation is carried out at atmospheric

pressure .The bottoms temperature is 1850C and the temperature at the top is

1150C. The distillate obtained at 1100C is then fed to the IST1001. From here

it is recycled to DST1001.

5. The bottoms form D1001 at 1850C, are fed to the distillation column D1002

where benzyl chloride is removed as distillate and the Benzal chloride as

bottoms.D1002 is a packed column with 16mm ceramic pall ring packing. The

operating pressure is again atmospheric. Bottoms temperature is 2150C while

the top temperature is 1860C. The distillate benzyl chloride is cooled to 250C

by HE1003 and pumped to DST1003. The Benzal chloride obtained as bottom

product is cooled by HE1004 to 250c and taken to DST1003.Products from

DST1003 and DST1004 are taken to MST 1002 and MST 1003 respectively.

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