Process for separating mixed monochlorotoluene isomers, a plant for carrying out the process, and the isomers separated in this manner

The invention relates to a process for separating mixed monochlorotoluene isomers, which comprises a first stage of adsorption carried out by passing said mixture through at least one column filled with a zeolite, and a second stage of deadsorption from said zeolite carried out by passing two deadsorbents in succession in the vapor phase through the column.

Generally, because of the complexity of the necessary processes and plants, 
the separation of compounds having very close chemical and physical 
properties, such as isomer mixtures, is one of the most difficult problems 
of industrial practice. In particular, with regard to the present 
invention, the availability of parachlorotoluene and orthochlorotoluene of 
high purity in excess of 99 weight % is very desirable industrially, for 
use as raw material in secondary chemical synthesis. However, in the 
synthesis of monochlorotoluene, for example by chlorination of the 
aromatic ring, the formation of a mixture of orthochlorotoluene and 
parachlorotoluene is inevitable even in the presence of a catalyst, their 
relative quantities depending on the type of catalysis. 
However, the metachlorotoluene isomer forms in negligible quantity (about 
0.3%). 
The separation of the orthochlorotoluene and parachlorotoluene isomers is 
very difficult to carry out industrially, because of the closeness of 
their boiling points (3.degree. C.), and because of the similarity of the 
other chemical and physical characteristics. In the most commonly used 
method, the orthochlorotoluene is separated by superfractionation with a 
very large number of theoretical stages, whereas the para isomer can be 
separated by fractional crystallisation, although this presents 
considerable problems because of the formation of eutectic mixtures, and 
the number of stages required. The combined use of the two aforesaid 
methods is also known. 
The difficulties connected with these known separation methods are 
apparent. In particular, the superfractionation requires excessively 
complicated and costly plant. 
One object of the present invention is to separate mixed monochlorotoluene 
isomers by a process which is considerably more simple and convenient than 
those proposed by the aforesaid known art, and thus substantially more 
economical. 
In this respect, it should be noted that separation methods have been 
recently developed based on selective adsorption on solids such as 
activated carbon and zeolite materials. 
Of those patents relating to separation by these methods, mainly regarding 
the separation of n-paraffins from branched paraffins and the separation 
of substituted aromatic isomers, U.S. Pat. No. 2,958,708 deals very 
generally with the interaction between mono and di-halo substituted 
isomers with certain zeolites. 
In order to better understand the invention, some general concepts 
regarding known separation methods utilising fluid-porous solid 
interactions will be discussed. 
Generally, a quantity of solid, normally granular, is brought into contact 
with the fluid mixture to be separated into its individual constituents, 
to form a liquid phase richer in the less adsorbable components and a 
solid phase which incorporates the more selectively adsorbed components, 
which can then be extracted for example by elution with a solvent or by 
changing the operating temperature and pressure variables. 
It is known to operate in a succession of equilibrium stages by reiterating 
the described elementary operation, with stages physically separated in 
different vessels, either with movement of solid, or with solid in a fixed 
bed. 
Recently, as an improvement on the prior art, the sequence of adsorption 
and elution stages has been carried out in a fixed bed by feeding the 
mixture to be separated or the eluent and drawing off the purified product 
at suitable flow rates and positions along the column, so as to render the 
concentration profile for the individual components stationary within the 
column. 
In order to recover the phase retained in the solid, it is eluted according 
to the known art by a solvent or diluent which does not interact with the 
absorbed component, or interacts only negligibly under the operating 
conditions, and certainly not as to be competitive with the individual 
components to be separated, i.e. not to become absorbed. 
It is known that the behaviour of the individual component between the 
external fluid phase and the absorbed phase can be described under 
equilibrium conditions by means of isotherm curves which are generally 
non-linear for separation problems. These curves, besides expressing the 
distribution between the solid phase and fluid phase at all points, 
contain other information such as the loading on the solid corresponding 
to saturation (at the indicated temperature and pressure), which 
hereinafter will be indicated by the symbol .GAMMA..sub.i .sup..infin. 
and expressed in moles of the i.sup.th component per gram of anhydrous 
solid, and also the quality of the bond which is expressed by the value of 
an equilibrium constant, hereinafter indicated by the symbol Keq.sub.i in 
liters/mole, this being the ratio of the kinetic constants, again at the 
indicated temperature, for the absorption process and the deadsorption 
process. High values of this constant indicate considerable ease of 
adsorption. 
It is also known they many of the adsorbed components simultaneously 
present interact to make the operation competitive. In other words, the 
quantity of the individual component present in the solid under 
equilibrium conditions depends, in a considerably non-linear manner, on 
the concentration of all the elements present and on the value of the 
specific Keq.sub.i constants for the previously described component-solid 
binary interaction. 
The interaction cannot therefore be considered as the simple removal of 
those various components which have greater affinity for the solid, but 
rather a competition between them, which cannot be described in terms of 
linearised equilibrium. 
The fluid used for extracting the adsorbed component from the solid can 
interact with said component either strongly or weakly, i.e. either 
competitively or negligibly with respect to the components to be 
separated. In the first case (strong interaction), the value of its 
equilibrium constant as heretofore defined will be either comparable with, 
i.e. of the same order of magnitude, or higher than the value of the 
constant for the components to be separated. In the second case (weak 
interaction) the value of its equilibrium constant is negligible or in any 
case less than the value of the constant for the components to be 
separated, as will be described hereinafter. 
Hereinafter, a fluid having interaction of the first type will be known as 
a deadsorbent, whereas a fluid having interaction of the second type will 
be known as a solvent, diluent or eluent. 
The considerable difference must be emphasised between the two types of 
extractors, in the first case there being competition towards the solid, 
with the substitution of components in selective cavities of the solid. In 
the second case, this is either only mildly present or is completely 
absent and in any case quite negligible. 
In the first case, the absorbed component is extracted by competition, 
while in the second case it is eluted into an inert phase and extracted by 
concentration difference between the adsorbed phase and external phase. In 
this case, it is more correct to speak of elution. 
The total void space available for the fluid phase can be divided into two 
sections, of which only the internal porosity representing the 
interstitial volume is selective and useful for separation purposes. The 
external void space, being the free space between the solid particles, is 
not selective and as it requires a large quantity of material for filling, 
does not enable high liquid phase final concentrations of the individual 
separated components in the deadsorbent or diluent to be obtained. As the 
components must be separated from these latter by known methods, for 
example distillation, increased dilution leads to increased process costs. 
In order to reduce them, the known art teaches the use of a diluent which 
does not interact in the adsorption process, this being fed after the 
deadsorbent in order to expel the deadsorbent-deadsorbed component mixture 
from the external cavities, i.e. from the external void space of the 
solid. The higher concentrations obtainable can compensate for the 
increased process complication. 
It should be emphasised that according to this method, the second fluid, 
the eluent, serves only to expel from the vessel the matter which has 
accumulated in the external void space between the solid particles, thus 
washing the bed. 
According to the known art, in order to separate any mixture, for example a 
binary mixture, the mixture to be separated and the extractor fluid are 
fed in cyclic succession to one end of a bed composed of granular 
adsorption material, with flow rates and for times such as to obtain 
enrichment in the required compounds. These beds can be connected together 
in order to process a continuous throughput. The succession, the flow 
rates and cycle duration are obviously related, for a fixed quantity of 
material to be treated and for a fixed final purity, to the form of the 
equilibrium relationship of the individual components, in particular 
whether they can be correlated by Langmuir relationships, and to the 
values of the equilibrium constants, this being valid without prejudicing 
the generalities of the problem. If the extractor fluid is a deadsorbent, 
the known art teaches that the operation should be carried out in such a 
manner as to obtain migrating concentration pulses in the column. 
The importance assumed by the deadsorbent is clear, its behavior, in 
particular its load at saturation and its equilibrium constant together 
with its facility for subsequent separation by known methods, completely 
defining the type of operation, the cycle duration, the percentage 
recoverable for each bed, the number of beds necessary for completing the 
operation, the concentration of useful product and the possibility of 
recycling a non-separated section. 
Let us assume that a multi-component mixture, which for simplicity of 
description will be taken as binary containing two substances A and B, is 
to be separated. If A and B are fed for a short time through the absorbent 
bed, this will become saturated preferentially with one of the two 
substances, for example B, while A will remain preferentially external to 
the adsorbent material. If a deadsorbent stream, D, is then fed, then its 
composition at the plant outlet will be of the following type: in a time 
a-b, A-D is obtained; in a time c-d, B+D is obtained; whereas in a time 
b-c, a mixture A+B+D is obtained, and this is recycled. Assuming that the 
nature of the chosen deadsorbent D is such as to allow its subsequent easy 
separation both from A and from B by known low-cost methods, the 
industrial process consists of repeating the described separation cycle, 
which can operate either in the liquid phase or in the vapour phase. 
In the specific case of monochlorotoluene, it has been found that if a 
single deadsorbent D is used, which may be a single compound or more 
compounds in mixture, the progress of the separation of the aforesaid type 
is unsatisfactory due to low plant yield, the plant being of too high 
dimensions and high operating costs. In this regard the EP-A- No. 0046068 
can be cited. 
Further objects of the invention, related specifically to the solution of 
the aforesaid problems are consequently to provide complete separation of 
the mixed monochloro toluene components, i.e. total component recovery, 
with a component purity which is not less than 99%, this being attained 
with high plant yield and low operating costs. 
According to the invention the aforesaid objects are attained by a process 
for separating mixed monochlorotoluene isomers, comprising a first stage 
of adsorption carried out by passing said mixture through at least one 
column filled with a zeolite, characterized by comprising a second stage 
of deadsorption from said zeolite carried out by passing two deadsorbents 
in succession in the vapour phase through the column. 
More particularly, according to the preferred embodiments of the present 
invention, the monochlorotoluene isomers are separated by vapour phase 
adsorption on wide pore zeolite materials, such as type CaX and KY 
faujasites, at a temperature of between 150.degree. and 350.degree. C. 
(preferably between 180.degree. and 250.degree. C.) at a pressure of 
between 0 and 5 atmospheres (preferably between 0.8 and 1.5 atm.), by 
feeding the orthochlorotoluene+parachlorotoluene mixture and two 
deadsorbents in the form of toluene and monochlorobenzene through a bed of 
solid in cyclic sequence. It should be noted that the two deadsorbents 
interact with the solid with the same mechanism, they having their 
equilibrium constants lying within the limits indicated hereinafter, these 
contacts being obtained from interpretation of the break-through curves as 
reported in the following examples. Deadsorption takes place by 
competition rather than by elution of the matter retained in the solid by 
a solvent. 
According to the invention, it is preferable to use zeolite X in which the 
cation present has been exchanged with calcium to the extent of more than 
98%, and zeolite Y in which the cation present has been exchanged with 
potassium to the extent of more than 98%. 
The isomer metachlorotoluene in binary mixture with the other isomers also 
has break-through curves and specific parameters analogous to those of the 
isomers orthochlorotoluene and parachlorotoluene. If it is present in 
non-negligible quantity in the starting mixture, its separation occurs 
without deviating substantially from that described with respect to the 
other isomers. 
The separation capacity of the mixtures used according to the invention was 
evaluated by expressing the selectivity as: 
##EQU1## 
(where .GAMMA..sub.p is the quantity of parachlorotoluene adsorbed in 
moles/g of zeolite, .GAMMA..sub.o is the quantity of orthochlorotoluene 
adsorbed in moles/g of zeolite, 
C.sub.o is the orthochlorotoluene concentration in the fluid phase in 
moles/liter at equilibrium, 
C.sub.p is the parachlorotoluene concentration in the fluid phase in 
moles/liter at equilibrium). 
Selectivity in the liquid phase was measured by bringing a known volume of 
ortho and parachlorotoluene solution in n-octane or other diluent 
solvents, into contact with known quantities of zeolite for the time 
necessary to reach equilibrium, and measuring the consequent variation in 
the concentration of the two components. Selectivity in the vapour phase 
was evaluated by interpreting break-through curves such as those reported 
in the examples given hereinafter. Table 1 shows as an example the 
selectivity ranges obtained at ambient temperature for variously exchanged 
Y zeolites. Table 2 shows as an example some selectivity data obtained for 
variously exchanged X zeolites, again at ambient temperature. 
TABLE 1 
______________________________________ 
Selectivity ranges at ambient temperature for variously exchanged 
Y zeolites. 
TYPE OF EXCHANGE SEATION FACTOR 
______________________________________ 
H 1.1-1.6 
Na 0.6-0.8 
K 1.8-3.0 
Rb 2.5-3.5 
Ca 1.17-1.5 
Ba 1.6-2.1 
______________________________________ 
TABLE 2 
______________________________________ 
Selectivity ranges at ambient temperature for variously exchanged 
X zeolites. 
TYPE OF EXCHANGE SEATION FACTOR 
______________________________________ 
Na 1.4-2 
K 1.15-1.6 
Ca 1.8-3.0 
______________________________________ 
The selectivity values vary little as the temperature varies, and in spite 
of the phase passage, the data reported in Tables 1 and 2 can be extended 
to behaviour in the vapour phase with limited variation, as will be shown 
in the examples given hereinafter. 
In addition to selectivity, the loading capacity of the tested zeolites 
also varies little in passing from the liquid phase to the vapour phase. 
For example, in the liquid phase there is generally an adsorption of 
1.5-1.7.times.10.sup.-3 moles per g of zeolite, whereas in the vapour 
phase this value falls to 1.3-1.5.times.10.sup.-3 moles/g of zeolite. 
It is important to note that in order to attain the objects of the 
invention, the value of the equilibrium constant Keq of the individual 
deadsorbents must lie around .+-.40% of the Keq value of the individual 
isomer. For comparison purposes, the Keq values must all be evaluated at 
the same temperature and pressure. 
The two deadsorbents are fed to the separation column pure in predetermined 
succession.

These figures show diagrams relating to the variation in the composition of 
the parachlorotoluene isomer (dashed curve) and the composition of the 
orthochlorotoluene isomer (continuous curve) at the column outlet. The 
ordinate axis represents concentration and the abscissa axis represents 
time. The concentrations are expressed as molar fractions, and the time is 
expressed in hours. 
EXAMPLE 1 
Separation of parachlorotoluene and orthochlorotoluene from a mixture of 
ortho and parachlorotoluene in equal parts. A stream deriving from 198 
cm.sup.3 /hour of a liquid 1:1 molar ratio mixture or ortho and 
parachlorotoluene, a stream of toluene, and a stream of monochlorobenzene 
are fed in the vapour phase at 230.degree. C. and 1 atmosphere, in cyclic 
sequence for 20 minutes, for 10 minutes and for 60 minutes respectively, 
to a steel column of inner diameter 2 cm and length 660 cm disposed in a 
temperature-controlled oven and filled with 1050 g of zeolite X in the 
form of extruded 1/8" pellets in which the initially present sodium had 
been exchanged with calcium to the extent of more than 98%. The entire 
system was temperature controlled at 230.degree. C. The composition of the 
emergent mixture, determined by gas chromatography, is shown in FIG. 1. 
It can be seen that the parachlorotoluene leaves first, and is considerably 
enriched. The adsorption of orthochlorotoluene, parachlorotoluene, toluene 
and monochlorobenzene obeys a Langmuir isotherm is a very satisfactory 
manner. For the type of zeolite described, Table 3 shows the values of 
.GAMMA..sub.i .sup..infin. and Keq.sub.i, determined by processing the 
data of FIG. 1. 
TABLE 3 
______________________________________ 
Data relating to adsorption on zeolite X exchanged with calcium 
to the extent of more than 98%, the data being obtained from 
break-through curves. Temperature 230.degree. C., pressure 1 
atmosphere. 
______________________________________ 
orthochlorotoluene 
1.25-1.35 .times. 10.sup.-3 
moles/g 
parachlorotoluene 
1.20-1.30 .times. 10.sup.-3 
moles/g 
toluene 1.25-1.35 .times. 10.sup.-3 
moles/g 
monochlorobenzene 
1.36-1.50 .times. 10.sup.-3 
moles/g 
Keq orthochlorotoluene 
1.5 .times. 10.sup.+3 
liters/mole 
Keq parachlorotoluene 
1.0 .times. 10.sup.+3 
liters/mole 
Keq toluene 2.1 .times. 10.sup.+3 
liters/mole 
Keq monochlorobenzene 
.7 .times. 10.sup.+3 
liters/mole 
______________________________________ 
EXAMPLE 2 
Separation of parachlorotoluene and orthochlorotoluene from a 9:1 molar 
mixture of para and orthochlorotoluene. 
The column of Example 1 is fed in the vapour phase, at 230.degree. C. and 1 
atmosphere, with a stream deriving from 190 cm.sup.3 /h of a liquid 9:1 
molar mixture of para and orthochlorotoluene for 20 minutes, a stream of 
toluene for 20 minutes, and a stream of monochlorobenzene for 40 minutes 
in cyclic sequence. The entire assembly is temperature controlled at 
230.degree. C. The composition of the emergent mixture determined by gas 
chromatography is shown in FIG. 2. The values of the adsorption constants 
are shown in Table 3. 
The process according to the invention enables the aforesaid objects to be 
effectively attained. In order to quantify the main advantage of the 
invention compared with separation of the same type of isomer mixture in 
which only one deadsorbent is used instead of two, it should be noted that 
in order to obtain 99% purity of both separated isomers, 250 kg zeolite/kg 
of mixture must be used in the case of a single deadsorbent, whereas with 
the process according to the invention 80 kg zeolite/kg of fed mixture are 
sufficient. 
The invention also relates to a plant for carrying out the aforesaid 
process. 
The flow diagram of a plant according to the invention is shown in FIG. 3 
by way of non-limiting example. 
With reference to FIG. 3, a fresh mixture of ortho and parachlorotoluene to 
be separated, and originating from the synthesis section through a line 
45, is added in the vessel 9 to a non-separated recycle fraction 
originating from the distillation section by way of a condenser 28, a pump 
46 and a line 39. The mixture is then fed by a pump 10 through an 
evaporator 12 and a line 11, to the separation plant composed of columns 
50, 51, 52, 53 containing solid absorbent of absolutely identical 
behaviour, the number of columns being greater than one so as to be able 
to treat a constant throughput. 
The cyclic behaviour of a single column, for example column 50 is described 
hereinafter, but this bevhaviour is absolutely identical for the other 
columns. 
The column 50 receives the mixture to be separated by way of the line 11, a 
valve 13 and a line 16. The valves 14 and 15 remain closed. Through a line 
7, the valve 14 and the line 16, the column subsequently receives the 
first deadsorbent, namely toluene, from a vesel 5 to which it arrives 
through a line 36 from the recovery and distillation section, plus any 
necessary make-up through a line 44, the toluene being vaporised in 8. The 
valves 13 and 15 remain closed. In the last stage of the cycle, the column 
receives monochlorobenzene through the line 16, the valve 15 and the line 
3, this monochlorobenzene being pumped by the pump 2 from the vessel 1 to 
which it is fed from a distillation section 57 by way of a condenser 37, 
pump 49 and line 42. The valves 13 and 14 are closed. 
Four fractions are obtained at the other end of the column 50 by way of a 
line 17 and valves 18, 19, 20 and 21, of which only one is open, these 
fractions being fed through lines 22, 23, 24, 25 to the section in which 
they are separated from the deadsorbents and the deadsorbents are 
separated from each other, this being carrid out in distillation columns 
54, 55, 56 and 57. The first fraction, collected through the valve 20 and 
line 24, contains parachlorotoluene in high concentration. It is separated 
in 55 from the mixture of toluene and monochlorobenzene, these latter 
being fed as overhead product through lines 30 and 22 to their separation 
stage. Pure parachlorotoluene is obtained at the bottom of the column and 
is cooled at 31, then fed by the pump 47 and line 40 to the storage tank. 
The third fraction containing orthochlorotoluene at high concentration is 
obtained in a like manner and is fed to the column 56, from which it is 
recovered pure in the liquid state by way of the line 32 and heat 
exchanger 34, and is fed by the pump 48 and line 41 to storage. The 
second section containing insufficiently separated ortho and 
parachlorotoluene is recovered by way of the valve 21 and line 25 and is 
separated from the deadsorbents in column 54, then condensed in 28, and 
fed by the pump 46 and line 39 to the feed vessel 9. As a modification, 
this section can be recycled to the column directly without the two 
deadsorbents being separated from it. 
The deadsorbent mixture, separated from the isomers in columns 54, 55 and 
56, is fed by way of lines 27, 30 and 33 to line 22 arriving from the 
column 50 by way of the valve 18, the deadsorbent mixture then being 
separated into its individual constituents in column 57. These are 
recycled through lines 42 and 36 to the feed vessels 1 and 5.