Simultaneous production of higher chloromethanes
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Masini, Jean-Jacques; Verot, Yvan; |
The higher chloromethanes, i.e., CH.sub.2 Cl.sub.2, CHCl.sub.3 and CCl.sub.4, are simultaneously produced by chlorinating methyl chloride with chlorine in a first reaction zone A, chlorinating at least one of the higher chloromethanes CH.sub.2 Cl.sub.2 and CHCl.sub.3 with chlorine in a parallel second reaction zone B, combining the reaction products from said first and said second reaction zones A and B, separating higher chloromethanes from said combined reaction products, and recycling at least one of said separated higher chloromethanes CH.sub.2 Cl.sub.2 and CHCl.sub.3 as chlorination feed to said second reaction zone B.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the preparation of higher chloromethanes by chlorination of methyl chloride, and, more especially, to the simultaneous such preparation of select amounts of the three higher chloromethanes CH.sub.2 Cl.sub.2, CHCl.sub.3 and CCl.sub.4.
2. Description of the Prior Art
Conventional processes for the preparation of higher chloromethanes by chlorination of CH.sub.3 Cl, as a practical matter, result only in the production of CH.sub.2 Cl.sub.2 and CHCl.sub.3. The CCl.sub.4 co-product constitutes less than 15% of the total production and is thus considered to be only a by-product of the reaction.
The production of CCl.sub.4 is typically carried out by so-called "Tetra-per" (carbon tetrachloride/perchloroethylene) processes from chlorinated residues and/or saturated or olefinic hydrocarbons having two or three carbon atoms. However, such techniques lead to the generation of a not inconsiderable amount of heavy by-products. In addition, they consume large amounts of energy.
Because of these disadvantages of conventional processes, the preparation of CC1.sub.4 from chloromethanes originating from the chlorination of CH.sub.3 Cl would be of interest, especially since the most recent improvements in this process permit free radical chlorination in the liquid phase at a low temperature, using a chemical or photochemical initiator. Such processes are economical due to the high yields obtained and the simplification of the separation operations, the amount of troublesome by-products being small.
However, in the current state of the art, the simultaneous production of prescribed amounts, which may be close to one another, of the three higher chloromethanes does not proceed without giving rise to problems in the conventional apparatus which consists of a reactor for the chlorination of CH.sub.3 Cl followed by downstream columns for purifying and separating the various products of reaction. Such a system lacks flexibility. In fact, in this type of unit, the principal constraint is the adjustment for the respective productions of CH.sub.2 Cl.sub.2 and CHCl.sub.3 ; the amount of CCl.sub.4 simultaneously produced is a function of the CH.sub.2 Cl.sub.2 -CHCl.sub.3 proportion effectively obtained and the operating conditions. This production of CCl.sub.4 can only be low with respect to that of the other two chloromethanes.
The production of prescribed amounts, which may be closed to one another, of the three higher chloromethanes in conventional apparatus comprising a single chlorination reactor must necessarily be balanced by the total amount of investment required for profitable operation. In fact, in this type of unit, the adjustment in the flexibility is obtained by limiting the degree of conversion of CH.sub.3 Cl and, if necessary, by appropriate recycling of CH.sub.2 Cl.sub.2. This manifests itself in a significant over-dimensioning in the entirety of the apparatus concerned, and concomitant marked increase in operating costs. Moreover, the productivity of a given reactor, all things otherwise being equal, is sensitive to the composition of the higher chloromethanes produced. It is known that the rate of chlorination of a chlormethane substantially decreases as the number of chlorine atoms contained in the molecule increases: a reactor having a productivity of 100 for respective compositions of CH.sub.2 Cl.sub.2, CHCl.sub.3 and CCl.sub.4 of about 45/45/10% by weight would only have, all things otherwise again being equal, a productivity of about 65 for a distribution of 37.5/47.5/15 and of 30 for 20/45/35. "Productivity" is defined as the amount of chlorine reacted per unit volume of the reactor for a fixed level of unreacted chlorine at the outlet of the reactor.
SUMMARY OF THE INVENTION
Accordingly, a major object of the present invention is the provision of improved chlorination apparatus/process, said apparatus comprising two reactors A and B in parallel, with reactor A operating in conventional manner for the chlorination of methyl chloride and reactor B, which is fed with chlorine and CH.sub.2 Cl.sub.2 and/or CHCl.sub.3 produced in reactor A, serving to produce either the higher chloromethanes CHCl.sub.3 and CCl.sub.4 simultaneously, or CCl.sub.4 by itself, and to ensure high flexibility respecting the overall apparatus.
Briefly, in the subject process adjustment in the production of CH.sub.2 Cl.sub.2, CHCl.sub.3 and CCl.sub.4 is effected by adapting, on the one hand, the distribution and amount of products formed in reactor A and, on the other hand, the production and the ratio of the amounts of CH.sub.2 Cl.sub.2 and CHCl.sub.3 converted in reactor B.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE of drawing is a schematic/diagrammatic illustration of apparatus suitable for carrying out the process according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
More particularly according to the present invention, with reference to the accompanying FIGURE of drawing, the reactor A is fed with Cl.sub.2 and CH.sub.3 Cl, respectively, via the lines 1 and 2. The reaction products are transferred via line 3 into a separation unit D: HCl/CH.sub.3 Cl/higher chloromethanes, with HCl being removed via line 4 and CH.sub.3 Cl and the higher chloromethanes being conveyed via line 5 into the separation unit E. Unreacted CH.sub.3 Cl is removed via line 8 or recycled via line 6 to reactor A. In order to limit the temperature at the base of the tank of unit D, it is also possible to recycle more or less CH.sub.3 Cl via line 7. The higher chloromethanes are conveyed via line 9 into the separation unit F for separating CH.sub.2 Cl.sub.2 from the other higher chloromethanes, it being possible for all or some of the CH.sub.2 Cl.sub.2 to be recovered at 12 or for all or some of it to be recycled via line 13 into reactor B. Finally, the remaining higher chloromethanes are conveyed via line 11 into the separation unit G for separating CHCl.sub.3 from crude CCl.sub.4, it being possible for all or some of the CHCl.sub.3 to be recovered at 16 or for all or some of it to be recycled via line 17 into reactor B, the crude CC1.sub.4 being recovered at 15. Provision may be made for recycling the higher chloromethanes into reactor B via lines 10 and 14. In this case, CCl.sub.4, which does not interfere with the reaction products, is recycled into reactor B. This CCl.sub.4 thus plays the role of diluent for the reactants and in this manner may decelerate the reaction; as a result, this recirculation should preferably be limited.
Before the higher chloromethanes are introduced into reactor B, it may be useful to first circulate them into a gas-scrubbing device C which enables the gaseous phase issuing from reactor B to be scrubbed.
The higher chloromethanes react, in reactor B, with Cl.sub.2 introduced via line 18. The reaction products are combined via line 19 with those of reactor A in the simplest manner, in the separation unit D.
It is recommended, although this is not essential, that the chloromethanes being charged to reactor B contain as little CH.sub.3 Cl as possible, such that this reactor at the same time maintains an acceptable selectivity and productivity for the given degrees of conversion of CH.sub.2 Cl.sub.2 and/or CHCl.sub.3.
The adjustment in the amounts of CH.sub.2 Cl.sub.2 converted and CHCl.sub.3 converted or produced is effected by adaptation of the composition of CH.sub.2 Cl.sub.2 and CHCl.sub.3 in the reactor, as well as the rate at which the reactor is fed. The reaction parameters of each of the reactors, and of course the reactors themselves, are adapted such that the reactions take place in the liquid phase. The pressures in each reactor are in general close to one another and should be selected such that the separation of HCl and the chloromethanes is facilitated. Good results are obtained using temperatures in the reactors ranging from 50.degree. to 120.degree. C., preferably from 60.degree. to 90.degree. C., under pressures of 10 to 50 bar, preferably 15 to 25 bar. The ratio in reactor A between the number of molecules of Cl.sub.2 and the number of molecules of CH.sub.3 Cl advantageously ranges from 0.3:1 to 1.5:1, preferably from 0.5:1 and 1:1. The amount of Cl.sub.2 introduced into reactor B is selected as a function of the amount of each of the higher chloromethanes introduced and of the higher chloromethane or chloromethanes desired to be ultimately obtained. The chlorination reactions in the reactors A and B are known per se, and are usually reactions of the free radical type, with initiation thereof being by means of a chemical or photochemical initiator.
The reaction products exit the reactors A and B in liquid or gaseous form. It is possible, although this is not essential, to scrub the gaseous phases with higher chloromethane in order to recover the trace amounts of chlorine which are contained therein and which constitute most of the unreacted chlorine. This scrubbing is of particular interest with respect to the products of reactor B. Such a scrubbing enables the excess chlorine to be recovered and as a result thereof significantly increases the productivity of the reactor.
The various reaction products of the reactors A and B are separated from each other in the downstream separation units, described above, under the typical distillation conditions appropriate for each of such products.
The process according to the invention has the advantage of concentrating the problems of limitation in productivity in a single auxiliary reactor B, which very substantially obviates same by reason of the fact that the reaction material in this reactor is formed from CH.sub.2 Cl.sub.2 and/or CHCl.sub.3. Another advantage concerns the flexibility, which is particularly significant in the reaction unit and which preserves, in addition, all the economic advantages of the conventional process. In fact, for a given production of CCl.sub.4, which may be up to 50% of the total production, the CH.sub.2 Cl.sub.2 -CHCl.sub.3 distribution in the complement of the production may vary continuously over wide ranges, and more particularly, from an economic point of view, between 75/25 and 25/75% by weight.
In order to further illustrate the present invention and the advantages thereof, the following specific example are given, it being understood that same are intended only as illustrative and in nowise limitative.
EXAMPLE 1
A reactor A was continuously charged with 0.15 kmoles/hour of liquid chlorine and 0.185 kmoles/hour of liquid CH.sub.3 Cl. This reactor was a perfectly stirred photochemical reactor operating under a pressure of 15 bar absolute and at 60.degree. C. and cooled by water circulation; the reaction products had the following approximate composition (mixture of liquid+vapor):
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% by weight
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HCl = 27.2
CH.sub.3 Cl =
22.7
Cl.sub.2 <0.5% of the mixture of the other
compounds
CH.sub.2 Cl.sub.2 =
22.1
CHCl.sub.3 =
22
CCl.sub.4 =6
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A reactor B was continuously charged with 0.021 kmoles/hour of liquid chlorine and 0.043 kmoles/hour of liquid CHCl.sub.3. This reactor was identical to the reactor A and operated under the same operating parameters. The reaction products therefrom had the following approximate composition (mixture of liquid+vapor):
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% by weight
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HCl = 11.8
Cl.sub.2 <1% of the mixture of HCL +
CHCl.sub.3 + CCl.sub.4
CHCl.sub.3 = 38.5
CCl.sub.4 = 49.7
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The two reaction mixtures were transferred into a distillation column operating under a pressure of 12 bar absolute, in which the HCl was separated from the other reaction products. A product having the following approximate composition was drawn off:
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% by weight
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CH.sub.3 Cl = 22.2
CH.sub.2 Cl.sub.2 =
21.7
CHCl.sub.3 = 34.1
CCl.sub.4 =22
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This mixture thus obtained was then separated by successive distillation such that each of the chloromethanes was obtained, i.e., the equivalent of:
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kmoles/hour
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CH.sub.3 Cl = 0.089
CH.sub.2 Cl.sub.2 =
0.051
CHCl.sub.3 = 0.058
CCl.sub.4 = 0.029
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was thus obtained.
Thus, taking into account the recycling of 0.043 kmoles/hour of CHCl.sub.3 into the reactor B, the production of higher chloromethanes was hence approximately:
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kmoles/hour
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CH.sub.2 Cl.sub.2 =
0.051
CHCl.sub.3 = 0.015
CCl.sub.4 = 0.029
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which gave an amount, by weight of CH.sub.2 Cl.sub.2 and CHCl.sub.3 in the CH.sub.2 Cl.sub.2 +CHCl.sub.3 mixture, of, respectively, 70.3% and 29.7%.
EXAMPLE 2
The experiment of Example 1 was repeated, but an amount of CCl.sub.4 corresponding to about 20% by weight of the chloroform was added to the chloroform charged to the reactor B. The only noticeable change in respect of the results obtained in Example 1 was the increase in unreacted chlorine in the reactor B.
EXAMPLE 3
Reactor A was operated under the same conditions as described in Example 1.
Reactor B was in this Example charged with 0.029 kmole/hour of liquid CH.sub.2 Cl.sub.2 and 0.05 kmole/hour of liquid chlorine. It operated under the same operating conditions as in Example 1. The reaction products had the following approximate composition (mixture of liquid+vapor):
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% by weight
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HCl = 30
Cl.sub.2 <95 of the total mixture
CH.sub.2 Cl.sub.2 =
2
CHCl.sub.3 = 14
CCl.sub.4 = 54
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After separation of the reaction products, the equivalent of:
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kmoles/hour
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CH.sub.2 Cl.sub.2 =
0.053
CHCl.sub.3 = 0.043
CCl.sub.4 = 0.029
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was thus collected.
Taking into account the recycling of 0.029 kmole/hour of CH.sub.2 Cl.sub.2 to the reactor B, the production of higher chloromethanes was thus approximately:
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kmoles/hour
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CH.sub.2 Cl.sub.2 =
0.024
CHCl.sub.3 = 0.043
CCl.sub.4 = 0.029
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which gave an amount, by weight of CH.sub.2 Cl.sub.2 and CHCl.sub.3 in the CH.sub.2 Cl.sub.2 +CHCl.sub.3 mixture, of, respectively, 27.8% and 72.2%.
While the invention has been described in terms of various preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions, and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims.
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