PROCESS FOR SEPARATING A CHLOROTOLUENE ISOMER

Abstract: 3,5-dichlorotoluene is separated from a mixture contain­ ing at least one isomer of 3,5-dichlorotoluene by adsorptive separation using a simulated moving bed with a faujasite type zeolite adsorbent in the presence of another substituted benzene compound. 2-chloro-m-xylene is similarly sepa­ rated from a chloroxylene isomer mixture. In each case the desired isomer is separated as the extract component. In the case of separation of 3,5-dichlorotoluene the said substituted benzene compound is either 1,2,4-­ trimethylbenzene or 2-chloro-p-xylene, and for separation of 2-chloro-m-xylene the said substituted benzene compound is preferably toluene or chlorobenzene.

Description: In our copending European patent application, published as EP-A-0125177, from which the present application is divided out, we have described and claimed a process for separating 2,6-dichlorotoluene by adsorptive separation in the liquid phase from a dichlorotoluene isomer mixture containing at least 2,6-dichlorotoluene and 2,3-dichlorotoluene. According to that patent application the separation is performed using a simulated moving bed and employing a faujasite zeolite of the X-type containing as the cation 50 to 100 mole % of sodium, 0 to 50 mole % of silver and 0 to 50 mole % of cesium, the separation being performed in the presence of another substituted benzene compound and the 2,6-dichlorotoluene being separated as the extract component. The inventors also found that 3,5-dichlorotoluene could be separated from a mixture of dichlorotoluenes also using a simulated moving bed, a faujasite type zeolite adsorbent and a substituted benzene compound. The faujasite type zeolite is not limited to the use of the X-type zeolite referred to above, but in the case of separation of 3,5-dichlorotoluene the inventors have found that 1,2,4-trimethylbenzene and 2-chloro-p-xylene are specifically suitable as the said substituted benzene compound. When a single isomer is prepared and recovered, the following problems inevitably arise in all conventional processes. In the first place, the formation ratio between an isomer formed as a by-product and an isomer to be recovered is not in agreement with the demanded ratio between the two isomers, and the by-product is a surplus and thus conventional processes are not preferred from the resource-saving viewpoint. In the second place, the difference of the boiling point between an isomer formed as a by-product and an isomer to be recovered is so small that it is difficult to separate these isomers from each other by ordinary rectification, with the result that the separation process becomes complicated and a large quantity of energy is consumed for the process. The first problem may be solved by adoption of a process in which an isomer formed as a by-product is isomerized under isomerization reaction conditions and is converted to an isomer to be recovered. The second problem may be solved by adoption of a recovery process utilizing an adsorptive separation technique or crystallizing separation technique. However, solutions of both the problems are still insufficient. Thus, 3,5-dichlorotoluene (dichlorotoluene is hereinafter referred to as "DCT" for brevity) is likely to be an intermediate for a novel agricultural chemical, but no industrial preparation process has been established because in the nuclear chlorination reaction of chlorotoluene the ortho- and para-orientations are strong and 3,5-DCT is hardly formed and because even if 3,5-DCT is formed by isomerization of other DCT isomers, the boiling point of 3,5-DCT is substantially equal to the other DCT isomers formed as by-products such as 2,4-DCT, 2,5-DCT and 2,6-DCT (having boiling points of about 200°C) and it is substantially impossible to separate 3,5-DCT having a high purity by rectification. U.S. Patent No. 4,254,062 proposes a process for separating a specific DCT isomer from a DCT isomer mixture by using a zeolite of the faujasite type as an adsorbent. However, the above mentioned U.S. patent does not refer to adsorptive separation of 3,5-DCT but discloses a separation technique to be applied to a system which does not substantially contain 3,5-DCT which is a nuclear chlorination product of toluene or chlorotoluene. Accordingly, it is not shown whether the adsorptivity of a faujasite type zeolite adsorbent to 3,5-DCT is higher or lower than that of the adsorbent to other DCT isomers or whether or not it is possible to substantially separate 3,5-DCT having a high purity from a DCT isomer mixture by using this adsorbent. In accordance with the present invention we provide a process for continuously separating 3,5-dichlorotoluene from a dichlorotoluene isomer mixture by adsorptive separation using a simulated moving bed, wherein a faujasite type zeolite adsorbent is used as the adsorbent and 3,5-dichlorotoluene is separated as the extract component in the presence of at least one compound selected from the group consisting of 1,2,4-trimethylbenzene and 2-chloro-p-xylene. In the simulated moving bed technique a plurality of adsorption chambers filled with an adsorbent is used. This adsorptive separation technique comprises as the basic operations an adsorption operation, a concentration operation, a desorption operation and a desorbent recovery operation, which are continuously repeated. These basic operations are as follows. (1) Adsorption Operation A starting mixture is brought into contact with a bed of an adsorbent and a highly adsorbable component is selectively adsorbed. The remaining weakly adsorbable component is withdrawn from the adsorbent bed together with a desorbent in the form of a raffinate flow. (2) Concentration Operation The adsorbent having the strongly adsorbable component selectively adsorbed therein is brought into contact with a part of an extract described hereinafter to expel the weakly adsorbable component left on the adsorbent and increase the purity of the strongly adsorbable component. (3) Desorption Operation The adsorbent bed containing the strongly adsorbable component having an increased purity is brought into contact with the desorbent and the highly adsorbable component having the increased purity is withdrawn from the adsorbent bed together with the desorbent in the form of an extract flow. (4) Recovery Operation The adsorbent bed containing the desorbent is brought into contact with a part of the raffinate flow and the desorbent is withdrawn from the adsorbent bed. A typical example of a simulated moving bed operation is described in Example 1(A) of the parent application EP-A-0125177 above referred to. The faujasite type zeolite used in the process of the present invention is a crystalline aluminosilicate represented by the following formula: 0.9±0.2M 2/n O:Al₂O₃:xSiO₂:yH₂O wherein M stands for a cation and n indicates the valency of the cation M. The faujasite type zeolite represented by the above formula is classified into a type X in which x in the above formula is in the range of 2.5 ± 0.5 and a type Y in which x in the above formula is from 3 to 6. The value of y in the above formula differs according to the degree of hydration. The faujasite type zeolite may be subjected to an aluminum-removing treatment. In the process for separating 3,5-DCT as the extract component from a DCT isomer mixture, it is preferred that the cation M be at least one member including sodium ion, and a most preferred cation comprises at least sodium and barium ions. The ion exchange process for the exchange of the cation M is well known to experts in the art of the preparation of crystalline aluminosilicates. Ordinarily, the ion exchange is accomplished by bringing a zeolite into contact with an aqueous solution containing a soluble salt of at least one cation to be introduced into the zeolite. The most important feature of the present invention resides in that a specific isomer is separated as the extract component in the presence of a substituted benzene compound. In the process for separating 3,5-DCT as the extract component from a DCT isomer mixture, at least one compound selected from 1,2,4-trimethylbenzene and 2-chloro-p-xylene is used as the substituted benzene compound. The substituted benzene compound used in the present invention may be added to the DCT isomer mixture or to the desorbent used in the simulated moving bed operation. It is preferable that the substituted benzene compound be used as the desorbent per se. This is because, in a simulated moving bed system as mentioned hereinbefore, an adsorbed component on an adsorbent is substituted by a desorbent in the desorption operation, and thus, a starting isomer mixture is brought into contact with the adsorbent having adsorbed thereon substantially only the desorbent. In other words, the absorption operation in a simulated moving bed system is always carried out in the presence of a desorbent, and therefore, when the desorbent is the substituted benzene compound itself, the adsorptive separation of the present invention can be advantageously effected. The concentration of the substituted benzene compound in the adsorptive separation which, in the case of separation of DCT isomers, is expressed by the following formula: is at least 5% by weight, preferably at least 15%. When the substituted benzene compound is used as the desorbent, it is preferred that the feed ratio between the starting mixture and the desorbent be adjusted so that local reduction of the concentration of the substituted benzene compound in the separation system below 15% by weight is prevented. In connection with the adsorptive separation conditions, the temperature is in the range of from room temperature to 350°C, preferably 50 to 250°C, and the pressure is in the range of from atmospheric pressure to 50 kg/cm².G (49 bar G), preferably from atmospheric pressure to 40 kg/cm².G (39 bar G). The isomer to be separated may be in the gas phase or in the liquid phase. However, it is preferred that the operation temperature be lowered and the operation be carried out in the liquid phase so as to control occurrence of undesirable side reactions of the starting mixture, substituted benzene compound and desorbent. The present invention is applied to a process in which 3,5-DCT is separated as the extract component from a DCT isomer mixture containing at least 3,5-DCT. Experiments of adsorption characteristics where isomers were separated from isomer mixtures in the presence of a zeolite adsorbent and a substituted benzene compound will now be described. In the Examples, the adsorption characteristic of the adsorbent is expressed by the adsorption selectivity α represented by the equation given below. If an isomer W is to be separated as the extract component from another isomer X of the compound W, the adsorption selectivity α W/X is represented by the following equation: wherein S stands for the adsorption phase and L stands for the phase equilibriated with the adsorption phase. If all the values of α W/X , taken for all the isomers X from which W is to be separated, are larger than 1, then W can be separated as the extract component, that is, the highly absorbale component. However, any substance X to which the α value is close to 1 can hardly be separated from W. It flows into the extract component together with W and W having a high purity cannot be obtained. Comparative Example 1 A powder of a zeolite of the Na-X type (SiO₂/Al₂O₃ molar ratio = 2.5) was mixed with alumina sol as a binder in an amount of 15% by weight as Al₂O₃ , and the mixture was kneaded and formed into granules having a size of 24 mesh to 32 mesh by extrusion molding. The granules were dried at 100°C and calcined at 500°C for 1 hour to obtain an adsorbent of the Na-X type. The adsorbent was subjected to the ion exchange treatment with an aqueous solution of a nitrate at a solid/liquid ratio of 5 at a temperature of about 90°C. After the ion exchange treatment, the adsorbent was dried and calcined under the same conditions as described above with respect to the Na-X type. In order to determine the adsorption selectivity of the adsorbent among DCT isomers, 2 g of the adsorbent and 3 g of a DCT isomer mixture were charged in an autoclave having an inner capacity of 5 ml, and the mixture was allowed to stand still at 130°C for 1 hour while the mixture was stirred now and then. The weight ratio of 2,3-DCT/2,4-DCT/2,5-DCT/2,6-DCT/3,4-DCT/3,5-DCT was 3.5/30.3/38.0/10.0/4.8/13.4. Furthermore, n-nonane was simultaneously charged as the reference substance for the gas-chromatography analysis in an amount of 20% by weight based on the DCT isomer mixture. Under the above adsorption conditions, n-nonane was substantially inert to the adsorptivity of the zeolite. After contact with the adsorbent, the composition of the liquid phase mixture was analyzed by gas chromatography, and the adsorption selectivities α of DCT isomers were determined. The obtained results are shown in Table 1. In Table 1, the adsorbent for which a single cation component is indicated is an adsorbent in which said single cation occupies at least 90 equivalent % of the cations contained in the adsorbent. For example, the adsorbent 0.2Ag-Na-X indicates an adsorbent formed by subjecting the adsorbent of the Na-X type to the ion exchange treatment with an aqueous solution of silver nitrate containing Ag in an amount corresponding to 20 equivalent % of the Na cation. The obtained results are shown in Table 1. Comparative Example 2 Adsorbents were prepared by using a powder of a zeolite of the Na-Y type (SiO₂/Al₂O₃ molar ratio = 5.2) in the same manner as described in Comparative Example 1. The adsorption selectivities α of the adsorbents of the Y type among DCT isomers were measured. The obtained results are shown in Table 2. Example 1 By using adsorbents prepared according to the method described in Comparative Example 1 or 2, the adsorption selectivities among DCT isomers were measured in the presence of a substituted benzene compound. The DCT isomer mixture used and experimental conditions adopted were the same as those described in Comparative Example 1 with the exception that one or other of the substituted benzene compounds shown in Table 3 were added to the mixture charged to the autoclave. The concentration of the substituted benzene compound was 50% by weight, the concentration being calculated according to the equation: The obtained results are shown in Table 3. In Table 3, the adsorption selectivity is shown by the α 3,5/X value (X indicates DCT other than 3,5-DCT). From the results shown in Table 3, it is seen that in the presence of 1,2,4-trimethylbenzene or 2-chloro-p-xylene, all of the α 3,5/X values are larger than 1.0 and 3,5-DCT has a strongest adsorbability among DCT isomers. That is, 3,5-DCT can be separated as the extract component. On the other hand, as shown in Tables 1 and 2, in the absence of a substituted benzene compound, the adsorbability of 3,5-DCT is weakest (the selective adsorbability is expressed by α X/3,5 ).

Claims: 1. A process for continuously separating 3,5-dichlorotoluene from a dichlorotoluene isomer mixture by adsorptive separation using a simulated moving bed, wherein a faujasite type zeolite adsorbent is used as the adsorbent and 3,5-dichlorotoluene is separated as the extract component in the presence of at least one compound selected from the group consisting of 1,2,4-trimethylbenzene and 2-chloro-p-xylene. 2. A separation process according to claim 1, wherein the faujasite type zeolite contains at least a sodium cation. 3. A separation process according to claim 1, wherein the faujasite type zeolite contains at least sodium and barium cations.

Inventors: INOUE, TAKEHISA, MIWA, KISHIO, NAGAOKA, YUKIKO, TADA, KUNIYUKI
Applicants: TORAY INDUSTRIES, INC.
Application year: 1984
Main classification: C07C25
IPC codes: C07C17, C07C25, C07C67