Use of an adsorbent for separating halogenated aromatic compounds and method of separation using it

Abstract: An adsorbent for separating halogenated aromatic compounds has at least one of a porosity of from 0.20 cc/cc to 0.37 cc/cc, a packing density of from 0.36 g/ml to 0.70 g/ml, and a grain size of from 0.1 mm to 1.0 mm. With the adsorbent, a halogenated aromatic compound isomer is separated and recovered; or a halogenated aromatic compound having a dissolved oxygen content of at most 15 ppm by weight is separated and recovered with an adsorbent. In the method, at least one high-purity isomer can be efficiently separated and recovered from a mixture of halogenated aromatic compound isomers.

Description: The present invention relates to the use of an adsorbent for separating halogenated aromatic hydrocarbons and to a method of using it for separating one isomer from a mixture of halogenated aromatic isomers. Halogenated aromatic compounds are of considerable industrial importance, for example, as intermediates for medicines and agricultural chemicals. At present, these intermediates are required to have a much higher purity. Except for monohalogenated benzenes, aromatic compounds with two or more substituents each have different isomers. For separating a mixture of those isomers into individual ones through ordinary industrial distillation, an ultra-precision distillation device is needed, since the difference in the boiling point between the isomers is small. However, at present, it is still extremely difficult to obtain high-purity products of a single isomer on an industrial scale. In addition, some types of halogenated aromatic compound could not be separated into individual isomers through distillation. Recently, for separating a mixture of isomers that have heretofore been difficult or impossible to separate into individual ones, methods of adsorbing separation have been developed. For example, JP-A 57-91933 and 58-131923 disclose a method of separating a single isomer from a mixture of chlorotoluene isomers through adsorption. Japanese Patent Application No. 9-335225 discloses a method of separating a single isomer from a mixture of chloroethylbenzene isomers through adsorption. JP-A 52-62229, 53-105434, 58-131924, 58-150524, 52-175433 and 4-330025 disclose a method of separating a single isomer from a mixture of dichlorobenzene isomers. JP-A 59-199542, 60-42340, 5-70383, 58-137795 and 3-20232 disclose a method of separating a mixture of dichlorotoluene isomers. JP-8 4-46933 discloses a process comprising removing HCl and water, which are formed in isomerization of a halogenated benzene derivative, through distillation, followed by separating a specific single isomer from the mixture of the resulting isomers by the use, as an adsorbing and separating agent of a zeolite. US-A-4 254 062 discloses a process for the separation of isomeric dichlorotoluenes using X or Y zeolites. Increasing the purity of the single isomer to be separated in those adsorbing separation methods and increasing the separation efficiency therein are extremely important criteria relevant to industrial applicability. However, the methods mentioned above are not satisfactory with respect to the purity of the single isomer separated therein. In the method of using an adsorbing and separating agent that comprises a zeolite for separating a single isomer from a mixture of halogenated aromatic compound isomers, the capabilities of the adsorbing and separating agent being used are lowered with the lapse of time. In the method, therefore, the adsorbing and separating agent used must be exchanged for a fresh one or must be regenerated through firing or the like. Therefore, prolonging the life of the agent and prolonging the regeneration cycle for the agent is extremely advantageous for improving the industrial applicability. However, the conventional techniques are unsatisfactory with respect to the prolongation of the life of the agent and to the prolongation of the regeneration cycle for it. The invention addresses the problems noted above, and seeks to provide an efficient method for separating halogenated aromatic compound isomers. In order to solve these problems, we, the present inventors have assiduously studied the capabilities of adsorbents for improving them. Preferably the adsorbents consist essentially of a zeolite. Zeolites includes natural zeolites and synthetic zeolites, of which a synthetic zeolite is especially preferably used. Such a synthetic zeolite is generally in the form of powder. For using a zeolite as an industrial adsorbent, it is usually shaped. Specifically, zeolite adsorbents are generally in the form of shaped articles. Having observed the role of shaped articles of zeolite as adsorbents, we investigated the factors that may govern the capabilities of an adsorbent such as a zeolite to adsorb and separate halogenated aromatic compounds. As a result, we have found that the packing density of the adsorbent, the porosity of the adsorbent and the grain size of the adsorbent are important factors. The packing density of an adsorbent depends, for example, on the porosity thereof and the shape thereof. An adsorbent having a higher packing density is preferred, since the amount thereof capable of being filled in a unit volume increases and since a larger amount of the adsorbent could be packed in an adsorbent tower. However, for an adsorbent having too high a packing density, it has been found that its adsorbing and separating capabilities are lowered. The porosity of an adsorbent is caused by the macro pores existing in the adsorbent. An adsorbent having too high a porosity is unfavorable, since its packing density is lowered. Contrary to this, for an adsorbent having too low a porosity, the volume of macro pores existing therein is too small. As a result, it has been found that, when a halogenated aromatic compound is applied to an adsorbent of zeolite of that type, it is prevented from being diffused in the adsorbent grains before it reaches the pores through which the compound is adsorbed by the adsorbent, zeolite, and therefore the adsorbing and separating capabilities of the adsorbent are poor. On the other hand, it has been found that a shaped adsorbent comprising smaller grains cculd have higher adsorbing and separating capabilities. It is believed that, in zeolite adsorbent grains having a small grain size, the pathway through which a halogenated aromatic compound to be adsorbed by the grains are diffused into the pores of the zeolite grains will be shortened, whereby the compound could be rapidly adsorbed by the grains. It is considered that the diffusion of halogenated aromatic compounds into adsorbent grains will be much influenced by the grain size of the grains, as compared with that of halogen-free aromatic hydrocarbons. This will be probably related to the fact that halogens, for example, chlorine and bromine have a large ionic radius or a large atomic weight. However, adsorbent grains having too small a grain size are unfavorable, since they will cause an increase in pressure loss. We, the present inventors have studied production methods for aromatic compounds, and, as a result, have found that using a halogenated aromatic compound having a dissolved oxygen content of at most 15 ppm by weight prevents the deterioration of an adsorbing and separating agent and prolongs the life of the agent, and that the halogenated aromatic compound can be efficiently separated. Specifically, in a first aspect the present invention provides the use of a zeolite adsorbent for separating a halogenated aromatic compound, which adsorbent is characterized by having a packing density of from 0.50 g/ml to 0.70 g/ml inclusive, a porosity of from 0.2 to 0.37 ml/ml inclusive, and a grain size of from 0.1 mm to 1.0 mm inclusive. In a second aspect the present invention provides a method of using the zeolite adsorbent for separating at least one halogenated aromatic compound isomer from a mixture of halogenated aromatic isomers. Such a method may comprise contacting a mixture of halogenated aromatic compound isomers having a dissolved oxygen content of at most 15 ppm by weight with the zeolite adsorbent for separating halogenated aromatic compounds. Preferred embodiments will now be described in more detail. It is especially preferred that the adsorbent used in accordance with the invention is a synthetic zeolite. Examples of the zeolite usable herein include faujasite-type zeolite, MFI-type zeolite and beta-type zeolite. Most preferred is faujasite-type zeolite. Faujasite-type zeolite is a crystalline aluminosilicate to be represented by the following general formula in terms of the molar ratio of the oxides constituting it: M 2/n O•AL 2 O•xSiO 2 •yH 2 O wherein M represents a metal ion or a hydrogen ion; n represents the valence of the metal ion or the hydrogen ion; x represents a molar ratio of silica/alumina. One in which x is smaller than 3 is referred to as X-type zeolite, and one in which x is 3 or more is referred to as Y-type zeolite. In the formula, y varies depending on the degree of hydration. Synthetic zeolite is generally obtained in the form of powder. For shaping the powder, for example, any method of kneading, rolling or compression my be employed. In accordance with the invention, a kneading method is preferred. In shaping the powder, a binder is often used for increasing the mechanical strength and the packing density of the shaped articles. As the binder, alumina, bentonite and kaolin, for example, are preferred. Depending on the intended mechanical strength and packing density of the shaped articles, the amount of the binder to be used may be from 5 to 40% by weight, but preferably from 10 to 20% by weight, inclusive. Having regard to the zeolite content of the shaped articles, it is desirable that the amount of the binder is smaller. However, the uppermost limit and the lowermost limit of the amount of the binder to be used shall be determined, depending on the type of binder and the intended mechanical strength of the shaped articles. The amount of the binder also depends on the crystal structure and the crystal morphology of the zeolite. For shaping the zeolite by a kneading method, zeolite powder, binder, water and optionally a surfactant that is to improve the shapability are well kneaded. The surfactant to be optionally used herein includes, for example, an anionic surfactant such as triethanolamine oleate or sodium oleate, a nonionic surfactant such as oleic acid, sorbitan trioleate as a trade name "Span 85", sorbitan monooleate as a trade name "Span 80", sorbitan monolaurate as a trade name "Span 20", polyoxyethylene sorbitan monostearate as a trade name "Tween 60", polyoxyethylene sorbitan monooleate as a trade name "Tween 80" or polyoxyethylene sorbitan monolaurate as a trade name "Tween 20" or a cationic surfactant such as N-cetyl-N-ethyl morpholinium Et sulfate as a trade name "Atlas G-251". The kneaded mixture is extruded through an extruder, for which the screen mesh is suitably selected to control the grain size of the intended adsorbent grains. The extruded adsorbent grains are then processed in a rounding machine (Marumerizer), in which their length is controlled or they are rounded. Through the rounding operation, the packing density of the adsorbent grains and the porosity can be controlled to some degree. The shaped adsorbent is then dried for removing water therefrom. The drying temperature may generally fall between 50 and 200° C. After having been dried, this is then calcined, thereby to provide an increased mechanical strength. The calcination temperature generally falls between 350 and 700° C. The adsorbent thus having been calcined to have an increased mechanical strength is thereafter optionally subjected to cation exchange treatment, depending on the type of the halogenated aromatic compound to be treated with it. For the ion exchange treatment, any and every cation generally employed in the art is employable. The packing density of the adsorbent may vary to some degree, depending on the measuring method used. The packing density of the adsorbent of the invention is measured according to the method mentioned below. (1) The empty weight of a measuring container is measured, and this is referred to as (A). The container is prepared by cutting a 250-ml messcylinder having an outer diameter of 40 mm, at a height of 200 mmH from its bottom, and its volume, V, is previously determined with water. (2) The measuring equipment is-as shown in Fig. 1. (see later). (3) About 300 ml of a calcined adsorbent (sample) is taken in a glass beaker. (4) The distance between the top of the measuring container and the top of the funnel is adjustedto be 200 mm, and the sample in the glass beaker is fed into the funnel, while being dropped into the container through the funnel to build up into a heap. (Note: The sample is dropped into the center of the container, messcylinder, and the measuring equipment must not be moved during the measurement.) (5) After the sample has been dropped into the container, its heaped part is gently scraped off with a scale to be flat. (6) The weight of the container with the sample therein is measured, and this is referred to as (B). (Note: As the sample is highly hygroscopic, the operation of (1) to (6) must be effected rapidly.) (7) The measurement is repeated twice, and the average value is obtained. (8) A 30-ml ceramic crucible that has been previously calcined at 500° C for 1 hour is weighed. About 2 g of the same sample as above is accurately weighed and put into the crucible, which is then calcined at 500° C for 1 hour. This is cooled in a desiccator for about 15 minutes. After having reached room temperature, this is weighed. From the thus-weighed dry weight of the sample, the water content (% by weight) of the non-dried sample is calculated. (9) The packing density (ABD) of the sample is calculated according to the following equation: ABD (g/ml) = B - A V x (1- 100 - water content 100 ) wherein; A indicates the weight of the empty container (g), B indicates the weight of the sample (adsorbent) + A (g), and V indicates the volume of the measuring container (ml). An adsorbent having a higher packing density is preferred, since the amount capable of being filled in a unit volume increases. However, an adsorbent having too high a packing density is unfavorable, since its adsorbing and separating capabilities for halogenated aromatic compounds are poor. The porosity of an adsorbent can be measured with ease, according to a method of mercury penetration, for which is used a porosimeter. Specifically, the porosity of the adsorbent of the invention is measured in the manner mentioned below. The adsorbent to be measured is placed in an electric furnace, calcined therein at 500°C for 2 hours, and then cooled in a desiccator to reach room temperature. This is placed in a sample cell, and is thereafter filled with mercury in vacuum. Then, this is set in a porosimeter device. Pressure is applied thereto, whereupon the volume of mercury to be penetrated into the pores of the adsorbent grains is electrically detected. Thus is obtained the relationship between the pressure Pm[psi] and the pore volume [cc]. This measurement is repeated under different pressures. The pressure Pm is converted into the pore radius, r, according to the following equation (1): r = -2 γcos/P wherein; r indicates the radius of pores into which mercury penetrates under a pressure P, [Å], P indicates a pressure, P=Pm[psi]×6.89476×10 3 [N/m·m], γ indicates the surface tension of mercury, 0.484 [N/m], and  indicates the contact angle of mercury to the sample (adsorbent), 141.3[° ]. The volume of the adsorbent grains is obtained as follows: The adsorbent grains to be measured are placed in a sample cell. The volume as expelled by the adsorbent grains to which is added mercury in vacuum, and the pore volume of the grains having a pore diameter of up to 3 µm to which is added mercury under pressure are summed up, and the total is the volume of the adsorbent grains. The pore volume is the accumulated volume of mercury as penetrated into the grains having a pore diameter of from 3 µm to 10 nm. Accordingly, the porosity (cc/cc) of the adsorbent grains is defined as follows: Porosity (cc/cc) = (pore volume (cc)) (adsorbent grain volume (cc)) Adsorbent grains having a larger porosity are preferred, since a halogenated aromatic compound can more easily diffuse inside the grains. However, too large grains are unfavorable, since the packing density of the adsorbent is lowered. The porosity of the adsorbent grains is from 0.20 to 0.37 cc/cc, more preferably from 0.25 to 0.35 cc/cc, inclusive. The grain size of the adsorbent grains can be easily measured with a scale or a scale equipped with a magnifying glass. At least any 10 adsorbent grains are measured for their grain size, and the data are averaged. The resulting mean value is the grain size of the grains. In accordance with the invention, the grain size of the adsorbent grains is from 0.1 to 1.0 mm, more preferably between 0.2 and 0.8 mm. Adsorbent grains having a smaller grain size are preferred, since the pathway in each grain through which the adsorbed molecules diffuse in the grains is shorter. However, too small grains are unfavorable, since they cause the increase in pressure loss, and since their industrial production is not economical. The halogen present the halogenated aromatic compounds to which the invention is directed includes fluorine, chlorine, bromine and iodide. Of those, favorable to the invention are chlorine and bromine, and more favorable thereto is chloride. It is also desirable that the halogenated aromatic compounds have at least one alkyl group. The alkyl group preferably has from 1 to 4 carbon atoms, and more preferred are methyl group, ethyl group and propyl group. Specific examples of the halogenated aromatic compounds include fluorotoluene, chlorotoluene, bromotoluene, dichlorobenzene, dibromobenzene, fluorochlorobenzene, chlorobromobenzene, trichlorobenzene, dichlorofluorobenzene, dichlorobromobenzene, dibromochlorobenzene, dichlorotoluene, dibromotoluene, chloroxylene, bromoxylene, chloroethylbenzene, bromoethylbenzene, chlorocumene, chloropropylbenzene, bromocumene, bromopropylbenzene, chlorobutylbenzene and bromobutylbenzene. In accordance with the invention, when the dissolved oxygen content of the halogenated aromatic compound to be processed is not higher than 15 ppm by weight, the adsorbent used for the compound may be prevented from being deteriorated. Smaller dissolved oxygen content is better. Preferably, the dissolved oxygen content is at most 5 ppm by weight, more preferably at most 1 ppm by weight. The dissolved oxygen content of the aromatic compound can be measured, for example, with a Beckman dissolved oxygen meter or a polarographic dissolved oxygen meter. To reduce the dissolved oxygen content of the aromatic compound to be at most 15 ppm by weight, for exemple, any treatment such as radiation, degassing, distillation or nitrogen sealing is employable. In accordance with the invention, in the radiation treatment a dissolved oxygen-containing aromatic compound in liquid form is contacted with an inert gas such as N 2 , thereby removing the dissolved oxygen from the aromatic compound. For the radiation, for example, N 2 , may be directly bubbled into an aromatic compound in liquid form present in a tank, or, alternatively, N 2 may be contacted with an aromatic compound in liquid form in a countercurrent flow in a plate column or a packed column. The pressure for the operation may be any of normal pressure, elevated pressure or reduced pressure. Regarding the temperature for the operation, the operation is generally effected at a temperature at which the aromatic compound to be processed could be in a liquid phase. The degassing treatment includes, for example, a method of reducing the pressure in a tank as filled with a dissolved oxygen-containing aromatic compound, by the use of, for example, a vacuum pump; and a method of reducing the pressure in a plate column or a packed column as filled with a liquid of a dissolved oxygen-containing aromatic compound to thereby remove the dissolved oxygen from the compound. The pressure for the operation may be any of vacuum to normal pressure, but a lower pressure is preferred. Regarding the temperature for the operation, the operation is generally effected at a temperature at which the aromatic compound to be processed can be in a liquid phase. The distillation treatment may be effected in any ordinary manner. For this, the distillation tower my be any of, for example, a plate column or a packed column, and the pressure may be any of normal pressure, elevated pressure or reduced pressure. In the nitrogen sealing treatment a dissolved oxygen-containing aromatic compound is placed in a nitrogen-sealed container, for example a tank, before the compound is subjected to isomerization or adsorbing separation, so that the dissolved oxygen is removed from the compound through vapor-liquid equilibration. If a halogenated aromatic compound to be subjected to adsorbing separation contains dissolved oxygen, the dissolved oxygen in the compound will form an oxygen-containing compound. Since the resulting oxygen-containing compound has strong polarity, and is therefore strongly adsorbed by the adsorbing and separating agent, the adsorbing and separating capabilities of the agent are much lowered. Therefore, reducing the dissolved oxygen content of the halogenated aromatic compound is effective for preventing the deterioration of the adsorbing and separating agent, and it is believed that the regeneration cycle for the agent can thereby be prolonged. Especially preferred embodiments of the invention will now be described with reference to the accompanying drawings and Examples. In the drawings: Fig. 1 is a schematic view showing a device for measuring the packing density of an adsorbent. Fig. 2 is a schematic view showing the adsorption and separation process of one embodiment of the invention in a simulated-moving bed line. Fig. 3 is a graph showing the pore volume distribution of the adsorbents in Example 1 and Example 2. Fig. 4 is a graph showing the adsorbing and separating capabilities of the adsorbents for a CEB isomer mixture in Example 1 and Comparative Example 1. Fig. 5 is a graph showing the adsorbing and separating capabilities of the adsorbents for a DCB isomer mixture in Examples 2 and 3 and Comparative Example 2. Fig. 6 is a graph showing the relationship between the amount of the isomer mixture processed and the adsorptive selectivity of the single isomer separated in a Reference Example. The technique of adsorbing and separating a single isomer from a mixture of halogenated aromatic compound isomers according to the invention may be attained in so-called paritioning chromatography, in partitioning adsorption using a simulated-moving bed line comprising a series of partitioning chromatography columns. In the simulated-moving bed line, the most easily adsorbable substance is collected in the extract flow, while the substance most difficult to adsorb is collected in the raffinate flow. The continuous adsorbing and separating technique using the simulated-moving bed line basically comprises adsorption, concentration, desorption and desorbent recovery, which are continuously repeated in that order. One example of a method of adsorbing and separating a mixture of chloroethylbenzene (hereinafter referred to as "CEB") isomers is mentioned below, in which m-CEB is adsorbed by the adsorbent used. (1) Adsorption: A mixture of CEB isomers is contacted with an adsorbent, and a strongly adsorbable component, m-CEB is selectively adsorbed by the adsorbent, while the other weakly adsorbable components, o- and p-CEBs are recovered along with the desorbent in the form of a raffinate flow that will be mentioned below. (2) Concentration: The adsorbent that has adsorbed the strongly adsorbable component, m-CEB is then contacted with a part of the extract that will be mentioned below, and the weakly adsorbable components remaining on the adsorbent are removed, whereby the strongly adsorbable component is concentrated. (3) Desorption: The adsorbent thus having the concentrated, strongly adsorbable component, m-CEB is contacted with a desorbent, whereby the strongly adsorbable component is removed from the adsorbent, and recovered along with the desorbent in the form of an extract flow. (4) Desorbent Recovery: The adsorbent thus having adsorbed substantially only the desorbent is contacted with a part of the raffinate flow, and a part of the desorbent having been adsorbed by the adsorbent is recovered in the form of a desorbent recovery flow. Fig. 2 shows a schematic view showing the adsorption and separation process according to the simulated-moving bed line mentioned above. As in Fig. 2, adsorption chambers 1 to 12 all filled with an adsorbent are continuously connected with each other, through which the isomer mixture to be separated is circulated. The adsorbing and separating operation for a halogenated aromatic compound is preferably effected in a liquid phase. A higher operation temperature is more preferred for higher diffusion rate of the compound being processed. Contrary to this, however, a lower operation temperature is more preferred for better adsorptive selectivity of the adsorbent used. Because of the contradictory temperature-dependent factors the operation temperature will be preferably between 50 and 200°C. EXAMPLES Example 1: 1.2 kg (dry weight) of Na-X zeolite powder (from Toso) having a molar ratio SiO 2 /Al 2 O 3 of 2.5, 0.96 kg of alumina sol (from Nissan Chemical, having an Al 2 O 3 content of 10 % by weight) and alumina gel (from Shokubai Kasei, having an Al 2 O 3 content of 75 % by weight) were mixed in a kneader (from Fuji Pawdal, Model KDHJ-10), and kneaded with a suitable amount of water added thereto, for about 1 hour. The resulting paste was extruded out through an extruder (from Fuji Pawdal, Model EXDS-60) equipped with a 0.3 mm-mesh screen, into pellets. In order to round them and to increase their packing density, the resulting pellets were processed in a rounding machine (Marumerizer, Model QJ-230 from Fuji Pawdal), at a revolution of 850 rpm for 3 minutes Next , these were dried overnight at about 120° C. After having been dried, these were classified through a 80-mesh sieve to remove fine grains and through a 48-mesh sieve to remove coarse grains. The thus-classified grains were then calcined at 500°C for 2 hours. The diameter of the adsorbent grains was measured with a scale equipped with a magnifying glass, and it was 0.3 mm. The length of the grains varied from about 0.3 mm to a about 0.6 mm. The packing density of the adsorbent was 0.54 g/ml. The pore volume of the adsorbent grains was measured, and the data are shown in Fig. 3. The adsorbent had a porosity of 0.27 cc/cc. The adsorbent was filled into 12 columns each having a size of 4.6 mm x 1 m. Of those columns, four were set in a desorption zone, three were in a concentration zone, three were in an adsorption zone, and two were in a desorbent recovery zone in a simulated-moving bed line. A mixture of chloroethylbenzene (CBB) isomers was applied to the simulated-moving bed line, and the adsorbing and separating capabilities of the adsorbent in the line were checked. The starting CEB isomer mixture applied had a composition of o-CEB/m-CEB/p-CEB = 33/47/20. As the desorbent, xylene was used. The operation temperature was 130° C. The capabilities curve is shown in Fig. 4. Comparative Example 1: An adsorbent was prepared in the same manner as in Example 1, except that the extruded grains were not rounded with a Marumerizer. The packing density of the adsorbent prepared herein was 0.49 g/ml. The pore volume of the adsorbent was measured, and the data are shown in Fig. 3. The adsorbent had a porosity of 0.38 cc/cc. Using the adsorbent, the CEB isomer mixture was processed in the same manner as in Example 1 to check the adsorbing and separating capabilities of the adsorbent. The data obtained are shown in Fig. 4. The data in Fig. 4 indicate that the most strongly adsorbable m-CEB was recovered in the extract flow and its purity in the flow was high. In addition, the data indicate that the adsorbent giving a higher ratio of m-CEB/ΣCEB (total of o-, m- and p-CEBs) in the extract flow and giving a lower ratio of m-CEB/ΣCEB in the raffinate flow has better adsorbing and separating capabilities. Obviously, it is known that the adsorbent having an increased packing density of 0.54 mg/ml from 0.49 mg/ml and having a decreased porosity of 0.27 cc/cc from 0.38 cc/cc has better adsorbing and separating capabilities. Example 2: An adsorbent paste was prepared in the same manner as in Example 1, except that Na-Y zeolite powder (from Toso) having a molar ratio SiO 2 /Al 2 O 3 of 5.5 was used herein, and this was extruded out through an extruder equipped with a 0.3 mm-mesh screen into adsorbent pellets. In order to round them and to increase their packing density, the resulting pellets were processed in a rounding machine, Marumerizer, at a revolution of 850 rpm for 3 minutes. Next, these were dried overnight at about 120°C. After having been dried, these were classified through a 80-mesh sieve to remove fine grains and through a 48-mesh sieve to remove coarse grains. The thus-classified grains were than calcined at 500° C for 2 hours. The calcined grains were treated in an aqueous solution containing 5 % by weight of KNO 3 , at a liquid/solid ratio (L/S) of 4 ml/g at about 80° C for 1 hour. Next, these were washed with distilled water at a liquid/solid ratio of 4. The K ion exchanging treatment followed by washing with water was repeated 8 times. After this treatment, the grains were washed with distilled water at a liquid/solid ratio of 8, which was repeated 5 times. Next, these grains were further subjected to ion exchanging treatment with an aqueous solution of 55 g of PbNO 3 dissolved in one liter of water, at room temperature for about I hour, and then washed with distilled water at a liquid/solid ratio of 4. The washing was repeated 5 times. The thus-prepared adsorbent was dried overnight at about 120°C, and then calcined at 500° C for 2 hours. The diameter of the adsorbent grains was measured with a scale equipped with a magnifying glass, and it was 0.3 mm. The length of the grains varied from about 0.3 mm to about 0.6 mm. The packing density of the adsorbent was 0.65 g/ml. The pore diameter of the adsorbent grains was measured, and the data are shown in Fig. 3, which indicates the pore diameter distribution. The adsorbent had a porosity of 0.26 cc/cc. The adsorbent was filled into 12 columns each having a size of 4.6 mm x 1 m. Of those columns, four were set in a desorption zone, three were in a concentration zone, three were in an adsorption zone, and two were in a desorbent recovery zone in a simulated-moving bed line. A mixture of dichlorobenzene (hereinafter referred to as "DCB") isomers was applied to the simulated-moving bed line, and the adsorbing and separating capabilities of the adsorbent in the line were checked. The starting DC3 isomer mixture applied had a composition of o-DCB/m-DCB/p-DCB = 37/40/23. As the desorbent, dichlorotoluene was used. The operation temperature was 130°C. The capabilities curve is shown in Fig. 5. Example 3: An adsorbent was prepared in the same manner as in Example 2, except that the rounding treatment with a Marumerizer was effected at a revolution of 400 rpm for I minutes. The packing density of the adsorbent was 0.60 g/ml. The pore volume of the adsorbent grains was measured, and the data are shown in Fig. 3. The adsorbent had a porosity of 0,35 cc/cc. The DCB adsorbing and separating capabilities of the adsorbent were checked in the same manner as in Example 2, and the data are shown in Fig. 5. Comparative Example 2 An adsorbent was prepared in the same manner as in Example 2, except that the extruder was equipped with a 1.2 mm-mesh screen and that a 24-mesh sieve and a 10-mesh sieve were used for the grain classification. The diameter of the adsorbent grains was measured with a scale equipped with a magnifying glass, and it was 1.2 mm. The length of the grains was 2.0 mm. The packing density of the adsorbent was 0.67 g/ml. The adsorbent had a porosity of 0.25 cc/cc. The DCB adsorbing and separating capabilities of the adsorbent were checked in the same manner as in Example 2, and the data are shown in Fig. 5. Fig. 5 indicates that the most weakly adsorbable m-DCB was recovered in the raffinate flow and its purity in the flow was high. In addition, the data in Fig. 5 indicate that the adsorbent giving a higher ratio of m-DCB/ΣDCB (total of o-, m- and p-DCBs) in the raffinate flow and giving a lower ratio of m-DCB/ΣDCB in the extract flow has better adsorbing and separating capabilities. Obviously, it is known that the adsorbents in illustrative Examples 2 and 3 have better adsorbing and separating capabilities. Reference Example, Part I. An adsorbent for separating 2,4-dichlorotoluene (hereinafter referred to as DCT) was prepared according to the method of Example 3 in JP-A 5-70383. This adsorbing and separating agent is or X-type zeolite containing, as cations, sodium and strontium ions. This was calcined at 500° C for 2 hours, and filled in a stainless steel column having an inner diameter of 4.6 mm and a length of 1 m. While m-xylene having been de oxidated through nitrogen bubbling (this m-xylene had a dissolved oxygen content of nearly 0 ppm, as measured with a polarographic dissolved oxygen meter) was introduced into the column at a constant flow rate, a small amount of a mixture of 2,4-, 2,5- and 2,6-DCTs and n-nonane that had been de oxidated in the same manner of nitrogen bubbling (this mixture had a dissolved oxygen content of almost 0 ppm, as measured with a polarographic dissolved oxygen meter) was applied to the column for an instant, and the liquid flowing out through the outlet was sampled at predetermined intervals, and analyzed for its composition through gas chromatography. All the starting liquid used herein were previously dewatered through a molecular sieve. Time-dependently plotting the data of the analysis gives the concentration peak of each component. The selectivity of the adsorbing and separating agent could be represented by the ratio of the difference in the retention time (hereinafter referred to as "RT") between one isomer and n-nonane to that between another isomer and n-nonane, as in the equation mentioned below. In general, adsorbents having a larger value for the selectivity to be obtained in that manner have better adsorbing and separating capabilities. N-nonane is substantially inactive to the adsorbing characteristic of zeolite. Adsorptive Selectivity of Component A relative to Component B, a (A/B) = (RT for Component A) - (RT for n-nonane) (RT for Component B) - (RT for n-nonane) The data obtained in this experiment are for the blank. The conditions for the measurement are shown in Table 1. Item Conditions Amount of Adsorbing and Separating Agent about 10 g Flow Rate of m-xylene 120 cc/hr Pulse Sample n-C9 10 wt.% 2,4-DCT 18 wt.% 2,5-DCT 45 wt.% 2,6-DCT 27 wt.% Amount 10 µl Temperature 150° C Next, a mixture of m-xylene and dichlorotoluene that had been deoxidated through nitrogen bubbling and dewatered through a molecular sieve (this mixture had a dissolved oxygen content of almost 0 ppm, as measured with a polarographic dissolved oxygen meter) was applied to the column. The amount of the mixture applied to the column was 520 times by weight the adsorbing and separating agent filled in the column. The liquid flowing out through the outlet of the column was sampled, and the adsorptive selectivity of the agent was determined in the same manner as above. The liquid flow conditions in this experiment are shown in Table 2; and the flow rate-dependent adsorptive selectivity of the adsorbent tested herein is shown in Fig. 6. Item Conditions WHSV (hr -1 ) 64 Mixture Composition m-xylene 80 wt.% 2,4-DCT 4 wt.% 2,5-DCT 10 wt.% 2,6-DCT 6 wt.% Temperature (150° C Reference Examples, Part II The adsorptive selectivity for the blank was determined in the same manner as in part I. The same experiment as in part I was repeated except that a mixture of m-xylene and dichlorotoluene that had been dewatered through a molecular sieve but not deoxidated through nitrogen bubbling (this mixture had a dissolved oxygen content of nearly 40 ppm, as measured with a polarographic dissolved oxygen meter) was applied to the column, and the adsorption selectivity of the adsorbent was determined in the same manner as above. The flow rate-dependent adsorptive selectivity of the adsorbent tested herein is shown in Fig. 6.

Claims: Use, for separating halogenated aromatic compounds, of a zeolite adsorbent, characterized in that the zeolite adsorbent has the following characteristics: (i) a porosity of from 0.2 cc/cc to 0.37 cc/cc inclusive; (ii) a packing density of from 0.50 g/ml to 0.70 g/ml; and (iii) a grain size of from 0.1 mm to 1.0 mm inclusive. Use, according to claim 1, wherein the adsorbent has a porosity of from 0.25 cc/cc to 0.35 cc/cc inclusive. Use, according to claim 1 or 2, wherein the adsorbent has a grain size of from 0.2 to 0.8 inclusive. Use, according to any one of claims 1 to 3, wherein the adsorbent comprises a synthetic zeolite. Use, according to any one of claims 1 to 4, wherein the adsorbent comprises a faujasite-type zeolite. A method for separating halogenated aromatic compounds, in which an adsorbent as defined in any one of claims 1 to 5 is used to separate at least one halogenated aromatic compound isomer from a mixture of halogenated aromatic compound isomers. A method according to claim 6, which comprises contacting an adsorbent for halogenated aromatic compounds with a mixture of halogenated aromatic compound isomers having a dissolved oxygen content of at most 15 ppm by weight to thereby separate at least one halogenated aromatic compound isomer from the mixture. A method according to claim 7, wherein the halogenated aromatic compound isomer mixture is, prior to being contacted with the adsorbent, processed to remove the dissolved oxygen therefrom through any of radiation, degassing, distillation or nitrogen sealing. A method according to any one of claims 6 to 8, wherein the halogenated aromatic compound contains at least one chlorine atom. A method according to any one of claims 6 to 9, wherein the halogenated aromatic compound contains at least one alkyl group.

Inventors: IWAYAMA KAZUYOSHI, KATO MASASHI, OKINO HIROHITO, YAMAKAWA SHINOBU, IWAYAMA, KAZUYOSHI, KATO, MASASHI, OKINO, HIROHITO, YAMAKAWA, SHINOBU
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1999
Main classification: B01D53
IPC codes: B01D53, C07C17, B01J20