PROCESS FOR SEPARATING METHYL DERIVATIVES OF NAPHTHALENE ACCORDING TO CRYSTALLIZATION UNDER PRESSURE

Abstract: The invention intends to separating a methyl derivative of naphthalene (2,6-dimethyl naphthalene or 2-methyl naphthalene) from a mixture material in a high purity more than 98 weight %. The starting material is preliminarily treated at a raised temperature up to 220°C at the highest and with LHSV of 0.1 - 6 Hr⁻¹ in the presence of an acid catalyst usually used for olefin polymerization. The treated material is distillated to remove the polymerized impurities and to raise the naphthalene derivative content at least up to 50 weight %. And then the obtained material is subjected to the crystallization under a pressure of 500 - 2500 kgf/cm² at a temperature of 80 - 150°C (for 2,6-DMN) or 10 - 35°C (for 2-MN).

Description: The invention relates to a process for separating 2,6-dimethylnaphthalene in a high purity of more than 98 weight % from a mixture containing this methyl derivative of naphthalene and a process for separating 2-methylnaphthalene in a high purity of more than 98 weight % from a mixture containing this methyl derivative of naphthalene. The dimethylnaphthalene and the methylnaphthalene are abridgedly called hereafter respectively DMN and MN. 2,6-DMN is oxidized and 2-MN is acylated and then oxidized to be naphthalene-2,6-dicarboxylic acid, which is an industrially important material for manufacturing polyester and as a component for a plasticizer. 2,6-DMN and 2-MN are contained in various fractions of petroleum and coal tar as mixtures together with other DMN and MN isomers. As for separation of 2,6-DMN and 2-MN from such fractions, various processes have been proposed. For instance, it is well known to those skilled in the art to cool the DMN fraction obtained by concentrating and extracting of petroleum or coal tar material so as to obtain a solid product containing 2,6- and 2,7-DMN, which is then subjected to recrystallization or partial melting in order to separate 2,6-DMN. 2-MN is separated by continuous crystallization or recrystallization of the MN fraction in order to separate 2-MN. The DMN compounds, however, generally form eutectic mixtures. For instance, 2,6-DMN and 2,7-DMN form a two-component eutectic mixture in the mole ratio of 41.5 : 58.5. 2,6- and 2,3-DMN form a two-component eutectic mixture in the mole ratio of 47.5 : 52.5. Therefore, the conventional process for separating 2,6-DMN of a high purity which relies on the recrystallization method can not attain a high separation yield, since the yield of 2,6-DMN is theoretically decided depending on the material composition. For instance the fraction of the boiling point of 250 - 270°C obtained by catalytically cracking petroleum contains 8 - 13% of 2,6-DMN and 8 - 13% of 2,7-DMN so that when separating and purifying by cooling, solidifying and recrystallizing or partially melting thereof, the yield for recovering of 2,6-DMN is to be about 30 % at the highest. It is possible to increase the 2,6-DMN content in the material up to 30% according to the rectification, but it is impossible to considerably change the ratio of 2,6-DMN and 2,7-DMN so that the yield of pure 2,6-DMN can not be raised. Various fractions from petroleum or coal tar contains 2,6-DMN and 2,7-DMN in the same amount in addition to which various components inclusive of DMN isomers are contained. 2-MN and 1-MN are contained in the concerned fraction in the ratio of 2 : 1. The boiling points of 2,6-DMN and 2,7-DMN as well as of 2-MN and 1-MN are very close respectively to each other so that the eutectic mixture and the solid solution thereof may be formed. Thus, separation of 2,6-DMN encounters the problems of decreased recovering yield, difficulty of raising the purity and considerably high cost of separation and purification. The same is applied to separation and purification of 2-MN. In order to solve the problems referred to above, it has been proposed to utilize a process involving crystallization under pressure, which is superior to recrystallization, partial melting and continuous crystallization, in that the apparatus used can be more compact, and it is possible to achieve lower costs, higher yield and higher purity. However, the process is disadvantageous in that impurities in the material are subjected to oxidative polymerization with a relatively small amount of oxygen due to localised superheating under high pressure in the pressure crystallizing apparatus and oxidized polymers mingle with the separated product crystals. The crystals of 2,6-DMN and 2-MN separated according to this pressure crystallization method are, thus, colored in black and the qualities thereof are considerably deteriorated due to the oxidized polymers of the impurities in the material so that the separated products are commercially less valuable. The method also has a disadvantage in that the discharged liquid can not be reused for the reason referred to above. SUMMARY OF THE INVENTION An object of the invention is, thus, to provide a process for separating methyl derivatives of naphthalene from a mixture containing such derivatives without the disadvantages referred to above of the prior art. It is a particular object to provide a process for separating 2,6-DMN and 2-MN by crystallization under pressure respectively in a high purity of more than 98% by weight without the disadvantages of the prior art referred to above with a lower cost and a higher yield. The other objects of the invention and advantageous effects attained thereby will be appreciated by those skilled in the art when studying the full explanation of the invention to be given hereafter. According to the present invention therefore there is provided a process for separating a methyl derivative of naphthalene comprising steps of preliminary treating a starting mixture material containing said derivative in the presence of an acid catalyst at a raised temperature for polymerizing impurities in the material, removing said polymerized impurities from the starting mixture material, and crystallizing under pressure of the material in which the derivative content is increased so as to obtain the methyl derivative of naphthalene in a high purity more than 98% by weight. The process of the invention is carried out in a preferred embodiment by heating a raw material containing 2,6-DMN or 2-MN in the presence of an acid catalyst to polymerize the impurities in the material, and then distilling the treated material to remove the polymerized impurities and to increase the 2,6-DMN or 2-MN content at least up to 50 weight %, and subsequently, subjecting the treated material to crystallization at a particular temperature and under a particular pressure. The reason why the 2,6-DMN or 2-MN content is increased to at least 50 weight %, lies in that the yield of the object compound is too low, and also the object compound with the high purity more than 98 weight % is not obtained, in the case that said content is less than 50 weight %. By removing the impurities in the starting material, the disadvantages caused by utilizing the crystallization under pressure as mentioned above, are removed, thereby obtaining the object compound with the high purity. DESCRIPTION OF ACCOMPANYING DRAWINGS Fig. 1 is a graph showing curves of solid-liquid equilibrium in relation to the temperature and pressure for explaining the crystallization under pressure, and Fig. 2 is a schematic view of the apparatus for carrying out the process according to the invention. EXPLANATION OF PREFERRED EMBODIMENTS OF THE INVENTION A fraction of a boiling point of 250 - 270°C, preferably 257 - 265°C (for separation of 2,6-DMN) or of 220 - 250°C, preferably 235 - 245°C (for separation of 2-MN), obtained by catalytically cracking petroleum, is used as the starting material which contains 2,6-DMN or 2-MN and other isomers thereof. This starting material is not subjected directly to crystallization under pressure but is heated in the presence of an acid catalyst in advance, so as to polymerize some impurities such as aromatic hydrocarbons and other unstable substances containing nitrogen, sulphur or oxygen which are apt to be oxidatively polymerized and difficult to be removed e.g. by a simple distillation, and is then distilled so as to increase the 2,6-DMN or 2-MN content to be at least 50% by weight, preferably higher than 70% by weight. The material thus treated in advance is then subjected to crystallization under pressure. Otherwise the yield of the objective compound is poor and a high purification of more than 98% can not be expected. As for raising the 2,6-DMN or 2-MN content in the concerned fraction itself, the cooling crystallization method and the zeolite method are already in public knowledge e.g. by U.S. Patent 3,590,091 and JP-B 27,578/1974. According to the former, the concerned cracking fraction is crystallized at a temperature of -15 - +5°C (for 2,6-DMN) or -40° - +10°C (for 2-MN). According to the latter, the DMN fraction is treated at 80 - 100°C, with SV 1g/g/hr in the presence of zeolite Na-Y, and as occasion demands further subjected to crystallization at a temperature of -35 - +10°C. These treatments, however, are practically not only troublesome but also difficult to sufficiently raise the concentration of 2,6-DMN or 2-MN in the material mixture so that even if the treated material is subjected to the crystallization under pressure, the defects referred to above can not be overcome. As for the above mentioned acid catalyst used for treating the raw material prior to the pressure crystallization this may be any of the acid catalysts such as sulfuric acid and phosphoric acid as are used e.g. for olefin polymerization. Solid acid catalyst such as silica, alumina, silica-alumina, chromia, titania, zirconia, chromia- alumina, clay, bauxite, zeolite, activated carbon and activated clay also may be used. The surface acidity of the catalyst is sufficient to be in such a degree as to discolor benzeneazodiphenylamine. Suitable properties of the solid catalyst are 100 - 500 m²/g, preferably 150 - 300 m²/g surface area, 30 - 300Å, preferably 50 - 100Å average porous diameter, 0.1 - 1.0 cc/g porous volume and 10 - 100 mesh, preferably 30 - 60 mesh particle dimension. Particularly desirable acid catalyst for the process of the invention is an activated clay comprising a main content of 2 : 1 type layer structure such as montmorillonite. The treatment is carried out at a temperature preferably in the range of 120 - 200°C. When the temperature is higher than 220°C, disproportionation, isomerization, demethylation and other undesirable side reactions of aromatic compounds contained in the treated mixture material occur. The treatment is done under such a pressure as to keep the material in a liquid phase in relation to the temperature, generally at the atomspheric. A liquid phase space velocity (LHSV) of the reaction column is generally 0.1 - 6 Hr⁻¹, and preferably 0.2 - 2 Hr⁻¹. The starting material to be treated according to the invention prior to the crystallization under pressure comprises aromatic compounds and other unstable substances having nitrogen, sulphur or oxygen which are readily oxidized or oxidatively polymerized with a small amount of oxygen at the treatment temperature so as to discolor the product or form undesirable sludge as referred to above. According to the treatment referred to above such impurities are polymerized to be readily removed from the material by distillation and consequently raise the DMN or MN content therein up to 70% by weight. The treated material is then subjected to the crystallization under pressure, according to which 2,6-DMN or 2-MN in the liquid mixture material is solidified under a pressure higher than the solid-liquid modification pressure thereof in a sealed vessel so as to exclude the liquid phase from the formed solid-liquid co-existing system and compress the solid phase to "squeeze out" the remaining liquid among solid particles and conglomerate the particles. Now expressing a concentration of impurities in the remaining liquid as X₂ (molar concentration), a treatment temperature as T (absolute), a solid (crystal)-liquid equilibrium pressure as P₁ (kg/cm²), a solid-liquid modification pressure of pure substance as Po (kg/cm²) and a difference between said P₁ and Po as ΔP (kg/cm²), the following relation is establised when X₂ is of a small value; in which R is a gas constant and ΔV means a volume change per mole caused by solidification (generally negative value). Further expressing a statisically average contact pressure of crystaline interface as P S , a solid-liquid equilibrium pressure of the residual liquid with impurity concentration X₂ as P₀ + ΔP and a pressure of excluding the residual liquid as P L , the relation of P O <P L <P O + ΔP<P S is the best for attaining the purpose. As P L nears close to P O + ΔP , the recovered solid amount is increased but the purifying effect is lowered. When P L nears to P O , the solid recovering efficiency is a little lowered but the purification can be efficiently made. It is possible, thus, to obtain the high purity of solid in a higher yield only by removing a relatively small amount of the residual liquid, when nearing P L from P O + ΔP closer to P O depending on the concentration and excluded amount of the residual liquid. In reference to Fig. 1 a curve gradient of solid-liquid equilibrium dP O /dT is generally larger than zero. When an absolutely pure substance in which the impurity concentration X₂ = 0 is in solid-liquid equilibrium state at a temperature T under a pressure P O , the solid-liquid equilibrium pressure in case of the substance of impurity concentration X₂ comes to P O + ΔP . When setting a residual liquid excluding pressure at P L , the solid in the vicinity of the liquid of impurity concentration X₂ is melted so as to attain the solid-liquid equilibrium to be made purer. The then statistically avarage pressure of crystalline interface P S is far higher than the pressures referred to above so as to affect a pressure on the crystal particles, whereby the residual liquid is "squeezed out". When the temperature is lowered down to T′ which is caused by melting some amount of crystals as a result of setting the excluding pressure at P L , it is possible to adjust the excluding pressure with considering the then pressure P O ′, newly as P O or to understand it as a variable to return to the initial value as a result of the temperature recovery. The invention will be more definitely explained in reference to Fig. 2 in which the apparatus for crystallizing under pressure according to the invention, is illustrated. In the case that 2,6-DMN is used for the starting material, the 2,6-DMN contained in 250 - 270°C fraction obtained by catalytically cracking petroleum is preliminarily concentrated (by thermally treating with the acid catalyst and distillating) to obtain a mixture of at least 50 weight %, preferably 70 weight % of 2,6-DMN and an another DMN isomer, and then said mixture is preliminarily adjusted to a temperature of 80 - 105°C to form a slurry containing the crystal of 2,6-DMN and the isomer thereof. In the case that 2-MN is used for the starting material, the 2-MN contained in 220 - 250°C fraction obtained by catalytically cracking petroleum is preliminarily concentrated to obtain a mixture of at least 50 weight %, preferably 70 weight % of 2-MN and an another MN isomer, and then, said mixture if preliminarily adjusted to a temperature of 10 - 35°C to form a slurry containing the crystal of 2-MN and the isomer thereof. Each said mixture is fed from a material tank 14 to a primary crystallization zone 2 through a conduit 3 by means of a motor driven pump 4 so as to form seed crystals therein. Otherwise not only higher pressure is necessitated for causing primary crystallization but also ultrafine crystals are formed under a supersaturated condition due to the rapid pressurization, which may lead to separation difficulty. Owing to formation of crystaline seeds, the supersaturated condition is not caused so that crystal growth can be started immediately upon a pressurization. The material containing crystaline seeds of 2,6-DMN or 2-MN is then fed to a pressure vessel 5 through a conduit 6 provided with a valve 7 by means of a pump 8. The pressure vessel 5 comprises a vertically movable piston 9 which is actuated by a hydraulic unit 10 so as to define a chamber 11 for crystallization under pressure between the free end of the piston 9 and the bottom wall of the chamber 11 in which pressure may be raised by lowering the piston down. The piston is preferably arranged with a conduit for overflow provided with a valve 13 and opened at the piston free end so that when the supplied material is filled in the chamber 11 to flow into the conduit 12, the overflow is detected to close the valves 7, 13 and the piston 9 is lowered to raise the pressure in the chamber. Thereby the material in the chamber 11 is made in the solid-liquid coexisting state as referred to above. The resulting 2,6-DMN or 2-MN in solid state is already of a high purity. As the solidification thereof is progressed, the temperature is raised, but it is generally preferable not to cool the system. The temperature after the pressure is raised adiabatically up to 500 - 2500 kgf/cm², at which the solid-liquid separation is started, affects on the purity and yield of the product. Thus, the temperature of the material to be supplied is controlled at 80 - 105°C in case of 1,2-DMN and at 10 - 35°C in case of 2-MN as referred to above with taxing into consideration, in advance, of a specific heat, a solidification latent heat and so on of the mixture material so that a desired temperature can be resultingly held. Then, a valve 14 is opened for discharging the liquid content in the crystallizing chamber 11 through a conduit via a decompressor 16 into a waste liquid tank 17, with keeping the pressure in the chamber 11 by lowering the piston 9 down. The crystal particles of 2,6-DMN or 2-MN are thus pressed so as to "squeeze out" the liquid contents remained thereamong to be exhausted out of the chamber 11 into the tank 17. As the piston 9 is further lowered, the crystal particles are further pressed to form a large mass in the form of the decreased volume of chamber 11. When the liquid content in the solid phase is almost completely excluded and discharged out of the chamber, the liquid phase pressure is correspondingly decreased so that crystalline surfaces are partially melted so as to increase the degree of purification by virtue of the so-called "sweating effect". Thereby the purity of the separated 2,6-DMN or 2-MN product reaches at 98 % or more. Then, the piston is raised up to be in the initial position and the product is taken out by opening the lid provided in the bottom wall of the pressure vessel 5. The invention will be explained in more detail in reference to following Examples. Example 1 The starting material comprising 67 weight % of 2,6-DMN, other DMN isomers inclusive of 9 weight % of 2,7-DMN and a small amount of impurities were preliminarily treated in the presence of the marketed activated clay as an acid catalyst. The properties thereof were as follows; (1) Particle dimension 30 - 60 mesh (2) Bulk density 0.63 (3) Specific surface area 281 m²/g (4) Porous volume 0.39 ml/g (5) Average porous diameter 55 Å (6) Free acid 2.6 mg KOH/g (7) Chemical composition (wt %) SiO₂ 76.0 Al₂O₃ 10.3 Fe₂O₃ 1.7 CaO < 0.1 MgO 1.5 Ignition loss 10.0 The starting material was passed through the activated clay at a temperature of 160°C with 0.5 Hr⁻¹ of LHSV and then subjected to distillation for removing polymerized impurities so as to obtain colorless oily material. This mixture material was continuously heated at 200°C for 200 hours in air in a sealed glass tube to recognize almost no change from the initial state. The mixture material was subjected to the preliminary crystallization so as to contain crystalline seeds of 2,6-DMN. The material in the slurry state was fed to a pressure vessel of the hydraulic piston-cylinder structure to be subjected to an adiabatic pressure of 1500 kgf/cm². Then the liquid phase in the pressure vessel was discharged therefrom with keeping the pressure and 2,6-DMN crystals pressed untill the liquid phase pressure fell down to 200 kgf/cm². Obtained crystaline 2,6-DMN was not colored and had a purity of about 98 %. Example 2 The starting material comprising 65 weight % of 2-MN, 7 weight % of 1-MN and a small amount of impurities were preliminarily treated similar to Example 1. The obtained oily colorless material was continuously heated similar thereto to find no substantial change. The material was subjected to the crystallization under pressure like as in Example 1 except the temperature of the supplied material was controlled to be of 5°C. Obtained crystaline 2-MN was not colored and had a purity of about 98 %. The residual mother liquors in Examples 1 and 2 were cyclically used in the subsequently repeated treatment.

Claims: A process for separating a methyl derivative of naphthalene comprising steps of preliminarily treating a starting mixture material containing said derivative in the presence of an acid catalyst at a raised temperature for polymerizing impurities in the material, removing said polymerized impurities from the starting mixture material, and crystallizing under pressure of the material in which the derivative content is increased so as to obtain the methyl derivative of naphthalene in a high purity more than 98 % by weight. The process as set forth in Claim 1, in which the 2,6-dimethyl naphthalene content in the preliminarily treated mixture material, which is to be subjected to the crystallization under pressure, is raised up to at least 50 % by weight. The process as set forth in Claim 1, in which the 2-methyl naphthalene content in the preliminarily treated mixture material, which is to be subjected to the crystallization under pressure, is raised up to at least 50 % by weight. The process as set forth in Claim 2, in which the preliminarily treated mixture material containing 2,6-dimethyl naphthalene, of which temperature is controlled to be of 80 - 105°C, is subjected to the crystallization under a pressure of 500 - 2500 kgf/cm². The process as set forth in Claim 3, in which the preliminarily treated mixture material containing 2-methyl naphthalene, of which temperature is controlled to be of 10 - 35°C, is subjected to the crystallization under a pressure of 500 - 2500 kgf/cm². The process as set forth in any of Claims 1 - 5, in which the starting mixture material is preliminarily treated in the presence of the acid catalyst at a raised temperature up to 220°C at the highest and with a liquid phase space velocity of 0.1 - 6 Hr⁻¹. The process as set forth in Claim 6, in which the starting mixture material containing dimethyl naphthalenes is a fraction with b.p. of 250 - 270°C obtained by catalytically cracking petroleum and subjected to the preliminary treatment in the presence of the acidic catalyst at a temperature of 120 - 200°C and with a liquid phase space velocity of 0.2 - 2Hr⁻¹ and then to a fractional distillation. The process as set forth in Claim 6, in which the starting mixture material containing methyl naphthalenes is a fraction with b.p. of 220 - 250°C obtained by catalytically cracking petroleum and subjected to the preliminary treatment in the presence of the acidic catalyst at a temperature fo 120 - 200°C and with a liquid phase space velocity of 0.2 - 2 Hr⁻¹ and then to a fractional distillation. The process as set forth in any of Claims 1 - 8, in which the acid catalyst is a solid catalyst and a liquid catalyst used generally for the olefin polymerization. The process as set forth in Claim 9, in which the liquid catalyst is sulfuric acid or phosphoric acid. The process as set forth in claim 9, in which the solid catalyst is silica, alumina, silica-alumina, chromia, titania, zirconia, chromia-alumina, clay, bauxite, zeolite, activated carbon or activated clay. The process as set forth in Claim 9, in which the surface acidity of the acid catalyst is in such a degree as to discolor benzeneazodiphenylamine. The process as set forth in Claim 9, in which the solid acid catalyst has a surface area of 100 - 500 m²/g, an average porous diameter of 30 - 300Å, a porous volume of 0.1 - 1.0 cc/g and a particle dimension of 10 - 100 mesh. The process as set forth in Claim 11, in which activated clay catalyst is of montmorillonite system having 2 : 1 type layer structure.

Inventors: AIHARA, SHINJI C/O SHOWA SHELL SEKIYU K.K., HATAKEYAMA, HITOSHI, KOIDE, SHUNICHI C/O SHOWA SHELL SEKIYU K.K., TAKAO, MASAMI, TAKESHITA, HIROSHI C/O SHOWA SHELL SEKIYU K.K., TANABE, HARUMASA
Applicants: KOBE STEEL LIMITED, SHOWA SHELL SEKIYU KABUSHIKI KAISHA
Application year: 1989
Main classification: C07C15
IPC codes: C07C7, C07C15, C07C67, C07C1