PROCESS FOR THE PREPARATION OF CYCLOHEXANEDIMETHANOL

Abstract: This invention provides a process for preparing cyclohexanedimethanol comprising hydrogenating a cyclohexanedicarboxylic acid dialkyl ester by a fixed-bed continuous reaction in the presence of a preformed copper-containing catalyst under the conditions of reaction temperature of 200 to 280°C, hydrogen pressure of 185 to 300 kgf/cm 2 and hydrogen gas feed rate of 1 to 40 cm/s in terms of superficial linear velocity, the cyclohexanedicarboxylic acid dialkyl ester being prepared, typically, by ring hydrogenating of an aromatic dicarboxylic acid dialkyl ester in the presence of a preformed supported ruthenium catalyst by a fixed-bed continuous reaction.

Description: Field of the Invention The present invention relates to a process for preparing cyclohexanedimethanol (hereinafter referred to as "CHDM"). When used as the diol component of polyester resins, polyurethane resins, polycarbonate resins or the like, CHDM is effective in improving the heat resistance, transparency, weatherability and molding properties of these resins. Particularly 1,4-cyclohexanedimethanol is drawing attention as a compound useful for improving the properties of polyethylene terephthalate. Background Art Conventional processes for preparing CHDM generally comprise ring hydrogenation of aromatic dicarboxylic acid dialkyl ester to give cyclohexanedicarboxylic acid dialkyl ester (first reaction) and hydrogenation of its ester groups to give CHDM (second reaction). Among these catalysts used for the respective reactions, known as effective for the first reaction are palladium, nickel, ruthenium, rhodium and the like US-A-3,334,149, JP-A-163554/1979 and 192146/1994, US-A-5,286,898 and 5,399,742); and known as effective for the second reaction are copper-chromite, copper oxide/zinc oxide, copper oxide/titanium oxide, copper oxide/iron oxide and catalysts prepared by modifying these copper-based catalysts with oxides of barium, magnesium and zinc and reducing the modified catalysts for activation (US-A- 3,334,149, JP-A-192146/1994 and 196549/1995, US-A-5,334,779, 5,030,771 and 4,929,777). As the mode of reaction, a fixed-bed continuous reaction process is considered to be advantageous over a suspended catalyst reaction process in terms of productivity and yield. Herein, the fixed-bed continuous reaction process includes a reaction process (downflow-type process) wherein a preformed catalyst is placed into a pressure-resistant reactor, into which hydrogen and a raw material are supplied to the top of the reactor at a predetermined temperature and hydrogen pressure, and the reaction product is withdrawn from the bottom of the reactor; and a reaction process (upflow-type process) wherein hydrogen and a raw material are supplied to the bottom of the reactor and the reaction product is removed from the top of the reactor. The suspended catalyst reaction process comprises suspending a catalyst powder in an aromatic dicarboxylic acid diester or a cyclohexanedicarboxylic acid diester and subjecting the suspension to a reaction with heating while being pressurized with hydrogen. As the examples of fixed-bed continuous reaction process, reported are processes comprising ring hydrogenation of a terephthalic acid dialkyl ester in the presence of a preformed supported ruthenium catalyst to give 1,4-cyclohexanedicarboxylic acid dialkyl ester in the first reaction (JP-A-163554/1979 and 192146/1994). Ruthenium catalysts are inexpensive compared with palladium catalysts, and exhibit a high activity even at a low pressure and a low temperature, but have the disadvantage of being likely to cause undesirable reactions involving high exothermic heat, such as hydrogenolysis of ester groups to hydroxymethyl group(s) or methyl group(s), in addition to the hydrogenation of the aromatic ring. It is advantageous to use a cyclohexane-dicarboxylic acid diester as a reaction solvent in order to avoid adverse effect due to reaction heat. However, the cyclohexanedicarboxylic acid diester used may be consumed due to the above-mentioned side reactions, not only resulting in a low yield but also, in extreme case, leading to a rapid generation of heat in a portion of the reactor which makes it difficult to continue the reaction. Therefore, JP-A-192146/1994 proposes the provision of a perforated plate in the reactor to improve the dispersibility of gas and liquid. However, even in this case, the concentration of terephthalic acid dialkyl ester in the feed to the reactor actually must be limited to an extremely low range of 5 to 20 weight %, and a large amount of reaction product is subjected to reaction by recycling, resulting in a low yield based on the terephthalic acid dialkyl ester used and leading to a low productivity. Examples of the fixed-bed continuous reaction process for the second reaction are those disclosed in JP-A-196549/1995; 196560/1995; 196558/1995; 196559/1995; 188077/1995; 188078/1995 and 188079/1995. These proposed processes are characterized in that the reaction is conducted under gas phase conditions of relatively low hydrogen pressure. However, the processes entail various disadvantages such as loss of thermal energy in vaporizing the raw material and necessity for removal of generated reaction heat in a gas phase of low heat conductivity, resulting in a need for complicated equipments. Furthermore, high-boiling-point by-products are deposited on the surface of catalyst, thereby markedly reducing the catalyst activity and thus necessitating frequent catalyst replacement or catalyst regeneration treatment. On the other hand, US-A-3,334,149, 5,030,771 and 5,334,779 and JP-A-192146/1994 disclose a gas-liquid mixed phase reaction. However, the disclosed processes include various problems. For example, the feed rate (F/V) of cyclohexanedicarboxylic acid dialkyl ester is as low as 1 /h or less, leading to a low productivity per reactor. Alternatively, cyclohexanedicarboxylic acid dialkyl ester is fed as diluted with the reaction product, i.e. CHDM or the like to a concentration of about 16% by weight or less, consequently involving complicated equipment and cumbersome operation relating to the reactor and resulting in increased by-products due to side reaction of CHDM. Disclosure of the Invention An object of the present invention is to provide a process for preparing high-quality CHDM, which is capable of producing CHDM with safety in a high yield by a simplified procedure at a high productivity per reactor and capable of diminishing costs for equipment. The present inventors found that in producing cyclohexanedimethanol a high yield can be achieved using simplified equipment when cyclohexanedicarboxylic acid ester is hydrogenated under specific reaction conditions, and completed the present invention. The present invention provides a process for preparing cyclohexanedimethanol represented by the formula (1) which comprises the steps of a) continuously feeding an aromatic dicarboxylic acid dialkyl ester represented by formula (3) wherein R is an alkyl group having 1 to 4 carbon atoms, and hydrogen to the top of a multi-tubular pressure-resistant reactor packed with a preformed supported ruthenium catalyst to effect hydrogenation under a gas-liquid mixed phase condition, and removing excess hydrogen and the obtained cyclohexanedicarboxylic acid dialkyl ester represented by formula (2) wherein R is a defined above, from the bottom of the reactor, wherein each tube of the multi-tubular reactor has an inner diameter of 2.5 to 10 cm and a length of 3 to 15 m, and (b) continuously feeding the cyclohexanedicarboxylic acid dialkyl ester obtained in step (a) above and hydrogen to the top of a multi-tubular pressure-resistant reactor packed with a preformed copper-containing catalyst to effect hydrogenation under a gas-liquid mixed phase condition, wherein the reaction temperature is 200 to 280°C, the hydrogen pressure is 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ) and the hydrogen gas feed rate is 1 to 40 cm/s in terms of superficial linear velocity, and removing excess hydrogen and the resulting cylcohexanedimethanol from the bottom of said reactor wherein each tube of the multi-tubular reactor has an inner diameter of 2.5 to 10 cm and a length of 3 to 15 m, wherein the number of tubes is 10 to 2000 in both of step (a) and step (b). In the specification and claims, superficial linear velocity has a generally recognized meaning and is defined as a value obtained by dividing the hydrogen gas flow rate (cm 3 /s) by a cross-sectional area (cm 2 ) of the tubular or columnar reactor in the case of a single-column reactor, or as a value obtained by dividing the hydrogen gas flow rate (cm 3 /s) by a total cross-sectional area (cm 2 ) of a plurality of tubes in the case of a multitubular reactor. Brief Description of the Drawing FIG. 1 a schematic view showing the reactor used in the process of the invention. Detailed Description of the Invention According to the present invention, there is provided a process characterized in that the process comprises the steps of: (a) hydrogenating an aromatic dicarboxylic acid dialkyl ester with use of a preformed supported ruthenium catalyst by a fixed-bed continuous reaction to give a corresponding cyclohexanedicarboxylic acid dialkyl ester (first reaction), and (b) hydrogenating the obtained cyclohaxanedicarboxylic acid dialkyl ester by a fixed-bed continuous reaction in the presence of a preformed copper-containing catalyst under the conditions of reaction temperature of 200 to 280°C, hydrogen pressure of 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ) and hydrogen gas feed rate of 1 to 40 cm/s in terms of superficial linear velocity to prepare cyclohexanedimethanol (second reaction), wherein a multitubular pressure-resistant reactor packed with each of the above specified catalysts is used as the reactor in each of the steps (a) and (b), and wherein hydrogen and each raw material are fed to the top of each reactor to effect hydrogenation under a gas-liquid mixed phase condition, and excess hydrogen and the reaction product are withdrawn from the bottom of each reactor. In other words, according to the invention, there is provided a process for preparing cyclohexanedimethanol, the process comprising the steps of: (a) continuously feeding an aromatic dicarboxylic acid dialkyl ester and hydrogen to the top of a multitubular pressure-resistant reactor packed with a preformed supported ruthenium catalyst to effect hydrogenation under a gas-liquid mixed phase condition, and removing excess hydrogen and the corresponding cyclohexanedicarboxylic acid dialkyl ester from the bottom of said reactor (first reaction), and (b) continuously feeding the cyclohexanedicarboxylic acid dialkyl ester obtained in step (a) above and hydrogen to the top of a multitubular pressure-resistant reactor packed with a preformed copper-containing catalyst to effect hydrogenation under a gas-liquid mixed phase condition and under the conditions of reaction temperature of 200 to 280°C, hydrogen pressure of 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ) and hydrogen gas feed rate of 1 to 40 cm/s in terms of superficial linear velocity, and removing excess hydrogen and the resulting cyclohexanedimethanol from the bottom of said reactor (second reaction). Desirably the following conditions are employed in embodiment II. In the first reaction, namely in step (a), it is preferable that the hydrogen pressure is 2.94 to 9.81 MPa (30 to 100 kgf/cm 2 ), the reaction temperature is 120 to 180°C, the superficial linear velocity of hydrogen gas is 1 to 15 cm/s, particularly 1 to 10 cm/s, and the concentration of the aromatic dicarboxylic acid dialkyl ester in the feed to the reactor is at least 30% by weight. In the second reaction, namely step (b), it is preferable that the hydrogen pressure is 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ), the reaction temperature is 200 to 280°C, the superficial linear velocity of hydrogen gas is 1 to 40 cm/s, more preferably 5 to 30 cm/s, and the concentration of cyclohexandedicarboxylic acid dialkyl ester in the feed to the reactor is at least 90% by weight. In the second reaction, cyclohexanedicarboxylic acid dialkyl ester is preferably fed at a feed rate (F/V) of 1.1 to 3.0 /h (F/V = feed rate per hour relative to the volume of the catalyst bed in the reactor; F is a feed rate (liter/h) of the cyclohexanedicarboxylic acid dialkyl ester and V is a volume (liter) of the catalyst bed in the reactor). Further, the reactor to be used preferably comprises a shell and a plurality of tubes arranged in parallel and housed in the shell, and a heat transfer medium is passed through the shell to heat or cool the tubes. Especially, it is preferable that the shell for the reactor for the first reaction is divided into at least two zones, and that the temperature of each of the zones can be independently controlled, whereby the temperature difference within the reactor will be not greater than 50 degrees (°C), particularly not greater than 30 degrees (°C). Preferably the reactor for step (a), namely the first reaction is heated or cooled with a shell having at least two zones, and the heat transfer media flowing through the zones are independently heated or cooled so that the temperature difference in the reactor will be not greater than 50 degrees (°C). The reactors to be used for the fixed-bed continuous reaction, for both of the first and second reactions, are multitubular reactors, in which a plurality of tubes having a small interior diameter are arranged in parallel, and which are equipped with means for heating and cooling a heat transfer medium flowing through a shell thereof. The interior diameter of each tube constituting the multitubular reactor is preferably 2.5 to 10 cm, in particular 3 to 6 cm. If tubes each having an interior diameter of less than 2.5 cm are used, the number of tubes required for achieving the desired production output will be excessive and additionally the productivity will be lowered. Conversely, if tubes each having an interior diameter of larger than 10 cm are used, the gas-liquid dispersion efficiency is lowered, making it difficult to achieve the high productivity, high quality and high yield as contemplated in the present invention. Preferably, each tube has a length of 3 to 15 m, particularly 5 to 10 m. The use of tubes less than 3 m or over 15 m in length results in pronouncedly reduced productivity and lowered yield of the desired CHDM. The number of tubes is 10 to 2000 in view of the cost for manufacturing the reactor. However, it should be noted that the upper limit of the number of tubes is not particularly restricted but may be suitably selected depending on the desired production output. The shell through which the heat transfer medium passes may be of a non-partition type or of a multi-partition type internally separated into plural zones, the temperature each of which can be independently controlled. Especially, the reactor for the first reaction preferably has a shell of a multi-partition type with at least 2 zones, preferably 3 to 6 zones. If the first reaction is conducted under a temperature control in a single-zone cell, there is a tendency that an intensive exothermic reaction can not be controlled, causing various side reactions and resulting in a low-quality product and a diminished yield. As the mode of the fixed-bed continuous reaction, a gas-liquid mixed phase downflow-type process is carried out, wherein hydrogen gas and raw material are fed to the top of the reactor packed with the preformed catalyst, and the reaction product and the excess hydrogen are removed from the bottom of the reactor, and wherein the reaction is carried out under the reaction temperature and hydrogen feed rate conditions such that the reaction temperature is lower than the dew point of at least one of the raw material and the product. Problems are raised if the reaction is conducted by an upflow-type process wherein a raw material and hydrogen are fed from the bottom of the reactor or by a countercurrent-type process wherein, for example, raw material is fed to the top of the reactor and hydrogen is supplied to the bottom of the reactor. In such case, the strength of catalyst is adversely affected by the friction caused due to the movement of the catalysts, necessitating frequent replacement of catalysts. Especially, ruthenium metal is separated from the catalyst used in the first reaction and is lost, whereby the activity of the catalyst is reduced within a short period of time. Also, under the gas phase conditions in which the reaction temperature is above a dew points of the raw material and the reaction product, high-boiling-point by-products are deposited on the surface of catalyst, significantly reducing the catalyst activity, thereby necessitating frequent replacement of catalysts and regeneration treatment thereof. Furthermore, under such conditions, it is necessary to remove the reaction heat in a gas phase of low heat conductivity so that extremely complicated equipment is required. First reaction In the first reaction, an aromatic dicarboxylic acid dialkyl ester is hydrogenated in the presence of a preformed supported ruthenium catalyst to give the corresponding cyclohexanedicarboxylic dialkyl ester. The aromatic dicarboxylic acid dialkyl ester to be used as the raw material is a diester prepared by esterifying terephthalic acid, isophthalic acid or phthalic acid with a monohydric aliphatic alcohol having 1 to 4 carbon atoms in the conventional manner. Among these diesters, preferred is a compound represented by the formula (II-1) wherein R is an alkyl group having 1 to 4 carbon atoms. Specific examples of the above alcohol ROH are methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, etc. Thus, examples of the aromatic dicarboxylic acid dialkyl ester are dimethyl terephthalate, diethyl terephthalate, di-iso-propyl terephthalate, di-n-butyl terephthalate, dimethyl isophthalate, diethyl isophthalate, di-iso-propyl isophthalate, di-n-butyl isophthalate, dimethyl phthalate, diethyl phthalate, di-iso-propyl phthalate, di-n-butyl phthalate, etc. Among them, the most preferred are commercially available dimethyl terephthalate and dimethyl isophthalate prepared from methanol. Useful preformed supported ruthenium catalyst include conventional preformed supported ruthenium catalysts which are known as catalysts for hydrogenating aromatic rings. Useful carriers or support can be any of alumina, silica, titania, magnesia, zirconia, silicon carbide and the like, among which alumina is preferred. The amount of ruthenium to be deposited on the support is preferably 0.05 to 10% by weight, more preferably 0.1 to 5% by weight, based on the support. As to the shape of the preformed supported ruthenium catalyst, generally recommended are those of cylindrical shape which are commercially readily available. The size of the preformed catalyst can be determined according to the interior diameter of the reactor to be used. Usually preferred are cylindrical catalysts having a diameter of 2 to 6 mm and a height of 2 to 6 mm. These preformed catalysts can be used as such or can be subjected to the reaction after being suitably activated as by reduction. The specific ruthenium catalyst to be described later in embodiment IV may be used in this embodiment II as well. Generally, the higher the hydrogen partial pressure, the more smoothly the hydrogenation reaction proceeds. If the hydrogen pressure is higher than necessary, a special pressure-resistant reactor is required, and hence uneconomical. The pressure to be employed in the first reaction is preferably in the range of 2.94 to 9.81 MPa (30 to 100 kgf/cm 2 ). At a pressure of below 2.94 MPa (30 kgf/cm 2 ), a low reaction rate results, hence undesirable. Above 9.81 MPa (100 kgf/cm 2 ), in addition to the above-mentioned equipment problem, various disadvantages result. For example, side reactions which involve large amount of exothermic heat tend to occur, such as hydrogenolysis of ester groups to hydroxymethyl group(s) or to methyl group(s). Further, low yields and difficulty in the control of reaction are entailed. A preferred reaction temperature is in the range of 120 to 180°C. At a reaction temperature of below 120°C, a reaction rate is markedly low, whereas at a reaction temperature of above 180°C, side reactions preferentially occur. Hence the reaction temperature outside said range is not practical. The first reaction is preferably carried out utilizing the above-mentioned mode of heating or cooling the heat transfer medium to maintain a temperature difference within 50 degrees (°C), preferably within 30 degrees (°C), in the longitudinal direction of tube within the reactor (namely in the direction of fluid flow). If there is a temperature difference beyond this range in the tubes of the reactor, there take place side reactions involving high exothermic heat, such as hydrogenolysis of ester groups to hydroxymethyl group(s) or to methyl group(s), which tend to induce lowered yield and markedly decreased productivity. The aromatic dicarboxylic acid dialkyl ester can be continuously fed to the top of the reactor either singly or in the form of a mixture with the reaction product obtained in the first reaction and mainly containing cyclohexane-dicarboxylic acid dialkyl ester. In the latter case, high-melting-point aromatic dicarboxylic acid dialkyl ester becomes easily melted and the reaction heat is easily controlled. As to the proportions of the aromatic dicarboxylic acid dialkyl ester and the first reaction's reaction product, since the above-mentioned multitubular reactor is used, the higher the concentration of the aromatic dicarboxylic acid dialkyl ester is, the more preferable from the standpoint of productivity and suppression of the formation of by-products. Accordingly, it is recommendable that the concentration of the aromatic dicarboxylic acid dialkyl ester in the mixture is at least 30% by weight, more preferably at least 40% by weight. At a low concentration of less than 30% by weight, a low productivity results, and the side reaction of cyclohexanedicarboxylic acid dialkyl ester circulating through the reaction system takes place, causing the decrease in yield and selectivity. The aromatic dicarboxylic acid dialkyl ester can be used as diluted with a solvent other than the product of the first reaction, which solvent will not adversely affect the reaction. The use of such solvent, however, requires additional procedures for separating and recovering the solvent and does not give a particularly favorable result. Recommended feed rate of the aromatic dicarboxylic acid dialkyl ester is 0.1 to 5 /h, preferably 0.2 to 3 /h, and particularly preferable feed rate is 0.5 to 3 /h, in terms of F/V (= feed rate of the aromatic dicarboxylic acid dialkyl ester per hour relative to the volume of the catalyst bed in the reactor). At a feed rate of lower than this range, productivity is lower, whereas at a feed rate of higher than this range, the resulting reaction product contains a large amount of unreacted aromatic dicarboxylic acid ester and is not suitable as the raw material in the second reaction. Herein, F stands for the feed rate (liter/h) of the aromatic dicarboxylic acid dialkyl ester per hour, and V represents the volume (liter) of catalyst bed in the reactor. The feed rate of hydrogen is 1 to 15 cm/s, preferably 1 to 10 cm/s in terms of superficial linear velocity under the reaction condition. At a superficial linear velocity lower than this range, it is difficult to achieve an effective contact between the gas and liquid on the surface of the catalyst with the result that the reaction rate is decreased and the quantity of by-products due to the above-mentioned side reactions increases. On the other hand, at a feed rate higher than the above range, there is no further appreciable improvement in the reaction, resulting in economical disadvantage. Hence it is undesirable. Second reaction The second reaction is a reaction to hydrogenate the hydrogenation product of the aromatic dicarboxylic acid dialkyl ester of the formula (II-1) obtained in the first reaction, namely cyclohexanedicarboxylic acid dialkyl ester, using a preformed copper-containing catalyst to give a corresponding CHDM. Examples of useful preformed copper-containing catalysts include conventional preformed copper-containing catalysts having ability to reduce esters, such as copper-chromite, copper oxide/zinc oxide, copper oxide/iron oxide, copper oxide/aluminum oxide, and these catalysts which contain oxide(s) of barium, manganese, aluminum, zinc or magnesium as a promoter. Further usable are those molded after addition of various binders in order to maintain an improved strength of the catalyst, and supported catalysts prepared by depositing said oxides on a support such as alumina, silica, silica-alumina, silicon carbide, zirconia, titania or zinc oxide. Among the above copper-containing catalysts, particularly preferred are commercially available copper-chromite catalysts of the so-called Adkins type, copper oxide/zinc oxide catalysts and these catalysts which contain one or more promoters such as barium oxide or manganese oxide. Generally, the above copper-containing catalysts contain copper in an amount, calculated as CuO, of 20-80 wt.%, preferably 30-70 wt.% and chromium in an amount, calculated as Cr 2 O 3 , of 15-70 wt.%, preferably 40-60 wt.%. The above Adkins type catalyst preferably contains copper in an amount, calculated as CuO, of 30-60 wt.% and chromium in an amount, calculated as Cr 2 O 3 , of 30-60 wt.%. Other copper/metal oxide catalysts mentioned above preferably contains copper in an amount, calculated as CuO, of 20-95 wt.% and other metal in an amount, calculated as the metal oxide, of 5-80 wt.%. These copper-containing catalysts preferably contain said one or more promoters mentioned above in an amount of up to 10 wt.%, calculated as said metal oxide. In the case of the supported catalysts, the percentage values are those calculated after excluding the amount of the support. In order to control an exothermic heat abruptly generated at the start of reaction and effectively exhibit a catalyst activity, it is effective that the preformed copper-containing catalyst is subjected to preliminary reduction treatment. The preliminary reduction treatment can be conducted in a conventional manner under a stream of hydrogen-nitrogen gas mixture at an atmospheric pressure or elevated pressure at a temperature in the range of 150 to 300°C while gradually increasing the concentration of hydrogen. As to the shape of copper-containing catalysts, catalysts of cylindrical shape which are commercially readily available are usually recommended. The size thereof can be determined according to the interior diameter of the reactor to be used, and usually preferred are cylindrical catalysts having a diameter of 2 to 6 mm and a height of 2 to 6 mm. The hydrogen pressure is 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ), preferably 19.61 to 24.52 MPa (200 to 250 kgf/cm 2 ). If the hydrogen pressure is below this range, various disadvantages are entailed, in addition to reduced reaction rate and decreased productivity, including increase in the quantity of high-boiling-point by-products such as ether compounds and wax esters and lowered yields and selectivity. A lower hydrogen pressure is also undesirable from the viewpoint of the duration of catalytic acitivity and strength of the catalyst. Conversely, the use of a hydrogen pressure of greater than the above range only increases the equipment cost without further appreciable improvement in the reaction rate and selectivity, hence undesirable. The reaction temperature is preferably 200 to 280°C, more preferably 225 to 280 °C. At a reaction temperature of below this temperature range, a markedly low reaction rate is exhibited, whereas at a reaction temperature higher than the above range, side reactions predominantly occur. Hence the reaction temperature outside said range tends to be impractical. In the second reaction, usually the reaction product obtained in the first reaction is continuously supplied, as it is, to the top of the reactor. The amount of unreacted aromatic dicarboxylic acid dialkyl ester in the reaction product obtained by the first reaction is preferably controlled to 5% by weight or less for producing the desired CHDM in a high yield. Also a portion of the reaction product obtained in the second reaction and predominantly containing CHDM may be fed as mixed with the reaction product obtained in the first reaction, but the concentration of cyclohexanedicarboxylic acid dialkyl ester in such feed to the second reaction reactor is preferably at least 90% by weight in view of the productivity and the increase in the amount of by-products due to relatively prolonged exposure of the desired CHDM to the reaction environment. The feed rate of the reaction product obtained in the first reaction and to be fed to the reactor for the second reaction is preferably 1.1 to 3.0 /h, in terms of F/V based on cyclohexanedicarboxylic acid dialkyl ester. If the F/V value is less than 1.1 /h, productivity is low with economical disadvantage, and also the yield of the desired CHDM is reduced due to side reactions, hence undesirable. Conversely, at a feed rate in excess of 3.0 /h, the obtained reaction product contains unreacted cyclohexanedicarboxylic acid dialkyl ester which brings about pronounced decrease in the yield and purity. Recommended feed rate of hydrogen gas is 1 to 40 cm/s, preferably 5 to 30 cm/s, particularly 10 to 20 cm/s, in terms of superficial linear velocity. The hydrogen gas superficial linear velocity has an extremely remarkable effect on the second reaction, and at a velocity of less than 5 cm/s, particularly less than 1 cm/s, the reaction rate significantly reduces, making it difficult to achieve the high productivity as contemplated in the present invention. On the other hand, at a superficial linear velocity of more than 30 cm/s, particularly more than 40 cm/s, a further advantage in terms of the reaction is not achieved, and the duration of activity and strength of the catalyst decreases, and only the costs for equipment such as a hydrogen circulator and for energy increases, hence undesirable. According to the present inventors' research, it has been found that when a chlorine content of the ruthenium catalyst used for hydrogenating an aromatic dicarboxylic acid dialkyl ester to produce cyclohexane-dicarboxylic acid dialkyl ester is 500 ppm or less, the desired cyclohexanedicarboxylic acid dialkyl ester is advantageously produced. Therefore, the desired CHDM is advantageously prepared by using this method in the first reaction of the process of the invention. Thus, according to this embodiment, the cyclohexanedicarboxylic acid dialkyl ester represented by the formula (IV-2) wherein R 1 and R 2 are the same or different and each represents an alkyl group having 1 to 4 carbon atoms, is prepared by hydrogenating an aromatic dicarboxylic acid dialkyl ester represented by the formula (IV-1) wherein R 1 and R 2 are as defined above in the presence of a preformed supported ruthenium catalyst by a fixed-bed continuous reaction, wherein the preformed supported ruthenium catalyst has a chlorine content of 500 ppm or less. The preformed supported ruthenium catalyst has a chlorine content of up to 500 ppm, preferably 50 to 300 ppm. By adjusting the chlorine content within the above range, reactions other than the ring hydrogenation reaction, namely side reactions, particularly hydrogenolysis reaction involving cleavage of carbon-oxygen bonds, is advantageously suppressed so that hydroxymethylcyclohexanecarboxylic acid alkyl ester and methylcyclohexanecarboxylic acid alkyl ester are produced as by-products in decreased amounts, resulting in a improved yield of the desired cyclohexanedicarboxylic acid dialkyl ester. Such supported ruthenium catalysts are readily prepared, for example, by depositing ruthenium chloride (RuCl 3 •3H 2 O) on a support in a conventional manner, sufficiently neutralizing the supported catalyst with an alkali and washing the neutralized catalyst with water until the chlorine content becomes up to 500 ppm, followed by drying and reduction. Among the preformed supported ruthenium catalysts with a chlorine content of up to 500 ppm, preferable are those having dispersion of ruthenium, surface distribution thereof and/or pore volume thereof that fall within specific ranges. That is, preferred catalysts are those which fulfil at least one of the following requirements (i), (ii) and (iii). (i) having a dispersion of ruthenium of at least 15%, preferably at least 20%. The preformed supported ruthenium catalyst which is at least 15% in the dispersion of ruthenium exhibits a high catalyst activity and achieves high reaction selectivity. The term "dispersion of ruthenium" used herein refers to the percentage of the ruthenium atoms exposed on the surface of the catalyst, relative to all the atoms of ruthenium deposited on a support. (ii) being at least 80 wt.%, preferably at least 90 wt.%, in the surface distribution of ruthenium. If a catalyst is at least 80 wt.% in the surface distribution of ruthenium, the catalyst has a high effectivity of the ruthenium metal on the surface of a support and exhibit an improved catalyst activity, hence more advantageous in practical use. The term "surface distribution of ruthenium" is used herein to mean the percentage (weight %) of the ruthenium located no deeper than 200 µm from the external surface of the support relative to the total amount of ruthenium. (iii) being at least 0.20 cc/g, preferably 0.25 to 0.35 cc/g, in the pore volume. If the catalyst has a pore volume of at least 0.20 cc/g, the catalyst is excellent in catalyst activity and in duration of activity. Hence such catalyst is improved in practical use. Herein, the pore volume is determined by the mercury intrusion porosimetry. In short, particularly recommendable preformed supported ruthenium catalysts are those which are not higher than 500 ppm in chlorine content, at least 15% in dispersion of ruthenium, at least 80 % by weight in surface distribution of ruthenium and at least 0.20 cc/g in pore volume. A preferred amount of ruthenium to be deposited on a support is 0.05 to 10% by weight, based on the support. If the amount is less than 0.05% by weight, the catalyst activity is low, hence the catalyst is unsuitable for use. If the amount is more than 10% by weight, it is difficult to impart a high dispersion of ruthenium, and results in low effectivity of expensive ruthenium and markedly low reaction selectivity. Useful support can be alumina, silica, titania, magnesia, zirconia, silicon carbide and the like, among which alumina is preferred. As to the shape of the preformed supported ruthenium catalyst, recommended are catalysts of cylindrical shape which are commercially readily available. The size of the preformed catalyst can be suitably selected according to the interior diameter of the fixed-bed reactor to be used. Usually preferred are preformed catalysts in the form of cylinders having a diameter of 2 to 6 mm and a height of 2 to 6 mm. These preformed supported ruthenium catalysts can be used as such, or can be used in the reaction after effecting a suitable activation treatment, such as reduction, in the conventional manner. Examples The following are examples which illustrate the present invention in further detail. In the following Examples and Comparative Examples, the term "L" means "liter". [Reactor] The reactor used in the following examples is a multitubular pressure-resistant vessel comprising 15 pressure-resistant tubes U each having an inner diameter of 43 mm and a length of 5 m, and a shell V for heating or cooling the tubes by means of a heat transfer medium is divided into three zones, wherein the temperature each of the zones are independently controllable. The raw material and hydrogen fed to the top of the reactor are evenly distributed among the tubes via a distributor W provided in a chamber at the top of the reactor. While monitoring the temperature with multi-point thermometers set in the tubes, the reaction temperature was maintained within the desired range by controlling the temperature of the heat transfer medium in each of the zones. The catalyst was uniformly packed into the tubes in such a manner that the total volume of the catalyst bed was 100 L. Fig. 1 shows a schematic view of the reactor. The symbols in Fig. 1 designate the following. A Raw material tank B Preheater C Multitubular reactor D Heat exchanger E Gas-liquid separator F Crude reaction product tank G Hydrogen gas condenser H Mist separator I Hydrogen gas circulator J-L Heat transfer medium heating/cooling units M Raw material feed pump N-P Heat transfer medium circulation pumps Q-S Cooling medium control valves T Multi-point thermometer U Tube V Shell (for heat transfer medium) W Distributor a-s Conduits Referring to Fig. 1, the reaction will be schematically described. The raw material is pumped from a raw material tank A to a preheater B , together with hydrogen, by means of a raw material feed pump M through a conduit a . The heated raw material and hydrogen are fed to the top of the reactor C through a conduit b . The crude reaction product and excess hydrogen are conveyed from the bottom of the reactor through a conduit c to a heat exchanger D where they are subjected to heat exchange with circulating hydrogen, and conveyed to a gas-liquid separator E through a conduit d . The liquid separated in this separator enters into a crude reaction product tank F through a conduit e . The gas separated in the separator E passes through a conduit f into a condenser G for cooling, and is conveyed through a conduit g to a mist separator H where the condensate is separated. The condensate enters into the tank F through a conduit h . The hydrogen gas from the mist separator H passes through conduits i and j into a hydrogen gas circulator I , and is conveyed to the heat exchanger D through a conduit k . The hydrogen gas heated in the heat exchanger D enters into the preheater B through a conduit l . The hydrogen gas is pressurized and fed through a conduit s to a conduit j . The heat transfer medium shell is divided into three zones. In the first zone at the upper part of the reactor, the heat transfer medium flowing out from the upper part of the first zone passes through a conduit m and is pumped to a heat transfer medium heating/cooling unit J by means of a heat transfer medium circulation pump N . The heat transfer medium cooled or heated in the unit J is fed through a conduit n to the lower part of the first zone and circulated. Similarly, in the second and third zones, the heat transfer medium drawn from the upper part thereof is pumped by means of a heat transfer medium circulation pump O or P to, and heated or cooled in, a heating/cooling unit K or L , fed to the lower part of each zone and circulated. In the following examples, crude liquid product of ring hydrogenation reaction (product of the first reaction) was prepared in the first reaction and stocked in an amount specified. Then, in the second reaction step, said crude liquid product of the ring hydrogenation reaction was placed in raw material tank A , and the catalyst used in the first reaction was replaced by the catalyst (copper-chromite catalyst or the like) to be used in the second reaction in the multitubular reactor C, and the second reaction was carried out. However, it is commercially advantageous to carry out the first reaction and second reaction in a continuous manner using a reactor for the first reaction containing the ruthenium catalyst and a reactor for the second reaction containing the copper-chromite catalyst or the like. [Composition analysis] The compositions of the raw materials and products of the first and second reactions in the examples were analyzed by gas chromatography. Example II-1 [First reaction] The reactor was charged with 95 kg of a tableted catalyst (3.2 mm in diameter and 3.2 mm in height) comprising 1.0 wt.% of Ru supported on alumina. To the top of the reactor, a solution consisting of: Dimethyl terephthalate 50.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 48.9 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.3 wt.% Methyl 4-methylcyclohexanecarboxylate 0.8 wt.% was fed at a rate of 300 L/h(F/V=1.5 /h), together with 207 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 5 cm/s), to continuously carry out ring hydrogenation at a pressure of 7.84 MPa (80 kgf/cm 2 ). The reaction temperature was adjusted to 144 to 150°C in the upper part of the reactor, 143 to 147°C in the middle part of the reactor and 139 to 144°C in the lower part of the reactor. The maximum temperature difference in the reactor was 11°C. The composition of the crude liquid reaction product obtained by the above fixed-bed continuous ring hydrogenation reaction during the period between 5 hours and 15 hours after the start of the reaction is shown below. Dimethyl 1,4-cyclohexanedicarboxylate 97.9 wt.% Methyl 4-hydroxymethylcyclohexane carboxylate 0.6 wt.% Methyl 4-methylcyclohexanecarboxylate 1.4 wt.% Dimethyl terephthalate 0.1 wt.% [Second reaction] The reactor was packed with 155 kg of tableted copper-chromite catalyst (3.5 mm in diameter and 3.5 mm in length) containing barium and manganese (47 wt.% of copper oxide, 48 wt.% of chromium oxide, 2.5 wt.% of barium oxide and 2.5 wt.% of manganese oxide). The catalyst was then subjected to preliminary activation treatment in a stream of hydrogen-nitrogen mixed gas at atmospheric pressure at a temperature of 180 to 200°C, while gradually increasing the hydrogen concentration. After the preliminary activation treatment, the crude liquid reaction product of ring hydrogenation obtained in the first reaction was fed to the top of the reactor at a feed rate of 188 L/h(F/V=1.84 /h) at a pressure of 24.52 MPa (250 kgf/cm 2 ), together with 2135 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 20 cm/s), to continuously carry out hydrogenation of the ester groups. The reaction temperature was independently adjusted to 232 to 235°C in the upper part of the reactor, 230 to 236°C in the middle part of the reactor and 228 to 232°C in the lower part of the reactor. The reaction product obtained by the above fixed-bed continuous hydrogenation of the ester groups during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 97.9 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.0 wt.% Low-boiling-point product 2.1 wt.% High-boiling-point product Trace Example II-2 [First reaction] The reactor was charged with 93 kg of a tableted catalyst (3.2 mm in diameter and 3.2 mm in height) comprising 0.5 wt.% of Ru supported on alumina. Dimethyl terephthalate was fed to the top of the reactor at a rate of 80 L/h(F/V=0.8 /h), together with 170 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 8 cm/s), to continuously carry out ring hydrogenation at a pressure of 3.92 MPa (40 kgf/cm 2 ). The reaction temperature was independently adjusted to 146 to 152°C in the upper part of the reactor, 138 to 142°C in the middle part of the reactor and 128 to 132°C in the lower part of the reactor. The maximum temperature difference in the reactor was 20°C. The crude liquid reaction product obtained by the above fixed-bed continuous ring hydrogenation reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 97.6 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.5 wt.% Methyl 4-methylcyclohexanecarboxylate 1.9 wt.% Dimethyl terephthalate 0.0 wt.% [Second reaction] Following the procedure of the second reaction in Example II-1, the crude liquid product of the above first reaction was fed to the top of the reactor at a rate of 120 L/h (F/V= 1.17 /h) at a pressure of 19.61 MPa (200 kgf/cm 2 ), together with 855 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 10 cm/s), to continuously carry out hydrogenation of ester groups. The reaction temperature was adjusted to 232 to 235°C in the upper part of the reactor, 230 to 236°C in the middle part of the reactor and 228 to 232°C in the lower part of the reactor. The crude reaction product obtained by the above fixed-bed continuous hydrogenation of the ester groups during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 97.8 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.0 wt.% Low-boiling-point product 2.2 wt.% High-boiling-point product Trace Example II-3 [First reaction] Following the general procedure of the first reaction of Example II-1, a solution consisting of: Dimethyl terephthalate 50.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 48.8 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.3 wt.% Methyl 4-methylcyclohexanecarboxylate 0.9 wt.% was fed to the top of the reactor at a rate of 200 L/h(F/V=1.0 /h), together with 192 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 6 cm/s), to continuously carry out ring hydrogenation at a pressure of 5.88 MPa (60 kgf/cm 2 ). The reaction temperature was adjusted to 133 to 139°C in the upper part of the reactor, 133 to 137°C in the middle part of the reactor and 131 to 135°C in the lower part of the reactor. The maximum temperature difference in the reactor was 8°C. The crude liquid reaction product obtained by the above fixed-bed continuous ring hydrogenation during the period between 5 hours and 15 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 98.2 wt.% Methyl 4-hydroxyroethylcyclohexanecarboxylate 0.4 wt.% Methyl 4-methylcyclohexanecarboxylate 1.4 wt.% Dimethyl terephthalate 0.0 wt.% [Second reaction] Following the general procedure of the second reaction in Example II-1, the crude product of the above first reaction was fed to the top of the reactor at a rate of 250 L/h(F/V=2.46 /h) at a pressure of 24.52 MPa (250 kgf/cm 2 ), together with 2567 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 25 cm/s) to continuously carry out the hydrogenation of the ester groups. The reaction temperature was adjusted to 248 to 261°C in the upper part of the reactor, 246 to 252°C in the middle part of the reactor and 248 to 252°C in the lower part of the reactor. The reaction product obtained by the above fixed-bed continuous hydrogenation of the ester groups during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 97.8 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.1 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.3 wt.% Low-boiling-point product 1.8 wt.% High-boiling-point product Trace Example II-4 [Second reaction] The second reaction in Example II-1 was repeated with the exception that the crude product of the following composition obtained by continuing the first reaction of Example II-1 was used as the raw material and that a copper-chromite catalyst (51 wt.% of copper oxide and 49 wt.% of chromium oxide) was used. The catalyst was in the form of cylinders with a diameter of 3.5 mm and a height of 3.5 mm. Dimethyl 1,4-cyclohexanedicarboxylate 97.8 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.7 wt.% Methyl 4-methylcyclohexanecarboxylate 1.4 wt.% Dimethyl terephthalate 0.1 wt.% The crude product obtained by the above reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 97.6 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.2 wt.% Low-boiling-point product 2.2 wt.% High-boiling-point product Trace Example II-5 [Second reaction] The second reaction in Example II-4 was repeated with the exception that a copper oxide/zinc oxide catalyst (41 wt.% of copper oxide, 50 wt.% of zinc oxide and 9 wt.% of aluminum oxide) was used. The catalyst was in the form of cylinders with a diameter of 3.5 mm and a height of 3.5 mm. The crude product obtained by the above reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 96.7 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.4 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.9 wt.% Low-boiling-point product 2.0 wt.% High-boiling-point product Trace Example II-6 [Second reaction] The second reaction in Example II-4 was repeated with the exception that a copper oxide/iron oxide catalyst (30 wt.% of copper oxide; 30 wt.% of iron oxide (FeO) and 40 wt.% of aluminum oxide as support) was used. The catalyst was in the form of cylinders with a diameter of 3.5 mm and a height of 3.5 mm. The crude product obtained by the above reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 94.2 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 1.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 2.2 wt.% Low-boiling-point product 2.5 wt.% High-boiling-point product 0.1 wt.% Example II-7 [Second reaction] The second reaction in Example II-4 was repeated with the exception that a copper oxide/aluminum oxide catalyst (60 wt.% of copper oxide, 6 wt.% of magnesium oxide and 34 wt.% of aluminum oxide as support) was used. The catalyst was in the form of cylinders with a diameter of 3.5 mm and a height of 3.5 mm. The crude product obtained by the above reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 93.6 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 1.4 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 3.0 wt.% Low-boiling-point product 1.9 wt.% High-boiling-point product 0.1 wt.% Example II-8 The influence of the length of the tubes in the reactor was determined in the following manner. The reaction was carried out under the same conditions as in Example II-1 with the exception that the feed rate of the raw material solution was doubled. Then, the obtained reaction product was fed again to the reactor under the same conditions. Thus, the reaction corresponding to the tube length of 10 m was carried out. [First reaction] The first reaction in Example II-1 was repeated with the exception that a solution consisting of: Dimethyl terephthalate 50.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 48.5 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.5 wt.% Methyl 4-methylcyclohexanecarboxylate 1.0 wt.% was fed at a rate of 600 L/h(F/V=3.0 /h), and the reaction was carried out for 5 hours. The obtained product was fed again at a rate of 600 L/h under the same conditions as in Example II-1 (F/V in total=1.5 /h) to carry out the reaction. The reaction product obtained by the above reaction during the period between 2 hours and 3 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 97.5 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.4 wt.% Methyl 4-methylcyclohexanecarboxylate 2.0 wt.% Dimethyl terephthalate 0.1 wt.% [Second reaction] The second reaction in Example II-1 was repeated with the exception that the above reaction product was used as the raw material and fed at twice the feed rate of Example II-1, i.e., at 378 L/h(F/V=3.68 /h). The reaction was carried out for 5 hours. The product obtained by this reaction was fed again at a rate of 378 L/h(F/V in total=1.84/h) under the same conditions as in the second reaction in Example II-1. The reaction product obtained by this reaction during the period between 2 hours and 3 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 97.6 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.1 wt.% Low-boiling-point product 2.2 wt.% High-boiling-point product Trace Example II-9 [First reaction] Ring hydrogenation was continuously carried out in the same manner as in Example II-2 with the exception that dimethyl isophthalate was used as the raw material in place of dimethyl terephthalate. The obtained product was of the following composition. Dimethyl 1,3-cyclohexanedicarboxylate 97.7 wt.% Methyl 3-hydroxymethylcyclohexanecarboxylate 0.8 wt.% Methyl 3-methylcyclohexanecarboxylate 1.5 wt.% Dimethyl isophthalate 0.0 wt.% [Second reaction] Ester reduction reaction was continuously carried out in the same manner as in Example II-2 with the exception that the above reaction product was used as the raw material. The obtained reaction product was of the following composition. 1,3-Cyclohexanedimethanol 97.3 wt.% Dimethyl 1,3-cyclohexanedicarboxylate 0.0 wt.% Methyl 3-hydroxymethylcyclohexanecarboxylate 0.1 wt.% Low-boiling-point product 2.6 wt.% High-boiling-point product Trace Example II-10 [First reaction] The first reaction in Example II-1 was repeated with the exception that a composition of the following make-up was used as the raw material and that the temperature difference in the tubes in the reactor was changed as described below. Dimethyl terephthalate 50.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 48.9 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.4 wt.% Methyl 4-methylcyclohexanecarboxylate 0.7 wt.% The temperature of the reactor was adjusted to 175 to 180°C in the upper part, 136 to 139°C in the middle part and 118 to 125°C in the lower part. The maximum temperature difference was 60°C. The crude liquid product obtained by this fixed-bed continuous reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 86.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 1.2 wt.% Methyl 4-methylcyclohexanecarboxylate 7.2 wt.% Dimethyl terephthalate 5.6 wt.% [Second reaction] The second reaction in Example II-1 was repeated with the exception that the crude liquid reaction product obtained above was used as the raw material. The reaction product obtained by this fixed-bed continuous ester group hydrogenation reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 84.7 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.1 wt.% Low-boiling-point product 15.2 wt.% High-boiling-point product Trace Example II-11 [First reaction] The first reaction in Example II-1 was repeated with the exception that the concentration of dimethyl terephthalate in the raw material feed was decreased to 20 wt.%. Stated specifically, a solution consisting of: Dimethyl terephthalate 20.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 78.2 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.5 wt.% Methyl 4-methylcyclohexanecarboxylate 1.3 wt.% was fed to the top of the reactor at a rate of 750 L/h(F/V=1.5 /h), together with 207 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 5 cm/s) to continuously carry out ring hydrogenation at a pressure of 7.84 MPa (80 kgf/cm 2 ). The temperature in the tubes was adjusted to 144 to 146°C in the upper part of the reactor, 143 to 146°C in the middle part of the reactor and 140 to 145°C in the lower part of the reactor. The maximum temperature difference in the reactor was 6°C. The crude liquid product obtained by this fixed-bed continuous ring hydrogenation reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 88.5 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 1.8 wt.% Methyl 4-methylcyclohexanecarboxylate 2.1 wt.% Dimethyl terephthalate 7.6 wt.% [Second reaction] The second reaction in Example II-1 was repeated with the exception that the above crude liquid product was used as the raw material. The reaction product obtained by this fixed-bed continuous hydrogenation of the ester groups during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 88.9 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.1 wt.% Low-boiling-point product 11.0 wt.% High-boiling-point product Trace Example II-12 [Second reaction] The second reaction in Example II-2 was repeated with the exception that the same raw material as used in Example II-4 (crude reaction product obtained by continuing the first reaction of Example II-1) was employed and that the reaction pressure was changed to 16.67MPa (170 kgf/cm 2 ). The crude product obtained by this reaction during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 84.1 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 0.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 9.2 wt.% Low-boiling-point product 2.2 wt.% High-boiling-point product 4.5 wt.% Example II-13 [Second reaction] The second reaction in Example II-2 was repeated with the exception that the same raw material as used in Example II-4 was employed and that the hydrogen gas superficial linear velocity was changed to 3 cm/s. The crude product obtained during the period between 5 hours and 10 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 87.8 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 1.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 5.8 wt.% Low-boiling-point product 2.2 wt.% High-boiling-point product 3.2 wt.% Example II-14 [First reaction] The first reaction in Example II-1 was repeated with the exception that the raw material feed was fed at four times the feed rate in Example II-1. The procedure of feeding the obtained product to the reactor under the same conditions was repeated three times. Thus, the reaction corresponding to the tube length of 20 m was carried out. Stated specifically, a solution consisting of : Dimethyl terephthalate 50.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 48.9 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.3 wt.% Methyl 4-methylcyclohexanecarboxylate 0.8 wt.% was fed at a rate of 1200 L/h(F/V=6.0 /h), and the reaction was carried out for about 5 hours under the same condition as in Example II-1. The obtained product was fed again to the reactor at a rate of 1200 L/h under the same conditions as in Example II-1. The procedure of feeding the obtained product to the reactor under the same conditions was repeated. The reaction product obtained in the fourth reaction of the above procedure (F/V in total=1.5 /h) during the period between 2 hours and 3 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 89.4 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.8 wt.% Methyl 4-methylcyclohexanecarboxylate 3.5 wt.% Dimethyl terephthalate 6.3 wt.% [Second reaction] The second reaction was carried out in the same manner as in the first reaction to effect the reaction corresponding to the tube length of 20 m. Thus, the reaction was carried out for 5 hours using the same raw material as used in Example II-4 under the same conditions as in the second reaction of Example II-1 with the exception that the raw material was fed at four times the feed rate in Example II-1, i.e., at 752 L/h (F/V=7.35 /h). The obtained reaction product was fed again to the reactor at a rate of 752 L/h under the same conditions as in the second reaction of Example II-1. The procedure of feeding the obtained product to the reactor under the same conditions was repeated three times. The reaction product obtained in the fourth reaction of the above procedure (F/V in total=1.84 /h) during the period between 2 hours and 3 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 84.8 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 3.5 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 7.8 wt.% Low-boiling-point product 1.8 wt.% High-boiling-point product 2.1 wt.% Comparative Example II-1 The reactor used was a pressure-resistant column reactor with an inner diameter of 159 mm and a length of 2 m, which comprises a shell for heating and cooling by a heat transfer medium. The raw material and hydrogen fed to the top of the reactor pass through perforated plate provided in the upper part of the reactor and are distributed through the zone to be charged with a catalyst. [First reaction] The reactor was packed with 35 L of tableted catalyst (3.2 mm in diameter and 3.2 mm in height) comprising 1.0 wt.% of Ru supported on alumina. To the top of the reactor, a solution consisting of: Dimethyl terephthalate 50.0 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 48.5 wt.% Methyl 4-methylcyclohexanecarboxylate 1.0 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 0.5 wt.% was fed at a rate of 105 L/h(F/V=1.5 /h), together with 182 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 5 cm/s), to continuously carry out ring hydrogenation at a pressure of 7.84 MPa (80 kgf/cm 2 ). The reaction temperature was measured with a thermometer set in the central part of the reactor, and found to be 144 to 178°C. The crude reaction product obtained by the above fixed-bed continuous ring hydrogenation during the period between 3 hours and 5 hours after the start of the reaction was of the following composition. Dimethyl 1,4-cyclohexanedicarboxylate 79.9 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 3.2 wt.% Methyl 4-methylcyclohexanecarboxylate 7.5 wt.% Dimethyl terephthalate 2.6 wt.% Low-boiling-point product 6.8 wt.% [Second reaction] The reactor of the same type as in the above first reaction was charged with 35 L of tableted copper-chromite catalyst (3.5 mm in diameter and 3.5 mm in height) containing barium and manganese (47 wt.% of copper oxide, 48 wt.% of chromium oxide, 2.5 wt.% of barium oxide and 2.5 wt.% of manganese oxide). The catalyst was then subjected to preliminary activation treatment under the same conditions as in Example II-1. After the preliminary activation treatment, the raw material of the same composition as in Example II-4 was fed to the top of the reactor at a rate of 66 L/h(F/V=1.84 /h) and a pressure of 24.52 MPa (250 kgf/cm 2 ), together with 1916 Nm 3 /h of hydrogen gas (superficial linear velocity under the reaction conditions = 20 cm/s), to continuously carry out hydrogenation of ester groups at 230 to 245°C. The reaction product obtained by the above reaction during the period between 5 hours and 6 hours after the start of the reaction was of the following composition. 1,4-Cyclohexanedimethanol 75.7 wt.% Dimethyl 1,4-cyclohexanedicarboxylate 5.8 wt.% Methyl 4-hydroxymethylcyclohexanecarboxylate 11.5 wt.% Low-boiling-point product 2.8 wt.% High-boiling-point product 4.2 wt.% Thus, the present invention provides a production process which is capable of remarkably improving the productivity per reactor and giving high-quality cyclohexanedimethanol by a simplified procedure in a high yield on a commercial scale.

Claims: A process for preparing cyclohexanedimethanol represented by the formula (1) which comprises the steps of (a) continuously feeding an aromatic dicarboxylic acid dialkyl ester represented by formula (3) wherein R is an alkyl group having 1 to 4 carbon atoms, and hydrogen to the top of a multi-tubular pressure-resistant reactor packed with a preformed supported ruthenium catalyst to effect hydrogenation under a gas-liquid mixed phase condition, and removing excess hydrogen and the obtained cyclohexanedicarboxylic acid dialkyl ester represented by formula (2) wherein R is a defined above, from the bottom of the reactor, wherein each tube of the multi-tubular reactor has an inner diameter of 2.5 to 10 cm and a length of 3 to 15 m, and (b) continuously feeding the cyclohexanedicarboxylic acid dialkyl ester obtained in step (a) above and hydrogen to the top of a multi-tubular pressure-resistant reactor packed with a preformed copper-containing catalyst to effect hydrogenation under a gas-liquid mixed phase condition, wherein the reaction temperature is 200 to 280°C, the hydrogen pressure is 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ) and the hydrogen gas feed rate is 1 to 40 cm/s in terms of superficial linear velocity, and removing excess hydrogen and the resulting cylcohexanedimethanol from the bottom of said reactor wherein each tube of the multi-tubular reactor has an inner diameter of 2.5 to 10 cm and a length of 3 to 15 m, wherein the number of tubes is 10 to 2000 in both of step (a) and step (b). The process of claim 1, wherein in step (a) the aromatic dicarboxylic acid dialkyl ester is dimethyl terephthalate, dimethyl isophthalate or dimethyl phthalate. The process of claim 1 or 2, wherein in step (a) the aromatic dicarboxylic acid dialkyl ester is fed as it is, or fed as admixed with a reaction product of step (a) predominantly comprising the cyclohexandedicarboxylic acid dialkyl ester; and wherein the hydrogenation in step (a) is carried out at a temperature of 120 to 180°C, a hydrogen pressure of 2.94 to 9.81 MPa (30 to 100 kgf/cm 2 ) and a hydrogen gas feed rate of 1 to 15 cm/s in terms of superficial linear velocity. The process of claim 3, wherein the aromatic dicarboxylic acid dialkyl ester is fed as admixed with the reaction product of step (a), and the concentration of the aromatic dicarboxylic acid dialkyl ester is at least 30% by weight. The process of any one of claims 1-4, wherein in step (b) the cyclohexanedicarboxylic acid dialkyl ester is fed as it is, or is fed as admixed with a reaction product of step (b) predominantly comprising cyclohexanedimethanol, the concentration of the aromatic dicarboxylic acid dialkyl ester in the mixture being at least 90 weight%, and wherein the hydrogenation in step (b) is carried out under the conditions of reaction temperature of 200 to 280°C, hydrogen pressure of 18.14 to 29.42 MPa (185 to 300 kgf/cm 2 ) and hydrogen gas feed rate of 5 to 30 cm/s in terms of superficial linear velocity. The process of any one of claims 1-5, wherein in step (b) the feed rate of the cyclohexanedicarboxylic acid dialkyl ester per hour per unit volume of the catalyst bed in the multi-tubular pressure-resistant reactor is 1.1 to 3.0 /h. The process of any one of claims 1-6, wherein the hydrogenation in step (a) is carried out while controlling the temperature difference within the multi-tubular reactor to not greater than 50°C. The process of any one of claims 1-7, wherein the multi-tubular reactor used in step (a) comprises a shell and a plurality of tubes contained in the shell, wherein the shell is divided into at least two separate zones through each of which a heat transfer medium is circulated for heating or cooling the tubes, each of the heat transfer media being independently heated or cooled in such a manner that the temperature difference within the reactor is controlled to not greater than 50°C.

Inventors: ITOH HIROSHI, IWAMURA TAIICHIRO, NAKAZAWA MIKIO, YOSHIDA YASUHISA, ITOH, HIROSHI, IWAMURA, TAIICHIRO, NAKAZAWA, MIKIO, YOSHIDA, YASUHISA
Applicants: SK NJC CO LTD, SK NJC CO., LTD.
Application year: 1997
Main classification: C07C31
IPC codes: B01J23, C07C29, C07C67, C07C69, C07C31