Title of Invention	PROCESS FOR CATALYTICALLY GENERATING ORGANIC SUBSTANCES BY PARTIAL OXIDATION.
Abstract	The process is performed in the gas phase in the presence of molecular oxygen at temperatures in the range from 200 to 500°C in at least one reactor, which constitutes a cooling- tube reactor and contains a catalyst. Cooling liquid flows through the cooling tubes of the reactor, and from the reac- tor a gaseous product mixture is withdrawn. 40 to 100 wt-% of the total amount of catalyst of the cooling-tube reactor are disposed as coating on the outside of the cooling tubes, wherein the feed mixture containing the feedstock and the mo- lecular oxygen gets in contact with the catalyst layers. Preferably, at least half the cooling tubes constitute ribbed tubes with ribs protruding on the outside, the ribs being at least partly coated with catalyst.

Title of Invention

PROCESS FOR CATALYTICALLY GENERATING ORGANIC SUBSTANCES BY PARTIAL OXIDATION.

Abstract

The process is performed in the gas phase in the presence of molecular oxygen at temperatures in the range from 200 to 500°C in at least one reactor, which constitutes a cooling- tube reactor and contains a catalyst. Cooling liquid flows through the cooling tubes of the reactor, and from the reac- tor a gaseous product mixture is withdrawn. 40 to 100 wt-% of the total amount of catalyst of the cooling-tube reactor are disposed as coating on the outside of the cooling tubes, wherein the feed mixture containing the feedstock and the mo- lecular oxygen gets in contact with the catalyst layers. Preferably, at least half the cooling tubes constitute ribbed tubes with ribs protruding on the outside, the ribs being at least partly coated with catalyst.

Full Text	Process for Catalytically Generating Organic Substances by Partial Oxidation This invention relates to a process for catalytically gener- ating organic substances by partial oxidation of an organic feedstock in the presence of molecular oxygen at temperatures in the range from 200 to 500°C in one or more series- connected reactors containing a catalyst for generating a gaseous product mixture. The conversion of the feedstock into the end product is ef- fected in one or more oxidation steps, where in each oxida-- tion step the individual molecules of the feedstock can re- lease one ore more atoms of hydrogen and/or carbon and also bind individual oxygen atoms. The carbon released combines with oxygen to form carbon dioxide and/or carbon monoxide, and the hydrogen released combines with oxygen to form water- Part of the feedstock usually is oxidized completely to form carbon dioxide and/or carbon monoxide, and another part is converted to other organic substances. The oxygen reguired for the oxidation reaction is withdrawn from the carrier gas (usually ambient air preheated in a heat exchanger) . Due to the various oxidation reactions, the entire process is highly exothermal. In accordance with the invention, e.g. phthalic anhydride, maleic anhydride is generated from butane or ben- zene, acrylic acid is generated from propylene, and acetic acid is generated from C4 hydrocarbons or anthraquinone. A typical product generated according to this principle in large amounts all over the world is phthalic anhydride from a feed mixture which contains orthoxylene or naphthalene and molecular oxygen in the concentration usual for ambient air or increased by additional enrichment. The generation of phthalic anhydride is effected at temperatures in the range from 200 to 500°C in a reactor containing a catalyst, through which reactor flows a cooling fluid and from which reactor gaseous product mixture containing phthalic anhydride vapor is withdrawn. This generation of phthalic anhydride will be represented below by way of example for all organic products to be generated by partial catalytic oxidation in the gas phase, where it is also possible to use a plurality of se- ries-connected reactors. Such process employing a multi-tube reactor is known from U.S. patent 4,592,412. The reactor comprises a multitude of vertical tubes in which the granular catalyst is disposed. The catalyst on the basis of vanadium pentoxide may also be formed of carrier elements coated with a catalytic mass. A cooling fluid flows around the outside of the numerous tubes containing the catalyst, in order to dissipate the heat re- leased by the exothermal reaction. In the known process mol- ten salt is preferably used as cooling fluid, which requires complex and therefore expensive apparatuses. Apart from high investment costs, the multi—tube reactor de- scribed above is also problematic for various other reasons. Due to the high weight and large dimensions of the reactor (150 to 250 t and 7 to 9 m diameter alone for the tube bundle with usual reactor sizes), cost-intensive special transports are required. Required pre-material in non-commercial dimen- sions as well as labor-intensive manufacture lead to long de- livery times in the manufacture of the reactor. The random bed of the catalyst in the individual tubes causes a large pressure loss on the gas side in conjunction with a high con- sumption of energy for the conveyance of the feed gas. in ad- dition, this type of construction involves a number of fur- ther, mostly process- or reaction-related problems which will be discussed in detail below. Modern high-performance catalysts require very uniform tem- peratures to evolve a high selectivity and to achieve a long service life. The admissible temperature difference of the cooling fluid inside the space surrounding the tubes there- fore is closely limited. In the known process, this requires large recirculation flow rates of the cooling fluid in con- junction with a high energy consumption and high investment costs for the recirculation pumps. Typically, in all known oxidation processes employing a reac- tor with stationarily incorporated catalyst (fixed bed) the conversion of the feedstock into the end product and the re- lated development of heat over the entire catalyst packing is effected very nonuniformly, as it is noted that for instance in the phthalic anhydride process more than 90 % of the reac- tion heat are produced on the inlet-side half of the catalyst packing (main reaction zone). In the reactor according to the known process, reaction temperatures far above the tempera- ture of the tube wall are obtained in the catalyst of the main reaction zone due to the poor heat transfer between catalyst and tube wall, whereas in the outlet-side half of the catalyst packing-(secondary reaction zone) the reaction temperature is very close to that of the cooling fluid. In the inlet-side half of the catalyst there is mostly formed what is called a hot spot, whose temperature can come close to the self-ignition temperature of the product. Even with minor variations of the reaction conditions there is thus a risk of an ignition of the gas with a partial or total damage of the catalyst due to overheating. At high temperatures in the catalyst a feedback effect can in addition accelerate the process of overheating, as experience has shown that with in- creasing reaction temperature the activity of the catalyst and hence the amount of oxidized feedstock is increasing, where the additional generation of heat can easily lead to the self-ignition temperature of the gas being reached. In addition, with increasing activity the selectivity of the catalyst is decreased, i.e. the product yield is decreased and more byproducts are formed. For an economic production of phthalic anhydride there should advantageously be used a reactor feed gas with a rather high concentration of feedstock. For plants of the same perform- ance smaller and thus less expensive apparatuses can be used. In the known process, however, the above-described risk of the over-reaction of the catalyst impedes an increase in con- centration to the extent desirable for such economy. It is the object underlying the invention to further develop the above-mentioned process and increase the product yield with improved operational safety and less expensive opera- tion. It should be possible to employ not only one reactor, but also a plurality of series-connected reactors, in order to be able to expand e.g. existing plants. In accordance with the invention this is achieved in that at least one reactor constitutes a cooling-tube reactor with cooling tubes through which flows a cooling fluid, where in the cooling-tube reactor 40 to 100 wt-% of the total amount of the catalyst are disposed as coating on the outside of the cooling tubes, and the feed mixture containing the feedstock and the molecular oxygen gets in contact with the catalyst layers. Part of the entire amount of catalyst may also be provided on uncooled surfaces in the cooling-tube reactor. Another essential aspect of the invention consists in that for dissipating the reaction heat in the cooling-tube reactor an evaporable liquid is used instead of the molten salt com- monly used in the known tubular reactor, e.g. diphyl may be used as cooling fluid. Subsequently, reference is made in part to the production of phthalic anhydride, but the expla- nations are analogously true for the production of other sub- stances. Preferably, at least half the cooling tubes in the cooling- tube reactor are designed as ribbed tubes with ribs protrud- ing on the outside, the ribs being at least partly coated with catalyst. By using ribbed tubes instead of smooth tubes, the catalytically active surface can be increased considera- bly without increasing the tube diameter or the number of tubes. Expediently, it is ensured that at least 10 wt-% of the entire amount of catalyst are applied on the ribs as coating. The catalyst coatings mostly have layer thicknesses in the range from 0.05 to 5 mm. Coating the cooling tubes with catalyst mass provides an im- proved heat transfer from the catalyst onto and through the tube wall to the cooling fluid. In the main reaction zone of the cooling-tube reactor, the high reaction temperatures un- favorable for the known process can thus be reduced consid- erably. Another advantage of the cooling-tube reactor consists in that by using tubes ribbed on the outside, the gas-side heat exchanging surface can be increased economically to a multi- ple of the usual surface area of the reactors used in the known process. By increasing the surface area, the reaction heat can be dissipated more easily and with a reduced tem- perature gradient, which leads to an additional decrease of the reaction temperatures. By decreasing the reaction tem- peratures, the selectivity of the catalyst and thus the prod- uct yield can usually be increased during oxidation reac- tions. The operational safety is increased to the extent in which the distance between catalyst temperature and self- ignition temperature of the gas can be increased. Another considerable advantage of the decrease of the reac- tion temperature as described in the two preceding paragraphs consists in the possibility of increasing the concentration of the feedstock in the carrier gas far above the concentra- tions possible in the known process without a risk of overre- action, as the reaction temperature can be kept at a suffi- ciently safe distance from the self-ignition temperature of the gas. The resulting reduction of the amount of carrier gas to be conveyed also results in a saving of energy for the op- eration of the carrier gas blower, for the preheating of the carrier gas and, when cooling the product mixture, to a depo- sition of product. An additional saving of energy is obtained by the inventive arrangement of the catalyst mass on the rib surfaces of the cooling tubes. This leads to a considerable reduction of the gas-side pressure loss over the entire cooling-tube reactor as compared to the principle of the random packing with cata- lyst bodies, which is commonly used in the known multi-tube reactor. Apart from the above-described saving of energy there is also obtained a reduction of costs in the blower and its drive required for conveying the carrier gas; due to the lower conveying pressure, the blower may be of a simpler and less expensive construction, and the drive may be designed smaller due to the lower demand of energy. In the cooling-tube reactor, the exchange of old, spent cata- lyst against new catalyst can be performed within a shorter period than in the multi-tube reactor, in which each tube must be filled individually with high accuracy, in the prin- ciple underlying the invention, the individual tube bundles can be designed withdrawable from the reactor, whereby tube bundles with spent catalyst can be replaced by newly coated tube bundles in a time- and cost-saving way. The with- drawability of the bundles also facilitates the removal of the spent catalyst mass and the subsequent new coating, which can be performed without a standstill of the reactor for in- stance in the factory of the catalyst manufacturer. The tube bundles can easily be fabricated in a standard size, whereby it is possible to reuse the newly coated bundles in a reactor of another plant operator. In view of the great reaction heat obtained in the inlet por- tion of the reactor, it is of considerable importance for the economy of the inventive process to keep the number of the cooling tubes at a minimum. This is achieved e.g. by using a material with a higher thermal conductivity than steel (e.g. copper) for the ribs in the main reaction zone. As a result, relatively large ribs of smaller thickness can be used, whereby small temperature differences between rib edge and inner tube can be achieved. In the secondary reaction zone of the cooling-tube reactor, where considerably less reaction heat is obtained, steel ribs of the same dimension as in the main reaction zone may, how- ever, be used. This has the additional advantage of equal geometric conditions in all parts of the reactor. For the heat transfer properties of a ribbed tube, the shape of the individual rib is also important apart from the dimen- sions and the material. A round rib for instance has a more favorable temperature distribution than a rib of square or rectangular shape. Therefore, this shape is particularly suited for zones of the reactor with a high thermal load. In the case of a square or rectangular shape, however, larger heat-exchanging surfaces per unit volume can be accommodated. This is in turn advantageous in zones with a lower thermal load. When using rectangular ribs, it may be advantageous to choose rib length and width for instance in a ratio 2 : 1, so that each rib can be connected with both legs of the U-tube. Under an extremely high thermal load, a rib of variable thickness may also be advantageous, i.e. the rib thickness then is largest close to the inner tube and constantly de- creases towards the edge. In particular in the outlet region of the cooling-tube reac- tor, it is not necessary at all points that the catalyst be cooled intensively by a cooling fluid. Therefore, it may be expedient and economic when 5 to 40 wt—% of the entire amount of catalyst are provided on uncooled surfaces of metal or other suitable materials. It may be advantageous to alter- nately pass the reaction gas over uncooled and cooled cata— lytically coated surfaces. One of the main advantages when using an evaporable cooling fluid in the cooling-tube reactor as compared to the molten salt commonly used in the known multi-tube reactor is the uniform temperature, which only depends on the pressure of the fluid, while at the same time having a better heat trans- fer. Since the absorption of heat is advantageously effected by evaporation and not by an increase in temperature as in the case of molten salt, the recirculation flow rate of the cooling fluid does not play a major role as long as at least so much liquid is supplied to each cooling tube as must be evaporated for the required dissipation of heat. When the recirculated cooling fluid is distributed over the individual cooling tubes, the behavior of the fluid described below may turn out problematic: Due to the different amount of cooling fluid to be evaporated in the cooling tubes of different reaction zones, different pressure losses are formed in the interior of the cooling tubes, where in zones of high heat: transfer with much genera- tion of vapor a high pressure loss is obtained, and in zones of low heat transfer with little generation of vapor a low pressure loss is obtained. When all cooling tubes on the side of the coolant are arranged in parallel, there is obtained less flow through the tubes with much generation of vapor than through tubes with little generation of vapor. For the process, a higher flow through tubes with much generation of vapor is, however, more advantageous than in the case of tubes with little generation of vapor. Expediently, this is achieved by using flow restrictors of a different flow resis- tance on the inlet or outlet side of each individual tube. The flow resistance of the flow restrictors is expediently adapted to the expected generation of vapor in the respective tube. Another advantage of using an evaporable cooling fluid con- sists in that upon separation of the liguid phase in the steam drum, the vapors of the cooling fluid formed in the cooling tubes can be used in other parts of the phthalic an- hydride plant. What is particularly advantageous is the high temperature level of the available vapor. Complex and expen- sive heating means, e.g. an electric heating or a fired, separate heat transfer system, which must be designed for temperatures of at least 300°C, can easily be replaced thereby. This results in considerable savings in the invest- ment and operating costs. It may be advantageous for the process to operate the various reaction zones at different temperatures. This may be ef- fected e.g. by connecting the reactor to two or more separate cooling systems. The cooling fluid may be of the same or of a different composition of substances. The feed mixture may flow through the cooling-tube reactor substantially horizontally or substantially vertically (from the top to the bottom or vice versa). When the reactor is de- signed with vertical flow, space is saved and the cooling tubes may be designed as U-tubes without a welding seam to- wards the gas space and be welded in the reactor such that in the case of a leakage of a welding seam no cooling fluid can get into the gas space. The cooling-tube reactor can be used alone or connected in series with one or more other reactors, which can be provided before and/or behind the cooling reactor. Only as an example, the multi-tube reactor, the fluidized-bed reactor and the liquid-phase reactor should be mentioned here. Embodiments of the process employing a cooling-tube reactor will be explained with reference to the drawing. Reference is made to the production of phthalic anhydride, but another substance may also be generated analogously. In the drawing Fig. 1 shows a flow diagram of the process, Fig. 2 shows a longitudinal section of a cooling tube por- tion in an enlarged representation, Fig. 3 shows a flow diagram of the process with two sepa- rate cooling circuits, and Fig. 4 shows a flow diagram of the process with two reac- tors. In the cooling-tube reactor (1) as shown in Fig. 1, the va- porous feed mixture, which contains orthoxylene or naphtha- lene and molecular oxygen, flows through the upper inlet (2). In the reactor, the catalyst (5) is disposed as coating on the outside of numerous cooling tubes (3) for at least 40 wt-%, cf. Figs. 3 and 4, onto which cooling tubes ring- shaped ribs (4) are welded. In Fig. 2, such tube portion is shown, which on the outside is provided with the catalyst, coating (5) indicated as dots. The reactor (1) of Fig. 1 has a preheating zone (6), a main reaction zone (7) and a secondary reaction zone (8), each having slightly different temperatures. Through these zones, the feed mixture flows downwards and past the catalyst-coated tubes. The vaporous product mixture, which contains phthalic anhydride vapor, leaves the reactor (1) through the outlet (10) and is then cooled in a manner known per se and not rep- resented here (e.g. U.S. patent 4,592,412). All zones (6, 7, 8) have bundles of ribbed tubes (3), through which flows cooling fluid. Beside the cooled tubes, there are also uncooled metallic or non-metallic surfaces (9) with catalyst coating, which above all belong to the secondary re- action zone (8). The bundles of the preheating zone (6) in the inlet region of the reactor (1) may, however, selectively be coated catalytically or be uncoated. The individual tube bundles may either be withdrawably incor- porated in the reactor or be welded to the same. The various parts of a tube bundle are for instance inseparably connected with each other, and the catalyst coating is performed at the finished bundle, which is not represented in detail in the drawing. The largest part of the length of the cooling tubes extends horizontally inside the reactor corresponding to a lying U-shape of the tubes, whereby welds in the gas space are avoided. The problem of thermal stresses due to thermal expansion is also largely avoided thereby. The cooling fluid comes as liquid from line (11) from a steam drum (13) and by means of the pump (12) is distributed over the various tubes via lines (lla to lle). Expediently, the cooling fluid enters the cooling tubes (3) from below as liq- uid and partly evaporates in the cooling tubes, whereby the heat of evaporation is utilized for cooling. As suitable cooling fluid, diphyl may for instance be used, a synthetic heat transfer oil. Partly evaporated cooling fluid leaves the tubes (3) through lines (14a to 14e) and flows through line (14) to the steam drum (13). Cooling fluid vapor escapes in line (15) through the throttle valve (16) and is cooled in a heat exchanger (17). The con- densate formed flows to an intermediate container (19) via line (18) and is then recirculated to the steam drum (13) through the pump (20) and line (21). A pressure control (22) monitors the pressure in the steam drum (13) and keeps it ap- proximately constant at a predetermined value, the throttle valve (16) being actuated if necessary via the signal line (23). By means of the approximately constant pressure, the temperature of the liquid cooling fluid in line (11) and of the partly evaporated cooling fluid in the cooling tubes (3) is kept at the desired temperature, which usually lies in the range from about 300 to 400°C, when diphyl is used as cooling fluid. When two reactors are employed, e.g. when a plant is ex- panded, the other reactor is disposed either before the inlet (2) or behind the outlet (10) of the cooling-tube reactor (1). Fig. 3 shows the same reactor as Fig. 1, but which now is connected to two separate cooling circuits. The secondary re- action zone (8) of the reactor can thereby be operated at temperatures which are independent of the temperatures of the main reaction zone (7). As is represented in Fig. 1, the cooling tubes (3) of the main reaction zone (7) are provided with cooling fluid via lines (lle to lie) from the steam drum (13). The cooling tubes (3) of the secondary reaction zone (8), however, contain cooling fluid from the second steam drum (26), and the temperatures of the cooling fluids in the two steam drums (13 and 26) may be different. Partly evapo- rated and unevaporated cooling fluid flows through lines (27a, 27b) into the collecting line (27) and from there into the steam drum (26). The pressurization of the steam drums (13 and 26), the condensation and recirculation of the con- densed cooling fluid are effected in the same way as de- scribed already in conjunction with Fig. 1. In accordance with another aspect of the process, part of the reaction heat generated is utilized for heating means of the phthalic anhydride plant which are disposed outside the reac- tor region, for instance in the thermal pretreatment and in the distillation of the crude phthalic anhydride. For this purpose, cooling fluid may selectively be withdrawn as vapor via line (15a) or as liquid via line (llf) and be supplied to the means to be heated. The condensed vapor or the cooled liquid is subsequently recirculated into the reactor cooling circuit via line (18a). Fig. 4 shows a similar cooling-tube reactor (1) as Fig. 1, but without the preheating zone (6) and the main reaction zone (7). Before this cooling-tube reactor, another reactor is provided, in the present case a multi-tube reactor of a conventional type, in which the preheating of the feed gas and the main reaction take place. This other reactor is re- ferred to as tubular reactor in the further description of Fig. 4 and in Example 2. In this tubular reactor (31), the vaporous feed mixture which contains orthoxylene or naphthalene and molecular oxygen flows through the inlet (32). In the tubular reactor (31), the catalyst (34) is disposed in granular form inside numer- ous indirectly cooled tubes (33). For dissipating heat and for temperature control a cooling fluid is used, for instance a molten salt which enters the lower part of the tubular reactor (31) via line (35), flows through said lower part on the outside of the tubes and leaves the same again at the upper end via line (36). A de- tailed representation and description of the means for con- veying and recooling the cooling fluid is omitted here, as it is known. The tubular reactor (31) has a preheating zone (6) and a main reaction zone (7). Through these zones, the feed mixture flows downwards inside the tubes (33) filled with catalyst (34). The vaporous product mixture, which contains phthalic anhydride vapor and also considerable amounts of unreacted feedstock as well as possible intermediate products, leaves the tubular reactor (31) through the outlet (37) and flows through line (38) to the cooling-tube reactor (1) for further reaction. Into the reactor (1), the vaporous mixture from the tubular reactor (31) flows through the upper inlet (2). In the reac- tor (1), the catalyst is at least partly disposed on the out- side of numerous cooling tubes (3), as described in conjunc- tion with Fig. 1. In contrast to Fig. 1, the reactor (1) of Fig. 4 only has the secondary reaction zone (8), as the pre- heating zone (6) and the main reaction zone (7) are already disposed in the tubular reactor (31). The vaporous product mixture, which contains phthalic anhydride vapor, but is vir- tually free from unreacted feedstock or possible intermediate products, leaves the reactor (1) through the outlet (10) and is then cooled in a manner known per se, which is not repre- sented here (e.g. U.S. patent 4,592,412). The dissipation of heat and the temperature control of the reactor (1) of Fig. 4 are effected in the same way as de- scribed in Fig. 1 by means of a partly evaporated cooling fluid. Example 1: There is employed a procedure comprising only one reactor corresponding to Fig. 1. The cooling-tube reactor (1) is de- signed for an hourly volume flow of air preheated to 180°C in an amount of 60,000 m3 in the normal condition, which air is mixed with 6000 kg orthoxylene as feedstock in the vaporous condition. The temperature of the gas mixture entering the reactor through the inlet (2) is 150°C. The orthoxylene used is reacted in the main reaction zone (7) for 90 %, and the rest is reacted in the secondary reaction zone (8) to obtain the end product phthalic anhydride and the byproducts. The reactor is disposed vertical, the gas entering from above. The reactor has a rectangular shape with a width of 3200 mm and a depth of 3100 mm, the height is about 6 m, the overall heights of the gas-side hoods at the inlet and outlet being included therein. Inside the reactor, there are each disposed two ribbed-tube heat exchanger bundles at the same level one beside the other. The entering gas mixture first of all flows through the preheating zone (6), which is made of a pair of U-shaped ribbed-tube heat exchanger bundles made of steel and arranged in parallel, and is thereby heated to 250°C. Liquid diphyl flows through the tubes (3) of the bun- dles, which diphyl enters the tubes (lle) with a temperature of 370°C and when flowing through the tubes is cooled to about 320°C at the outlet (14e). The preheated feed gas subsequently flows through the main reaction zone (7), which consists of 15 pairs of U-shaped ribbed-tube heat exchanger bundles arranged in parallel, which on their outer surface are coated with a catalytic mass. The same chiefly consists of vanadium pentoxide and ti- tanium dioxide. During the contact of the gas mixture with the catalyst, the orthoxylene contained therein reacts with the oxygen likewise contained therein to form phthalic anhy- dride and other byproducts, reaction heat being released. Through the inside of the tubes (3), diphyl is flowing, which absorbs the reaction heat by evaporation, whereby the cata- lyst temperature can be maintained. Each bundle is equipped with 21 U-tubes (3) arranged in parallel. The U-shaped ribbed tubes have an outside diameter of 30 mm and a wall thickness of 2 mm, the straight tube length is 3.0 m. The ribs have an outside diameter of 60 and a thickness of 0.5 mm. The ma- terial for ribs and inner tubes is copper. The distance be- tween two adjacent ribs is 1.5 mm, so that each U-tube has 3000 ribs. The rib surface area of each bundle provided with a catalytic coating is 273.1 m2. The thermal output of all ribbed tubes of the main reaction zone (7), which will be dissipated in the case of an expected 90 % conversion of the orthoxylene used, is about 22.600 kw. Due to the required tenperature differences with respect to the heat transfer inside the tube wall and in the rib as well as between tube wall and the evaporating diphyl, the tempera- ture increases from the inside of the tube to the rib edge. In this example it is 370°C for the evaporating liquid, 374°C for the rib at the rib base, and 384°C at the rib edge. Due to a favorable arrangement of each pair of bundles, each off- set by 180°C, with respect to the one preceding in the gas stream and the succeeding one, the overall height of the en- tire main reaction zone is 1950 mm. The reaction gas mixture subsequently flows through the sec- ondary reaction zone (8), in which the remaining orthoxylene is converted. The reaction heat generated is about 2500 kW. The secondary reaction zone consists of 20 ribbed-tube heat exchanger bundles and 10 elements without cooling of honey- comb-shaped catalytically coated monolithic ceramic bodies (9). As regards design, dimension and arrangement, the ribbed-tube heat exchanger bundles are largely identical with the bundles used for the main reaction zone (7) with the dif- ference that for the secondary reaction zone the thickness of the ribs is 1 mm and the bundles are completely made of steel. Like in the main reaction zone, the bundles are offset in pairs. After the bottommost 5 pairs of bundles there is each provided a layer of uncooled elements of monolithic bod- ies. The diphyl recirculation pump (12) sucks in the liquid diphyl required for the individual reactor zones from a steam drum (13), which is mounted at a level of 11 m above the pump. As required, it supplies different amounts of liquid diphyl to the individual reactor zones, namely 100 m3/h to the preheat- ing zone, 2000 m3/h to the main reaction zone and 200 ma/h to the secondary reaction zone. The diphyl emerging from the bundles of the individual reac- tor zones partly in liquid form, partly in vaporous form is recirculated to the steam drum (13) via a common collecting line (14). In the steam drum, vapor phase and liquid phase are separated from each other, and the liquid is available for another recirculation. The pressure in the steam drum is 7.5 bar, which corresponds to a saturated steam temperature of 370°C. The amount of diphyl vapor discharged via line (15) is 400.000 kg/h. The vaporous diphyl discharged from the steam drum (13) through line (15) is largely reliquefied in several condens- ers (17), collected in the container (19) and recirculated to the steam drum (13) via the pump (20). In one of the condensers, the condensation heat is used for generating saturated steam of 51 bar. In another condenser connected in parallel thereto, the saturated steam thus gen- erated is superheated to a temperature of 330°C and is thus available for driving a steam turbine. By means of the heat transferred per hour by the condensation of 400,000 kg diphyl vapor, about 37 tons of superheated steam are thus generated. For heating the distillation of the crude phthalic anhydride with a heat requirement of 2000 kW, another part of vaporous cooling fluid is withdrawn in an amount of 32,000 kg/h. The cooling fluid condensed in the heat—consuming devices flows back through line (18a) into the coolant system of the reac- tor. Example 2: There is employed a procedure corresponding to Fig. 4. The two reactors (31) and (1) are designed for a volume flow of carrier gas (preheated ambient air) of 60.000 Nm3/h, which carrier gas is loaded with orthoxylene in an amount of 7200 kg/h as feedstock in the vaporous condition. The temperature of the gas mixture entering the tubular reactor (31) through the inlet (32) is 143°C. The tubular reactor (31) has 15,000 vertically arranged steel tubes (33) with an inside diameter of 25 mm and a length of 3.4 m. The catalyst (34) inside the tubes has a filling level of 3.1 m and consists of ring-shaped carrier bodies of an in- ert ceramic material with a diameter of 7 mm and a length of 7 mm, which are provided with a thin layer of catalytic mate- rial, chiefly consisting of vanadium pentoxide. The gas mixture of preheated air and vaporous orthoxylene, which is used as feedstock, enters the tubes (33) from above with a temperature of 144"C. When flowing through the upper, catalyst-free part of the tubes with a length of about 150 mm and the uppermost 100 mm of the catalyst packing, the gas is heated in the preheating zone (6) to a reaction temperature of 330°C. Due to the subsequently starting reaction, the gas is heated even more and upon exceeding the cooling fluid temperature of 350°C dissipates heat to the tube wall. At a level of 500 mm below the inlet of the catalyst bed, the gas reaches its highest temperature of 410°C (hot spot). Below the hot spot, the gas is cooled again and upon leaving the tubes (33) reaches a temperature of 360°C. Upon contact of the gas mixture used with the catalyst (34) in the main reaction zone (7) , the orthoxylene contained therein reacts to form phthalic anhydride and other byprod- ucts, reaction heat being released. The reaction conditions are such that about 10 % of the orthoxylene used leave the tubular reactor (31) unreacted together with the reaction gas via line (38). The thermal output to be dissipated from the reactor (31) by the cooling fluid in the case of an expected 90 % conversion of the orthoxylene used is about 18.000 kW. The cooling-tube reactor (1) is arranged vertically, as is represented in Fig. 4, the gas mixture entering from above. This reactor has a rectangular shape with a width of 3.2 m and a depth of 3.1 m, and the height is about 4 m, the over- all heights of the gas-side hoods at the inlet and at the outlet being included. Inside the reactor (1), two ribbed- tube heat exchanger bundles (3) or two uncooled elements (9) are each disposed one beside the other at the same level, as will be described in detail further below. The reaction gas from the tubular reactor (31) flows around the bundles (3) and the uncooled elements (9) in the cooling- tube reactor (1). Both the bundles and the elements are coated on their surface with a catalytic mass. When the gas mixture gets in contact with the catalyst, the orthoxylene contained therein and the intermediate products formed in the tubular reactor (31) react with the oxygen likewise contained therein to form phthalic anhydride and other byproducts, re- action heat being released. The reaction heat generated in the reactor (1) is about 3000 kW. The reactor (1) has 20 ribbed-tube heat exchanger bundles (3) and 10 elements (9) formed of honeycomb-shaped monolithic ce- ramic bodies without cooling. The U-shaped ribbed tubes (3) of the bundles are provided on their outer surface with a catalytic coating, the nonolithic bodies of the elements (9) being provided on the inside of the honeycombs. In both of them, the catalytic mass largely consists of vanadium pentox- ide. The bundles are arranged in pairs. After the bottommost 5 pairs of bundles there is each provided a layer of uncooled elements (9) of monolithic bodies, which are likewise ar- ranged in pairs. Diphyl flows through the inside of the tubes (3) of the bun- dles, which diphyl absorbs the reaction heat by evaporation, whereby the catalyst temperature can be maintained. Each bundle is fitted with 21 U-tubes (3) arranged in paral- lel. The U-shaped ribbed tubes have an outside diameter of 30 mm and a wall thickness of 2 mm, the straight tube length is 3.0 m. The ribs have an outside diameter of 60 mm and a thickness of 1 mm. The material for ribs and inner tubes is steel. The distance of two adjacent ribs is 1.5 mm, so that each U-tube has 2400 ribs. The catalytically coated rib sur- face of each bundle is 218.5 m2. Due to the required temperature differences for heat transfer inside the tube wall and in the rib as well as between tube wall and the diphyl to be evaporated, the temperature in- creases from the inside of the tube to the rib edge. In this example it is 350°C for the evaporating liquid, 350.5°C for the rib at the rib base, and 360°C at the rib edge. The uncooled elements consist of catalytically coated honey- comb-shaped monolithic elements with an edge length of 150 x 150 x 150 mm. Since the reaction heat released in the honey- combs is not dissipated, the gas is heated by 1 to 5°C when flowing through a monolithic element. This elevated tempera- ture is decreased again when the gas flows through the subse- quent cooled bundle by releasing heat at the ribbed tubes. Due to a favorable arrangement of each pair of bundles offset by 180° with respect to the one preceding and succeeding in the gas stream, the overall height of the entire reaction zone (8) is 2.1 m. The liquid diphyl required for cooling the reactor tubes is sucked in by the diphyl recirculation pump (12) from a steam drum (13), which is mounted at a level of 10 m above the pump. It conveys 250 m3/h. The diphyl emerging from the bundles via lines (14a) and (14b) partly in liquid form, partly in vaporous form is re- circulated to the steam drum (13) via a common collecting line (14). In the steam drum, vapor phase and liquid phase are separated from each other, and the liquid is available for another recirculation. By means of a throttle valve (16) in the vapor outlet line (15), which is actuated by a pres- sure controller (22) via a pulse line (23), the pressure in the steam drum is constantly maintained at a pressure of 5.6 bar, which corresponds to the saturated steam temperature of 350°C. The amount of diphyl vapor discharged via line (15) is about 45.000 kg/h. The discharge pressure of the pump for overcoming the flow resistances in the bundles with the in- corporated throttle bodies as well as in the tube lines sup- plying and discharging diphyl is 1 bar. The vaporous diphyl discharged from the steam drum (13) through line (15) is largely religuefied in a condenser (17), collected in the container (19) and recirculated to the steam drum (13) via the pump (20). The condensation heat is used for generating saturated steam of 51 bar. By means of the heat transferred by the condensation of 45.000 kg/h diphyl vapor, 4 t/h steam.are thus generated. WE CLAIM: 1. A process for catalytically generating organic substances by partial oxidation of an organic feedstock in the presence of molecular oxygen at temperatures in the range from 200 to 500°C in one or more series-connected reactors containing a catalyst for generating a gaseous product mixture, characterized in that at least one reactor constitutes a cooling-tube reactor with cooling tubes through which flows a cooling fluid, where in the cooling-tube reactor 40 to 100 wt-% of the total amount of the catalyst are disposed as coating on the outside of the cooling tubes and the feed mixture containing the feedstock and the molecular oxygen gets in contact with the catalyst layers, and that a plurality of cooling systems are provided, which supply cooling fluid to the cooling tubes of the cooling-tube reactor and withdraw said cooling fluid therefrom. 2. The process as claimed in claim 1, wherein in the cooling-tube reactor at least half the cooling tubes constitute ribbed tubes with ribs protruding on the outside, the ribs being at least partly coated with catalyst. 3. The process as claimed in claim 2, wherein at least 10 wt-% of the total amount of catalyst of the cooling-tube reactor are applied as coating on the ribs. 4. The process as claimed in claim 1 or any of the preceding claims, wherein 5 to 40 wt-% of the total amount of catalyst of the cooling- tube reactor are provided on uncooled metal surfaces. 5. The process as claimed in claim 1 or any of the preceding claims, wherein the feed mixture flows through the cooling-tube reactor substantially vertically and the cooling tubes are in part horizontally arranged in the reactor. 6. The process as claimed in any of claims 1 to 5, wherein the cooling fluid enters the cooling tubes of the cooling-tube reactor as liquid and partly evaporates in the cooling tubes. 7. The process as claimed in claim 1 or any of the preceding claims, wherein the catalyst coatings in the cooling-tube reactor have layer thicknesses in the range from 0.05 to 5 mm. 8. The process as claimed in claim 1 or any of the preceding claims, wherein in the cooling-tube reactor one of the substances phthalic anhydride, maleic anhydride, acrylic acid, acetic acid or anthraquinone is at least partly generated. 9. The process as claimed in claim 1 or any of the preceding claims, wherein the cooling-tube reactor cooperates with at least one other reactor, which is disposed before or behind the cooling-tube reactor. 10.The process as claimed in claim 9, wherein the other reactor is a multi-tube reactor with a catalyst bed in the tubes, a fluidized-bed reactor or a liquid-phase reactor. The process is performed in the gas phase in the presence of molecular oxygen at temperatures in the range from 200 to 500°C in at least one reactor, which constitutes a cooling- tube reactor and contains a catalyst. Cooling liquid flows through the cooling tubes of the reactor, and from the reac- tor a gaseous product mixture is withdrawn. 40 to 100 wt-% of the total amount of catalyst of the cooling-tube reactor are disposed as coating on the outside of the cooling tubes, wherein the feed mixture containing the feedstock and the mo- lecular oxygen gets in contact with the catalyst layers. Preferably, at least half the cooling tubes constitute ribbed tubes with ribs protruding on the outside, the ribs being at least partly coated with catalyst.

Full Text

Process for Catalytically Generating Organic
Substances by Partial Oxidation
This invention relates to a process for catalytically gener-
ating organic substances by partial oxidation of an organic
feedstock in the presence of molecular oxygen at temperatures
in the range from 200 to 500°C in one or more series-
connected reactors containing a catalyst for generating a
gaseous product mixture.
The conversion of the feedstock into the end product is ef-
fected in one or more oxidation steps, where in each oxida--
tion step the individual molecules of the feedstock can re-
lease one ore more atoms of hydrogen and/or carbon and also
bind individual oxygen atoms. The carbon released combines
with oxygen to form carbon dioxide and/or carbon monoxide,
and the hydrogen released combines with oxygen to form water-
Part of the feedstock usually is oxidized completely to form
carbon dioxide and/or carbon monoxide, and another part is
converted to other organic substances. The oxygen reguired
for the oxidation reaction is withdrawn from the carrier gas
(usually ambient air preheated in a heat exchanger) . Due to
the various oxidation reactions, the entire process is highly
exothermal. In accordance with the invention, e.g. phthalic
anhydride, maleic anhydride is generated from butane or ben-
zene, acrylic acid is generated from propylene, and acetic
acid is generated from C4 hydrocarbons or anthraquinone.
A typical product generated according to this principle in
large amounts all over the world is phthalic anhydride from a
feed mixture which contains orthoxylene or naphthalene and
molecular oxygen in the concentration usual for ambient air
or increased by additional enrichment. The generation of
phthalic anhydride is effected at temperatures in the range
from 200 to 500°C in a reactor containing a catalyst, through
which reactor flows a cooling fluid and from which reactor
gaseous product mixture containing phthalic anhydride vapor
is withdrawn. This generation of phthalic anhydride will be
represented below by way of example for all organic products
to be generated by partial catalytic oxidation in the gas
phase, where it is also possible to use a plurality of se-
ries-connected reactors.
Such process employing a multi-tube reactor is known from
U.S. patent 4,592,412. The reactor comprises a multitude of
vertical tubes in which the granular catalyst is disposed.
The catalyst on the basis of vanadium pentoxide may also be
formed of carrier elements coated with a catalytic mass. A
cooling fluid flows around the outside of the numerous tubes
containing the catalyst, in order to dissipate the heat re-
leased by the exothermal reaction. In the known process mol-
ten salt is preferably used as cooling fluid, which requires
complex and therefore expensive apparatuses.
Apart from high investment costs, the multi—tube reactor de-
scribed above is also problematic for various other reasons.
Due to the high weight and large dimensions of the reactor
(150 to 250 t and 7 to 9 m diameter alone for the tube bundle
with usual reactor sizes), cost-intensive special transports
are required. Required pre-material in non-commercial dimen-
sions as well as labor-intensive manufacture lead to long de-
livery times in the manufacture of the reactor. The random
bed of the catalyst in the individual tubes causes a large
pressure loss on the gas side in conjunction with a high con-
sumption of energy for the conveyance of the feed gas. in ad-
dition, this type of construction involves a number of fur-
ther, mostly process- or reaction-related problems which will
be discussed in detail below.
Modern high-performance catalysts require very uniform tem-
peratures to evolve a high selectivity and to achieve a long
service life. The admissible temperature difference of the
cooling fluid inside the space surrounding the tubes there-
fore is closely limited. In the known process, this requires
large recirculation flow rates of the cooling fluid in con-
junction with a high energy consumption and high investment
costs for the recirculation pumps.
Typically, in all known oxidation processes employing a reac-
tor with stationarily incorporated catalyst (fixed bed) the
conversion of the feedstock into the end product and the re-
lated development of heat over the entire catalyst packing is
effected very nonuniformly, as it is noted that for instance
in the phthalic anhydride process more than 90 % of the reac-
tion heat are produced on the inlet-side half of the catalyst
packing (main reaction zone). In the reactor according to the
known process, reaction temperatures far above the tempera-
ture of the tube wall are obtained in the catalyst of the
main reaction zone due to the poor heat transfer between
catalyst and tube wall, whereas in the outlet-side half of
the catalyst packing-(secondary reaction zone) the reaction
temperature is very close to that of the cooling fluid. In
the inlet-side half of the catalyst there is mostly formed
what is called a hot spot, whose temperature can come close
to the self-ignition temperature of the product. Even with
minor variations of the reaction conditions there is thus a
risk of an ignition of the gas with a partial or total damage
of the catalyst due to overheating. At high temperatures in
the catalyst a feedback effect can in addition accelerate the
process of overheating, as experience has shown that with in-
creasing reaction temperature the activity of the catalyst
and hence the amount of oxidized feedstock is increasing,
where the additional generation of heat can easily lead to
the self-ignition temperature of the gas being reached. In
addition, with increasing activity the selectivity of the
catalyst is decreased, i.e. the product yield is decreased
and more byproducts are formed.
For an economic production of phthalic anhydride there should
advantageously be used a reactor feed gas with a rather high
concentration of feedstock. For plants of the same perform-
ance smaller and thus less expensive apparatuses can be used.
In the known process, however, the above-described risk of
the over-reaction of the catalyst impedes an increase in con-
centration to the extent desirable for such economy.
It is the object underlying the invention to further develop
the above-mentioned process and increase the product yield
with improved operational safety and less expensive opera-
tion. It should be possible to employ not only one reactor,
but also a plurality of series-connected reactors, in order
to be able to expand e.g. existing plants.
In accordance with the invention this is achieved in that at
least one reactor constitutes a cooling-tube reactor with
cooling tubes through which flows a cooling fluid, where in
the cooling-tube reactor 40 to 100 wt-% of the total amount
of the catalyst are disposed as coating on the outside of the
cooling tubes, and the feed mixture containing the feedstock
and the molecular oxygen gets in contact with the catalyst
layers. Part of the entire amount of catalyst may also be
provided on uncooled surfaces in the cooling-tube reactor.
Another essential aspect of the invention consists in that
for dissipating the reaction heat in the cooling-tube reactor
an evaporable liquid is used instead of the molten salt com-
monly used in the known tubular reactor, e.g. diphyl may be
used as cooling fluid. Subsequently, reference is made in
part to the production of phthalic anhydride, but the expla-
nations are analogously true for the production of other sub-
stances.
Preferably, at least half the cooling tubes in the cooling-
tube reactor are designed as ribbed tubes with ribs protrud-
ing on the outside, the ribs being at least partly coated
with catalyst. By using ribbed tubes instead of smooth tubes,
the catalytically active surface can be increased considera-
bly without increasing the tube diameter or the number of
tubes. Expediently, it is ensured that at least 10 wt-% of
the entire amount of catalyst are applied on the ribs as
coating. The catalyst coatings mostly have layer thicknesses
in the range from 0.05 to 5 mm.
Coating the cooling tubes with catalyst mass provides an im-
proved heat transfer from the catalyst onto and through the
tube wall to the cooling fluid. In the main reaction zone of
the cooling-tube reactor, the high reaction temperatures un-
favorable for the known process can thus be reduced consid-
erably.
Another advantage of the cooling-tube reactor consists in
that by using tubes ribbed on the outside, the gas-side heat
exchanging surface can be increased economically to a multi-
ple of the usual surface area of the reactors used in the
known process. By increasing the surface area, the reaction
heat can be dissipated more easily and with a reduced tem-
perature gradient, which leads to an additional decrease of
the reaction temperatures. By decreasing the reaction tem-
peratures, the selectivity of the catalyst and thus the prod-
uct yield can usually be increased during oxidation reac-
tions. The operational safety is increased to the extent in
which the distance between catalyst temperature and self-
ignition temperature of the gas can be increased.
Another considerable advantage of the decrease of the reac-
tion temperature as described in the two preceding paragraphs
consists in the possibility of increasing the concentration
of the feedstock in the carrier gas far above the concentra-
tions possible in the known process without a risk of overre-
action, as the reaction temperature can be kept at a suffi-
ciently safe distance from the self-ignition temperature of
the gas. The resulting reduction of the amount of carrier gas
to be conveyed also results in a saving of energy for the op-
eration of the carrier gas blower, for the preheating of the
carrier gas and, when cooling the product mixture, to a depo-
sition of product.
An additional saving of energy is obtained by the inventive
arrangement of the catalyst mass on the rib surfaces of the
cooling tubes. This leads to a considerable reduction of the
gas-side pressure loss over the entire cooling-tube reactor
as compared to the principle of the random packing with cata-
lyst bodies, which is commonly used in the known multi-tube
reactor. Apart from the above-described saving of energy
there is also obtained a reduction of costs in the blower and
its drive required for conveying the carrier gas; due to the
lower conveying pressure, the blower may be of a simpler and
less expensive construction, and the drive may be designed
smaller due to the lower demand of energy.
In the cooling-tube reactor, the exchange of old, spent cata-
lyst against new catalyst can be performed within a shorter
period than in the multi-tube reactor, in which each tube
must be filled individually with high accuracy, in the prin-
ciple underlying the invention, the individual tube bundles
can be designed withdrawable from the reactor, whereby tube
bundles with spent catalyst can be replaced by newly coated
tube bundles in a time- and cost-saving way. The with-
drawability of the bundles also facilitates the removal of
the spent catalyst mass and the subsequent new coating, which
can be performed without a standstill of the reactor for in-
stance in the factory of the catalyst manufacturer. The tube
bundles can easily be fabricated in a standard size, whereby
it is possible to reuse the newly coated bundles in a reactor
of another plant operator.
In view of the great reaction heat obtained in the inlet por-
tion of the reactor, it is of considerable importance for the
economy of the inventive process to keep the number of the
cooling tubes at a minimum. This is achieved e.g. by using a
material with a higher thermal conductivity than steel (e.g.
copper) for the ribs in the main reaction zone. As a result,
relatively large ribs of smaller thickness can be used,
whereby small temperature differences between rib edge and
inner tube can be achieved.
In the secondary reaction zone of the cooling-tube reactor,
where considerably less reaction heat is obtained, steel ribs
of the same dimension as in the main reaction zone may, how-
ever, be used. This has the additional advantage of equal
geometric conditions in all parts of the reactor.
For the heat transfer properties of a ribbed tube, the shape
of the individual rib is also important apart from the dimen-
sions and the material. A round rib for instance has a more
favorable temperature distribution than a rib of square or
rectangular shape. Therefore, this shape is particularly
suited for zones of the reactor with a high thermal load. In
the case of a square or rectangular shape, however, larger
heat-exchanging surfaces per unit volume can be accommodated.
This is in turn advantageous in zones with a lower thermal
load. When using rectangular ribs, it may be advantageous to
choose rib length and width for instance in a ratio 2 : 1, so
that each rib can be connected with both legs of the U-tube.
Under an extremely high thermal load, a rib of variable
thickness may also be advantageous, i.e. the rib thickness
then is largest close to the inner tube and constantly de-
creases towards the edge.
In particular in the outlet region of the cooling-tube reac-
tor, it is not necessary at all points that the catalyst be
cooled intensively by a cooling fluid. Therefore, it may be
expedient and economic when 5 to 40 wt—% of the entire amount
of catalyst are provided on uncooled surfaces of metal or
other suitable materials. It may be advantageous to alter-
nately pass the reaction gas over uncooled and cooled cata—
lytically coated surfaces.
One of the main advantages when using an evaporable cooling
fluid in the cooling-tube reactor as compared to the molten
salt commonly used in the known multi-tube reactor is the
uniform temperature, which only depends on the pressure of
the fluid, while at the same time having a better heat trans-
fer. Since the absorption of heat is advantageously effected
by evaporation and not by an increase in temperature as in
the case of molten salt, the recirculation flow rate of the
cooling fluid does not play a major role as long as at least
so much liquid is supplied to each cooling tube as must be
evaporated for the required dissipation of heat.
When the recirculated cooling fluid is distributed over the
individual cooling tubes, the behavior of the fluid described
below may turn out problematic:
Due to the different amount of cooling fluid to be evaporated
in the cooling tubes of different reaction zones, different
pressure losses are formed in the interior of the cooling
tubes, where in zones of high heat: transfer with much genera-
tion of vapor a high pressure loss is obtained, and in zones
of low heat transfer with little generation of vapor a low
pressure loss is obtained. When all cooling tubes on the side
of the coolant are arranged in parallel, there is obtained
less flow through the tubes with much generation of vapor
than through tubes with little generation of vapor. For the
process, a higher flow through tubes with much generation of
vapor is, however, more advantageous than in the case of
tubes with little generation of vapor. Expediently, this is
achieved by using flow restrictors of a different flow resis-
tance on the inlet or outlet side of each individual tube.
The flow resistance of the flow restrictors is expediently
adapted to the expected generation of vapor in the respective
tube.
Another advantage of using an evaporable cooling fluid con-
sists in that upon separation of the liguid phase in the
steam drum, the vapors of the cooling fluid formed in the
cooling tubes can be used in other parts of the phthalic an-
hydride plant. What is particularly advantageous is the high
temperature level of the available vapor. Complex and expen-
sive heating means, e.g. an electric heating or a fired,
separate heat transfer system, which must be designed for
temperatures of at least 300°C, can easily be replaced
thereby. This results in considerable savings in the invest-
ment and operating costs.
It may be advantageous for the process to operate the various
reaction zones at different temperatures. This may be ef-
fected e.g. by connecting the reactor to two or more separate
cooling systems. The cooling fluid may be of the same or of a
different composition of substances.
The feed mixture may flow through the cooling-tube reactor
substantially horizontally or substantially vertically (from
the top to the bottom or vice versa). When the reactor is de-
signed with vertical flow, space is saved and the cooling
tubes may be designed as U-tubes without a welding seam to-
wards the gas space and be welded in the reactor such that in
the case of a leakage of a welding seam no cooling fluid can
get into the gas space.
The cooling-tube reactor can be used alone or connected in
series with one or more other reactors, which can be provided
before and/or behind the cooling reactor. Only as an example,
the multi-tube reactor, the fluidized-bed reactor and the
liquid-phase reactor should be mentioned here.
Embodiments of the process employing a cooling-tube reactor
will be explained with reference to the drawing. Reference is
made to the production of phthalic anhydride, but another
substance may also be generated analogously.
In the drawing
Fig. 1 shows a flow diagram of the process,
Fig. 2 shows a longitudinal section of a cooling tube por-
tion in an enlarged representation,
Fig. 3 shows a flow diagram of the process with two sepa-
rate cooling circuits, and
Fig. 4 shows a flow diagram of the process with two reac-
tors.
In the cooling-tube reactor (1) as shown in Fig. 1, the va-
porous feed mixture, which contains orthoxylene or naphtha-
lene and molecular oxygen, flows through the upper inlet (2).
In the reactor, the catalyst (5) is disposed as coating on
the outside of numerous cooling tubes (3) for at least
40 wt-%, cf. Figs. 3 and 4, onto which cooling tubes ring-
shaped ribs (4) are welded. In Fig. 2, such tube portion is
shown, which on the outside is provided with the catalyst,
coating (5) indicated as dots.
The reactor (1) of Fig. 1 has a preheating zone (6), a main
reaction zone (7) and a secondary reaction zone (8), each
having slightly different temperatures. Through these zones,
the feed mixture flows downwards and past the catalyst-coated
tubes. The vaporous product mixture, which contains phthalic
anhydride vapor, leaves the reactor (1) through the outlet
(10) and is then cooled in a manner known per se and not rep-
resented here (e.g. U.S. patent 4,592,412).
All zones (6, 7, 8) have bundles of ribbed tubes (3), through
which flows cooling fluid. Beside the cooled tubes, there are
also uncooled metallic or non-metallic surfaces (9) with
catalyst coating, which above all belong to the secondary re-
action zone (8). The bundles of the preheating zone (6) in
the inlet region of the reactor (1) may, however, selectively
be coated catalytically or be uncoated.
The individual tube bundles may either be withdrawably incor-
porated in the reactor or be welded to the same. The various
parts of a tube bundle are for instance inseparably connected
with each other, and the catalyst coating is performed at the
finished bundle, which is not represented in detail in the
drawing. The largest part of the length of the cooling tubes
extends horizontally inside the reactor corresponding to a
lying U-shape of the tubes, whereby welds in the gas space
are avoided. The problem of thermal stresses due to thermal
expansion is also largely avoided thereby.
The cooling fluid comes as liquid from line (11) from a steam
drum (13) and by means of the pump (12) is distributed over
the various tubes via lines (lla to lle). Expediently, the
cooling fluid enters the cooling tubes (3) from below as liq-
uid and partly evaporates in the cooling tubes, whereby the
heat of evaporation is utilized for cooling. As suitable
cooling fluid, diphyl may for instance be used, a synthetic
heat transfer oil. Partly evaporated cooling fluid leaves the
tubes (3) through lines (14a to 14e) and flows through line
(14) to the steam drum (13).
Cooling fluid vapor escapes in line (15) through the throttle
valve (16) and is cooled in a heat exchanger (17). The con-
densate formed flows to an intermediate container (19) via
line (18) and is then recirculated to the steam drum (13)
through the pump (20) and line (21). A pressure control (22)
monitors the pressure in the steam drum (13) and keeps it ap-
proximately constant at a predetermined value, the throttle
valve (16) being actuated if necessary via the signal line
(23). By means of the approximately constant pressure, the
temperature of the liquid cooling fluid in line (11) and of
the partly evaporated cooling fluid in the cooling tubes (3)
is kept at the desired temperature, which usually lies in the
range from about 300 to 400°C, when diphyl is used as cooling
fluid.
When two reactors are employed, e.g. when a plant is ex-
panded, the other reactor is disposed either before the inlet
(2) or behind the outlet (10) of the cooling-tube reactor
(1).
Fig. 3 shows the same reactor as Fig. 1, but which now is
connected to two separate cooling circuits. The secondary re-
action zone (8) of the reactor can thereby be operated at
temperatures which are independent of the temperatures of the
main reaction zone (7). As is represented in Fig. 1, the
cooling tubes (3) of the main reaction zone (7) are provided
with cooling fluid via lines (lle to lie) from the steam drum
(13). The cooling tubes (3) of the secondary reaction zone
(8), however, contain cooling fluid from the second steam
drum (26), and the temperatures of the cooling fluids in the
two steam drums (13 and 26) may be different. Partly evapo-
rated and unevaporated cooling fluid flows through lines
(27a, 27b) into the collecting line (27) and from there into
the steam drum (26). The pressurization of the steam drums
(13 and 26), the condensation and recirculation of the con-
densed cooling fluid are effected in the same way as de-
scribed already in conjunction with Fig. 1.
In accordance with another aspect of the process, part of the
reaction heat generated is utilized for heating means of the
phthalic anhydride plant which are disposed outside the reac-
tor region, for instance in the thermal pretreatment and in
the distillation of the crude phthalic anhydride. For this
purpose, cooling fluid may selectively be withdrawn as vapor
via line (15a) or as liquid via line (llf) and be supplied to
the means to be heated. The condensed vapor or the cooled
liquid is subsequently recirculated into the reactor cooling
circuit via line (18a).
Fig. 4 shows a similar cooling-tube reactor (1) as Fig. 1,
but without the preheating zone (6) and the main reaction
zone (7). Before this cooling-tube reactor, another reactor
is provided, in the present case a multi-tube reactor of a
conventional type, in which the preheating of the feed gas
and the main reaction take place. This other reactor is re-
ferred to as tubular reactor in the further description of
Fig. 4 and in Example 2.
In this tubular reactor (31), the vaporous feed mixture which
contains orthoxylene or naphthalene and molecular oxygen
flows through the inlet (32). In the tubular reactor (31),
the catalyst (34) is disposed in granular form inside numer-
ous indirectly cooled tubes (33).
For dissipating heat and for temperature control a cooling
fluid is used, for instance a molten salt which enters the
lower part of the tubular reactor (31) via line (35), flows
through said lower part on the outside of the tubes and
leaves the same again at the upper end via line (36). A de-
tailed representation and description of the means for con-
veying and recooling the cooling fluid is omitted here, as it
is known.
The tubular reactor (31) has a preheating zone (6) and a main
reaction zone (7). Through these zones, the feed mixture
flows downwards inside the tubes (33) filled with catalyst
(34). The vaporous product mixture, which contains phthalic
anhydride vapor and also considerable amounts of unreacted
feedstock as well as possible intermediate products, leaves
the tubular reactor (31) through the outlet (37) and flows
through line (38) to the cooling-tube reactor (1) for further
reaction.
Into the reactor (1), the vaporous mixture from the tubular
reactor (31) flows through the upper inlet (2). In the reac-
tor (1), the catalyst is at least partly disposed on the out-
side of numerous cooling tubes (3), as described in conjunc-
tion with Fig. 1. In contrast to Fig. 1, the reactor (1) of
Fig. 4 only has the secondary reaction zone (8), as the pre-
heating zone (6) and the main reaction zone (7) are already
disposed in the tubular reactor (31). The vaporous product
mixture, which contains phthalic anhydride vapor, but is vir-
tually free from unreacted feedstock or possible intermediate
products, leaves the reactor (1) through the outlet (10) and
is then cooled in a manner known per se, which is not repre-
sented here (e.g. U.S. patent 4,592,412).
The dissipation of heat and the temperature control of the
reactor (1) of Fig. 4 are effected in the same way as de-
scribed in Fig. 1 by means of a partly evaporated cooling
fluid.
Example 1:
There is employed a procedure comprising only one reactor
corresponding to Fig. 1. The cooling-tube reactor (1) is de-
signed for an hourly volume flow of air preheated to 180°C in
an amount of 60,000 m3 in the normal condition, which air is
mixed with 6000 kg orthoxylene as feedstock in the vaporous
condition. The temperature of the gas mixture entering the
reactor through the inlet (2) is 150°C. The orthoxylene used
is reacted in the main reaction zone (7) for 90 %, and the
rest is reacted in the secondary reaction zone (8) to obtain
the end product phthalic anhydride and the byproducts.
The reactor is disposed vertical, the gas entering from
above. The reactor has a rectangular shape with a width of
3200 mm and a depth of 3100 mm, the height is about 6 m, the
overall heights of the gas-side hoods at the inlet and outlet
being included therein. Inside the reactor, there are each
disposed two ribbed-tube heat exchanger bundles at the same
level one beside the other. The entering gas mixture first of
all flows through the preheating zone (6), which is made of a
pair of U-shaped ribbed-tube heat exchanger bundles made of
steel and arranged in parallel, and is thereby heated to
250°C. Liquid diphyl flows through the tubes (3) of the bun-
dles, which diphyl enters the tubes (lle) with a temperature
of 370°C and when flowing through the tubes is cooled to
about 320°C at the outlet (14e).
The preheated feed gas subsequently flows through the main
reaction zone (7), which consists of 15 pairs of U-shaped
ribbed-tube heat exchanger bundles arranged in parallel,
which on their outer surface are coated with a catalytic
mass. The same chiefly consists of vanadium pentoxide and ti-
tanium dioxide. During the contact of the gas mixture with
the catalyst, the orthoxylene contained therein reacts with
the oxygen likewise contained therein to form phthalic anhy-
dride and other byproducts, reaction heat being released.
Through the inside of the tubes (3), diphyl is flowing, which
absorbs the reaction heat by evaporation, whereby the cata-
lyst temperature can be maintained. Each bundle is equipped
with 21 U-tubes (3) arranged in parallel. The U-shaped ribbed
tubes have an outside diameter of 30 mm and a wall thickness
of 2 mm, the straight tube length is 3.0 m. The ribs have an
outside diameter of 60 and a thickness of 0.5 mm. The ma-
terial for ribs and inner tubes is copper. The distance be-
tween two adjacent ribs is 1.5 mm, so that each U-tube has
3000 ribs. The rib surface area of each bundle provided with
a catalytic coating is 273.1 m2.
The thermal output of all ribbed tubes of the main reaction
zone (7), which will be dissipated in the case of an expected
90 % conversion of the orthoxylene used, is about 22.600 kw.
Due to the required tenperature differences with respect to
the heat transfer inside the tube wall and in the rib as well
as between tube wall and the evaporating diphyl, the tempera-
ture increases from the inside of the tube to the rib edge.
In this example it is 370°C for the evaporating liquid, 374°C
for the rib at the rib base, and 384°C at the rib edge. Due
to a favorable arrangement of each pair of bundles, each off-
set by 180°C, with respect to the one preceding in the gas
stream and the succeeding one, the overall height of the en-
tire main reaction zone is 1950 mm.
The reaction gas mixture subsequently flows through the sec-
ondary reaction zone (8), in which the remaining orthoxylene
is converted. The reaction heat generated is about 2500 kW.
The secondary reaction zone consists of 20 ribbed-tube heat
exchanger bundles and 10 elements without cooling of honey-
comb-shaped catalytically coated monolithic ceramic bodies
(9). As regards design, dimension and arrangement, the
ribbed-tube heat exchanger bundles are largely identical with
the bundles used for the main reaction zone (7) with the dif-
ference that for the secondary reaction zone the thickness of
the ribs is 1 mm and the bundles are completely made of
steel. Like in the main reaction zone, the bundles are offset
in pairs. After the bottommost 5 pairs of bundles there is
each provided a layer of uncooled elements of monolithic bod-
ies.
The diphyl recirculation pump (12) sucks in the liquid diphyl
required for the individual reactor zones from a steam drum
(13), which is mounted at a level of 11 m above the pump. As
required, it supplies different amounts of liquid diphyl to
the individual reactor zones, namely 100 m3/h to the preheat-
ing zone, 2000 m3/h to the main reaction zone and 200 ma/h to
the secondary reaction zone.
The diphyl emerging from the bundles of the individual reac-
tor zones partly in liquid form, partly in vaporous form is
recirculated to the steam drum (13) via a common collecting
line (14). In the steam drum, vapor phase and liquid phase
are separated from each other, and the liquid is available
for another recirculation. The pressure in the steam drum is
7.5 bar, which corresponds to a saturated steam temperature
of 370°C. The amount of diphyl vapor discharged via line (15)
is 400.000 kg/h.
The vaporous diphyl discharged from the steam drum (13)
through line (15) is largely reliquefied in several condens-
ers (17), collected in the container (19) and recirculated to
the steam drum (13) via the pump (20).
In one of the condensers, the condensation heat is used for
generating saturated steam of 51 bar. In another condenser
connected in parallel thereto, the saturated steam thus gen-
erated is superheated to a temperature of 330°C and is thus
available for driving a steam turbine. By means of the heat
transferred per hour by the condensation of 400,000 kg diphyl
vapor, about 37 tons of superheated steam are thus generated.
For heating the distillation of the crude phthalic anhydride
with a heat requirement of 2000 kW, another part of vaporous
cooling fluid is withdrawn in an amount of 32,000 kg/h. The
cooling fluid condensed in the heat—consuming devices flows
back through line (18a) into the coolant system of the reac-
tor.
Example 2:
There is employed a procedure corresponding to Fig. 4. The
two reactors (31) and (1) are designed for a volume flow of
carrier gas (preheated ambient air) of 60.000 Nm3/h, which
carrier gas is loaded with orthoxylene in an amount of 7200
kg/h as feedstock in the vaporous condition. The temperature
of the gas mixture entering the tubular reactor (31) through
the inlet (32) is 143°C.
The tubular reactor (31) has 15,000 vertically arranged steel
tubes (33) with an inside diameter of 25 mm and a length of
3.4 m. The catalyst (34) inside the tubes has a filling level
of 3.1 m and consists of ring-shaped carrier bodies of an in-
ert ceramic material with a diameter of 7 mm and a length of
7 mm, which are provided with a thin layer of catalytic mate-
rial, chiefly consisting of vanadium pentoxide.
The gas mixture of preheated air and vaporous orthoxylene,
which is used as feedstock, enters the tubes (33) from above
with a temperature of 144"C. When flowing through the upper,
catalyst-free part of the tubes with a length of about 150 mm
and the uppermost 100 mm of the catalyst packing, the gas is
heated in the preheating zone (6) to a reaction temperature
of 330°C.
Due to the subsequently starting reaction, the gas is heated
even more and upon exceeding the cooling fluid temperature of
350°C dissipates heat to the tube wall. At a level of 500 mm
below the inlet of the catalyst bed, the gas reaches its
highest temperature of 410°C (hot spot). Below the hot spot,
the gas is cooled again and upon leaving the tubes (33)
reaches a temperature of 360°C.
Upon contact of the gas mixture used with the catalyst (34)
in the main reaction zone (7) , the orthoxylene contained
therein reacts to form phthalic anhydride and other byprod-
ucts, reaction heat being released. The reaction conditions
are such that about 10 % of the orthoxylene used leave the
tubular reactor (31) unreacted together with the reaction gas
via line (38).
The thermal output to be dissipated from the reactor (31) by
the cooling fluid in the case of an expected 90 % conversion
of the orthoxylene used is about 18.000 kW.
The cooling-tube reactor (1) is arranged vertically, as is
represented in Fig. 4, the gas mixture entering from above.
This reactor has a rectangular shape with a width of 3.2 m
and a depth of 3.1 m, and the height is about 4 m, the over-
all heights of the gas-side hoods at the inlet and at the
outlet being included. Inside the reactor (1), two ribbed-
tube heat exchanger bundles (3) or two uncooled elements (9)
are each disposed one beside the other at the same level, as
will be described in detail further below.
The reaction gas from the tubular reactor (31) flows around
the bundles (3) and the uncooled elements (9) in the cooling-
tube reactor (1). Both the bundles and the elements are
coated on their surface with a catalytic mass. When the gas
mixture gets in contact with the catalyst, the orthoxylene
contained therein and the intermediate products formed in the
tubular reactor (31) react with the oxygen likewise contained
therein to form phthalic anhydride and other byproducts, re-
action heat being released. The reaction heat generated in
the reactor (1) is about 3000 kW.
The reactor (1) has 20 ribbed-tube heat exchanger bundles (3)
and 10 elements (9) formed of honeycomb-shaped monolithic ce-
ramic bodies without cooling. The U-shaped ribbed tubes (3)
of the bundles are provided on their outer surface with a
catalytic coating, the nonolithic bodies of the elements (9)
being provided on the inside of the honeycombs. In both of
them, the catalytic mass largely consists of vanadium pentox-
ide.
The bundles are arranged in pairs. After the bottommost 5
pairs of bundles there is each provided a layer of uncooled
elements (9) of monolithic bodies, which are likewise ar-
ranged in pairs.
Diphyl flows through the inside of the tubes (3) of the bun-
dles, which diphyl absorbs the reaction heat by evaporation,
whereby the catalyst temperature can be maintained.
Each bundle is fitted with 21 U-tubes (3) arranged in paral-
lel. The U-shaped ribbed tubes have an outside diameter of 30
mm and a wall thickness of 2 mm, the straight tube length is
3.0 m. The ribs have an outside diameter of 60 mm and a
thickness of 1 mm. The material for ribs and inner tubes is
steel. The distance of two adjacent ribs is 1.5 mm, so that
each U-tube has 2400 ribs. The catalytically coated rib sur-
face of each bundle is 218.5 m2.
Due to the required temperature differences for heat transfer
inside the tube wall and in the rib as well as between tube
wall and the diphyl to be evaporated, the temperature in-
creases from the inside of the tube to the rib edge. In this
example it is 350°C for the evaporating liquid, 350.5°C for
the rib at the rib base, and 360°C at the rib edge.
The uncooled elements consist of catalytically coated honey-
comb-shaped monolithic elements with an edge length of 150 x
150 x 150 mm. Since the reaction heat released in the honey-
combs is not dissipated, the gas is heated by 1 to 5°C when
flowing through a monolithic element. This elevated tempera-
ture is decreased again when the gas flows through the subse-
quent cooled bundle by releasing heat at the ribbed tubes.
Due to a favorable arrangement of each pair of bundles offset
by 180° with respect to the one preceding and succeeding in
the gas stream, the overall height of the entire reaction
zone (8) is 2.1 m.
The liquid diphyl required for cooling the reactor tubes is
sucked in by the diphyl recirculation pump (12) from a steam
drum (13), which is mounted at a level of 10 m above the
pump. It conveys 250 m3/h.
The diphyl emerging from the bundles via lines (14a) and
(14b) partly in liquid form, partly in vaporous form is re-
circulated to the steam drum (13) via a common collecting
line (14). In the steam drum, vapor phase and liquid phase
are separated from each other, and the liquid is available
for another recirculation. By means of a throttle valve (16)
in the vapor outlet line (15), which is actuated by a pres-
sure controller (22) via a pulse line (23), the pressure in
the steam drum is constantly maintained at a pressure of 5.6
bar, which corresponds to the saturated steam temperature of
350°C. The amount of diphyl vapor discharged via line (15) is
about 45.000 kg/h. The discharge pressure of the pump for
overcoming the flow resistances in the bundles with the in-
corporated throttle bodies as well as in the tube lines sup-
plying and discharging diphyl is 1 bar.
The vaporous diphyl discharged from the steam drum (13)
through line (15) is largely religuefied in a condenser (17),
collected in the container (19) and recirculated to the steam
drum (13) via the pump (20). The condensation heat is used
for generating saturated steam of 51 bar. By means of the
heat transferred by the condensation of 45.000 kg/h diphyl
vapor, 4 t/h steam.are thus generated.
WE CLAIM:
1. A process for catalytically generating organic substances by partial
oxidation of an organic feedstock in the presence of molecular
oxygen at temperatures in the range from 200 to 500°C in one or
more series-connected reactors containing a catalyst for generating
a gaseous product mixture, characterized in that at least one
reactor constitutes a cooling-tube reactor with cooling tubes
through which flows a cooling fluid, where in the cooling-tube
reactor 40 to 100 wt-% of the total amount of the catalyst are
disposed as coating on the outside of the cooling tubes and the
feed mixture containing the feedstock and the molecular oxygen
gets in contact with the catalyst layers, and that a plurality of
cooling systems are provided, which supply cooling fluid to the
cooling tubes of the cooling-tube reactor and withdraw said cooling
fluid therefrom.
2. The process as claimed in claim 1, wherein in the cooling-tube
reactor at least half the cooling tubes constitute ribbed tubes with
ribs protruding on the outside, the ribs being at least partly coated
with catalyst.
3. The process as claimed in claim 2, wherein at least 10 wt-% of the
total amount of catalyst of the cooling-tube reactor are applied as
coating on the ribs.
4. The process as claimed in claim 1 or any of the preceding claims,
wherein 5 to 40 wt-% of the total amount of catalyst of the cooling-
tube reactor are provided on uncooled metal surfaces.
5. The process as claimed in claim 1 or any of the preceding claims,
wherein the feed mixture flows through the cooling-tube reactor
substantially vertically and the cooling tubes are in part
horizontally arranged in the reactor.
6. The process as claimed in any of claims 1 to 5, wherein the cooling
fluid enters the cooling tubes of the cooling-tube reactor as liquid
and partly evaporates in the cooling tubes.
7. The process as claimed in claim 1 or any of the preceding claims,
wherein the catalyst coatings in the cooling-tube reactor have layer
thicknesses in the range from 0.05 to 5 mm.
8. The process as claimed in claim 1 or any of the preceding claims,
wherein in the cooling-tube reactor one of the substances phthalic
anhydride, maleic anhydride, acrylic acid, acetic acid or
anthraquinone is at least partly generated.
9. The process as claimed in claim 1 or any of the preceding claims,
wherein the cooling-tube reactor cooperates with at least one other
reactor, which is disposed before or behind the cooling-tube
reactor.
10.The process as claimed in claim 9, wherein the other reactor is a
multi-tube reactor with a catalyst bed in the tubes, a fluidized-bed
reactor or a liquid-phase reactor.
The process is performed in the gas phase in the presence of
molecular oxygen at temperatures in the range from 200 to
500°C in at least one reactor, which constitutes a cooling-
tube reactor and contains a catalyst. Cooling liquid flows
through the cooling tubes of the reactor, and from the reac-
tor a gaseous product mixture is withdrawn. 40 to 100 wt-% of
the total amount of catalyst of the cooling-tube reactor are
disposed as coating on the outside of the cooling tubes,
wherein the feed mixture containing the feedstock and the mo-
lecular oxygen gets in contact with the catalyst layers.
Preferably, at least half the cooling tubes constitute ribbed
tubes with ribs protruding on the outside, the ribs being at
least partly coated with catalyst.

Documents:

114-KOLNP-2003-FORM 27 1.1.pdf

114-KOLNP-2003-FORM 27.pdf

114-KOLNP-2003-FORM-27.pdf

114-kolnp-2003-granted-abstract.pdf

114-kolnp-2003-granted-claims.pdf

114-kolnp-2003-granted-correspondence.pdf

114-kolnp-2003-granted-description (complete).pdf

114-kolnp-2003-granted-examination report.pdf

114-kolnp-2003-granted-form 1.pdf

114-kolnp-2003-granted-form 18.pdf

114-kolnp-2003-granted-form 26.pdf

114-kolnp-2003-granted-form 5.pdf

114-kolnp-2003-granted-priority document.pdf

114-kolnp-2003-granted-reply to examination report.pdf

114-kolnp-2003-granted-specification.pdf

114-kolnp-2003-granted-translated copy of priority document.pdf

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Patent Number

224447

Indian Patent Application Number

114/KOLNP/2003

PG Journal Number

42/2008

Publication Date

17-Oct-2008

Grant Date

14-Oct-2008

Date of Filing

29-Jan-2003

Name of Patentee

MG TECHNOLGIES, AG

Applicant Address

BOCKENHEIMER LANDSTR. 73-77, 60325 FRANTFURT MAIN

Inventors:

#	Inventor's Name	Inventor's Address
1	FRANZ, VOLKEV	KIESSTR.29, D-60486 FRANKFURT AM MAIN
2	DOMES, HELMUTH	KOLNER STR. 19,D-63179 OBERTSHAUSEN

PCT International Classification Number

C07C 51/265

PCT International Application Number

PCT/EP01/08002

PCT International Filing date

2001-07-11

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	100 38 755.1	2000-08-09	Germany
2	101 04 406.2	2001-02-01	Germany