Patent Application
20070270597
The process of the invention is advantageously carried out isothermally, with a temperature which increases over the length of the reactor or with a combination of a temperature which increases over the length of the reactor and isothermal operation.
The process of the invention is advantageously carried out at an oxygen partial pressure of from 0.6 bar to 50 bar, preferably from 2 bar to 50 bar, particularly preferably from 3 bar to 50 bar, in particular from 4 bar to 50 bar.
The oxygen partial pressure, which is above that in the prior art, can advantageously be achieved either by increasing the total pressure to from 3 bar to 50 bar, preferably from 5 bar to 50 bar, in particular from 10 bar to 50 bar, or by introducing an increased amount of oxygen of from 20% by volume to 98% by volume, preferably from 40% by volume to 98% by volume, in particular from 60% by volume to 98% by volume, or by a combination of increasing the total pressure and increasing the amount of oxygen introduced.
In the process of the invention, the ratio of oxygen to hydrocarbon in the feed stream is advantageously from 10 to 50, preferably from 20 to 50, in particular from 30 to 50.
The n-butane concentration of the feed stream fed to the reactor unit is from 0.5 to 10% by volume, preferably from 0.8 to 10% by volume, particularly preferably from 1 to 10% by volume and very particularly preferably from 2 to 10% by volume.
A high n-butane concentration of from 2 to 10% by volume, preferably from 2.5 to 10% by volume, is preferred, particularly at high ratios of oxygen to butane of advantageously greater than 10, preferably greater than 20.
The n-butane conversion per pass through the reactor is from 40 to 100%, preferably from 50 to 95%, particularly preferably from 70 to 95% and in particular from 85 to 95%, of the n-butane in the stream at the inlet.
In the process of the invention, a GHSV (gas hourly space velocity) of preferably from 2000 to 10 000 h−1 and particularly preferably from 3000 to 8000 h−1, based on the volume of the stream fed at the inlet standardized to 0° C. and 0.1013 MPa abs and based on the reaction volume which is filled with catalyst or whose geometric surface is coated, is set in the reactor unit via the amount of the stream at the inlet.
The process of the invention can be carried out in two preferred process variants, viz. the variant with a “single pass” and the variant with “recirculation”. In the case of a “single pass”, maleic anhydride and, if appropriate, oxygenated hydrocarbon by-products are removed from the output from the reactor and the remaining gas mixture is discharged and, if appropriate, utilized thermally. In the case of “recirculation”, maleic anhydride and, if appropriate, oxygenated hydrocarbon by-products are likewise removed from the output from the reactor but part or all of the remaining gas mixture, which comprises unreacted hydrocarbon, is recirculated to the reactor. A further variant of “recirculation” is the removal of unreacted hydrocarbon and recirculation of this to the reactor.
The reaction products or the product stream can, if appropriate, be diluted at the end of the reactor or at the reactor outlet by addition of substances which are inert under the respective reaction conditions, for example water or nitrogen, so that a nonexplosive product stream is obtained. A nonexplosive product stream can also advantageously be obtained by means of a pressure increase. This product stream can then be worked up using conventional work-up units.
When using n-butane, a volatile phosphorus compound is advantageously added to the gas in the process of the invention to ensure a long catalyst operating life and to achieve a further increase in conversion, selectivity, yield, space velocity of the catalyst and space-time yield. Its concentration at the beginning, i.e. at the reactor inlet, is from 0.2 to 20 ppm by volume of the volatile phosphorus compounds based on the total volume of gas at the reactor inlet. A content of from 0.5 to 5 ppm by volume is preferred. For the purposes of the present invention, volatile phosphorus compounds are all phosphorus-comprising compounds which are in the gaseous state at the desired concentration under the use conditions. Preference is given to using triethyl phosphate or trimethyl phosphate as volatile phosphorus compound.
Generally known microchannel reactors are suitable for carrying out the process of the invention. In contrast to conventional reaction apparatuses, e.g. tube/shell-and-tube or fluidized-bed reactors, microchannel reactors offer, owing to the very small dimensions of the reaction channels (dimension in at least one spatial direction of <3 mm, preferably about 1 mm and less), inherent safety, i.e. propagation of flames or explosions is not possible (the diameter is below the minimal quench diameter). In terms of the way in which the process is carried out, there is increased freedom in terms of the choice of the organic/oxygen or air ratio, since explosion limits within the reactor do not have to be taken into account or adhered to. Design of the reactor for maximum explosion pressures is not necessary. Furthermore, short diffusion paths within the microstructures lead to greatly improved mass transfers and heat transfers which can be many times greater than those of conventional reaction apparatuses. Transport limitations which frequently occur in conventional shell-and-tube reactors are accordingly largely absent. Furthermore, the high heat removal potential of microchannel reactors makes more precise temperature control possible, so that, for example, the formation of hot spots can be suppressed and operation with an optimally selected axial temperature profile can be made possible. A runaway reaction in the reactor is effectively prevented.
Comprehensive descriptions of the configuration of microchannel reactors which in terms of their basic structure are suitable for carrying out the process of the invention may be found, for example, in US 2006/0036106 A1 and also in WO 02/18042 A1, which are hereby incorporated by reference.
For the purposes of the present invention, microchannel reactors or microreactors are reactors in general whose characteristic dimensions of the reaction channels, i.e. the dimensions in at least one spatial direction, e.g. height or width or diameter, are in the range from a few microns to a few millimeters, preferably <3 mm.
In large-scale industrial applications, too, the characteristic dimensions of the reaction space are retained. The increase in capacity is achieved by numbering-up, so that costly and time-consuming scale-up is dispensed with. The size of a production plant is thus flexible and can be inexpensively matched to requirements.
To introduce the catalysts into the microchannel reactor, it is possible to use all methods known to those skilled in the art. A comprehensive description of the prior art relating to this may be found in WO 01/12312 A2 and the references cited therein. The catalyst can be present, for example, as a wall coating which is bound firmly to the wall of the microreactor (cf. WO 01/12312 A2, pages 1 and 2, and references therein), or be introduced in the form of crushed material or shaped bodies as a fixed bed into the channels of the microreactor (cf. Tonkovich et al. (reference in WO 01/12312 A2, page 2)). Furthermore, the catalyst can be present as an insert, for example in the form of a metal foil or a metal mesh or metal gauze which is advantageously provided with a surface which is advantageous for the catalytic properties (e.g. a metal oxide surface). The active component is advantageously applied to or fixed on the surface (cf. WO 01/12312 A2, in particular pages 6 and 7, brief descriptions of the drawings).
In the case of a fixed bed, the catalyst which can be used in the process of the invention advantageously comprises shaped bodies having an essentially spherical geometry.
As catalysts in microchannel reactors, it is possible to use all catalysts which are generally suitable for the preparation of maleic anhydride, if appropriate with a suitable support material. Preference is given to using catalysts which are suitable for the conversion of n-butane, propane or benzene into maleic anhydride.
In the preparation of maleic anhydride from n-butane, it is advantageous to use vanadium-, phosphorus- and oxygen-comprising catalysts having a phosphorus/vanadium atomic ratio of from 0.9 to 1.5, preferably from 0.9 to 1.2, in particular from 1.0 to 1.1. The mean oxidation state of the vanadium is advantageously from +3.9 to +4.4 and preferably from 4.0 to 4.3. The catalysts used according to the invention advantageously have a BET surface area of >15 m2/g, preferably from >15 to 50 m2/g and in particular from >15 to 40 m2/g. They advantageously have a pore volume of >0.1 ml/g, preferably from 0.15 to 0.5 ml/g und in particular from 0.15 to 0.4 ml/g. The bulk density of the catalysts used according to the invention is advantageously from 0.5 to 1.5 kg/l and preferably from 0.5 to 1.0 kg/l.
The catalysts can comprise the vanadium-, phosphorus- and oxygen-comprising active composition in, for example, pure, undiluted form as “all-active catalyst” or diluted with a preferably oxidic support material as “mixed catalyst”. Suitable support materials for the mixed catalysts are, for example, aluminum oxide, silicon dioxide, aluminosilicates, zirconium dioxide, titanium dioxide or mixtures thereof. Preference is given to all-active catalysts.
The catalysts can further comprise additional promoters. Suitable promoters are the elements of groups 1 to 15 of the Periodic Table and their compounds. Suitable promoters are described, for example, in WO 97/12674 and WO 95/26817 and in U.S. Pat. No. 5,137,860, U.S. Pat. No. 5,296,436, U.S. Pat. No. 5,158,923 and U.S. Pat. No. 4,795,818. Preferred further promoters are compounds of the elements molybdenum, iron, zinc, hafnium, zirconium, titanium, chromium, manganese, nickel, copper, boron, silicon, tin, niobium, cobalt, lithium, antimony and bismuth, in particular molybdenum, iron, zinc, bismuth. The total content of promoters in the finished catalyst is generally not more than about 5% by weight, in each case calculated as oxide.
The catalysts can also comprise auxiliaries such as tableting aids or pore formers.
Furthermore, heteropolyacids known to those skilled in the art can also be used as catalytically active composition. Selectivities to maleic anhydride of 90% are usually achieved at n-butane conversions of 15% and selectivities of 46% to maleic anhydride are usually achieved at conversions of 62% (Davis et al., Angew. Chem. (2002) 114, 886-888; Holles et al., J. Catal. (2003) 218, 42-66).
The catalysts mentioned can be produced by all methods known to those skilled in the art. The catalysts used according to the invention can, for example, be produced as described in the patents U.S. Pat. No. 5,275,996 and U.S. Pat. No. 5,641,722 or the published specification WO 97/12674. Shaping is preferably effected by tableting.
In the prior art, the production of the catalyst is generally described as a multistage process in which a catalyst precursor is firstly produced and this is subsequently converted into the active form by calcination. The catalyst precursors which can be used in the process of the invention can be produced, for example, as described in the documents U.S. Pat. No. 5,275,996, U.S. Pat. No. 5,641,722, WO 97/12674, WO 01/68626, WO 01/68245, WO 02/22257, WO 02/34387, DE 102 11 449 A1, DE 102 11 445 A1, DE 102 11 447 A1, DE 102 11 446 A1 and DE 102 35 355 A1.
In the process of the invention, the activation can, in contrast to the prior art, take place directly in the microchannel reactor.
In the preparation of maleic anhydride from propane, it is advantageous to use catalysts based on vanadium and molybdenum mixed oxides (Tang et al., Appl. Catal. A 287 2005 197). The V/Mo ratio advantageously ranges from 1/9 to 3/7, preferably from 1/4 to 3/7. The active composition can advantageously comprise doping components in addition to the two main components vanadium and molybdenum in order to increase the activity or selectivity of the catalyst. Ag, Cu and Zn are particularly preferably used as doping components. The production of the catalyst is carried out by methods known to those skilled in the art, which are described, for example, in Tang et al., Appl. Catal. A 287 2005 197.
In the preparation of maleic anhydride from benzene, it is advantageous to use coated catalysts having supported active compositions based on molybdenum and vanadium mixed oxides. The MoN ratio advantageously ranges from 1/2.5 to 1/5. The proportion of active composition in the coated catalyst is advantageously from 10 to 20% by weight. The active composition can advantageously comprise doping components in addition to the two main components molybdenum and vanadium in order to increase the activity or selectivity of the catalyst. Advantageous doping components are Ag, Na (Bielanski et al., Bull. Acad. Pol. Sci., Ser. Sci. Chim. (1976) 24(5), 415-23), rare earths such as, for example, Tb, Dy, Gd or Er (Khiteeva et al., Zh. Fiz. Kihm. (1981) 55(8), 2121-2), Cr, Co (Bielanski et al., Bull. Acad. Pol. Sci. Ser. Sci. Chim. (1976) 24(6), 485-92, Zn, Cu (Ionita et al., Rev. Chim. (Bucharest) (1968) 19(2), 105-7). Other promoters which are used in the oxidation of o-xylene to phthalic anhydride, e.g. Cs and P, can also be used advantageously. The production of the catalyst is carried out by methods known to those skilled in the art, which are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2000 electronic release, Chapter 2.2.1.
The process of the invention gives a yield which is improved by 10% compared to the prior art. Furthermore, the separate process step of activation can be saved, since activation can be carried out directly in the microchannel reactor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06114165.1 | May 2006 | EP | regional |
