JPS5929679A - Manufacture of maleic acid anhydride - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the catalytic gas phase oxidation of n-methane to maleic anhydride. More specifically, the present invention relates to an improved method for producing maleic anhydride, in which (1) an oxidizing feedstock with a high n-butane content is treated with a catalyst graded for reactivity; (2) oxidize the reactor effluent to maleic anhydride at a relatively low co-flow conversion of n-butane in a heat transfer medium cooled tubular reaction zone containing one fixed bed; (3) removing the bulk of the maleic anhydride from it and recycling the bulk of the effluent remaining after separation to the reaction zone.

Catalytic gas phase oxidation of n-methane to maleic anhydride in a tubular reaction zone cooled with a heat transfer medium is well known.

Typically, molecular oxygen, n-methane and balas) (
A gaseous feedstock consisting of a ballast) gas is placed over a fixed bed of oxidation catalyst in a single or multi-tube reaction tube for about 30 minutes.
Temperatures from 0°C to about 650°C and about 10 to about 75 ps
ia (about 0.07 to about 5.3 Kq/2). This oxidation reaction is highly exothermic and
To maintain the desired reaction zone temperature, a heat transfer medium such as oil or molten salt is circulated around the reaction tube. Typically, the temperature of the heat transfer medium is adjusted to provide adequate cooling of the hottest point in the reaction zone. Maleic anhydride, by-product oxidized hydrocarbon,
An effluent typically containing inert gas and unreacted n-butane and oxygen is removed from the reaction zone and the maleic anhydride is substantially separated therefrom.

Currently, a known economical method for producing maleic anhydride from n-butane uses air as the source of molecular oxygen;
Therefore, due to the nitrogen content of the air, the level of nitrogen in the reaction zone effluent is such that even when nitrogen is separated from unreacted n-butane, recycling of the effluent is not economically possible. It can be characterized as a flow-through air oxidation process. In view of the explosive nature of the mixture of n-methane and air, the concentration of n-butane in the flowthrough is limited to approximately 1.8 molar, which is even slightly within the explosive limits. As a result of these constraints on n-butane in the feed, the amount of maleic anhydride that can be produced per unit volume of reaction zone is limited. Additionally, because it is not possible to recycle unreacted n-methane, it is typically wasted, resulting in a lower amount of maleic anhydride produced per amount of n-butane consumed than desired.

To improve productivity and consumption of n-butane, use an oxidation feedstock containing n-methane at a higher concentration than that typically used in the continuous flow air oxidation process described above (7); Alternatively, it has been proposed to recycle the reaction zone effluent. For example, U.S. Patent Nos. 3 and 8
No. 99,516 (Ditzkason) states that the space-time yield and catalyst selectivity for maleic anhydride are determined by combining n-butane and substantially pure (at least 95%) molecular oxygen in a ratio of at least 1:4. It is disclosed that improvements are achieved by the use of feedstocks containing at least 20 molar parts of n-butane and less than 80 molar parts of oxygen. Co-assigned U.S. Pat. No. 3,904,652 (Frank) discloses that 1.7 molar or higher n-butane, 6 to 16 moles of oxygen, and 70 to 9
Using a feed containing 5 molar parts of an inert gas, preferably nitrogen, with a continuous flow conversion of 60 parts to 70 parts of n-butane, and recycling the reaction effluent after maleic anhydride separation. Discloses that it can be used in combination with Similarly, U.S. Pat. No. 4.044.027 (Andern et al.)
8) is primarily aimed at improving product quality and yield by rapid cooling of the reactor effluent to avoid decomposition of maleic anhydride and avoid the formation of colored products. However, using a feedstock containing at least 1.5 molar n-butane and less than 20 molar oxygen,
The use of a once-through conversion of less than 0% in combination with recycling of a portion of the reactor effluent after separation of maleic anhydride and at least some unreacted D-butane is disclosed.

German Patent No. 2.544.972 (Höggist) discloses a process for the production of maleic anhydride from a mixture of butane isomers, in which a feedstock of butane and air is oxidized, followed by to remove the effluent from the reaction zone.
The maleic anhydride is separated from the effluent, 75% to 98% of the effluent remaining after separation is recycled to the reaction zone with the addition of make-up air and butane, and the remaining 2% to 25% of the effluent is transferred onto activated carbon. Butane is adsorbed, fresh air is used to desorb the butane, and the butane-containing air is fed to the reaction zone.

Despite the improvements reported in the above-mentioned patents, the improvements are not entirely satisfactory from the point of view of the operation of the reaction zone cooled by a heat transfer medium. As mentioned above, the temperature of the heat transfer medium is determined with reference to the temperature at the hottest point of the reaction zone.

The point, also called the "hot spot", exists at the point where the oxidation rate is maximum and the reaction is largely exothermic. If the dependence of the reaction rate on the reactant concentration is positive, the reaction zone hotspot typically exists near the feed end of the reaction zone because it is where the reactant concentration is greatest. In the butane-rich process described above, the feed end reaction rate is even greater than in typical air oxidation processes due to the increased n-butane concentration. Above a certain n-butane concentration, the reaction rate Heat cannot be removed due to poor heat transfer in the reaction zone. As a result, catalyst damage and/or thermal runaway of the oxidation reaction may occur. The n-butane concentration is not high enough to cause thermal runaway. Even so, attempts to remove the heat of reaction from the reaction zone feed end by adjusting the heating medium temperature lead to problems downstream from the feed end.Thus, the reactants are consumed as they go from the reaction zone feed end to the exit, and thus the reaction The heat carrier provides adequate cooling in the vicinity of the reaction zone hot spot, but the reduced amount of heat released downstream from the feed end reduces the degree of cooling provided near the feed end. Therefore, downstream from the feed end of the reaction zone, the reaction rate is limited not only by the reduced reactant concentration, but also by excessive cooling, resulting in a loss of productivity.

It has been proposed to improve temperature control and operation in various processes involving exothermic reactions by varying the catalytic activity along the length of the reaction zone, and such proposals are in connection with the present invention. It's interesting. British Patent No. 721,412 (Chembatents) discloses that an olefin, particularly ethylene, is placed in a reaction tube immersed in a cooling medium and filled with a supported silver catalyst such that the activity increases from the inlet to the outlet of the reaction tube. It is disclosed that it oxidizes.

Diluting the effluent remaining after product removal and then recycling the diluted effluent is also disclosed.

According to the patentee, the graded catalyst results in increased productivity and allows for sustained operation without the formation of sporanes, but it does not limit the grading of the catalyst bed to explosive range carbonization. There is no suggestion that it could be used to enable operation on the hydrogen rich side, nor is there any disclosure of the use of graded catalysts as a means of improving productivity. Furthermore, the results in the silver-catalyzed oxidation of ethylene are not readily transferable to the process for the oxidation of n-butane to maleic anhydride.

G, F, Froment is Industria] and
Engineering Chemistry, 59 (
2k pp, 18-27 (1967), a binary scheme (
proposed a two-dimensional model) and proposed that by appropriately diluting the catalyst with inert packing in the early stage of the catalyst bed, the average reaction zone temperature can be increased to eliminate hot spots. There is.

Calvirael and Calderbank are British Ch.
chemical engineer, ng, 14
(9), pp-1199-1201 (1969), discusses the above-mentioned problem of temperature regulation in tubular reactors and attempts to increase conversion without making the reaction zone more sensitive to variations in operating conditions. Proper dilution of the catalyst in the vicinity of the reaction zone hot spot is proposed as a means of achieving this. The authors also assume that the catalyst dilution equation required to achieve the desired temperature gradient is a function of temperature and conversion.

Calderbank, Caldwell and Ross Press
, Oxford, pp. 9ro-106 (1971
), the grading is based on one-dimen
sional ) method or six-di
(mensional) method.

Calderbank and quad 9 wells are Chθmj-Cal
Reaction Engineering, Adv
ances in ChemistryF3erjθB
, 1 09 AC8, Washington
D, C, pp. 38-43 (1972), Rating discusses the oxidation of 00-xylene to phthalic anhydride in a tubular/reaction tube containing a catalyst.

Smith and Carvelli, The Canadian Jo
urnalof Chemical Engineer
ing, 53. pp, Ro47-349 (19
75) discloses the use of partially impregnated catalysts primarily in connection with the oxidation of naphthalene to phthalic anhydride;
It is concluded that improved yields, higher average reaction zone temperatures, and reduced catalyst consumption are obtained.

Although mathematically related to the teachings of the aforementioned British patents and the theories presented in the cited references, they do not permit smooth operation on the hydrocarbon-rich side of the explosion range, or The ratings do not disclose or suggest the use of catalyst beds as an economically practical means of recycling in the production of maleic anhydride from -butane.

These references also fail to disclose the actual reactivity gradients that are useful for oxidizing a given feedstock to a given product under given conditions.

Furthermore, the specific processes named by each document, such as the oxidation of ethylene to ethylene oxide, the oxidation of O-xylene to phthalic anhydride, and the oxidation of naphthalene to phthalic anhydride, are The teachings and theory are sufficiently different to predict direct application to n-butane oxidation.

For example, for a given volume of gas, operation at a hydrocarbon concentration at the lower explosive limit results in twice the exotherm in the case of n-butane oxidation than in the case of oxidation of 0-xylene to phthalic anhydride. cause Therefore, in the former case, the requirements for heat removal are more severe.

From the foregoing, it can be recognized that it would be desirable to provide a process for producing maleic acid that can better utilize the advantages of oxidized Kawahara O's with a high n-butane content. One object of the invention is to provide such a method. Yet another object of the present invention is to provide increased productivity per unit of reaction zone volume in the production of maleic anhydride from n-butane. A further object of the present invention is to provide an improved use of n-butane. Another object of the invention is to make recycling the reaction zone effluent economically practicable. A further object is to provide a method that allows safe operation on the hydrocarbon-rich side of the explosive range. A further object is to provide a process that can be carried out in a conventional continuous flow air oxidation facility without substantial waste of capacity. Other objects of the invention will be apparent to those skilled in the art from the following description and claims.

We now contact the n-butane-rich oxidation feedstock with the catalyst at a relatively low n-butane-through-flow conversion in a heat carrier-cooled tubular reaction zone containing a catalyst graded for reactivity. It has now been discovered that the objects of the present invention can be achieved by simultaneously recycling a large portion of the reaction zone effluent free of maleic anhydride to the reaction zone. Advantageously, the use of n-butane-rich feedstocks with graded catalysts compared to typical air oxidation processes and n-butane-rich processes using ungraded catalysts. Improves productivity of maleic anhydride.

Furthermore, the grading of the catalyst bed provides a more uniform temperature distribution over the effective length of the reaction tube so that the reaction zone capacity is better utilized. Additionally, operation at relatively low co-flow conversions results in improved selectivity to maleic anhydride, which, in combination with recycling of unreacted n-butane, results in improved yields of maleic anhydride. The final conversion of n-butane is also improved. Compared to conventional integrated flow air oxidation processes, the operation according to the invention can increase the yield for a given reaction zone capacity by up to a factor of two;
Catalyst consumption is thereby reduced by grading the catalyst bed.

In summary, the present invention provides a method for converting a feedstock consisting of (A) about 2 to about 10 moles of n-butane, about 8 to about 20 moles of molecular oxygen, and the remainder of an inert gas into n-butane. The oxidation catalyst is catalyzed in a heat transfer medium-cooled tubular reaction zone maintained under oxidizing conditions effective to provide a relatively low coherent flow conversion, the catalyst extending along at least a portion of the effective length of the reaction zone. (B) from the outlet end of the reaction zone to maleic anhydride; Remove the effluent consisting of by-product oxidized hydrocarbons, inert gas, and unreacted oxygen and n-butane; separate maleic anhydride and most of the by-product oxidized hydrocarbons from the reaction zone effluent.1. (D) removing from the effluent remaining after recovery of maleic anhydride and oxidized hydrocarbon by-products a rate discharge stream substantially corresponding to the rate of inert gas accumulation in the reaction zone; (:E) discharge stream; A process for producing maleic anhydride is provided, comprising: recycling the effluent remaining after withdrawal to the reaction zone together with the addition of a make-up gas consisting of n-butane and molecular oxygen.

More specifically, a typical heat medium cooled tubular reaction zone used in accordance with the present invention has a length-to-diameter ratio of about 25.
to about 500, preferably in the range of about 500 mm, such tubes being enclosed within a shell containing a circulating heat transfer medium. The tube is preferably made of carbon or stainless steel, but other materials with high mechanical strength, corrosion resistance and chemical inertness are also suitable. The shell surrounding the tube may be of any material, but is preferably made of carbon steel. The shell is equipped with a heating device, for example an external electric coil or a heater, to heat the heating medium to the desired starting temperature. A cooling device such as a steam boiler is also provided to maintain the heating medium at the desired temperature during oxidation. Circulation of the heating medium around the tubes can be conveniently accomplished by the use of suitable means such as stirrers, pumps and baffles.

Heat carriers useful in accordance with the present invention are well known to those skilled in the art and are generally materials that maintain a liquid state at the process temperature and have relatively high thermal conductivity. Examples of useful media include various heat transfer oils and salts such as alkali metal nitrates and nitrites, with salts being preferred due to their high boiling points. A particularly preferred heat transfer medium is a eutectic mixture of potassium nitrate, sodium nitrate, and sodium nitrite, which not only has a desirable high boiling point but also has a sufficiently low freezing point that it remains in a liquid state even during reaction zone shutdowns. .

The catalyst is such that the reactivity increases over at least a portion of the length of the effective reaction zone from a minimum reactivity closest to the feed end of the reaction zone to a maximum reactivity closest to the outlet end. Then, it is loaded into the reaction tube. For this purpose,
The reaction zone effective length is defined as the catalyst-containing portion of the reaction zone. Preferably, the effective length is at least about 75% of the total length, with the remaining length being in one or two substantially dead zones, such as at the feed end and/or outlet end of the reaction zone. Forming a catalyst-free zone. More preferably, the reaction zone effective length is about 80% to about 100% of the total length. As for the feed end fractional band and the exit end dead zone, the former is preferred because the feed end dead zone can serve as a preheating zone for the oxidizing raw material introduced into the reaction zone.

Preferably, the total effective length of the reaction zone is graded from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end. However, it is also conceivable to grade only a portion of the effective length of the reaction zone from minimum reactivity closest to the feed end to maximum maximum reactivity closest to the outlet end. For example, when proceeding from the feed end to the outlet end of a reaction zone, the first portion of the reaction zone effective length has high reactivity or medium reactivity and the remaining portion of the effective length is graded from minimum reactivity to maximum reactivity. be able to.

Preferably, this initial high- or medium-responsive zone is relatively short and can advantageously serve as a preheating zone for the feedstock for oxidation. Such a reactivity gradient is the fourth
It is depicted in the figure and will be further explained later. It is conceivable to grade the length from minimum to maximum reactivity over the first section, giving medium or low reactivity at the reaction zone outlet end, but it is also possible to This is not a preferred example because the benefits associated with providing this method are diminished. These configurations can also be combined to grade only the central portion of the effective length from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end.

In that case, the reactivity decreases from the part of the effective length closest to the reaction zone feed end to the least reactive part in the central part;
It increases across the central portion and decreases from the most reactive portion in the central portion to the less reactive portion closest to the outlet end.

The portion of the reaction zone effective length graded from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end may be graded continuously and/or graded stepwise. be able to. Ideally, the reactivity increases continuously from minimum to maximum reactivity; however, from a practical point of view, two or more reactive zones or stages may be used along a portion of the effective length of the reaction zone. It is most convenient to assign and grade from least reactivity to most reactivity. The number of zones is preferably in the range of 2 to about 20, but from the point of view of obtaining a relatively uniform temperature distribution and thus high productivity without excessive catalyst loading costs, about 3 is preferred. 8 to about 8 is more preferred.

For a given process, the exact location of the various bands,
The length and relative reactivity will vary depending on a variety of factors, such as total and effective reaction zone length, production rate, catalyst selection, and reaction conditions, and may be determined experimentally according to the examples that appear below. can. Preferably, the least reactive zone closest to the feed end extends from about 10% to about 50 inches of the effective length of the reaction zone and has a reactivity about 10% of the reactivity in the most reactive zone closest to the outlet end. % to about 75%, with the remaining effective length including one or more bands of increased reactivity.

For this purpose, reactivity is calculated by adding a value of 100 to the maximum reactivity portion of the reaction zone effective length graded from the minimum reactivity closest to the feed end to the maximum reactivity closest to the exit end. Assigned and expressed on a relative basis. Therefore,
Based on the n-methane conversion and relative n-butane conversion in the 100-inch band, higher or lower values can be assigned to other parts.

Any suitable means for providing the desired reactivity gradient is contemplated in accordance with the present invention. Most simply, a pelletized oxidation catalyst is used, and the desired grading is achieved by mixing the catalyst pellet with an inert solid. Suitable inert solids do not adversely affect catalyst performance during oxidation, and the catalyst is at least roughly similar in size and shape to the catalyst bed to ensure substantially uniform gas flow through the catalyst bed. It includes such substances. Examples of useful solid diluents include silica, alumina, and carborundum pellets. A continuous reactivity gradient can be provided by feeding catalyst pellets and inert to the reaction tube using separate variable speed feeders. A stepwise gradient can be provided by batchwise mixing the catalyst R-let and the inert in appropriate amounts.

One related method of obtaining the desired reactivity gradient is to use supported catalysts, where the proportion of support decreases from the least reactive zone to the most reactive zone and thus increases with the proportion of active catalyst. Partial impregnation of the support with catalyst gives particularly good results in that the selectivity is typically improved. Suitable carriers are discussed in detail below.

A third method of grading catalyst beds involves using different catalysts in the individual reactive zones, with the most reactive catalyst in the most reactive zone closest to the outlet end and the least reactive catalyst closest to the feed end. using the least reactive catalyst in the zone;
One or more intermediately reactive catalysts are then used in the intermediately reactive zone. Particularly good results are obtained when relatively low reactivity catalysts have high selectivity.

Another way to obtain the desired gradient is to use catalyst mixtures of various reactivity containing various proportions of individual catalysts in zones of different reactivity.

Of course, various combinations of the above techniques or others may be used to obtain reactivity gradients depending on the particular process equipment or requirements.

Oxidation catalysts useful in accordance with the present invention are known to those skilled in the art and are generally materials that can catalyze the oxidation of n-butane to maleic anhydride under oxidizing conditions. Examples of useful catalysts are discussed below, but it will be understood that they are for purposes of illustration and guidance in practicing the present invention.

One type of catalyst useful in accordance with the present invention is a phosphorus-vanadium-oxygen composite catalyst. Such catalysts typically contain from about O15 to about 5 atoms of phosphorus per atom of vanadium and are made by reaction of a compound of phosphorus and vanadium in an aqueous or organic medium followed by heating of the resulting solid. Suitable phosphorus compounds include phosphorus pentoxide, phosphoric acid, orthophosphorous acid, phosphorus trichloride, phosphorus trioxide and triethyl phosphate;
Suitable vanadium compounds include vanadium oxalate, vanadium formate, ammonium metavanadate, vanadyl trichloride,
Contains metavanadate, vanadium sulfate and vanadium phosphate. The reaction of such compounds is carried out in an aqueous or organic medium to form a solid reaction product, which is isolated by evaporation of the supernatant. This solid is then heated to about 300°C.
Activated by heating to about 600° C. and milled and/or pelletized. Optionally, prior to use, the catalyst is coated with a suitable inert support such as alpha alumina,
or mixed with silicon carbide. Further details regarding this type of phosphorus-vanadium-oxygen composite catalyst can be found in U.S. Pat.
No. 95.268 (Berbman et al.) and No. 3,90Z70
No. 7 (Raffelson et al.), incorporated herein.

The phosphorus-vanadium-oxygen composite catalyst is also disclosed in U.S. Pat.
They can also be made by melting phosphorus pentoxide into a matrix of vanadium pentoxide, as disclosed in Slinked et al.

A second type of useful catalyst is a metal promoted phosphorus-vanadium-oxygen complex. Catalysts of this type typically contain about 0.5 to about 5 atoms of phosphorus per atom of vanadium;
Additionally, it contains at least one metal promoter at about o, ooi to about 5 atoms per vanadium atom.

Useful promoters include the alkali and alkaline earth metals, scandium, lanthanum, yttrium, cesium, neodymium, samarium, titanium, zirconium, hafnium, niobium, tantalum, chromium, tungsten, mol.
J7' ten, manganese, rhenium, iron, cobalt, nickel, palladium, copper, silver, zinc, cadmium, aluminum, gallium, indium, silicon, germanium,
Contains tin, antimony, hismuth, tellurium, thorium and uranium. Generally, one or more of these promoters are incorporated into the phosphorus-vanadium-oxygen complex before, during, and after the complex-forming reaction. Such catalysts can be supported or unsupported. Further details of this type of catalyst can be found in the following patents cited herein: U.S. Pat. No. 3,862,146 (Zr, Bi
PVO promoted with , Gu, Li); U.S. Pat. No. 3,862,359 CPVOFθ complex): U.S. Pat. No. 3,905,914; 3,931,
046; and 6.962.305 (PVOZr
complex); U.S. Patent No. 3,980,585 (T
e, Zr, Ni, Ce.

W, Pa, Ag, Mn, Or, Zn, Mo
, Re, Sm, La.

Hf, Ta, 'I'h, Go, U, or Sn
, and other alkali metals and alkaline earth metals); U.S. Pat. Nos. 4.062.873 and 4.064.07
Specification No. 0 (PVO8i complex); U.S. Patent No. 4,147.661 (W, Sb,
Nb and/or Mo promoted pvo); US Pat. No. 4,151,116 (post-deposited M
g, Ca, Se, Y, La, U, Ce,
Or, Mn, Fe.

Co, Ni, Cu, Zn, AI, Ga,
In, Si, Ge.

PVO promoted by 3n, Bi, Sb, or Te: optionally Ti, Zn, Hf, Lj, Mg,
, Ca.

Fe, Go, Ni, Cu, Sn, Bi, U
, rare earth metals Cr, Ca, or AI are added into the composite prior to post-promoter deposition); US Pat. No. 4,152,638 (Nb, Cu
, Mo, Ni.

promoted with Go, and Or, preferably in addition to Ce, N
a, Ba, Hf, U, Ru, Re, Li,
or Mg-containing pvo); U.S. Pat. No. 4,152,339 (Te, Zr
, N1. Ce.

W, Pd, Ag, Mn, Or, zn, Mo
, Re, Sm, La.

PVOCu promoted with Hf, Ta, Th, co, IJ, or 3n, preferably still containing an alkali metal or alkaline earth metal; US Pat.
．． No. 153.577 and No. 4,158,671 (Cu
, Mo, Ni, Go, Cr, Na, Ce,
PVO promoted with Ba, Y or Sm): British Patent No. 1.403.395 (optionally WO3
and/or PVOT1 containing MoO3).

A third group of catalysts are those in which the primary metal is other than sodium. For example, U.S. Pat. No. 2,691.
No. 660 contains oxides of molybdenum and cobal) or nickel, optionally promoters selected from oxides of boron, phosphorus, and vanadium, silicon, tungsten, titanium, beryllium, zirconium, chromium, and uranium. Discloses a butane oxidation catalyst comprising: US Patent No. 3
．． No. 92.8,392 discloses a methane oxidation catalyst containing antimony, molybdenum, and oxides of nickel, co-Glut, copper or zinc. U.S. Patent No. 4.
O65,468 specifies antimony, molybdenum,
and a combined oxide of vanadium or iron, optionally with one or more oxides of aluminium, boron, telnopechrome, cobalt, nickel, copper, hismuth, phosphorus, titanium or tungsten. discloses a butane oxidation catalyst comprising:

A presently preferred catalyst for use in accordance with the present invention is a metal promoted phosphorus-vanadium-oxygen complex catalyst, such as in accordance with U.S. Pat. No. 3,862,146.

The oxidation according to the present invention is carried out in a heat medium cooled tubular reaction zone as described above containing a graded catalyst, about 2 to about 10 moles of butane, about 8 to about 20 moles of molecular oxygen,
and the remaining inert gas, n
- carried out by introducing butane into the reaction zone feed end maintained under reaction conditions such as to achieve a relatively low through-flow conversion of maleic anhydride, by-product oxidized hydrocarbons, unreacted An effluent consisting of n-butane, oxygen, and an inert gas is removed from the reaction zone outlet end, and the maleic anhydride and oxidized hydrocarbon byproducts are substantially separated therefrom, after which the remaining effluent is removed. , an effluent stream that is removed at a rate that substantially offsets the buildup of inerts in the reaction zone, and a recycle that is recycled to the reaction zone together with a make-up gas consisting of n-butane and molecular oxygen. Separate into two parts.

The composition of the oxidizing feedstock used in accordance with the present invention is important not only from the standpoint of obtaining the desired yield of maleic acid, but also from the standpoint of safety of reaction zone operation. As mentioned above,
Preferably, the feedstock contains a level of n-butane that is on the hydrocarbon-rich side of the explosive range. Regardless of the concentration of n-butane, the feed gas concentration should be adjusted to avoid the formation of explosive mixtures. Generally, oxygen concentrations in the upper part of the above range are used, with n-butane concentrations in the upper part of the above range. Conversely, at lower n-butane concentrations, lower oxygen levels should be used.

Further details regarding the feed composition are given in FIG. 1, which illustrates the explosion range for a mixture of molecular oxygen, n-methane and the balance nitrogen at room temperature and atmospheric pressure. Point 1 represents a typical continuous flow air oxidation, while points 2-4 illustrate the feed compositions used in accordance with the present invention.

Points 6 and 4 illustrate feedstocks with preferred levels of n-butane and oxygen that are on the n-butane-rich side of the explosive range and thus provide improved productivity. It must be recognized that the explosion curve of FIG. 1 is specific to a gaseous mixture in which the inert gas is nitrogen. When the carbon dioxide is present in the inert gas, as in the preferred embodiment of the invention, the explosion range is somewhat reduced. Increasing temperature and pressure increases the explosion range.

It is preferred that the n-butane used in accordance with the present invention be substantially pure, ie, at least about 96% pure. but,
One or more other oxidizable C4s, such as isobutane, butenes, or butadienes, are present in the feedstock at about 20%
It is also conceivable that it may even include the mole staff.

The molecular oxygen source used in accordance with the present invention has a substantial impact on productivity. Substantially i.e. at least 95 g of pure oxygen is preferred, since the amount of parasto gas required to avoid the formation of explosive mixtures in the reaction zone is supplied entirely by the circulation stream; In some cases, the effluent stream removed from the reaction zone effluent prior to recycling need only be large enough to offset the level of carbon oxides produced during oxidation. On the other hand, when lower concentrations of oxygen are used, a larger discharge flow is required to compensate not only for the formation of carbon oxides as a result of oxidation but also for the inert gas introduced along with the molecular oxygen.

The parasitic gas supplied to the reaction zone is varied across the source of molecular oxygen. When using substantially pure oxygen, the parasto gas supplied by the recycle stream is primarily a mixture of carbon monoxide and carbon dioxide. - The molar ratio of carbon oxide to carbon dioxide is a function of catalyst selection, but is generally about 0.
．． It ranges from 5:1 to about 2.5:1. In addition to minimizing effluent flow requirements, the use of substantially pure oxygen and the resulting presence of carbon dioxide in the parast gas facilitates temperature regulation due to the high heat capacity of carbon dioxide. If the molecular oxygen source used is air, the last gas will of course contain a substantial amount of nitrogen. Similarly, if a mixture of molecular oxygen and one or more inert gases constitutes the source of molecular oxygen, the palust gas comprises the inert gas introduced along with the oxygen. If parasitic gases other than those supplied in the recirculation stream and/or other than those introduced into the reaction zone together with oxygen have to be introduced into the reaction zone in order to adjust the feed composition, nitrogen, carbon oxides, Preference is given to using carbon dioxide or mixtures thereof. More preferred is a mixture of carbon monoxide and carbon dioxide that contains sufficient carbon dioxide to reduce the explosive range.

Preferably, the oxidizing feedstocks include about 3 to 8 moles of n-butane, about 10 to about 18 moles of molecular oxygen, and about 74 moles of molecular oxygen.
Contains approximately 87 moles of palastogas. When substantially pure oxygen is used as the source of molecular oxygen, the palast gas consists of about 30 to about 50 molar parts of carbon monoxide, about 65 to about 55 molar parts of carbon dioxide, and less than about 1 molar part of nitrogen. It is preferable. When air is the source of molecular oxygen, the pallast gas preferably includes about 70 to about 85 molar parts of nitrogen, up to about 5 molar parts of carbon dioxide, and about 1 to about 10 molar parts of carbon monoxide. . 5.5
% n-butane, 14 momoles of oxygen, and the remainder ca.
．． For feedstocks containing a mixture of carbon monoxide and carbon dioxide in a 1:10 molar ratio, the explosion limit is 1.2.
A safety margin of molar n-butane or 1.6 molar oxygen exists.

The oxidizing feedstock is introduced into the feed end of the reaction zone maintained under effective reaction conditions to obtain a relatively low consistent flow conversion of n-butane. For this purpose, the through-flow conversion is defined as the number of n-butane molecules oxidized divided by the number of n-butane molecules in the feedstock times 100%. Preferably the continuous flow conversion ranges from about 10 to about 70%, more preferably from about 60 to about 55%, to achieve high final conversions with good selectivity to maleic anhydride. It is. Reaction conditions include temperature, pressure, space velocity, and others detailed below.

The reaction zone temperature is high enough to obtain reasonable reaction rates, but not so high as to damage the oxidation catalyst or promote undesirable side reactions. Preferred temperatures range from about 300°C to about 650°C, and from about 650°C to about 500°C.
is more preferable. It is desirable to preheat the oxidation feed to within about 100° C. of the reaction temperature before passing the feed over the portion of the catalyst bed that is graded from least reactive to most reactive.

The reaction zone pressure is not critical, but from a practical standpoint it ranges from about 10 to about 7 psia (about 0.7 to about "" cm2
) is preferable.

The oxidizing feedstock is fed to the reaction zone at a rate such that a relatively low continuous flow conversion of n-butane is achieved.

Preferably, the volumetric space velocity of the feedstock ranges from about 1,000 to about 30,004. More preferably, the volumetric space velocity is from about 15 DO to about 2500/R=, as this provides desirable productivity without excessive pressure drop from the feed end to the outlet end of the reaction zone. At volume hourly velocities of about 1,500 to about 25,004, the consistent flow conversion of n-butane is typically about 60 to about 50%. Contact time in the reaction zone depends on space velocity and pressure and typically ranges from about 0.5 to about 4 seconds.

From the outlet end of the reaction zone, maleic anhydride, by-produced oxidized hydrocarbons, inert gas and unreacted n-butane and oxygen,
Remove any effluent containing. The major by-product oxidized hydrocarbons include acetic acid and acrylic acid, which are typically produced in slightly higher amounts than in conventional continuous flow air oxidation. The effluent is passed to a separator zone where maleic anhydride and by-product oxidized hydrocarbons are substantially recovered. The remaining effluent is divided into a discharge stream and a recycle stream, the latter being returned to the reaction zone together with make-up gas containing oxygen and n-butane.

Separation of maleic anhydride and by-product oxidized hydrocarbons from the reaction zone effluent can be accomplished by any suitable means. for example,
The effluent is washed with an aqueous liquid, such as an aqueous solution of maleic acid, and the resulting scrubber solution is dehydrated to convert the maleic acid to maleic anhydride.

Acetic acid and acrylic acid, which are also removed from the effluent by washing, can be passed overhead during dewatering and recovered by fractional distillation, extraction, or other suitable means.

Another method of recovering maleic anhydride from the effluent is to condense a portion of the anhydride from the effluent prior to washing the effluent and dehydrating the maleic acid. Partial condensation prior to washing is
in that less anhydride is converted to acid and therefore less acid is available for irreversible isomerization to fumaric acid.
Favorable results can be given in terms of maleic anhydride yield. Additionally, lower levels of maleic and fumaric acids are advantageous since these acids are solids under recovery conditions and can clog process equipment.

Another suitable method for recovering maleic anhydride and by-product oxidized hydrocarbons is to contact the effluent with an organic solvent that has a low absorption capacity for water and a high absorption capacity for maleic anhydride, and then removes the maleic anhydride from the solvent. It is to drive out. In this way, hydrolysis of maleic anhydride to acid is substantially avoided, thus resulting in an improved yield of maleic anhydride. Useful absorbents include various organic solvents,
Specific examples thereof are found in the specifications of US Pat. No. 3,891,680 (Kamoto et al.) and No. 4,118,403 (White).

The effluent remaining after substantially recovering the maleic anhydride and by-product oxidized hydrocarbons, consisting of inert gas, unreacted n-butane and oxygen, is divided into a discharge stream and a recycle stream. The effluent stream is removed at a rate that substantially offsets the buildup of inerts in the reaction zone. The exact rate of release depends on a variety of factors that will occur to those skilled in the art, such as the molecular oxygen source, production rate, reaction zone volume, catalyst selection, and oxidation conditions. When substantially pure oxygen is used as the source of molecular oxygen, the preferred rate of release ranges from about 1 to about 20 molar parts of the effluent gas.

If air is the source of molecular oxygen, about 20 to about 7
A release rate of 5 molar is preferred.

After removal of the effluent, the remaining effluent is recycled to the reaction zone. As mentioned above, when substantially pure oxygen is used as the source of molecular oxygen, only n-butane and oxygen need be supplied as make-up gases to the reaction zone, since the recycle flow is suitable for the oxidizing feedstock. This is because it provides a level of inert gas. If air or low purity oxygen is the source of molecular oxygen, an inert gas is entrained with the makeup oxygen.

A preferred mode of operation according to the invention will be described in more detail in connection with the drawings.

Referring to FIG. 2, for start-up, n-butane and air are metered from respective sources (not shown) and supplied to line 16 via lines 5 and 7, respectively.

This feedstock is introduced into the feed end of reactor 15. The reactor is shown with a portion of its wall cut away to show reaction tubes 17 and 19. The reactor also circulates molten salt (
(not shown). A portion of the wall of tube 19 has been cut away to show temperature measuring tube 21 extending centrally. Tubes 17 and 19 contain catalysts graded for reactivity (
(not shown) is loaded.

The preferred reactivity gradient distribution is shown in Figures 6 and 4, where the reaction zone length is plotted on the horizontal axis, with 0% corresponding to the feed end and 100% corresponding to the outlet end, and the relative Reactivity is plotted on the vertical axis.

According to the distribution diagram of FIG. 3, the overall effective length of the reaction zone is graded from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end.

The dead zone has a reactivity of 0% and is the first one of the total reaction zone length.
0% and serves as a preheating zone for the raw material for oxidation. The reactivity distribution illustrated in Figure 4 is such that the total reaction zone length is equal to the effective reaction zone length, and only a portion of the effective length ranges from the minimum reactivity closest to the feed end to the maximum reactivity closest to the outlet end. It differs from the distribution in Figure 3 in that it is graded into reactivity.The first 15% of the reaction zone is a meso-responsive zone, and like the dead zone in Figure 6, it serves as a pre-war zone. Helpful.

Referring again to FIG. 2, the air and n-butane feeds are passed through tubes 17 and 19 under oxidizing conditions to contact the catalyst, whereby the n-butane is converted into maleic anhydride and by-products. oxidized to From the feed end to the outlet end, the reactor effluent exits the outlet end of the reactor and passes through line 26 to scrubber 2.
There is a pressure drop as it passes to 5.

In the scrubber, the effluent is contacted in countercurrent fashion with an aqueous maleic acid solution, which flows from the scrubber solution tank 29 through lines 61 and 36, pump 35, line 67, and cooler filter 9.
, and piping 40 to the top of the scrubber. Maleic anhydride and by-product oxidized hydrocarbons from the effluent are absorbed by the scrubber solution and pass by gravity through line 27 to the scrubber solution tank. The use of an aqueous scrubber solution involves the net hydrolysis of maleic anhydride to maleic acid. Most of the maleic acid collected in the scrubber solution tank passes via line 31 to a product recovery and purification section (not shown).

The gaseous effluent from scrubber 25, which includes inert gas, unreacted n-butane and oxygen, and maleic anhydride and by-product oxidized hydrocarbons not removed by the scrubber liquid, is piped from scrubber 25. 41 and passes as overhead to condenser 43 where volatile scrubber liquid is condensed from the effluent.

From the condenser, the effluent passes via line 45 to filter 53 which also leads to discharge line 47.

The discharge line has a back pressure valve 49 Dtfft which regulates the pressure in the product recovery section.

The discharge stream passes from the backpressure valve through line 51 to an incinerator or other disposal device (not shown). The remaining effluent from line 45 passes to filter 53 where solids such as maleic acid and fumaric acid are removed from the gas stream.

The filtered effluent is passed through pipe 55 to compressor 57
Pass through. The compressed effluent is sent to the circulation supply line 11 via the circulation line 59.

As the process progresses, the level of inert gas fed to the reactor in the cycle gas accumulates until the inert is present in an appropriate proportion of the total feed to the reactor, preferably from about 70 to about 95 moles. When reached, the air supply is interrupted and oxygen is supplied to the reactor via lines 9 and 16 in order to reduce the discharge flow requirement and thereby increase the amount of n-butane recycled to the reactor. Alternatively, simply continue the air supply. It is also conceivable to use oxygen instead of air during startup.

Although the following examples illustrate the invention, it is understood that they are for illustrative purposes and are not intended to be limiting.

General Experimental Method (A) Molten Salt-Cooled Tubular Reaction Zone The reaction zone used in all experiments is 8 feet long)
(2,44m) with an inner diameter of 1.049 inches (266t-n)
l) was a vertical stainless steel pipe. A temperature measuring tube with an outside diameter of 0.375 inches (0.953 z) is attached along the axis of the reaction tube. The temperature measurement tube contained a movable thermocouple assembly consisting of eight individual thermocouples spaced one foot apart. During oxidation experiments, the reaction tube is immersed in a molten salt bath, consisting of a eutectic mixture of nitric acid, sodium nitrite, and potassium nitrate, 8 feet long (2,44
m) and was housed in a stainless steel shell with an internal diameter of 4 inches (10,2 crn).

The salt bath was heated with an electric coil attached to the outside of the shell. Nitrogen was bubbled through the salt bath at a rate of about 1 5 CFH (28,3 t4) to provide circulation around the reaction tube.

(B) Graded Catalyst Bed The catalyst used in all experiments contained 1,19 atoms of phosphorus and 0.2 atoms of zinc per atom of vanadium and was similar to the example of U.S. Pat. No. 3,862,146. 1, a zinc-promoted phosphorus-vanadium-oxygen complex prepared substantially in accordance with No. 1. The catalyst has a length of % inches (0,48 m
) in the form of a cylindrical R-let with a diameter of % inch (0.48 c1n). When loaded, the bulk density of the catalyst is 67.
4 ponton cubic fee) (1,082/3).

Catalyst grading was performed by mixing the catalyst with a basis weight of an alumina pellet of the same shape and size (TwoTone trademark "Dens"). The bulk density of the catalyst-denstone mixture was the same as that of the pure catalyst.

The reaction zone had a 1 foot (0.3 m) feed end dead zone and a % foot ('0.15 m) exit end dead zone, both of which contained only dense pellets. Effective length is 6
% feet (1,98 m), and the whole was graded according to the following form: from the lowest reactivity closest to the feed end to the highest reactivity closest to the outlet end. ) 208 2
08 50m 1.3 (0.40m)
131 77 63m 1.3
(0,40m) 1/)6 42
80(C) Feed Gas Commercial grade n-butane was used in all experiments.

Typical analytical values expressed in mulch are as follows.

Ethane 0-0.05% Propane 0.09-0.54 J-Butane
1.63-1.77 n-butane 96.7
-96.84% Butene-10-0,038% 1-Butene O-0,028% J-Notene 0.06-1.04% T-Notene-2
0-0.067% C-butene-20,040-0,74%C5 or more
0-0.480% oxygen was used with a minimum purity of 996% and special dry edge, and air was used with a maximum moisture content of 0.3%.

CD) Operation For all experiments, the integral orifice (integ
A DP cell with a ral orifice was used to supply oxygen, air and circulation gas. n-Butane was metered into the reaction tube using a capillary tube with an internal diameter of 0,18,-m and a length of 90 feet (27,4 m).
This capillary helped maintain laminar flow and adequate pressure drop. The n-butane chamber was set up in a 150F (66C) bath to ensure repeatability of the feed rate.

The feed gas is mixed and heated for 350' prior to introduction into the reaction tube.
7” (177°C). This feedstock was then heated to 1.
The feed composition, reaction conditions and feed rate are shown in the table below.

The effluent is 14 I) Big (0,98Kr/crn2
) at the bottom of the reaction tube and passed through a filter to remove entrained catalyst particles. The filtered effluent was then passed through a pressure-controlled pulp, from which it was passed through an 8-inch (1,6α) ceramic tube in a 4-inch (10,2 crn) internal diameter glass tube with a length of 2% fee (0,76 m). through a first scrubber consisting of a glass tube filled with a saddle (5add, 1e). In this first scrubber, the effluent contacts the aqueous maleic acid solution in a countercurrent manner. Most of the maleic anhydride in the effluent is removed in the first scrubber. The scrubbered effluent is collected from the first scrubber in a 2 ft (0.61 m') long and 4 in. (1.0 m') internal diameter.
, 2 cm) glass tube is passed through a second varnish scrubber consisting of a glass tube filled with an 8 inch (0.95 crn) ceramic saddle. The lower half of this scrubber is filled with water, and the heroic half serves as an anti-entanglement chamber. The effluent from the first scrubber is bubbled through this water to further remove maleic anhydride and by-product oxidized hydrocarbons.

A portion of the effluent that was scrubbed was approximately 598.
A wet test make where the discharge flow rate is measured through a back pressure valve maintained at 1g (0.35?/cm2).
The flow was divided into a discharge flow and a circulation flow. In the circulation experiment, the circulation flow passed offshore, and 25
psig (1,76 Kq/cm2) in a diaphragm compressor and returned to the reaction tube.

- In a comparative experiment carried out in flow-through mode, the entire scrubbered effluent was passed through this wet test meter. The space velocity through the tube was adjusted by adjusting the circulating gas flow rate.

(E) Analysis and Yield Calculations For all experiments, feed gases and effluent gases were analyzed using a dual column Fisher-Hamilton model 1200 gas chromatograph.

Volumetric space velocity (hereinafter abbreviated as "vsv") was determined by dividing the volumetric feed rate by the reaction zone volume at 0° C. and 1 atm.

The yield of maleic anhydride was determined by potentiometric amine titration of a sample of the scrubber solution.

Maleic anhydride productivity was determined by dividing the calculated weight of recovered maleic anhydride by the product of (1) auration and (2) the total weight of catalyst and diluent.

Selectivity to maleic anhydride was calculated by dividing the molar maleic anhydride yield by the number of moles of n-butane converted and multiplying by 100. The selectivity based on the total hydrocarbon feed can be calculated by multiplying the selectivity based on n-methane by 0.96.

EXAMPLE The above series of oxidations were carried out over several days. The feed composition, operating mode, reaction conditions, and results are shown in Table 1.

(1) In this table and the following tables, "OTA" stands for once-through air oxidation.

(2) In this table and the following tables, "RA" represents a circulation experiment using air as the molecular oxygen source.

(3) In this table and the following tables, "MAN" represents maleic anhydride.

(4) In this table and subsequent reports, MAN productivity is MAN
Report as 2 of N/[7 of (catalyst + diluent)] (hours).

(5) In this table and the following tables, "MAN" selectivity is reported as molti.

Example 2゜ A series of four experiments were conducted over four days.

Details are reported in Table 2.

EXAMPLE A third series of continuous flow and circulation experiments using 6° air were conducted over several days. Details are reported in Table 6.

- Comparison of the experiments of once-through type one air (○TA) and circulating type - air CRA) in Examples 1-6 and Tables 1 to 6 shows that the productivity of the latter is substantially higher than that of the continuous flow type. large, but otherwise indicate that the results are generally equivalent.

Due to incomplete activation of the catalyst in Examples 1-3, the reaction rate was low and several short temperature excursions occurred. Runaway was easily controlled by slightly reducing the n-butane feed rate and salt bath temperature.

After sufficiently activating the catalyst (Experiment 4 of Example B),
There was no runaway.

Example 4 In this example, a series of circulation experiments using oxygen were conducted, followed by a continuous flow air oxidation experiment. Details are reported in Table 4.

(1) In the tree table and the following tables, "R○" represents a circulation experiment using 96 t pure oxygen.

Example 5 After the experiment in Example 4, the oxidation catalyst was heated to 404°C.
onto the catalyst of 0.077 SCFM C2,2t/f'j
) was regenerated by passing 9-carbon tetrachloride in nitrogen for 15 minutes. Carbon tetrachloride and nitrogen were introduced into the outlet end of the reaction tube.

Following regeneration, four cycling experiments were performed with oxygen. Details are reported in Table 5.

Example 5 and Table 5 illustrate one preferred mode of operation in accordance with the present invention. As you can see, about 94 to 97%
Final conversions of n-butane in the range of 90% were obtained at sufficiently high selectivities that maleic anhydride yields of about 90% were achieved. The productivity in Runs 6 and 4 was approximately twice that of the continuous flow air oxidation in the previous example.

Example 6. Another series of circulation experiments using oxygen was carried out over a period of 5 days, followed by continuous flow air oxidation. Details are reported in Table 6.

Example Z After regenerating the catalyst according to the method of Example 5, - once-through air oxidation was carried out and then five circulation experiments with oxygen were carried out over a period of 5 days. Details are shown in Table 7.

From the table, it can be seen that the selectivity varies somewhat depending on the n-butane concentration. For the catalysts used in these examples, the selectivity was about 5 molar n-butane concentrations.

EXAMPLE 8 Another series of circulation experiments using oxygen was conducted followed by a continuous flow experiment using air. Details are reported in Table 8.

It can be seen from the foregoing Examples and Tables that operation according to the present invention provides a substantial improvement over typical continuous flow air oxidation operations. Among these improvements are an increase in reactor productivity of more than 100 inches in some cases and a decrease in n-butane consumption per unit weight of product.

[BRIEF DESCRIPTION OF THE DRAWINGS] Figure 1 shows the explosion limits for the n-butane, oxygen and nitrogen system, Figure 2 shows the process diagram, and Figures 3 and 4 show the preferred Figure 2 illustrates a catalytic reactivity gradient. In Fig. 2, 15...reactor, 17, 19...-2 reaction tubes. 25... Scrubber, 29... Scrubber liquid tank. 46... Condenser, 49... Back pressure pulp. 56...Filter, 57...
compressor. FIG, 1 oxygen (mono V'/)

Claims (1)

Claims: 1. (A) A feedstock consisting essentially of about 2 to about 10 moles of n-butane, 8 to about 20 moles of molecular oxygen, and the remainder at least one inert gas. is contacted with an oxidation catalyst in a tubular reaction zone cooled with a heat transfer medium and maintained under oxidizing conditions effective to produce a relatively low consistent flow conversion of n-butane, the most of the feed end of the reaction zone being The effective length of the reaction zone is small (
(B) removing from the reaction zone an effluent consisting of maleic anhydride, by-product fermented hydrocarbons, carbon oxides, n-butane, and oxygen; (C) separating the majority of the maleic anhydride and oxidized hydrocarbon by-products from the effluent; s) separating the bulk of the maleic anhydride and oxidized hydrocarbon by-products from the effluent remaining after recovering the maleic anhydride and oxidized hydrocarbon by-products; remove the effluent stream at a rate substantially corresponding to the rate of accumulation of inert gas; and (g) combine the effluent remaining after removal of the effluent stream with the addition of make-up gas consisting of n-butane and oxygen into the reaction zone. A method for producing maleic anhydride, comprising: circulating it to; 2. The method of claim 1, wherein the molecular oxygen is substantially pure and the inert gas consists essentially of a mixture of carbon monoxide and carbon dioxide. 3. Only a portion of the effective length of the reaction zone is graded from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end, with the remainder of the effective length being located at the feed end and feeding. The method according to claim 2, which serves as a pre-heating zone for the raw material. 4. The method of claim 2, wherein the total effective length of the reaction zone is graded from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end. 5 Feedstock is about 6 to about 8 molar n-butane, about 8
consisting essentially of about 18 to 18 moles of molecular oxygen, about 60 to about 50 moles of carbon oxide, about 65 to about 55 moles of carbon dioxide, and less than about 1 mole of nitrogen;
A method according to claim 4. 6 Minimum Reaction Closest to the Feed End 1 The gradient zone extends over about 10 to about 50% of the effective length of the reaction zone, such that the reactivity in this zone is equal to the reactivity at the maximum reactivity closest to the outlet end. 6. The method of claim 5, in the range of about 10% to about 75%. 7. The method of claim 1, wherein the oxygen source is air and the inert gas contained in the feed consists essentially of nitrogen. 8 Only a portion of the reaction zone effective length is graded from minimum reactivity closest to the feed end to maximum reactivity closest to the outlet end, with the remainder of the effective length being located at the feed end and graded from the feed source. 8. The method according to claim 7, which serves as a preheating zone for a 100% carbon fiber. 9. The total effective length of the reaction zone is graded from minimum reactivity closest to the feed end to maximum reactivity closest to the exit end. Method: 10. The feedstock is about 6 to 8 molar n-butane, about 1
Consists essentially of 0 to about 18 molar parts of molecular oxygen, about 1 to about 10 molar parts of carbon monoxide, up to about 5 molar parts of carbon dioxide, and about 70 to about 85 molar parts of nitrogen. , the method according to claim 9. 11. The least reactive zone closest to the feed end extends from about 10 to about 50% of the reaction zone effective length such that the reactivity in that zone is approximately equal to the reactivity in the most reactive zone closest to the outlet end. 11. The method of claim 10, in the range of 10% to about 75%. 12. The method according to claim 1, wherein the acid catalyst is a phosphorus-vanadium-oxygen complex. 13. The method according to claim 1, wherein the oxidation catalyst is a metal-activated phosphorus-vanadium-oxygen complex. ]40 The grading of the catalyst with respect to reactivity is achieved by mixing the catalyst with an inert body, Claim 1-1
The method described in any of Section 3. 15. A method according to any of claims 1 to 13, wherein the grading of the catalyst with respect to reactivity is achieved by partial impregnation of the catalyst support. 16. A method according to any of claims 1-16, wherein the grading of catalysts with respect to reactivity is achieved through the use of catalysts with different degrees of activity. 17. A method according to any of claims 1 to 16, wherein the grading of the catalysts with respect to reactivity is achieved by mixing catalysts with different reactivities.