Process for the production of maleic anhydride

Disclosed is a process for the production of maleic anhydride, which comprises: reacting a hydrocarbon with an oxygen-containing gas in the presence of a catalyst; recovering maleic anhydride from the reaction gas; recovering unreacted hydrocarbon from the remaining gas; and returning the hydrocarbon thus recovered to the reactor for re-use, wherein said reaction is effected under the conditions such that the hydrocarbon concentration X (vol %) and oxygen concentration Y (vol %) in all the gases to be fed into the reactor, the hydrocarbon conversion Z (%) in the reactor and the oxygen concentration W (vol %) in all the effluent gases from the reactor satisfy the following relationships: EQU Y.gtoreq.20, EQU X+Y.ltoreq.70, EQU 1.ltoreq.Y/X.ltoreq.5, and EQU 20(Y-10)/X.ltoreq.Z.ltoreq.25Y/X or 2.ltoreq.W.ltoreq.10.

TECHNICAL FIELD OF THE INVENTION
 The present invention relates to a process for the production of maleic
 anhydride by subjecting a hydrocarbon to catalytic oxidation in a gas
 phase. More particularly, the present invention relates to a process for
 the production of maleic anhydride which comprises recovering a
 hydrocarbon left unreacted in a reactor, and then returning the
 hydrocarbon thus recovered to the reactor where it is then subjected to
 catalytic oxidation under specific reaction conditions.
 BACKGROUND ART
 It is well known to produce maleic anhydride by subjecting a hydrocarbon to
 catalytic oxidation in a gas phase. Heretofore, the production of maleic
 anhydride has been accomplished by the reaction of benzene and air as raw
 materials in the presence of a vanadium pentoxide-based catalyst. In
 recent years, processes involving the use of a straight-chain hydrocarbon
 having four carbon atoms such as butane, butene and butadiene have been
 developed. Among these processes, one involving the reaction of n-butane,
 which is a saturated hydrocarbon, as a raw material in the presence of a
 catalyst comprising a vanadium-phosphorus mixed oxide as an active
 component has been mainly employed. As the active component to be
 incorporated in such a catalyst, divanadyl pyrophosphate ((VO).sub.2
 P.sub.2 O.sub.7) has been known to exhibit excellent performance. Many
 references concerning this compound have been published (e.g., Chem. Rev.
 88, p. 55-80 (1988)).
 The foregoing reaction is effected in a fluidized bed process or a fixed
 bed process. In some detail, a hydrocarbon and an oxygen-containing gas,
 normally air, are fed as raw material into a reactor in such a manner that
 the concentration of the hydrocarbon reaches from about 1.5 to 10%. The
 reaction mixture is then allowed to undergo reaction at a temperature of
 from 300.degree. C. to 600.degree. C. The reaction gas coming out of the
 reactor contains maleic anhydride as well as carbon monoxide, carbon
 dioxide, water and other reaction products. The separation and recovery of
 maleic anhydride from the reaction gas is accomplished by a process which
 comprises cooling the reaction gas to condense maleic anhydride, a process
 which comprises allowing the reaction gas to come in contact with water so
 that maleic anhydride is collected as maleic acid in water, a process
 which comprises allowing the reaction gas to come in contact with an
 organic solvent such as phthalic acid ester or alkyl ester of hydrogenated
 phthalic acid so that maleic anhydride is collected in the organic
 solvent.
 In the commercially practiced process for the production of maleic
 anhydride, the hydrocarbon conversion in the reactor [number of mole of
 hydrocarbon consumed in the reaction per pass/number of mols of
 hydrocarbon supplied into the reactor.times.100 (mol %)] is kept as high
 as possible. This is required to minimize the amount of hydrocarbon as a
 raw material required to produce maleic anhydride. In general, the
 hydrocarbon left unreacted in the reactor is incinerated in a waste gas
 burning apparatus.
 On the other hand, it has been known to reduce the hydrocarbon conversion,
 making it possible to reduce the proportion of carbon monoxide or carbon
 dioxide to be produced as a by-product and hence enhance the maleic
 anhydride selectivity [number of mols of maleic anhydride produced by the
 reaction/number of mols of hydrocarbon consumed in the reaction.times.100
 (mol %)]. Accordingly, if the hydrocarbon conversion can be kept low and
 the hydrocarbon left unreacted can be recovered and again supplied for
 reaction as a raw material, the unreacted hydrocarbon which would
 otherwise be incinerated and the hydrocarbon which would otherwise be
 converted to carbon monoxide or carbon dioxide can be partly converted to
 maleic anhydride, making it possible to drastically reduce the amount of
 hydrocarbon to be consumed as a raw material in the production of a unit
 amount of maleic anhydride. Therefore, this process is an extremely
 fascinating on an economical basis.
 The foregoing process is also advantageous in that the recovery of the
 unreacted hydrocarbon which would be otherwise incinerated makes it
 possible to drastically reduce the amount of gas to be wasted during the
 production of maleic anhydride, particularly the emission of carbon
 dioxide, which is one of the greenhouse effect gases the emission of which
 has recently faced a growing demand for reduction, and hence drastically
 reduce the influence on the environment.
 In practice, JP-A-49-81314 (The term "JP-A" as used herein means an
 "unexamined published Japanese patent application"), JP-A-54-151910 and
 JP-A-59-29679 propose a process which comprises reducing the hydrocarbon
 conversion in the reactor to keep the maleic anhydride selectivity high
 while the unreacted hydrocarbon is being partly recovered and returned to
 the reactor.
 However, none of these proposals have ever been commercially practiced.
 This is because the hydrocarbon concentration needs to be higher than ever
 to prevent the drop of the productivity of maleic anhydride while keeping
 the conversion in the reactor low. If the hydrocarbon concentration is
 higher than ever, high temperature portions called "hot spot" occur in the
 reactor, causing degradation of catalyst. This is also because when the
 unreacted hydrocarbon is recovered, carbon monoxide or carbon dioxide
 produced as by-product, too, is recovered, making it necessary to use
 large amount of pure oxygen or oxygen enriched air, which is an expensive
 oxygen source, due to restrictions on material balance.
 Further, economically favorable reaction conditions differ greatly from
 that of the conventional once through reaction. Thus, the criteria of
 explosion safety of the feed gas to, or the effluent gas from the reactor,
 product recovering apparatus or hydrocarbon recovering apparatus greatly
 differ. The foregoing proposals contain reference to the safety of the
 reactor feed gas but have no reference to the safety of the entire recycle
 process.
 On the other hand, JP-A-1-165564 proposes a process which comprises
 returning unreacted hydrocarbon recovered by an apparatus for selectively
 separating hydrocarbon to a reactor wherein the content of flame
 suppressor is regulated to prevent a mixture of hydrocarbon and oxygen
 from producing a flammable mixture. However, this proposal regulates the
 safety of the stream from the reaction apparatus to the hydrocarbon
 recovering apparatus and back to the reaction apparatus but doesn't
 suffice for the safety of the entire process for the production of maleic
 anhydride. In other words, it is substantially difficult to completely
 recover hydrocarbon by the hydrocarbon recovering apparatus. Thus, the
 exhaust gas after recovering hydrocarbon is a mixed gas containing
 flammable gases such as hydrocarbon and carbon monoxide and oxygen.
 Accordingly, the explosion safety of the mixed gas must be considered.
 In accordance with economically favorable conditions under which a high
 productivity can be realized, that is, the concentration of maleic
 anhydride in the reaction gas can reach not less than 2 vol %, the
 concentration of carbon monoxide in the reaction gas, too, is higher than
 under the conventional conditions. Accordingly, the exhaust gas after
 recovering hydrocarbon has a higher carbon monoxide concentration than the
 conventional composition the safety of which has heretofore been known.
 Thus, it is likely that the explosive region of the exhaust gas is
 expected to be wider. Nevertheless, no specific methods for controlling
 the explosion safety have been known.
 In other words, some methods have been proposed which comprise recovering
 and recycling unreacted hydrocarbon to the rector while keeping the
 hydrocarbon conversion in the reactor low to enhance the maleic anhydride
 selectively for the purpose of efficiently producing maleic anhydride.
 However, all the foregoing proposals are disadvantageous in that the use
 of pure oxygen or oxygen enriched air, which is expensive, adds to the
 production cost and the enhancement of productivity is accompanied by the
 generation of enormous heat that deteriorates the performance of the
 catalyst. These proposals are also disadvantageous in respect to safety
 control. Thus, these proposals are not necessarily excellent methods. In
 actuality, these proposals have never been commercially practiced.
 SUMMARY OF THE INVENTION
 The present invention has been worked out for the purpose of providing
 reaction conditions required for the production of economically excellent
 maleic anhydride and conditions required for securing safety in a process
 which comprises allowing a hydrocarbon and an oxygen-containing gas to
 undergo reaction in the presence of a catalyst to produce maleic
 anhydride, recovering maleic anhydride from the reaction gas, recovering
 the hydrocarbon left unreacted from the remaining gas, and then returning
 the unreacted hydrocarbon to the reactor for re-use.
 The inventors made extensive studies of the foregoing problems. As a
 result, it was found that when the reaction is effected with the
 concentration of hydrocarbon and oxygen in the gas to be fed into the
 reactor and the conversion of hydrocarbon in the reaction combined under
 predetermined conditions, the productivity of maleic anhydride can be
 enhanced while keeping the amount of hydrocarbon to be consumed as a raw
 material low, making it possible to produce maleic anhydride on an
 economical basis. Further, paying attention to the concentration of
 oxygen, hydrocarbon and carbon monoxide in the exhaust gas after
 recovering hydrocarbon at the hydrocarbon recovery step, a gas explosion
 experiment was repeated. As a result, it was found that when the
 relationship between these gas concentrations is kept under predetermined
 conditions, safety can be secured. The present invention has been worked
 out on the basis of this knowledge.
 The present invention provides a process for the production of maleic
 anhydride which comprises a reaction step for reacting a hydrocarbon with
 an oxygen-containing gas in the presence of a catalyst, a maleic anhydride
 recovering step for recovering maleic anhydride from the reaction gas, a
 hydrocarbon recovering step for recovering unreacted hydrocarbon from the
 remaining gas and a recycling step for returning the hydrocarbon thus
 recovered to the reactor for re-use, characterized in that said reaction
 is effected under the conditions such that the hydrocarbon concentration X
 (vol %) and oxygen concentration Y (vol %) in all the gases to be fed into
 the reactor, the hydrocarbon conversion Z (%) in the reactor and the
 oxygen concentration W (vol %) in all the effluent gases from the reactor
 satisfy the following relationships:
EQU Y.gtoreq.20,
EQU X+Y.ltoreq.70,
EQU 1.ltoreq.Y/X.ltoreq.5, and
EQU 20(Y-10)/X.ltoreq.Z.ltoreq.25Y/X or 2.ltoreq.W.ltoreq.10.
 In another embodiment of the present invention, a process for the
 production of maleic anhydride is provided which comprises a reaction step
 for reacting a hydrocarbon with an oxygen-containing gas in the presence
 of a catalyst, a maleic anhydride recovering step for recovering maleic
 anhydride from the reaction gas, a hydrocarbon recovering step for
 recovering unreacted hydrocarbon from the remaining gas and a recycling
 step for returning the hydrocarbon thus recovered to the reactor for
 re-use, characterized in that the concentration of maleic anhydride in the
 reaction gas is not less than 2 vol % and the oxygen concentration A (vol
 %), hydrocarbon concentration B (vol %) and carbon monoxide concentration
 C (vol %) in the exhaust gas after recovering unreacted hydrocarbon at the
 hydrocarbon recovering step, satisfy the following requirements:
 D=C/(B+C),
 E=100A/(100-B-C), and
 0&lt;.alpha.&lt;10, in which .alpha.=-10.51+51.22D-35.35D.sup.2 -E.
 In a preferred embodiment of the present invention, a process for the
 production of maleic anhydride is provided which comprises a reaction step
 for reacting a hydrocarbon with an oxygen-containing gas in the presence
 of a catalyst, a maleic anhydride recovering step for recovering maleic
 anhydride from the reaction gas, a hydrocarbon recovering step for
 recovering unreacted hydrocarbon from the remaining gas and a recycling
 step for returning the hydrocarbon thus recovered to the reactor for
 re-use, characterized in that (a) said reaction is effected under the
 conditions such that the hydrocarbon concentration X (vol %) and oxygen
 concentration Y (vol %) in all the gases to be fed into the reactor, the
 hydrocarbon conversion Z (%) in the reactor and the oxygen concentration W
 (vol %) in all the effluent gases from the reactor satisfy the following
 relationships:
EQU Y.gtoreq.20,
EQU X+Y.ltoreq.70,
EQU 1.ltoreq.Y/X.ltoreq.5, and
EQU 20(Y-10)/X.ltoreq.Z.ltoreq.25Y/X or 2.ltoreq.W.ltoreq.10, and
 (b) the oxygen concentration A (vol %), hydrocarbon concentration B (vol %)
 and carbon monoxide concentration C (vol %) in the exhaust gas after
 recovering unreacted hydrocarbon at the hydrocarbon recovering step
 satisfy the following requirements:
 D=C/(B+C),
 E=100A/(100-B-C), and
 0&lt;.alpha.&lt;10, in which .alpha.=-10.51+51.22D-35.35D.sup.2 -E.
 DETAILED DESCRIPTION OF THE INVENTION
 Preferred Embodiment
 The present invention will be further described hereinafter.
 As mentioned above, the present invention provides a process for the
 production of maleic anhydride which comprises a reaction step for
 reacting a hydrocarbon with an oxygen-containing gas in the presence of a
 catalyst, a maleic anhydride recovering step for recovering maleic
 anhydride from the reaction gas, a hydrocarbon recovering step for
 recovering unreacted hydrocarbon from the remaining gas and a recycling
 step for returning the hydrocarbon thus recovered to the reactor for
 re-use, characterized in that said reaction is effected with the
 concentration of hydrocarbon and oxygen in the gas to be fed into the
 reactor, the hydrocarbon conversion in the reaction and the oxygen
 concentration in all the effluent gases from the reactor combined under
 predetermined conditions and the relationship among the concentration of
 oxygen, hydrocarbon and carbon monoxide in the exhaust gas after
 recovering unreacted hydrocarbon at the hydrocarbon recovering step
 satisfies predetermined conditions.
 In the production process of the present invention, maleic anhydride is
 produced at a reaction step for reacting a hydrocarbon with an
 oxygen-containing gas in the presence of a catalyst in a reactor.
 As the hydrocarbon to be used as a raw material at the foregoing reaction
 step there is preferably used a hydrocarbon having four carbon atoms such
 as butane, butene and butadiene. Particularly preferred among these
 hydrocarbons is n-butane, which is a saturated hydrocarbon having four
 carbon atoms. As the oxygen-containing gas there is normally used air.
 Further, air diluted with an inert gas, air enriched with oxygen or the
 like may be used.
 As the catalyst there is preferably used one comprising as an active
 component a mixed oxide containing vanadium and phosphorus as main
 constituents (hereinafter occasionally referred to as "vanadium-phosphorus
 mixed oxide-based catalyst"). These catalysts themselves are well known
 and commonly used. For example, such a catalyst can be prepared by the
 method described in U.S. Pat. Nos. 4,520,127 and 4,472,527, and
 JP-A-7-68179.
 As the reactor there may be used a commonly used fixed bed reactor or
 fluidized bed reactor. However, the fluidized bed reactor is preferable
 due to its insusceptibility to problem of explosion of the reactor feed
 gas and generation of hot spots.
 The fluidized bed reactor to be used herein may be in the form of ordinary
 structure comprising a gas dispersing plate provided at the bottom of the
 reactor defining the lower end of the catalyst fluidized bed, a raw
 material gas feed port provided in the lower zone of the catalyst
 fluidized bed and a particle recovering apparatus such as cyclon collector
 and/or filter system provided at the top or outlet of the reactor.
 Preferably, the reactor is further provided with an indirect heat
 exchanger for cooling the reaction product gas such as cooling coil at the
 position where the catalyst fluidized bed is to be formed.
 In the process of the present invention, the vanadium-phosphorus mixed
 oxide-based catalyst on the gas dispersing plate becomes fluidized by the
 gas which has been fed from below the gas dispersing plate in the reactor
 to form a dense fluidized bed above the gas dispersing plate. The heat
 generated by the reaction is removed by the heat exchanger provided in the
 fluidized bed to control the reaction temperature. The reaction
 temperature is normally from about 330.degree. C. to 500.degree. C.,
 preferably from about 360.degree. C. to 460.degree. C. In this arrangement
 of catalyst fluidized bed, the hydrocarbon as raw material undergoes
 catalytic oxidation in a gas phase to produce maleic anhydride in the
 reaction product gas.
 The reaction product gas contains maleic anhydride as desired compound as
 well as unreacted oxygen and hydrocarbon and by-products, including carbon
 dioxide, water and carbon monoxide, in various concentrations. The
 reaction product gas comes out of the catalyst fluidized bed together with
 the catalyst, and then is introduced into the particle recovering
 apparatus such as cyclon provided at the top or outlet of the reactor
 where it is then separated from the entrained catalyst and withdrawn. The
 catalyst separated from the reaction product gas in the particle
 recovering apparatus is returned to the fluidized bed, if desired. Maleic
 anhydride is separated and recovered from the reaction product gas thus
 withdrawn (maleic anhydride recovering step).
 The concentration of maleic anhydride in the reaction product gas is not
 specifically limited. However, it is preferably not less than 2.0 vol %,
 more preferably not less than 2.5 vol %, even more preferably not less
 than 3.0 vol %. By keeping the concentration of maleic anhydride in the
 reaction product gas high, the amount of gas to be circulated in the
 recycle process can be reduced.
 The separation and recovery of maleic anhydride can be accomplished by any
 commonly used method known as such, e.g., method which comprises cooling
 the reaction gas to condense maleic anhydride, method which comprises
 allowing the reaction gas to come in contact with water to collect maleic
 anhydride in water as maleic acid, method which comprises allowing the
 reaction gas to come in contact with an organic solvent such as phthalic
 acid dialkyl ester or alkyl ester of hydrogenated phthalic acid (e.g.,
 tetrahydrophthalic acid, hexahydrophthalic acid) to collect maleic
 anhydride in the organic solvent.
 The raw material hydrocarbon left unreacted is then recovered from the
 remaining gas after separating and recovering maleic anhydride
 (hydrocarbon recovering step). The hydrocarbon thus recovered at the
 hydrocarbon recovering step is then returned to the reactor for re-use
 (recycling step). A fresh oxygen-containing gas and a hydrocarbon are fed
 into the reactor in such an amount that the total amount of gases to be
 fed into the reactor and the concentrations of oxygen and hydrocarbon in
 all the feed gases are kept at predetermined values.
 If gases other than the raw material hydrocarbon are recovered at the
 hydrocarbon recovering step, the amount of air which can be used as an
 oxygen source to be fed into the reactor must be restricted. In other
 words, if nitrogen, carbon monoxide or carbon dioxide is recovered
 together with hydrocarbon at the hydrocarbon recovering step, the amount
 of inert gases (i.e., gases other than hydrocarbon and oxygen) to be
 returned to the reactor is increased so much. The total amount of inert
 gases which can be fed into the reactor is determined depending on the
 concentration of hydrocarbon and oxygen. The value obtained by subtracting
 the amount of inert gases to be recycled to the reactor from the total
 amount of inert gases which can be fed into the reactor is the amount of
 inert gas which can be freshly fed into the reactor. Thus, when air is
 used as an oxygen source, the amount of nitrogen accompanied with air to
 be fed is restricted. As a result, the amount of air which can be fed into
 the reactor is remarkably restricted, making it necessary to use pure
 oxygen or oxygen enriched air as an oxygen source.
 The foregoing problem can be solved by the use of a method enabling the
 selective recovery of hydrocarbon as a method for recovering the raw
 material hydrocarbon. The term "selective" as used herein is meant to
 indicate that more than about half the amount of gases other than
 hydrocarbon is not recovered while recovering the majority (more than
 about 90%) of hydrocarbon. This selective recovery of hydrocarbon makes it
 possible to maximize the utilization of air, which is an inexpensive
 oxygen source to be fed into the reactor. However, even this method can
 hardly eliminate the necessity of using pure oxygen or oxygen enriched
 air. This is because the process of the present invention unavoidably
 requires the use of some amount of pure oxygen or oxygen enriched air to
 meet the requirements for high concentration of hydrocarbon and oxygen in
 all the gases to be fed into the reactor for the purpose of remarkably
 enhancing the productivity of maleic anhydride as described later.
 However, in accordance with the process of the present invention, the
 economical advantage developed by a drastic drop of the amount of
 hydrocarbon consumed as a raw material overwhelms the disadvantage
 developed by the use of a small amount of pure oxygen or oxygen enriched
 air. Further, the cost reduction developed by the enhancement of the
 productivity of maleic anhydride that allows the drop of the recycled
 amount of gas and the load on the waste gas incinerator overwhelms the
 cost rise developed by the introduction of the apparatus for selectively
 separating and recovering hydrocarbon. Accordingly, the entire production
 cost of maleic anhydride can be reduced.
 As the apparatus for selectively separating and recovering hydrocarbon
 there may be used any commonly used membrane type separating apparatus or
 adsorption-separation type apparatus known as such, e.g., PSA (pressure
 swing adsorption apparatus), VSA (vacuum swing adsorption apparatus), TSA
 (temperature swing adsorption apparatus). If such an adsorption-separation
 type apparatus is used, a method may be employed involving the use of an
 adsorbent capable of selectively separating and recovering hydrocarbon
 such as zeolite or silicalite as disclosed in JP-A-8-325256 and U.S. Pat.
 No. 4,987,239.
 The remaining gas after separating and recovering maleic anhydride can
 partly be directly returned to the reactor without being passed through
 the hydrocarbon separating/recovering apparatus. This arrangement causes
 gases other than hydrocarbon such as carbon dioxide, carbon monoxide and
 nitrogen to be recycled to the reactor but makes it possible to reduce the
 size of the hydrocarbon separating/recovering apparatus.
 The point which should be noted to produce maleic anhydride more
 economically than ever in the foregoing process for the production of
 maleic anhydride which comprises returning the unreacted hydrocarbon gas
 to the reactor for re-use is how much the consumed amount of raw material
 hydrocarbon and the amount of gas to be recycled in the system can be
 reduced. In order to meet the foregoing requirements, it is effective to
 keep the conversion in the reactor low, thus enhancing the maleic
 anhydride selectivity, and at the same time to increase the concentration
 of maleic anhydride in the reaction product gas.
 In the case where the concentration of raw material hydrocarbon in all the
 gases to be fed into the reaction system is the same, if the conversion of
 raw material hydrocarbon is reduced, the concentration of maleic anhydride
 in the reactor effluent gas (hereinafter referred to as "productivity") is
 reduced even if the maleic anhydride selectivity is enhanced. This means
 that it is necessary to raise the concentration of hydrocarbon in all the
 gases to be fed into the reaction system higher than ever in order to
 obtain a productivity which is equal to or higher than the conventional
 value in the process for the reduction of the hydrocarbon conversion in
 the reactor.
 In the case where maleic anhydride is prepared from n-butane, which is a
 saturated hydrocarbon having four carbon atoms, carbon monoxide, carbon
 dioxide, etc. are normally produced as by-products. As a result, oxygen is
 consumed in a total amount of from 3.8 to 4.9 mols per mol of n-butane in
 the entire reaction involving the production of maleic anhydride, carbon
 monoxide, carbon dioxide, water, etc. from n-butane.
 However when the butane concentration is high to enhance the productivity,
 if oxygen is not present in an amount high enough for reaction, oxygen is
 earlier consumed completely, making it impossible to enhance the
 concentration of maleic anhydride in the reaction gas. In other words, it
 is necessary to supply oxygen into the reactor in an amount of at least
 from 3.8 to 4.9 times the amount of butane to be consumed in the reaction
 while increasing the butane concentration in order to enhance the
 productivity.
 On the other hand, the inventors found that the maleic anhydride
 selectivity is greatly effected not only by the hydrocarbon conversion but
 also by the concentration of hydrocarbon and oxygen in the gas to be
 supplied into the reaction. The results show that the reduction of the
 hydrocarbon conversion causes the enhancement of the maleic anhydride
 selectivity while the increase of the hydrocarbon concentration in the
 reactor feed gas as well as the decrease of the oxygen concentration in
 the reactor feed gas cause the drop of maleic anhydride selectivity. This
 demonstrates that the ratio of oxygen concentration to hydrocarbon
 concentration in the reactor feed gas is an important factor determining
 the maleic anhydride selectivity.
 Accordingly, it is extremely important to select optimum conditions from
 various combinations of the concentration of hydrocarbon and oxygen in the
 reactor feed gas and the hydrocarbon conversion in the reaction in order
 to enhance the productivity while keeping the consumed amount of raw
 material hydrocarbon low.
 In other words, the inventors succeeded in finding the optimum conditions
 by measuring the maleic anhydride selectivity over various hydrocarbon
 concentrations, oxygen concentrations and hydrocarbon conversions and
 evaluating the economical efficiency of the entire process under the
 various conditions.
 Thus, in the present invention, when the hydrocarbon concentration in all
 the gases to be fed into the reactor is sufficiently high, the oxygen
 concentration is an important factor determining the productivity.
 Accordingly, in order to enhance the productivity higher than ever, it is
 necessary that the oxygen concentration (Y: vol %) be not less than 20 vol
 %, preferably not less than 25 vol %. The higher the oxygen concentration
 is, the higher can be enhanced the productivity. In order to increase the
 oxygen concentration, it is necessary to use increased amount of pure
 oxygen or oxygen enriched air, which is expensive. Thus, it is required
 that the oxygen concentration be substantially kept not greater than 50
 vol %, preferably not greater than 40 vol %.
 On the other hand, the hydrocarbon concentration in all the gases to be fed
 into the reactor (X: vol %) needs to be arranged such that the ratio of
 the oxygen concentration to the hydrocarbon concentration (Y/X) is kept at
 a range of from 1 to 5. If Y/X falls below 1, that is, the hydrocarbon
 concentration is greater than the oxygen concentration, the hydrocarbon
 conversion is decreased but the maleic anhydride selectivity drops. On the
 contrary, if Y/X exceeds 5, it is necessary to increase the hydrocarbon
 conversion in order to reduce the oxygen concentration at the outlet of
 the reactor so as to prevent the effluent gas from being kept in an
 explosive state. This, too, reduces the maleic anhydride selectivity. As a
 result, it is necessary that Y/X range from not less than 1 to not more
 than 5, preferably from not less than 1.2 to not more than 4.5, more
 preferably from not less than 1.5 to not more than 4 to keep the maleic
 anhydride selectivity high.
 The higher the sum of the content of hydrocarbon and oxygen in all the
 gases to be fed into the reactor (X+Y) is, the higher is the productivity.
 However, it is not practical to increase the sum (X+Y) high than 70 vol %.
 This is because the increase in the sum of the content of hydrocarbon and
 oxygen means the reduction of the proportion of other gases such as
 nitrogen or carbon dioxide that limits the consumable amount of air, which
 is the most inexpensive oxygen source, resulting in the increase in the
 required amount of pure oxygen or oxygen enriched air, which is relatively
 expensive, and hence increasing the economical disadvantage. Therefore,
 the nitrogen concentration in all the gases to be fed into the reactor is
 at least 30 vol %, preferably at least 35 vol %.
 Further, when hydrocarbon is recovered from at least a part of the
 remaining gas after recovering maleic anhydride from reactor effluent gas,
 and then returned to the reactor, it is not practical to recover only
 hydrocarbon even using an apparatus capable of selectively separating
 hydrocarbon. As a result, some amount of carbon dioxide or nitrogen must
 be returned to the reactor together with the hydrocarbon. The amount of
 carbon dioxide or nitrogen to be returned to the reactor together with the
 hydrocarbon restricts the sum of the amount of hydrocarbon and oxygen in
 the reactor feed gas.
 For these reasons, the economically and practically possible range of the
 composition of the reactor feed gas was extensively studied. As a result,
 it was found that the sum (X+Y) needs to be not more than 70 vol %,
 preferably not more than 60 vol %, even more preferably not more than 50
 vol %.
 When the gas having the composition satisfying the foregoing requirements
 is fed into the reactor, the resulting hydrocarbon conversion must be from
 not less than 20(Y-10)/X to not more than 25Y/X. If the hydrocarbon
 conversion (Z: %) exceeds the above defined range, the maleic anhydride
 selectivity id dropped more than required. On the contrary, if the
 hydrocarbon conversion falls below the above defined range, the oxygen
 concentration in all the reactor effluent gases is too high, possibly
 causing the explosion of the effluent gas. By controlling the hydrocarbon
 conversion to keep the oxygen concentration in all the reactor effluent
 gases (W: vol %) to a range of from not less than 2 vol % to not more than
 10 vol %, the same effect can be exerted.
 In this recycle process, it is important to minimize the loss of
 hydrocarbon for the purpose of efficiently produce maleic anhydride. The
 percent recovery of hydrocarbon [number of mols of hydrocarbon recovered
 by the recovering apparatus/number of mols of hydrocarbon supplied into
 the recovering apparatus.times.100 (%)] is preferably not less than 90%,
 more preferably not less than 95%, even more preferably not less than 98%.
 In the recycle process satisfying the foregoing requirements, the exhaust
 gas left after the hydrocarbon recovering step involving the recovery of
 unreacted hydrocarbon from the remaining gas after separating and
 recovering maleic anhydride from reactor effluent is a mixture gas
 containing carbon monoxide, hydrocarbon, oxygen, carbon dioxide and
 nitrogen. If hydrocarbon is selectively separated and recovered under
 preferred condition, the major flammable component in the exhaust gas is
 carbon monoxide with a slight amount of hydrocarbon.
 In general, the safety of the mixture gas containing oxygen and a plurality
 of flammable gas components against explosion is determined by the
 concentration of oxygen, the concentration of the various flammable gas
 components, and the temperature and pressure of the mixture gas. However,
 no methods for accurately predicting the safety of a mixture of e.g.,
 butane and carbon monoxide against explosion have been known. On the other
 hand, the composition of the gas to be fed into the hydrocarbon recovering
 step varies with the reaction conditions or results of reaction. Further,
 the composition of the exhaust gas after recovering hydrocarbon at the
 hydrocarbon recovering step varies with the hydrocarbon recovering
 conditions or results of recovery. In particular, if an
 adsorption-separation type hydrocarbon recovering apparatus is used, the
 composition of the effluent from the hydrocarbon recovering apparatus
 shows a cyclic variation. Thus, in order to invariably secure the
 explosion safety of the mixture gas, it is essential to invariably confirm
 by some means that the gas composition is in a safe state.
 As methods for securing the explosion safety of a mixture of
 oxygen-containing gas and flammable gas there have normally been known
 three methods, i.e., method which comprises keeping the concentration of
 flammable gas at lower than the lower explosive limit, method which
 comprises keeping the concentration of flammable gas at higher than the
 upper explosive limit, method which comprises keeping the oxygen
 concentration at lower than the minimum oxygen concentration.
 The foregoing method which comprises keeping the concentration of flammable
 gas component at lower than the lower explosive limit has heretofore been
 actually used in the process for the production of maleic anhydride. In
 other words, the lower explosive limit of mixed gas can be predicted by Le
 Chateliers law, which has heretofore been widely known to those skilled in
 the art (see "Encyclopedia of Chemical Processing and Design Volume 22",
 published by Marcel Dekker, Inc., page 120 (1985)). Thus, by comparing the
 monitored concentration of hydrocarbon and carbon monoxide with the lower
 explosive limit in the monitored composition calculated using Le
 Chateliers law, a predetermined safety margin can be invariably secured.
 This method not only has heretofore been employed to secure the safety of
 the remaining gas after recovering maleic anhydride from the reactor
 effluent but also can be applied to secure the safety of the exhaust gas
 after recovering hydrocarbon at the hydrocarbon recovering step in the
 recycle process depending on some conditions.
 However, the study made by the inventors shows that the concentrations of
 flammable gases (sum of the concentration of hydrocarbon and carbon
 monoxide) in the exhaust gas after recovering hydrocarbon at the
 hydrocarbon recovering step can often exceed the lower explosive limit.
 This phenomenon becomes remarkable particularly under the conditions such
 that the concentration of maleic anhydride in the reaction gas increases.
 Under the conditions that the productivity is not less than 2 vol % as
 calculated in terms of maleic anhydride concentration, the method which
 comprises keeping the concentrations of flammable gases at lower than the
 lower explosive limit is substantially made difficult. This is because
 such condition produces carbon monoxide in a high concentration at the
 reaction step, thus the concentration of carbon monoxide in the exhaust
 gas from the hydrocarbon recovery step increases and when a small amount
 of unrecovered hydrocarbon is introduced into this stream, the
 concentration of flammable gas becomes equal to or greater than the lower
 explosive limit. In addition, if there occurs a composition change with
 time, particularly a cyclic or unexpected change in the concentration of
 hydrocarbon in the adsorption-separation type recovering apparatus, it is
 necessary to reduce the concentration of maleic anhydride at the reaction
 step and hence reduce the concentration of carbon monoxide co-produced or
 recover hydrocarbon in a higher percent recovery at the hydrocarbon
 recovering step in order to invariably secure sufficient safety margin. It
 is necessary at the same time to invariably monitor the gas composition
 accurately without delay in measurement.
 Further, the exhaust gas after recovering hydrocarbon at the hydrocarbon
 recovering step is mainly composed of carbon monoxide as a flammable gas
 and thus has an extremely high upper explosive limit. Accordingly, it is
 substantially difficult to keep the concentration of flammable gas higher
 than the upper explosive limit.
 On the other hand, it was made obvious that the method which comprises
 keeping the oxygen concentration in the exhaust gas after recovering
 hydrocarbon at the hydrocarbon recovering step lower than the minimum
 oxygen concentration can be sufficiently used as a safety control method
 even under the conditions such that the productivity of maleic anhydride
 is high.
 The oxygen concentration of the exhaust gas can be indirectly controlled by
 controlling the conditions in the reaction step to control the reactor
 effluent oxygen concentration even under such a high maleic anhydride
 productivity conditions. Thus the oxygen concentration of the exhaust has
 can be easily kept lower than the minimum oxygen concentration at all the
 time.
 The minimum oxygen concentration, too, is somewhat affected by the change
 in the flammable gas composition. However, the effect of the change in the
 flammable gas concentration on the minimum oxygen concentration is smaller
 than that on the lower explosive limit. Further, the object to be directly
 monitored is not the flammable gas concentration, the fluctuation of which
 is large, but the oxygen concentration, which is little liable to
 fluctuate even using the adsorption-separation type apparatus.
 Accordingly, the explosion safety of this stream can be easily managed by
 controlling the oxygen concentration always below the minimum oxygen
 concentration.
 Then, the inventors experimentally determined the minimum oxygen
 concentration in the mixture of carbon monoxide and hydrocarbon, which has
 never been known, and found that explosion safety control can be made on
 the basis of this value. Thus, another embodiment of implication of the
 present invention has been attained.
 In the present invention, in order to secure the safety of the exhaust gas
 after recovering hydrocarbon at the hydrocarbon recovering step, it is
 required that the concentration of oxygen, hydrocarbon and carbon monoxide
 in the exhaust gas (A (vol %), B (vol %) and C (vol %), respectively)
 satisfy the following requirements.
 The safety coefficient .alpha. shall indicate a value calculated by the
 following equation (1):
EQU .alpha.=-10.51+51.22 D-35.35 D.sup.2 -E (1)
 where D indicates a value calculated by the following equation (2):
EQU D=C/(B+C) (2)
 and E indicates a value calculated by the following equation (3):
EQU E=100 A/(100-B-C) (3)
 The safety coefficient also shall satisfy the following relationship:
EQU 0&lt;.alpha.&lt;10
 The safety coefficient .alpha. is a coefficient related to the minimum
 oxygen concentration in the oxygen-containing flammable gas mixture. The
 greater the safety coefficient .alpha. is, the more the current
 composition from the minimum oxygen concentration deviates, this means the
 condition is safer. Thus, when the safety coefficient .alpha. indicates a
 positive value, it can be said that the gas is safe. However, it is
 preferred that some margin be taken to the safety coefficient .alpha.
 taking into account the error in the analysis of various components or the
 lag in measurement. On the other hand, however, the productivity, catalyst
 life or operation cost needs to be sacrificed for increasing the safety
 coefficient .alpha.. Thus, it is not practical to take an excessive
 margin. Accordingly, the safety coefficient .alpha. is preferably kept at
 a range of from more than 0 to less than 10, more preferably from more
 than 1 to less than 5.
 The foregoing equation (1) is an empirical equation obtained by explosion
 experiments under various conditions. The temperature and pressure on
 which this equation is based range from normal temperature to 100.degree.
 C. and atmospheric pressure to 0.1 MPa (gauge), respectively. The equation
 (1) is substantially effective, when the value of D in the foregoing
 equation (2) is from not less than 0.7 to not more than 1.0 as a result of
 the explosion experiments.
 The oxygen concentration in the exhaust gas after recovering hydrocarbon at
 the hydrocarbon recovering step is preferably measured continuously by any
 commonly used oxygen analysis means known as such, e.g., on-line oxygen
 analyzer. The concentration of hydrocarbon and carbon monoxide in the
 exhaust gas are preferably measured continuously by any commonly used
 on-line monitor means known as such, e.g., infrared analyzer. The gas
 composition can be also measured by a discontinuous measuring means such
 as gas chromatograph. However, such a discontinuous measuring means is not
 desirable from the standpoint of safety control if the gas composition
 varies with time.
 In particular, if an adsorption-separation type apparatus is used as the
 hydrocarbon recovering apparatus, the composition of the exhaust gas after
 recovering hydrocarbon shows a cyclic variation. Thus, it is preferred to
 confirm that the safety coefficient .alpha. falls within a predetermined
 range by invariably calculating the safety coefficient .alpha. on a real
 time basis using an analyzer capable of continuously measuring the
 concentrations of each component.
 If the exhaust gas doesn't show a large composition change, the safety
 coefficient .alpha. can be calculated, e.g., from the newest value of
 hydrocarbon concentration and carbon monoxide concentration obtained by a
 discontinuous measuring means such as gas chromatograph and continuous
 measurement values of oxygen concentration obtained by an on-line oxygen
 analyzer. In this case, however, it is preferred that the larger safety
 coefficient .alpha. is to be kept taking into account the possible change
 in the minimum oxygen concentration with the composition change between
 the measurements.
 In the method for controlling the explosion safety using the safety
 coefficient .alpha., when the safety coefficient .alpha. thus calculated
 becomes smaller than the predetermined safety margin, the safety
 coefficient .alpha. can be increased by a method which comprises changing
 the operation conditions of the reaction apparatus to lower the effluent
 oxygen concentration, a method which comprises changing the operation
 conditions of the hydrocarbon separator to lower the oxygen concentration,
 a method which comprises adding an inert gas such as nitrogen to the
 stream to be fed into the hydrocarbon separator to lower the oxygen
 concentration or the like. The reduction of the oxygen concentration at
 the outlet of the reaction apparatus can be accomplished by any method
 well known to those skilled in the art such as method involving the
 reduction of the flow rate of an oxygen-containing gas to be fed into the
 reaction apparatus, method involving the increase of the flow rate of
 hydrocarbon to be fed into the reaction apparatus and method involving the
 rise in the reaction temperature of the reaction apparatus.
 On the contrary, if the safety coefficient .alpha. exceeds the
 predetermined margin, the safety coefficient .alpha. can be lowered to the
 desired range by operating the system counter to the foregoing procedures
 taking into account the economical efficiency of the process.
 It is most preferred that the foregoing method involving the safety control
 over the exhaust stream from the hydrocarbon recovering step using the
 safety coefficient .alpha. be applied to the process for the production of
 maleic anhydride in combination with the foregoing reaction conditions
 (combination of concentration of hydrocarbon and oxygen to be fed into the
 reactor and the hydrocarbon conversion in the reactor or the effluent
 oxygen concentration). However, in the process for the production of
 maleic anhydride by a recycle process under reaction conditions different
 from those described above, too, if the reaction is effected under the
 conditions such that the productivity is as high as not less than 2.0 vol
 % as calculated in terms of concentration of maleic anhydride in the
 reaction product gas, the safety control over the exhaust stream from the
 hydrocarbon recovering step is preferably effected using the foregoing
 safety coefficient .alpha..
 It goes without saying that the foregoing safety coefficient .alpha. can be
 used in the explosion safety control not only over the exhaust stream from
 the hydrocarbon recovering apparatus but also over the mixture of carbon
 monoxide and hydrocarbon having the similar composition. For example, the
 effluent gas from the maleic anhydride recovering step in the present
 process, too, is a mixed gas containing carbon monoxide and hydrocarbon as
 main flammable gases. Thus, if the temperature, pressure and composition
 of such an effluent gas fall within the range to which the foregoing
 conditions can be applied, such an effluent gas can undergo similar safety
 control using the safety coefficient .alpha..
 In accordance with the process of the present invention, the productivity
 can be drastically enhanced while reducing the amount of hydrocarbon to be
 consumed in the production of maleic anhydride. Further, the amount of
 gases to be wasted, particularly carbon dioxide gas, can be drastically
 reduced. Moreover, the possibility of explosion of the exhaust gas after
 recovering hydrocarbon at the hydrocarbon recovering step can be
 eliminated, making it possible to provide a safe and efficient process for
 the production of maleic anhydride.

EXAMPLES
 The present invention will be further described in the following examples,
 but the present invention should not be construed as being limited
 thereto.
 Examples 1-3
 Comparative Examples 1-4
 A fluidized bed catalyst comprising a vanadium-phosphorus-based mixed oxide
 as an active component was prepared in accordance with the method
 described in JP-A-7-068179 (corresponding to U.S. Pat. No. 5,498,731). The
 catalyst thus obtained was then measured for the results of reaction with
 different compositions of the mixture of n-butane, oxygen and nitrogen in
 the following manner.
 A quartz glass reaction tube was filled with 0.001 l of the catalyst. A
 mixed gas having a predetermined concentration was then allowed to flow
 through the reaction tube at a rate of 1.0 Nl/hr while the temperature of
 the reactor was being kept at a predetermined value by means of an
 electric furnace. After a predetermined period of time, the reactor
 effluent gas was sampled, and then analyzed by means of a gas
 chromatograph on-line connected to the reactor. The results are set forth
 in Table 1, which indicates the maleic anhydride concentration of the
 reactor effluent as a measure of productivity. As can be seen in Table 1,
 under the conditions of Examples 1, 2 and 3, the maleic anhydride
 selectivity can be kept high and the maleic anhydride concentration of the
 reactor effluent can be drastically enhanced as compared with the
 comparative examples. Comparative Example 1 exhibits a sufficiently high
 maleic anhydride concentration of the reactor effluent but exhibits very
 low maleic anhydride selectivity due to considerably high butane
 conversion. Comparative Example 2 exhibits a sufficiently high selectivity
 and maleic anhydride concentration but exhibits too high effluent oxygen
 concentration, giving an effluent gas falling within an explosive range.
 Thus, Comparative Example 2 is disadvantageous in respect to safety in
 operation.
 TABLE 1
 Butane oxygen Reaction Remaining
 MA
 concen- concen- temper- Butane MA oxygen
 concen- concen-
 tration X tration Y ature Conversion: selectivity:
 tration: W tration
 Example No. (vol %) (vol %) (.degree. C.) Z (mol %) S (mol %)
 (vol%) Y/X (vol %)
 Example 1 7.9 30.0 414 66.4 66.6 10.9
 3.8 2.8
 431 68.7 62.5 7.0
 3.2
 448 79.2 58.1 3.0
 3.3
 454 81.8 56.2 2.0
 3.3
 Example 2 12.0 39.4 418 54.1 64.4 11.9
 3.3 3.9
 436 66.7 60.6 6.3
 4.4
 454 76.5 56.3 0.0
 4.6
 Example 3 20.6 24.2 365 13.0 67.7 13.0
 1.2 1.8
 382 18.0 66.1 8.6
 2.3
 400 23.5 63.6 3.7
 2.9
 Comparative 7.9 40.0 451 89.2 56.6 8.8
 5.1 3.6
 Example 1 464 94.8 51.5 6.3
 3.5
 481 98.7 42.4 3.9
 2.9
 Comparative 8.0 58.5 430 84.0 62.2 28.9
 7.3 3.8
 Example 2 446 96.1 50.5 23.3
 3.4
 Comparative 40.7 19.9 366 7.0 60.7 7.4
 0.6 1.7
 Example 3 384 9.7 58.3 2.6
 2.2
 Comparative 16.0 17.6 380 17.2 67.4 6.1
 1.1 1.8
 Example 4 396 22.2 64.8 2.7
 2.2
 Butane conversion = (Number of n-butane consumed in the reaction)/(Number
 of n-butane supplied) .times. 100 (mol %)
 MA selectivity = (Number of mols of maleic anhydride produced)/(Number of
 mols of n-butane consumed in the reaction) .times. 100 (mol %)
 MA concentration = Concentration of maleic anhydride in the reactor
 effluent (vol %)
 Example 4
 A small-sized fluidized bed reactor having a length of 2.0 m and a diameter
 of 0.1 m longitudinally partitioned by 10 sheets of a metal mesh was
 filled with 0.85 kg of the same catalyst as used in Examples 1 to 3. A
 fluidized bed reaction was then effected with a mixture of oxygen and
 nitrogen being fed through the bottom of the reactor and butane being fed
 at a position 0.1 m above the bottom of the reactor. Each gas flow rate
 was controlled so that the total feed flow rate was 0.48 Nm.sup.3 /hr and
 the concentrations of butane, oxygen and nitrogen were 15 vol %, 25 vol %
 and 60 vol %, respectively.
 Referring to the results obtained just after the start up of the reaction,
 the butane conversion and the maleic anhydride selectivity were 33 mol %
 and 63 mol %, respectively, at a reaction temperature of 405.degree. C.
 The reaction continued at the same temperature. The results obtained after
 2,000 hours were as stable as 31 mol % for butane conversion and 62 mol %
 for maleic anhydride selectivity.
 Example 5
 Mixtures of n-butane, carbon monoxide, oxygen and nitrogen having the
 composition set forth in Table 2 were prepared. The mixed gas thus
 prepared was then introduced into a preheated explosion vessel having a
 capacity of 1 l. In the explosion vessel, ignition was made with a 15 kV
 a.c. spark (0.01 sec., spark gap: 3 mm). With the rise in the pressure in
 the vessel, the mixed gas was checked to evaluate whether it exploded or
 not. The temperature of the explosion vessel was 80.degree. C.
 The results are set forth in Table 2.
 TABLE 2
 Total concen-
 Oxygen
 n- Carbon CO/BTA tration of
 concentration Judgment
 Example Butane monoxide Oxygen Nitrogen (vol flammable gas in
 atmosphere on
 No. (vol %) (vol %) (vol %) (vol %) /vol) (vol %)
 (vol %) explosion
 1 0.41 7.70 9.65 82.25 95/5 8.1
 10.5 .largecircle.
 2 0.43 8.08 6.13 85.37 95/5 8.5
 6.7 .largecircle.
 3 0.47 8.84 5.90 84.80 95/5 9.3
 6.5 .largecircle.
 4 0.46 8.65 5.82 85.08 95/5 9.1
 6.4 X
 5 0.47 8.84 5.80 84.90 95/5 9.3
 6.4 X
 6 0.48 9.03 5.79 84.71 95/5 9.5
 6.4 X
 7 0.75 4.81 9.89 84.54 87/13 5.3
 10.5 .largecircle.
 8 0.78 4.98 7.52 86.72 87/13 6.0
 8.0 .largecircle.
 9 0.81 5.15 7.04 87.01 87/13 6.2
 7.5 .largecircle.
 10 0.83 5.31 6.93 86.93 87/13 6.4
 7.4 .largecircle.
 11 0.83 5.31 6.83 87.02 87/13 6.4
 7.3 X
 12 0.86 5.48 6.82 86.65 87/13 6.6
 7.3 X
 13 0.88 5.64 6.80 86.67 87/13 6.8
 7.3 X
 14 1.14 2.66 10.10 86.10 70/30 3.8
 10.5 .largecircle.
 15 1.20 2.80 8.06 87.94 70/30 4.0
 8.4 .largecircle.
 16 1.26 2.94 7.95 87.85 70/30 4.2
 8.3 .largecircle.
 17 1.26 2.94 7.86 87.94 70/30 4.2
 8.2 .largecircle.
 18 1.23 2.87 7.77 88.13 70/30 4.1
 8.1 X
 19 1.26 2.94 7.76 88.04 70/30 4.2
 8.1 X
 20 1.29 3.01 7.75 87.95 70/30 4.3
 8.1 X
 O/BTA (vol/vol) = Carbon monoxide concentration (vol %)/n-butane
 concentration (vol %)
 Total concentration of flammable gas (vol %) = n-Butane concentration (vol
 %) + carbon monoxide concentration (vol %)
 Oxygen concentration in the atmosphere (vol %) = (Oxygen concentration (vol
 %)/(Oxygen concentration (vol %) + nitrogen concentration (vol %)) .times.
 100
 Judgment on explosion: .largecircle.: Exploded; X: Not exploded
 The maximum oxygen concentration in the atmosphere [oxygen
 concentration/(oxygen concentration+nitrogen concentration).times.100 (vol
 %)] at which no explosion is observed when the total concentration of
 flammable gas [sum of n-butane concentration and carbon monoxide
 concentration (vol %)] varies from a value lower than the expected lower
 explosive limit to a value higher than the expected upper explosive limit
 corresponds to the minimum oxygen concentration of the mixed gas having
 the foregoing carbon monoxide/butane proportion.
 Accordingly, the minimum oxygen concentration of mixed gases having various
 carbon monoxide/butane proportions can be determined from the results
 shown in Table 2. The results are set forth in Table 3.
 TABLE 3
 CO/BTA (vol/vol) minimum oxygen concentration (vol %)
 95/5 6.4
 87/13 7.3
 70/30 8.1
 Example 6
 Comparative Example 5
 An embodiment of implication of the present invention was calculated as
 follows. In some detail, the material balance in the production of maleic
 anhydride at a rate of 1,000 kg per hour was determined.
 In this embodiment of implication of the present invention, the
 concentrations of butane (X) and oxygen (Y) in all the gases to be fed
 into the reactor were 15 vol % and 25 vol %, respectively, the butane
 conversion (Z) was 35%, and the maleic anhydride selectivity was 63 mol %.
 The recovery of unreacted butane was accomplished by means of PSA
 (pressure swing adsorption apparatus) described in U.S. Pat. No. 4,987,239
 at a percent recovery of 95% for butane, 84% for carbon dioxide and 20%
 for others. In this arrangement, butane, oxygen and air must be freshly
 fed at a flow rate of 397.3 Nm.sup.3 /hr, 1,102.1 Nm.sup.3 /hr and 2,882.6
 Nm.sup.3 /hr, respectively. The gas composition and flow rate at various
 positions were as set forth in Table 4. In this case, X+Y was 40 vol %,
 Y/X was 1.67, and the reactor effluent oxygen concentration (W) was 3.0
 vol %.
 As a comparative example, a conventional fluidized bed reaction involving
 no recovery of unreacted hydrocarbon was calculated. In some detail,
 calculation was made on the supposition that the concentration of butane
 and oxygen in all the gases to be fed into the reactor were 4.5 vol % and
 20 vol %, respectively, the butane conversion was 83% and the maleic
 anhydride selectivity was 60 mol %. In this arrangement, butane, oxygen
 and air must be freshly fed at a flow rate of 458.7 Nm.sup.3 /hr, 0.0
 Nm.sup.3 /hr and 9,733.0 Nm.sup.3 /hr, respectively. The gas composition
 and flow rate at various positions were as set forth in Table 5.
 As can be seen in Tables 4 and 5, in accordance with the foregoing
 embodiment of implication of the present invention, the amount of raw
 material butane, the amount of gas to be fed into the reactor, the amount
 of exhaust gas and the amount of carbon dioxide to be exhausted for the
 production of the same amount of maleic anhydride can be drastically
 reduced, i.e., to about 87%, about 69%, about 29% and about 73%,
 respectively, as compared with the conventional case. On the other hand,
 the ratio of the oxygen amount from the pure oxygen to the total oxygen
 amount in the reactor feed among oxygen sources was kept at about 65%.
 This demonstrates that the economical efficiency of the entire process is
 extremely enhanced.
 TABLE 4
 Reactor inlet Reactor inlet
 (fresh Feed) (total feed) Reactor effluent
 Component Nm.sup.3 /hr Vol % Nm.sup.3 /hr Vol % Nm.sup.3 /hr Vol %
 Butane 397.3 9.1 1,051.5 15.0 688.7 9.2
 Oxygen 1,707.2 39.0 1,752.5 25.0 226.3 3.0
 Nitrogen 2,249.9 51.3 2,812.4 40.1 2,812.4 37.6
 Argon 26.8 0.6 33.5 0.5 33.5 0.4
 Maleic 228.6 3.1
 anhydride
 Carbon 73.2 1.0 366.1 4.9
 monoxide
 Carbon 0.9 0.0 1,286.8 18.4 1,530.9 20.5
 Dioxide
 Water 1,585.5 21.2
 Total 4,382.1 100.0 7,009.9 100.0 7,472.0 100.0
 TABLE 4
 Reactor inlet Reactor inlet
 (fresh Feed) (total feed) Reactor effluent
 Component Nm.sup.3 /hr Vol % Nm.sup.3 /hr Vol % Nm.sup.3 /hr Vol %
 Butane 397.3 9.1 1,051.5 15.0 688.7 9.2
 Oxygen 1,707.2 39.0 1,752.5 25.0 226.3 3.0
 Nitrogen 2,249.9 51.3 2,812.4 40.1 2,812.4 37.6
 Argon 26.8 0.6 33.5 0.5 33.5 0.4
 Maleic 228.6 3.1
 anhydride
 Carbon 73.2 1.0 366.1 4.9
 monoxide
 Carbon 0.9 0.0 1,286.8 18.4 1,530.9 20.5
 Dioxide
 Water 1,585.5 21.2
 Total 4,382.1 100.0 7,009.9 100.0 7,472.0 100.0
 The safety coefficient .alpha. calculated from the composition of the
 exhaust gas after recovering hydrocarbon at the hydrocarbon recovering
 step in Table 4 is 0.32. This demonstrates that this composition has lower
 oxygen concentration in the atmosphere than the minimum oxygen
 concentration and thus falls outside of explosive range. The lower
 explosive limit of this gas is calculated by Le Chateliers law to be about
 6.6 vol %. This gas has a total flammable gas concentration of 10.8 vol %,
 which is higher than the lower explosive limit. Accordingly, it is made
 obvious that the process of the present invention which comprises keeping
 the oxygen concentration in the atmosphere lower than the minimum oxygen
 concentration must by employed to control the explosion safety of this
 composition.