The present invention is related to a process for treating a catalyst used in a process for the preparation of maleic anhydride by oxidizing a hydrocarbon having four carbon atoms in a fluidized bed reactor when the activity of the catalyst has decreased during use.

Background of the Invention

It has been known that maleic anhydride is prepared by oxidizing a hydrocarbon having four carbon atoms using a vanadium - phosphorus oxide catalyst, often called a V-P-O catalyst, in a fluidized bed reactor. In this process, a problem is that the catalytic activity decreases with time. Then, if the reaction temperature is raised to maintain the same level of conversion, the yield of maleic anhydride decreases. Accordingly, various processes have been tried for the regeneration of the catalyst. For example, the known methods include increasing the valence of the vanadium to 3.9 to 4.6 using sulfur trioxide, by which the catalytic activity is partly regenerated (US Patent No. 4, 1 23,442); removing inactive vanadium from the catalyst using the action of a halogen or an organic halide (US Patent No. 4,020, 1 74); treating the catalyst with a reducing gas such as hydrogen, carbon monoxide, etc. (UK Patent No. 4,020, 1 74); bringing the catalyst into contact with aqueous ammonia or an amine (UK Patent No. 1 ,51 2,305); and adding a phosphorus compound to the catalyst (US Patents No. 3,296,282 and No. 3,474,041 and UK Patent No. 1 ,291 ,354). However, these methods are unsatisfactory. Japanese Patent Application Laid-Open No. Hei-5-329381 discloses a method where the catalyst particles used in a fluidized bed reactor are de-agglomerated or a method where the surface of the catalyst particles in a fluidized bed reactor is re-exposed, wherein a high speed gas is blown onto the catalyst particles while they are in a

fluidized state in a fluidized bed reactor whereby the surface of the particles is polished due to the collision of the particles. In Japanese Patent Application Laid-Open No. Hei-4-316567, a method is disclosed where the catalyst is taken out of a fluidized bed reactor and then crushed to expose the active surface of the catalyst which is then put back into the reactor.

Summary of the Invention

A purpose of the present invention is to provide an improved process for the efficient regeneration of a V-P-O type catalyst to improve conversion, yield and selectivity. The present invention provides a process for improving the activity of a catalyst used in a process for the preparation of maleic anhydride by oxidizing a hydrocarbon having four carbon atoms, wherein the catalyst comprises oxide compounds of vanadium and phosphorus, and wherein at least a part of the catalyst particles are removed from the reactor, the less desirable catalyst particles having a flake-like configuration which have accumulated are separated and the desirable catalyst particles having the normal, generally spherical-like configuration are returned to the reactor. The process may also include the step of replacing the reaction gases in the reactor with nitrogen or air before the catalyst particles are removed from the reactor or the step of replacing the reaction gas that has been removed from the reactor with the catalyst particles also with nitrogen or air prior to the separation step.

Brief Description of the Drawings

Fig. 1 is a graph illustrating the particle size distributions of the catalyst particles.

Fig. 2 shows the structure of the catalyst particles before the classification. Fig. 3 shows the structure of the classified coarse powder.

Fig. 4, 5 and 6 all show the structure of classified fine powders.

Description of the Preferred Embodiments

The catalyst used in the invention comprises oxide compounds of vanadium and phosphorus, hereinafter referred to as a V-P-O catalyst, where the activity of the catalyst has decreased during use.

The catalyst contains crystalline vanadium - phosphorus mixed oxides as an active component where an atomic ratio of phosphorus to vanadium (P/V) is preferably 0.8 to 2.0 /1 , more preferably 1 to 1 .5 /1 . For example, one such catalyst has a main crystalline component of divanadyl pyrophosphate. The catalyst may or may not contain carriers such as SiO ₂ , A1 ₂ O ₃ , and TiO ₂ . In addition, the catalyst may also contain elements such as Li, B, Si, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Sn, Hf and Bi as a co-catalyst component.

These V-P-O catalysts may be prepared by known methods, for example, a method where a catalyst precursor is prepared by reducing divanadium pentaoxide with hydrochloric acid, oxalic acid, hydrazine, etc. in the presence of phosphoric acid, which is then calcined (JP Application Laid-Open No. Sho-54-120273/1979 and USP No. 4,085, 1 22); and a method where divanadium pentaoxide is reduced in a substantially anhydrous organic solvent, which is then heated in the presence of phosphoric acid to obtain a precursor which is then calcined (JP Publication Sho-57-8761 /1982 and JP Publication Hei-1 - 50455/1 989).

It is known that in a heterogeneous catalytic reaction using a solid catalyst, diffusion into the inside of the pores of the catalyst has a large influence on the catalytic activity. In the case where the resistance to diffusion is large, the contribution of the catalyst to the reaction is less near the center of the particle than near the surface. A smaller catalyst particle has a larger proportion of the volume near the outer surface relative to the total volume. Accordingly, in general, the

catalyst having a smaller particle size of the generally spherical configuration has a higher reaction activity. However, catalyst particles which are flaky in configuration have a lower activity which reduces the overall activity and selectivity of the catalyst mass even though the flaky particles are much smaller than the initial size of the fluidized bed catalyst particles.

Separation of the flaky catalyst particles from spherical catalyst particles is carried out preferably in a dry process. In a wet process, the quality of the catalyst may be changed because the catalytic component is eluted during the separation treatment. The dry process for the separation is carried out preferably using the force of air, more preferably using air streams in combination with centrifugal force, inertial force or gravity. Particularly preferred is a classifier where classification is carried out using the action of centrifugal force and air streams. Such classifiers include those generally known as turbo classifiers, microprex, multiprex zigzag classifiers, super separators, accurecut, etc. In classification using a screen, it is difficult to separate the flaky catalyst efficiently and selectively.

In the above treatment, fine particulate, spherical catalyst may be simultaneously separated and removed together with the flaky catalyst. The separation equipment is preferably operated such that most of the flaky catalyst is removed, as observed in microscopic photographs as described below. Also, the proportion of fine spherical particles removed to the flaky particles removed should be minimized. If the flaky catalyst is insufficiently removed, there is less of an increase in the activity and selectivity of the catalyst. Meanwhile, if too large a proportion of the spherical catalyst particles are removed, a large proportion of the desired catalyst is discarded, which is uneconomical.

The composition of the gas flow used in the separation process is not particularly limited as long as it does not cause adverse effects, such as a decrease in the activity of the catalyst. Also, a gas mixture

may be used. Air is the least expensive but nitrogen, oxygen, rare gases, carbon dioxide, steam or hydrocarbons alone or as a mixture thereof can be used. A gas mixture of butane and air or a reactor outlet gas may also be used. For the sake of safety, the reaction gas in the reactor may be replaced with other gases such as the aforesaid gases before the catalyst is taken out of the reactor. Alternately, reaction gas accompanying the catalyst particles taken out of the reactor may be replaced with other gases before the separation and removal treatment is carried out.

The pressure in the separation treatment is not particularly limited. Operations are easier with a pressure of at least atmospheric pressure. The temperature is not limited and may be room temperature. Alternatively, the treatment may be carried out at a temperature which is approximately equal to the reaction temperature so the treated catalyst is already at the reaction temperature when it is reintroduced into the reactor. The step of taking the catalyst out of the fluidized bed reactor, the step of treating it for separation and the step of putting it back into the fluidized bed reactor may each be carried out continuously or batchwise.

The present invention will be illustrated more specifically by means of the following examples. Maleic anhydride is represented as "MAH" hereinafter.

In the examples, the conversion of n-butane, the yield of MAH and the selectivity to MAH are calculated as follows:

Conversion of n-butane = (molar concentration of butane at the inlet of the reactor minus the molar concentration of butane at the outlet of the reactor) ÷ (the molar concentration of butane at the inlet of the reactor) x 100

Yield of MAH = (moles of MAH generated per unit time) ÷ (moles of butane feed per unit time) x 100

Selectivity to MAH = (yield of MAH) ÷ (conversion of n-butane) x 100

Example 1

A sample catalyst was a V-P-O catalyst which had been used for about two years in a reaction where n-butane was air oxidized into maleic anhydride in a fluidized bed reactor whereby its activity had decreased over time. The catalyst was sampled from an outlet gas from the fluidized catalyst bed after passing through a cyclone.

One hundred grams of the sample catalyst were classified using a precision air classifier TC-1 5N available from Nisshin Engineering Company at room temperature, a rotor rotation of 3500 rpm, air flow in the amount of 2.9 m ³ /min. and channel air pressure of 2.0 kgf/cm ² . There was obtained 85 g of coarse powder and 15 g of fine powder.

Using a wet type particle size distribution measuring apparatus, a MELVERNSystem 3601 based on a laser light diffraction method, the particle size distributions were determined for the catalyst before the classification and for the coarse powder and the fine powder obtained from the classification. The results are as shown in Figure 1 . The coarse powder obtained by the classification contained almost no particles smaller than 1 3 micron and had an average particle size somewhat larger than that before the treatment. Meanwhile, most of the fine powder obtained by the classification had a particle size of 1 to 13 microns.

Each sample was observed with a scanning electron microscope and recorded in a photograph. The catalyst before the classification was shown in Figure 2; the coarse powder in Figure 3; and the fine powder in Figure 4. Although spherical particles, flaky particles and fine particles were present as a mixture in the catalyst before the treatment,

the fine particles were removed almost completely and the proportion of the flaky particles was reduced in the coarse powder obtained by the classification treatment. In the fine powder obtained by the classification, there were observed a large amount of flaky particles together with fine particles, but almost no particles with spherical shapes were observed. These results show that the sample catalyst with the decreased activity contained the flaky particles together with the spherical particles and that the flaky particles were selectively classified into the fine powder using the air classifier. The activity test was then carried out as follows:

Comparison 1

One gram of the above sample catalyst before the classification was loaded into a fixed bed flow reactor. The reaction was carried out at atmospheric pressure, a GHSV of 1 500 hour ^"1 , a reaction temperature of 430 °C and a n-butane concentration in air of 1 .5 mole %. Normal butane concentrations in the inlet gas and the outlet gas were determined quantitatively using gas chromatography showing a conversion of butane of 36 mole %. The resultant MAH was absorbed in water by introducing the outlet gas in 20 to 50 ml of water for 30 to 60 minutes, which was then titrated with an aqueous 0.1 N sodium hydroxide solution to obtain the yield of MAH. The yield of MAH was 24 mole %. The selectivity to MAH was 66 mole %.

Comparison 2

The activity was determined as in Comparison 1 with the exception that 1 g of the fine powder obtained in the above classification was used in place of 1 g of the sample catalyst before the classification. The conversion of n-butane was 22 mole %, the yield of

MAH was 14 mole% and the selectivity to MAH was 63 mole %.

It is seen that the conversion was very low with the fine powder, compared to the sample catalyst before the classification, even though the average particle size of the fine powder was smaller. In the reaction for forming MAH, there is a general tendency that the selectivity is higher as the conversion is lower. However, in this Comparison, the selectivity was low, compared to that with the catalyst before the treatment even though the conversion was lower.

Comparison 3

The activity was determined as in Comparison 1 with the exception that 1 g of the coarse powder obtained in the above classification was used in place of 1 g of the sample catalyst before the classification. The conversion of n-butane was 39 mole %, the yield of

MAH was 28 mole % and the selectivity to MAH was 71 mole %. That is, the conversion was improved, compared to the result with the catalyst before the classification shown in Comparison 1 . Moreover, it is seen that the selectivity to MAH was also improved, even though the conversion was high compared to that with the catalyst before the classification. Compared to the results in Comparison 2 for the fine powder, the conversion and the selectivity in the present invention are much higher and the yield of MAH was doubled.

These results show that the portion of a catalyst mass which is causing decreased catalytic activity may be selectively removed by classification and thereby increase the activity and the selectivity per unit weight of the catalyst.

Example 2

A V-P-O catalyst which had been used for about two years in a reaction where n-butane was air oxidized to prepare maleic anhydride in a fluidized bed reactor whereby its activity had decreased was sampled from a sampling opening located approximately at the center

of the catalyst bed. One hundred grams of this catalyst were classified using the same apparatus as in Example 1 at room temperature, a rotor rotation of 3500 rpm, air in the amount of 2.0 m ³ /min. and a channel air pressure of 2.0 kgf/cm ² . There was obtained 99 g of coarse powder and 1 g of fine powder.

A scanning electron microscopic photograph of the fine powder obtained is shown in Figure 5. A large amount of flaky particles as well as fine particles were observed in the fine powder obtained by the classification, similar to Figure 4. The photograph in Figure 6 is of a lower magnification than that in Figure 5. It can be seen that the amount of particles with spherical shape is very small. Therefore, in the sample catalyst used in this Example, the flaky particles were selectively separated in the fine powder in the classification using the air classifier.