WO1997039300A1

WO1997039300A1 - Introduction of fluidizable inerts into reactors containing fluidized catalyst

Info

Publication number: WO1997039300A1
Application number: PCT/US1997/002646
Authority: WO
Inventors: Robert E. Trubac; Rudolf A. Overbeek
Original assignee: Abb Lummus Global Inc.
Priority date: 1996-04-17
Filing date: 1997-02-20
Publication date: 1997-10-23

Abstract

A gas phase chemical reaction is carried out in a fluidized bed reactor (10) to which a quantity of fluidizable 'inert' particles has been added. The chemical reaction is one in which the fluidized particles in the reactor tend to become sticky and agglomerate, which has a negative impact on the hydrodynamics of the fluidized bed and the effectiveness of the catalyst. By employing inert particles having a low surface area, i.e., a BET surface of less than 1 m2/g, these problems are reduced or eliminated. The inert particles may range from fines, smaller than the catalyst particles, to particles larger than the catalyst. In an embodiment, fine catalyst particles are omitted, avoiding the loss of expensive catalyst fines. The particles size distribution for proper fluidization and reactor performance, is provided by the addition of inert particles, which includes fines to replace the omitted fine catalyst particles. Any loss of fines due to the inability to separate them or by contamination is in the form of inexpensive inert particles rather than expensive catalyst particles. The presence of low surface area inert particles substantially removes or prevents the adverse impact on hydrodynamics and reaction performances created by the stickiness and/or agglomeration.

Description

Introduction of Fluidizable Inerts Into Reactors Containing Fluidized Catalyst

The present invention relates to reactors containing fluidized particles, and more particularly to the introduction of fluidizable inert particles into a reactor containing fluidized catalyst particles.

Fluidized bed reactors and related systems using fluidized particles are being commonly employed to accomplish various chemical reactions. In many of these applications, the reaction is carried out in the presence of a catalyst. In these situations, the fluidized particles in the reactors comprise, at least in part, fluidizable catalyst particles. There are a number of factors which affect the operation of such reactors. One of the factors which has been known to influence the proper hydrodynamics of the fluidized particles is the size distribution of particles in the reactor. To obtain proper fluidization, it is necessary to maintain a mixture of fine, medium and large particles. In commercial reactors with fluidized catalyst, especially those where the catalyst particles have the desirable attribute of high resistance to attrition, it has been observed that there is a depletion of fine catalyst particles due to a number of factors. First, it is more difficult to separate the fine particles than the larger particles from the gaseous products, and the fine particles usually can only be removed in a filter or scrubber downstream from the primary separator such as a system with one or more cyclones. Second, the fine particles that are ultimately removed by a scrubber, and even by a filter, are often contaminated such as by sticky material that makes them unsuitable for recycle. This stickiness problem occurs primarily in partial oxidation reactions such as the selective partial oxidation of butane with air to produce maleic anhydride or ammoxidation reactions such as the reaction of propylene with ammonia and air to produce acrylonitrile. The fine catalyst particles, which have been cooled somewhat by the time they reach the filter, serve as sites for the condensation of various compounds. For example, in a maleic anhydride reactor using VPO (vanadium- phosphorous-oxide) catalyst, some of the phosphorous is slowly volatilized from the catalyst and carried out of the reactor to the effluent system where it eventually is condensed and adsorbed onto the fine catalyst particles deposited in the off-gas filter. To replenish the phosphorous content of the catalyst, phosphorous compounds are injected into the reactor. A portion of these compounds is not adsorbed/reacted onto the catalyst in the reactor and contributes further to phosphorous condensation and adsoφtion on the fine catalyst particles in the off-gas filter. The condensation of the phosphorous compounds on the fine particles makes the fine particles sticky when they are recycled back to the reactor. Deposits of maleic acid and various byproducts can also contribute to this phenomenon. The sticky particles which are returned to the reactor tend to agglomerate with the other particles in the reactor, thus reducing the content of fine particles that are circulating and also impairing the fluidity of the sticky and/or agglomerated particles. As an alternative, these fine particles are all discarded rather than being recycled. In any of these cases, there is a depletion of fine particles which has a detrimental effect on reactivity and selectivity due to the decreased fluidization or hydrodynamics in the reactor and increased bed density. In order to improve fluidization, bed expansion, height, residence time, and the resultant reactor performance, an optimal range of the particle size distribution must be maintained and the stickiness and agglomeration problem must be reduced. The maintenance of fine particles in the bed and the elimination of agglomeration increases the bed height, the residence time of the contact between the reactants and catalyst, the effectiveness of the contact, and the maximum utilization of the catalyst.

In some cases, after time there is the development of a "shell" or other coating on the catalyst. Removal of this coating can improve both hydrodynamics and catalyst performance, sometimes to the point where the catalyst that has had its coating totally or partially removed is more active and/or selective than before the shell was initially observed. However, this improvement can be temporary, with the catalyst gradually returning to its condition of inferior fluidization and performance and requiring repeated removal of the recurring coating.

The stickiness and resulting particle agglomeration is a problem particularly encountered in fluid bed partial oxidations, such as the partial oxidation of butane to maleic anhydride and the reaction of propylene with ammonia and air to form acrylonitrile. Although the prior art teaches the introduction of small inert particles into a poorly fluidizing bed to improve fluidization performance, in such systems as partial oxidation systems with a stickiness problem, the simple introduction of conventional fine inerts is not sufficient and may even have a negative effect.

SUMMARY OF THE INVENTION

The present invention relates to a method for improving the performance of reactors containing fluidized catalyst particles, and more particularly involves the introduction of materials into the reactor which will tend not only to maintain a desired particle size distribution of the solid particles within a range for proper fluidization hydrodynamics but are of a nature which will improve catalyst performance. Even more particularly, the invention involves the introduction into the reactor of particles of fluidizable "inert" material which have a particle size range selected to produce the desired particle size distribution of the mixed fluidized solids (catalyst plus added inerts) but which also prevent the agglomeration of the catalyst particles present in the reactor. Specifically, the inert material which is added has a low surface area of less than 1 m²/g, more preferably less than 0.75 m²/g and most preferably less than 0.5 m²/g. The presence of the low surface area inert particles in the reactor has been found to improve the hydrodynamics and reactor performance of catalyst particles which were previously impaired by becoming sticky and/or agglomerated.

A further aspect of the invention is that the inert particles may have a particle size distribution equivalent to that of the catalyst particles or may even be limited to larger particle sizes. In these cases, the larger inert particles act as heat sinks and dilute the concentration of catalyst particularly in the bottom of the reactor. This tends to distribute the reaction throughout the height of the reactor rather than being concentrated at the bottom.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a simplified flow scheme illustrating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In many cases, it is known to be advantageous to carry out catalytic reactions in a reactor containing fluidized catalyst particles. However, in certain types of these catalytic reactions, the problem of particle agglomerization has a detrimental effect on the operation and effectiveness of the process. This occurs with reactions where sticky materials are formed on the surface of the particles causing the agglomerization. This is particularly true of partial oxidation reactions such as the process of producing maleic anhydride from butane, butylene or benzene, by oxidation with air in the vapor phase which has been carried out in a fluidized bed reactor with an oxidation catalyst of mixed oxides of vanadium and phosphorous. Other catalyst compositions can also be used for this reaction such as shown in US Patents 4,594,433 and 4,654,425. Another example of a partial reaction where stickiness occurs is the previously mentioned ammoxidation of propylene with ammonia and air to produce acrylonitrile. Although references may be made herein to the fluidized bed production of maleic anhydride and to the specific catalysts used in that process, such references are not to be taken as a limitation on the present invention.

The drawing illustrates the general arrangement of a fluidized bed catalytic reactor system, in which reactor 10 contains a bed of particles. This bed of particles normally comprises primarily catalyst particles, but in the present invention it also includes "inert" particles as will be explained and defined hereinafter. In the bottom of the reactor 10 is a support grid 12, below which is the gas plenum 14. A fluidizing gas supply 16 is introduced into the gas plenum 14 and the gas flows up through the bed of particles at a rate such that the particles in the bed are fluidized. Other designs for introducing and distributing the fluidizing gas can also be utilized, as is known in the art. The materials to be reacted are commonly fed into the reactor 10 at the feed location 18, or can alternatively be included with the gas flow at 16. In the case of a maleic anhydride reactor, the gas flow 16 would be primarily air and the feed 18 would be primarily the butane, butylene or benzene. The catalyst would be, for example, a mixed oxide catalyst of vanadium and phosphorous as described in U.S. patents 4,594,433 and 4,653,425.

The reactor 10 may be one of a variety of types of fluidized bed reactors such as circulating, turbulent, and bubbling bed reactors. As used herein, the term fluidized bed reactors also includes dilute phase transport reactors where the solid phase is much less dense and might not ordinarily be considered to be a "bed" as in other fluidized reactor systems. Although bubbling beds do not elutriate the entire spectrum of solid particle sizes, there is still a carry-over and potential loss of fines. The rate of feed and air flow, as well as the particle size distribution of the catalyst, are all coordinated not only for a proper reaction, but also for the particular type of fluidized bed reactor as well known in the art.

The gaseous reaction products carrying the elutriated solids exit the reactor 10 overhead at 20. The mixture of reaction products and solids is introduced into a solids separator system 22, such as a typical cyclone separator system, which often consists of several cyclones. Alternatively, the cyclone separator system can be inside the reactor so that separation is accomplished before the gaseous products exit the reactor. In the cyclone separator system 22, the cyclonic action separates the solids from the gas, with the solids leaving through the bottom at 24 and the gas through the top at 26. However, the separator system is not capable, as a practical matter, even with two or three stages, of removing the finest particles from the gas stream. Merely as an example, particles below about 20 microns are difficult to remove. In a conventional fluidized bed reactor system, the circulating solids are essentially all catalyst particles which, for example purposed only, might initially have a particle range from 10 to 60 microns for some types of reactors and perhaps from 10 to 200 microns for other types of reactors. A portion of the smallest particles is not returned to the reactor by the cyclones. In addition, during the operation of the reactor, the catalyst is subject to attrition and fracture, which produce fines of less than 20 microns. In such a conventional system, the larger catalyst particles removed at 24 are recycled to the reactor 10 while the finest particles in the gas would be removed at 28 from the product gas 30 by some form of filter 32 (or a scrubber) and then either recycled to the reactor or discarded. Fresh make-up catalyst to replace any lost catalyst is added at 34. In the present invention, fluidizable "inert" particles are added to the reactor at 36 along with the catalyst, or by some other means. The term "inert" in the context of the present invention is defined as particles which are inert with respect to the reactants, the reaction product and the catalyst such that they do not adversely affect or interfere with the basic reaction and catalyst. On the other hand, the inert particles may be reactive with respect to contaminants or by-products in the system or at least adsorb or abrade away such contaminants or by-products. In addition, their hydrodynamic benefits result in improving the reactor performance. Some of the inert fluidizable particles which may be employed in the present invention are alumina, silica, titania, magnesia, zirconia, metal carbides, silicon carbide, carbon and zeolites, or mixtures thereof. The inert material should be sufficiently attrition resistant so that it maintains its integrity during the process. The density of the inert particles may be selected to closely match the density of the catalyst particles or the density may be less or even more than that of the catalyst particles. Also, the size range and distribution of the inert particles may match that of the catalyst particles (including fine particles) or it may be adjusted to be different as desired. In the preferred embodiment of the present invention, the catalyst that is introduced into the reactor, including the initial load and the make-up catalyst added during operation, has fewer fine particles than the optimum for hydrodynamics. The fine particles that are desirable to maintain the proper particle size distribution in the bed and which are not provided as catalyst particles, are instead provided by the inert particles. In this case, the particle size distribution of the inert particles would be weighted toward the fine particles and contain a larger proportion of particles below about 45 microns. It can now be seen that all or most of the losses of fine particles, except for attrited or broken catalyst particles, will comprise inexpensive inert particles rather than expensive catalyst. In this case, the fine particles which are collected in the off-gas filter 32 need not be recycled to the reactor since they are now inexpensive inerts which can be discarded and replaced by addition to the reactor of fresh inert fine particles. In the alternative, if the contaminants on the inert fine particles such as the sticky phosphorous material can be removed by some appropriate process, or if they will not have the same adverse agglomerative and hydrodynamic impacts as catalyst fines when they are recycled, the inert fines can be recycled.

In another embodiment of the present invention, the particle size distribution of the inert particles may match that of the catalyst particles (before removal of fine catalyst particles) or even be weighted toward the larger particles. The inert particles, particularly the larger particles, act as a heat sink and dilute the concentration of catalyst. By using larger and/or heavier inert particles, especially if there are internals which contribute to segregation of large/heavy vs small/light particles, the heat/reaction moderation at the bottom can be enhanced, distributing the reaction throughout the height of the bed rather than being concentrated at the bottom as occurs without inerts being present. According to the present invention, the inert particles have a low surface area as compared to conventional fines which might have previously been added to fluidized catalyst beds. Specifically, the inert particles have a BET surface area of less than 1 m²/g, more preferably less than 0.75 m²/g and most preferably less than 0.5 m²/g. (BET surface area is the surface area measured by the standard Brunauer, Emmet and Teller technique in square meters per gram).

As previously indicated, a problem encountered in fluid bed partial oxidations is stickiness and particle agglomeration, resulting in a loss of fluidity. This agglomeration can be caused by a coating on the particle surface due to desired product or byproduct formation, possibly in combination with a migration and concentration of catalyst components on the surface. As expected the loss in fluidity results in poor performance, characterized by lower activity and selectivity, along with difficulty in heat management in these highly exothermic systems.

Although the prior art teaches the introduction of small inert particles into a poorly fluidizing bed as a means of improving performance, it has been found that in partial oxidation systems, particularly butane partial oxidation to maleic anhydride, that simply introducing inerts may have no benefit, and may even result in the loss of product yield. According to the present invention, it has been found that only inerts that have very low surface areas can be used without resulting in the loss of selectivity that may negate any benefit accomplished by better fluidization.

It has also been found, contrary to prior teachings, that small particles are not necessarily required to achieve benefits in fluidization and performance. A low surface area inert with a particle size range that is the same as or even larger than the catalyst particle size range, with no particles below 40 microns, will also result in improved fluidization in a "sticky" catalyst bed, accompanied by an increase in activity with no loss in selectivity.

Summarized below are test results obtained using fixed-bed micro- reactor tests on an aged vanadium phosphorus oxide catalyst mixed with various inert materials in the selective oxidation of n-butane with air to selectively produce maleic anhydride (MA). A fixed bed reactor was used in the tests in order to investigate the effect of surface area alone without any impact from fluidization variations. As inert materials, two different α-AI₂0₃'s were used: 1) α-AI₂O₃-1 having a BET surface area of 2 m²/g and 2) α-AI₂0₃-2 with a BET surface area of 0.2 m²/g. In both cases 2.22 gram of VPO catalyst was physically mixed with 1.11 gram inert material. As can be seen, alumina-1 with a higher surface area had a negative influence on the selectivity. The specific activity of the catalyst, represented by the Frequency Factor (which is a well known measure of the activity of the catalyst), did not increase significantly with alumina-1. The low surface area alumina-2, on the other hand, had a negligible influence on the selectivity while significantly increasing the measured Frequency Factor (2700 vs. 2100).

From the above it can be concluded that dilution with extremely low

BET surface area inerts does not harm catalyst performance, even in a fixed bed reactor. It may even increase the activity somewhat, because of the larger residence time in the bed. However, dilution with inerts with a larger specific surface area decreased the performance considerably. It is therefore necessary to use an inert which has a very low surface area.

Using the example of the production of maleic anhydride employing a catalyst bed of the mixed oxides of vanadium and phosphorous, inert fluidizable particles of low surface area α-alumina with a higher particle density and essentially the same size range as the catalyst particles were added to a catalyst which exhibited sticky fluidization behavior and impaired/deteriorating reaction performance. After the introduction of 25 wt% α-alumina, even without introducing an increase in the content of fine particles, the specific activity relative to the catalyst increased by 50% and maintained stable operation and performance. In addition, the temperature profile, which had shown a tendency toward a hot spot due to the poor fluidity, became very uniform, confirming improved hydrodynamics. Even after the removal of a weight of catalyst equal to the weight of added inerts, the specific activity relative to the catalyst (i.e. reaction per weight of catalyst) was maintained at the increased level of 50% above the level before the addition of the inerts. In other words, the 75/25 mixture of catalyst and low surface area inerts had 25% higher activity per combined weight of the catalyst plus inerts than the same weight of catalyst by itself. The higher activity can be utilized to perform the reaction at a lower temperature, which may increase selectivity for the desired product, or to increase the feed and product rates of an existing reactor at a given temperature, or to allow some other process changes such as the introduction or increase of a selectivity-enhancement agent which might otherwise be an activity suppressant. Another beneficial feature of adding the inert material can be for the inerts to adsorb contaminants such as the excess phosphorous previously discussed. If the contaminants are adsorbed onto the inert material, they will not be available to form a sticky coating or otherwise contaminate the catalyst. The α-alumina is active for phosphorous adsorption. Other inert materials for adsorbing potential contaminants are, for example, calcium and magnesium oxides and, in some cases, zirconium and vanadium oxides.

The present invention improves the contact of the reactant gases with the catalyst by improving fluidization, decreasing bed density, increasing bed height and increasing the residence time of the reactants in contact with the catalyst in the reactor. It will also reduce the cost of losing fine particles and the amount of catalyst fines generated by attrition or breakage, when fine inerts are utilized in place of fine catalyst. In addition, the use of large or heavy inerts can decrease the adverse impact of localized exothermic reactions at the bottom of the reactor.

With respect to the optimum particle size of the inert material which is added, it will vary for each particular situation and will depend upon a variety of factors such as the type and density of the catalyst, the velocity of the gas mixture in the reactor, the need to dilute the catalyst at the bottom, and other relevant factors which all determine the optimum particle size distribution for a specific fluidized catalytic reactor. However, in each case, the inerts have a low surface area.

Claims

1. A process for carrying out a gas phase chemical reaction in a reactor containing fluidized catalyst particles, comprising the steps of: a. providing fluidizable catalyst particles and fluidizable inert particles in said reactor, said catalyst particles having a first selected particle size range and said inert particles having a second selected particle size range and a BET surface area less than 1 m²/g, and wherein said second selected particle size range includes particles smaller than said first selected particle size range; b. introducing a fluidizing gas into the bottom of said reactor and fluidizing said catalyst and inert particles; c. introducing gaseous reactants into said reactor and reacting said reactants in the presence of said catalyst particles and producing a reaction product; d. removing said reaction product and a portion of said catalyst particles and at least a portion of said inert particles from the reaction zone, including at least a portion of said smaller inert particles; e. separating said catalyst particles from said removed reaction product and recycling said separated catalyst particles to said reactor, thereby leaving said removed reaction product containing said smaller inert particles; and f. separating said smaller inert particles from said reaction product.

2. A process as recited in claim 1 and further including the step of recycling said separated smaller inert particles into said reactor.

3. A process as recited in claim 1 and further including the step of adding to said reactor fresh catalyst particles falling within said first selected particle size range.

4. A process as recited in claim 2 and further including the step of adding to said reactor fresh catalyst particles falling within said first selected particle size range.

5. A process as recited in claim 4 and further including the steps of providing a supply of catalyst particles, removing from said supply of catalyst particles any particles falling below said first selected particle size range prior to adding said supply to said reactor as fresh catalyst particles.

6. A process as recited in claim 3 and further including the steps of providing a supply of catalyst particles, removing from said supply of catalyst particles any particles falling below said first selected particle size range prior to adding said supply to said reactor as fresh catalyst particles.

7. A process for carrying out a gas phase catalyzed chemical reaction in a reactor containing fluidized catalyst particles comprising the steps of: a. providing fluidizable catalyst particles and fluidizable inert particles in said reactor, said catalyst particles having a first selected particle size range including both small and large particles sizes and said inert particles having a second particles size range and wherein said second range is selected from within an overall range extending from above said first range to below said first range and said inert particles further having a BET surface area less than 1 m²/g; b. introducing a fluidizing gas into said reactor and fluidizing said catalyst and inert particles; c. introducing gaseous reactants into said reactor and reacting said reactants in the presence of said catalyst particles and producing a reaction product; d. removing said reaction product together with a portion of said catalyst particles from said reactor; e. separating at least a part of said removed portion of said catalyst particles from said removed reaction product; and f. recycling said separated catalyst particles to said reactor.

8. A process as recited in claim 7 wherein said second particle size range includes said large particle sizes.

9. A process as recited in claim 8 wherein said second range extends above said first range.

10. A process as recited in claim 7 wherein said second range is above said first range.

11. A process as recited in claim 7 wherein said second range extends below said first range.

12. A process for carrying out a gas phase catalyzed chemical reaction in a reactor containing fluidized catalyst particles and fluidized inert particles and wherein said chemical reaction results in sticky particles and particle agglomeration comprising the step of: a. providing fluidizable catalyst particles and fluidizable inert particles having a BET surface area less than 1 m²/g in said reactor; b. introducing a fluidizing gas into said reactor and fluidizing said catalyst particles and inert particles; c. introducing gaseous reactants into said reactor and reacting said reactants in the presence of said catalyst particles and producing a reaction product; d. removing said reaction product together with a portion of said catalyst particles from said reactor; e. separating said removed catalyst particles from said removed reaction product; and f. recycling at least a portion of said separated catalyst particles to said reactor.

13. A process as recited in claim 12 wherein said catalyst particles have a first selected particle size range and said inert particles have a second selected particle size range.

14. A process as recited in claim 13 wherein said second selected particle size range is selected from within a particle size range extending both above and below said first selected particle size range.

15. A process as recited in claim 13 wherein said second selected particle size range is selected to coincide with said first selected particle size range.

16. A process as recited in claim 13 wherein said second selected particles size range is selected within said first particle size range.

17. A process as recited in claim 13 wherein said second selected particle size range is selected to extend from above said first selected particle size range to within said first selected particle size range.

18. A process as recited in claim 13 wherein said second selected particle size range is selected to be above said first selected particle size range.

19. A process as recited in claim 13 wherein said second selected particle size range is selected to extend from below said first selected particle size range to within said first selected particle size range.

20. A process as recited in claim 13 wherein said second selected particle size range is selected to be below said first selected particle size range.

21. A process for carrying out a gas phase catalyzed chemical reaction in a reactor containing fluidized catalyst particles comprising the steps of: a. establishing a desirable particle size range and a desirable distribution of particle sizes within said range to produce desirable fluidizing hydrodynamics including a quantity of fine particle sizes and a quantity of larger particle sizes; b. providing fluidizable catalyst particles in said reactor, said fluidizable catalyst particles being essentially said larger particle sizes; c. providing fluidizable inert particles in said reactor, said fluidizable inert particles having a

BET surface area less than 1 m²/g and including said quantity of fine particles of sizes; d. introducing a fluidizing gas into the bottom of said reactor and fluidizing said catalyst and inert particles; e. introducing gaseous reactants into said reactor and reacting said reactants in the presence of said catalyst particles and producing a reaction product; f. removing said reaction product from said reactor together with a portion of said catalyst particles and a portion of said inert particles including a portion of said larger catalyst particles and said fine inert particles; g. separating said larger catalyst particles from said reaction product leaving said reaction product together with said fine inert particles; h. recycling said larger catalyst particles to said reactor; and i. separating said fine inert particles from said reaction product.

22. A process as recited in claim 1 wherein said BET surface area is less than 0.5 m²/g.

23. A process as recited in claim 7 wherein said BET surface area is less than 0.5 m²/g.

24. A process as recited in claim 12 wherein said BET surface area is less than 0.5 m²/g.

25. A process as recited in claim 21 wherein said BET surface area is less than 0.5 m²/g.