WO2007080374A1

WO2007080374A1 - Process and apparatus for reducing the probability of ignition in fluid bed-catalysed oxidation reactions

Info

Publication number: WO2007080374A1
Application number: PCT/GB2007/000011
Authority: WO
Inventors: Derek Alan Colman; Austin James Dunn; Merion Evans
Original assignee: Bp Chemicals Limited
Priority date: 2006-01-13
Filing date: 2007-01-04
Publication date: 2007-07-19
Also published as: TW200732038A

Abstract

A reactor and process for reducing the probability of ignition in fluid bed-catalysed oxidation reactions in which a molecular oxygen-containing gas is fed into a region of a fluidised catalyst bed having a higher time-averaged catalyst particle density higher than the fluidised catalyst bed as a whole.

Description

PROCESS AND APPARATUS FOR REDUCING THE PROBABILITY OF IGNITION IN FLUID BED-CATALYSED OXIDATION REACTIONS

This invention relates to fluidised bed catalysed processes, more specifically to reducing the probability of ignition of potentially explosive mixtures in fluid bed-catalysed oxidation processes.

In a fluidised bed catalysed process, particles of solid catalyst are maintained in a fluidised state by an upwards flow of fluidising gas. Generally, the fluidising gas is fed into the base of a reactor, and flows through a grid-plate, which disperses and distributes the fluidising gas. The fluidising gas may comprise one or more of the reactants of the fluid bed catalysed reaction.

US 5,980,782 describes a process in which oxygen and methane reactants are premixed and fed upwardly into a catalyst bed, and which function as the fluidising gas. To minimise the chance of autoignition of such a potentially explosive gas mixture, the length of time that the methane and oxygen are maintained in contact with each other before coming into contact with the catalyst is less than the autoignition time.

To minimise the possibility of an explosive combination of gases forming in the first place, the reactants may instead be mixed within the fluidised catalyst bed, as described for example in EP-A-I 163 953, which relates to a process for the production of vinyl acetate. In the process of EP-A-I 163 953, one of the reactants, ethylene, is used as the fluidising gas, and the other reactants, oxygen and acetic acid, are separately injected directly into the fluidised catalyst bed. The oxygen is injected in a downwards direction, counter to the flow of fluidising gas, which is stated to reduce chance of the inlet becoming blocked by catalyst particles. For fluidised bed-catalysed reactions employing potentially explosive mixtures of gases, such as the ammoxidation of propylene to acrylonitrile, or the production of vinyl acetate from ethylene, acetic acid and oxygen, it is important to minimise the probability of creating potentially explosive gas mixtures, particularly in catalyst-free or catalyst- deficient regions of the reactor. It is therefore desirable to inject oxygen into a fluidised catalyst bed so as to avoid catalyst-free or catalyst-deficient regions.

According to a first aspect of the present invention, there is provided a reactor for fluid bed catalysed reactions, which reactor comprises a fluidising gas inlet, an oxygen inlet and a reactor outlet, in which the oxygen inlet is disposed between the fluidising gas inlet and the reactor outlet such that, when in use, a molecular oxygen-containing gas is fed into a fluidisation zone defined by the region of the reactor between the fluidising gas inlet and the reactor outlet that is occupied by a fluidised bed of catalyst, characterised in that the region of the fluidisation zone into which the molecular oxygen-containing gas is fed has a time-averaged catalyst particle density higher than the time-averaged catalyst particle density of the fluidisation zone as a whole.

When the reactor is in use, some regions of the fluidised catalyst bed will have a time-averaged catalyst particle density of greater than the time-averaged catalyst particle density of the fluidised bed taken as a whole. It has surprisingly been found that the time- averaged density of catalyst particles around an object within a fluidised catalyst bed is greater on the downstream side of the object compared to the upstream side of the object, with respect to the direction of flow of fluidising gas. Thus, for an upwardly flowing fluidising gas, the density of catalyst particles is greater above the object compared to below the obj ect.

The reactor of the present invention ensures that a molecular oxygen-containing gas is fed into regions of high time-averaged catalyst particle density. When molecular oxygen is mixed with a combustible material, for example one or more organic compounds, a potentially explosive mixture can result. As catalyst particles are capable of removing heat from the surrounding gases, due to their higher heat capacity, then a higher concentration of catalyst particles will enable more heat to be removed from the surrounding gases, which will reduce the probability of ignition. Additionally, in the presence of catalyst, molecular oxygen and the reactant(s) are consumed, which lowers the partial pressure of the molecular oxygen, which can also act to reduce the probability of a potentially explosive mixture forming. Thus, by feeding molecular oxygen into a region of the fluidisation zone having a high density of catalyst particles, the process may be operated more safely, and may even be operated at a higher oxygen mole ratio, which can benefit productivity and process efficiency.

Thus, in a preferred embodiment of the reactor of the present invention, the oxygen inlet is adapted so that, in use, a molecular oxygen-containing gas is fed into the fluidisation zone co-currently with the fluidising gas. A co-current flow can be defined as an angle (θ) of less than 90° between the overall, averaged direction of flow of fluidising gas and the direction of feeding of molecular oxygen-containing gas. Preferably, θ is less than 60°, and most preferably is less than 45°. As the oxygen inlet is within the fluidisation zone; the catalyst particle concentration above the inlet (i.e. on the downstream side in relation to the flow of fluidising gas) will have a higher time-averaged density of catalyst particles than the fluidisation zone as a whole.

The fluidisation zone is the region within the reactor between the fluidising gas inlet and reactor outlet which is occupied by the fluidised catalyst bed when in use.

The oxygen inlet is an opening through which a molecular oxygen-containing gas can be fed into the fluidisation zone of the reactor. The opening may be a nozzle or sparger connected, for example, to a pipe that protrudes into the reactor from the reactor walls. The inlet is preferably made of a non-porous material, to prevent combustible material permeating into the oxygen line and risking a possible combustion or explosion event. Suitable inlet materials include metals or metal alloys, for example selected from one or more of nickel, copper, chromium, cobalt, stainless steel, Ni-Cu alloys such as Monel™, Ni-Cr-Mo-Fe alloy such as Inconel™, and Hastelloy™.

Preferably the oxygen inlet is disposed so that molecular oxygen is not fed into areas of low gas flow, such as near the walls of the reactor. This ensures that oxygen is not fed into relatively stagnant regions of the reactor, which improves mixing between the reactant gases and the fluidised catalyst particles. In addition, the oxygen inlet is preferably positioned so that oxygen is not directly injected onto other surfaces or structures in the reactor, such as inlets for other reactants.

The oxygen inlet may comprise a restriction, which prevents back-flow of reagents, products and catalyst into the oxygen line, which could result in a fire or explosion. The restriction also acts to increase the oxygen velocity through the opening which further reduces the possibility of back-flow. The restriction also reduces the chance of flame back-flow into the inlet should an ignition event occur.

There is preferably more than one oxygen inlet which may, for example, be located on or within one or more pipes within the fluidisation zone. The one or more oxygen inlets, and/or the pipes or other apparatus having the one or more oxygen inlets, may additionally be surrounded by a supply of inert fluid, as described for example in EP-A-I 163 905.

In embodiments of the invention comprising more than one oxygen inlet, the oxygen inlets are preferably positioned sufficiently far apart so that flame propagation will not occur between them should an ignition event take place. Therefore, the distance between the oxygen inlets is preferably significantly in excess of the potential flame length, which will be dependent, inter alia, on factors such as the oxygen inlet diameter and the oxygen gas velocity.

The reactor comprises an inlet for a fluidising gas. The fluidising gas is used to maintain a catalyst in a fluidised state. The fluidising gas inlet is preferably situated towards the base of the reactor, which creates an upwards flow of gas through the reactor when in use. The reactor has an outlet, which allows egress of reactor contents such as unreacted reactants, products and catalyst fines, from the reactor. The outlet is preferably positioned at or towards the opposite end of the reactor compared to the fluidising gas inlet, beyond the one or more oxygen inlets and any other inlets, such as inlet for other reactants or coolants. The reactor optionally, and preferably, also comprises a grid-plate. The grid-plate acts to distribute and disperse the fluidising gas as it passes through the grid-plate. The grid-plate is preferably situated within the reactor between the fluidising gas inlet and the oxygen inlet to distribute the fluidising gas before addition of the molecular oxygen- containing gas. The grid-plate is preferably situated at or below the base of the fiuidisation zone, such that most or all of the fluidised catalyst bed is above the grid-plate when the reactor is in use.

Additionally, and optionally, the reactor may also comprise a means for removing heat from the reactor contents, such as one or more cooling rods located within the fiuidisation zone which are connected to a supply of cooling water. The molecular oxygen-containing gas that is fed into the fiuidisation zone through the oxygen inlet when the reactor is in use may be, for example, pure oxygen, air, or a gas richer or poorer in molecular oxygen than air.

According to a second aspect of the present invention, there is provided a process for reducing the probability of ignition of potentially explosive mixtures in fluid-bed catalysed reactions, which process comprises the steps of;

(a) introducing a fluidising gas into a reactor having a fluidised bed of catalyst particles, a fluidising gas inlet, an oxygen inlet, and a reactor outlet, which oxygen inlet is disposed between the fluidising gas inlet and the reactor outlet, the fluidising gas being introduced into the reactor through the fluidising gas inlet; .

(b) feeding a molecular oxygen-containing gas into a fluidisation zone of the reactor through the oxygen inlet, which fluidisation zone is defined by the region of the reactor between the fluidising gas inlet and the reactor outlet that is occupied by the fluidised bed of catalyst particles;

(c) removing a product stream comprising one or more reaction products from the reactor through the reactor outlet; characterised in that the molecular oxygen-containing gas is fed through the oxygen inlet into a region of the fluidisation zone having a higher time-averaged catalyst particle density compared to the time-averaged catalyst particle density of the fluidisation zone as a whole.

The reactor is preferably a reactor as hereinbefore described. The fluidising gas is used to fluidise the fluid bed catalyst. Typically, and preferably, the fluidising gas is one of the gaseous reactants other than oxygen. Thus, for example, in the ammoxidation of propylene, propylene may be used as the fluidising gas, while in the production of vinyl acetate ethylene may be used as the fluidising gas. The fluidising gas is fed into the reactor through the fluidising gas inlet at a rate sufficient to maintain the fluid bed catalyst in a fluidised state. The molecular oxygen-containing gas fed into the fluidisation zone may be, for example, pure oxygen, air, or a gas richer or poorer in molecular oxygen than air. The concentration of molecular oxygen is preferably greater than 20% by volume, more preferably from 30 to 100 % by volume. Most preferably, the concentration of impurities in the oxygen is 0.4% by volume or less. A product stream comprising reaction products, and optionally unconverted reactants and entrained catalyst particles are removed from the reactor through the reactor outlet. Entrained catalyst particles comprise small catalyst particles, or fines, which typically result from attrition of the catalyst particles due to the abrasive nature of the fluidisation process. The product stream is typically fed to a cyclone and/or filter system to remove fines from the product stream, which are preferably returned to the reactor. Further fresh catalyst may also be added to the reactor to replenish any losses that occur from the process. The product stream after removal of the catalyst fines is then typically fed to a purification zone to recover and recycle unreacted reactants, and to purify the product.

The catalyst particles are preferably microspheroidal in nature. Typically, a microspheroidal catalyst will have particle sizes of lmm or below. Preferably, at least 60% of the catalyst particles will have a diameter of below 200μm. More preferably, at least 70% of the catalyst particles have a particle diameter of below 105μm. Preferably, no more than 40% of the catalyst particles have a particle diameter of less than 45 μm.

In a preferred embodiment of the invention, a molecular oxygen-containing gas is fed into the fluidisation zone co-currently with the fluidising gas, such that angle θ is less than 90°, at a velocity of preferably greater than 20m/s. In such an embodiment, the velocity is preferably maintained at a rate sufficient to prevent back-flow of catalyst particles and/or reactants into the oxygen inlet, but is not so high that catalyst attrition becomes unacceptably high. Therefore, the molecular oxygen-containing gas is preferably fed into the reactor at a velocity of 200m/s or below. More preferably, the velocity is maintained in the range from 40 to 120m/s, and even more preferably from 80 to 120m/s.

The present invention is advantageously applied to the fluid bed-catalysed production of vinyl acetate from the reaction of ethylene, oxygen and acetic acid as described, for example, in EP-A-O 685 449, in which ethylene, oxygen and acetic acid are the reactants. Examples of suitable fluid bed vinyl acetate catalysts include those typically comprising palladium and gold supported on silica, as exemplified in EP-A-O 685 449, and shell-impregnated microspheroidal catalysts, as described for example in EP-A-I 175 939. The reactor is preferably maintained at a temperature in the range from 100 to 400⁰C, preferably from 140 to 21O⁰C. The pressure of the reaction is preferably in the range from 1 to 20 barg (0.2 to 2.1 MPa), more preferably from 6 to 15 barg (0.7 to 1.6 MPa). The ethylene concentration in the combined feed to the reactor is preferably at least

60 mol%. Preferably, the concentration of oxygen in the combined feed to the reactor is in the range from 4 to 15 mol%, preferably from 4 to 12 mol%. The combined feed to the reactor includes any component fed to the reactor, including recycled and freshly fed components, but does not include catalyst. Acetic acid may be introduced into the reactor in liquid or gaseous form. Preferably it is introduced into the fluidisation zone. In one embodiment, it is introduced as a liquid, as evaporation of the liquid reactant removes heat from the exothermic reaction taking pace within the fluidisation zone of the reactor. The acetic acid may be introduced to the fluidisation zone through one or more inlets, preferably within the fluidisation zone. Preferably, the acetic acid concentration in the combined feed to the reactor is in the range from 10 to 20mol%. Water may additionally be fed into the fluidisation zone. It may be introduced as a separate feed, or together with one or more of the reactants, for example as a mixture with acetic acid. Water is fed into the reactor at a concentration of preferably less than 15 wt%, more preferably less than 10wt%, based on the combined quantities of water and acetic acid. The invention will now be illustrated by reference to the figures, in which:

Figure 1 is a longitudinal section of a reactor according to the present invention, showing how angle θ is defined;

Figure 2 shows the calculated concentration of catalyst particles in a fluid bed reactor for a reactor with no inlet pipes (A) and a reactor with an oxygen inlet pipe (B). Figure 1 shows a reactor 1 in which the overall, averaged direction of flow of a fluidising gas 2 between the fluidising gas inlet 3 and the reactor outlet 4 is represented by vector 5, and the direction of molecular oxygen-containing gas fed into the reactor through oxygen inlet 6 via pipe 7 is represented by vector 8. Thus, for co-current flow, the angle θ between the fluidising gas vector 5 and the molecular oxygen-containing gas vector 8 is less than 90°.

Figure 2 is a particle density profile in a cross section of fluid bed reactor 10 as calculated using Fluent 6.2 software employing the Eulerian Granular multi-phase model in which the interphase drag coefficient is adjusted to fit experimental observations. Reactor A has no oxygen inlet pipe within the fluidisation zone 11. Reactor B additionally has an oxygen inlet pipe 12 within the fluidisation zone 11 (oxygen inlets on the pipe are not shown). In reactor B, there is a (darker-shaded) region of high catalyst particle density above the oxygen inlet pipe 13, and a (lighter-shaded) region of low catalyst density 14 below the oxygen inlet pipe. The calculation is based on a 0.8m diameter and Im tall fluidised catalyst bed with a superficial vapour velocity of 14.6m/s (the mean upward velocity of gas in the absence of any particles), a vapour density of 7.4kg/m³, a solids density of 1230 kg/m³ with particles of 70μm diameter, and a solids phase bulk viscosity of 4 Pa.s.

Claims

1. A reactor for fluid bed catalysed reactions, which reactor comprises a fluidising gas inlet, an oxygen inlet and a reactor outlet, in which the oxygen inlet is disposed, between the fluidising gas inlet and the reactor outlet such that, when in use, a molecular oxygen- containing gas is fed into a fluidisation zone defined by the region of the reactor between the fluidising gas inlet and the reactor outlet that is occupied by a fluidised bed of catalyst, characterised in that the region of the fluidisation zone into which the molecular oxygen- containing gas is fed has a time-averaged catalyst particle density higher than the time- averaged catalyst particle density of the fluidisation zone as a whole.

2. A reactor as claimed in claim 1, in which the oxygen inlet is disposed so that, when in use, molecular oxygen-containing gas is fed into the fluidisation zone at an angle (θ) of less than 90°, wherein θ is the angle between the direction of feeding of molecular oxygen- containing gas and the overall, averaged direction of flow of fluidising gas.

3. A reactor as claimed in claim 2, in which the angle (θ) is less than 60°.

4. A reactor as claimed in any one of claims 1 to 3, in which there is more than one oxygen inlet.

5. A reactor as claimed in any one of claims 1 to 4, in which the reactor also comprises a grid-plate situated between the fluidising gas inlet and the oxygen inlet.

6. A reactor as claimed in any one of claims 1 to 5, in which the oxygen inlet is made of non-porous material.

7. A reactor as claimed in claim 6, in which the non-porous material is a metal or metal alloy.

8. A reactor as claimed in claim 7, in which the non-porous material is selected from one or more of nickel, copper, chromium, cobalt, stainless steel, Ni-Cu alloys such as

Monel™, Ni-Cr-Mo-Fe alloy such as Inconel™, and Hastelloy™.

9. A process for reducing the probability of ignition of potentially explosive mixtures in fluid-bed catalysed reactions, which process comprises the steps of;

(a) introducing a fluidising gas into a reactor having a fluidised bed of catalyst particles, a fluidising gas inlet, an oxygen inlet, and a reactor outlet, which oxygen inlet is disposed between the fluidising gas inlet and the reactor outlet, the fluidising gas being introduced into the reactor through the fluidising gas inlet; (b) feeding a molecular oxygen-containing gas into a fluidisation zone of the reactor through the oxygen inlet, which fluidisation zone is defined by the region of the reactor between the fluidising gas inlet and the reactor outlet that is occupied by the fluidised bed of catalyst particles ; (c) removing a product stream comprising one or more reaction products from the reactor through the reactor outlet; characterised in that the molecular oxygen-containing gas is fed through the oxygen inlet into a region of the fluidisation zone having a higher time-averaged catalyst particle density compared to the time-averaged catalyst particle density of the fluidisation zone as a whole.

10. A process as claimed in claim 9, in which the reactor is a reactor according to any one of claims 1 to 8.

11. A process as claimed in claim 9 or claim 10, in which the velocity of the molecular oxygen-containing gas fed into the reactor is 200 m/s or below.

12. A process as claimed in claim 11, in which the velocity of the molecular oxygen- containing gas fed into the reactor is in the range from 40 to 120 m/s.

13. A process as claimed in any one of claims 9 to 12, in which at least 60% of the catalyst particles have a diameter of below 200 μm. 1,4. A process as claimed in any one of claims 9 to 13, in which the reaction is the production of vinyl acetate from ethylene, acetic acid and oxygen, wherein ethylene is the fluidising gas, and acetic acid is also fed into the fluidisation zone.