Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2001-06-11
2002-09-10
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reexamination Certificate
active
06448432
ABSTRACT:
The present invention relates to a process for the production of vinyl acetate and in particular to a method of controlling the production rate of a fluid bed process for the manufacture of vinyl acetate.
BACKGROUND OF THE INVENTION
It is known that vinyl acetate can be produced in a fluid bed reactor by contacting acetic acid and ethylene with molecular oxygen in the presence of a fluid bed catalyst active for the production of vinyl acetate. Such a process is described for example in EP-A-0672453, EP-A-0685449, EP-A-068545 1, EP-A-0985655 and EP-A-1008385.
It is generally known in such catalytic processes that catalytic activity will decrease with time for various reasons such as metal sintering and as a result, changes in operating conditions are required to maintain the rate of production. Typically, such a decrease in catalytic activity may be partially countered by increasing the temperature of the catalyst bed. However, increasing the temperature of the catalyst bed may result in a decrease in the selectivity of the process. In addition, there is a maximum operating temperature at which the process can be safely carried out. Thus, there is a constraint on the production rate which can be achieved by increasing the temperature of the catalyst bed. Consequently, periodically, it is necessary to shut down the plant and replace the deactivated catalyst with fresh catalyst.
EP-A-0 672 453 describes a process for the fluid bed production of vinyl acetate from ethylene, acetic acid and an oxygen-containing gas in the presence of a palladium promoted catalyst. EP-A-0 672 453 discloses that the continuous addition of make-up catalyst to such a fluid bed vinyl acetate process can maintain peak performance and virtually eliminate catalyst change-outs.
In co-pending application EP 99309222.0 (EP-A-1008385) which relates to a fluid bed process for the manufacture of vinyl acetate it is disclosed that continuous addition of make-up catalyst maintains catalyst performance and eliminates complete change-out and shut-downs.
Thus, there remains a need for an improved fluid bed process for the manufacture of vinyl acetate which avoids or at least mitigates the disadvantages associated with the prior art. It has now been found that the production rate of a fluid bed process for the manufacture of vinyl acetate may be controlled by the addition of fresh catalyst to the fluid bed and/or removal of older catalyst from the fluid bed wherein the activity and/or the rate of addition of any added catalyst is adjusted so as to achieve a desired overall catalyst activity of the fluid bed and hence a desired production rate.
SUMMARY OF THE INVENTION
Thus, according to the present invention there is provided a process for the fluid bed production of vinyl acetate which comprises reacting ethylene, acetic acid and an oxygen-containing gas in a fluid bed reactor at elevated temperature in the presence of a fluidised bed of catalyst, in which process, catalyst is added to said fluidised bed of catalyst, wherein the overall catalytic activity of the fluidised bed of catalyst is controlled to a pre-determined value by adjusting the activity and/or adjusting the rate of addition of said added catalyst.
Generally, if the overall activity of a catalyst bed in a fluid bed process for the manufacture of vinyl acetate is controlled, the production rate of the process may thereby be controlled. The desired production rate in a fluid bed process for the manufacture of vinyl acetate may over time remain constant or may increase or decrease, for example, in response to the market demand for vinyl acetate product.
In accordance with the present invention the overall catalytic activity of a fluidised bed catalyst and hence the production rate of the vinyl acetate process is adjusted to a pre-determined overall level by adjusting the activity and/or adjusting the rate of addition of the added catalyst. Preferably, the overall catalytic activity of a fluidised bed catalyst and hence the production rate of the vinyl acetate process is adjusted to a pre-determined overall level by adjusting the activity and optionally adjusting the rate of addition of the added catalyst.
The added catalyst may have a different activity to that of the initial activity of the catalyst in the fluidised bed. Suitably, the overall catalytic activity of the catalyst bed may be increased by adding catalyst which has a higher activity than that of the initial activity of the catalyst in the fluidised bed. Alternatively, the overall catalytic activity of a fluid bed process for the manufacture of vinyl acetate may be decreased by adding a catalyst which has a lower activity than that of the initial activity of the catalyst in the fluidised bed.
For example, a catalyst having a higher activity than the initial activity of the catalyst in the fluidised bed may be achieved by using a higher active metal loading in the catalyst. Similarly, a vinyl acetate catalyst of lower activity than the catalyst bed may be achieved by using a lower active metal loading. Catalyst having a reduced activity may be added in which the catalyst is diluted with inert support material.
In accordance with the present invention the overall catalytic activity of a fluidised bed of catalyst may be increased, by increasing the rate of addition of any added catalyst to the catalyst bed. Optionally, and in addition to increasing the rate of addition of catalyst to the fluidised bed of catalyst the rate at which deactivated catalyst is removed from the bed may be decreased. Similarly, to decrease the overall catalytic activity of a fluidised bed of catalyst, the rate of addition of any added catalyst to the bed may be decreased. Optionally, and in addition to decreasing the rate of fresh catalyst addition to the catalyst bed the rate at which deactivated catalyst is removed from the catalyst bed may be increased. When the changed rates of addition and optionally removal of the catalyst have achieved the required change in the amount of catalyst in the fluidised bed and hence the required change in overall catalytic activity of the fluidised bed, the rates of addition and removal may be returned to their earlier levels. The amount of catalyst in the reactor should not be reduced below a certain minimum beyond which the reactor will not function. Conversely, the amount of catalyst should not be increased beyond the maximum working level.
The overall catalytic activity of the fluidised bed may also be adjusted by increasing the rate of addition of added catalyst and also increasing the rate of removal of deactivated catalyst thereby reducing the time the catalyst resides in the fluidised bed and hence is subject to deactivation with use. Conversely the rates of addition and removal may be reduced together to adjust the overall catalytic activity.
Rapid changes in the overall activity of the fluidised bed and the overall rate of production may be achieved by changing the reaction conditions such as temperature in addition to adjusting the activity and/or adjusting the rate of addition of the added catalyst. Temperature changes may be achieved more rapidly than catalyst changes and as the catalyst is changed the temperature may be readjusted back to its previous value.
Typically, the catalyst added to the catalyst bed may be fresh catalyst.
Vinyl acetate is generally prepared on a commercial basis by contacting acetic acid and ethylene with a molecular oxygen containing gas in the presence of a catalyst active for the production of vinyl acetate. Hitherto such processes have been operated with a fixed bed of catalyst. Recently, a fluid bed process has been introduced.
In a fluid bed reactor system for the manufacture of vinyl acetate, the particles of the catalyst are maintained in a fluidized state by a suitable gas flow through the system. Excess flow rate may cause channeling of the gas through the reactor which decreases conversion efficiency.
A typical catalyst useful in this invention may have the following particle size distribution:
0 to 20 microns
0-30 wt %
20 to 44 microns
BP Chemicals Limited
Forohar Farhad
Killos Paul J.
Nixon & Vanderhye
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