Method and device for the two-step production of acrylic acid

Abstract

α,β-ethylenically unsaturated aldehydes and/or carboxylic acids are prepared by means of a two-stage catalytic gas-phase oxidation in which a gas mixture comprising at least an alkane or alkene having from three to six carbon atoms and oxygen is subjected to a catalytic oxidation reaction, in the first stage ( 1 ). Oxygen is added to the gases produced in the first stage ( 1 ) and the resulting mixture is introduced into the second stage ( 2 ) in which it is subjected to a further catalytic oxidation reaction. In the process of the present invention, a signal which correlates with the oxygen content of the reaction gases before and/or after the addition of oxygen between the first and second stages ( 1, 2 ) is generated and the oxygen addition is regulated as a function of the signal. Also provided are an apparatus for regulating a process for preparing α,β-ethylenically unsaturated aldehydes and/or carboxylic acids and an apparatus for preparing these substances using the regulating apparatus of the present invention.

1 - 4 . (canceled)  
   
   
       5 . An apparatus for regulating a process for preparing α,β-ethylenically unsaturated alkehydes and/or carboxylic acids by means of a two-stage catalytic gas-phase oxidation, comprising a gas feed facility ( 9 ,  10 ,  11 ) through which oxygen can be added to the gases produced in the first stage ( 1 ) before they are introduced into the second stage ( 2 ), wherein the apparatus comprises a measurement facility ( 3 ,  5 ,  6 ,  7 ) by means of which a signal which correlates with the oxygen content of the reaction gases before and/or after the addition of oxygen can be generated in which measurement facility ( 3 ,  5 ,  6 ,  7 ) comprises laser ( 3 ), two windows ( 5 ,  6 ) located opposite one another in a line ( 12 ) connecting the two stages ( 1 ,  2 ) or in a bypass around this and a detector ( 7 ) for measuring the intensity of the laser beam ( 4 ) after passage through the line ( 12 ) and wherein the measurement facility ( 3 ,  5 ,  6 ,  7 ) is connected to a regulating device ( 8 ) by means of which the amounts of oxygen added by the gas feed facility ( 9 ,  10 ,  11 ) can be regulated as a function of the signal generated.  
   
   
       6 . An apparatus as claimed in  claim 5 , wherein the point of exit of the beam from the laser ( 3 ) is connected in a gastight manner via a tube ( 15 ) to one window ( 5 ) and the point of entry of the beam into the detector ( 7 ) is connected in a gastight manner via a tube ( 16 ) to the other window ( 6 ) and gaseous nitrogen can be passed into and out of the tubes ( 15 ,  16 ).  
   
   
       7 . An apparatus as claimed in  claim 6 , wherein one of the tubes ( 15 ,  16 ) is configured as a calibration cell ( 17 ) through which or into which a gas having a defined oxygen ccntent can be passed.  
   
   
       8 . An apparatus as claimed in  claim 5 , wherein the surfaces of each of the two windows ( 5 ,  6 ) are not parallel to one another and/or the laser beam ( 4 ) does not form a right angle with these surfaces.  
   
   
       9 . An apparatus as claimed in  claim 5 , wherein the laser ( 3 ) is a modulatable diode laser whose wavelength is able to be set to one of the rotational fine structure bands of molecular oxygen in the range from 759.5 nm to 768 nm and the modulation range is ±0.05 nm.  
   
   
       10 . An apparatus for preparing α,β-ethylenically unsaturated aldehydes and/or carboxylic acids comprising a first reaction stage ( 1 ) and a second reaction stage ( 2 ) connected via a connecting line ( 12 ) to the first reaction stage ( 1 ) and also a regulating apparatus as claimed in  claim 5.