Anthraquinone process hydrogenator

Working Solution Composition. The working solution in an anthraquinone process is composed of the anthraquinones, the by-products from the hydrogenation and oxidation steps, and solvents. The solvent fraction usually is a blend of polar and aromatic solvents which together provide the needed solubiUties and physical properties. Once the solution has been defined, its composition and physical properties must be maintained within prescribed limits for achieving optimum operation. 

Interest has continued in on-site manufacture of hydrogen peroxide from the elements, particularly for remote sites located considerable distances from wodd-scale anthraquinone processes. However, no commercial-scale direct combination plants have been constmcted as of this writing. 

The alkylated anthraquinone process accounts for over 95% of the world production of H202, mainly because the it operates under mild conditions and direct contact of 02 and H2 is avoided. In this process, 2-alkylanthraquinone (the alkyl group is typically an ethyl, terf-butyl or amyl group) is dissolved in a mixture of a non-polar solvent (C9-Cn alkylbenzene) and a polar solvent [Trioctyl phosphate (TOP), or tetrabutyl urea (TBU) or diisobutyl carbinol (DIBC)] and then hydrogenated over a precious metal (Pd or Ni) catalyst in a three-phase reactor (trickle bed or slurry bubble column) under mild reaction conditions (<5bar, <80 °C) to generate 2-alkylanthrahydroquinone [1-3, 5], The latter is then auto-oxidized with air in a... 

The hydrogenation step in the anthraquinone process of AKZO-Nobel is an industrial realization of a monolithic reactor and includes a lot of pioneering work from the Anderson group (59-63). More examples of the use of monoliths can be found in Refs. 5 and 64. 

Of these processes, the first has only historical interest the plants which produced 15 000 t/a of hydrogen peroxide and 30 000 t/a of acetone were shut down in 1980. Only in the former States of the USSR are such plants still in use. The electrochemical oxidation process is also of limited importance. Over 95% of the hydrogen peroxide is produced with the anthraquinone process. Electrochemical... 

Formation of byproducts - particularly during hydrogenation - complicates the anthraquinone process... 

In the first step of the process the anthraquinone is hydrogenated to the hydroquinone with palladium as the preferred catalyst on carriers, such as gauze, or in suspension. The reaction is carried out at about 40°C and at pressures up to ca. 5 bar with cooling and only to ca. 50% hydrogenation to suppress side reactions (see below). 

In practice, the anthraquinone process is much more complicated than has been de.scribed above, in that byproducts such as 1,2,3,4-tetrahydroanthraquinone are formed, particularly in the hydrogenation step. These behave similarly to anthrahydroquinones, but their further hydrogenation leads to octahydroanthrahydroquinones which are unusable in this process. Other byproducts such as oxanthrones and anthrones can only be partially regenerated. These unusable byproducts have to be removed from the process. 

J. M. Campos-Martin, G. Blanco-Brieva, J. L. G. Fierro, Hydrogen peroxide syntliesis, an outlook beyond the anthraquinone process, Angew. Chem. Int. Ed. 45, 6962-6984 (2006). 

Fig. 1 (a) Schematic representation of the "indirect" anthraquinone process for the production of hydrogen peroxide using 2-alkyl-anthraquinone (1) and 2-all lhydroan-thraquinone (2). (b) Schematic representation of the direct synthesis of hydrogen peroxide from H2 and O2 (route A). Other reactions that decrease the selectivity of direct synthesis reaction are decomposition of hydrogen peroxide (route B) to water and hydrogenation of hydrogen peroxide (route C) to water. 

Hydrogen peroxide, which is produced worldwide in quantities of 800,000 tpa is manufactured by wet-chemical processes based on barium peroxide, electrochemical processes and organic autoxidation processes. Hydrogen peroxide production by autoxidation was developed to industrial viability by BASF in the 1930 s, in the form of the hydrazobenzene process. The hydrazobenzene process, however, was characterized by the disadvantage that sodium amalgam had to be used to reduce azobenzene to hydrazobenzene. Georg Pfleiderer and Hans-Joachim Riedl then used alkylated anthraquinones in place of azobenzene. The first plant to use this process to produce H2O2 was started in Memphis, Tenn. by Du Pont in 1953. 

In the anthraquinone process, an alkylanthraquinone is catalytically reduced to the corresponding hydroquinone with hydrogen the hydroquinone reacts with oxygen (air), becoming re-oxidized to the anthraquinone derivative, with the formation of hydrogen peroxide. 

Pure hydrogen and oxygen mixtures are highly explosive. Reactions that involve such mixtures are carried out safely in microchannel reactors. For example, the direct preparation of hydrogen peroxide is obtained with a special catalyst, avoiding the circuitous anthraquinone process, used at the industrial scale. Calculations of explosion limits clearly demonstrate that there is a considerable shift when explosive reactions are carried out in microchannels. The safety of the process is not only due to the avoidance of thermal runaway (because of large surface-area-to-volume ratio), but also due to the fact that radical chains are broken down due to the increased wall collision in the small channels of the reactor. 

Tunmen, I., 1997, Intesification of the anthraquinone process for production of hydrogen peroxide. Proceedings of 2 International Conference on Process Intensification, BHR Group Conference Series No.28,99-107. 

Figure 2.15. The anthraquinone process (AO process) to produce H2O2 [30]. (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Electrochemical reduction of 2-ethyl-9,10-anthraquinone (EAQ) and mediated formation of hydrogen peroxide in a two-phase medium Part I Electrochemical behaviour of EAQ on a vitreous carbon rotating disc electrode (RJ5E) in the two-phase medium, 29, 2004, 11-16, A. Huissoud, Scheme 1, 3Springer.)...

At the industrial scale, hydrogen peroxide is produced almost exclusively by the alternate oxidation and reduction of alkylanthraquinone derivatives. This anthraquinone process, or AO (from autoxidation) process, was originally developed... 

Campos-Martin, J., Blanco-Brieva, G. and Fierro, J. (2006). Hydrogen Peroxide Synthesis An Outlook Beyond the Anthraquinone Process, Angewandte Chemie International Edition, 45, pp. 6962-6984. 

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