U.S. Pat. No. 2,070,263 describes a method for obtaining aqueous solutions of hydrobromic acid which consists, in a first step, in passing hydrogen through liquid bromine maintained at a temperature of between 37.degree. C. and 42.degree. C. in order to form a mixture of bromine and hydrogen gas which is burnt at a temperature of between 600.degree. C. and 850.degree. C. By working in this way, it is difficult to have an intimate mixture of bromine and hydrogen in stoichiometric amounts on account of the difficulties in rigorously maintaining the temperatures and the thermodynamic equilibria.
An instability of the combustion flame has also been observed, in devices based on the direct combustion of bromine in hydrogen according to an H.sub.2 /Br.sub.2 molar ratio of greater than 1, and for which no technique for mixing the reactants is mentioned, this instability being manifested in particular by strong vacillation of the flame at the burner outlet, going as far as a detachment of the flame ("blow off") from the said burner, which may entail a risk of explosion and a fluctuating quality of the hydrogen bromide gas produced.
Furthermore, such flames become extended forming cones at the base of which are regions from which the reactants are liable to escape without being burnt.
This disrupts the combustion of the bromine in the hydrogen and results especially in residual bromine in the combustion gases, this being of a nature to bring about a considerable decrease in the lifetime of the burners, limit the range of materials which may be used and degrade the quality of the hydrogen bromide gas, thereby preventing it from being used as a reactant for downstream syntheses (secondary reactions, colorations of the products) or for the preparation of hydrobromic acid solutions.
Patent FR 2,365,516 proposes a process which improves the stability of the flame resulting from the combustion of bromine in hydrogen by establishing a helical stream of bromine in a cylindrical chamber, then injecting the hydrogen radially towards the outside in the helical stream of bromine and continuously supplying a flame close to the chamber with the helical stream of bromine and hydrogen.
This process, using a molar excess of hydrogen of 2.6%, leads to an HBr gas containing 300 ppm of bromine by volume, which still gives rise to colorations of the downstream synthesis products as well as the drawbacks mentioned above.
In addition, the complexity of the burner entails a lack of flexibility. Thus, in particular, when it is desired to increase the capacity of the said device, several burners are arranged side-by-side in the same chamber. In such an arrangement, it cannot be avoided that the flames from different burners mounted in parallel will interfere with each other, and furthermore this arrangement is unacceptable with regard to obtaining good distribution of the reactants. This configuration inevitably leads to a lowering in the degree of conversion of the bromine, complicates the control of the cooling of the HBr formed and increases the risks of explosion.