1. Field of the Invention
This invention relates to methods and an apparatus for producing decabromodiphenyl alkanes. More specifically, the field of the invention is that of producing decabromodiphenyl ethane.
2. Description of the Related Art
Halogenated aromatic compounds are often employed as flame retardant agents. Flame retardants are substances applied to or incorporated into a combustible material to reduce or eliminate its tendency to ignite when exposed to a low-energy flame, e.g., a match or a cigarette. The incorporation of flame retardants into the manufacture of electronic equipment, upholstered furniture, construction materials, textiles and numerous other products is well known.
Brominated aromatic compounds are often utilized as flame retardant agents in polymer compositions such as the outer housing of computers, television sets, and other electronic appliances. One group of halogenated flame retardants are decabromodiphenyl alkanes. The manufacture of decabromodiphenyl alkanes is known. Conventionally, decabromodiphenyl alkanes are prepared by reacting a diphenyl alkane with bromine in the presence of a bromination catalyst, such as AlCl3 or FeCl3.
For example, U.S. Pat. No. 5,030,778 to Ransford discloses a process for producing decabromodiphenyl alkanes in which bromine and a bromination catalyst are charged to a reaction vessel. Liquid diphenyl alkane is fed by a dip tube into the reaction vessel at a point which is beneath the level of the charged liquid bromine and catalyst. The stated advantages of this sub-surface addition method are that (1) a product with a high average bromine number is obtained faster when the diphenyl alkane is fed below the surface of the charged liquid bromine and catalyst; and (2) splattering of the reaction mass associated with the addition of the diphenyl alkane into the vessel is reduced.
One disadvantage to adding the diphenyl alkane to the vessel at a location below the surface of the charged bromine and catalyst is that the dip tubes used for adding the diphenyl alkane to the vessel are prone to plugging. It is believed that the sub-surface addition dip tubes become plugged when a small amount of diphenyl alkane remains at the tip of the tube and reacts in place, thereby forming insoluble, high melting point material. It is believed that this is more likely to occur at the end of the addition or if the diphenyl alkane addition is interrupted. This susceptibility to plugging prevents the manufacturer from being able to stop and start the diphenyl alkane addition, which is sometimes desirable for controlling the evolution HBr gas.
It is also believed that the agitation of the reaction mass may create a vortex within the tip of the sub-surface addition dip tube. This vortex may pull solids from the reaction mass into the tube, thereby creating a blockage. Additionally, because some diphenyl alkanes, such diphenylethane (“DPE”), are solids at room temperature and are fed to the reaction vessel as liquids, they may begin to crystallize in an unheated dip tube if the feed is interrupted for any reason. In the event that the sub-surface dip tube does become plugged, regardless of the reason, the diphenyl alkane feed must be stopped and the tube must be pulled out of the reactor in order to remove the blockage. It is desirable to avoid the need to remove the dip tube, as the vapor space of the reaction vessel is filled with toxic and corrosive bromine vapors which may escape during removal.
The above-surface diphenyl alkane addition technique of the present invention reduces dip tube plugging, thereby providing a more efficient method of adding diphenyl alkane to a reactor charged with bromine and catalyst.