In nitration processes to produce nitrated aromatic products, nitro-phenolic species and other oxidative decomposition compounds are formed as by-products. For example, nitration of benzene produces nitrophenols, nitration of toluene produces nitrocresols, and nitration of xylene produces nitroxylenols. The crude nitrated aromatic product requires purification to remove these by-products before it is suitable as a feedstock to downstream hydrogenation processes, where these impurities are believed to adversely affect the catalyst performance.
The purification of nitrated aromatic products is commonly practiced industrially by contacting the crude nitrated organic with an alkaline water stream in a multi-stage counter-current washing system. The nitro-phenolics and other oxidation compounds, which are organic acids, are neutralized into their respective organic salt form and extracted into the alkaline water phase to produce a water effluent stream rich in nitro-phenolics and other oxidation species. This process is well known, and is described in the patent literature: U.S. Pat. No. 6,288,289 (Boyd et al.); U.S. Pat. No. 4,604,214 (Carr et al.); U.S. Pat. No. 6,506,948 (Sawicki); US 2007/0088183 (Hermann et al.); and WO 2012/156095 (Polmann et al.).
The generated effluent stream is commonly known as strong effluent or red water, due to its high nitro-phenolic concentration and its strong characteristic color. The treatment of this effluent stream is particularly challenging since some nitro-phenolics are highly bio-toxic, such that biological wastewater treatment plants can only tolerate very low concentrations. Therefore, this effluent stream usually requires pre-treatment to reduce its toxicity before it can be discharged to a biological treatment facility.
There are several effluent treatment technologies currently in use to treat this strong effluent stream. These include: thermal destruction (U.S. Pat. No. 4,230,567), wet oxidation (U.S. Pat. No. 5,250,193, U.S. Pat. No. 8,801,932), solvent extraction (U.S. Pat. No. 4,597,567 and U.S. Pat. No. 4,925,565), ozonation (U.S. Pat. No. 4,604,214) and incineration/thermal oxidation (U.S. Pat. No. 6,288,289). The optimum treatment technology is dependent on many factors, including local site conditions, economics and operator preference. In addition, the selected treatment technology is dependent on the type of base used to provide alkalinity in the nitrated product purification system.
In most industrial plants, caustic soda (sodium hydroxide) is used as the washing base to provide the required alkalinity for the nitrated product purification process; however, aqueous ammonia is used in a small fraction of industrial nitration processes. The reason for caustic soda being favored is that it is a stronger base than ammonia, and as such, results in superior washing efficiency and leads to higher product quality, with lower nitro-phenolic and oxidation species in the final washed nitrated aromatic product. Ammonia, being a weaker base, is not able to effectively neutralize and thereby extract some of the weaker organic acid by-products (i.e., those which have higher pKa values). In addition, caustic is non-volatile whereas ammonia exerts a significant vapor pressure and may deposit unstable ammonium nitrite salts formed by reacting ammonia with NOx present in the nitration plant vent.
However, ammonia may be the preferred washing base if incineration/thermal oxidation is chosen as the strong effluent treatment option. In general, inorganic salts produce ash in the incinerator which can attack the refractory lining and require the installation of a slag collection system; whereas, ammonia is reduced to nitrogen in a thermal oxidizer so there are no ash deposits to deal with.
Operating costs of incineration can be fairly high due to the large amount of water that has to be vaporized. However, in some circumstances incineration may be an attractive option as it has the benefit of not requiring further treatment, such as a biological treatment, and can reduce total capital investment costs of a project. It can also achieve competitive operating costs where there is a low cost of energy or alternatively where the effluent stream can be used to offset water already used in an incineration process.
In summary, selecting caustic soda as the washing base results in improved product quality but requires additional treatment processes to treat the generated strong effluent. Alternatively, selecting ammonia allows for incineration/thermal oxidation which can eliminate the requirement for further treatment processes but results in reduced product quality, which may negatively affect the downstream processes.