PTQ Q2 2024 Issue

Steam, o-gas, nitrogen, air, water Caustic Sulphuric acid Brine Acid oils

PDC

O-gas

Acid oils

Nitrogen

Scrubbing phase separation

Neutralisation

Process water

Oxidation

Cooling water

Alkaline solution

Oxidation air

Fresh caustic

Cooling water

MP steam

Neutralised brine

Cooling water

Spent caustic

Sulphuric acid

Figure 1 Spent caustic treatment system employing wet air oxidation

demand (COD) and biochemical oxygen demand (BOD). This allows the treated brine to be sent to a wastewater treatment facility for final polishing before release to the environment. Treatment technologies typically utilise sulphuric acid, atmospheric oxygen, nitrogen, fuel gas, electricity, steam, and/or cooling water – all elements readily available at most refineries. After treatment, a neutral pH, saltwater effluent is produced that has minimal odour, presents no health and safety issues, and is easily manageable in wastewater treatment plants. The most frequently used technologies include sim - ple neutralisation, deep neutralisation, wet air oxidation (WAO) and incineration. Although caustic neutralisation and oxidation systems have been used for decades, their seemingly simple processes are surprisingly complex. The type and concentration of impurities affect the temperature and pressure requirements, how many treatment steps are required, the mixing and residence times for the reaction and separation steps, and the selection of the metallurgy to protect equipment from severe corrosion. Simple neutralisation Simple neutralisation is the most straightforward and least expensive treating option for regenerated caustic streams coming from light hydrocarbon streams such as LPG where most of the toxicity and COD result from excess NaOH, thiosulphate, and dissolved organics. A simple acid, such as sulphuric or hydrochloric acid, is added to the spent caustic to neutralise any free NaOH remaining and drop the pH to an acceptable wastewater discharge value, within the preferred range of 6-9, although some publicly owned treatment works allow higher or lower pH ranges. The acids are conveniently available and add no COD to the treated brine. If the resulting sulphate or chloride levels are too high, organic acids, such as formic or acetic acid, can be substituted.

The simple neutralisation process is streamlined and efficient. Spent caustic and acid are injected continuously into a treatment unit, mixed, and then entered into a three- phase separator. The acid flow is regulated by sensing the pH of the released brine. Any acid gases or oils generated by the reaction will separate. If necessary. to lower the COD and odour, nitrogen or fuel gas can be added to strip away dissolved acid gases. When low levels of unregenerated sulphide, hydro - sulphide, and mercaptide salts are present, a variation of simple neutralisation adds a stripping column to the three- phase separator where the nitrogen or fuel gas can effi - ciently remove these dissolved acid gases. The degree of removal of these dissolved acid gases is highly dependent on the treatment pH, with a lower pH releasing more of the gases. Where high levels of these impurities are pres - ent, the pH must be lowered beyond the normal discharge range, requiring the need for ‘deep neutralisation’. Deep neutralisation When spent caustic contains high concentrations of unre - generated sulphides, mercaptides and organic compounds, or the brine product specifications are more demanding, deep neutralisation is utilised. This treatment method can treat most sulphidic, phenolic, and naphthenic spent caus - tics. ‘Deep’ refers to acidification of the spent caustic to a low pH followed by re-neutralisation with caustic. This process reconstitutes all the original acids that were present in the treated hydrocarbon streams: sulphides, mercaptides, and organic compounds, such as phenolics, cresylics, and naphthenic acids. The organics form an oil phase that separates from the brine. If deemed beneficial or re-blended with another hydrocarbon stream, the organics can be decanted for reclamation. The deep neutralisation treatment system generally con - sists of a low pH reactor, a three-phase separator, acid gas

PTQ Q2 2024