PTQ Q2 2024 Issue

stripping, and re-neutralisation. As with simple neutralisa- tion, sulphuric acid is added – except in excess – which is needed to drive the pH to a low value to liberate the acidic impurities. Any acid gases or oils liberated at the low pH conditions separate in the three-phase separation drum. If sulphide or mercaptide salts are present in significant amounts, nitrogen or fuel gas is used to strip these away to low ppm levels. The acidic brine is then re-neutralised back to a neutral pH with fresh caustic. The effluent streams become neutralised brine, a phenolic or naphthenic oil phase, and an acid gas stream containing H 2 S and mer- When treating high concentrations of impurities, deep neutralisation has the unique and distinct advantage of utilising low temperature and pressure operation captans. If there is a significant acid gas stream, an amine treater or Claus unit processes the H 2 S. When treating high concentrations of impurities, espe- cially sulphides and organics, deep neutralisation has the unique and distinct advantage of utilising low temperature and pressure operation, and it can recover the organics. Wet air oxidation (WAO) WAO is a well-established technology with great efficacy for treating all spent caustic types. It is particularly suited for high flow rate sulphidic caustic streams from ethylene production plants. WAO destroys impurities through oxidation. It converts sulphides to sulphate and organic compounds to car- bonate and low molecular weight carboxylic acids. During

the process, no acid gas is released, so the off-gas stream contains mere traces of volatile organic compounds (VOC). Highly coloured, malodorous liquors treated with wet air oxidation become crystal clear and can be effectively man- aged by a biological wastewater treatment system. In the WAO process, hot caustic containing suspended or dissolved pollutants is brought into contact with air at elevated temperatures and pressures in a reactor unit. The molecular oxygen dissolved in the caustic becomes an oxidising agent that causes a chain reaction of free rad- icals formed from organic and inorganic compounds in the reaction mixture (see Figure 1 ). The organic pollutants are mineralised or converted into biodegradable substances, and the inorganic compounds are degraded. The degree of oxidation hinges on the compound, the reactor residence time, and the prescribed temperature and pressure. Upon withdrawal from the reactor, the off- gas separates from the oxidised caustic and flows to a downstream incinerator. The oxidised caustic then flows to a neutralisation system where sulphuric acid is added to bring the pH within a desired final range, yielding a neutral brine and off-gas (see Figure 2 ). A very small number of providers offer WAO with varying techniques and efficacy, with temperature being the most critical variable in WAO system design. As the temperature rises, there is a higher degree of oxidation. WAO reactor temperatures range from as low as 100°C to as high as 320°C, with pressures ranging from 5 barg up to 120 barg. The temperature and pressure selected depend on the feed type and the product specifications required. Most sulphidic systems operate in the middle of the range at 200°C and 30 barg, where all sulphides are fully oxidised to sulphate. High-temperature systems (260-320°C) are applied where organics such as phenolics, naphthenic acids, and amines are prevalent. Although amines are difficult to oxidise, at 290°C and higher they are partially destroyed, depending on the amine type.

HP steam

Oxidation

O-gas

Spent caustic

Sulphuric acid

Gas/liquid separation

Air

Cooling water

Neutralisation

Steam, air, o-gas Caustic Oxidised caustic Acid Brine

Cooling water

Neutralised brine

Figure 2 Merichem Mericon II spent caustic treatment system employing wet air oxidation

PTQ Q2 2024