Solving the Sour Water Challenge: Process Details and Case Reference
1. Sources of sour water:
In the production of petroleum and its derivatives, the sulfur and nitrogen compounds present pass through atmospheric and vacuum distillation, catalytic cracking, delayed coking and hydrotreating, where high-temperature cracking, catalytic conversion and hydrocracking reactions convert them into hydrogen sulfide and ammonia nitrogen that enter the product stream. Once these streams are cooled and dewatered or water-washed, wastewater rich in sulfur and ammonia is formed, commonly called sour water or sulfur-bearing wastewater.
2. Characteristics and hazards of sour water:
High coke fines concentration with a fine particle size distribution, high oil content with small droplets and severe emulsification; the large amount of emulsified oil and suspended solids (coke fines) affects stripping performance and liquid ammonia quality.
It readily plugs the stripper and, in severe cases, can cause unplanned shutdowns and significant economic loss.
Its chemical composition is complex and pollutant concentrations fluctuate markedly.
Leakage of irritating odours from sulfides and ammonia nitrogen seriously affects the environment and employee health.
Discharging it directly to the wastewater treatment plant places enormous environmental pressure on the company.
3. Conventional sour water processes and their difficulties:
(1) Gravity settling for sour water: Most refineries in China still use this method, with settling periods of 50 to 70 hours. Because the oil in sour water is deeply emulsified and the coke fines are very fine, even prolonged settling gives poor separation. Storage tanks are mostly open or semi-enclosed, so acid gases escape, worsening the environment and occupational health conditions. The process needs a large area, heavy civil works and large capital investment. It relies entirely on manual operation with a low level of automation, and cleaning and maintenance require much manpower and are inefficient.
(2) Cyclonic removal of oil and coke fines: Some plants have tried tank-in-tank cyclones or two-stage cyclones to remove coke fines and oil, but the separation problem remains unsolved. Better than gravity settling, cyclonic separation is still limited because the emulsified droplets are tiny and their density is close to that of water, so oil and coke removal remains poor. The system tolerates load fluctuations badly and separation stability is insufficient to cope with large changes in sour water composition. Cyclones plug easily and in severe cases bring the system to a halt. Operating pressure is high, lift pumps and other equipment consume much energy, and operating costs are high.
(3) Flotation plus inorganic ceramic membrane: The high-precision filtration of the membrane gives good effluent quality in the short term, but system stability is poor: under shock loads membrane fouling quickly occurs, reducing capacity and water recovery and producing large volumes of concentrate and chemical cleaning waste that are difficult to dispose of. Operating pressure is high, energy consumption significant, and investment and maintenance costs remain high.
4. The SINOKLE sour water treatment solution:
We adopt a combined process of “CDFU + SiC filter + KHC coalescing oil separator” to achieve oil-water separation of sour water.
The technology has the following features:
Process flow diagram
5. How the sour water treatment process works:
Sour wastewater first enters the CDFU unit, where, under the combined action of the swirling flow field and dissolved-gas microbubbles, free oil, dispersed oil, emulsified oil and suspended particles rapidly collide with and attach to the micro- and nanobubbles. Surface tension breaks the emulsified oil film purely physically, and oil on the surface of suspended solids is adsorbed by the bubbles. The demulsified oil phase, suspended solids and microbubbles form a multiphase mixture that gathers in the CDFU and floats up to the oil collection zone at the top, from where it is drawn off automatically through the oil outlet by pressure difference. The partially treated effluent is pumped to the SiC filter, which controls the absolute particle size of suspended solids in the effluent to below 10 μm. The separated oily water is discharged by its own pressure to an oily water collection tank and then pumped to the oil storage system.CDFU Off-gas is routed to the flare knock-out drum.
CDFUThe effluent is pumped to the SiC filter, where water enters from the centre of the SiC filter element and leaves around the periphery, intercepting suspended solids and gelatinous agglomerates; the absolute particle size of suspended solids in the effluent can reach 1 μm. Filter effluent then enters the coalescing oil separator. When the pressure drop across the filter system is large, backwashing is required to wash impurities and oil from the filter element and prevent plugging; the backwash wastewater is discharged to the oily water collection tank.
The sour wastewater enters the coalescing oil separator, where special coalescing media coalesce small oil droplets into large ones for physical demulsification. After demulsification the oil layer separates from the water layer by density difference; the oily waste is drawn off from the oil collection point at the top with a water content below 10%, and the wastewater leaves through the bottom drain, achieving oil-water separation with oil content in the effluent below 50 ppm, dissolved oil excluded.
CDFU, the SiC filter, the coalescing separator effluent, oil skimming, sludge discharge, venting and air makeup are all automatically controlled, and the air feed to the dissolved-gas system is automatically regulated.
This process is fully automatic and runs enclosed and pressurised, with off-gas routed to the flare knock-out drum; there are no safety or environmental risks.
6. Influent and effluent specifications of the process:
7. Case study:
