Sour Water from Atmospheric and Vacuum Units: When a Sulphur Recovery Unit Is Overkill

Sour water from the atmospheric and vacuum distillation units of a refinery contains small amounts of sulphides, ammonia nitrogen and trace oil. By long-standing practice the stream is sent wholesale to the sulphur recovery unit for high-specification, full-process purification, regardless of how mild its actual composition turns out to be. At some refineries that habit costs far more than it should.

SINOKLE examined the assumption at a large refinery and reached a different conclusion: the sour water from these units was considerably cleaner than the design assumption. Treating it in a unit built for high-concentration, highly toxic pollutants was a mismatch that consumed energy and chemicals for no environmental gain.

A two lows and one high water quality

Before redesigning anything, the SINOKLE team ran three months of continuous monitoring and sampling. The profile that emerged had two low values and one high one.

• Low sulphide. Sulphide content held steady at around 30 ppm, far below the load conventional high-concentration sour wastewater presents, which is usually 100 ppm and above. Deep removal through a high-temperature, high-pressure sulphur recovery process was unnecessary.
• Low ammonia nitrogen. Ammonia nitrogen stayed around 20 ppm, a mild concentration a conventional wastewater treatment plant can accept, so no separate deep denitrification was needed.
• Relatively high oil. This was the real difficulty. Dispersed and emulsified oil droplets in the water are fine; sent straight to the wastewater plant, they would shock the biological system, cut treatment efficiency and push the effluent out of compliance. Targeted oil removal upstream was the one step genuinely required.

The cost of treating clean water in a heavy-duty unit

Routing this lightweight stream into the sulphur recovery unit is like using an anti-aircraft gun on a mosquito. Treatment cost stays high, because the unit consumes large amounts of steam, electricity and chemicals to hold high-temperature reaction and catalytic conversion stages steady - consumption that does not fall just because the feed is mild. High-quality resources are wasted too: water purified in the sulphur unit meets indicators far beyond an ordinary wastewater plant's acceptance standards, yet with no reuse channel it is simply sent to the wastewater plant for another pass. And the low-concentration volume occupies the unit's effective capacity, so genuinely difficult, high-concentration streams wait in line.

The conclusion was straightforward: liberate this sour water from the sulphur unit's long treatment chain and give it a targeted on-site pre-treatment step so it can go directly to the wastewater plant once it meets acceptance criteria.

The process: buffer, filter, coalesce

The chosen flow is deliberately simple and fully automatic: sour water feed, regulating buffer tank to even out flow and quality, filtration unit to take out suspended solids and mechanical impurities, a coalescing oil separator as the core oil-removal stage, effluent testing, and discharge to the wastewater plant once compliant. The buffer tank matters because refinery production swings; evening out the shock protects everything downstream.

Why pre-filtration matters

A precision filtration unit with high-efficiency media intercepts suspended particles, mechanical impurities and a small amount of larger oil, with filtration precision down to below 10 μm. That does two things: it hands the coalescing stage a cleaner feed, and it keeps fine impurities from clogging the coalescer's internals, which extends service life and trims maintenance. The unit backwashes automatically on a timer, so media are not replaced often and O&M workload stays low.

Coalescing as the core step

For dispersed and emulsified oil in the 0.1 to 10 μm range, natural separation is not realistic. The SINOKLE coalescing oil separator uses a specially modified oleophilic, hydrophobic material as the medium. As oily water passes through, tiny droplets are adsorbed and gathered, then coalesce into larger droplets - up to over 100 μm - which rise quickly on the oil-water density difference to a collection trough at the top of the vessel and leave through the automatic oil discharge system. The whole pre-treatment train runs under PLC automatic control, watching influent quality, effluent quality and equipment status in real time and alarming or adjusting when quality drifts.

Trial-run results

A one-month continuous trial run met every target. Oil content fell from about 80-120 ppm to a stable level below 10 ppm, a removal rate above 92%. Suspended solids fell from about 50-80 ppm to below 20 ppm, a removal rate above 75%. Sulphides and ammonia nitrogen settled at below 5 ppm and below 15 ppm respectively, both inside the plant's acceptance limits. There were no equipment failures during the run, effluent compliance held at 100%, and all of the sour water bypassed the sulphur recovery unit.

Where the economics land

Pulling low-concentration sour water out of an energy-intensive unit saves substantial steam, electricity and chemical cost each year; at this refinery the annual operating saving approached one million yuan with a payback period of only 1.5 years. The sulphur unit regains capacity for the streams it was designed for, and purified water from the sulphur unit can be redirected to lower-grade uses such as greening, floor washing and circulating water make-up instead of being sent to the wastewater plant for nothing.

The point is not a new process so much as a right-sized one. Precise targeting, a simple and efficient flow and full use of the resource resolve the high spec, low use problem the traditional route had been quietly carrying for years.