Sour Water Treatment Solution
After light fractions are extracted in atmospheric and vacuum distillation units, the heavy fractions of crude oil generally require secondary processing such as catalytic cracking, catalytic pyrolysis or delayed coking. Because most heavy oils cannot meet the stringent feed quality requirements of catalytic cracking (carbon residue no more than 10%, metal mass fraction no more than 32 μg/g, and so on), many refiners choose delayed coking — which tolerates poorer feed — to process hydrogen-poor, low-grade heavy oils such as vacuum residue, cracked residue and asphalt.
Delayed coking heats the feed rapidly to coking temperature (about 763 K) and sends it to the coke drum, where cracking and condensation reactions convert it into light products such as raw gasoline and diesel plus coke. Delayed coking of heavy oil, however, generates large volumes of wastewater, mainly oily wastewater (coke cooling water), high-oil wastewater (blow-down condensate) and sour water. The sour water must pass through a stripper to remove ammonia nitrogen and recover hydrogen sulfide before being used as desalter wash water or discharged after treatment to standard.
Because low-grade heavy oils contain natural surfactants (asphaltenes, resins, naphthenic acids and so on), production chemicals (alkylbenzene sulfonates, fatty alcohol polyoxyethylene ethers and so on), desalter demulsifiers and corrosion inhibitors, the sour water forms emulsions with light oil that are hard to separate. It also carries large amounts of coke powder and light oil, which not only means product loss but creates major difficulties downstream, for example:
① It disrupts the gas-liquid balance of the stripper: surfactants contacting steam inside the column readily generate heavy foaming, which impairs mass transfer between phases, raises steam consumption, causes operating fluctuations and reduces stripper capacity;
② It plugs trays and reboilers: oily agglomerates in the water accumulate at trays, reboilers and other critical points, causing blockages, reducing mass-transfer efficiency, creating local overheating and shortening equipment performance and service life;
③ It contaminates by-products: during stripping the sour water produces black sulfur, which affects liquid ammonia quality. Removing oil and coke powder from sour water is therefore of real importance.
Technologies for oil and coke-powder removal from sour water include mechanical filtration, gravity settling, chemical demulsification and cyclonic separation. Single technologies, however, have limited effect, so many research institutes combine and integrate them to develop new sour water processes with high capacity, high separation efficiency and easy maintenance.
Against this background, SINOKLE's integrated cyclonic flotation technology emerged. Combining it with other technologies not only removes coke powder completely but brings all sour water indicators within specification. The technology has already been applied industrially at a Sinopec refinery.
Because sour water produced in the small steam-blowing stage is harder to treat, the oil and suspended-solids removal performance of the "CDFU + coalescing de-oiling" combined process was tested separately on sour water from the normal coking stage and from the small steam-blowing stage. Results for the normal stage are shown in Table 2.
As Table 2 shows, the average oil mass concentration in normal-stage sour water is 45,000 mg/L; after treatment by the "CDFU + coalescing de-oiling" combined process it falls to 175.11 mg/L. The process therefore removes oil very effectively from normal-stage sour water, with an average oil removal rate of 98.33%. Meanwhile, with an average inlet suspended solids concentration of 353 mg/L, the outlet averages 7 mg/L and the water is clear and bright, confirming excellent coke-powder removal.
Economic benefit analysis:
The packaged de-oiling unit treats 15 m³/h of sour water and uses only a small amount of electricity. Assuming 8,400 h/a of operation, the running cost is about CNY 115,670; taking light oil recovery in the normal coking stage and a light oil price of CNY 7,000/t, the annual benefit from saved light oil is at least 564.8 t/a × 7,000 = CNY 3.9536 million.
Conclusion:
The combined coking sour water process built around "cyclonic dissolved air flotation + coalescing de-oiling" has the advantages of a small footprint and low operating cost, and provides an industrial reference of real economic value for resource recovery from refinery oily wastewater.
