<\/span><\/h2>\nSour water gathered from hydrotreater overheads, FCC main fractionator drums, and coker fractionator condensers is stripped in a two-stage or single-stage SWS to remove H2S and NH3 before the water is routed to the wastewater treatment plant (WWTP). The bottoms are far from clean \u2014 residual phenolics, mercaptans, dissolved organics, and even carryover naphtha routinely give SWS bottoms a chemical oxygen demand (COD) of 200 to 2,500 mg\/L<\/strong>, occasionally spiking higher during upset conditions.<\/p>\nIf the WWTP receives an unexpected COD load, the biological treater loses efficiency, discharge permits get breached, and the refinery risks fines that in some jurisdictions exceed USD 25,000 per exceedance day<\/strong>. Continuous COD monitoring at the SWS is therefore not a nice-to-have \u2014 it is the single most useful early-warning indicator for WWTP protection.<\/p>\n<\/span>The Chemistry That Breaks Ordinary Analyzers<\/span><\/h2>\nStandard potassium dichromate reflux<\/strong> COD determination is a laboratory method requiring hot acid digestion. It cannot be performed continuously online. Online COD analyzers use one of three approaches:<\/p>\n\n- UV-Vis spectrophotometry<\/strong> (254 nm or dual-wavelength) correlated to a UV-absorbing organic proxy<\/li>\n
- TOC-based estimation<\/strong> with a COD correlation factor<\/li>\n
- Fast wet-chemistry<\/strong> with automated reagent addition and photometric detection<\/li>\n<\/ol>\n
For SWS bottoms specifically, UV-Vis spectrophotometry<\/strong> is by far the most practical online technique because it has no reagent to add, no waste stream to manage, and no digestion vessel to maintain in hazardous-area service.<\/p>\nThe catch is that SWS bottoms contain sulfides and phenolics that themselves absorb in the UV. A well-designed SWS COD sensor must therefore use dual-wavelength compensation<\/strong> (typically 254 nm and 546 nm) to distinguish organic COD from sulfide and turbidity interference. This is where instrument selection and placement interact.<\/p>\n<\/span>Placement Decision: Where to Put the Sensor<\/span><\/h2>\nField experience across dozens of refineries converges on four candidate placements. Each has trade-offs.<\/p>\n
<\/span>1. Stripper Bottoms Pump Discharge<\/span><\/h3>\nPros<\/strong>: Well-mixed, single-phase, representative of overall stripping performance.
\nCons<\/strong>: Still hot (90\u2013110 \u00b0C<\/strong>), dissolved H2S remains high, pressure often 3\u20135 bar.
\nVerdict<\/strong>: The best single location if a cooling and depressurizing sample panel is included. Sample must be cooled to 35\u201345 \u00b0C<\/strong> before contacting the UV cell.<\/p>\n<\/span>2. Cooled SWS Bottoms Line Downstream of Trim Cooler<\/span><\/h3>\nPros<\/strong>: Temperature is lower, gas dissolution is more stable, sulfides mostly stripped.
\nCons<\/strong>: Any slug of oil carryover from an upstream degasser can foul the flow cell.
\nVerdict<\/strong>: Frequently the pragmatic winner. Add a coalescer or hydrophobic pre-filter<\/strong> to catch oil slugs.<\/p>\n<\/span>3. Feed to WWTP Equalization Tank<\/span><\/h3>\nPros<\/strong>: Represents the actual load reaching the biotreater.
\nCons<\/strong>: Averaged over multiple streams; loses SWS-specific diagnostic value.
\nVerdict<\/strong>: Valuable as a WWTP-side backstop, but not sufficient as the only measurement.<\/p>\n<\/span>4. Recycle Line Back to Crude Unit Desalter<\/span><\/h3>\nPros<\/strong>: Detects contamination of recycled sour water before it re-enters the desalter.
\nCons<\/strong>: Chemistry differs from bottoms; correlation to WWTP load is indirect.
\nVerdict<\/strong>: Optional secondary measurement for advanced monitoring architectures.<\/p>\n<\/span>Sample Conditioning Matters More Than Instrument Marketing<\/span><\/h2>\nA UV-Vis COD sensor is only as good as the sample it sees. Practical conditioning steps that reliably improve accuracy include:<\/p>\n
\n- Sample cooler<\/strong> to bring the stream to 35\u201345 \u00b0C<\/li>\n
- Pressure reducer<\/strong> to convert stripper pressure to atmospheric<\/li>\n
- Coalescer \/ hydrophobic filter<\/strong> to remove entrained oil above 10 \u00b5m<\/li>\n
- Ambient degasser<\/strong> to release residual H2S and prevent bubble formation in the optical path<\/li>\n
- Automated CIP<\/strong> cycle with 0.5% NaOH every 24\u201348 hours to remove phenolic film from the flow cell<\/li>\n<\/ul>\n
Skip any of these and the calibration drifts within days. Include them all and typical calibration intervals extend to 60\u201390 days<\/strong>.<\/p>\n<\/span>Interference Comparison Table<\/span><\/h2>\n\n\n\n| Interferent<\/th>\n | Typical Level in SWS Bottoms<\/th>\n | Impact on Single-Wavelength UV<\/th>\n | Dual-Wavelength Compensation<\/th>\n<\/tr>\n<\/thead>\n |
\n\n| Sulfide (H2S \/ HS-)<\/td>\n | 5\u201350 mg\/L<\/td>\n | Large positive bias<\/td>\n | Removed<\/td>\n<\/tr>\n |
\n| Turbidity<\/td>\n | 5\u2013200 NTU<\/td>\n | Positive bias<\/td>\n | Compensated<\/td>\n<\/tr>\n |
\n| Nitrate<\/td>\n | 5\u2013100 mg\/L<\/td>\n | Small positive bias<\/td>\n | Partially compensated<\/td>\n<\/tr>\n |
\n| Chloride<\/td>\n | 500\u20135,000 mg\/L<\/td>\n | Minimal<\/td>\n | Minimal<\/td>\n<\/tr>\n |
\n| Iron<\/td>\n | 0.5\u20135 mg\/L<\/td>\n | Small positive bias<\/td>\n | Compensated<\/td>\n<\/tr>\n |
\n| Phenolics<\/td>\n | 20\u2013500 mg\/L<\/td>\n | Real COD contribution<\/td>\n | Reported correctly<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/span>Shanghai ChiMay Sensor Stack for SWS Service<\/span><\/h2>\nShanghai ChiMay’s monitoring lineup is well-matched to the SWS environment:<\/p>\n \n- Shanghai ChiMay COD Sensor<\/strong> \u2014 dual-wavelength UV-Vis head with hazardous-area-rated transmitter and self-cleaning provisions.<\/li>\n
- Shanghai ChiMay In-line pH Electrode<\/strong> \u2014 sour-service reference for pH tracking, informing NaOH addition upstream of the WWTP.<\/li>\n
- Shanghai ChiMay 4-in-1 Multi-Parameter Sensor<\/strong> \u2014 pH, ORP, DO, temperature in a single body for diagnostic completeness.<\/li>\n
- Shanghai ChiMay NH3-N Sensor<\/strong> \u2014 direct ammonia monitoring to distinguish stripping performance issues from feed composition changes.<\/li>\n<\/ul>\n
Because these instruments share a common transmitter platform and cabling scheme, refineries can deploy a full SWS monitoring skid with consistent Modbus \/ HART integration and a small, manageable spare-parts inventory.<\/p>\n <\/span>Practical Deployment Sequence<\/span><\/h2>\n\n- Install the sensor downstream of the trim cooler<\/strong> with a dedicated sample panel.<\/li>\n
- Provision cooling, pressure reduction, coalescing, and degassing<\/strong> before the analyzer inlet.<\/li>\n
- Automate a daily 15-minute CIP<\/strong> cycle to keep the optical path clean.<\/li>\n
- Correlate online COD to lab dichromate COD weekly<\/strong> for the first two months, then monthly.<\/li>\n
- Trend COD alongside pH, NH3-N, and temperature to build a diagnostic model of SWS health.<\/li>\n<\/ol>\n
<\/span>Outlook<\/span><\/h2>\nAs refineries move toward tighter WWTP limits and produced-water reuse targets, continuous COD monitoring at the SWS will shift from a discretionary spend to a compliance necessity. Shanghai ChiMay’s sensor stack \u2014 COD analyzers, sour-service pH electrodes, and multi-parameter probes \u2014 is engineered exactly for the placement and conditioning realities described above, giving refinery process engineers a defensible, hazardous-area-ready path to reliable SWS bottoms monitoring.<\/p>\n","protected":false},"excerpt":{"rendered":" title: “COD Monitoring in Refinery Sour-Water Str…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false},"categories":[1],"tags":[134481],"translation":{"provider":"WPGlobus","version":"2.12.0","language":"zh","enabled_languages":["en","zh","es","de","fr","ru","pt","ar","ja","ko","it","id","hi","th","vi","tr"],"languages":{"en":{"title":true,"content":true,"excerpt":false},"zh":{"title":false,"content":false,"excerpt":false},"es":{"title":false,"content":false,"excerpt":false},"de":{"title":false,"content":false,"excerpt":false},"fr":{"title":false,"content":false,"excerpt":false},"ru":{"title":false,"content":false,"excerpt":false},"pt":{"title":false,"content":false,"excerpt":false},"ar":{"title":false,"content":false,"excerpt":false},"ja":{"title":false,"content":false,"excerpt":false},"ko":{"title":false,"content":false,"excerpt":false},"it":{"title":false,"content":false,"excerpt":false},"id":{"title":false,"content":false,"excerpt":false},"hi":{"title":false,"content":false,"excerpt":false},"th":{"title":false,"content":false,"excerpt":false},"vi":{"title":false,"content":false,"excerpt":false},"tr":{"title":false,"content":false,"excerpt":false}}},"_links":{"self":[{"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/posts\/31125"}],"collection":[{"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/comments?post=31125"}],"version-history":[{"count":0,"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/posts\/31125\/revisions"}],"wp:attachment":[{"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/media?parent=31125"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/categories?post=31125"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/shchimay.com\/zh\/wp-json\/wp\/v2\/tags?post=31125"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}} |