Petrochemical Process Solutions · Case study
Northshore GCD2673 + GCD2676: Replacing DNBP in styrene distillation
A 120 kt/a styrene unit replaced its inhibitor-plus-DNBP program with Northshore GCD2673 and GCD2676. During a monitored production trial, tar yield fell 14% and polymer in the T-6402 column bottoms fell 25%, with no polymerisation-related shutdown.
Request Full Case StudyOperating context and objective
A 120 kt/a styrene unit used DNBP as a retarder alongside a commercial true inhibitor. Washing the DNBP injection system at turnaround produced DNBP-bearing effluent, while nitrogen left in the tar could contribute to NOx when the tar was burned. The plant wanted a DNBP-free substitute that would control tar and polymer without new injection hardware or a polymerisation-related shutdown.
The technical agreement set four acceptance measures: styrene in tar at or below 11%, tar yield at or below 10 kg/t of styrene, combined chemical cost no higher than RMB 31.28/t of styrene, and no unplanned polymerisation shutdown.
Product and monitored application
Northshore GCD2673 is a true inhibitor, while GCD2676 is a retarder. A compatibility test showed that GCD2673 could enter the existing inhibitor system without a displacement wash; only the former DNBP line to column T-6401 required an ethylbenzene flush.
GCD2673 started on 16 October 2023 at 0.35 kg/t of styrene. GCD2676 started two days later with a short high-rate charge, then 1.35 kg/t. Daily residual and polymer analysis supported four retarder dose reductions and two inhibitor reductions, reaching 0.28 and 0.64 kg/t by 17 November. The monitored period covered 11,989 t of styrene over 35 days of retarder treatment and 37 days of inhibitor treatment.
Reported results
- Tar yield fell 14%. The reported rate decreased from 7.66 to 6.58 kg/t of styrene, below the agreement's 10 kg/t ceiling. On the unit's 120 kt/a basis, the plant valued the difference as about 130 t/yr of additional styrene, or approximately RMB 1.11 million per year at its stated RMB 8,500/t valuation.
- Bottoms polymer fell 25%. T-6402 bottoms averaged 955 mg/kg during the trial, compared with 1,267 mg/kg in the July-September baseline. A single excursion to 1,935 mg/kg was attributed in the report to an unnoticed change in tar recycle ratio.
- Styrene in tar rose but the average remained within the stated limit. The reported average was 7.8%, compared with 6.3% before the switch. It peaked at 15% early in the trial, above the 11% acceptance limit, while conditions were held unchanged for comparison; the plant attributed the response to thin-film evaporator temperature.
- No shutdown or product-quality upset was reported. The report records no polymerisation-related shutdown and no off-spec product during the 35-day retarder trial. A coupon in the T-6402 bottoms pump filter showed 0.0007 mm/a corrosion with no visible attack.
| Measure | Reported result |
|---|---|
| Tar yield | 7.66 to 6.58 kg/t of styrene (−14%) |
| T-6402 bottoms polymer | 1,267 to 955 mg/kg (−25%) |
| Trial chemical cost | RMB 50.52/t of styrene |
| Optimised chemical cost | RMB 42.81/t, compared with RMB 31.28/t agreed |
| Optimised cost after plant's styrene credit | RMB 33.56/t, compared with RMB 34.00/t actual prior spend |
Cost interpretation. The optimised chemical rate remained above the technical agreement's cost target. The plant's net comparison reached RMB 33.56/t only after crediting the lower tar yield at RMB 9.25/t of styrene. The report states that dose reduction was still in progress when the trial ended.
Operational value
The plant's own accounting connected lower tar yield with about 130 t/yr of additional styrene, valued at RMB 1.11 million per year. Applying that credit to the optimised chemical cost gave a net RMB 33.56/t, or RMB 0.44/t below the RMB 34.00/t actually spent on the previous program during July-September 2023.
This economic comparison depends on the plant's production rate, styrene valuation and cost records. Another unit should use its own tar balance, netback and treatment cost rather than transfer this result directly.
Applying the findings
Check miscibility with the incumbent inhibitor before switching. Establish retarder residual with an initial charge, then reduce each dose only as residual and polymer analysis allow. Hold each setting long enough to separate a treatment response from normal process variation.
Track tar recycle ratio, thin-film evaporator temperature, styrene in tar, column-bottoms polymer and tar yield together. This case's polymer excursion and early styrene-in-tar peak show why operating conditions need to accompany the chemical data.
Discuss a similar application
Request the full illustrated case study and share your current inhibitor and retarder program, injection points, tar yield, styrene-in-tar and column-bottoms polymer history. Cestoil can help define a monitored DNBP-replacement evaluation.
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Source: Customer evaluation report from the participating 120 kt/a styrene unit, 16 October-22 November 2023, as summarized in Cestoil case study CS-STY-GCD2673-GCD2676-2026A, v1.0. Baselines are those stated by the plant. All figures are reported by the customer, and performance may vary with feed quality and operating conditions.