Chemical Fiber Industry Thermal Oil System Cleaning: Descaling & Decarbonization for Polyester Production

1. Special Operating Conditions of Chemical Fiber Thermal Oil Systems

Across the chemical fiber chain — polyester chip, polyester filament/staple fiber, nylon and spandex production — almost every step depends on thermal oil heaters (organic heat carrier heaters) for heat supply. Taking polyester (PET) production as an example, from esterification and polycondensation of purified terephthalic acid (PTA) with ethylene glycol, to melt conveying and spinning box insulation, the entire process requires a stable and uniform heat source, typically at 280~320°C, and is extremely sensitive to temperature fluctuation. Once heat supply becomes unstable, the intrinsic viscosity of the polyester melt drifts, directly increasing yarn breakage and lowering product grade.

Although thermal oil (organic heat carrier) is more energy-efficient and precise than steam, its degradation at high temperature is an unavoidable chemical process. Chemical fiber enterprises commonly report that after 1-2 years of operation, furnace tube pressure differential rises noticeably, exhaust temperature climbs, and spinning temperature fluctuates — all pointing to the same culprit: thermal oil coking and carbon deposition.

2. Thermal Oil Coking Mechanism and Distribution

2.1 Thermal Cracking and Condensation Polymerization

Operating continuously at 280~320°C, some carbon chains of the thermal oil undergo C-C bond cleavage, producing low-molecular-weight hydrocarbons; meanwhile aromatic components undergo dehydrogenation condensation, progressively polymerizing into high-molecular-weight tar and coke. The higher the temperature and the longer the residence time, the deeper the condensation, ultimately forming graphitized hard coke on tube walls. The radiant section, exposed directly to flame radiation with the highest heat flux, is where hard coke concentrates most.

2.2 Oxidative Degradation and Iron Ion Catalysis

Residual trace oxygen and moisture in the system react with the thermal oil to form organic acids, aldehydes and gum. Acids corrode metal surfaces, and the resulting iron ions (Fe³⁺, Fe²⁺) are precisely the active catalytic centers for condensation coking — creating a vicious "oxidation-corrosion-coking" cycle. Controlling thermal oil acid value and moisture, and replenishing antioxidants in time, is therefore the first line of defense against coking.

2.3 Coking Distribution Characteristics

3. Impact of Coking on Chemical Fiber Production

Thermal oil coking is far from a mere energy issue — its impact on chemical fiber production is multifaceted:

1. Markedly higher energy consumption: a 1 mm coke layer reduces heat transfer efficiency by 15%~20%, raising fuel consumption for the same output.
2. Prominent safety hazards: hard coke has poor thermal conductivity; radiant section tubes overheat locally and lose strength, risking tube rupture or even fire.
3. Product quality fluctuation: unstable heat supply causes polyester viscosity drift and higher yarn breakage, directly affecting product grade and profitability.
4. Accelerated thermal oil degradation: existing coke catalyzes further coking, forming a self-accelerating cycle that shortens thermal oil service life.

4. Thermal Oil System Decoking and Cleaning Solutions

4.1 Shutdown Chemical Cleaning (Primary Solution)

For severely coked systems, sectional chemical cleaning after shutdown is recommended, using a combined "alkaline degreasing — acid derusting — decoking agent — passivation" process. The chemicals and parameters for each step are shown below:

4.2 Process Flow

1. Drain the used thermal oil; sectionalize and isolate the system;
2. Alkaline wash to degrease and emulsify gum on tube walls;
3. Rinse with clean water to neutral;
4. Acid wash to remove rust scale and carbonate;
5. Rinse with clean water to neutral;
6. Circulate the specialized decoking agent to dissolve and peel coke;
7. Rinse with clean water to neutral;
8. Passivate to form a protective film;
9. Neutralize and discharge the cleaning effluent to standard;
10. Refill with fresh thermal oil and resume operation.

4.3 Online Non-Stop Cleaning (Emergency Option)

For continuous lines that cannot be shut down, an online decoking agent can be circulated with the thermal oil while running. It slowly softens and disperses coke, which is then discharged from the system with the oil. It suits lightly coked systems dominated by soft coke and gum, but has a longer cleaning cycle and limited effect on hard coke — typically used as a transition before shutdown cleaning.

So when should cleaning be scheduled? Enterprises can judge by three indicators: first, thermal oil acid value — once it exceeds 0.5 mg KOH/g, oxidative degradation is already evident; second, kinematic viscosity — 15% higher than fresh oil indicates coke-derived gum is affecting fluidity; third, carbon residue — above 1.5% signals obvious coking. In addition, continuously rising exhaust temperature and increasing furnace tube pressure differential are direct signs of worsening coking. These indicators should be included in monthly inspections; once two or more exceed limits, decoking should be initiated.

5. Case Study: Polyester Plant Thermal Oil Heater Decoking

Equipment: A chemical fiber enterprise operated a 6-million kcal organic heat carrier heater with over 200 m of melt tracing pipes, at a thermal oil operating temperature of 305℃.

Symptoms: After 20 months of operation, exhaust temperature rose about 30℃ above design, furnace tube pressure differential increased, spinning box temperature fluctuated by ±5℃, and polyester yarn breakage increased.

Inspection: Endoscopic inspection confirmed radiant section tube coking thickness of 2~3 mm, classified as moderate hard coke.

Solution: A combined "alkaline wash — acid wash — specialized decoking agent — passivation" process after shutdown, with a total duration of about 48 hours.

Results: Decoking rate above 95%, furnace tube pressure differential recovered to design value, exhaust temperature returned to normal, thermal oil heat transfer efficiency recovered to 92% of design, spinning temperature stabilized, and yarn breakage returned to normal levels.

6. Acceptance Criteria

All acceptance criteria follow GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment. Key acceptance points for thermal oil system decoking include: descaling rate no less than 95%; carbon steel corrosion rate no more than 6 g/(m²·h); an intact passivation film free of secondary rust; and post-cleaning pressure differential and exhaust temperature recovered to the design allowance.

7. Summary and Recommendations

Coking of chemical fiber thermal oil systems is an inevitable progressive process; the key is early detection and early cleaning. Enterprises are advised to combine online monitoring of thermal oil acid value, viscosity and carbon residue, and to include shutdown decoking in the annual preventive maintenance plan — generally once every 1-2 years. For lightly coked systems, online cleaning can be used first as a transition. Scientific cleaning not only restores heat transfer efficiency and reduces energy consumption, but also prevents tube rupture risks and stabilizes spinning quality — delivering both cost reduction and production safety for chemical fiber enterprises.

8. FAQ

Q1: How often should a chemical fiber thermal oil system be cleaned?

Generally a shutdown decoking cleaning is scheduled every 1-2 years, depending on coking severity and thermal oil monitoring indicators. If acid value, viscosity or carbon residue exceed limits, or exhaust temperature rises noticeably, cleaning should be scheduled earlier.

Q2: Can a thermal oil heater be cleaned without shutdown?

Yes. For continuous production lines that cannot be shut down, an online decoking agent can be circulated with the thermal oil. It suits lightly coked systems dominated by soft coke and gum; severe hard coke still requires shutdown chemical cleaning.

Q3: Will decoking cleaning corrode the equipment?

A proper process adds corrosion inhibitor and controls cleaning temperature and time, keeping the carbon steel corrosion rate within 6 g/(m²·h). Passivation after cleaning forms a protective film, so the equipment is not damaged.

Q4: How to judge whether the thermal oil system needs cleaning?

Check three indicators — acid value above 0.5 mg KOH/g, viscosity 15% higher than fresh oil, or carbon residue above 1.5%; combine with rising exhaust temperature and increasing furnace tube pressure differential.

Q5: How to verify acceptance after cleaning?

Per GB/T 25146-2010, descaling rate no less than 95%, corrosion rate no more than 6 g/(m²·h), and an intact passivation film; meanwhile furnace tube pressure differential and exhaust temperature should recover to the design allowance.

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