Vacuum pyrolysis carries out tire thermal decomposition under reduced pressure (sub-atmospheric conditions, typically 10–50 kPa absolute pressure), still within the 450–600 °C pyrolysis temperature range. Compared with atmospheric-pressure pyrolysis (inert gas purged), the low-pressure environment accelerates vapor escape from the solid char matrix, which changes tar retention, aggregate sintering, sulfur/PAH content and the final quality of crude char and downstream rCB, as referenced at rcb-mill.com.
1. Reduced tar adsorption on crude char (primary benefit)
At vacuum, hydrocarbon vapors generated from rubber cracking boil at lower temperatures and diffuse out of the char pores much faster.
- Less heavy tar remains trapped and deposited onto carbon aggregate surfaces.
- Lower residual tar means less blocking of aggregate pores and voids.
- Outcome: crude char requires lighter thermal devolatilization downstream. The finished rCB retains higher BET surface area and higher DBP oil absorption (better aggregate structure and rubber reinforcement performance).
- Atmospheric pyrolysis: vapors linger longer inside the char matrix; heavy tar condenses onto aggregates and fills pores.
2. Lower sintering tendency of carbon aggregates
Tar acts as a reactive carbonaceous binder during heating. Removing tar quickly under vacuum reduces carbon bridge formation between primary particles.
- Hard sintered agglomerates are less severe.
- De-agglomeration grinding becomes easier, less energy is required to break fused aggregates.
- Risk: If vacuum level is too high and residence time too short, rubber decomposition may be incomplete, leaving partially cracked rubber polymer in char.
3. Impact on sulfur and PAH content
- Organic sulfur and PAH compounds are mostly in the tar fraction. Fast tar removal under vacuum reduces organic sulfur and PAH loading on crude char.
- Limitation: Vacuum pyrolysis cannot remove inorganic sulfur (ZnS) or mineral ash (ZnO, silica, CaCO₃). These solids stay in the char and still need magnetic separation, air classification or acid leaching.
- Result: Vacuum pyrolysis produces char with lower volatile organic contaminants, which is favorable for premium rubber-grade rCB.
4. Effect on temperature and energy balance
Vacuum lowers the boiling point of hydrocarbons, so full rubber pyrolysis can sometimes be achieved at slightly lower temperatures.
- Operating at marginally lower temperature further minimizes aggregate sintering.
- Disadvantage: Vacuum systems require heavy-duty vacuum pumps, sealed reactor vessels, and leak-tight feed/discharge ports. Capital and maintenance costs rise substantially. Air ingress risk must be strictly controlled; even small air leaks under vacuum can cause explosive vapor mixtures and carbon oxidation.
5. Product yield trade-offs
- Vacuum pyrolysis typically increases pyrolysis oil yield and reduces char yield slightly, because more hydrocarbon material vaporizes and leaves the solid phase.
- The char that remains is cleaner, but the total mass of crude char per ton of waste tire is lower than atmospheric pyrolysis. This is an economic consideration for rCB-focused plants.
6. Limitations for mass production of rCB
- Vacuum reactors are harder to scale up for continuous large throughput. Most large commercial rCB facilities use atmospheric continuous rotary kilns.
- Vacuum batch pyrolysis is more common for small or pilot lines targeting high-quality char.
- Vacuum does not eliminate downstream purification steps: magnetic separation, grinding, air classification are still required to remove iron and mineral ash.
Comparison table
| Parameter | Atmospheric Pyrolysis | Vacuum Pyrolysis |
|---|---|---|
| Tar retention on char | Higher | Lower |
| Aggregate sintering | More severe | Reduced |
| DBP and BET of final rCB | Lower, tar-blocked pores | Higher, better preserved aggregate structure |
| Organic sulfur / PAH | Higher | Lower |
| Ash & inorganic sulfur | Unchanged | Unchanged |
| Capital & maintenance cost | Lower | Higher |
| Scale-up capability | Easy for large continuous plants | Difficult for large throughput |
Vacuum pyrolysis improves rCB quality mainly by rapidly removing tar vapors from the char matrix. This preserves carbon black aggregate structure, keeps higher DBP and BET, reduces organic sulfur and PAH, and makes de-agglomeration easier. However, vacuum treatment cannot remove mineral ash or ZnS inorganic sulfur, and it comes with higher equipment cost and scale-up challenges. It is an attractive route for premium-grade rCB, but not widely adopted for very large industrial rCB production.
