What are the common operational errors of vertical fluidized bed tube furnace？

The operation of a vertical fluidized bed tube furnace involves complex processes such as high temperature, gas flow, and material suspension. Incorrect operation may lead to equipment damage, experimental failure, and even safety accidents. The following are common operational errors and detailed analysis, covering core modules such as gas control, temperature management, fluidization regulation, and safety protection:

1. Gas control errors
a. Insufficient gas purity or impurities mixed in
Error manifestation: Using industrial grade nitrogen gas (purity 99.5%) instead of high-purity nitrogen gas (≥ 99.999%) for high-temperature oxidation sensitivity experiments (such as metal nitride synthesis).
consequence:
Residual oxygen (O ₂) leads to metal oxidation and a decrease in product purity;
Water vapor (H ₂ O) reacts with high-temperature carbon materials to generate CO/CO ₂, which destroys the carbon structure.
Case: When preparing TiN thin films at 1200 ℃, if the nitrogen oxygen content is>50ppm, the TiN layer will mix with TiO ₂ phase, resulting in a 30% decrease in hardness.

b. Mismatch between gas flow rate and pressure
Error manifestation:
Low flow rate: unable to maintain fluidized state, particle deposition leads to local overheating;
High flow rate: Severe particle entrainment, pollution of gas pipelines, and even blockage of exhaust gas treatment equipment.
consequence:
Fluidized bed collapse or spraying;
The wear and tear of the gas distribution plate intensifies, leading to a shortened lifespan.
Data reference: For quartz sand with an average particle size of 50 μ m, the minimum fluidization velocity (Umf) is approximately 0.15m/s (based on 20 ℃ air standard), which needs to be converted according to the actual gas (such as nitrogen) density.

c. Incorrect gas switching sequence
Error manifestation:
Heat up first and then ventilate: residual air in the furnace reacts violently with high-temperature materials (such as carbon combustion);
Failure to cool down before gas stop: The material oxidizes in the air (such as when metal powder is rapidly cooled after sintering).
consequence:
Explosion risk (such as direct exposure to air without replacement after hydrogen reduction);
The oxidation loss rate of the product is over 50%.
Correct operation:
Before heating up, nitrogen should be introduced for at least 10 minutes to ensure that O ₂<0.1%;
Cool down to below 200 ℃ before switching to air or stopping the air supply.

2. Temperature management error
a. Uncontrolled heating/cooling rate
Error manifestation:
Heating up too quickly: Thermal stress causes furnace tube rupture (such as sudden temperature change of corundum tube<100 ℃/h); Slow cooling: prolonging the experimental period and increasing energy consumption; Rapid cooling: internal stress cracking of the material. consequence: The service life of the furnace tube is shortened (such as SiC furnace tube may crack after 3 rapid cooling and heating cycles); Abnormal grain growth and performance degradation of ceramic materials. Recommended speed: Heating up: ceramic materials ≤ 5 ℃/min, metal materials ≤ 10 ℃/min; Cooling: Natural cooling or programmed cooling (such as 5 ℃/min to 500 ℃ followed by furnace cooling). b. Uneven temperature field Error manifestation: Untreated gas: Cold nitrogen gas directly enters the high-temperature zone, causing a sudden drop in local temperature; Uneven gas distribution: The particle distribution in the fluidized bed is uneven, forming a “hot spot”. consequence: Thin film growth thickness deviation>20%;
The density difference of metal sintering is greater than 15%.
Solution:
Preheat the gas to above 200 ℃ before passing it into the furnace body;
Use porous gas distribution plates (porosity of 30% -40%) or optimize the fluidized bed structure.

c. Temperature measurement and control deviation
Error manifestation:
Improper position of thermocouple (such as close to the furnace wall): The measured value is lower than the actual temperature;
PID parameters not optimized: temperature fluctuation>± 5 ℃.
consequence:
The deposition temperature deviation of silicon carbide film is 10 ℃, and the resistivity changes by 2 orders of magnitude;
Inaccurate metal phase transition temperature leads to abnormal microstructure.
Optimization suggestions:
Insert the thermocouple into the middle of the furnace, avoiding the gas jet zone;
Adjust PID parameters through self-tuning function (such as P=5%, I=120s, D=30s).

3. Flow regulation error
a. Poor fluidization quality (gully flow or surging)
Error manifestation:
Gully flow: Gas short circuits along a certain channel in the bed, and most particles are not suspended;
Tengyong: The bed fluctuates violently, and particles aggregate into blocks.
consequence:
Uneven particle mixing reduces the reaction conversion rate by more than 30%;
The wear and tear on the inner wall of the furnace tube intensifies, resulting in a shortened service life.
Diagnostic method:
Observe the fluctuation of bed pressure drop (normal pressure drop fluctuation<5%);
Auscultation of gas flow sound (dull sound during groove flow, accompanied by a “plop” sound when surging).
Adjustment measures:
Increase the porosity of gas distribution plates or switch to double-layer distribution plates;
Adjust the particle size distribution (D10/D90<3). b. Particle entrainment and sedimentation Error manifestation: Carrier: Fine particles are discharged with the exhaust gas, polluting the filter; Sedimentation: Coarse particles accumulate at the bend of the furnace tube, hindering gas flow. consequence: Poor experimental repeatability (such as particle coating rate fluctuation>10% in powder modification experiments);
The exhaust treatment system is blocked and requires frequent shutdown for maintenance.
Solution:
Install a cyclone separator to recover entrained particles (collection efficiency>95%);
Install baffles or increase the curvature radius (R/D ≥ 5) at the bend of the furnace tube.

4. Security protection errors
a. Gas leakage and suffocation risk
Error manifestation:
Failure to install oxygen concentration alarm: Nitrogen leakage causes O ₂<19% in the environment;
Direct exhaust: Combustible gases such as hydrogen accumulate in enclosed spaces.
consequence:
The operator may experience dizziness within 30 seconds and lose consciousness within 5 minutes;
When the hydrogen concentration reaches 4%, it will explode upon contact with an open flame.
Protective measures:
Forced ventilation (air exchange rate ≥ 6 times/h) in parallel with oxygen concentration alarm device;
The exhaust gas is discharged after combustion treatment or catalytic oxidation.

b. Overpressure and explosion risk
Error manifestation:
Close the exhaust emission valve: the pressure inside the furnace suddenly rises to above 0.1MPa;
Combustible gas mixture: Hydrogen and air form an explosive mixture in the furnace.
consequence:
Explosion of furnace tube, splashing of fragments causing injury;
The explosion shock wave caused the equipment to overturn as a whole.
Safety design:
Install bursting discs (bursting pressure 0.08-0.12MPa) and safety valves;
Gas pipelines are equipped with flame arresters (explosion-proof rating in accordance with IEC 60079).

c. Misoperations and emergency deficiencies
Error manifestation:
Unattended during heating: temperature out of control without timely intervention;
No emergency plan has been developed: the exhaust system was not activated in the event of a nitrogen leak.
consequence:
Equipment burning or causing a fire;
The risk of personnel injury and death has increased.
Management requirements:
Install remote monitoring and alarm system;
Conduct emergency drills every quarter to ensure a response within 3 minutes.

5. Summary and Suggestions
Standardization of operations:
Develop SOP (Standard Operating Procedure) to clarify key steps such as gas switching, temperature control, and fluidization regulation;
Operators need to undergo specialized training and hold certification before taking up their posts.
Equipment maintenance:
Monthly inspection of gas pipeline sealing (helium mass spectrometry leak detection, leakage rate<1 × 10 ⁻⁹ Pa · m ³/s);
Calibrate temperature sensors and flow meters every six months (accuracy ± 0.5% FS).
Risk prevention and control:
Using HAZOP (Hazard and Operability Analysis) to identify potential risks;
Configure a Safety Instrumented System (SIS) to achieve triple interlocking protection for temperature, pressure, and oxygen concentration.
By standardizing operations and scientific management, the accident rate of vertical fluidized bed tube furnaces can be significantly reduced, and the experimental reliability and equipment service life can be improved.