Tech Blog

How to Slow Down Melamine Reactor Temperature Difference Rise Rate

Melamine production via high-pressure processes (3rd-5th generation) relies heavily on reactor long-cycle operation. However, the abnormal rise of reactor temperature difference is a critical bottleneck—once it reaches 55~60℃, the reactor must shut down for acid cleaning, disrupting production.

Core Causes of Abnormal Temperature Difference Rise

The temperature difference (Δθ = outlet molten salt temperature – average reactor internal temperature) rises due to multiple interconnected factors:

Feed Quality & Condition Issues

Urea Solution: Impurities from small urea plants or melted granular urea (e.g., oil, debris) accelerate fouling; unstable temperature (too high/low) disrupts reaction efficiency and circulation.

Ammonia: Catalyst dust, high oil content, insufficient temperature (<380℃), or unstable flow (too low/high) hinder material circulation, leading to local fouling on bayonet tubes.

Equipment & Operation Factors

Electric Tracing: Low power of the inlet valve electric tracing causes heat loss, increasing molten salt heat supply demand.

Load Fluctuations: Frequent load reductions/increases or rapid load-up disrupt internal circulation, causing crystallization on bayonet tube walls.

Heat Preservation: Poor insulation material, insufficient thickness, or gaps in aluminum cladding result in excessive heat loss.

Bayonet Tube Fouling: Inadequate acid cleaning (improper concentration, temperature, or duration) leaves residual scale, accelerating the rise in temperature difference.

Practical Methods to Slow Down Temperature Difference Rise

Optimize Feed Quality & Stability

Urea Solution Control

Source & Purity: Prioritize high-quality urea from large plants; limit use of urea from small plants. Control oil content <20×10⁻⁶.

Impurity Removal: Add dual filters (one online, one standby) at the urea inlet; install filters at pump inlets and clean screens regularly.

Melting Process: Use demineralized water for granular urea melting; avoid debris (e.g., bag fragments) and contaminated urea.

Temperature Stabilization: Maintain urea concentration section temperature; control evaporator liquid level at 50% and secondary vacuum <94 kPa.

Ammonia System Optimization

Purity Improvement: Add ammonia impurity filters; clean ammonia heater tubes during overhauls to remove black-gray deposits.

Temperature Control: Upgrade electric superheaters to silicon-controlled rectifier (SCR) components (DC heating), ensuring ammonia temperature ≥380℃.

Flow Stabilization: Maintain ammonia mass flow at 450~550 kg/h (full load) to promote internal circulation and suppress polymer formation.

Upgrade Equipment & Reduce Heat Loss

Electric Tracing Upgrade: Replace inlet valve resistance wires with 3 MW high-power models to maintain temperature at 380℃.

Insulation Enhancement: Use high-quality insulation cotton, increase thickness in areas with surface temperatures>50℃, and seal aluminum cladding to prevent rainwater infiltration.

Stabilize Operating Conditions

Minimize Load Fluctuations: Avoid unnecessary load reductions; limit the rate of load increases to 1% per minute. Pre-raise molten salt temperature before load-up.

Cycle Optimization: Ensure ammonia acts as an effective carrier to drive material circulation between the central tube and bayonet tube annulus, preventing local fouling.

Improve Bayonet Tube Acid Cleaning

Acid Solution Preparation: Use nitric acid at an appropriate concentration, determined by the scale thickness and operating time.

Process Control: Heat acid lotion to 50℃ (monitor with multiple thermometers), control temperature <65℃, and circulate with air agitation; use nitrogen to protect bayonet tube seals.

Duration Adjustment: Tailor cleaning time to actual temperature difference and fouling status—shorten if temperature rises excessively.

FAQ

Q1: Why does load fluctuation affect temperature difference?

A1: Frequent load reduction/increase or rapid load-up disrupts internal material circulation, causing local crystallization on bayonet tubes and reversing temperature gradients—accelerating fouling and temperature difference rise.

Q2: What are the key parameters for bayonet tube acid cleaning?

A2: Acid concentration (customized by fouling), temperature (50~65℃), and duration (adjusted by real-time temperature monitoring); air agitation and nitrogen protection improve cleaning efficiency and protect equipment.

Q3: How to ensure ammonia temperature meets requirements?

A3: Stabilize steam/condensate pressure to maintain ammonia heater outlet ≥190℃; upgrade electric superheaters to DC heating for stable ammonia temperature ≥380℃.

Q4: Why is the urea solution source critical?

A4: Large urea plants produce higher-purity urea with fewer impurities (e.g., oil, debris), reducing bayonet tube fouling; small-plant urea or melted granular urea introduces more contaminants, accelerating temperature difference rise.

conclusion-How to Slow Down Melamine Reactor Temperature Difference Rise Rate

The abnormal rise of the melamine reactor temperature difference is driven by feed quality, equipment performance, and operational stability. By integrating feed purification (filters, impurity control), process optimization (temperature/flow stabilization), equipment upgrades (high-power electric tracing), and improved maintenance (acid cleaning procedures), plants can achieve ultra-low temperature difference rise rates.

The 267-day full-load operation results prove that these methods are industrially feasible and cost-effective. For further optimization, continue monitoring filter efficiency and extend the oil-free passivation air compressor’s service life—laying the foundation for even longer reactor cycles and reduced production costs.