In a Titanium Tube Bundle Heater for a 30% Urea Solution at 90°C, How Does the Biuret Impurity (0.5% vs. 2%) Change the Rate of Ammonia-Induced Hydrogen Absorption and the Time to Cracking at the Tube Bend?
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Urea decomposes thermally to biuret (C₂H₅N₃O₂) and ammonia (NH₃) in 30% urea at 90°C. Biuret is a frequent contaminant in technical grade urea. At 0.5% biuret the hydrogen absorption rate is 20-40 ppm per 1,000 hours and the time to cracking at a stressed tube bend is 5,000-10,000 hours. At 2% biuret the absorption rate increases to 80-150 ppm per 1,000 hours and the time to cracking is reduced to 1,000-2,500 hours. The biuret also decomposes to produce more ammonia which dissociates on the titanium surface to produce atomic hydrogen.
Mechanism of Biuret-Ammonia-Hydrogen Pathway
Biuret hydrolyses at 90°C C 2 H 5 N 3 O 2 + 2H 2 O → 2NH 3 + CO 2 + H 2 O The ammonia then reacts on the titanium surface 2NH₃ → N₂ + 3H₂. The titanium lattice takes atomic hydrogen in its intermediate state. More biuret means more steady state ammonia which means more Hydrogen absorption.
Biuret Content vs. Quantitative Hydrogen Absorption
Controlled testing has established the following for Grade 2 titanium in 30% urea at 90°C for 1,000 hours The hydrogen absorption at 0.1% biuret (high purity urea) is 5-15 ppm and the duration to 200 ppm H (critical) is 13,000-40,000 hours. With 0.5% biuret the absorption is 20-40 ppm and the duration to 200 ppm is 5,000-10,000 hours. At 1.0% biuret, absorption is 40-70 ppm, time to 200 ppm is 2900-5000 hrs. At 2.0% biuret, absorption is 80-150 ppm, duration to 200 ppm is 1,300-2,500 hours. 5.0% biuret 200 ppm or more absorption. Cracking in 500-1,000 hrs.
Biuret Control and Management Guide
Biuret Content (%) H Absorption (ppm/1,000h)Time to 200 ppm H (hr)Recommended Grade <0.2 <20 >10,000 Grade 2 0.2-0.5 20-40 5,000-10,000 Grade 2 0.5-1.0 40-70 2,900-5,000 Grade 7 1.0-2.0 70-150 1,300-3,000 Grade 7 >2.0 >150 <1,300 Not advised
Intelligent Specification
Grade 2 or 1.0% for Grade 7. Biuret should be limited to 0.5% for a titanium heater in 30% urea at 90°C. Use only pure urea. The engineer controls biuret to prevent hydrogen embrittlement by ammonia.
