A Reactor for Biomass Pretreatment by Rapid Compressed-Air Decompression (0–300 bar): Design, Safety, and Cycle Energetics

Rapid decompression is a physical route to disrupting lignocellulose, but its gas-based variants operate below 60 bar, release through valves, and have not been documented on the millisecond scale of the pressure drop. We present the design, safety analysis, and cycle energetics of a laboratory reactor in which compressed air at up to 300 bar and up to 90 °C is released through the full chamber bore by a press-clamped lid that lifts when the clamping force is removed. The 1.13 dm3 chamber of 316L steel was assessed with the Lamé solution, thick-plate theory, and the EN 13445-3 and ASME VIII-1 rules and verified by a 320-bar leak test and approximately 150 decompressions. The plain wall has safety factors of 1.9 and 1.5 at 20 and 90 °C, whereas the edges of the Ø11.8 mm ports reach the yield strength at 255–285 bar and should set the allowable pressure. Fifteen wheat-straw decompressions recorded at 1 ms resolution took 84–780 ms (median 177 ms), 100 times longer than the 1.5 ms predicted for the lid opening; the decay corresponds to a throttling area of about 2% of the bore, pointing to the two-phase outflow of the chamber contents as the rate-limiting step. The electrical energy of compression (0.29–0.33 MJ per cycle at 160 bar) exceeds the methane-energy gain by about fourfold (benefit-to-input ratio 0.22–0.26), so at laboratory scale the process is energetically unfavorable; reversing the balance would require a fourfold denser charge or industrial compression efficiency with energy recovery.