Process intensification writ large with microchannel absorption in nitric acid production

• Published in: Chemical engineering science Vol. 169; pp. 140 - 150
• Main Authors: Lee, Jessy J.L., Haynes, Brian S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 21-09-2017
• Subjects: Microchannel; Microreactor; Narrow passage; Nitric acid; Process intensification; Process intensification; Narrow passage; Microreactor; Microchannel; Nitric acid
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2017.01.015

•Process microfluidics coupled with process innovation.•Steam ballast in nitric acid production from ammonia provides profound process intensification.•Annular flow model shows condensation of steam in microchannels is extremely rapid.•Residual gas-phase NO in O2 is rapidly oxidised and absorbed to form acid.•Experiments show ∼1000-fold process intensification with 99% absorption.
This paper presents experimental studies on the absorption of nitrous gas in microchannels. Liquid and gas phase products have been analysed for a wide range of nominal residence times (0.03–1.4s), gas compositions (5–10% NO, 5–49% O2, 46–82% H2O, balance argon), system pressures (2.0–10.0bar), mass fluxes (1.5–30kgm−2s−1), and coolant temperatures (23–51°C) in circular tubes with internal diameters of 1.4 and 3.9mm.
Condensation of water vapour is very rapid in the microchannel, thereby enhancing the concentrations of NO and O2 and leading to highly intensified production of nitric acid. The presence of inerts inhibits the degree of enhancement. A simple model of condensing two-phase annular flow, in which vapour-liquid interface is assumed to be smooth, describes the main features of the observations. The interfacial area between the phases is the single most sensitive parameter in the model.
Under the best conditions studied experimentally, an absorption efficiency of 99% was achieved at 2bar, with an intensification factor relative to a conventional tower absorber of the order of 1000. The NOx concentration in the tail gas is of the order of a few percent under these conditions but the tail gas flow is very small. While further work remains to develop a complete process with acceptable emissions, these results indicate that a microstructured system employing passages ∼1m long could replace the ∼70m absorption tower in a conventional nitric acid plant.