Design and operational considerations of catalytic membrane reactors for ammonia synthesis

Abstract

Production of ammonia using hydrogen derived from renewable electricity instead of hydrocarbon reforming would dramatically reduce the carbon footprint of this commodity chemical. Novel technologies such as catalytic membrane reactors (CMRs) may potentially be more compatible with distributed ammonia production than the conventional Haber–Bosch process. A reactor model is developed based on integrating a standard industrial iron catalyst into a CMR equipped with an inorganic membrane that is selective to NH3 over N2/H2. CMR performance is studied as functions of wide ranges of membrane properties and operating conditions. Conversion and ammonia recovery are dictated principally by the ammonia permeance, and the benefits by using membranes become significant above 100 GPU = 3.4 × 10−8 mol m−2 s−1 Pa−1. To be effective, the CMR requires a minimum selectivity for ammonia of 10 over both nitrogen and hydrogen and purity scales with the effective selectivity. Increasing the pressure of operation significantly improves all metrics, and at P = 30 bar with a quality membrane, ammonia is almost completely recovered, enabling direct recycle of unreacted hydrogen and nitrogen without need for recompression. Temperature drives conversion and scales monotonically without thermodynamic limitations in a CMR. Alternatively, the temperature may be reduced as low as 300°C while achieving conversion levels surpassing equilibrium limits at T = 400°C in a conventional reactor.

Energetics of Reaction Pathways Enabled by N and H Radicals during Catalytic, Plasma-Assisted NH3 Synthesis

“…However, as green hydrogen is likely to be intermittent in nature, compatible ammonia synthesis processes need to be easily turned “on and off” for on-demand production. Given that the harsh HB conditions are incompatible with the above scenario, it is critical to develop methods to synthesize ammonia under mild conditions. − …”

Optimizing Reaction-Absorption Process for Lower Pressure Ammonia Production

“…Another field focuses on the need for intensified ammonia separations as the conventional phase-changing separation imposes elevated reaction conditions to achieve viable single-pass conversion . To resolve this challenge, different types of separations are proposed, including membrane-based separations, , membrane reactors, absorption on inorganic metal halides, , etc. Previous reports have shown the promise of using absorber columns packed with metal halides that can enable ammonia production at lower pressure.…”

Selective aqueous ammonia sensors using electrochemical stripping and capacitive detection

“…[10][11][12][13] Twenty-first century exigencies, including environmental restoration and curbing greenhouse gas emissions, demand additional chemical engineering breakthroughs, such as reducing the environmental impacts of Haber-Bosch. 14,15 In addition, accelerating nitrogen removal from wastewater and monitoring nitrogen discharges will play vital roles in improving the management of the global nitrogen cycle.…”

“…However, much of the nitrogen in fertilizer is lost to aquatic ecosystems through several process inefficiencies, including fertilizer runoff, emissions from coal‐fired power plants, and insufficient removal from wastewater treatment plants 10–13 . Twenty‐first century exigencies, including environmental restoration and curbing greenhouse gas emissions, demand additional chemical engineering breakthroughs, such as reducing the environmental impacts of Haber–Bosch 14,15 . In addition, accelerating nitrogen removal from wastewater and monitoring nitrogen discharges will play vital roles in improving the management of the global nitrogen cycle.…”