(716e) Microporous Inorganic Membranes for Ammonia Separation

Padinjarekutt, S., Rensselaer Polytechnic Institute
Li, H., Rensselaer Polytechnic Institute
Yu, M., Rensselaer Polytechnic Institute
Jiang, J., Rensselaer Polytechnic Institute
Ammonia (NH3) is synthesized by the Haber-Bosch process where nitrogen (N2) and hydrogen (H2) gases are reacted at high pressure (150-200 bar) and temperature (400-500 oC). The process has two drawbacks: i) low NH3 yield (~10-15%) due to the tradeoff between reaction kinetics and thermodynamics; ii) energy intensive processes are required for NH3 removal and recycling of unreacted gases. Considering the harsh separation conditions for NH3 recovery, microporous inorganic membranes, such as polycrystalline zeolite membranes, composed of zeolites with uniform molecular-sized pores, are a promising candidate due to their tunable surface properties and excellent thermal and chemical stabilities. The goal of this project is to develop a zeolite membrane that can separate NH3 from a mixture gas containing NH3, H2 and N2 at high pressure and elevated temperature to drastically improve the energy efficiency of NH3 synthesis.
Considering the differences in size and polarity of NH3 (kinetic diameter: 0.260 nm), H2 (kinetic diameter: 0.289 nm) and N2 (kinetic diameter: 0.364 nm), membrane separation can be accomplished by two mechanisms: i) adsorption-based separation at relatively low temperature and ii) molecular-sieving separation at relatively high temperature. Preliminary data at low temperature and low pressure shows that the developed microporous membrane has great potential for NH3 separation. At 50 psig, NH3/H2 selectivity of >250 and NH3 permeance of >400 GPU were obtained at > 200 oC. High pressure experiments at 200 oC resulted in minimum NH3/H2 selectivities of 201, 213 and 209 and NH3 permeances of 173 GPU, 163 GPU and 151 GPU for 100, 300 and 500 psig, respectively. The long-term testing results also indicated good membrane stability. Our results suggest the potential use of the developed membrane for NH3 separation.