Comparison between carbon molecular sieve and Pd-Ag membranes in H2-CH4 separation at high pressure

Maria Nordio,Jon Melendez,Martin Van Sint Annaland,D Alfredo Pacheco Tanaka,Margot Llosa Tanco,Fausto Gallucci

doi:10.1016/j.ijhydene.2020.07.191

Maria Nordio, Jon Melendez + Show 4 more

Open Access

https://doi.org/10.1016/j.ijhydene.2020.07.191

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Abstract

From a permeability and selectivity perspective, supported thin-film Pd–Ag membranes are the best candidates for high-purity hydrogen recovery for methane-hydrogen mixtures from the natural gas grid. However, the high hydrogen flux also results in induced bulk-to-membrane mass transfer limitations (concentration polarization) especially when working at low hydrogen concentration and high pressure, which further reduces the hydrogen permeance in the presence of mixtures. Additionally, Pd is a precious metal and its price is lately increasing dramatically. The use of inexpensive CMSM could become a promising alternative. In this manuscript, a detailed comparison between these two membrane technologies, operating under the same working pressure and mixtures, is presented.First, the permeation properties of CMSM and Pd–Ag membranes are compared in terms of permeance and purity, and subsequently, making use of this experimental investigation, an economic evaluation including capital and variable costs has been performed for a separation system to recover 25 kg/day of hydrogen from a methane-hydrogen mixture. To widen the perspective, also a sensitivity analysis by changing the pressure difference, membrane lifetime, membrane support cost and cost of Pd/Ag membrane recovery has been considered. The results show that at high pressure the use of CMSM is to more economic than the Pd-based membranes at the same recovery and similar purity.

Highlights

Environmental concerns regarding climate change are driving the world towards the search of clean and sustainable energy sources
The difference in hydrogen pure gas permeance is quite remarkable between PdeAg membranes, which is in the order of 10À6 mol/ s/m2/Pa, and Carbon molecular sieve membranes (CMSM) of 10À8 mol/s/m2/Pa
The transport mechanism for CMSM takes place according to one of three mechanisms [18,28,29]: Knudsen diffusion dominates for the largest pores, molecular sieving for the smallest. (i) Molecular sieving is often referred to as a configurational diffusion, and it is an activated diffusion like surface selective flow. (ii) For Knudsen diffusion to take place, the lower limit for the pore diameter is usually set to d > 20 A

Summary

Introduction

Environmental concerns regarding climate change are driving the world towards the search of clean and sustainable energy sources. To further explain the focus of this work is to compare from an economic approach two types of membranes at different operating temperatures and showing the hydrogen purity is the key parameter of gas separation.

Results

Conclusion