Hybrid Membrane - Pressure Swing Adsorption (PSA) Hydrogen Purification Systems
Key Objectives of the project
The main goal of the proposed work is the design and testing of hybrid separation schemes that combine Membrane and Pressure Swing Adsorption (PSA) technology for the purification of H2 from a reformate stream that also contains CO2, CO, CH4, and N2. A hybrid process should combine the very high throughput and purity of a PSA process with a membrane separation process which has lower operating costs. The project focus is on small-scale PSA units, which have to operate at relatively low feed pressures (<10 atm, compared to ~30 atm for large-scale units), and as a result there is less flexibility in operating modes in order to maintain high H2 purity without sacrificing recovery. Various possible configurations of the hybrid system are envisioned depending on the material (H2 or CO2-selective membrane & adsorbent) properties, as well as on the operating conditions (membrane upstream or downstream the PSA unit).
Methane steam reforming is currently the major route of hydrogen production. The cost for hydrogen-derived unit of energy is higher than the cost of the same amount of energy derived from hydrocarbon fuels. One reason for the increased production cost is the low recovery rates at the PSA separation unit, which is employed for hydrogen separation and purification. Furthermore, one of the major greenhouse gases (CO2), is the dominant by-product of this process. Currently it is usually released in the atmosphere since the existing processes are not cost effectively designed for CO2 capture. A hybrid process combining PSA with membrane separation is expected to have lower operating costs and to increase the overall H2 recovery without sacrificing its purity, especially for small scale units. Furthermore, it provides the means for co-producing a CO2 stream ready for capture and sequestration.
The technical approach incorporates both materials development and process/system modelling. In terms of material development, the main activity refers to optimization of procedures for synthesis of carbon membranes on ceramic tubular supports tuned to being either H2 or CO2-selective. The adopted protocols should be easily transferable to scaled-up production. An additional activity refers to the assessment of alternative, newly developed adsorbents for the PSA unit. At the same time, the study of membranes and adsorbents will generate transport & adsorption data, which will be used in the course of the project. Mathematical models will be employed in the conceptual design and optimization of a membrane – PSA hybrid system with goal to maximize H2 recovery without sacrificing purity. Dynamic optimization of the overall system will be carried out for selected flow sheet configurations. Design and control issues will be considered simultaneously. The performance of optimized membranes will be evaluated under actual industrial conditions. The final stage of the project refers to the construction and testing of a membrane-PSA hybrid system at the scale of 1 Nm3 H2 h-1.
Expected socio and economic impact
The expected impact of the project is in the field of “Hydrogen Production & Distribution” by enabling the more efficient clean hydrogen production in small scale units through the design of an advanced hydrogen separation and purification system. The proposed hybrid process should in principle be also able to produce hydrogen with almost no CO2 emissions if it is optimized appropriately and, as a consequence, aims to improve, make more effective and reduce the cost of the fossil fuel decarbonization process.