Higher H2 Recovery Research Articles

Chemical looping preferential oxidation of CO over ceria-supported γ-Fe2O3

A novel chemical looping preferential oxidation (CL-PROX) process is proposed and demonstrated for eliminating trace CO from a H2-rich stream. The proposed process exhibited an excellent CO removal performance (CO concentration < 100 ppm) and a high H2 recovery (>∼96 %) with a ceria-supported γ-Fe2O3 oxygen carrier. It was obtained by activating the fresh sample that was synthesized through a co-precipitation method and the innovative activation process comprising a partial reduction step at 400 °C followed by an oxidation process at 260 °C. The structure and evolution of iron species in the oxygen carrier were investigated through experimental characterizations during activation and redox cycles. TPR characterizations illustrated a significant difference in the reducibility of the active species, γ-Fe2O3, under H2 and CO atmospheres, which could be the evidence for its relatively high selectivity to CO oxidation. In-situ FTIR characterization also pointed out that the desorption of reaction intermediates to CO2 is facilitated via the reduction of the surface γ-Fe2O3, which might contribute to the outstanding CO oxidation performance of the oxygen carrier. The successful application of chemical looping process to selective oxidation of CO in H2-rich gas not only illustrates a new strategy for H2 purification, but also provides insights into the redox reaction mechanism with the newly-prepared γ-Fe2O3-based oxygen carrier.

Carbon Dioxide Capture from Biomass Pyrolysis Gas as an Enabling Step of Biogenic Carbon Nanotube Synthesis and Hydrogen Recovery.

Utilization of renewable raw materials as feedstock defossilizes industrial manufacturing while subsequent carbon capture reduces carbon footprint. We applied this concept to design a new pyrolysis-based process for synthesis of biogenic multi-walled carbon nanotubes (MWCNTs) and H2 from biomass. It was demonstrated that the conversion of hydrocarbon compounds in pyrolysis gas into MWCNTs and H2 is detrimentally influenced by accompanied CO2 released from biomass decomposition. Capturing CO2 with a calcium sorbent upgraded the pyrolysis gas into a suitable gaseous precursor for downstream production of MWCNTs and H2 -rich gas. Furthermore, the results suggest that CO2 capture with the sorbent has a potential to outperform a liquid alkaline scrubber owing to avoided liquid organic waste generation, sorbent regenerability and higher H2 recovery from biomass pyrolysis gas.

High efficient separation of H2/CH4 using ZIF-8/glycol-water slurry: Process modelling and multi-objective optimization

Hydrogen (H2) is expected to play a vital role in future global energy system. The efficient and low-energy consumption process for H2/CH4 separation from hydrogen rich industrial off-gas is still a key challenge. The absorption-adsorption process for H2/CH4 separation using ZIF-8/glycol-water slurry is a promising alternative technique due to its high separation efficiency, mild operation conditions and continuous operation mode. We proposed two process configurations: a decompression desorption process A to obtain 99.5 mol% H2; and a process B combined with decompression and H2 stripping desorption to attain 99.99 mol% H2. The detailed process modelling and multiple objective optimizations for two processes are conducted to determine optimal operation conditions, stream characteristics, and unit energy requirements. Results show that the H2 recovery ratio and total unit energy consumption reaches 99.70% and 0.3876 kW·h/Nm3 for Process A; 99.47% and 0.4608 kW·h/Nm3 for Process B, respectively. It indicates the novel process can simultaneously achieve high purity and high H2 recovery with low energy consumption.

Active and stable Co catalysts supported on La-Si binary systems for H2 production through ethanol steam reforming

Co catalysts supported on La2O3-SiO2 binary systems were used in the steam ethanol reforming reaction for the production of hydrogen. Calcined and used catalysts were characterized by different techniques, in situ Raman spectroscopy, XPS and HRTEM to determine the Co species present in the solids before and after the reaction. The Co catalysts were active and stable for H2 production. In order to improve catalyst stability and H2 production, the contact time (W/F) effect was studied (4.9 10−3 g h L−1 to 3.3 10−2 g h L−1). In the case of the catalytic measurements with lower W/F, the catalyst with 15 wt% of Co presented higher selectivity to H2. It was observed that a higher content of Co0 species increased the hydrogen yield, suggesting that metallic cobalt could be the active species in most of the parallel reactions that occur in the reforming of ethanol. In the solid with lower Co content, the low formation of carbon nanotubes produced a slight decrease of ethanol conversion with a decrease in the hydrogen yield. On the other hand, in the catalyst with higher cobalt content the carbonaceous deposits are of the amorphous type and the hydrogen yield equal to 3.5 remained constant with time on stream. The most active and stable catalyst with 15 wt% of Co was tested in a membrane reactor, obtaining a high H2 recovery of 54% under moderate reaction conditions (H2O/ethanol = 5, 500 °C and 1 bar).

Biochar-supported magnetic noble metallic nanoparticles for the fast recovery of excessive reductant during pollutant reduction

The catalytic reduction of diverse pollutants by noble metal catalysts in the presence of reductants is a highly effective and widely used method. However, the considerable cost of noble metal catalysts impedes the practical application of this method, and the recovery of excessive reductants has not been reported previously. In this work, we prepared inexpensive biochar-supported magnetic noble metallic nanoparticles (NPs) and efficiently recovered the excessive reductants in the form of H2. The as-synthesized biochar-supported noble metallic NPs exhibited high H2 recovery during the 4-nitrophenol reduction reaction. Results showed that the catalysts with low noble metallic content have higher H2 recovery rate than commercial Pd/C, Ag/C, and Pt/C. The catalytic mechanism of magnetic biochar-supported noble metallic NPs was demonstrated to be a “synergetic effect”, where biochar and Fe3O4 acted as accelerants that enable noble metallic NPs to produce active hydrogen for the reduction reaction, and the excess active hydrogen atoms combined to form H2.

Sulfur inhibition of PdCu membranes in the presence of external mass flow resistance

Coal gasification products typically contain less than 40% H2 and less than 40ppm H2S after desulfurization with ZnO beds at operation temperatures around 773K. Hence, both sulfur poisoning and external mass flow resistance must play a major role in separation of such mixtures with Pd-type membranes. We have investigated the separation of 1:1 and 9:1 H2/N2 mixtures contaminated with 7–35ppm H2S to elucidate the relative importance of these transport resistances using a ca. 5µm thick PdCu membrane supported on a ceramic substrate. Sulfur inhibition depended strongly on H2 recovery (10–80%) and temperature (673–773K) in the investigated range. Sulfur poisoning of the membrane dominated H2 permeation rates at lower temperatures and in the 9:1 H2/N2 mixture especially. However, its impact declined rapidly with increasing H2 recovery in the 1:1 H2/N2 mixture. As a consequence H2 recovery was only slightly reduced from 75% to 70% at 773K even after adding 35ppm H2S to that mixture. This demonstrates that concentration polarization is a stronger limitation to H2 permeation than sulfur inhibition in practical separation situations where very high H2 recovery will be an economic necessity. Exposure to H2S for altogether 75h had no lasting effect on H2 permeability of the membrane but the N2 leak rate doubled presumably due to sulfide formation at defect sites at lower temperatures.

Parametric analysis and potential prediction of membrane processes for hydrogen production and pre-combustion CO2 capture

This study primarily discusses the recommended membrane properties and potential usages of membrane systems for hydrogen purification and pre-combustion CO2 capture. Single-stage and two-stage systems for CO2 selective membranes (CSMs) and H2 selective membranes (HSMs) are investigated. To achieve the different goals for hydrogen purification, determining the possible operating range for different membrane systems is urgently required. It is found that CSMs are more appropriate for high purity H2 acquisition and that HSMs are more advantageous in terms of economic cost and H2 recovery over a relatively low purity range. The two-stage CSM has higher H2 recovery than the single-stage CSM membrane process. Although HSM is less competitive than CSM when extremely high product purity and high recovery are required, the hydrogen purity could be increased to more than 99.99% by increasing the HSM selectivity. Additionally, the recovery and purity analysis are restricted by the mass balance of the membrane system. Especially for HSM systems, a boundary that prevents the acquisition of high purity H2 is observed, and because of this boundary, two-stage HSM is unsuitable for stable operation over a low selectivity range. Additionally, for a two-stage CSM system, a cost-based optimization procedure is calculated and analyzed. With fixed membrane permeability, along with the product purity, the minimum cost selectivity is found. The moderation and cooperation between selectivity and permeability will reach a minimum cost for the unit product with different product purities. When the membrane permeance is 59.5 GPU and CO2/H2 selectivity is 63.5, the two-stage CSM system will acquire ultra-high purity (99.999%) hydrogen with a 99% hydrogen recovery, and under the optimum conditions, the CO2 purity is above 98.5%. Therefore, the increase in permeability requires an appropriate improvement of selectivity to reach the optimized conditions, which will reduce the hydrogen purification cost for integrated gasification combined cycle (IGCC) syngas. Moreover, the membrane selectivity must be enhanced during the development of gas separation membranes.

Evaluation of Pd composite membrane for pre-combustion CO2 capture

Carbon capture with hydrogen-selective Pd membranes can be realized as highly integrated process and CO2 can be separated at high pressure without employing environmentally harmful sorption agents. This study investigates the technical viability of using ultrathin Pd/ceramic composite membranes with innovative ceramic-to-metal connections under conditions that are practically interesting for natural gas-fired power stations, i.e. high feed and permeate pressures and high H2 recovery at elevated temperature. The 90% CO2 capture target was well exceeded during a 150 day test in water gas-shifted reformate at 673K. After system stabilization 98% carbon were held back by the membrane while 98% H2 were separated at nominally 99.5% H2 purity. Special attention has been paid to long-term operation effects on the 2–4μm thick Pd layer. These include the development of a striking, cavernous Pd morphology which increases mechanical stability of the composite membrane, a somewhat reduced but steady H2 permeability, and the disappearance of most membrane defects. The latter is attributed to carbon deposits as exit flow analyses point to CO2 reduction preserving effectively membrane selectivity. Altogether, carbon capture with supported Pd membranes projects as feasible from a technological vantage point.

Development of a Dynamic Model of a Palladium Membrane Reactor for Water Gas Shift

A dynamic model of a palladium membrane reactor for the Water Gas Shift (WGS) reaction has been developed by using the ISAAC Dynamic© platform. The mono-dimensional, time depending model has been developed by coupling the simple models of WGS reaction and Pd-membrane, e.g. considering that separation and reaction occur simultaneously in a single cell. The model of the composite Pd-membrane includes the presence of defects in the palladium film and a multilayered support, while in the WGS model kinetics of the reaction is considered. Membrane model has been first validated by comparison with permeation measurements, in the 300-445°C temperature range, of two Pd-membranes 12 and 29μm thick, respectively, obtained by electroless plating on a macroporous stainless steel support. The whole model has been then validated by using results of WGS tests performed at RSE at 410°C in a tubular palladium membrane reactor fed with a syngas mixture with a 7.6% CO concentration and H2O/CO ratio in the 2.7-3.6 range. Model results fit quite well experimental data; the model, however, slightly overestimates CO conversion at high H2 recovery (up to the 10%), mainly due to the fact that concentration polarization effects are not considered.

Separation of bulk hydrogen sulfide–hydrogen mixtures by selective surface flow membrane

AbstractThe separation performance of binary hydrogen sulfide–hydrogen mixtures by a nanoporous carbon membrane called selective surface flow (SSF) membrane is described. The membrane selectively permeates H2S from H2, and a H2‐enriched stream is produced at the feed‐gas pressure. A two‐stage embodiment of the SSF membrane is described for production of high‐purity H2 with high H2 recovery from an equimolar H2S‐H2 feed gas. A novel protocol for operation of the two‐stage membrane is needed to achieve that separation goal.