Sorbent-enhanced Reforming

By mixing a solid absorbent for carbon dioxide with the reforming catalyst, it is possible to combine the steam reforming of methane [reaction (2.1)] and its subsequent shift reaction [reaction (2.5)] into a single step and, at the same time, reduce the temperature of the former from about 900 to 400—500 °C. Removal of the carbon dioxide from the reaction zone causes a shift in the equilibrium of the combined reaction so that the production of hydrogen is enhanced, while the carbon monoxide is oxidized to carbon dioxide. A typical product gas is composed of 90% hydrogen and about 10% unreacted methane, with a small percentage of carbon dioxide and a trace of carbon monoxide. 

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Ding and Alpay also studied sorption-enhanced reforming with K-HTC as sorbent [28], using a commercial Ni-based catalyst. They found that the SER process benefits from higher pressures and that lower steam to methane ratios can be used than in ordinary reforming. Reijers et al. [25] have shown that K-HTC is an effective sorbent between 400 and 500 °C, with an C02 uptake of approx. 0.2 mmol g 1. This capacity is low compared with calcium oxides and lithium zirconates. Above 500 °C, the C02 sorption capacity of K-HTC decreases rapidly to zero [36]. 

PBRs have been also investigated for sorption enhanced reforming by using Ca-based sorbent for CO2 combined with CuO to supply the heat of reaction during the regenerative stage [64, 65]. 

Sorbents that are considered in the literature for sorption-enhanced reforming process are potassium promoted hydrotalcite (K-HTC), lithium orthosilicate (LiSi04), lithium zir-conate (LiZr03), sodium zirconate (Na2Zr03) and calcium oxide (CaO). The affinity of a sorbent to a molecule can be expressed by the equilibrium partial pressure at different temperatures. For example, the equilibrium partial pressure of carbon dioxide for different sorbents is shown in Figure 6.2. 

In sorption-enhanced reforming (SER) reactors, one of the products is extracted from the reaction zone, thus shifting the reaction equilibrium to the product side. In SER, the methane steam-reforming catalyst is mixed with a CO2 sorbent ( acceptor ). The CO2 produced during the reaction is adsorbed and the reverse... 

Chen ZX, Po F, Grace JR, et al Sorbent-enhanced/membrane-assisted steam-methane reforming, Chem Eng Sd 63 170-182, 2008. 

Broda M, Kierzkowska AM, Baudouin D, Imtiaz Q, Coperet C, Muller CR (2012) Sorbent-enhanced methane reforming over a Ni—Ca-based, bifunctional catalyst sorbent. ACS Catal 2 1635-1646... 

Reijers, H.Th.J., S.E.A. Valster-Schiermeier, P.D. Cobden, and R.W. van den Brink, Hydrotalcite as C02 Sorbent for Sorption-Enhanced Steam Reforming of Methane, ECN Report RX-05-122, July 2005. 

The UMR process can be improved by introducing calcium oxide, a carbon dioxide sorbent, into the packed bed. The potential advantages of using calcium oxide as a carbon dioxide sorbent have been previously recognized. The use of calcium oxide to enhance the steam reforming process has been patented by Gluud et al.( 1931). More recently Harrison and coworkers (Han and Harrison, 1994) have reported laboratory-scale data for the steam reforming of methane in the presence of calcium oxide. 

Recently, Comas et al.219 performed the thermodynamic analysis of the SRE reaction in the presence of CaO as a C02 sorbent. The equilibrium calculations indicate that the presence of CaO in the ethanol steam reforming reactor enhances the H2 yield while reducing the CO concentrations in the outlet of the reformer. Furthermore, the temperature range at which maximum H2 yield could be obtained also shifts from above 700 °C for the conventional steam reforming reaction without CaO to below 700 °C, typically around 500 °C in the presence of CaO. It appears that the presence of CaO along with ethanol reforming catalyst shift the WGS equilibrium in the forward direction and converts more CO into C02 that will be simultaneously removed by CaO by adsorption. 

Ochoa-Fernandez, E., Rusten, H.K., Jakobsen, H.A., R0nning, M., Holmen, A., and Chen, D. Sorption enhanced hydrogen production by steam methane reforming using Ii2Zr03 as sorbent Sorption kinetics and reactor simulation. Catalysis Today, 2005, 106 (1—4), 41. 

High-temperature carbon dioxide sorbents can also find applications in fuel reforming process to enhance fuel to hydrogen conversion efficiency. It was reported... 

In recent years, new concepts to produce hydrogen by methane SR have been proposed to improve the performance in terms of capital costs reducing with respect to the conventional process. In particular, different forms of in situ hydrogen separation, coupled to reaction system, have been studied to improve reactant conversion and/or product selectivity by shifting of thermodynamic positions of reversible reactions towards a more favourable equilibrium of the overall reaction under conventional conditions, even at lower temperatures. Several membrane reactors have been investigated for methane SR in particular based on thin palladium membranes [14]. More recently, the sorption-enhanced steam methane reforming (Se-SMR) has been proposed as innovative method able to separate CO2 in situ by addition of selective sorbents and simultaneously enhance the reforming reaction [15]. 

For both the reforming and WGS processes, various methods have been proposed and tested that utilise a sorbent material for removing the CO2 from the gas phase as the reactions occurs. These have been given various names in the literature but are generically known as Sorption Enhanced Reaction Processes (SERF) and more specifically for fuel... 

Another type of sorbent is based on hydrotalcite, which shows good cyclic stability, even if lower than competitive sorbents, and fast sorption kinetics. On the other hand, CO2 sorption on hydrotalcite has a low heat of reaction, making it necessary to sustain the endothermic reforming reaction by an external source of heat or the application of an autothermal system. In addition, these materials are suitable for operations at 400-450 °C where methane conversion is rather low. For these reasons, hydrotalcites are more promising for the sorption-enhanced water-gas shift concept, which is described in the following section. 

For the last 10-12 years, the development of sorption-enhanced fuel conversion has mainly focused on the further improvement of sorption-enhanced water-gas shift using hydrotalcite-based sorbents. Starting with the sorption-enhanced steam reforming work of Air Products, hydrotalcite-based materials were identified as potentially attractive sorbents for a pressure swing-based sorption-enhanced water-gas shift reactor system. 

Reijers, H.T.J., Valster-Schiermeier, S.E.A., Cobden, RD. and Van den Brink, R.W. (2006) Hydrotalcite as CO2 sorbent for sorption-enhanced steam reforming of methane. Industrial Engineering Chemistry Research, 45, 2522-2530. 

More recently, Solsvik and Jakobsen [140] performed a numerical study comparing several closures for mass diffusion fluxes of multicomponent gas mixtures the Wilke, Maxwell-Stefan, dusty gas, and Wilke-Bosanquet models, on the level of the single catalyst pellet and the impacts of the mass diffusion flux closures employed for the pellet, on the reactor performance. For this investigation, the methanol synthesis operated in a fixed packed bed reactor was the chemical process adopted. In the mathematical modeling study of a novel combined catalyst/sorbent pellet. Rout et al. [121] investigated the performance of the sorption-enhanced steam methane reforming (SE-SMR) process at the level of a single pellet. Different closures... 

Solsvik J, Jakobsen HA (2011) A numerical study of a two property catalyst/sorbent pellet design for the sorption-enhanced steam-methane reforming process modeling complexity and parameter sensitivity study. Chem Eng J 178 407-422... 

Li ZS, Cai NS (2007) ModeUng of multiphase cycles for sorption-enhanced steam methane reforming and sorbent regeneration in fixed bed reactor. Energy Euels 21 2909-2918... 

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