In recent years, one of the most promising fields of silica MRs application was that of greenhouse gases reduction combined to hydrogen production. Hence, in the following paragraph, a literature overview about the theoretical approach on silica MRs to evaluate their performance in the field of hydrogen generation from reforming reactions is detailed, reported, and discussed.
Modeling of Silica MRs
It is generally accepted that the main benefit deriving from the modeling of such a process is due to cost saving, originated by the reduction of the experimental tests number. Concerning silica membranes and their applications in MRs, computational fluid dynamics (CFD), black box and molecular dynamic (MD) models were applied in various studies as addressed in literature [32
], but most of them are based on 1-D model with a mass balance equation; in other cases, 2-D models were based on the utilization of CFD method. Table 4
reports some of the most representative modeling studies on silica MRs, highlighting the main details about them.
Koukou et al. [36
] developed two mathematical models with different levels of complexity as tools for designing and optimizing silica MRs, adopted for operations at industrial scale. In particular, they theoretically evaluated the design of a silica MR with high hydrogen perm-selectivity, integrated in a gasification combined cycle unit for better controlling CO2
emissions and the whole process energy efficiency, adopting both a simple and a CFD-based model. These authors reached a good understanding of the factors determining the silica MR performance during a water gas shift (WGS) process, observing that:
the simple model represents a useful tool for a preliminary evaluation of the WGS-MR performance.
The CFD-based model can be adopted for designing the WGS-MR and for optimising the silica MR performance.
The desired MR performance can be provided with a high perm-selective silica membrane.
Prabhu et al. [37
] proposed a 1-D mass balance-based model to evaluate the performance of three reactor configurations: a fixed-bed reactor, a partially selective MR, and a totally selective MR. Methane dry reforming process was carried out at ambient pressure over a Rh/Al2
catalyst. The experimental data were obtained by using a perm-selective silica membrane (Nanosil) and a semipermeable Vycor glass membrane, observing that methane conversion increased in the following order: plug-flow < Vycor glass < Nanosil. Furthermore, simulation results highlighted that pressure drop and axial temperature gradients across the catalytic bed were not noticeable.
In particular, Table 5
reports the good agreement between theoretical and experimental values for the reverse WGS reaction.
Hwang and Onuki [38
] theoretically investigated hydrogen iodide decomposition in a silica MR to produce hydrogen for further utilization in a thermochemical iodine-sulfur process. The simulations evidenced a conversion higher than 90%. In particular, Figure 8
shows HI conversion versus H2
perm-selectivity at different h values (h value represents the ratio between reaction zone volume and silica membrane surface area).
At h = 0.05, 0.01, 0.005, 0.001, and 0.0005, HI conversions were simulated to be comparable by increasing H2/I2 permselectivity. At h = 0.0001, HI conversion would be high at very low selectivity (H2/I2 perm-selectivity ~1). At higher h values (>0.0005), the permeation rate of the products becomes lower than the reaction rate due to DH2 being lower than 1. In summary, these authors theoretically demonstrated that H2/I2 perm-selectivities above 100 do not substantially induce further shift effect on the MR conversion, confirming that silica membranes could represent a better solution than more hydrogen perm-selective membranes.
Yu et al. [39
] used a 1-D mathematical model to study methane steam reforming reaction performed in a silica MR, analyzing the effect of various parameters, such as reaction temperature, feed flow rate, typology of sweep gas, and its flow rate as well as its flow pattern configuration.
A counter-current flow pattern was indicated as better modality to carry out this process, while Figure 9
a,b points out how steam, when used as sweep gas, allowed better methane conversion and hydrogen recovery over nitrogen. This was described with the different partial pressure profiles of components on the reaction and permeation sides.
For the same reaction, Oyama and Hacarlioglu [40
] developed both 1-D and 2-D models without adjustable parameters, useful for describing the performance of another silica MR. These authors evaluated when a 2-D model should be applied instead of a 1-D model for MR performance evaluation, stating that a 2-D model is essential when both deviations from plug-flow MR behavior and permeation rate are higher than reaction rate take place. Table 6
shows the predicted production yields of H2
, CO and CO2
using a 2-D model versus experimental data, simulating both a conventional packed-bed reactor (PBR) and a silica MR, operated at 600 and 650 °C. At higher pressures, the theoretical results were slightly higher than the experimental values. The 2-D model effectively predicted an increase of production yields as a consequence of the reaction temperature increase from 600 to 650 °C.
Tsuru et al. [41
] studied the effects of various parameters such as silica membrane perm-selectivity, permeation, and reaction rates on WGS reaction carried out in a silica MR. According to their theoretical results, when the Damkhöler number was approximatively equal to the permeation number, maximum CO conversion improvement was achieved. However, enhancements in CO conversion may be attained even when silica membranes show low perm-selectivity values (~10).
Moparthi et al. [42
] theoretically analysed the economic feasibility of silica and palladium MRs used for performing dehydrogenation reactions. Hence, they used a theoretical design-based simulation strategy for the comparative economic assessment of MRs and TRs. In details, the propylene production process was studied to provide 60–70% extra profits using MRs in comparison to TRs. The gross profit profiles for both MR and TR schemes were found to be similar to the case of styrene production. In both cases, it was estimated that the cost contribution of membranes and other auxiliary equipments did not exceed 20% of the total costs. It was concluded that the industrial applicability of silica/Pd-based MRs was economically feasible for those dehydrogenation reactions that enable higher conversions than those of the equivalent TRs. Figure 10
illustrates an example of a comparative economic performance of TR, silica, and Pd-based MRs at different temperatures. As indicated, the TR configuration provided an optimal gross profit of −0.52 M$
at 500 °C, which increased to 4.4 M$
at 550 °C, and 11.2 M$
at 600 °C, with the recommendation that industrial operations should not consider temperatures higher than 550 °C due to possible coking process effects. Both silica and palladium MRs provided higher optimal gross profit values, varying from 4.5 (at 500 °C) to 18 M$
(at 600 °C). In other words, it can be detected from this figure that both silica and Pd MRs perform better than the TR.
Furthermore, the economic evaluation of both silica and palladium membranes performance indicated that, while palladium membrane provided higher values of hydrogen flux and selectivity at higher cost, silica membrane provided moderate combinations of hydrogen flux and selectivity at lower costs.
Ghasemzadeh et al. [43
] developed a 1-D isothermal model to compare, from a modeling point of view, a silica MR with a Pd-Ag MR, both used in a methanol steam reforming process for producing hydrogen. The simulations indicated that silica MR performance was comparable to that of the Pd-Ag MR in terms of methanol conversion operating at low temperature (200 °C) and high space velocity (>2000 h−1
). As reported in Table 7
, at a higher silica membrane permselectivity, both higher methanol conversion and hydrogen recovery were reached. For example, adopting a silica membrane showing H2
ideal selectivity = 600, ~75% methanol conversion and ~67% hydrogen recovery could be obtained, whereas ~81% methanol conversion and ~52% hydrogen recovery could be reached using a dense hydrogen fully perm-selective Pd-Ag membrane.
Furthermore, Ghasemzadeh et al. [45
] presented a further detailed quantitative analysis of a silica MR performance during methanol steam reforming reaction for hydrogen production. Figure 11
shows how higher hydrogen perm-selectivities of silica membrane can improve the hydrogen recovery, leaving a substantial constant trend of methanol conversion. Nevertheless, higher hydrogen selectivity in lower ranges of hydrogen permeance could not be more effective on silica MR performance during the reaction process.
Finally, these authors modeled a silica MR during an WGS reaction, theoretically comparing its performance with a Pd-Ag MR [46
]. For this purpose, a 1-D isothermal mathematical model was developed and its validation was carried out by using experimental data coming from literature, achieving a good matching between simulation and experimental results. After model validation, the effects of some significant operating parameters on the performance of both MRs were studied in terms of hydrogen recovery and CO conversion. The simulations showed lower performance for the silica MR in terms of CO conversion and hydrogen recovery with respect to those of the Pd-Ag MR, while the reaction temperature evidenced dual effects at various space velocities for both MRs (Figure 12
and Figure 13
Nevertheless, this theoretical study evidenced how the silica MR during WGS reaction presented an acceptable performance in comparison with a Pd-Ag MR in the case where its hydrogen permselectivity is higher than 400 and possessing a hydrogen permeance higher than 5 × 10−7 mol/m2·Pa·s.