Hydrogen Production Via Steam Reforming: a Critical Analysis of MR and RMM Technologies

membranes

Article Hydrogen Production via Steam Reforming: A Critical Analysis of MR and RMM Technologies

Giovanni Franchi 1,* , Mauro Capocelli 1 , Marcello De Falco 1, Vincenzo Piemonte 2 and Diego Barba 1

1 Unit of Process Engineering, Department of Engineering, Università Campus Bio-Medico di Roma, via Álvaro del Portillo 21, 00128 Rome, Italy; [email protected] (M.C.); [email protected] (M.D.F.); [email protected] (D.B.) 2 Unit of Chemical-physics Fundamentals in Chemical Engineering, Department of Engineering, Università Campus Bio-Medico di Roma, via Álvaro del Portillo 21, 00128 Rome, Italy; [email protected] * Correspondence: [email protected]

Received: 11 November 2019; Accepted: 31 December 2019; Published: 3 January 2020

Abstract: ‘Hydrogen as the energy carrier of the future’ has been a topic discussed for decades and is today the subject of a new revival, especially driven by the investments in renewable electricity and the technological efforts done by high-developed industrial powers, such as Northern Europe and Japan. Although hydrogen production from renewable resources is still limited to small scale, local solutions, and R&D projects; steam reforming (SR) of natural gas at industrial scale is the cheapest and most used technology and generates around 8 kg CO2 per kg H2. This paper is focused on the process optimization and decarbonization of H2 production from fossil fuels to promote more efficient approaches based on membrane separation. In this work, two emerging configurations have been compared from the numerical point of view: the membrane reactor (MR) and the reformer and membrane module (RMM), proposed and tested by this research group. The rate of hydrogen production by SR has been calculated according to other literature works, a one-dimensional model has been developed for mass, heat, and momentum balances. For the membrane modules, the rate of hydrogen permeation has been estimated according to mass transfer correlation previously reported by this research group and based on previous experimental tests carried on in the first RMM Pilot Plant. The methane conversion, carbon dioxide yield, temperature, and pressure profile are compared for each configuration: SR, MR, and RMM. By decoupling the reaction and separation section, such as in the RMM, the overall methane conversion can be increased of about 30% improving the efficiency of the system.

Keywords: membrane; methane; hydrogen permeation; mathematical model; conversion

1. Introduction Hydrogen is widely used in industrial sector such as oil reﬁning, ammonia and methanol synthesis, iron and steel production [1], and its production has fourfold from 1975 to 2018, reaching 115 Mton/y. Nowadays, over 95% of hydrogen is obtained from fossil fuels, consequently releasing about 830 million tons of carbon dioxide per year [2]. The 48% of current hydrogen production is via steam reforming of natural gas (SR), 30% via petroleum fraction, 18% via coal gasiﬁcation, and only 4% via electrolysis due to the still high cost of production (2.50–5.30 US$/gge against 1.33–2.30 US$/gge of SR) [3–6]. Moreover, the production through electrolysis requires the use of electricity, still and for many years to come, heavily linked to the combustion of fossil fuels. Other processes able to produce hydrogen from renewable resources such as aqueous phase reforming, photoelectrolysis and thermochemical water splitting are at laboratory

Membranes 2020, 10, 10; doi:10.3390/membranes10010010 www.mdpi.com/journal/membranes Membranes 2020, 10, 10 2 of 20 scale [7–10]. Biomass gasification has been developed at commercial scale, but hydrogen production is still expensive compared to SR (reaching in some cases 3.5 US$/gge) [11–13]. To meet improved targets of costs and efficiency of the decarbonization pathways, this group has contributed to find an attractive process scheme for high-performance and CO2-free hydrogen production. The results from this R&D group, mainly in collaboration with ENEA and KT-Technologies, focused on two main research lines:

(i) separation technology to produce both a high purity hydrogen and a high-pressure CO2 stream at the basis of the pre-combustion capture schemes [14,15]; (ii) innovative process scheme to couple the heat demanding thermochemical conversion with renewable energy [16–18].

As a recent example, CoMETHy (Compact Multifuel-Energy to Hydrogen converter) Project, co-funded by the European Commission under the Fuel Cells and Hydrogen Joint Undertaking (FCH JU), has showed the feasibility of a process scheme, powered by concentrating solar power (CSP) plants using molten salts as heat transfer fluid, to produce pure hydrogen (chemical storage of solar energy) [17,19]. The main innovative process scheme, also adopted by this research group in the cited R&D projects, includes hydrogen separation through palladium membranes to increase the reaction yield [20,21] in two different configurations: the membrane reactor (MR) and the reformer and membrane module (RMM). Both schemes can overcome the thermodynamic limit of SR by removing the produced hydrogen and enabling strategies towards CO2 capture [15,16,20–24]. In the MR configuration, the hydrogen is continuously collected from the reaction environment to the permeate side of a membrane put inside the reactor. In the simplest configuration, the catalyst is packed in the annular zone while the membrane represents an inner concentering tube. A sweeping gas may be fed through the inner tube, co-currently or counter-currently, in order to increase the driving force and to promote the hydrogen separation. The reformer and membrane module (RMM) configuration presents the alternation of multiple reaction stages with multiple separation stages: an hydrogen selective membrane is interposed between two reaction units. By decoupling reaction and separation, it is possible to optimize the heat transfer in the reaction zone and the mass transfer in the separation zone separately. As an example, to adopt milder operating conditions for the membranes can increase membrane durability and enable the use of thinner membranes, achieving higher hydrogen separation efficiency. Although less compact than MR, since it requires two devices to carry out reaction and separation, RMM appears more feasible from an engineering point of view. The open architecture of RMM allows to obtain easier maintenance for both the membrane modules and catalyst replacement, which makes this configuration more suitable for industrial scale applications [16,25–27]. For both configurations, most scientific works are realized at laboratory scales with very few exceptions, as the pilot plant implementing RMM configuration previously studied by this research group [15,25–28]. Several mathematical models have been developed to describe the reaction-permeation phenomena in different geometries at different operating conditions [27–37]. Ward et al. [30] analyzed, for the first time, all the elementary steps regarding hydrogen permeation through Pd self-supported membrane: diffusion in the gas phase, adsorption on the surface, dissociation in atomic hydrogen, diffusion within the metal, recombination of the diatomic molecule and, finally, the desorption from the membrane. Caravella et al. [31,32], studied Pd-alloy membranes, introducing an ‘adjusted’ pressure exponent in the Sievert law, accounting for different operating conditions, non-ideal behaviors and limiting permeation step. Barbieri et al. [33] considered the effect of CO in the gas mixture by means of a Sieverts-Langmuir law, where the reduction of hydrogen permeation through the membrane was a function of the surface coverage by the inert gas. Also, Abir et al. [34] estimated the effect of adsorbents on the membrane considering Langmuir isotherms. Gallucci et al. [35] compared fluidized bed reactors and packed Membranes 2020, 10, 10 3 of 20 one showing a reduction of gradients concentration and thermal stability with a consequently low membrane surface of about 20–25%. However, due to fluidization, erosion issues and membrane sealing could affect the mechanical stability of the system. Murmura et al. [36] proposed an elegant sensitivity analysis carried out through dimensionless number, addressed the effect of radial gradient in membrane reactors and shown the existence of boundary layer near the membrane surface, whose dimension depend on fluid dynamics conditions. Whereas, Marin et al. [37] implemented a 2D model for mass, heat and momentum balances in case of high hydrogen flux through the membrane. In our previous work [27], we applied a physical-mathematical model of the mass transfer, in three different geometries, considering both the concentration polarization and the membrane permeation, to simulate the experimental results of more than 70 tests obtained at the Chieti Pilot Plant [25,26]. In this work, we use the know-how acquired through the aforementioned experimental and modeling works to propose a kinetic comparison between the three configurations indicated: SR, MR, and RMM. According to the defined mass transfer correlation, the hydrogen permeation, through the membrane and the hydrogen production due to reaction, are compared. Since the two velocity are equal only at the end of the reactor, radial gradients are neglected and one-dimension model for mass, heat, and momentum balances has been proposed for both SR and MR. The commercial steam reformer is simulated according to Xu and Froment data [38,39], but several feedstocks can be considered. In order to make a comparison between the two configurations the same void fraction of the bed, the same geometrical characteristics and the same operating conditions have been considered. The membrane permeability has been estimated taking into account the activation energy for the diffusion of hydrogen atoms, the standard enthalpy and the entropy change due to the dissociation reaction [27]. Only for RMM, where the membrane is outside of the reformer, the separation module is modeled as an isothermal and isobar material exchanger [27]. In Sections 3.1–3.3 at fixed feed composition, GHSV, S/C, HRF, and membrane area, a comparison between SR, MR and RMM is performed (‘base case’). In Section 3.4, instead, the effect of the main parameters (GHSV, S/C, and HRF) affecting the efficiency of the system in the three configurations is analyzed.

2. Mathematical Modeling The present section describes the diﬀerential equations used to model the steam reformer (SR), the membrane reactor (MR), and the reformer and membrane module (RMM), depicted in Figure1. Membranes 2020, 10, 10 4 of 20 Membranes 2020, 10, x FOR PEER REVIEW 4 of 21

FigureFigure 1. Schematic representation of steam reformer (SR), membrane reactor (MR), and reformer and membranemembrane modulemodule (RMM).(RMM).

TheThe SRSR isis simulatedsimulated asas ﬁxedfixed bedbed reactorreactor neglectingneglecting radialradial temperaturetemperature gradients,gradients, radialradial andand intraparticleintraparticle concentrationconcentration [[40].40]. These hypotheses allow to developdevelop aa one-dimensionalone-dimensional modelmodel onon thethe axialaxial directiondirection forfor mass,mass, heat,heat, andand momentummomentum balances.balances. The overall heat transfer coecoefficientﬃcient andand frictionfriction factorfactor areare workedworked outout accordingaccording toto thethe followingfollowing expressionsexpressions [[38,41]38,41]

" !# 1 1 𝑑 𝑑 𝑈=1 + di ln de − (1) U = ℎ + 2𝑘 ln 𝑑 (1) hi 2kt di 1−𝜀 1−𝜀 𝑓 = 1 ε1.75 + 150 ∙1 ε (2) f = 𝜀− 1.75 + 150 −𝑅𝑒 (2) ε3 · Re where the void fraction of the bed is estimated by means of Haughey and Beveridge correlations [42]. where the void fraction of the bed is estimated by means of Haughey and Beveridge correlations [42]. The kinetics, instead, are described by means of Xu and Froment equations, where steam reformer, The kinetics, instead, are described by means of Xu and Froment equations, where steam reformer, water gas shift, and the overall steam reformer reactions are considered respectively [39]. The rate of water gas shift, and the overall steam reformer reactions are considered respectively [39]. The rate of reactions, the kinetic parameters, the equilibrium and adsorption constants are listed in Table 1. reactions, the kinetic parameters, the equilibrium and adsorption constants are listed in Table1.

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Table 1. Xu and Froment kinetic parameters.

Reactions Rates of Reactions Kinetic Parameters

 3  p pCO k1  H2    28879 2.5 pCH4 pH2O K  + + p  − 1  15 T CH4 H2O CO 3H2 H2 k1 = 4.225 10 e− r1 = 2 · (DEN) p p k2 H2 CO2 pCOpH O 8074.3 pH 2 K 6 CO + H2O CO2 + H2 2 − 2 k = 1.955 10 e T r2 = 2 2 − (DEN) ·  4  p pCO k3  2 H2 2    29336 3.5 pCH4 pH O K  + + p  2 − 3  5 T CH4 2H2O CO2 4H2 H2 k3 = 1.02 10 e− r3 = 2 · (DEN) Equilibrium Constant Adsorption Constant

53717 60.25 T 8497.7 2 1.987− T · 5 T K1 = 1.01325 e− · KCO = 8.23 10− e 8514· +7.11 T · 4604.3 − 1.987 T · 4 T K2 = e− · KCH4 = 6.65 10− e 45203 52.54 T · 9971.13 2 1.987− T · 9 T K3 = 1.01325 e− · KH2 = 6.12 10− e · 5 10666.35 - K = 1.77 10 e T H2O · − K p = + + + + H2O H2O where DEN 1 KCOpCO KH2 pH2 KCH4 pCH4 p . H2

For the MR, considered for this study in a double-pipe configuration enabling the contemporary heat and material exchange, two configurations are possible: one in which the catalyst is packed inside the inner tube (MR1) and, the other one, where the catalyst is in the annulus section (MR2) [16]. In order to develop a proper mathematical model, the rate of hydrogen production and the rate of hydrogen permeation through the membrane, are compared as ! dFH2 = rH2 ηH2 ρcΩ (3) dz prod. ! dFH2 R P = F p p πODt (4) OG H2 H2 dz perm. − where the overall mass transfer coefficient contains the contribution of mass transfer coefficient on the R retentate side FG and the membrane permeability PH2 , neglecting the presence of sweeping gas. The overall mass transfer coefficient and membrane permeability are calculated according to the expressions   1 P p −  H2 δ q q  F =  − ML + pLI + pRI  OG  R H H  (5)  PH 2 2  FG 2     ∆S0 E + ∆H0 1  R   D R  0 Ea PH2 = D0,H exp  exp  = P exp (6) 2  R  − RT  H2 −RT where the activation energy for the diffusion of hydrogen atoms ED, the standard enthalpy of the surface 0 0 dissociation reaction ∆HR, and the entropy change of the dissociation reaction ∆SR are considered [27]. The RMM scheme, as shown in Figure1, consists of two reactors and separation modules. The reaction section is simulated according to the previous considerations for SR, whereas the membranes are considered as an isothermal and isobar material exchanger. The mathematical models are developed in MATLAB for both steam reformers and membrane modules. The domain is divided into 400 elements and the methane conversion, carbon dioxide yield, temperature and pressure profile have been estimated by means of the explicit Runge–Kutta of fourth order method and the Levenberg–Marquardt algorithm respectively (Figure2). Membranes 2020, 10, 10 6 of 20 Membranes 2020, 10, x FOR PEER REVIEW 7 of 21

Figure 2. Figure 2. FlowFlow chart chart of of simulation. simulation.

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In the following section, the numerical results obtained for the three configurations and the related pros and cons of the three options are discussed for a fixed gas hourly space velocity (GHSV), steam to carbon ratio (S/C), and hydrogen recovery factor (HRF). While in Section 3.4 the feed composition is fixed, and the effect of aforementioned parameters is analyzed.

3. Results and Discussion A commercial steam reformer tube has been simulated according to Xu and Froment data and the main geometrical characteristics and operating conditions are summarized in the Table2. In order to make a comparison between the three conﬁgurations, the same operating conditions, geometrical length, and void fraction of the bed are considered.

Table 2. Data [27,38,39].

Parameters SR MR RMM (Reformer and Membrane Module) PR, bar 29 29 29 PP, bar - 1.3 1.3 T, K 793.15 793.15 793.15 Tw, K 1073 1073 1073 Lr, m 11.12 11.12 11.12 LM, m 11.12 11.12 11.12 ODs, m - 0.28 0.28 IDs, m - 0.25 0.25 IDt, m 0.1016 0.1016 0.1016 ODt, m 0.1322 0.1322 0.1322 dc, m 0.02 0.02 0.02 3 ρc, kgc mr− 2355.2 2355.2 2355.2 1 · 1 1 kc,J s− m− K− 3.8 3.8 3.8 · 1· 1· 1 kt,J s− m− K− 43 43 43 · · · 1 Ea, kJ mol− - 20.2 20.2 0 ·1 1 0.5 4 4 P , kmol h− m− bar− - 1.69 10− 1.69 10− H2 · · · × × K , kmol h 1 m 2 bar0.5 - 1.92 1.92 H2 · − · − ·

The feed, a mixture of CH4 (21.28 mol %), CO2 (1.19 mol %), H2 (2.60 mol %), H2O (71.45 mol %), and N2 (3.49 mol %), enters the SR, MR, and RMM. The S/C is equal to 3.5. Whereas the GHSV is the 1 1 same for SR, MR1, and RMM and equal to 11,600 h− . However, for MR2 is about 2660 h− . In this type of reactor, indeed, the catalyst is stacked in the annulus section. Therefore, assuming the same void fraction of the bed and the same geometrical length, the catalyst volume increases. In order to have the same GHSV the shell diameter should be reduced from 0.25 m (of the present case) to 0.16 m with an annulus section of 3 cm. Hence, the simulation has been performed at lower GHSV parameter.

3.1. SR and MR Comparison In this section the SR and MR have been analyzed. It is worth to highlight that the methane conversion reaches about 67%, 68%, and 70% for SR, MR1 and MR2 with a carbon dioxide yield of 33%, 34%, and 35% respectively (Figure3). Membranes 2020, 10, x FOR PEER REVIEW 9 of 21 Membranes 2020, 10, x FOR PEER REVIEW 9 of 21 Membranes 2020, 10, 10 8 of 20

Figure 3. Methane conversion and carbon dioxide yield in SR, MR1, and MR2. FigureFigure 3.3. Methane conversion and carbon dioxide yield inin SR,SR, MR1,MR1, andand MR2.MR2. As shown shown in in F Figureigure 4,, t thehe temperature temperature profile profile is is almost almost the the same same for for SR, SR, MR1 MR1,, and and MR2 MR2 with with an As shown in Figure 4, the temperature profile is almost the same for SR, MR1, and MR2 with an averagean average temperature temperature for MR2 for MR2 higher higher than than the other the other configurations. configurations. The temperature The temperature drops drops at the at inlet the average temperature for MR2 higher than the other configurations. The temperature drops at the inlet ofinlet the of reactor the reactor is ordinary is ordinary in packed in packed bed bedreactor reactor and and can canaffect aff ectthe themembrane membrane stability. stability. Therefore, Therefore, a of the reactor is ordinary in packed bed reactor and can affect the membrane stability. Therefore, a prea pre-reformer-reformer is isrecommended recommended [35]. [35 ]. pre-reformer is recommended [35].

FigureFigure 4. 4. TemperatureTemperature profile profile in SR, MR1, and MR2. Figure 4. Temperature profile in SR, MR1, and MR2. The simulation has worked out at temperature higher than the maximum allowable for for a membraneThe simulation reactor in order has workedto make make outa a comparison comparison at temperature between between higher the the SR SR than and and MR the MR in maximum in homogenous homogenous allowable conditions conditions. for a. Amembrane temperature reactor drops in orderin a SR to frommake 800a comparison ◦°CC to 550 ◦°C,betweenC, indeed, the would SR and reduce MR in the homogenous methane conversion conditions. fromfromA temperature 67% toto 16%16% drops andand the inthe hydrogena hydrogenSR from partial 800 partial °C pressure, to pressure, 550 °C, at indeed, theat the outlet outlet would of theof reducethe reactor, reactor, the of aboutmethane of about one-third conversion one-third from from11.36from 11.36 bar67% to tobar 4.32 16% to bar. 4.32and Lower thebar. hydrogen L temperatureower temperature partial promotes pressure, promotes the waterat the the gasoutlet water shift of gas reactionthe shiftreactor, reducingreacti ofon about reducing the eonefficiency-third the offrom the 11.36 system. bar Figure to 4.325 showsbar. Lower the cited temperature temperature promotes e ffect onthe XCH waterand gas XCO shift .reaction reducing the efficiency of the system. Figure 5 shows the cited temperature effect4 on XCH42 and XCO2. efficiency of the system. Figure 5 shows the cited temperature effect on XCH4 and XCO2.

Membranes 2020, 10, x FOR PEER REVIEW 10 of 21 MembranesMembranes 20202020,, 1010,, 10x FOR PEER REVIEW 109 of 2021

FigureFigure 5.5. MethaneMethane conversion conversion and and carbon carbon dioxide dioxide yield yield in in a aSR SR operating operating at at800 800 °C ◦andC and 550 550 °C ◦Figurerespectively.C respectively. 5. Methane conversion and carbon dioxide yield in a SR operating at 800 °C and 550 °C respectively. InIn FigureFigure6 6,, instead,instead, the pressure profile proﬁle decreases decreases from from 29 29 bar bar at at the the inlet inlet of ofthe the reactor reactor to 27 to 27bar barat the atIn outlet the Figure outlet for 6, SR forinstead, and SR MR and the1. MR1. pressurePressure Pressure profiledrops drops aredecreases lower are lowerfor from MR 29 for2 inbar MR2 which at the in the whichinlet catalyst of the thecatalyst isreactor in the toisannulus in27 thebar annulusatsection. the outlet section. Furthermore, for SR Furthermore, and MR1. in this Pressure in configuration this conﬁguration drops are membrane lower membrane for MR2 works works in which under under the compression compression catalyst is in avoiding avoiding the annulus the the detachmentsection.detachment Furthermore, ofof membranemembrane in support.support.this configuration membrane works under compression avoiding the detachment of membrane support. P[bar]

Figure 6. Pressure proﬁle in SR, MR1, and MR2. Figure 6. Pressure profile in SR, MR1, and MR2. However, as shown in FigureFigure7 6., thePressure rate ofprofile production in SR, MR1, of hydrogenand MR2. is about ten times the rate of permeationHowever, and, as shown only atinthe Figure end 7 of, the the rate reactor, of production the two velocities of hydrogen are equal. is about This ten justify times the userate ofof one-dimensionalpermeationHowever, and as, modelonly shown at alsothe in Figureend for MRof the7, conﬁguration the reactor rate of, the production andtwo thevelocities necessity of hydrogen are toequal. put is theThisabout membrane justify ten times the outsideuse the ofrate one the of- reactionpermeationdimensional section and, model such only also asat RMMthe for end MR scheme. of configuration the reactor, Moreover, the and two as the shown velocities necessity in Barbaare to equal. put et al.,the This the membrane justify MR is the mechanically outside use of one- the complex,dimensionalreaction section requires model such greater also as RMMfor heat MR transfer scheme. configuration surface Moreover, [28 and] andas the shown the necessity reactor in Barba shouldto put et al., workthe themembrane at MR lower is mechanically temperature outside the ofreactioncomplex, about 850section requires K to such allow greater as membrane RMM heat transfescheme. thermalr surface Moreover, stability [28] andas [43 shown, the44]. reactor in Barba should et al., work the at MR lower is mechanically temperature complex,of about 850 requires K to allow greater membrane heat transfer thermal surface stability [28] and [43 the,44]. reactor should work at lower temperature of about 850 K to allow membrane thermal stability [43,44].

Membranes 2020, 10, x FOR PEER REVIEW 11 of 21 Membranes 20202020,, 1010,, 10x FOR PEER REVIEW 1011 of 2021

1.4 rate of production rate of permeation

1.2

0.8

0.6 dFH2/dz [mol s-1 m-1] s-1 [mol dFH2/dz

0.4

0.2

0 024681012 L [m]

FigureFigure 7. 7.Comparison Comparison betweenbetween thethe raterate of of production production and and rate rate of of permeation permeation of of hydrogen hydrogen in in a a commercialFigurecommercial 7. Comparison steam steam reformer. reformer. between the rate of production and rate of permeation of hydrogen in a commercial steam reformer. 3.2.3.2. RMM RMM Conﬁguration Configuration 3.2. RMM Configuration InIn this this paragraph paragraph the the same same simulation simulation has has been been performed performed with with RMM. RMM. In In this this way, way, the the reformer reformer workedworkedIn this as as aparagraph a commercial commercial the one sameone and and simulation the the membrane, membrane, has been out out performed of of the the reaction reaction with RMM section, section,. In hasthis has beenway been, simulatedthe simulated reform ater at lowerworkedlower temperature temperature as a commercial of of about about one 750 750and K. K. the The The membrane syngas syngas at at the, theout outlet outletof the of ofreaction the the steam steam section reformer reform, haser SR1 beenSR1 is is routedsimulated routed to to the atthe membranelowermembrane temperature M1 M1 at at about ofabout about 27 27 bar.750 bar. TheK The. The separator separator syngas at has has the been beenoutlet considered considered of the steam as as an reformeran isobar isobar materialSR material1 is routed exchanger. exchanger. to the Hence,membraneHence, the the retentateM retentate1 at about enters enters 27 the bar. the steam Thesteam reformerseparator reformer SR2 has SR2 at been the at samethe considered same pressure. pressure. as Thean isobarThe removal removal material of a productof exchanger.a product in the in feedHencethe allowsfeed, the allows retentate to shift to the entersshift equilibrium the the equilibrium steam of reformer the reaction of the SR 2 andre ataction the to convertsame and pressure.to the convert residual The the methane.removal residual of The methane.a product hydrogen Thein producedthehydrogen feed allows at produced 25 bar to in shift SR2 at 25 the is furtherbar equilibrium in SR2 recovered is further of the in membrane recovered reaction and M2.in membrane to For convert both the M2. the membranes For residual both the methane. the membranes sweeping The gashydrogenthe is sweeping neglected produced gas and is theat neglected 25 permeate bar in and SR side2 the is worksfurther permeate at recovered about side 1.3works in bar. membrane at about 1.3 M bar.2. For both the membranes the sweepingAsAs shown shown gas in in is Figure Figureneglected8, 8, the the and methane methane the permeate conversion conversion side reached worksreached at 68% 68%about in in the 1 the.3 SR1 bar. SR1 and and a furthera further 75% 75% in in the the SR2SR2 withAs with shown an an overall overall in Figure methane methane 8, the conversion methaneconversion conversion of of about about 92% reached92% (Figure (Figure 689%). 9).in The theThe hydrogen SR hydrogen1 and a recovery further recovery 75 factor %factor in forthe for membranesSRmembranes2 with an M1overall M1 and and M2methane M2 is is about about conversion 88% 88% and and of 84%, 84%, about respectively. respectively. 92% (Figure 9). The hydrogen recovery factor for membranes M1 and M2 is about 88% and 84%, respectively.

Figure 8. Methane conversion and hydrogen partial pressure in RMM scheme. Figure 8. Methane conversion and hydrogen partial pressure in RMM scheme. Figure 8. Methane conversion and hydrogen partial pressure in RMM scheme.

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FigureFigure 9.9. OverallOverall methanemethane conversionconversion inin RMM.RMM. 3.3. Feed and Gas Composition for SR, MR, and RMM 3.3. Feed and Gas Composition for SR, MR, and RMM The feed used for the simulations is listed in Tables3 and4, where the gas composition at the The feed used for the simulations is listed in Tables 3 and 4, where the gas composition at the outlet of the reactors and membranes for the three conﬁgurations are summarized. In Figures 10 outlet of the reactors and membranes for the three configurations are summarized. In Figures 10 and and 11, instead, the gas ﬂowrates in SR, MR, and RMM are shown. 11, instead, the gas flowrates in SR, MR, and RMM are shown. Table 3. Gas composition for SR and MRs. Table 3. Gas composition for SR and MRs FEED SR MR1 MR2 Component FEED SR MR1 MR2 In Out Out Out Component In Out Out Out kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol CH4 5.17 21.28% 1.71 5.48% 1.65 5.26% 1.54 4.88% COCH4 0.005.17 0.00%21.28% 1.771.71 5.67%5.48% 1 2.05.65 5. 6.53%26% 1.54 2.07 4.88 6.56%% CO2 CO 0.290.00 1.19%0.00% 1.981.77 6.34%5.67% 2 1.77.05 6. 5.63%53% 2.07 1.85 6.56 5.86%% H 0.63 2.60% 12.70 40.68% 12.95 41.33% 13.30 42.14% 2 CO2 0.29 1.19% 1.98 6.34% 1.77 5.63% 1.85 5.86% H2O 17.36 71.45% 12.21 39.12% 12.08 38.55% 11.95 37.87% H2 0.63 2.60% 12.70 40.68% 12.95 41.33% 13.30 42.14% N2 0.85 3.49% 0.85 2.72% 0.85 2.71% 0.85 2.69% H2O 17.36 71.45% 12.21 39.12% 12.08 38.55% 11.95 37.87% N2 0.85 3.49Table% 4. Gas0.85 composition 2.72% for RMM.0.85 2.71% 0.85 2.69%

FEEDTable SR1 4. Gas composition M1 for RMM SR2 M2 Component In Out Out Out Out FEED SR1 M1 SR2 M2 kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol kmol/h % mol Component In Out Out Out Out CH4 5.17 21.28% 1.71 5.48% 1.71 8.54% 0.42 1.87% 0.42 2.47% COkmol/h 0.00 % 0.00%mol kmol/h 1.77 % 5.67% mol kmol/h 1.77 8.84%% mol 1.88kmol/h 8.31%% mol 1.88kmol/h 10.97% % mol CHCO4 2 5.170.29 21 1.19%.28% 1 1.98.71 5 6.34%.48% 1.981.71 9.88%8.54% 3.160.42 13.97%1.87% 3.160.42 18.44%2.47% COH 2 0.000.63 0. 2.60%00% 1 12.70.77 40.68%5.67% 1.511.77 7.54%8.84% 6.551.88 28.98%8.31% 1.071.88 6.27%10.97% H2O 17.36 71.45% 12.21 39.12% 12.21 60.97% 9.75 43.11% 9.75 56.90% 2 CON 2 0.290.85 1. 3.49%19% 1 0.85.98 6 2.72%.34% 0.851.98 4.23%9.88% 0.853.16 3.75%13.97% 0.853.16 4.95%18.44% H2 0.63 2.60% 12.70 40.68% 1.51 7.54% 6.55 28.98% 1.07 6.27% H2O 17.36 71.45% 12.21 39.12% 12.21 60.97% 9.75 43.11% 9.75 56.90% N2 0.85 3.49% 0.85 2.72% 0.85 4.23% 0.85 3.75% 0.85 4.95%

Membranes 2020, 10, x FOR PEER REVIEW 13 of 21 Membranes 20202020,, 10,, 10x FOR PEER REVIEW 1213 of 2021 Fi [kmol/h] Fi

Figure 10. Gas flowrate ﬂowrate in SR. Figure 10. Gas flowrate in SR.

Figure 11. Gas flowrate flowrate in MRs and RMM. 3.4. Sensitivity Analysis Figure 11. Gas flowrate in MRs and RMM. 3.4. Sensitivity Analysis In this section, the feed composition and geometrical characteristics are fixed and equal to the 3.4. SensitivityIn this section, Analysis the feed composition and geometrical characteristics are fixed and equal to the previous case. The main steam reformer and membrane reactor parameters (GHSV, S/C, and HRF), previous case. The main steam reformer and membrane reactor parameters (GHSV, S/C, and HRF), instead,In this have section been, varied. the feed As composition discussed in and the geometrical previous paragraph, characteristics the SR are and fixed MR1 and have equal a similar to the instead, have been varied. As discussed in the previous paragraph, the SR and MR1 have a similar behavior.previous case. Therefore, The main from steam this pointreformer forward, and membrane only the MR2 reactor will parameters be taking into (GHSV, account S/C, and and will HRF) be, behavior. Therefore, from this point forward, only the MR2 will be taking into account and will be renamedinstead, have generically been varied MR. Regarding. As discussed the increasing in the previous GHSV values,paragraph both, the SR andSR and MR MR show1 have a reduction a similar in renamed generically MR. Regarding the increasing GHSV values, both SR and MR show a reduction methanebehavior. conversion Therefore, from from 68% this to point 64% forforward SR and, only from the 70% MR to2 67% will for be MRtaking due into to unfavorable account and residence will be in methane conversion from 68% to 64% for SR and from 70% to 67% for MR due to unfavorable timerenamed and generically temperature MR. profile. Regarding The carbon the increasing dioxide GHSV yield isvalues, almost both the SR same; and whereasMR show the a reduction pressure residence time and temperature profile. The carbon dioxide yield is almost the same; whereas the profilein methane drops conversion owing to increasing from 68% volumetric to 64% for flowratesSR and from from 70 28.5% to bar 67 to% 24.3 for barMR for due SR to and unfavorable from 28.9 pressure profile drops owing to increasing volumetric flowrates from 28.5 bar to 24.3 bar for SR and barresidence to 28.7 time bar forand MR temperature (Figures 12 profile. and 13 ).The carbon dioxide yield is almost the same; whereas the pressurefrom 28.9 profile bar to 28.7drops bar owing for MR to increasing(Figures 12 volumetric and 13). flowrates from 28.5 bar to 24.3 bar for SR and from 28.9 bar to 28.7 bar for MR (Figures 12 and 13).

Membranes 2020, 10, x FOR PEER REVIEW 14 of 21

Membranes 2020,, 10,, x 10 FOR PEER REVIEW 1413 of of 21 20

Figure 12. Methane conversion, carbon dioxide yield, temperature, and pressure profile in SR for −1 GHSVFigure = 12. 5800,Methane 11,600, conversion,17,400 h . carbon dioxide yield, temperature, and pressure proﬁle in SR for Figure 12. Methane conversion,1 carbon dioxide yield, temperature, and pressure profile in SR for GHSV = 5800, 11,600, 17,400 h− . GHSV = 5800, 11,600, 17,400 h−1.

Figure 13. Methane conversion, carbon dioxide yield, temperature, and pressure profile in MR for Figure 13. Methane conversion,1 carbon dioxide yield, temperature, and pressure profile in MR for GHSV = 2660, 3990, 5320 h− . GHSV = 2660, 3990, 5320 h−1. FigureBoth steam 13. Methane reformer conversion, and water carbon gas shiftdioxide reactions yield, te aremperature, promoted and by pressure increasing profile S/C. in As MR shown for in FiguresGHSVBoth 14 steam and= 2660, 15 reformer ,3990, the methane 5320 and h−1. water conversion gas shift and reaction carbons dioxide are promoted grow from by increasing 53% to 78% S/C. and As from shown 20% in to Figures45% respectively 14 and 15, for the SR. methane Whereas conversion for MR from and 56%carbon to 82%dioxide for methanegrow from conversion 53% to 78% and and from from 22% 20% to to45% 45%Both for respectively carbon steam dioxide.reformer for SR. The and Whereas profile water temperaturegasfor MRshift from reaction can56% bes toare considered 82% promoted for methane stable, by increasing conversion while the S/C. pressure and As fromshown profile 22% in Figurestodecreases 45% 14for fromand carbon 15, 29 barthe dioxide. methane up to 25The barconversion profile for SR temperat and and from carbonure 28.9 can dioxide bar be to considered 28.8 grow bar from for stable,MR.53% to while78% and the from pressure 20% toprofile 45% respectivelydecreases from for 29SR. bar Whereas up to 25 for bar MR fo rfrom SR and 56% from to 82% 28.9 for bar methane to 28.8 barconversion for MR. and from 22% to 45% for carbon dioxide. The profile temperature can be considered stable, while the pressure profile decreases from 29 bar up to 25 bar for SR and from 28.9 bar to 28.8 bar for MR.

Membranes 2020, 10, x FOR PEER REVIEW 15 of 21 Membranes 2020, 10, 10 14 of 20 Membranes 2020, 10, x FOR PEER REVIEW 15 of 21

Figure 14. Methane conversion, carbon dioxide yield, temperature, and pressure profile in SR for S/C = 2, S/C = 3.5, S/C = 5. FigureFigure 14.14.Methane Methane conversion,conversion, carbon carbon dioxide dioxide yield, yield, temperature, temperature, and and pressure pressure proﬁle profile in in SR SR for for S/C S/C= 2,= S2,/C S/C= 3.5,= 3.5, S/ CS/C= 5.= 5.

Figure 15. Methane conversion, carbon dioxide yield, temperature, and pressure proﬁle in MR for S/C

=Figure2, S/C 15.= 3.5, Methane S/C = conversion,5. carbon dioxide yield, temperature, and pressure profile in MR for S/C = 2, S/C = 3.5, S/C = 5. TheFigure HRF 15. Methane has been conversion, examined carbon for the dioxide RMM yield, configuration. temperature, and By decreasingpressure profile the in pressure MR for S/C in the = 2, S/C = 3.5, S/C = 5. retentateThe sideHRF of has the been M1 membrane examined from for 27the bar RMM to 11 configuration. bar, the HRF fallsBy decreasing from 88% to the 55%. pressure Furthermore, in the theretentate methane side conversion of the M1 membrane in the SR2 from drops 27 from bar to 75% 11 bar, to 51% the andHRF the falls overall from 88% conversion to 55%. inFurthermore, the system The HRF has been examined for the RMM configuration. By decreasing the pressure in the fromthe methane 92% to 84% conversion (Figures in 16 the and SR2 17 ).drops The reductionfrom 75% ofto methane51% and conversionthe overall atconversion the inlet of in the the SR2 system are retentate side of the M1 membrane from 27 bar to 11 bar, the HRF falls from 88% to 55%. Furthermore, relatedfrom 92% to etoffi 84%ciency (Figures factors 16 defined and 17). in The the reduction Xu and Froment of methane equations. conversion Table at5 liststhe inlet the methaneof the SR2 and are the methane conversion in the SR2 drops from 75% to 51% and the overall conversion in the system carbonrelated dioxideto efficiency conversion factors for defined the ‘base in the case’ Xu analyzed and Froment in Section equations.3. Whereas, Table the5 lists e ff ectthe ofmethane GHSV, Sand/C, from 92% to 84% (Figures 16 and 17). The reduction of methane conversion at the inlet of the SR2 are andcarbon HRF dioxide on the conversion process effi forciency the ‘base is shown case’ in analyzed Tables6– in8. Section 3. Whereas, the effect of GHSV, S/C, related to efficiency factors defined in the Xu and Froment equations. Table 5 lists the methane and and HRF on the process efficiency is shown in Tables 6–8. carbon dioxide conversion for the ‘base case’ analyzed in Section 3. Whereas, the effect of GHSV, S/C, and HRF on the process efficiency is shown in Tables 6–8.

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Figure 16. Methane conversion and HRF in RMM at different pressure on retentate side for M1 and M2. Figure 16. Methane conversion and HRF in RMM at diﬀerent pressure on retentate side for M1 and M2. Figure 16. Methane conversion and HRF in RMM at different pressure on retentate side for M1 and M2.

Figure 17. Overall methane conversion in RMM as function of HRF. Figure 17. Overall methane conversion in RMM as function of HRF. Table 5. Methane conversion, carbon dioxide yield, and hydrogen recovery factor at ﬁxed GHSV Table1 5. Methane conversion,1 carbon dioxide yield, and hydrogen recovery2 factor at fixed GHSV (11,600 h− SR and 2660 h− MR), S/C (3.5) and membrane area (4.6 m ). (11600 h−1 SR and 2660 h−1 MR), S/C (3.5) and membrane area (4.6 m2)

KEY PARAMETERS SR MR RMM KEY PARAMETERS SR MR RMM Figure 17. OverallXCH4 methaneXCH4 conversion67%67% in RMM 70% as function92% 92% of HRF. XCO 33% 35% 55% 2 XCO2 33% 35% 55% Table 5. Methane conversion,HRF carbonHRF dioxide yield, - an-d hydrogen- recovery88% 88% factor at fixed GHSV −1 −1 2 (11600 h SR and 2660 h MR), S/C (3.5) and membrane area (4.6 m ) Table 6. Eﬀect of GHSV in SR and MR. KEY PARAMETERS SR MR RMM 1 1 1 SR. GHSV (5800XCH h− )4 GHSV (11,60067% h70%− ) 92% GHSV (17,400 h− ) XCH 68%XCO2 67%33% 35% 55% 64% 4 XCO2 32%HRF 33%- - 88% 33% 1 1 1 MR GHSV (2660 h− ) GHSV (3900 h− ) GHSV (5320 h− ) XCH4 70% 69% 67% XCO2 34% 34% 33%

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Table 7. Eﬀect of S/C in SR and MR.

SR S/C = 2 S/C = 3.5 S/C = 5

XCH4 53% 67% 78% XCO2 20% 33% 45% MR S/C = 2 S/C = 3.5 S/C = 5 XCH4 56% 70% 82% XCO2 22% 34% 45%

Table 8. Eﬀect of pressure on retentate side in RMM.

RMM Pret (11 bar) Pret (15 bar) Pret (27 bar)

XCH4 84% 87% 92% XCO2 46% 49% 55% HRF 55% 67% 88%

4. Conclusions Hydrogen production has increased four-fold in the last 40 years. Steam reformer of natural gas is still the most common and cheapest way to produce hydrogen; indeed, hydrogen from renewable resources is expensive compared to fossil fuels and except for biomass gasification, the current technologies are at laboratory scale. Moreover, the steam reforming is a key technology for promoting decarbonization of fossil fuels pathways. Pre-combustion carbon-capture strategies based on the reforming of hydrocarbons appear to be the most ready and affordable solution to reduce CO2 emissions while waiting for a future energy transition. In this paper, the commercial steam reformer (SR) has been compared with two emerging technologies: the membrane reactor, with catalyst packed in the tube section (MR1) or in the annuls section (MR2), and the reformer and membrane module (RMM). These architectures allow to increase the production yields, to couple the steam reforming to solar energy harvesting and to realize pre-combustion capture schemes including the separation of CO2-rich currents from those rich in hydrogen. A one-dimensional mathematical model has worked out for mass, heat and momentum balances for the three configurations considering the same operating conditions, void fraction of the bed and geometrical length. The benchmarking between the rate of production and rate of permeation of hydrogen highlighted that only at the end of the reactor the two velocities are equal with an average rate of production ten times greater than the other one. The RMM configuration allows to match this phenomenology; indeed, by decoupling the reaction section and permeation one is possible to optimize the two equipment independently. Therefore, the steam reformer has been simulated as commercial one according to Xu and Froment equations; whereas the membrane module has been developed as an isobar and material exchanger working at 750 K. The mass transfer coefficient on retentate side has been estimated according to a previous work where more than 70 tests have been analyzed in the RMM Pilot plant (Chieti, Italy). The results shown an overall methane conversion of 92% with two reactors and membrane modules and a hydrogen recovery factor of 88% and 84% for the two membranes. As shown in the paper, the HRF rises with partial pressure on retentate side of the membrane. This allows to enhance the methane conversion in the second stage of RMM, improving the efficiency of the system. The XCH4, indeed, passes from 84% to 92% increasing the pressure on retentate side from 11 bar to 27 bar. In the MR, instead, the methane conversion reaches 68% for MR1 and 70% for MR2. Moreover, for higher GHSV and lower S/C the efficiency of this configuration, in term of methane conversion, decreases up to 56% when S/C = 2. Furthermore, the MR is mechanically complex and requires a pre-reforming section to allow a thermal stability at the inlet of the reactor. Temperature drops, typical of fixed bed reactor, will affect the sealing and the mechanical stability of the membrane. In the RMM, on the contrary, the separator Membranes 2020, 10, 10 17 of 20 can be designed as a shell and tube configuration instead of an embedded membrane in a catalyst tube. Hence, the RMM could be a starting point for increasing steam reform efficiency and to produce hydrogen, separating carbon dioxide from fossil fuels before combustion.

Author Contributions: Conceptualization and methodology, G.F., D.B., and M.C.; Resources, data curation, and validation, G.F., All authors contributed to the remaining activities regarding the paper production. All authors have read and agreed to the published version of the manuscript. Funding: Part of this work was carried out within the framework of the project “Pure hydrogen from natural gas reforming up to total conversion obtained by integrating chemical reaction and membrane separation”, financially supported by MIUR (FISR DM 17/12/2002)-Italy. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

1 1 cp specific heat (J kg K ); · − · − dc catalytic pellets diameter (m); di internal shell/tube diameter for MR and SR respectively (m); de outside shell/tube diameter for MR and SR respectively (m); 2 1 D0,H diffusion pre-exponential factor (m s− ); · 1 Ea activation energy for hydrogen permeation through metallic membranes (J mol− ); 1 · ED activation energy for the diffusion of hydrogen atoms (J mol− ); dF · H2 1 1 dz rate of production of hydrogen (mol m− s− ); prod. · · dF H2 1 1 dz rate of permeation of hydrogen (mol m− s− ); perm. · · f friction factor; 1 2 FG mass transfer coefficient (kmol h− m− ); · · 1 2 FOG overall mass transfer coefficient (kmol h− m− ); 1 · · GHSV gas hourly space velocity (h− ); h convective heat transfer coefficient in packed bed (J m 1 s 1 k 1); i · − · − · − IDs internal shell diameter (m); IDt internal tube diameter (m); 2 JH2 hydrogen molar flux (mol m− s) · · 1 1 1 kT thermal conductivity of tube (J s− m− K− ); · · 1 · 1 1 kc thermal conductivity of catalyst (J s m K ); · − · − · − k1 steam reformer reaction rate constant; k2 water gas shift reaction rate constant; k3 overall steam reformer reaction rate constant; 2 K1 steam reformer equilibrium constant (bar ); K2 water gas shift equilibrium constant; 2 K3 overall steam reformer equilibrium constant (bar ); KCO carbon monoxide adsorption equilibrium constant;

KCH4 methane adsorption equilibrium constant;

KH2 hydrogen adsorption equilibrium constant;

KH2O steam adsorption equilibrium constant; K average hydrogen permeance (kmol h 1 m 2 bar 0.5); H2 · − · − · − L reactor, membrane geometrical length (m); ODs outside shell diameter (m); ODt outside tube diameter (m); P operating pressure (bar); pCO carbon monoxide partial pressure (bar); pCH4 methane partial pressure (bar); pCO2 carbon dioxide partial pressure (bar); pH2O steam partial pressure (bar); pH2 hydrogen partial pressure (bar); Membranes 2020, 10, 10 18 of 20

P0 permeability pre-exponential factor (kmol h 1 m 1 bar 0.5); H2 − − − 1 1 · 0.5 · · PH2 hydrogen permeability (kmol h− m− bar− ); · · ·1 1 r1 steam reformer reaction rate (mol kgc− s− ); · 1 · 1 r2 water gas shift reaction rate (mol kgc− s− ); · · 1 1 r3 overall steam reformer reaction rate (mol kgc− s− ); · · 1 1 ri rate of disappearance of i-th reactions (kmoli-th kgc− h− ); · 1 1· rCH4 rate of disappearance of methane (kmolCH4 kgc− h− ); · · 1 1 rCO2 rate of production of carbon dioxide (kmolCH4 kgc− h− ); 1· 1 · r rate of production of hydrogen (kmol kgc h ); H2 CH4· − · − Re Reynolds number; S/C steam to carbon ratio; T operating temperature (K); Tw tube wall temperature (K); u superﬁcial velocity of gas mixture (m3 m 2 s 1); · − · − U overall mass transfer coeﬃcient (J m 2 K 1 s 1); · − · − · − XCH4 methane conversion;

XCO2 carbon dioxide yield; Apices and Subscripts c relative to the catalyst; LI relative to the left interface in the film theory; M relative to the membrane; ML logarithm mean; P relative to permeate side; R relative to the retentate side; RI relative to the right interface in the film theory; r relative to reactor; s relative to the shell-side; t relative to tube-side; zi axial coordinates of mass/heat/momentum balances; Greek Letters ε void fraction of the bed; δ membrane thickness, (m); ∆H molar enthalpy of i-th reactions (J mol 1); i · − ∆H0 standard enthalpy of the surface dissociation reaction (J mol 1); R · − ∆S0 entropy change of the dissociation reaction (J mol 1 K 1); R · − · − ηi efficiency factor of i-th reactions;

ηCH4 methane eﬃciency factor;

ηCO2 carbon dioxide eﬃciency factor; 3 ρg density of gas mixture (kg m− ); · 3 ρc density of catalytic bed (kgc m ); · − Ω cross section of the reactor (m3)

References

1. Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G. The U.S. Department of Energy’s National Hydrogen Storage Project: Progress towards Meeting Hydrogen-Powered Vehicle Requirements. Catal. Today 2007, 120, 246–256. [CrossRef] 2. International Energy Agency. The Future of Hydrogen: Seizing Today’s Opportunity; Report Prepared by the IEA for the G20, Japan; IEA: Paris, France, 2019. 3. Zeng, K.; Zhang, D. Recent progress in alkaline water electrolysis for hydrogen production and application. Prog. Energy Combust. Sci. 2010, 36, 307–326. [CrossRef] 4. Decourt, B.; Lajoie, B.; Debarre, R.; Soupa, O. Hydrogen-Based Energy Conversion, More Than Storage: System Flexibility; SBC Energy Institute: Paris, France, 2014. Membranes 2020, 10, 10 19 of 20

5. Dixon, R.K.; Li, J.; Wang, M.Q. Progress in hydrogen energy infrastructure development-addressing technical and institutional barriers. Compend. Hydrogen Energy 2016, 2, 323–343. 6. IRENA. Hydrogen from Renewable Power: Technology Outlook for the Energy Transition; International Renewable Energy Agency: Abu Dhabi, UAE, 2018.

7. Shabaker, J.W.; Dumesic, J.A. Kinetics of Aqueous-Phase Reforming of Oxygenated Hydrocarbons: Pt/Al2O3 and Sn-Modified Ni Catalysts. Ind. Eng. Chem. Res. 2004, 43, 3105–3112. [CrossRef] 8. Kechagiopoulos, P.N.; Voutetakis, S.S.; Lemonidou, A.A.; Vasalos, I.A. Hydrogen Production via Steam Reforming of the Aqueous Phase of Bio-Oil in a Fixed Bed Reactor. Energy Fuels 2006, 20, 2155–2163. [CrossRef] 9. Jia, J.; Seitz, L.C.; Benck, J.D.; Huo, Y.; Chen, Y.; Ng, J.W.D.; Bilir, T.; Harris, J.S.; Jaramillo, T.F. Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%. Nat. Commun. 2016, 7, 13237. [CrossRef] 10. Wang, Z.; Roberts, R.; Naterer, G.; Gabriel, K. Comparison of thermochemical, electrolytic, photoelectrolytic and photochemical solar-to-hydrogen production technologies. Int. J. Hydrogen Energy 2012, 37, 16287–16301. [CrossRef] 11. Bocci, E.; Di Carlo, A.; Vecchione, L.; Villarini, M.; De Falco, M.; Dell’Era, A. Technical-Economic Analysis of an Innovative Cogenerative Small Scale Biomass Gasification Power Plant. In Computational Science and Its Applications—ICCSA 2013; Springer: Berlin/Heidelberg, Germany, 2013; Volume 7972. 12. Kalamaras, C.M.; Efstathiou, A.M. Hydrogen Production Technologies: Current State and Future Developments. Conf. Pap. Energy 2013, 2013, 690627. [CrossRef] 13. El-Shafie, M.; Kambara, S.; Hayakawa, Y. Hydrogen Production Technologies Overview. J. Power Energy Eng. 2019, 7, 107–154. [CrossRef]

14. Capocelli, M.; Luberti, M.; Inno, S.; D’Antonio, F.; Di Natale, F.; Lancia, A. Post-combustion CO2 capture by RVPSA in a large-scale steam reforming plant. J. CO2 Util. 2019, 32, 53–65. [CrossRef] 15. De Falco, M.; Salladini, A.; Palo, E.; Iaquaniello, G. Pd-Alloy Membrane Reactor for Natural Gas Steam

Reforming: An Innovative Process Design for the Capture of CO2. Ind. Eng. Chem. Res. 2015, 54, 6950–6958. [CrossRef] 16. De Falco, M.; Salladini, A.; Palo, E.; Iaquaniello, G. Reformer and Membrane Modules (RMM) for Methane Conversion Powered by a Nuclear Reactor. Nuclear Power Deploy. Oper. Sustain. 2011.[CrossRef] 17. Giaconia, A.; De Falco, M.; Caputo, G.; Grena, R.; Tarquini, P.; Marrelli, L. Solar steam reforming of natural gas for hydrogen production using molten salt heat carriers. AIChE J. 2008, 54, 1932–1944. [CrossRef] 18. De Falco, M.; Piemonte, V. Solar enriched methane production by steam reforming process: Reactor design. Int. J. Hydrogen Energy 2011, 36, 7759–7762. [CrossRef] 19. CoMEThy—Compact Multifuel-Energy to Hydrogen Converter. Available online: https://cordis.europa.eu/ project/rcn/101421/factsheet/en (accessed on 17 October 2019). 20. De Falco, M.; Piemonte, V.; Di Paola, L.; Basile, A. Methane membrane steam reforming: Heat duty assessment. Int. J. Hydrogen Energy 2014, 39, 4761–4770. [CrossRef] 21. Pieterse, J.; Boon, J.; Van Delft, Y.; Dijkstra, J.; Brink, R.V.D. On the potential of nickel catalysts for steam reforming in membrane reactors. Catal. Today 2010, 156, 153–164. [CrossRef] 22. Boon, J.; Pieterse, J.; Dijkstra, J.; Annaland, M.V.S. Modelling and systematic experimental investigation of mass transfer in supported palladium-based membrane separators. Int. J. Greenh. Gas Control. 2012, 11, S122–S129. [CrossRef] 23. Tosti, S. Overview of Pd-based membranes for producing pure hydrogen and state of art at ENEA laboratories. Int. J. Hydrogen Energy 2010, 35, 12650–12659. [CrossRef] 24. Van Berkel, F.; Hao, C.; Bao, C.; Jiang, C.; Xu, H.; Morud, J.; Peters, T.; Soutif, E.; Dijkstra, J.W.; Jansen, D.; et al.

Pd-membranes on their Way Towards Application for CO2-capture. Energy Procedia 2013, 37, 1076–1084. [CrossRef] 25. De Falco, M.; Iaquaniello, G.; Salladini, A. Experimental tests on steam reforming of natural gas in a reformer and membrane modules (RMM) plant. J. Membr. Sci. 2011, 368, 264–274. [CrossRef] 26. De Falco, M.; Salladini, A.; Iaquaniello, G. Reformer and Membrane Modules for Methane Conversion: Experimental Assessment and Perspectives of an Innovative Architecture. ChemSusChem 2011, 4, 1157–1165. [CrossRef][PubMed] Membranes 2020, 10, 10 20 of 20

27. Barba, D.; Capocelli, M.; De Falco, M.; Franchi, G.; Piemonte, V. Mass Transfer Coefficient in Multi-Stage Reformer/Membrane Modules for Hydrogen Production. Membranes 2018, 8, 109. [CrossRef][PubMed] 28. Barba, D.; Giacobbe, F.; De Cesaris, A.; Farace, A.; Iaquaniello, G.; Pipino, A. Membrane reforming in converting natural gas to hydrogen (part one). Int. J. Hydrogen Energy 2008, 33, 3700–3709. [CrossRef] 29. Patrascu, M.; Sheintuch, M. On-site pure hydrogen production by methane steam reforming in high flux membrane reactor: Experimental validation, model predictions and membrane inhibition. Chem. Eng. J. 2015, 262, 862–874. [CrossRef] 30. Ward, T.L.; Dao, T. Model of hydrogen permeation behavior in palladium membranes. J. Membr. Sci. 1999, 153, 211–231. [CrossRef] 31. Caravella, A.; Scura, F.; Barbieri, G.; Drioli, E. Sieverts Law Empirical Exponent for Pd-Based Membranes: Critical Analysis in Pure H2Permeation. J. Phys. Chem. B 2010, 114, 6033–6047. [CrossRef] 32. Caravella, A.; Hara, S.; Drioli, E.; Barbieri, G. Sieverts law pressure exponent for hydrogen permeation through Pd-based membranes: Coupled influence of non-ideal diffusion and multicomponent external mass transfer. Int. J. Hydrogen Energy 2013, 38, 16229–16244. [CrossRef] 33. Barbieri, G.; Scura, F.; Lentini, F.; De Luca, G.; Drioli, E. A novel model equation for the permeation of hydrogen in mixture with carbon monoxide through Pd-Ag membranes. Sep. Purif. Technol. 2008, 61, 217–224. [CrossRef]

34. Abir, H.; Sheintuch, M. Modeling H2 transport through a Pd or Pd/Ag membrane, and its inhibition by co-adsorbates, from first principles. J. Membr. Sci. 2014, 466, 58–69. [CrossRef] 35. Gallucci, F.; Van Sintannaland, M.; Kuipers, J. Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming. Int. J. Hydrogen Energy 2010, 35, 7142–7150. [CrossRef] 36. Murmura, M.A.; Cerbelli, S.; Annesini, M.C. Transport-reaction-permeation regimes in catalytic membrane reactorsfor hydrogen production. The steam reforming of methane as a casestudy. Chem. Eng. Sci. 2017, 162, 88–103. [CrossRef] 37. Marin, P.; Patino, Y.; Diez, F.; Ordonez, S. Modelling of hydrogen perm-selective membrane reactors forcatalytic methane steam reforming. Int. J. Hydrogen Energy 2012, 37, 18433–18445. [CrossRef] 38. Xu, J.; Froment, G.F. Methane steam limitations and reforming: II. Diffusional reactor simulation. Am. Inst. Chem. Eng. J. 1989, 35, 97–103. [CrossRef] 39. Xu, J.; Froment, G.F. Methane steam reforming, methanation and water-gas shift: I. Intrinsic kinetics. AIChE J. 1989, 35, 88–96. [CrossRef] 40. De Deken, J.; Devos, E.F.; Froment, G.F. Steam Reforming of Natural Gas: Intrinsic Kinetics, Diffusional Influences and Reactor Design Chemical Reactor Engineering. Chem. React. Eng. Boston 1982, 196, 181–197. 41. Ergun, S. Fluid flow through packed columns. Chem. Eng. Prog. 1952, 48, 89–94. 42. Haughey, D.P.; Beveridge, G.S.G. Structural properties of packed beds—A review. Can. J. Chem. Eng. 1969, 47, 130–140. [CrossRef] 43. Bredesen, R. Thin Pd–23w%Ag membranes for hydrogen separation. In Proceedings of the CASTOR/CACHET/DYNAMIS/ENCAP Workshop, Lyon, France, 22–24 January 2008. 44. Okazaki, J.; Ikeda, T.; Pacheco, D.A.; Sato, K.; Suzuki, T.M.; Mizukami, F. An investigation of thermal stabilityof thin palladium-silver alloy membranes for high temperature hydrogen separation. J. Membr. Sci. 2011, 366, 212–219. [CrossRef]

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