CHAPTER 1: Membrane Reactors for Hydrogen Production
Published:06 Oct 2017
A. Brunetti, A. Caravella, E. Drioli, and G. Barbieri, in Membrane Engineering for the Treatment of Gases: Volume 2: Gas-separation Issues Combined with Membrane Reactors, ed. E. Drioli, G. Barbieri, A. Brunetti, E. Drioli, G. Barbieri, and A. Brunetti, The Royal Society of Chemistry, 2017, pp. 1-29.
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Today, the use of membrane reactors for hydrogen production is becoming more and more a reality with various pilot installations all over the world and increasing literature studies focused not only on the development of mechanically and chemically stable membranes with high permselectivity, but also on integrated processes able to maximize the productivity and reduce the equipment size and energy consumption. The possibility to combine reaction and separation processes in the same unit, reducing the whole volume of the plant and increasing its efficiency, is the main asset motivating the development of this technology. The scope of this chapter highlights the main findings on membrane reactors used in hydrogen production from reforming processes. A thorough discussion is presented on the main membrane reactor configurations used for various processes (from packed bed to fluidized bed to micro-reactors), with a short overview of some representative results on the upgrading of syngas via the water–gas shift reaction. In addition, by considering the water–gas shift as a reference reaction, process intensification metrics are detailed. These parameters, together with the traditional variables commonly used in the evaluation of the performance of a process, supply additional and important insight for the selection of the type of technology and the identification of the operating conditions that make a process more profitable.
In the last decade, the energy demand has grown by 1.2% a year and fossil fuels still maintain a production share of ca. 75%. However, the ever stricter problems connected to sustainable growth and lower environmental impact lead to the conclusion that the times of easy oil consumption are over. Nowadays, the necessity to produce energy from oil and natural gas as primary energy sources is becoming more and more pressing. Indeed, more generally, the diversification of said sources in order to ensure a constant supply makes the interest in membrane reactor (MR) technology more urgent. Moreover, the increasing efforts dedicated to the reduction of environmental problems has recently led to the development of clean technologies, designed to enhance both the efficiency and environmental acceptability of energy production, storage, and use, in particular for power generation.1 Among these technologies, the exploitation of light hydrocarbons is surely the main realistic energy source, since they allow both power generation and environmentally friendly fuel production. Specific reference should be made to hydrogen in this context.
At present, the global hydrogen production relies mainly on processes that extract hydrogen from fossil fuel feedstocks. About 96% of hydrogen is directly produced from fossil fuels and about 4% is produced indirectly using electricity generated through them.2 The stream coming out from a reformer or a coal gasification plant contains around 50% hydrogen (on a dry basis) that must be recovered and between 40–45% CO that is usually reduced in an upgrading stage, producing more hydrogen at the same time. In traditional applications (Figure 1.1), the upgrading of reformate streams is performed using a multi-stage CO-shift process based on a series of catalytic reactors: the first one operates at high temperatures (about 350–400 °C) and takes advantage of the high reaction rate, converting a large portion of CO into hydrogen and CO2; the other one operates at lower temperature (around 220–300 °C) and refines the carbon monoxide conversion, thus allowing a lower final concentration of CO (less than 1% molar).3 This H2-rich stream coming out from the last reactor is fed to a pressure swing adsorption (PSA) unit for H2 separation from other gases. It should be pointed out that the new utilization of H2 as feed in fuel cells for mobile power sources requires the anode inlet gas to have a CO concentration below 10–20 ppm4 in order to avoid catalyst poisoning with subsequent drops in fuel cell efficiency. Hence, the purification step for the H2 produced from hydrocarbons must be very efficient to fulfil said fuel cell requirements. Because of this, in some cases, another reaction unit is added to oxidize CO into CO2.
One of the main challenges in the next few years will be the identification of new technologies able to provide better exploitation of fossil fuels, e.g., hydrocarbons, in order to improve the yield, energy savings, and so on. The reduction of the number of reaction/separation/purification stages, which translates into a lower footprint area occupied by the whole plant, fewer auxiliary devices, reduction of the energetic load, and so forth, is a fundamental issue to consider when redesigning hydrogen production processes. A promising approach for concretizing these technological aspects in the field of hydrogen production is the use of MRs, combining the reaction and H2 separation by means of selective membranes. Many studies are now focused on the analysis of MR performance, where light hydrocarbon reforming or water–gas shift (WGS) reactions are carried out. In these cases, for both reactions, the presence of a membrane allows the recovery of a hydrogen-rich stream that does not require further separation/purification. Moreover, the removal of H2, the reaction product, from the reaction volume shifts the reaction toward further conversion. This means the possibility of having an intensified process with a reduced plant size and higher yield. The traditional process can thus be redesigned in a more compact and efficient manner (Figure 1.2), following the logic of the Process Intensification Strategy, which is an innovative methodology for process and plant design proposing a new design philosophy to achieve significant reductions (by factors of 10 to 100 or more) in plant volume at the same production capacity or to improve the overall efficiency.
Figure 1.2 shows an integrated membrane system constituted by fewer reaction/separation units than the conventional one (Figure 1.1). A first MR can be used to carry out the reforming of light hydrocarbons and another reactor for the WGS reaction.
The presence of a membrane in both reactors allows the separation of a hydrogen-rich stream from the two reaction volumes, as well as improvements in the conversion of the two stages. Obviously, the H2 purity level strictly depends on the membrane type used in each MR. In fact, membranes can be distinguished by their selectivity, which can be infinite or finite. The first ones, traditionally Pd-based, allow a pure hydrogen stream to be obtained, whereas the others provide a hydrogen-rich stream of variable purity. If the recovered H2 stream does not have the purity required, the latter can be increased by adding another purification unit depending on the final use of the H2 stream. Selective CO oxidation is known as an interesting and economical approach for CO removal from H2-rich gas streams. Also in this field, new studies proposed in the literature have demonstrated how the use of MRs can improve the process by increasing the CO conversion as well as the purity of the hydrogen stream.
In this context, membrane engineering plays a fundamental role in the integration of these units into a single plant and, at the same time, in the definition of the knowledge necessary to drive the process by maximizing the gains, both in terms of efficiency and plant size reduction. The synergic effects offered by MRs by combining reaction and separation processes in the same unit, their simplicity, and the possibility of advanced levels of automation and control offer an attractive opportunity to redesign industrial processes.5–8
1.2 Membranes for Hydrogen Production
MRs represent the most significant class of the so-called multifunctional reactors,9 which integrate reaction and separation processes in the same unit. Membranes for hydrogen separation should exhibit high selectivity toward hydrogen and high flux, being in the meantime highly mechanically and chemically stable. These aspects are very well addressed in Chapter 4 of this volume.
Most of the membranes used in hydrogen production allow the selective removal of H2 from the reaction volume under the effect of a driving force. This is a function of the species partial pressure on each membrane side and can be created by means of an inert sweep gas in the permeate compartment (nitrogen, helium, water, etc.), or by application of a pressure difference between the retentate and permeate sides.
For Pd-alloy membranes, Sieverts’ law (eqn (1.1)) is used worldwide for the mathematical description of H2 permeating fluxes in this type of membranes. Accordingly, the hydrogen permeating flux is a linear function of the permeability and driving force and a reverse function of the membrane thickness. The permeation driving force in Sieverts’ law is the difference of the square root of the hydrogen partial pressure on both membrane sides.
The removal of a product such as hydrogen from the reaction volume implies a series of advantages:
conversion enhancement of equilibrium-limited reactions,
depletion of undesired secondary reactions,
recovery of concentrated rich streams: pure H2 in the permeate, CO2 concentrated and compressed in the retentate,
coupling of two or more reactions, e.g., dehydrogenation (endothermic) with hydrogenation (exothermic) on the two membrane sides,
more desirable operating conditions (e.g., temperature).
The thermodynamic equilibrium limit of a traditional reactor (TR) can be exceeded owing to the removal of the product from the reaction volume, obtaining higher conversion under analogous operating conditions. In other words, for endothermic reactions, this allows the MR to achieve the same conversion of a TR at significantly lower temperatures. Another interesting aspect of MR usage is the positive effect that the reaction pressure can have on the process, also for reactions taking place without mole number variation (e.g., WGS) or with a mole number increase (e.g., methane steam reforming, SMR).
In hydrogen production, dense or microporous membranes can be used depending on the role of the membrane, whether it is aimed at H2 separation or purification. Most studies reported in the open literature show that membranes can be separated into dense metallic Pd-based membranes and ceramic membranes (silica, zeolite, etc.). The former show permselective transport governed by a solution-diffusion mechanism. Microporous ceramic membranes can present both permselective and non-permselective transport, depending on the size of the permeating molecules with respect to the membrane pore size as well as the chemical nature of the permeating molecules and the membrane material.10
1.3 MR Configurations for Hydrogen Production
In the past, MRs were studied by carrying out several gaseous phase reactions with different membrane types, in particular for high temperature operations, as firstly proposed by Prof. Gryaznov in the late 1960s.11 Since then, many papers have been published on the use of MRs for hydrogen production via various reactions.
Most of the studies carried out to date on MRs have focused on equilibrium-limited reactions, where the permeation of the product enhances the conversion with respect to that of a TR. Other new applications propose the use of membranes as contactors between catalysts and reactants. However, even though MR studies on pilot plants have returned promising results supporting the wide-ranging potential of this technology, there are currently no large-scale applications of MRs. Different types of MRs for hydrogen production have been proposed in the literature. Most works refer to packed bed MRs; however, other configurations such as fluidized bed MRs and micro-MRs have also been recently introduced.
1.3.1 Packed Bed MRs
Usually, packed bed MRs have a tubular configuration where the outer tube is the shell side and the inner tube is the membrane. The catalytic bed can be confined in the core of the membrane or in the annulus between the two tubes (Figure 1.3), while the permeate stream is recovered on the other side of the membrane. In the case of multi-tubular configurations, the catalyst is packed in the shell side both for construction reasons as well as for reduced heat and mass transfer limitations. One of the mains drawbacks claimed for packed bed MRs is their external mass transfer limitations, such as the limitations to hydrogen transport between the bulk of the catalytic bed (where hydrogen is produced) and the membrane wall, especially for high flux membranes.
Most recently, Caravella et al.12 investigated the concentration gradient distributions in Pd-based MRs for the WGS reaction by considering a 3.6 µm-thick membrane. The already developed and validated multicomponent-based permeation model13 was updated to account for the presence of the particle (catalyst) bed. It was demonstrated that the velocity field between particles and membranes contributes to the enhancement of the mass transfer toward the membrane surface and that the particle size does not provide an appreciable contribution toward changing the concentration polarization level in the reactor, at least for mono-disperse particles. The simulation results indicated that the maximum concentration polarization in the reactor was ca. 20%. This high value, present at the reactor end, is caused by the low hydrogen concentration, which implies a larger resistance to mass transport owing to non-permeating species. However, the weight of this reactor section on the overall concentration polarization was not so high, ca. 10.5% in average, which is significantly lower than the maximum value.
1.3.2 Fluidized Bed MRs
A typical fluidized MR for hydrogen production consists of hydrogen-selective membranes immersed in a catalytic bed operated in the bubbling or turbulent regime. The main advantage of this reactor is the negligible pressure drop, which allows using small particle sizes resulting in no internal mass and heat transfer limitations. Moreover, fluidized beds are also suitable for isothermal operations even if a highly exothermic reaction is occurring, as demonstrated by Deshmukh et al.,14,15 who carried out the oxidative dehydrogenation of methanol in lab-scale membrane fluidized bed reactors. This important aspect allows the auto-thermal reforming of methane (and other hydrocarbons) by feeding oxygen directly into the MR, preventing the formation of hot spots and subsequent damage to the membranes. An example of a fluidized bed MR is shown in Figure 1.4.16,17
Membrane micro-reactors or micro-MRs can be defined as micro-reactors reinforced by membrane separation/purification or MRs miniaturized into characteristic dimensions of 1–1000 µm, combining the advantages of both MRs and micro-reactors, leading to greatly intensified operation units.18–21 The improvement of mass/heat transfer owing to the reduction of the scale length and the enhancement of the surface area-to-volume ratio owing to the extremely high intensification are the main advantages of micro-MRs.
A lot of research is currently devoted to the study of micro-MRs with hydrogen separation function as they have found a number of applications, such as hydrogen production from the water–gas shift (WGS) reaction, hydrogen production from the methanol steam reforming reaction, on-board fuel processing for portable PEMFCs (Polymer Electrolyte Fuel Cells), production of moisture-free formaldehyde by the dehydrogenation of methanol, and dehydrogenation of cyclohexane to benzene.18,22–28 Three different configurations can be found for these reactors: planar, hollow-fiber, and monolithic.
Microchannels in a planar configuration based on microelectromechanical systems were the first micro-MRs investigated. Mejdell et al.29 constructed a microchannel MR in a planar configuration from thin defect-free Pd/23 wt% Ag membranes. As shown in Figure 1.5, this microchannel MR consisted of a stainless steel feed channel plate with six parallel channels with dimensions of 1 mm × 1 mm × 13 mm. The Pd/23 wt% Ag membrane was placed between the channel housing and a stainless steel plate with apertures corresponding to the channel geometry. Such stainless steel plate was employed for mechanical support.
In the hollow fiber configuration, the diameter of a membrane tube is reduced to below 1000 µm. Catalysts can be coated on the inner surface of the hollow fibers or impregnated inside the porous wall, whilst the separation can be achieved by the porous hollow fibers themselves or by a membrane formed on the outer surface of the hollow fibers.20
Honey-comb or straight-channel monoliths provide an inexpensive and rapid means for constructing scalable two-dimensional arrays of identical square microchannels with diameters of 500–5000 µm and wall thicknesses of 200–2000 µm.30 This kind of structures can be prepared from a variety of porous ceramic materials, such as cordierite, mullite, and alumina, which afford large networks of micro-MRs. Monolithic micro-MRs provide much better mechanical stability than hollow-fiber micro-MRs and much higher intensification than planar microchannel MRs (Figure 1.6).
1.4 Current and Potential Applications of MRs for Hydrogen Production
This section will focus on providing an overview on MR technology applications, considering only the main reaction stages constituting the traditional hydrogen production plants via reforming of light hydrocarbons (Figure 1.1).
1.4.1 Steam Reforming of Methane and Other Light Hydrocarbons
Steam methane reforming (SMR) is the most common and cost-effective reaction for hydrogen production:
SMR on a very large scale is commercially carried out in packed bed reactors at 700–900 °C using supported nickel catalysts.31 Owing to mass and heat transfer issues, packed bed SMR suffers from significant disadvantages, such as low catalyst effectiveness factors and large temperature gradients. In addition, in most cases, the reactor off-gas must go through a series of treatments, such as high and low WGS reactions and PSA, to obtain high grade purity hydrogen.
In the last 20 years, membrane technology has been repeatedly proposed as an alternative to improve the performance of conventional processes. Compared to TRs, MRs can achieve higher conversions at the same temperature or the same conversion at lower temperatures.32 Moreover, the use of dense Pd-based membranes allows pure hydrogen streams to be obtained so that there is no need for additional purification as in conventional processes. Up to now, many researchers have proposed the use of Pd-based MRs for the SMR reaction operating under milder conditions than traditional reactors.33–46 The operating temperature of 500–550 °C used in most experiments is a compromise of several factors. Both the membrane permeance and SMR thermodynamics and kinetics are favored by high temperatures, but the membrane is more stable at lower temperatures. The promising results obtained at laboratory scale encouraged its application on a larger scale. MRT Inc.47 has developed a proven technology based on a patented fluidized bed MR for high-purity hydrogen. The process combines hydrocarbon reforming, shift conversion, and hydrogen purification in a single step with capacities in the 15–50 Nm3 h−1 range.
The Shell Oil Company has patented48 a process for the production of pure hydrogen by steam reforming integrating the steam reforming and shift reactions to produce pure hydrogen with minimal production of CO and virtually no CO in the hydrogen stream.
Hydrogen production by steam reforming of methanol, ethanol,49 and other light hydrocarbons has become an attractive alternative to traditional operations. Especially attractive is their use in the decentralized production of clean electrical energy from fuel cells. The main differences against the reforming of light hydrocarbons are the catalyst types used and the product distribution in the two reaction systems. Recent studies concern the use of MRs in these reactions.50 In both conventional and membrane systems, the main reaction products are hydrogen, carbon monoxide, and carbon dioxide. Depending on the fuel used (e.g., ethanol, bioethanol, methane), acetaldehyde and ethylene can also be present.
220.127.116.11 Autothermal Reforming
Autothermal reforming (ATR) can be obtained if oxygen (or air) is supplied to the reactor, providing the necessary heat for the reforming of methane. In order to avoid hot-spot formation,51–54 ATR with integrated CO2 capture can be obtained in fluidized beds when heat is supplied either by burning a small part of the recovered hydrogen55,56 or by burning part of the feed with oxygen being fed to the reactor via oxygen selective membranes.57,58
Methane ATR is conventionally performed at high temperatures (>850–900 °C). However, this reaction can be carried out in MRs below 600 °C. A way to achieve ATR is by using an external heat carrier. In this case, molten salts at temperatures up to 550 °C can be used to supply the required heat for methane reforming.59 Consequently, a tube-in-shell configuration reactor can be used with molten salts flowing in the reactor shell to supply the necessary heat to drive the reforming reaction.
Other possibilities to supply the necessary heat use indirect coupling reactors. Exothermic and endothermic reactions take place in different chambers, separated by heat conductive walls. Heat transfer happens via the dividing wall from the chamber where the exothermic reaction (methane oxidation) is taking place to another chamber where the endothermic reaction (methane reforming) proceeds.60 Two important parameters need to be finely tuned. One is the ratio between the oxidation of methane and steam reforming reaction, which defines the temperature profile inside the membrane reformer. However, because the two reactions generally occur with different kinetic rates, the flow rates and residence times in the two chambers need to be controlled to avoid hot/cold spots along the reactor.
1.4.2 Water–Gas Shift (WGS) Reaction
Syngas upgrading by means of the WGS reaction has been widely investigated both experimentally and by simulation since the 1990s61–74
The WGS reaction is industrially carried out in two fixed bed adiabatic reactors connected in series by a cooler (heat exchanger). The first reactor operates at high temperature (HT-WGS) ranging from 300 to 500 °C using Fe/Cr-based catalysts. The second reactor (LT-WGS, low temperature water–gas shift) uses CuO/ZnO-based catalysts and operates at lower temperatures (180–300 °C) in order to displace the equilibrium, since the WGS reaction is exothermic. The whole cycle has the big disadvantage of being accompanied by large emissions of CO2. MRs can replace these three/four stages of reaction/purification with a single stage, in which reaction and separation occur in the same vessel, reaching conversions significantly higher than those in traditional systems (Figure 1.7).
Comparison of the performance of an MR with a traditional system under the same operating conditions (Figure 1.8) revealed that the CO conversion achieved by the MR was around 10% higher than the overall conversion of the traditional process, also significantly exceeding (ca. 25–30%) the traditional reactor equilibrium conversion (TREC). Hydrogen removal from the reaction side by permeation shifts the reaction toward further conversion. This effect is successfully achieved in MRs since the reaction pressure of 15 bar promotes the permeation of hydrogen. This advantage is even more obvious considering that the MR conversion is ca. 33% higher than that achieved by the first stage of a traditional process (HT-WGS).
The upgrading of a syngas stream was recently investigated by Brunetti et al.75 in a WGS-MR operated at high temperature with an ultra-thin supported membrane (3.6 micron-thick).
Figure 1.9 shows the CO conversion and hydrogen recovery as a function of the gas hourly space velocity (GHSV) at different feed pressures. The conversion decreases with the GHSV because of the lower residence time, whereas it is higher at the higher feed pressure owing to the positive effect of the pressure on the hydrogen removal through the membrane. The MR CO conversion exceeds the TREC, the maximum conversion achievable by TR, operating at GHSVs up to 10 700 h−1 for both values of feed pressure considered. Moreover, at 5000 h−1 and 5 bar, the CO conversion is quite close to the MR equilibrium conversion (MREC), indicating a very good performance of the MR. This corresponds to around 90% of hydrogen recovered in the permeate as a pure stream. Higher GHSVs mean lower CO conversions, and, thus, less hydrogen produced that can be recovered. However, it has to be pointed out that, even under the worst conditions (i.e., 15 000 h−1 and 4 bar), around 50% of hydrogen is recovered.
The use of sweep gas can further promote the MR performance, reducing the H2 partial pressure in the permeate side, thus enhancing its removal from the reaction volume. This is reflected in an improvement of the CO conversion of around 16% with respect to the case without sweep gas, gaining a hydrogen recovery of an additional 28%, increasing from 52% to 74% (Figure 1.10).
As in the case of methane steam reforming, the interesting results achieved at laboratory level were reported in various patents, including industrial ones.76–82 Among them, United Technologies Corp.76 patented the use of a WGS-MR, comprising a WGS reaction region and a permeate volume separated by an H2-separation membrane allowing the H2 formed over the catalyst in the reaction region to cross selectively to the permeate for delivery to a point of use (such as the fuel cell of a fuel cell power plant). ExxonMobil77 developed and patented a heat exchange MR for electric power generation comprising an MR system using catalytic or thermal SMR, with the WGS on one side of the membrane and hydrogen combustion on the other side for the production of electricity. In addition, the General Electric Company78 patented a polygeneration system including: (a) a syngas generator to produce syngas, (b) a syngas enrichment unit to remove undesired species from the syngas (that is, to enrich the syngas), and (c) a syngas utilization system that uses the enriched syngas to produce useful products. In certain embodiments, the polygeneration system includes an MR, a catalytic burner, and a power generation unit.
1.4.3 MRs for Dehydrogenation Reactions
The recent literature contains also works related to the production of hydrogen from higher hydrocarbons in MRs. In most cases, hydrogen is considered a by-product of a reaction involving a higher hydrocarbon that is dehydrogenated to a higher valuable product. An example is the dehydroisomerization of n-butane to isobutene.83 Isobutene is an important material for the production of chemicals and polymers. It can take part in various chemical reactions, such as hydrogenation, oxidation, and other additions owing to the presence of a reactive double bond. It is usually obtained as a by-product of petroleum refinery by FCC (Fluid Catalytic Cracking) of naphtha or gas-oil. However, an interesting alternative to isobutene production is n-butane dehydroisomerization, which allows the direct conversion of n-butane via dehydrogenation and successive isomerization. An interesting alternative to the two-step process is a direct one-step process, allowing the direct conversion of n-butane to isobutene. Various bi-functional catalytic systems have been recently reported in the literature, usually comprising Pt-supported zeolites, as successful catalysts for such direct conversion.84 At the same time, the use of MRs has also been proposed. This integrated approach, combining the reaction and separation steps in a single unit, fits well the targets of process intensification. Furthermore, the possibility of exceeding the equilibrium constraint of traditional reactors for reversible reactions such as dehydrogenations is quite appealing. For dehydrogenation-type reactions, the application of MRs, where hydrogen can be removed with high selectivity from the reaction mixture, is an interesting strategy. In these reactions, which are thermodynamically favored by low temperatures, the removal of a product shifts the equilibrium, thereby improving significantly the conversion. Various researchers have reported the dehydrogenation of isobutane using catalytic MRs85–87 by considering many types of membranes, such as γ-alumina, zeolite MFI, Pd/Ag and Pd, dense silica, and carbon molecular sieve membranes.88–91 In most studies, a conversion above the equilibrium of a TR was obtained owing to hydrogen removal through the membrane.
In particular, Pd-based membranes, owing to their infinite selectivity toward hydrogen, promote the recovery of pure hydrogen, whose removal from the reaction volume shifts the equilibrium conversion according to Le Châtelier’s principle. Recently, Al Megren et al.83 analyzed the n-butane dehydroisomerization reaction equilibrium at a wide range of temperatures, reaction pressures, and equilibrium hydrogen partial pressures by means of a simplified reaction model. This analysis revealed that the MREC achievable with a Pd/Ag MR can be up to seven times greater than the TR one, operated under the same conditions.
Figure 1.11 shows the MREC (red line) and TREC (black line) as a function of the temperature for different reaction pressures. The analysis was carried out for an n-butane/hydrogen feed mixture, with a 0.80 initial molar fraction of n-butane. As discussed, n-butane dehydroisomerization is an endothermic reaction that occurs with an increase of the mole number; therefore, it is favored by high temperatures and low pressures. As shown in Figure 1.12, the equilibrium conversion of both MR and TR increased with the temperature. However, a negative effect of the pressure appears quite evidently on the TR conversion. For instance, at 550 °C, the TR conversion is ∼0.3 at a reaction pressure of 5 bar, but it decreases to 0.21 at a pressure of 10 bar. Conversely, the MREC does not depend on the reaction pressure (Figure 1.11) at an equilibrium hydrogen partial pressure of 0.1 bar. Figure 1.11 shows that, under the considered operating conditions, the MREC is always higher than the TREC owing to the hydrogen permeation through the Pd/Ag membrane, which boosts the n-butane equilibrium conversion by removing one of the products from the reaction volume.
In a recent work, Melone et al.92 proposed an integrated membrane system for butene production, with a multistage membrane separation system constituted by various GS membrane units for gas separation placed at the downstream of the MR. Four different case studies were investigated, analyzing different options in terms of the membranes used and the operating conditions to maximize certain separations. Globally, higher butene recoveries were achieved at lower C4 olefin concentrations (55.5%). Contrarily, at a high C4 concentration of 99.3%, a low C4 recovery was observed (14%).
1.5 New Indexes for the Comparison of Membrane and Traditional Reactors
In the last decade, many efforts have been performed to transform the traditional industrial growth into sustainable growth. The Process Intensification Strategy, an alternative design philosophy introduced to bring drastic improvements in manufacturing and processing, aims to pursue this growth in a competitive but sustainable way, reducing the energy consumption, exploiting better the raw materials, minimizing waste, increasing the plant efficiency, reducing the plant size and capital costs, increasing the safety, improving remote control, etc.93–97
A deep understanding of the process intensification principles gives membrane technology and membrane engineering a crucial role for the implementation of this strategy.98 Among other new unit operations involving membranes, MRs are expected to play a decisive role in the sustainable growth scenario. They represent a solution for several processes involving the petrochemical industry,99,100 energy conversion,101,102 or hydrogen production,103–108 and fulfil the requirements of process intensification, offering better performance, lower energy consumption, and lower volume occupied with respect to those of conventional operations. The synergic effects offered by MRs by combining reaction and separation processes in the same unit, their simplicity, and the possibility of advanced levels of automation and control offer an attractive opportunity to redesign industrial processes.109–111 However, to make the use of a new technology more attractive, it is fundamental to define a new way of analyzing its performance and highlighting its potentialities with respect to the well-consolidated traditional technologies. Hand in hand with the redesign of new processes comes the identification of new indexes, so-called metrics, that, together with the traditional parameters commonly utilized to analyze a process, can supply additional and important information to support decision-making processes on the type of operation and identification of the operating condition windows that make a process more profitable. Up to now, many efforts have been made to define indicators of industrial processes112,113 and most of them are calculated in the form of appropriate ratios that can provide a measurement of the impact independent of the scale of operation, or to weigh the costs against benefits and, in some cases, they can also allow comparison between different operations.114 The use of these indexes can lead to innovation in the analysis of the performance of unit operations and, in the case of membrane technology, can clearly and easily show the advantages and drawbacks the choice of a specific technology present in comparison with traditional units. In light of the above considerations, the upgrading of syngas via the WGS reaction by means of an MR is considered a case study for the introduction of a non-conventional analysis of the performance of alternative unit operations. In particular, referring to the evaluation of MR performance, the following indexes are defined:65,115,116
Volume index (eqn (1.2)), defined as the ratio of catalytic volume of an MR and a TR to reach a set conversion,
Conversion index (eqn (1.3)), the ratio between the conversion of an MR and a TR for a set reaction volume,
Mass intensity (eqn (1.4)–(1.6)), defined as the ratio between the total H2 fed to the MR plus that produced by the reaction over the total mass entering the reactor,
Energy intensity (eqn (1.7)–(1.9)), defined as the ratio between the total energy involved in the reactor and, similarly to the mass intensity, the total H2 fed to the MR and produced by the reaction, that is, the whole hydrogen exiting the system,
The volume index is an important parameter for the installation of new plants, which must be characterized by a small size and high productivity. It is an indirect indicator of the MR productivity, comparing the MR reaction volume with that of a TR necessary to achieve the same conversion. The volume index ranges from 0 to 1. A low volume index means that the reaction volume required by an MR to reach a set CO conversion is much lower than that necessary for a TR. As a consequence, the catalyst weight necessary in the MR is significant reduced.
Considering the WGS reaction as an example, it can be seen that the volume index is a decreasing function of the feed pressure, owing to the positive effect that the latter has in an MR for CO conversion. The MR reaction volume is 75% of the TR one at 600 kPa and is further reduced to 25% at 1500 kPa when an equimolecular mixture is fed and a final conversion of ∼80% is considered (corresponding to 90% of the traditional reactor equilibrium conversion). This means a reduction in plant size (Figure 1.12) and hence related costs.
Figure 1.13 shows the volume index calculated as the ratio between the reaction volume required by an MR with respect to that necessary for a whole traditional process (high temperature and low temperature reactors) to achieve the same conversion as a function of the feed pressure for inlet temperatures of 300 and 325 °C. The huge difference between the two reaction systems mainly depends on the low-temperature WGS reaction requiring a significant higher volume, since it operates at 220–300 °C and at low GHSVs (3000 h−1) owing to the slow kinetics of the CuO/ZnO catalyst. This means a much greater amount of catalyst required to convert a relatively small feed flow rate, which it is crucial for the determination of the reaction volume of the whole traditional process. As expected, the reaction volume required by the MR results always smaller than that of the whole traditional process and it decreases with the increasing feed pressure. At 5 bar, for an inlet temperature of 300 °C, the MR reaction volume is around 90% that of the traditional process, owing to the limited H2 permeation making the MR work in similar terms to those of the TR. This value is drastically reduced at higher feed pressures, becoming ca. 13% at 15 bar. Furthermore, at temperatures above 325 °C, it is further reduced from 55% at 5 bar to ca. 10% at 15 bar.
The Conversion Index, defined as the ratio between the conversion achieved in an MR and that of a TR for a set reaction volume gives an evaluation of the gain offered by the MR in terms of conversion, and its use is particularly indicated when the feed mixture also contains reaction products. A high conversion index implies a relevant gain in terms of the conversion achieved in an MR with respect to a conventional reactor with the same reaction volume, meaning better raw material exploitation and lower waste. MRs are pressure-driven systems; therefore, the conversion index is an increasing function of the feed pressure, as shown in Figure 1.14. In particular, a conversion index of ca. 2 is achieved at 200 kPa, whereas one of ca. 6 is reached at 1500 kPa when feeding a formate stream (CO : H2O : H2O : CO2 = 20 : 20 : 50 : 10%molar). However, already at 500 kPa, a conversion index of 4 can be obtained.117
Most specifically for H2 production MRs, the mass intensity is defined as the ratio between the total H2 fed to the MR plus that produced by the reaction over the total mass entering the reactor. The higher its value, the more intensified is the process. In any case, it cannot be higher than one when pure hydrogen is fed to the system. The value of this index depends on the conversion and on the composition of the feed stream. In this example, the nominator of mass intensity consists of the H2 fed to the reactor plus the hydrogen given by the reactor, since the reaction stoichiometry says that one mole of H2 is produced by one mole of CO converted by the WGS reaction. The maximum or ideal value of mass intensity is that at the reactor equilibrium conversion.118
The energy intensity is defined as the ratio between the total energy involved in the reactor and, similarly to the mass intensity, the total H2 fed to the MR and produced by the reaction, that is, the whole hydrogen exiting the system. For this index, the higher its value, the more intensified is the process. The value of this index depends also on the conversion and composition of the feed stream and the ideal energy intensity is achieved under equilibrium conditions. High energy intensities (considering the absolute value when an exothermic reaction such as the WGS is considered) mean more energy developed by the system and, thus, the best performance of the reactor.
Upon comparing TRs and MRs, the latter are always more energy and mass intensive than traditional reactors, particularly at high feed pressures, indicating that MRs require less material as feed, thus making available more energy for the production of the same amount of H2. For instance, looking at Figure 1.15, for a GHSV of 30 000 h−1, the temperature range of 350–380 °C appears the most suitable, implying the achievement of a more intensified process, since both mass and energy indexes for the TR and MR show the highest values. In particular, at 350 °C and 1500 kPa, the MR achieves a mass intensity = 0.031 and energy intensity = −12.6 kJ gH2−1, whereas the values of mass and energy intensities for the TR are only 0.023 and −9.00 kJ gH2−1. These results are interesting since they can also be considered from a different point of view. To get the same indexes achieved by the TR at 350 °C, it would be sufficient for the MR to operate at 320 °C and 5 bar or at 300 °C and 10 bar. This means milder temperature conditions with indirect gains also in terms of catalyst lifetime, etc.
The advantages offered by MRs with respect to traditional reaction units are clearly highlighted in Figure 1.16, where the ratios between the actual indexes of MRs and the corresponding ideal (calculated at equilibrium, TREC) indexes of TRs are shown.
In the graph, two zones can be identified; the first relative to values of both MI and EI higher than 1 and the second for values below 1. A ratio equal to 1 means that the MR reaches the best/ideal performance achievable by a TR in equilibrium under the same conditions. Values greater than 1 indicate that the process carried out in an MR results more intensified; this condition can never be achieved by a TR. In general, the higher the ratio, the more intensified the process. An MR process is always more intensified than a TR process operated under real conditions and exceeds the ideal performance of a TR at temperatures higher than 350 °C. This temperature can be reduced by increasing the feed pressure, as this promotes the conversion of the reaction. The mass and energy intensities demonstrate, in line with the process intensification strategy, the assets of MR technology also in terms of the enhanced exploitation of raw materials (reduction up to 40%) and superior energy efficiency (up to 35%).
Nowadays, membrane reactors are a promising innovative technology in the field of hydrogen production from light hydrocarbons. Their use allow better performances than those of conventional reactors to be achieved in terms of high recovery of pure hydrogen streams, higher conversions, and reduced catalyst loadings. Traditional processes can thus be redesigned into more compact and efficient ones, thereby obtaining intensified processes with reduced plant size and higher yields.
Membrane reactors have been demonstrated to be multifunctional units able to significantly increase the conversion (up to 5 times) with respect to that of traditional reactors, significantly reducing the reaction volume required (down to 15% of a traditional reactor). Moreover, the analysis of their performance in terms of mass and energy intensities highlights a region to which only membrane reactors have access, demonstrating the assets of this technology also in terms of enhanced exploitation of raw materials (reduction up to 40%) and superior energy efficiency (up to 35%).
The present work was performed within the framework of activities under the ITM-CNR-Hanyang University International Joint Laboratory on Membrane Technology, established in Seoul on June 14, 2011.