Membrane Reactors for Hydrogen Production

A. BRUNETTI, A. CARAVELLA, E. DRIOLI AND G. BARBIERI

1.1 Introduction

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. 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.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). 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 ppmin 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 H 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 H, 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 H 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...