Alessio Caravella

Pd-based membrane reactors for one-stage process of water gas shift

The water gas shift (WGS) reaction is the upgrading stage in the cycles of hydrogen production by, for example, steam reforming of light hydrocarbons from fossil or renewable sources. This is a thermodynamics limited reaction and CO conversion is furthermore depleted by the presence of products, such as hydrogen as often/always happens in industrial applications. WGS industrial processes consist of two reactors in series: the first one operates at high temperature (300–400 °C), exploiting the advantages offered by a fast kinetics; the second one works in the low temperature range (200–300 °C) to the benefit of the higher thermodynamic conversion. This work proposes the use of one Pd-based membrane reactor (MR) operating at the same temperature range as the high temperature WGS reactor as a suitable alternative to the whole traditional reactor (TR) process. The hydrogen permeation allows the increase of the equilibrium conversion close to the total value and thus to operate in the higher temperature range exploiting the greater kinetics offered by Fe–Cr based catalysts. The values of gas hourly space velocity (GHSV), temperature, H2O/CO feed molar ratio, feed composition, etc. used in the simulations are those typical of an industrial application of a WGS upgrading stage. A reference value of 15 bar of feed pressure was assumed since this is the strength limit of the self supported Pd–Ag membrane considered in the simulations. However, a feed pressure of 30 bar was also considered as that used in industrial processes. The pressure proves to be one of the most interesting variables of MR processing. The proposed analysis demonstrates how only one stage based on a Pd-alloy MR can replace the two reactors of the traditional process, which also gives better performance for, e.g., CO conversion, pure hydrogen recovered on the permeate stream, etc. An intensified process with a smaller reaction volume, higher conversion and GHSV, etc. is the outcome.

3.2 Modeling and Simulation of Membrane Reactors and Catalytic Membrane Reactors

Models of membrane reactors (MRs) and catalytic membranes are presented by taking into account the advantages of the separation provided by the membranes. In particular, a 1D mathematical model is presented for tubular reactors considering a cylindrical symmetry, which is modeled by first- or second-order differential equations depending on the importance played by the axial diffusion. The permeation through the membrane is described by a specific transport model. Progresses in estimating the effect of external mass transfer in the gaseous films and inhibition phenomena limiting the permeation through the membranes are described, also introducing the so-called polarization and inhibition maps allowing the immediate evaluation of the influence of these phenomena on MR performance. Moreover, process intensification metrics are presented as additional nonconventional parameters useful for the identification of the operating condition ranges making a process more profitable.

Chapter 14:Polarization and Inhibition by Carbon Monoxide in Palladium-based Membranes

The topic of this chapter concerns the effect of inhibition by CO and concentration polarization in Pd-based membranes used for hydrogen purification processes. In particular, the decrease of membrane performances due to a reduced permeance and driving force with respect to the pure-hydrogen case is analyzed as a function of several operating conditions. This study is carried out by using a coefficient recently introduced in the literature – the overall Permeation Reduction Coefficient(PRC)– whose definition allows the coupled effects of concentration polarization and inhibition by CO to be analyzed by identifying and separating their respective contributions. According to this approach, it is clearly shown that the decrease of effective permeance due to polarization and that due to inhibition cannot be simply added to each other for the overall permeance to be evaluated. Differently, a non-linear (quadratic) dependence is obtained, showing the notable result that the concentration polarization is reduced in presence of inhibition. Such behaviour is described by some Permeation Reduction Maps, which are graphical representations of the mixture-membrane system complexity. From these maps, useful information on membrane performance can be directly read without the need of solving numerically the complex mathematical model behind them, allowing a more precise design of Pd-membrane systems for hydrogen purification.

Coupling Newton‐Raphson and Bisection Solving Methods to Simulate Hydrogen Permeation through Pd‐based Membranes with Inhibition by CO and Concentration Polarization

In this work, hydrogen permeation through Pd‐based membranes is modeled and simulated taking into account the presence of both concentration polarization and inhibition by CO. Hydrogen permeation is described as a series of several elementary steps, which are numerically solved by means of a method combining a Newton‐Raphson and bisection routines. The results are expressed in terms of maps of the permeation reduction coefficient (PRC), which is a coefficient taking into account the permeance reduction due to polarization and inhibition. This model can be very useful to improve the design of the Pd‐based membrane separation modules, being able to be fully integrated as a separated numerical routine in CFD commercial software.