Membrane Reactor Engineering
Applications include water treatment, wastewater recycling, desalination, biorefineries, agro-food production
Membrane reactors can bring energy saving, reduced environmental impact and lower operating costs
Membrane Reactor Engineering
Membrane Reactors: The Technology State-of-the-Art and Future Perspectives
Gaetano Iaquaniello 1, Gabriele Centi 2, Marcello De Falco 3 and Angelo Basile 4
1 Processi Innovation SRL, KT - Kinetics Technology S.p.A., Rome, Italy
2 Department of MIFT University of Messina, ERIC AISBL and INSTM/CASPE, Messina, Italy
3 Department of Engineering, University Campus Bio-Medico of Rome, Italy
4 Institute of Membrane Technology of the Italian National Research Council (ITM-CNR), Rende, Italy
1.1 Selective Membranes: State-of-the-Art
IUPAC  defines membranes as structures having lateral dimensions much greater than their thickness, with the mass transfer regulated by a driving force, expressed as gradient of concentration, pressure, temperature, electric potential, and so on. In other words, a membrane is a permeable phase between two fluid mixtures, which allows a preferential permeation to at least one species of the mixture. So, the membrane acts as a barrier for some species whereas for other species it does not. In effect, the main function of the membrane is to control the relative rates of transport of the various species through its matrix structure giving a stream (permeate) concentrated in (at least) one species and another stream (retentate) depleted with the same species.
The performance of a membrane is related to two simple factors: flux and selectivity. The flux through the membrane (or permeation rate) is the amount (mass or molar) of fluid passing through the membrane per unit area of membrane and per unit of time. Selectivity measures the relative permeation rates of two species through the membrane, in the same conditions (pressure, temperature, etc.). The fraction of solute in the feed retained by the membrane is the retention. Generally, as a rule, a high permeability corresponds to a low selectivity and, vice versa, a low permeability corresponds to a high selectivity and an attempt to maximize one factor is compromised by a reduction of the other one. Ideally membrane with a high selectivity and with high permeability is required.
Membranes are used for many different separations: the separation of mixtures of fluids (gas, vapor, and miscible liquids such as organic mixtures and aqueous/organic ones) and solid/liquid and liquid/liquid dispersions, and dissolved solids and solutes from liquids .
Membrane processes are a well-established reality in various technology fields, as testified, for example, by Figure 1.1 , which describes the trend in scientific publications regarding "membranes" in the last 15 years.
Figure 1.1 Number of publications about "membranes" versus time. (Scopus database: www.scopus.com )
Membranes are applied to fluid treatment and they can be involved in different processes such as Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF), Pervaporation (PV), Gas Permeation (GP), Vapor Permeation (VP), and Reverse Osmosis (RO) processes.
To briefly summarize :
MF is related to the filtration of micron and submicron size particulates from liquid and gases.
UF refers to the removal of macromolecules and colloids from liquids containing ionic species.
PV refers to the separation of miscible liquids.
The selective separation of mixtures of gases and vapor and gas mixtures are called GP and VP, respectively.
RO refers to the (virtual) complete removal of all material, suspended and dissolved, from water or other solvents.
The selective separation of species among others is related, in the aforementioned cases, to molecular size dimensions (see Figure 1.2 ). Furthermore, as reported in Figure 1.3 , the pore size of useful membranes sets which kind of processes they