Introduction It is well understood today that our society has to face the challenge of modifying the traditional industrial growth to a sustainable growth, if we want to keep developing for generations. In principle, adverse environmental impact can be notably reduced by optimizing the existing activities. Present design methods are effectively devoted, in most cases, to managing wastes better, to introducing methods for pollution abatement, and to realizing cleaner processes for cleaner products. Nevertheless, these positive effects are expected to be offset by the on going growth. Traditional environmental management and pollution prevention will not suffice in the long run; newer approaches, which are radically innovative and integrated, are needed. The chemical and engineering community is already paying significant attention to the request for technologies that would lead us to the goal of technological sustainability. A promising example with a lot of interest by process engineers is the strategy of process intensification. It consists of innovative equipment, design and process development methods that are expected to bring substantial improvements in chemical and any other manufacturing and processing, such as decreasing production costs, equipment size, energy consumption, waste generation, and improving remote control, information fluxes and process flexibility. How to implement this strategy is, however, not obvious. An interesting and important case is the continuous growth of modem membrane engineering whose basic aspects satisfy the requirements of process intensification. Membrane operations, with their intrinsic characteristics of efficiency and operational simplicity, high selectivity and permeability for the transport of specific components, compatibility between different membrane operations in integrated systems, low energetic requirement, good stability under operating conditions and environment compatibility, easy control and scale-up, and large operational flexibility, represent an interesting answer for the rationalization of chemical productions. Many membrane operations are practically based on the same hardware (materials), only differing in their software (methods). The traditional 2 Introduction membrane separation operations (reverse osmosis, micro-,ultra- and nanofiltration, electrodialysis, pervaporation etc.), already largely used in many different applications, are today conducted with new membrane systems such as catalytic membrane reactors and membrane contactors. At present, redesigning important industrial production cycles by combining various membrane operations suitable for separation and conversion units, thus realizing highly integrated membrane processes, is an attractive opportunity because of the synergic effects that can be attained. Interesting examples already exist in water desalination plants, in downstream processing of biological and biotechnological productions, etc. This strategy starts to penetrate also in new areas such as the petrochemical industry, the electronic industry. Limits however exist to the traditional membrane operations, as p.e. the level of feed concentrations which can be reached in a RO system or on the recovery factors in the same RO desalination units. New unit operations moreover might be invented and/or developed in same cases which better satisfy the requirement of the process intensification strategy. Among other new unit operations involving membranes, membrane contactors are expected to play a decisive role in this scenario. The key concept is to use a solid, microporous, hydrophobic (or hydrophilic) polymeric matrix in order to create an interface for mass transfer and/or reaction between two phases: large exchange area and independent fluid dynamics allow an easily controlled operation. These membrane systems, in the form generally of low cost hollow fibres, provide a high interfacial area significantly greater than most traditional absorbers between two phases to achieve high overall rates of mass transfer. In addition, whereas the design of the conventional devices is restricted by limitations in the relative flows of the fluid streams, membrane contactors give an active area, which is independent of the liquid fluid dynamics. Membrane crystallizers, membrane emulsifiers, membrane strippers and scrubbers, membrane distillation systems, membrane extractors, etc. can be designed and integrated in the production lines together with the other existing membranes operations for advanced Introduction 3 molecular separation, and chemical transformations conducted using selective membranes and membrane reactors, overcoming existing limits of the more traditional membrane processes (for example the osmotic effect of concentration by reverse osmosis). It is amazing to note that, although the above mentioned systems are quite "young", the potentialities of membrane systems have been already discovered and suggested at the beginning of the XX Century 1 . In Table 1 are summarized the most traditional membrane contactors developped in these last years. Table .1 Membrane contactors systems Membrane strippers Membrane scrubbers Membrane extractors Supported liquid membranes Membrane distillation Osmotic distillation Membrane emulsifiers Phase transfer catalysis A first example might be considered the supported liquid membranes where the microporous hydrophobic membranes act as support to the liquid phase containing appropriate carriers for the selective transport of the species dissolved in the solutions facing the membrane; other most recent examples are membrane distillation contactors. In all the operations mentioned the role of the membranes is crucial; they not only serve as an ideal contactors between the two phases they separates, but contribute more to the efficiency of the overall processes. 4 Introduction The relative simplicity of the hardware of these systems is combined with a certain complexity on the contrary of their software. A multidisciplinary background is certainly necessary for a deep basic knowledge of the membrane contactors properties in their various configurations and in their various applications. Transport phenomena in porous media, interphacial phenomena in liquid-.liquid, gas- liquid, in gas-gas phases, basic properties of polymeric materials, as also of colloids and gels, are necessary and must be well integrated with a knowledge of fundamentals of chemistry as of the thermodynamics and kinetic aspects. In this book we will present the basic aspects of the various membrane contactors already existing, and their applications. The overall potentialities of these new technologies will be also temptatively discussed. References 1 .P .A Kober. Pervaporation, perstillation and percrystallization., Contribution read at the meeting of the Soc. Expt. Biol. Med., Feb. 12 (1917) Chapter I. Basic principles of membrane contactors .1 Generalities on membrane contactors operations The term "membrane contactor" is used to identify membrane systems that are employed to "keep in contact" two phases. On the contrary of the more "traditional" idea of membranes as media for performing separations thanks to their selectivity, membrane contactors do not offer any selectivity for a particular species with respect to another, but simply act as a barrier between the phases involved, by allowing their contact in correspondence of a well defined interfacial area [ 1-9]. Being the two phases separate by the membrane, there is no mix of them and dispersion phenomena do not occur. The species are transferred from one phase to the other by only diffusion. The membranes are usually microporous and symmetric and can be both hydrophobic and hydrophilic. In the case of hydrophobic materials, the membrane can be wetted by non polar phases (e.g., non polar organics) or filled by gas, while the aqueous/polar phase can not penetrate into the pores (see Figure .)1 6 Chapter 1 Figure .1 Interface between a non polar/gas phase and a polar phase in a hydrophobic membrane. In this way, it is possible to define the area of contact in correspondence of the pores mouths. In order to avoid the mixing of the two phases, it is important to carefully control the operating pressures. First of all, the pressure of the aqueous/polar phase has to be equal to or higher than the pressure of the wetting/filling phase. This permits to eliminate any possibility of dispersion as drops of one phase into the other phase. Moreover, the interfacial area can be established at the pore mouth only if the penetration of the aqueous/polar phase into the membrane pores is prevented. The hydrophobicity of the material is not, in fact, a warranty for keeping the pores aqueous/polar phase-free. If a critical value of pressure, called generally breakthrough pressure, is exceed, the membrane loses its hydrophobic character and the aqueous/polar phase starts to wet it [10-12]. For a particular material the breakthrough Basic Principles of Membrane Contactors 7 pressure depends on the pore radius, surface/interfacial tension, contact angle between the membrane and the fluid, and can be calculated by using the Laplace's equation (see Chapter 2). In figure ,1 as well as in all the other figures, for simplicity, straight pores are considered for symmetric membranes. In practice, membrane pores have an un-defined shape, mainly related to the tortuosity of the membrane along its thickness. With asymmetric membranes in which the pore size reduces along the thickness, it is possible to keep in non-dispersive contact the two phases also by working, at the bigger pores side, at pressures higher than the breakthrough value. In fact, being the breakthrough pressure inversely dependent on the pore size, there is a partial wetting of the membrane for the bigger pores, whereas the smaller pores continues to be aqueous/polar phase free. The interfacial area is now established within the pores (see Figure 2). Figure .2 Interface between a non polar/gas phase and a polar phase in a partially wetted asymmetric membrane. 8 Chapter 1 The hydrophobicity of the membrane can also vary because of the interactions with the phases involved that lead to changes in the membrane structure and morphology. This last aspect can be minimized by using composite membranes with a non-porous thin layer coated on the microporous surface that prevents the penetration of the aqueous/polar phase (Figure 3) [13-17]. Figure .3 Composite membrane with a dense thin layer coated on the microporous surface. The non-porous thin layer allows also to enlarge the range of the operating pressures, but, in order to do not increase too much the resistance to the mass transport, it has to be highly permeable for the trasferred species. The membrane wetting can be partial or complete; in the first case the two phases are in contact somewhere in the membrane pores, whereas for complete wetting the two phases are mixed and the membrane contactor loses its function. Basic Principles of Membrane Contactors 9 When hydrophilic materials are used, the aqueous/polar phase wets the membrane pores while the non polar/gas phase is blocked at the pore mouth. In this configuration the interface is established at the pore mouth at the non polar/gas phase side and the dispersion as drops between the phases is avoided by working with pressures of the non polar/gas phase equal to or higher than the wetting phase pressure (Figure 4). Figure .4 Interface between a non polar/gas phase and a polar phase in a hydrophilic membrane. As for the hydrophobic membranes, the interface is kept at the pore mouth until the breakthrough pressure is not exceed. As reported by Sirkar [10], two liquid phases can be in contact also by means of a composite hydrophobic-hydrophilic membrane where the polar phase wets the hydrophilic 10 Chapter 1 part and the non polar phase enters the hydrophobic one (Figure 5). The interface is now located at the hydrophobic-hydrophilic interface and can be well defined by operating with one of the two phases at higher pressure, taking care in not exceeding the critical pressure value. Figure .5 Interface between a non polar/gas phase and a polar phase in a composite hydrophilic - hydrophobic membrane. Until now, we did not consider any reaction between the phases involved. When the species present into the two phases react, an interface where the reaction occurs can be formed and it can correspond with the phase interface or can be located into one phase.