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Membranes and membrane processes
Membranes are barriers that restrict partially or totally the transport of one or more species of interest. Their development started in the 1940s, as stated by Bhave and Ramesh (1991), with the nuclear industry and the membranes for uranium isotopes enrichment by gaseous diffusion, a difficult operation to be reached through other processes. After that, the membranes started to be used in liquid processes in ultrafiltration and microfiltration industries and, more recently, are being used in a big range of industrial applications (BAKER, 2004).
Widely applied, the membrane processes can be seen in chemical, food and pharmaceutical industries and sectors like water treatment, medicine, and biotechnology. The use of membrane technology for separation processes in the industry is a clean and energetically efficient alternative to conventional methods as distillation, physical and chemical adsorption, and crystallization (BISSETT, 2005).
Membranes are compact systems, where separation occurs without phase change, making the separation energetic and economically interesting. Membrane processes can be applied in mild temperature and pressure conditions; however, membranes are also indicated to separate or purify compounds unable to experience variations on these parameters, preventing losses during critical process conditions.
The transport across the membrane occurs due to a driving force expressed as the chemical potential difference and electrical potential difference. The gradient of chemical potential depends on pressure, temperature, and chemical composition. In the processes at constant temperature, the driving force can be expressed in terms of the pressure or concentration gradient.
Membranes morphology
According to the morphology, the membranes can be classified into porous or dense, symmetric or asymmetric. Symmetric (or isotropic) membranes possess homogeneous structure along the cross-section that can be porous or dense. The asymmetric (or anisotropic), on the other hand, present gradual changes along the cross-section, being porous, dense, or composite, as exemplified in Figure 3.
Porous membranes contain pore diameters in the order of 0.01 to 10 μm, and the separation in this type of membrane occurs due to considerably differences in molecular size (BAKER, 2004). The restriction to the components larger than the maximum pore size results in fractionation. The dense membranes are constituted by a dense layer also called nanoporous layer since only nanometric pores are present. The dense symmetric membranes generally present very low fluxes that limit their application. To overcome this limitation, were developed the asymmetric membranes consisting of a very dense top layer supported by a porous sublayer. If the top layer and sublayer are from different materials, the membrane is known as composite; each layer can be optimized independently. The support is an asymmetric membrane (porous asymmetric membrane), on which a selective dense layer is deposited. This deposition can be performed by different methods as dip-coating, interfacial, in-situ, or plasma polymerization (MULDER, 1996). The porous asymmetric membranes can be constituted by a single material or by different materials and can present more than two porosity levels along the cross-section. The support in the asymmetric composite membranes gives mechanical resistance to the dense layer, responsible for the selectivity. Figure 4 shows the micrograph of the cross-section of a composite membrane, in which different materials, silica and γ-alumina, constitute the thin, dense skin and the porous support, respectively.
Membrane processes
Membrane technologies differ according to the membrane structure and driving force and are applied to the separation of solutions, liquid or gas mixtures. As quoted by Baker (2004), the main membrane processes industrially developed are microfiltration, ultrafiltration, reverse osmosis, and electrodialysis. Developing technologies in the membrane separation include gas separation and pervaporation.
• Microfiltration (MF) – the microporous membrane is used to filter particulates from liquids, retaining particles bigger than 100 nm. It possesses as driving force the pressure 35 gradient and presents low hydrodynamic resistance, i.e., small pressures are sufficient to obtain high fluxes (MULDER, 1996). Filters small particles and bacteria.
• Ultrafiltration (UF) – the membrane is used to separate macromolecules from aqueous solutions with the sizes between 2-100 nm. The driving force is the pressure gradient (∆P ranging from 1.0 – 5.0 bar). The main applications of UF comprise protein fractionation and concentration, pigment and recovery, and oil recovery.
• Nanofiltration (NF) – the membrane is used to separate molecules with a molar mass in the range between 500 and 2,000 Da. The driving force is the pressure gradient (∆P ranging from 5.0 – 20 bar). Mainly used for enzyme purification.
• Reverse Osmosis (RO) – dense membrane with pores between 3 and 5 Å. The
driving force is the pressure gradient (∆P from 10 – 100 bar), with high hydrodynamic resistance. The separation occurs by solution-diffusion mechanism, and the main application is seawater desalination.
• Electrodialysis – electrically charged membranes are used to separate ions from aqueous solutions. The driving force for ionic transport is the electrical potential difference. The main application is brackish groundwater desalination.
• Gas separation – the dense membrane is used to separate gas mixtures at elevated pressures. The solution and diffusion of one component occur selectively on the membrane surface. The permeate side is enriched in this species with more affinity by the membrane. One of the main applications is hydrogen separation from mixtures of hydrogen-nitrogen.
• Pervaporation – the membrane is dense with an asymmetric structure, the separation occurs with phase change, and the driving force is the vapor pressure gradient. A liquid mixture is permeated through solution-diffusion mechanism, and vapor is collected in the permeate side. Therefore, pervaporation offers the possibility of separating closely boiling mixtures or azeotropes that are difficult or non-possible to separate by distillation (BAKER, 2004). The main application is the organic solvents dehydration.
Materials for manufacturing membranes
Membranes can be produced from inorganic materials – carbonic, metallic, ceramic, or mineral – and organic materials, polymers, such as polyamide, polyetherimide, and polyvinyl alcohol. Usually, membranes of an organic nature have a lower production cost than inorganic ones.
Most membranes used commercially nowadays are organic due to both the production cost and the variety of materials available. In recent years, however, the application of other types of membranes has increased, mainly in processes requiring thermal stability and high resistance to solvents or an aggressive cleaning protocol (BAKER, 2004).
The use of polymers for membranes manufacturing is based on specific characteristics from structural factors of these polymers, such as the molecular weight, chain flexibility and chain interactions. These factors can determine the thermal, chemical and mechanical attributes and also the permeability of the polymeric material (MULDER, 1996). The commercially applied polymers are the polyethylene and the polysulfone, both hydrophobic, low-surface-energy materials, poly(vinylidine fluoride), polyamides, and the cellulose acetate, a hydrophilic material that often carries charged surface groups.
The criteria for selecting a membrane depend on the application: the characteristics of the feed solution and the component to be separated, as well as parameters such as permeability, selectivity, mechanical and chemical resistance, and desired thermal stability. Table 1 shows a general comparison between ceramic and polymeric membranes. Inorganic membranes are the focus of this work and will therefore be presented in greater detail, and more specifically ceramic and zeolite membranes.
Table of contents :
CHAPTER 1 INTRODUCTION
General Objective
Specific Objectives
CHAPTER 2 LITERATURE REVIEW
2.1 Membranes and membrane processes
2.2 Ceramic membranes
2.3 Centrifugal casting
2.4 Composite membranes
2.5 Zeolites
2.6 Zeolite membranes
2.7 Zeolite Membranes Applications
2.8 Membrane reactors
2.9 Xylene isomers separation and reaction
2.10 Conclusion
CHAPTER 3 EXPERIMENTAL
3.1. Raw materials
3.2. Ceramic membrane characterization techniques
3.3. Zeolite membrane characterization
3.4. Zeolitic membrane permeation and catalytic experiments
CHAPTER 4 SYNTHESIS AND PROPERTIES OF ALUMINA-COAL FLY ASH COMPOSITE TUBULAR ASYMMETRIC MEMBRANES
4.1. Raw-materials characterizations
4.2. Ceramic Membranes Preparation
4.3. Ceramic membrane characterizations
4.4. Conclusion
CHAPTER 5 DESIGN AND PROPERTIES OF ALUMINA-CFA COMPOSITE MEMBRANES COATED WITH ZSM-5 ZEOLITE
5.1 Zeolite Coating
5.2 Ion Exchange
5.3 Zeolite membrane characterization
5.4 Separation of aromatic molecules over zeolite coated alumina-CFA membranes
5.5 Separation and isomerization tests using ortho-xylene
5.6 Conclusion
CHAPTER 6 CONCLUSIONS AND FUTURE PERSPECTIVES
