New separation overview in chemical engineering

Summary

1. Membrane Separation Processes Membranes keep things separated in the living world. They pass materials selectively. In every membrane separation process there is a membrane that is placed between two phases. One phase is called feed and the other is called permeate.

2. Membrane The word membrane originates from the Latin word membrane which means a skin. A  membrane  is a  selective barrier  that permits the separation of certain species in a fluid by combination of sieving and sorption diffusion mechanism. Separation is achieved by  selectively passing (permeating)  one or more components of a stream through the membrane while  retarding  the passage of one or more other components. I n other word, a membrane is defined as a structure having lateral dimensions much greater than its thickness, through which mass transfer may occur under a variety of driving forces.

3. The membrane can be a selective or a contacting barrier. In former case, it controls the exchange between the two regions adjacent or it in a very specific manner, whereas in the latter case, its function is mainly to contact the two regions between which the transport occurs. C ontrary to the conventional mass transfer operations, the two phases between which transfer of one or more species occurs are not in direct contact in membrane separation. It is basically either a polymeric phase; are in a organic phase, that separate two phases and it at preferential transport particular species through it. Therefore, one that cause is separation, if you want species is preferential is more transport; if the rate of transport is more compare to the other species, through the membrane. Then, the downstream it will be least, in the in that particular species that cause is the separation.

4. Basic principle of membrane separation : Some mechanism must exist in the membrane which facilitates the transport of one component and impedes the transport of other. In a first approximation, membranes can be classified according to means of such impediment. Porous membranes discriminate according to size of particles or molecules. Non-porous membranes discriminate according to chemical affinities between components and membrane materials. A flow of mass is induced from feed to permeate by applying a driving force. When the feed consists of two or more than two components, and some of those components flow faster than others through the membrane, separation of the feed mixture takes place. Different driving forces can be conceived, but the difference in pressure, concentration and electrical potential between the feed and the permeate phase are by far the most popular in industrial applications.

5. Membrane separation processes can be classified according to their driving forces as follows. Pressure is the driving force : Reverse osmosis Nanofiltration Ultrafiltration Microfiltration b. Partial pressure is the driving force : Gas separation Vapor permeation Pervaporation c. Concentration is the driving force : Dialysis Membrane extraction d. Electrical potential is the driving force : Electrodialysis Membrane electrolysis.

6. The flux through the membrane can be generally expressed as : Flux = (Membrane permeability / membrane thickness) * driving force This mean that the flux depends linearly on both the permeability (a constant which indicates how tight the membrane is) and the driving force. This driving force may be difference in pressure, concentration, temperature, chemical potential and so on. The flux also depends inversely upon the thickness of the membrane. The thinner the membrane, the higher will be the flux.

7. Key properties determining membrane performance are   High selectivity and fluxes, Good mechanical, chemical and thermal stability under operating conditions, Low fouling tendencies and good compatibility with the operating environment, Cost effective and defect-free production.

8. Three different mechanisms by which membrane can perform separations By having holes or pores which are of such a size that certain species can pass through and others cannot. This mechanism is called size exclusion . By selective retardation by the pores when the pore diameters are close to molecular sizes. This mechanism is called pore flow . By dissolution into the membrane, migration by molecular diffusion across the membrane, and re-emergence from the other side. This is called solution diffusion.

9. Principal characteristics of various commercialized membrane separation processes can be specified based on following seven aspects Separation goal Nature of species retained (size of the species) Nature of species transported through membrane, electrolytic or volatile Minor or major species of feed solution transported through membrane Driving force Mechanism for transport / selectivity Phase of feed and permeate streams.

10. Advantages of membrane processes : Appreciable energy savings Clean technology with operational ease Replaces the conventional processes Recovery of high value products Can be Easily coupled with other processes and operations Membrane processes present basically a very simple flowsheet . Greater flexibility in designing systems Membranes can be produced with extremely high selectivities  for the components to be separated. Generally, values of these selectivities are much higher than typical values for relative volatility for distillation operations.

11. Because of the fact that a very large number of polymers and inorganic media can be used as membranes, there can be a great deal of control over separation selectivities . Membrane processes are able to recover minor but valuable   components from a main stream without substantial energy costs. Environmentally friendly: Membrane processes are potentially better for the environment since the membrane approach require the use of relatively simple and non-harmful materials. Clean technology Produces high quality products with variable operating parameters No additives and chemicals

12. Disadvantages / limitations : Membrane processes seldom produce 2 pure products, that is, one of the 2 streams is almost always contaminated with a minor amount of a second component. In some cases, a product can only be concentrated as a retentate because of osmotic pressure problems. In other cases the permeate stream can contain significant amount of materials which one is trying to concentrate in the retentate because the membrane selectivity is not infinite. Membranes can have chemical incompatibilities with some process solutions. This is especially the case in typical chemical industry solutions which can contain high concentrations of various organic compounds.

13. Against such solutions, many polymer-based membranes (which comprise the majority of membrane materials used today), can dissolve, or swell, or weaken to the extent that their lifetimes become unacceptably short or their selectivities become unacceptably low. Membrane modules often cannot operate at much above room temperature. This is again related to the fact that most membranes are polymer-based, and that a large fraction of these polymers do not maintain their physical integrity at much above 100 C. This temperature limitation means that membrane processes in a number of cases cannot be made compatible with chemical processes conditions very easily.

14. Membrane processes often do not scale up very well to accept massive stream sizes. Membrane processes typically consist of a number of membrane modules in parallel, which must be replicated over and over to scale to larger feed rates. Membrane processes can be saddled with major problems of fouling of the membranes while processing some type of feed streams. This fouling, especially if it is difficult to remove, can greatly restrict the permeation rate through the membranes and make them essentially unsuitable for such applications. Concentration polarisation Fouling Short membrane life-time Expensive Upper solid limits

15. Classification of membranes Membrane is nothing more than a discrete, thin interface that moderates the permeation of chemical species in contact with it. Homogeneous membrane Heterogeneous membrane Symmetrical membrane Isotropic membrane Anisotropic membrane Non-porous dense membrane Electrically charged membrane Ceramic membranes Organic and inorganic membrane

16. Membrane materials Membranes generally comprise a thin surface layer which provides the required perm-selectivity on top of a more open, thicker porous support which provides mechanical stability. Membranes are usually fabricated to: have a high surface porosity, or % total surface pore cross-sectional area, have a narrow pore size distribution to provide as high a throughput and selectivity as possible, and be mechanically strong to give structural integrity.

17. Membrane materials Organic membranes Typical materials used in formulating organic membranes   - polyethylene (PE) - polytetrafluoroethylene (PTFE) - polypropylene (PP) - cellulose acetate (CA), Polyacrylonitrile (PAN) - polyimide (PI) - polysulfone (PS) - polyethersulfone (PES)

18. Inorganic membranes The primary materials used for inorganic membranes : - aluminum oxide (Al 2 O 3 ) - zirconium oxide (ZrO 2 )  Also being tried as membranes are materials such as: - stainless steel (SUS) - glass ( Sirasu Porous Glass, SPG) In comparison with organic membranes, inorganic membranes excel in heat and chemical resistance, as well as in mechanical strength. These advantages often offset their inherent higher capital cost.

19. General methods of preparation : Generally, various techniques have been extensively used to prepare synthetic membranes either inorganic membranes (e.g. ceramics, metals, glass, Zeolites ) or organic membranes that includes all sorts of polymers. The purpose of preparation is to modify material using an appropriate technique to obtain a membrane structure with an appropriate morphology for a specific separation. The preparation method is limited the materials used, the membrane morphology obtained and the separation principle applied. The techniques that are being employed for the preparation of synthetic membrane are phase inversion, stretching of films , irradiation and etching of films , sintering of powders, track-etching, sol–gel process, microfabrication vapour deposition and coating. One of the most important methods is phase inversion.

20. Phase Inversion One of the most common and versatile technique used to prepare all sorts of morphologies (both symmetric and asymmetric types) due to the significance of immersion precipitation. It is a process whereby a polymer is transformed in a controlled manner from a liquid state to a solid state. Throughout this technique, a thermodynamically stable polymer solution with a multiple component is subjected to a liquid-liquid demixing whereby the cast polymer film separates into a polymer—rich phase (membrane matrix) and a polymer—lean phase (membrane pores).

21. The mechanism of phase inversion during membrane formation can be concisely described by a polymer/solvent/ nonsolvent system, as explained typically ternary phase diagram. The corners of the triangle act the three components (polymer, solvent, nonsolvent ), whilst at any point within the triangle represents a mixture of three components (starting compositions of the casting solution); The binodal curve divides the triangle into two phase regions; a one-phase region where all components are miscible to and a two-phase region where the system is separated into a polymer-rich, generally a solid phase and a polymer-poor which is generally liquid phase (although the one-phase region in the phase diagram is continuous thermodynamically).

22. The general concept of phase inversion method covers a variety of different techniques, including: (a) Precipitation by solvent evaporation (b) Precipitation by controlled evaporation (c) Thermal precipitation (d) Precipitation from the vapour phase (e) Immersion precipitation or nonsolvent induced phase inversion

23. Characterization of membranes : Membrane processes can cover a wide range of separation problems with a specific membrane being required for every problem. Membranes may differ significantly in their structure and consequently in their functionality. To know what membrane to use in a particular separation process, different membranes must be characterized in terms of structure and mass transport properties. Because very different membranes are used, different techniques are required for characterization. The following sections briefly discuss the following: 1. Characterization of  Porous  Membranes 2. Characterization of  Dense, Homogeneous  Membranes 3. Characterization of  Charged  Membrane s