Membrane dehumidification

Membrane dehumidification method is advantageous over the conventional methods since the running cost is low, the construction of the apparatus is simple, and the dry gas can be produced continuously. The key to membrane driven dehumidification process is to provide a moisture removing membrane having both high water vapour permeation rate and high selectivity of water over other gases [24]. 

Partial Pressure Pinch An example of the hmitations of the partial pressure pinch is the dehumidification of air by membrane. While O9 is the fast gas in air separation, in this apphcation H9O is faster still. Special dehydration membranes exhibit a = 20,000. As gas passes down the membrane, the pai-dal pressure of H9O drops rapidly in the feed. Since the H9O in the permeate is diluted only by the O9 and N9 permeating simultaneously, p oo rises rapidly in the permeate. Soon there is no driving force. The commercial solution is to take some of the diy air product and introduce it into the permeate side as a countercurrent sweep gas, to dilute the permeate and lower the H9O partial pressure. It is in effect the introduction of a leak into the membrane, but it is a controlled leak and it is introduced at the optimum position. 

Flat-membrane contactors have been specifically designed and commercialized by GVS SpA (Italy) for air dehumidification processes [23]. 

In Table 22.1 the characteristics of a membrane contactor housing 1 m2 of flat membrane and used for the demonstration tests related to air dehumidification are... 

The membrane contactor is placed at the inlet of the evaporator of the refrigerating unit and handles 10 - 20% of the total recycled air flow. This value defines the by-pass factor (BF). In the demonstrations the BF was 80-90%. After dehumidification, the air is mixed with the recycled air. 

In conclusion, the demonstration runs have shown that the membrane contactor dehumidification system is effective when the total loads of the refrigerated cell do not require an evaporative temperature of the cooling coil below —10 °C. In these conditions it has been shown that energy, operating costs and C02 emissions can be decreased up to 20% when optimized systems are built. The optimization is related mostly to the selection of adequate pumps, fans, heat exchangers. 

Wagner B, Jehle W, Steinwandel J, Staneff T, and Herczog J, Continuous membrane supported cabin air dehumidification process. Euromembrane 97, University of Twente, Enschede, The Netherlands, June 26, 1997. 

Includes solvent recovery, dehumidification, pervaporation, and helium recovery membranes. 

Isetti C, Nannei E, and Magrini A. On the application of a membrane air-hquid contactor for air dehumidification. Energ. Build. 1997 25 185-193. 

A group of operations for separating the components of mixtures is based on the transfer of material from one homogeneous phase to another. Unlike purely mechanical separations, these methods utilize differences in vapor pressure, solubility, or diffusivity, not density or particle size. The driving force for transfer is a concentration difference or a difference in activity, much as a temperature difference or a temperature gradient provides the driving force for heat transfer. These methods, covered by the term mass transfer operations, include such techniques as distillation, gas absorption, dehumidification, adsorption, liquid extraction, leaching, crystallization, membrane separations and a number of others not discussed in this book. 

Y. Sugaya, M. Nakao, H. Horie and H. Wakabayashi, Multi-layer Dehumidification membrane, Jpn. Pat. JP 8-4705 (examined application). 

Membranes are used to dehydrate process air streams as replacement for desiccant dryers or adsorption systems. Such membrane units have been on the market for many years, but they are mainly for small gas streams. The manbranes being used have very high water to air selectivity. The dehumidification units are usually connected to a compressed air line, and loss of pressurized air through the membrane may be a major cost. 

Desalination of saline water (sea and brackish waters) is a well-established means of water supply in many countries. Basically, desalination processes in this area can be divided into two groups (1) phase-change/thermal, and (2) membrane-based separation processes. Phase-change processes include multi-stage flash,multiple effect boiling, vapour compression,freezing,humidi-fication/dehumidification and solar stills. RO, ED and membrane distillation (MD) are typical membrane separation processes (Charcosset, 2009). 

Modified polyether-ether-ketones are of eonsiderable interest due to their exeellent mechanical toughness, thermo-oxidative stability, solvent resistanee and high transition temperature. In the last deeade eonsiderable effort has been spent to introduce chemical modifications in this class of polymers in order to obtain better physical properties and to build up membranes for eleetro-dialysis, gas dehumidification and gas separation. Relatively few atomistie simulations have been performed on this class of polymers. The monomer strueture, experimental density and the glass transition temperature of the four poly(ether ether ketone)s is reported in Figure 1.5. 

Separation of a gas from the gas mixture is a key issue in the various industrial fields hydrogen recovery in petroleum refinery process, oxygen removal to prevent flame, air dehumidification to prevent moisture absorption, dehydration of fine chemical products, for example. Although there are various methods to perform gas separation [membrane separation, pressure swing adsorption (PSA) separation, cryogenic separation], the method using membranes attracts much attention. 

Chen, G., Zhang, X., Wang, J., Zhang, S., Synthesis and characterization of soluble poly(amide-imide)s bearing triethylamine sulfonate groups as gas dehumidification membrane material, J. Appl. Polym. ScL, 2007,106, 3179-3184. 

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