Humidification effect

The temperature of operation has one of the strongest influences on durability and contamination effects. It can also have a strong influence on performance when coupled with humidification effects. Typical fuel cell operation is in the range of 60 to 80°C, but the overall drive in the fuel cell industry is to run at higher temperatures in order to reduce system cooling requirements and catalyst contamination issues. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

Humidity can be a problem. Whereas it was shown (284) that 33% RH was best for spore inactivation, and that at least 30% RH was needed for effective sterilisation (285), dried spores are difficult to kill, and the spore substrate material and wrappings compete with the spore for the available moisture (286). Therefore, the relative humidity is adjusted to 50—70% to provide sufficient moisture for the spores to equiUbrate. The exposure time depends upon the gas mixture, the concentration of ethylene oxide, the load to be sterilised, the level of contamination, and the spore reduction assurance requited. It may be anywhere from 4—24 hours. In a mn, cycles of pre-conditioning and humidification, gassing, exposure, evacuation, and air washing (Fig. 9) are automatically controlled. 

Green, G. H. (1982). Positive and negative effects of building humidification.. ASHRAE Trans, 88(1), 1049-1061,... 

In the processing of materials it is often necessary either to increase the amount of vapour present in a gas stream, an operation known as humidification or to reduce the vapour present, a process referred to as dehumidification. In humidification, the vapour content may be increased by passing the gas over a liquid which then evaporates into the gas stream. This transfer into the main stream takes place by diffusion, and at the interface simultaneous heat and mass transfer take place according to the relations considered in previous chapters. In the reverse operation, that is dehumidification, partial condensation must be effected and the condensed vapour removed. 

Humidification of inspired air may promote the hydration (liquefaction) of tenacious secretions, allowing for more effective sputum production. The use of mucolytic aerosols (e.g., N-acetylcysteine deoxyribonuclease) is of questionable therapeutic value. Mucolytics may have the greatest benefit... 

The objective of these Aerosol Contribution Procedures (ACP) is to determine humidifier solids residue contributions, x,t> to apparent workplace respirable dust levels so feasibility OT engi-neering control or exclusion strategies may be evaluated. By engineering control of x, meant that low solids water is used for humidification. This can be a very cost-effective means of control. Generally, x, is the component which should receive first attention when co5t-prioritiling the various components of multi constituent, respirable cotton dust. 

Choose an inlet air temperature that is high enough to negate weather effects (outside air humidity or inside room conditions). The air temperature should not be detrimental to the product being granulated. (To achieve consistent process year round, a dehumidification/humidification system is necessary, which provides the process air with constant dew point and, hence, constant drying capacity.)... 

Desflurane does not have a marked bronchodilator effect and in cigarette smokers it is associated with significant bronchoconstriction. In clinical practice, both humidification of inspired gases and opioids are thought to reduce airway irritability but even at moderate concentrations (2 MAC), desflurane is more likely to cause coughing than sevoflurane. In common with other volatile agents, desflurane causes dose-related respiratory depression. Tidal volume is reduced and respiratory rate increases, initially. As inspired concentrations of desflurane increase, the trend is to hypoventilation and hypercardia and apnoea is to be expected at concentrations of 1.5 MAC or greater. 

A variety of desalting technologies has been developed over the last 40 years. Based on their commercial success, they can be classified into major (viz., multistage flash distillation, MSFD multiple-effect distillation, MED vapor compression, VC ED RO) and minor (i.e., freezing, membrane distillation solar humidification) processes. 

In distillation systems, the improvement of tray efficiency due to taller weirs is small (5). Koch Engineering (8), Kreis and Raab (28), and Kalbassi et al. (184) observed little effect of weir height on distillation tray efficiency for weirs 1.5 to 3 in, 1 to 2 in, and 0.5 to 1 in tall, respectively. Finch and Van Winkle (185) reported an efficiency increase of the order of 5 to 10 percent as weir height is raised from 1 to 3 in a similar increase was reported by Prado and Fair (110,144) in humidification and stripping tests. 

For the low-frequency arc, many studies and explanations have been published. Fieiie and Gonzalez [29] studied the effects of temperature, membrane thickness, and humidification conditions on the impedance response. They suggested that the low-frequency arc has two causes the effect of liquid water formed at the anode, which affects the transport of oxygen and the hydration effect, which limits the transport within the membrane. The relative weight of these two effects depends on the thickness of the membrane and the working conditions of the fuel cell. They... 

Figure 5.32. Impedance plots for single cells at ambient temperature, a Nation 117. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A) 0.70 V (0.76 V) ( ) 0.6 V (0.74 V) ( ) 0.5 V (0.74 V). b Nafion 112. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A ) 0.70 V (0.73 V) ( ) 0.6 V (0.67 V) ( ) 0.5 V (0.61 V). Plots were corrected for the high-frequency resistances. Left detail of the high-frequency regions [29]. (Reprinted from Journal of Electroanalytical Chemistry, 503, Freire TJP, Gonzalez ER. Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells, 57-68, 2001, with permission from Elsevier.)...

Figure 5.42 shows an example of the effect of humidity on the EIS spectra. It can be seen that the cut-off for anode humidification does not affect the spectra too much compared with the cut-off for cathode humidification. As we know, the fuel cell EIS primarily represents the cathode behaviour. Therefore, cathode humidification can greatly affect the whole impedance spectra. The humidification cut-off at the cathode causes a large difference in both the membrane resistance and the kinetic resistance. Dehydration of the anode also brings about a substantial increase in cathode impedance because a dry anode pulls water away from the cathode and across the membrane, which makes it hard to keep the cathode well hydrated [18],... 

The sensitivity of the impedance to humidification of the reactant gases also depends on the membrane thickness. It has been reported that a thicker membrane is much more sensitive to humidification conditions. For Nafion 117, membrane resistance normally decreases when the reactant gases are better humidified because the conductivity of Nafion increases almost linearly with its water content [60]. If the fuel cell is operated with Nafion 112, the effect of humidifying the reactant gases is much less significant because this thin membrane remains well hydrated by the water produced in the fuel cell reaction [29],... 

Freire TJP, Gonzalez ER (2001) Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells. J Electroanal Chem 503 57-68... 

Figure 6.11. Nyquist plots for MEAs containing different proton-conducting ionomers at 0.85 V without external humidification catalyst loading = 0.4, 0.7 mg Pt/cm2 for anode and cathode, respectively TceU = 25°C Pressure = 1 atm and H2/02 flow = 400 cmVmin [8]. (Reprinted from Electrochimica Acta, 50(2-3), Ahn SY, Lee YC, Ha HY, Hong SA, Oh IH. Effect of the ionomers in the electrode on the performance of PEMFC under non-humidifying conditions, 673-6, 2004, with permission from Elsevier.)...

By using the impedance method, Freire et al. also demonstrated that thinner membranes not only show better performance but also are much less sensitive to humidification conditions, cell temperature, and current density. The dependence of the real resistance at high-frequency intercepts on membrane thickness at different current densities is illustrated in Figure 6.14. Linear dependence of the high-frequency resistance, RhJ. on the membrane thickness was observed at 80°C, whereas non-linear dependence of RhJ on membrane thickness was shown at 40°C and 60°C. They explained that this was due to better hydration of the membrane at higher temperatures. It is also observable that Rhf almost does not depend on current density at 80°C and at low current densities rather, Rhf dependence on membrane thickness is almost linear, which indicates that for thicker membranes at high current densities it no longer behaves as a pure resistor due to a capacitive effect caused by less effective back transport of water [9],... 

As acid paper ages, the amorphous regions of the cellulose fiber that are plasticized by water and/or humectant tend to disappear. The sorbi-tol/Kymene treatment becomes ineffective with degraded paper. This suggests that somewhat the same effect would be observed between new and degraded papers when humidified. Adrian Sclawy of the Library of Congress Preservation Research and Testing Office, carried out the experiments on humidification which are reported in Chapter 16 of this volume. 

The shape of particles is normally that of more or less regular spheres, dense or hollow, with smooth surfaces and sometimes cracks. This is related to the composition and the rate of solvent evaporation, with possible existence of internal pressure inside the drops when a rigid surface layer is being formed (Walton and Mumford 1999). All these characteristics will have some effect on handling properties of powders such as bulk and tapped densities, particle density, (mixing with other powders, storage) wettability and solubility, porosity, specific area (rehydration, instantisation) flowability (size, surface asperities), friability and creation/existence of dust, stability in specific atmosphere and medium (oxidation, humidification, active component release) (Huntington 2004). 

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