Humidification efficiency

Humidification efficiency The ratio of the actual mass of water evaporated by a humidifier to the theoretical mass of water needed to achieve saturation at a given temperature. 

Raw Gas Preconditioning In order to ensure the required destruction efficiency and continued biofilter life, the raw gas stream must be adjusted to a preselected range of particulate loading, temperature, and humidification before the gas can be safely introduced into the biofilter. 

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. 

The transfer of mass from one phase to another is involved in the operations of distillation, absorption, extraction, humidification, adsorption, drying, and crystallization. The principal function of the equipment used for these operations is to permit efficient contact between the phases. Many special types of equipment have been developed that are particularly applicable for use with a given operation, but finite-stage contactors and continuous contactors are the types most commonly encountered. A major part of this chapter, therefore, is devoted to the design aspects and costs of stagewise plate contactors and continuous packed contactors. 

Often it is necessary to transport excess water from the cathode side to the anode side, where it is used either for humidification of the hydrogen stream in the PEFC or to dilute the methanol fuel. (In order to increase energy and power density, methanol, while being used in solution, will usually be stored as the pure liquid.) Water management needs additional aggregates or devices. This adds to the cost of the fuel cell system and further reduces its efficiency. All these transport limitations give rise to diffusion overpotentials which lead to the rapid breakdown of the cell at high current densities in Fig. 2. 

Since PEM, like living organisms, need water to function and the amount and state of water are critical for an efficient operation, secondary requirements on this type of fuel cell membranes emerge. These include the necessity of sufficient humidification and the ability to retain water under operation conditions. Associated problems under fuel cell operation include the electroosmotic transport of water to the cathode side accompanied by dehydration at the anode side [45]. In the cathode the accumulation of water at high current densities, typically greater than 1 A cm-2, causes performances losses due to blocking of catalytically active sites and restriction of oxygen transport. 

The operative parameters which can be regulated to optimize the stack performance are MEA humidification, reactant pressure, stack temperature, and stoichiometric ratio. While the role of membrane humidification, already partially discussed in Sect. 3.2, is closely considered in Sect. 4.5 and in case studies (Chaps. 6 and 7), the influence of the other parameters is examined here with reference to the stack of Fig. 3.5. These effects have already been described from a thermodynamic point of view (see Sect. 3.1), while kinetic implications are considered in this section for their importance in determining the stack efficiency. 

Thus, the optimization of PEMFC performance in terms of efficiency and reliability requires a proper design and management of reactant feeding sections, as well as of cooling and humidification sub-systems [4]. 

The water balance inside FCS requires at least a water condenser located at the outlet of the stack. This device has the dual role to condense a significant part of the water content of the warm and wet cathodic outlet stream (a mixuire of nitrogen, oxygen, and water vapor with the probable presence of small water liquid droplets) for re-cycling in humidification devices, and to transfer/recover a part of significant thermal energy content of the cathode stream. The condenser could also collect, but in minor quantities, the excess water content of the anode compartment. The condenser is then aimed at maintaining the vehicle endurance and to cooperate to efficiency maximization. 

In order to clarify the effectiveness of the external humidification strategy in stack managing, a further experiment is shown in Figs. 6.18 and 6.19. This test is effected in a steady-state condition corresponding to the maximum efficiency of the FCS at 1.2 kW and R — 1.9. Stack current, voltage, and temperature are reported as function of time in Fig. 6.18, while Cy and hydrogen pressure are shown in Fig. 6.19. 

The analysis of the main aspects to be faced in design and realization of a fuel cell system as power source of an electric drive train is described in Chap. 4. Here the problems connected to the choice of auxiliary components, their energy consumption and integration in the overall system are discussed, paying particular attention to the management of membrane humidification, hydrogen purge and air supply as a way to optimize system efficiency and reliability. 

The efficiency of the automotive proton exchange membrane (PEM) fuel cell is dependent on many factors, one of which is the humidification of the inlet air. If the inlet air is not sufficiently humid (saturated), then the stack can develop dry spots in the membrane and efficiency and voltage will drop. Therefore, it is necessary to ensure that humid inlet air at the proper elevated temperature is supplied to the stack. Current methods involve utilizing a spray nozzle to atomize water droplets onto a cloth or wire mesh substrate. As the ambient inlet air passes over the cloth it picks up moisture however, the relative humidity drops as the air is heated in the fuel cell. If heat could be supplied to the water efficiently, the system would become independent of the ambient conditions, the inlet air could become more humid at the proper temperatures, and the overall stack could maintain a high level of efficiency. Previous work with power electronic heat sinks and automotive radiators has demonstrated the high efficiency of carbon foam for heat transfer. Utilizing the carbon foam in the PEM fuel cell may reduce the inlet air humidification problems. 

Air-cooled heat exchangers are employed on large scale as condensers of distillation columns or process coolers. The approach temperature - the difference between process outlet temperature and dry-bulb air temperature - is typically of 8 to 14 °C above the temperature of the four consecutive warmest months. By air-humidification this difference can be reduced to 5 °C. Air cooled heat exchangers are manufactured from finned tubes. Typical ratio of extended to bare tube area is 15 1 to 20 1. Finned tubes are efficient when the heat transfer coefficient outside the tubes is much lower than inside the tubes. The only way to increase the heat transferred on the air-side is to extend the exchange area available. In this way the extended surface offered by fins increases significantly the heat duty. For example, the outside heat transfer coefficient increases from 10-15 W/m K for smooth tubes to 100-150 or more when finned tubes are used. Typical overall heat transfer coefficients are given in Table 16.10. The correction factor Ft for LMTD is about 0.8. 

The fly ash that was used for all experiments came fixim a single homogenous 200 kg batch. This batch showed variation in metal concentiation of up to 15% (sample size 0.Sg). To compensate for this variation a homogenous sample (0.5 g) of untreated ash was taken prior to each extraction and analysed. The variation of those samples was ftien about 5%. An average of the analyses served as a reference for eveiy set of extraction experiments, allowing a more reliable expression of the extraction efficiency. For the studies of wet ashes, homogeneous humidification was desired. The ash was mixed with neutral water (pH=7) prior to SFE, an excess of water was removed after sedimentation, and the ash was dried to a humidity of 38wt.%. 

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