Water humidification operations

For the air—water system, the Lewis relation shows that r = 1. Under these conditions, the two parenthetical terms on the right-hand side of equation 33 ate enthalpies, and equation 33 becomes the design equation for humidification operations ... 

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. 

In gas absorption a soluble vapor is absorbed by means of a liquid in which the solute gas is more or less soluble, from its mixture with an inert gas. The washing of ammonia from a mixture of ammonia and air by means of liquid water is a typical example. The solute is subsequently recovered from the liquid by distillation, and the absorbing liquid can be either discarded or reused. When a solute is transferred from the solvent liquid to the gas phase, the operation is known as desorption or stripping. In dehumidification a pure liquid is partially removed from an inert or carrier gas by condensation. Usually the carrier gas is virtually insoluble in the liquid. Removal of water vapor from air by condensation on a cold surface and the condensation of an organic vapor such as carbon tetrachloride out of a stream of nitrogen are examples of dehumidification. In humidification operations the direction of transfer is from the liquid to the gas phase. In the drying of solids, a liquid, usually water, is separated by the use of hot, dry gas (usually air) and so is coupled with the humidification of the gas phase. 

DEFINITIONS. In humidification operations, especially as applied to the system air-water, a number of rather special definitions are in common use. The usual basis for engineering calculations is a unit mass of vapor-free gas, where vapor means the gaseous form of the component that is also present as liquid and gas is the component present only in gaseous form. In this discussion a basis of a unit mass of vapor-free gas is used. In the gas phase the vapor will be referred to as component A and the fixed gas as component B. Because the properties of a gas-vapor mixture vary with total pressure, the pressure must be fixed. Unless otherwise specified, a total pressure of 1 atm is assumed. Also, it is assumed that mixtures of gas and vapor follow the ideal-gas laws. 

In humidification or dehumidification (depending upon the direction of transfer) the liquid phase is a pure liquid containing but one component while the gas phase contains two or more substances. Usually the inert or carrier gas is virtually insoluble in the liquid. Removal of water vapor from air by condensation on a cold surface and the condensation of an organic vapor such as carbon tetrachloride out of a stream of nitrogen are examples of dehumidification. In humidification operations the direction of transfer is from the liquid to the gas phase. 

It has been observed that for the system air-water vapor at near-ambient conditions, Le = 1.0 (Treybal, 1980). This observation, called the Lewis rela -tion, has profound implications in humidification operations, as will be seen later. Based on the Lewis relation, estimate the diffusivity of water vapor in air at 300 K and 1 atm. Compare your result with the value predicted by the Wilke-Lee equation. For air at 300 K and 1 atm C = 1.01 kJ/kg-K, k = 0.0262 W/m-K, x = 1.846 x 10-5 kg/m-s, and p = 1.18 kg/m3. 

In order to remove the excess water, an operator can either lower the humidification temperature, increase the stack outlet temperature, or increase the air flow rate. For example, for the above case, if the dry gas flow rate is increased to 1000 L min 1,116 L min i water vapor will be taken into the stack (1000 x 0.104/0.896) from humidification, leading the total water vapor at the stack cathode to 185.6 L min (116 + 69.6). The total air leaving the stack will be 965.2 L min taking away water vapor at 197.7 L min" ... 

After approximately 10 years of development, PBI chemistries and the concomitant manufacturing processes have evolved to produce commercially available MEAs. PBI MEAs can operate rehably without complex water humidification hardware and are able to run at elevated temperatures of 120-180 C due to the physical and chemical robustness of PBI membranes. These higher temperatures improve the electrode kinetics and conductivity of the MEAs, simplify the water and thermal management of the systems, and significantly increase their tolerance to fuel impurities. Membranes cast by a newly developed PPA Process possessed excellent mechanical properties, higher PA/PBI ratios, and enhanced proton conductivities as compared to previous methods of membrane preparation. 

All the systems described above require liquid water to operate. This means that the exit air must be treated so that a good deal of the water content is condensed out as liquid water, stored, and then pumped to where it is needed for the humidification process. This clearly adds to the system size, cost, and complexity. There are some systems that use the water in the exit gas to humidify the inlet air, but without actually condensing it out as liquid water. One way to do this is to use a rotating piece of water-absorbing material. It is put into the path of the exit air, and it absorbs water. It is then rotated so that it is in the path of the dry entry air, which will at least partly dry it out. If the piece is made circular this can be done on a continuous basis - constantly transferring water from exit to entry gases. This method has the disadvantage that it will still be fairly bulky and will require power and control to operate. 

Packed columns are used primarily in gas absorption and liquid extraction and in air-water contact operations such as humidification and water cooling, which we take up in Chapter 9. They are found less frequently in distillation operations where their use is confined mostly to small-scale processes involving high-efficiency packing. 

The situation is different when the use in DMFC is concerned. Here, one of the main issues is low methanol crossover, and, related to this, low swelling of the membrane in methanol/water mixtures. In this field, membranes based on aromatic polymers have an advantage over Nation due to their generally low methanol crossover rate. In addition, the chemical structure of hydrocarbon polymers can be adjusted relatively easily (at least in the lab), allowing for optimization of swelling in methanol/water mixtures. Humidification, operation under dry conditions, and freezing are not as much of a problem, since methanol/water mixtures can be used as fuel. If the intended use is for portable electronic devices, cost per kW power is less critical (compare your laptop battery it delivers probably approximately 20 W for 3 h at a price of 150, amounting to 7500 /kW). The system complexity and, hence, the size are much more of a problem. 

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. 

A Simple example will clarify the point. Consider wintertime operation, having all inside air and heavy evaporation in order to control conditions. Contrast this with summertime operation having a large fraction of outside air with outside environmental conditions such that little humidification is required. (Furthermore, solids levels in the humidifier water tend to be higher in winter than summer, in some cases.)... 

The utilities check is used to verify that the utility supplied fulfills the chamber requirements as specified by the manufacturer. During IQ, the as found parameters are verified against the as specified parameters on the checklist. If the as found results are significantly different from the as specified parameters, it will be necessary to determine the cause of this discrepancy and to implement corrective actions. The utilities check is a part of the site preparation for the installation of the chamber and should include verification of the power supply, such as the voltage, amperage, and wire size and the quality, pressure, and flow rate of the feed water supply. The quality of the feed water supply is a critical component of proper operation of the chamber. Experience has shown that if the quality of the feed water does not meet the manufacturer s specifications, this will lead to premature corrosion within the humidification and/or dehumidification system and subsequent problems with maintaining the humidity in the chamber within the specified limits. 

The key properties of mixtures of air and water vapor are described in Section 9.1. Here the interactions of air and water in packed towers under steady flow conditions will be analyzed. The primary objectives of such operations may be to humidify or dehumidify the ait as needed for particular drying processes or other processes, or to cool process water used for heat transfer elsewhere in the plant. Humidification-dehumidification usually is accomplished in spray towers, whereas cooling towers almost invariably are filled with seme type of packing of open structure to improve contacting but with minimum pressure drop of air. 

The use of these relationships in constructing and applying humidity charts is best illustrated by examining a simplified case, that of adiabatic cooling or humidification. Figure 5.4 illustrates this process between air and water that is recycled through the cooling tower. In this operation air is both cooled and... 

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