Humidification process

Humidification processes also occur in spray contactors often used to scmb minor components from a gas stream. Here the gas passes through successive sprays of hquid. The hquid is often water but may be specially compounded to enhance absorption of the component to be removed. 

Good bed drainage characteristics are necessary to ensure that reaction products are easily transported out of the filter media. The leachate is generally recycled through the humidification process to reduce the wastewater stream. 

Many different types of humidifiers are in common use the humidification process is simply achieved by adding moisture into the air to be conditioned. 

In this type of humidifier, the water required for the humidification process is stored within the unit and is normally fed automatically from the water mains. However, smaller portable units do not have this automatic facility and have to be manually filled with water as required. 

The reverse of the humidification process is that of dehumidification. In this process the water content of air, gases, or fluids is reduced. 

The solar humidification process is represented schematically in Fig. 6. This process generally imitates a part ol the natural hydrologic cycle in that the saline water is healed by the sun s rays to enhance vapor production. 

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. 

Since 1904, maintaining the climatic status quo in the tomb of Nefertari has proved difficult, as has been described in previous sections. In addition to the humidification processes occurring in the tomb, well-meaning plaster repairs are a major source of additional water. Threatened murals have been fortified periodically with new plasters, the most recent repairs having been made in 1983. By March 1984, the humidity had increased considerably to an estimated 50 10% RH compared with 31 1% RH in December 1977 (28) and 16 1% RH in March 1981. 

The relative humidity of air at 27"C and 1 atm is increased from 30 percent to 90 percent during a humidification process at constant lemperntuie and pressure. Determine the percent enor involved In assuming the density of air to have remained constant Answer 2.1 percent... 

CONTINUOUS HUMIDIFICATION PROCESSES 10.5A Introduction and Types of Equipment for Humidification... 

Secondly, the humidification process involves evaporating water in the incoming gas. This will cool the gas, as the energy to make the water evaporate will come from the air. In pressurised systems this is positively helpful. It will be seen in Chapter 9, when compressors are considered, that a result of compressing the air is considerable heating, and the humidification process can be an ideal way of bringing the air temperature to a suitable value. 

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. 

The difference between these two figures, that is, 0.0557 — 0.0392 = 0.0165kg s is an estimate of the rate at which water must enter the cell. Some of this water would have been in the air anyway, while the rest is added by the humidification process. If the humidity of the entry air is about 70%, then it can be shown that approximately two-thirds of the water is added via humidification. This example illustrates the beneficial effect of... 

The exact amount of water may be dosed by a metering pump. It is important that water is injected in the form of fine mist, so that a large contact area between water and air facilitates evaporation. However, simple injection of liquid water in the gas stream may not be sufficient to actually humidify the gas, because humidification also requires heat for evaporation. The enthalpy of water, even if the water is hot, is usually not sufficient and additional heat is required. The sources of heat may be the air compressor (obviously applicable only in pressurized systems) and the fuel cell stack itself. In most of the operating conditions, there is sufficient amount of heat generated in the fuel cell stack. It is the duty of the system to transfer a portion of that heat to the humidification process (Figure 9-20b). 

In addition to taking the heat away from the fuel cell stack, the cooling loop also cools down the reformate gas coming from the fuel processor and provides heat, if needed, in the air humidification process. Moreover, temperature control of the preferential oxidation process is needed (as shown in Figures 9-37 and 9-38). The heat is then rejected from the system to the environment by the heat exchanger, typically of a radiator type. Some heat may be rejected from the system in the condenser just before the exhaust gases leave the system. The condenser is needed to condense and save the water in the exhaust to maintain a neutral water balance in the system. 

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