Dew-point depression

Anhydrous calcium chloride absorbs water to a capacity of 3.5 kg/kg of calcium chloride and forms a nonreuseable brine. This technique is best suited for remote appHcations where modest dew point depressions are required and gas processing volumes are small. 

Glycerol, Glycol, and Other Polyhydnc Alcohols. These alcohols (Class 3, regenerative) are widely used to dry gases (20,21). However, they can only produce dew points in the range of —15 to 0°C (see Antifreezes and deicing fluids). They have somewhat lower capacity than sulfuric acid (Fig. 2) but are effective when either injected or employed in a multistage contactor to achieve dew point depression. 

Dew point depression The difference in temperature between the wet and dry bulb readings. 

To keep water from condensing as the gas is processed, it is necessary to dehydrate the gas (that is, remove water vapor) until the amount of water vapor remaining in the gas is less than that required to fully saturate the gas at all conditions of temperature and pressure. Since the dehydrated gas will have a lower dew point, dehydration is sometimes called dew point depression. For example, if the amount of water vapor in the 3,000 psig gas stream referred to earlier were reduced from 105 Ib/MMscf to 50 Ib/MMscf, the dew point would be reduced from 150°F to 127°F. That is, its dew point will be depressed by 23°F. 

I riethylene glycol—Most common. Reconcentrate at 340 F to 4()0°F for high purity. At contactor temperatures in excess ot 1 2(1 F tends to have high vapor Ilis.scs to gas. Dew point depressions up to 150 F are possible with stripping gas. 

The temperature of the lean glycol entering the contactor has an el tec on the gas dew-point depression and should be held low to minimi/c required crrculation rate. High glycol losses to the gas exiting the contac... 

Figure 8-8. The number of trays can have a greater effect on dew-point depression than the circulotion rate.

This is defined as the percentage ratio of the water vapour pressure in the atmosphere compared to that which would saturate the atmosphere at the same temperature. Alternatively, the difference in temperature between the ambient atmosphere and that to which it would have to be cooled before moisture condensed from it, is also used as a measure of moisture content. This difference in temperature is called the dew point depression. The actual temperature at which condensation takes place is known as the dew point. The relative humidity is then expressed as ... 

Fig. 2.13 Dew point depression below ambient temperature as a function of the relative humidity of the ambient atmosphere over a range of temperature...

Favor glycol absorption for large-scale desiccation operations requiring dew point depressions of <28° C (50°F) initial and operating costs of high volume glycol absorbers are typically much lower for small—medium dew point depressions than the corresponding costs of solid-phase desiccation. 

However, it is now recommended that thermometers be calibrated in °C. More recently, thermistors have been used instead of mercury thermometers. Cryoscopes based on dew point depression have also been approved for use. These latter instruments also use thermistors and are based on changes in osmotic pressure. Thermistor cryoscopes are now used more widely than Hortvet instruments. 

Favor glycol absorption for large-scale desiccation operations requiring dew-point depression of 27°C or less. 

Favor adsorption for processes that require essentially complete removal of water vapor. This is capable of achieving dew-point depression of more than 44 °C. 

DEW POINT DEPRESSION - The difference between dry bulb and dew point temperatures. 

The relative humidity of a water vapor-air mixture is defined as 100 times the partial pressure of water divided by the saturation vapor pressure of water at the same temperature. The relative humidity may be determined from the dew point which is the temperature at which liquid water first condenses when the mixture is cooled from an initial temperature t. This table gives relative humidity as a function of the dew point depression... 

Figures 11-14 and 11-15, which are based on the work of Polderman (1957) and Parrish et al. (1986), present data on the water dew points of gases in equilibrium with diethylene and triethylene glycol solutions at various temperatures. The TEG curves are based on the Parrish et al. data rather than the widely used Worley (1967) data because the Parrish et al. data cover a wider range of dew-point depressions and TEG concentrations are thermodynamically consistent and generally result in more conservative designs.

Figim 11-29. Predicted dew-point depression for TEG at 100°F and 600 psia, one equilibrium stage. Manning and Wood, 1991)... 

Rflure 11-34. Calculated dew-point depression vs. circulation rate for triethylene glycol sohrtkms in a cocurrent contactor basis 96S approach to equilRiriiJiii. From Baker md Hagers (198S)... 

The reboiler temperatures of plants listed in Table 11-5 are well below the recommended maximums however, higher temperatures are often used. Worley (1967) reported that up to 10 years experience with several hundred TEG units utilizing a reboiler temperature of 4(X)°F has failed to indicate any evidence of measurable losses by degradation. Dew-point depressions in excess of 100°F are reported for these units. A dew point of-9S°F is reported by Smith and Skiff (1990) for a plant employing only 1.5 gal TEG/lb water removed. This represents a dew-point depression of at least 150°F (assuming a minimum contact temperature of 55°F). and was attained by using the Drizo process to regenerate the TEG to 99.999% concentration (10 ppm H2O). 

Inability to maintain proper dew-point depression with the existing glycol charge... 

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