Dew-point surface

From the three distinct 2D cross-sectional views (7.41a), (7.42), (7.43) of the P-T-x surface, we can now visualize the full 3D form of the surface as shown in Fig. 7.8. The surface is seen to resemble a curved envelope, clipped at each end to reveal the inside of the envelope through the hatched holes. Viewed toward the P—T plane, only the curved edge of the envelope is seen, as in (7.41a). However, viewed toward the P-xB plane or the T-xB plane, the inside of the envelope is seen as the hatch marks in (7.42) or (7.43), respectively. The upper P-T-x surface of the envelope is called the bubble-point surface, in reference to the first vapor bubbles that are seen as the liquid is heated to its boiling point. The P-T-xBap underside of the envelope is correspondingly called the dew-point surface, in reference to the first dewy droplets of liquid as the vapor is cooled to its condensation temperature. Although we normally see only the flat P-T, P-xB, or T-xb projections on the blackboard or book page, it is useful to keep in mind the full 3D form of the P-T-xB surface that underlies these 2D projections of the / = 3 system. 

As the pressure is further reduced along line FG, more and more liquid vaporizes until at W the process is complete. Thus W lies on the under surface and represents a state of saturated vapor having the mixture composition. Since W the point at which the last drops of liquid (dew) disappear, it is a dew point, and the lower surface is the dew-point surface. Continued reduction of pressure merely leads into the superheated vapor region. 

The points Ci and are the critical points of pure methane and ethane, respectively. The line connecting these two points, which is the intersection of the bubble point and dew point surfaces, is the critical locus. This is the set of critical points for the various mixtures of methane and ethane. The black curve connecting points A and Ci is the vapor pressure curve of pure methane, and the violet curve connecting points B and C2 is the vapor pressure curve of pure ethane. 

Fig. 5.2-18 Boiling point and dew point surfaces of the system acetone/chloroform/metha-nol at a pressure of 1 bar...

Boihng point and dew point surfaces touch each other only at the pure constituents and at the azeotropes. They do not touch at the ridges and the valleys, i.e., at the boundary distillation lines. Hence, boundary distillation hues do not have exactly the same features as azeotropes have with respect to rectification... 

These azeotropy expressions 2.19 state that in the space of transformed composition variables the bubble-point and dew-point surfaces are tangent at an azeotropic state (Barbosa and Doherty, 1988a), allowing the azeotropes to be found easily by visual inspection in the reactive phase diagram for the case of Uc — rirx < 3 (figure 2.4). For systems beyond this space, a graphical determination of azeotropes might not be feasible. 

Equations of state (ES) may be divided between those that are analytic and those that are not. Analytic equations of the form P(p,T,[Zi]) cannot provide an accurate description of thermodynamic properties in the critical region whether for the pure components or their mixtures. Scaled ES are non-analytlc in the usual P (p,T) coordinates but assume analyticlty in y(p,T) for pure components. The choice of variables for a scaled ES for a mixture is not well-defined although Leung and Griffiths (1 ) have used P(T,[uil) with success on the 3He- He system. Phase diagrams are simplier in such coordinates as the bubble-point surface and dew-point surface collapse into a single sheet. 

Gas is produced to surface separators which are used to extract the heavier ends of the mixture (typically the components). The dry gas is then compressed and reinjected into the reservoir to maintain the pressure above the dew point. As the recycling progresses the reservoir composition becomes leaner (less heavy components), until eventually it is not economic to separate and compress the dry gas, at which point the reservoir pressure is blown down as for a wet gas reservoir. The sales profile for a recycling scheme consists of early sales of condensate liquids and delayed sale of gas. An alternative method of keeping the reservoir above the dew point but avoiding the deferred gas sales is by water injection. 

Dehumidification. Dehumidification may be accompHshed in several ways (see Drying). Moderate changes in humidity can be made by exposing the air stream to a surface whose temperature is below the dew point of the air. The air is cooled and releases a portion of its moisture. Closed cycle air conditioning systems normally effect dehumidification also. The cooled air may require reheating to attain the desired dry-bulb temperature if there is insufficient sensible load in the space. 

Compounds having low vapor pressures at room temperature are treated in water-cooled or air-cooled condensers, but more volatile materials often requite two-stage condensation, usually water cooling followed by refrigeration. Minimising noncondensable gases reduces the need to cool to extremely low dew points. Partial condensation may suffice if the carrier gas can be recycled to the process. Condensation can be especially helpful for primary recovery before another method such as adsorption or gas incineration. Both surface condensers, often of the finned coil type, and direct-contact condensers are used. Direct-contact condensers usually atomize a cooled, recirculated, low vapor pressure Hquid such as water into the gas. The recycle hquid is often cooled in an external exchanger. 

Dew-Point Method For many applications, the dew point is the desired moisture measurement. VHien concentration is desired, the relation between water content and dew point is well-known and available. The dew-point method requires an inert surface whose temperature can be adjusted and measured, a sample gas stream flowing past the surface, a manipulated variable for adjusting the surface temperature to the dew point, and a means of detecting the onset of con-densation. 

Dew-Point Method The dew point of wet air is measured directly by observing the temperature at which moisture begins to form on an artificially cooled polished surface. The polished surface is usually cooled by evaporation of a low-boihng solvent such as ether, by vaporization of a condensed permanent gas such as carbon dioxide or liquid air, or by a temperature-regulated stream of water. 

Although the dew-point method may be considered a fundamental technique for determining humidity several uncertainties occur in its use. It is not always possible to measure precisely the temperature of the polished surface or to eliminate gradients across the surface. It is also difficult to detect the appearance or disappearance of fog the usual practice is to take the dew point as the average of the temperatures when fog first appears on cooling and disappears on heating. 

Low humidity also affects comfort and health. Comfort complaints about dry nose, throat, eyes, and skin occur in low-humidity conditions, typically when the dew point is less than 0 °C. Low humidity can lead to drying of the skin and mucous surfaces. On respiratory surfaces, drying can concentrate mucus to the extent that ciliary clearance and phagocytic activities are re-... 

By allowing moisture-laden, relatively warm gas to come into intimate contact with a cold surface which is below the dew point of the gas, moisture is condensed from the gas. 

Acid dew point The temperature at which a vapor containing an acid appears as condensate on a cool surface, causing corrosion. 

Acidic smuts Solid and liquid conglomerates formed by the condensation of water vapor and sulfur trioxide on a cold surface. A typical case is combustion products in a flue, which come into contact with surfaces at temperatures below the flue gas dew point temperature. These products contain metallic sulfate and carbon aqueous particles approximately 1-3 mm in size. 

Apparatus dew point For practical purposes, the average temperature of a cooling coil surface. 

Dew point, acid The temperature at which acid vapor in a gas stream condenses out of the flow onto a cold surface or in a cold gas stream. 

After each reaction stage, sulfur is removed by condensation so that it does not collect on the catalyst. The temperature in the catalytic converter should be kept over the dew point of sulfur to prevent condensation on the catalyst surface, which reduces activity. 

Moisture precipitation Apart from wetting by sea-spray, moisture may either be deposited on a surface by rainfall or dew formation. For a known ambient humidity the dew point can be calculated, using the expression given previously, from standard tables giving the saturated vapour pressure of... 

If air at 21°C dry bulb, 50% saturation, is brought into contact with a surface at 12°C, it will give up some of its heat by convection. The cold surface is warmer than the dew point, so no condensation will take place, and cooling will be sensible only (Figure 24.3). 

A similar effect occurs if the air is brought into contact with a solid surface, maintained at a temperature below its dew point. Sensible heat will be transferred to the surface by convection and condensation of water vapour will take place at the same time. Both the sensible and latent heats must be conducted through the solid and removed. The simplest form is a metal tube, and the heat is carried away by refrigerant or a chilled fluid within the pipes. This coolant must be colder than the tube surface to transfer the heat inwards through the metal. 

The process is indicated on the chart in Figure 24.9, taking point B as the tube temperature. Since this would be the ultimate dew point temperature of the air for an infinitely sized coil, the point B is termed the apparatus dew point (ADP). In practice, the cooling element will be made of tubes, probably with extended outer surface in the form of fins (see Figure 7.3). Heat transfer from the air to the coolant will vary with the fin height from the tube wall, the materials, and any changes in the coolant temperature which may not be constant. The average coolant temperature will be at some lower point D, and the temperature difference B — D will be a function of the conductivity of the coil. As air at condition A enters the coil, a thin layer will come into contact with the fin surface and will be cooled to B. It will then mix with the remainder of the air between the fins, so that the line AB is a mix line. 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

In order to obtain a high rate of humidification, the area of contact between the air and the water is made as large as possible by supplying the water in the form of a fine spray alternatively, the interfacial area is increased by using a packed column. Evaporation occurs if the humidity at the surface is greater than that in the bulk of the air that is, if the temperature of the water is above the dew point of the air. 

Dehumidification of air can be effected by bringing it into contact with a cold surface, either liquid or solid. If the temperature of the surface is lower than the dew point of the gas, condensation takes place and the temperature of the gas falls. The temperature of the surface tends to rise because of the transfer of latent and sensible heat from the air. It would be expected that the air would cool at constant humidity until the dew point was reached, and that subsequent cooling would be accompanied by condensation. It is found, in practice, that this occurs only when the air is well mixed. Normally the temperature and humidity are reduced simultaneously throughout the whole of the process. The air in contact with the surface is cooled below its dew point, and condensation of vapour therefore occurs before the more distant air has time to cool. Where the gas stream is cooled by cold water, countercurrent flow should be employed because the temperature of the water and air are changing in opposite directions. 

Measurements at abrasive surface temperatures between the dew point and 70°C... 

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