FIG 12.12. Enough solvent must be supplied to ex tract the heavy component from the feed. in Visual Studio .NET

Generator QR Code 2d barcode in Visual Studio .NET FIG 12.12. Enough solvent must be supplied to ex tract the heavy component from the feed.

FIG 12.12. Enough solvent must be supplied to ex tract the heavy component from the feed.
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Other Mass Transfer Operations
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both; if too little is withdrawn, some of the lighter product will be carried into the second tower. As solvent is introduced above the feed tray, the separation beyond that point is principally between solvent and the light component, which does not require many trays. A large number of trays is required below, however, to sufIicient,ly reduce the content of the light component in the ternary bott,om mixkrre. Separation in the second tower is easy, if the solvent is not especially volatile.
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DRYING OPERATIONS
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The drying of solids has historically defied control principally because a continuous measurement of product moisture has been lacking. A n y kind of measurement on solids-even flow rate-is fraught with problems, but on-line analyt,ical determinations are virt ually impossible. Consequently environment~al measurement,s must be relied upon, but their successful employment hinges entirely on how capably they represent the true state of the process.
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The Rate of Drying
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Drying is similar in many ways t o other mass transfer operations, particularly to humidification. If the surface of a solid is completely covered wit h liquid, the rate of its evaporation is controlled by the same mechanism as humidification. In order for equilibrium to exist under this condition, the gas must be 100 percent saturated with moisture, because the solid is. If t,he surface is free of this LLunbound moisture, however, the moisture content will vary with the relative humidity of the surrounding gas, at equilibrium. Figure 12.13 shows an equilibrium curve for a typical solid. l\Iass transfer between a solid and a gas is slow, particularly if agitation is lacking, making the approach to equilibrium very gradual. As a con-
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FIG 12.13. The equilibrium moisture content of most solids varies with relative humidity.
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Moisture content of solid
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1 Applications
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sequence, for any significant, rate of drying to t ake place, there must be a sizable depart urc from cquilihrium. In t,hc presence of unbound moisture, the rat,e of drying is constant, if the relative humidity of the gas is maintained constant. As bound moisture is removed, however, the rate of drying falls, approaching zero as equilibrium is approached. Again these examples apply only to a condition of const ant humiditjy, which does not exist in most dryers. In a contJinuous dryer, the welt solids t,rnvcl horizontally in contact with a moving stream of gas. If (UK> gas t~ravels cou~lfcrcurrcnt,ly to the solids, the hottest gas will cnwunter the driest solids, which tends to distribute the rate of evaporation somewhat, cvcnly through the dryer. But if the product is sensitive to high temperatures, cocurrent gas flow is safer, in that t he wet solids arc less likely to bc damaged by hot gas. Food products or fine chemicals are ordinarily dried by air which is steam hent,ed. Heavy chemicals are more often dried by direct exposure t,o a burning fuel gas. Evaporation of moisture from a solid requires the application of heat, sufficient to convert it, from the solid to the vapor state. I f external heat is not applied, the tjcmpcrature of both solid and gas will fall, increasing the relative humidity and thus relarding drying. Enormous quantities of heat arc found necessary so that, the product, may leave t hc dryer at a much higher tcmpcrature t,han it, ent ered. IYgain, because of low rates of mass transfer, t,he gas leaving is also quite hot. As the gas travels from inlet to outlet,, its temperat ure falls and its humidity increases. As long as the temperature of the gas exceeds the boiling point of the evaporating liquid, however, its relative humidity contributes little to the driving force. Instead, the rate of drying in this region is principally determined by the rate of heat) transfer between the two phases.5 As a result, t his part of t,he dryer can be compared to a fired boiler. Where the gas temperature is belo\v the boiling point of the evaporating liquid, humidity assumes a controlling posit ion. It may be recalled from the section on humidification, that in an adiabatic syst,em the difference between dry-bulb and wet-bulb tcmperat<ure is a measure of the rate of evaporation. EquntJion (12.12) is act,ually formulated from heat and mass transfer relationships at the surface of an evaporating liquid. Therefore, as the tcmpcrature difference between gas and solid is proportional to the driving force above the boiling point, dry-bulb minus wet-bulb temperat ure assumes the same role below the boiling point. The velocity of the gas naturally affects mass transfer, by reducing t hc resistance of the film at the surface of the solids. But more significantly, increasing flow reduces the temperature gradient of the gas through the dryer, thereby increasing the net driving force. Since increasing gas
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