Vapor pressure control

Vapor pressure control is normally used when satisfactory pressure compensation is difficult to achieve or when product vapor pressure is 

A popular vapor pressure transmitter is Foxboro s differential vapor pressure (DVP) cell (Fig. 18.9). A bulb filled with a reference liquid is inserted in the column and is connected to one end of a differential pressure transmitter. The other end of the transmitter is connected directly to the column in the same elevation as the bulb. The reference liquid is selected so that it has the same vapor pressure as that on the tray, and is often a sample of the desired tray composition. The same vapor pressure in the bulb as on the tray signals a satisfactory tray composition. A rise in tray vapor pressure compared to the reference liquid signals an excessive presence of lights a fall in tray vapor pressure compared to the reference liquid signals depletion of lights. 

The vapor pressxire controller has a far greater sensitivity to composition changes than a temperature controller (68, 332, 362), gives fast response, provides an accurate measurement in binary systems, and is relatively inexpensive. This technique is popular in services where pressure compensation of the control temperature is needed and is difficult to achieve, such as low-pressure (particularly vacuum) distillation and close separations. A typical example is ethanol-water columns (361). The technique is also useful where a good sensitivity of control temperature to composition and good correlation between product composition and control temperature are difficult to meet simultaneously (Sec. 18.2). 

The DVP cell is generally unsuitable for controlling Reid vapor pressure (RVP) of petroleum products, because the RVP is measured at 100°F. This temperature is usually about 200 F lower than the column temperature, resulting in poor correlation between the tray va- 

Ftgura 18.9 The Foxboro differential vapor pressure (DVP) cell (From F. G. Shinskey, Distillation Control, second edition. Copyright by McGraw-HUl, Inc. Reprinted by permission.) 

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The mechanisms that control dmg deUvery from pumps may be classified as vapor-pressure, electromechanical, or elastomeric. The vapor-pressure controlled implantable system depends on the principle that at a given temperature, a Hquid ia equiUbrium with its vapor phase produces a constant pressure that is iadependent of the enclosing volume. The two-chamber system contains iafusate ia a flexible beUows-type reservoir and the Hquid power source ia a separate chamber (142). The vapor pressure compresses the dmg reservoir causiag dmg release at a constant rate. Dmg maybe added to the reservoir percutaneously via a septum, compressing the fluid vapor iato the Hquid state. 

By limiting the amount of hydrocarbons that are lower boiling than the main component, the vapor pressure control is reinforced. Tests are available for vapor pressnre 100°F (38°C) (ASTM D1267) and at 113°F (45°C) (IP 161). The limitation on the amonnt of higher-boiling hydrocarbons supports the volatility clause. The vapor pressure and volatility specifications will often be met automatically if the hydrocarbon composition is correct. 

C and D by the direct return of uhcooled first stage reooopresslon vapors from 1, and the use of a heat exchanger (Indicated as H) to heat the liquids from 2 which are pumped to C. The gas conditioning unit A serves as the reflux condenser for this fractionation system. Two vapor pressure controls are available, the first stage pressure (on 1 and D), and the temperature of C and D which can be controlled, within limits, by adjusting the heat input, H. 

Figure 3.12. Fractionator for separating ethylene and ethane with a refrigerated condenser. FC on feed, reflux, and steam supply. LC on bottom product and refrigerant vapor. Pressure control PC on overhead vapor product.

Hi) Reduced Releases, Product Recovery. Vapors that remain in the tank can be recovered as product, provided their recovery does not violate product vapor pressure limits. Since the more volatile compounds typically concentrate in emissions, their recovery may require removing some butane, a light hydrocarbon normally added to gasoline for vapor pressure control. Where butane can be sold or consumed, recovery should not present a serious problem. But in some refineries, excess butane is a low-valued product and cannot be sold economically. 

By limiting the amount of hydrocarbons that are lower boUing than the main component, the vapor pressure control is reinforced. Tests are avail-... 

The hydrocarbon yields shown represent those expected averaged over the useful life of the ZSM-5 catalyst. Finished gasoline contains C4 s for vapor pressure control. For an 82.7 kPa (12 psi) RVP (Reid Vapor Pressure) finished gasoline, the yield is 86 wt% of hydrocarbons and the clear Research octane number is 93. Additional gasoline could be made by alkylating the propene and butenes produced with isobutane. As the amount of alkylate would be low, its manufacture would most likely be considered only for very large plants. 

Vapor pressure control Here we can have loops that react quite fast or are relatively slow. Consider, for example, the two configurations shown in Figure 16.5. The loop in Figure 16.5a measures the pressure and manipulates the flow of vapor, thus affecting directly and... 

Figure 4-25. General schematic of InP vapor pressure controlled Czochrasiki boule growth system.

For chemical effects in the surface water, the only realistic possibility is to renew the water from time to time. In the case of a cell with a membrane film cover, this requires movement of the membrane and flushing, which probably will require a retuning of the sample position. For the closed chamber vapor-pressure control approach, the constant re-equilibrium of the thin layer of water on the surface should be sufficient if the vapor volume is satisfactorily large compared to the surface water volume. Beam effects on surface species themselves have also been observed, with reduction of both metals and anion complexes. If samples are studied ex situ in air, ozone production can cause oxidation. These effects will be most severe with third generation sources, and need to be taken quite seriously in the design of experiments. 

The three remaining streams, ethanol, water, and fusel oil, are treated as prominent products. The prime product specs are alcohol in water and water in alcohol these are composition-controlled (in this case, using vapor pressure controllers, see Sec. 18.9). The flywheel in this system is the fusel oil stream, and its ethanol content is allowed to vary somewhat. This bears a much lower economic penalty than allowing alcohol to escape in the water or permitting water to dilute the alcohol. In this specific case, the fusel oil is cooled and decanted, and the water phase returned to the column. The column in Fig. 19.9 can be viewed as two merged columns—the top section controlled using scheme 16.4d, the bottom using scheme 16.4a. 

Normal butane for gasoline-vapor pressure control... 

Fig. 3.3 Principle of vapor-pressure-controlled Czochralski (VCz) method without boric oxide encapsulation in order to control the melt composition and, hence, the solid stoichiometry of GaAs crystals by the partial...

Isotherms and isobars are drawn from thermal gravimetric measurements using a Me Bain balance well suitable to impose water vapor pressures controlled by means of a "cold point" (ref. 16). The curves are constructed in graduated steps by increasing (or decreasing) pressure or temperature in small successive increments. Before each experiment, the zeolite is activated in situ at 400°C at 10 1 Pa. For adsorption the initial state is the activated state at 350 C, either at a pressure p under isobaric conditions, or at a pressure of 101 Pa under isothermal conditions. The final state is a state close to saturation. For desorption measurements the previous defined initial and final states are reverse. 

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