Differential vapor pressure cell

The Foxboro Company Differential Vapor Pressure Cell Transmitter, Type 13VA, Technical Information Sheet 37-91a. 

Another early effort to compensate for pressure variations in binary distillations resulted in the development of the differential vapor pressure cell. The version made by Foxboro is known as the DVP Cell. As shown in Figure 10.1, a bulb filled with liquid whose composition is the same as that desired on a particular tray is installed on that tray. It is connected to one side of a differential-pressure transmitter. The other side of the AP transmitter is connected direcdy to the same tray. When the liquid on the tray has the same composition as the liquid in the bulb, pressure in the bulb will be the same as pressure on the tray. For that condition the AP transmitter is normally set to read midscale. Deviations in the tray composition from that in the bulb are then reflected by AP transmitter output signals above or below midscale. 

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. 

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.)... 

The problem was recognized many years ago and solutions were proposed and applied. One solution discussed by Shinskey is to use a differential vapor-pressure transmitter. This device is a differential-pressure cell with one side of the diaphragm open to pressure of the column at the control tray and the other side connected to a bulb inserted on the same tray. The bulb contains hquid with a composition the same as the desired composition on the tray. A zero differential pressure means that the composition on the tray is equal to the desired composition. 

Valedyne differential pressure cell. 91 vapor phase hydrogenation... 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Gas formation volume factors are calculated with z-factors measured with the gases removed from the cell at each pressure step during differential vaporization. Equation 6-2 is used. Usually Bg values as calculated are listed in the report. 

Figure 9.4 Left Adsorption isotherm for benzene (CeLL) adsorbing to graphitized thermal blacks at 20° C. The insert shows the adsorption isotherm for low coverages in more detail. Dotted lines indicate mono- or multilayer coverages at multiples of 4.12 //mol/m2. The equilibrium vapor pressure of benzene at 20°C is Po = 10.2 kPa. Right Differential heat of adsorption versus adsorbed amount. The dashed line corresponds to the heat of condensation of bulk benzene. Redrawn after Ref. [369].

SIMS. Secondary Ion Mass Spectrometry is particularly suited for ionization of nonvolatile, polar, and thermally labile molecules. Liquid SIMS, using liquid glycerol matrices, is best done in the differentially-pumped external ion source, because matrix effects and the high vapor pressure of glycerol make liquid SIMS unsuitable for single cell low-pressure FTMS. 

The temperature in a vacuum crystallizer is normally controlled by an absolute pressure recorder controller that purges air into the vacuum system or bleeds off vent gas from a condenser if the vessel is operated above atmospheric pressure. It should be capable of maintaining the temperature in the vessel to within 1 /2 C of the set point. Typically, this is done through an absolute differential pressure cell mounted on top of the vessel so that drainage can be back into the vapor space and the control signal is transmitted to a remote recorder controller. 

Humidity-induced pressurization is the result of vapor pressure differential between the leaf and atmosphere separated by a porous partition (plant cell membrane) (Figure 7.16). The total pressure will be greater on the more humid side. Humidity-induced diffusion is more important than thermal transpiration because it can be increased with temperature and can function at a constant temperature, as well as across temperature gradient (Armstrong et al., 1991a, 1991b). 

The existence of this vapor pressure differential will cause water to leave the cell at a... 

Both the water manometer and the differential pressure cell exhibited fluctuations of approximately 0.2 to 0,4 inches of water as internal vapor formation and collapse in the tank and the momentary egress of liquid in the warm pressure sensing lines produced an unstable pressure differential. In investigating these fluctuations, we found that ... 

Fig. 340. Preparation of active metal oxides by oxidation of metal vapor, a funnel for addition of metal b observation port c side port d circular nozzle for air intake e first chamber with lateral observation ports (these are not shown) f illuminating device g glass tubes (the remaining parts of the apparatus are made from sheet iron) h carbon electrodes i flow meter activated by differential pressure n precipitation cell s movable carbon electrode.

Samples in the gaseous or vapor state require cells with x-ray-transparent windows that can withstand high pressure-differentials. Some type of pressure regu-... 

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