Vaporizers float valves

A float valve is provided to control the supply of makeup water to replace the water vapor that has flashed off, Tne flash tank should be insulated. 

Various types of valve arrangements are used for the valve-tray towers. Some of these are liltable disk-type valves which come as a cap with each riser. As the vapor rate increases, the valve lifts to provide a larger opening until it reaches a limit point which still directs the vapors horizontally into the liquid. Float-valve trays are designed to operate on the same principle as the liltable disk-type valves except that a floating pivotal valve is used hi each cap. 

Manufacturers of valve trays, such as Koch-Glitsch, Inc., of Wichita, Kansas (Ballast trays and Flexitrays), and Sulzer-Chemtech (formerly Nutter Engineering Co.), of Tulsa, Oklahoma (Float Valve Trays), have prepared proprietary design manuals. Hence, only limited discussion will be given here. As for bubble-cap trays, design methods follow those for sieve trays. The vapor capacity chart (Figure 12.29) covers valve trays, as does the alternate method of Kister and Haas. Information on liquid entrainment is proprietary, but measurements have been made by Fractionation Research, Inc. Because of the vapor flow reversal, one would not expect entrainment from valve trays to be greater than that from sieve trays. Liquid capacity considerations follow exactly those for sieve trays. 

Since some of the reagents and products have both a relatively high vapor pressure and a high toxicity, a standard high-vacuum system is recommended for carrying out the reactions and separating the products. Because some of the reactants and products attack mercury on long exposure, it is desirable to avoid mercury float valves and to protect manometers with stopcocks until pressure measurements are to be made. Essentially the same basic procedure is used for the preparation of chloro-, bromo-, and iododifluorophosphine. 

Figure 4.4.8. Isopiestic vapor-sorption apparatus with built-in manometer using a quartz spring 1 - connection to the vacuum, 2-9 -stop corks, 10, 11, 12 - connections to nitrogen, 13 - degassing flask for the pure solvent, 14, 18 - buffers, 15 - cold trap, 16,19 - Hg-ma-nometers, 17,20 - mercury float valves, 21 -pure solvent reservoir at temperature Ti provided by 22 - thermostat, 23 - temperature controlled air box, 24 - measuring cell, 25 - quartz spring (four quartz springs can be inserted into the equilibrium cell, only one is shown), 26 - pan with the polymer solution, 27 - closing plug sealed with epoxy resin, 28 - heating to avoid solvent condensation.

More elaborate arrangements are the familiar bubble-plate (Fig. 16-6) and such modifications as tunnel trays (vapor slots and vapor troughs extend across the plate) and Uniflux trays, float-valve bubble trays, perforated trays with downcomers for the liquid, and Kaskade-type (Bentuii and Flexitray) trays. 

Setting of float valves on vaporizers. The fiuid that is boiling inside the vaporizer is not entirely liquid, but a mixture of liquid and vapor. As a result, its density is usually appreciably lower than the density of the liquid column, which acts upon the float in the fioat chamber, which is not subject to such vigorous boiling. See Figs. V-32 through V-39. 

FIG. V-34 Typical control hookup for routine fluid vaporizing. Note presence of shellside float valve. This operates to stop liquid feed when flow level gets too high due to excessive draw of fluid from the vaporizer. The valve shuts down flow of the liquid feed to the vaporizer to avoid carryover of liquid Into the exit line. (Source Armstrong Engineering Associates.)... 

To lower the tower pressure, the hot-vapor bypass pressure recorder controller (PRC) valve is closed. This forces more vapor through the condenser, which, in turn, lowers the temperature in the reflux drum. As the liquid in the reflux drum is at its bubble point, reducing the reflux drum temperature will reduce the reflux drum pressure. As the stripper tower pressure floats on the reflux drum pressure, the pressure in the tower will also decline. 

Optimization and vacuum control strategies (a) minimizing (floating) pressure by maximizing coolant valve opening, (b) floating pressure control of partial condenser with vapor distillate, (c) floating pressure control when the distillate is both vapor and liquid. 

The objective of the preheat control system is to supply the column with a feed of consistent specific enthalpy (enthalpy per unit mass). With a single-phase feed, this translates into a constant feed temperature with a partially vaporized feed, this translates into a constant fractional vaporization. Maximizing feed temperature (if desired) is usually performed manually, by an advanced control system, or by a valve position controller similar to that used in floating pressure control (Sec. 17.2.4). 

Types of trays (a) bubble-cap tray vapor rises through openings in the plate, reverses direction, and escapes through the slots of an inverted cup (b) sieve tray (c) valve tray tray openings are adjusted by means of floating disks that rise and fall with the vapor flow rate. 

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