Vapor space

This eliminates the vapor space but sealing the edge can be a problem. Double seals can help and sometimes a fixed roof is also added above the floating roof to help capture any leaks from the seal. However in this case, the space between the fixed and floating roof now breathes and an inert gas purge of this space would typically be used. The inert gas would be vented to atmosphere after treatment. 

Flexible membrane. Another method to stop the vapor space breathing to atmosphere is to use a tank with a flexible membrane in the roof, Fig. 9.26. 

When process tanks, road tankers, or rail tank cars are filled, material in the vapor space is forced out of the tank and lost to atmosphere. 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

Phosphine has an 8-h time-weighted average exposure limit of 0.3 ppm (13). Under alkaline conditions the rate of PH formation is high. At neutral or acidic pH, the PH generation is slow but stiU very ha2ardous if the PH is allowed to accumulate in a confined vapor space. The safest commercial handling conditions for molten phosphoms are generally considered to be from pH 6 to 8 at 45—65°C. 

Storage of Flammable Materials. The preferred storage for flammable Hquids or gases is in properly designed tanks. Floating roof tanks frequently are used in the petroleum industry for flammable cmdes and products (see Tanks and pressure vessels). The vents on cone roof tanks should either be equipped with flame arrestors or the vapor space above the contents should be inerted with a nonflammable gas or vapor, unless the flash point is weU above the maximum ambient temperature, the contents are not heated above the flash point, and the tank is not exposed to other tanks containing flammable Hquids. 

For tank work, inches water column (in. wc) or ounces per square inch (osi) are commonly used to express the value of pressure or vacuum in the vapor space of a tank. These pressures are usually very low relative to atmospheric pressure. The common measures of pressure are compared as follows ... 

Interna.la.nd External Pressure. The difference la pressure between the iaside of a tank or its vapor space and local barometric or atmospheric pressure is called internal pressure. When the internal pressure is negative it is simply called a vacuum. The pressure is measured at the top of the hquid ia the tank because the Hquid itself exerts hydrostatic pressure, thus increa sing to a maximum value at the base of the tank. 

Internal pressure may be caused by several potential sources. One source is the vapor pressure of the Hquid itself. AH Hquids exert a characteristic vapor pressure which varies with temperature. As the temperature iacreases, the vapor pressure iacreases. Liquids that have a vapor pressure equal to atmospheric pressure boH. Another source of internal pressure is the presence of an iaert gas blanketing system. Inert gas blankets are used to pressuri2e the vapor space of a tank to perform speciali2ed functions, such as to keep oxygen out of reactive Hquids. The internal pressure is regulated by PV valves or regulators. 

EFR tanks have no vapor space pressure associated with them and operate strictly at atmospheric pressure. IFR tanks, like fixed-roof tanks, can operate at or above atmospheric pressure in the space between the floating roof and the fixed roof. 

Skin and pontoon roofs are usually constmcted of an aluminum skin supported on a series of tubular aluminum pontoons. These have a vapor space between the deck and the Hquid surface. 

Ethylene oxide storage tanks ate pressurized with inert gas to keep the vapor space in a nonexplosive region and prevent the potential for decomposition of the ethylene oxide vapor. The total pressure that should be maintained in a storage tank increases with Hquid temperature, since the partial pressure of ethylene oxide will also increase. Figure 5 shows the recommended minimum storage pressures for Hquid ethylene oxide under nitrogen or methane blanketing gas. 

Liquid Column Density may be determined by measuring the gauge pressure at the base of a fixed-height hquid column open to the atmosphere. If the process system is closed, then a differential pressure measurement is made between the bottom of the fixed height liquid column and the vapor over the column. If vapor space is not always present, the differential-pressure measurement is made between the bottom and top of a fixed-height column with the top measurement being made at a point below the liquid surface. 

Foams Two excellent reviews (Shedlovsky, op. cit. Lemlich, op. cit.) covering the literature pertinent to foams have been published. A foam is formed when bubbles rise to the surface of a liquid and persist for a while without coalescence with one another or without rupture into the vapor space. The formation of foam, then, consists simply of the formation, rise, and aggregation of bubbles in a hquid in which foam can exist. The hfe of foams varies over many magnitudes—from seconds to years—but in general is finite. Maintenance of a foam, therefore, is a dynamic phenomenon. 

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

A major portion of the reaction is found to occur in the vapor space between trays. A unit in which most of the trays are replaced oy empty space is called a. flash roaster its mode of operation is like that of a spray dryer. 

Sparging may not condense all the vapor. The injection of cold liquid spray in the vapor space should be considered. 

---