The Flash

The equilibrium ratios are not fixed in a separation calculation and, even for an isothermal system, they are functions of the phase compositions. Further, the enthalpy balance. Equation (7-3), must be simultaneously satisfied and, unless specified, the flash temperature simultaneously determined. 

The temperature and composition of each feed stream and the stream ratios are specified along with a common feed pressure (significant only for the vapor stream) and the flash pressure. For an isothermal flash the flash temperature is also specified. Resulting vapor and liquid compositions, phase ratios, vaporization equilibrium ratios, and, for an adiabatic flash, flash temperature are returned. 

Figure 3.1a shows a flash drum used to separate by gravity a vapor-liquid mixture. The velocity of the vapor through the flash drum must be less than the settling velocity of the liquid drops. Figure 3.11) shows a simple gravity settler for removing a... 

Flash point. The flash point of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air. The flash point generally increases with increasing pressure. 

The flash point measures the tendency of a petroleum material to form a flammable mixture with air. It is one of the properties to be considered when evaluating the flammability of a petroleum cut. 

The flash point is measured in the laboratory following procedures that depend on the sample being tested. 

Calculating the flash point starting from the mixture s composition is not very accurate however an estimation can be obtained if 7) is determined as-the temperature for which the following relation holds true [ 9 )... 

The flash points obtained experimentally according to the different procedures differ slightly. The present estimation refers to the flash point called the closed cup method. 

The flash curve of a petroleum cut is defined as the curve that represents the temperature as a function of the volume fraction of vaporised liquid, the residual liquid being in equilibrium with the total vapor, at constant pressure. 

The calculation of the flash curve is achieved using the models given earlier. 

The flash curve at atmospheric pressure can be estimated using the results of the ASTM D 86 distillation by a correlation proposed by the API. For the same volume fraction distilled one has the following relation ... 

The flash point of a petroleum liquid is the temperature to which it must be brought so that the vapor evolved burns spontaneously in the presence of a flame. For diesel fuel, the test is conducted according to a closed cup technique (NF T 60-103). The French specifications stipulate that the flash point should be between 55°C and 120°C. That constitutes a safety criterion during storage and distribution operations. Moreover, from an official viewpoint, petroleum products are classified in several groups according to their flash points which should never be exceeded. 

The flash point depends closely on the distillation initial point. The following empirical relationship is often cited ([, 1 ... 

It should be noted finally that adding gasoline to diesel fuel which was sometimes recommended in the past to improve cold behavior conflicts with the flash point specifications and presents a serious safety problem owing to the presence of a flammable mixture in the fuel tank airspace. Adding a kerosene that begins to boil at 150°C does not have the Scune disadvantage from this point of view. 

Volatility can be characterized either indirectly, by measurement of the flash point (the temperature to which the oil must be heated for inflammation of its vapor to become possible) or by direct measurement, following the Noack method. 

Performance can be illustrated for example by the time necessary for deaeration or de-emulsification of oils, anti-rust properties, copper strip corrosion test, the flash point in closed or open cup, the cloud and pour points, the foaming characteristics, etc. 

Table 8.4 gives the vapor pressures and the flash points of some crude... 

The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

The second indication is a faint smoke-like cloudiness in the zone of the tube which is being heated by the Bunsen this is readily visible as the interior of the tube is normally quite clear and bright. This is a later stage of development of the flash-back than the rise of pressure, already mentioned, and should be counteracted by moving the Bunsen immediately to the point of the combustion tube where heating was commenced. In either case the Bunsen should then be moved slowly forwards as before. A flash-back is attended by the deposition of carbon particles, carried back by the explosion wave, on the cold walls of the tube. Care should be taken that these are completely burnt off as the Bunsen is slowly moved forward again. 

The most dangerous solvent in the laboratory is carbon disulphide, the flash-point of which is so low that its vapour is ignited, e.g., by a gas-ring 3 4 minutes after the gas has been turned out. CarlK>n disulphide should therefore never be used in the laboratory unless an adequate substitute as a solvent cannot be found. Probably the next most dangerous liquid for general manipulation is ether, which, however, has frequently to be employed. If the precautions described on pp. 79, 163, are always followed, the manipulation of ether should however quite safe. 

If the flash lamp is pulsed very rapidly, the emergent beam appears at a rate governed by the lifetime of the inverted population. The resulting laser beam becomes almost continuous because the pulses follow each other so rapidly. However, such a solid-state laser should not be pulsed too rapidly because, if it is, the rod heats to an unacceptable extent, causing distortion and even fracture. Generally, solid-state lasers are not used in continuous mode because of this heating aspect. Liquid or gas lasers do not suffer from this problem. 

The flash point of furfural is 143°F by Tag Closed Cup. Because of its chemical reactivity, furfural should be kept away from strong acids, alkaHes or strong oxidi2ing chemicals. When furfural is stored for long periods in contact with air, there is a gradual darkening of color, increase in acidity, and formation of a soluble polymer. 

In methanol—formaldehyde—water solutions, increasing the concentration of either methanol or formaldehyde reduces the volatility of the other. Vapor-hquid-equihbrium data (8,27) for several methanolic formaldehyde solutions ate given in Table 2. The flash point varies with composition, decreasing from 83 to 60°C as the formaldehyde and methanol concentrations increase (17,18). 

Properties of the principal hydrocarbons found in commercial hexane are shown in Table 9. The flash point of / -hexane is —21.7 °C and the autoignition temperature is 225°C. The explosive limits of hexane vapor in air are 1.1—7.5%. Above 2°C the equiUbrium mixture of hexane and air above the Hquid is too rich to fall within these limits (42). 

The rich oil from the absorber is expanded through a hydrauHc turbiae for power recovery. The fluid from the turbiae is flashed ia the rich-oil flash tank to 2.1 MPa (300 psi) and —32°C. The flash vapor is compressed until it equals the inlet pressure before it is recycled to the inlet. The oil phase from the flash passes through another heat exchanger and to the rich-oil deethanizer. The ethane-rich overhead gas produced from the deethanizer is compressed and used for produciag petrochemicals or is added to the residue-gas stream. 

Reduction to Liquid Metal. Reduction to Hquid metal is the most common metal reduction process. It is preferred for metals of moderate melting point and low vapor pressure. Because most metallic compounds are fairly insoluble in molten metals, the separation of the Hquified metal from a sohd residue or from another Hquid phase of different density is usually complete and relatively simple. Because the product is in condensed form, the throughput per unit volume of reactor is high, and the number and si2e of the units is rninimi2ed. The common furnaces for production of Hquid metals are the blast furnace, the reverberatory furnace, the converter, the flash smelting furnace, and the electric-arc furnace (see Furnaces, electric). 

The lower flammable limit (LEL) or lower explosive limit (LEL) is the minimum concentration of vapor in air below which a flame is not propagated when an ignition source is present (61—64). Below this concentration, the mixture is considered too lean to bum. The lower flammable limit and the flash point of a flammable Hquid are closely related by the Hquid s vapor pressure characteristics. 

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. 

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