True vapor pressure

The true vapor pressure of hydrocarbons is expressed in bar and represents the vapor pressure directly above a saturated liquid in equilibrium with the vapor that rises over it. 

The Reid vapor pressure is generally barely different from the true vapor pressure at 37.8°C if the light gas content —methane, ethane, propane, and butane— of the sample is small, which is usually the case with petroleum products. The differences are greater for those products containing large quantities of dissolved gases such as the crude oils shown in Table 4.13. 

Relation between the true vapor pressure at 37.8 C and the Reid vapor pressure. R = True vapor pressure/ Reid" vapor pressure. --------------------------------------------------------------------------------i... 

The criterion retained up to now in the specifications is not the true vapor pressure, but an associated value called the Reid vapor pressure, RVP. The procedure is to measure the relative pressure developed by the vapors from a sample of motor fuel put in a metallic cylinder at a temperature of 37.8°C. The variations characteristic of the standard method are around 15 millibar in repeatability and 25 millibar in reproducibility. 

Since a portion of the liquid was vaporized to the vapor space, the hq uid will have lost some of its lighter components. This effectively changes the composition of the liquid and yields a slightly lower s apor pressure than the true vapor pressure of the liquid at 100°F. Figure 6-6 can be used to estimate true vapor pressure at any temperature from a known RVP. 

The inherent error between true vapor pressure and RVP means that a stabilizer designed to produce a bottoms liquid with a true vapor pressure equal to the specified RVP will be conservatively designed. The a[>or pressures of various hydrocarbon components at 100°F are given in Table 6-1. 

Milton et al. [1.136] used this methods and refer to it as manometric temperature measurement. They used times of pressure rises of up to 30 s. During this time, the ice temperature will increase, mainly due to continued heat flow. Therefore, an equation has been developed to transform the experimental pressure data, including three other corrections, into the true vapor pressure of the ice. If the valve is closed for only a very short time, < 3 s, and the pressure is measured and documented 60 to 100 times/s, these data can be recorded as shown in Fig. 1.78.1. The automatic pressure rise measurements (1) can then be plotted... 

Reid Vapor Pressure (RVP) - The pressure caused by vaporized part if a liquid and the enclosed air and water vapor as measured under standardized conditions in standardized apparatus, the result given in psi at 100 °F, although normally reported as RVP in lbs. RVP is not the same as the true "vapor pressure" of the liquid, but provides a relative index of the volatility of a liquid. 

In the time scale covered by the calculation, the 137I in the particles results from in situ decay of 137Te. The low surface concentration is associated with the high volatility of iodine in an oxidizing atmosphere. In this same figure the 137Xe curves reflect their parent 137I curves to some extent. The surface concentration of this element probably does not represent a true vapor pressure equilibrium but just the thermody-... 

Crude oil vapor pressure is the specification which most influences the design of oil-gas separation systems. For offshore tanker loading, the oil may be limited to a vapor pressure In the range of 8 to U pounds KVP (Reid Vapor Pressure). RVP refers to a standard method of vapor pressure testing utilizing a specific test cylinder assembly. and determined at a temperature of 100°F. RVP is (not Identical with TVP (true vapor pressure), which is the actual vapor pressure exerted by a liquid in equilibrium with a vapor at any given temperature. 

It can also be seen from Table 1 that the small amounts of light components have an even greater effect on true vapor pressure than on Reid vapor pressure. True vapor pressure more accurately measures the tendency of a crude oil to cause problems in the offshore loading of tankers, in pump stations of open system oil pipelines, and in increased vaporization losses in storage and in transit. True vapor pressure of crude oil will be fixed by the operating temperature and pressure of the last stage of oil-gas separation, or the bottom conditions of any crude stabilization unit, if no other liquids are added back to the crude stream. 

Vapor Pressure—Equilibrium pressure exerted by vapors over a liquid at a given temperature [2.1, 2.3]. The Reid vapor pressure (RVP) is typically used to describe the vapor pressure of petroleum fuels without oxygenates at 100°F (ASTM Test Method D 323, Test Method for Vapor Pressure of Petroleum Products) [2.5]. The term true vapor pressure is often used to distinguish between vapor pressure and Reid vapor pressure. The Reid vapor pressure test involves saturating the fuel with water before testing and cannot be used for gasoline-alcohol blends or neat alcohol fuels a new procedure has been developed which does not use water and is called Dry Vapor Pressure Equivalent, or DVPE (see ASTM D 4814-95c under Additional Information section). 

Problem The true vapor pressure of water is 23.76 mm. at 25 C. Calculate the vapor pressure when water vaporizes into a space already containing an insoluble gas at 1 atm. pressure, assuming ideal behavior. 

The Reid vapor pressure test method (ASTM D-323, IP 69) measures the vapor pressure of volatile petroleum. The Reid vapor pressure differs from the true vapor pressure of the sample because of some small sample vaporization and the presence of water vapor and air in the confined space. 

Correlative methods are also available for application to aviation fuels. Such methods include the use of viscosity-temperature charts (ASTM D-341), calculation of the cetane index (ASTM D-976, ASTM D-4737), calculation of the viscosity index (ASTM D-2270), calculation of the viscosity gravity constant (ASTM D-2501), calculation of the true vapor pressure (ASTM D-2889), and estimation of the heat of combustion (ASTM D-3338). 

If the chemical of interest is a solid, it sublimates to pose its vapor pressure. The sublimation process can be viewed as the sum of processes of a hypothetical fusion (melting) to liquid and vaporization from liquid to gas at given temperature and pressure. Such liquid, of course, is hypothetical because it may not exist at the given condition. The vapor pressure of this hypothetical liquid, Pp, rather than the true vapor pressure of the solid, Pg, is often generated by various vapor pressure estimation models. Pg can be converted to liquid vapor pressure Pp using the fugacity ratio F ... 

Insoluble liquids steam distillation The mutual solubility of some liquids is so small that they can be considered substantially insoluble points K and M (Fig. 9.8) are then for all practical purposes on the vertical axes of these diagrams. This is the case for a mixture such as a hydrocarbon and water, for example. If the liquids are completely insoluble, the vapor pressure of either component cannot be influenced by the presence of the other and each exerts its true vapor pressure at the prevailing temperature. When the sum of the separate vapor pressures equals the total pressure, the mixture boils, and the vapor composition is readily computed, assuming the applicability of the simple gas law,... 

This is the situation in, for example, a Knudsen cell in which the size of the orifice is very small compared to the area of the sample. In this experiment the true vapor pressure of the sample is measured and, as the amount of gas escaping from the cell is very small, the composition of the sample is fixed. It is also usually assumed that the solid is in equilibrium with its surroundings in a flow system when the rate of flow of gas is small. 

Absolute Bias—Since there is no accepted reference material suitable for determining the bias for this test method, bias cannot be determined. The amount of bias between this test vapor pressure and true vapor pressure is unknown. 

Note I—Because the external atmospheric pressure is counteracted by the atmospheric pressure initially present in the air chamber, this vapor pressure is an absolute pressure at 37.8 C (lOO F) in kilopascals (pounds-force per square inch). This vapor pressure diflers from the true vapor pressure of the sample due to some small vaporization of the sample and air in the confined space. 

Evaporation losses are a function of so many variables such as. (1) liquid or surface temperature, (2) variation in atmospheric temperature, (3) true vapor pressure of liquid, (4) volume of vapor space (how full), (5) color or reflectiveness of surface, (6) frequency of filling, (7) construction of tank, (8) degree of saturation attained in gas space, etc., that only the gross approximation indicated in Table 8-1 for a 10 R.v.p. gasoline... 

Suction Lift of Pumps. Figure 13-7 serves to illustrate pump suction conditions. The maximum lift that a pump can produce is equal to barometric pressure less the vapor pressure of the liquid. The customary Reid vapor pressure is not usually satisfactory because the small amount of air or gas dissolved in oils often causes the true vapor pressure to be 3 to 60 per cent higher (Table 4-12) than the Reid vapor pressure. In addition, the pump must lift the suction valve against its spring, which seldom... 

The net suction lift possible with a pump amounts to the barometric pressure less the following items (1) true vapor pressure of liquid at the pumping temperature, (2) pressure caused by weight and spring of suction valve, and (3) entry friction losses. Should this computation come out to be negative it means that a net positive suction head is necessary, that the pump can produce no lift. Centrifugal pumps should always be provided with a flooded suction (positive head), because there is no convenient way to prime or fill them with liquid and they cannot operate without liquid in the case. 

The final gasoline, a debutanized total of 41.7 -1- 7.33 or 49.0 per cent, will have a vapor pressure of only about 5 psia. Butane will be added to produce a total gasoline of 10 R.v.p. Most approximately the amount of mixed butanes required will be a little more than the difference in vapor pressures, namely, 5 to 6 per cent, but more specifically, using the true vapor pressures of Table 4-12, and a mole basis ... 

In computing vapor pressures, the vapor pressure of the heaviest or hexane plus fraction is always troublesome. In estimating its vapor pressure, boiling point is the most accurate basis, specific gravity is fairly accurate, and molecular weight means little. The approximate relationship between the true vapor pressure and the specific gravity of hexane plus fractions is... 

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