Flash point calculation methods

Flash points calculated by the proposed sum-of-indexes method are compared in Table III with experimentally determined flash points. To place this comparison in context, Table III also includes several other comparisons between sets of data. When earlier experimentally determined data were compared with flash points redetermined for this study, the differences were almost evenly distributed around the zero point. The median absolute difference between the two sets of data was 5°F while the largest difference between flash points determined for a single blend was 27°F. Some skew is apparent in the differences between either set of experimental data and the calculated flash points. The median absolute differences between experimental and calculated data were either 7° or 8°F depending upon which set of experimental data was used in the comparison. When the flash point of the lowest flashing component was taken as an indication of blend flash point, the median differences from the two sets of experimental data were 13° and 18 °F, respectively. 

Semi-empirical formulae, based only on molecular structure, have been derived which allow flammability limits to be calculated for hydrocarbons and alcohols. Flash points, autoignition temperatures and boiling points may also be calculated from molecular structure for these classes. Quoted examples indicate the methods... 

Evaluation of the vapor pressure method with normal alkanes disclosed favorable agreement of estimates and data for small, intermediate and large size molecules. Evaluation with other compound types was not performed. If the lower explosive limit (LEL) used in the calculations is estimated, the estimates for flash point should be considered as rough values. 

A suitable method is described in Gmehling and Rasmussen (Ind. Eng. Chem. Fundament, 21, 186, (1982)). For a mixture containing non-volatile components, e.g. polymers or additives, the flash point is calculated from the volatile components. It is considered that a non-volatile component only slightly decreases the partial pressure of the solvents and the calculated flash point is only slightly below the measured value. 

A —> group contribution method was also proposed for the calculation of the flash point of chemicals [Albahri and George, 2003]. 

Flash points of mixtures of oxygenated and hydrocarbon solvents cannot be predicted simply. A computer based method is proposed which exhibits satisfactory prediction of such Tag Open Cup flash points. Individual solvent flash point indexes are defined as an inverse function of the component s heat of combustion and vapor pressure at its flash point. Mixture flash points are then computed by trial and error as the temperature at which the sum of weighted component indexes equals 1.0. Solution nonidealities are accounted for by component activity coefficients calculated by a multicomponent extension of the Van Laar equations. Flash points predicted by the proposed method are compared with experimental data for 60 solvent mixtures. Confidence limits of 95% for differences between experimental and predicted flash points are +8.0-+3.0°F. 

A further approach to estimating flash points of mixed hydrocarbons is to sum volume fraction weighted flashing indexes which are calculated directly from the flash points of the individual components (5). Blend flash points estimated by the latter two methods generally agreed with experimental values within the limits of reproducibility of experimental data. 

The flash point of a solvent mixture is not identical to that of its most flammable component. When solvents with widely differing hydrogen bond parameters are mixed (e.g., alcohol hydrocarbon), tbe flash point is significantly reduced. On the other hand, the flash point of a mixture of chemically related solvents lies between those of the individual components [14.87]-[14.89]. Methods have been developed for calculating the flash point of solvent mixtures and solutions, activity coefficients are used to account for nonideal behavior [14.90]. 

For rapid determination of closed cup flash points between 5 and 105 °C, rapid equilibrium method, ISO 3679-1983, ASTM D 3941-1980, ASTM D 3828-1981), the Setaflash tester is recommended (Fig. 3.15). An aluminium block is heated to 3 °C below the expected flash point then 2 cm of the sample is injected into the sample hollow by a syringe. After 60 s, ignition is attempted by a 3.5-mm long test flame with a momentary opening of the sliding cover on the sample hollow. With repeated trials, the flash point is bracketed at first within 5 then within 1 C, using fresh sample portions in each test. For calculation of the closed cup flash point, the above barometric correction is again used. 

Shortcut calculation methods. In the remainder of this chapter, shortcut calculation methods for the approximate solution of multicomponent distillation are considered. These methods are quite useful to study a large number of cases rapidly to help orient the designer, to determine approximate optimum conditions, or to provide information for a cost estimate. Before discussing these methods, equilibrium relationships and calculation methods of bubble point, dew point, and flash vaporization for multicomponent systems are covered. 

Bubble points or dew points can be determined by flash calculations with yr in Equations 2.7 or 2.13 set to either zero or one. Although the general flash calculation method described in the previous section may be applied to bubble points and dew points, phase boundary calculations are handled more efficiently by other methods. At the bubble point, the liquid composition equals the feed composition since all the feed remains in the liquid phase. The composition of a vapor bubble at equilibrium with the liquid is given by Equation 2.13b, which reduces to... 

Verify the performance of the manual apparatus at least once per year by determining the flash point of a certified reference material (CRM), such as those listed in Annex A2, which is reasonably close to the expected temperature range of the samples to be tested. The material shall be tested according to the procedure of this test method and the observed fla point obtained in 9.5 shall be corrected for barometric pressure (see Section 13). The flash point obtained shall be within the limits stated in Table A2.1 for the identified CRM or within the limits calculated for an unlisted CRM (see Annex A.2). 

A2.1.1 Typical values of the flash point corrected for baromdiic pressure for some reference materials and their typical limits are given in Table A2.1 (see Note A2.3). Suf liers of CRMs will provide certificates stating the m od-spedfic fiash point for each material of the current production batch. Calculation of the limits for these other CRMs can be determined fix>m the reprodudbility value of this test method, reduced by interlaboratory effect and then multiplied by 0.7 (see Research Report RR S1S-1007). 

Another set of key indicators are the product properties of the liquid fuel from the FCC. The properties of interest to refiners are density, flash point (volatility), RON/MON (for gasoline), sulfur content and aromatic content. This is one of the areas where our model is different from other published work described earlier. We discussed a method to transition from kinetic lumping to fractionation lumping in Section 4.8. Not only does this method allow the user to observe the results directly, we can also see the effect of the reactor conditions on fractionated properties. Using the results from the fractionator model, we can calculate the distillation... 

The calculation for a point on the flash curve that is intermediate between the bubble point and the dew point is referred to as an isothermal-flash calculation because To is specified. Except for an ideal binary mixture, procedures for calculating an isothermal flash are iterative. A popular method is the following due to Rachford and Rice [I. Pet. Technol, 4(10), sec. 1, p. 19, and sec. 2, p. 3 (October 1952)]. The component mole balance (FZi = Vy, + LXi), phase-distribution relation (K = yJXi), and total mole balance (F = V + L) can be combined to give... 

Feed analyses in terms of component concentrations are usually not available for complex hydrocarbon mixtures with a final normal boihng point above about 38°C (100°F) (/i-pentane). One method of haudhug such a feed is to break it down into pseudo components (narrow-boihng fractions) and then estimate the mole fraction and value for each such component. Edmister [2nd. Eng. Chem., 47,1685 (1955)] and Maxwell (Data Book on Hydrocarbons, Van Nostrand, Princeton, N.J., 1958) give charts that are useful for this estimation. Once values are available, the calculation proceeds as described above for multicomponent mixtures. Another approach to complex mixtures is to obtain an American Society for Testing and Materials (ASTM) or true-boihng point (TBP) cui ve for the mixture and then use empirical correlations to con-strucl the atmospheric-pressure eqiiihbrium-flash cui ve (EF 0, which can then be corrected to the desired operating pressure. A discussion of this method and the necessary charts are presented in a later subsection entitled Tetroleum and Complex-Mixture Distillation. ... 

Omega is a correlating parameter in an "equation of state" (EOS) which links the specific volume of a two-phase mixture flowing in a relief system with the pressure at any point. Such an EOS is required to evaluate the HEM without performing repeated flash calculations. The EOS used by the Omega method is ... 

High-pressure systems in the vicinity of critical points, such as synthesis gas and air separation systems, remain a challenge. Our flash algorithm has difficulty in identifying the correct phase state, or converging to the correct vapor-Uquid solutions. This problem may be exacerbated by the difficulty in obtaining the equation of state volume root in the vicinity of the critical points. Further work to improve the algorithm and the equation of state volume root determination is required. It is believed that the homotopy continuation methods are probably better suited for calculations near the critical points. 

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