The determination of the critical parameters of a gasoline petroleum fractionSB-180 Trinity-Anastasievskoe oil

Annotation. The article determines the critical parameters of the oil fraction-temperature, pressure and density. The experimental results are compared with the calculated results, the calculation method closest to the experimental results is chosen, the calculation error is estimated. The main results and conclusions are presented. The use of the theory of thermodynamic similarity in the method of density calculation and DNP required first of all knowledge of the parameters of the state at the critical point. The accuracy of the calculation of critical parameters affects the reliability of the results.

In addition, the nomogram method of the American petroleum Institute API is used to calculate critical temperatures [1]. As the initial data for the nomogram, the density of the product at t=20⁰C and AVBP is used.
We calculated the critical temperatures of the studied products by various methods, and found that the most accurate calculation is given by the Nokei formula. To describe the experimental data, we used the Nokei formula used to calculate the critical temperatures of pure hydrocarbons in the form of -. / = 0 + 1-.2 • 3 + 4-. 5 (6) The relative density of the product at 20⁰C and atmospheric pressure was used as the value S * ω, and the average volume boiling point of the oil product (K) was used as Tb [2,3,4].
The coefficients A, B, C obtained during the processing of experimental data by the least squares method have the following form: A=1,1004405; B=0.2498864; C=0.652894.
The obtained coefficient values are close to the coefficients used in the Nokei equation for calculating the critical temperatures of olefin hydrocarbons. The table shows that the values of critical temperatures calculated by the formula (6) are the closest to the experimental ones. The mean square error of calculation according to the formula (1) and the nomogram method is an order of magnitude higher than according to the formula (6).

Pressure
To calculate the critical pressures of the studied oil fraction we used: The formula for calculation of critical pressures of the products of cracking: where T b is the average boiling point; : $ # -density at 20⁰C. Formula for calculating critical pressures of pure hydrocarbons: where T CR is the critical temperature, K; T S -boiling point; < ?@ ?A > 97 -selected for a specific hydrocarbon group.
The formula used for the calculation of critical pressures of jet fuels: where T CR is the critical temperature, K; ρ CR -critical density, kg / m3; C and D are constants. Expressions such as the Nokia equation: The table shows the calculated values of RCR according to the above formulas and their comparison with the experimental ones. The standard error of calculation is given. The maximum error is obtained when calculating by equation (9), the best result is obtained by using an expression of the form Nokei, where A=2,11216; B=0,796456; C=0,235387.

Density (specific volume)
For calculation of critical values of density of oil products are used: Filippov formula for calculation of critical values of oil fraction density: where ρ and ρ 0 , P And P 0 -refer to two points of the isotherm; R-universal gas constant; M-molar mass.
Formula for calculation of critical values of density of jet fuels and pure hydrocarbons: The authors indicate that the error of density calculation for (11) is ±1%, for (12) ±2%.
To calculate the critical values of the density of the oil fraction studied by us an expression of the type of the Nokay equation was used The calculation results are given in the table. The mean square error of the calculation was 0.04%. Coefficients of equation (13) A=0,04838144; B=0,92861442; C=-0,21308121.
Thus, using expressions such as the Nokei equation, one can calculate the T КР , P КР and ρ КР of the gasoline oil fraction.
The generalized dependence of density on saturation lines in the critical region is presented in Fig. 1. Table 4 shows the values of the critical parameters of the studied fraction.

Estimation of the error of pressure measurement
When calculating the error of the pressure measurement was based on the working formula: where P EP -experimental pressure (MPa * 10-1); B -barometric pressure (MPa * 10-1); 0,0417ρ Q R -correction for pressure associated with the difference in liquid levels in the piezometer and the measuring column of the pressure gauge (bar); P M -manometric pressure (kgf / cm2).
The confidence interval of each variable is less than the instrumental error, so to characterize the total error of the measured value P, we use the instrumental errors of all variables: where ΔPM-the error of measurement of the piston gauge (MO-ΔPM = 0.0016 P; MP6-ΔPM = 0.0002 P; MP60, MP600-ΔPM = 0.0005 P).
Accuracy of mercury barometer pressure measurement: ∆6 W = 0,0001, 6 W = 0,1 • 10 8$ МПа The error introduced by inaccuracy of measurement of heights of a liquid in system: The sensitivity of the membrane separator: Then the total error of the pressure measurement:

Estimation of temperature measurement error
Systematic and random errors were taken into account when estimating the temperature measurement error.
The random error of temperature measurement δt With a model platinum resistance thermometer is calculated in detail in [5,6,7]. The systematic error Δtpts and the thermostatic error Δtt and is determined from the expression: The authors [1,8] calculated the error ΔtPTS for a standard measuring circuit at different temperatures. Since our measurement scheme used a similar measuring potentiometric setting, we will use the results [5,8] to determine the error of temperature measurement. The data of temperature measurement errors are presented in table 3.4.

Evaluation of measurement uncertainty of specific volume
The specific volume of the investigated liquid was calculated by the expression: where V 0-is the internal volume of the piezometer at P = Pat, t = 20 ºC; V j k -volume of capillaries in the zone of variable temperatures; V l k -volume of capillaries in the thermostatic zone; ∆V m -correction for thermal expansion of the piezometer; ∆V R -correction for isothermal expansion of the piezometer; m , m j k , m l k -masses of liquid in corresponding volumes; ∑ m o j is the mass of the liquid released from the piezometer. We find in this formula the corresponding coefficients of influence of each term separately for the numerator and denominator: Formulas (18) and (19) can be written in the form: ∆f = 1,1 • V 1 • q w ! # + 1 • q w r s # + 1 • q w t s # + 1 • q xw y # (21) The confidence limit of systematic error of measurement of specific volume can be calculated by the formula: To estimate the random error, data on the measurement of specific volumes of water at six isotherms were used. The obtained values were compared with the literature data [8,9]. As an approximate estimate of the random error of our experimental data we can take the total for all isotherms estimate of the standard deviation of the measured specific volumes compared with the literature data: As an example, we give an estimate of the measurement error of the specific volume for the fraction NK-180ºC of the Troitsko-anastasievskaya oil at the following parameters: The systematic error in determining the volume of the piezometer and capillaries are equal respectively: q F ! = 0,011 }f~ ; q F r s = 0,002 }f~ ; q F t s = 0,008 }f~ The correction for the isothermal expansion of the piezometer at P = 2 MPa is 0.006 cm3 and is made with an error of 1% (data error δ and E for steel X18H10T): The error of correction for the thermal expansion of the piezometer can be found from the approximate dependence: where SΔV is an estimate of the standard deviation of the experimental values ΔV from the approximating curve.
For temperature range 20÷300 ºC: The correction for the thermal expansion of the piezometer at 200 ºC is 1,195 cm3, then: q F u = 0,039 }f~ Systematic errors in determining the mass of the test substance in the piezometer and capillaries coincide with the error in determining the corresponding volumes.
When assessing the mass error of the liquid merged from the piezometer, the requirements of GOST 19491-74, the error from the nonequilibrium of the weights, the mass error of the weights were taken into account.
Losses due to evaporation lay in the range of 0.004-0.006 g during the measurement on the isotherm, which was 14-16 hours.
As an absolute error in determining the mass of the evaporated liquid, a double variation of the weights ADV-200 -0.0002 was adopted. the Correction was distributed proportionally to the time from the first drain to the last point on this isotherm.
The confidence limit of the systematic error of measuring the mass of the liquid at discharge was: q X Y w y = 0,001 .
Substituting the obtained values into formulas (13) The systematic error in determining the specific volumes of the substance at the filling parameters was determined by the error in determining the specific volumes at the plant by the method of hydrostatic weighing and was accepted according to [3]: Substituting the obtained values into the formula (15), we obtain: We define the boundary of the confidence interval of the random error by the formula (4). At α = 0.95 and the number of degrees of freedom K = 10, the percentage point of the student's distribution tg = 2.23, whence Ψ = 0.04%.
The standard deviation of the systematic error : 2 ‚ 3 = ƒ 1 3 • 0,69 • 10 8~= 0,015% Then the standard deviation of the total error: 2 x 3 = "0,018 # + 0,015 # = 0,023% The value of tΣ is determined : T x = 0,029 + 0,04 0,018 + 0,015 = 2,09 Then the confidence limit of the total error of the measurement result is determined: ∆= 0,023 • 2,09 = 0,048% Similar calculations were performed for other areas of state parameters. For the studied fraction, the confidence limit of the total error of the measurement result lies in the range of 0.03÷0.1%. At temperatures close to critical, the influence of the reference errors and the error associated with the introduction of an amendment to the thermal expansion of the piezometer increases.
In the two-phase region, the confidence limit of the total error increases and lies within 0.08÷0.15%. This is due to a sharp increase in this area of the error of the pressure reference and the error of determining the mass of the substance in the piezometer [10].