2) The vapour pressure of the (numerically) degassed sample – by skipping the Pvap contribution.
In the TVP mode, due to time constraints, analysers do not perform such a detailed multipoint expansion as shown in the graphs, but use much fewer points as well as a proprietary extrapolation procedure to obtain the vapour pressure at zero expansion [2]. In the TVP mode, three D6377 measurements at different V/L ratios are performed and the TVP at a V/L = 0/1 is extrapolated (see Fig. 4).
Summary
From the data shown, it can be stated that a vapour pressure analyser can be employed to measure true vapour pressure. By using robust hardware (expansion type measuring chamber) and proven extrapolation, a TVP measurement can be performed within minutes, reducing the API calculation to a cross-check.
Fig 2. Total vapour pressure of different hydrocarbon mixtures as a function of vapour/liquid ratio.
Where Ptot = the total vapour pressure of the sample, which is comprised of Pliquid and Pvap
Pliquid = the absolute pressure of the liquid sample Pvap = the pressure contribution of the dissolved gas A1 = representation of the Ideal Gas Law (nRT) V = the actual gas-volume in the test chamber Va = the volume of the dissolved gas in the sample
Assuming that Va and nRT=const.=A1 are constants that only depend on the substance under consideration, will allow to have three independent and constant parameters (Pliquid, A1 and Va) to be fi tted to the measured curve.
Testing this model against real measurements however reveals that decomposition of the sample happens, which can be corrected by introducing a linear increase of the vapour pressure of the liquid as the volume increases.
Assuming validity of Raoult’s Law [5], it becomes evident that the more volatile components evapourate to a higher extent, shifting the composition stoichiometrics of the mixture. The larger the volume of the vapour space, the larger the shift. As Raoult’s Law assumes a linear dependence of the vapour pressure versus mixing ratio, this results in a linear dependence of the vapour pressure with volume.
Fig 4. Schematic of the 3-step expansion sequence. Validation of the TVP model
In the following section, actual measurements of the TVP analyser will be compared with the theoretical model and with results generated by the API TVP calculation.
Table 1 shows
• Results derived from the previously discussed theoretical TVP model, using a 28-point fi t • Results from TVP analyser measurements, using a 3-point fi t • Results from API calculation
The fi rst section shows measured RVPE values. For crude oils, the measurement procedure and RVPE conversion as described in ASTM D6377 was used. For refi ned products, ASTM D5191 was used, using the conversion between Ptot and RVPE as described in CARB regulations.
The second section shows theoretical (28-point curve fi t) and measured TVP results for not degassed samples. It can be seen, that inclusion of gases and air in the samples signifi cantly increases the TVP.
(2)
Where P liquid = the modifi ed vapour pressure of the liquid sample, written as the sum of the vapour pressure of the liquid (Pliquid) at a given V/L Ratio, and a term comprising of the vapour space volume of the test chamber (V) times a fi t constant (A2). By introducing this into the equation of total vapour pressure, the new equation reads:
The third section compares results calculated from the 28-point curve-fi t (with numeric-degassing) and compares them to the results that can be expected when using API Chapter 19 Section 2 to convert from RVP to TVP [2, 3]. As these results are signifi cantly lower, it is clear that the API conversion assumes degassing of the sample.
The following conclusions can be drawn from the results listed in Table 1:
1. The results from the theoretical 28-point curve fi t model compare excellent to actually measured TVP data.
(3)
When performing a Curve-Fit using a Levenberg-Marquardt algorithm of above function to the measured vapour pressure versus volume plots, the differences between calculated and measured values can be shown (see Figure 3).
2. So it is not necessary to employ a 28-point curve fi t to get accurate results. A 3-point curve fi t yields comparable results to the more complex theoretical model.
3. Air saturation has a signifi cant infl uence on the results, even for pure substances.
4. Assuming degassing, the results of the theoretical 28-point curve fi t model are comparable to results from API Chapter 19 Section 2 conversion from RVP to TVP.
Fig 5. Laboratory TVP analyser with mounted Floating Piston Cylinder
References
1. International Chamber of Shipping, London and Oil Companies International Marine Forum, Bermuda. “International Safety Guide for Oil Tankers and Terminals.” (ISGOTT). 5th edition, Witherby & Co. Ltd., London. 2006.
2. Lord, David L., and Rudeen, David K., “Strategic Petroleum Reserve Crude Oil Equation of State Model Development” – Current Performance against Measured Data, Research Report, Sandia National Laboratories, Albuquerque, NM, and Livermore, CA, 2010.
3. “Organic Liquid Storage Tanks”, In: Compilation of Air Pollutant Emission Factors, Emission Factor Documentation for AP-42, Fifth Edition, Volume I, Section 7.1, Offi ce of Air Quality Planning and Standards Offi ce of Air and Radiation, U.S. EPA, Durham, NC, 2006.
4. “Evaporative Loss Measurements”, In: Manual of Petroleum Measurement Standards, Chapter 19, section 2, Former API Pub 2517 and 2519, second Edition; 2003.
5. Silbey, Robert J., Alberty, Robert A., Bawendi, Moungi G. ; “Physical Chemistry, 4th Edition”, Wiley, 2004.
GRABNER INSTRUMENTS, a subsidiary of AMETEK Inc., is considered one of the world´s leading developers and manufacturers of automatic petroleum testing equipment. Grabner Instruments’ success is based on the development of portable, rugged and easy-to-operate fuel and oil analysers for accurate quality control in the laboratory as well as for fast on- site tests in mobile lab facilities.
AMETEK, Inc. is a leading global manufacturer of electronic instruments and electromechanical devices with annual sales of 4.0 billion US $.
One of the main advantages of the TVP analyser is that the TVP can be measured for degassed and not degassed samples. This is critical for the accurate measurement of today’s highly volatile crude oils. As shown it is also of highest importance to prevent air saturation of the sample prior to the test, as full or partly air saturation will signifi cantly alter test results. Air saturation can be prevented, if samples are measured directly in process or are transported in a sealed fl oating piston cylinder (FPC) or manual piston cylinder (MPC) from the sample source to the TVP analyser (see Fig. 5).
Fig 3. Difference (in kPa) between predicted and measured vapour pressure as a function of V/L ratio.
Using this theoretical curve, two parameters can be calculated easily:
1) The vapour pressure of the complete (not degassed) sample at V/L=0/1.
Table 1: Comparison of experimental and calculated data for different hydrocarbon mixtures and methods
Contact Details Hannes Pichler • Tel: 0043 / 1 / 282 16 27-213 • Email:
hannes.pichler@ametek.com
ANNUAL BUYERS GUIDE 2017 •
WWW.PETRO-ONLINE.COM
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