Your petroleum fraction can be re -defined by adjusting the properties of the petroleum fractions to reproduce known experimental data, e.g. the dew point or bubble point of a mixture.
The reflected changes of the properties of a petroleum fraction to re -produce the dew point or bubble point experimental data depend on the equation of state models used when matching. If the RKSA, PRA or CPA models are used, the vapour pressures of each of the pseudo components are modified. The
modifications are defined by two Mathias Copeman parameters. The keywords saved to the .mfl file are model-dependent. For example the keywords
MCRKSA1 and MCRKS2 are for RKSA model, MCPRA1 and MCPRA2 for PRA model. If other equation of state models are used (such a standard RKS) the acentric factor of the pseudo components are adjusted to match the dew point or bubble point experimental data.
Matching is carried out using the Tools/Matching menu option. The matching function works with any number of fractions. In earlier versions of Multiflash
the user was limited to matching single dew or bubble points or a single viscosity value. In Multiflash 3.2 this was expanded to allow the option of entering a table of multiple values up to 10. From 3.4 this was increased to 20.
Typically you may know an experimental upper retrograde dew point. You define your stream including one or more petroleum fractions and, if you wish, check the calculated dew point pressure or temperature using a fixedphase fraction flash, see “Fixed phase fraction flashes ” on page 122.
Our example has 9 petroleum fractions, the properties of the heaviest, C20+, are shown below.
If the calculated dew point does not match the experimental values, activate the matching facility using Tools/Matching/Dew point menu option.
and enter the values for one or more experimental dew points and the type of dew point you are calculating (the Upper retrograde solution is the default setting for dew point). Click on the Match button and the vapour pressure of the fractions will be adjusted until the calculated and experimental values match.
A plot will show a comparison of the matched data to the experimental data - and the unmatched calculations.
The two adjusted Mathias Copeman parameters will be reported in the main window together with the critical temperature and pressure.
match table dewpoint upper retrograde temperatures 305.35 340.95 398.55 442.55;pressures 462.8 401.6 303.5 229.6;;;
PETROLEUM FRACTIONS PHYSICAL PROPERTIES:
TC/K PC/bar MC1 MC2 11P 515.61 31.293 0.83985 0.31249 12P 545.10 31.030 0.87122 0.33002 13P 570.76 29.346 0.90788 0.35784 14P 597.63 27.167 0.95284 0.39556 15P 617.81 25.482 0.98986 0.42828 16P 643.74 23.564 1.0377 0.47243 17P 687.08 20.486 1.1244 0.55600 18P 747.39 16.427 1.2626 0.69403 19P 811.89 13.044 1.4141 0.85153
EXPERIMENTAL AND CALCULATED VALUES: DEWPOINT
T(exp)/K P(exp)/bar P(calc)/bar 305.35 462.800000 455.265209 340.95 401.600000 407.966543 398.55 303.500000 315.039181 442.55 229.600000 215.530477 where C20+ is the 19th component. The adjusted Mathias Copeman parameters MCRKS1 and MCRKS2 for the RKSA model will be reflected in the pure component record.
The new fraction properties can be saved using Fi le/Save Problem Setup. The second Mathias Copeman Parameter MCRKS2 values may not be displayed if they do not result in a better fit to the dew point line.
Bubble points can be matched in a similar manner and similar plots generated, either for the whole phase envelope or over a selected temperature or pressure range.
A recent upgrade was to expand the matching facility to match bubble points, GOR and a liquid density together. This is accessed through the
Tools/Matching/Bubble point option.
If only bubble point data are available then these are entered using the bubble point data table. To simultaneously fit the GOR then you must enter the conditions at which the supplied GOR were measured and the GOR itself. The units may either be standard cubic feet per barrel of liquid at the T,P specified or the equivalent in standard m3/ m3. The liquid density is optional but, if provided, must be at the same T,P conditions as the GOR. A choice of units is provided for liquid density including specific gravity in relation to water. The GOR and liquid density units in the bubble point matching form are not affected if the Units option is changed. The T,P units may be changed but in GOR conditions and bubble point units must be the same. The plot will show t he phase envelope before and after matching,
The output in the results window will show the adjusted property values for each of the properties matched and the comparative values to experimental data. For the bubble point matching the Mathias Copeman parameters are changed as described for the dew point matching. For fitting to liquid density the Peneloux shift parameters are altered as described below for density/volume matching.
The GOR is matched by changing the fluid composition. The output shows the original and adjusted composition for each fluid component and the ratio of the two.
Component amounts adjusted to match GOR:
original adjusted ratio NITROGEN 0.35 0.358497 0.9763 CO2 3.14 3.21454 0.9768 METHANE 54.26 55.5711 0.9764 ETHANE 8.57 8.77073 0.9771 PROPANE 5.72 5.84139 0.9792 ISOBUTANE 0.76 0.772932 0.9833 N-BUTANE 2.45 2.48441 0.9862 ISOPENTANE 0.75 0.748821 1.002 N-PENTANE 1.2 1.19059 1.008 C6 1.53 1.45224 1.054 C7 2.6 2.42592 1.072 C8 3.02 2.7916 1.082
Although some warning messages are triggered, e.g. entering a GOR but without T,P conditions, it is not possible to trap inconsistencies in the entered data. The best way of spotting major inconsistencies is the comparison of the matched and unmatched phase envelopes. If, for instance, the GOR is entered in the wrong units then there may be a good match for bubble point, density and GOR but the phase envelope change is significantly different and merits closer inspection of the experimental data.
The ratio of adjusted composition to original data may also indicate possible inconsistencies.
Similarly any GOR entered as part of the PVT Analysis and used to calculate the recombined fluid composition is ignored for the purposes of Bubble point/GOR matching and is not checked for consistency. Major differences in the phase envelopes may indicate that it is worth checking and eliminating inconsistencies.