A major addition to Multiflash was the ability to calculate and plot phase envelopes. It is possible to trace any phase boundary including gas, liquid and solid on pressure-temperature co-ordinates. A VLE Autoplot function traces the primary dew and bubble point curves. You should note that the phase envelope should only be used for models based on an equation of state (or at low pressures for activity models).
Since the first release of the phase boundary plotter we have added
• the ability to trace selected phase boundaries on the basis of mass or volume fraction in addition to mole fraction and the nucleation boundary.
• the flexibility to include the plotting of constant enthalpy, entropy, volume or free energy boundaries.
• improvements to handle cases where the line crosses a phase boundary at which there are no degrees of freedom according to the phase rule.
• A generalisation of the plotting facility so that for any chosen phase boundary it is possible to plot any phase or property against another.
In MF3.6 we have changed the program used to generate the plots, giving more clarity and flexibility to the plotting functions.
The Phase envelope is activated through the Calculate/Phase Envelope menu or using the phase envelope button, .
The resulting window has three sections. The first
allows you to generate either the VLE phase envelope automatically or to set a specific phase boundary e.g. WATER in zero amount to define the water dew point line. You should also choose the appropriate “Type of Solution” as discussed previously, e.g. unspecified if you want to trace the asphaltene flocculation boundary above the bubble point pressure. If the VLE AutoPlot fails to trace the full phase envelope the liquid and gas phase boundary lines can be plotted individually.
You may then select a variety of properties of that phase to be plotted, including phase fraction.
If you choose to plot any of these then you will be reminded of the current unit setting for the property when you enter the value.
The second section provides the initial conditions for any phase boundary and the conditions and direction for the boundary search. Experience has shown that using increasing pressure is a useful default. If the user does not set an initial value for the pressure a default value of 1 bar, or the equivalent in the units chosen, will be set. This value is only shown in the t ext box once the boundary has been plotted.
While one bar is usually an appropriate default to start the plot you should also be aware that on some occasions there may not be a solution for a particular boundary at this pressure and it may be necessary to vary initial conditions to start the tracing of the phase boundary.
If the boundary still proves difficult to trace, starting values for the temperature or pressure as appropriate can be provided. For the phase envelope calculator the starting value is provided by entering a “guess” in the text box, i.e. a value for
the temperature if you are defining pressure and you should check the “Use starting value box.
The final section allows you to set P,T boundaries for the calculation and to set the properties you wish to see displayed on the x and y axis. For normal phase envelopes these will be temperature and pressure.
In the main results window T,P conditions, volume, enthalpy, entropy, internal energy and phase fractions of any other phases present as s pecified in the model specification for the problem are displayed.
Any of these properties plus temperature and pressure can be plotted against any other. The choice is available from the drop down menu.
A typical phase envelope plot might be a VLE plot for a hydrocarbon fluid. The axes will be pressure versus temperature.
The grid may be deleted if you prefer.
It is also possible to look at the liquid mole fraction as a function of temperature.
Before the new plot is displayed you will be reminded that you have altered at least one of the axis properties and that boundaries plotted on the previous axes will be lost.
If you wish to return to the standard P,T axes to plot you must remember to alter the axes yourself in the Frame tab, they will not be reset automatically.
The Frame tab allows you to change the maximum number of points calculated at any one time. The default is 100. If, when plotting any phase boundary, it is possible to calculate more points, then a displayed message will allow the phase envelope calculator to continue or to stop at any point.
The phase envelope will be shown at the same time as this message so that you can make an informed decision as to whether more points would be beneficial.
The phase diagram window also remains open even when you return to the main window to carry out further calculations. When you want to close it click on the Close button.
If the calculations have reached any limit within the 100 points the message will not appear.
Any critical points are labelled with a C and discontinuities, where phase boundaries cross, are marked with a D.
Similarly, the individual flash results which appear in the first three columns in the window are marked with C or D so that the numeric values at these points can be determined.
Phase envelope output:
T/K P/bar
39 294.300 110.80
40D 294.300 110.80
41 294.304 110.95
42 294.306 111.03
The first column is the number of the point plotted.
N.B. When plotting phase boundaries with a high level of salt component in the stream, to obtain sensible results you should allow a solid salt phase to form. If you are using the Salt Component model, set up a solid salt component using Select/Freeze-out Comp onent option, see “The Freeze-out model” on page 48. If you are using the Electrolyte salt component model then switch on the Chloride scale option.