Many engineering applications involve a wide range of flash calculations, not just those related to determining the phase envelope. For example, an isenthalpic flash at fixed pressure can be used to simulate the expansion of a stream through a valve
Basing this case study o n the simple hydrocarbon stream (and model) we first defined
Methane 0.45
Ethane 0.20
Propane 0.10
Butane 0.10
Hexane 0.10
Decane 0.05
we must initially carry out a P,T flash at the upstream conditions to determine the enthalpy and then a P,H flash at the exit pressure.
Having loaded the model set and stream information Enter the upstream temperature, 300K and pressure, 50 bar Click on the P,T flash button
The calculated total enthalpy is -10912.3 J/mol
The stream is then throttled isenthalpically to 10 bar, by Entering the new pressure, 10 bar under Conditions Entering the calculated enthalpy under Conditions Clicking on the P,H flash button
The calculated temperature at outlet has dropped to 276.468K.
You can also add the isenthalpic boundary for -10912.3 J/mol to your phase envelope.
PVT Analysis
Many users will receive a PVT Analysis for the composition of an oil or gas from one of the PVT laboratories and wish to use this as input to Multiflash.
These reports follow a fairly standard format and the PVT Lab Analysis form endeavours to reproduce this to make entering information as easy as possible.
The facility to add or delete components from the generated list is also useful.
The form is discussed in detail in section “PVT Analysis ” on page 86.
The case study we are considering here is based on a problem setup file called pvt_anal2.mfl, which uses the Revised Analysis method.
To enter a PVT Analysis when you have no measured n -paraffin distribution either choose the Select/PVT Lab Input menu option or click on the icon.
The Lab Analysis form will then be displayed.
Initially we will consider a case where you only have a single fluid composition.
First select the datasource for your discrete (i.e. well-defined) pure components.
This can be Infodata or DIPPR and we have chosen Infodata. Next at the top of the column headed Single fluid choose either mass or mol % as appropriate by clicking on the down arrow. If your PVT report offers a choice of mole or mass
%, it is the mass % that is the experimentally measured data and should be given preference for separator oils. Next enter the compositions of the discrete components and the compositions of the petroleum cuts. In the form the
pseudocomponents or single carbon number (SCN) cuts are labelled C6, C7 etc.
In your PVT Laboratory report they may be referred to as hexanes, heptanes etc., with the heaviest being labelled as a plus fraction such as C20+ or eicosanes+.
In our example the heaviest SCN is C20.
The overall percentage will be totalled as you enter the compositions. If the final total is not 100 you will be offered the opportunity to normalise the compositions when you characterise the fluid.
You can enter further information to define the stream, such as the molecular weight of the Stock Tank Oil (STO), the total fluid or the heaviest SCN or the specific gravity of either the heaviest SCN or the STO. We have provided general advice on when such data should be supplied in “Fluid composition” on page 90. As the fluid in question has a heavy end (C6+) which comprises more than 50% of the stream we should supply this information if possible. We have therefore entered the molecular weight of the heaviest SCN but if you have the molecular weight of the total fluid available this may be preferable as this is again the measured quantity.
We will use the default distribution method, Infoanal2.
You are now ready to define the basis of your characterisation by choosing where in your existing analysis you want to start redistributing the remaining fluid into new pseudocomponents and how many pseudocomponents you want to split this heavy end into. We’ve started with the simplest case where we have chosen to start the split at the heaviest SCN and only allocate one
pseudocomponent. Effectively we are only allocating physical properties to the existing SCNs. Click on the Do Characterisation button and you will see a message box such as
followed by a screenshot of the experimental data and the fitted distribution
Click on OK and Close to return to the main window where the new fluid composition will be reported
Properties of the individual pseudocomponents may be viewed using Tools/Pure Component Data as usual and further calculations can be carried out on the basis of this characterisation.
At this point, having successfully characterised the fluid, you can also save the input as an .mfl file.
A useful way of seeing how changing characterisations alter the results of phase calculations is to use the phase envelope generator. For instance, plot the phase envelope of this fluid.
You can investigate various aspects of the characterisation and the sensitivity of the phase envelope to changing these. For instance you can include a n -paraffin distribution by ticking the Estimate Wax Content box. Set the starting point for the n-paraffin to N6 with 15 n-paraffins. In the this case the names and compositions of the fraction cuts will differ,
Change the distribution function to Infoanal1 and repeat the process. In neither case is the phase envelope significantly affected.
If you return to the PVT Lab Analysis form and instead of the heaviest SCN choose total liquid and enter a MW of 68. Do the characterisation and plot the phase envelope. Then see what the effect is of extending the heaviest SCN to further fractions, by leaving C20 as the start of the pseudocomponents but choosing to split it into 5 pseudocomponents. Alternatively you can group the components by starting the pseudocomponent split at C8 and grouping the plus
fraction into 15 pseudocomponents. You can see that this alters the cricondenbar but the major effect is on the cricondentherm.
With Infoanal1 you cannot choose a starting point that is above the highest cut for which experimental data are entered. With Infoanal2 you can set the start ing point to be any pseudocomponent cut provided this is lower than the highest cut in the Component column. The highest default cut is C100.
Next, return to the original fluid definition and re-plot the phase envelope, then in the PVT Analysis form enter a watercut. This is defined in terms of the volume percentage of the total fluid that is water. In this case choose 3 %. In the main window plot the new phase envelope and the water phase. boundary.
Finally, return to the original fluid analysis again and this time add a separator gas. Here we will look at a simple problem where the gas is 100 % methane added at a GOR of 100 m3/m3. Move to the Liquid + Gas tab and enter 100 next to methane in the left hand column headed separator gas and in the Re combined fluid section of the PVT form set the GOR units to m3/ m3 and enter 100. Do the characterisation and return to the main window and plot the new phase envelope.
Black Oil Analysis
The black oil analysis offers the user an opportunity to take a very limited input specification (known as Black Oil input) for a condensate or oil and from this generate a normal compositional analysis. Our example is based on the blackoil.mfl file.
The minimum required input is the gas gravity(relative to air), the STO specific gravity(relative to water) at 60F and 14.7 psi and the solution GOR. The latter is the volume of gas produced at surface standard conditions divided by the volume of oil entering the stock tank at standard conditions. It is often referred to as Rs.
The remainder of the form is the standard PVT, except that you do not provide molecular weight or specific gravity. You can choose the pseudocomponent distribution as normal, depending on the final application. In this case the split is fifteen fractions from C6+. Clicking on Do Characterisation generates the message that the characterisation has been successfully completed – in this case there is no compositional information to generate the compositional plot. The new composition is echoed in the main window and the phase envelope can be plotted as before.
Additional data can be added such as the Watson K-factor and/or the Gas analysis. Plotting the phase envelopes shows the effect of including this data.