Oil and gas systems

Initially we will look at the phase envelope of a system which contains six components: methane, ethane, propane, butane, hexane and decane.

As with the pure component data system discussed earlier the case study can be set up interactively or by using a problem setup file. Only the former will be discussed in detail, but the setup file, hycbvle.mfl, is provided and may be used to load models and components or as an example for writing or editing your own files for handling similar cases.

Specify the model

Any of the equation of state models would be suitable for handling this problem. We have chosen the advanced version of RKS but very similar results would be expected using advanced PR, PR78 or RKS.

Select Select, then

Select Model set from the sub-menu, followed by

Selecting Equations of state from the Select Model set dialogue box and again,

Selecting RKS (Advanced) with the default option for transport property models.

Finally, click on OK in the message box and Close in the Model set dialogue box.

Specify the components

The INFODATA databank will be the default data source and this is acceptable for this case study, so we can move directly to specifying the components. You can activate the Select Components dialogue box by either

Clicking on the select components button

or

Selecting Select from the menu bar and

Selecting Components from the sub-menu

The various methods for selecting or searching for components have been shown before, see “Selecting components” on page 39. As our current system contains simple well known compounds they have been selected by

Highlighting the Name option button

Typing the component name in the Enter name text box

and pressing the enter key after the name to load it for Multiflash

or

Clicking on Add to load the component.

Click on Close to load the components

Define the composition

Click on Compositions in the Conditions section, and

Type in the compositions in the drop down table. For our example they are:

Calculating the bubble point curve

You can calculate the bubble point curve using a series of flash calculations or by generating the phase envelope. Individual calculations may be a series of bubble point calculations at either fixed temperature or fixed pressure. Technically there is no particular reason to prefer one over the other; for this particular example we will fix temperature and have our output pressure in bar.

To change the pressure units, click on the Select input and output units button , then in the Tab control click on Pressure and click in the Output option button box against bar. (You can also change the input units for pressure if you wish.)

Having specified the model, components, compositions and units,

Enter the first temperature, 250K, in the Conditions section and

Click on the T Bubble button .

Repeat the last two steps increasing the temperature by 25K each time. At 375K you still have a stable solution reported but by 400K the solution is reported to be “marginally stable” but by 425K you will notice a failure message

Bubble point at fixed T:
*** WARNING  -20323 ***
Possible instability in solution (constrained flash)
*** ERROR   20334 ***
Constrained flash solution unstable
*** ERROR   20028 ***
Cannot solve flash problem - other unspecified error
*** ERROR     344 ***
The flash calculation has not converged

Investigating the bubble point curve using reduced temperature steps you will see that at the solution is marginally stable up to 404K and fails at 405K. The compositions of the liquid and gas phases are very similar, indicating that we are probably close to the critical region. You can either investigate this area of the phase envelope further using P,T flashes or move to calculating the dew point curve to formulate a view of the phase envelope.

Calculating the dew point curve

As with the bubble point curve, the dew point curve can be calculated from a series of dew point calculations at either fixed temperature or pressure. Again we will use dew point calculations at fixed temperature for this example

As with the bubble point curve, start at 275K and calculate the dew point at 25K intervals using the T dew point button .

This time the dew point calculation fails to converge at 475K. Returning to the last successful convergence at 450K, increase the temperature in 5K steps. This time the first failure to converge occurs at 470K.

We know that it is probable that the critical point is around 400K so it is a reasonable assumption that this system has a significant retrograde region. Check this by repeating the dew point calculation at 450K but this time by using the fixedphase fraction route and looking for the upper retrograde solution.

To do this:

Click on the fixedphase flash at fixed temperature button, ,

In the dialogue box which is then activated,

Click on "Do flash".

The calculated dew point pressure is now 114.527 bar for the retrograde region, whereas for the “normal” dew point the calculated pressure was 28.1796 bar. This confirms that we have a retrograde region for this system.

To calculate the full dew point curve you therefore need to increase the temperature at 1K intervals above 460K, using the normal T, dew point flash, until you meet the first convergence failure, at which point you are just beyond the cricondenbar. You should now switch to fixedphase fraction flashes at fixed temperature, set the options as described and reduce the temperature in small steps. This will define the retrograde dew point curve to 403K.

Phase envelope

The same problem can be investigated more easily using the phase envelope calculator. Set up the problem as before, but instead of carrying out individual dew and bubble point calculations

Select Calculate/Phase Envelope or click on the Phase Envelope button

Click on V/L Autoplot and Click on Yes when the message box appears asking if more points should be calculated. The resulting plot includes the dew and bubble point lines and the critical point is labelled.

The output in the results window will allow you do identify the critical point explicitly

     53C	     402.732	  147.11

Adding water to the system

It is quite common in the oil and gas industry to find some water present with hydrocarbon systems, which will probably form a separate liquid phase. In this case study we can add water and look for the water dew point line.

To add water to the input stream

With the hydrocarbon case study defined,

Click on the Select component button

In the Select components dialogue box, enter water in the Enter name text box then press the enter key or click on Add

Click on Close

In the main window, click on composition and in the drop down table enter a composition for water, say 0.2.

Calculating the water dew point line

If there are more than two liquid phases present then using Multiflash to calculate the dew point, defining either temperature or pressure, will result in finding the primary dew point, in this case the hydrocarbon liquid dew point. To find the dew point for the second liquid phase, in this case water, you must use the fixed phase fraction flash and look for the temperature or pressure where there is a zero amount of that phase. Therefore, for the water dew point,

Enter 300K in the Conditions section of the main window.

Click on the fixedphase fraction flash at fixed temperature button, .

In the resulting dialogue box, click on the arrow under Select phase and select water. Under Select basis choose Mole Fraction and enter a phase fraction of 0.0. The “normal” type of solution should be satisfactory for the water dew point.

Click on Do flash

Repeat the calculation at increasing temperatures to obtain the water dew point line.

Alternatively, plot the water dew point line using the Phase Envelope calculator by selecting the water phase at 0.0 molar phase fraction.

Including a petroleum fraction

The heaviest component in our hydrocarbon stream is decane, but often the heavier end of oil or gas condensate systems is defined as a petroleum fraction rather than as a single specified component. Each petroleum fraction will consist of a mixture of components and the fraction as a whole will be defined in terms of its molecular weight, density and possibly boiling point, although the first two properties are the most likely to be reported. Often the heavy end will be reported as a single fraction, e.g. C7+, although sometimes a more detailed analysis may be available breaking the heavy end down into several fractions.

Multiflash includes petroleum fraction correlations which may be used to predict the thermodynamic and transport properties of the fraction based on the data available.

For this case study we will remove decane and water from our stream and replace decane with a petroleum fraction of molecular weight 234 and specific gravity 0.838.

To delete components

Assuming the stream definition for the last case study is loaded

Click on Select components button

In the Select Components dialogue box, select water in the list of components selected for Multiflash, then click on Delete. This will remove water from the list. Repeat this for decane. You should now be left with methane, ethane, propane, butane and hexane.

To add the petroleum fraction

In the Select components dialogue box

Click on the arrow to the right of the Data source list box, then select Petroleum fractions correlations.

In the dialogue box then activated:

Enter C7+ in the Name box, enter 234 for Molecular Weight and enter 0.838 for specific gravity.

Click on Add to include the fraction in the defined stream.

Alternatively you could use the Replace option to replace decane with C7+.

Click on Close

In the main window click on Composition and enter 0.05 for the amount of C7+.

The petroleum fraction is now included in the stream definition and the phase envelope calculation may be repeated with the new stream, although the cricondenbar, cricondentherm and retrograde regions will now be different.

If you only know that the petroleum fraction is C7+ but do not have a reported MW or specific gravity you can simply fill in 7 as the Carbon number

and Multiflash will determine the properties from a set of standard tables.

Other flash calculations

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 on the simple hydrocarbon stream (and model) we first defined

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.