Alph - A Little Process Helper

Hydrate and Phase Envelope Tutorial

This is the second part of the Alph introduction tutorial. If you did not go through the first part, you might want to start there first, but a completed tutorial one case is available.

Alph tutorial pages:

In this part we shall look at determining hydrate conditions and phase envelopes for a fluid.


If you are continuing on from part one, you should see the page at the right on the iPhone and iPod Touch. On an iPad, the Session menu will probably be on top of the diagram. Even if you quit and restarted Alph, it will remember what you were working on. For demonstration purposes though, let's pretend you need to load the tutorial from the saved file.

If necessary, tap the Session button at the left of the title bar (if for some reason you are on a different page than shown, tap whatever button is at the left of the title bar until you get to the Session page shown below).


Now tap the Case line and then the Load Case line on the resulting Case page.


You should now see a page similar to this one, although you probably only have a single case on the spinner.

In any event select the case you saved in part one and tap the Load button at the upper right.


Alph will warn us that we are about to replace any unsaved work.

Go ahead and tap OK.


Now let's add a tool to check the hydrate temperature of the feed fluid.

Tap on the Add Tool button on the bottom tool bar.


Alph has several standard tools:

Set the spinner on Hydrate and tap OK.


Replace the dull name with hydrate and type feed into the Fluid field and tap Done on the pop up keyboard.


Alph tells us that, if free water were present, the fluid would start forming hydrates at just under 15 C at the fluid pressure of 4000 kPa. Since we have set our temperature at 10 C, we better hope the fluid is indeed dry.

If water had been defined in the fluid composition, the water quantity and phase characteristics would have been taken into account in determining the hydrate condition, rather than making the free water assumption.

Tap on the button with T on it to change the calculation so it determines the hydrate pressure at the fluid temperature instead.


It turns out that, if free water were present, we would have to lower the pressure to about 2000 kPa to avoid hydrate formation at the fluid temperature of 10 C.

Tap on the button at the upper left of the title bar (should be labeled wit the case name) to return to the diagram. Note you can also tap the
ifd icon
button that is available on the bottom tool bar of most pages to return directly to the diagram.

iPad On the iPad, you can usually close the current window and return to the diagram by just tapping on any blank spot on the background screen. A few windows require you to explicitly save or cancel them and on those you can also use the
ifd icon
icon to cancel and close the window.


An icon for the hydrate tool has been added to the Information Flow Diagram (IFD) and a line running from the right side of the feed icon to the left side of the hydrate icon shows that the hydrate tool uses information from the feed fluid.

The icon has most likely been automatically positioned as shown, but if you tapped the background of the diagram before tapping the Add Tool button, it would be positioned at that point.

You can also add an object by tapping and holding on the background. If you add an object from the resulting context menu, it will be positioned at the tap.

Tap and hold on the background somewhere until the context menu appears.


Tap the Add Tool line and this time select Phase Envelope on the resulting selection wheel and then tap OK.


You should see an envelope input screen such as this.

Change the name to envelope and enter feed in the fluid field.


Now tap the Done button at the upper right of the screen to have Alph generate the phase envelope. There will be a brief delay (about 2 or 3 seconds on my 3GS) while the calculation is done.


The resulting phase envelope is displayed with the specified quality lines. Note that each specified quality line also includes its counterpart on the other side of the critical point. Thus the specification of 1.0 not only draws the dew point line, but also the bubble point quality line (0.0) as well. Similarly the 0.75 specification means that both the 0.75 and 0.25 quality lines are calculated and plotted.

The small circle with a cross in it represents the current temperature and pressure of the fluid.

The temperature and pressure shown are for the point under the intersection of the full width horizontal and vertical 'cross hairs'.

Try dragging the plot around see how this changes.


Here I have dragged the envelope so the cross hairs are over the critical point.

You can also use two finger pinch and spread gestures to change the size of the plot.


Here I have expanded the plot quite a bit and again centered the crosshairs on the critical point.

To return to the initial scaling and position at any point, simply double tap on the envelope.

Tap the Edit button at the top right to return to the envelope definition page.


Go to the composition field and add a .v to the end of feed as shown. This will designate that we want the feed's vapour composition, rather than its bulk composition. The phase descriptors are:

  • v  - vapour
  • l  - liquid

These are not case sensitive, so they can be entered in upper case if you wish.

Note the phase envelope can only be used for two phase fluids.

While on this page, also add a quality of .95 to the Qualities field, also as shown. Note you can separate the qualities with spaces or commas or both as desired.

Normally you can probably ignore the Start Pressure field and leave it at its default value of 500 kPa. A quality curve calculation starts on the dew point side and this is the pressure it starts from. If you want the dew point side of the curve to start at a lower pressure, you can reduce this value, but be aware that some compositions may start to predict a second liquid phase at colder temperatures. The phase envelope cannot handle multiple liquid phases and will quit at that point with an error message. Any quality lines already calculated will be displayed though. Although it will almost always contain a constant, this is actually a formula field (which will be discussed in the next tutorial section).

Tap the Done button at the top right to see the vapour phase envelope.


The 0.95 quality line illustrates how a small heavy fraction greatly increases the dew point.

Since this composition represents the equilibrium vapour of the feed at 10 C and 4000 kPa, we would expect those conditions to fall on this fluids dew point line, and indeed that is where the current conditions point is shown.

Drag the envelope over to confirm this.


As expected the current conditions point is at 10 C and 4000 kPa, or at least as close as my fat fingers can manoeuvre at this scale, and the dew point line runs through it.

Continue with the next tutorial page to learn more about what you can do with fluids and interactions between them.