Alph is no longer for sale.

I am afraid revenue generated from Alph has not proven to be sufficient to justify its continued sale. I greatly appreciate the support of the folks who have purchased Alph in the past and hope that it will continue to serve you well in the future.

This and related pages will be maintained for archival purposes only.


Alph Deep Cut Turbo Expander Example

From your device, you can download the completed example


This example demonstrates the use of a turbo expander to cryogenically recover ethane from a natural gas feed.

Turbo Expander Demethanizer

The inlet gas is cooled by a network of heat exchangers after which the vapour portion is fed to an expander, while the liquid is flashed through a Joule Thompson valve. The liquid portion of the expander outlet is fed to the top of a demethanizer, while the entire outlet of the J-T values is fed in slightly lower in the tower.

The heat for the reboiler and two side exchangers is supplied by the cooling of the feed fluid, with 40% of the heat going to the reboiler and 30% to each of the side exchangers. To ensure high purity ethane after the heavier compounds are removed in later processing, the methane to ethane ratio in the bottom liquids should be no more than 0.02.

The power generated by the expander is used to recompress the sales gas as much as possible.


The natural gas feed stream.

The cooled feed fluid. A temperature of -50 C has been chosen and the subsequent calculation shows there is enough "cold" to achieve this, however some optimization is probably possible, especially if exchanger temperature approaches are examined.

The vapour phase of the lts fluid is expanded in the expander tool to become this fluid, the liquid phase of which is fed to the top of the demethanizer.

The liquid phase of the lts fluid is flashed through a valve to the same pressure as expout to become this fluid, which is fed in its entirety to stage 2 (the third stage) of the demethanizer.

The vapour leaving the top of the demethanizer.

The demethanizer liquid product from the tower.

The combination of the vapour phase of expout and the ovhds fluid. This is fed back into the heat exchanger network to help cool the feed fluid.

The is the mixedovhd after being warmed in the heat exchanger network. It's enthalpy is calculated by a heat balance, where the total heat removed from the feed fluid, minus the heat used by the side exchangers, is added to the warmovhd heat flow. The resulting temperature must be less than the feed temperature for the process to be feasible.

The recompressed warmovhd fluid.



This is the sum of heat flows of the reboiler and two side exchangers used in the tower.


A pressure drop value used in a few places.



The lts vapour is expanded and chilled by this tool. For this example the pressure is arbitrarily set to 2000 kPa, but could be subject to optimization. An adiabatic efficiency of 80% was used.


The demethanizer is the heart of this process. It is configured, rather arbitrarily, with 11 stages, the feeds going to stages 0 and 2 and with energy inputs on stages 5, 8 and 10. The top pressure is set to be the same as the expander outlet, while the bottom is that plus the dp variable.

The top temperature estimate is given as -90 C (close to the expout temperature), while the bottom temperature is estimated (fairly poorly as it turns out) as 15 C. The overhead flow rate is estimated at 1000 kgmole/h (the total methane in the feed to the tower is about 1100 kgmole/h) and the reflux estimate is just left at 1.

Each energy input has a specification:

  • (#._1.l.x:0 / #._1.l.x:1 0 0.2) * 10
    the ratio of methane to ethane in the liquid from the bottom stage should be 0.02. The calculated error is multiplied by 10, just to tighten the convergence to a more precise value.
  • #.8.q / (#._1.q + 1e-10) - .75
    the ratio of the heat flow of the side exchanger on stage 8 to the reboiler heat flow should be 0.75. This is in keeping with the 40%, 30%, 30% ratio mentioned earlier (30/40 = 0.75). The addition of the 1e-10 term is just to prevent the possibility of a division by zero during the solution.
  • #.5.q / (#._1.q + 1e-10) - .75
    the stage 5 side exchanger will also be 0.75 times the reboiler duty.


A mixer that combines the ovhds fluid from the tower with the vapour phase of the expout fluid from the expander.


A compressor tool that compresses the warmovhd fluid to become the salesgasfluid. The outlet pressure is given by the formula:

{fromunit "kpa" "p" 2500} + #equatepower.0 * $dp

where equatepower is a function solver tool that solves when the compressor power requirement matches the power produced by the expander. The {fromunit "kpa" "p" 2500} sets 2500 kpa, in a unit system independent manner, as the base value at the initial solver output of 0. The solver output values can range from -10 to 10, so its value is scaled up using the $dp variable as a convenient value (dubious, but easy).


The solver tool mentioned above. The error calculation is simply the sum of the expander and compressor power values, keeping in mind that an expanders power will be negative.