Using HYSYS to check sales gas specifications

A process engineer supporting gas operations will often need to use HYSYS to calculate sales gas properties such as water dewpoint, hydrocarbon dewpoint, heating value and water content.

Typical sales gas specifications are as follows:

Water dewpoint not to exceed -10C at 8275 kPa
Hydrocarbon dewpoint not to exceed -10C at operating pressure
Shall not contain more than two (2) percent by volume of carbon dioxide
Shall not contain more than 4 lb H2O/MMSCF
Shall have a gross heating value of not less than 36 MJ/m3

Sales gas specifications vary by region and by recipient; these are typical gas specifications in Alberta, Canada.

HYSYS is well-suited to calculate these properties - in particular much work has gone into fitting the Peng Robinson equation of state interaction parameters for standard gas operations.

Calculating Dewpoints

Most HYSYS users know that a stream dewpoint can be calculated by setting the vapour fraction to 1. However, this calculation does not differentiate between a hydrocarbon or water dewpoint (unless you look at the incipient phase).

Here are two ways to easily calculate hydrocarbon and water dewpoints in HYSYS:

Aspentech developed a unit operation extension called "Gas Properties" which calculates both the hydrocarbon and water dewpoints for the stream to which it is attached. To use the Gas Properties unit operation extension, it must first be downloaded from the Aspentech Technical Support site, then registered in HYSYS. Once registered, it can be added as a unit operation extension.
HYSYS 3.1 introduced gas property correlations which enable dewpoints to be calculated in the stream. Gas property correlations can be enabled by selecting the "Append New Correlation" button on the Properties tab of a stream, and are available under the "Gas" heading.

If you wish to calculate a dewpoint at a pressure which is different than (e.g.) the sales gas pressure, we recommend using a Balance operation to transfer the composition to a new stream, and then specify a new pressure in that stream.

Pitfalls of Dewpoint prediction

Usually, HYSYS does a reasonable job calculating stream dewpoints. However, there are some pitfalls to watch out for:

Streams which have small amounts of TEG (e.g., dry gas from a TEG contactor) can predict an incorrect dewpoint. We recommend one of the following:
- use a component splitter to remove all TEG from the dry gas, and then calculate the dewpoint
- use a different property package for TEG dehydration/regeneration which includes the component TEG. When the dry gas leaves the TEG subflowsheet, it should transfer to the main flowsheet property package which does not contain TEG. All TEG is therefore removed at the flowsheet boundary, and the dewpoint can be calculated in the main flowsheet.
Be careful when comparing stream dewpoints to those calculated by the envelope utility. By default, the envelope utility is constructed on a dry basis and therefore the dewpoint line corresponds to the hydrocarbon dewpoint. Note that in recent versions of HYSYS a "COM Thermo Three-Phase" envelope can be plotted which shows both the water and hydrocarbon dewpoint lines.
Hydrocarbon dewpoint calculations are very sensitive to trace amounts of heavier hydrocarbons.
The hydrocarbon dewpoint for an analysis which shows an amount of "C7+" can have a very different dewpoint depending on whether the heaviest component is assumed to be (e.g.) n-C7 or n-C10.
One should be aware of the maximum pressure for the phase envelope (cricondenbar). In the supercritical (dense phase) region, the concept of a dewpoint is not applicable. However, it should be noted that HYSYS may apply a vapour fraction of 0 or 1 in the supercritical region as an indication of which correlations are being used for the stream.

Calculating Water Content and Heating Values

As with the dewpoints, water content can be calculated in HYSYS using the "Gas Properties" extension or by adding the corresponding gas property correlation to a HYSYS stream. The Gas Properties extension calculates the water content in two sets of units (lb/MMSCF and mg/m3) while the stream correlation only calculates the water content in mg/m3.

Alternatively you could set up a spreadsheet calculation in HYSYS to calculate water content in lb/MMSCF units by importing the water mass flowrate and stream flowrate in MMSCFD.

Higher and Lower heating values can be calculated using the "Gas Properties" extension or by adding the corresponding gas property correlation to a HYSYS stream. Results are usually very similar for typical gas analyses but may differ in some cases due to treatment of "non-standard" components.

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