Written by James Holoboff, Process Ecology
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/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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