As material enters a holdup, the liquid and vapor feeds can associate in dif-ferent proportions with the existing material already in the holdup. For
instance, a separator’s vapor and liquid feeds can enter the column differently.
It is very likely that the liquid feed mixes well with the liquid already in the hol-dup.
The vapor feed is not mixed as well with the existing material in the vessel since the residence time of the vapor holdup is much smaller than that of the liquid. If the feed nozzle is situated close to the vapor product nozzle, it is pos-sible that even less mixing occurs. In the physical world, the extent of mixing the feeds with a holdup depends on the placement of the feed nozzles, the amount of holdup, and the geometry of the piece of equipment.
Efficiencies
In HYSYS, you can indirectly specify the amount of mixing that occurs between the feed phases and the existing holdup using feed, recycle, and product effi-ciencies. These feed efficiency parameters can be specified on the Efficiencies tab of the unit operation’s Advance property view. Click the Advance button on the Holdup page under the Dynamics tab to open the Advance property view.
Essentially, the efficiencies determine how rapidly the system reached equi-librium. If all efficiencies are 1, then all feeds reach equilibrium instant-aneously. If the values are lower, it takes longer and the phases cannot be in equilibrium and can have different temperatures.
A flash efficiency can be specified for each phase of any stream entering the hol-dup. A conceptual diagram of the non-equilibrium flash is shown for a two
phase system in the figure below:
As shown, the flash efficiency, η, is the fraction of feed stream that participates in the rigorous flash. If the efficiency is specified as 1, the entire stream par-ticipates in the flash; if the efficiency is 0, the entire stream bypasses the flash and is mixed with the product stream.
The recycle stream (and any streams entering the holdup) participates in the flash. You can specify the flash efficiency for each phase of the recycle stream and any feed entering the holdup. The flash efficiency can also be specified for each phase of any product streams leaving the holdup.
Note: Product flash efficiencies are only used by the holdup model when reverse flow occurs in the product flow nozzles. In such cases, the product nozzle effectively becomes a feed nozzle and uses the product flash efficiencies that you provided.
The default efficiencies for the feed, product, and recycle streams is 1, and this value is sufficient in the vast majority of cases. The flash efficiencies can be changed to model non-equilibrium conditions. For example, the efficiency of vapor flowing through a vessel containing liquid can be reduced if the residence time of the vapor is very small and there is little time for it to reach ther-modynamic equilibrium with the liquid. Also, in some narrow boiling systems,
lower efficiencies can be used to reduce the rate at which material can con-dense or evaporate. This can help to stabilize the pressure in certain difficult cases such as narrow boiling systems like steam.
For example, a water system is heated by pure steam (no inerts) can encounter problems if the stream efficiency is specified as 1. If the holdup material is sig-nificantly larger than the stream flow, all the steam condenses and the holdup temperature increases accordingly. No vapor is present which can complicate pressure control of the system. In the physical world, typically not all of the steam condenses in the water and there are also some inerts (e.g., nitrogen or air) present in the system. Using lower efficiencies can help to model this sys-tem better.
Nozzles
In HYSYS, you can specify the feed and product nozzle locations and diameters.
These nozzle placement parameters can be specified in the unit operation’s Nozzles page under the Rating tab.
The placement of feed and product nozzles on the equipment has physical mean-ing in relation to the holdup. The exit stream’s composition depends partially on the exit stream nozzle’s location in relation to the physical holdup level in the vessel. If the product nozzle is located below the liquid level in the vessel, the exit stream draws material from the liquid holdup. If the product nozzle is loc-ated above the liquid level, the exit stream draws material from the vapor hol-dup. If the liquid level sits across a nozzle, the mole fraction of liquid in the product stream varies linearly with how far up the nozzle the liquid is.
Static Head Contributions
When the Static Head Contributions check box is selected on the
Options tab of the Integrator property view, Aspen HYSYS calculates static head using the following contributions:
l Levels inside separators, Towers, and so forth
l Elevation differences between connected equipment
Note: Including static head contributions in the modeling of pressure-flow dynamics is an option in Aspen HYSYS.
For unit operations with negligible holdup, such as the valve operation, Aspen HYSYS incorporates only the concept of nozzles. There is no static head con-tributions for levels, unless the feed and product nozzles are specified at dif-ferent elevations.
You can specify the elevation of both the feed and product nozzles. If there is a difference in elevation between the feed and product nozzles, Aspen HYSYS uses this value to calculate the static head contributions. It is recommended that static head contributions not be modeled in these unit operations in this way since this is not a realistic situation. Static head can be better modeled in these unit operations by relocating the entire piece of equipment.
Static head is important in vessels with levels. For example, consider a vertical separator unit operation that has a current liquid level of 50%. The static head contribution of the liquid holdup makes the pressure at the liquid outlet nozzle higher than that at the vapor outlet nozzle. Nozzle location also becomes a factor. The pressure-flow relationship for the separator is different for a feed nozzle which is below the current liquid holdup level as opposed to a feed which is entering in the vapor region of the unit.
It is important to notice that exit stream pressures from a unit operation are cal-culated at the exit nozzle locations on the piece of equipment and not the inlet nozzle locations of the next piece of equipment.