Hydrate Calculation Models
The Hydrate Formation Analysis can handle three types of hydrates: Type I, Type II,
and Type H. The hydrate type that will form is determined automatically.
The models used in predicting the hydrate formation conditions are based on fundamental
thermodynamic principles and the original hydrate equilibrium model proposed by van der
Waals and Platteeuw, coupled with a modification suggested by Parrish and Prausnitz (van
der Waals-Platteeuw; see, for example, W.R. Parrish, J.M. Prausnitz, Ind. Eng. Chem. Proc.
Des. Dev. 11 (1) (1972) 26–35.).
Two models/methods are available for Type I and Type II hydrates:
•
Ng & Robinson
•
CSM
These models have been incorporated and enhanced by AspenTech in the model used for its
hydrate predictions. In addition, the equation of state is used to predict properties of the
hydrate-forming components that are in equilibrium with the solid hydrate phase. This model
is referred to as the Ng & Robinson model.
The Ng & Robinson model is based on the following references:
•
H.-J. Ng and D.B. Robinson. “The prediction of hydrate formation in
condensed systems.” AIChE Journal, 23, 4 (1977).
•
H.-J. Ng and D.B. Robinson. “The measurement and prediction of hydrate
formation in liquid hydrocarbon-water system.” Ind. Eng. Chem. Fundam.15, 4
(1976).
•
H.-J. Ng and D. B. Robinson. “A method for predicting the equilibrium gas
phase water content in gas-hydrate equilibrium.” Ind. Eng. Chem. Fundam. 19
(1980).
A new hydrate model developed by the Colorado School of Mines (CSM) is also available.
This work also follows the formulation of van der Waals and Platteeuw but with very
significant enhancements to thermodynamic models for the hydrate and aqueous phases.
This model is referred to as the CSM model and is described in further detail below.
The Ng & Robinson model consists of three submodels: 2-Phase, 3-Phase, and Assume Free
Water, as described below. By default, the most appropriate submodel is selected
automatically based on the process conditions. Since the CSM model is more general, it does
not require a submodel. For structure H hydrate calculations, the model developed by Mehta
and Sloan is used (E.D. Sloan, C. A. Koh. Clathrate Hydrates of Natural Gases, Third Edition.
Taylor & Franci Group, 2007).
The hydrate calculation model can be specified on the Design tab | Connections page using
the Model drop-down list. By default, the Ng & Robinsonmodel is used.
Currently, the Hydrate Formation Analysis can be used with the following property
packages: Peng-Robinson, SRK, Glycol Package, and CPA.
•
CPA: When methanol, MEG, DEG, or TEG is used as the hydrate
inhibitor, CPA is recommended. The CPA property method has built-in pure
component and binary parameters for use in modeling processes that contain
methanol, MEG, DEG, or TEG. CPA is also recommended when using
CSM models, since it is based on an SRK-based equation of state.
A quick introduction to the CPA and methanol partitioning can be accessed at:
o
Methanol Partition Using the Cubic Association Fluid
Package
•
Peng-Robinson: Peng-Robinson has the largest applicable range for
temperature and pressure and can be used for Hydrate Formation Analysis.
•
SRK: When using the CSM model in the Hydrate Formation Analysis,
the SRK property package can be used to achieve better results. The CSM model
was developed based on SRK.
•
Glycol Package: When Glycols (MEG, DEG, and TEG) are used as the
hydrate inhibitor, starting in V10, we recommend that you use the
improved CPA property package to model MEG, DEG, and TEG dehydration
processes. The V10 CPA package includes updated validated parameters that
cover a broader range of components and operating conditions. You can continue
to use the Glycol property package for legacy TEG dehydration cases or for
situations where faster performance is required.
Hydrate Calculation Modes
The hydrate prediction submodel (the Calculation Mode), which is automatically selected for
the calculation, is reported in the Calculation Mode field on
the Design and Performance tabs in the Hydrate Formation Analysis.
The calculation modes reported are as follows:
For Ng & Robinson Model
•
Use 2-Phase Model
•
Use 3-Phase Model
•
Assume Free Water
The Ng-Robinson model is based on the following references:
•
H.-J. Ng and D.B. Robinson, “The prediction of hydrate formation in
condensed systems,” AIChE Journal, 23, 4 (1977).
•
H.-J. Ng and D.B. Robinson, “The measurement and prediction of hydrate
formation in liquid hydrocarbon-water system,” Ind. Eng. Chem. Fundam., 15, 4
(1976).
•
H.-J. Ng and D. B. Robinson, “A method for predicting the equilibrium gas
phase water content in gas-hydrate equilibrium,” Ind. Eng. Chem. Fundam., 19
(1980)
For the CSM Model
Use CSM
For Type H Hydrate
Use SH Model
These hydrate calculation modes and the appropriate model treatments are described as
follows.
2-Phase Model
For scenarios that result in the absence of a free aqueous phase after an equilibrium flash (in
other words, Vapor only, Liquid only, Vapor-Liquid, Liquid-Liquid, and Vapor-Liquid-Liquid,
where Liquid refers to a hydrocarbon liquid), the 2-Phase model is used for hydrate
predictions of the Types I and II.
The 2-Phase model is based on the work of Ng and Robinson. The fugacity of water, as a
function of pressure and temperature in the empty lattice (MT), is determined by data
reduction. Plots of lnfw,o vs. l/T and of (dlnfw) / (dP) vs. T show linear relationships.
where:
fw,o = fugacity of the water at zero pressure over the unfilled Type II lattice
The empty lattice water fugacity at any pressure is represented by the following expression:
(1)
where:
= empty lattice fugacity at any pressure
fw,o = fugacity of the water at zero pressure
P = pressure
By combining this expression with the linear regressed plots, the fugacity of water over the
unfilled hydrate lattice as a function of temperature and pressure is obtained. The
relationships depend on hydrate structure but are independent of the composition of the
examined mixture.
For hydrates of Type I, the fugacity relationships are found to be:
(2)
(3)
where:
T = temperature in Kelvin
For hydrates of Type II, the fugacity relationships are found to be:
(4)
(5)
where:
T = temperature in Kelvin
3-Phase Model
For scenarios that result in the presence of a free aqueous phase after an equilibrium flash (in
other words, Aqueous only, Vapor-Aqueous, Liquid-Aqueous, and Vapor-Liquid-Aqueous,
where Liquid refers to a hydrocarbon liquid), the 3-Phase model is used for hydrate
predictions of the Types I and II.
The 3-Phase model is based on the work of Ng and Robinson. The Parrish-Prausnitz
algorithm is modified to allow for the prediction of hydrates in aqueous-containing systems. All
fluid properties including phase behavior, volumetric behavior, and fugacities are calculated
with the selected equation of state (Peng-Robinson, Soave Redlich Kwong, or Glycol
package). The Kihara parameters for each hydrate-forming component are recalculated
based on the work by Ng and Robinson.
Assume Free Water Model
In the absence of water as a component in the simulation or when the amount of water in the
stream being analyzed equals zero, either the 3-Phase model or the SH model is used for
hydrate predictions.
The Assume Free Water model uses either the 3-Phase model or the SH model and
calculates the hydrate formation by assuming the stream is at the saturation point of water at
hydrate conditions, neglecting the amount of water present in the stream.
Hydrate results for a waterless stream that uses the Assume Free Water in the default
calculation model will be very similar to the hydrate results for the same stream with water
that has had the calculation model manually changed to Assume Free Water. The difference
is due only to the composition difference between the streams when water is removed.
Asymmetric Model
The Asymmetric model is equivalent to the default calculation model/method previously
described; however, it does not include the Structure H calculation. This Asymmetric model
automatically selects:
•
The 2-Phase model for scenarios that result in the absence of free aqueous
phase after an equilibrium flash
•
The 3-Phase model for scenarios that result in the presence of free aqueous
phase after the equilibrium flash
•
The Assume Free Water model when water is not traceable in a given
stream.
Symmetric Model
The Symmetric model is the 3-Phase model. (For more information, refer to the Use 3-Phase
Model section).
Vapor Only Model
The Vapor Only model is identical to the 2-Phase model (for more information, refer to
the Use 2-Phase Model section). This model was originally developed for the Vapor only
case. With an extension proposed by Sloan, the implemented model can be applied for any
scenarios that result in the absence of free aqueous phase after an equilibrium flash; for
example, the Liquid-only case (E.D. Sloan, C. A. Koh. Clathrate Hydrates of Natural Gases,
Third Edition. Taylor & Franci Group, 2007).
Most parameters used in the hydrate models in HYSYS were fitted from experimental data
obtained at the saturation point of water at hydrate conditions. Therefore, the models will
provide reasonable hydrate predictions at the saturated water condition. A saturated water
condition can be obtained by:
•
Not incorporating water in a given stream (in other words, forcing the Hydrate
Formation Analysis to use the Assume Free Water model).
-or•
Adding a sufficient amount of water in the given stream so that a free
aqueous phase appears at the stream condition.
When HYSYS cases containing Hydrate Formation Analyses (without the override
specification) are loaded from previous versions, the default calculation method is
automatically selected and is used for hydrate predictions in the current version. If you want to
have control over the model selection (namely Assume Free Water, Asymmetric
Model, Symmetric Model, or Vapor Only Model), you can override the model by accessing
the Model Override page and then selecting the desired model.