Case studies - Hydrate dissociation, formation and inhibition
Introduction............................................................................................................................................... 1
Defining the hydrate models................................................................................................................... 2
Fluid phase model..................................................................................................................... 2
Hydrate model........................................................................................................................... 2
Nucleation model...................................................................................................................... 2
Ice model.................................................................................................................................... 3
Phases........................................................................................................................................ 3
Hydrate calculations with Multiflash.................................................................................................... 3
Will hydrates form at given P and T ?................................................................................... 4
Hydrate formation and dissociation temperature at given pressure................................. 5
Hydrate formation and dissociation pressure at given temperature................................. 7
Hydrate phase boundary......................................................................................................... 8
Other flash calculations with hydrates.................................................................................. 9
Maximum water content allowable before hydrate dissociation....................................... 9
Calculations with inhibitors.................................................................................................................. 10
Can hydrates form at given P and T ?................................................................................. 10
Hydrate dissociation temperature at a given pressure..................................................... 11
Hydrate dissociation pressure at a given temperature..................................................... 12
Hydrate phase boundary....................................................................................................... 12
Amount of inhibitor required to suppress hydrates......................................................... 13
Salt inhibition.......................................................................................................................... 14
CPA model............................................................................................................................................... 16
This section is only applicable if your copy of Multiflash includes the hydrates option.
Natural gas hydrates are solid ice-like compounds of water and the light components of natural gas. Also, some heavier hydrocarbons found in gas condensates and oils are now known to form hydrates if smaller molecules such as methane or nitrogen are present to stabilise the structure. Hydrates may form at temperatures above the ice point and are therefore a serious concern in oil and gas processing operations. The phase behaviour of systems involving hydrates can be very complex because up to seven phases must normally be considered. The behaviour is particularly complex if there is significant mutual solubility between phases, e.g. when inhibitors or CO2 are present. Multiflash offers a powerful set of thermodynamic models and calculation techniques for modelling gas hydrates.
The models used in Multiflash for hydrates and hydrate inhibition have been briefly described, see “Hydrate model” on page 55 or our separate guide to models and physical properties.. Components known to form hydrates are also listed.
To ensure that reliable results are obtained it is particularly important that the correct set of models and phase descriptors is used. The Hydrate model sets contain a complete description of model and phase specifications (as do the relevant hydrate model configuration files).
To define a hydrate model interactively, select Model Set from Select option in the menu bar and click the Hydrates tab to activate the hydrates dialog box. The Hydrate model is then defined by selecting the relevant hydrate phases, i.e. Hydrate 1, Hydrate 2 or Hydrate H; the default is for hydrate1 and hydrate2 to be selected. The thermodynamic hydrate model will calculate the hydrate dissociation temperature or pressure, i.e. the point at which hydrates can form. To predict the temperature or pressure at which hydrates will definitely form you need to calculate hydrate nucleation. To do this you should also select Phase Nucleation. Phase Nucleation in the list of phase descriptors always works in conjunction with one of the solid phases such as any hydrate phase or the ice phase. Therefore the selection of Phase Nucleation does not increase the number of phases associated with the model used.
The hydrate model sets and the hydrate model configuration files have the following definitions.
Fluid phase model
To carry out the full range of hydrate calculations with all the available inhibitors the recommended fluid phase model is the advanced RKS equation of state with the a parameter fitted to the pure component vapour pressure, the Peneloux density correction and the Infochem mixing rule. The required binary interaction parameters for hydrocarbons, light gases, water and inhibitors are available from the OILANDGAS BIP dataset. However, for inhibition with methanol, MEG, DEG or TEG CPA may be preferable as it reproduces the partitioning of methanol, and MEG between water and hydrocarbon vapour and liquid phases more accurately than RKSA plus NRTL style mixing rule. As a result it will usually predict less conservative results for the amount of methanol required for a fixed inhibition. The differences between the model predictions will be most marked for systems with low water content and/or significant amounts of C6+.
Hydrate model
The thermodynamic hydrate model consists of lattice parameters for the empty hydrate and parameters for the interaction of gas molecules with water in the hydrate. There are different parameter values for each hydrate structure, Hydrate 1, 2 and H. In addition the hydrate must be associated with a liquid phase model that is used to obtain the properties of water. It is important that this is the same model that is used for water as a fluid phase.
Nucleation model
This model can be used to predict the nucleation of any hydrate phases and provides an estimate of the temperature or pressure at which hydrates can be realistically expected to form. The nucleation model is based on the statistical theory of nucleation in multicomponent systems.
With the Infochem hydrate model described above and the nucleation model, the hydrate formation and dissociation boundaries can be predicted. Between these two boundaries is the area of hydrate risk.
Ice model
Ice is treated as a pure solid phase. The Infochem freeze-out model can be used to model the solidification of any component. As with the hydrate phase it is necessary to associate the solid phase model with a liquid phase model that is used to obtain the properties of water. It is important that this is the same liquid model that is associated with the hydrate phase. The nucleation model can also be used to predict the temperature or pressure at which ice starts to nucleate.
Phases
In most cases six phase descriptors (PDs) are required: gas, hydrocarbon liquid, aqueous liquid, hydrate 1, hydrate 2 and ice. At high pressures and/or low temperatures the “gas” phase may become liquid-like and a second non-aqueous liquid PD is needed. This is also the case if there is a significant amount of CO2 or H2S present. When considering structure H hydrates an additional phase descriptor is needed for hydrate H.
In most practical cases a natural gas contains propane and the stable hydrate structure will be hydrate 2, although for very lean gases at higher pressures hydrate 1 may be the most stable form. Key components are defined to distinguish between the hydrocarbon and aqueous liquid phases.
The phase names used in the hydrate models are: GAS, LIQUID1, LIQUID2, Water, Ice, HYDRATE1, HYDRATE2 and HYDRATEH. You can apply Phase Nucleation to any of the solid phases (hydrates or ice) defined by the hydrate model. If Phase Nucleation is selected, this means that the nucleation model is defined and can be used to predict the nucleation of any of the hydrate phases or ice.
In contrast to the thermodynamic hydrate model which allows all possible phases to be present when carrying out calculations, the nucleation model considers only the nucleation of the specified phase. At low pressures this can lead to predictions that the hydrate formation temperature is higher than the dissociation temperature. However, this is not a real situation as ice is not being allowed to form.
In principle, hydrate calculations with Multiflash are no different from flash calculations for fluid phases alone. Multiflash treats fluid and solid phases on the same basis and the full range of flashes can be carried out for streams with hydrates.
An important point to note is that you must include water in the mixture explicitly if you wish to do hydrate calculations. Unlike some other programs Multiflash does not assume that water is present unless you specify it. The amount of water may influence the results of the calculations, particularly when inhibitors or water-soluble gases are present.
It may be useful for this case study to look at the results alongside the phase diagram for the system, shown below
Will hydrates form at given P and T ?
To find out whether a mixture will start to form hydrates at a given pressure and temperature it is simply necessary to define your mixture, specify a hydrate model set and do a P, T flash. If you wish to start from a problem setup file we have provided hyd1.mfl
To define the case study interactively:
Select Select, then select Model Set, followed by selecting Hydrates tab to activate the Hydrates dialog box. In the dialog box, select the relevant phases required and initially specify RKSA (Infochem) as the fluid phase model.
Click on OK once the hydrate model set has been successfully defined and loaded.
Specifying the components and composition
We will use a simple binary mixture plus water for this case study, but in practice it is common to calculate hydrate dissociation and inhibition with large multicomponent streams including several petroleum fractions. The mixture chosen consists of 0.5 mol methane, 0.5 mol butane plus water.
Define the components in the usual way; click on the Select components button, enter the component name in the Enter name text box and press the enter key or click on Add to select them for loading into Multiflash. Click on Close
Click on composition and enter the correct number of moles for each component. Add 1.0 mol of water.
Enter the temperature, 250K and the pressure, 10 bar (remember to change the standard pressure units from Pa to bar). The input units are defined in moles but the outputunits for this example are in g.
Click on the P,T flash button
You will see the following results in the results window.
Hydrate 2 is formed at the specified conditions, and you can see that this is in agreement with the phase diagram. Note that the output shows the amount of hydrate formed just as it does for other phases.
Hydrate formation and dissociation temperature at given pressure
The hydrate formation or dissociation temperature calculation is an example of a fixed phase fraction flash. The dissociation temperature is the point below which hydrates can form (also known as the equilibrium hydrate formation curve). The formation temperature is the point at which the nucleation of hydrate occurs and hydrate will form. Between these two points is the area of hydrate risk where hydrates may or may not form depending on the time scale.
To calculate the hydrate dissociation temperature at given pressure
Retain the pressure at 10 bar
Either
Click on the Hydrate dissociation T at fixed P
button, 
or the Fixed
Phase Fraction Flash at specified pressure button, 
In the first case Multiflash will determine the most stable hydrate structure and report the dissociation temperature for this. In the second case a dialogue box will be activated, click on the button next to Select phase and from the list select Hydrate2. Select Normal from the Type of solution and enter 0.0 for the molar phase fraction
Click on Do flash
The results,
show that the hydrate first begins to form at 273.236K. It is important with the fixed phase fraction flash to specify the correct hydrate structure to search for. If Hydrate1 was specified in the above example the calculation would fail because there is no solution where hydrate1 is more stable than hydrate2. In most cases of practical interest hydrate2 is the structure formed, although hydrate1 may be more stable at high pressures for streams containing a high concentration of methane or H2S. If hydrate1 is more stable and if there are enough components present in the mixture, hydrate1 should be present in non-zero amount in the list of phases formed. If in doubt you can check with the P,T flash option which reports which structure is stable.
Hydrate formation temperature at given pressure
To calculate the hydrate formation temperature at 10 bar, make sure the nucleation model has been defined. Then select Nucleation from “Select basis” in the Fixed Phase Fraction Flash – at specified P dialog box., Set the phase fraction text box to zero as before and then click Do flash button.
The calculated results with the nucleation model are displayed in the main screen. Note that the hydrate formation temperature at 10 bar is now 263.42K, about 10 Kelvin lower than the hydrate dissociation temperature, 273.2K.
If you try to calculate the hydrate formation temperature without first defining the nucleation model, then the calculation will not converge and error messages will occur in the screen:
Fixed phase fraction flash (nucleation): Pfracflash;
*** ERROR 20292 ***
Cannot find converged point - max. iterations
*** ERROR 20024 ***
Cannot find starting point for calculation - there may be no solution.
*** ERROR 344 ***
The flash calculation has not converged
If this happens, define the nucleation model by selecting Phase Nucleation in the Hydrates model dialog box and repeat the calculation.
Hydrate formation and dissociation pressure at given temperature
The hydrate formation or dissociation pressure calculation is analogous to the formation or dissociation temperature calculation, but is carried out with the fixed phase fraction flash at specified T option (using the appropriate button or menu option). The following example finds the hydrate dissociation pressure for the above mixture at 250K.
The hydrate first forms at 3.6 bar. Under these conditions the hydrate forms from the ice phase rather than the liquid water phase. The hydrate formation pressure at the same temperature is 0.783 bar.
Hydrate phase boundary
You can also use the phase envelope calculator to plot the hydrate phase boundaries for formation and dissociation for this stream by using the thermodynamic hydrate model and nucleation model.
You will notice that at low pressures hydrate formation apparently occurs at higher temperatures than hydrate dissociation. You are referred to “Phases” on page 205 for a discussion on the reasons for this.
Other flash calculations with hydrates
Once the hydrate model has been specified it is possible to do the same flash calculations as for other fluid phases. For example, an isenthalpic flash calculation can be carried out in the same way as shown for the oil and gas system, see “Other flash calculations” on page 188.
Maximum water content allowable before hydrate dissociation
Multiflash can determine the maximum amount of water that may be present in a mixture at a given pressure and temperature before hydrates can form.
This is an example of a Multiflash tolerance calculation. The overall compositions must be specified on a water-free basis. A second mixture composition is then specified using the Composition of Second Fluid tab in the Tolerance Calculation dialogue box.
For a water tolerance calculation this would be pure water. The fixedphase and phase fraction can be specified using the Select phase and Enter phase fraction boxes, zero molar phase fraction of hydrate2 in this case. The tolerance calculation combines the two mixtures in different ratios until the specified condition is met. The following example finds the maximum water content for the above mixture at 250K and 10bar.
As the overall composition must be specified on a water free basis, first remove water from the mixture by:
Clicking on Composition and entering 0.0 for the amount of water. Water must remain in the components list.
Select Calculate from the menu bar, then select Tolerance Calculation.
Select the required phase from Select phase box by clicking the downward-arrow on the right side of the box, then set phase fraction to zero in Enter phase fraction box.
Click the Composition of Second Fluid tab to obtain the second stream of the mixture, then set the composition of water to 1.0 mole and leave the rest to zero. Click Calculate to carry out the tolerance calculation. Click Close back to the main window.
In the results window you will see, with output units set to mole,
The first column shows the overall composition at the hydrate dissociation point. The predicted maximum water content is 38.3 PPM on a molar basis. The amount of second fluid added is the number of moles of water specified by the tolerance calculation which must be mixed with the original water-free stream to meet the condition of zero hydrate phase at the chosen P and T.
There is no fundamental difference between calculations with and without inhibitors. To investigate the effect of an inhibitor you can either add it to the list of components in the mixture and specify the amount in the total mixture just as for any other component or you can use the Inhibitor Calculator (see “Inhibitor calculator” on page 101) to add the amount of inhibitor relative to water. However, the inhibitor will not, of course, remain solely in the water phase but will partition between the different phases present at equilibrium and the amount in a particular phase will depend on the conditions and the amounts of other components. This is exactly what happens in reality.
All the calculations described above can be carried out in the presence of inhibitors.
Can hydrates form at given P and T ?
This is based on a P,T flash calculation. The following example illustrates the calculation for an equimolar methane/butane mixture with water plus 35% by mass of methanol relative to the water. Using Tools/Inhibitor Calculator bring up the Inhibitor calculator window and add 35 mass% methanol to the 1 mole of water in the system.
Alternatively 35 wt% methanol is approximately equivalent to adding 0.3 moles to 1 mole of water. If you still have methane, butane and water loaded from the previous example then add methanol using the Select Components option as before. In the Composition drop down table enter 1.0 mol of water and 0.3 mol of methanol.
With the temperature at 250K and a pressure of 10 bar
Click on the P,T flash button.
The results show that even with the addition of inhibitor hydrates can still form. However, there is now an additional free water phase which contains most of the methanol.
Hydrate dissociation temperature at a given pressure
With the same mixture, calculate the hydrate dissociation temperature using the Hydrate dissociation T at given P button or the fixedphase fraction flash at fixed P with the hydrate 2 phase at 0.0 phase fraction.
You can see that, compared to the earlier calculation in the absence of methanol, the addition of methanol has reduced the hydrate dissociation temperature from 273K to 252K. Virtually all the methanol is in the aqueous phase at these conditions.
Hydrate dissociation pressure at a given temperature
Again this is analogous to the
calculation above but you use the Hydrate dissociation at given T button, 
, or specify a fixedphase
fraction flash at fixed T. The hydrate dissociation pressure increases from
3.6 bar to 7.73 bar. The anti-freeze effect of methanol means that the hydrate
forms from liquid water rather than ice as previously.
Hydrate phase boundary
You can compare the hydrate phase boundary with and without inhibitor by plotting the new phase boundary with methanol present.
Amount of inhibitor required to suppress hydrates
Multiflash can determine the amount of inhibitor that must be added to the system at a given pressure and temperature in order to suppress hydrates. This is another example of a tolerance calculation and is therefore specified using the Tolerance Calculation from the Calculate menu.
The overall compositions must be specified on an inhibitor-free basis. The inhibitor is entered as a second stream using the tolerance calculation. The phase required to be fixed and phase fraction can be specified in the Select phase and Enter phase fraction boxes, zero phase fraction of hydrate2 in this case. The tolerance calculation combines the two mixtures in different ratios until the specified condition is met. The following example finds the amount of methanol that must be added to suppress hydrates for the above mixture at 250K and 10bar.
Remove methanol from the main stream by clicking on Composition and entering 0.0 mol for methanol.
Select Calculate, then Tolerance Calculation to activate the Tolerance Calculation dialogue box.
Select the required phase from Select phase box.
Set phase fraction to zero.
Click the Composition of Second Fluid tab to specify the composition of methanol as 1.0 mole and leave the remainder zero.
Click Calculate to carry out the tolerance calculation.
Click Close to go back to the main window.
The first column shows the overall composition at the hydrate dissociation point. The predicted methanol concentration required is 14.35% on a molar basis with respect to the total stream, approximately 24.9 mol % with respect to water. The amount of second fluid added is the number of moles of the mixture specified by the tolerance calculation which must be mixed with the original inhibitor-free stream to meet the condition of zero hydrate phase.
If the methanol concentration is required on a mass basis it is simple to change the output units and recalculate the results.
Salt inhibition
Multiflash models for hydrate inhibition include the inhibiting effect of saline water. The original salt model represents the salts as a single salt pseudocomponent which can be loaded from INFODATA. As sodium chloride is usually the dominant component, the model reduces other salt components to a sodium chloride equivalent basis and the databank stores the molecular weight of sodium chloride. The original Electrolyte salt model treats the salt as an electrolyte composed of Na+ and Cl- ions only. The model extension in MF3.5 allows for the salt to be described in terms of Na+, K+, Ca++ and Cl- ions.
Unfortunately the information supplied for the amount of salt in brine, formation or production water is not usually specified in the input format required. To help you with the conversion we have provided a Salinity Calculator, see “Salt calculator” on page 103 that converts various analyses into either the equivalent amount of salt component or sodium, potassium, calcium and chloride ions.
To repeat the calculation for the hydrate dissociation temperature at fixed pressure using salt, rather than methanol, inhibition:
Initially, select the RKSA(Infochem) model and 0.5 moles of methane, 0.5 moles of butane and 1.0 moles of water as before.
Set the pressure units to bar and enter 10 bar for the pressure
Select the Inhibitor Calculator from the Tools menu and the tab for the Salt Component Model
Enter 15 mass% NaCl in the Salt Analysis table
The Salinity Calculator will then determine that this is equivalent to adding 0.0544 mole of salt component to the 1 mole of water in the mixture.
and the salt component and this amount will be entered in the Composition drop down table.
Specify the fixed phase flash at constant pressure, setting hydrate2 to 0.0, and click on Do flash
The output shows that the hydrate dissociation temperature at 10 bar for this stream is reduced from 273K to 265K.
To compare this to the results using the new salt model, return to Select Model Set and this time define RKSA(Infochem) + Electrolyte as the model. Defining this model will automatically remove Salt component from the list of components. With no salt present the hydrate dissociation temperature at 10 bar is 273.2K, exactly the same as with RKSA(Infochem). Now from the Inhibitor Calculator select the Electrolyte Model tab and enter 15 mass% NaCl in the Salt Analysis table. This time .0544 moles of Na+ and .0544 moles Cl- are added to the component list. K+ and Ca++ ions are also added to the component list but are present in zero amount as the Salt Analysis was defined in terms of NaCl only. If other salts had been present then positive amounts of these ions would be included. The new salt model also predicts that the hydrate dissociation temperature at 10 bar is reduced to 265.1K. Larger differences may occur at higher salt concentrations.
You can repeat the above calculations using the CPA model for the fluid phase. The chosen mixture is not one where we might expect to see significant differences between model predictions.
The predictions of hydrate dissociation temperatures and pressures are virtually identical. The hydrate dissociation temperature at 10 bar with RKSAINFO was 273.236K, with CPA it is 273.238K. Similarly the hydrate dissociation pressure at 250K was 3.6092 bar for both RKSAINFO and CPA.
The partitioning results do show differences between RKSAINFO and CPA. The amount of water required before hydrates form at 250K and 10 bar changes from 3.8299e-5 mol (RKSAINFO) to 4.959e-5 (CPA) and the amount of methanol required to inhibit hydrate formation at these conditions with 1 mole water present reduces from .3352 mole (RKSAINFO) to .3155 mole (CPA).
Although ethanol has been added as a possible hydrate inhibitor for the CPA model ethanol has not yet been added to the Inhibitor/Salt Calculator form. The amount of ethanol to be added to the overall composition must be calculated and entered in the composition list manually.
CPA can only be used with the new electrolyte salt model. With 15 mass% NaCl the hydrate dissociation temperature at 10 bar is reduced to 265.1K.