Hydrate dissociation, formation and inhibition
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 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, even without considering the possibility of scale formation. 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 hydrates.
Defining the hydrate models
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.
If you have a high concentration of salt then you may need to consider the possibility of salt precipitation. In MF3.6 we added the possibility of considering the formation of chloride scales and this has now been extended to bromide scales As this may not be a problem for many systems this option is not considered by default. If you think you may have a problem you should check the Halide Scales box. This will increase the number of phases that must be considered but the additional phases will be added automatically when the box is checked, the user does not have to do anything.
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, ethanol, 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 the Infochem 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.
Scale model
In its general form, the freeze-out model can be applied to any solid phase of fixed composition, which must be defined. The model can for example be applied to hydrated salts such as monoethylene glycol (MEG) monohydrate or to crystalline mineral salts, i.e. scales.
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 both 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 nucleation temperature is higher than the dissociation temperature. However, this is not a real situation as ice is not being considered except for nucleation.
If Halide scales are to be considered then further phase descriptors are required. These must represent the correct fixed composition of the scale, these are: NaCl, NaCl.2H2O, KCl, CaCl2.2H2O, CaCl2.4H2O, CaCl2.6H2O, NaBr, NaBr.2(H2O), KBr, CaBr2.6(H2O).
Hydrate calculations with Multiflash
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.
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 hydrate.mfl, which describes a gas condensate.
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 CPA as the fluid phase model.
Click on OK once the hydrate model set has been successfully defined and loaded.
Specifying the components and composition
The fluid for this case study is defined in the following table:
| Component | Moles |
|---|---|
| Methane: | 85.93 |
| Ethane: | 6.75 |
| Propane | 3.13 |
| Isobutane | 0.71 |
| Butane | 0.88 |
| Pentane | 0.57 |
| CO2 | 1.31 |
| N2 | 0.72 |
| Water | 10 |
Define the normal 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. Close to go back to the main window. Click on composition and enter the correct number of moles for each component.
Alternatively. Load the hydrate.mfl input file.
Enter the temperature, 270K and the pressure, 1 MPa (remember to change the standard pressure units from Pa to MPa). 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.
Flash at fixed P and T:
T (K ) = 270.000 P (MPa ) = 1.00000
NO. PHASES = 2 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2
GAS HYDRATE2
fractions fractions fractions
METHANE 0.653016 0.721475 5.731124E-02
ETHANE 9.614705E-02 0.105377 1.583481E-02
PROPANE 6.537901E-02 6.531767E-02 6.591280E-02
ISOBUTANE 1.954786E-02 1.845111E-02 2.909125E-02
BUTANE 2.422833E-02 2.637440E-02 5.554173E-03
PENTANE 1.948061E-02 2.171936E-02 0.00000
CO2 2.730986E-02 3.021736E-02 2.009991E-03
NITROGEN 9.554287E-03 1.062828E-02 2.088432E-04
WATER 8.533709E-02 4.398631E-04 0.824077
Total(g ) 2111.06 1893.46 217.600
Hydrate2 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 1 MPa.
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,
Fixed Phase Fraction Flash - at specified P (Mole Fraction):
T (K ) = 276.145 P (MPa ) = 1.00000
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.653016 0.713434 3.043895E-04 5.324528E-02
ETHANE 9.614705E-02 0.105040 7.168518E-05 1.536530E-02
PROPANE 6.537901E-02 7.142692E-02 4.172151E-05 6.568016E-02
ISOBUTANE 1.954786E-02 2.135657E-02 7.870856E-06 3.079266E-02
BUTANE 2.422833E-02 2.646976E-02 1.352633E-05 5.300713E-03
PENTANE 1.948061E-02 2.128363E-02 2.068753E-06 0.00000
CO2 2.730986E-02 2.980834E-02 3.180291E-04 1.861692E-03
NITROGEN 9.554287E-03 1.043849E-02 1.964002E-06 1.939799E-04
WATER 8.533709E-02 7.422721E-04 0.999239 0.827560
Total(g ) 2111.06 1932.21 178.854 0.00000
show that the hydrate2 is the most stable form and first begins to form at 276.1K.
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 were to be more stable it would 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 hydrate structures are stable at any T and P.
Hydrate formation temperature at given pressure
To calculate the hydrate formation temperature at 1 MPa, make sure the nucleation model has been defined. There is no button for nucleation calculations so 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 1 MPa is now 268K, about 8 Kelvin lower than the hydrate dissociation temperature, 276.1K.
Note that the nucleation calculation is, in the thermodynamic sense, inherently unstable, as reported.
Fixed Phase Fraction Flash - at specified P (Nucleation):
T (K ) = 268.044 P (MPa ) = 1.00000
NO. PHASES = 3 CONVERGED UNSTABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.653016 0.713678 3.813484E-04 5.637848E-02
ETHANE 9.614705E-02 0.105075 1.000482E-04 1.438212E-02
PROPANE 6.537901E-02 7.145022E-02 6.226387E-05 6.707607E-02
ISOBUTANE 1.954786E-02 2.136371E-02 1.212217E-05 3.128501E-02
BUTANE 2.422833E-02 2.647833E-02 2.179693E-05 5.112691E-03
PENTANE 1.948061E-02 2.129112E-02 2.400343E-06 0.00000
CO2 2.730986E-02 2.980938E-02 4.188658E-04 1.963835E-03
NITROGEN 9.554287E-03 1.044214E-02 2.381014E-06 2.038157E-04
WATER 8.533709E-02 4.120615E-04 0.998999 0.823598
Total(g ) 2111.06 1931.53 179.536 0.00000
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 appear:
Fixed Phase Fraction Flash - at specified T (Nucleation): *** 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 270K.
Hydrate dissociation pressure:
T (K ) = 270.000 P (MPa ) = 0.598190
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS ICE HYDRATE2
fractions fractions fractions fractions
METHANE 0.653016 0.713399 0.00000 4.727385E-02
ETHANE 9.614705E-02 0.105038 0.00000 1.444882E-02
PROPANE 6.537901E-02 7.142445E-02 0.00000 6.735604E-02
ISOBUTANE 1.954786E-02 2.135540E-02 0.00000 3.197482E-02
BUTANE 2.422833E-02 2.646866E-02 0.00000 5.314598E-03
PENTANE 1.948061E-02 2.128194E-02 0.00000 0.00000
CO2 2.730986E-02 2.983513E-02 0.00000 1.682924E-03
NITROGEN 9.554287E-03 1.043775E-02 0.00000 1.687177E-04
WATER 8.533709E-02 7.604461E-04 1.00000 0.831780
Total(g ) 2111.06 1932.38 178.683 0.00000
The hydrate first forms at 0.598 MPa. 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 1.26 MPa.
Fixed Phase Fraction Flash - at specified T (Nucleation):
T (K ) = 270.000 P (MPa ) = 1.25858
NO. PHASES = 3 CONVERGED UNSTABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.653016 0.713703 4.491446E-04 5.921085E-02
ETHANE 9.614705E-02 0.105078 1.126342E-04 1.476136E-02
PROPANE 6.537901E-02 7.145279E-02 6.781422E-05 6.646315E-02
ISOBUTANE 1.954786E-02 2.136456E-02 1.293252E-05 3.071116E-02
BUTANE 2.422833E-02 2.647937E-02 2.293214E-05 5.070174E-03
PENTANE 1.948061E-02 2.129201E-02 2.710224E-06 0.00000
CO2 2.730986E-02 2.980466E-02 4.833198E-04 2.042363E-03
NITROGEN 9.554287E-03 1.044255E-02 2.853770E-06 2.164527E-04
WATER 8.533709E-02 3.830115E-04 0.998846 0.821524
Total(g ) 2111.06 1931.44 179.620 0.00000
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 - and add experimental data if available.
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.
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. Under the Phase Specified tab 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 270K and 1 MPa before hydrates will form.
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, hydrate2, 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,
Tolerance Calculation:
T (K ) = 270.000 P (MPa ) = 1.00000
NO. PHASES = 2 CONVERGED STABLE
AMOUNT OF SECOND FLUID ADDED 0.840386 g
COMPONENT OVERALL PHASE1 PHASE2
GAS HYDRATE2
fractions fractions fractions
METHANE 0.713631 0.713631 5.707630E-02
ETHANE 0.105072 0.105072 1.459914E-02
PROPANE 7.144771E-02 7.144771E-02 6.664019E-02
ISOBUTANE 2.136235E-02 2.136235E-02 3.112184E-02
BUTANE 2.647728E-02 2.647728E-02 5.151819E-03
PENTANE 2.128887E-02 2.128887E-02 0.00000
CO2 2.984485E-02 2.984485E-02 1.988588E-03
NITROGEN 1.044115E-02 1.044115E-02 2.067955E-04
WATER 4.350384E-04 4.350384E-04 0.823215
Total(g ) 1931.75 1931.75 0.00000
The first column shows the overall composition at the hydrate dissociation point. The amount of second fluid added is the number of grams 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.
Calculations with inhibitors
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 Multiflash Manual for more details) 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 the gas defined previously with water plus 20% by mass of methanol relative to the water. Using Tools/Inhibitor Calculator bring up the Inhibitor calculator window and add 20 mass% methanol to the 10 mole of water in the system.
Alternatively 20 wt% methanol is approximately equivalent to adding 1.4 moles of methanol to 10 mole of water.
With the temperature at 270K and a pressure of 1 MPa
Click on the P,T flash button.
Flash at fixed P and T:
T (K ) = 270.000 P (MPa ) = 1.00000
NO. PHASES = 2 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2
GAS WATER
fractions fractions fractions
METHANE 0.639375 0.712999 3.926927E-04
ETHANE 9.413867E-02 0.104973 1.070123E-04
PROPANE 6.401333E-02 7.138176E-02 6.224186E-05
ISOBUTANE 1.913953E-02 2.134328E-02 1.304444E-05
BUTANE 2.372223E-02 2.645279E-02 2.352782E-05
PENTANE 1.907369E-02 2.127088E-02 4.126220E-06
CO2 2.673939E-02 2.977815E-02 3.658298E-04
NITROGEN 9.354711E-03 1.043231E-02 2.168072E-06
WATER 8.355452E-02 4.171200E-04 0.805110
METHANOL 2.088863E-02 9.520436E-04 0.193920
Total(g ) 2156.10 1933.34 222.759
The results show that the addition of this concentration of methanol is sufficient to prevent hydrate forming even though some has been lost to the gas phase.
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.
Hydrate dissociation temperature:
T (K ) = 266.490 P (MPa ) = 1.00000
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.639375 0.713218 4.191679E-04 5.921029E-02
ETHANE 9.413867E-02 0.105005 1.173156E-04 1.415768E-02
PROPANE 6.401333E-02 7.140325E-02 6.885396E-05 6.718022E-02
ISOBUTANE 1.913953E-02 2.134977E-02 1.445391E-05 3.129951E-02
BUTANE 2.372223E-02 2.646069E-02 2.653702E-05 5.062451E-03
PENTANE 1.907369E-02 2.127751E-02 4.132071E-06 0.00000
CO2 2.673939E-02 2.978403E-02 3.943057E-04 2.057184E-03
NITROGEN 9.354711E-03 1.043555E-02 2.252377E-06 2.137903E-04
WATER 8.355452E-02 3.192770E-04 0.803784 0.820819
METHANOL 2.088863E-02 7.474072E-04 0.195169 0.00000
Total(g ) 2156.10 1932.74 223.362 0.00000
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 276.1 K to 266.5 K. 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 0.56 MPa to 1.51 MPa. The anti-freeze effect of methanol means that the hydrate forms from liquid water rather than ice as previously.
Hydrate dissociation pressure:
T (K ) = 270.000 P (MPa ) = 1.51072
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.639375 0.713314 5.853841E-04 6.379932E-02
ETHANE 9.413867E-02 0.105017 1.542533E-04 1.488228E-02
PROPANE 6.401333E-02 7.141270E-02 8.716872E-05 6.610791E-02
ISOBUTANE 1.913953E-02 2.135284E-02 1.786033E-05 3.022806E-02
BUTANE 2.372223E-02 2.646434E-02 3.205426E-05 4.977173E-03
PENTANE 1.907369E-02 2.128081E-02 5.488956E-06 0.00000
CO2 2.673939E-02 2.977252E-02 5.349878E-04 2.176003E-03
NITROGEN 9.354711E-03 1.043713E-02 3.286434E-06 2.353849E-04
WATER 8.355452E-02 2.820936E-04 0.802979 0.817594
METHANOL 2.088863E-02 6.659840E-04 0.195600 0.00000
Total(g ) 2156.10 1932.43 223.676 0.00000
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 270K and 1 MPa.
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.
Tolerance Calculation:
T (K ) = 270.000 P (MPa ) = 1.00000
NO. PHASES = 3 CONVERGED STABLE
AMOUNT OF SECOND FLUID ADDED 28.5147 g
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.644313 0.713235 3.618571E-04 5.707093E-02
ETHANE 9.486567E-02 0.105009 9.423738E-05 1.459909E-02
PROPANE 6.450769E-02 7.140616E-02 5.392019E-05 6.664066E-02
ISOBUTANE 1.928734E-02 2.135051E-02 1.069433E-05 3.112260E-02
BUTANE 2.390543E-02 2.646197E-02 1.921201E-05 5.151826E-03
PENTANE 1.922099E-02 2.127790E-02 2.913969E-06 0.00000
CO2 2.694589E-02 2.979196E-02 3.545098E-04 1.985987E-03
NITROGEN 9.426955E-03 1.043570E-02 2.085587E-06 2.067829E-04
WATER 8.419978E-02 4.373411E-04 0.866809 0.823222
METHANOL 1.332726E-02 5.944838E-04 0.132292 0.00000
Total(g ) 2139.58 1932.72 206.859 0.00000
The first column shows the overall composition at the hydrate dissociation point. The predicted methanol concentration required is 1.3% on a mass basis with respect to the total stream, approximately 13.6 mass % with respect to water in the feed. 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.
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. This has now been expended to allow for the salt to be described in terms of Na+, K+, Ca++ , Cl- and Br- 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 35 that converts various analyses into either the equivalent amount of salt component or sodium, potassium, calcium, chloride and bromide ions.
Load the hydrate.mfl file,
Change the Model set from Association to Association + Electrolyte
Select the Inhibitor Calculator from the Tools menu and the tab for the Electrolyte Model
For this particular example there is information on the composition of the formation water.
| mass % | |
|---|---|
| NaCl | 6.993 |
| CaCl2 | 0.735 |
| MgCl2 | 0.186 |
| KCl | 0.066 |
| SrCl2 | 0.099 |
| BaCl2 | 0.036 |
Enter this data into the Salt Calculator
By clicking on ADD the Salinity Calculator will determine the ion concentration that needs to be added to the 10 mole of water in the mixture.
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 1 MPa for this stream is reduced from 276.1K to 272.77K.
Hydrate dissociation temperature:
T (K ) = 272.771 P (MPa ) = 1.00000
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER HYDRATE2
fractions fractions fractions fractions
METHANE 0.648154 0.713568 2.049058E-04 5.535855E-02
ETHANE 9.543126E-02 0.105061 4.301075E-05 1.494584E-02
PROPANE 6.489228E-02 7.144113E-02 2.310176E-05 6.621013E-02
ISOBUTANE 1.940233E-02 2.136074E-02 3.439576E-06 3.097696E-02
BUTANE 2.404796E-02 2.647509E-02 6.106020E-06 5.220234E-03
PENTANE 1.933559E-02 2.128753E-02 7.705965E-07 0.00000
CO2 2.710654E-02 2.981069E-02 3.208288E-04 1.929922E-03
NITROGEN 9.483159E-03 1.044040E-02 1.241347E-06 2.010920E-04
WATER 8.470178E-02 5.553163E-04 0.918209 0.825157
NA+ 2.535925E-03 0.00000 2.765536E-02 0.00000
K+ 3.190391E-05 0.00000 3.479259E-04 0.00000
CA++ 3.384629E-04 0.00000 3.691085E-03 0.00000
CL- 4.538397E-03 0.00000 4.949319E-02 0.00000
Total(g ) 2126.90 1931.87 195.031 0.00000
Scale precipitation
The precipitation of Halide Scales is a recent feature. The model allows for precipitation of NaCl, NaCl.2(H2O), KCl, CaCl2.6(H2O), CaCl.4(H2O), CaCl.2)H2O), NaBr, NaBr.2(H2O), KBr and CaBr2.6(H2O). This is activated by ticking the Halide Scales box in the Hydrates Model Set but can only be defined with an “Electrolyte” salt model option. If you have not specified such an option a warning message is generated.
For our example the salt concentration is not high enough to trigger the precipitation of a scale for hydrate calculations at 1 MPa. In principle, you can use fixed phase fraction flashes to see when any of the scales will form
But the temperatures may well be below those of operational interest.
Fixed Phase Fraction Flash - at specified P (Mole Fraction):
T (K ) = 259.968 P (MPa ) = 1.00000
NO. PHASES = 4 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3 PHASE4
GAS WATER HYDRATE2 NACL.2(H2O)
fractions fractions fractions fractions fractions
METHANE 0.648154 0.719568 1.151857E-04 6.318138E-02 0.00000
ETHANE 9.543126E-02 0.105527 1.988749E-05 1.422313E-02 0.00000
PROPANE 6.489228E-02 6.681123E-02 9.316947E-06 6.772564E-02 0.00000
ISOBUTANE 1.940233E-02 1.913782E-02 8.781810E-07 3.008965E-02 0.00000
BUTANE 2.404796E-02 2.645507E-02 1.922306E-06 5.191261E-03 0.00000
PENTANE 1.933559E-02 2.162665E-02 1.278885E-07 0.00000 0.00000
CO2 2.710654E-02 3.011456E-02 4.789803E-04 2.205412E-03 0.00000
NITROGEN 9.483159E-03 1.058740E-02 5.925149E-07 2.276358E-04 0.00000
WATER 8.470178E-02 1.730699E-04 0.749288 0.817156 0.381383
NA+ 2.535925E-03 0.00000 8.518802E-02 0.00000 0.243348
K+ 3.190391E-05 0.00000 1.071731E-03 0.00000 0.00000
CA++ 3.384629E-04 0.00000 1.136981E-02 0.00000 0.00000
CL- 4.538397E-03 0.00000 0.152456 0.00000 0.375268
Total(g ) 2126.90 1901.58 63.3147 162.003 0.00000
A similar calculation looking for the formation of CaCl2.6(H2O) produces seven phases but at 240K.
Fixed Phase Fraction Flash - at specified P (Mole Fraction):
*** WARNING -20021 ***
Unstable solution, more phases exist.
T (K ) = 240.049 P (MPa ) = 1.00000
NO. PHASES = 7 ? CONVERGED UNSTABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3 PHASE4 PHASE5 PHASE6 PHASE7
GAS LIQUID1 WATER HYDRATE2 NACL KCL CACL2.6(H2O)
fractions fractions fractions fractions fractions fractions fractions fractions
METHANE 0.648154 0.738343 2.077449E-02 1.029737E-04 7.288859E-02 0.00000 0.00000 0.00000
ETHANE 9.543126E-02 0.107822 3.079703E-02 2.438776E-05 1.256837E-02 0.00000 0.00000 0.00000
PROPANE 6.489228E-02 6.381951E-02 0.104107 1.354106E-05 7.199782E-02 0.00000 0.00000 0.00000
ISOBUTANE 1.940233E-02 1.649580E-02 0.104603 3.252140E-06 2.836284E-02 0.00000 0.00000 0.00000
BUTANE 2.404796E-02 2.178155E-02 0.226114 8.333141E-06 4.018957E-03 0.00000 0.00000 0.00000
PENTANE 1.933559E-02 9.956510E-03 0.509566 7.460706E-08 0.00000 0.00000 0.00000 0.00000
CO2 2.710654E-02 3.085601E-02 3.967638E-03 8.258672E-04 2.575226E-03 0.00000 0.00000 0.00000
NITROGEN 9.483159E-03 1.090395E-02 6.824214E-05 2.940736E-07 2.565356E-04 0.00000 0.00000 0.00000
WATER 8.470178E-02 2.174014E-05 3.154079E-06 0.648259 0.807332 0.00000 0.00000 0.493399
NA+ 2.535925E-03 0.00000 0.00000 2.257260E-02 0.00000 0.393031 0.00000 0.00000
K+ 3.190391E-05 0.00000 0.00000 3.667810E-03 0.00000 0.00000 0.517922 0.00000
CA++ 3.384629E-04 0.00000 0.00000 0.104080 0.00000 0.00000 0.00000 0.182942
CL- 4.538397E-03 0.00000 0.00000 0.220442 0.00000 0.606969 0.482078 0.323659
Total(g ) 2126.90 1844.36 44.6682 6.91657 217.541 13.3260 8.203484E-02 0.00000
A more likely scenario is if the salt concentration is higher, e.g. 30wt% equivalent of NaCl. A flash at temperatures higher than hydrate dissociation conditions will show NaCl forming
Flash at fixed P and T:
T (K ) = 280.000 P (MPa ) = 1.00000
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS WATER NACL
fractions fractions fractions fractions
METHANE 0.629975 0.713440 5.727859E-05 0.00000
ETHANE 9.275458E-02 0.105044 7.712658E-06 0.00000
PROPANE 6.307217E-02 7.142886E-02 3.385715E-06 0.00000
ISOBUTANE 1.885813E-02 2.135681E-02 2.630345E-07 0.00000
BUTANE 2.337345E-02 2.647040E-02 4.623809E-07 0.00000
PENTANE 1.879326E-02 2.128337E-02 7.298586E-08 0.00000
CO2 2.634625E-02 2.980748E-02 2.360708E-04 0.00000
NITROGEN 9.217173E-03 1.043841E-02 3.691739E-07 0.00000
WATER 8.232605E-02 7.311716E-04 0.735778 0.00000
NA+ 1.387987E-02 0.00000 0.103818 0.393375
CL- 2.140423E-02 0.00000 0.160098 0.606625
Total(g ) 2188.27 1932.25 242.925 13.0994
whereas at the lower temperatures where a hydrate phase is present you will see NaCl.2(H2O) being formed.
RKSA(Infochem) model
You can repeat any of the above calculations using the RKSA(Infochem) 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 1 MPa with RKSAINFO was 276.144K, with CPA it is 276.142K. Similarly the hydrate dissociation pressure at 270K was 0.598 MPa for both RKSAINFO and CPA.
The partitioning results do show some differences between RKSAINFO and CPA. The amount of water required before hydrates form at 270K and 1 MPa changes from .800g (RKSAINFO) to .840g (CPA) for the composition specified and the amount of methanol required to inhibit hydrate formation at these conditions with 10 mole water present reduces from 30.45g (RKSAINFO) to 28.5g (CPA).
Using the Electrolyte salt model with CPA or RKSAINFO gives results with trivial differences. To compare these to the results from the old salt model, you have to return to Select Model Set and this time define RKSA(Infochem) as the model. Defining this model will automatically remove the ions from the list of components. Now from the Inhibitor Calculator select the Salt Component Model tab and add the same salt composition to the water phase. This time .266 moles of Salt component are added. The predicted hydrate temperature at 1 MPa changes from 277.78K to 272.83K. Larger differences may occur at higher salt concentrations.