Mercury Partitioning
Infochem has developed a mercury model in order to predict the solubility of mercury in natural gases and condensates, and the distribution of mercury between gas, condensate and water phases. An advanced form of the Redlich-Kwong-Soave equation of state (RKS), widely used in the oil and gas industry, has been used as the basis for the model.
In condensates and petroleum liquids, organomercury compounds are significant and may be the predominant form of mercury, although it is generally agreed that mercury in natural gas is almost all in elemental form. To correctly predict the phase behaviour of both mercury and organomercury compounds, data was collected for solubility in hydrocarbons and water. Dimethylmercury and diphenylmercury are chosen to represent light and heavy organomercury compounds respectively. Additional measurements on mercury solubility, including that in TEG, provide access to data not in the public domain.
The mercury model is supported by a special version of the Infodata databank and BIP correlations.
Defining the mercury model
The mercury model my be defined by selecting the mercury tab in the Select/Model Set box
The phases defined for this model are gas, liquid1 (hydrocarbon liquid), mercury (liquid mercury), water and solidmercury. If you do require a second hydrocarbon liquid phase then you must define an additional phase descriptor using Tools/Command as described in the Multiflash commands section in the User Guide.
Calculating mercury partitioning
Once the mercury model has been defined the fluid components are defined using Select\Components as usual and entering the fluid composition in the Composition drop down menu. The example file for this case study is supplied as Hg_Example.mfl.
In this particular example we have only specified mercury but the principle is the same if components dimethyl and diphenyl mercury are present.
To demonstrate the partitioning we envision a simplified flowsheet where the feed gas goes through a warm separator, the gas phase exiting this separator then enters a cold separator and the exiting gas is compressed for export. If the mercury model is used in an Excel spreadsheet or a third party simulator the streams can be merged and recycled but the GUI only allows for one calculated stream composition to be used for the any flash.
Once Hg_Example.mfl has been loaded enter the flash conditions 4 °C and 77 bar and carry out a P,T flash.
Flash at fixed P and T:
T (degC) = 4.000 P (bar ) = 77.0000
NO. PHASES = 3 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3
GAS LIQUID1 WATER
fractions fractions fractions fractions
NITROGEN 1.003450E-02 1.007390E-02 1.712982E-03 2.740420E-08
CO2 2.114841E-02 2.116796E-02 2.107836E-02 3.157117E-06
METHANE 0.863377 0.865980 0.363744 3.839148E-08
ETHANE 6.087664E-02 6.081565E-02 9.294088E-02 2.722893E-11
PROPANE 2.457653E-02 2.434675E-02 9.397172E-02 1.483804E-14
ISOBUTANE 3.917853E-03 3.831513E-03 2.865773E-02 0.00000
BUTANE 7.465909E-03 7.240048E-03 7.148564E-02 0.00000
ISOPENTANE 2.098850E-03 1.961530E-03 4.040915E-02 0.00000
PENTANE 2.288746E-03 2.102863E-03 5.400884E-02 0.00000
HEXANE 1.918949E-03 1.572865E-03 9.762536E-02 0.00000
HEPTANE 9.494798E-04 6.350576E-04 8.770092E-02 0.00000
OCTANE 2.298740E-04 1.070769E-04 3.407516E-02 0.00000
NONANE 5.996714E-05 1.500216E-05 1.244712E-02 0.00000
TEG 0.00000 0.00000 0.00000 0.00000
WATER 1.057329E-03 1.493793E-04 1.416890E-04 0.999997
MERCURY 3.498083E-09 3.413234E-09 2.738684E-08 1.337185E-09
Total(mole ) 100.055 99.6019 0.361992 9.086137E-02
Z (Fug. Model) 0.763620 0.765938 0.302438 6.003744E-02
Av.Mol.Wt. 19.2688 19.1616 49.0744 18.0153
Den/V(m3/mol ) 2.285246E-04 2.292183E-04 9.050916E-05 1.796709E-05
H ( J/mol ) -3036.76 -2939.92 -19065.5 -45339.1
S ( J/mol/K ) -39.6824 -39.5114 -65.5625 -123.970
U ( J/mol ) -4796.40 -4704.90 -19762.4 -45477.4
G ( J/mol ) 7961.21 8010.68 -894.856 -10980.9
The mercury partitions between the exiting streams from the warm separator.
The gas phase compositions can be highlighted, copied and then pasted into the Composition drop down menu to provide the feed for the P,T flash for the cold separator. In the first case the conditions are set to -25. °C and 41 bar.
Flash at fixed P and T:
T (degC) = -25.000 P (bar ) = 41.0000
NO. PHASES = 4 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3 PHASE4
GAS LIQUID1 MERCURY ICE
fractions fractions fractions fractions fractions
NITROGEN 1.007390E-02 1.028776E-02 9.848831E-04 0.00000 0.00000
CO2 2.116797E-02 2.114042E-02 2.246629E-02 0.00000 0.00000
METHANE 0.865980 0.879799 0.279882 0.00000 0.00000
ETHANE 6.081567E-02 5.941965E-02 0.120865 0.00000 0.00000
PROPANE 2.434676E-02 2.098150E-02 0.168410 0.00000 0.00000
ISOBUTANE 3.831515E-03 2.613939E-03 5.592623E-02 0.00000 0.00000
BUTANE 7.240051E-03 4.264691E-03 0.134531 0.00000 0.00000
ISOPENTANE 1.961531E-03 6.862826E-04 5.651215E-02 0.00000 0.00000
PENTANE 2.102864E-03 5.896009E-04 6.683364E-02 0.00000 0.00000
HEXANE 1.572866E-03 1.717423E-04 6.150467E-02 0.00000 0.00000
HEPTANE 6.350579E-04 2.391026E-05 2.677606E-02 0.00000 0.00000
OCTANE 1.070769E-04 1.216981E-06 4.635076E-03 0.00000 0.00000
NONANE 1.500217E-05 3.042433E-08 6.553949E-04 0.00000 0.00000
TEG 0.00000 0.00000 0.00000 0.00000 0.00000
WATER 1.493794E-04 1.981524E-05 1.743695E-05 0.00000 1.00000
MERCURY 3.413235E-09 4.832536E-10 3.706910E-09 1.00000 0.00000
Total(mole ) 1.00000 0.977025 2.284491E-02 2.856399E-09 1.296210E-04
Z (Fug. Model) 0.801205 0.816256 0.161849 2.933633E-02 3.883237E-02
Av.Mol.Wt. 19.1616 18.5545 45.1330 200.590 18.0152
Den/V(m3/mol ) 4.031861E-04 4.107599E-04 8.144661E-05 1.476276E-05 1.954140E-05
H ( J/mol ) -3543.00 -3131.41 -20868.4 -63580.6 -52446.3
S ( J/mol/K ) -37.7216 -36.8503 -74.3475 -107.587 -149.909
U ( J/mol ) -5196.07 -4815.53 -21202.3 -63641.1 -52526.4
G ( J/mol ) 5817.61 6013.00 -2419.08 -36882.9 -15246.5
At these conditions the separator is cold enough for a separate liquid mercury phase to form. If the temperature is even colder,-52°C, then mercury forms as a solid and slightly more mercury is formed.
Flash at fixed P and T:
T (degC) = -52.000 P (bar ) = 41.0000
NO. PHASES = 4 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2 PHASE3 PHASE4
GAS LIQUID1 ICE SOLIDMERCURY
fractions fractions fractions fractions fractions
NITROGEN 1.007390E-02 1.088966E-02 1.452085E-03 0.00000 0.00000
CO2 2.116797E-02 1.976610E-02 3.605055E-02 0.00000 0.00000
METHANE 0.865980 0.907698 0.425672 0.00000 0.00000
ETHANE 6.081567E-02 4.936538E-02 0.182182 0.00000 0.00000
PROPANE 2.434676E-02 1.027567E-02 0.173406 0.00000 0.00000
ISOBUTANE 3.831515E-03 7.699538E-04 3.626091E-02 0.00000 0.00000
BUTANE 7.240051E-03 1.015837E-03 7.316872E-02 0.00000 0.00000
ISOPENTANE 1.961531E-03 1.120298E-04 2.155165E-02 0.00000 0.00000
PENTANE 2.102864E-03 8.649504E-05 2.346041E-02 0.00000 0.00000
HEXANE 1.572866E-03 1.822761E-05 1.803963E-02 0.00000 0.00000
HEPTANE 6.350579E-04 1.978559E-06 7.340642E-03 0.00000 0.00000
OCTANE 1.070769E-04 7.946262E-08 1.240395E-03 0.00000 0.00000
NONANE 1.500217E-05 7.617385E-10 1.738972E-04 0.00000 0.00000
TEG 0.00000 0.00000 0.00000 0.00000 0.00000
WATER 1.493794E-04 1.062181E-06 1.415600E-06 1.00000 0.00000
MERCURY 3.413235E-09 1.093640E-11 6.392073E-11 0.00000 1.00000
Total(mole ) 1.00000 0.913585 8.626630E-02 1.482868E-04 3.397728E-09
Z (Fug. Model) 0.686973 0.737965 0.148056 4.353157E-02 3.145013E-02
Av.Mol.Wt. 19.1616 17.7944 33.6429 18.0152 200.590
Den/V(m3/mol ) 3.080875E-04 3.309560E-04 6.639899E-05 1.952265E-05 1.410448E-05
H ( J/mol ) -5457.31 -4305.30 -17575.1 -53375.5 -67299.3
S ( J/mol/K ) -45.9105 -43.0128 -76.4119 -153.879 -123.456
U ( J/mol ) -6720.47 -5662.22 -17847.4 -53455.5 -67357.1
G ( J/mol ) 4695.78 5206.98 -676.638 -19345.1 -39997.1
Calculating mercury dropout
As can be seen from the examples above, the mercury model can calculate mercury dropping out either as a liquid or as a solid. The fixedphase fraction flash can also be used to calculate the temperature or pressure at which a pure mercury phase will drop out.
If, for the example we are using, the gas from the cold separator at -25°C and 41 bar is compressed to 100bar, such as for gas export, then the fixed phase fraction calculation can be used to determine the temperature at which liquid mercury will drop out.
Fixed Phase Fraction Flash - at specified P (Mole Fraction):
T (degC) = -19.470 P (bar ) = 100.000
NO. PHASES = 2 CONVERGED STABLE
COMPONENT OVERALL PHASE1 PHASE2
GAS MERCURY
fractions fractions fractions
NITROGEN 1.028776E-02 1.028776E-02 0.00000
CO2 2.114043E-02 2.114043E-02 0.00000
METHANE 0.879799 0.879799 0.00000
ETHANE 5.941968E-02 5.941968E-02 0.00000
PROPANE 2.098151E-02 2.098151E-02 0.00000
ISOBUTANE 2.613940E-03 2.613940E-03 0.00000
BUTANE 4.264693E-03 4.264693E-03 0.00000
ISOPENTANE 6.862829E-04 6.862829E-04 0.00000
PENTANE 5.896012E-04 5.896012E-04 0.00000
HEXANE 1.717424E-04 1.717424E-04 0.00000
HEPTANE 2.391027E-05 2.391027E-05 0.00000
OCTANE 1.216982E-06 1.216982E-06 0.00000
NONANE 3.042434E-08 3.042434E-08 0.00000
TEG 0.00000 0.00000 0.00000
WATER 1.981525E-05 1.981525E-05 0.00000
MERCURY 4.832538E-10 4.832538E-10 1.00000
Total(mole ) 1.00000 1.00000 0.00000
Z (Fug. Model) 0.619421 0.619421 7.004612E-02
Av.Mol.Wt. 18.5545 18.5545 200.590
Den/V(m3/mol ) 1.306484E-04 1.306484E-04 1.477415E-05
H ( J/mol ) -5054.02 -5054.02 -63231.6
S ( J/mol/K ) -49.8305 -49.8305 -106.543
U ( J/mol ) -6360.50 -6360.50 -63379.4
G ( J/mol ) 7587.00 7587.00 -36203.7
If the gas from the cold separator at -52 °C and 41 bar is used for this calculation the mercury concentration in the gas phase is much lower and a separate mercury phase will not form in the gas export line until solid mercury forms at -48.6 °C.
Other calculations
The mercury model can be used for any flash calculation such as P,S or P,H. The model can also be used for mercury partitioning in a gas dehydrator.
TEG has been included in the component list for the example provided, but was present at zero concentration. If the TEG is added, e.g. 0.3 moles, then the calculations can be repeated but in this case starting with a simple dehydrator flash at 10 °C and 110 bar. You can then see the mercury partitioning into the aqueous TEG phase and the consequent effects.