Concentration of elemental mercury in selected plant streams calculated by UMR-PRU and SRK-Twu. Concentration of elemental mercury in selected plant streams as calculated by UMR-PRU and SRK-Twu under scenario 3 with liquid phase reaction (Keq from data of Robie et al. [127]).
Introduction
Motivation
Another point to be highlighted is the difficulty encountered in determining the amount of elemental mercury and its compounds in different systems via various analytical methods. This, combined with the often confidential nature of the oil industry data, makes the search for reliable data on the distribution and concentrations of mercury and its compounds in oil and gas processing plants challenging.
Objectives
Implementation of developed thermodynamic models in the simulation of mercury distribution in an existing natural gas processing plant.
Thesis structure
In Chapter 10, the UMR-PRU model is used to simulate the distribution of mercury in an existing natural gas processing plant. In Chapter 11, the developed algorithm for simultaneous chemical and phase equilibria is applied to calculations in complex mixtures involving reactive and nonreactive azeotropes.
Mercury: A toxic pollutant
- The properties of mercury
- Mercury related health and safety issues
- Mercury in nature
- Mercury in fossil fuels
- Hg partitioning during NG processing
- Mercury reactions in hydrocarbons
- Analytical methods for Hg measurement
- Overview
- Analytical techniques
- Challenges for experimenters
Natural sources include volcanic activity, soil erosion, natural fires and dissolution of mercury in water. Finally, mercury is found in amine solutions with high pH in the form of HgS2H-, whereas the dominant forms of mercury in glycol dehydrators are of the Hg(SR)2 type [3].
Natural gas processing
Introduction
Especially after the oil crisis of the 1970s, natural gas gained momentum and its role in the energy sector became increasingly important. To date, the consumption of natural gas shows an increasing trend over time, its share in the global primary energy sector was 24.2% in 2019 [30].
The natural gas value chain
Therefore, natural gas is expected to play an important role in this transition and gradually replace the share of oil and coal in the electricity sector. In the case of offshore fields, natural gas undergoes a first treatment either in subsea templates or topside on platforms or floating vessels.
Basic processes
- Inlet separation
- Condensate stabilization
- NGL recovery & fractionation
- Acid gas removal
- Water removal
- Mercury removal
Removal of water from natural gas (also known as “dehydration”) can be accomplished by absorption, adsorption, or cooling. The glycol exiting the contactor is called "rich glycol" because it is rich in water and can be regenerated and recycled back to the contactor.
Phase & chemical equilibria
- Thermodynamic equilibrium
- Phase equilibrium
- Vapor – liquid equilibrium
- Liquid – liquid equilibrium
- Solid solubility
- Chemical reaction equilibrium
- Gas phase reactions
- Liquid phase reactions
- Stability
The fugacity of the hypothetical supercooled liquid can be approximated to be equal to that of the saturated liquid. For liquids, the standard state fugacity, 𝑓𝑖𝑜, is usually taken to be equal to the fugacity of the pure component at 1 bar or 1 atm.
Algorithms for chemical & phase equilibria
Phase equilibrium algorithms
- Bubble point pressure calculation
- Dew point temperature calculation
- Two-phase PT flash algorithm
- Multiphase PT flash algorithm
During the calculations in the inner loop it may be necessary to deactivate or reactivate phases. More specifically, the difficulty lies in deciding how many and which trial phases to introduce into the system during the stability analysis. One of these corresponds to the search for a vapor phase and the rest 𝑁𝐶trial phases correspond to liquid phases rich in the respective components.
Calculation time is thus saved by avoiding explicit calculation of the fugacity coefficients for the other components in the mercury phase.
Simultaneous chemical & phase equilibrium algorithm
- Working equations
- Reference state chemical potential
- Initialization
- Procedure
For convenience, the reduced Gibbs energy (𝐺/RT) can be minimized since its minimum coincides with that of the Gibbs energy at constant temperature. It should be noted that the solution is a minimum of the Gibbs energy and not a minimum of the Lagrangian, but a saddle point. The mole fractions can be expressed as a function of the Lagrange multipliers by replacing the chemical potential with fugacity in Eq.
After outer loop convergence, stability analysis is performed to check if an additional phase can lower the Gibbs energy of the system, as described in Section 4.4.
Thermodynamic models
- Equations of state
- Virial equation of state
- Cubic equations of state
- a-functions
- Activity coefficient models
- The NRTL model
- The UNIQUAC model
- Advanced models
- Mixing rules
- Virial EoS for mixtures
- The van der Waals one fluid theory
- The Universal Mixing Rules
- The UMR-PRU model
- The PPR78 model
- Thermodynamic models for mercury
The parameters of the a functions are usually fitted to the pure vapor pressures of the component and extrapolated to the supercritical region. The resulting models offer the "best of both worlds": they can be used for both low and high pressures, for all types of phase equilibria, even in complex mixtures involving asymmetric or polar components with size. The most popular among these theories is the Statistical Associating Fluid Theory (SAFT), which originated from the turbulence theory by Wertheim [63].
𝐺𝐴𝐶𝐸,𝑆𝐺, 𝐺𝐴𝐶𝐸,𝑟𝑒𝑠: Staverman-Guggenheim terms for the combinatorial and residual part of the excess Gibbs energy (GE), respectively.
Thermodynamic modelling of elemental mercury solubility in natural gas components
- Introduction
- Pure Hg 0 vapor pressure
- Description of Hg 0 solubility in binary mixtures
- Database with experimental Hg 0 solubilities & evaluation
- Results & discussion
- Prediction of Hg 0 solubility in multicomponent mixtures
- Conclusions
The estimated UNIFAC group interaction parameters for the UMR-PRU model are presented in Table 7.2. Optimum and generalized kij values for SRK-Twu and PR-MC along with their corresponding deviations in Hg0 mole fraction compared to deviations derived from the UMR-PRU model. Estimated kij values between Hg0 and polar compounds for the SRK-Twu and PR-MC models along with the corresponding deviations in Hg0 mole fraction compared to the deviations produced by the UMR-PRU model.
Hg0 solubility in mixtures 1-3 as calculated with UMR-PRU, SRK-Twu and PR-MC models against exp.
Thermodynamic modelling of hydrogen vapor-liquid equilibrium with oil & gas components
- Introduction
- a-function consistency and pure H 2 property estimation
- VLE description in binary mixtures of hydrogen
- Prediction of hydrogen VLE in multicomponent systems
- Conclusions
UNIFAC interaction parameters between hydrogen and other groups determined in this work for the UMR-PRU model. VLE results with UMR-PRU and PPR-78 models. a 𝐴𝐴𝐷𝑦 = 100/𝑁𝐷 ∑𝑁𝐷𝑖=1|𝑦𝑖𝑒𝑥𝑝− 𝑦𝑖 vapor phase; b Excluding MeOH, MEG and TEG. In this chapter, the UMR-PRU model was used to study the phase equilibria of hydrogen with oil and gas components.
Moreover, the UMR-PRU model was successfully extended to mixtures of hydrogen with hydrocarbons, gases (CO2, N2, H2S) and polar compounds (H2O, MeOH, MEG, TEG) by introducing a new set of UNIFAC (H2) and assessing the temperature. -dependent binary interaction parameters between different groups.
Mercury chemical & phase equilibria in natural gas
- Introduction
- Mercury solubility in typical natural gas and condensate mixtures
- Mercury reaction with H 2 S in natural gas and condensate mixtures
- Thermodynamic analysis of the reaction
- CPE calculations in typical natural gas and condensate fluids
- Conclusions
Again, this can be attributed to compositional changes in the vapor and liquid phases under pressure. The initial amount of reaction products (β-HgS and H2) in the liquids was assumed to be zero. In the case of the liquid phase reaction things are not so straightforward because of the concavity of the conversion curves.
However, this effect can be encountered in the case of the liquid phase reaction, which exhibits lower conversions.
Simulation of mercury distribution in an offshore natural gas processing plant
Introduction
Process description
Simulation of mercury distribution in the overall process
- Mercury mass balance calculations
- Description of the studied scenarios
- Simulation of Hg distribution in the overall process without reaction
- Investigation of the effect of fluid characterization
- Simulation of Hg distribution with reaction
From field campaign measurements, it is possible to compile a total mass balance of mercury in the plant. In the test separator (10-001), the concentration of mercury in the vapor phase calculated by UMR-PRU and SRK-Twu is lower than the measured value. The overall distribution of mercury in the plant calculated by SRK-Twu and UMR-PRU under scenario 3 is shown in Figure 10.5 and Figure 10.6, respectively.
It is shown that UMR-PRU with the TTPL characterization method produces a higher Hg0 distribution in the output gas.
Simulation of Hg distribution in TEG dehydration & regeneration process
- Process description
- Mercury mass balance calculations
- Simulation of Hg distribution
In the case of no response, it was also shown that UMR-PRU generally predicts more mercury in the liquid phase. However, the Hg mass balance in the vessel is not satisfied according to the measured concentrations, as shown in Table 10.18. Regarding train B, UMR-PRU predicts that the Hg0 concentration in dry gas is the same as in wet gas, while SRK-Twu predicts a slight increase in the dry gas.
The results with UMR-PRU show a higher mercury concentration in rich TEG compared to SRK-Twu, which also leads to higher Hg0 content in streams involved in the regeneration part of the process.
Mercury distribution in MEG regeneration process
- Process description
- Simulation of Hg distribution
Finally, examination of other process parameters shows that UMR-PRU produces water content in dry gas close to the normal values expected for the process. The results of the simulations with SRK-Twu and UMR-PRU are shown in Table 10.19. Furthermore, all models and scenarios show that the mercury concentration in lean MEG is negligible, which is consistent with the findings of the field campaign.
Hg0 concentration in MEG regeneration process streams calculated by SRK-Twu and UMR-PRU.
Validation of the simulation
- Fluid compositions
- Stream flowrates
- H 2 S and CO 2 in process gases
The flow rates of gas, condensate and water streams as calculated with SRK-Twu and UMR-PRU are presented in Table 10.20 to Table 10.22. The results with UMR-PRU and SRK-Twu are shown in Table 10.23 together with the measured values. It is observed that both models can accurately predict the concentrations of H2S and CO2 in the investigated gases.
H2S and CO2 concentrations in selected plant streams as calculated with UMR-PRU and SRK-Twu.
Conclusions
However, in the test separator (10-001), the model predictions varied significantly from the distributions indicated by the field campaign measurements. Therefore, the calculated distributions in the export condensate are closer to the values calculated from the measurements, but deviate further in the export gas. Both models predict a very high Hg0 concentration in the vapor product from the glycol flash drums, which is directed to the vent, in both trains.
The stream flows as calculated in the simplified simulation were also close to the actual process data.
Phase and chemical equilibrium in systems involving non-reactive and reactive azeotropes
- Introduction
- Thermodynamic modelling
- Cyclohexane synthesis
- MTBE synthesis
- Acetic acid/isopropanol esterification
- Conclusions
This was necessary to account for dimerization of acetic acid in the vapor phase. To describe the phase equilibrium in the MTBE synthesis system, the UNIQUAC, UNIFAC, UMR-UNIQUAC and UMR-UNIFAC thermodynamic models were used. The last step is to examine the simultaneous chemical and phase equilibria involved in the reactive quaternary mixture.
Calculated and experimental reactive azeotropic composition in the acetic acid/isopropanol esterification system at 1 atm.
Conclusions & future work
Conclusions
The effect of pressure on mercury solubility in the different phases was also investigated, and results showed a weak dependence in the liquid hydrocarbon and aqueous phases. On the other hand, Hg0 solubility in the vapor phase was found to decrease with pressure, until a plateau was reached. The effect of pressure on conversion was also studied, and it was found that in the vapor reaction the conversion increases with pressure.
UMR-PRU was also found to provide the most accurate results for the distribution of Hg in aqueous streams, as well as for the processes involved in TEG dehydration.
Future work
We connected the algorithm with the classic activity coefficient models UNIQUAC and NRTL and with UMR-PRU. The results showed that the models, as well as the algorithm, can successfully describe the chemical and phase equilibria involved in these systems, which provides important information about the feasibility of the separation processes.