October 2024
Process Optimization
Thermal decomposition of particulate mercury sulfides in petroleum—Part 2
Traces of particulate mercury sulfide (HgS) in stabilized crude oils transform to elemental mercury below 400°C in refinery distillation units. The authors evaluated this transformation and measured reaction rates and activation energies in crude oil and Hg-spiked mineral oil, using glass vessels at atmospheric pressure and a microunit at 1,000 psig.
Traces of particulate mercury sulfide (HgS) in stabilized crude oils transform to elemental mercury below 400°C in refinery distillation units. The authors evaluated this transformation and measured reaction rates and activation energies in crude oil and Hg-spiked mineral oil, using glass vessels at atmospheric pressure and a microunit at 1,000 psig. The results showed that HgS decomposes irreversibly above ˜150°C. The data indicated the presence of two Hg species. Approximately 88% of the HgS decomposed with an activation energy of 56±7 kJ mol-1, while the remaining 12% decomposed with an activation energy of 130±10 kJ mol-1. Near-identical kinetic parameters were found with differential measurements at atmospheric pressure and with integral measurements in the liquid phase at > 1,000 psig, indicating that mild pressures do not change the reaction kinetics. The results showed that > 99% of HgS in crude oil decomposes in a typical crude distillation unit. Part 1 of this article—published in the September issue—covered the background, methods and experimental studies at atmospheric pressure. Part 2 covers the results at 1,000 psig, the model of kinetics, data availability and discussion.
Materials and methods. A stainless-steel microunit was also used to study the thermal decomposition of Hg in crude oil using integral kinetic analysis. This was operated at 1,000 psig, which is above the bubble point of crudes. The equipment is shown in FIG. 12. Nitrogen was injected into the base of the glass stripper. An 8.67-cm3 empty wide zone in the reactor was held at constant temperature. This volume was significantly greater than the volumes of the preheat and post-heat sections and was used in space rate calculations. Because this unit operated continuously, it could evaluate the decomposition at one temperature and space rate, and then continue to the next experiment under different conditions. The results from the microunit are recorded in supplemental materials—available upon request.
FIG. 12. Microunit for 1,000-psig thermal decomposition.
The batch atmospheric pressure measurements provided measurements of the rate over time. In contrast, the continuous microunit studies only reported an integral of the rate; however, both measurements have utility.
The data handling for the microunit’s integral data followed this procedure:
- A target space rate and temperature were selected.
- Once the unit had lined out, measurements of the outlet Hg content were obtained
- The rate for each test was determined by the standard first-order rate equation for flow microreactors (Eq. 1): Rate (min-1) = LHSV * ln (Inlet Hg/Outlet Hg) / 60 (1)
- The natural log of the first-order rates was then plotted vs. the reciprocal of the absolute temperature. The slope of this line provides the activation energy, and the intercept provides the preexponential.
- Comparisons were made between the two measurement methods.
Details of both methods are described in supplemental materials—available upon request.
Results. As shown in FIG. 13, the rate constant at 1,000 psig under liquid-phase flow conditions was the same as the rate constant measured at atmospheric pressure in batch conditions.
FIG. 13. First-order rate constants for atmospheric pressure batch and 1,000-psig microunit reactions.
In these high-pressure studies, the crude was heated to decompose the HgS. Then, the mixture was cooled to strip the produce elemental Hg from the crude. If the reaction was reversible, the non-volatile HgS would have reformed, and no Hg would have been stripped from the crude. The failure of the HgS to reform during the cooling was evidence that the reaction was not reversible. Furthermore, the rates obtained from batch glass atmospheric pressure reactors and high-pressure microreactors were in agreement. The HgS decomposition appeared to be independent of pressure between the atmospheric pressure and 1,000 psig. This indicates that the reaction is independent of pressure and is not reversible to any significant extent. If it were reversible, this agreement would not have been observed. Additional pressure bomb experiments were performed to show that decomposed elemental Hg remains volatile after several days at room temperature, indicating that the reaction was not reversible. Data is available in supplemental materials.
Since the microunit studies measured an integrated rate, it was not possible to distinguish the reactive and refractory rates measured in the batch atmospheric pressure studies.
Discussion. The data showed that particulate HgS decomposes at temperatures from 150°C–250°C. This is also well below the temperature reported in literature.5,20 Leckey and Nulf noted this discrepancy with their own experimental work, and analyzed the high temperatures reported in the literature.12 They found that the high temperatures were consistent with a calculated thermodynamic equilibrium value for cinnabar decomposition. In a thermodynamic calculation, the elemental Hg and sulfur products would be present in contact with the cinnabar at a total of 1 atmosphere pressure, which is significantly higher than the partial pressures encountered during most experimental and commercial decompositions of cinnabar and metacinnabar. These high values reported in literature are not representative of what would likely be seen in practice. Similar thermodynamic calculations are shown in supplemental materials and support the conclusions of Leckey and Nulf.12
Particulate HgS decomposes well below the typical furnace outlet temperature of ~350°C used in crude distillation. The particulate HgS is therefore expected to be converted to elemental Hg, which can accumulate in the overhead sections of the atmospheric distillation unit due to its volatility and may be present in light products (C1–C8).
During batch distillation, the Hg decomposition appears to show the presence of two distinct forms: reactive and refractory. The two kinetic expressions for the reactive and refractory Hg compounds derived from the experimental data are shown below, where the gas constant is 0.0083 kJ mol-1 K-1. A typical amount of refractory mercury in crude can be assumed to be 12%:
- Rate constant of reactive mercury conversion, min-1 = Exp(10.49-56/RT)
- Rate constant of refractory mercury conversion, min-1 = Exp(28.14-130/RT)
Using these equations, approximately 90% of the Hg will decompose in about 3 min, and 99% of the Hg will decompose in approximately 6 min at typical furnace outlet temperatures of around 350°C (FIG. 14). The residence time at the elevated temperatures in the furnace and in the bottoms and reboiler of the main column are more than sufficient for these levels of decomposition.
FIG. 14. Remaining Hg vs. residence time and temperature.
SUPPLEMENTAL MATERIALS
Supplemental materials are available from the corresponding author(s) upon request.
LITERATURE CITED
5 Weast, R. C., Handbook of Chemistry and Physics, The Chemical Rubber Co., January 1969.
12 Leckey, J. H. and L. E. Nulf, “Thermal decomposition of mercuric sulfide,” U.S. Department of Energy, October 28, 1994, online: https://www.osti.gov/servlets/purl/41313
20 Wikipedia, “Mercury sulfide,” online: https://en.wikipedia.org/wiki/Mercury_sulfide
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