Process for removing mercury from crude oil

Abstract

Methods, systems and designs are provided for removing mercury from crudes. Crude oil containing a synthetic reducing agent is heated to a temperature above 100 C. and held at that temperature for a specified period of time to convert all of the forms of mercury in the oil into the elemental mercury form. The elemental mercury is then stripped from the crude oil by e.g., flashing the hot oil and/or contacting it with a gas phase.

Claims

1. A method of removing mercury from crude oil, comprising: a) mixing an organic phosphite with a crude oil comprising mercury in various forms to form a crude oil mixture; b) heating said crude oil mixture to at least 100 C. and less than 200 C. until at least 95% of the mercury in various forms is converted to elemental mercury; c) converting the elemental mercury to gaseous elemental mercury; and d) removing the gaseous elemental mercury from said crude oil.

2. The method of claim 1, wherein said organic phosphite is selected from a group consisting of triphenyl phosphite, tributyl phosphite, dibutyl phosphite, triethyl phosphite, diethyl phosphite, trimethyl phosphite, dimethyl phosphite, and combinations thereof.

3. The method of claim 2, wherein said organic phosphite is dimethyl phosphite.

4. The method of claim 1, wherein said heating is between 100 C.-180 C.

5. The method of claim 1, wherein said heating is between 120-150 C.

6. The method of claim 1, where said conversion of the elemental mercury to gaseous elemental mercury is by flashing.

7. The method of claim 1, where said conversion of the elemental mercury to gaseous elemental mercury is by gas stripping.

8. The method of claim 1, where said removing step is by condensation, precipitation, absorption, adsorption, or combinations thereof.

9. An improved method of removing mercury from a liquid hydrocarbon stream, the method comprising contacting a liquid hydrocarbon stream having mercury contaminants with a gas stream, wherein the gas stream strips mercury from the liquid hydrocarbon stream to thereby form a treated liquid stream and a mercury rich gas stream, wherein the improvement comprises first mixing the liquid hydrocarbon stream with an organic phosphite before heating the mixed liquid hydrocarbon stream having mercury contaminants at 100-200 C. until 90% of said mercury contaminants are converted to elemental mercury, and then contacting said heated liquid hydrocarbon stream with a gas stream to strip mercury from the heated liquid hydrocarbon stream to thereby form the treated liquid stream and the mercury rich gas stream.

10. The improved method of claim 9, wherein said organic phosphite is selected from a group consisting of triphenyl phosphite, tributyl phosphite, dibutyl phosphite, triethyl phosphite, diethyl phosphite, trimethyl phosphite, dimethyl phosphite, and combinations thereof.

11. The method of claim 9, wherein said organic phosphite is dimethyl phosphite.

12. The improved method of claim 9, wherein said gas stream is nitrogen, methane, ethane, propane, butane, or natural gas.

13. The improved method of claim 9, wherein said gas stream is a natural gas stream.

14. The improved method of claim 9, further comprising removing mercury from said mercury rich gas stream.

15. The improved method of claim 9, further comprising removing mercury from said mercury rich gas stream by precipitation as HgS, wherein said gas stream contains hydrogen sulfide.

16. The improved method of claim 9, further comprising removing mercury from said mercury rich gas stream by precipitation as HgO.

17. A method of removing mercury from crude oil, comprising: a) determining the mercury speciation in a crude oil; b) calculating a reaction rate expression, wherein said reaction rate expresses the conversion of ionic mercury into elemental mercury; c) mixing an organic phosphite with said crude oil comprising mercury in various forms to form a crude oil mixture; d) heating said crude oil mixture to about 100 C.-200 C. until at least 90% of the mercury in various forms is converted to elemental mercury, wherein said 90% conversion is calculated using said reaction rate expression; e) converting said elemental mercury to gaseous elemental mercury; and f) removing said gaseous elemental mercury.

18. A method of removing mercury from crude oil, comprising: a) feeding a crude oil comprising mercury in various forms into a mixer at a predetermined flow rate; b) feeding an organic phosphite in said mixer at a predetermined flow rate; c) mixing said crude oil and said organic phosphite to form a crude oil mixture; d) feeding said crude oil mixture into a thermal soak vessel; e) heating said crude oil mixture a first time period in said thermal soak vessel to at least 100 C. and less than 200 C. until at least 90 wt % of the mercury in various forms is converted to elemental mercury; f) converting the elemental mercury to gaseous elemental mercury; and g) removing the gaseous elemental mercury from said crude oil.

19. The method of claim 18, wherein said organic phosphite is selected from a group consisting of triphenyl phosphite, tributyl phosphite, dibutyl phosphite, triethyl phosphite, diethyl phosphite, trimethyl phosphite, dimethyl phosphite, and combinations thereof.

20. The method of claim 18, wherein said organic phosphite is dimethyl phosphite.

21. A method of removing mercury from crude oil, comprising: a) mixing dimethyl phosphite with a crude oil comprising mercury in various forms to form a crude oil mixture; b) heating said crude oil mixture to at least 100 C. and less than 200 C. until at least 95% of the mercury in various forms is converted to elemental mercury; c) converting the elemental mercury to gaseous elemental mercury; and d) removing the gaseous elemental mercury from said crude oil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1. Literature values of concentrations of mercury in crude oil, from Hollebone 2007.

(2) FIG. 2. Schematic of a process for removal of mercury from crude oil.

(3) FIG. 3. Ionic to elemental mercury conversion in crude oil with various concentrations of dimethyl phosphite as the reducing agent. The reaction time was 5 minutes.

(4) FIG. 4. Decrease in the ionic mercury reduction reaction temperature in crude oil with increasing amounts of dimethyl phosphite as the reducing agent.

(5) FIG. 5. Comparison of relative reaction rate in direct response to increasing dimethyl phosphite concentration.

DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

(6) The invention provides a novel method of removing all forms of mercury from hydrocarbon sources. Specifically, a synthetic reducing agent comprising a phosphite is added to a hydrocarbon source containing various forms of mercury. The hydrocarbon/phosphite mixture is then heated to convert the various forms of mercury to elemental mercury, which can then be removed from the hydrocarbon source.

(7) The present methods includes any of the following embodiments in any combination(s) of one or more thereof: A method of removing mercury from a hydrocarbon wherein a synthetic reducing agent is mixed with a hydrocarbon comprising mercury in various forms, then heated to at least 100 C. and less than 200 C. until at least 95% of the mercury in various forms is converted to elemental mercury. The elemental mercury can then be converted to gaseous elemental mercury before removal from the hydrocarbon/reducing agent mix. A method of removing mercury from a hydrocarbon that comprises various forms of mercury, wherein the mercury speciation is first determined and a reaction rate expression is created to express the conversion of ionic mercury to elemental mercury. Once the conversion information is determined, the hydrocarbon can be mixed with a synthetic reducing agent, then heated to about 100-200 C., depending on the conversion information, until at least 90% of the mercury in various forms is converted to elemental mercury, as calculated using the reaction rate expression. The elemental mercury can then be converted to gaseous elemental mercury before removal from the hydrocarbon/reducing agent mix. An improved method of removing mercury from a liquid hydrocarbon stream that is mercury rich, wherein a synthetic reducing agent is mixed with the liquid hydrocarbon stream, then heating the reducing agent/liquid hydrocarbon stream to at least 100 C. and less than 200 C. until at least 90% of the mercury in various forms is converted to elemental mercury. The elemental mercury can then be converted to gaseous elemental mercury.

(8) The gaseous elemental mercury can then be stripped from the hydrocarbon stream by contacting the stream with gas stream, such as nitrogen, methane, ethane, propane, butane, natural gas or combinations thereof. This results in a treated liquid hydrocarbon stream and a mercury rich gas stream. The mercury can there be removed from the mercury rich gas stream by precipitation as HgS if the gas stream contains hydrogen sulfide. Alternatively, mercury can there be removed from the mercury rich gas stream by precipitation as HgO. A method of removing mercury from a hydrocarbon that contains various forms of mercury, wherein the hydrocarbon is fed into a mixer at a predetermined flow rate, along with an organic phosphite that is also fed into the mixer at predetermined flow rate. The hydrocarbon and organic phosphite can then be mixed, and fed into a thermal soak vessel. The mixture can be heated in the thermal soak vessel to at least 100 C. and less than 200 C. until at least 90 wt % of the mercury in various forms is converted to elemental mercury. The elemental mercury can then be converted to gaseous elemental mercury before removal from the hydrocarbon/reducing agent mix. In any of the above methods, the synthetic reducing agent is an organic phosphite and can be selected from a group including, but not limited to, triphenyl phosphite, tributyl phosphite, dibutyl phosphite, triethyl phosphite, diethyl phosphite, trimethyl phosphite, dimethyl phosphite, ethyl methyl phosphite, or combinations thereof. Dimethyl phosphite is preferred. The heating range in the above methods is expected to fall between 100 and less than 200 C. Depending on the type of hydrocarbon, heating ranges can include 100 C.-180 C. or 120-150 C. However, it can also be less than 100 C., e.g, 95, 90, 85, or even 80, depending on crude components, amount of mercury, and plant design considerations. In any of the above methods, the elemental mercury can be converted to gaseous elemental mercury by flashing or gas stripping. The gaseous elemental mercury can then be removed by condensation, precipitation, or absorption, adsorption, or combinations thereof. In any of the above methods, the hydrocarbon can be most hydrocarbon matrices, including but not limited to crude oil, natural gases, condensates, naphthas, middle distillates, and waxes.

(9) In crude oil, the elemental mercury redox equilibrium, Hg.sup.0.Math.Hg.sup.2++2e.sup., is shifted towards the oxidized state (Hg.sup.2++2e.sup.) at temperatures below 100 C. The equilibrium begins to shift towards the reduced state at temperatures above 100 C. Although the Hg.sup.2+ reduction rate is too small at 100 C. to be commercially useful, the conversion to Hg.sup.0 will be complete in a petroleum reservoir at that temperature because of the geologic timescale that applies to that environment (>10 million years). As such, the mercury concentration and speciation in wellhead crude oil is a function of reservoir geology and temperature.

(10) Additionally, mercury speciation undergoes predictable changes as the physical and chemical conditions change during oil production and transport. In crude oil reservoirs at temperatures above 100 C., mercury is present only as Hg.sup.0. After the crude is extracted from the reservoir and its temperature falls below 100 C., the spontaneous oxidation of Hg.sup.0 to Hg.sup.2+ will occur.

(11) Hg.sup.2+ is very soluble in crude oils and is a non-volatile form of mercury, making its removal more difficult. Thus, preheating oils to about 100 C. or 100-200 C. with a phosphite will convert Hg.sup.2+ to Hg.sup.0, and simplify extraction because processes to remove elemental mercury already exist.

(12) For example, U.S. Pat. Nos. 4,962,276 and 8,080,156 disclose processes that employ gas stripping to remove mercury from condensates and crude oils. These processes, however, only work if the mercury is already in the gas strippable elemental form. As noted above, a significant portion of the mercury in a crude oil can be present in the non-volatile ionic form, and the non-volatile ionic mercury cannot be removed from a crude oil by gas stripping. Each of these methods can be used however, if proceeded by the preheat stage described herein, which converts various forms of mercury to elemental mercury.

(13) U.S. Pat. No. 5,384,040 discloses a catalytic process for transforming mercury compounds contained in a gas condensate liquid into elemental mercury. Although not the preferred embodiment, a non-catalytic heat treatment process in the absence of hydrogen is also disclosed. The elemental mercury formed by the catalytic process is removed from the gas condensate liquid using a solid phase sorbent.

(14) In this disclosure, a process is described for converting the various forms of mercury in a crude oil to the elemental form using a reducing agent and heat so that the mercury can be subsequently removed from the oil by gas stripping.

(15) A range of different compounds with antioxidant/reducing properties was tested for their ability to reduce Hg.sup.2+ and other forms of mercury in crude oil. Not all reducing agents were effective for converting ionic mercury to elemental mercury in crude oil matrices. A variety of amines, phenols, and phosphites were evaluated. Phosphites demonstrated good performance. However, the phenol and amine reducing agents did not prove to be as effective.

(16) To further evaluate the use of phosphites as a reducing agent, a series of di- and tri-substituted alkyl and aryl phosphites with varying chain lengths or phenyl substituents were evaluated using a continuous flow reactor. The performance tests revealed that the rate of ionic mercury conversion increased as the size of the hydrocarbon substituents on the phosphite molecule decreased. In other words, the methyl-substituted phosphites reacted faster than the ethyl, butyl, and phenyl-substituted compounds. However, increasing the concentrations of the large alkyl-substituted phosphites narrowed the gap between reaction rates.

(17) The performance tests also revealed that the di-substituted phosphites performed better than the tri-substituted phosphites. This appears to be due to the fact that the di-substituted phosphites have significantly better thermal stability compared to their tri-substituted counterparts.

(18) Thus, the extent of mercury reduction in a given hydrocarbon matrix is therefore a function of the reaction temperature, the chemical composition of the reducing agent, the reducing agent concentration, and the length of time that the oil is allowed to react.

(19) The present methods are exemplified with respect to crude oil in FIG. 2-5. However, this is exemplary only, and the invention can be broadly applied to any type of hydrocarbon matrix. In view of the initial findings on the reducing agent, Applicant performed a series of experiments using a di-substituted phosphite with smaller alkyl chains: dimethyl phosphite. However, other organic phosphites are expected to work well, too.

(20) The following examples are intended to be illustrative only, and not unduly limit the scope of the appended claims.

(21) A block flow diagram of the disclosed mercury removal process is shown in FIG. 2. A mercury-containing crude oil and the reducing agent is introduced into an in-line mixer to be mixed. From there, the mixed composition is introduced into a heater to quickly and efficiently preheat the crude oil to at least 100 C.

(22) The heated oil is then moved into a thermal soak vessel that is heated to a pre-determined temperature above 100 C. The crude remains in the heated soak vessel while the mercury species are being converted into elemental mercury.

(23) After conversion, the crude oil flows into a gas-stripping vessel with an optional packing therein to facilitate contact between a stripping gas and crude oil. As shown in FIG. 2, the stripping gas flows from the bottom of the vessel through the oil. Any gas, such as nitrogen, methane, ethane, propane, butane, or natural gas, can be used.

(24) As the stripping gas contacts the crude oil, the elemental mercury is removed in the form of mercury gas. The stripping gas plus mercury vapor is drawn from the top of the vessel and passed through a mercury removal unit, wherein the mercury can be removed from the stripping gas using an adsorption method (filter or scrubber). Alternative, mercury can be removed from the stripping gas via precipitation with a filter containing selenium or a gas containing hydrogen sulfide.

(25) The mercury-free stripping gas can then be recycled. The stripped crude oil will be discharged for further processing.

(26) Obtaining mercury speciation and kinetic information, per the methods described in U.S. Pat. No. 9,574,140, are beneficial first steps in the mercury conversion process. Speciation of the mercury provides information for understanding the fate and distribution of mercury throughout the petroleum system from reservoir rock to consumer products and how to structure the conversion. Each of the mercury species is characterized by a unique set of properties that define its toxicity, solubility, volatility, thermal stability, and reactivity. Thus, the amount of reducing agent, mixing times, and conversion temperature and time used in the process depicted in FIG. 2 should be augmented for the relative concentrations of various mercury species in a given hydrocarbon source to improve efficient conversion inexpensively.

(27) The process for obtaining kinetic data for the conversion of mercury to the elemental form was previously described in U.S. Pat. No. 9,574,140. The rate at which mercury is thermally reduced to elemental mercury is also strongly influenced by the composition of the crude oil. Therefore, for process design purposes, it is beneficial to experimentally determine the kinetics of the mercury reduction reaction for the specific oil feed to the process.

(28) Kinetic data for the mercury reduction reaction were obtained by spiking the oil with an enriched stable isotope of ionic mercury (e.g. .sup.198Hg.sup.2+ or .sup.201Hg.sup.2+). To accomplish this, an enriched isotope, in the form of HgCl.sub.2 or HgO for example, is dissolved in the oil and the rate of conversion of this ionic mercury standard to elemental mercury is monitored as a function of time and temperature. The use of an enriched isotope allows the reduction reaction to be monitored accurately even though naturally-occurring mercury may also be present in the oil.

(29) The conversion of Hg.sup.2+ to Hg.sup.0 was monitored using enriched isotopic tracers and the mercury speciation procedure that was described above.

(30) The results of the kinetic measurements can be used to define a reaction rate expression for a specific oil that might have a form such as:
[Hg.sup.2+].sub.t=[Hg.sup.2+].sub.ie.sup.kt
k=Ae.sup.Ea/RT
where: k=apparent first-order rate constant; t=time; [Hg.sup.2+].sub.i=concentration of ionic mercury at time zero; [Hg.sup.2+].sub.t=concentration of ionic mercury at time t; Ae.sup.Ea/RT is the Arrhenius equation used to calculate the effect of temperature (T) on the reaction rate constant.

(31) The kinetics, fluid flow and heat transfer of a process are important when upscaling for large-scale designs. To retain the same reaction rate, the other variables in the process design must be decreased or increased as necessary. For instance, increasing vessel sizes could decrease the rate, such that the temperature of the conversion must be increased to return the rate to its original value.

(32) In the presently described method, the reaction rate is accelerated through the use of a reducing agent. FIG. 3 depicts the increase in mercury conversion at lower reaction temperatures and increasing concentrations of the reducing agent, dimethyl phosphite. Each reaction was 5 minutes long. As expected, the higher concentration reducing agent (200 ppm) reached at least a 90% conversion efficiency at a much lower temperature than the reaction without any reducing agent.

(33) Even when a small amount of reducing agent was used, the reaction temperature was at least 40 C. less. This is very useful because higher reaction temperatures can lead to degradation of hydrocarbon components or the loss of lower molecular weight hydrocarbons due to evaporation.

(34) As an example, in U.S. Pat. No. 9,574,140, increasing reaction temperature increased the amount and rate of mercury conversion. However, a balance had to be struck to prevent thermal degradation of other components in the crude oil or destruction of processing equipment.

(35) Here, the reducing agent can be utilized to reduce the temperature needed for the conversion process, such that thermal degradation of hydrocarbons and/or evaporation of lighter weight hydrocarbons less problematic.

(36) FIG. 4 illustrates the decrease in reaction temperature after the addition of the synthetic reducing agent for a conversion efficiency of 95%. The reducing agent significantly lowers the reaction temperature compared to crude oil with no addition. At a reducing agent addition of 200 ppm for example, the reaction temperature is lowered by about 70 C. FIG. 5 illustrates the relative effect of reducing agent concentration on the rate of the ionic mercury conversion reaction.

(37) Though shown with crude oil, the method can be applied to any hydrocarbon source. Lower molecular weight hydrocarbons such as those contained in condensates have inherently slow mercury conversion rates. By adding a reducing agent to these feedstocks, commercially attractive processing rates can be achieved.

(38) Further, the ability to convert ionic mercury to the elemental form at lower temperatures has significant advantages in terms of reducing the capital and operating expenses that would be required for building and operating a mercury removal unit.

(39) The reducing agent's ability to accelerate the reaction and not be significantly affected by oil compositional variations is an unexpected advantage. Thus, by adding an organic phosphite reducing agent to a hydrocarbon source, a simple, robust and cost-effective method for removing all forms of mercury is obtained.

(40) The following references are incorporated by reference in their entirety.

(41) Salv et al (2010) SPE 138333.

(42) Hollebone, B. P. and C. X. Yang, Mercury in Crude Oil Refined in Canada, Environment Canada, Ottawa, ON, 2007.

(43) U.S. Pat. No. 3,194,629

(44) U.S. Pat. No. 4,962,276

(45) U.S. Pat. No. 5,384,040

(46) U.S. Pat. No. 6,350,372

(47) U.S. Pat. No. 6,537,443

(48) U.S. Pat. No. 6,685,824

(49) U.S. Pat. No. 6,806,398

(50) U.S. Pat. No. 8,080,156

(51) U.S. Pat. No. 9,574,140