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The present invention relates to a process for the elimination of heavy metals, and more particularly mercury, that are present in a liquid or gaseous feedstock.
STATE OF THE ART
Mercury is a metallic contaminant that is found in gaseous or liquid hydrocarbons produced in many regions of the world, such as the Niger Delta, South America or North Africa.
The elimination of mercury from hydrocarbon cuts is desirable in an industrial context for several reasons:
for reasons of the safety of operators: elemental mercury is volatile and presents serious risks of neurotoxicity via inhalation, while its organic forms present similar risks via skin contact;
for reasons of preventing de-activation of the heterogeneous catalysts serving to upgrade these liquid hydrocarbon cuts: mercury amalgamates very readily with the noble metals such as platinum or palladium used for various catalytic operations, and in particular the selective hydrogenation of the olefins produced by steam cracking or catalytic cracking of liquid hydrocarbons.
Industrially, the elimination of heavy metals, in particular mercury, from the liquid or gaseous hydrocarbon cuts is carried out by circulating them through beds of capture material. By capture material is meant in the present invention any type of solid in bulk or supported form containing within it or on its surface an active element capable of reacting irreversibly with an impurity such as mercury contained in the feedstock to be purified. The elimination of mercury from the liquid or gaseous hydrocarbon-containing cuts is generally carried out by circulating said feedstock to be treated through beds of capture materials containing an active phase capable of reacting with the mercury. It is in particular known to a person skilled in the art that mercury capture can be carried out easily by reacting the latter with an active phase based on sulphur or a sulphur-containing compound, in particular metallic sulphides, the mercury then forming with the sulphur the chemical species HgS called cinnabar or metacinnabarite. These different chemical reactions are generally implemented in a process by using contact of the feedstock to be treated with a capture material that is either bulk in which in particular particles of the active phase can be bonded together via binders, or supported in which the active phase is dispersed within or on the surface of a porous solid support.
However, it is not possible to carry out such a purification operation directly on crude oil cuts or gas condensates for several reasons. The first is that the porosity of these capture materials would very quickly become clogged by the heavy compounds present in said feedstock, which would be deposited on the surface of the materials. Moreover, these crude oil cuts or gas condensates contain mercury in different forms. In fact, unlike the gas phases, they contain not only elemental mercury but also mercury in complexed or ionic and organic form. Now, these complexed or ionic and organic compounds of mercury are called refractory, as they are stable under normal operating conditions and are not reactive with the capture materials of heavy metals. It therefore appears to be necessary to convert the refractory mercury compounds to elemental mercury.
Numerous means have been developed in order to convert the refractory forms of mercury to elemental mercury (also called mercury in atomic form Hg0). For example, U.S. Pat. No. 4,911,825 discloses a process for the transformation of the refractive species of mercury from the feedstock to elemental mercury in the presence of a catalyst and under high hydrogen pressure and at a high temperature.
U.S. Pat. No. 5,384,040 discloses a process for the elimination of the mercury from a hydrocarbon-containing feedstock comprising a stage of transformation of the mercury contained in the compounds of the feedstock to elemental mercury, the transformation stage being carried out between 120 and 400° C. and under pressure of 0.1 to 6.0 MPa. Preferably, the transformation stage is carried out in the presence of a catalyst comprising at least one metal M selected from the group formed by iron, nickel, cobalt, molybdenum, tungsten and palladium. Alternatively, the transformation stage can be carried out in the absence of a catalyst.
In the latter case, the temperature must be set at 180° C. as a minimum. In fact, in the article by Masatoshi Yamada et al. entitled “Mercury removal from natural gas condensate” in the journal Studies in Surface Science and Catalysis, volume 92, pages 433-436, 1995, it is shown that the conversion of diethyl mercury starts at 180° C. and reaches 100% conversion at 240° C. At the same time, it is shown that it is possible to reduce the transformation temperature in the presence of a catalyst. In fact, the conversion of the refractory species of mercury starts at 130° C. and reaches over 90% from 200° C. However, the problem with the use of a catalyst, apart from its cost, is that there is a tendency to promote the cracking of molecules and therefore the formation of coke. Furthermore, in the case of highly clogging feedstocks such as crude oil, a very rapid de-activation of the porous catalyst is noted, due to the deposition of heavy compounds such as asphaltenes, within the pores of said catalyst. Such a process is thus more suitable for the treatment of hydrocarbons originating from a first fractionation.
The Applicant discovered, surprisingly, that it is possible to eliminate heavy metals, and more particularly mercury, contained in a gaseous or liquid feedstock, and more particularly a crude oil feedstock, by carrying out upstream of the main fractionation unit, a stage of heating said feedstock at a target temperature and during a residence time sufficient to allow the transformation of the refractory spaces containing heavy metals, present in different forms, to metals in the atomic (or elemental) form, even in the absence of a catalyst or hydrogen, and by carrying out upstream of the main fractionation unit, a stage of capture of the heavy metals, and more particularly mercury. In fact, although the crude oil feedstocks comprise a very great diversity of molecules, bringing said feedstock up to a temperature during a sufficient residence time upstream of the main fractionation unit makes it possible to convert the majority of the refractory compounds to metallic compounds (also called elemental compounds) that can be captured by a single capture material.
SUBJECTS OF THE INVENTION
The present invention relates to a process for the elimination of mercury contained in a heavy hydrocarbon-containing feedstock upstream of a main fractionation unit, in which process:
a) the non-elemental mercury contained in the compounds of said feedstock is transformed to elemental mercury, said stage being carried out in a conversion unit at a target temperature during a fixed residence time and adapted to said target temperature so that at least 90% by weight of non-elemental mercury contained in the compounds of said feedstock are converted to elemental mercury, said stage of transformation being carried out in the absence of hydrogen and in the absence of a catalyst, it being understood that:
when the target temperature of said feedstock is comprised between 150° C. and 175° C., the residence time of said feedstock in the conversion unit is comprised between 150 and 2700 minutes; and/or
when the target temperature of said feedstock is greater than 175° C. and less than or equal to 250° C., the residence time of said feedstock in the conversion unit is comprised between 100 and 900 minutes; and/or
when the target temperature of said feedstock is greater than 250° C. and less than or equal to 400° C., the residence time of said feedstock in the conversion unit is comprised between 5 and 70 minutes; and/or
when the target temperature of said feedstock is greater than 400° C., the residence time of said feedstock in the conversion unit is comprised between 1 and 10 minutes;
b) a separation of the feedstock obtained in stage a) is carried out in a separation unit, in order to produce a liquid effluent and a gaseous effluent comprising elemental mercury;
c) the gaseous effluent originating from stage b) comprising the elemental mercury is brought into contact with a mercury capture material contained in a unit for the capture of mercury, in order to produce an effluent that is at least partially de-mercurized.