Mineral oils: processes and products – Refining – Metal contaminant removal
Reexamination Certificate
2001-03-08
2003-09-23
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Refining
Metal contaminant removal
C208S253000, C208S295000, C208S299000, C585S820000, C585S823000
Reexamination Certificate
active
06623629
ABSTRACT:
The subject matter of this invention concerns a process for capturing arsenic using a mass of lead deposited on alumina, wherein the active phase is in the oxide form regarding a portion of the catalytic bed, preferably the major portion of the catalytic bed, and in the pre-sulphurised form regarding the other portion. Mercaptans are known to be powerful arsenic capture inhibitors. However, it has been observed that pre-sulphurising a portion of the catalytic bed can result in a very good arsenic capture gradient over the remainder of the bed even when mercaptans are present in the feed, in contrast to that observed in the absence of a pre-sulphurisation step.
PRIOR ART
Processes for cracking heavy petroleum cuts, for example catalytic cracking, visbreaking or cokefaction, produce light cuts that are strongly contaminated with various compounds containing sulphur, nitrogen and oxygen. Arsenic is often detected alongside those impurities.
The sulphur-containing compounds are usually hydrogen sulfide and mercaptans. The nitrogen-containing compounds, present in the light cuts, are principally ammonia or light amines. Arsenic is itself also present in the form of compounds with the general formula AsR
3
, R being a hydrocarbon radical such as CH
3
or a hydrogen atom.
The term “light cuts” as used here means those that are gaseous under normal pressure and temperature conditions, i.e., C
2
, C
3
or C
4
cuts. Such cuts are generally treated to eliminate sulphur-containing compounds. In particular, C
3
and C
4
cuts usually undergo an amine washing treatment, followed by washing with sodium hydroxide. Those different washes eliminate almost all of the H
2
S, only a portion of organic sulphur-containing compounds such as mercaptans, and only extract COS in a very incomplete fashion.
In general, C
3
and C
4
cuts, which contains a large proportion of olefins, constitute a high value starting material for the production of fuels or chemical products such as certain polymers. These transformations involve a variety of catalytic treatments in which the catalysts are poisoned at varying rates by sulphur-containing or arsenic-containing compounds.
Recently, a process for capturing the arsenic contained in hydrocarbons in the gas phase (U.S. Pat. No. 3,782,076) or in the liquid phase (U.S. Pat. No. 4,849,577) have been described, carried out at relatively high pressures (more than 2 MPa) in the presence of sulphur-containing compounds. That process uses an absorbent mass comprising lead oxide and a “non acidic” support, i.e., it does not catalyse reactions known to the skilled person to be catalysed by acidic solids, namely hydrocarbon skeletal isomerisation, cracking and polymerisation.
Following the washing treatments cited above, the C
3
cut from fluidised bed catalytic cracking (FCC) still contains COS and/or mercaptans in amounts of the order of 1 to 50 ppm by weight, and arsenic in amounts of the order of 0.1 to 5 ppm by weight.
Thus, providing an arsenic capture mass that can decontaminate a feed even when said feed contains compounds such as mercaptans which may contaminate that mass, would be desirable.
This capture mass is employed in any method that can bring the fluid to be decontaminated into contact with the lead mass.
SUMMARY OF THE INVENTION
The invention concerns a process for eliminating arsenic from a hydrocarbon cut, in which said cut is brought into contact with an absorption mass that is at least partially pre-sulphurised and comprises a support and lead oxide.
The support for said mass preferably has a specific surface area in the range 10 to 300 m
2
/g, a total pore volume in the range 0.2 to 1.2 cm
3
/g and a macroporous volume in the range 0.1 to 0.5 cm
3
/g. The lead content of said mass, expressed as lead oxide, is preferably in the range 5% to 50% by weight. The fraction of the sulphurised mass preferably represents at least {fraction (1/20)}
th
of the total volume of the absorption mass.
DETAILED DESCRIPTION OF THE INVENTION
The present invention concerns a process for eliminating arsenic from a hydrocarbon cut in which said cut is brought into contact with an absorption mass that is at least partially pre-sulphurised and comprises a support and lead oxide.
The support used in the present invention can be any support that is known to the skilled person. As an example, and preferably, alumina, silica or magnesia is used, more preferably alumina, which can produce both relatively large specific surface areas and sufficient mechanical strength.
The recommended support for the invention is an alumina with a surface area in the range 10 to 300 m
2
/g, preferably in the range 50 to 200 m
2
/g. Its total pore volume is preferably in the range 0.2 to 1.2 cm
3
/g, more preferably in the range 0.5 to 1.2 cm
3
/g. The macroporous volume, defined as that corresponding to pores over 100 nm, is preferably in the range 0.1 to 0.5 cm
3
/g, more preferably in the range 0.15 to 0.45 cm
3
/g.
The absorbent mass containing lead oxide can be prepared using any technique that is known to the skilled person. It is prepared by mixing a lead compound with the support, using known techniques. One preparation procedure that routinely leads to a high performance mass is “dry” impregnation, i.e., filling the pores of the support with an aqueous solution of a lead salt by a volume equal to the pore volume of the support. Any sufficiently soluble lead salt can be used, such as lead nitrate or lead acetate. Preferably, lead acetate is used, as it has a satisfactory solubility and can produce a capture mass with a high efficiency.
After impregnating the support with the solution of the lead compound, the mass is heated to a temperature in the range 300° C. to 700° C., preferably in the range 400° C. to 550° C., to convert the lead compound into lead oxide. Preferably, an atmosphere containing oxygen is employed.
The masses obtained advantageously comprise 5% to 50% by weight of lead, preferably 10% to 45% by weight, more preferably 15% to 40% by weight of lead, these percentages being expressed as lead oxide.
Absorption is carried out at a temperature that is preferably in the range 5° C. to 150° C., ore preferably in the range 10° C. to 100° C., at a pressure that can maintain the cut to be treated either in the gas phase or in the liquid phase, for example, in the range 0.1 MPa to 4 MPa, preferably in the range 0.5 MPa to 2.5 MPa. The absorbent mass is thus in different chemical forms: an oxide form and a pre-sulphurised form.
When the amount of sulphur-containing contaminants (COS, H
2
S, mercaptans) is low, a capture mass is preferably used in the process of the invention that preferably comprises only a very small proportion of pre-sulphurised mass, for example {fraction (1/20)}
th
to {fraction (1/10)}
th
of the total volume of the absorption bed or beds, more preferably {fraction (1/20)}
th
to {fraction (1/15)}
th
of the total volume of the absorption bed or beds.
When the feed to be treated has a higher sulphur-containing contaminant content, more particularly mercaptans, then preferably the capture mass used is in an at least partially pre-sulphurised form, preferably in a proportion of at least {fraction (1/15)}
th
of the total volume of the catalytic bed, more preferably at least {fraction (1/10)}
th
and very preferably at least ⅕
th
.
It has been shown that the absorbent mass has a poor arsine capture gradient when the feed to be decontaminated has a non negligible mercaptan content. Mercaptans usually cause severe inhibition of arsenic capture.
In contrast, pre-sulphurising the capture mass can produce a very good capture gradient in the remainder of the catalytic bed, which remains in the form of lead oxide PbO, deposited on the support.
This first layer of pre-sulphurized absorbent can decompose mercaptans into sulphur-containing compounds that are not poisons and do not impair arsenic capture over the remainder of the catalytic bed, which remains in the oxide form.
Pre-sulphurisation of the capture mass is preferably carried out with any sulphur-containing c
Didillon Blaise
Savary Laurent
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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