Process for regenerating a catalytic redox solution, to...

Details Process for regenerating a catalytic redox solution, to... Process for regenerating a catalytic redox solution, to...

2001-12-21

2003-12-30

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Sulfur or compound thereof

Elemental sulfur

C423S576400, C423S576500, C423S576600, C423S576700, C423S576800

Reexamination Certificate

active

06669921

ABSTRACT:

This invention pertains to a process for regenerating a catalytic solution in a process for desulfurization by oxidation-reduction of a gaseous feedstock that contains hydrogen sulfide in order, on the one hand, to optimize the operation of the process based on the quantity of H
2
S to be eliminated and, on the other hand, to reduce the degradation of the chelating or complexing agent used in the process.
The process according to the invention is applied in particular to the regeneration in air of the catalytic solution during the course of a “redox” desulfurization process for a gas that contains at least hydrogen sulfide. During this process, a catalytic solution is used that contains at least one polyvalent chelated metal (Fe
3+
, V
5+
), by at least one chelating agent under conditions that are suitable for ensuring the oxidation of hydrogen sulfide into elementary sulfur and the simultaneous reduction of the polyvalent chelated metal from a higher level of oxidation to a lower level of oxidation. On the one hand, a gaseous effluent that is essentially devoid of hydrogen sulfide and, on the other hand, an elementary sulfur-containing catalytic solution that is at least partially reduced are recovered. The solid elementary sulfur may or may not be separated from the partially reduced catalytic solution. At least a portion of said partially reduced catalytic solution, which may or may not be stripped of the majority of the solid elementary sulfur, is expanded. In general, said solution is regenerated with air in a regeneration zone. At the outlet from this regeneration stage, the regenerated solution is recycled to the stage where the hydrogen sulfide-containing gas is oxidized.
The prior art describes many redox processes and associated devices that make it possible to eliminate hydrogen sulfide and to recover the elementary sulfur that is formed during the process.
The desulfurization process includes, for example, the following two oxidation-reduction stages:
In a first stage (stage of absorption, oxidation-reduction reaction) the hydrogen sulfide that is present in the gas to be treated reacts with chelated ferric ions according to the reaction:
H
2
S+2 Fe
3+
(chel.)→S+2 H
+
+2 Fe
2+
(chel.)  (1)
in a second stage (regeneration stage), the ferrous ions are reoxidized by oxygen from the air according to the reaction:
O
2gas
→O
2liquid
  (2)
2 Fe
2+
(chel.)+2 H
+
+½O
2
→2 Fe
3+
(chel.)+H
2
O  (3)
Thus, the overall reaction is:
H
2
S+½O
2
→⅛S
8
+H
2
O
In the case where the metal is vanadium, the regeneration reaction is as follows:
In this reaction, secondary reactions may lead to a degradation of the ADA.
It is known that redox processes are flexible with regard to their treatment capacity in terms of flow rates and H
2
S concentrations, but this flexibility is generally achieved without oxygen control. Conversely, it is also known that it is during the second stage that the degradation of the chelating agent occurs; this makes the process expensive because it is then necessary to re-add said agent. Moreover, the degradation of the chelating agent leads to the formation of products that accumulate in the catalytic solution and can precipitate out. These organic or inorganic products are entrained by the sulfur and reduce its quality, thus increasing costs. This degradation involves highly reactive radicals such as free radicals that are initiated by the presence of dissolved oxygen in the catalytic solution.
The catalytic solutions are thus complex because they are charged with organic or inorganic compounds and have more or less elevated concentrations, depending on the formulations. This presence of compounds has a direct effect on the first stage of the regeneration reaction, reaction (2), which is the transfer of gaseous oxygen into the catalytic solution, and consequently on the presence of dissolved oxygen in the heart of the solution.
In order to mitigate the degradation of the chelating agent, U.S. Pat. No. 5,223,173 claims to run the process with an excess of the complexed polyvalent metal at a lower level of oxidation, regardless of whether it is at the absorption or the oxidation stage.
By operating with an excess of complexed metal at the lowest level of oxidation, however, this process will be less flexible in the face of fluctuations in the flow rate or H
2
S concentration in the gas to be treated. The treatment capacity of the process will thus be smaller with equivalent sizing.
In U.S. Pat. No. 5,422,086, the degradation of the chelating agent is assumed to be in relation to, on the one hand, the dwell time of the solution in the reactor and, on the other hand, to be inversely proportional to the metal concentration at the lowest level of oxidation. In order to reduce the degradation of the chelating agent, the concept is not to convert the iron(II) completely into iron(III) at a total iron(II)/iron(III) ratio of less than 0.1. In order to avoid back-mixing phenomena, this patent claims to use two reactors, whereby the first operates in co-current and the second in counter-current. As described, the process makes it possible to have an Fe(II) concentration of less than 0.1 mole per mole of total iron at the outlet from the regeneration stage.
This device, which requires two reactors for the catalytic solution regeneration stage, turns out to be very expensive, however.
Finally, using an anti-oxidizing agent such as thiosulfate can also help to reduce the degradation of the chelating agent. U.S. Pat. No. 6,083,472 claims a method for generating thiosulfate ions using a draw-off of the gas to be treated. This fraction is treated with soda or potassium, and HS is transformed into HS
−
ions. This solution is then injected with the reduced catalytic solution directly into the oxidizer where the HS
−
ions are oxidized into thiosulfates, while the reduced catalytic solution is reoxidized. It is thus necessary to insert an additional reactor, along with managing chemical products that are added to the chemical products already required in the redox processes.
Patent No. FR-A-2 794 665 also describes a device for dispersing into very small bubbles in said solution the gas that is used to regenerate the catalytic solution.
Patent FR-A-2 771 945 also teaches the regeneration of an aqueous catalytic phase by an oxidizing agent.
Finally, the technological background is illustrated by patents U.S. Pat. No. 4,859,436, U.S. Pat. No. 4,532,118, U.S. Pat. No. 5,753,189 and DE 3 444 252.
One of the objects of the invention is to remedy the deficiencies of the prior art.
Another object of the invention is to propose a new approach to operating the catalytic solution regeneration stage that makes it possible, on the one hand, to ensure optimum adjustment of oxygen consumption in the process and, on the other hand, to reduce the degradation of the complexing agent during the regeneration stage.
More specifically, the invention pertains to a process for regenerating a catalytic redox solution that is at least partly reduced, whereby it comprises at least one chelating agent of a polyvalent metal associated with said solution in which, in at least one regeneration zone, said solution is circulated in the presence of an oxygen-containing gas under appropriate regeneration conditions, and a regeneration effluent that is at least partially oxidized is recovered; this is done by measuring the concentration of oxygen dissolved in the regeneration effluent and adjusting the operating parameters so as to minimize the degradation of the chelating agent and to regenerate at least a portion of the catalytic solution.
In order to accomplish this function, the residual dissolved-oxygen concentration is usually measured at the outlet of the regeneration stage by means of a specific sensor. It is advantageous for this measurement to be made on line, and this measurement can do its monitoring by controlling, for example, the fl

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