Pyrophoric iron sulfide treatment using sodium nitrite

Abstract

Pyrophoric material such as iron sulfide is frequently found in refinery equipment. When the equipment is opened to the atmosphere for maintenance, an exothermic reaction can take place that may cause injury to personnel and catastrophic damage to equipment. A process used to treat pyrophoric material uses sodium nitrite injected into a gaseous carrier stream to oxidize iron sulfides to elemental sulfur and iron oxides. The sodium nitrite solution may be buffered to a pH of about 9 with disodium phosphate or monosodium phosphate. A chemical additive that provides a quantitative measure of reaction completion may be added to the treatment solution.

Claims

1. A method for reducing the amount of iron sulfide in chemical process equipment containing deposits of iron sulfide, the method comprising: injecting a gaseous stream of nitrogen containing a solution of sodium nitrite into the chemical process equipment; reacting the deposits of iron sulfide with the sodium nitrite; injecting a solution of lauryl dimethylamine oxide; monitoring the progress of the method by testing samples withdrawn from the chemical process equipment for the presence of lauryl dimethylamine oxide while continuing the injection of the solution of lauryl dimethylamine oxide; and continuing the injection of a gaseous stream of nitrogen containing a solution of sodium nitrite and a solution of lauryl dimethylamine oxide into the chemical process equipment until the lauryl dimethylamine oxide is no longer being consumed as indicated by the presence of unreduced lauryl dimethylamine oxide in the test samples.

2. The method recited in claim 1 wherein the sodium nitrite is in a buffered solution.

3. The method recited in claim 2 wherein the sodium nitrite solution is buffered to a pH of about 9.

4. The method recited in claim 3 wherein the buffer is selected from the group consisting of disodium phosphate and monosodium phosphate.

5. The method recited in claim 2 wherein the buffer comprises disodium phosphate or monosodium phosphate.

6. The method recited in claim 2 wherein the buffer comprises disodium phosphate and monosodium phosphate.

7. The method recited in claim 2 wherein the buffer comprises substantially equal concentrations of disodium phosphate and monosodium phosphate.

8. The method recited in claim 1 wherein the sodium nitrite oxidizes iron sulfide to ferric oxide and elemental sulfur.

9. The method recited in claim 1 wherein the solution of lauryl dimethylamine oxide comprises an aqueous solution of about 1.2 percent by weight lauryl dimethylamine oxide.

10. The method recited in claim 1 wherein the solution of lauryl dimethylamine oxide is injected with the solution of sodium nitrite.

11. The method recited in claim 1 wherein the solution of lauryl dimethylamine oxide additionally comprises sodium nitrite.

12. The method recited in claim 1 further comprising: chemically cleaning hydrocarbons from the chemical process equipment prior to injecting the sodium nitrite solution into the chemical process equipment; purging the chemical process equipment with a gas selected from the group consisting of steam and nitrogen; applying steam to the chemical process equipment to heat the equipment to between about 213 F. and about 275 F. at a pressure between about 1 psig and about 20 psig.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) According to one embodiment, sodium nitrite may be combined with steam or nitrogen to oxidize iron sulfide. The method may be used to solve the aforementioned problems of iron sulfide treatment and catalyst wet dumping described above.

(2) Sodium nitrite is able to oxidize iron sulfides to elemental sulfur and iron oxide. A balanced chemical equation for such a reaction is shown below.
6H.sub.2O+3NaNO.sub.2+6FeS.fwdarw.3NH.sub.3+6S+3Fe.sub.2O.sub.3+3NaOH

(3) When only small amounts of sulfide in the form of solid FeS are reacted, the reaction proceeds irreversibly. Experiments conducted on extremely sour brines produce a reaction limit, possibly the result of a reverse reaction, schematically illustrated below.

(4) ##STR00001##

(5) The reaction of NaNO.sub.2 with S.sup.2 produces elemental sulfur (S.sup.0) but also produces NH.sub.3 and NaOH as by-products, and in the presence of high concentrations of these two alkaline materials, the equilibrium persists. NaNO.sub.2 promotes the forward reaction; the reverse reaction is promoted by NH.sub.3 and NaOH.

(6) Sodium nitrite stock solutions must be prepared and used at a slightly alkaline pH (e.g., pH=8.75). This may be accomplished by buffering the stock solution with disodium phosphate (Na.sub.2HPO.sub.4). In blending a stock concentrate, the phosphate buffering chemical may first be blended with water, and then the sodium nitrite may be added at the end. Sodium nitrite must never experience an excursion into acidic conditions, under which conditions nitric oxide gasses are formed and evolved from solution (NO and NO.sub.2).

(7) Pyrophoric iron sulfide will produce heat when introduced to oxygen. Hence, it is impossible to judge whether a vessel has been made safe by simply opening the vessel and making an observation. Thus, an important aspect of the present invention is inclusion of a chemical additive that provides a quantitative measure of reaction completion. This may be accomplished by adding a 1.2% by weight solution of lauryl dimethylamine oxide (LAO) in water, for example, to provide an easily detectable marker during treatment. The solution of lauryl dimethylamine oxide may additionally comprise sodium nitrite. Laboratory tests have confirmed that: 1. Sodium nitrite is compatible with LAO 2. A 5% solution by weight of sodium nitrite in water effectively oxidizes iron sulfide 3. A 1.2% solution of LAO allows the reaction progress to be easily measured.

(8) In one exemplary method, after the process equipment is taken out of service and chemically cleaned for hydrocarbons, the entire system remains oxygen free. The system is then put under a positive pressure with nitrogen and/or steam purging through the equipment. Steam may be injected into the chemical process equipment to heat the equipment to between about 213 F. and about 275 F. at a pressure between about 1 psig and about 20 psig.

(9) Once the oil and chemical is removed, the sodium nitrite solution with LAO described above is injected into the steam. The solution is vaporized and becomes entrained in the steam entering the vessel. Bleeder valves are opened at prescribed locations on the vessel and samples tested. When it is determined that a suitable concentration exists at the sample point(s) (e.g., greater than about 400 ppm), the steam is removed and the vessel is allowed to cool.

(10) Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.