GB2266306A

GB2266306A - Method for inhibiting stress corrosion cracking of carbon steel equipment in ethylene oxide plant carbon dioxide removal systems

Info

Publication number: GB2266306A
Application number: GB9305917A
Authority: GB
Inventors: Hans Uwe Schutt
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1992-03-23
Filing date: 1993-03-22
Publication date: 1993-10-27
Also published as: GB9305917D0

Abstract

Stress corrosion cracking of the carbon steel equipment of carbon dioxide removal systems in ethylene oxide plants is inhibited by adding a sulfiding agent chosen from elemental sulphur or sulphur-yielding compounds to the carbonate solvent solutions used to remove carbon dioxide from recycle ethylene streams.

Description

METHOD FOR INHIBITING STRESS CORROSION CRACKING OF CARBON STEEL EQUIPMENT IN ETHYLENE OXIDE PLANT CARBON DIOXIDE REMOVAL SYSTEMS The present invention relates to a method for inhibiting stress corrosion cracking of carbon steel equipment utilized in the carbon dioxide removal systems of ethylene oxide plants by adding a sulfiding agent to the carbonate solvent solution used to remove carbon dioxide from the ethylene stream which is recycled to the ethylene oxide reactor.

In an ethylene oxide plant, fresh ethylene mixed with recycled ethylene from the carbon dioxide removal system is reacted with oxygen in the presence of a silver catalyst to produce ethylene oxide. In addition to ethylene oxide, carbon dioxide and water are produced as by-products. Ethylene oxide is scrubbed out with water.

Thereafter, carbon dioxide is removed from the recycle ethylene stream by chemical reaction with an aqueous carbonate solvent solution in a section of the ethylene oxide plant referred to herein as the "carbon dioxide removal system". This section consists of a CO2 absorber, a CO2 stripper, heat exchange equipment and associated tubing, pumps, and valves, all of which are typically constructed of carbon steel.

Due to the corrosive nature of carbonate solutions, a high degree of corrosion is experienced in the carbon dioxide removal system. This corrosion, often referred to as stress corrosion cracking or carbonate cracking, is the main failure mechanism for carbon steel equipment in ethylene oxide plant carbon dioxide removal systems.

In the past, the equipment of the carbon dioxide removal system was protected against stress corrosion cracking by the addition of chromate or vanadate inhibitors to the carbonate solvent solution. Environmental concerns over the use of heavy metal inhibitors and the disposal of heavy metal waste materials make it desirable to eliminate these inhibitors and replace them with less objectionable inhibitors. The present invention provides an environmentally safer alternative to heavy metal inhibitors in which the stress corrosion cracking of carbon steel equipment in the carbon dioxide removal system of an ethylene oxide plant is inhibited by adding a sulfiding agent to the carbonate solution.

The present invention is directed to a method for inhibiting stress corrosion cracking of carbon steel equipment utilized in the carbon dioxide removal systems of ethylene oxide plants. Stress corrosion cracking is inhibited by adding sufficient amounts of a sulfiding agent selected from elemental sulphur or a sulphur-yielding compound to the carbonate solvent solution used to remove carbon dioxide from the recycle ethylene stream.

The present invention provides an environmentally safer alternative for inhibiting stress corrosion cracking of carbon steel equipment used in carbon dioxide removal systems of ethylene oxide plants than do the prior art heavy metal inhibitor systems.

The addition of elemental sulphur or sulphur-yielding compounds to the carbonate solvent solution used to remove carbon dioxide from the recycle ethylene stream protects the carbon steel equipment against stress corrosion cracking.

The use of sulfiding agents in ethylene oxide plant carbon dioxide removal systems is especially attractive since the high pH of the aqueous environment in the carbon dioxide removal system prevents the transfer of sulphur species into the vapor phase recycle ethylene stream which is fed back into the ethylene oxide reactor where such species have the potential of poisoning the silver catalyst.

In the method according to the present invention, the sulfiding agent added to the carbonate solvent solution may consist of elemental sulphur, one or more sulphur-yielding compounds and mixtures thereof. Sulphur-yielding compounds include volatile and non-volatile compounds and also include soluble sulphur compounds such as hydrogen, alkali metal or alkaline earth metal sulfides, disulfides, and polysulfides, all kinds of soluble thiosulfates, organic sulfides, disulfides, and polysulfides, sulfates, and equivalent kinds of sulphur-yielding compounds. Volatile sulphuryielding compounds will be totally absorbed by the carbonate solution due to its high pH.Non-limiting examples of the materials which can be added to reduce stress corrosion cracking include sodium sulfides, disulfides or polysulfides, calcium sulfides, disulfides or polysulfides, sodium thiosulfates, and organic sulfides, disulfides and polysulfides containing up to about 20 carbon atoms and which can be aliphatic, cycloaliphatic, aryl or (hetero)aryl in nature, such as dimethyl disulfide, phenyl disulfide, sodium thiosulfate and the like. Any sulphur-yielding compound present in a sufficiently high concentration which would accomplish the objectives of the invention is equivalent to those sulphur-yielding compounds specifically illustrated herein. The preferred sulfiding agent is an alkali sulfide. It is even more preferable if the sulfiding agent is either sodium sulfide or potassium sulfide.

The amount of sulfiding agent to be added to the carbonate solvent solution may vary depending upon the sulfiding agent utilized. When the sulfiding agent is sodium sulfide, the amount of sodium sulfide added is such that the amount of sulphur present is at least about 0.04% on the basis of the carbonate solution, preferably at least about 0.10% sulphur and even more preferably at least about 0.18% sulphur.

The sulfiding agent is preferably added to the carbonate solvent solution in such a manner and at such a time as to insure that the sulphur or sulphur-yielding compounds are well dissolved in the aqueous carbonate solvent solution. Those skilled in the art will recognize that numerous devices and arrangements are available to allow for this addition.

The carbonate solvent solution utilized comprises any conventional carbonate known in the art which contributes to stress corrosion cracking conditions because of the presence of carbon dioxide, carbonate and chlorides. In the preferred embodiment of the invention, a saturated mixture of a carbonate and a bicarbonate in water is used. It is even more preferred if the carbonate solution consists of a saturated mixture of potassium carbonate and potassium bicarbonate in water.

The ranges and limitations provided in the instant specification and claims are those which are believed to particularly point out and distinctly claim the instant invention. It is, however, understood that other ranges and limitations that perform substantially the same function in substantially the same way to obtain the same or substantially the same result are intended to be within the scope of the instant invention as defined by the instant specification and claims.

The invention will be described by the following examples which are provided for illustrative purposes and are not to be construed as limiting the invention.

A series of slow strain rate tests, applying strain rates of about 10 6 -l about 10 sec 1 to ASTM A516-70 tensile specimens, were chosen as the principal investigative tool. In addition, potentiodynamic polarization curves were recorded in support of slow strain rate tests and for guidance in potential controlled experiments.

All test solutions simulated the carbon dioxide removal system environment. Their basic composition was: 7.1 Fwt Bicarbonate 5.4 %wt Carbonate 0.047 %wt Chloride 1.0 %vol Ethylene Glycol The pH of the solution was 10.5. The chloride content was chosen to be higher than normally found in commercial units in order to enhance the potentially negative impact this contaminant might have on stress corrosion test results, since chlorides generally impede the function of metal surface films as barriers to continued corrosion.

The base solutions were saturated with carbon dioxide for twenty minutes at room temperature before specified amounts of sodium sulfide were added. The system was then pressured with 50 psig carbon dioxide and heated to 110 OC (230 "F). Since carbonate stress corrosion cracking is very sensitive to the surface potential of the exposed steel, the most crack enhancing potential, namely -670 mV versus the saturated calomel electrode (SCE), was applied to the tensile specimen being strained.

Table I lists the results of adding varying amounts of sodium sulfide to carbonate solutions on carbonate stress corrosion cracking.

TABLE I Sodium Sulfide Time to Appearance Addition, Failure, Tensile Crack %wt Sulphur Hours Solution Specimen Morphology 0 17 Clear Shiny Black Heavy Intergranular 0.0025 16 Slight Red Bluish, Heavy Deposit Rusty Intergranular 0.01 25 Black Shiny Black Medium * Suspension Intergranular 0.02 26 Black Shiny Black Very Light Suspension Intergranular 0.04 29 Clear Shiny Black No Cracks Green 0.1 35 Clear Shiny Black/ No Cracks Green Silver 0.18 40 Clear Shiny Black No Cracks Green * Black suspension consists of elemental sulphur and iron sulfide.

Claims

1. A method for reducing stress corrosion cracking of carbon steel equipment in an ethylene oxide plant carbon dioxide removal system in which a carbonate solvent solution is used to remove carbon dioxide from the recycle ethylene stream, the method comprising adding a stress corrosion cracking inhibitor consisting of a sulfiding agent to the carbonate solvent solution.

2. The method of claim 1 wherein the sulfiding agent is selected from the group consisting of elemental sulphur, one or more sulphur-yielding compounds and mixtures thereof.

3. The method of claim 2 wherein a sulphur-yielding compound is used.

4. The method of claim 2 wherein the sulphur-yielding compound is an alkali sulfide.

5. The method of claim 4 wherein the alkali sulfide is sodium and/or potassium sulfide.

6. The method of claim 5 wherein the amount of sodium sulfide added is such that the amount of sulphur present is at least about 0.04% on the basis of the carbonate solvent solution.

7. The method of claim 5 wherein the amount of sodium sulfide added is such that at least about 0.10% sulphur is present.

8. The method of claim 5 wherein the amount of sodium sulfide added is such that at least about 0.18% sulphur is present.

9. The method of claim 1 wherein the carbonate solvent solution consists of a mixture of potassium carbonate and potassium bicarbonate in water.

10. A method for reducing stress corrosion cracking of carbon steel equipment in ethylene oxide plant carbon dioxide removal systems in which a carbonate solvent solution consisting of a mixture of potassium carbonate and potassium bicarbonate in water is used to remove carbon dioxide from recycle ethylene streams, which method consists of adding sodium sulfide to the carbonate solvent solution in an amount to provide at least about 0.18% sulphur on the basis of the carbonate solution.