API PUBL 948
A STUDY OF VARIABLES THAT AFFECT THE CORROSION OF SOUR WATER STRIPPERS
|Publication Date:||1 May 1996|
FOREWARD: The Battelle report, "A Study of Variables that Affect the Corrosion of Sour Water Strippers", covers the results of an 18-month laboratory study initiated and monitored by the Corrosion Research Panel, Subcommittee on Corrosion, the Refining Department, API. This is a complex corrosion problem principally concerned with the inconsistent corrosion behavior of overhead condenser tubes.
The primary purposes of the present study were to isolate the chemical constituents responsible for corrosion and to identify the conditions under which corrosion would occur. The study was limited to a modest, low-pressure simulation of a condensed liquid environment, dealing only with liquid compositions capable of being contained in atmospheric pressure equipment.
Some members of the Subcornittee feel strongly that cyanides are the primary cause of corrosion, and support for this can be found in the Battelle report. One fact that quickly became obvious during the test program was that it was extremely difficult to control or even measure the concentration of free cyanide in the test solutions. The occurrence of corrosion was often determined by whether cyanides were present or absent. Cyandies could escape from the systems, react with corrosion products to form ferrocyanide, or react with air to form thiocyanate. Air leakage into the system not only consumed cyanide, but formed extraneous materials like elemental sulfur which complicated the solution chemistry. Because of the inability to control cyanide content, it was never possible to establish a reproducible set of base conditions from which we could set out to systematically evaluate the effect of other variables . The elusive nature of cyanides and problems in analyzing for them at low concentrations may explain some of the apparent inconsistencies in sour water stripped corrosion.
Further work is needed to evaluate the effect of higher ammonia concentrations. At the October 1975 Subcommittee meeting, the possibility of corrosive ammonium bisulfide deposits forming on tube surfaces during operation was discussed. These would disappear on flushing out and opening the condensers in line with a similar problem noted on page 14 of the report. clearly, questions as to the exact mechanism of sour water stripped corrosion have not been totally resolved. However, it is generally agreed that titanium tubes provide a satisfactory solution to the problem.