As part of an ongoing sourwater corrosion study, 15 major refiners and engineering service companies have identified two seldom considered parameters in sourwater corrosion: wall shear stress and process creep.
These same companies formally launched a 2-year industry-wide study at the beginning of this year to determine the causes and solutions to refinery sourwater corrosion. Pooling their corrosion data, they hope to better predict and control ammonium bisulfude corrosion via material selection, process-unit operations control, and chemical treatments.
Sourwater is ammonium bisulfide, a corrosive produce resulting from sulfur removal in refineries that can cost US refineries up to $50 million/incident.
New environmental regulations have increased the level of corrosion-causing ammonium bisulfide in refinery sourwater systems, commonly found in refinery hydrocracker and hydrotreater applications, specifically in the reactor effluent air cooler (REAC) systems.
InterCorr International Inc., a corrosion solutions provider based in Houston, and Equilon Enterprises LLC, also based in Houston, are heading the program.
Since the program's inception in March 2000, sponsor companies, such as Equilon, Fluor Daniel, and Koch Petro leum Group, have already recaptured their participation fees in a single application of collected data. Sponsor companies also currently include Chevron Corp., Conoco Inc., ExxonMobil Corp., Petroleo Brasileiro SA, Saudi Aramco, Sunoco Inc., Syncrude Canada Ltd., Tosco Corp., TotalFinaElf, UOP LLC, and Valero Energy Corp.
Data
The joint industry program provides a medium for the accumulation of quantitative data on ammonium bisulfide corrosion for a variety of materials under varying refinery service conditions. The program has verified its laboratory corrosion rate data with documented corrosion rates obtained from its sponsors.
The sponsors have made efforts to provide operational parameters (for example, temperature, flow rate, and ammonium bisulfide concentration) that accurately describe the process conditions where the corrosion data were monitored. This diligence has allowed the program to map these in-plant data back to the laboratory data for similar conditions.
With relevant parameters included, the corrosion rates from the testing program provide better correlation with unit corrosion than possible with the accepted sourwater corrosion control guidelines.
Parameters
An important aspect of this program, says Russell Kane, senior consultant of InterCorr, is it is the first major effort to emphasize fluid flow as a critical corrosion parameter, represented in terms of "wall shear stress." Wall shear stress reflects the mechanical force produced on the inner pipe by flowing media.
This approach allows linkage between laboratory conditions from the program and actual multiphase flow conditions in real sourwater units, irrespective of pipe size and flow regime.
After completion of the first of two parts of this study, the program has identified a new parameter not currently used in the generally accepted guidelines for sourwater corrosion. This parameter suggests that some units have moved from non-corrosive conditions to corrosive ones via process creep.
Process creep refers to process conditions that have slowly changed with time without serious consideration for the effect of the changes on corrosion, which can lead to unforeseen operational or mechanical problems.
Experimental capabilities
InterCorr and Equilon have developed experimental and analytical capabilities to analyze the multiphase flow conditions, which are then used as a basis for experimental simulation using three-phase emulation techniques.
These techniques establish multiphase conditions (water and gas; water, oil, and gas) in a reservoir autoclave of a laboratory flow loop using rigorous ionic modeling of test environments. To simulate the alkaline sourwater conditions in the REAC system, the companies dissolve the primary corrodent (H2S) in an ammonium hydroxide solution.
The laboratory simulates the mechanical effects of the flowing media based on a hydrodynamic analysis of the flow regime and the wall shear stress produced by the flowing multiphase media. With the wall shear stress for the actual conditions, the flow loop can produce equivalent levels of wall shear stress by flowing a sourwater solution in the loop.
