By Dr. Michael J. Humphries
Because of their economic attractiveness, there is widespread interest in processing corrosive crudes, particularly those with high TAN. Many refiners have conducted studies to assess the amount of corrosive crude that can be processed with their existing equipment (i.e., without changing its metallurgy). Also, they must often determine the extent to which their equipment and piping construction materials must be upgraded in order to be able to process the desired crude. It has become clear from a number of studies that no single approach is best for all sites - the answer is different for each site.
Many refiners have opted to install high alloy equipment and piping systems that are resistant to corrosion by Napthenic Acids. While effective, this approach is only economically attractive if a secure long-term supply of corrosive crude is available at attractive prices. Money spent on alloy material can only be recovered by processing corrosive crudes at a good margin. Experience has shown that the most effective approach is to establish a series of limits on TAN (or Neutralization Number), that are related to progressive corrosion control actions. Preferred corrosion control actions include the following:
The advantage of this approach is that inhibitor injection can be discontinued and monitoring can be reduced when high TAN crude is not available. This reduces the total cost for the program.
Crude corrosivity is a function of TAN and sulfur content. Crudes that are high TAN and low sulfur are particularly corrosive. It is possible to develop a series of operating envelopes in terms of TAN and sulfur (i.e., corrosive sulfur compounds). For each operating envelope, a specific corrosion control action is required. The economically best option is to run to the limit of a chosen corrosion control level. This approach maximizes the amount of corrosive crude that can be processed for a given level of corrosion control.
Experience has shown that Napthenic crudes generally first affect the vacuum transfer line and VGO sidestream. As feed TAN is further increased, corrosion will affect the atmospheric transfer line and HAGO circuits and the bottoms of both towers.
Predictive models of considerable complexity exist. However, experience has proven that it is most cost-effective and reliable to apply a monitoring program using hot corrosion probes and hot UT/RT to confirm the predictions of any model, and to adjust the predicted limits as necessary. By this method, a simple model can be used, and this approach has proven to be very cost-effective at many locations. While high TAN crudes primarily affect the hot parts of the Atmospheric and Vacuum units, they also affect downstream units as well (e.g., hydrotreater preheat trains). Some crudes have also caused overhead system corrosion because of their poor desaltability.
Phase I - Background Information (usually provided by the refinery)
Phase II - Determine the predicted equipment life for both the current crude slate and future crude options. Establish “pinch points” of equipment/piping that would limit the run length due to corrosion.
Phase II would be done by Carmagen Engineering, Inc., with assistance from the refinery inspection and supply specialists and input from a vendor of proven Napthenic Acid corrosion inhibitor.
This approach to evaluating processing options for high TAN or corrosive crudes has proven to be faster and more cost-effective than more analytical and data-intensive approaches that have been used. Carmagen Engineering staff has extensive experience in developing corrosion mitigation methodologies for high TAN and corrosive crudes, and has actively implemented in-plant programs in many refineries worldwide.