By Winston K. Robbins, Ph.D.
As noted in a previous article, the applied chemist is an individual who draws on experience with petroleum chemistry and process engineering to solve refinery problems. In the following example, an applied chemist combines oxidation chemistry with analytical methodology and field observations to direct process modifications.
A problem arose when a petroleum company encountered erratic results in color and failures in engine tests for several batches of pre-production (i.e., demonstration) runs of a specialty oil. The engine test failures did not appear to correlate with the color of the product; both off-color and water-white oils would fail. Attempts to isolate and characterize the color bodies at a research lab were unsuccessful. However, a high performance liquid chromatography (HPLC) under development at the lab suggested the presence of hydroperoxides in the oil. Hydroperoxides were confirmed by isolation, FTIR and M techniques. Although hydroperoxides are not colored, they are thermally unstable, leading to discoloration or engine-test failure.
With this information in hand, the production protocol was examined with the process engineers. Because the wax hydroisomerization process was being run in a refinery but on small scale, the protocol called for two steps in the process to be run in a blocked operation. After hydroisomerization, the product oil was accumulated in a heated carbon steel storage tank to prevent wax precipitation. Because the tank was not inerted, it was hypothesized that the 60°C temperature and prolonged exposure to air was sufficient to form hydroperoxides. Because the latter were known to be the types of compounds responsible for color formation and engine test failures, additional review of the sample histories were followed.
Attempts were made to rationalize variations in color and severity of engine test failures with process conditions. Two key factors were found. First, the storage tank was maintained at temperature between runs for different periods of time. Second, the configuration of the storage tank allowed the tank heel to be strenuously agitated at higher temperatures than realized. (The control thermocouple was positioned 3 feet above the tank bottom, the heel liquid level was only 18 inches deep, and the tank agitated with a 36 inch diameter stirrer mounted on the side of the tank. Thus, during the periods between runs, full heat was applied to the tank with a half-submerged propeller while beating air into the heel of the previous run.)
With this knowledge, the tank was cleaned, a nitrogen inerting system was installed, and a new demonstration run was commissioned. In addition, a local lab was trained with a method for monitoring for hydroperoxides. The first several samples of intermediate product were caught as it flowed into the tank; these samples were free of hydroperoxides. The following day, hydroperoxides were found but at levels that decreased over time. Rapid consultation with the engineering group revealed that these samples were being taken with a sampling loop that had not been cleaned. The run was aborted, the tank and lines cleaned, and a run completed with a product that met color and engine test specifications.