Recent refinery industry trends are to optimize crude unit operation with advanced control technology such as real-time-optimization. One potential crude unit optimization objective could be maximizing diesel product yields and minimizing the quantity of diesel boiling range material in the FCC feed. Appropriately designed advanced process control technology for a crude unit can be used to fully utilize existing equipment performance. The advanced process control scheme (or operator) can adjust the appropriate process variables to optimize the diesel yields against the current unit limitations.
Nevertheless, depending on the crude unit equipment limitations, process and equipment design changes may be required to fully implement the diesel product optimization. This paper will address crude unit process variable optimization and the potential equipment design issues that can materially improve the refinery diesel production. While each refinery crude units design, operation, and equipment constraints are different, the fundamental operating variables and the process and equipment design issues are common to all crude units.
Optimization of the refinery No. 2 Oil production begins with an evaluation of the current crude unit operation. Crude unit baseline operation should be determined by a controlled unit test run. The test run will measure process variable and equipment performance issues such as atmospheric column cutpoint, diesel boiling range material in the AGO product, and diesel boiling range material in the vacuum column feed. A proper test run will also provide field measured data such as atmospheric column tray pressure drop which is not typically monitored by the process instrumentation. This data can be used to better quantify equipment performance. Unfortunately design equipment performance is not a measure of the true equipment performance. In spite of the sophisticated process control technology, periodic crude unit test runs are required to measure performance.
Advanced process control optimization can only vary the parameters that the existing equipment performance will allow the variable to affect. Often, the existing process configuration and equipment is not designed to meet our optimization objective of increased atmospheric column distillate yields. For instance, the atmospheric tower bottom product stripping section must accomplish its intended function. Increasing stripping steam to an atmospheric crude column to optimize performance requires properly operating trays to allow the stripping steam to function. If the section has been damaged or incorrectly designed trays, stripping provides minimal benefit.
While theory shows that increased stripping steam increases stripout, the actual column equipment performance must reflect the assumed theory. Often, theory and practice diverge. Typically, the design and operation of the atmospheric crude column can be modified with minimal capital to significantly increase the yield of diesel and kerosene. Specific crude unit process and equipment designs vary significantly depending on when the unit was built, design product specifications, and original design charge rate.
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