LP Modeling - Past, Present and Future

More documents

Recommendations

Info

efinery in the same case. Also, there are limitations on the use of multi-refinery. In general, no more than about 3 refineries should be attempted with recursed LP multirefinery models. Large recursed multi-refinery models tend to be unstable as represented in the following graph. Recursed Pools 600 500 400 300 200 100 1000 3000 5000 7000 9000 Model Maintenance 1 2 Stable Matrix Rows 3 Refineries Unstable This paper is mainly about LP model structure. However, there are some important points to be made about generating the LP data and maintaining the LP model. In the past, we occasionally (every few months) compared the actual plant process data with the LP predicted yields. We looked at combinations of test run results, historical process data, and yield accounting data when updating LP models and sub-models. LP sub-models were not frequently updated (maybe once every couple of years) and the methodology for updating the LP sub-model was not very well defined. Currently, there are two main best practice methods for monitoring LP performance and maintaining and updating LP models. First, an overall refinery backcasting process may be used to identify overall model weaknesses (e.g. refinery gain higher than actual) and profit opportunities. Backcasting is comparing a previous month’s actual results against the plan and against an LP-actual case (where the LP case attempts to match actual results). Many refineries conduct backcasting on a regular basis (at least several times each year). LP maintenance activities are frequently based on the backcast results. The second major activity is for process engineers to monitor their process units on a weekly or bi-weekly basis. Lab samples, along with plant historian data are used to compare the actual process unit results (yields and properties) against the LP predicted CC-01-153 Page 8
esults for the same snapshot period. Process engineers and planners monitor and compare the actual against the LP predicted and decide if the LP sub-model needs updating. In most cases, a process simulator model may be re-calibrated and used to generate the LP base and delta correctors on an as-needed basis. With this technique, process unit sub-models may be updated as frequently as several times a year using plant data and tuned simulator models. Future LP Developments The LP system vendors are now touting the virtues of the latest major LP system enhancement: simulator interfaces. With the simulator interface, the LP system may be directed to an outside application during each recursion pass. This outside application may be as simple as a single non-linear equation or as complex as several large complex process simulation models. The concept is that model coefficients that were previously static (i.e. unchanged during recursion) now become dynamic (changing after each recursion pass). For example, the feed % naphthene and % aromatics linear yield shift correctors commonly used in reformer LP models are generally based on particular feed qualities and severities. These linear correctors are calculated externally during the model development and then built into the model. These yield correctors are not updated during recursion. The LP model predicted yields become less accurate if the LP optimized feed deviates significantly from the base feed quality assumptions. With the simulator interface option, these linear feed property correctors may now be calculated and updated during each recursion pass, and then used for the next optimization pass. As the model optimizes the feed quality and severity after each pass, the naphthene and aromatics correctors are then adjusted to match the last pass results. This improves the accuracy since the final converged solution of the LP more closely matches the results of the simulation prediction. However, the risk of a local optimal solution may increase, since more and more coefficients are recursed and the initial starting assumptions for feed quality and severity become increasingly important as they may influence the final converged solution. A number of refiners have successfully developed useful and reliable applications for the simulator interface. There are a number a relatively simple non-linear applications being used where previously unsolvable non-linear relationships have now been handled with the simulator interface. For instance, in one application, we needed to control the percent of a particular feed to a process unit. In this particular case, the percent was an unknown and the process unit feedrate was also an unknown. This was previously unsolvable with DR. However, with the simulator interface, it was now possible to externally calculate the correct percent (based on the first optimized solution) and then update the percent following each recursion pass. CC-01-153 Page 9
Page 1 and 2: Introduction LP Modeling - Past, Pr
Page 3 and 4: The downstream process unit sub-mod
Page 5 and 6: 1. Some modelers “offset” the l
Page 7: For models that contain a moderate
Page 11: Every refinery LP model situation i

LP Modeling - Past, Present and Future

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?