Crude switches are automatically detected and factored into the inferential product property calculations that form the soft sensor signals. The true boiling point (TBP) curve in red below shows crude switching from heavy to light crude and the resultant product quality control changes.
Generalized Cutpoint Control Technology
Detects changes in crude composition or "crude switch"
Dynamically calculates product qualities in real-time
Decouples the effects of feed quality disturbances
Decreases crude switch time from several hours to 1 hour
Downgrading of more valuable products minimized
Decreases the number of alarms
This trend serves the engineer, checking mass balance on the unit, comparing crude feed (magenta line) against the sum of products (purple line).
The chart above shows mass balance disturbances caused by crude switches and possibly other disturbances, but the general behavior is acceptable. The slope (blue line) of the crude TBP curve, calculated by GCC, showing a number of crude switches during the period and there is no mass balance change from crude to crude.
GCC health is monitored by comparing heat balance against measurements.
The distillate flows (plus overflash) are calculated by heat balance to speed up crude switch detection and it is important to monitor that the heat balance calculation trends well against measured distillate product flows.
The chart above illustrate a healthy trend where the difference between heat balance distillates and measured distillates remain constant while the unit goes through several crude switches. The blue trend is the slope of crude TBP curve as calculated by GCC, showing several mild and some severe crude switches.
Crude Switch Control
The chart above shows a trend of key MVs and CVs during a 24 hour period.
The red trend here is the slope of crude TBP curve and it shows that a switch from heavy to light crude started about 8:30 am. The yellow trends are MVs, manipulated by the APC controller to keep product properties at target.
The blue trends are some of the product properties (90% points) under control. It can be seen that by and large these properties are kept constant before and after the switch.
Minor quality disturbances are to be expected because of equipment limitations as to how quickly flows or temperatures may change.