Optimize with Expert System Advisor

The distillate and bottoms streams from a distillation column can be sampled and analyzed once per shift, and then an expert system can be used to recommend adjustments in setpoints for the separation power base and the MRT using a set of backward chaining logic rules. For the case of an upper specihcation limit of 1,000 ppm heavy key impurity in the distillate stream and a standard deviation of 100 ppm impurity, the desired impurity concentration may be 700 ppm, that is, 3 standard deviations below the upper specification limit. The expert decision rules can be set to make adjustments only if the impurity concentrations are outside of the range from 0.6 to 0.8 times the upper specification limit (USL). For example, the hrst priority is to check the key impurity separation power. If the key heavy impurity concentration in the distillate is above 0.8 times its upper specification limit and the key light impurity in the bottoms is above 0.8 times its upper specification limit, then increase the separation power base (Rule 1 in Table 7.2). An initial guideline is to increase the separation power base, steam/feed, or reflux/feed by 1%. 

Finally, check to see if the separation power is too high and can be reduced to save energy and increase tower capacity. If the key heavy impurity concentration in the distillate is less than 0.6 times its upper specification limit and the key light impurity in the bottoms is less than 0.6 times its upper specification limit, then reduce the separation power base (Rule 6 in Table 7.2). An initial guideline is to reduce the separation power base, steam/feed, or reflux/feed by 1%. 

The trigger points and the size of incremental adjustments can be changed to meet the needs of each distillation column. For the case of a distillate that forms an azeotrope with 30% heavy key impurity, the column may be designed for an upper 

In some distillation columns, the temperature movement from concentration changes is too small to be measured. For example, an off-spec product with 5 ppm of light key impurity is stripped down to 10 ppb in the bottoms and concentrated to 50 ppm in the distillate. In that case, then, the distillate flow rate can be set to be 10% of the feed rate to get the distillate concentration to increase by 10 from 5 to 50 ppm. The recommended action in Table 7.2 can be to increase or decrease the distillate, or distillate/feed, by 1%. 

An expert system will run a set of backward chaining rules in the same order each time the system is run starting with Rule No. 1. If Rule No. 1 is not true, then Rule No. 2 is run. If Rule No. 2 is true, then Rule No. 2 is said to fire. When Rule No. 2 fires, the expert system stops, and the action for Rule No. 2 is recommended. If none of the first six rules fire, then Rule No. 7 fires, and the recommendation is No Change Necessary.  

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