PRESERVING FEED COMPOSITION EFFECTS DOWNSTREAM OF AN OPEN LEVEL IN A DMCPLUS CONTROLLER

摘要

The FS-1 Fractionator is a natural gas fractionation unit which separates natural gasrnliquids (C3P) into propane, butane and C5P. The process consists of two distillation towers inrnseries. DC3 is a depropanizer and DC4 is a debutanizer. DC3 is fed by three different feedrnpipelines, characterized by having fast and frequent changes to both flow and composition.rnThe FS-1 Fractionator does not have significant feedstock or product storage, and plant feedrnrates are closely coupled to desired product flow rates sent by pipeline to customers. Thernproduct compositions and the product flows must be controlled to specified targets. Therncapability of meeting the production targets depends strongly on the compositions of the feedrnstreams and the ability of the DMC controller to manipulate the ratio between the various feedrnsources.rnA key design consideration is whether to open the DC3 level between DC3 and DC4.rnThe advantage is that the delay and oscillation created by the level PID loop is removed andrnsharper, clearer models are created for DC4 CVs. However, opening the level breaks thernrelationship between the DC4 CVs and the feed compositions. The DC4 butane product flowrnwould have models with the DC3 bottom flow, but not with the three different feed flows andrntheir compositions.rnThe project team overcame this trade-off by opening the DC3 level, and re-building thernfeed composition effects around the open level. At steady-state, the problem can be treatedrnas a mass/component balance. All propane that is added to the column must be removed asrnpropane product or as an impurity in the butane product and so on. Steady state predictorrnmodels are created between the feed flows and compositions, with unity models to thernproduct flows (ignore the effect of impurities). The DC3 bottom flow has a dynamic-only modelrnto the butane product flow. The models are identified from open-loop data, but assembled inrnthe way that we know they will perform in closed loop. The DC3 bottom flow is dynamicallyrnindependent, but the need to control the DC3 bottom level within constraints means that it isrnnot truly independent at steady state. The dynamic behaviour of the DC3 bottom flow must bernaccounted for, but the feedstock flows can be connected directly to the product flows atrnsteady state, because the DC3 bottom level must remain in control.