Inventory reset

EPA (2008b). TSCA inventory reset and inorganic high production volume challenge programs notice of pubhc meeting. Fed Reg 73, 70640—70643. 

Control objectives related to the operation of the process units and the process itself (production rate, product quality, unit-level, and total inventory) should be addressed in the fast time scale. For instance, when a multi-loop linear control strategy is considered, the reset time for the controllers should be of the order of magnitude of the time constants of the individual process units. 

The flowsheet is completed with P-type controllers for level in the reflux drum and bottoms, and PI controller for pressure. These controllers ensure the basic Inventory control, but are not sufficient for quality control. Therefore, we are interested by distillate flow rate and purity faced with disturbances in the feed. Fig. 4.7 presents the open loop response to feed variation of +/- 10%. Increasing the feed to 110 kmol/hr gives an increase in purity over 99%, but a decrease of the distillate rate to less than 47.5 kmol/hr. After reset to initial conditions, the feed is reduced to 90 kmol/h. This time the distillate rate increases at 52.5 kmol/hr, but the purity drops dramatically to 86%. This behaviour seems somewhat strange, so the reader is encouraged to find a physical explanation. The need for quality control in a distillation column is obvious. This issue will be treated in the Examplel2.2. 

EPA overheads entitled Thought Starter Resetting the TSCA Chemical Inventory (May 13, 2008), available at http //www.epa.gov/CHAMP/ pubs/inventoryreset.pdf... 

The advantage of this method was its simplicity and flexibility. TLC could reset inventory up or down by just changing the expected sales per week as it accumulated sales history. This didn t remove the need for a forecast, of course, but did assure that inventory levels would track actual demand. 

The use of high or low limits for process variables represents another type of selective control called an override, where a second controller can override or take over from the first controller. This is a less extreme action than an interlock, which is used for emergency shutdown of the process (see Chapter 10). The anti-reset windup feature in feedback controllers (cf. Chapter 8) is a type of override. Another example is a distillation column that has lower and upper limits on the heat input to the column reboiler. The minimum level ensures adequate liquid inventory on the trays, while the upper limit exists to prevent the onset of flooding (Buckley et al., 1985 Shinskey, 1996). Overrides are also often used in forced draft combustion control systems to prevent an imbalance between air flow and fuel flow, which could result in unsafe operating conditions (Singer, 1981). 

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