Quality control feed rate adjustment

Cold feed extruders (Figure 14.25) have much larger length-to-diameter ratios because of the requirements of reducing green compound plasticity, heat buildup in the compound, and pressure buildup required to produce the extruded profile. The length-to-diameter ratio is typically 24 1. The modem cold feed extruder is also computer controlled, which enables adjustment of the compound temperature profile through the barrel, pressure control, flow rate, and feed rate. This provides accurate control over die swell, extrudate surface quality, and buildability of the extruded product. 

For example, suppose we set the sidestream flow rate at 3 kmol/h instead of the design 1.21kmol/h, then this reduces the concentration of the MeOH in the sidestream from 81.7 mol% to 33.3 mol% under design conditions where the feed composition is 1 mol% MeOH. Let us consider a control structure in which the temperature on Stage 17 is controlled by manipulating reboiler heat input and reflux flow rate is fixed. Now if the feed composition is increased to 2 mol% MeOH, the sidestream composition only changes to 42 mol% MeOH. This is not enough to remove all the additional MeOH in the feed, so the distillate purity is severely affected (increases to 1.55 mol% MeOH). Thus, a simple control structure with a fixed sidestream flow rate does not provide effective product quality control. The control structure must be able to adjust the sidestream flow rate in some manner so that MeOH cannot drop out of the bottom or go overhead. 

Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If on-line M WD is available, initiator flow rate or reactor temperature can be used to adjust MW [38]. In emulsion polymerization, initiator feed rate can be used to control monomer conversion, while bypassing part of the water and monomer around the first reactor in a train can be used to control PSD [39,40]. Direct control of surfactant feed rate, based on surface tension measurements also can be used. Polymer quality and end-use property control are hampered, as in batch polymerization, by infrequent, off-line measurements. In addition, on-line measurements may be severely delayed due to the constraints of the process flowsheet. For example, even if on-line viscometry (via melt index) is available every 1 to 5 minutes, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the MW develops, and the viscometer where the MW is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data is old. There are two approaches possible in this case. One is to do open-loop, steady-state control. In this approach, the measurement is compared to the desired output when the system is believed to be at steady state. A manual correction to the process is then made, based on the error. The corrected inputs are maintained until the process reaches a new steady state, at which time the process is repeated. This approach is especially valid if the dominant dynamics of the process are substantially faster than the sampling interval. Another approach is to connect the output to the appropriate process input(s) in a closed-loop scheme. In this case, the loop must be substantially detuned to compensate for the large measurement delay. The addition of a dead time compensator can... 

For polymerization or copolymerization processes performed in semi-batch reactors, the feed rate can also be adjusted to meet quality criteria [44-48]. Advanced techniques are proposed to control the feed rate by taking into account simultaneously productivity and safety criteria [49, 50]. 

If feed rate and composition are invariant, there seems to be no purpose for a forward loop. Although feed rate to a column may be on flow control, this does not mean that it is invariant-it means that the stream is only subject to intentional disturbances. Supply of feed stock must come from somewhere, and its source cannot, have infinite capacity. The smaller the supply capacity, the more often feed rate will have to be adjusted. And whether feed rate is subject to random variations or intentional set-point adjustments, it can change rapidly-far more rapidly than a feedback loop on product quality can respond. 

Total unit heat dnty will typically be in the range of 500-1000 BTU per pound of feed to the unit. This set of process heat requirements establishes the amount of heat that must be supplied by combustion of coke. Because of the process control schemes that are normally employed in FCCUs, the unit operation will automatically adjust itself so that the energy produced via coke combustion equals the heat requirements of the process. If the balance is shifted by changes to the feed quality or operating conditions, shifts in catalyst circulation rate and regenerator temperature will occur until a new equilibrinm set of conditions is established. 

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