Transport delay

Selecting the Sampling Point The selection of the sampling point is based primarily on supplying the analyzer with a sample whose composition or physical properties are pertinent to the control function to be performed. Other considerations include selecting locations that provide representative homogeneous samples with minimum transport delay, locations that cohect a minimum of contaminating material, and locations that are accessible for test and maintenance procedures. 

Automobile companies are major users of just-in-time methods, coordinating the delivery of thousands of parts from many different suppliers. A transportation delay of a few hours for one part could shut down an assembly line for half a day. 

Continuous Transfer functions in polynomial or pole-zero form state-space models transport delay... 

A similar problem affects the heat exchange jacket and may be reduced by using a coil, in which a plug flow can be assumed for the heat exchange fluid. When the reaction temperature is controlled by an external heat exchanger or condenser [17], the recirculation of the fluids introduces a transport delay that may strongly affect the control action. 

Whenever a sample has to be brought to an analyzer, a transportation delay and a potential for interference with the integrity of the sample are inevitable. If an automatic controller maintains the measured composition, the transportation lag can seriously deteriorate the closed-loop control stability of the loop. An even more serious consequence of the use of sampling systems is the potential for interference with the integrity of the sample. This can occur due to filtration, condensation, leakage, evaporation, and so on, and these operations cannot only delay, but also change information and measurement. 

The sample system is responsible for collecting a representative sample of the process and delivering it to the analyzer for analysis. Obviously, the reliability of the sample system directly affects the reliability of the overall composition analysis system. The transport delay associated with the sample system contributes directly to the overall deadtime associated with an on-line composition measurement. This difference in sampling deadtime can have a drastic effect on the performance of a control loop. Table 15.2 summarizes the dynamic characteristics and repeatability of typical control valve systems and several different types of sensors. 

Determining the time constant of the sensor is usually more difficult than estimating the repeatability. To determine the time constant of the sensor system, one needs to know the actual process measurement. Consider a temperature measurement. A measurement of the actual process temperature is required to estimate the time constant of a sensor. Instead, the thermal resistance, which causes the excessive thermal lag of the temperature sensor, can be evaluated. The location of the thermowell should be checked to ensure that it extends far enough into the line that the fluid velocity past the thermowell is sufficient the possibility of buildup of insulating material on the outside of the thermowell should be assessed, and the thermal contact between the end of the temperature probe and the thermowell walls should be evaluated. In this manner, an indirect estimate of the responsiveness of the temperature sensor can be developed. The velocity of a sample in the line, which delivers a sample from a process line to a GC, can indicate the transport delay associated with the sample system. A low velocity in the sample line from the process stream to a GC can result in excessive transport delay, which can greatly reduce controller effectiveness. 

Excessive transport delay for an analyzer Sample drawn from wrong process point Plugged sample system ... 

LUNG LAG - 5 SECONDS (5TH ORDER) FLOW DELAY - 20 SECONDS VARIABLE 02 CONSUMPTION NITROGEN PULSE - 90 SECONDS CONTROLLER TIME CONSTANT - 10 SEC. CONTROLLER TRANSPORT DELAY - 20 SEC. 

Stabilization time Time of transportation delay Time constant Time of overshoot Value of maximum overshoot Time of the query... 

Informally, the transport delay signal assignment aij s <= TRANSPORTeAFTERn... 

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... 

A variable that affects the plastification rate should have some impact on the processing rate. The mixing quality and the average time that the polymer stays in the extruder will direcdy affect the final product properties. The mean residence time is related to the transport delay time, which has enormous impact on the stability of feedback loops over the length of the extruder [18-20]. 

Integrator State-space Transfer Fen Transport delay Variable transport delay Zero-pole... 

The choice of the manipulated variables imposes a constraint on all incoming material to a particular node. The PID controllers are tuned to allow for fast set-point tracking and good disturbance rejection dynamics, taking into consideration the transportation delay between nodes. 

Feed flow rate may also affect process deadtime. If the prime cause of deadtime is transport delay than an increase in feed will cause the residence time to fall and a reduction in deadtime. At worst, deadtime may be inversely proportional to feed rate. If so then the maximum turndown limit of 1.5 will apply. In fact controllers are more sensitive to increases in deadtime than decreases. Rather than design for the average deadtime, a value should be chosen so that it varies between —30 % and 10 %. Techniques for accommodating excessive variation in deadtime are covered in Chapter 7. 

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