Process line rheometers

As discussed in Chapters 10 and 11, rheology can be very sensitive to the microstructure of liquids. For example, the viscosity of entangled polymer melts depends on molecular weight to the 3.4 power, T]o Ml . Equilibrium creep compliance is very sensitive to molecular weight distribution. The yield stress and low frequency G are good indicators of the flocculation state of colloids. Extensional viscosity can be an important indicator of bread dough quality (Padmanabhan, 1993). 

There are two basic types of process line rheometer those that can operate directly in the process and those that pull a side stream off the process for analysis. The side stream may be either dumped or recycled back to the process. 

The simplest approach to in-line measurement is to monitor the pressure drop and flow rate over a flow channel of constant cross section in the process. This is more difficult than it might first appear. Getting a uniform section with a reasonable Ap can be difficult because of temperature gradients. A low pressure gradient in the process means that a longer section is needed for accurate measurement. Rather than trying to infer the shear stress by the 

On-line concentric cylinder rheometer with torque sensor on the rotating bob. Similar to commercial designs from Brookfield, Haake, and Mettler. 

It is also possible to mount an oscillating probe in the process line. As discussed in Section 8.4, these probes operate at relatively high frequency, which may not be sensitive to the process variable. These siuface loading devices typically measure material within a few micrometers of the surface. Thus they should be located to have good flow over the surface. However, such high flow rates may damage the probe. 

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For mobile liquids, the use of this kind of controllable instruments is practically impossible. For these liquids, the non-controllable measurement techniques are available only and in general an apparent transient viscosity will be obtained. Nevertheless these measurements are still of great value, because in many cases they approximate industrial process conditions. Mostly used is the spinning line rheometer, where an elastic liquid is pressed through a spinneret and the liquid is pulled from the die by winding the filament around a rotating drum or by sucking the tread into a capillary tube. This is schematically shown in Fig. 15.25. A serious problem is the translation of the obtained data to the extensional viscosity. Many other non-controllable devices are discussed by,... 

An in-line measurement is performed in a process line an on-line measurement is performed in a bypass loop from the main process line and the food may be returned to the main process line after measurement is performed. A near-line measurement is performed on a sample taken from a process line which is often discarded after measurement. Because foods are complex materials (e.g., suspensions, emulsions, gels), stmctural changes may take place during sampling (e.g., flow through a valve) for on-line and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, and rotational concentric cylinder, plate-cone, and mixer viscometers may be used for in-line, on-line, and near-line measurements. In this respect, Tamura et al. (1989) proposed a helical screw rheometer as an on-line viscometer. The empirical measurement methods described previously are used primarily in near-line measurements. 

Rheometry is the most widely used on-line monitoring technique. Rheometers of different types have been developed for use in closed loop process control. Commercially available instm-ments all use steady state shear deformation. One commercial instrument attempted to operate in the dynamic mode, Rheometrics On-line Rheometer (ROR) but is not currently commercialized. In dynamic mode, the frequency dependence... 

A closed-loop extruder control scheme loosely based on this mechanism has been described by Curry et al. (8). They used an on-line rheometer at the discharge of a ZSK 30 twin-screw extruder to generate a signal related to melt flow index to a programmable logic controller (PTC). The PLC compared this value with a set point and manipulated the ratio of feeders, where one feeder contained a master batch of polypropylene and peroxide. Their control scheme featured a process lag time on the order of 4-7 min. 

On-line rheometers are also available, suitable for use with continuous compounding lines, such as co-rotating twin-screw extrusion processes. Exploratory work has been... 

The organization of this chapter parallels that of its predecessors in Part II, describing first the design of drag flow rheometers, then pressure-driven ones. Separate sections are devoted to analysis of data, particularly sinusoidal oscillations, and to special designs for process-line rheological measurements. The section on exten-sional rheometry is brief because most of the design issues were discussed in Chapter 7. 

Until recently, the rheological measurements were confined to test labs for quality control, research, and development purposes. New techniques for on-line rheological measurements have been developed, bringing the analytical instrumentation to the production floor. This is accomplished by employing a closed-loop instrument which diverts a portion of the process stream to a measurement head and then returns it to the process. Within the measurement head, most of the material flows through a bypass channel and a fraction of the stream is pumped across a capillary die, using precision gear pumps. A similar concept, employed by another manufacturer, utilizes a slit die to measure viscosity. An open-loop on-line rheometer, which does not return the diverted melt stream to the process after measurement, splits the diverted melt stream to two capillary dies of different LID ratios... 

When the rheometer is retrofitted with a single-screw extruder one can measure rheological properties and extrusion processing characteristics to differentiate lot-to-lot variance of polymer stocks. It also enables the process engineer to simulate a production line in the laboratory and to develop processing guidelines. 

Analysis with Scanning Probe Microscopy U.S. 5,543,594, Nuclear Magnetic Resonance Imaging Rheometer U.S. 6,220,083, Elongational Rheometer and On-Line Process Controller U.S. 5,520,042, Apparatus and Method for the Simultaneous Measurements of... 

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