Monitoring Polymerization in Batch Reactors

Table of Content

Introduction
Batch Reaction Process
Standard Instrument Configuration
Simplified Sensor for Batch Reactors
Hazardous Environments Around Batch Reactors
Explosion-Proof Enclosures
Implementing Intrinsically Safe Barriers into a Batch Reaction Process
Conclusion

Introduction

Dielectric Analysis (DEA), or the measurement of dielectric properties, is widely used to monitor and control polymerization in a batch reactor. Earlier work has revealed that DEA could monitor the free-radical polymerization of methyl methacrylate,1 where the change of log(ion viscosity) matched with physical viscosity and fractional monomer conversion.

In contrast to standard laboratory gravimetric techniques that must be performed off-line, dielectric cure monitoring has the advantage of in-process monitoring of polymerization in real time.

Both the movement of ions and mechanical viscosity are influenced by the degree of polymerization, and this impacts electrical resistivity. Consequently, the term ‘ion viscosity’ was coined to highlight the relationship between resistivity and mechanical viscosity.

Batch Reaction Process

Resins processed in batch reactors usually start with a high concentration of monomers (Figure 1). At this stage, mobile ions can travel easily via the material, resulting in very low ion viscosities (low resistivities).

Figure 1. Unreacted monomers

When these monomers are heated, they react and bond to form polymer chains (Figure 2). During this phase, the molecular weight of the molecules increases, while the number of molecules decreases. Both mechanical viscosity and resistance to the flow of mobile ions in an electric field increases.

This electrical resistance is measured by dielectric cure monitoring, which allows direct monitoring of material state in the reaction vessel.

Figure 2. Early stage polymerization

As more new monomers are polymerized, log(ion viscosity) increases in a similar manner to the plot shown in Figure 3. It must be noted that only the alteration in log(ion viscosity) correlates with fractional monomer conversion, and the actual relationship should always be experimentally determined.

The value of log(ion viscosity) is based on the level of impurities and ions, and may differ from one batch to another. Therefore, only the change is useful and log(ion viscosity) alone is not a reliable measure of resin polymerization.

Figure 3. Change in log (IV) vs. Fractional Monomer Conversion

Standard Instrument Configuration

A computer with data acquisition software can be connected to a dielectric cure monitor, such as the Lambient Technologies LT-451, through an USB serial port or RS-232. The instrument connects to a thermocouple and a dielectric sensor, which are immersed in the batch reactor. Figure 4 shows this standard configuration.

A dielectric sensor typically comprises of two interdigitated electrodes on a ceramic or polyimide substrate. The process temperature is measured by a thermocouple, which is an important parameter because dielectric properties differ with both the material’s temperature and degree of conversion.

Figure 4. Dielectric cure monitoring system for a batch reactor

Simplified Sensor for Batch Reactors

In many cases, the resin in a batch reactor has low ion viscosity during the whole process. As a result, it is often not necessary to use a sensor with interdigitated electrodes. This type of sensor is highly sensitive because the electrodes are tightly packed and occupy a large area.

At the end of cure, thermosets have very high ion viscosities, so analyzing materials at this stage requires a sensor with the equivalent high sensitivity. However, when there is low ion viscosity, the large signal from interdigitated electrodes can surpass the range of the dielectric cure monitor, leading to faulty measurements.

For batch reactors, a simple electrode design with an output level more suitable to the instrument is recommended. Figure 5 shows how two wires with exposed ends becomes a low sensitivity dielectric sensor.

Figure 5. Simplified sensor for use with a batch reactor process

The best configuration must be determined by trial and error, but a logical design could start with 5 mm of exposed wire and 5 mm separation between the bare ends (Figure 6). A solid wire 24 AWG or thicker should be used, so that it easily forms and remains rigid after shaping.

The insulation must be Teflon to ensure chemical resistance and ruggedness, and the ends should be secured so their separation does not change.

Figure 6. Simplified dielectric sensor for low ion viscosity, example configuration

By making a measurement in the material under test at the process temperature, the sensor can be tested. If required, the sensor’s configuration can be adjusted using the following guidelines:

  • If the signal level is too low, increase the length of the exposed wire or decrease the separation
  • If the signal level is too high, decrease the length of exposed wire or increase the separation

Hazardous Environments Around Batch Reactors

Batch reaction processes use large volumes of resin and may generate volatile, flammable gases. In hazardous locations, manufacturers must ensure that electrical failure does not cause explosion or ignition. Dielectric sensors and thermocouples in batch reactors may require intrinsically safe barriers in-line with their instrument connections to remove this risk.

Intrinsically safe barriers (I.S. barriers) are protective circuits designed to limit voltage and current to electrical devices. The restrictions prevent dangerous energy release and depend on the requirements of the device and the categorization of the hazardous environment.

An intrinsically safe barrier contains a resistor, one or two zener diodes, and a fuse, and must be connected to a ground point. I.S. barriers may handle AC, positive, or negative signals, with configurations shown in Figure 7.

Figure 7. Intrinsically safe barrier configurations

Explosion-Proof Enclosures

I.S. barriers ensure that the available energy is insufficient to ignite the dangerous substance that may be present, removing the need for special explosion-proof enclosures around measurement circuitry. However, users must always consult with the company or local safety authority to verify that equipment meets safety standards.

Implementing Intrinsically Safe Barriers into a Batch Reaction Process

The initial step to implement dielectric cure monitoring in a batch reaction process is to identify the category of dangerous environment. Then, suitable I.S. barriers for use with dielectric sensors and thermocouples should be selected. These barriers should be installed in an enclosure to support connectors to the sensors and connectors for cabling to the instrumentation.

Important: For safe operation, the Intrinsically Safe Interface and the Dielectric Cure Monitor must both be connected to the same electrical ground.

Figure 8. Apparatus for intrinsically safe process monitoring in a batch reactor

Conclusion

Dielectric cure monitoring can provide on-line, real time data about the polymerization in a batch reaction process. However, batch reactors usually function in dangerous environments with flammable gases.

I.S. barriers can be used with dielectric sensors and thermocouples to restrict energy below the ignition point of these gases and stop explosion. A dielectric cure monitoring system with an Intrinsically Safe Interface can enable valuable process control of batch reactions in large scale resin manufacture.

References

1. Crowley, Timothy J. and Choi, Kyu Yong, In-line dielectric monitoring of monomer conversion in a batch polymerization reactor, Journal of Applied Polymer Science, Feb. 28, 1995, pp 1361-1365

This information has been sourced, reviewed and adapted from materials provided by Lambient Technologies.

For more information on this source, please visit Lambient Technologies.

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