Special challenges in NIR monitoring of polymer melts

Polymers (both for plastics and for fibers) are a major part of the chemical industry. The composition of polymer melts can be of interest in several kinds of systems, including polymerization reactors, polymer melt transfer lines, and extruders. However, there are additional challenges involved when using NIR analyzers in such systems due to a number of unique factors sustained high temperatures, sudden changes in temperature, pressure extremes (high or low), polymer flow issues, and fouling. 

Polymer flow issues are concerns on the part of plant operations personnel that can arise when one proposes to put an in-line NIR probe (or pair of probes) into a polymer reactor or transfer line. These concerns tend to be plant or process-specific. Plant personnel are likely to be concerned if the probe will change the pressure drop in the line, create a cold spot in the reactor or line, protrude into the flow stream, or create dead spots (e.g. recessed probes or the downstream side of protruding probes). There may also be plant- or process-specific resfricfions on where probes (or analyzers) can be located, on pipe sizes, on the use of welds, and on materials of construction. It is critical to involve plant operations personnel (including process operators) as early as possible in discussions about probe design and location. 

The applications described in this second part of the chapter are intended to illustrate the wide range of uses for NIR in the chemical industry. The selection of examples was intentionally limited to work done within industry and published in the open literature in order to keep the focus on work that has demonstrated business value. However, it has been the authors experience that for every industrial NIR application published in the open literature there are at least two others practiced as trade secrets for business reasons. This is especially true for on-hne applications that have progressed beyond the feasibility stage, since the NIR results can reveal a great deal of information about the chemical process itself - information which industry managers are often reluctant to share. Pubhshed industrial applications should therefore be considered merely the tip of the iceberg. 

High pressures (hundreds or thousands of psig) can break probe windows, or break or loosen the seal between the window and the probe body. This is especially true when high pressures are combined with high temperatures. High pressures can also, in a worst case, 

---