Optical sensing keeps PVC manufacture from going awry

March 25, 2010
Concord, MA--In finding out how optical instruments are used in the manufacture of polyvinyl chloride (PVC), one gets a peek into how finicky the process of plastic-making really is.

Concord, MA--In finding out how optical instruments are used in the manufacture of polyvinyl chloride (PVC), one gets a peek into how finicky the process of plastic-making really is: a series of finely tuned, immense, and continuously operating chemical reactions that need to be closely watched to keep them free of ruinous contamination.

Optical-analysis instrument maker Applied Analytics (AAI) notes that the production of the PVC that carries our water, insulates our power cables, lines our kitchen floors, and entertains our children is a process which has much to gain from the use of the company's analyzers. (Of course, PVC is only one of the many types of plastics in use worldwide.)

IR analysis, spectrophotometry
The first intermediate in the manufacturing of PVC is ethylene dichloride (EDC), which is created from the raw materials chlorine (Cl2) and ethylene (C2H4). More than 90% of all EDC produced is used for PVC manufacture. Actually, the EDC is used to synthesize the second intermediate in the process: vinyl chloride monomer (VCM), the chemical precursor to PVC. The EDC is heated to around 500°C in a cracking furnace, eventually splitting into the desired VCM and recyclable dry hydrochloric acid (HCl). The products are quickly cooled to avoid recombination, after which unreacted EDC is returned to the furnace. The HCl byproduct is reserved for oxychlorination, a second method for creating EDC -- meaning that all organics that don't end up as VCM the first time around can be recycled in the process.

Applied Analytics instruments are used in three applications within this chemical process.

First, AAI's Microspec analyzer looks for the presence of water. The threat of corrosion is always present, and monitoring the water level to a specificity of zero to 20 parts per million (ppm) is the first line of defense. The Microspec, which has an IR sensing range of 2 to 10 microns, is designed to monitor low and high water contents in various solvents; it consists of an IR light source, a dual pyroelectric detector with a filter that isolates a specific IR wavelength, and a transmission flow cell.

Second, the company's OMA-300 analyzer measures the concentrations of Cl2 and ferrous tetrachloride (FeCl3), in the ranges of zero to 50 ppm for Cl2 and zero to 100 ppm for FeCl3. The OMA-300 is a diode-array fiber-optic process spectrophotometer, continuously measuring a high resolution (wavelength range of 190 to 800 nm at a 1 nm resolution) spectrum of the sample. The absorbance signals are correlated to concentrations via a multiwavelength method. FeCl3 is an efficient catalyst in the direct chlorination of C2H4 to form EDC.

Third, while FeCl3 is needed to produce EDC, its presence in the product is highly undesirable. EDC that has not been washed of all its FeCl3 threatens to clog the cracking furnace that heats and breaks EDC down into VCM and dry HCl. Monitoring the level of ferrous tetrachloride in EDC is thus of critical importance in protecting against fouled mechanisms.

Both FeCl3 and free Cl2 can potentially pollute the EDC. PVC made from impure EDC has different characteristics that make it less useful than high-quality PVC. Maintaining EDC purity is imperative if the product is not to be compromised, and such quality assurance necessitates monitoring of both components. While conventional analyzers struggle with measuring interfering components like Cl2 and FeCl3, AAI says its OMA-300 is specifically designed for selectivity and accuracy in such use.

So, starting with two gases (chlorine and ethylene), and monitoring the ensuing chemical processes finely enough, one ends up with water pipes and kids' toys. Remarkable.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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