Analysis of Impurities in Polymer-grade Ethylene, Propylene and 1,3-Butadiene
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Analysis of Impurities in Polymer-grade Ethylene, Propylene and 1,3-Butadiene by Surinder (Sandy) Thind Current Detection Techniques – A Case Study with Arsine: Abstract: Producers of high purity monomers need to measure, not only, more and more impurities in their high purity product, but also, they need to measure these impurities at lower and lower detection limits. This paper describes the latest technique available to perform these analyses at low ppb levels with high levels of accuracy and speed. Both laboratory and on-line detection is possible. The Critical Nature of Impurity Analysis: The production of poly-ethylene and poly-propylene has become a very competitive business. Producers utilize more selective and more sensitive polymerization catalysts all the time. These catalysts are very expensive, and frequent replacements may lead to a loss. During catalyst replacement, the plant is shut down, which further adds to losses. In order to avoid frequent shutdowns due to catalyst poisoning, these companies insist that monomer producers meet tight specifications regarding such poisonous impurities. Failure to comply can result in lawsuits and loss of business. For this reason, impurity analysis has achieved critical importance. Take as a case study the detection of one impurity: arsine in olefins. Even at low ppb levels, arsine can adversely affect certain polymerization catalyst properties and also lead to polymer contamination. Detection of arsine in ethylene, propylene, or 1,3butadiene is very difficult at the desired 5 ppb level. Normally, a complicated GC column system is used to separate arsine from propylene. After this separation of low ppb levels of the impurity from almost 99.9% propylene, expensive systems such as GC-AED, are used to these ppb levels of arsine. Since GC-AED will also respond to ppb or ppm levels of other impurities, such as hydrogen cyanide, nitric oxide, ammonia, or hydrogen chloride. This being the case, with many other impurities present at low ppb levels, it is easy to misidentify the arsine peak. Thus, the individual working with GC-AED must be highly skilled. Even the most experienced chemists experience difficulty in positively identifying and accurately quantifying low (1-10 ppb) levels of this impurity. As a result, most companies have stopped using this technique for arsine detection and have gone back to the ‘old’ wet technique method, which requires hours of bubbling followed by detection via atomic absorption. The manual laboratory technique and the GC-AED technique for ppb-level arsine detection are only available for laboratory testing applications to date. Further, both of these techniques are not only impractical for on-line applications, but they are also very expensive to install and maintain. It is clear that, in today’s monomer-production facilities, nonspecialist technicians must be able to quickly, efficiently, and accurately perform arsine analysis without GC separations both either in the lab or on-line. It is at this point that the time-proven analytical technique of Dry Colorimetry draws new attention. The application of the dry colorimetric technique to the measurement of trace arsine in the hydrocarbon streams has been independently demonstrated by several petroleum/petrochemical companies. Figure 1 indicates potential monitoring points in an olefin/polyolefin plant for trace arsine measurements. With prompt, reliable on-line results, the process control engineer has the opportunity to develop strategies to minimize the effect of a periodic excursion in arsine levels. In one field test, the concentration of arsine was monitored before and after an arsine scrubber.Wet chemical analysis was used to obtain the concentration of arsine, 25 ppb and 0 ppb, respectively. The results obtained using Dry Colorimetry technology corresponded to the wet chemical results within experimental error. A New Look at Dry Colorimetry: INTRODUCTION Classical colorimetry utilizes an impinger to collect gas in a liquid medium. Chemical reagents are then added to the medium to cause it to change color in proportion to the concentration of gas present. The resulting color change is measured by a laboratory spectrophometer and compared to known standards. Ultra-sensitive "tape" detectors, are also colorimetric based, but these are dry reaction substrates that serve as gas collecting and analyzing media. Individually formulated for a specific gas or family of gases, each detection tape is a nontoxic, proprietary chemical reagent system.When exposed to a target gas, the tape will change color in proportion to the amount of gas: the higher the target gas concentration, the darker the stain that will appear. The change in color, or stain, on the tape is read by a photo-optical system in the analysis instrument, and the intensity of this stain is then compared to a standard response curve preprogrammed into the instrument’s data system. Non-Hydrocarbon Impurities That Require Monitoring: Just one of the impurities listed in Table 1 has the power to adversely affect many polymerization catalysts, yet more than one of these may be present during the process. For this reason, monomer producers must be able to accurately detect and measure one or more of the specified impurities at very low levels. Typical Specification Hydrogen Sulfide Carbonyl Sulfide Nitrogen Dioxide Sulfur Dioxide Table 1: Typical Impurities That Require Monitoring in Monomer Production Figure 1: Some Potential Monitoring Points for Arsine in an Olefin /Polyolefin Plant

MEASUREMENT AND TESTING Accuracy of Dry Colorimetry The dry colorimetric detection technique, as outlined above, gives accurate and extremely precise results. Factory calibration of instruments and the detection tape is referenced against NIOSH approved and analytical methods. Both laboratory and field tests have verified that analyzers using Dry Colorimetry give data in agreement with standard reference methods, as typified by the examples in Table 2. Key Benefits of Dry Colorimetry for Impurity Analysis in Monomer Production: It has been demonstrated that low-level impurity analysis is of...