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Part One: The Importance of Measuring Water in Ethylene and Propylene Production

This is Part One of a three-part blog series.

Ethylene is a primary petrochemical building block. It is used for the manufacture of many derivative products such as polyethylene, vinyl chloride, ethylene oxide, ethylene dichloride, acetaldehyde, and alpha olefins. These products can be converted into many industrial materials including surfactants, detergents, solvents, and synthetic polymers such as polyethylene, polyvinyl chloride, polystyrene, and synthetic rubber.

Propylene is mostly used to make polypropylene. It is the second most important product in the petrochemical industry, after ethylene, and is the raw material for a wide variety of products. Two thirds of propylene production is for polypropylene. Propylene is also used for production of propylene oxide, acrylonitrile, cumene, butyraldehyde, and acrylic acid.

It is important that the petrochemical industry conduct ethylene and propylene purity analysis. The precision, reliability, and speed of these measurements are important to identifying process upsets and to assure optimum throughput. The faster a reliable purity measurement can be achieved, the larger quantity of product to market. For this reason, any significant advance in end product purity analysis that increases measurement speed and lowers installation and operational costs without sacrificing accuracy can have a major impact on the profitability of ethylene and propylene production plants.

Most ethylene and propylene production processes use catalysts to improve production quality and process yield. Impurities in ethylene and in propylene can affect catalyst performance and as a result, production quality. Ethylene and propylene have strong purity requirements that must be verified in production and at custody transfer points. One critical impurity is moisture. Moisture content in ethylene and propylene reduces the activity of the catalysts used at the polymerization stage, thereby decreasing the overall yield of polyethylene and polypropylene reactors. For this reason, it is desirable to measure moisture in ethylene and propylene streams, which are used in the production of polyethylene and polypropylene [1].

Over the past several years, near-infrared TDLAS has gained attention for use in industrial applications due to the technique’s specificity for the analyte, high sensitivity, and fast response speed. This technology’s specificity is the result of the extremely high spectral resolution achievable. Emission bandwidths for tunable diode lasers are on the order of 10-4 – 10-5 cm-1, which results in the ability to isolate a single rovibrational transition line of an analyte species. A second advantage of TDLAS is the ability to rapidly tune the lasers, so techniques like wavelength modulation spectroscopy (WMS), which yield dramatic sensitivity enhancements over a direct absorption approach, are easily implemented. Because TDLAS is an optical technique, it also offers a very fast response speed. These three features make TDLAS technology very suitable for a variety of process measurements. 

Using a water-vapor absorption line to measure water vapor in ethylene and propylene products with a TDLAS requires a means of compensating for the absorption spectrum of ethylene, which has several small peaks overlapped with the water line. Propylene as more heavy gas has no resolved rotational structure in the vicinity of wavelength used for measurements and from this point of view can be considered as zero gas approximation. By implementing a multivariate calibration in the TDLAS instrument, it is possible to accurately measure the water vapor concentration in ethylene and in propylene.

To learn more about AMETEKs TDLAS solutions for this application, click here.

Read Part Two of this series - Instrument and Testing Methodology for Measuring Water in Ethylene and Propylene Production


Reference

  1. Pieter R Wiederhold “Water Vapor Measurements: Methods and Instruments,” Marcel Dekker, Inc., 1997.

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