Moisture measurement for quality monitoring and production optimization
Ethylene and propylene are perhaps the two most important chemical compounds used in industry. They are key building blocks for the manufacture of a wide range of chemicals and plastics. Ethylene, for example, is a feedstock for the production of polyethylene, which is the most widely used plastic in the world. It is also used for ethylene glycol, vinyl chloride (a precursor to PVC) and solvents, adhesives and cleaning agents. Similarly, propylene forms the basis of polypropylene, which is another widely used plastic. It is also a key component in polyurethane foams and is used for the production of refrigerants and solvents; when converted to propylene glycol, it acts as a moisturizer, solvent and stabilizer in cosmetics, pharmaceuticals and food processing.
Thermal cracking, quenching and drying
Ethylene and propylene are produced using similar processes, with a feedstock – normally ethane, propane or naphtha – being heated, mixed with steam and then processed through a furnace or tubular reactor at temperatures up to 950 °C. The steam acts as a diluent and helps to prevent the formation of unwanted by-products, while the high temperatures break apart, or crack, the hydrogen molecules in a process known as pyrolysis, forming smaller molecules that include ethylene or propylene, plus co-products.
The cracked gases exit the furnace at elevated temperatures and need to be quenched using water or oil to preserve the composition of the gases and prevent secondary reactions from taking place. Quenching is followed by gas compression, scrubbing to remove acid gas components and then drying over molecular sieves to reduce the moisture content of the saturated gases. These are then passed through a series of distillation columns to separate the different products, before being prepared for subsequent pipeline distribution.
Moisture monitoring is the key to gas quality
The moisture content of ethylene and propylene being used as a feedstock for polymerization is critical. For example, the catalysts used in the polymerization of polyethylene – LDPE, HDPE and LLDPE – are extremely sensitive to the presence of moisture, which reduces the efficiency of the catalyst and thus the yield from polyethylene and polypropylene reactors.
Accurate and consistent measurement of moisture content is therefore one of the most important determinants of gas quality; this is especially true for custody transfer, as out-of-specification ethylene or propylene will be rejected by downstream processors or possibly sent to flare – in each case, there will be significant cost implications and reputational damage.
It should also be noted that the presence of moisture will not only affect gas quality and polymerization yield; it can also react with the process gas to form acids that cause corrosion in pipelines and downstream equipment; in turn, this can increase maintenance costs and lead to potential safety issues.
Dependable moisture measurement
The solution is to monitor the moisture content at key stages in the production, distribution and end-use polymerization processes. This helps with process optimization and allows producers to prove gas product quality.
Traditionally, single-channel moisture sensors and controllers have been used for this process. Although this arrangement is effective, the necessity to use multiple devices across the process facility means it is expensive. It also leads to a large installed base, high maintenance and calibration costs and, if instruments are sourced from multiple suppliers, the challenge for operational staff of learning to use and service often differing equipment.
By comparison, we’re able to offer a range of moisture and gas measurement and sensing technologies, complemented by remote multichannel control units. This gives production or plant managers the option of standardizing on equipment from one supplier and thereby reducing the capital and operational costs of moisture measurement.
Our choice of sensing technologies is suitable for moisture measurement in both the liquid and gas phases. The product line-up includes the Liquidew I.S. intrinsically safe analyzer, which incorporates proven ceramic metal-oxide sensors providing measurement capabilities from 0.01ppmw to saturation point. This system is ideal for use with naphtha feedstock through to isomerization, and in ethylene and polyethylene feed for process applications.
Similarly, our Promet EExd moisture analyzer is designed for use with ethylene feedstocks. It is flameproof certified by IECEx, ATEX and UKCA, accurate to ± 1 °Cdp, with a resolution of 0.1 °C.
We also offer advanced multi-channel process monitoring and control units – the MCU and MCPM – that are designed to be combined for use with multiple moisture and gas input sensors and sample conditioning units. They are easy to configure and use, providing real-time measurement and data logging of critical process parameters.
Ethylene (C2H4) is one of the simplest olefins, or compounds formed from carbon and hydrogen atoms. It is believed to have been discovered in 1669 by the German chemist Johann Becher, who experimented by heating ethanol with sulfuric acid to release ethylene as a gas. By the late eighteenth century, scientists had discovered how to synthesize the gas, which was then known as the ‘olefiant gas’ or oil-forming gas, as it could be combined with chlorine to produce a substance that was subsequently used as a solvent for fats and waxes. The name ethylene came into common use in the mid-nineteenth century when the substance was thought to be part of the functional group ethyl and had the suffix ‘ene’, meaning ‘daughter of’, added.
Not only is ethylene crucial for plastics and chemical production, it also plays a key role in plant biology, triggering leaves to change color, wilt and drop, and prompting flowers to open and fruit to ripen.
Related Information
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Moisture in Liquid Analyzer - Michell Liquidew I.S.
Process Moisture Analyzer - Michell Promet EExd
Process Moisture Analyzer
Michell Multi-Channel Process Monitor
Process Moisture Analyzer - Michell QMA601
Modular Process GC – LDetek MultiDetek3
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