Lubrication's Critical Role in Low Density Polyethylene Production

 1st Jul 2015

Polyethylene (PE) is one of the most common polymers used to manufacture goods today. It is found in products ranging from plastic bags to artificial human hips to cable insulation.

While PE demand is already high, it is expected to grow even more in the coming years. Research group Freedonia forecasts that global PE demand will grow to nearly 100 million metric tons by 2018, a sizeable increase from the 81.8 million tons produced in 2013.*

To keep pace with growing market demand, producers of PE are focused on optimizing the productivity of their plants and equipment.  This is particularly true for producers of low-density polyethylene (LDPE), a specific class of PE used to make familiar commercial and consumer products such as medical equipment, plastic bags, beverage cartons, and playground slides.

LDPE production involves the free radical polymerization of ethylene at temperatures exceeding 200°C (392°F) and at pressures of up to 3,500 bar. LPDE production utilizes two compression stations: a low pressure and a high pressure station. The low pressure station is comprised of two reciprocating compressors ­ the booster and the primary compressor ­ while the high pressure station has only one main component, the hyper compressor. This is usually a two-stage, multi-cylinder reciprocating compressor.

In these harsh conditions, lubricant performance directly influences equipment reliability and the quality of finished products.  That’s why it’s critical that operators learn how to select the right lubricant(s).

For example, LPDE is used in food packaging and medical products, so lubricants used in production must adhere to the most stringent incidental food contact and pharmacopeia requirements. This is particularly important since, on average, it’s estimated that about 0.1 percent of the lubricant becomes part of the finished product. A common reason for this is due to oil loss related to the lubrication of hyper compressor cylinders.

In addition to meeting regulatory requirements, the lubricants used must be formulated to withstand the specific viscosity and pressure needs of each piece of equipment. For example, in a low pressure station, the booster and primary compressors’ function is to raise the ethylene pressure from 1.5 to 250 bar. The units usually have two separate lubrication systems.

  • Crankcase lubrication: Typically, industrial circulating oils are used in crankcases. The most common viscosity grades are ISO VG 150 and 220.
  • Cylinder lubrication: To meet incidental-food contact legislation standards, cylinders require a lubricant with low reactivity and high purity.

The hyper compressor’s function is to raise the pressure of ethylene from 250 to 3500 bar. Under these extreme conditions, the lubrication of the cylinder/plunger system is critical. Usually, hyper compressors have three separate lubrication systems:

  • Crankcase lubrication: Often, the same lubricant used in the primary and booster compressors can be used in crankcases, which is usually an industrial circulating oil.
  • Cooling system: For cooling systems, typically a pure white oil with low viscosity, such as an ISO VG 68 lubricant, is appropriate. Alternatively, the same cylinder lubricant can be used, but it must meet the requirements of incidental food contact legislation.
  • Cylinder lubrication: As with cooling systems, cylinders must be lubricated with an oil that complies with incidental-food contact legislation. The lubricant also must be formulated specifically for high pressure environments, with low reactivity and high purity to prevent reactor fouling and to protect the quality of the final product.

Numerous factors must be considered when selecting PE compressor lubricants, but there are a range of lubricants that are tailored to meet the specific challenges facing PE producers. For example, ExxonMobil offers an entire series of lubricants formulated specifically for PE compressor applications.

The Mobil Rarus PE Series is a comprehensive lubricant product line formulated to comply with incidental-food contact legislation and designed for the lubrication of critical LDPE plant equipment. These lubricants combine high purity white oils and friction reducing additives, resulting in a formulation that can help increase the reliability and productivity of polyethylene plants.

Lubricants play an important role in helping optimize the productivity of PE operations. Moreover, their role in ensuring the quality of the end product cannot be overstated, particularly considering that a tiny portion of the lubricant actually becomes a part of the end product.

Consequently, PE producers should work with their lubricant suppliers to select a mix of products specifically formulated to meet the needs of the PE industry. If done properly, this can help producers meet their productivity and profitability goals and help provide them with a competitive boost.

"Lubrication's Critical Role in Low Density Polyethylene Production" by, Alessandro di Maio used with permission of the Mobil SHCTM Club (all rights reserved.)

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