Marcellus Natural Gas Liquids

In the Western half of the Marcellus Shale Play, more than just methane flows from gas wells. In this region, hydrocarbons known as “natural gas liquids” (NGLs), are also present with the methane extracted from these wells. If harvested, these NGLs can be very valuable co-products of natural gas processing. The price of NGLs typically follow oil prices which have decoupled from lower natural gas prices. NGLs include ethane, propane, butane, iso-butane, and natural gasoline. These NGLs from “wet” gas have a variety of different uses from providing raw materials for oil refineries or petrochemical plants to other sources of energy (propane for home heating).

Some of the necessary processing can be accomplished near the wellhead. However, the complete processing of “wet” natural gas takes place away from a wellhead at a fractionation plant. The extracted natural gas is transported to these processing plants through a network of gathering pipelines, which are smaller diameter, and at lower pressures. A gathering system can consist of thousands of miles of pipes, interconnecting the processing plant to hundreds of wells in the area.

There are two basic steps to the treatment of natural gas liquids (NGLs). First, the liquids must be extracted from the natural gas. Second, the NGLs must be separated into their base components.

The Cryogenic Expansion Process

Cryogenic processes (dropping the temperature of the gas stream to around -120 deg F) are often used to extract ethane and other lighter hydrocarbons from natural gas when high recovery rates are desired. There are a number of different ways of chilling a gas to these temperatures, but one of the most effective is known as the turbo expander process. In this process, external refrigerants are used to cool the natural gas stream. Then, an expansion turbine is used to rapidly expand the chilled gases, causing the temperature to drop significantly. This rapid temperature drop causes ethane and other hydrocarbons in the gas stream to change into a liquid phase, while methane remains in the gas phase. This process recovers about 90 to 95 percent of the ethane originally in the gas stream. In addition, the expansion turbine is able to convert some of the energy released when the gas stream is expanded (Joule–Thomson Effect) into recompressing the gaseous methane effluent, thus increasing the process efficiency of extracting ethane.

Natural Gas Liquid Fractionation

Once NGLs have been removed from the natural gas stream, they are separated by constituent. The process used to accomplish this task is called fractionation. Fractionation is based on the different boiling points of the hydrocarbons in the NGL stream. Essentially, fractionation occurs in steps corresponding to the boiling off of hydrocarbons one by one. The name of a fractionator generally refers to its purpose, as it is usually named for the hydrocarbon that is boiled off. The entire fractionation process starts with the removal of the lighter NGLs from the stream. The fractionators are used in the following order:

• Deethanizer — separates the ethane from the NGL stream.
• Depropanizer — separates the propane.
• Debutanizer — boils off the butanes, leaving the pentanes and heavier hydrocarbons in the NGL stream.
• Butane Splitter or Deisobutanizer — this step separates the iso and normal butanes.

To date, fractionation plants in the Marcellus have produced propane for home heating use that is transported by truck. Soon modifications will be made to existing plants to enable full fractionation capability for production of ethane and butane. In addition, there are several ethane pipeline projects under construction in the Marcellus to pipe ethane to Gulf coast processing plants for conversion to ethylene then polyethylene.

Will an ethane to ethylene conversion plant be built in the Marcellus?

Transporting ethane to the Gulf Coast and then shipping polyethylene resin back to the Northeast should open the door economically for a local ethane cracker to produce ethylene. This ethylene could be converted to polyethylene resin locally and shipped to the converters throughout the Northeast US.

Another economic advantage is the low price of US natural gas based feedstocks compared to the price of crude oil. Oil (naphtha) based production of polyethylene is widespread in Asia, Europe, and Latin America. As a result, global demand for US based resin is strong as roughly 25% of the domestically produced polyethylene is exported.

Conventional ethane cracking utilizes steam and is energy intensive. However, new technology exists to more efficiently crack ethane to ethylene by reducing the amount of energy required and improving the yield of the process. The lower capital cost of new conversion technology combined with relatively low feedstock prices should attract investment in a new plant in the Marcellus. A team of investors consisting of drillers to end users would further reduce the risk of investment. A better question may be when will a new ethane cracker be built?

For more information on Marcellus Natural Gas Liquids processing, please contact Travis Buggey at 412-231-5890, ext. 325. Or visit us on the web at www.ventureengr.com .