What is a Cracker and Why Should I Care?

What is a Cracker and Why Should I Care?

What are petrochemicals and how are they used?

Ethylene is one of the most important chemicals in American manufacturing.  Along with propylene, butenes, butadiene, benzene, toluene and xylenes, ethylene and other petrochemicals are used as building block materials that affect most manufacturing supply chains.  Petrochemicals are unique substances that allow industrial scientists to create innovative products that are used every day throughout the world, from aspirin and medical equipment, to football helmets and bulletproof vests.  Because ethylene is distinctive in its usefulness throughout chemistry, it is manufactured in greater amounts than any other chemical.

The green double bond on the ethylene molecule gives it unique properties for chemistry.   Graphic courtesy of Conrod Associates for AFPM

The green double bond on the ethylene molecule gives it unique properties for chemistry.
Graphic courtesy of Conrod Associates for AFPM

What makes ethylene unique is a part of the molecule called a double bond.  That area is very reactive and particularly well-suited for many different chemical reactions, which makes ethylene one of the most important chemicals in all chemistry.The method for producing ethylene depends on high tech engineering and uses a tremendous amount of energy.  The common term for this engineering feat is called cracking, because the energy is used to break apart (or crack) molecules and form new molecules, and the petrochemical plant is called a cracker.

The most common feedstocks, or raw materials, for petrochemical manufacturing are naphtha and light gas oil, which are derived from the oil refining process, and individual gases such as ethane, propane and butane, which come from a complex mixture of hydrocarbons known as natural gas liquids, or NGLs.  NGLs exist under the ground in a liquid state along with methane (natural gas), but several substances in the liquid hydrocarbon mixture are also gases above the ground at normal temperature and pressure.

Each petrochemical facility is custom designed and engineered from the ground up. Photo courtesy of NOVA Chemicals

Each petrochemical facility is custom designed and engineered from the ground up.
Photo courtesy of NOVA Chemicals

The engineering design for the cracking process differs according to the type of feedstock used.  Because oil is a global commodity and is easy to ship long distances, many crackers around the world are configured to handle naphtha as a feedstock.  Since ethane is more difficult – and expensive – to ship long distances, it can be considered more of a regional commodity.  The regions that have an abundance of NGLs tend to also have the capacity to crack ethane.  Those regions that do not have access to ethane usually do not have ethane crackers.

Crackers in the US can often handle either naphtha or ethane.  During the 1990s and early 2000s the instability and uncertainty over the price of oil and natural gas eventually led to technological advancements in domestic petrochemical processes.  High tech American petrochemical manufacturers were able to achieve greater flexibility by reengineering their facilities to process a wider variety of feedstocks, whether derived from oil or NGLs.

Feedstocks make up anywhere from 60 to 70 percent of the cost to manufacture petrochemicals; therefore, the feedstock flexibility of domestic petrochemical processes has enabled US producers to remain competitive.  As both oil and gas became more expensive in the US during the early 2000s, however, coupled with increased energy costs, the competitive advantage that came with feedstock flexibility all but disappeared.  Petrochemical companies began to seek joint ventures in other parts of the world where energy and feedstock costs were cheaper.  All that changed around 2008 when shale development led to a dramatic decrease in the cost of industrial energy and feedstocks here in the US.

This modern gas well is directly connected to a gas processing plant, which is seen in the background.

This modern gas well is directly connected to a gas processing plant, which is seen in the background.

Shale development is providing manufacturers with an unprecedented opportunity in low cost energy; however, that is only part of the picture.  Along with the natural gas and oil coming from shale plays like the Marcellus, Bakken, Utica and Eagle Ford, these areas contain an abundance of NGLs.  Ethane is a major component of NGLs, especially in the Marcellus, Utica and Eagle Ford plays.  The economic law of supply and demand states that an increase in supply, other things being equal, results in a decrease in price.  This newfound bounty of NGLs has significantly reduced the price of ethane in the US.  The low cost of energy and ethane has prompted American petrochemical manufacturers to expand ethane cracking capacity.  Over the past three years, petrochemical companies have announced plans to invest over $80 billion in new manufacturing infrastructure.

How is ethylene made?

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The manufacturing supply chain begins with the extraction of natural resources.  In the case of ethane, it begins with shale development, through a highly technical process that combines multi-dimensional seismic imaging, horizontal drilling and hydraulic fracturing.  What comes out of the ground is a mixture of different liquid and gaseous hydrocarbons, so the next steps in the process are treating and processing the mixture to separate methane, which is also called natural gas or dry gas, from the liquids.  Methane is used as fuel and as a feedstock for methanol and fertilizer production.  The NGLs go to another type of separation facility called a fractionator, which separates the ethane, propane and butane.

5Once the ethane is separated it is shipped by pipeline to a cracker facility, which is a very sophisticated series of processes that convert the ethane to ethylene.  The first process is using steam to transport a mixture of ethane and a small amount of propane to a series of industrial furnaces and heating it to approximately 1500 degrees Fahrenheit, which requires a lot of energy.  At that temperature single bond of the ethane molecule is loosened to the point that it loses two of the hydrogen atoms.  The two hydrogen atoms combine and form a stable hydrogen molecule known as H2.  In addition to ethylene, a number of other molecules are formed, but ethylene is in much greater abundance (about 80 percent) than the other substances.

The next step is sending the mixture of very hot gases – liquids also become gases at that temperature – to a series of heat exchangers that use steam to cool the gaseous mixture.  Once cooled, the mixture of steam and gases go to a tower where cold water is poured onto it from above to force all the different liquids to the bottom.  These liquids are usually hydrocarbons with more than five carbon atoms.  This process is also called quenching and the tower is referred to as a quenching tower.

Two separate product streams come out of the quenching tower.  One product stream is water and a mixture of heavier hydrocarbons commonly referred to as pyrolysis gasoline.  The water is cleaned and recycled back into the quench tower.  The pyrolysis gasoline is sent to a separation unit that extracts the petrochemical aromatics (benzene, toluene and xylenes) for use in making plastics and other chemicals.  The remaining liquid hydrocarbon mixture is used to blend in automotive fuels or sent to a refinery for further processing.

The other product stream is a mixture of hydrogen (formed in the furnace when ethane is destroyed) and light hydrocarbons, including:

  • Industrial distillation towers are giant high tech versions of the distillation columns found in chemistry labs.

    Industrial distillation towers are giant high tech versions of the distillation columns found in chemistry labs.

    Methane

  • Ethane
  • Ethylene
  • Propane
  • Propylene
  • Butane
  • Butenes
  • Butadiene

After leaving the quench tower, the gaseous mixture (including the hydrogen) goes to a compressor that squeezes the molecules together.  From there the compressed mixture of gases goes to a condenser where it is further cooled, then to a high tech refrigeration unit called a cold box, which reduces the temperature to the point at which the mixture, other than methane and hydrogen, turns into a liquid.  Since ethane remains a liquid down to minus 128.2 degrees Fahrenheit (minus 89 C), the cold box has to be very, very cold.  Once the gases are liquefied, the mixture goes through a series of tall towers called distillation units that are precisely controlled for temperature.  Since liquefied gases boil (turn back into gases) at different temperatures, the distillation towers can control the isolation and removal of specific gases during this process.

What happens to the ethylene?

As stated earlier, ethylene is very reactive.  It is also very flammable.  For safety reasons, ethylene is usually transported by a special dedicated pipeline to other industrial chemistry facilities to make a variety of products called ethylene derivatives.  One of the most common products made directly from ethylene is a plastic called polyethylene.  The ethylene goes to a facility, usually in close proximity to the ethane cracker, and it gets converted through a chemistry process referred to as polymerization.  The output of that facility takes the form of small plastic pellets called polyethylene resin.  The polymer units are also custom-designed and engineered to control the specific physical properties of the resulting plastic resin.  That is why polyethylene is used in such diverse products ranging from sophisticated, high tech military helmets.

Polyethylene resin takes the form of plastic pellets and can be colored prior to processing into a specific product.

Polyethylene resin takes the form of plastic pellets and can be colored prior to processing into a specific product.

Ethylene is also used to make a variety of other chemicals.  For example, if chlorine is added to the ethylene molecule it becomes vinyl chloride, which is the petrochemical derivative (building block) used to make polyvinylchloride or PVC.  PVC is used as a corrosion-resistant material for modern plumbing, siding for houses, decking and many other applications in construction material supply chains.  Flexible PVC is also used in medical tubing to replace glass and rubber because PVC is more resistant to other chemicals than rubber and, unlike glass, it does not break.

Ethylene also reacts with other hydrocarbons in a chemical process called alkylation.  If benzene (an aromatic petrochemical mentioned earlier) is mixed with ethylene, it reacts to form ethylbenzene.  Ethylbenzene is a derivative petrochemical building block found in the manufacturing supply chains for Styrofoam and automobile tires.

If oxygen is added to ethylene, it reacts to form the chemical ethylene oxide.  Ethylene oxide is used to make surfactants and detergents in the manufacturing supply chain for cleaning products.  To take it even further, if water is added to ethylene oxide, it reacts in a process called hydrolysis and forms the product ethylene glycol.  Ethylene glycol is used as an ingredient in antifreeze.  It is also used to make a fiber called polyethylene terephthalate, otherwise known as polyester and can be found under the brand name Dacron®.  Polyester resin is also used to make beverage bottles and a whole plethora of other useful products.

The pervasiveness and importance of ethylene in manufacturing supply chains is undeniable.   The significant increase expected in US ethylene production capacity will provide other manufactures with dependable raw materials at low cost all along the various supply chains.  Manufacturers around the world are keeping a close eye on the petrochemical industry.  Considering the low energy and raw material costs due to increased shale production, companies are beginning to realize that in the near future the United States may be the most competitive place in the world for the production of finished goods and consumer products.