INDUSTRIAL CHEMISTRY AND ENGINEERING

Benzene’s Unusual Supply–Demand Dilemma

by Jeffrey S. Plotkin

November 30, 2015

Benzene is one of the most important building-block molecules in the petrochemical industry. The 2015 global demand for benzene for chemical use is estimated at 46 million tonnes.

As shown in Figure 1, benzene is the starting point for a wide variety of products. In 2015, about half of all benzene was converted to ethylbenzene, the precursor to styrene monomer. Styrene is used to produce a family of polymers called styrenics that includes polystyrene, expandable polystyrene, acrylonitrile-butadiene-styrene (ABS) high-impact plastic, styrene-butadiene rubber (SBR), and unsaturated polyester resin (UPR). 

Another key benzene derivative is cumene, which is the starting material for making phenol and acetone. Phenol and acetone are then converted to bisphenol A, the backbone monomer for making polycarbonate and epoxy resins.

Globally, about 11% of benzene is hydrogenated to cyclohexane, the starting point for producing nylon-6,6 and nylon-6. Another key benzene derivative is nitrobenzene, which, in a series of processing steps, is ultimately converted to methylene diphenyl diisocyanate, the largest-volume diisocyanate used for polyurethanes production. Lesser derivatives of benzene include alkylbenzenes for producing detergents, maleic anhydride, and chlorobenzenes.

Another key benzene derivative is cumene, which is the starting material for making phenol and acetone. Phenol and acetone are then converted to bisphenol A, the backbone monomer for making polycarbonate and epoxy resins.

Globally, about 11% of benzene is hydrogenated to cyclohexane, the starting point for producing nylon-6,6 and nylon-6. Another key benzene derivative is nitrobenzene, which, in a series of processing steps, is ultimately converted to methylene diphenyl diisocyanate, the largest-volume diisocyanate used for polyurethanes production. Lesser derivatives of benzene include alkylbenzenes for producing detergents, maleic anhydride, and chlorobenzenes.

Supply–demand balancing act

Despite benzene’s role as the starting material for many diverse products, for the most part it is not made intentionally. This situation makes balancing supply and demand for benzene very challenging. The three primary sources for benzene are

  • oil refineries, from the catalytic reforming of naphtha, primarily for gasoline production;
  • chemical plants, from naphtha steam cracking, primarily for ethylene production; and
  • coal processing plants, from coke oven light oil, primarily for steel production.

Benzene production for use in the chemical industry is not the primary objective of any of the three processes. Thus, benzene supply is not influenced by benzene demand but instead by the demands for gasoline, ethylene, and steel. Typically, the chemical value of benzene to these businesses is not great enough to influence their operating rates. So the chemical industry must cope with supply fluctuations that are out of its control.

A Brief History of Benzene

In 1825, Michael Faraday isolated and identified benzene from the oily residue from the production of illuminating gas. He called the molecule “bicarburate of hydrogen”; and he determined that its empirical formula is C6H6. In 1833, Eilhard Mitscherlich distilled benzoic acid and lime to produce what he termed “benzin”. By the mid-1840s, Charles Mansfield had isolated benzene from coal tar; he patented his production process and began industrial scale production.

Throughout the19th century, chemists studied benzene to identify benzene’s nature and structure. Friedrich Kekulé suggested the six-sided ring of alternating single and double bonds; the story goes that he had a daydream about a snake biting its tail to form a ring. Kekulé later supported the ring-structure hypothesis by showing that disubstituted benzenes exist in exactly three isomers. It was not until 1929 that crystallographer Kathleen Lonsdale confirmed the cyclic nature of benzene.

Commercially, benzene was originally produced as a solvent. However, in the late 19th and early 20th centuries, it was sold as an aftershave cologne for men because of its sweet aroma. Perhaps Ludwig Roselius found the most obscure use for benzene—to decaffeinate coffee, specifically Sanka. This practice was later abolished.

As the 20th century progressed, the horrific health effects of benzene were recognized. In 1948, the American Petroleum Institute reported, "it is generally considered that the only absolutely safe concentration for benzene is zero." Benzene is now known to cause several cancers and other illnesses. 

To further complicate the supply–demand balancing act, regional benzene capacities do not necessarily match regional demands. The mix of benzene-yielding process technologies is very different across regions, as shown in Figure 2. (TDP, TA, and HAD are defined below.) Asia is by far the world’s largest benzene-producing region; it is the key supplier to North America, which is woefully short of benzene. 

In 2014, North America imported more than 1.8 million tonnes of benzene. This shortfall is the result of four factors:

  • the shift to a very light feedstock slate in steam crackers in the United States because of the availability of inexpensive shale-gas–derived ethane, which reduces the supply of byproduct benzene;
  • regulations limiting the amount of toxic benzene in gasoline, which have forced US refiners to change the operation of catalytic reformers, thus reducing the amount of benzene produced;
  • flat gasoline demand in the United States; and
  • the increasing use of ethanol in gasoline. (Ethanol has a very high octane number, which decreases the need for aromatics as octane boosters.)

On-purpose benzene

This unusual supply–demand pattern has prompted researchers to develop “on-purpose” routes to benzene. Several aromatic interconversion technologies are available. In one strategy, toluene is converted to equimolar amounts of benzene and xylenes, as shown in Figure 3. This technology is known as toluene disproportionation (TDP).

Other technologies used to balance supply are toluene hydrodealkylation (HDA) and transalkylation (TA). In HDA, toluene reacts with hydrogen to give benzene and methane (Figure 4). In TA, toluene and polymethylated benzenes disproportionate to produce benzene, xylenes, and other aromatics.

In addition to TDP, HAD, and TA, researchers continue to develop economical routes to benzene. For example, in a recent patent issued to Shell Oil, Ann Marie Lauritzen and Ajay Madhav Madgavkar disclose a process and catalysts that promote the dehydroaromatization of ethane to aromatics, mostly benzene. Total aromatics selectivity ranges from 48 to 70%; benzene selectivity ranges from 30 to 39%. Toluene selectivity is also good: ≈13% to almost 20%. (US Patent 9,144, 790, Sept 29, 2015)

The outlook for benzene

The expectation is that researchers will increasingly investigate economical routes to benzene that use low-cost feedstocks such as shale gas–derived natural gas and the accompanying natural gas liquids. As regulatory limits on benzene levels in gasoline become stricter around the world, the petrochemical industry will need to rely less and less on refineries for benzene production. This shortfall will make the industry increasingly dependent on on-purpose processes.

Breaking News! 

On November 23, 2015, just before Cutting-Edge Chemistry went to press, Garret Ellison at mlive.com in Michigan reported that the US Coast Guard had successfully unloaded10,000 gal of benzene from a 78-year-old shipwrecked chemical tanker in Lake Erie. When the Argo sank in 1937, the ship was reported to be illegally carrying 4700 barrels, or about 197,400 gal of chemicals. A recreational diver discovered the ship in about 45 ft of water in August 2015. There was a strange odor in the water around the boat.

Coast Guard testing identified benzene, toluene, xylene, and trace elements of petroleum; its first priority was to remove the benzene. The 10,000 gal was removed from one of eight holds in the ship. Each of the seven remaining holds must be tested before the contents can be removed safely. The Coast Guard hopes to finish the task before hard winter sets in.