WATCHING WATER LINE DANCE

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    17-Feb-2017

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NEWS OF THE WEEK UNWANTED Too little demand has led Chevron Phillips to close this ethylene cracker in Sweeny, Texas. C O S T C U T T I N G SAFE PASSAGE Water molecules (red and gray) are guided one by one through protein pores (blue) to cross bilayer membrane. CHEVRON PHILLIPS SHUTTERS PLANTS Commodity petrochemical units are latest victims of economic downturn CHEVRON PHILLIPS CHEMI-cal is making a rare move: It is shutting down an ethyl-ene cracker. In addition, the com-pany is closing benzene, cyclo-hexane, and polyethylene capacity The cracker is a small, 400 million-lb-per-year unit at the firm's Sweeny Texas, petrochemicals complex that has been idle for ayear because of the weak ethylene mar-ket. The company is al-so idling, but not clos-ing, a 650 million-lb ethylene cracker at the complex. Chevron Phillips isn't alone in slating ethylene closures. Dow Chemical recently told investors that it may close older ethylene units in Seadrift and Texas City Texas, and replace them with a new cracker in Seadrift (C&EN, Nov. 19, page 17). In addition, Equistar has had an 850 million-lb cracker in Lake Charles, La., idle since February Earl H. Armstrong, director of olefins and derivatives for con-sultancy De Witt & Co., says crackers like the Chevron Phillips unit that were built in the 1960s and 70s are not competitive with newer units. "Producers would just as soon put the cost of up-grading into a new plant," he adds. In addition, Armstrong says profitability and demand in the industry have been particularly poor this year. Operating rates, he says, are barely more than 80%. "Those are the lowest Fve seen in the last 15 or 16 years." Chevron Phillips is also shut-ting indefinitely benzene and cy-clohexane plants in Guayama, PR., that have been idle since March. And it is delaying the restart of 0-xylene and -xylene units at the site that were due back on-line early next year. The company is also closing two loop polyethylene reactors at its Orange, Texas, complex over the next couple of months. This will reduce polyethylene capaci-ty at the site by 25%, to 900 mil-lion lb per year.ALEX TULL0 M O D E L I N G N A T U R E WATCHING WATER LINE DANCE To pass through a membrane pore quickly, H 2 0 must 'change partners' R ESEARCHERS AT THE MAX Planck Institute for Bio-physical Chemistry in Got-tingen, Germany, have used mo-lecular dynamics simulations to solve a biological mystery: how water molecules can pass through a protein pore in a cell membrane as rapidly as they do without ferry-ing extra protons across with them. Theoretical biophysicist Hel-mut Grubmuller and chemist Bert L. de Groot com-bined the atomic resolution structure of the protein aqua-porin with a virtual bilayer mem-brane surrounded by a large num-ber of water molecules to create one of the largest and most com-plex computer simulations of the movement of molecules ever generated [Science, 294, 2353 (2001)}. The researchers were able to accurately calculate the movements of some 100,000 atoms for a period of 10 nanosec-onds. That's long enough to "watch" 16 water molecules pass through the channel of the protein. "This is one of a very few, if not the first, complete biological processes to be fully visualized by a simulation," Grubmuller says. In nature, aquaporin rapidly fil-ters water through membranes. Larger molecules are kept from passing through the protein pore by the pore's small size. But tiny protons would be expected to skim through easily by hopping along the network of hydrogen bonds that inevitably forms when water molecules are near one another. The simulation reveals a deli-cately choreographed dance of the water molecules, directed by care-fully positioned amino acid res-idues throughout the channel in-terior. "The water molecules are handed off from one residue to the next," Grubmuller explains. "Each time you break a hydrogen bond between two water mole-cules, you form another between water and the protein." The re-sult is an energy-efficient process that allows the water molecules to move rapidly through the membrane and leaves the protons behind-REBECCA RAWLS U C&EN / DECEMBER 17. 2001 H T T P : / / P U B S . A C S . O R G / C E N HTTP://PUBS.ACS.ORG/CENMODELING NATUREWATCHING WATER LINE DANCE

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