Investigators have pinpointed the cause of a February 1990 blast that crippled Shell Chemie's Berre, France, olefins complex steam cracker.
The 420,000 ton/year unit was knocked out for 5 months.
A Shell commission working with outside specialists cited the buildup of brown red gum deposits in the cracker's cold box as the cause of the explosion.
They concluded the gum was formed by nitrogen oxide contamination of noncondensable gases in the unit's catalytic cracker-used since 1982 as makeup load.
Shell Chemie plans to make a presentation to the American Institute of Chemical Engineers in April to inform other steam cracker operators about the problem.
ACCIDENT'S CAUSE
Investigations showed the explosion occurred when a malfunction caused compressors in ethylene and propylene refrigeration cycles to stop.
The temperature rose in the cold box causing throughput of cracked feedstock to fall.
The malfunction occurred when aluminum within the low pressure methane section in one of the two plate exchangers reached fusion temperature at several points.
Inside an undamaged exchanger, a deposit of brown red gum was found containing 15-20% nitrate and the presence of numerous nitrated/nitrosed clusters on short hydrocarbonated chains.
Shell said levels of 30-300 parts per billion for NOx and less than 10 parts per million for oxygen is considered typical by industry under these circumstances.
As a result, when the unit underwent its fourth annual turnaround in 1988, the usual recommended measures for steam crackers were applied, in particular for reheating the cold box.
Shell said before the 1988 shutdown, no abnormal temperature or pressure profile had been noted, so no special measures for the elimination of possible deposits had been taken.
HOW IT HAPPENED
The investigation showed that during the unit's malfunctioning, and particularly during reheating of the cold box small quantities of mono-olefins and diolefins came into contact with nitrate oxides, most likely N2O3.
This component is formed at very low temperatures from NOx and oxygen and concentrates on the coldest exchangers of the cold box.
When NOx comes in the presence of 1-3 butadiene or cyclopentadiene, formation of unstable gums can occur, Shell noted. It was shown that such deposits can ignite spontaneously below ambient temperatures.
Investigators concluded that might have served as a detonator during reheating of the cold box leading to an explosion.
To prevent another such accident, Shell has eliminated all traces of NOx in the cold box. Cart cracker gases will not be used as makeup loads for the Berre steam cracker until there is a way to eliminate NOx buildup.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.