TECHNOLOGY
Capacity Increased in Claus Sulfur Removal An improved method for removing the sulfur from acid gases (sour gases) has been introduced by Air Products & Chemicals. It substantially increases capacity of conventional Claus recovery units. The new technology, Claus oxygen-based process expansion (COPE), replaces air with oxygen and adds recycle gas to the combustion furnace to control temperature. As a result, the capacity of conventional Claus units that treat hydrogen sulfide-rich sour gas streams is doubled. The COPE method is a joint effort of Air Products and Goar, Arrington & Associates. The first unit went on stream at Conoco's Lake Charles, La., refinery in March 1985 and three more are being readied. Cost will vary with circumstances, but Air Products claims savings of as much as 90% of the construction costs for additional Claus capacity. For new plants designed to use the process, savings may be as much as 25%, mostly because of the reduction in equipment size. The COPE technology was developed because of the need to treat heavier crudes containing more sulfur and to process gas from new fields, which is inherently more sour. Conventional Claus plants may use oxygen-enriched air to obtain modest capacity increases with lean gas streams. Because of flame temperature limitations, the conventional process can use enriched air with no more than 30% oxygen. In contrast, the COPE method uses up to 100% oxygen for acid-rich gases from the refinery and up to 80% oxygen for acid-gas streams from gas-field processing. The oxygen-based process permits control of flame temperatures in the combustion furnace below 2800 °F, which is a practical limit for refractory furnace linings. Use of oxygen reduces or eliminates diluent nitrogen and thereby allows for increased capacity. Overall acid conversion in the sulfur recovery unit is increased
Oxygen replaces air in COPE unit Recycle
1 Condenser
Oxygen Sour feed gas
WSXÊÈÈÊÊ
MHHI
Catalytic converter
" " 'r Reheater
Alternate air
Catalytic converter
Condenser|
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Reheater
((•••I converter
Elemental sulfur
Tail gas cleanup
Exhaust
at the same time. In most cases, utility savings downstream partially offset the cost of the oxygen. In a conventional Claus process, the acid-gas feed is mixed with air and burned in a combustion furnace under site-specific conditions. Sulfur-containing gases are then passed through a series of catalytic converters to yield elemental sulfur. Typically, three converters are used in train, although four may be found on occasion. In the principal chemical reactions involved, hydrogen sulfide reacts with oxygen to form sulfur dioxide and water. The sulfur dioxide reacts with hydrogen sulfide to form elemental sulfur and water. Typical sulfur removal in a Claus plant is about 97%. Any sulfurous gases remaining following the converters are removed in a tail gas cleanup unit. Tail gases from Claus plants once were simply exhausted to the atmosphere, before or after incineration. However, tail gas cleanup is now a necessity, since most Claus plants would have difficulty meeting air pollution regulations without them.
The major equipment difference in the COPE technology is a proprietary design for the burner—which handles oxygen, air, fresh acid gas, and recycle gas—and the insertion of a hot-gas recycle blower following the first sulfur condenser. A portion of this gas effluent is recycled to control combustion temperature. This redistributes the pressure drop throughout the unit, placing the greater portion of the load on the front end of the train. Nitrogen removal results in lower pressure drops downstream, which allow smaller processing units to be used. Air Products claims that, in a retrofitted unit, capital cost savings approach 90% of the cost of a new sulfur recovery unit. In gas fields, COPE technology also offers the possibility of recovering carbon dioxide for use in enhanced oil recovery. The developers believe that the COPE process will be used mostly to retrofit existing Claus plants. Air Products is now aiming for expanding applications in the world market. Joseph Haggin, Chicago March 10, 1986 C&EN
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