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Revised: June, 2004

SILICA REDUCTION IN SECONDARY REFORMERS

Hydrogen produced in the primary reformer at 1800EF passes through a transfer line to the secondary reformer where it is mixed with steam at or about 2450EF, passes through a catalyst bed and through a waste heat boiler at 1800EF. From there it passes through the high temperature shift converter (900EF), the low temperature shift converter (450EF) and into the methonator, emerging as anhydrous ammonia (NH3). When ammonia plants had a capacity of less than about 350 to 400 T/D, alumina-silica refractories were used in the transfer line, the secondary reformer and the waste heat boiler. As capacities increased, it was observed that there was a breakdown of the alumina-silica with the silica being leached out as SiO and, picking up oxygen from the steam, deposited as a fine SiO2 dust on the tubes in the waste heat boiler, thereby reducing its thermal efficiency. Dr. Wygant of Amoco first identified this leaching out of silica as a reaction caused by nascent hydrogen (H). Molecular hydrogen (H2) above 2100EF changes to mono-atomic hydrogen (H). Later, Charlie Venable of Phillips Petroleum identified the same reaction by steam above 1500EF. Of the two reactions which were occurring, the steam reaction is probably the more important. These identifications led to the decision to use high alumina refractory material in the secondary reformer and transfer line. High alumina support media and catalyst carriers were also chosen for use in the secondary reformer. What is really required is a product containing low levels of silica. High alumina materials are used because they are the most readily available and economical product on the market to achieve low silica (SiO2)levels. It was for this reason that Christy developed T-46 Alumina PROX-SVERS®, which are approved for use in secondary ammonia reformers by Kellogg Brown & Root (KBR), Süd Chemie and others. Some users prefer to exclusively utilize T-46 balls in their ammonia plant so that balls containing silica do not inadvertently get placed into the secondary reformer. T-46 2" or 3” balls are used over the perforated dome in the bottom of the secondary. KBR specification calls for a 9" bed of 2" balls over the dome, topped off with a 6" to 9" bed of 1” balls. On top of the catalyst bed, a disperser is needed to prevent churning of the bed and to aid in uniform gas distribution. The KBR specification calls for high alumina Hex-Tile, but T-46 2” or 3" balls can also be used. These products typically have enough mass to resist the force of the gas stream. 1" balls do not have enough mass to prevent being churned by the stream and as a result tend to be worked down into the catalyst bed. Please see our technical bulletin on Hexagonal Target Tile for more information.