Octanizing reformer options
Staged investment and reformer technology improvement strategies are available for increasing hydrogen production, cycle time and reliability
Bruno Domergue and Pierre-Yves le Goff, Axens
Jay Ross, Axens
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The refining industry is investing heavily in new processing units to produce ultra-low-sulphur (ULS) fuels. As a result, hydrogen utilisation is increasing, on-stream factor and hydrogen reliability are becoming more important, and resources for other investments are scarce. Catalytic reforming is the preferred technology for producing high-octane gasoline and is usually the refinery’s main source of hydrogen. Although existing reformers in North America are generally not fully utilised, many are older semi-regenerative (SR) or cyclic units with cycle times that are incompatible with continuous ULS fuels production. They produce less gasoline and hydrogen than newer ultra-low-pressure continuous catalytic regeneration (CCR) units.
More than 35 Octanizing and Aromizing CCR reforming processes for gasoline- and aromatics-orientated catalytic reforming have been licensed worldwide. Ten new units were licensed in 2005. The Axens CCR reforming process is schematically represented in Figure 1, including key features for producing high-octane gasoline or aromatics-rich petrochemical streams from naphtha.
The catalyst circulation systems of these reformers are designed for long and active catalyst service as well as ease of operation and maintenance. To ensure low catalyst attrition, the lift system must be designed for continuous, smooth, non-pulsating and gentle lifting. Catalyst is continuously transferred to the regenerator, where the coked catalyst undergoes a sequence of steps involving controlled coke combustion, oxychlorination and calcination to restore the catalyst activity and metals redispersion. The proprietary RegenC-2 dry burn loop regeneration system is able to perform complete catalyst activity restoration under mild conditions to maintain catalyst activity and mechanical strength. The catalyst circulation and regeneration operations are highly automated and require minimal operator attention.
The reformer’s side-by-side reactor arrangement, as shown in Figure 1, has several advantages over the stacked design. Access for construction, inspection and future modifications to the reactors, as well as to the internals, is greatly increased. In addition, thermal expansion problems are minimised and the reactor structure is lighter and lower to the ground. This enables an optimal radial reactor design (L/D) without height constraints and a simplified internals structure that is less prone to mechanical problems due to thermal expansion. The reactor placement also provides for shorter catalyst transfer lines, shorter hot transfer lines between reactors and heaters, plus minimal non-flowing heel catalyst volume due to the use of spherical heads (less than 0.5% of the catalyst inventory compared to many times this in other designs). These advantages translate into significant immediate and longer-term savings in investment, construction and maintenance costs.
The key to unit performance and long catalyst life in CCR reforming is the RegenC-2 catalyst regenerator technology. Combined with recently developed and commercialised catalysts, regenerators incorporating this technology can provide sustained catalyst performance over hundreds of regeneration cycles. Significant technology and monitoring improvements in the coke burn and catalyst oxychlorination zones result in increased catalyst life and improved operating flexibility.
RegenC-2 consists of four independent zones, depicted in the block flow diagram in Figure 2. These zones include:
• A primary burn zone equipped with a dry burn loop to minimise moisture during combustion
• A finishing zone with oxygen and temperature control (no sharp exotherms or carbon breakthrough)
• An oxychlorination zone for metals redispersion
• A calcination zone to dry the catalyst.
Coke burning is the principal function of a catalyst regeneration system. It is essential that this step be carried out to completion. However, the coke burning step is the primary contributor to three negative factors concerning catalyst performance and life:
• Metallic phase sintering, which lessens catalyst performance; in particular, stability
• Partial dechlorination of the carrier, which reduces catalytic activity
• Hydrothermal sintering of the carrier, which decreases mechanical strength and ultimate catalyst life.
The hydrothermal sintering of the alumina carrier, which occurs during regeneration and, in particular, during coke burning, results in a decrease of the specific surface area of the catalyst. The leading factors involved in carrier ageing are the moisture level, temperature and combustion time. It is therefore critical that the water content is kept as low as possible in the combustion gases. This observation has led to the incorporation of a dry burn loop in the RegenC-2 regenerator to dry the recirculating combustion gas.
The benefits of a dry burn loop are shown in Figure 3, where catalyst surface area decay (carrier degradation) is plotted against a number of regeneration cycles for the same catalyst in three regeneration systems:
• Hot burn loop: recirculating burning gas is hot and wet
• Cold burn loop: recirculating gas is washed but not dried
• Dry burn loop: recirculating gas is washed and dried.
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