Two Retrofit Cases for Shifting Bottlenecks

Overall, prevention of fuel gas long is the best strategy. In the scenario of fuel gas long, a mitigation road map must be developed well in advance so that the plant has options in short, middle, and long terms, and can be successful in turning an unfortunate fuel gas long situation into a profitable opportunity. 

Zhu et al. (2011) reported a successful retrofit feasibility assessment for a North American refiner during 2010. The global economy was still in recession but there was increased diesel demand in the market. The refinery wanted to capture the market opportunity, but it was so difficult to obtain loans from investors. The refinery management decided to investigate the possibihty of capacity expansion by 15% in the hydrocracking unit with a stretch goal of 20% at minimum capital cost. If it can be done, they wanted to implement the modifications fast. Therefore, a feasibility project was conducted to look into the possibihty. 

In the kick-off meeting, the plant management emphasized the retrofit objective that increasing the throughput by 15% must be met with minimal capital investment because it was difficult to obtain large investments during the recession. 

In this project, energy optimization was used as an enabler to move the bottleneck from expensive-to-fix locations to cheaper ones, allowing optimization of process conditions and designs. Actually achieving and implementing this synergy is difficult as one needs to pay attention to every detail of practical feasibility. 

The assessment first looked into the opportunities associated with design margin. By optimizing process conditions and pushing the equipment to the absolute limits, the plant could make majority of the spare capacity and push plant capacity by 10% with some minor modifications. However, beyond a 10% expansion, several major limitations were identified from detailed simulations, which included the feed heater flux limit before the main fractionator column, jet flooding limit in the main fractionation column, and reactor space velocity limit in the hydrocracking unit. 

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