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A Small Matter of Cleapip g

A single idea, if it is right, saves us the labor of an infinity of experiences.

...Jacques Maritain, French philosopher.

Author: V. Narayan

Location: 2.1.3 Petroleum Refinery

Background

In a Fluid Catalytic Cracking Unit (FCCU), the waxy long-chain hydrocarbon molecules are split into shorter-chain, high-value products such as gasoline. At the heart of the FCCU are two pressure vessels working in a loop configuration: the Reactor and Regenerator. The Regenerator is a large vessel operating at about 15 psig., and at temperatures over 1000°F. In the Reactor, the catalyst powder gets coated with carbon released in the cracking process. This carbon is burnt off the catalyst surface by combustion in the Regenerator. A large axial-flow compressor provides air for combustion. Air is distributed inside the Regenerator with a set of pipes called an air-spider. There is a large and continuous fire inside the vessel. The steel wall of this vessel is protected on the inside surface by two layers of thick refractory insulation, keeping the shell relatively cool.

In the FCCU we are discussing here, the lower part of the Regenerator was about 20' diameter and 30' high, while the upper part was about 35' diameter and 60' high. The two parts are joined together by a swaged section, making the vessel about 120' tall. In Chapter 30, Ma- hen discussed cyclone replacement in this vessel. You can see a crosssectional view with some internal details in Figure 30.4. The combustion inside the vessel heats it up significantly and it grows as a result. This causes flexing of the refractory lining, especially at the swaged section. Such flexing occurs mainly during the startup and shutdown phases. The lining damage due to such flexing could be quite large, requiring a fair amount of repair work.

During the planned shutdown (called turnaround in North America), we were to renew the cyclones fitted at the top of the vessel (see Chapter 30, Figures 30.2 and 30.4). Another job was the renewal of the air distribution spider, fitted at the bottom of the vessel.

 
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