NH3 Slip Monitoring in Fluidized-bed Catalytic Cracking Using the LDS 6 Gas Analyzer – Case Study

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Courtesy of Siemens Industry, Inc. - Process Analytics

Fluidized catalytic cracking (FCC) units are used in refineries to convert low value heavy oils into more valuable lighter compounds. Cracking units are sources of emissions of pollutants (e.g. NOx) and must comply with concentration permits set by local or federal authorithies. Modern technologies are applied to reduce the content of pollutants and to monitor certain gas compounds (here: NH3) in the exhaust gas.

The LDS 6 in-situ laser gas analyzer offers the best possible capabilities for this application. It is installed directly in the process gas flow and delivers fast and accurate ammonia concentration data. The data are used to prove compliance with the permits as well as to control and optimize the denitrification process.

This Case Study presents details of the corresponding LDS 6 application in the petrochemical industry. Fluidized-bed Catalytic Cracking (FCC) Fluidized-Bed Catalytic Cracking (FCC) is the most important and widely used refinery process for converting low value heavy oils into more valuable gasoline and lighter products. The typical FCC process will convert 75 % or more of the heavy oils to gasoline and lighter products. Originally, chemical cracking was accomplished by heating the oil to high temperatures (thermal cracking), which nowadays has been almost completely replaced by catalytic cracking.

FCC units are sources of NOx (and SOx) emissions. Emissions from refineries are regulated in many parts of the world and their reduction and monitoring is of increasing concern worldwide. Even though petroleum refining represents only a few percent of the total NOx emission rate, these emissions are typically concentrated in a small area and, therefore, the subject of advanced gas cleaning and gas monitoring measures.

Various techniques for flue-gas denitrification can be applied. The suppression of NOx formation directly in the boiler or NOx reduction by postcombustion abatement will involve use of an ammonia compound (e.g. ammonium hydroxide or urea). Ammonia is mostly used as a reducing agent to convert nitrogen oxides to nitrogen and water at high temperatures. Under real conditions, a small amount of NH3 will remain unused during the process and slip through to the atmosphere. This NH3 slip concentration must be monitored continuously to comply with environmental regulations. Additionally, minimizing the NH3 slip means optimizing the denitrification and provides a possible cost reduction to the process operation.

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