6.+Real+World+Examples

__Real World Examples __
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Catalyst fouling has been a major problem on all industries that use catalyst to speed up their processes. In this section, we will tackle some of the most common real life examples where catalyst fouling occurs such as automotive catalytic converters and hydrocracking processes. This section includes an overview about each example, why it is important, how fouling occurs and ways to prevent or lessen the fouling effect.

=Automotive Catalytic Converters =

Catalytic converter is an anti-pollution device installed on automotive exhaust systems to reduce the toxicity of emissions from an internal combustion engine. It mostly uses a metal based catalyst like platinum, palladium or rhodium to oxidize pollutants like hydrocarbons and carbon monoxide into carbon dioxide and water vapor and to reduce nitrogen oxide to just nitrogen (Bryant et al 2000). Catalyst's task in the catalytic converter can be summarized into three main tasks:


 * 1) 2NO x  → xO 2  + N 2  (Reduction of nitrogen oxides to nitrogen)
 * 2) 2CO + O 2  → 2CO 2  (Oxidation of carbon monoxide to carbon dioxide)
 * 3) C <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; vertical-align: sub;">x <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">H <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; vertical-align: sub;">2x <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">+2 + [(3x+1)/2]O <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; vertical-align: sub;">2 <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;"> → xCO <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; vertical-align: sub;">2 <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;"> + (x+1)H <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; vertical-align: sub;">2 <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">O (Oxidation of excess hydrocarbons into carbon dioxide and vapor)



<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Catalytic converter plays an important role in reducing toxic pollutants like nitrogen oxide and carbon monoxide from the atmosphere, considering that there has been a great increase in the number of vehicles used yearly. The primary cause of reduced efficiency of this device is fouling. This is consider to be a costly problem with automotive maintenance, so finding out how fouling occurs and ways to prevent it is considered to be very important.

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Fouling of catalytic converters mainly happens when there is an incomplete combustion. The unburned air and fuel components exhausted from the combustion chambers may foul the converter. Also, the presence of lead content in gasoline can cause a plugged catalyst. Lead is a known catalyst poison and when it is mixed with alkyl halides, it results in the formation of lead halides that penetrates deeply on the catalysts surface causing high degree of site blockage (Rostrup- Nielsen 1998).

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">There are practical ways of preventing and minimizing fouling catalytic converters which are widely used nowadays. The honeycomb structure of auto catalytic converters are mostly used to minimize the effects of fouling. Other than that, the most practical solution focuses on the use of lead free gasoline for auto exhaust system (Koike et al. 1999).



=<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 15px;">Hy﻿drocracking =

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Hydrocracking is a process used in petroleum refinery industry to convert low value petroleum feedstock into high value transportation products and petrochemical feedstock. It has become one of the major processes used for refinery operations because of its great flexibility as a process which permits a wide variety of product to be produced more economically. It also produces more environmentally acceptable products that are low in sulfur, nitrogen and other unsaturated compounds (Ward 1993).

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">The﻿ major causes of hydrocracking catalyst fouling are organometallic compounds depositing their metals, carbonaceous species forming coke and other materials present in the feedstock which deposit on the catalyst. Fouling on hydrocracking catalyst can be prevented to various techniques and proper selection of based material for the catalyst depending on the desired product for each operation. The use of intermixed layers of catalysts have been found to provide surprisingly good selectivity for liquid products and stability against fouling (Habib et al 1995). Also an increase in the catalyst surface area and pore size is said to lessen the effects of fouling on the process while prolonging the life of the catalyst (Ternan et al. 1985). Lastly, hydrocracking catalyst based on zeolitic materials are adapted to techniques like ammonia ion exchange and various calcination to improve the performance of the catalyst regardless of fouling(Habib et al 1995).

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">For those who would like to have a more detailed idea of how the hydrocracking process works, the following diagram should oleviate most questions on how the process works.