Reaction catalyzed by catalyst … Link to heading
Chemical reactions engineering is a sub-area of Chemical Engineering and deals with the design of reactors based on theoretical and experimental principles, where the main objective is to determine the operating variables and dimensions of reactors. For this, one of the first steps of a study of this type is to know the reaction rate expressions.
Rate expressions for chemical reactions describe how fast a number of moles of one chemical species is being consumed to form another chemical species. 1 The rate law of a chemical reaction is given in reacted moles per volume per unit of time, or in SI units: mol/(m³*s). Here comes the role of catalysis, because catalysts are compounds or materials capable of decreasing (or increasing!) the energy barrier that must be overcome for a chemical reaction to occur; and this alters the time required for reactions to happen and, consequently, the rate of reaction. Everything connected.
Catalysis is divided into two major branches:
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Homogeneous catalysis: when the catalyst and reagents occupy the same phase (example: two liquids);
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Heterogeneous catalysis: when the catalyzed reaction occurs at the interface between two phases (example: in a reaction catalyzed by a solid catalyst, chemical transformations occur only on the surface of the solid).
The qualities of catalysts can be assessed for selectivity, activity, stability and regenerability, mechanical and thermal properties. 2
Selectivity Link to heading
It is the reaction rate of the desired product in relation to the formation of the secondary products, therefore, catalysts with high selectivities are always desired.
Activity Link to heading
Activity refers to the rate at which a reaction is catalyzed, the higher the rate, the greater the activity (under identical experimental conditions).
Regenerability and stability Link to heading
How easy is it to recover the catalyst? Does the catalyst degrade over time?
Many solid catalysts accumulate deposits of impurities or reaction products that stick to their surfaces and isolate the catalytic surface from the reaction medium, an example is coke formation in catalytic reforming.
Another problem is sintering, which occurs in some solid catalysts exposed to high temperatures.
Sintering is the change in shape and coalescence of small particles forming large particles, due to high temperatures. 2
Size of a reactor Link to heading
A tubular reactor of only 2.28 m³ (602.3 US galons) is capable of producing up to 300 million pounds (136.08 million Kg) of ethylene per year! 1