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1 UNIVERSITY OF CALIFORNIA, LOS ANGELES Civil & Environmental Engineering Department Fundamentals of Chemical Reactor Theory Michael K. Stenstrom Professor Diego Rosso Teaching Assistant Los Angeles, 2003 Introduction In our everyday life we operate chemical processes, but we generally do not think of them in such a scientific fashion. Examples are running the washing machine or fertilizing our lawn. In order to quantify the efficiency of dirt removal in the washer, or the soil distribution pattern of our fertilizer, we need to know which transformation the chemicals will experience inside a defined volume, and how fast the transformation will be. Chemical kinetics and reactor engineering are the scientific foundation for the analysis of most environmental engineering processes, both occurring in nature and invented by men. The need to quantify and compare processes led scientists and engineers throughout last century to develop what is now referred as Chemical Reaction Engineering (CRE). Here are presented the basics of the theory and some examples will help understand why this is fundamental in environmental engineering. All keywords are presented in bold font. Reaction Kinetics Reaction Kinetics is the branch of chemistry that quantifies rates of reaction. We postulate that an elementary chemical reaction I is a chemical reaction whose rate corresponds to a stoichiometric equation. In symbols: A + B Æ C + D [1] I for our purpose we will limit our discussion to elementary reactions
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Stenstrom, M.K. & Rosso, D. (2003) Fundamentals of Chemical Reactor Theory 2 and the reaction rate will be defined as: - r = k · ( c A ) D · ( c B ) E ,, >±@ where k is referred as the specific reaction rate (constant). The overall order of reaction III is defined as: n = D ² E³ [3] T h e t e m p e r a t u r e d e p e n d e n c y o f k is described by the Arrhenius equation: / () a ER T kT Ae ´µ µ [4] where, A = p r e e x p o n e n t i a l o r f r e q u e n c y f a c t o r E a = activation energy [J/mol, cal/mol] R = g a s c o n s t a n t = 8 . 3 1 4 J /m o l · K = 1 . 9 8 7 c a l o l · K T = a b s o l u t e t e m p e r a t u r e [ K ] The Mass Balance Mass is a conservative entity IV , hence given a control volume V the sum of mass flows entering the system will equal the sum exiting minus (plus) the consumed (generated) or accumulated fractions: rate of rate of rate of rate of rate of mass mass mass mass mass in out generated consumed accumulated ­½ ­ ½ ­ ½ ­ ½ °° ° ° ° ° ° ° ´² ´ ®¾ ® ¾ ® ¾ ® ¾ ° ° ° ° ° ° ¯¿ ¯ ¿ ¯ ¿ ¯ ¿ [ 5 ] shortly: IN – OUT + PROD – CONS = ACC [6] E q u a t i o n [ 6 ] r e p r e s e n t s t h e k e y p o i n t i n mass transfer : analogously to the force balance in statics, the mass balance allows us to quantify and verify mass flows in our system.
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