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Unformatted text preview: ChE 101 Chemical Reaction Engineering Final Exam Review Winter 2012 Equation and Concept Review Remember to review the equations and concepts from the midterm exam review! Chapter 5: Nonisothermal Reactors Heat removal and generation The rate of enthalpy generation H in any flowing or closed system can be written as H = H o- H H = j F jo H oj- j F j H j where the first term on the right is the rate of enthalpy flow into the system and the second is the enthalpy flow out of the system. If a reaction is exothermic ( H R < 0), then the reaction generates heat and tends to increase the reactor temperature. If the reaction is endothermic ( H R > 0), the reaction absorbs heat and tends to cool the reactor. Heat can be removed or added to an integral reactor through heat exchange across the walls. We write the rate of heat removal from the reactor Q as Q = UA c ( T- T c ) where U is the heat transfer coefficient in energy per area A c per temperature difference, and A c is the area across which heat exchange occurs between the reactor at temperature T and the coolant at temperature T c . For an adiabatic reaction, Q = 0. Shaft work W s is heat generated by stirring within a reactor. We will generally neglect this term in this course. Heat can also be exchanged with the surroundings in a flow reactor thought the inflow and outflow of heat with fluid entering and leaving the reactor. If the fluid is flowing into the reactor with 1 volumetric flow rate v o and has a heat capacity per reactor volume C po or with total molar flow rate F o with a heat capacity per mole C po , then the rate of enthalpy flow into the reactor is H = v o C po T o = F o C po T o and enthalpy carried out is H = vC p T = F C p T We frequently assume that the molar heat capacity, which can be approximated as c p ( T ) = S j = 1 y j C pj ( T ) is independent of temperature and composition, so that C po = C p and C po = C p . Assumptions for energy balances In the following energy balances, we assume constant density so that we can use concentration as the composition variable. We also assume that the parameters in these systems ( H R , C p , and U ) are independent of temperature and composition. Note that the general form of the energy balance is given by [accumulation of heat] = [heat flow in] - [heat flow out] + [heat generation by reaction] - [heat removal to surroundings] Energy balance in a CSTR An enthalpy balance on the contents of a CSTR gives dH dt = C p V dT dt = vC p ( T o- T ) + V (- H R ) r- UA c ( T- T c ) + W s dH dt = C p V dT dt = vC p ( T o- T ) + V R i = 1 (- H Ri ) r i- UA c ( T- T c ) + W s for single and multiple reactions, respectively....
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