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Chapter 2 Conversion and Reactor sizing

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Overview In the first chapter the general mole balance was derived for different reactors In this chapter, these equations are used to size CSTR and PFR using “Conversion” Value and overall conversion of CSTR and PFR arranged in series

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2.1 Definition of Conversion Conversion is the number of moles of reactant A (limiting reactant) that has been reacted per mole of A fed to the system For irreversible X A =1 complete conversion For reversible X max =X e equilibrium conversion

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2.2 Batch Reactor Design Equation After time t, the number of moles of A remaining is differential form Batch reactor design equation used for reaction rate data analysis Integral form This equation gives the time required to achieve a specified conversion X The longer the reactants are left in the reactor, the greater the conversion

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2.3 Design equation for flow reactor Mole balance for reactant A around the reactor In liquid phase C A0 is the solution molarity (moles/volume) In gas phase

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CSTR The mole balance for CSTR yields This equation calculates the CSTR volume necessary to achieve a specified conversion X Because of perfect mixing, the exit conc is identical to the conc inside the reactor and the reaction rate is evaluated at the exit conditions F A0 FAFA

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Levenspiel CSTR Plot Volume = Area of rectangle

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2.3.2 Tubular Flow Reactor (PFR) No gradient change in T, CA & -Ra The reactants are consumed as they enter and flow axially down the reactor Differential form of design for PFR Integral form used to calculate volume required to achieve specified conversion X

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Levenspiel PFR Plot Volume= area under the curve

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2.3.3 Packed Bed Reactor Packed bed reactors are analogous to PFR Differential form of the design equation used to analyze the reactor pressure drop Integral form used to determine the catalyst weight in the absence of pressure drop

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Applications of the design equations We can size the reactor from the reaction rate, as a function of conversion For the first order For irreversible reactions of greater than zero order For reversible reactions

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2.5 Reactors in series For reactors in series where no side stream either fed or withdrawn, the conversion at point i is defined as The molar flow rate at point i is given by F A0 F A1 F A2 F A3 i=1 X 1 i=3 X 3 i=2 X 2

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2.5.1 CSTR in Series For 2 CSTR in series F A0 F A1 F A2 i=1 X 1 i=2 X 2 -r A1 -r A2

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Levenspiel CSTR Plot Volume of CSTR1 Volume of CSTR2 For the same overall conversion, the total volume for 2 CSTRs in series is less than that required for one CSTR

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CSTR and PFR Comparison PFR can be modeled with a large number of CSTRs in series. This concept can be used in – Catalyst decay in packed bed reactors – Transient heat effects in PFRs V1V1 V2V2 V3V3 V4V4 V5V

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Reactor Mole Balance Summary

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