CHE 404 Chemical Reaction Engineering. Chapter 8 Steady-State Nonisothermal Reactor Design

Transcription

1 Textbook: Elements of Chemical Reaction Engineering, 4 th Edition 1 CHE 404 Chemical Reaction Engineering Chapter 8 Steady-State Nonisothermal Reactor Design

2 Contents 2 PART 1. Steady-State Energy Balance and Adiabatic PFR Applications Energy Balance User Friendly Energy Balance Equations Adiabatic Operation Adiabatic Equilibrium Conversion and Reactor Sizing PART 2. Flow Reactors with Heat Exchange Steady-State Tubular Reactor with Heat Exchange Balance on the Heat Transfer Fluid Algorithm for PFR/PBR Design with Heat Effects CSTR with Heat Effects Multiple Steady State (MSS) Non-isothermal Multiple Chemical Reactions

3 8.1 Rationale ( 이론적배경 ) 3 Let s calculate the volume necessary to achieve a conversion, X, in a PFR for a first-order, adiabatic, exothermic reaction (liquid phase & elementary rxn). A B The temperature profile might something like this. 1. Mole balance (design equation) 2. Rate law

4 4 3. Stoichiometry (liquid phase) 4. Combining 5. Energy Balance Function (X vs. T) or (T vs. V) is necessary to solve this equation. If the reaction is adiabatic, the temperature-conversion relationship is

5 8.2 Energy Balance First Law of Thermodynamics for Open Systems UNIT

6 8.2 Energy Balance Evaluating the Work Term often referred to as the shaft work (stirrer in a CSTR or a turbine in a PFR) Eq. 8-3 FPV [=] (mol/s)*(n/m 2 )*(m 3 /mol) = (N m)/s = J/s = Watts = internal energy + kinetic energy + potential energy + other

7 7 In almost all chemical reactor situations, the kinetic, potential, and other energy terms are negligible in comparison with the enthalpy, heat transfer, and work terms. Unit = J/mol H i H i This equation is the most convenient starting point as we proceed to develop the relationship between T, X and ( r A ).

8 8.2.3 Energy Balance Overview of Energy Balances

9 Energy Balance Overview of Energy Balances Nomenclature

10 Energy Balance Overview of Energy Balances Examples on How to Use Table 8-1 Case 1. Adiabatic reactor Elementary rxn: A B Choose X calculate T calculate k calculate r A A calculate 0 r A Case 2. Cool along the length of a PFR F /F A0

11 User Friendly Energy Balance Equations Dissecting the Steady-State Molar Flow Rates to Obtain the Heat of Reaction Steady-state energy balance Generalized Reaction F F ( X) i i0 i i F / F i i0 A0

12 12 Heat of Reaction at temperature T (unit = Joules per mol A reacted) Steady-State Energy Balance in a more usable form If a phase change takes place during the course of a reaction, this form of the energy balance must be used.

13 User Friendly Energy Balance Equations Dissecting the Enthalpies (No phase change) (No phase change)

14 User Friendly Energy Balance Equations Relating HRx ( T), HRx ( TR ), and Cp

15 Example 8-2. Heat of Reaction 15 Calculate the heat of reaction for the synthesis of ammonia from hydrogen and nitrogen at 150 C in kcal/mol of N 2 reacted and also in kj/mol of H 2 reacted. N 2 + 3H 2 2NH 3 Heats of formation of H 2 and N 2 are zero at 25 C. Exothermic

16 16

17 Summary on Energy Balance 17 0 (S-S EB) negligible

18 Adiabatic Operation Adiabatic Energy Balance Adiabatic (no heat flow) For adiabatic exothermic rxns In many instances, the ΔC p (T T R ) term in the denominator is negligible with respect to ΔH RX term, so that a plot of X vs. T will usually be linear. Conversion from the energy balance This equation applies to a CSTR, PFR, PBR and also a batch when Q = 0 and W s =0.

19 Adiabatic Operation Adiabatic Tubular Reactor Rearrange to solve for temperature as a function of conversion Eq. (8-30) dx F r X T dv A0 A(, ) If pure A enters and ΔC p = 0, then T T 0 Rx X[ H ( TR )] C pa Use Equation (8-30) to construct a table of T as a function of X. Once we have T as a function of X, we can obtain k(t) as a function of X and hence -r, as a function of X alone.

20 표 8-2A 단열 PFR/PBR 알고리듬 20

21 21

22 TABLE 8-2B 22 Rxn: A B (T8-2.9) (T8-2.3) (T8-2.4) (T8-2.7) (T8-2.11)

23 Ex 8-3. Liquid-Phase Isomerization of n-butane 23 Feed: a mixture of 90 mol% n-butane + 10 mol% i-pentane

24 24 Why necessary?

25 For example, at X = 0.2, 25

26 26 PFR

27 27 CSTR at 40% conversion,