Module 1: Mole Balances, Conversion & Reactor Sizing (Chapters 1 and 2, Fogler)

Transcription

1 CHE 309: Chemical Reaction Engineering Lecture-2 Module 1: Mole Balances, Conversion & Reactor Sizing (Chapters 1 and 2, Fogler)

2 Module 1: Mole Balances, Conversion & Reactor Sizing Topics to be covered in Module 1 (Lectures 2-6): General definitions for homogeneous/heterogeneous reactions Reaction rate. Common Industrial and Laboratory Reactor types and their key characteristics Development of general mole balance and its application to common industrial reactors (batch and continuous) Development of reactor design equations in terms of conversions Application of design equations in reactor sizing

3 Topics to be covered in today s lecture Homogeneous and Heterogeneous reactions Reaction Rates and Definitions [Fogler Section 1.1] General Mole Balance Equation (GMBE) [Fogler Section 1.2] Common Reactor Types and Their Characteristics GMBE for a Batch Reactor [Fogler Section 1.3]

4 Homogeneous & Heterogeneous Reactions Homogeneous Reactions: reactions that occur in a single phase (gas or liquid) NOx formation NO (g) + O 2 (g) NO 2 (g) Ethylene Production C 2 H 6 (g) C 2 H 4 (g) + H 2 (g) Heterogeneous Reactions: reactions that require the presence of two distinct phases Coal combustion C (s) + O 2 (g) CO 2 (g) SO 3 (for sulphuric acid production) SO 2 (g) + 1/2 O 2 (g) SO 3 (g) Vanadium catalyst (s)

5 Classification of chemical reactions useful in rxn design Non-catalytic Catalytic Homogeneous Most gas-phase rxn Most liquid phase rxn Heterogeneous Fast rxns such as burning of flame Burning of coal Roasting of ores Attack of solids by acids Gas-liquid absorption with rxn Reduction if iron ore to iron and steel Rxns in colloidal systems Enzyme and microbial rxns Ammonia synthesis Oxidation of ammonia to produce nitric acid Cracking of crude oil Oxidation of SO 2 to SO 3

6 On Reaction Rates Design of a reactor requires that reaction rates of participating species be specified.

7 Reaction Rate (-r A ) for Homogeneous Reactions ( r A ) = rate of consumption of species A = moles of A consumed per unit volume (mass) per unit time (r A ) = rate of formation of species A Note: minus sign denotes consumption or disappearance. Units of (r A ) or ( r A ) moles per unit volume (mass) per unit time mol/l-s or kmol/m 3 -s, mol/g-s, kmol/kg-s Intrinsic Specific * Caution Customarily, we use r A = dc A /dt. But r A & dc A /dt come from different concepts. As will be shown later, r A = dc A /dt is valid for only a constant volume batch reactor system with perfect mixing.

8 Reaction Rate for Heterogeneous Reactions For a heterogeneous reaction, rate of consumption of species A is denoted as (-r A ') Heterogeneous reactions of interest are primarily catalytic in nature. Consequently, the rates are defined in term of mass of catalyst present. Units of (-r A ') mol per unit time per mass of catalyst mol/s-g or kmol/hr-kg catalyst

9 Is (-r A ) = dc A /dt always true? Let us consider the example of this flow reactor and evaluate if dc A /dt is equal to (r A ). Ethylene Oxide C AO C A Neither C AO nor C A are changing with time C AO (mol/l) C A (mol/l) 10:00 am :00 pm :00 pm :00 pm Steady State Operation - no change with time

10 More on. Reaction Rate Reaction rate (intrinsic) function of temperature and reactant concentrations independent of reactor type described by a kinetic expression or rate law. Rate Law (Rate Equation) rate law is an algebraic equation that relates reaction rate to species concentration via a reaction rate constant --- a constitutive relationship. (-r A ) = k [concentration terms] e.g. (-r A ) = k C A or (-r A ) = k C A 2 where, k is rate constant [k=f(t)] Note: a more appropriate description of functionality should be in terms of activities rather than concentration.

11 Reactor Types

12 Common Industrial & Laboratory Reactor Types Batch Reactor Continuous-Flow Reactors Continuous-Stirred Tank Reactor (CSTR) Tubular Reactor Plug Flow Reactor (PFR) Packed Bed Reactor (PBR) Other Reactor Types Membrane Reactor Fluidized Bed Reactor

13 Characteristics of Key Reactor Types Batch Reactor mainly used for small scale operation suitable for slow reactions mainly used for liquid-phase reaction charge-in/clean-up times can be large Loading (t < 0) CSTR (Continuous-Stirred Tank Reactor) steady state operation; used in series good mixing leads to uniform conc. and temp. mainly used for liquid phase reaction suitable for viscous liquids PFR (Plug Flow Reactor) suitable for fast reaction gas phase reaction temperature control is difficult there are no moving parts Reactants Reaction (t 0) C AO Discharge (t = t f ) Products C A

14 Characteristics of Other Reactor Types Membrane Reactor Perfect solid-liquid separation Short hydraulic retention time Long solid retention time Facility compactness Suitable for coating, combustion process

15 Characteristics of Other Reactor Types Fluidized Bed Reactor The smooth, liquidlike flow of particles Easy handling Isothermal condition Large-scale operation Suitable for coating, combustion process Spraying Wetting Solidifying coating droplets particle coated particle Drag force by upward moving gas = Weight of particles [Ref.] Daizo Kunii & Octave Levenspiel, Fluidization Engineering, John Wiley & Sons, Inc

16 General Mole Balance w.r.t. RATE ( Amount / Time)

17 Element of reactor volume Reactant enters Reactant leaves Reactant accumulates within the element Reactant disappears by chemical rxn within the element

18 General Mole Balance Equation (GMBE) General mole balance equation is the foundation of reactor design. You have used the following form of mole balance INPUT Rate OUTPUT Rate + Rate of GENERATION Rate of Consumption = Rate of ACCUMULATION { rate of reactant flow into element of volume} {rate of reactant flow out element of volume} +{rate of reactant loss due to chemical rxn within the element of volume} ={rate of accumulation of reactant in element of volume} { rate of heat flow into element of volume} {rate of heat flow out element of volume} +{rate of disapperance due to chemical rxn within the element of volume} ={rate of accumulation of heat in element of volume}

19 General Mole Balance Equation INPUT Rate - OUTPUT Rate + Rate of GENERATION = Rate of ACCUMULATION FAO F A + G A = dn A dt F A F AO Control Volume = V In case of perfect mixing G A = (rate of Generation of A) V = (r A ) V In a general case G A = M lim G ji = lim rji Vi = M i= 1 M M i= 1 V r A dv

20 General Mole Balance for a Batch Reactor Reaction: A Products General Mole Balance Equation (GMBE) for Batch Reactor may be F A0 = F A = 0 and G A = (r A ) V (as a result of perfect mixing) written in differential form as written in integral form as 1 Key information: Time to reduce N A from N A0 to N A1.