Chemical Reactor Design-CHEM-E7135

Chemical Reactor Design-CHEM-E7135

Yongdan Li

The field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place

Professor of Industrial Chemistry Department of Chemical and Metallurgical Engineering School of Chemical Technology Aalto University

Email: [email protected] Kemistintie 1, E404

Date/time Place Topic Lecturers

Mon 7th of Jan 10:15-12:00 Ke 5 D 311 Lecture 1: Introduction to the course and basic kinetics

Yongdan Li

Mon 21th of Jan 10:15-12:00 Ke 5 D 311 Lecture 2: Ideal reactor design Yongdan Li Mon 28th of Jan 10:15-12:00 Ke 5 D 311 Lecture 3: Non-ideal flow patterns Yongdan Li

Mon 4th of Feb 10:15-12:00 Ke 5 D 311 Assignment 1: Lecture 1-2 Assign the project

Reetta Karinen/Tiia Viinikainen

Yingnan Zhao/Yongdan Li

Mon 11th of Feb 10:15-12:00 Ke 5 D 311 Lecture 4: Typical catalytic reactors Yongdan Li

Mon 25th of Feb 10:15-12:00 Ke 5 D 311 Assignment 2: Lecture 3-4 Reetta Karinen/Tiia Viinikainen

Fri 1th of Mar 10:15-12:00 Ke 5 D 311 Lecture 5: Typical non-catalytic reactors Yongdan Li Mon 4th of March 10:15-12:00 Ke 5 D 311 Lecture 6: Micro-structured reactors Yongdan Li

Fri 8th of March 10:15-12:00 Undetermined Feedback of project Yingnan Zhao/Yongdan Li

Mon 11th of March 10:15-12:00 Ke 5 D 311 Lecture 7: Biochemical reaction systems Yongdan Li Fri 15th of March 10:15-12:00 Ke 5 D 311 Lecture 8: Reactors with ion transfer through

interfaces

Zhengze Pan/Yongdan LI

Course Timetable

 Professor Yongdan Li

– Office hours whenever office door is open, room E404

–

[email protected]



University lecturer Reetta Karinen

– Office hours whenever office door is open, room E406

–

[email protected]

 University teacher Tiia Viinikainen

– Office hours whenever office door is open, room E406

–

[email protected]

Text Book

Chemical

Reaction

Engineering

Third Edition Octave Levenspiel

Department of Chemical Engineering Oregon State University

Online version of the textbook available in Aalto University:

https://app.knovel.com/web/toc.v/cid:kpCREE0005/viewerTy pe:toc/root_slug:viewerType%3Atoc/url_slug:root_slug%3Ac hemical-reaction-engineering?kpromoter=federation

A teacher will guide you to do assignments

Solution

A. Several examples are demonstrated to teach you how to

calculate the related problems

B. Assignments should be completed by you with the help of

teachers

A. Examples

B. Assignments

Project

Design a

Non-catalytic Reactor for Olefins Production by Pyrolysis

Some related materials will be given in Mycourse

Submit a design report

:

Detailed requirements will be listed after the first

assignment

• Background

• Reactor selection

• Mass balance

• Heat balance

• Flow pattern

• Reactor volume

………

- MyCourses is used during the course

–

mycourses.aalto.fi/

– General information and time table

– Lecture slides

– Exercises and assignments

– Project materials

Handling

 Submission of assignments and project in MyCourses

Content Given Accepted DL

First assignment Mon 04th _{of Feb Thu 14}th _{of Feb}

Second assignment Mon 25th _{of Feb Thu 07}th _{of Mar}

Third assignment Mon 18th _{of Mar Thu 28}th _{of Mar}

Project assignment Mon 04th _{of Feb Mon 01}st _{of Apr}

 Completed assignments are marked by teachers at the end

 Total number of exercises in the assignment is 10

Points distribution Number of exercises to be completed 20 P (7.5-10] 15 p (5-7.5] 10 p (2.5-5] 5 P [0.5-2.5]

Evaluation

Assignment - 20%

Project - 80%

Overview of Chemical Reactor Design

Typical chemical process

Chemical reaction engineering (or reactor design) is the engineering practice

concerned with the exploitation of chemical reactions on a commercial scale. Its goal is the successful design and operation of chemical reactors.

Thermodynamics Chemical kinetics Fluid mechanics Heat transfer

Input _Output

Performance equation

relates input to output

Contacting pattern or how materials

flow through and contact each other in the reactor, how early or late they mix, their clumpiness or state of aggregation. By their very nature some materials are very clumpy, for instance, solids and noncoalescing liquid droplets.

Kinetics or how fast things happen. If

very fast, then equilibrium tells what will leave the reactor. If not so fast, then the rate of chemical reaction, and maybe heat and mass transfer too, will

determine what will happen.

Information needed to predict what a reactor can do

Output = f [input, kinetics, contacting] (1)

Develop appropriate performance equations by reaction types

It depends on how we choose to treat them, and this in turn depends on which description we think is more useful.

Overview of Chemical Reactor Design

Reaction rate is the key issue

If the rate of change in number of moles of component i due to reaction is

dN_i/dt, the rate of reaction is defined as follows.

Based on unit volume of reacting fluid,

𝑟_𝑖 = 1 𝑉 𝑑𝑁_𝑖 𝑑𝑡 = 𝑚𝑜𝑙𝑒 𝑖 𝑓𝑜𝑟𝑚𝑒𝑑 (𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑙𝑢𝑖𝑑)(𝑡𝑖𝑚𝑒) (2)

Overview of Chemical Reactor Design

Based on unit mass of solid in fluid-solid systems,

𝑟_𝑖′ = 1 𝑊 𝑑𝑁_𝑖 𝑑𝑡 = 𝑚𝑜𝑙𝑒 𝑖 𝑓𝑜𝑟𝑚𝑒𝑑 (𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑)(𝑡𝑖𝑚𝑒) (3)

Based on unit volume of solid in gas-solid systems, 𝑟_𝑖′′′ = 1 𝑉_𝑠 𝑑𝑁_𝑖 𝑑𝑡 = 𝑚𝑜𝑙𝑒 𝑖 𝑓𝑜𝑟𝑚𝑒𝑑 (𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑)(𝑡𝑖𝑚𝑒) (5)

Overview of Chemical Reactor Design

Based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems,

𝑟_𝑖′′ = 1 𝑆 𝑑𝑁_𝑖 𝑑𝑡 = 𝑚𝑜𝑙𝑒 𝑖 𝑓𝑜𝑟𝑚𝑒𝑑 (𝑠𝑢𝑟𝑓𝑎𝑐𝑒)(𝑡𝑖𝑚𝑒) (4)

Based on unit volume of reactor, if different from the rate based on unit volume of fluid, 𝑟_𝑖′′′′ = 1 𝑉_𝑟 𝑑𝑁_𝑖 𝑑𝑡 = 𝑚𝑜𝑙𝑒 𝑖 𝑓𝑜𝑟𝑚𝑒𝑑 (𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑜𝑟)(𝑡𝑖𝑚𝑒) (6)

Relationship between these definitions:

Overview of Chemical Reactor Design

Variables Affecting the Rate of Reaction

In homogeneous systems the temperature, pressure, and composition are obvious variables.

Heat and mass transfer may play important roles in determining the rates

Broad classification of reactor types

Overview of Chemical Reactor Design

(a) The batch reactor. (b) The steady-state flow reactor. (c), (d), and (e) Various forms of the semibatch reactor

Broad classification of reactor types

Overview of Chemical Reactor Design

Batch Ideal for small-scale experimental studies on reaction kinetics

or small amounts of material are to be treated industrially.

Steady-state flow

Ideal for industrial purposes when large quantity of materials is to be processed and when the rate of reaction is fairly high to extremely high. Good product quality control can be obtai-ned (oil industry).

Semibatch

It offers good control of reaction speed because the reaction proceeds as reactants are added. It was used from the calor-imetric titrations in the laboratory to the large open hearth furnaces for steel production.

Batch Reac tor Fl ow Reactor

Example I: Ammonia Synthesis

20～35 MPa 470~520 o_C

Ammonia is the initial chemical material for a variety of industries. Ammonia synthesis is therefore a very important process in chemical world.

The reaction features High temperature

High pressure Exothermic process

The reactor must bear high temperature and high pressure The heat generated by the reaction must be removed in time

The requirements for reactors

N₂ and H₂ The reactor shell bare the high pressure

The core layer of the reactor bare the high temperature

The heat generated by the reaction was removed by the cool N₂ and H₂, and the feeding N₂ and H₂was

preheated

NH₃

Example II: Fluid Catalytic Cracking (FCC)

Heavier fractions are converted into naphtha and middle distillates

AlCl3 Earthly 20th _century Acid-treated clay 1930 1940 silica-alumina Zeolites 1963-Nowadays  FCC is an endothermic process  Coke deposits on the catalyst,

so the catalyst easily deactivates The reaction features

Catalyst

20% Zeolite Y 80% Matrix

The catalyst and coke

Coke was burned, and the catalyst was heated

The hot catalyst

Example III: Hydrocarbon Thermal Cracking

The raw material is heated to 750-900 o_{C for pyrolysis without catalyst}

Naphtha oil, but natural gas, refinery gas, light oil, diesel, heavy oil etc. are also occasionally used

Raw Material

Ethylene, propylene, butadiene Products

The reaction features

 The reaction is strongly endothermic. Increasing the temperature is advantageous for the formation of olefins

 The residence time of the feedstock in the reactor should be as short as possible. If reaction reaches equilibrium, large amounts of hydrogen and carbon will be formed.  Reducing the pressure helps to improve the ethylene balance composition and

Hydrocarbon Thermal Cracking

Tubular reactor

The reactor is placed at the center of the furnace and the heat is adsorbed in the flame.

Diameter 75 ~ 133 mm

length: 80~90 m

Wall temperature 1050 ~1100 o_C

Flow rate 277 m/s

Residence time 0.09s

Outlet gas temperature 875 o_C

Hydrocarbon Thermal Cracking

 The volume of gas in the tube increases greatly. The pressure drop caused by small diameter is obvious

 The conversion of the reaction becomes high, and the demand for heat is moderated

 The coking is serious and the large diameter can reduce the risk of coke blockage

Variable diameter At the later period of the reaction

Lecture 1.1 Basis of Kinetics

The Rate Equation

Suppose a single-phase reaction：

The most useful measure of reaction rate for reactant A is

The rates of reaction of all molecules are related by

Experience shows that the rate of reaction is influenced by the composition and energy Temperature

Lecture 1.1 Basis of Kinetics

The Rate Equation

Suppose a single-phase reaction：

The most useful measure of reaction rate for reactant A is

The rates of reaction of all molecules are related by

Experience shows that the rate of reaction is influenced by the composition and energy of the material.

Single and Multiple Reactions

When a single stoichiometric equation and single rate equation are chosen to represent the progress of the reaction, we have a single reaction.

When more than one stoichiometric equation is chosen to represent the observed changes, then more than one kinetic expression is needed to follow the changing composition of all the reaction components, and we have multiple reactions.

Series reactions,

Parallel reactions,

more complicated,

Elementary and Nonelementary Reactions

The rate-controlling mechanism involves the collision or interaction of a A molecules with b B molecules The number of collisions of molecules A with B is proportional to the rate of reaction

The number of collisions is proportional to the concentration of

reactants in the mixture (T constant)

Such reactions are called elementary reactions.

Otherwise, the ones are called nonelementary reactions.

Lecture 1.1 Basis of Kinetics

aA + bB

cC + dD

C

_Aa

=

-r

_A

_k

C

_Bb

Mass interaction law

Representation of an Elementary Reaction

the order unchanged, but k different

any measure equiva-lent to concentration

AMBIGUITY:

correct -r expression?

k₁ refers to ?

1) write the stoichiometric equation followed by the complete rate expression. 2) give the units of the rate constant

I

II However,

Representation of a Nonelementary Reaction

Stoichiometry: Rate:

Develop a multistep reaction model to explain the kinetics

unobserved intermediates

Determined by experiments

Suggested by chemistry of the materials

Lecture 1.1 Basis of Kinetics

Br₂ → 2Br ·

Br · + H₂ → HBr + H · H · + Br₂ → HBr + Br ·

Molecularity and Order of Reaction

 The molecularity of an elementary reaction (must be an elementary reaction) is the number of molecules taking part in the reaction.

 This has been found to have the values of one, two, or occasionally three.

 For non-elementary reaction: a, b, . . . , d are not necessarily related to the stoichiometric coefficients.

 We call the powers to which the concentrations are raised the order of the Reaction. Must be integer

ath order with respect to A bth order with respect to B nth order overall

k: rate constant, (time)-1_{(concentration)}1-n

Kinetic Model Development

 Type 1

 Type 2

For unseen and unmeasured intermediate X

Pseudo-steady-state approximation

Quasi-equilibrium approximation

Lecture 1.1 Basis of Kinetics

A X X B

=

-r_X 0 A B : A + B C : _{A + B} X X C X C is rate-determining step A + B k1 X K=k₁/k₂=[X]/([A][B]) k₂

Kinetic Model Example

Lecture 1.1 Basis of Kinetics

(8) (9) (10) (11) Type 1, steady-state approximation (13) (14) (12) Michaelis-Menten type d[X]/dt ≈ 0

Temperature Dependency from Arrhenius' Law

Arrhenius’ Law Frequency factor Activation energy J/mol Same concentration Actually, Mask pre-exponential term sensitive

Collision and transition state theories

Activation Energy and Temperature Dependency

1, Reactions with high activation energies are very temperature-sensitive.

2, Reactions are much more temperature-sensitive in low temperature range than in a high temperature range.

Constant-Volume Constant-density reaction system Constant-Volume

of reaction mixture

Most liquid-phase reactions and all gas-phase reactions occurring in a constant-volume bomb

For gas reactions with

changing numbers of moles

r_i is to follow the change in total pressure π

(15) (16)

The Conversion

X_A: the conversion of A

(17)

(18)



Irreversible Unimolecular-Type First-Order Reactions

(19)

Suppose the first-order rate equation,

(20)

Separating and integrating,

(21)

In terms of conversion ( Eqs. 17 and 18) and the rate equation Eq. 20,

(22)

Rearranging and integrating,

Fig 1.2 Test for the first-order rate equation

Lecture 1.2 Constant-Volume Batch Reactor



Irreversible Bimolecular-Type Second-Order Reactions

(23)

Note: The reacted amounts of A and B at any time t are equal, i.e., C_A0X_A= C_B0X_B,

Let M = C_B0/C_A0 be the initial molar ratio of reactants,

After separation and integration it becomes

After breakdown into partial fractions, integration, and rearrangement, the final result in a number of different forms is

(24)

Fig 1.3 Test for the bimolecular mechanism A + B → R with C_A0≠ C_B0

C_A0 C_B0

Reactants are introduced in their stoichiometric ratio

go back to the original diff-erential rate expression

For a second-order reaction with equal initial C_A0 and C_B0 or for the reaction

the defining second-order differential equation becomes

(25)

On integration it yields

(26)

Rate Equations of nth Order reaction

When the mechanism of reaction is not known

(27)

On separation and integration it yields

(28)

Trial-and-error solution select a value for n and calculate k. The value of n which minimizes the variation in k is the desired value of n

Curious features

the reaction never goes to completion

the reactant concentration will fall to zero and then become negative

n > 1

n < 1

Zero-Order Reactions

high concentration

(29)

Integrating and noting that C_Acan never become negative

(30)

concentration independent radiation intensity,

available surface

Lecture 1.2 Constant-Volume Batch Reactor

30

Overall Order of Irreversible Reactions from the Half-Life t

_1/2

If C_B0/C_A0 = β/α…, at any time C_B/C_A = β/α…

(31)

Integrating for n ≠ 1 gives

Half-Life t_1/2 (Time needed for C_A /C_A0=1/2) is

(32a)

Lecture 1.2 Constant-Volume Batch Reactor

Irreversible Reactions in Parallel

(33)

(34)

(35)

Eq. 33, which is of simple first order, is integrated to give

(36)

dividing Eq. 34 by Eq. 35 we obtain the following

(37)

Fig 1.6 Plotting for Eqs. 36, 37 Fig 1.7 Concentration-time curves for Parallel reactions

Lecture 1.2 Constant-Volume Batch Reactor

36

Irreversible Reactions in Series

First consider consecutive unimolecular type first-order reactions

(38)

(39)

(40)

Start with a concentration C_A0 of A, no R or S present. Integrate Eq. 38,

(41)

Substitute C_A in Eq. 39

(42) (43)

Because there is no change in total number of moles,

(44)

In general, for any number of reactions in series it is the slowest

step that has the greatest influence on the overall reaction rate

Differentiate Eq. 43 and set dC_R/dt = 0, C_{R, max} occurs

(45) ₍₄₆₎

Evaluate k₁ and k₂

Lecture 1.2 Constant-Volume Batch Reactor

43 41

44

46

First-Order Reversible Reactions

Irreversible reactions can be considered as reversible ones with large equilibrium constants.

(47)

Starting with M = C_R0/C_A0

equilibrium constant

(48)

Now at equilibrium dC_A/dt = 0, Hence

and Combining the above three equations (48, 49, 50)

Lecture 1.2 Constant-Volume Batch Reactor

(49) (50)

(51) (21)

Reversible

Irreversible

(22) special case C_Ae=0 or X_Ae=1 or K_C= ∞ i ii

Lecture 1.2 Constant-Volume Batch Reactor

51

Second-Order Reversible Reactions

For the bimolecular-type second-order reactions

(52a) (52b) (52c) (52d) When C_A0=C_B0 and C_R0=C_S0=0 (53)

Fig 1.10 Test for the reversible bimolecular reactions

Lecture 1.2 Constant-Volume Batch Reactor

Lecture 1.3 Varying-Volume Batch Reactor

Fig 1.11 A varying-volume batch reactor

The progress of the reaction is followed by noting the movement of the bead with time

Isothermal constant pressure operations

Volume is linearly related

to the conversion (54)

(55) Fractional change in volume of the system between no

conversion and complete conversion of reactant A

Noting that (56)

On combining with Eq. 54

(57)

(isothermal varying-volume systems)

In general

Replace V (Eq. 54) and N_A (Eq. 56)

in terms of volume (Eq. 54)

(58)

(59)



Zero-Order Reactions

(60)

Lecture 1.3 Varying-Volume Batch Reactor



First-Order Reactions

Replace X_A by V from Eq. 54 and integrate it gives

(61)



Second-Order Reactions

Yongdan Li

Professor of Industrial Chemistry Department of Chemical and Metallurgical Engineering School of Chemical Technology Aalto University

Email: [email protected] Kemistintie 1, E404

Chemical Reactor Design

The field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place