Mohammad Kamil , PhD Prof. & Ex-Chairman - PowerPoint Presentation

Slide1

Mohammad

Kamil

, PhD

Prof. & Ex-Chairman

Department of Petroleum Studies

AMU Aligarh- 202002, UP, India

Email: sm_kamil@rediffmail.com

Slide2

PK 605: Reactor Analysis and Design (3L-1T, 4 Cr.)

Unit I

: Characteristics of RTD, RTD in ideal reactors, Reactor modeling with RTD, Zero parameter, one parameter and two parameter models, Other models of non ideal reactors using CSTRs.

Unit II

: External diffusion effects on heterogeneous reactions, Mass transfer to a single particle, mass transfer limited reaction in packed beds and metallic surfaces, Diffusion and reaction in porous catalysts, Estimation of diffusion and reaction limited regimes.

Unit III

: Non isothermal design of Chemical reactors, Maximum temperature in tubular reactor with heat exchange, control of hot spot temperature, Multiple steady states in CSTR,

vanHeerden

criterion for stability, Ignition and extinction of adiabatic CSTR, Hysteresis,

Autothermal

reactor operation, Unsteady state operation of tubular reactor,

Unit IV

: Catalyst deactivation, Types of deactivation, order of deactivation, temperature-time trajectories, Moving bed reactors. Design of Slurry and trickle bed reactors.

Books

:

1

.

 

Elements of Chemical Reaction

Engineering

(3

rd

or

4th

Edition)

 

,

Prentice Hall International Series, H. Scott

Fogler

.

2.

Chemical Reaction Engineering, 3rd Edition,

Octave

Levenspiel

,

 

Wiley

.

3.

Chemical Reactor Theory: An introduction, second edition, K. G. Denbigh and J. C. R. Turner, Cambridge University Press, England (1971).

4. Fundamentals of

Chemical Reaction Engineering

, Charles D. Holland and

Rayford

G. Anthony, Prentice-Hall International Series.

Slide3

Introduction

Almost all chemical engineering process contains three operations.

What does chemical reactor design means ?

Unit

operation

(cleaning )

Chemical reactor

Unit

operation

(separation)

Raw

material

Product

Slide4

Reactor System

Homogenous

( If the catalyst does not constitute a separate phase from the reactants or the products)

Heterogeneous (

If the catalyst constitutes a separate phase from the reactants or the products)

Slide5

Types of reactors

1. Batch- uniform composition

everywhere in reactor but changes with time. All molecules are exposed for same time and undergo same history of change of temperature and conc. In the absence of flow.

2. Semi batch- in semi-batch one reactant will be added when reaction will proceed.

3. Continuous reactor

Mixed flow- this is uniformly mixed , same composition everywhere, within the reactor and at exit

Plug flow- flow of fluid through reactor with order so that only lateral mixing is possible.

Slide6

Reactor design

Reactor design basically means which type and size of reactor and method of operation we should employ for a given conversion

Parameters

Volume of reactor

Flow rateConcentration of feed

Reaction kineticsTemperaturepressure

Slide7

"An ounce of careful plant design is worth ten pounds of reconstruction."

LABORATORY AND INDUSTRIAL CATALYTIC REACTORS: SELECTION, APPLICATIONS, AND DATA ANALYSIS

I.

Introduction

A. Why study reactors?

B. Definition and classification of reactors

C. Reactor/process design perspective: from the laboratory to the full-scale plant

D. Selection of reactors in the laboratory and plantII. Laboratory and Bench Scale Reactors

A. Kinds

B. Criteria for selection of lab/bench scale reactors; applications

III. Plant Reactors

A. Common types B. Fixed catalyst bed reactors: characteristics, advantages, limitations

C. Fluidized beds: characteristics, advantages, limitations

D. Criteria for selectionIV. Collecting, Analyzing and Reporting Data from Laboratory Reactors

A. General approach and guidelines

B. Criteria for choosing catalyst form and pretreatment, reaction conditions

C. Choosing mode of reactor operation; differential and integral reactors

D. Analyzing and reporting data from laboratory reactors

1. Analysis of rate data: objectives and approach

2. Integral analysis

3. Differential analysis

Slide8

New HDS Unit, ARCO Carson, CA Refinery

Slide9

I. Introduction

Why study reactors?

The design of catalyst and reactor are closely interrelated.

Design of catalytic processes requires a knowledge reactor design, operation optimization and selection

Progress in improving our standard of living depends on our ability to design reactorsOur personal existence depends on controlling cellular reactions in our body while that of the human race hangs on the outcome of enormous global reactions.

Slide10

B. Definition and Classification of Reactors

What is a Reactor?

A device that encloses the reaction space, and which houses the catalyst and reacting media.

A container to which reactants are fed and products removed, that provides for the control of reaction conditions

.

Classification of Reactors

SizeMethods of charging/discharging:

batch or steady-state flow Motion of particles with respect to each other

Fluid flow type: tubular or mixed-fluid

Slide11

Table 1

Classification of Catalytic Reactors

 

Basis for Classification

Classes

Examples

Size

Laboratory

Bench scale

Pilot scale

Plant scale

0.5 cm diam. tubular microreactor (0.1-1 g catalyst)

2.5 cm diam. x 30-50 cm long tubular reactor (50-200 g catalyst)

7.5 cm diam x 6-10 m long tubular reactor (20-100 kg catalyst)

1-6 m diam x 20-50 m long tubular reactor (20-100 metric tons cat.)

Methods of charging and discharging

Batch

Flow, steady state

Stirred liquid and solids

a. tubular, fixed catalyst bed

b. slurry, mixed fluid, mixed solids

Motion of catalyst particles relative to each other

Fixed

Relative motion

Tubular fixed solids (fixed bed)

a. fluidized bed

b. slurry bubble column

Fluid flow

Tubular, plug flow

Mixed fluid flow

Turbulent gas in tubular fixed bed Slurry reactor with mechanical stirring

Slide12

Reactor/process design perspective

Fig. 1

Structure of Catalytic Process Development [adapted from J. M. Smith, Chem. Eng. Prog., 64, 78 (1968)].

Slide13

D. Choosing reactors in the lab and plant

Reactors are used for many different purposes:

to study the mechanisms and kinetics of chemical reactions to provide data for validation of process simulations

to investigate process performance over a range of process variables

to obtain design data

to produce energy, materials and products.

Choosing the right reactor is critical to the engineering process and is dictated by many different variables such as reaction type

rate of deactivationeconomics

other process requirements

Slide14

II. Common Lab and Bench Scale Reactors

fixed bed tubular

stirred gas, fixed bed

stirred liquid/gas, stirred catalyst

fluid bed

fixed bed, transient gas flow

Laboratory and bench-scale reactors vary greatly in size, complexity, cost, and application.

Slide15

Table 2

Laboratory and Bench-Scale Catalytic Reactors

Classes

Class Examples

Features

Fixed bed tubular

Laboratory

differential/integral

Bench-scale integral

0.5 cm diam tubular microreactor (0.1-1 g catalyst, solid catalyst, gas fluid; glass or metal

2.5 cm diam. x 30-50 cm long tubular reactor (50-200 g catalyst); solid catalyst, gas or liquid fluid; metal

Stirred gas, fixed bed

Stirred batch

Batch recycle

Berty

Carberry

microreactor, 1 g catalyst, glass or met.

microreactor, 1 g catalyst, glass or met.

bench-scale, 2-200 g cat., 10-100 atm, stainless steel, circulating gas

bench-scale, 2-200 g cat., 10-100 atm, stainless steel, spinning catalyst basket

Stirred liquid/gas, stirred catalyst

Stirred batch

Bubble slurry

bench-scale, 2-50 g cat., 1-200 atm, glass or metal heterogeneous or homogeneous catalyst

Fluid bed

Laboratory

Bench-scale transport

Recirculating transport

microreactor, 1-5 g cat, 1 atm, glass bench-scale, 50-200 g catal, 1-10 atm, metal

Fixed bed, transient gas flow

Pulse flow

TPD/TPSR

Radio tracer exchange

MS/Transient response

Frequency response

microreactor, 0.1-1 g catalyst, glass or metal, 1 atm

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Fig. 2

Features of representative laboratory reactors [Levenspiel, 1979].

Slide17

Figure 3

Laboratory Pyrex FBR reactor (courtesy of the BYU

Catalysis Laboratory).

Slide18

Figure 4

Berty internal recycle reactor

.

Slide19

Gas-Liquid CSTR (UCSB)

Batch Reactor (UCSB)

Slide20

II. Laboratory and Bench Scale Reactors

Criteria

for selection of lab and bench-scale reactors; applications

Satisfying intended application

Avoiding deactivation

Avoiding inter- and intra- particle heat and mass transport limitationsMinimizing temperature and concentration gradients

Maintaining ideal flow patternsMaximizing the accuracy of concentration and temperature measurementsMinimizing construction and operating costs

Slide21

Table 2

Seven Criteria for Selection of Laboratory and Bench-Scale Catalytic Reactors

Criterion

Issues Involved/Measures of/Methods to Meet Criterion

1. Satisfy purpose of measurement (i.e., application)

Measure: (1) intrinsic activity/selectivity, (2) kinetics of reaction and deactivation

Obtain mechanistic understanding

Simulate process

2. Avoid catalyst deactivation where possible; where not, decide if fast or slow

See Chap. 5 (B&F) on avoiding different kinds of catalyst deactivation

Fast decay causes activity and selectivity disguises and requires use of

transient or transport reactor

Slow decay best studied using CSTR or differential reactor

3. Avoid inter- and intra-particle heat and mass transport limitations

Thiele modulus less than 0.5; small particles or thin catalyst layer

Minimize film thickness with high flow rates, turbulence

Operate at low conversions

Use CSTR or differential reactor

4. Minimize temperature and concentration gradients

Gradients cause activity and selectivity disguises

Maximize mixing in batch reactor and CSTR; use inerts

Use CSTR or differential reactor where possible

5. Maintain ideal flow patterns

Minimize mixing and laminar flow in tubular reactors;

Maximize mixing and minimize gradients in CSTR

Avoid gas or liquid holdup in multi-phase reaction systems

6. Maximize accuracy of concentration and temperature measurements

Sensitive analytical methods and well-placed, sensitive probes

Sufficiently high product concentrations

7. Minimize construction and operating costs

Select the least expensive reactor that will satisfy the other criteria

Consider ways of minimizing size of catalyst and volume of reactant gas

Slide22

Table 3

Applications of Lab/Bench Test Reactors

Reactor Type

Catalyst Selection

Activity/Selectivity

Reactor/Design

Fundamental

Mechanism

Process

Simulation

Life

Kinetics

Integral

Adiabatic

X (overall avg. conv.)

X

X

Isothermal

X (overall conv. at T)

X

X

Differential

Single Pass

X (intrinsic)

X (intrinsic)

X (eliminate)

Recycle

X (intrinsic)

X (intrinsic)

X (eliminate)

Stirred gas

X (intrinsic)

X (kinetics)

X (intrinsic)

X (eliminate)

X (model)

Fluid bed/ Transport

X (fast deact.)

X (fast deact.)

X (fast deact.)

X

Micro-pulse

X (comparative, initial)

X

Transient

X (elem. steps)

X

X (model)

Slide23

Common Types of Catalytic

Plant

Reactors

Fixed-bed Reactors

Packed beds of pellet or monoliths

Multi-tubular reactors with cooling

Slow-moving pellet bedsThree-phase trickle bed reactors

Fluid-bed and Slurry Reactors“Stationary” gas-phase

Gas-phaseLiquid-phase

SlurryBubble Column

Ebulating bed

Slide24

Table 4

Characteristics of Plant-Scale Fixed Bed Reactors

Advantages

1. Ideal plug (or mixed) flow

2. Simple analysis

3.

Low cost

,

low maintenanc

e

4. Little loss or attrition

5. Greater variation in operating conditions and contact times is possible

6. Usually a high ratio of catalyst to reactants

long residence time complete reaction

7 Little wear on catalyst and equipment

8. Only practical, economical reactor at very high pressures

Disadvantages

1. Poor heat transfer in a large fixed bed.

a. Temp. control and measurement difficult

b. Thermal catalyst degradation

c. Non uniform rates.

2. Non uniform flow patterns e.g. channeling

3. Swelling of the catalyst; deformation of the reactor

4. Regeneration or replacement of the catalyst is difficult - shut down is required.

5. Plugging, high pressure drop for small beads or pellets - ∆P is very expensive.

6. Pore diffusional problems intrude in large pellets

Overcoming the Disadvantages

1. Monolithic supports overcome disadvantages 2, 5 & 6

2. Temperature control problems are overcome with:

a. Recycle

b. Internal and external heat exchanges

c. Staged reactors

d. Cold shot cooling

e. Multiple tray reactor - fluid redistributed & cooled between stages.

Catalyst is easily removed - varied from tray to tray.

f. Use of diluents

g. Temperature self regulation with competing reactions, one endo and one exothermic.

h. Temp control by selectivity and temporarily poisoning the catalyst

Slide25

B. Fixed-bed reactors: characteristics, advantages, limitations

Advantages:

Flexible- large variation in operating conditions and contact times is possible

Efficient- long residence time enables a near complete reaction

Generally low-cost, low-maintenance reactors

Disadvantages:

Poor heat transfer with attendant poor temperature controlDifficulty in regenerating or replacing spent catalyst

Slide26

Fig. 6

Commercial fixed-bed reactor designs for controlling temperature: (a) multi-tubular heat-exchange reactor, (b) series of fixed-bed, adiabatic reactors with interstage heating or cooling

.

Figure 5

Commercial fixed-bed, adiabatic catalytic reactor.

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Advantages

1. Frequent regeneration of the catalyst possible.

2. Rapid mixing of solids in fluid beds means uniform gas composition.

3. Isothermal operation and efficient temperature control is practical.

4. Small-diameter particles in fluid minimize pore diffusional resistance.

5. Improved thermal efficiency because of high heat transfer rates.

6. In the case of highly exothermic, liquid phase reactions, slurry reactors are less complex and less expensive than heat-exchange-tubular systems.

Disadvantage

s

1. Fluidized beds are complicated systems involving multiple reactors, heat exchangers, extensive valving and piping to provide continuous system.

2. $$ Extensive investment. Maintenance is high.

3. Fluid flow is complex in fluidized and slurry bubble columns - less than ideal contacting. Product distribution is changed - less intermediate formed in a series reaction.

4. Only a small variation in residence time possible. Low residence times. Conversion may be limited.

5. Attrition & loss of Catalyst.

Table 5

Characteristics of Plant-Scale Fluidized and Slurry Bed Reactors

Slide28

Figure 7

Liquid-phase slurry reactors: (a) forced-circulation, slurry-bed reactor, (b) bubble-column, slurry-bed reactor.

Slide29

Figure 8

Batch-slurry reactor for hydrogenation of specialty chemicals

.

Slide30

Fig. 9

Design of typical FCC transfer-line (riser) reactor with fluidized-bed regenerator.

Slide31

Figure 10

Commercial FCC riser reaction designs (a) Exxon, (b) UOP

.

Slide32

Fluid Cat Cracker (Chevron)

Stacked Fluid Cat Cracker (UOP)

Slide33

Shell Cat-Cracker

All-riser Cracking FCC Unit

Slide34

Reactor Types

Ideal

PFR

( no attempt is made to introduce mixing between elements of fluid at different points in the direction of flow. No attempt is made to introduce mixing between elements of fluid at different points in the direction of flow.

CA0

OR No deliberate mixing in the direction of flow . Distributed parameter model . C

AeCSTR (due to intense stirring effluent stream approaches the same composition and temperature as the contents of the reactor ).

Lumped parameter model CA0

Actual reactors are some where in between these two RealUnique design geometries and therefore RTD

CAe

MultiphaseVarious regimes of momentum, mass and heat transfer

Slide35

Reactor Cost

Reactor is

PRF

Pressure vessel

CSTRStorage tank with mixerPressure vesselHydrostatic head gives the pressure to design for

Slide36

Reactor Cost

PFR

Reactor Volume (various L and D) from reactor kinetics

hoop-stress formula for wall thickness:

t= vessel wall thickness, in.P= design pressure difference between inside and outside of vessel, psigR= inside radius of steel vessel, in.S= maximum allowable stress for the steel.

E= joint efficiency (≈0.9)tc

=corrosion allowance = 0.125 in.

Slide37

Reactor Cost

Pressure Vessel –

Material of Construction gives

ρ

metalMass of vessel = ρmetal (VC

+2VHead)Vc = πDL

VHead – from tables that are based upon DCp= FM

Cv(W)

Slide38

Reactors in Process Simulators

Stoichiometric Model

Specify reactant conversion and extents of reaction for one or more reactions

Two Models for multiple phases in chemical equilibrium

Kinetic model for a CSTRKinetic model for a PFRCustom-made models (UDF)

Used in early stages of design

Slide39

Kinetic Reactors - CSTR & PFR

Used to Size the Reactor

Used to determine the reactor dynamics

Reaction Kinetics

Slide40

PFR – no backmixing

Used to Size the Reactor

Space Time = Vol./Q

Outlet Conversion is used for flow sheet mass and heat balances

Slide41

CSTR – complete backmixing

Used to Size the Reactor

Outlet Conversion is used for flow sheet mass and heat balances

Slide42

Review : Catalytic Reactors – Brief Introduction

Major Steps

A

 B

A

Bulk Fluid

External Surface

of Catalyst Pellet

Catalyst

Surface

Internal Surface

of Catalyst Pellet

C

Ab

C

As

2. Defined by an Effectiveness Factor

External Diffusion

Rate = k

C

(C

Ab

– C

AS

)

3. Surface Adsorption

A + S <-> A.S

4. Surface Reaction

5. Surface Desorption

B. S <-> B + S

6 . Diffusion of products

from interior to pore

mouth

B

7 . Diffusion of products

from pore mouth to

bulk

Slide43

Catalytic Reactors

Various Mechanisms depending on rate limiting step

Surface Reaction Limiting

Surface Adsorption Limiting

Surface Desorption LimitingCombinationsLangmuir-Hinschelwood Mechanism (SR Limiting)H

2 + C7H8 (T)

CH4 + C6

H6(B)

Slide44

Catalytic Reactors – Implications on design

What effects do the particle diameter and the fluid velocity above the catalyst surface play?

What is the effect of particle diameter on pore diffusion ?

How the surface adsorption and surface desorption influence the rate law?

Whether the surface reaction occurs by a single-site/dual –site / reaction between adsorbed molecule and molecular gas?

How does the reaction heat generated get dissipated by reactor design?

Slide45

Problems

Managing Heat effects

Optimization

Make the most product from the least reactant

Slide46

Optimization of Desired Product

Reaction Networks

Maximize yield,

moles of product formed per mole of reactant consumed

Maximize SelectivityNumber of moles of desired product formed per mole of undesirable product formedMaximum Attainable Region – see discussion in Chap’t. 7.Reactors (pfrs &cstrs in series) and bypass

Reactor sequencesWhich come first

Slide47

Managing Heat Effects

Reaction Run Away

Exothermic

Reaction Dies

EndothermicPreventing ExplosionsPreventing Stalling

Slide48

Temperature Effects

On Equilibrium

On Kinetics