ii choice of reactor - ydhermawan's blog · pdf file02.01.2017 · chemical...

Chemical Plant Design – 1210384 Chapter-2

Department of Chemical Engineering - UPN “Veteran” Yogyakarta of 48

IIChoice of Reactor

Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

Outline

1. Introduction2. Reaction Path3. Types of Reaction System4. Reactor Performance5. Rate of Reaction6. Idealized Reactor Models7. Reactor Configuration8. Design Guideline for Reactor




II.1.INTRODUCTION


Introduction

Choice of Reactor involves:1. Type of Reactor2. Reaction Conditions (P, T, C, phase)

Two Types of Reactor:1. Mixed-flow: CSTR, Fluidized2. Plug-flow: PFR, Fixed-Bed, Column

Type of Reactor depends on:1. Type of reaction: single, parallel, series2. Heat effect (heat exchanger): adiabatic, direct/indirect heating

and/or cooling3. Reaction conditions: T, P, phase, catalyst

Most Reaction conditions are Limited by research’s results




Introduction

Temperature and Pressure affect to:1. Reaction rate: Arrhenius equation, concentration2. Reaction equilibrium: endothermic / exothermic (mole

ratio of reactant)

Reaction phase:1. Single phase ( gas, liquid, solid)2. Two phases or more (with or without catalyst)

Catalyst:1. Homogen2. Heterogen


II.2.REACTION PATH




Introduction to choice of Reactor(Smith, R., 2005)

Reactors can be broadly classified as chemical or biochemical.

Most reactors, whether chemical or biochemical, are catalyzed.

The strategy will be to choose the catalyst, if one is to be used, andthe ideal characteristics and operating conditions needed for thereaction system.

The issues that must be addressed for reactor design include:

Reactor type Catalyst Size Operating conditions (temperature and pressure) Phase Feed conditions (concentration and temperature).


Reactor Path(Smith, R., 2005)

Preferred:

Reaction paths that use the cheapest raw materials andproduce the smallest quantities of byproducts are to bepreferred.

Avoided:

Reaction paths that produce significant quantities of unwantedbyproducts should especially be avoided, since they cancreate significant environmental problems.




Example 2.2.1.Given that the objective is to manufacture vinyl chloride, there are atleast three reaction paths that can be readily exploited.


Molar masses and values of materials

Oxygen is considered to be free at this stage, coming from theatmosphere. Which reaction path makes most sense on thebasis of raw material costs, product and byproductvalues?




Solution:

Decisions can be made on the basis of the economic potential ofthe process. At this stage, the best that can be done is to define theeconomic potential (EP) :

EP = (value of products) - (raw materials costs)

Path 1EP = (62 × 0.46) - (26 × 1.0 + 36 × 0.39)

= – 11.52 $·kmol-1 vinyl chloride product

Path 2EP = (62 × 0.46 + 36 × 0.39) - (28 × 0.58 + 71 × 0.23)

= 9.99 $·kmol-1 vinyl chloride productThis assumes the sale of the byproduct HCl. If it cannot be sold, then:

EP = (62 × 0.46) - (28 × 0.58 + 71 × 0.23)= –4.05 $·kmol-1 vinyl chloride product


Path 3EP = (62 × 0.46) - (28 × 0.58 + 36 × 0.39)

= –1.76 $·kmol-1 vinyl chloride product

Paths 1 and 3 are clearly not viable. Only Path 2 shows a positiveeconomic potential when the byproduct HCl can be sold. In practice,this might be quite difficult, since the market for HCl tends to belimited. In general, projects should not be justified on the basis of thebyproduct value.

The preference is for a process based on ethylene rather than themore expensive acetylene, and chlorine rather than the moreexpensive hydrogen chloride. Electrolytic cells are a much moreconvenient and cheaper source of chlorine than hydrogen chloride. Inaddition, it is preferred to produce no byproducts.




Example 2.2.2.Devise a process from the three reaction paths in Example 2.2.1that uses ethylene and chlorine as raw materials and produces nobyproducts other than water. Does the process look attractiveeconomically?

Solution:A study of the stoichiometry of the three paths shows that this canbe achieved by combining Path 2 and Path 3 to obtain a fourth path.

Path 2 and 3


These three reactions can be added to obtain the overallstoichiometry.

Now the economic potential is given by:EP = (62 × 0.46) - (28 × 0.58 + 1/2 × 71 × 0.23)

= 4.12 $·kmol-1 vinyl chloride product

In summary, Path 2 from Example 2.1 is the most attractive reactionpath if there is a large market for hydrogen chloride. In practice, ittends to be difficult to sell the large quantities of hydrogen chlorideproduced by such processes. Path 4 is the usual commercial route tovinyl chloride.

Path 4




II.3.TYPES OF REACTION SYSTEM


Reaction systems can be classified into sixbroad types (Smith, R., 2005):

1. Single Reaction

2. Multiple reactions in parallel producing byproducts.

3. Multiple reactions in series producing byproducts.

4. Mixed parallel and series reactions producingbyproducts.

5. Polymerization reactions.

6. Biochemical reactions.




1. Single Reaction

FEED PRODUCTorFEED PRODUCT + BYPRODUCTorFEED1 + FEED2 PRODUCT

Examples:

Does not produceby product:

Produce by product:


2. Multiple Reactions in Parallel Producing Byproducts

FEED PRODUCTFEED BYPRODUCT

or

FEED PRODUCT + BYPRODUCT1FEED BYPRODUCT2 + BYPRODUCT3

or

FEED1 + FEED2 PRODUCTFEED1 + FEED2 BYPRODUCT

and so on




Examples of a parallel reactions system occurs in the production ofethylene oxide


3. Multiple Reactions in Series Producing Byproducts

FEED PRODUCTPRODUCT BYPRODUCT

or

FEED PRODUCT + BYPRODUCT1PRODUCT BYPRODUCT2 + BYPRODUCT3

or

FEED1 + FEED2 PRODUCTPRODUCT BYPRODUCT1 + BYPRODUCT2

and so on




Examples of series reactions system occurs in the production offormaldehyde from methanol


4. Mixed Parallel and Series Reactions Producing Byproducts

FEED PRODUCTFEED BYPRODUCTPRODUCT BYPRODUCT

or

FEED PRODUCTFEED BYPRODUCT1PRODUCT BYPRODUCT2

or

FEED1 + FEED2 PRODUCTFEED1 + FEED2 BYPRODUCT1PRODUCT BYPRODUCT2 + BYPRODUCT3

and so on




Examples of mixed parallel and series reactions is the production ofEthanolamines by reaction between Ethylene Oxide and Ammonia:


5. Polimerization Reactions

• monomer molecules are reacted together to produce ahigh molar mass polymer.

• Depending on the mechanical properties required of thepolymer, a mixture of monomers might be reactedtogether to produce a high molar mass copolymer.

• Two broad types of polymerization reactions:

those that involve a termination step

those that do not




An example of polimerization reaction that involves a terminationstep:

Polymerization of Vinyl Chloride from a free-radical initiator •R

Initiation step:

Propagation step:

and so on, leading to molecules of the structure:


Eventually, the chain is terminated by steps such as the unionof two radicals that consume but do not generate radicals:




An example of a polymerization without a termination step ispolycondensation

Here the polymer grows by successive esterification withelimination of water and no termination step. Polymers formedby linking monomers with carboxylic acid groups and those thathave alcohol groups are known as polyesters. Polymers of thistype are widely used for the manufacture of artificial fibers. Forexample, the esterification of terephthalic acid with ethyleneglycol produces polyethy-lene terephthalate.


6. Biochemical Reaction

often referred to as fermentations can be divided into two broad types, promoted by:

1. microorganisms2. enzymes

the advantages1. operating under mild reaction conditions of

temperature and pressure2. usually carried out in an aqueous medium rather than

using an organic solvent.




an example of the reaction exploits the metabolic pathwaysin selected microorganisms

In such reactions, the microorganisms reproduce themselves. In addition to the feed material, it is likely that nutrients (e.g.

a mixture containing phosphorus, magnesium, potassium,etc.) will need to be added for the survival of themicroorganisms.

Reactions involving microorganisms include: hydrolysis oxidation esterification reduction


An example of an oxidation reaction is the production ofcitric acid from glucose:




Enzymes are the catalyst proteins produced bymicroorganisms that accelerate chemical reactions inmicroorganisms.

The biochemical reactions employing enzymes are of thegeneral form:

An example of the reaction that promoted by enzymes

An example in the use of enzymes is the isomerization ofglucose to fructose:


II.4.REACTOR PERFORMANCE




Reactor Performance(Smith, R., 2005)

Three important parameters to describe reactor performance:

The stoichiometric factor is the stoichiometric moles of reactant required per mole ofproduct. When more than one reactant is required (or more than one desired productproduced) three Equations above can be applied to each reactant (or product).


Example 2.4.1: Benzene is to be produced from toluene accordingto the reaction

Reactor feed and effluent streams:

Calculate the conversion, selectivity and reactor yield with respect to the:a. Toluene feedb. Hydrogen feed




Solution:


However, the principalconcern is performancewith respect to toluene,since it is more expensivethan hydrogen.




II.5.RATE OF REACTION


Rate of Reaction(Smith, R., 2005)

To define the rate of a reaction, one of the components mustbe selected and the rate defined in terms of that component.

The rate of reaction is the number of moles formed withrespect to time, per unit volume of reaction mixture:

dt

dN

Vr ii

1

whereri = rate of reaction of Component i (kmol·m -3·s-1)Ni = moles of Component i formed (kmol)V = reaction volume (m3)t = time (s)

… (2.5.1)




If the volume of the reactor is constant (V = constant):

dt

dC

dt

VdN

dt

dN

Vr iiii

1

where Ci = molar concentration of Component i (kmol·m-3)

The rate is negative if the component is a reactant andpositive if it is a product. For example, for the generalirreversible reaction:

bB + cC + ··· → sS + tT + ···

The rates of reaction are related by:

t

r

s

r

c

r

b

r TSCB

… (2.5.2)

… (2.5.4)

… (2.5.3)


If the rate-controlling step in the reaction is the collision ofthe reacting molecules, then the equation to quantify thereaction rate will often follow the stoichiometry such that:

cC

bBBB CCkr

cC

bBCC CCkr

cC

bBSS CCkr

cC

bBTT CCkr

whereri = reaction rate for component i (kmol·m-3·s-1)ki = reaction rate constant for component i ([kmol·m-3]NC – b – c-... s-1)NC = is the number of components in the rate expressionCi = molar concentration of component i (kmol·m-3)

The exponent for the concentration (b, c,...) is known as the orderof reaction.

… (2.5.5)

… (2.5.6)

… (2.5.7)

… (2.5.8)




The reaction rate constant is a function of temperature, aswill be discussed next.

t

k

s

k

c

k

b

k TSCB

Reactions for which the rate equations follow thestoichiometry are known as elementary reactions.

If there is no direct correspondence between the reactionstoichiometry and the reaction rate, these are known asnon-elementary reactions and are often of the form:

TSCBBB CCCCkr

TSCBCC CCCCkr

TSCBSS CCCCkr

TSCBTT CCCCkr

where , , , = order of reaction

… (2.5.9)

… (2.5.13)

… (2.5.10)

… (2.5.11)

… (2.5.12)


If the reaction is reversible, such that:

tTsScCbB

then the rate of reaction is the net rate of the forward andreverse reactions. If the forward and reverse reactions areboth elementary, then:

tT

sSB

cC

bBBB CCkCCkr '

tT

sSC

cC

bBCC CCkCCkr '

tT

sSS

cC

bBSS CCkCCkr '

tT

sST

cC

bBTT CCkCCkr '

where

'ik = reaction rate constant for Component i for the reverse reaction

ik = reaction rate constant for Component i for the forward reaction

… (2.5.14)

… (2.5.15)

… (2.5.16)

… (2.5.17)

… (2.5.18)




II.6.IDEALIZED REACTOR MODELS


Idealized Reactor Models(Smith, R., 2005)

the reactants arecharged at thebeginning of theoperation.

Ideal Batch Reactor

The contents are subjected to perfect mixing for a certainperiod, after which the products are discharged.

Concentration changes with time, but the perfect mixingensures that at any instant the composition andtemperature throughout the reactor are both uniform.




Ideal Batch Model

wheret = batch timeNi0 = initial moles of Component i

Nit = final moles of Component i after time t

dt

dN

Vr ii

1

converted

reactantofmoles

itN

iN i

i

Vr

dNt

0

… (2.6.1)

Integration of (2.6.1): … (2.6.2)

Vr

dt

dXN

dt

XNd

dt

dNi

ii

iii

00 1

In term of reactor conversion (Xi)

… (2.6.3)


Ideal Batch Model

iX

i

ii Vr

dXNt

00

… (2.6.4)Integration of (2.6.3):

from the definition of reactor conversion, for the special case of aconstant density reaction mixture:

0

0

0

0

i

iti

i

itii C

CC

N

NNX

Ci =molar concentration of Component i

Ci0 = initial molar concentration of Component i

Cit = final molar concentration of Component i at time t

Substitution of (2.6.5) into (2.6.3) ii r

dt

dC

itC

iC i

i

r

dCt

0

… (2.6.5)

where

… (2.6.6)

… (2.6.7)Integration of (2.6.6):





Feed and producttakeoff are bothcontinuous.

Mixed-Flow or Continuous Well-Mixed or Continuous-Stirred-Tank Reactor (CSTR)

The reactor contents are assumed to be perfectly mixed.

This leads to uniform composition and temperaturethroughout the reactor.

Because of the perfect mixing, a fluid element can leave theinstant it enters the reactor or stay for an extended period.

The residence time of individual fluid elements in the reactorvaries.


Material Balance for Component i per unit time

unit timeperproduct

inreactantofmoles

unit timeperconverted

reactantofmoles

unit timeperfeed

inreactantofmoles

outiiini NVrN ,,

Ni,in = inlet moles of Component i per unit timeNi,in = outlet moles of Component i per unit time

VrNN iiniouti ,,Rearrange (2.6.9):

Substituting Ni,out = Ni,in (1-Xi) into (2.6.10):i

iini

r

XNV ,

… (2.6.8)

… (2.6.9)

where

… (2.6.10)

inii

outiiniini

Cr

CCNV

,

,,,

… (2.6.11)

… (2.6.12)

For the special case of a constantdensity system, (2.6.5) can besubstituted to give:




Analogous to time as a measure of batch process performance,space–time () can be defined for a continuous reactor:

ini

outi

N

VC

F

V

,

,

where F = volumetric flowrate of the feed (m3.s-1)

The reciprocal of space–time is space–velocity (s):

unit timeainprocessedlumereactor voofnumber1

s

… (2.6.13)

… (2.6.14)

Combining Equations (2.6.12) for the mixed-flow reactor withconstant density and (2.6.13) gives:

i

outiini

r

CC

,, … (2.6.15)


This figure is a plot of (2.6.15), from Ci,in to Ci,out the rate of reactiondecreases to a minimum at Ci,out . As the reactor is assumed to beperfectly mixed, Ci,out is the concentration throughout the reactor, thatis, this gives the lowest rate throughout the reactor. The shaded areain the figure represents the space–time (V /F ).

Mixed-Flow Reactor

Concentration vs Reaction Rate





A steady uniform movement of reactant is assumed, withattempt to include mixing along the direction of flow

Like the ideal-batch reactor, the residence time in a PFR isthe same for all fluid elements.

Plug-Flow Reactor


Plug-flow operation can be approached by using a numberof mixed-flow reactors in series.

The greater the number of mixed-flow reactors in series,the closer is the approach to plug-flow operation




Plug-flow Model

unit timepervolume

lincrementaleaving

reactantofmoles

unit timeperconverted

reactantofmoles

unit timepervolume

lincrementaentering

reactantofmoles

… (2.6.16)

iiii dNNdVrN … (2.6.17)

(2.6.16) can be written per unit time as:

Ni = moles of Component i per unit timewhere

Rearrange (2.6.17): dVrdN ii … (2.6.18)

Substituting reactor conversion into (2.6.17):

dVrXNddN iiinii 1, … (2.6.19)

Ni ,in= inlet moles of Component i per unit timewhere


Rearrange (2.6.19): … (2.6.20)dVrdXdN iiini ,

Integration of (2.6.20): … (2.6.21)

iX

i

iini r

dXNV

0,

Writing (2.6.21) in termof the space time:

… (2.6.22)

iX

i

iini r

dXC

0,

For the special caseof constant densitysystems, substitution(2.6.13) gives:

… (2.6.23)

outiC

iniC i

i

ini

ini

r

dC

C

NV

,

,,

,

… (2.6.24)

outiC

iniC i

i

r

dC,

,




This Figure is a plot of (2.6.24). The rate of reaction is high at Ci,in

and decreases to Ci,out where it is the lowest. The area under thecurve now represents the space–time.

Plug-Flow Reactor

Concentration vs Reaction Rate


Use of mixed-flow and plug-flow reactors.




Example 2.6.1:Benzyl acetate is used in perfumes, soaps, cosmetics and householditems where it produces a fruity, jasminelike aroma, and it is used toa minor extent as a flavor. It can be manufactured by the reactionbetween benzyl chloride and sodium acetate in a solution of xylene inthe presence of triethylamine as catalyst.

or A + B C + D

The reaction has been investigated experimentally by Huang andDauerman in a batch reaction carried out with initial conditions givenin Table as follows:


The solution volume was 1.321 × 10-3 m3 and the temperaturemaintained to be 102 ◦C. The measured mole per cent benzyl chlorideversus time in hours are given as follows:

Experimental data for theproduction of benzyl acetate.

Derive a kinetic model for thereaction on the basis of theexperimental data!Assume the volume of thereactor to be constant.




Solution:

Solution The equation for a batch reaction is given by (2.6.2):

AtN

AN A

A

Vr

dNt

0

Initially, it could be postulated that the reaction could be zero order,first order or second order in the concentration of A and B. However,given that all the reaction stoichiometric coefficients are unity, and theinitial reaction mixture has equimolar amounts of A and B, it seemssensible to first try to model the kinetics in terms of the concentrationof A. This is because, in this case, the reaction proceeds with thesame rate of change of moles for the two reactants. Thus, it could bepostulated that the reaction could be zero order, first order or secondorder in the concentration of A. In principle, there are many otherpossibilities.


Substituting the appropriate kinetic expression into (2.6.11) andintegrating gives the expressions in Table as follows:

Expressions for a batch reaction with different kinetic models.




The experimental data have been substituted into the threemodels and presented graphically in Figure as follows:

From Figure, all three models seem to give a reasonablerepresentation of the data, as all three give a reasonable straightline. It is difficult to tell from the graph which line gives the best fit.The fit can be better judged by carrying out a least squares fit to thedata for the three models.


The difference between the values calculated from the model and theexperimental values are summed according to:

Results of a least squares fit for the threekinetic models.

the best fit is given by afirst order reaction model:

rA = kA CA

with kA = 0.01306 h-1 .




Consider now which of the idealized models is preferred for thecategories of reaction systems introduced in Section 2.3.

1. Single reaction:

Clearly, the highest rate of reaction is maintained by the highestconcentration of feed (CFEED, kmol·m-3).

in the mixed-flow reactor the incoming feed is instantly diluted bythe product that has already been formed.

The rate of reaction is thus lower in the mixed-flow reactor thanin the ideal-batch and plug-flow reactors, since it operates at thelow reaction rate corresponding with the outlet concentration offeed.

Thus, a mixed-flow reactor requires a greater volume than anideal-batch or plug-flow reactor. Consequently, for singlereactions, an ideal-batch or plug-flow reactor is preferred.


2. Multiple reactions in parallel producing byproducts:

The ratio of the rates:

Maximum selectivity requires a minimum ratio r2/r1

A batch or plug-flow reactor maintains higher averageconcentrations of feed (CFEED ) than a mixed-flow reactor, in whichthe incoming feed is instantly diluted by the PRODUCT andBYPRODUCT.

If a1 > a2 : the primary reaction to PRODUCT is favored by ahigh concentration of FEED: use batch or PFR

If a1 < a2 the primary reaction to PRODUCT is favored by a lowconcentration of FEED: use a mixed-flow reactor





3. Multiple reactions in series producing byproducts:

For a certain reactor conversion, the FEED should have acorresponding residence time in the reactor.

In the mixed-flow reactor, FEED can leave the instant it enters orremains for an extended period. Similarly, PRODUCT can remainfor an extended period or leave immediately. Substantial fractionsof both FEED and PRODUCT leave before and after what shouldbe the specific residence time for a given conversion. Thus, themixed-flow model would be expected to give a poorer selectivityor yield than a batch or plug-flow reactor for a given conversion.

A batch or plug-flow reactor should be used for multiplereactions in series.




4. Mixed parallel and series reaction producing byproducts:

a1 > a2: use a batch or plug-flow reactor

a1 < a2: use a mixed-flow reactor Series of mixed-flow reactors Plug-flow reactors with a recycle Series combination of plug-flow and mixed-flow

reactors


Mixed parallel and series reactions producing byproducts

As far as the parallel byproduct reaction is concerned, for highselectivity, if:• a1 > a2, use a batch or plug-flow reactor

• a1 < a2, use a mixed-flow reactor




ifa1 < a2


II.7.REACTOR CONFIGURATION




there is steady movement only in one direction. If heat needs to be added or removed as the reaction

proceeds, the tubes may be arranged in parallel, in aconstruction similar to a shell-and-tube heat exchanger.

Tubular reactors can be used for multiphase reactions.However, it is often difficult to achieve good mixingbetween phases, unless static mixer tube inserts are used.

One mechanical advantage tubular devices have is whenhigh pressure is required. Under high-pressure conditions,a small-diameter cylinder requires a thinner wall than alarge-diameter cylinder.

1. Tubular Reactor


Tubular reactor




2. Stirred Tank Reactor

Application include: homogeneous liquid-phase reactions heterogeneous gas–liquid reactions heterogeneous liquid–liquid reactions heterogeneous solid–liquid reactions heterogeneous gas–solid–liquid reactions.

Can be operated: Batch Semi batch Continuous


Heat Transfer to and from Stirred Tank




3. Fixed-bed Catalytic Reactor

the reactor is packed with particles of solid catalyst. Most designs approximate to plug-flow behavior. If the catalyst degrades (e.g. as a result of coke formation on the

surface), then a fixed-bed device will have to be taken off-line toregenerate the catalyst. This can either mean shutting down theplant or using a standby reactor.

If a standby reactor is to be used, two reactors are periodicallyswitched, keeping one online while the other is taken offline toregenerate the catalyst. Several reactors might be used in this wayto maintain an overall operation that is close to steady state.

However, if frequent regeneration is required, then fixed beds arenot suitable, and under these circumstances, a moving bed or afluidized bed is preferred.

Gas–liquid mixtures are sometimes reacted in catalytic packedbeds.


Heat transfer arrangements for fixed-bed catalytic reactors.

The simplest form of fixed-bedcatalytic reactor uses anadiabatic arrangement




Heat transfer arrangements for fixed-bed catalytic reactors

If adiabatic operation is notacceptable because of a largetemperature rise for anexothermic reaction or a largedecrease for an endothermicreaction, then cold shot or hotshot can be used



a series of adiabatic beds withintermediate cooling or heatingcan be used to maintaintemperature control





Tubular reactors similar to ashell-and-tube heat exchangercan be used, in which the tubesare packed with catalyst. Theheating or cooling mediumcirculates around the outsideof the tubes.


4. Fixed-bed Non-catalytic Reactor

Fixed-bed noncatalytic reactors can be used to react a gas and asolid.

For example, hydrogen sulfide can be removed from fuel gases byreaction with ferric oxide:

The ferric oxide is regenerated using air:




5. Moving-bed Catalytic Reactor

If a solid catalyst degrades inperformance, the rate ofdegradation in a fixed bed mightbe unacceptable. In this case, amoving-bed reactor can be used.

Here, the catalyst is kept inmotion by the feed to the reactorand the product. This makes itpossible to remove the catalystcontinuously for regeneration.

An example is a refineryhydrocracker reactor


6. Fluidized-bed Catalytic Reactor

In fluidized-bed reactors, solid material in the form of fineparticles is held in suspension by the upward flow of thereacting fluid.

The effect of the rapid motion of the particles is good heattransfer and temperature uniformity. This prevents theformation of the hot spots that can occur with fixed-bedreactors.

The performance of fluidized-bed reactors is notapproximated by either the mixed-flow or plug-flow idealizedmodels.

The solid phase tends to be in mixed-flow, but the bubbleslead to the gas phase behaving more like plugflow.

Overall, the performance of a fluidized-bed reactor often liessomewhere between the mixed-flow and plugflow models.





7. Fluidized-bed Non-catalytic Reactor

Fluidized beds are also suited to gas–solid noncatalyticreactions.

All the advantages described earlier for gas–solid catalyticreactions apply here.

As an example, limestone (principally, calcium carbonate)can be heated to produce calcium oxide in a fluidized-bedreactor according to the reaction

Air and fuel fluidize the solid particles, which are fed to thebed and burnt to produce the high temperatures necessaryfor the reaction.




8. Kiln Reactions involving free-flowing solid, paste and slurry materials

can be carried out in kilns. In a rotary kiln, a cylindrical shell is mounted with its axis making

a small angle to the horizontal and rotated slowly. The solid material to be reacted is fed to the elevated end of the

kiln and it tumbles down the kiln as a result of the rotation.


Rotary Kiln The behavior of the reactor usually approximates plug-flow. High-temperature reactions demand refractory lined steel shells

and are usually heated by direct firing. An example of a reaction carried out in such a device is the

production of hydrogen fluoride.




II.8.DESIGN GUIDELINE

FOR REACTOR


Design Guideline for Reactor:

I. Single irreversible reaction (not autocatalytic)A. Isothermal – always use a plug-flow reactorB. Adiabatic

1. Plug-flow if the reaction rate monotonically decreasewith conversion

2. CSTR operating at the maximum reaction ratefollowed by a plug-flow section

II. Single reversible reaction – adiabaticA. Maximum temperature if endothermicB. A series of adiabatic beds with a decreasing

temperature profile if exothermic




Design Guideline for Reactor:

III. Parallel reactions – composition effectsA. for A B (desired) and A S (waste), where the ratio of

the reaction rates is:

1. if a1 > a2, keep CA higha. Use batch or plug-flowb. High pressure, eliminate innertsc. Avoid recycle of productsd. Can use a small reactor

2. if a1 < a2, keep CA lowa. Use a CSTR with a high conversionb. Large recycle of productc. Low pressure, add innertsd. Need a large reactor


Design Guideline for Reactor:B. for A +B R (desired) and A + B S (waste), where the

ratio of the reaction rates is:

1. if a1 > a2 and b1 > b2 , both CA and CB high

2. if a1 < a2 and b1 > b2 , then CA low and CB high

3. if a1 > a2 and b1 < b2 , then CA high and CB low

4. if a1 < a2 and b1 < b2 , both CA and CB low

5. See fig below:

IV. Consecutive reactions – composition effects:A R (desired); R S (waste) : minimize the mixing ofstreams with different compositions





Design Guideline for Reactor:V. Parallel reactions – temperature effects:

A. if E1 > E2, use a high temperatureB. if E1 < E2, use an increasing temperature profile

VI. Consecutive reactions – temperature effects:

A. if E1 > E2, use a decreasing temperature profile – not verysensitive

B. if E1 < E2, use a low temperature

Sumber: bahan ajar PPK TK UGM




Choosing heattransfer in thereactor:


Operating temperature for favorable product distribution







Note: See Denbigh (1958) for discussion of this reaction

Sumber: bahan ajar PPK TK UGM

ii choice of reactor - ydhermawan's blog · pdf file02.01.2017 · chemical...

Documents