reactors simulation 2014-agb.pdf

8/11/2019 Reactors Simulation 2014-AGB.pdf

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REACTOR SIMULATION

Simulación y Optimización de Procesos Químicos

Content

• Review of fundamental concepts•

Types of reactor models in processsimulators (ASPEN)• Specific characteristics of reactor models in

ASPEN• Example



REACTOR SIMULATION


Conversion•

Conversion is amount of reactant reacted. p

G

reactant consumed in reactor single-pass conversion=X =

reactant fed to reactor reactant consumed in process

overall conversion=X = reactant fed to process• Defined in terms of limiting reactant• Most processes recover and recycle unreacted material to

provide a high overall conversion.• High reactor conversions are neither necessary nor

desirable for optimum reactor performance. At low reactorconversions , high overall conversions can be achievedwith increased recycle.



REACTOR SIMULATION


Selectivity and Yield• Selectivity is amount of reactant converted to desired

product.mol of desired product

selectivity=S=mol of reactant consumed (limitant)

*A high selectivity is always desirable.

• Yield is the amount of reactant fed converted todesired product.

moles of desired productYield=Y= X×S

moles of reactant fed



REACTOR SIMULATION


Reaction Kinetics• Reaction rate for single phase reactions

1 moles of formed

volume of reactor timei

i

dN ir

V dt

• Reaction rate for solid catalyzed reactions

1 1 moles of formed

volume of reactor timei i

i bdN dN i

r V dt W dt

• For solid catalyst systems, reactor performance isusually controlled by resistances to masstransfer .



REACTOR SIMULATION


Reactor Models in ASPENEquilibrium reactors :

• RGIBBS• REQUIL

Conversion reactor:• RSTOIC

Kinetic Models• RCSTR • RPLUG• RBATCH

Other: RYIELD, specific reactors



REACTOR SIMULATION


Characteristics of reactor models



REACTOR SIMULATION


Inputs and outputs:

Degree of freedom analysis• The model reactors calculates:

• the flow rates of all species leaving the reactor,• the energy balance (optional) and either the duty on

the reactor or the temperature of the exit stream.• Degree of freedom analysis (mass and thermal)

• Nc+2 (inlet stream)• 2 further specifications (typically: heat duty and

pressure drop)• Other: depends on reactor model



REACTOR SIMULATION




REACTOR SIMULATION


RSTOIC• The fractional conversion, X, of key reactant k, is

specified.0 1

moles of entering reactor

moles of leaving reactor

in out

in

in

out

k k k k

k

k

k

n n X X

n

n k

n k

• The model calculates the flow rates of all speciesleaving the reactor, the energy balance, and either the

duty on the reactor or the temperature of the exit stream.• Besides k fractional conversions Nc+2 (inlet stream), 2

more specifications are required . Typically: heat dutyand pressure drop.



REACTOR SIMULATION


Rstoic: specifications• Use Setup Reactions sheet to specify the

Stoichiometry and molar extent or conversion foreach reaction

• Use Setup Specifications sheet to specify thereactor outlet operating conditions

• – Two of temperature, pressure, duty and vapor

fraction• – The phases to consider in flash calculations

(Default: Vapor-Liquid)



REACTOR SIMULATION




REACTOR SIMULATION


REQUIL



REACTOR SIMULATION


REQUIL(1)

exp exp ...i

i i

ieq ii

G B

K c A RT T

•

Useful when there are many components, a few knownreactions, and when relatively few components take part inthe reactions

• EQ Models provide a better description than Conversion Model for many industrial reactors.

• The equilibrium reactor model solves simultaneously theenergy balances and the following equation for eachreaction.

(the coefficients A, B are obtained from the simulatordatabase)



REACTOR SIMULATION


REQUIL(2)• Performs single -phase property calculations or twophase

flash calculations nested inside a chemical equilibriumloop

• Cannot perform three-phase calculations• Also perform solid (conventional) calculations



REACTOR SIMULATION




REACTOR SIMULATION


REQUIL (optional)• User can restrict equilibrium by specifying one of:

– Molar extent of the reaction – A temperature approach to chemical equilibrium

* Temperature approach is the number of degrees above thereactor temperature at which chemical equilibrium is determined

TEquil =TR + Δ T.

•Entrainment sheet to specify the fraction

of the liquid or solid phase in the productthat is entrained in the vapor outlet stream



REACTOR SIMULATION


REQUIL (Results)• Summary

– Outlet temperature and pressure – Heat duty – Net heat duty – Molar vapor fraction

• Mass, mole and enthalpy balance

• Equilibrium constants. Not reported for reactions whererestricted molar extent specified



REACTOR SIMULATION


Example and tips• Gas-phase synthesis of methanol from syngas (exot. React)(Ex: Stoichiometrically fed)

2 3

213

3 2

2

3 24.8 10 exp 11,458 /

4 1eq

CO H CH OH

X X K T

X P

• Tips:•

Think about the results first!• The calculation must fit the expected results!• A separate analysis of the reactor can be helpful to

set/understand the operation condition in the process



REACTOR SIMULATION




REACTOR SIMULATION


Input Data : None (Stoichiometry optional)

RGIBBS



REACTOR SIMULATION


RGIBBS (1)•

It consider all possible reactions by minimizing theGibbs free energy of the mixture subject to a material balance.

• Reactions (stoichiometry) do not require (optional) to

be specified, but feed and product streams do. Allcomponents for the current flowsheet are considered.• Useful

• if one wishes to examine the thermodynamics

associated with converting a given stream to desired products.• to solve problems at first stages of the synthesis

process when not much data are available.



REACTOR SIMULATION


RGIBBS (2)• Use model for:

– Single phase (vapor or liquid) chemical equilibrium – Phase equilibrium with no chemical reactions – Phase and/or chemical equilibrium with solid solution

phases – Simultaneous phase and chemical equilibrium

• This is the only Aspen Plus Unit Operation modelthat calculates solid-liquid-vapor equilibrium .



REACTOR SIMULATION


RGIBBS (Specifications)

Specified on the Setup Specification sheet the: – Reactor Conditions

• Pressure and either Duty or Temperature

– Calculation options for phase, chemical, andrestricted chemical equilibrium – Maximum number of fluid phases to consider in

the equilibrium calculations

REACTOR SIMULATION



REACTOR SIMULATION


RGIBBS (Specifications 2)

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REACTOR SIMULATION


RGibbs: Phase & Chemical Equilibrium• By default, all components entered on the Components SpecificationSelection sheet are possible fluid phase or solid products• You can limit the number of possible products by using the SetupProducts sheet• Tries to distribute all species among the specified solution phasesby default• Use Setup Products sheet to assign different sets of species to eachsolution phase• You can assign different thermodynamic property methods to eachphase

REACTOR SIMULATION



REACTOR SIMULATION


Rgibbs (Results)

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REACTOR SIMULATION


Ejercicio

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REACTOR SIMULATION


Se pide:• Using RGibbs and either a Design Specification or Sensitivity

Analysis to determine: – Maximum temperature at which the reactor can operate

to produce a mixture of ethylene and ethane (not more than1% acetylene by mole). What is the conversion of ethane? – Minimum temperature at which the reactor can operate to produce a mixture of ethane and acetylene (not more than1% ethylene by mole). What is the conversion of ethane?

– Temperature at which equilmolar quantities of acetyleneand ethylene are produced* [Hint: use T range of 500-2000 C]

REACTOR SIMULATION



REACTOR SIMULATION


Basic Kinetic Models

0

0

0

0

0

RCSTR ,

RPLUG , , 0

1RCSTR , , 0

volumetric flow rate inlet vol. flow rate

: mole avilable at given instantvector of molar concentrations vecto

Q Q V T

d QT V

dV d N

T t V dt

Q Q

N

0c c r c

cr c c c

r c c c

c c r of inlet concentrations

temperature vector of reaction rates reactor volumeT V r

REACTOR SIMULATION



REACTOR SIMULATION


Input Data : Stoichiometry and kinetics

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REACTOR SIMULATION



REACTOR SIMULATION



REACTOR SIMULATION


Kinetic Reactor Models• It needs kinetics• The allow determination of volume• kinetic reactor models are useful only if kinetic data

are available• They allow introduction of LH kinetics, multiple

reactions, etc• Besides k set of reaction data and Nc+2 specification

of inlet streams, 3 more specifications are required.Typically: heat duty, pressure drop and reactor volume.

•

In RPLUG you will have to provide the length anddiameter of the tubes.• In RPLUG, optionally, temperature profile can be

specified.

REACTOR SIMULATION



REACTOR SIMULATION


Specifying the simulation

involving reactors1. Specification of reactions

– Species – Stoichiometry (except to RGIBSS) – Other (reactor type specific)

• Conversion•

Kinetics• Other

2. Specification of reactor

REACTOR SIMULATION



REACTOR SIMULATION


Specifications of Reactions• It is necessary to specify R chemical

reactions involving C chemical species.

1

0 1,....,

chemical formula for species

stoichiometric coefficient for species in reaction(negative for reactants, positive for products, 0 for inerts)

C

ji ii

i

ji

A j R

A i

i j

REACTOR SIMULATION



REACTOR SIMULATION


ExampleHydrodealkylation of toluene to produce benzene

• Rewrite in stoichiometric form.

7 8 2 6 6 4

6 6 12 10 2

primary reaction

2 side reaction

C H H C H CH

C H C H H

7 8 2 6 6 4

6 6 12 10 2

=0

2 =0

C H H C H CH

C H C H H

• What are the chemical species A j and thestoichiometric coefficients ij?

REACTOR SIMULATION




Evolution of Reactor Models During theDesign Process

• Ideal reactor models are often used early in the process design.

– Reactor effluents and heat duties are needed – Details of the reactor design are less important.

• Ideal models are replaced by custom-made modelsas the details gain significance.

• All commercial flowsheet simulators provide forinsertion of user-generated models.

– Refined as data from laboratory or pilot plant becomeavailable

– ASPEN can estimate parameters for custom modelsthrough non-linear regression.

REACTOR SIMULATION




Summary• All species (components) involved in the reactions

have to be previously specified• Stoichiometry have to be specified (ex: RGIBBS)• Heat balance is optional• Profiles (C,T) only possible in RPLUG• DOF : (inlet streams + 2+r specifications)•

The choice of reactor model depends on the dataavailable, and the required output for yoursimulation (detailed vs. first estimates)

REACTOR SIMULATION




Example:

Pyrolysis of benzene in a Plug flowreactor

Objetive: To study the pyrolysis of benzene at T=1400 ºF and 1 atm

REACTOR SIMULATION




REACTOR SIMULATION

reactors simulation 2014-agb.pdf

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