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© 2013 Aspen Technology, Inc. All rights reserved

Reaction in Fluidized Beds

Guide to the Fluidized Bed Reactor Demo

Aspen Technology

Burlington, MA

2013

© 2013 Aspen Technology, Inc. All rights reserved |

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Why Model a Fluidized Bed Reactors?

Problem: Yield below expectations, loss of fines, unknown particle size distributions or flow rates, high operating costs

Benefits:

– Optimize reactor yield and selectivity

– Gain a better understanding of particle size distributions and flow rates throughout process

– Minimize loss of fines due to optimal designed gas-solid separation sections

– Reduce operating costs due to optimal gas and solids flow rates


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Fluidization in Aspen Plus

Aspen Plus fluidized bed model

– describes isothermal fluidized bed

fluid mechanics (one-dimensional)

entrainment of particles

– considers

particle size and density / terminal velocity

geometry of the vessel

additional gas supply

impact of heat exchangers on bed temperature and fluid mechanics

chemical reactions and their impact on the fluid-mechanics and vice-versa

– provides different options/correlations to determine

minimum fluidization velocity

transport disengagement height

entrainment of solids from the bed

distributor pressure drop (porous plate / bubble caps)


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Model short description - fluid-mechanics

Model of the fluidized bed considers two zones

– Bottom zone

high solids concentration

fluid mechanics according to Werther and Wein.

– considers growth and splitting of bubbles

– Freeboard

comparable low solids concentration

fluid mechanics according to Kunii and Levenspiel

User defines bed inventory by specifying the pressure drop or the

solids hold-up

– height of the bottom zone and the freeboard can be determined

– bubble related profiles (e.g. bubble diameter, bubble rise velocity etc.), interstitial gas velocity, pressure and solids volume concentration profile can be calculated

– by use of selected entrainment correlation the solids mass flow and PSD at the outlets can be calculated


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Model short description - chemical reactions

Model allows to consider chemical reactions

– assumptions:

gas in plug flow

solids ideally mixed

each balance cell is considered as CSTR

– model considers

impact of volume production/reduction on the fluid mechanics

change in PSD due to reaction

Use reaction object to define

– stoichiometry

– reaction kinetics


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Model short description - Change in particle size

Particle size distribution may change due to chemical reaction

– available options that allow to calculate or set the bed PSD


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Fluidization in Aspen Plus - Fluidized Bed GUI

• Define bed inventory by defining bed pressure drop or bed mass

• Define voidage at minimum fluidization

• Select Geldart group for the bed material

• Select correlation used for the determination of the entrainment flow

• Overwrite correlation parameter if necessary

• Select correlation used for the calculation of the TDH

• Specify gradient used for determination of TDH based on calculated solids volume concentration profile

Specifications Tab

• Define decay constant for the freeboard

• Specify minimum fluidization velocity or select a correlation to determine it


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Define temperature in the vessel by specifying either: • heat duty • temperature

Operation Tab


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Define the vessel diameter as function of height

Specify the location of additional gas inlets

Remarks: - All locations are relative to the vessel height (0 bottom, 1 top)

- Table for additional gas inlets is only active if streams are connected to the additional gas inlet port

Geometry Tab

Specify Dimensions • Height of the vessel • Solids outlet location (relative to the

height) • Cross-section (circular or rectangular) • If the vessel diameter changes with

height or remains constant


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Select distributor type • Perforated plate • Bubble caps

Define distributor pressure drop method • Constant pressure drop • Calculated based on geometry and given

orifice discharge coefficient

Define distributor geometry

Gas Distributor Tab


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Define heat exchanger geometry

Define heat transfer coefficient

Select if arithmetic or logarithmic temperature difference should be used

Heat Exchanger Tab

Remark: - Heat exchanger input form is only active if streams are

connected to the heat exchanger inlet and outlet


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Reactions Tab

Select or remove reaction sets

Add new reaction set

Defined reaction sets can be edited via the reactions section in the Navigation Pane

Shows list of selected reaction sets

Shows list of available reaction sets


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PSD Tab

Remark: - PSD input form is only active if a reaction set is

selected on the reactions input form

Select method that should be use to determine the PSD after the reaction occurred


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Define solver tolerance and maximum number of solver steps

Define number of cells used for the discretization of the bottom zone and the freeboard

Define minimum relative deviation used by the solver to recalculate the height of the zones

Convergence Tab

Define a flash parameter


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Fluidized Bed Reactor - Application Example

Task: Setup a Aspen Plus model to simulate the synthesis of organosilanes as monomer for silicone polymers

Reaction (simplified): Si + 2CH3Cl + (Cat.) (CH3)2SiCl2 Silicone Chloromethane Dimethyldichlorosilane

Chloromethane is used a fluidization

gas

Entrained particles are

separated with a gas cyclone and

recycled

Silicon is mixed with

copper (catalyst)


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Fluidized Bed Reactor Example - Custom Table & Layouts

Open file “fluidized bed reactor demo.bkp”

A custom table is used to show the main input and output parameters of the model

Several layouts have been defined to more easy use the model and review the calculation results

– To navigate through the layouts, use the “Swtich Layout” option in the “View” Ribbon


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Fluidized Bed Reactor Example - Feed Definitions

PSD mesh

– type: Logarithmic

– number of intervals 100

– lower limit: 0.0001 mm

– upper limit: 10 mm

Chloromethane (CH3Cl) feed

– 108 kmol/hr CH3Cl

Remark: We will use the constant number of particles model in the fluidized bed and therefore the silicones particles will shrink need enough classes in the fine range to

get a good resolution


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Fluidized Bed Reactor Example - Feed Definitions

Silicone feed

– 54 kmol/hr silicone

– PSD described by RRSB distribution with d63,3 = 85 mu an dispersion parameter n = 2

Copper feed

– 0.1 kmol/hr copper

– PSD described by RRSB distribution with d63,3 = 200 mu an dispersion parameter n = 2


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Fluidized Bed Reactor Example - Heater and Mixer Setup

Heater

– Outlet temperature of 200 C is specified

– No pressure change

Mixer

– Specify outlet pressure of 2 bar

Remark: By default the mixer sets the outlet stream to the lowest inlet pressure. Since the stream from the cyclone (RECYCLE) will have a lower pressure as the solids inlet stream (TO-REAC) due to the pressure drop of the cyclone, we need the set the pressure in the mixer.


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Fluidized Bed Reactor Example - Gas Cyclone Setup

– Simulation mode is used (separation efficiency is calculated based on given geometry and stream data)

– Efficiency calculation according to Muschelkanutz is used to predict the grade efficiency curve

– Geometry of the gas cyclone is described by use of a geometry concept according to Stairmand. All measurements (e.g. vortex finder length etc.) are related to the main diameter of the cyclone.


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Fluidized Bed Reactor Example - Fluidized Bed Setup

Specifications

– Bed inventory is defined by given bed pressure drop of 60 mbar

– Minimum fluidization velocity is determined by use of the correlation according to Wen & Yu

– TDH model according to George and Grace is used

– Entrainment is modeled according to the correlation from Tasirin & Geldart (the default parameters of the correlation are used)


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Operating conditions

– Temperature in the vessel is set to 573 K

Geometry

– Height of the vessel is set to 4 meters

– Relative solids discharge location is 0.1 (0.4 meters from the bottom)

– Cross-section is circular with a height dependent diameter

0 – 2 meters:

1.5 meter diameter

2-3 meters:

extension of the diameter from 1.5 meter to 2 meter

3-4 meters:

2 meter diameter


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Gas Distributor

– Perforated plate with 6000 openings each 2 mm diameter is used

– Pressure drop is calculated based on geometry of the gas distributor and given orifice discharge coefficient

Reactions

– Shows the available and selected reaction sets

– For the time being no reaction set is selected no reaction will occur in the reactor

Remark: The predefined reaction set R-2 will be used later in the example

Remark: Heat exchanger tab is inactive since no heat exchanger streams have been connected to the block ( heat exchanger will not be considered in this example)


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PSD

– The PSD tab is inactive since no reaction sets have been selected

Convergence

– Use default parameters for all settings except the number of cells for bottom zone and dilute zone (freeboard)

– Set number of cells for bottom and dilute zone to 10 to speed up calculation


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Fluidized Bed Reactor Example - Calculator Setup

– Calculator is used to calculate CH3Cl conversion

– Switch to layout “calculator” for details


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Fluidized Bed Reactor Example - Run the Model and Review Results

Run the model

Open the layout flowsheet-results

Remark: For now the model does not consider any chemical reactions


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Fluidized Bed Reactor Example - Review Results

Results summary shows main results (e.g. height of bottom zone, pressure drops) of the fluidized bed

• Plots show solids volume concentration and superficial gas velocity as function of the vessel height

• Further plots (e.g. bubble diameter, pressure etc.) can be generated by use of the plot gallery

Custom table shows: - CH3Cl flow in the

FB exhaust gas - Superficial gas

velocity on the bottom and the top of the fluidized bed

- CH3Cl conversion


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Conversion of CH3Cl is zero, since no reactions have been defined

Superficial gas velocity on the top of the vessel is smaller than on the bottom due to extension of the vessel with height


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Fluidized Bed Reactor Example - Add Reactions

1 Open reactions input form of the fluidized bed

Click “New…” 2 3 Enter ID and click “OK”

4 Select “POWERLAW” as type

5 New reaction set is shown in the list of selected reaction sets

Remark: This adds a power law based kinetic to the reaction set. Other types available are GENREAL, USER etc.

6 Open input forms of the reaction set from the navigation pane


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For power law type the kinetic (with concentration basis molarity) is given as: ai are the exponents that are defined on the form on the left


7 Click “New…” to start defining the stoichiometry

8 Select the educts and products and enter the stoichiometry coefficients

Reaction: Si + 2CH3Cl (CH3)2SiCl2 9 Enter exponents for the kinetics and click close

𝑟 = 𝑘 ∙ 𝑇𝑛 ∙ 𝑒−𝐸𝑅∙𝑇 ∙ 𝐶𝑖

𝛼𝑖

Remark: The kinetic used here is only for demonstration purposes and NOT based on measured data or data from literature

Remark: Copper is used as the catalyst and therefore included in the kinetic but not in the stoichiometry of the reaction


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10 Open the “Kinetic” input form

11 Enter the kinetic parameters


• Select vapor as reacting base • Select reactor volume as rate basis • Enter values for parameter k and

activation energy • Select molarity as rate basis


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12 Click “Solids” button 13 Make solids specific settings

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Close reaction input forms and make sure that R-1 is selected as reaction set for the fluidized bed

15 Select “Constants number of particles” the PSD tab

Remark: Since silicone is consumed in the reaction the silicone particles will shrink, while the copper particle size will be unchanged


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Fluidized Bed Reactor Example - Run the Model and Review Results

Reinitialize & Run the model

Open the layout flowsheet-results reaction


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• In case of defined chemical reactions a plot of the gas phase composition is available in the plot gallery

Results summary shows main results (e.g. height of bottom zone, pressure drops) of the fluidized bed


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no chemical reaction chemical reaction

- CH3Cl conversion is 69.6% - Superficial gas velocity at the top of

the vessel dropped from ~0.24 m/s to 0.15 m/s due to reduction in volume as a result of the chemical reaction


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Investigate influence of Copper flow rate

Change copper flow rate from 0.1 kmol/hr to 0.3 kmol/hr and run the model

0.1 kmol/hr Cu 0.3 kmol/hr Cu

Increased copper flow rate leads to increased CH3CL

conversion (based on the defined kinetics)


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Add more reactions

The predefined reaction set R-2 contains the following reactions and corresponding kinetics

– 2 Si + 4 CH3Cl (CH3)3SiCl + CH3SiCl3

– Si + 3 CH3Cl (CH3)3SiCl + Cl2

– Si + 2 Cl2 SiCl4

– 2 CH3Cl C2H4 + 2 HCl

– Si + 2 HCl HSiCl3 + H2

Add the reaction set R-2 to the selected reaction sets in the fluidized bed, reinitialize and run the simulation


1

2 3


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Review Results

- CH3Cl conversion increased to 99.4%

- Superficial gas velocity at the top of the vessel dropped to 0.13 m/s due to reduction in volume as a result of the chemical reactions


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Summary

The fluidized bed model in Aspen Plus v8.4 allows to consider chemical reactions and their impact on the fluid-mechanics and the particle size of the material in the vessel

The reactions are defined by use of a reaction object

Download - Fluidized bed reactor demo.pdf

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