Transcript
© 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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Fluidization in Aspen Plus - Fluidized Bed GUI
Define temperature in the vessel by specifying either: • heat duty • temperature
Operation Tab
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Fluidization in Aspen Plus - Fluidized Bed GUI
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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Fluidization in Aspen Plus - Fluidized Bed GUI
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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Fluidization in Aspen Plus - Fluidized Bed GUI
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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Fluidization in Aspen Plus - Fluidized Bed GUI
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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Fluidization in Aspen Plus - Fluidized Bed GUI
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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Fluidization in Aspen Plus - Fluidized Bed GUI
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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Fluidized Bed Reactor Example - Fluidized Bed Setup
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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Fluidized Bed Reactor Example - Fluidized Bed Setup
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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Fluidized Bed Reactor Example - Fluidized Bed Setup
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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Fluidized Bed Reactor Example - Review Results
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
Fluidized Bed Reactor Example - Add Reactions
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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Fluidized Bed Reactor Example - Add Reactions
10 Open the “Kinetic” input form
11 Enter the kinetic parameters
Remark: The kinetic used here is only for demonstration purposes and NOT based on measured data or data from literature
• 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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Fluidized Bed Reactor Example - Add Reactions
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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Fluidized Bed Reactor Example - Review Results
• 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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Fluidized Bed Reactor Example - Review Results
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
Remark: The kinetic used here is only for demonstration purposes and NOT based on measured data or data from literature
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
top related