Fluidized bed reactor demo.pdf

Reaction in Fluidized Beds Guide to the Fluidized Bed Reactor Demo

Aspen Technology Burlington, MA 2013

Why Model a Fluidized Bed Reactors? 

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

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

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

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)

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

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

Model short description - Change in particle size

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Particle size distribution may change due to chemical reaction – available options that allow to calculate or set the bed PSD

Fluidization in Aspen Plus - Fluidized Bed GUI •

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Define bed inventory by defining bed pressure drop or bed mass Define voidage at minimum fluidization Select Geldart group for the bed material Specify minimum fluidization velocity or select a correlation to determine it

Define decay constant for the freeboard

Specifications Tab

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Select correlation used for the calculation of the TDH Specify gradient used for determination of TDH based on calculated solids volume concentration profile

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Select correlation used for the determination of the entrainment flow Overwrite correlation parameter if necessary

Fluidization in Aspen Plus - Fluidized Bed GUI Operation Tab Define temperature in the vessel by specifying either: heat duty temperature • •

Fluidization in Aspen Plus - Fluidized Bed GUI 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 • •

• •

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

Fluidization in Aspen Plus - Fluidized Bed GUI Define distributor pressure drop method Constant pressure drop Calculated based on geometry and given orifice discharge coefficient

Gas Distributor Tab

• •

Select distributor type Perforated plate Bubble caps • •

Define distributor geometry

Fluidization in Aspen Plus - Fluidized Bed GUI Heat Exchanger Tab

Define heat exchanger geometry

Define heat transfer coefficient Select if arithmetic or logarithmic temperature difference should be used Remark: - Heat exchanger input form is only active if streams are connected to the heat exchanger inlet and outlet

Fluidization in Aspen Plus - Fluidized Bed GUI Reactions Tab

Shows list of available reaction sets

Shows list of selected reaction sets

Select or remove reaction sets Add new reaction set

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

Fluidization in Aspen Plus - Fluidized Bed GUI PSD Tab

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

Remark: - PSD input form is only active if a reaction set is selected on the reactions input form

Fluidization in Aspen Plus - Fluidized Bed GUI Define solver tolerance and maximum number of solver steps

Convergence Tab

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

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

Define a flash parameter

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.) Silicone

Chloromethane

Silicon is mixed with copper (catalyst)

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(CH3)2SiCl2

Dimethyldichlorosilane

Entrained particles are separated with a gas cyclone and recycled

Chloromethane is used a fluidization gas

Fluidized Bed Reactor Example - Custom Table & Layouts 

Open file “fluidized bed reactor demo.bkp”

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

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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

Fluidized Bed Reactor Example - Feed Definitions 

PSD mesh – – – –

type: Logarithmic number of intervals 100 lower limit: 0.0001 mm upper limit: 10 mm

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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Chloromethane (CH3Cl) feed – 108 kmol/hr CH3Cl

Fluidized Bed Reactor Example - Feed Definitions 

Silicone feed – 54 kmol/hr silicone – PSD described by RRSB distribution with d 63,3 = 85 mu an dispersion parameter n = 2

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Copper feed – 0.1 kmol/hr copper – PSD described by RRSB distribution with d63,3 = 200 mu an dispersion parameter n = 2

Fluidized Bed Reactor Example - Heater and Mixer Setup

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Heater – Outlet temperature of 200 C is specified – No pressure change

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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.

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.

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)


Operating conditions – Temperature in the vessel is set to 573 K

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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


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

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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)

Fluidized Bed Reactor Example - Fluidized Bed Setup

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

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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

Fluidized Bed Reactor Example - Calculator Setup – Calculator is used to calculate CH3Cl conversion – Switch to layout “calculator” for details

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

Fluidized Bed Reactor Example - Review Results

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

Results summary shows main results (e.g. height of bottom zone, pressure

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Plots show solids volume concentration and superficial gas velocity as function of the vessel height Further plots (e.g. bubble diameter, pressure etc.) can

Fluidized Bed Reactor Example - Review Results 

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

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

Fluidized Bed Reactor Example - Add Reactions 1

Open reactions input form of the fluidized bed

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Select “POWERLAW” as type

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

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Click “New…”

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

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Enter ID and click “OK”

Open input forms of the reaction set from the navigation pane


Click “New…” to start defining the stoichiometry

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Enter exponents for the kinetics and click close

Select the educts and products and enter the stoichiometry coefficients

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Reaction: Si + 2CH3Cl

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(CH3)2SiCl2

For power law type the kinetic (with concentration basis molarity) is given as:

 −  ∙  =  ∙  ∙  ∙    the exponents that are defined on the form on the left a

i are

Remark: Copper is used as the catalyst and therefore included in the kinetic but not in the stoichiometry of the reaction Remark: The kinetic used here is only for demonstration purposes and NOT based on measured data or data from literature


Open the “Kinetic” input form

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Enter the kinetic parameters

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Select vapor as reacting base Select reactor volume as rate basis Enter values for parameter k and activation energy Select molarity as rate basis

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


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

Fluidized Bed Reactor Example - Run the Model and Review Results

Reinitialize & Run the model

Open the layout flowsheet-results reaction

Fluidized Bed Reactor Example - Review Results

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

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

Fluidized Bed Reactor Example - Review Results no chemical reaction

chemical reaction

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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

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

 Increased

0.3 kmol/hr Cu

copper flow rate leads to increased CH3CL conversion (based on the defined kinetics)

Add more reactions 

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

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2 Si + 4 CH 3Cl  (CH 3 )3SiCl + CH 3SiCl 3 Si + 3 CH 3Cl  (CH 3 )3SiCl + Cl 2 Si + 2 Cl 2  SiCl 4 2 CH 3Cl  C 2H 4 + 2 HCl Si + 2 HCl  HSiCl 3 + H 2

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

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

Review Results

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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

Fluidized bed reactor demo.pdf

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