fluidized bed reactor demo.pdf

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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 |

    2

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    3

    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)

  • 2013 Aspen Technology, Inc. All rights reserved |

    4

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    5

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    6

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    7

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    8

    Fluidization in Aspen Plus - Fluidized Bed GUI

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

    Operation Tab

  • 2013 Aspen Technology, Inc. All rights reserved |

    9

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    10

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    11

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    12

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    13

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    14

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    15

    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)

  • 2013 Aspen Technology, Inc. All rights reserved |

    16

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    17

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    18

    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

  • 2013 Aspen Technology, Inc. All rights reserved |

    19

    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.

  • 2013 Aspen Technology, Inc. All rights reserved |

    20

    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 effici

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