Section 04 - Fractionation

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    FractionationFractionation

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    Task Checklist

    X Introduction

    TOWER SIMULATION

    Z Basic Concepts

    Z Specifications

    DEVICE SELECTION

    Z Contacting Devices

    Z Tray Hardware Definitions

    Z Tray Hydraulics

    Z Packing Hydraulics

    Z Other Process Considerations

    Z Other Tower Internals

    Z Tower Revamps

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    Did You Know?

    Oldest and most important petrochemical manufacturing process

    Equipment accounts for 30% of all investments today

    Consumes 70% of all energy at plant

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    More on Fractionation

    Separate feed into products

    according to boiling point

    Products sent to finished

    product tank or further

    processed by other units

    Multiple products and feeds

    common

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    Objective Questions

    How can you improve separation efficiency?

    How do you decide on the optimum number of trays for a new

    tower?

    What sets a towers operating pressure?

    What are my options to increase a towers vapor / liquid handlingcapacity?

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    Designers Role

    Perform HMB calculations to develop tower loadings

    New Towers Select optimum tower size, contacting device, and internals to meet

    process requirements

    Existing Towers

    Select optimum contacting device and internals to improve

    performance or expand capacity

    Screening - Apply Concepts to Troubleshoot Problems

    Prepare Design Package (Consult with Specialist)

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    Engineers Toolbox

    XOM DP Section III

    (Intranet)

    Simulation Tool (PRO/II or

    HYSIS)

    Pegasys ExxonMobil TowerInternals Program - EMoTIP

    Fractionation Specialists

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    Task Checklist

    X Introduction

    TOWER SIMULATION

    XBasic ConceptsZ Specifications

    DEVICE SELECTION

    Z Contacting Devices

    Z Tray Hardware Definitions

    Z Tray Hydraulics

    Z Packing Hydraulics

    Z Other Process Considerations

    Z Other Tower Internals

    Z Tower Revamps

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    Conventional Tower

    Reflux Ratio = R / D

    R

    D

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    Rules of Thumb

    More Reflux

    More Trays More Separation

    Lower Pressure

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    BINARY DISTILLATION: MCCABE - THIELE DIAGRAM

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    MINIMUM REFLUX

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    TOTAL REFLUX - MINIMUM STAGES

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    Stages Versus Reflux

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    Optimum Reflux Ratio

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    ESTIMATING ACTUAL REFLUX RATIO AND ACTUAL NUMBER OF STAGES

    General Rule of Thumb--

    RACTUAL = (1.1 TO 1.35) x R MIN

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    Task Checklist

    X Introduction

    TOWER SIMULATION

    X Basic ConceptsXSpecifications

    DEVICE SELECTION

    Z Contacting Devices

    Z Tray Hardware Definitions

    Z Tray Hydraulics

    Z Packing Hydraulics

    Z Other Process Considerations

    Z Other Tower Internals

    Z Tower Revamps

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    Factors to consider:

    Temperature of available condensing fluid (air, water, etc.)

    Wheres the overhead product go?

    Is the overhead totally condensed?

    Possible Limitations:+ Bottoms temperature (cracking / color)

    + Reboiler

    + Critical Point (poor separation)

    How much pressure drop is acceptable?

    Operating Pressure

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    Operating Temperature

    Fixed once product specifications and

    pressure are set

    Dew Point Calculation

    Overhead temp.

    Bubble Point Calculations

    Bottoms temp.

    Flash Calculations

    Reflux temp.

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    Feed Condition

    If not specified, start with bubble

    point feed - Then optimize.

    Adjust temp. to balance tower

    loading

    Other Considerations:

    Reflux vs. Reboiler Duty / Costs

    Quenching

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    HIGH FEED TEMPERATURE

    L

    RECYCLE GAS

    COMPRESSORPRODUCT

    FUEL GAS CONTROL:

    FEED RATE

    PRODUCT VAPOR PRESSURE

    PRODUCT SULFUR

    OVERHEAD DRUM LEVEL

    FEED

    OVERRIDE

    F

    A VAPOR PRESSURE

    SULFUR

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    Simulation Inputs

    Feed Rate and Composition

    VLE Data Method

    Specifications & Control

    Variables

    Operating Pressure

    Initial Guess (If Required)

    For Rating: Plant Test Data

    Pressure, Temperature Profile

    Lab Data (Complete Set of Samples)

    Tray Efficiencies

    See DP III-I Table 2; Estimates

    from Past Designs

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    Tower Specif ications

    Specifications: usually on product quality or temperatures Can be mathematical

    Avoid specifying rate. Better to specify % recovery or fraction.

    Condenser temperature

    Variables: usually condenser / reboiler duty

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    ProII Condensers

    Stage 1, if present

    Different Types

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    ProII Reboilers

    Last Stage(s)

    Kettle or Thermosiphon

    Kettle

    Thermosiphon specify if have baffle or not

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    KETTLE REBOILER

    Advantages:

    + Additional fractionation stage

    Disadvantages:

    - Low NPSH to downstream pump- Very low hold-up

    - Tubes can dry out

    - High temperature increase

    - Large plot space

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    VERTICAL THERMOSIPHON REBOILER

    -Advantages:

    + Partial fractionation stage

    + Low plot space

    + Low temperature increase

    Disadvantages:

    - Partial fractionation stage- High circulation rate

    - Pressure drop critical

    - Limited duty

    - Difficult to clean

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    HORIZONTAL THERMOSIPHON REBOILER

    -Advantages:+ Partial fractionation stage

    + Large duty

    + Low temperature increase

    + LowerP than vertical

    Disadvantages:- Partial fractionation stage

    - High circulation rate

    - Pressure drop critical

    - Large plot space

    - Large piping

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    Column Stage Summary

    Number of Stages Condenser (1)

    Theoretical Trays (#)

    Can also use Actual Trays * Tray

    Efficiency Efficiency will be different above and

    below feed

    Reboiler (1 or 2)

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    Simulation Strategy

    Optimize Reflux Ratio, Stages, Feed Condition, and Feed

    Location

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    Simulation Pitfalls

    Save / Backup cases often.

    Viscosity (high temperature), otherproperties may need to be verified.

    Others?

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    SIMULATION RESULTS

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    Task Checklist

    X Introduction

    TOWER SIMULATION

    X Basic Concepts

    X Specifications

    DEVICE SELECTION

    XContacting DevicesZ Tray Hardware Definitions

    Z Tray Hydraulics

    Z Packing Hydraulics

    Z Other Process ConsiderationsZ Other Tower Internals

    Z Tower Revamps

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    Device Categories

    Conventional Trays

    High Capacity Trays(Proprietary)

    Random Packing

    Structured Packing

    Baffles, Sheds, and Grid

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    Valve Up to 5:1 Turndown

    5-10% higher capacity over sieve

    Marginally higher cost

    Not recommended for fouling

    services

    Jet hExxon Design for High Liquid

    Loads

    Efficiency ~20% less than sieve

    Mainly used in pumparounds

    Conventional Trays

    Sieve Most Widely Used

    2:1 Turndown

    Low Cost (Nonproprietary)

    Bubble Cap Standard until 1950s

    High Cost

    Maintenance / Inspection Difficult

    Excellent Turndown

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    Pumparound Definition

    Method to provide towerreflux by cooling tower liquid

    Draw-off intermediate l