Design of Columns for Distillation, Absorption, Stripping,

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Design of Columns for Distillation, Absorption,
Stripping, Liquid Extraction

• Dr. Avinash R. Sirdeshpande
• Process Solutions Engineer
• BOC Gases
• Murray Hill, NJ

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Separation Process Principles - 1Mixtures

• Mixtures 2 or more components
• Mixture classification
• Homogeneous - single phase, gas or liquid
• Air - N2, Ar, O2
• Dry syngas CO/H2/CH4/CO2
• Heterogeneous - multiple, immiscible phases
• Liquid liquid, e.g. oil water
• solid liquid
• solid gas
• liquid gas
• Mixing - spontaneous, irreversible

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Separation Process Principles - 2Separation Types

• Separation
• Mixture ? Products that differ in composition
• Classification
• Force field (mechanical)
• Gravity, pressure, electrical field, centrifugal
  force
• Heterogeneous mixtures
• Physical - exploit differences in physical
  properties
• vapor pressure, solubility, molecular geometry,
  surface activity, freezing point
• Chemical - based on differences in reactivity

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Separation Process Principles - 3Design

• Select appropriate separation process
• Distillation differences in boiling points
• Absorption/Stripping/Liquid Extraction
  solubility
• Mass or Energy Separating Agent
• Solvent for absorption, e.g. H2O for NH3/air
• Reflux, reboiling by adding/removing heat in
  distillation
• Design equipment for process
• Proposal/Budgetary stage
• Letter of intent
• Construction

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Column Design Process
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Column Process DesignFour Basic Principles

• Extent of separation
• of equilibrium stages, predicted with high
  confidence
• Hand calculation - McCabe-Thiele method
• Approximate methods - Kremser equation, FUG
  method
• Rigorous methods - computer simulation
• Time of phase contact
• Rate (Driving force)/Resistance
• Driving force departure from equilibrium
• Resistance (difficult to predict)
• operating conditions T, P, composition
• physical properties of both phases
• velocity, flow regime

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Column Process DesignFour Basic Principles

• Permissible pressure drop
• Fixes diameter of column
• Energy requirements
• Heat
• producing temperature changes
• creating a new phase
• overcoming heat of solution effects
• Mechanical/Electrical
• moving fluids from one location to another
• dispersing liquids and gases
• operate moving parts of machinery

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Efficiency

• Actual Trays or Height Theoretical
  Stages/Efficiency
• (Murphree) efficiency for tray columns
• Apply efficiency factor for actual trays
• Apply safety factor for final number of trays
• Height equivalent of a theoretical stage (HETP)
  for packed columns
• Multiply by HETP to get total height of packed
  section
• Difficult to predict from first principles
• Depends on contacting device, system, hydraulics
• Experimental data and correlations
• Vendor information (see brochure)

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Characteristics of Tray Columns

• Predictable hydraulic and mass transfer behavior
• Moderate to high pressure drop per tray
• Can be scaled to large diameters
• Low cost
• Suitable for fouling service
• Feed point flexibility is easy

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Characteristics of Packed Columns

• Low pressure drop / smaller diameter
• Random packing scale-up for HETP is difficult
  structured packing scale-up is predictable
• HETP prediction less well developed than for
  trays
• Low to moderate cost for random packing high
  cost for structured packing
• Not suitable for fouling service
• Feed point flexibility is difficult

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Specification of a Tray Column

• Number of actual stages
• Feed tray location
• Type of trays
• Tray spacing
• Tray layout
• Column diameter
• Column height
• Feed / Offtake arrangements / Nozzle sizes
• Reboiler / Condenser details

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Packed Column Specification

• Type of packing
• Random packing
• Structured packing
• Trade-off pressure drop vs. HETP vs. cost
• Column diameter
• Height of packing
• Feeds and offtakes

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Tray Column Height

• Column Height
• actual stages x tray spacing space allowance
  for feed/draws sump top volume
• Tray spacing for most applications is 18-24
  inches
• Rule of thumb could be to add 1- 2 nozzle
  diameters to the total height for feeds and draws
• Sumps sized on liquid residence time. Two to
  five minutes is typical.

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Packed Column Height

• HETP Replaces Tray Efficiency
• Bed Height No of Theoretical Stages x HETP
• HETP height equivalent to a theoretical plate
• HETP is difficult to generalize and is a function
  of the type of packing, the system being
  separated and the hydraulics of the column.
  Experimental or vendor-supplied values are used.

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Flooding and Weeping
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Column Diameter

• Column diameter is chosen to provide a
  comfortable range of operating between flooding
  and weeping. A typical operating range is about
  70 of the flooding velocity.
• The flooding velocity is determined by
  correlations
• An approach to flooding is used to get the actual
  superficial velocity (based on column diameter).
• Downcomers take up about 5-20 of column area
• Diameters should be rounded to standard dimensions

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Additional Topics in Column Design

• Process Control - Steady state vs. Unsteady state
• PID development
• Synthesis of separation sequences
• Separation of difficult mixtures
• Azeotropes
• Multiple liquid phases
• Novel distillation designs