Distillation of Binary Mixtures

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Distillation of Binary Mixtures
Chapter7
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• Purpose and Requirements
• Know the importance and mechanism of separation
• Learn to select feasible separation process for
  an industry process

• Key and Difficult Points
• Key Points
• Mechanism of Separation
• Component Recoveries and Product Purities
• Separation Power
• Selection of Feasible Separation Processes
• Difficult Points
• Mechanism of Separation
• Selection of Feasible Separation Processes

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Outline

• 6.1 EQUIPMENT
• 6.2 GENERAL DESIGN CONSIDERATIONS
• 6.3 GRAPHICAL EQUILIBRIUM-STAGE METHOD FOR TRAYED
  TOWERS
• 6.4 ALGEBRAIC METHOD FOR DETERMINING THE NUMBER
  OF EQUILIBRIUM STAGES
• 6.5 STAGE EFFICIENCY
• 6.6 TRAY CAPACITY, PRESSURE DROP, AND MASS
  TRANSFER
• 6.7 RATE-BASED METHOD FOR PACKED COLUMNS
• 6.8 PACKED COLUMN EFFICIENCY, CAPACITY, AND
  PRESSURE DROP
• 6.9 CONCENTRATED SOLUTIONS IN PACKED COLUMNS

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Absorption (Gas Absorption/Gas Scrubbing/Gas
Washing??)

• Gas Mixture (Solutes or Absorbate)
• Liquid (Solvent or Absorbent)
• Separate Gas Mixtures
• Remove Impurities, Contaminants, Pollutants, or
  Catalyst Poisons from a Gas(H2S/Natural Gas)
• Recover Valuable Chemicals

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• Physical Absorption

• Chemical Absorption
• (Reactive Absorption)

Figure 6.1 Typical Absorption Process
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Absorption Factor(A????)

• A L/KV
• Component A L/KV K-value
• Water 1.7
  0.031
• Acetone 1.38
  2.0
• Oxygen 0.00006 45,000
• Nitrogen 0.00003 90,000
• Argon 0.00008 35,000

• Larger the value of A,Fewer the number of stages
  required
• 1.25 to 2.0 ,1.4 being a frequently recommended
  value

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Stripping (Desorption??)

• Stripping
• Distillation
• Stripping Factor(S????)
• S 1/ A KV/L

High temperature Low pressure is
desirable Optimum stripping factor 1.4.
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6.1 EQUIPMENT
trayed tower
packed column
bubble column
spray tower
centrifugal contactor
Figure 6.2 Industrial Equipment for Absorption
and Stripping
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Trayed Tower(Plate Clolumns???)
Figure 6.3 Details of a contacting tray in a
trayed tower
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(b) valve cap
(c) bubble cap
(a) perforation
(d) Tray with valve caps

Figure 6.4 Three types of tray openings for
passage of vapor up into liquid
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Froth
Liquid carries no vapor bubbles to the tray
below Vapor carries no liquid droplets to
the tray above No weeping of liquid through the
openings of the tray Equilibrium between the
exiting vapor and liquid phases is
approached on each tray.
(a) Spray(b) Froth(c) Emulsion(d)
Bubble(e)Cellular Foam
Figure 6.5 Possible vapor-liquid flow regimes for
a contacting tray
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Packed Columns
Figure 6.6 Details of internals
used in a packed column
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Packing Materails

• More surface area for mass transfer
• Higher flow capacity
• Lower pressure drop

(a) Random Packing Materials
(b) Structured Packing Materials

• Expensive
• Far less pressure drop
• Higher efficiency and capacity

Figure 6.7 Typical materials used in a packed
column
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6.2 ABSORBER/STRIPPER DESIGN

• 6.2.1 General Design Considerations
• 6.2.2 Trayed Towers
• 6.2.2.1 Graphical Equilibrium-Stage
• 6.2.2.2 Algebraic Method for Determining
• the Number of Equilibrium
• 6.2.2.3 Stage Efficiency
• 6.2.3 Packed Columns
• 6.2.3.1 Rate-based Method
• 6.2.3.2 Packed Column Efficiency, Capacity,
• and Pressure Drop

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6.2.1 General Design Considerations
Design or analysis of an absorber (or stripper)
requires consideration of a number of factors,
including

• 1. Entering gas (liquid) flow rate, composition,
  temperature, and pressure
• 2. Desired degree of recovery of one or more
  solutes
• 3. Choice of absorbent (stripping agent)
• 4. Operating pressure and temperature, and
  allowable gas pressure drop
• 5. Minimum absorbent (stripping agent) flow rate
  and actual absorbent (stripping agent) flow rate
  as a multiple of the minimum rate needed to make
  the separation

6. Number of equilibrium stages 7. Heat effects
and need for cooling (heating) 8. Type of
absorber (stripper) equipment 9. Height of
absorber (stripper) 10. Diameter of absorber
(stripper)
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SUMMARY

• 1. A binary liquid and/or vapor binary mixture
  can be separated into two nearly pure products
  (distillate and bottoms) by distillation,
  provided that the value of the relative
  volatility of the two components is high enough,
  usually greater than 1.05.
• 2. Distillation is the most mature and widely
  used separation operation, with design procedures
  and operation practices well established.
• 3. The purities of the products from distillation
  depend on the number of equilibrium stages in the
  rectifying section above the feed entry and in
  the stripping section below the feed entry, and
  on the reflux ratio. Both the number of stages
  and the reflux ratio must be greater than the
  minimum values corresponding to total reflux and
  infinite stages, respectively. The optimal
  reflux-to-minimum-reflux ratio is usually in the
  range of 1.10 to 1.50.
• 4. Distillation is most commonly conducted in
  trayed towers equipped with sieve or valve trays,
  or in columns packed with random or structured
  packings. Many older towers are equipped with
  bubble-cap trays.
• 5. Most distillation towers are equipped with a
  condenser, cooled with cooling water to provide
  reflux, and a reboiler, heated with steam, to
  provide boilup.

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• 6. When the assumption of constant molar overflow
  is valid in each of the two sections of the
  distillation tower, the McCabe-Thiele graphical
  method is convenient for determining stage and
  reflux requirements. This method facilitates the
  visualization of many aspects of distillation and
  provides a procedure for locating the optimal
  feed-stage location.
• 7. Miscellaneous considerations involved in the
  design of a distillation tower include selection
  of operating pressure, type of condenser, degree
  of reflux subcooling, type of reboiler, and
  extent of feed preheat.
• 8. The McCabe-Thiele method can be extended to
  handle Murphree stage efficiency, multiple feeds,
  side streams, open steam, and use of
  interreboilers and intercon-densers.
• 9. Rough estimates of overall stage efficiency,
  defined by (6-21), can be made with the Drickamer
  and Bradford, (7-42), or O'Connell, (7-43),
  correlations. More accurate and reliable
  procedures use data from a small Oldershaw column
  or the same semitheoretical equations for mass
  transfer in Chapter 6 that are used for
  absorption and stripping.
• 10. Tray diameter, pressure drop, weeping,
  entrainment, and downcomer backup can all be
  estimated by the procedures in Chapter 6.

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• 11. Reflux and flash drums are sized by a
  procedure based on avoidance of entrainment and
  provision for adequate liquid residence time.
• 12. Packed column diameter and pressure drop are
  determined by the same procedures presented in
  Chapter 6 for absorption and stripping. , .
• 13. The height of a packed column may be
  determined by the HETP method, or preferably from
  the HTU method. Application of the latter method
  is similar to that of Chapter 6 for absorbers and
  strippers, but differs in the manner in which the
  curved equilibrium curve must be handled, as
  given by (7-47).
• 14. The Ponchon-Savarit graphical method removes
  the assumption of constant molar overflow in the
  McCabe-Thiele method by employing energy balances
  with an enthalpy-concentration diagram. However,
  the Ponchon-Savarit method has largely been
  supplanted by rigorous computer-aided methods.

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REFERENCES

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• Chan tf and j R Fair /nd ng Chem Pfocess Des
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• TJST' J A A'B- Hil!- N-H- Hochgrof, and D.B.
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• 21. Ponchon, M., Tech. Moderne, 13, 20, 55
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• T" m Distillation Columns, Final Report from the
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• fttlaware, AIChE, New York (1958).
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