Chapter 15 Distillation Systems ZHOU Xiantao ( ) E-mai

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Chapter 15 Distillation Systems
ZHOU Xiantao (???) E-mail Robert_zhoucn_at_163.com
Date04/11/2005
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1. terms in this chapter

1. Azeotropic mixture(???) with similar boiling
   points.
2. Bottoms product and distillation
3. Feed distributor
4. Final boiling point the heaviest component
   boils.
5. local flooding(????) excessive liquid flowing
   down a column blocks vapor flow up the column in
   one section.
6. All flooding local flooding expands to entire
   column.

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1. terms in this chapter

1. Overlap incomplete separation of a mixture.
2. Overloading operating a column at maxium
   condition.
3. Puking what happen is when the vapor is so great
   that it forces liquid up the column or out the
   overhead line.
4. Rectifying section(???) the upper section above
   the feed line. Lighter.
5. Stripping section(???)The section below the feed
   line.heavier.
6. Vacuum distillation

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2. overview of distillation

1. Definition distillation is a process that
   separates a substance from a mixture by its
   boiling point, which includes a feed system,a
   preheat system,the distillation column,the
   overhead system,and the bottom system.
2. Application petroleum products...
3. Process during the distillation process, lighter
   components with lower boiling point rise up the
   column, while heavier components with higher
   boiling point drop down.

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3. Three Distinct Operations3.1. Batch
Distillation
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3.2. Continuous Batch Distillation
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3.3. Modern Fractional Distillation
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4 Principles of Distillation4.1.Effected by
temperature4.1.1 Kinetic energy (Temperature)

1. Molecules of liquids are closer together than
   that of gases
2. The speed of molecules is proportional to the
   amount of heat
3. The molecules begin to speed up and move farther
   apart when it is been heated
4. The molecular motion is referred to as kinetic
   energy.
5. Its temperature determines the kinetic energy.

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4.1.2 sensible and latent heat

1. Sensible heat can be measured and
   observedtemperature rises when heated.
2. Latent heat cannot be measured, but can be
   observed---temperature stops rising and a great
   deal of vapor is produced when heated.

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4.1.Effected by temperature4.2.1 pressure
related to vaporization

1. Vapor pressure exerted by molecules escaping
   from the the liquid, which linked to the strength
   of the molecular bonds of substanceif it
   lower,the boiling point is high.
2. External pressure exerted by gravity, which
   force the molecules back into solution
   difficult.
3. Vacuum allow vapor molecules to escape the
   surface of the liquid

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4.2.2 boiling point

• Boiling point is the temperature at which
• Vapor pressure exceeds atmospheric pressure
• Bubbles become visible in the liquid
• Vaporization.

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4.2.3 boiling point effected by pressure

1. When the pressure increases the boiling point
   increases
2. The escape of molecules from the liquid is
   reduced proportionally
3. The vapor molecules are forced closer together
4. The vapor above the liquid could be forced back
   into solution.

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4.2.4 vacuum distillation

1. Vacuum distillation is process which distillates
   under any pressure below atmospheric.
2. vacuum affects the boiling point in the opposite
   way that positive pressure does
3. It encourages molecules to move apart.

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4.2.5 Useful condition and used situation of
Vacuum distillation

• Useful condition lower the boiling point,enhance
  the molecular to escape from the liquid,reduce
  energy cost,reduce damage by overheating.
• Used situation large heavy hydrocarbons, boiling
  point is high enough to damage equipment and
  product.

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4.2.6 considerations for pressure calculations

1. The liquid level on each tray puts pressure on
   the tray
2. Rising vapors encounter resistance at each
   plate,which slows them down,creating pressure
3. Each tray has a different mixture on it
4. Vapor pressure vary among trays
5. inlet feed tray pressure varies if feed
   composition changes

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4.2.6 considerations for pressure calculations

1. Flow rate changes affect tower pressure
2. Downcomer liquid level puts pressure on the
   plate
3. Reboiler discharge increases pressure where where
   it enters the tower
4. Cool reflux slows activity
5. Each tray has pressure from above,below,and with
   the stage.

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4.3.1 Daltons law

1. Daltons law
2. Application
3. fraction should be mole fraction

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4.3.2 Errors

1. Please revise following equitation on page 294
2. Percent component in vapor partial pressure 100
3. Here, the partial pressure should be the rate of
   partial pressure to total pressure

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4.3.3 Raoults law (?????)

• ???????????,???????????????,?????????????????????
  ????????????
• ??Pi???????????????Pig???????,??????????xi?????
  ?????????

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4.3.4 ??

1. ????????,???????????,??????C???P????F??????FCP
   2?
2. ?????????????,???C2???F2,?F2-222????????????
   ????????????????2???(????????????)??????????
3. ??,????????????,???t,??p,????y?????x?????,????????
   ??,???????????

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4.3.5???????

• ??????????,?????????,??????????.????????
• ??T?????X(?????y)????????
• ?????y?x????????
• ??,?????????,?????????,??t?????x?????y????
  ??????????t-x-y?t??y-x??????

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4.3.6 an example of equilibrium cures
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4.3.7 quality of a operator

1. Can understand the theory of distillation.
2. Can understand the key components of a
   distillation system
3. Years of field field experiment.

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4.3.8 ????????????

1. ??Dalton???????????(?????),?
2. ?????????K??????,??????,K?????,?????????????????,
   ????????,??????K???????,????????????
3. Revision

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4.4.1 heat balance and material balance
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5.1 basic components of a plate columns5.1.1
plate columns
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5.1.2 sieve tray-plate
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5.1.3 basic components of a plate columns

1. ??feed preheater, feed inlet, feed tray
   sidestream?
2. ???rectifying or enriching section above the
   feed tray has overhead, reflux line, condenser,
   outlet?
3. ??? stripping sectionbelow the feed tray
   bottom outlet, reboiler, outlets?
4. ????downcomer,weir(?),riser(??),control
   loops(??),pipes,pumps,valves, instrumentation,
   and computers?

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5.2 three basic designs

1. Bubble cap tray commonly

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5.2 three basic designs

• b. Sieve traythe simplest type of tray. Hole
  range 1-1/8 inch

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5.2 three basic designs

• c. Valve tray

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5.3 reboiler or steam injection

• 5.3.1 horizontal(????)

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5.3 reboiler or steam injection

• 5.3.2 kettlel

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5.3 reboiler or steam injection

• 5.3.3 stab-in reboiler

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5.3 reboiler or steam injection

• 5.3.4 natural circulation

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5.3 reboiler or steam injection

• 5.3.5 forced circulation

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5.4 overhead condenser

• 1. Total condenservapor is converted liquid
  100,one steam is to product tank or further
  processing the other is pumped back into the
  column as reflux.
• 2. Partial condenser is designed to return all
  of the condensed liquid back to the top traythe
  vapor is sent for further processing.

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5.5 reflux line

1. Aim to control or limited the overlap by
   returning part of the overhead product to the
   tower.
2. Total reflux all of the overhead product is
   returned into the tower. This type of operation
   is commonly used to test the tower or to put the
   system in standby.
3. It is easier and cheaper to run the tower on
   total reflux than to shut it down and then
   restart it.

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5.6 feed preheater

1. Aim avoid thermal shock and match the
   temperature set set point of the feed tray.
2. Temperature should be controlled.
3. The flow is adjusted to the feed ,or in some
   cases the preheaters steam rate is regulated.

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5.7 Variables should be controlled

1. Flux
2. Temperature
3. Level
4. Pressure
5. composition

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6. packed columns

1. Definition Packed towers use specially designed
   packing materials to provide more surface area
   for maximum contact between gases and liquids and
   less pressure drop as well as high efficiency.
2. Process In packed columns, liquid travels down
   through feed distributors, hold-down gird, random
   and structured packing,support plates,structured,
   girds,and finally into the bottom section hot
   vapors move countercurrent to the downward flow
   of liquid.
3. Demand Packed columns are designed for pressure
   drops between 0.2-0.6 inch of water per foot of
   packing medium

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6.1 packed column
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6.2.1 types of packed distillation columns

• Random packing(including discrete pieces of
  packing) which packing is randomly dumped or
  poured into a packed column.
• Structured packing has specific geometric shapes,
  like a mesh. It works best in columns requiring
  high loadings.
• Stacked packing is uniformly arranged inside a
  distillation column.
• Demand packing columns are designed for pressure
  drops between 0.2 and 0.6 inch of water per foot
  of packing medium

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6.2.2 Common packing
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6.3 material of shell

1. Material of shell is determined by the type of
   chemicals.
2. Typical material is metal, carbon steel,
   steel,stainless steel, special alloy,or
   nonferrous.
3. The other glass,ceramic,plastic,or wood
4. Or some overlap lined with brock,acid-resistant
   mortar.

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6.4 heat balance in a packed system6.4.1 systems
to maintain heat balance

1. Kettle or thermosyphon reboiler system
2. Direct injection of system.
3. Elactric heaters on the column
4. Steam tracing on the column
5. Furnace system

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6.5 packing supports

1. What It should hold up the combined weight of
   the packing and liquid under a variety of
   abnormal conditions such as pressure
   surges,localized flooding,and temperature and
   flow variations.
2. Packing support plates come in the following
   designs flat sieve plates, gas-injection support
   plates, gird supports,cone supports,and
   corrugated supports.

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6.5.1 picture of some packing supports
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6.6 bed limiters and hold-down
plates

1. Rolekeep fixed bed packing from migrating or
   fluidizing out of the sectionkeep the internal
   components of the column stable and in operating
   condition.
2. Bed limitersattached to the inside wall of the
   column, used on metal or plastic packing.
3. Hold-down plates rest on top of the packing.
   Used on carbon and ceramic packing,

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Hold-down plates and bed limiters
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6.7 vertical slant of packing column

1. Definition Vertical slant is the angle between
   axial of the column and the vertical line.
2. Cause of inclination soil movement, wind
   sway,poor foundation construction,or uneven
   thermal expansion. that causes uneven
   distribution of the wetting process and unbalance
   the internal vapor-liquid flow

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6.7 vertical slant of packing column

• Result forms channeling(liquid flow toward the
  downward-sloping end of the packed section)which
  decrease efficiency 5-10 for each degree of
  inclination
• Allowable value Structured packing provides a
  more stable efficiency environment with
  recommended tolerances between 0.2-0.5 degree.

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6.8 liquid distribution and redistribution

1. Common liquid distributor design are ladder
   pipe,spray distributor,perforated ring
   distributor,notched-trough distributor.tunnel
   orifice distributor,weir-riser distributor,and
   orifice pan distributor.
2. Redistributor pull the liquid off the walls and
   redirect it into the center.

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Liquid distributor
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7 plate distillation system

• A typical plate distillation system can involves
  five subsystems
• Feed system
• preheat system
• distillation column
• overhead system
• bottom system.

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7.1 feed system
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7.2 preheat system
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7.2.1 pay attention during preheating

1. The mixture flows throng the shell side of each
   exchanger.
2. A low-pressure steam system is used as the
   heating medium for the tube side of Ex703.
3. The hot pentane flowing from the bottom of c-702
   is used as the heating medium for Ex-702. The
   mixture enters Ex-702 at 70-80F, and exits at
   120F.
4. A product analyzer continuously monitors feed
   composition and its temperature when it goes out.

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7.3 process during distillation column

1. A 10-psi nitrogen blanket kept on the unit while
   the system is down.
2. During startup, cold feed is sent to the column
   at a quarter of normal flux to allow the steam
   heating systems to be engaged.
3. As cold feed enters the column, it drops from the
   feed tray to the bottom of the column.
4. The bottom outlet line feeds the reboiler.

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7.3 process during distillation column

1. As the temperature starts to increase. The flow
   rate is increased to 200GPM. At the same
   time,vapors begin to rise up the column and cool
   off, liquid begins to accumulate on each tray.
2. When the column first starts operating, product
   purity will be off spec and will continue until
   each set point and process variable matches unit
   specification.

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7.4 overhead system
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7.4.1 overhead system

1. Basic components the top of column, the overhead
   condenser, a drum, pumps, control loops.
2. FIC-703 maintains 420GPM cooling water to
   condenser Ex-704
3. LIC-703 maintained 50 level of liquid in D-702
4. P 709 transfers feed from D-702 to the tank farm
   (tk710) or back to the top of the column as
   reflux

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7.4.1 overhead system

1. An analyzer monitors reflux product purity to
   keep it at 98
2. TIC-703 is cascaded to the reflux flow control
   loop FIC-705 to control the top temperature at
   159 degree.
3. Controlling temperature is more important than
   controlling the flow rate. loop can

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7.5.1 Bottom system
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7.5.1 Bottom system

1. Temperature in the bottom of C-702 are controlled
   at 222C,Ex-705 utilizes a natural circulation
   feed system.
2. The levels are held at 50 by LIC-702 and allows
   the 92 hot pentane to flow to Ex-702 and then to
   TK-730.

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7.5.1 Bottom system

1. FIC-700 controls steam flow rates to the kettle
   reboiler,TIC-702 has a thermocouple lined-out
   conditions, and operates as the primary or maste
   controller for FIC-700.
2. An analyzer monitors the composition of the
   pentane bottoms product.

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8 trouble shooting

• 8.1 trouble shooting To check the difference
  between the set point and the process variable

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8 trouble shooting

• 8.2 the difference between the set point and the
  process variable may be a result of instrument
  error, pump problems, low bottom level,or vale
  problems

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8 trouble shooting 8.3 a full-scale process
upset is occurring.

• The initial problem was the partial failure of
  the steam valve(TIC2). look at the other process
  variable affected by the higher bottom
  temperature on the column.

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8.4 typical problems

1. Feed composition changes which should be
   controlled to ensure uniform operation
2. Pukingwhich can upset trays,push products up or
   down the column or out the overhead line.
3. Flooding which can block vapor flow up the
   column.
4. Equipment or instrumentation failure which can
   damage the operation of a distillation column.
5. temperature,flows,pressure,level and composition
   variables are closely tracked.

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9 summary

1. Principle distillation separates the basic
   components or fractions of a mixture by their
   individual boiling points. During the
   distillation process, a mixture is heated until
   it vaporizes, then is re-condensed on the trays
   or at various stages of column, where it is drawn
   off and collected in a variety of overhead,
   side-stream,and bottom receivers. The condensed
   liquid is referred to as the distillatethe
   liquid that does not vaporize in a column is
   called the residue

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9 summary

1. Process as vaporization occurs,the lighter
   component goes out of the tower in a vapor state
   and is passed over the cooling coils of a shell
   and tube condenser. As the hot vapor comes in
   contact with the coils, it condenses and is
   collected in the overhead accumulator.part of
   this product is sent to storage while the other
   is returned to the tower as reflux.

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9 summary

1. Heat balance is from a reboiler,the heaviest
   components of the tower are pulled into the
   reboiler and stripped of smaller molecules. The
   stripped vapors are returned to the column to
   provide heat and mass transfer to the separation
   process.
2. Equilibrium curves are used to determine the
   vapor composition above a boiling points and how
   one mixture differs from another. The larger the
   difference between boiling points ,the easier it
   is to separate the fraction by boiling point.
3. The temperature in a tower can be affected by
   pressure,flow,level,feed composition,reflux rate
   and upset trays.

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Thanks for Your Attention.