chemistry and technology of petroleum

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chemistry and technology of petroleum

• By Dr. Dang Saebea

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REFINING CHEMISTRY
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Introduction

• Petroleum refining plays an important role in our
  lives.
• Most transportation vehicles are powered by
  refined products such as gasoline, diesel,
  aviation turbine kerosene (ATK) and fuel oil.

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the refining industry in three ways

• An increased search for fuel products from
  non-fossil sources such as biodiesel and alcohols
  from vegetable sources.
• The development of better methods to process tar
  sand, coal gasification and synthesis of fuels by
  new technology.
• The initiation of long-term plans to look for
  renewable energy sources.

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Refining Means. . .
1. To a pure state, to remove impurities 2. To
improve product
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Refining is carried out in three
main steps

• Step 1 Separation
• Step 2 Conversion
• Step3 - Purification

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Refining is carried out

• Step 1 - Separation
• The oil is separated into its constituents by
  distillation, and some of these components (such
  as the refinery gas) are further separated with
  chemical reactions and by using solvents.

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Refining is carried out

• Step 2 - Conversion
• The various hydrocarbons produced are then
  chemically altered to make them more suitable
  for their intended purpose.
• For example, naphthas are "reformed" from
  paraffins and naphthenes into aromatics.

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Refining is carried out

• Step3 - Purification
• The hydrogen sulfide gas which was extracted from
  the refinery gas in Step 1 is converted to
  sulfur, which is solid in liquid form to
  fertiliser manufacturers.

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Refinery-petrochemical integration
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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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3. Thermal Chemical Conversion Processes
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Refinery-petrochemical integration
1. Physical Separation Processes
Crude Distillation
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• Crude Distillation
• Crude oils are first desalted and then introduced
  with steam to an atmospheric distillation column.
  
• The atmospheric residue is then introduced to a
  vacuum distillation tower operating at about 50
  mmHg, where heavier products are obtained.

Atmospheric distillation
Vacuum distillation
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Refinery-petrochemical integration
1. Physical Separation Processes
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• Solvent Deasphalting
• This is the only physical process where carbon is
  rejected from heavy petroleum fraction such as
  vacuum residue.
• Propane in liquid form (at moderate pressure) is
  usually to dissolve the whole oil, leaving
  asphaltene to precipitate.
• The deasphalted oil (DAO) has low sulphur and
  metal contents since these are removed with
  asphaltene. This oil is also called Bright
  Stock and is used as feedstock for lube oil
  plant.
• The DAO can also be sent to cracking units to
  increase light oil production.

Solvent deasphalting process
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Refinery-petrochemical integration
1. Physical Separation Processes
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• Solvent Extraction
• In this process, lube oil stock is treated by a
  solvent, such as phenol and furfural, which can
  dissolve the aromatic components in one phase
  (extract) and the rest of the oil in another
  phase (raffinate).
• The solvent is removed from both phases and the
  raffinate is dewaxed.

Solvent Extraction
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Refinery-petrochemical integration
1. Physical Separation Processes
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• Solvent Dewaxing
• The raffinate is dissolved in a solvent (methyl
  ethyl ketone, MEK) and the solution is gradually
  chilled, during which high molecular weight
  paraffin (wax) is crystallized, and the remaining
  solution is filtered.
• The extracted and dewaxed resulting oil is called
  lube oil.
• In some modern refineries removal of aromatics
  and waxes is carried out by catalytic processes
  in all hydrogenation process

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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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• Catalytic Reforming
• In this process a special catalyst (platinum
  metal supported on silica or silica base alumina)
  is used to restructure naphtha fraction (C6C10)
  into aromatics and isoparaffins.
• The produced naphtha reformate has a much higher
  octane number than the feed. This reformate is
  used in gasoline formulation and as a feedstock
  for aromatic production (benzenetoluenexylene,
  BTX).

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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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• Hydrotreating
• This is one of the major processes for the
  cleaning of petroleum fractions from impurities
  such as sulphur, nitrogen, oxy-compounds,
  chloro-compounds, aromatics, waxes and metals
  using hydrogen.
• The catalyst is selected to suit the degree of
  hydrotreating and type of impurity. Catalysts,
  such as cobalt and molybdenum oxides on alumina
  matrix, are commonly used.

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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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• Catalytic Hydrocracking
• For higher molecular weight fractions such as
  atmospheric residues (AR) and vacuum gas oils
  (VGOs), cracking in the presence of hydrogen is
  required to get light products.
• In this case a dual function catalyst is used. It
  is composed of a zeolite catalyst for the
  cracking function and rare earth metals supported
  on alumina for the hydrogenation function.
• The main products are kerosene, jet fuel, diesel
  and fuel oil.

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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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• Catalytic Cracking
• Fluid catalytic cracking (FCC) is the main player
  for the production of gasoline. The catalyst in
  this case is a zeolite base for the cracking
  function.
• The main feed to FCC is VGO and the product is
  gasoline, but some gas oil and refinery gases are
  also produced.

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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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• Alkylation
• Alkylation is the process in which isobutane
  reacts with olefins such as butylene (C4 ) to
  produce a gasoline range alkylate.
• The catalyst in this case is either sulphuric
  acid or hydrofluoric acid. The hydrocarbons and
  acid react in liquid phase.
• Isobutane and olefins are collected mainly from
  FCC and delayed coker

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Refinery-petrochemical integration
2. Chemical Catalytic Conversion Processes
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• Isomerization
• Isomerization of light naphtha is the process in
  which low octane number hydrocarbons (C4, C5, C6)
  are transformed to a branched product with the
  same carbon number. This process produces high
  octane number products.
• One main advantage of this process is to separate
  hexane (C6) before it enters the reformer, thus
  preventing the formation of benzene which
  produces carcinogenic products on combustion with
  gasoline.
• The main catalyst in this case is a Pt-zeolite
  base.

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3. Thermal Chemical Conversion Processes
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• Delayed Coking
• This process is based on the thermal cracking of
  vacuum residue by carbon rejection forming coke
  and lighter products such as gases, gasoline and
  gas oils.
• The vacuum residue is heated in a furnace and
  flashed into large drums where coke is deposited
  on the walls of these drums, and the rest of the
  products are separated by distillation.

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• Flexicoking
• In this thermal process, most of the coke is
  gasified into fuel gas using steam and air.
• The burning of coke by air will provide the heat
  required for thermal cracking.
• The products are gases, gasoline and gas oils
  with very little coke.

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3. Thermal Chemical Conversion Processes
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• Visbreaking
• This is a mild thermal cracking process used to
  break the high viscosity and pour points of
  vacuum residue to the level which can be used in
  further downstream processes.
• In this case, the residue is either broken in the
  furnace coil (coil visbreaking) or soaked in a
  reactor for a few minutes (soaker visbreaker).
• The products are gases, gasoline, gas oil and the
  unconverted residue.

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• The End

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Crude Distillation
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Crude Distillation

• Crude distillation unit (CDU) is at the front-end
  of the refinery, also known as topping unit, or
  atmospheric distillation unit.
• It receives high flow rates hence its size and
  operating cost are the largest in the refinery.
• This involves the removal of undesirable
  components like sulphur, nitrogen and metal
  compounds, and limiting the aromatic contents.

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Typical products from the unit are
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Crude Oil Desalting

• The crude oil contains salt in the form of
  dissolved salt in the tiny droplet of water which
  forms a water-in oil emulsion.
• This water cannot be separated by gravity or
  through mechanical means.
• It is separated through electrostatic water
  separation. This process is called desalting.

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Crude Oil Desalting
In the electrostatic desalter, the salty water
droplets are caused to coalesce and migrate to
the aqueous phase by gravity. It involves mixing
the crude with dilution water (56 vol) through
a mixing valve.
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Poor desalting has the following effects

• 1. Salts deposit inside the tubes of furnaces and
  on the tube bundles of heat exchangers creating
  fouling, thus reducing the heat transfer
  efficiency
• 2. Corrosion of overhead equipment.
• 3. The salts carried with the products act as
  catalyst poisons in catalytic cracking units.

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Types of Salts in Crude Oil

• Salts in the crude oil are mostly in the form of
  dissolved salts in fine water droplets emulsified
  in the crude oil. The salts can also be present
  in the form of salts crystals suspended in the
  crude oil.
• These are mostly magnesium, calcium and sodium
  chlorides with sodium chloride being the abundant
  type.

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Types of Salts in Crude Oil

• These chlorides, except for NaCl, hydrolyze at
  high temperatures to hydrogen chloride
• Hydrogen chloride dissolves in the overhead
  system water, producing hydrochloric acid, an
  extremely corrosive acid

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Desalting Process

• The process is accomplished through the following
  steps
• 1. Water washing
• - Water is mixed with the incoming crude oil
  through a mixing valve.
• - The water dissolves salt crystals and the
  mixing distributes the salts into the water,
  uniformly producing very tiny droplets.
• - Demulsifying agents are added at this stage to
  aide in breaking the emulsion by removing the
  asphaltenes from the surface of the droplets.

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Desalting Process

• 2. Heating
• - The crude oil temperature should be in the
  range of 49-54 C (120130 F) since the wateroil
  separation is affected by the viscosity and
  density of the oil.

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Desalting Process

• 3. Coalescence
• - The water droplets are so fine in diameter in
  the range of 110 mm that they do not settle by
  gravity. Coalescence produces larger drops that
  can be settled by gravity.
• - This is accomplished through an
  electrostatic electric field between two
  electrodes.
• - The electric field ionizes the water
  droplets and orients them so that they are
  attracted to each other.
• - Agitation is also produced and aides in
  coalescence.

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Desalting Process

• 4. Settling According to Stocks law the
  settling rate of the water droplets after
  coalescence is given by
• where is the density
• is the viscosity,
• d is the droplet diameter
• k is a constant.

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Description of Desalter
Two electrodes
Simplified flow diagram of an electrostatic
desalter
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Description of Desalter
A primary field of about 600 V/cm
This field helps the water droplets settle faster.
Simplified flow diagram of an electrostatic
desalter
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Description of Desalter
A secondary field of about 1000 V/cm
The ionization of the water droplets and
coalescence takes place here
Simplified flow diagram of an electrostatic
desalter
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Two-stage desalting

• - The desalter of this design achieves 90 salt
  removal. However 99 salt removal is possible
  with two-stage desalters.
• A second stage is also essential since desalter
  maintenance requires a lengthy amount of time to
  remove the dirt and sediment which settle at the
  bottom.
• The crude unit can be operated with a one stage
  desalter while the other is cleaned.

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Desalter Operating Variables

• For an efficient desalter operation, the
  following variables are controlled
• Desalting temperature The settling rate depends
  on the density and viscosity of the crude
• Desalting temperature can vary between 50 and 150
  C
• Washing water ratio Adding water to the crude
  oil helps in salt removal.

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Desalter Operating Variables

• Water level
• - Raising the water level reduces the settling
  time for the water droplets in the crude oil
• However, if the water level gets too high and
  reaches the lower electrode, it shorts out the
  desalter.
• it is always better to keep the level constant
  for stable operation.

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Desalter Operating Variables

• Washing water injection point
• Usually the washing water is injected at the
  mixing valve.
• However, if it is feared that salt deposition
  may occur in the preheat exchangers, part or all
  of the washing water is injected right after the
  crude feed pump.

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Desalter Operating Variables

• Type of washing water
• Process water in addition of fresh water is used
  for desalting. The water should be relatively
  soft in order to prevent scaling.
• It should be slightly acidic with a pH in the
  range of 6. It should be free from hydrogen
  sulphide and ammonia so as to not create more
  corrosion problems.

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Atmospheric Distillation
330 C
Process flow diagram of an atmospheric
distillation unit
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Component of atmospheric distillation
Rectifying section
Flash zone
Stripping section
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Description of atmospheric distillation

• The vapor from pipestill furnace discharge as a
  foaming stream into distillation tower.
• The partially vaporized crude is transferred to
  the flash zone.
• The vapour goes up the tower to be fractionated
  into gas oil that is called the overhead product
  .
• liquid portion of feed go down to bottom of tower

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Description of stripping section

• Steam reboilers may take the form of a steam
  coil in the bottom of the tower or a separate
  vessel.
• The bottom product from the tower enters the
  rebolier where part is vaporized by heat from
  steam coil.
• The hot vapor is directed back to the bottom of
  the tower and the nonvolatile leaves the
  rebolier and passes through a heat exchanger,
  where its heat is transferred to the feed to the
  tower.

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Description of stripping section

• Steam is also injected into the column
• To strip the atmospheric residue of any light
  hydrocarbon.
• To lower the partial pressure of the hydrocarbon
  vapours in the flash zone. This has the effect of
  lowering the boiling point of the hydrocarbons
  and causing more hydrocarbons to boil and go up
  the column to be eventually condensed and
  withdrawn as side streams.

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Description of rectifying section

• As the hot vapours from the flash zone rise
  through the trays up the column, they are
  contacted by the colder reflux
• down the column.
• In the overhead condenser, the vapours are
  condensed and part of the light naphtha is
  returned to the column as reflux.
• Reflux is provided by several pumparound streams
  along the column.

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Improvement of distillation efficiency with
pumparound

• Vapours Cold liquid condenses
  Reflux
• To compensate for the withdrawal of products
  from the column.
• The addition to the heat removal from the
  condenser. The thermal efficiency of the column
  is improved and the required furnace duty is
  reduced.

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Description of rectifying section

• Liquid collects on each tray to a depth, and
  the depth controlled by a dam or weir. As the
  liquid spills over the weir into a channel, which
  carries the liquid to the tray below.

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Important of stripping and Rectifying section

• Stripping section
• The more volatile component are stripped from
  the descending liquid
• Rectifying section
• The concentration of the less volatile
  component in the vapor is reduced

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Straight-Run Naphtha and Gases
Heavy Naphtha
Kerosene
Gas Oil
Crude Oil

• The Temperature of tray is progressively cooler
  from bottom to top

Residuum
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The efficient operation of the distillation

• Tower requires the rising vapors to mix with
  liquid on each tray.
• This is usually achieved by installing a bubble
  caps.
• The cap forces the vapor to go below the surface
  of the liquid and to bubble up through it.

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Limiting temperature of atmosphereric distillation

• It is important not to subject the crude oil to
  temperatures above 350 C because the
  high molecular weight components in the crude oil
  will undergo thermal cracking and form petroleum
  coke .
• Formation of coke would result in plugging the
  tubes in the furnace and the piping from the
  furnace to the distillation column as well as in
  the column itself.
• The constraint imposed by limiting the column
  inlet crude oil to a temperature of less than 350
  C yields a residual oil from the bottom of the
  atmospheric distillation column.

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Vacuum Distillation

• To further distill the residual oil from the
  atmospheric distillation column, the distillation
  must be performed at absolute pressures as low as
  10 to 50 mmHg  so as to limit the operating
  temperature to less than 350C.
• Vacuum distillation is the reduced temperature
  requirement at lower pressures.
• Vacuum distillation increases the relative
  volatility of the key components.

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• Vacuum distillation can improve a separation by
• Prevention of product degradation or polymer
  formation because of reduced pressure leading to
  lower tower bottoms temperatures.
• Reduction of product degradation or polymer
  formation because of reduced mean residence time
  especially in columns using packing rather
  than trays.
• Reduction of capital cost because of reduced the
  height and diameter.

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Fractions obtained by vacuum distillation
used as catalytic cracking stock or, after
suitable treatment, light lubricating oil
Gas Oil
Atmospheric Residuum
Vacuum Distillation
Light
Lube oil
Medium
Heavy
used directly as asphalt or converted to asphalt
Residuum, Nonvolatile
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Operation of Crude Distillation Units

• The factors affect the design and operation of
  the unit are explored
• 1. Fractionation
• The degree of fractionation in a crude unit is
  determined by the gap or overlap between two
  adjacent side stream products.
• Example The gap or overlap in the boiling point
  range between kerosene and LGO.

Lighter product kerosene
end boiling point
initial boiling point
Heavier product LGO
In the ideal case there would be no overlap
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• However, if we compare the ASTM distillation
  boiling points, and since ASTM distillation does
  not give perfect fractionation. Since determining
  the initial and end point on the laboratory test
  is not always possible or accurate.
• The fractionation gap is defined as the
  difference between the ASTM 5 boiling point of
  the heavier product and the 95 point of the
  lighter product.

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a gap indicating good fractionation
some of the light product is still in the heavier
product
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2. Overflash

• The partially vaporized crude is transferred to
  the flash zone. The furnace outlet temperature
  should be enough to vaporize all products
  withdrawn above the flash zone plus about 35
  vol of the bottom product.
• This overflash has the function of providing
  liquid wash to the vapours going up the column
  from the flash zone.
• The overflash improve fractionation on the trays
  above the flash zone, thereby improving the
  quality of the HGO and reducing the overlap with
  the bottom products below the flash zone.

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3. Column Pressure

• The pressure inside the CDU column is controlled
  by the back pressure of the overhead reflux drum
  at about 0.20.34 bar gauge (35 psig).
• The top tray pressure is 0.40.7 bar gauge (610
  psig) higher than the reflux drum.
• The flash zone pressure is usually 0.340.54 bar
  (58 psi) higher than the top tray.

Pflash zone gt PTop tray gt Preflux drum
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4. Overhead Temperature

• The overhead temperature must be controlled to
  be 1417 C higher than the dew point
  temperature for the water at the column overhead
  pressure so that no liquid water is condensed in
  the column.
• This is to prevent corrosion due to the
  hydrogen chloride dissolved in liquid water
  (hydrochloric acid).

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Example

• If the overhead stream contains 8.5 mol water
  at a pressure of 34.7 psia (2.36 bars), calculate
  the overhead temperature for safe operation.

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Example

• If the overhead stream contains 8.5 mol water
  at a pressure of 34.7 psia (2.36 bars), calculate
  the overhead temperature for safe operation.

Solution The saturation temperature of water at
the partial pressure of water in the overhead
vapour. Water partial pressure 0.085 x 2.36
0.2 bars From the steam tables Saturated steam
temperature at 0.2 bars 61 C Safe overhead
operating temperature 6117 C
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• The End

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