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Atmospheric  Vacuum Distillation in U.S

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Crude Stills​

  • Historically the oldest refining     process

  • Only the first step in crude oil processingOnly the first step in crude oil processing


Purpose

  • Torecover light materials

  • Fractionate into sharp light fractions

Configuration — May be as many asthreecolumns in series
  Crude Stabilizer/Preflash Column

  • Reduce traffic in the Atmospheric Column
     

   Atmospheric Column
   Vacuum Column

 

  • Reduced pressure to keep blow cracking temperatures

Reduct Yield Curves-Cut Point,Overlap,Tails

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Atmospheric Vacuum
Tower Complex

Atmospheric Vacuum
Tower Complex

Atmospheric Column With Preflash

Preflash Options- Tight Oil Example With No AGO

Feed Preheat Train Desalter

Feed Preheat Train

  • Initial heatexchange withstreams from within the tower

    • Heat recovery important to distillation economics!

      • First absorb part of the overhead condensation load

      • Exchange with one or more of the liquid sides streams, beginning with the top (coldest)side stream

    • Require flexibility

      • Changes in crude slate

      • Temperature at desalter

      • Limits on two‐phaseflowthroughnetwork

  • Final heating in a direct fired heater

    • Heat enough to vaporize light portions of the crude but temperature kept low tominimize thermal cracking

      • Inlet typically 550oF, outlet 600 to 750oF.

      • Heavier crudes cannot be heated to the higher temperatures

Desalter

  • Temperature carefully selected — do not let water vaporize

    • Lighter crudes (> 40oAPI) @ 250oF

    • Heavier crudes (< 30oAPI) @ 300oF

  • All crudescontain salts (NaCl, MgCl, …)

    • Saltpresent in theemulsified water

  • Treated in thefield with heat & chemicals to break oil wateremulsions.

    • Salt can cause damage to equipment

      • Scale in heat exchangers

      • HCl formation can lead to corrosion

      • Metals can poison refinery catalysts

  • Remove salts & dissolved metals & dirt

  • Oil mixed with fresh wash water & demulsifiers.

    • Separation in electrostatic settling drum

  • Wash water up to 10% of crude charge

    • ~ 90% of the water can be recovered

  • Effluent water treated for benzene

Crude Electrostatic Desalting

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Crude Desalting

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Breaking the crude oil/wateremulsion important to minimizedownstream problems 

Performance of additives may be crude specific

Direct Fired Heater

Atmospheric Distillation Summary

Condenser …

§  Partial condenser if no Stabilizer Column.

  • Totalcondenser if Stabilizer Column to remove light ends.

… but no reboiler.

Feed preheat exchanger train

  • All of the heat to drive the column comes from the hot feed.

    • As much as 50% of the incomingcrude may be flashed.

    • “Overflash”

      • Extraamount of material vaporized to ensurerefluxbetween flashzone & lowest side draw

      • Typically 2 vol% of feed

 

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Wash Zone

  • Couple trays between flash zone & gas oil draw.

  • Reflux to wash resins & other heavy materials that may contaminate the products.

Condenser

  • Typically 0.5 to 20 psig.

  • Balancing act

  • Low pressures reduce compression on overhead system

  • High pressures decrease vaporization but increase flash zone temperatures & furnace duty;affects yields

Pumparounds

  • Reduces overhead condenser load & achieves more uniform tower loadings

  • Provides liquid reflux below liquid draws

Pumparounds

§  Move cooling down column.

  • Liquid returned above draw tray

Side draws Side strippers

  • “Clean up” side products

Stripping steam

  • Reduce hydrocarbon partial pressure

  • Condensed & removed as a second liquid phase.

    • Conditions set so it doesn’tcondense within the column – can lead tofoaming

      • Must be treated as sour water

Side Draws & Strippers

  • Sidestrippersremovelightcomponent“tail” & return to main column

  • Steam strippers traditional

    • Reboiled strippers reduce associated sour water & may reduce steam usage

Trays & Pressure Profile

  • Typically 32 trays in tower

  • 0.1 psi per tray for design & target for operation

    • May find as high as 0.2 psi per tray, but probably flooding!

      • Condenser & accumulator

  • 3 to 10 psi across condenser

    • Liquid static head in accumulator

  • Typically 6 to 16 psi across entire column.

Vacuum Distillation
 

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Packing used in vacuum towers instead of trays

  •  Lower pressure drops across the tower – vapor “slides by” liquid instead of pushing through the layer on the tray

  • Packing also helps to reduce foaming problems

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Vacuum Distillation-Trays vs. Packing
 

Vacuum Distillation Summary
 

Column Configuration

 Vacuum conditions to keep operating temperatures low

  • Large diametercolumn

  • Very low density gases

  • Condenser only for water vapor

  • Liquid reflux from pumparounds

  • No reboiler

  • Stripping steam may be used

  • Needed for deep cuts (1100oF)

Common problem – coking in fired heater & wash zone

  • Fired heater – high linear velocities to minimize cokeformation

  • Washzone – sufficient wash oilflow to keep the middle of the packed bed wet

Feed

Atmospheric residuum

  • All vapor comes from the heated feed

  • Under vacuum (0.4 psi)

  • Separate higher boiling materials at lower temperatures

    • Minimize thermal cracking

Products

  •  May have multiple gas oils

  • Usually recombined downstream to FCCU after hydrotreating

Vacuum resid

  • Blended — asphalt, heavy fuel oil

    • Further processing — thermal, solvent

o Depends on products & types of crude

Vacuum Distillation Summary

Dry System

  • 1050oF+ cut temperature & no stripping steam

  • Smaller tower diameters

  • Reduced sour water production

  • Pressure profile

  • Flash zone: 20‐25 mmHg abs & 750 to 770oF.

    • Top of tower: 10 mmHgabs

Deep Cut System

  • 1100oF+ cut temperature & strippingsteam

  • Steam reduces hydrocarbon partial pressures

  • Pressure profile

    • Flash zone: 30 mmHg abs

    • HC partial pressure 10‐15 mmHg abs

    • Top of tower: 15 mmHg abs

Steam Ejectors & Vacuum Pumps

  • Vacuum maintained on toweroverhead

  • Steam systems considered more reliable

    • Waste steam is sour & must be treated

  • Combinations systems — Last steam stage replaced with a vacuum pump

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''Composite Curve'' For Preheat Train

Compare amount of heat available at what temperatures
Goal is to shift the hot cold composite curves as close aspossible 

 “Pinch” technology

  • This will reduce the amount of “excess” heat to be “thrown away”to the environment

  • This will also reduce the amount of “fresh” heat added to thesystem

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Example-Exisiting Preheat Train

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Example - Improved Preheat Train

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Product Yield Curves- Cut Point,Overlap ''Tails''

Industrial distillation columns do not provide perfectly sharp separations
§Initial calculations using crude oil assays assume that all materials at a certain boiling point goes to one product or another

  • Imperfect separations result in light‐ends & heavy‐ends “tails” in adjacent products

  • Presence of tails complicate the definition of “cut point”

Analysis

  • Scale distillation curves to represent the volume removed

  • “Cut point“ temperature represents the feed’s TBP corresponding the cumulative volume removed

  • “Tail” represents the light fraction’s amount above the cut point & the heavy fraction’s amount below the cut point

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Example- Atmospheric Tower Products

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Example- Scale to Fraction of Crude Charge

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Scale to Fraction of Crude Charge

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Cut Points Overlaps for Example

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Summary

Reported refinery capacity tied to charge to crude distillation complex

§Increase capacity with Pre‐flash column

Complex column configurations

  • No reboilers, heat from feed furnaces

•Reuse heat via heat exchange between feed & internal column streams

  • Side draws, pumparounds, side strippers

    • Pumparounds ensure proper liquid reflux within the column

  • Stripping steam

  • 3‐phase condensers

    • Condensed water will have hydrocarbons & dissolved acid gases

  • Pre‐heat train recycles heat

    • Products & internal streams heat the feed

Feed cools the internal streams & products

Vacuum column to increase the effective cut points

  • Vacuum columns large diameter to keep vapor velocities low

  • Vacuum gas oils recombined – only separated for operating considerations

Pressure drops are important, especially in the vacuum column

Steam stripping aids in separation without cracking

Metals are undesirable. Can remove some metals via desalters.

Crude Distillation Unit Costs
 

Atmospheric column includes

  • Side cuts with strippers

  • All battery limitsprocessfacilities

  • Heatexchangetocool productstoambient temperature

  • Central control system

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Vacuum column includes

  • Facilities for single vacuum gas oil

  • 3-stage vacuum jet system at 30-40 mmHg

  • Heat exchange to cool VGO to ambient temperature

''Typical''Distillation Column​

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Top of column – condenser to remove heat
  • Provides liquid reflux through top of column
  • Partial condenser may have vapor but no liquiddistillate product
  • Coldest temperature – cooling media must be evencolder
  • Lowest pressure
  • Top section strips heavy components from the risingvapors
Feed
  • Vapor, liquid, or intermediate quality
  • Introduced in vapor space between trays
Internals
  • Trays to contact rising vapors with falling liquids
  • Pressure drop across trays – overcome static head ofliquid on tray, …
Bottom of column – reboiler to add heat
  • Provides vapor traffic in bottom of column
  • Highest temperature – heating media must be evenhotter
  • Highest pressure
  • Bottom section strips light components from thefalling liquid
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