
INTERFACE ENERGY
Atmospheric Vacuum Distillation in U.S

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Crude Stills
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Historically the oldest refining process
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Only the first step in crude oil processingOnly the first step in crude oil processing
Purpose
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Torecover light materials
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Fractionate into sharp light fractions
Configuration — May be as many asthreecolumns in series
Crude Stabilizer/Preflash Column
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Reduce traffic in the Atmospheric Column
Atmospheric Column
Vacuum Column
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Reduced pressure to keep blow cracking temperatures
Reduct Yield Curves-Cut Point,Overlap,Tails
Feed Preheat Train Desalter
Feed Preheat Train
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Initial heatexchange withstreams from within the tower
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Heat recovery important to distillation economics!
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First absorb part of the overhead condensation load
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Exchange with one or more of the liquid sides streams, beginning with the top (coldest)side stream
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Require flexibility
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Changes in crude slate
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Temperature at desalter
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Limits on two‐phaseflowthroughnetwork
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Final heating in a direct fired heater
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Heat enough to vaporize light portions of the crude but temperature kept low tominimize thermal cracking
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Inlet typically 550oF, outlet 600 to 750oF.
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Heavier crudes cannot be heated to the higher temperatures
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Desalter
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Temperature carefully selected — do not let water vaporize
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Lighter crudes (> 40oAPI) @ 250oF
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Heavier crudes (< 30oAPI) @ 300oF
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All crudescontain salts (NaCl, MgCl, …)
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Saltpresent in theemulsified water
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Treated in thefield with heat & chemicals to break oil wateremulsions.
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Salt can cause damage to equipment
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Scale in heat exchangers
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HCl formation can lead to corrosion
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Metals can poison refinery catalysts
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Remove salts & dissolved metals & dirt
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Oil mixed with fresh wash water & demulsifiers.
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Separation in electrostatic settling drum
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Wash water up to 10% of crude charge
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~ 90% of the water can be recovered
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Effluent water treated for benzene
Crude Electrostatic Desalting



Crude Desalting

Breaking the crude oil/wateremulsion important to minimizedownstream problems
Performance of additives may be crude specific
Atmospheric Distillation Summary
Condenser …
§ Partial condenser if no Stabilizer Column.
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Totalcondenser if Stabilizer Column to remove light ends.
… but no reboiler.
Feed preheat exchanger train
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All of the heat to drive the column comes from the hot feed.
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As much as 50% of the incomingcrude may be flashed.
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“Overflash”
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Extraamount of material vaporized to ensurerefluxbetween flashzone & lowest side draw
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Typically 2 vol% of feed
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Wash Zone
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Couple trays between flash zone & gas oil draw.
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Reflux to wash resins & other heavy materials that may contaminate the products.
Condenser
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Typically 0.5 to 20 psig.
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Balancing act
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Low pressures reduce compression on overhead system
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High pressures decrease vaporization but increase flash zone temperatures & furnace duty;affects yields
Pumparounds
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Reduces overhead condenser load & achieves more uniform tower loadings
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Provides liquid reflux below liquid draws
Pumparounds
§ Move cooling down column.
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Liquid returned above draw tray
Side draws Side strippers
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“Clean up” side products
Stripping steam
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Reduce hydrocarbon partial pressure
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Condensed & removed as a second liquid phase.
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Conditions set so it doesn’tcondense within the column – can lead tofoaming
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Must be treated as sour water
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Side Draws & Strippers
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Sidestrippersremovelightcomponent“tail” & return to main column
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Steam strippers traditional
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Reboiled strippers reduce associated sour water & may reduce steam usage
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Trays & Pressure Profile
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Typically 32 trays in tower
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0.1 psi per tray for design & target for operation
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May find as high as 0.2 psi per tray, but probably flooding!
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Condenser & accumulator
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3 to 10 psi across condenser
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Liquid static head in accumulator
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Typically 6 to 16 psi across entire column.
Vacuum Distillation


Packing used in vacuum towers instead of trays
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Lower pressure drops across the tower – vapor “slides by” liquid instead of pushing through the layer on the tray
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Packing also helps to reduce foaming problems

Vacuum Distillation-Trays vs. Packing
Vacuum Distillation Summary
Column Configuration
Vacuum conditions to keep operating temperatures low
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Large diametercolumn
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Very low density gases
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Condenser only for water vapor
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Liquid reflux from pumparounds
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No reboiler
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Stripping steam may be used
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Needed for deep cuts (1100oF)
Common problem – coking in fired heater & wash zone
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Fired heater – high linear velocities to minimize cokeformation
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Washzone – sufficient wash oilflow to keep the middle of the packed bed wet
Feed
Atmospheric residuum
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All vapor comes from the heated feed
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Under vacuum (0.4 psi)
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Separate higher boiling materials at lower temperatures
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Minimize thermal cracking
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Products
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May have multiple gas oils
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Usually recombined downstream to FCCU after hydrotreating
Vacuum resid
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Blended — asphalt, heavy fuel oil
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Further processing — thermal, solvent
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o Depends on products & types of crude
Vacuum Distillation Summary
Dry System
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1050oF+ cut temperature & no stripping steam
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Smaller tower diameters
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Reduced sour water production
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Pressure profile
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Flash zone: 20‐25 mmHg abs & 750 to 770oF.
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Top of tower: 10 mmHgabs
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Deep Cut System
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1100oF+ cut temperature & strippingsteam
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Steam reduces hydrocarbon partial pressures
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Pressure profile
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Flash zone: 30 mmHg abs
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HC partial pressure 10‐15 mmHg abs
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Top of tower: 15 mmHg abs
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Steam Ejectors & Vacuum Pumps
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Vacuum maintained on toweroverhead
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Steam systems considered more reliable
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Waste steam is sour & must be treated
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Combinations systems — Last steam stage replaced with a vacuum pump

''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
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This will reduce the amount of “excess” heat to be “thrown away”to the environment
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This will also reduce the amount of “fresh” heat added to thesystem

Example-Exisiting Preheat Train

Example - Improved Preheat Train

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
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Imperfect separations result in light‐ends & heavy‐ends “tails” in adjacent products
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Presence of tails complicate the definition of “cut point”
Analysis
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Scale distillation curves to represent the volume removed
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“Cut point“ temperature represents the feed’s TBP corresponding the cumulative volume removed
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“Tail” represents the light fraction’s amount above the cut point & the heavy fraction’s amount below the cut point

Example- Atmospheric Tower Products


Example- Scale to Fraction of Crude Charge

Scale to Fraction of Crude Charge

Cut Points Overlaps for Example

Summary
Reported refinery capacity tied to charge to crude distillation complex
§Increase capacity with Pre‐flash column
Complex column configurations
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No reboilers, heat from feed furnaces
•Reuse heat via heat exchange between feed & internal column streams
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Side draws, pumparounds, side strippers
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Pumparounds ensure proper liquid reflux within the column
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Stripping steam
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3‐phase condensers
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Condensed water will have hydrocarbons & dissolved acid gases
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Pre‐heat train recycles heat
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Products & internal streams heat the feed
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Feed cools the internal streams & products
Vacuum column to increase the effective cut points
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Vacuum columns large diameter to keep vapor velocities low
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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
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Side cuts with strippers
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All battery limitsprocessfacilities
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Heatexchangetocool productstoambient temperature
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Central control system


Vacuum column includes
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Facilities for single vacuum gas oil
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3-stage vacuum jet system at 30-40 mmHg
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Heat exchange to cool VGO to ambient temperature
''Typical''Distillation Column

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