Crude Distillation Chapter 4

Transcription

1 Crude Distillation Chapter 4

2 Gases Gas Sat Gas Plant Polymerization LPG Sulfur Plant Sulfur Alkyl Feed Alkylation Butanes Fuel Gas LPG Gas Separation & Stabilizer Light Naphtha Heavy Naphtha Isomerization Naphtha Hydrotreating Naphtha Reforming Isomerate Polymerization Naphtha Alkylate Reformate Naphtha Aviation Gasoline Automotive Gasoline Solvents Atmospheric Distillation Crude Oil Desalter Vacuum Distillation AGO LVGO HVGO Distillate Gas Oil Hydrotreating Kerosene Fluidized Catalytic Cracking Hydrocracking Cat Distillates Cycle Oils Cat Naphtha Fuel Oil Distillate Hydrotreating Treating & Blending Jet Fuels Kerosene Solvents Heating Oils Diesel Residual Fuel Oils DAO Solvent Deasphalting Coker Naphtha SDA Bottoms Naphtha Asphalts Vacuum Residuum Visbreaking Coking Heavy Coker Gas Oil Light Coker Gas Oil Distillates Fuel Oil Bottoms Solvent Dewaxing Lube Oil Waxes Lubricant Greases Waxes Coke

3 Crude Distillation Technologies Provider Foster Wheeler Shell Global Solutions TECHNIP Uhde GmbH Features Complex of atmospheric & vacuum distilation for initial separation of crude oil. May include preflash column. Vaccum distillation

4 Typical Crude Tower Cut Points Cut Light Naphtha (LSR Gasoline) Heavy Naphtha TBP IBP ( F) 80 to to 220 TBP EP ( F) 180 to to 380 Distillate (Kerosene) AGO (Atm Gas Oil) LVGO (Light Vac Gas Oil) HVGO (Heavy Vac Gas Oil) 330 to to to to 1100 Vacuum Resid 950 to 1100

5 Overview Crude Stills Historically the oldest refining process Only the first step in crude oil processing Purpose To recover light materials Fractionate into sharp light fractions Configuration May be as many as three columns in series Crude Stabilizer/Preflash Column Reduce traffic in the Atmospheric Column Atmospheric Column Vacuum Column Reduced pressure to keep temperatures below 650 F

6 Feed Preheat Train & Desalter Feed Preheat Train Initial heat exchange with streams 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. Final heating in a direct fired heater Inlet typically 550 F Heated no higher than 750 F» Minimize thermal cracking Desalter Temperature must be carefully selected care taken not to let the water vaporize Lighter crudes (> F Heavier crudes (< F All crudes contain salts (NaCl, MgCl, ) Salt present in the emulsified water Treated in the field with heat & chemicals to break oil water emulsions. Salt can cause damage to equipment Scale in heat exchangers can drastically reduce heat transfer HCl formation can lead to corrosion Metals can poison refinery catalysts Remove salts & dissolved metals & dirt Oil mixed with fresh wash water & demulsifiers. Mixed & heated followed by settling 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

7 Atmospheric Distillation Summary Column Configuration Condenser Partial condenser if no Stabilizer Column. Total condenser 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. o As much as 50% of the incoming crude may be flashed. 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 t condense within the column. o Can lead to foaming.» Must be treated as sour water

8 Atmospheric Distillation Summary 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 Side Draws & Strippers Cut point related to final boiling point of draw stream Side strippers remove light component tail & return to main column Steam strippers traditional Reboiledstrippers reduce steam usage & associated sour water Trays & Pressure Profile Typically 32 trays in tower 0.1 to 0.2 psi per tray Condenser & accumulator 3 to 10 psi across condenser Liquid static head in accumulator Typically 6 to 16 psi across entire column.

9 Typical Configuration Atmospheric Column w/o Preflash Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD-ROM, 2000

10 Crude Electrostatic Desalting Drawings from: Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD-ROM, 2000

11 Overview Crude Stills Historically the oldest refining process Only the first step in crude oil processing Purpose To recover light materials Fractionate into sharp light fractions Configuration May be as many as three columns in series Crude Stabilizer/Preflash Column Reduce traffic in the Atmospheric Column Atmospheric Column Vacuum Column Reduced pressure to keep temperatures below 650 F

12 Vacuum Distillation Drawings from: Refining Overview Petroleum Processes & Products, by Freeman Self, Ed Ekholm, & Keith Bowers, AIChE CD-ROM, 2000

13 Vacuum Distillation Summary Column Configuration Vacuum conditions to keep operating temperatures low Large diameter column Very low density gasses Condenser for water vapor Liquid reflux from pumparounds No reboiler Stripping steam may be used Needed for deep cuts (1100 F) Feed Atmospheric residuum All of the vapor comes from the heated feed stream Under vacuum (0.4 psi) Separate higher boiling materials at lower temperatures Minimize thermal cracking Products May have multiple gas oils Separate products to increase the heat recovery in the column Products recombined downstream Usually FCCU via hydrotreating Vacuum resid Blended asphalt, heavy fuel oil Further processing thermal, solvent Choice depends on products & types of crude

14 Vacuum Distillation Summary Dry System 1050 F+ cut temperature & no stripping steam Smaller tower diameters Reduced sour water production Pressure profile Flash zone: mm Hg abs & 750 to 770 F. Top of tower: 10 mm Hg abs Deep Cut System 1100 F+ cut temperature & stripping steam Steam reduces hydrocarbon partial pressures Pressure profile Flash zone: 30 mm Hg abs Hydrocarbon partial pressure mm Hg abs Top of tower: 15 mmhg abs Steam Ejectors & Vacuum Pumps Vacuum maintained on tower overhead Steam systems considered more reliable Waste steam is sour & must be treated Combinations systems Last steam stage replaced with a vacuum pump

15 Summary Off spec product can either reprocessed or blended differently at the end. Pressure drops are important, especially in the vacuum column. Reuse the heat that you put via heat exchange between the feed & internal column streams. Refinery capacity is based on how much the atmospheric column can process. Metals are bad. Can remove some metals via desalters. Complex column configurations. No reboilers, heat from feed furnaces. Side draws, pumparounds, side strippers. Stripping steam. 3-phase condensers.

16 Summary Increase capacity with a Pre-flash column. Steam stripping aids in separation without cracking. Condensed water will have hydrocarbons & dissolved acid gases. Must be cleaned before discharged. Vacuum gas oils recombined only separated for operating considerations. Pumparoundsmay be required to ensure liquid reflux within the column. Vacuum columns very wide (short & fat).