Understand vacuum-system fundamentals
Crude vacuum unit heavy vacuum gas-oil (HVGO) yield is significantly impacted by ejector-system performance, especially at conditions below 20 mmHg absolute pressure. A deepcut vacuum unit, to reliably meet the yields, calls for proper design of all the major pieces of equipment. Understanding vacuum ejector system impacts, plus minimizing their negative effects equals maximum gas yield. Ejector-system performance may be adversely affected by poor upstream process operations.
The impacts of optimum ejector performance are more pronounced at low flash-zone pressures. Gas-oil yield improvements for small incremental pressure reductions are higher at 8 mmHg than at 16 mmHg. Commercial operation of a column with a 4.0 mmHg top pressure and 10 mmHg flash-zone pressure is possible. Designing a deepcut vacuum unit calls for a balance between practical limits of furnace design, column diameter, utility consumption and ejector-system size. Commercial performance of a deepcut vacuum unit operating at a HVGO true boiling point (TBP) cutpoint of 1,150°F highlights the impact of off-design ejector performance on gas-oil yield. Understanding the vacuum ejector-system fundamentals is critical to maintaining gas-oil yields.
Ejector-system performance at deepcut vacuum column pressures may be independently or concurrently affected by:
• Atmospheric column overflash, stripper performance or cutpoint
• Vacuum column top temperature and heat balance
• Light vacuum gas-oil (LVGO) pumparound entrainment to the ejector system
• Cooling-water temperature
• Motive steam pressure
• Non-condensible loading, either air leakage or cracked light-end hydrocarbons
• Condensible hydrocarbons
• Intercondenser or aftercondenser fouling
• Ejector internal erosion or product build-up
• System vent back pressure....
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