(325a) Atmospheric Preflash Drums

Current trends in the crude petroleum supply are making large quantities of lighter crudes available. These crudes have API gravities of 40° or higher (SG of less than 0.8251). Over the last 20 years the refining industry invested heavily in capability for processing heavy crudes. Running light crudes stresses the capacity of the atmospheric crude unit in these plants. The most common method of debottlenecking light crude capacity adds a preflash drum or preflash column to the crude unit. This presentation examines adding a preflash drum and how to connect this to the existing unit.

Four major topics covered are:

• Influence of soluble water and entrained water on crude vaporization.
• Connection of the preflash to the atmospheric column in integrated units.
• Basic preflash drum configurations.
• Consequences of crude composition changes.

Atmospheric preflash towers and drums are the third most common type of crude distillation tower. In spite of the number of preflash installations relatively little public discussion has focused on them.

Most preflash application add a drum an existing crude unit. Some units have preflash drums drums installed during initial construction. General cases that make preflash drum installations attractive include:

• To improve heat integration – add to existing unit.
• Prevent vaporization upstream of heater pass balancing valves.
• To increase capacity – add to existing unit.
• Constraint in crude preheat hydraulics.
• Constraint in fired heater pressure drop.
• Constraint in atmospheric tower capacity.
• Constraint in atmospheric condenser capacity, duty, or pressure drop.
• To dehydrate crude – add to existing unit or include in a new unit.
• Helps prevent vaporization upstream of fired heater.
• Reduces fired heater pressure drop.
• To reduce preheat fouling – add to existing unit or include in a new unit.
• Remove light paraffinic material to reduce asphaltene fouling in the hot train exchangers.

As the different preflash configurations are examined, the discussion will highlight the benefits and costs of each option.