(20d) Atmospheric Distillation Process Design - Fundamental Concepts

Crude oil processing is the first step in the petroleum refinery. This distillation occurs in the refinery Crude Unit. The objectives of the unit include rejection of major contaminants and an initial cut of the crude oil into streams for further processing. Understanding the fundamentals of crude unit operation is critical to effective design, modification, operation and troubleshooting refinery crude units.

Fundamentals of the unit operation with focus on two areas is included. The areas are:

• Yield and separation measures for analysis and comparison
• Inherent unit performance balancing heat recovery versus product separation

The basic approach in yield analysis is the cut-point evaluation. This links unit performance to standard measures used in crude assays. Cut-point analysis is useful for comparing different crudes in the same unit or the same crude in different units. Cut-point analysis is also a key in economic evaluation and linear program modeling for crude selection. Gap-overlap is the major concept in separation analysis. Separation between products is a key to meeting specification requirements. How to evaluate both of these areas, and how they interact are explored.

Inherent unit performance in the atmospheric crude distillation is a balance of heat recovery versus product separation.  Steps that improve heat integration degrade product separation. Steps that improve product separation increase heat losses. However, depending upon the constraints imposed by the overall refinery objectives, some operating envelope will be available for balancing heat recovery versus product separation.

The heat balance versus product separation ties back to the gap-overlap analysis. In most units, the heat balance tightly links to separation performance. Understanding how these link together is key to understanding crude unit constraints.