Design Issues

There are many issues to be decided during the design of a new plant. Some firms want plants to run exactly at the design rate- no more and no less. Other firms want units designed conservatively so that the unit can be debottlenecked relatively easily to higher than design rates. Design conservatism also increases the processing flexibility for different feeds or for different product specifications. Some important design philosophies that maximize the debottlenecking flexibility of a new plant follow:

• Provide flexibility for other operating scenarios and conditions (e.g., start of run conditions, high conversion, higher impurity levels).

• Oversize specific critical equipment (vacuum systems, FCCU compressors, critical towers, etc.).

• Over-design utility systems (flare, cooling water, steam, etc.) if future expansion is desired.

• Design new towers and internals for less than 80% of flood.

• Install 2 pass trays instead of 1 pass trays when borderline.

• Do not install high capacity trays or packing.

• Do not install extended surface shell and tube exchangers.

• Purchase pumps and compressors that meet the design conditions with smaller than the maximum impeller.

• Install drivers and delivery systems that can handle the maximum power required by the driven equipment.

• Specify new equipment design pressures high enough so that they will not be violated at above design rates.