Background

Vacuum towers are one of the simpler refinery units since they are not a conversion unit. However, vacuum units are very important since crude and vacuum units process all of the incoming crude. The operation and yields from crude and vacuum units affect all of the downstream operations.

Vacuum units have been improved over the years. Originally, many vacuum units had trays for mass transfer. In fuels type vacuum towers where low pressures improve HVGO recovery and profitability, trays were replaced with random packings. The packings had lower pressure drops than trays reducing the flash zone pressures. Structured packings were successfully installed in many units. Structured packing had even higher capacity than random packings. Structured packing is the dominant contacting device and vacuum tower improvements have allowed further reductions in operating pressure.

The improved vacuum tower operation has met the demands of the ever heavier world crude slate. Increasing HVGO recovery not only provides a large economic incentive per barrel, but it also may reduce the need for capital expenditure. Reduced resid production resulting from deep cut vacuum tower operation helps mitigate the effects of heavier crude slates reducing the need for additional coking capacity. Many refineries can increase their FCCU capacity by taking advantage of the latest riser designs.

At lower tower pressures, the design of the packing and distributors has become even more critical. Poor vapor or liquid distribution can lead to premature flooding, or coking of the structured packing. Design of the vacuum tower feed internals is also critical because of the high velocity of the two phase inlet. The feed inlet design should promote good separation of the vapor and liquid feed, and also provide good distribution of the vapor leaving the flash zone.

Fully evaluating the benefits of deep cut vacuum tower operation requires an engineering study. The crude type, furnace, tower, exchanger, and vacuum jet equipment all play important roles in the ultimate capacity, low pressure capability, and revamp cost of each specific unit. Specific conclusions cannot be reliably made without studying all of these and other factors. However, scoping economic judgments can be made to determine project feasibility's and potential project payouts.