The objective in this case is to produce maximum distillate and minimum pitch, which is used for fuel oil blending. In this case, the TBP cut point between the distillate and pitch has to be set by unit design, generally around 1100°F. From the crude assay data, the total distillate yield from the crude up to the cut point is known; deducting the total distillates yield in the atmospheric column, the total yield of vacuum distillate can be estimated. The light vacuum distillate yield is set at approximately 30% of the total vacuum gas oil, to facilitate heat recovery at two levels of heat.
The unit design has to specify the amount of overflash, depending on the purity of the heavy vacuum gas oil (HVGO) required. If the color requirements or level of metal contaminants is not severe, l-2vol% (volume %) of vacuum feed is taken as overflash.
Vacuum column design calculation is similar to atmospheric column design with some differences in technique as follows:
• A material balance is made for vacuum feed vs. the products— vacuum bottoms, sidestream products (vacuum gas oils), and overhead condensable hydrocarbons. The assumed quantity of noncon-densables is not carried in the material balance nor considered in the flash zone calculations but must be estimated for vacuum ejector calculations.
• The construction of flash vaporization curve (AFVC, atmospheric flush vaporization curve) of the reduced crude, feed to vacuum distillation unit is done in the same manner as for the whole crude.
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