(pitch or asphalt)
Figure 3.1. Lube type vacuum tower with pumpback reflux heat removal.
VAC. BOTTOMS f AS PH AIT P ITCH I
Figure 3.3. Fuels type vacuum tower.
VAC. BOTTOMS f AS PH AIT P ITCH I
that all trays are true fractionating trays since all internal liquids provide equilibrium reflux whereas the liquids in the pumparound sections are foreign with respect to the composition of the vapors passing through these sections. For this reason, a pumparound section tray is usually considered to be only 30 to 50 percent as effective as other tiays in the tower. Thus, a tower with pumparound reflux must either be provided with more trays, usually one in each section, or sacrifice a certain amount of fractionation capability. Since trays require pressure drop to be effective, anything which minimizes tray requirements is directionally correct.
Note the two different types of draw trays shown in the illustrations. Figure 3.1 uses the chimney type in which the vapor flows through risers, and the liquid is collected on the tray deck. Figure 3.2 utilizes a fractionating tray with a sealed downcomer in which the liquid is collected. Obviously, a Figure 3.2 arrangement will provide more trays for a given pressure drop although a Figure 3.1 arrangement is a better way of collecting and removing the liquid. Detailed design considerations for chimney trays have been presented in a recent article (1). Tray design principles for low pressure drop applications are adequately discussed in various manufacturer's bulletins (2,3). In recent years, the trend has been toward the greater use of sieve trays in vacuum towers because of their lower cost and inherently lower ultimate minimum pressure drop. Bubble-caps are rarely used any longer.
The number of trays between draw trays is set rather arbitrarily. Nelson (4) recommends using 3 to 5 trays between the draws. If one employs fractionating type draw trays, this would indicate four trays between draws at the maximum since the draw counts as a tray. If chimney type draw trays are used, one more tray in the section would be required to provide the equal degree of fractionation.
The manufacture of distillates either directly for fuels or for feed to downstream processing units ordinarily does not require any particular degree of fractionation between cuts. Also, wide cuts are usually acceptable. For these reasons, the distillates can be condensed by cooled pumparound reflux, grid type contacting sections and chimney draw trays. For all practical purposes, the operation of the main condensing sections can be described as a single-stage equilibrium condensation.
Figure 3.3 shows a typical fuels type tower. Note that the overflash liquid is condensed by cooled pumpback reflux rather than by another pumparound circuit. There are two reasons for this. First, another pumparound circuit would require significant additional investment. Secondly, it is generally believed that the materials which contaminate the heavy vacuum gas oil are vaporized upward from the flash zone rather than being entrained upward. Accordingly, product quality is more easily maintained by refluxing this section rather than by a high density washing with cool oil.
The grid materials used in these towers have been developed only recently. The process design of the grid sections is discussed thoroughly in the manufacturers' literature (5).
In the design of any vacuum tower, the first question to be settled is the selection of the optimum operating pressure of the system. In order to simplify this discussion, let us consider certain facts, assuming that a maximum allowable flash zone temperature has been set.
Was this article helpful?