Vacuum tower foaming

Pressure drop. Packed towers are designed so that the pressure drop at any point in the tower does not exceed a recommended maximum value. Maximum pressure drop criteria for packed towers are listed in Table 8.4. For vacuum distillation, foaming systems, and where fan horsepower needs to be minimized, the pressure drop criteria frequently set tower diameter. 

In the vacuum tower, heavy liquids from the atmospheric tower, referred to as reduced crude, are fed to a vacuum furnace and heated to about 750—800 °F. To suppress coking in the furnace tubes, steam is added to increase the velocity of the hydrocarbon in the tubes. The reduced crude enters the vacuum tower flash zone, where the pressure is maintained at 20—30 mmHg absolute (2600-4000 Pa). When foaming occurs, fouling of the demister pads (sets of grids designed to minimize entrainment of heavier liquids into the upper sections of the tower) above the flash zone can occur, side stream gas oil products will be discolored, and gas oil end-point specification cannot be met. Again, silicone additives are used. The antifoam is normally injected into the feed to the... 

Packed vs Plate Columns. Relative to plate towers, packed towers are more useful for multipurpose distillations, usually in small (under 0.5 m) towers or for the following specific appHcations severe corrosion environment where some corrosion-resistant materials, such as plastics, ceramics, and certain metaUics, can easily be fabricated into packing but may be difficult to fabricate into plates vacuum operation where a low pressure drop per theoretical plate is a critical requirement high (eg, above 49,000 kg/(hm ) (- 10, 000 lb/(hft )) Hquid rates foaming systems or debottlenecking plate towers having plate spacings that are relatively close, under 0.3 m. 

Higher vaporizer temperatures can be maintained. Whereas vaporizer temperatures in lube towers are usually limited to a maximum of about 710 F, the vaporizer temperature in asphalt towers sometimes exceeds 780 F. However, vacuum decomposition, which causes violent foaming, begins to occur at about 800 F, and at such a temperature the material in the bottom of the tower must be removed quickly. Means of rapid cooling by recirculation have been employed. 

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