Startup vacuum columns

Once inert gas purging and leak testing are completed (in either a pressure or a vacuum tower), the column is usually depressxired to a small positive pressime (5 to 10 psig) until the plant is ready to proceed with the startup. 

Once steaming is completed, the column should be opened to let air or inert gas in on shutdown, or placed under gas pressure (inert gas such as nitrogen or fuel gas, or process gas) at startup. The alternative of leaving small steam purges to keep the column pressure up at the conclusion of steaming is often unsatisfactory and resulted in creation of vacuum in some instances (3). Unless the column is fully open to the atmosphere, so that no vacuum can be created, a pressure of at least 5 to 10 psig should be kept in it (3). 

An alternative technique that prevents excessive bottom temperatures and permits reboiler startup before vacuum is fully drawn is routing the column feed directly into the reboiler inlet (Fig. 12.4). This technique may generate substantial quantities of off-spec bottom product a provision for recycling bottom (and possibly also top) products into the feed tank may minimize product loss. 

When the column feed passes through a heater (e.g., a refinery fractionator or vacuum tower), any water lying at low points in the coils must be blown out prior to startup. In multipass coils, water must be separately blown out of each pass block valves are sometimes installed on each pass to permit this (7). If blowing into the tower, it must be performed when the tower can still tolerate water. The coils must be kept hot and/or purged from then on to prevent condensation. One pressure surge incident (7) occurred when water accumulated in one heater pass entered a refinery vacuum tower which was under full vacuum and circulating 280°F oil. 

Late one night, a brand new distillation column was wrecked by vacuum and toppled as a water filled flooded column was being pumped out. The column was designed and equipped with sufficient under/over pressure protection during normal operations, but the system was ill equipped to deal with flooded columns at startup. This was a newly constmcted plant, which had been in the pre-startup phases. 

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