Problem: The existing partially trayed column with low tray efficiency and high pressure drop resulted in low recovery and wide boiling ranges of lube cuts. And the poor column performance also led to deep color and high CCR content of the heaviest lube cut.
History: The vacuum column that used to be the largest one in China was originally designed with P-K trays and with a capacity of 3.5 million tons/year (crude oil base). At the end of 1980's, the column was partially constructed with structured packings by a famous US company to improve the quality of its heavy side cuts. After several years an accident during repairs caused burning of packings and deformation of the internals. Another company was commissioned to supply new packings and restore the internals, but the column performance was not completely recovered.
Solution: To minimize the cost of reconstruction, two collector trays and the random packing on top bed are remained without modification. Flash zone vapor distributor was slightly modified for better performance. The number of side cuts of the lube column was kept as before. High efficient structured packings were used for all the other six beds to provide 23 theoretical stages. The packings were specially treated on the surface to form thin liquid films and provide more NTS than conventional structured packings. Trough type distributors were specially designed for all the seven packing beds to provide uniform liquid distribution across the packing beds. The latest developed collector trays were designed for five beds to improve vapor distribution uniformity with minimum pressure drop. Specific liquid downcomer boxes were used to guarantee the fluid level inside main troughs of liquid distributors. An internal cylinder was designed for the wash section to increase specific liquid rate in the bed.
Results: Pressure drop between the top and flash zone was measurably reduced to less than 20 mm Hg at designed capacity and the boiling range of all side cuts became much narrower. The color value and CCR content of heavy side cuts were markedly improved. The distillate fraction lighter than 500¡æ in vacuum residue was reduced from 7% to 4%.