Design of Vacuum Systems for Crude Oil Vacuum Tower Distillation
Paper will discuss in detail design considerations involved in selecting steam ejector/liquid ring pump vacuum system for crude tower service. Materials of construction, energy utilization, packaging, system performance and flexibility will be addressed. Crude Oil Vacuum Towers require reliable, trouble free vacuum systems that operate for months on end, between scheduled shutdowns. The key to a well designed system is the utilization of dependable vacuum equipment combined to achieve optimum energy consumption.
Steam jet ejectors have historically been accepted as being the logical and economical means of pulling vacuum on towers in refinery service. To produce the high vacuum required, they are staged together in two, three, or four stages depending on the level of vacuum. Ejectors can be single element or multi element systems (i.e., twin 50% element, twin 1/3 - 2/3 elements, triple element with each 1/ 3 capacity), or triple element with each 50% capacity. Multi element systems allow for flexibility in varying load situations. Along with the ejectors, special designed shell and tube heat exchangers are utilized to condense the steam and hydrocarbons, and in addition cool the gases at the various optimum interstage pressures. (Refer to Fig. 1 and 2 for typical schematics of equipment.) The primary stage(s) vary in size and are generally in the range of 15 to 70 feet long. They are either mounted on top of the tower pointing vertically down or located at the same platform elevation as the intercondensers, which is approximately 45 feet minimum above the condensate seal pot liquid level. Steam jet ejectors have no moving parts and are probably one of the most trustworthy pieces of equipment that function in a vacuum system. This does not mean they can be ignored indefinitely. Routine inspection, maintenance, and repair is usually taken care of during the normal turnaround.