Raffinate splitter tower

Part of the petroleum refinery, the isomerization unit (ISOM) unit, was in the process of being started up just prior to the destmctive explosion and fire. This part of the unit had been shut down for about a month for a maintenance mmaround. During this startup a tall (164 ft.) distillation column (which separates blends of petroleum) called a raffinate splitter tower was inadvertently overfilled [ 14]. 

At 0215, operators had started to introduce raffinate into the raffinate Splitter Tower, which is used to distil and separate gasoline components. (The word raffinate means a product in the refining process. In this case the raffinate was naphtha -raw gasoline - from the crude distillation column.) The tower was more than 30 m tall. A single instrument (shown as LT) was available for liquid level indication at the bottom of tower which had a maximum indicated level 9 feet (about 3 m). Above this level the instrument just indicated 9 feet . However, operators routinely filled above this level during start-ups to avoid the possibility of low level causing furnace damage. 

The shift log left by the nightshift was unclear about the level in the raffinate Splitter Tower and the general state of start-up. All that was recorded was ISOM Isomerization Unit), brought in some raff to unit. ... 

At 1000, the furnace under the raffinate Splitter Tower was lit to start feed heating. Raffinate feed was still going on, although the only level instrument still showed its maximum of about 9 feet. 

At 0951, the start-up was resumed. The day shift began to put more feed into the aheady over-filled Splitter Tower. An Auto-level control valve on the raffinate feed to the Splitter Tower was disabled because of conflicting instructions . So the Splitter Tower just kept filling up higher and higher... 

At 1241, an alarm appeared in the control room to say there was high pressure at the top of the Splitter Tower. (This was caused by compression of gases as the liquid raffinate level rose. The Splitter Tower - a distillation column - was now almost completely full of liquid raffinate.) The control room operator instructed plant operators to respond to this alarm as follows ... 

To recap the accident involved the Splitter Tower becoming completely filled with hot liquid raffinate (naphtha or gasoline), when it should have been less than one-tenth full. Hot raffinate then overflowed into the blowdown drum and out through its vent. The Splitter Tower had been receiving raffinate feed for several hours without any apparent concern that it might be overfilling. The ineffective level instrumentation at the... 

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