Catalytic fractionation shutdown
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During shutdown, after the top circulation pump is emptied and stopped, is fresh water fed through the cold return flush line using the bulk gasoline pump inlet? My question is: if water is added through cold reflux only into the fractionation tower, how can it reach the entire top circulation system? Newcomers don’t understand.
In the case of crude vapor top-feed circulation, it generally reaches the inlet of the top circulation pump; by closing the valve, flow can be directed in the reverse direction to the distillation column. During cold reflux, the flow goes directly to the valve assembly of the top circulation return column, and this setup can also be used to flush the return line.
I have never done this before; procedurally, it should involve closing the valve on the top circulation return wall, using cold return flow to pump water into the top circulation system, and then using various sealed pressure-relief points to remove the oil.
Our process involves having two additional lines at the crude gasoline pump outlet, besides the cold reflux line; these two lines connect to the outlet and inlet of the overhead recycle pump, respectively. The outlet line is also connected to the waste oil line and the crude gasoline tank. Additionally, there is a bypass line for drawing off and returning material before the valve on the tower wall for overhead recycle extraction. In this way, there are several possible methods for completely removing the oil from the overhead recycle line using water. If your top circulation process has no other branches besides the cold reflux, then there are only two methods: one is to use cold reflux to inject water into the distillation tower, fill the top circulation extraction tank, and then start the top circulation pump to push the oil in the top circulation pipeline into the distillation tower. The second method is to close the top circulation tower valve, allow cold reflux water to flow upward, and push the fluid back into the fractionation tower (provided that there is a bypass valve at the outlet of the top circulation pump); after that, close the pump outlet valve and open the valve in the return line to send the fluid back to the tower. Top circulation pumps and top circulation heat exchangers generally have oil return lines, allowing oil to be drained first before water is pumped in.