Raise Pumparound To Save Energy

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Operators are usually more interested in making on-spec products than in saving energy. In the field one will often find pumparound rates cut back to 50% of their proper level. Try increasing pumparound circulation until the product separation is adversely affected. Perhaps an end-pont spec can be extended. Up to the point at which the trays flood, increasing pumparound flow will save furnace fuel.

On one unit preheat was increased by 8°F simply by opening up the two discharge valves on the pumparound circuit. The operators could not recall when or for what reason these valves had been pinched back.

Note that as the pumparound duty is decreased, the vapor and liquid loads on the trays above the pumparound return tray will increase. This should ordinarily enhance fractionation. However, the reduction in pumpa­round duty could cause trays 3, 4, and 5 shown in Figure 1-3 to flood. This would reduce the separation efficiency between FCCU feed and furnace oil.

The question then is: As an operator reduces pumparound duty, how can he tell if fractionation between two adjacent products is getting better (due to increased internal reflux on the trays) or worse (due to tray deck flooding and entrainment)? The simple answer is to observe the difference in draw temperatures (AT), between the adjacent cuts.

An increase in this AT between FCCU feed and furnace oil accompanying a reduction in pumparound duty indicates enhanced fractionation. A reduc­tion in this AT indicates trays 3, 4, and 5 in Figure 1-3 are flooding.

pumparound duty

 

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