In normal distillation columns, heat is added to the column from a reboiler and removed in an overhead condenser. A part of the distillate condensed in overhead condenser is returned to the column as reflux to aid fractionation. This approach is not feasible in crude distillation because the overhead temperature is too low for recovery of heat. Also the vapor and liquid flows in column increase markedly from bottom to top, requiring a very large-diameter tower. To recover the maximum heat and have uniform vapor and liquid loads in the column, intermediate refluxes are withdrawn, they exchange heat with incoming crude oil before entering the furnace and are returned to the plate above in the column (Figure 1-3).
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