Figure 11.30 Stage 2 temperature control.
pipestill. The preflash column has removed the light hydrocarbons from the crude feed as a vapor product and produced a light naphtha product as a distillate overhead product. The bottoms from the preflash column is pumped through a furnace in which about 70% of the material is vaporized (depending on the assay of the crude) and fed into a very large column. This column produces an overhead distillate product (heavy naphtha) and three sidestreams: kerosene, diesel, and atmospheric gasoil. The bottoms is called "reduced crude" and is fed to another downstream pipestill operating under vacuum so that more gasoil can be recovered.
Two "pumparounds" are used to recover some of high-temperature energy in the vapor stream flowing up the column. A pumparound takes hot liquid from a tray and pumps it through a heat exchanger that cools the liquid. The cooled liquid is returned back to a tray higher in the column. These pumparound trays are direct-contact heat exchangers. Typically the heat is used to preheat the crude feed to the unit with the objective of reducing furnace fuel consumption. The design of a pipestill incorporates an interesting tradeoff between energy consumption and product purity and yield. If no pumparounds were used, all the vapor in the stream from the furnace would pass up the column and be condensed in the water-cooled or air-cooled condenser. No heat would be recovered, so furnace firing would be much larger. A large amount of reflux would be required, which would give higher L/V (liquid to vapor) ratios in the column and therefore better fractionation. This improves the separation between the sidestream products. So the tradeoff is between energy consumption and separation.
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