(a)	The main column consists of 45 trays and the secondary columns (side strippers) consist of 4 trays each. Three side strippers are used to strip the light ends from kerosene, LGO and HGO products.
(b)	The main column has two sections that are distinguished with respect to a flash zone. The flash zone is where the crude oil partially vaporized is fed to the main column. There are about 4 trays below the flash zone and 41 trays above the flash zone of the main column. The bottom most tray (residue stripping tray) is numbered as 1 and the top tower tray is numbered as 45. Trays 1 to 4 process the atmospheric residue portion of the crude in the section below the flash zone.
(c)	Trays 5 to 10 (6 trays above the flash zone) process the HGO product portion of the crude. From tray 10, HGO draw off product is taken out (as liquid) and enters the HGO side stripper unit. From tray 10 as well, the liquid stream is drawn and sent to tray 12 via a bottom pump around unit that enables cooling of the liquid stream. The steam + light ends from the HGO side stripper enter tray 11 of the main column.
(d)	Trays 13 to 22 (10 trays above the HGO processing zone) process the LGO product portion of the crude. From tray 22, LGO draw off product is taken (as liquid) and sent to the LGO side stripper unit. Also, from tray 22, another liquid stream is taken out and sent to tray 24 via a top pump around unit (TPA) that enables cooling of the liquid stream. The steam + light ends from the LGO side stripper enter tray 23 of the main column.
(e)	Trays 24 to 34 (10 trays above the LGO processing zone) process the kerosene product portion of the crude. From tray 34, the kero draw off stream is taken and sent to the kerosene side stripper unit. The steam + light ends of the kerosene side stripper enter tray 35.
(f)	Trays 34 to 45 (12 trays above the Kerosene processing zone) process the naphtha product portion of the crude. It is interesting to note that tray 34 is regarded as a tray processing both LGO as well as naphtha processing zone. This is because there is no pump around associated to the tray 34. Where pump around is associated, that tray is often ignored in counting, as it affects to a large extent the tray hydraulics and contributes less towards the separation of the components.
(g)	It is interesting to note that steam enters main column at trays 1, 11, 23, 35 and therefore is present along with the vapor stream along with the hydrocarbons. Therefore, steam balances throughout the column are very important.
(h)	The cold naphtha stream obtained from the phase separator is sent back to the main column as reflux stream.

5.1.6 Heat exchanger networks

• Two heat exchanger networks exist in the crude distillation unit, one before the crude desalter and one after the crude desalter.
• The heat exchanger networks facilitate energy recovery from hot product, naphtha+steam vapor and reflux streams to heat the crude oil in an indirect heat transfer mode i.e, using heat exchangers.
• Therefore, the design and operation of a heat exchanger network is very important in the crude distillation unit.
• Further, it needs to be understood that the naphtha heat integrated condenser is a partial condenser where as all others are heat exchangers without any phase change streams.
• The heat exchanger networks enable to increase the crude oil stream temperature to about 200 – 230°C which is significantly higher than the crude oil source temperature (about 20 – 30°C).
• Crude distillation units without heat exchanger networks have higher furnace load targets. They also cause more pollution due to burning more fuel oil and fuel gas streams.