**3. ADU-VDU plant sustainable energy efficient design**

Oil refineries are vital to the global economy and at the identical time major consumers of energy. Petroleum refineries are under increased pressure to reduce emissions of greenhouse gases, mainly CO2 to accommodate with the upcoming stricter environmental regulations. Energy efficiency optimization may be a means solution to GHG emissions reduction because of its impact on energy consumption

at the source. Heat exchangers play a significant role in crude oil refineries in energy saving, in general. Distillation is that the main consumer of energy in a crude oil refinery, and heat exchangers connected together in what's called a preheat train are essential to dramatically reduces the thermal duty of the atmospheric crude unit furnace. Crude distillation could be a primary processing operation in refineries throughout the planet and requires heat, steam and cooling to work. Although the crude distillation unit (CDU), that consists of both atmospheric distillation unit (ADU) and also the vacuum distillation unit (VDU), is not the foremost energyintensive plant within the petroleum refinery, in terms of energy per barrel, every barrel of oil that is processed within the petroleum refinery passes through this unit/ plant, making it the most important energy consumer, of the entire energy consumed, in fossil oil refineries.

Crude distillation process separates fossil oil into fractions in keeping with boiling point so that down-streams processing units/plants are often charged with feedstock that meets their particular specifications.

Crude oil separation process is accomplished by first fractionating crude petroleum at essentially atmospheric pressure and then feeding the high-boiling fraction, called topped crude or reduced crude, from the atmospheric distillation tower bottoms to a second fractionation tower that's operated under vacuum conditions.

The petroleum oil vacuum distillation unit is employed to avoid the high temperatures necessary to vaporize topped crude at atmospheric pressure. This unit reduces the hazard of thermal cracking, product discoloration and equipment fouling because of coke formation.

Before entering the atmospheric distillation tower flash zone, the petroleum oil charge is heated to the required desalting temperature, desalted, heated again to separate light fractions vapor during a pre-flash drum or pre-flash tower, heated again before the atmospheric unit furnace using product streams and column reflux streams, referred to as pump-arounds. The desalted and pre-flashed petroleum oil charge is heated up within the atmospheric distillation furnace(s) to about 375 C.

Topped crude from the atmospheric tower bottom sometimes called reduced crude is mixed with steam and pre-heated to about 390 C to 450 C before routed to the vacuum distillation tower. A system of vacuum pumps or steam ejectors is employed to form vacuum within the vacuum distillation column for the separation of high boiling temperatures cuts without its chemical degradation.

Crude oil distillation plants design including pre-heat train (PHT) is well established. However, the design and retrofit of the crude oil pre-heat train remains up to now the topic of many research and development because of its importance in any crude oil refinery, since the crude oil distillation system is among the largest energy consumers in industry, and therefore the problem is not trivial with many decision variables and constraints because of the high interaction between the pre-heat train and also the distillation columns of the crude distillation plant.

Nowadays, the retrofit of the crude distillation plant including PHT may be a task that may be conducted a minimum of 4 to 5 times along the fossil oil refinery lifetime not only because of the requirement for energy saving, GHG emissions reduction but also more importantly for throughput increase, change; for product mix/specification; (more gasoline than Diesel or vice versa) and permanent change in the API of the processed crude. Since the atmospheric and vacuum crude distillation towers designs are highly interlinked to the crude distillation plant pre-heat train (PHT), any retrofit of one system goes to severely impact the other. All of those objectives require heat duties within the PHT to be changed, surface areas to be changed, pressure
