Component vacuum distillation unit

The atmospheric reduced crude is the feedstock for the vacuum distillation unit. To prevent thermal decomposition (cracking) of the higher boiling point hydrocarbons in the crude oil, the pressure in the vacuum distillation fractionation column is reduced to about one-twentieth of an atmosphere absolute (one atmosphere pressure is 14.7 psia or 760 mm Fig). This effectively reduces the boiling points of the hydrocarbons several hundred degrees Fahrenheit. The components boiling below about 1050°F (565°C) are vaporized and removed as vacuum gas... 

In the 1970 s, heavy fuel came mainly from atmospheric distillation residue. Nowadays a very large proportion of this product is vacuum distilled and the distillate obtained is fed to conversion units such as catalytic cracking, visbreaking and cokers. These produce lighter products —gas and gasoline— but also very heavy components, that are viscous and have high contaminant levels, that are subsequently incorporated in the fuels. 

Other components in the feed gas may react with and degrade the amine solution. Many of these latter reactions can be reversed by appHcation of heat, as in a reclaimer. Some reaction products cannot be reclaimed, however. Thus to keep the concentration of these materials at an acceptable level, the solution must be purged and fresh amine added periodically. The principal sources of degradation products are the reactions with carbon dioxide, carbonyl sulfide, and carbon disulfide. In refineries, sour gas streams from vacuum distillation or from fluidized catalytic cracking (FCC) units can contain oxygen or sulfur dioxide which form heat-stable salts with the amine solution (see Fluidization Petroleum). 

The atmospheric bottom, also known as reduced oil, is then sent to the vacuum unit where it is further separated into vacuum gas oil and vacuum residues. Vacuum distillation improves the separation of gas oil distillates from the reduced oil at temperatures less than those at which thermal cracking would normally take place. The basic idea on which vacuum distillation operates is that, at low pressure, the boiling points of any material are reduced, allowing various hydrocarbon components in the reduced crude oil to vaporize or boil at a lower temperature. Vacuum distillation of the heavier product avoids thermal cracking and hence product loss and equipment fouling. 

An existing lube hydrocracker can be operated at higher severity to make this special product, but the sharp reduction in yield may not be attractive for the base oil plant economics. However, an alternative source of hydrocracked base oil is available from some of the many existing fuel crackers. These hydrocrackers are important refinery conversion units and are used to make a range of fuel products from vacuum distillate feedstocks. Some plants do not fully convert the feed in one pass to low-boiling products and the limited amount of residue which remains, 5-10%, can be recycled within the plant, used as a fuel oil blending component or upgraded to make the special base oils. 

Vacuum distillate undergoes hydrogenation in the VGO hydrogenation unit before being fed to the catalytic cracking (FCC), whilst vacuum residue passes to the visbreaker for further conversion. FCC residue is used as a fuel component in the refinery s own power station, and it supplies the refinery, in return, with steam and electrical power. It includes three oil- and one gas-fired boilers and is designed for 110 MW output. 

A vacuum tower is a major component of most crude distillation units. The vacuum in these towers is usually created by a series of steam jet ejectors that typically use 100-psi steam. 

Atmospheric distillation is least effective in converting heavier products into lighter components. A second distillation column under vacuum is needed to further separate the heavier parts of crude oil into lighter fractions. Some fractions from the vacuum units have better quality than atmospheric distillation cuts because the metal-bearing compounds and carbon-forming materials are concentrated in the vacuum residue. 

Description Extractive distillation is used to separate close-boiling components using a solvent that alters the volatility between the components. An ED Sulfolane unit consists of two primary columns they are the ED column and the solvent recovery column. Aromatic feed is preheated with lean solvent and enters a central stage of the ED column (1). The lean solvent is introduced near the top of the ED column. Nonaromatics are separated from the top of this column and sent to storage. The ED column bottoms contain solvent and highly purified aromatics that are sent to the solvent recovery column (2). In this column, aromatics are separated from solvent under vacuum with steam stripping. The overhead aromatics product is sent to the BT fractionation section. Lean solvent is separated from the bottom of the column and recirculated back to the ED column. 

From the beginning, FCC was employed to upgrade atmospheric (or long) residue. By distillation under vacuum, FCC feedstock (also called VGO) was separated from vacuum (or short) residue. Distillation techniques have been extended to increase the yield of FCC feedstock, and this development (revamps of vacuum units) is still taking place at various refineries. This deeper distillation does sufficiently restrict metals (Ni, V, etc.) and nonreactive and coke-producing components (CCR) in the distillate FCC feedstock. In some cases, deasphalting was and is employed to further increase the yield of acceptable FCC feedstock. 

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