Thermal vacuum distillation unit

Where is naphthenic acid corrosion found Naphthenic acid corrosion occurs primarily in crude and vacuum distillation units, and less frequently in thermal and catalytic cracking operations. It usually occurs in furnace coils, transfer lines, vacuum columns and their overhead condensers, sidestream coolers, and pumps. 

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... 

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. 

High temperature deep flashing and vacuum flashing of thermally cracked distillate also contributes to increasing the percentage of the crude, which can be routed to the FCC unit. 

Vacuum distillation is suitable for the separation of thermally sensitive liquids, being a gentle distillation process which operates at low pressure. Due to the reduced operating pressure, the boiling point of the mixture is also reduced. A low residence time and a narrow residence time distribution are maintained, by careful selection of the distillation unit to be used. 

Crude fatty acid is predried and degassed under vacuum and fed to the distillation unit, which is operated at a vacuum of 1.2 kPa or less and a temperature of approximately 200°C. Modem stills use thermal oil or high-pressure steam as the heat source. Stripping... 

A schematic of the EDS process is shown in Fig. 17.27. The coal is ground and mixed with a hydrogenated distillate recycle solvent before entering the liquefaction reactor. Hydrogen is added to the slurry in a tubular thermal reactor, which operated at 800 to 880"F (425-470"C) and 1500 to 2000 psig (10-14 MPa). The total reactor product is set to a distillation unit, where it is separated into gas, naphtha, distillates, and a vacuum... 

Figure 17.28 shows a schematic of the SRC-I process. The feed coal is crushed and mixed with a recycle solvent and hydrogen prior to entering the preheater. The preheater effluent, at 700 to 750°F (370-400°C), then is fed to the dissolver unit, or thermal liquefaction unit (TLU), which operates at 840 to 870°F (450-465°C). There is no upgrading step, as the desired product is a solid at room temperature and not a distillate. The solids removal from the liquid slurry is accomplished by critical solvent de-ashing (CSD). The solids-free resid from the CSD was separated by vacuum distillation into a recycle solvent (the light fraction) and a solvent refined coal product (the bottoms). 

Thermal decomposition increasingly occurs at higher temperatures and further separation by distillation of the atmospheric residue into lubricant base oil is carried out in a vacuum unit. Fig. 1.8. Atmospheric residue feedstock is injected with steam and pre-heated in a furnace before entering the lower part of the vacuum column. Inside the column, a variety of different mechanical arrangements are used to assist separation of different boiling range fractions ... 

Consideration of the nature of the petrochemical refinery itself gives some clues as to another source of its profit problem. In the simple, basic unit depicted in Figure 2 thermal cracking dominates the operation. Over 90% of the crude input is consumed without regard to relative values. Thus, it is an indiscriminate cracker of butanes, light naphtha, heavy naphtha, kerosene, distillate, atmospheric gas oil, and vacuum gas oil. Since acceptably similar product slates can be obtained from many of these fractions, it is obvious that the economics suffer when the high valued naphtha and kerosene fractions are thermally cracked. 

Low-value distillates, including heavy-cycle oils from FCC units, thermal and coker gas oils, and other heavy-vacuum gas oils, are cracked to produce naphtha, jet fuel, and diesel oils. The reaction mechanism is the same as in catalytic cracking and some aromatic products are also hydrogenated. 

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