Light vacuum distillation unit

Atmospheric and vacuum distillation units (Figures 4.3 and 4.4) are closed processes, and exposures are expected to be minimal. Both atmospheric distillation units and vacuum distillation units produce refinery fuel gas streams containing a mixture of light hydrocarbons, hydrogen sulfide, and ammonia. These streams are processed through gas treatment and sulfur recovery units to recover fuel gas and sulfur. Sulfur recovery creates emissions of ammonia, hydrogen sulfide, sulfur oxides, and nitrogen oxides. 

The atmospheric residuum is then fed to the vacuum distillation unit at the pressure of 10 mmHg where light vacuum gas oil, heavy vacuum gas oil, and vacuum residue are the products (Fig. 13.4). 

Atmospheric residue oil (Residue), containing compounds that boil above about 340°C (650°F). This is normally sent to a vacuum distillation unit to recover more light products, but parts of it may be blended into high sulfur fuels such as heating oil or bunker fuel (marine fuel). 

The current oil sands bitumen upgrading processes for the production of synthetic crude oil (Table 4) begin with diluted bitumen being processed through the diluent recovery units. The diluent recovery units are atmospheric distillation units that serve three purposes 1) distill off diluent naphtha and return it to the froth treatment process 2) distill off light gas oil and send it directly to a light gas oil hydrotreater and 3) produce hot atmospheric topped bitumen as feedstock for vacuum distillation unit and downstream bitumen conversion processes. 

The vacuum distillation unit cuts deeper into the atmospheric topped bitumen. It distills off the remaining light gas oil and heavy gas oil that are sent directly to hydrotreaters. The remaining vacuum topped bitumen can be blended with atmospheric topped bitumen and then sent to bitumen conversion units. 

Middle distillates are withdrawn and brought up to specification with separate processes, e.g., hydrodesulfurization. The heavy oil from the bottom of the crude unit can be used for oil blending or can be processed further in vacuum distillation units to recover a light distillate, used in blending diesel oils. 

In present-day refineries, the fluid catalytic cracking (FCC) unit has become the major gasoline-producing unit. The FCC s major purpose is to upgrade heavy fractions, that is, gas oil from the atmospheric and vacuum distillation columns and delayed coker, into light products. Atmospheric gas oil has a boiling range of between 650-725°F.9... 

The fractions obtained by vacuum distillation of the reduced crude (atmospheric residuum) from an atmospheric distillation unit depend on whether or not the unit is designed to produce lubricating or vacuum gas oils. In the former case, the fractions include (1) heavy gas oil, which is an overhead product and is used as catalytic cracking stock or, after suitable treatment, a light lubricating oil (2) lubricating oil (usually three fractions—light, intermediate, and heavy), which is obtained as a side-stream product and (3) asphalt (or residuum), which is the nonvolatile product and may be used directly as, or to produce, asphalt, and which may also be blended with gas oils to produce a heavy fuel oil. 

The heavy bottoms from vacuum distillation may be sent to a FLEXICOKING unit along with air and steam to produce additional distilled liquid products and a low quality fuel gas for process furnaces. Light hydrocarbon gases coming from the distillation unit are steam reformed to produce hydrogen. The total liquid yield is thus a blend of streams from liquefaction and flexi-coking. 

Liquid feeds can be atmospheric or vacuum gas oils from crude distillation units gas oils (light and heavy) from delayed cokers, fluid cokers, or visbreakers and cycle oils (light and heavy) from FCC units. 

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). 

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