Vacuum refinery column

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... 

Refinery Crude residuum vacuum distillation column Tray effidemy was lower than expected. Cdunm contained valve trrys and operated at low liquid loads and with wide variations in veqxir loads. Beware of overpredicting effidenry at very low liquid loads. 

Fractionation columns in tar-acid refineries are generally operated under vacuum and heated by high pressure steam or circulating hot oil. Calandtia in the reboders, condensers, mndown lines, and receiving tanks are constmcted of stainless steel, or, in the case of the condensers, of tin or nickel. 

In many refineries the atmospheric column bottoms are passed to vacuum distillation. This produces vacuum gas oils used primarily to produce lubricating oils. These boil in the range 350 C to about 550°C. The residua from this coluimi, often referred to as short residua or heavy fuel oil, concentrates all of the contaminant metals. 

Several of the commercial simulation programs offer preconfigured complex column rigorous models for petroleum fractionation. These models include charge heaters, several side strippers, and one or two pump-around loops. These fractionation column models can be used to model refinery distillation operations such as crude oil distillation, vacuum distillation of atmospheric residue oil, fluidized catalytic cracking (FCC) process main columns, and hydrocracker or coker main columns. Aspen Plus also has a shortcut fractionation model, SCFrac, which can be used to configure fractionation columns in the same way that shortcut distillation models are used to initialize multicomponent rigorous distillation models. 

In atmospheric or straight-run distillation the crude oil is first pumped into the fractional distillation unit. This is the refinery s tallest unit and some of its columns are used for atmospheric distillation while others are for vacuum distillation. Heated to about 680°F in the gas furnaces, the petroleum reaches the first atmospheric column, which is divided into compartments for fractional distillation. The lighter and more volatile hydrocarbons rise to the upper part. Those that are heavier and less volatile collect in the lower part. While rising, a volatile mass tends to shed its less volatile elements. 

The spray nozzle is not widely used. If more than one nozzle is used, it is difficult to obtain a uniform spray pattern because of overlap and underlap of the patterns. Also, liquid entrainment from the sprays is a problem. Spray distributors are sometimes used in petroleum refinery vacuum columns. The full cone nozzle is normally used, singly or in banks. 

Typically, an internal demister in the intertray space is about 2 in thick and contains wire mesh. Attention must be paid to correctly specifying the mesh type, size, and materials. In a refinery vacuum column, it is also important to keep the mesh wet (eg, by spraying wash oil above the demister) in order to inhibit coking. AVhen the service has a high plugging tendency, chevrons rather than wire-mesh may be the best demister choice (270). 

In a complex column (e.g., a refinery vacuum tower), it may be beneficial to conduct the test by sections (5, 345). For instance, the tower is tested first, then the connecting lines are tested one at a time by opening block valves. 

In some instances, installing automatic stripping steam shutoff trips on high bottom level has been highly successful and saved many trays in refinery vacuum columns (296). Unfortunately, this technique is only suitable for the few services where cutting out steam, boilup, or bottom feed is unlikely to greatly upset the column. 

When the column feed passes through a heater (e.g., a refinery fractionator or vacuum tower), any water lying at low points in the coils must be blown out prior to startup. In multipass coils, water must be separately blown out of each pass block valves are sometimes installed on each pass to permit this (7). If blowing into the tower, it must be performed when the tower can still tolerate water. The coils must be kept hot and/or purged from then on to prevent condensation. One pressure surge incident (7) occurred when water accumulated in one heater pass entered a refinery vacuum tower which was under full vacuum and circulating 280°F oil. 

Direct-contact condensers (Fig. 15.14f). These are used for minimizing pressure drop in vacuum condensation. To accomplish this, the direct-contact zone contains low-pressure-drop internals such as packings, or is a spray chamber. Another common application is intermediate heat removal ("pumparounds ) in refinery fractionators. Here the main purpose is to maximize heat recovery at the highest possible temperature levels. A third common application is intermediate heat removal from absorbers or reactive distillation columns in which an exothermic reaction takes places. In all these applications, condensation... 

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