Vacuum tower bottoms

Vacuum resid 1050T-H 5GG°C-H Vacuum tower bottoms (VRC) Heavy and bunker fuel oils, asphalts... 

A possible solution is to gasify the more dilute vacuum tower bottoms product in an oxygen blown gasifier and to convert the excess synthesis gas to methanol. In those cases where a Flexicoker is used the heavy scrubber liquids could be recycled to extinction. Therefore, the plant products are SNG, naphtha, 300-800°F distillate and methanol. All of these products are of high quality or can be hydrotreated to achieve high quality. As a result, they could be easily integrated into the utility fuel mix with a minimum amount of disruption or special product handling facilities. 

In the development of coal liquefaction processes considerable effort has been concentrated on the coal liquefaction part of the process. In contrast, less effort has been directed toward utilization of the coal liquefaction residue or vacuum tower bottoms. 

Activities in the propane deasphalting (PDA) unit submatrix represent the operations on vacuum tower bottoms from the base crude mix and from the incremental crudes. The de-asphalter overhead streams from all activities enter one stream balance row with common properties. Each deasphalter bottoms enter a separate row for No. 6 fuel oil blending. 

Figure 6. Microscopic pictures (500X) of >5 m solids for (a) Vacuum tower charge and (b) vacuum tower bottom.

A recent GRACE survey of the European refining industry showed that over 40 percent of European refiners add various amounts of resid to their FCC unit feeds. The term "Resid" covers a broad range of feeds boiling above 350°C such as Long Resid or Atmospheric Tower Bottoms, Short Resid or Vacuum Tower Bottoms, Heavy Vacuum Gas Oil, Aromatic Extracts and Deasphalted Oil. 

Delayed coking is a thermal cracking process used in refineries to upgrade and convert crude oil residue known as vacuum tower bottom product (i.e. the bottoms fraction from a vacuum rectification tower) into liquid and gas product streams leaving behind a solid concentrated carbon material, coke. The vacuum towers referred to are generally used to further fractionate virgin atmospheric-... 

Molybdenum occurs in petroleum, and has been found in some crudes, atmospheric tower bottoms and vacuum tower bottoms by emission analysis. We used this method in the beginning, but later switched to atomic absorption, which is more reliable. The smallest amount of molybdenum that can be determined by this method is 0.4 ppm. Therefore, it can be found more readily in vacuum or atmospheric tower bottoms of crudes with a low content of molybdenum. The analytical methods are described at the end of this chapter. 

Residual fuel oil is a complex blend of hydrocarbons derived from various refinery streams, usually residue, and can contain hydrogen sulfide and polycyclic aromatic hydrocarbons. Typical streams include atmospheric tower bottoms and vacuum tower bottoms. Fuel oil is used in many countries... 

The heavy vacuum gas oil (HVGO) produced in most refineries is the principal component charged to an FCCU. The catalyst at the FCCU is adversely affected by the nickel, vanadium, and sodium that become concentrated in the vacuum-tower bottoms resid. When resid is entrained into the HVGO, the metals in FCCU charge will dramatically increase. 

Gas oil left in resid will often have a value equivalent to fuel oil. Gas oil recovered for FCCU feed will have a value equivalent to crude. It follows, then, that every barrel possible ought to be boiled out of the vacuum tower bottoms. The best troubleshooting tool for judging overall vacuum-tower performance is to vacuum distill off in the lab the 1,000°F- gas oil in the resid. Based on plant data, the following rules of thumb may then be applied ... 

Increase the number or efficiency of resid stripping trays. Increasing the superheat of the exhaust stripping steam will also multiply the amount of gas oil stripped out of the vacuum-tower bottoms resid product. 

All of the components in the slurry oil coker charge are produced as a vapor from the FCCU reactor fresh feed. Even if vacuum tower bottoms are charged to the FCCU, none of the asphaltenes in the resid will vaporize, and therefore, the asphaltenes will not be in the coker charge. 

Providing net positive suction head (NPSH) for any centrifugal pump can be a tricky business. For vacuum tower bottoms pumps, the difficulties are magnified. A few pounds of vapor that another pump might pass corn-... 

FIGURE 13-2 Providing suction head for a vacuum tower bottoms pump is difiicult. 

I was all set to land a nice contract to revamp a vacuum column in a West Coast refinery. The revamp of the tower was intended to recover an incremental 3,000 B/SD of gas oil from the vacuum tower bottoms. 

The process conditions shown in Figure 13-7 were the design-basis operating parameters. Note that the 715°F flash-zone temperature and the 25-in. Hg flash-zone pressure (128 mm Hg) are indicative of an operation that results in excessive gas oil left in the vacuum tower bottoms. This downgrades virgin gas oil from FCCU feedstock to delayed coker feed at a penalty of 5/bbI. A properly designed and operated vacuum column that employs steam stripping of the heater coils operates at 27-in. Hg flash-zone pressure and 760°F flash-zone temperature. 

After these moves, pressure decreased and the heater temperature was increased another 5°F. Finally, the vacuum tower bottoms stripping steam rate, heater outlet temperature, and heater pass flow were optimized to obtain maximum gas oil production. These last items usually take some trial and error. 

The fuel oil flows from a circulating hot oil system kept at several hundred °F, depending on what is being burned. In most refineries, visbreaker or vacuum tower bottoms provide the fuel oil, which is circulated at about 500°F. Burner tip plugging of the oil orifices shown in Fig. 29.11 is a routine problem. 

Heavy hydrocarbons, such as vacuum tower bottom products, tar, and pitch, are frequently pumped and stored in tanks above their auto-ignition temperatures. The process reason for this is to prevent these fluids from "setting up" or solidifying in the pipelines or tanks. At ambient temperatures, even in hot climates, these fluids would normally be either solid or extremely viscous. 

It is true that in oil refineries and many chemical plants the fluids conveyed in pipes frequently fall within this low-viscosity category (less than 2 or 3 cP). Even materials we might expect to be more viscous, like the heavier hydrocarbon fractions such as atmospheric and vacuum-tower bottom products, do have quite low viscosity when pumped because it is normal practice to heat them up until the viscosity is low in order to make it easier to pump them. The pipelines are then insulated and "steam traced" to keep these heavy fluids hot and at acceptably low viscosity. 

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