Vacuum tower

Cmde oil containing about 30% asphalt can be refined completely in an atmospheric unit to an asphalt product. However, most cmde oil cannot be distilled satisfactorily to an asphalt product at atmospheric pressure because of the presence of substantial proportions of high boiling gas oil fractions. Thus, as a supplement to the atmospheric process, a second fractionating tower (a vacuum tower) is added (Fig. 1). 

FIG. 13-87 Crude unit with atmospheric and vacuum towers. [Kleinsch7 odt and Hammer, Exchange Networks for Cixide Units . Chem. Eng. Prog., 79(7),. 3.3 (1983).]... 

Graf states For spray type distributors in vacuum towers the main header of the distributor will be 18-36 in. above the top of the packing. ... 

Graf, Kenneth, Correlations for Design Evaluation of Packed Vacuum Towers, Oil and Gas Journal, May 20, 1985. 

Problem A structured packing vacuum tower had too much heavy key in a vapor side-draw between the feed and the column bottom. The side-draw heavy key concentration was several times the design value. 

Petroleum products may be treated with various solvents for the removal by selective solubility of undesirable constituents or for the recovery of by-products. The solvent and solute must be separated to yield the desired product and to recover the solvent for reuse. The solvents normally boil at a lower temperature than the products from which they are to be removed and so are generally distilled off as overhead products. The pipe stills used for this service may be single-stage or multi-stage units, depending on the service involved. Some solvents can be removed by the use of steam heated stills. In other cases, the high temperature required necessitates the use of fired heaters and vacuum towers. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

Figure 2 shows a simplified flow plan for a typical conversion type refinery. The atmospheric P/S residuum can be fed to a vacuum pipestill. The vacuum tower enables the refiner to cut deeper into the crude, at the same time avoiding high temperatures (above about 750 °F) which cause thermal cracking with resultant deposition of coke and tarry residues in the equipment. 

Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. 

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

The method proposed by Bolles fits the average design problem quite satisfactorily. However, for low pressure drop designs as in vacuum towers, it may well require checking by the more detailed method of Dauphine [13]. 

Figure 3-1. Flow diagram of atmospheric and vacuum distillation units (1,3) heat exchangers (2) desalter, (3,4) heater (5) distillation column, (6) overhead condenser, (7-10) pump around streams, (11) vacuum distillation heater (12) vacuum tower.

The feed is first preheated by exchange with the products, charged to the vacuum unit heater, and then passed to the vacuum tower in an atmosphere of superheated steam. Using superheated steam is important it... 

Figure 10-6. The Mitsui Petrochemical Industries process for producing phenol and acetone from cumene (1) autooxidatlon reactor, (2) vacuum tower, (3) cleavage reactor, (4) neutralizer, (5-11 ) purification train.

The gas oil feed for the conventional cat cracker comes primarily from the atmospheric column, the vacuum tower, and the delayed coker. In addition, a number of refiners blend some atmospheric or vacuum resid into the feedstocks to be processed in the FCC unit. 

Principal component regression (PCR) is an extension of PCA with the purpose of creating a predictive model of the Y-data using the X or measurement data. For example, if X is composed of temperatures and pressures, Y may be the set of compositions that results from thermodynamic considerations. Piovoso and Kosanovich (1994) used PCR and a priori process knowledge to correlate routine pressure and temperature measurements with laboratory composition measurements to develop a predictive model of the volatile bottoms composition on a vacuum tower. 

An additional mechanism affects the deposits formation from the H-Oil reactor, rejection of vanadium and nickel sulfides from the catalyst. In the vacuum tower, asphaltene precipitation was found to be the prevalent fouling mechanism. In asphaltene... 

Fractions from the atmospheric and vacuum towers are often used as feedstocks to these second-stage refinery processes that break down the fractions or bring about a basic chemical change in the nature of a particular hydrocarbon compound to produce specific products. 

Solvent deasphalting Treatment Absorption Remove asphalt Vacuum tower residual, propane Heavy lube oil, asphalt... 

Solvent dewaxing Treatment Cool/filter Remove wax from Inbe stocks Vacuum tower lube oils Dewaxed lube base stock... 

Chemical and Physical Properties Petroleum fuels contain paraffins, isoparaffins, naphthenes, and aromatics, plus organic sulfur, oxygen, and nitrogen compounds that were not removed by refining. Olefins are absent or negligible except when created by severe refining. Vacuum-tower distillate with a final boiling point equivalent to 730 to 840 K (850 to 1050°F) at atmospheric pressure may contain from 0.1 to... 

The chief engineer of a West Texas process plant had decided to replace the main condenser. Colder weather always coincided with a vastly improved vacuum, in their vacuum tower. It seemed as if colder air to the condenser really helped. So the engineering concluded, that a bigger condenser would also help during warm weather. 

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