Vacuum pipestills

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. 

Achieves low viscosity and volatility using efficiently fractionated solvent neutrals from the lubes vacuum pipestill via one base stock per feed. 

The feed is partially vaporized 2278 lb mol/h of vapor with a feed of 3644 lb mol/h. It is introduced into the flash zone on Stage 22. There are three stages below the flash zone that are used to strip out any light material that is in the liquid leaving the flash zone. Open steam is fed to the bottom of the column at a rate of 12,000 Ib/h. The bottoms stream from the pipestill ( reduced crude ) goes to a downstream vacuum pipestill in which more gas oil is recovered. The low pressure in the vacuum furnace produces more vapor for the same furnace temperature. 

Cmde is distilled and the bottoms, atmospheric resid, is sent to a vacuum distillation unit (VDU) sometimes called a vacuum pipestill (VPS) for further fractionation. Vacuum fractionation is used to separate the atmospheric resid into several feed streams or distillates. Conventional solvent processing uses selected solvents in physical processes to remove undesirable molecules... 

Lube Vacuum Distillation Unit (VDU) or Vacuum Pipestill (VPS) - Viscosity and Volatility Control... 

Vacuum distillation equipment is often referred to as the vacuum distillation unit (VDU) or vacuum pipestill (VPS). 

Pumparounds are used to remove heat from the tower and to adjust the vapor-liquid flow in the tower. They condense vapors rising in the tower and create an internal reflux for the fractionation stages below the pumparound. They also reduce vapor loads in sections of the tower above the pumparound. A pumparound takes liquid from the tower, cools it, and returns it higher up in the tower. The liquid condenses the vapors in the pumparound section creating liquid reflux for fractionation lower in the tower. Vacuum pipestills do not use overhead reflux seen in other distillation towers, a top pumparound is used instead. 

To provide raw material for this comparative study of untreated and heat-treated oils, asphaltenes from Cold Lake crude (crude asphaltenes) and from Cold Lake vacuum residuum (residuum asphaltenes) were prepared by n-heptane precipitation as described in the Experimental section. The Cold Lake residuum fraction was prepared by Imperial Oil Enterprises, Ltd. at Sarnia, Ontario, Canada. The distillation history of this bottoms fraction indicates that the pot material was subjected to temperatures as high as 314-318°C during atmospheric and vacuum distillation. The length of time at 300°C or higher was about two hours. This is well in excess of what would be experienced in a pipestill and should have provided ample time for any decomposition. It should be noted, however, that since it was possible to maintain the system vacuum at 0.35 mm, the maximum temperature experienced by the residuum was not quite as high as it might be during refinery distillation (e.g. ca 350°C). 

My own technical experience has pretty much followed this history of distillation simulation. My practical experience started back in a high-school chemistry class in which we performed batch distillations. Next came an exposure to some distillation theory and running a pilot-scale batch distillation column as an undergraduate at Penn State, learning from Arthur Rose and Black Mike Cannon. Then, there were five years of industrial experience in Exxon refineries as a technical service engineer on pipestills, vacuum columns, light-ends units, and alkylation units, all of which used distillation extensively. 

Crude oil was first distilled in batch distillation, like a lab technique, beginning in the 1850s. Advancements were made by increasing the size of the batch vessel. A continuous process was developed by using a series of batch stills - called a battery. The first continuous pipestill appeared in the 1920s and the modem pipestill came on the scene in the 1930s. A typical lube vacuum distillation unit is shown below (Fig 6). 

Refiners usually limit the temperature of redistillation in continuous pipestill and fractionator plants to 325 F by the use of steam, although some plants have been operated at 375 F. The tendency has been to decrease the temperature because the treating operation is much simplified if the temperature is low. Several companies have installed vacuum systems in which the temperature is limited to 275°F. The vacuum redistillation systems are usually of the two-stage type, in which part of the distillateJs distilled at atmospheric pressure and the heavier part is vaporized in 3 iecond fractionator which operates at a reduced pressure. 

The pipestill and vent chamber (Fig. 10-14) of the contact process are increasingly being replaced by a vacuum rerun distillation so that light lubricating oils leave the contact process as distilled clay-free stocks and only the heaviest stock contains clky. Such operations are particularly suited to the rerunning of solvent-treated oils which are so free from asphaltic material that they need not be add treated but must be further decolorized. The use of vacuum during the contacting step is helpful. 

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