Distillation: processes – separatory – Plural distillations performed on same material – One a distillation under positive pressure or vacuum
Patent
1995-02-27
1997-05-27
Manoharan, Virginia
Distillation: processes, separatory
Plural distillations performed on same material
One a distillation under positive pressure or vacuum
196 1452, 196 99, 202152, 202176, 202168, 203 80, 203DIG9, 208 87, 208100, 208347, B01D 300
Patent
active
056328672
DESCRIPTION:
BRIEF SUMMARY
This invention, in its broadest sense, relates to apparatus and a method for recovering a relatively volatile component of a solution from a less volatile component of the solution. However, it finds particular application to recovering solvent from the extract solution from a solvent extraction process.
Solvent extraction processes are very well known, for example in the oil industry. Examples of solvent extraction methods are lubricating oil extraction, de-asphalting and lubricating oil de-waxing processes. Usually, economics dictate that the solvent used in the solvent extraction process be recovered and recycled for re-use in the extraction process.
Numerous techniques are known for recovering the solvent from the extract solution. Typically, the extract solution is passed through a separator, for example a recovery tower, in which the solvent is recovered from the extract product. However, due to inevitable separation inefficiency and carryover within the separator, the recovered solvent contains residual amounts of the extract product. Usually, because of the presence of these residual amounts of the extract product in the recovered solvent, this solvent cannot merely be recycled to the solvent extraction plant: rather, measures have to be taken to remove as much of the residual extract product as possible beforehand. Furthermore, trace quantities remaining after as much residual extract product as possible has been removed effectively reduces the capacity of the solvent extraction plant.
Examples of solvent recovery processes of background interest to the present invention are disclosed in U.S. Pat. No. 4,482,453 (Coombs et al), U.S. Pat. No. 4,664,783 (Preusser et al) and U.S. Pat. No. 4,997,547 (Emmrich et al).
In a typical lubricating oil extraction process, a distillation feed, comprising a fraction boiling in the lubricating oil range from a vacuum distillation tower, and solvent are separately fed to a solvent extraction tower, from which raffinate, i.e. lubricating oil and wax, is produced as overhead and extract solution (i.e. a solution of aromatics in solvent) is produced as bottoms. One form of plant known to the Applicants for recovering the solvent from the extract solution from a lubricating oil extraction plant will be described in more detail hereinbelow with reference to FIG. 1. Essentially, however, this plant comprises a recovery tower to which the solvent extract is fed, via a pre-heater, so that most of the solvent is vaporised as it enters the tower, the remainder vaporising within the tower. The tower is a conventional distillation tower provided with trays which are often refluxed with solvent or part of the solvent/solute mixture. The ascending vapor is contacted with descending cooler liquid, for example branched off from the solvent extract feed before it is pro-heated, together with any reflux liquid. In order to improve the solvent recovery, residual quantities of solvent in the liquid bottoms product from the recovery tower can be removed in a stripper and, together with strip gas, introduced into the recovery tower. Solvent vapor produced overhead from the recovery tower is passed, along an overhead line, to a condenser, in which the solvent vapor is condensed to form liquid solvent which can then be recycled for re-use in the lubricating oil extraction plant. A small amount of solvent vapor may condense to form liquid solvent in the overhead line.
There are three main problems with existing solvent recovery facilities such as exemplified above in a lubricating oil extraction plant. Firstly, the solvent/feed ratios needed for such processes are typically large and this in turn often results in flooding limitations in the recovery tower. Thus, the plant capacity cannot be increased unless the tower is replaced with a larger one. Such tower replacements are undesirable because of their high cost. Secondly, high vapor velocities or flooding can result in oil entrainment from the recovery tower and oil contamination of the solvent recovered overhead. Thus, the treat ratio
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Davis Michael B.
Dugan John J.
Eagan James D.
Welmers Adrianus
Allocca Joseph J.
Exxon Research and Engineering Company
Manoharan Virginia
Takemoto James H.
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