This invention relates to fractionation processes in which two or more compounds are separated.
The separation of compounds by fractionation is an old and well-developed art. Fractionation is one of the most widely used processes in petroleum refining and chemical processing. Separation of chemical compounds is frequently carried out in a fractionating tower containing a stripping section and a rectifying section located below and above the feed point, respectively. These two sections commonly operate at the same pressure. Heat is normally applied at the bottom of the stripping section to reboil the bottoms liquid and heat is removed at the top of the rectifying section. The separation occurs in the fractionation column due to volatility differences between the compounds. The light components vaporize and the heavier or higher-boiling compounds condense so that the lighter components go overhead as a vapor and the heavier components go out the bottom as liquid. A condenser is normally used to condense at least a portion of the rectifying section overhead product so it can be returned to the top as reflux. The condensed reflux liquid passes down through the tower with heat and mass transfer occurring. Typically, a large portion of the heat removed from the overhead vapors is wasted in a condenser where it is heat exchanged with air or cooling water. This happens because the overhead vapors are normally at a low temperature level not suitable for generation of steam or other use. Also, all the heat input to the reboiler is normally from an external source. As a result of these factors, most fractionating towers are somewhat thermally inefficient.
It is known in the art that in certain situations compressors, which are often referred to as heat pumps in this context, may be used to advantage to reduce the utility cost of operating a fractionation process. This recognition has led to the utilization of a compressor (heat pump) to reboil the stripping section of a fractionation column by heat exchange of bottoms liquid and compressed overhead vapors of the same column. The prior art method of utilizing a heat pump in a fractionation process is described in an article starting at page 85 of the Sept. 1, 1975 edition of the Oil & Gas Journal. In this prior art system, the overhead vapor is compressed to raise its pressure and temperature and is then passed to the reboiler located at the bottom of the column. The heat exchange in the reboiler causes the condensation of some of the overhead vapor to produce a product liquid. Part of this condensed liquid is used as reflux. The heat given off in the condensation of the overhead vapors is transferred to the bottoms liquid from the stripping section to provide reboil heat.
A variation of the heat pump fractionation process described in the above-mentioned Oil & Gas Journal article is described in U.S. Pat. No. 4,137,129. The prior art fractionation processes utilizing heat pumps all use a single distillation column operated at a common pressure in both the stripping and rectifying sections.
In many fractionation processes, there are advantages in conducting the stripping portion of the fractionation at a low temperature and pressure. The tendency of the bottom materials to polymerize is reduced at lower temperatures, thus reducing fouling in the lower section of the distillation column. Also, the stripping operation is more efficient at lower pressures.
Similarly, there are advantages in operating the rectifying section at higher pressure, as this enables condensation of overhead vapors at more readily available temperatures, and the rectification process is more efficient at higher pressures.
A fractionating process utilizing a dual pressure fractionation tower is described in U.S. Pat. No. 3,783,126. The process described in that patent obtains the advantages of low temperature stripping operation and high pressure rectifying operation, but involves condensing overhead vapors from the top of the stripping section and pumping them at higher pressure into the rectifying section. Also, this process does not utilize heat from the rectifying section overhead vapors to reboil the bottoms in the stripping section.
A process for separating hydrocarbons in which a compressor is utilized between a stripping section and a rectifying section is described in U.S. Pat. No. 2,277,387. The process described in that patent does not utilize heat from rectifying section vapors to reboil the stripper.
Thus, there has been a need in the fractionation art for a process which combines the advantages of operating the stripping section of a distillation column at a low pressure and temperature while operating the rectifying section of the distillation column at a higher pressure, and simultaneously obtaining the efficiencies available from utilization of a heat pump in the distillation process.
In some cases, depending on the thermal condition of the feed, the heat wasted in the overhead condenser will be essentially equal to the heat input to the reboiler. If some means is available to use the condenser heat to heat the reboiler, the thermal efficiency could be greatly increased and the energy operation cost reduced. This is accomplished to some extent by prior art fractionation processes utilizing heat pumps.
This invention achieves this objective by operating the stripping and rectification section at dramatically different conditions to permit using the heat in the overhead from the rectification section to heat the reboiler at the bottom of the stripping section, and by using a heat pump compressor to raise the temperature and pressure of vapors passing from the stripping section to the rectifying section of a fractionator. Further, the use of a compressor between the stripping and rectifying sections, with resultant higher pressure in the rectifying section, enables a given separation to be accomplished more efficiently, with fewer stages or trays in the unit, or smaller vessel sizes, or both.