Mixtures of low-boiling gases are readily separated by phase change at cryogenic temperatures. One type of process for separating such mixtures is partial condensation in which a feed gas stream is cooled at a relatively high pressure to a temperature below the dew point of the mixture in order to condense a heavy fraction. The condensed liquid and the uncondensed vapors can be separately recovered as products. Typically, the condensed liquid fraction is rewarmed and at least partially revaporized, usually at a lower pressure. Such a process can provide high recovery of higher boiling components, but since there is no purification step the process cannot recover these products at high purity. Similarly, it is difficult to obtain good separation of lower boiling components if these are to be recovered. In order to increase the purity of recovered fractions, multiple warming zones and additional separators have been incorporated into the revaporization step, but this significantly increases the complexity of the process with only a small increase in product purity. Processes of this type have been suggested for the separation and recovery of C.sub.2.sup.+, C.sub.3.sup.+ or C.sub.4.sup.+ hydrocarbons.
Simple partial condensation processes have been described by L. M. Lehman in "Cryogenic Purification of Hydrogen", Energy Progress, Vol. 3, No. 1, pp 7-12 (March 1983) and in U.S. Pat. 4,559,069. More complex partial condensation processes utilizing multiple warming and separator steps in the revaporization of the heavier products especially C.sub.2.sup.+, C.sub.3.sup.+ or C.sub.4.sup.+ hydrocarbons, are described in U.S. Pat. Nos. 3,373,574; 4,256,476; and 4,726,826.
U.S. Pat. 2,994,966 discloses a method for separating hydrocarbon mixtures using a vertical shell-and-tube heat exchanger with internal baffle trays in which countercurrent liquid-vapor flow occurs with vaporization and stripping within the tubes and countercurrent liquid-vapor flow occurs with condensation and absorption on the outer surface of the tubes. Heat is exchanged across the tube walls while mass transfer occurs simultaneously inside and outside of the tubes. In another embodiment, a liquid is subcooled in upward flow which transfers heat through the wall to a region of countercurrent liquid-vapor flow in which vaporization and stripping occur simultaneously.
Higher boiling products can be recovered at higher purity by distillation in multi-stage distillation columns. While such a technique increases product purity, there is a large increase in the capital cost for the distillation column and related equipment such as reboilers, condensers, reflux drums and pumps. U.S. Pat. Nos. 4,695,303, 4,698,081, and 5,275,005 describe the incorporation of de-methanizer distillation columns for the production of relatively high purity C.sub.2.sup.+ product streams. Similarly, processes with de-ethanizer columns to produce high purity C.sub.3.sup.+ product streams are described in U.S. Pat. Nos. 4,666,483, 4,710,214, 4,711,651, 4,714,487, 4,752,312, 4,854,955, and 4,921,514.
Another approach to separating low-boiling gas mixtures is the lean oil absorption process in which a heavier hydrocarbon oil (the lean oil) is used to absorb C.sub.2.sup.+ and/or C.sub.3.sup.+ hydrocarbons from a feed gas. While such a process can provide a higher product recovery, it is generally more power intensive than processes which rely on partial condensation of the heavy, higher boiling hydrocarbons, since the absorbed C.sub.2.sup.+ and/or C.sub.3.sup.+ product components must subsequently be separated from the absorption oil fraction as well as from the co-absorbed light impurities. U.S. Pat. No. 4,272,269 describes one such process which utilizes a refrigerated C.sub.5 absorption oil to scrub C.sub.3.sup.+ components from a natural gas feed. A similar process is suggested in U.S. Pat. No. 4,698,081, which describes a process wherein a C.sub.3 -C.sub.5 hydrocarbon fraction is recirculated to the top of a demethanizer column as an absorption oil to increase the recovery of C.sub.2 from a natural gas feed. Other scrubbing processes are described in U.S. Pat. Nos. 4,942,305, 4,881,960, and 4,966,612.
All of the techniques described above have common disadvantages. When it is desired to increase the recovery of the heavier, higher boiling components (e.g., C.sub.2 and heavier hydrocarbons), more light components are condensed as impurities along with the additional quantities of the heavier components. This results in a lower product purity which may require additional separation and/or distillation equipment to remove the additional impurities and to produce a high purity product. Such additional equipment obviously increases the capital cost of the process.
The present invention addresses these problems by combining heat transfer and mass transfer in a single step in which a liquid stream is simultaneously heated and stripped of dissolved light impurities by indirect heat transfer from one or more cooling process streams. The stripped liquid stream can be separated further into individual high-purity products requiring no further process steps to remove lower-boiling impurities.