The main condenser or reboiler in a double column cryogenic air separation plant has heretofore generally been of the thermosyphon type. With this type of heat exchanger nitrogen vapor from the higher pressure column condenses, exchanging heat by indirect heat exchange with the lower pressure vaporizing liquid oxygen of the lower pressure column. The liquid oxygen is drawn up through the heat exchanger by the thermosyphon effect and the heat exchange is carried out by the countercurrent flow of the liquid oxygen against the downflowing gaseous nitrogen.
A problem with the conventional thermosyphon configuration is that because of the head of liquid oxygen required to drive the circulation, the oxygen pressure at the base of the heat exchanger is increased. The oxygen liquid is therefore subcooled as it enters the heat exchanger. As the liquid rises, its temperature increases by sensible heat transfer and the pressure falls until eventually the temperature reaches the saturation temperature and boiling occurs. The net consequence is that the thermal performance of thermosyphon reboilers is diminished and the pressure of the condensing nitrogen cannot be reduced below a limiting value.
Those skilled in the art have addressed this problem by employing a downflow heat exchanger wherein both the gaseous nitrogen and the liquid oxygen flow downwardly in cocurrent fashion during the heat exchange. The downflow configuration reduces the nitrogen pressure in the higher pressure column resulting in power savings.
It is important in the operation of a downflow heat exchanger to ensure that the boiling liquid oxygen does not boil to dryness. Boiling liquid oxygen to dryness reduces the heat exchange efficiency and can increase the hydrocarbon concentration in localized areas within the heat exchange passages so as to reach a flammable concentration in such pockets, raising the danger of ignition. Accordingly, in the practice of cryogenic air separation with downflow heat exchangers, it is important that the liquid oxygen be evenly distributed to each of the liquid oxygen passages and that the liquid oxygen be uniformly distributed along each passage. This uniform distribution is generally done in two stages above the boiling heat exchange passages, a first rough distribution stage spaced from and followed by a second fine distribution stage. The well distributed liquid then flows through the heat exchange passages. The first stage has generally employed either orifices, openings or sparger tubes and the second stage has generally employed hardway fins. A disadvantage is that relatively expensive components are used for the first stage and it would be preferable to use only lower cost hardway fins to completely distribute the liquid. Furthermore, conventional first stage distributions require rather precise tolerances for the orifices or openings used to effect the first stage distribution.
Accordingly, it is an object of this invention to provide a downflow heat exchanger and heat exchange method, which may be effectively employed in cryogenic air separation, and which can reduce the problems of conventional downflow heat exchangers such as uneven liquid distribution.