This arrangement is very similar to that employed in the so-called double towers for the separation of oxygen and nitrogen. In this case the feed is relatively rich in the lower boiling component, nitrogen, and the first tower is used to produce an overhead that contains a high concentration of nitrogen and an impure bottoms containing 40 to 50 per cent oxygen. The liquid nitrogen overhead product from the high-pressure tower is added to the top of the low-pressure tower and serves as the only reflux for it. There is no other source of refrigeration to produce additional reflux. The impure bottoms from the high-pressure tower is introduced into the middle portion of the low-pressure tower. Such a two-tower system will give high-purity nitrogen and oxygen low in nitrogen although it will contain appreciable quantities mm of noble gases. It is common practice in this case to build the low-pressure tower on top of the high-pressure tower, and the condenser-reboiler is a common unit of both columns.
A further modification of the split-tower system is given in Fig. 7-26 in which the feed is rectified into impure bottoms and an impure distillate, and these are then retreated in the lower pressure tower. In general, this system gives slightly lower heat requirements than the previous system, and it utilizes the same principle, namely, reusing the heat several times in the middle region concentrations where the heat requirements are the highest. The systems of Figs. 7-22 to 7-26 can be utilized with three or more towers to obtain still further heat reductions.
In order to show the comparison between the different systems, the various equations have been plotted in Fig. 7-27 for the cases of feed concentrations equal to 0 and 1.0. The equation for the multieffect system has been plotted for n - 1, i.e., a single tower. If a two-stage system is used, the values should be divided by 2, etc. For the vapor-reuse system the maximum vapor requirement for the two columns is given, and no credit has been applied for the low-temperature heat that could be withdrawn for the cases with the relative volatility greater than 2.0. It will be noted that none of the modifications is so effective in reducing the heat requirements as the multistage system, although some of them are equal to it for specialized conditions. Of the specialized arrangements, vapor reuse would not appear to be so attractive as the split-tower system shown although, in cases where the waste heat was of real utility, it could be attractive. The relative attraction of the systems will be shifted somewhat as they are compared at reflux ratios lower than the minimum (or greater than total reflux). It still appears that, for a given number of towers, the multi-effect system is the most attractive from the heat viewpoint. Likewise, for a given total quantity of heat supplied to the system and for the same number of towers, the total plate area required is less for the
Comparison of vapor requirements.
multieffect system than for the modifications. This greater efficiency is due to the more efficient utilization of the heat over a wide temperature range.
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