Two methods have been developed to purify large batches of technical-grade carbon tetrachloride. The relatively impure carbon tetrachloride that had been purchased for use in infrared analysis of dissolved hydrocarbons proved that it could be purified. One method is spinning-band distillation, which is a special type of fractional distillation. The other method involves the use of a molecular sieve, which is a material that is formulated to have special porosity and adsorbent properties and can physically lock substances (e.g., the impurities that one seeks to remove from CCl4) into its pores.
To be useful as a solvent for infrared analysis, CCl4 must be very pure. The two methods were investigated as alternatives to simple distillation, which was known to be insufficient for achieving the required purity for two reasons: (1) in general, simple distillation does not remove impurities (e.g., other solvents) that have boiling temperatures near that of the solvent that one seeks to purify and (2) the technical-grade CCl4 that was to be purified had been produced by simple distillation.
Spinning-band distillation involves the use of a thin band rotating rapidly in a narrow-bore distillation column. The condensate returning to the pot is wiped by the band in a very thin layer along the bore. The returning condensate makes contact with the counter-flowing, rising vapors. The net result, in comparison with that achievable by simple distillation, is better separation of constituents with higher boiling temperatures from constituents with lower boiling temperatures. The improvement in separation is attributable to enrichment of the vapor in the more volatile constituents.
Although the spinning-band distillation method was effective, it was decided that an attempt should be made to develop a faster and perhaps even more effective method. Gas chromatography coupled with mass spectrometry (GC-MS) and infrared spectrometry had indicated that most, if not all, of the impurities were polar molecules. So it was decided to try to develop a method that would preferentially remove polar molecules from this nonpolar solvent (CCl4). It was reasoned that molecular sieves might be able to achieve such preferential adsorption. Several were tried and one was found that adsorbed the impurities as desired.
In this method, an activated molecular sieve is immersed in the solvent in a glass container. Optionally, one can stir the solvent to bring more of the solvent into contact with the molecular sieve within a given time and thereby accelerate the purification process. By experimentation with several different molecular sieves, the Linde 13X (or equivalent) molecular sieve was found to be exceptionally effective in removing impurities from CCl4. The Linde 13X molecular sieve has a pore size of 10 Å.
In comparison with spinning-band distillation, purification by use of this molecular sieve was found to be faster. In addition, the concentration of impurities (1 part per million) remaining after contact with the molecular sieve was found to be less than that remaining after spinning-band distillation. Yet another advantage of the molecular sieve is that it can be regenerated and reused. Recent advances in the synthesis of molecular sieves suggest that it may be possible to tailor the pores and/or constituents of a molecular sieve to remove selected impurities from a liquid.
This work was done by Raymund P. Skowronski and Carl J. Schack of the Rocketdyne Division of Boeing North American for Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Materials category, or circle no. 166 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).