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DISTILLATION' OF FORMALDEHYDE SOLUTIONS 61
hyde melts in the range 120 to 15Q°C, forming a boiling liquid. The boiling
point of the solution approaches this figure at high concentrations (70
per cent and higher).
Fractional Condensation. When the mixture of formaldehyde and water
vapor obtained by boiling a formaldehyde solution is subjected to partial
condensation, the formaldehyde content of the uncondensed vapor is
greater than that of the original gas mixture. In other words, the vapors
behave as a mixture of water and a volatile gas, as formaldehyde would be
expected to behave. By making use of this phenomenon, concentrated
solutions of formaldehyde can be obtained from dilute solutions by a process
involving distillation and fractionation at atmospheric pressure. The
apparently anomalous results obtained by Ledbury and Blair 15 on distilling
formaldehyde with the use of a fractionating column were due to this
phenomenon.
In 1927, Zimmerh 25 reported that concentrated distillates containing up
to 55 per cent formaldehyde could be obtained by distilling comparatively
Table 14. Concentration of Formaldehyde in Liquid and Gas Phase for Solutions
Boiling at Approximate Atmospheric Pressure.
Concn- of CHiO Pressure Boiling Temp. Ctmcn. of CHsO
Solution (%) (mm Hg) CO) in Vapors
6.82 740 98,0 6.95
17'. 34 742 98.1 14,96
27.67 751 9S.5 22.79
dilute formaldehyde solutions at atmospheric pressure. This was accomplished
by the use of a five-foot distilling column surmounted by an efficient
reflux condenser. The writer 20 later obtained equivalent results
with an eighteen-inch tube in which the vapors from a boiling formaldehyde
solution were partially condensed with a water-cooled bayonet-type condenser.
In this apparatus the reflux condensate was returned to the still
with minimum exposure to the hot ascending vapors. By this procedure
a substantial portion of the formaldehyde in a 28 per cent solution was
collected as a 53 per cent distillate.
The fractional condensation process gives the best results when the
condensate is chilled to a low temperature and removed promptly from
further exposure to the hot gases. Apparently the rate at which formaldehyde
dissolves in the condensate decreases with temperature and the
liquid is removed before it has had the opportunity to come to equilibrium
with the gas phase. Although the efficiency of the fractional condensation
process falls off considerably when an attempt is made to carry it to its
extreme limit, German investigators 11 have succeeded in producing paraformaldehyde
by polymerization of the end gases obtained iu a thoroughgoing
process of this sort. Paraformaldehyde obtained in their process