marked color changes that the end point is readily determined by the color.18
In a solution containing approximately 4.5 per cent methyl alcohol and 10 per cent acetic acid, there must be, in accordance with the equilibrium of the reversible reaction, a certain amount of methyl acetate. When the acetic
CHsOH + CHsCOOH CH8COOCH8 + HaO
acid is neutralized, the methyl acetate slowly hydrolyzes to methyl alcohol and acetic acid until equilibrium is again attained, the solution again becoming acid. If more lime is added the methyl acetate will finally be completely hydrolyzed, but more lime is not commonly added and the hydrolysis is so slow that methyl acetate is generally a prominent constituent of the distillate from the neutralized liquor.
A similar effect is due to the presence of formaldehyde in the pyro-ligneous acid. In slightly acid solution formaldehyde is stable, but when the solution is made slightly alkaline and heated it undergoes self-contained oxidation and reduction forming methyl alcohol and formic acid (see page 56). Here too a completely neutralized solution tends to become acid again, due to reactions started by the neutralization.
The most important and noticeable effect of neutralization is the formation of non-volatile organic compounds by polymerization, some of which are insoluble and help to form the sludge, others being soluble and thus becoming impurities in the acetate of lime. A few guesses have already been made that the insoluble polymerization products were mostly of the class of aldehyde resins and the soluble of the class of xylochinon, but these are only good guesses, and the organic part of the sludge and the organic impurities in the acetate are really of unknown composition. That the sludge contains organic material can be shown by boiling with hydrochloric acid to dissolve the lime and other inorganic constituents. A thick heavy oil is left which on cooling solidifies to a hard pitchy substance. The inorganic portion of the sludge probably consists of unburned calcium carbonate and other insoluble impurities in the lime.
The neutralized pyroligneous acid on distillation now yields the volatile constituents not fixed by the lime, consisting mainly of water, methyl alcohol, acetone, methyl acetate and allyl alcohol. By this distillation also some of the oils, especially the phenol ethers, which are apparently more soluble in the acid solution than in the neutral solution, are brought to notice by separating from the distillate. On further concentration of the alcohol constituents still more of these
M Johlin Uour. Ind. Eng. Chem. 7, ss>6 (1917)] says that this color change 1b largely due to traces of iron in the lime reacting with the di-'metliyl ether of pyrogallic acid or other oils separate, probably on account of their greater solubility in methyl alcohol than in water. On account of the different solubility of these oils in different fractious of the distillate, it is advantageous to separate them continuously from the distillate, otherwise oils which are insoluble in some of the later parts of the distillate and 'can be separated from them may be redissolved if allowed to mix with the first part of the distillate in which they are more soluble.
The preparation of pure methyl alcohol from the crude concentrated wood alcohol is a more complicated process than a simple separation of various constituents according to boiling points. This is due to the facts that some of the constituents form constant boiling mixtures which can not be separated by distillation, that certain volatile bases, probably pyridines, require an acid washing for removal, and that oils soluble in the crude are not soluble in certain fractions and must be removed at the proper place.
According to Haywood 14 methyl alcohol and acetone form a constant boiling mixture consisting of 86.5 per cent by weight of acetone and 13.5 per cent methyl alcohol and boiling at 55.90 C. Bergstrom16 found the mixture to consist of 90 per cent by weight of acetone and 10 per cent methyl alcohol, but he did not determine the boiling point. According to Ryland10 methyl alcohol and methyl acetate form a constant boiling mixture consisting of 18 per cent methyl alcohol and 82 per cent methyl acetate, boiling at 53.5°-54.5° C. He also states that methyl acetate and acetone form a constant boiling mixture, but the boiling points of the pure compounds are so close together that the composition of the constant boiling mixture could not be determined. From these figures it can be seen that methyl acetate and acetone can not be separated completely from methyl alcohol by distillation alone, but that methyl alcohol can be separated in pure state from the other constituents by losing a certain amount of methyl alcohol in the constant boiling mixtures.
It is not known whether a triple constant boiling mixture is formed, but if not, the results of distilling a mixture of these three constituents is much the same as if the mixture consisted of the three components, (1) methyl alcohol B.P. 66.5°,.(2) constant boiling mixture of methyl acetate and alcohol with B.P. 53.5°, and (3) constant boiling mixture of acetone and alcohol with B.P. 54.5°. That this is the case is indicated by the fact that the first fractions on distillation have a specific gravity higher than the later fractions (methyl acetate sp. gr. 0.96, methyl alcohol and acetone sp. gr. both about 0.79). This is also indicated by the presence on the market of commercial products from wood alcohol refining, one containing 50-60 per cent methyl acetate,
Bihang Ml Jemkomtorits Amaler Amt Chm. Jour. »1, 384 <1899).
Bihang Ml Jemkomtorits Amaler Amt Chm. Jour. »1, 384 <1899).
20 per cent acetone and the rest presumably methyl alcohol, the other containing 80 to 90 per cent acetone. The former of these products has a composition very near to that of 3 parts of low boiling mixture (2) and one part of mixture (3). The latter is almost the pure constant boiling mixture of acetone and methyl alcohol.
The separation of these constant boiling mixtures from methyl alcohol is difficult on account of the small differences in boiling points and this results in the production of large quantities of refined wood alcohol containing small amounts of acetone, since for many purposes small amounts of acetone are no disadvantage. The methyl acetate constant boiling mixture is more readily separated by distillation because there is less of it and it has a lower boiling point.
Many chemical methods for separating acetone from methyl alcohol have been suggested but they are apparently not as efficient as the separation by distillation, since none of them are used commercially. The formation of chloroacetone and subsequent separation by distillation is the favorite suggestion and a method based on the insolubility of acetone in caustic soda solution has also been described.17
The solution containing calcium acetate and organic impurities from which the alcohol constituents have been removed by distillation is now ready for the preparation of the acetate. On account of the fact that calcium acetate is very soluble in water, and is more soluble in cold water than in hot, ho rational method of concentration and crystallization has been, developed, and instead evaporation and drying are the methods used for preparing the final acetate product. This method results in an acetate contaminated by organic impurities. Commercial acetates contain 80-83 per cent calcium acetate and this variation is probably due largely to the conditions of neutralization, although the exact effect of these conditions is not known. There is a certain amount of calcium formate also present. No data on the calcium formate content of American made samples are available, but Berg-strom18 shows 2.3 per cent formic acid on two samples of acetate produced in Sweden from hardwoods. Just how much of the to 20 per cent impurity is water of crystallization, hygroscopic water or inorganic impurity is unjcnown, although calcium acetate is supposed to crystallize with one molecule of water from hot solutions,19 and this would give about 9 per cent water and 10 per cent organic impurity in an 81 per cent acetate.
Combinations of Steps in Refining
The simple refining process which has been discussed in some detail consists of numerous separate operations during which the main part
11 U. S, Pat. No. 1,106,707. u Bihang tilt Jernkontorets Annaler, 191 a. u Lumsdai, Jour. Chtm. Soc. Si, 355 (190a).
of the crude distillate is repeatedly condensed and redistilled. This is an inefficient process from the standpoint of consumption of steam and condenser water and many methods are now in use for combining certain operations so that the vapors from one. may be used as the heating medium for the other with corresponding economy in steam and condenser water. One suggested method is to use the heat in the vapors and gases as tliey come from the retort for a continuous refining process in which the products are separated by fractional condensation. The use of multiple effect evaporators is also becoming common for the distilling operations in which fractionation between volatile liquids is not required, such as the distillation of the crude pyroligneous and the evaporation of the acetate solution. The use of column stills on the neutralized pyroligneous acid for separating the alcohol in concentrated form in one operation is another combination of steps which increases efficiency in steam consumption.
The first operation in the ordinary refining process, the distillation of the crude pyroligneous to remove the dissolved tar, requires a distillation of the entire watery distillate with immediate condensation, and nothing is accomplished except to prepare the pyroligneous acid for neutralization, after which it is immediately redistilled. If the soluble tar could be prevented from reaching the condenser by some device between the retort and the condenser, this first distillation could be- avoided. Different methods for accomplishing this have been suggested and tried but there is only one plant in the United States where such an operation is even attempted and it is reported that only about half of the pyroligneous acid comes from the condenser free enough . from tar so that it can be neutralized directly without redistillation.
The removal of soluble tar from the hot gases and vapors is not entirely a problem of fractional condensation because the soluble tar is not volatile. It must occur in the vapors in the form of a fog or mist which is so fine that it does not settle out in the short distance from the retort to the condenser. A partial cooling of the vapors until the first condensate is formed does not, therefore, cause a condensation of pitch (soluble tar), but a condensation of the highest boiling volatile constituents of the tar (heavy oils).30 The condensation of a small part of the vapors may, however, be of assistance in the precipitation
In tlie vapors coming from a wood distillation retort at a temperature of say aoo* C. everything is probably in the form of a true vapor except the actually non-volatile pitch. On cooling the first condensate consists of the highest boiling oils with boiling points around 370° C. If these oils were in vapor from below their boiling points only on account of the presence of saturated steam, water would condense along with them in the proportion required for their volatilization, but the steam ie in superheated condition and there Is also a con* siderable quantity of pas present and both the superheated steam and the gas carry the oil vapors the same as air or other inert gas carries water vapor below ioo° C. It haa been found that cooling the vapors to about 105°. C. condenses most of the tar oils. (Hawley and Ptsr, Cham. 15- Mrt Eng. 36. 1031, 193a.) The only oils which remain in vapor fonn above ioo° C, are those which constitute the maximum amount that could be distilled with the amount of saturated steam present plus those carried in vapor form by the gaa at that temperature.
of the tar because the first drops of condensate are very likely to form on the suspended pitch particles and increase their size so that they more rapidly settle from the vapors. This is probably the principle upon which the tar separators work which cool the vapors until condensation just begins, and then pass the vapors through towers or columns where they are scrubbed with liquid tar. Klar 21 and Bergstrom and Wessl6n 22 have described such tar separators but it is not known to what extent they are used in Europe or what quality of acetate can be made by direct neutralization of the pyroligneous acid passed through them.
Some preliminary experiments have been reported on another type of tar separator.28 On a small retort a Cottrell precipitator was used with the idea that if the soluble tar existed in the vapors in the form of a fog it could be separated from the true vapors by electrostatic precipitation. This attempt was successful in stopping most of the pitch before it reached the condenser, but a small amount of soluble tar was found in the pyroligneous acid and the acetate made by direct neutralization was only 78 per cent pure calcium acetate in comparison with the 80 to 83 per cent obtained in practice by separating the tar by distillation. Either on account of poor insulation in the precipitator or because some of the soluble tar exists as vapor before condensation and polymerizes to non-volatile products afterward the results were not entirely satisfactory.
Two difficulties always encountered in any combination of steps in refining wood distillation products are caused by the continuous formation of "tar" throughout the process and by the separation of oils which should be removed from the products of a distillation before redistillation takes place. Apparently there are volatile products in the crude pyroligneous acid which slowly polymerize to non-volatile products—so slowly that they may not be removed in the first distillation but remain to deposit on evaporator tubes and in distilling columns during subsequent distillations. A continuous fractional condensation process of refining has been mentioned in which the heat of the vapors is sufficient for most of the refining process. After the vapors leave the tar separator they would be passed into milk of lime for separation of the acetic acid, and the temperature of the container would be maintained at such a point that a concentrated acetate solution would remain behind, while the alcohol vapors would pass over to a continuous column still where the water would be separated by fractional condensation and the concentrated alcohol would pass to a condenser. Such a process is perfectly possible and is ideai from the standpoint of fuel consumption, but there are difficulties to be over-
M0m TrSkeining, p. aoi.
• Hawley and Pier Chem. & Met, Eng. 36, 1031 (1923).
come. The oils which get through the tar separator and would be removed from the pyroligneous acid before neutralization in the ordinary process now come in contact with the lime and probably form impurities in the acetate. Part of the oils would also remain uncondensed in the vapors from the acetate solution and would go to the alcohol still. Here most of them would probably be condensed with the water but some would carry through and contaminate the alcohol solution. Probably the greatest difficulty of a process of this kind, however, would lie in the non-condensible gas from the retort, which would carry through all the apparatus and finally pass out the condenser with the concentrated alcohol solution. We have already seen that there is a slight loss of alcohol in this gas even when it is separated by the retort condenser in the first step of the refining process. The loss would be very much greater in this case where the alcohol is much more concentrated and its vapor pressure is, therefore, much higher. In order to control this loss very efficient gas scrubbers would have to be used with large amounts of water for scrubbing, so that the gain in the fractional condensation process would probably be offset by the additional expense of recovering the alcohol from the wash water.
It is hoped that this discussion of the chemical properties of wood, its decomposition by heat, the products of the decomposition and the processes of separating the valuable products, has been successful in bringing out the complexity of the subject, the small amount of detailed information available on important points and the opportunities for research on scientific details and on practical application.
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