The Decomposition of Resinous Wood by Heat

The decomposition of softwood by heat follows much the same course as the complicated reaction described for hardwood and, in the case of the highly resinous wood used commercially, the process is further complicated by the presence of considerable quantities of resin and the products from the wood are mixed with the distillation products of the resin. Knowing the properties of the constituents of the resin it is possible to describe the manner in which the mixture of wood and resin distils when subjected to gradually increasing temperatures.

The volatile oils distil mostly below the temperature at which wood begins to decompose strongly, say 250° C., so that it might be a fairly simple matter to distil these oils before the wood distils and, therefore, obtain them free from contamination by the decomposition products of the wood. This seems even more readily possible when we consider that these volatile oils in the presence of steam distil below ioo° C., and if there is enough water in the wood to furnish the steam they might be completely distilled below ioo° C. . There are, however, some difficulties in the way of accomplishing this result. We have seen that wood is a poor conductor of heat, and when distilling large charges of wood the temperature gradient between the outside and inside of a charge is very rapid, so that slow and careful heating . would be required to obtain the distillation of thé volatile oils without decomposing a part of the wood. The presence of the rosin causes even more difficulty since it begins to decompose at even lower temperatures than the wood and since it lowers the vapor pressure of the turpentine. Accurate figures are not available for the minimum temperature of the decomposition of rosin, but in the steam distillation of wood it has been shown1 that there is some contamination of the turpentine when distilled, from the wood at 75 pounds steam pressure (1600 C.) and that this is probably due to the beginning of decomposition of the rosin. The decomposition at this temperature is only slight and it is believed that even up to 200° C. the volatile products given off by rosin are still small in amount. Higher than this, however, the rosin begins to decompose rapidly and 200° C. is usually con-

1 Forest Service Bull. 169, P. 25.

sidered the highest temperature to which resinous wood can he heated without serious contamination of the turpentine by rosin decomposition products.

It would be expected that considerable turpentine would be driven off from resinous wood at temperatures near ioo° C., since a mixture of water and turpentine will boil below this temperature but it is not until about i6o° C. is reached that much distillate is actually obtained. The reason for this is probably that the water and the oil are not in contact in the wood and it is not until the resin becomes a thin fluid with the heat and begins to flow in the wood that much distillation can take place. That such a flow does take place is shown by the fact that with rich wood and carefully controlled temperature considerable quantities of rosin can be drawn off from the bottom of the retort after the temperature has been held at 180 to 200° C. for some time. If the temperature is carefully controlled all the turpentine and part of the pine oil can be distilled off before the rosin begins to decompose. Then as the temperature is raised the rosin begins to distil rapidly, and finally the distillation point of the wood is reached. Commonly, however, these separate stages are not readily distinguished, and a mixture of volatile oils, rosin products and wood distillates may come over together from the start.

The presence of the resin makes it possible to obtain a liquid product from the bottom of the retort by drawing it off at some stage in the distillation instead of distilling it off through the condenser. It was noted above that with proper care an only slightly decomposed rosin could be drawn off during the first stages of the distillation. If the retort is tapped at a later stage or if the temperature is not controlled, a tar may be drawn off. If this tar is allowed to remain until the last stages of the process, a hard pitch may be drawn off.

Before discussing the products of distillation it will be necessary to state what little is known about the composition of rosin distillation products, since rosin is such a large part of the raw material. Rosin has been distilled commercially for many years and many publications have been made on the subject but yet little is known on the actual composition of the products. A few indefinite compounds have been identified or at least have been named, but not even a table giving percentages and boiling points or specific gravity of the oils has been published. The usual terms in which the products are described are the trade terms, such as "rosin spirits," "kidney oil" and "blue oil" * It is known m a general way that on distillation rosin furnishes a very complex series of oils with boiling points all the way from 50° to 170° C., thus overlapping completely the boiling range of wood tar oils, of turpentine and pine oil > The oils are only partly soluble in or polymerized by alkali and this characateristic can be varied largely by the

* Shuey, Naval Stores Rtvkw, Special Number, p. 333 (igai).

conditions of distillation* This is about all that can be said on rosin oils and indeed is all that is necessary for the further discussion.

The process in which the turpentine and pine oil are distilled first at low controlled temperatures will not be discussed further since this is not a common commercial practice, but instead the more common process of distilling in which all the products come off more or less mingled and are collected together for further refining. The discussion will be further limited to the process in which all the products are distilled and none is drawn off the bottom of the retort. The total oily distillate will then consist of the turpentine and pine oil originally present in the wood, together with the wood tar and the rosin oils.

There is a difference of opinion without data on either side in regard to the effect of different systems of distillation on the yield, composition and value of the oily distillate. It is commonly believed that certain retorts with poorly arranged heating flues may "burn up" the distillate giving low yields and poor quality products, but from what we have seen of the decomposition and distillation of the different constituents of the raw material, volatile oils, rosin, and wood fiber, it is difficult to understand how there can be any noticeable diminution in yields of total oil even with considerable superheating during the distillation process.

To take a concrete instance, suppose the maximum temperature to which the bottom of the retort was heated was 500° C., what would be exacted in the way of loss of total oily distillate? Only a small part o'f the distillate would necessarily be subjected to a temperature anywhere near as high as this, and even this small part would be decomposed into nonliquid products such as coke and gas only to a small extent at this temperature. Suppose 10 per cent of the products are subjected to the maximum temperature and thereby 10 per cent of this portion are decomposed into coke and. gas, the total loss would be only I per cent or a practically negligible quantity. With any reasonable skill in designing the heating system of a retort and reasonable care in operation it should be possible to keep the maximum temperature below 500° C. and obtain full yields of products.

There is more to be said in regard to quality of products, however. It has been stated that the composition of the distillation products of rosin varied with the condition of distillation and that unnecessarily high temperatures would give more decomposition of the original rosin and

* This is for the reason tljat rosin is slightly volatile at the temperature of its destructive distillation and, therefore, varying quantities of undecoraposed rosin are vaporized with the volatile decomposition products. If rofrin is carefully distilled in vacuo a very large proportion can be obtained as solid undecomposed rosin, if distilled slowly at atmospheric pressure a considerably smaller proportion of unchanged rosin is obtained and if distilled rapidly a still smaller proportion. In practice frequently the unchanged rosin ib not wanted and the "first run'* oil is redistilled in order to decompose it further: the "second run," "third run," eto.F oils resulting are thinner and contain less "rosin acids." We have often wondered why a single distillation under pressure might not accomplish the same results as repeated distillations at atmospheric pressure.

a product more like "second" or "third run" oils. It is possible, therefore, that the composition of that part of the total distillate which comes from the rosin may be varied by the amount of .superheating during the distillation. So little is known of the detailed requirements of composition for these distillation products that it can not be foretold whether such a superheating would cause an improvement of quality or otherwise.

The effect of slight superheating on the composition of the pine oil constituents is not known, but it is believed that the effect is not-serious. It is known, however, that at temperatures above 250 ° C. pinene may be transformed into dipentene. That this does take place to a certain extent in commercial wood distillation processes has been shown by the composition of the refined wood turpentine. One wood turpentine contained about 40 per cent dipentene instead of the 10 per cent normally present.8 This change of part of the pinene into dipentene does not result in any considerable reduction in value, however.

Softwoods are somewhat different from hardwoods in the distribution of methoxyl groups in the distillation products.4 There is. less methoxyl in the pyroligneous acid, corresponding to the lower yield of methyl alcohol. There is also less methoxyl in the dissolved tar and more in the settled tar from softwoods. The effect of treating the softwood with sodium carbonate before distillation is much the same as in the case of oak: the methoxyl in the pyroligneous acid is increased and that in the dissolved tar, the charcoal, and the gas is decreased. With the softwood, however, the methoxyl in the settled tar is decreased very considerably by the sodium carbonate.

Yields of Products

The question of yields of turpentine, pine oil, tar oils, tar and pitch or, in short, of total oily products is one that is apparently not properly comprehended and that is the cause of many mistakes and failures. It has already been stated that the yields depend almost entirely on the amount of resin m the wood distilled and that this is a very variable factor. Ordinary unselected sawmill waste or green limbs from long-leaf pme may contain only about 5 per cent resin and give tar products only a little greater in quantity and a little different in quality than

III !n T hariW00Ad' »X 27 S*. per ton of a black tar with about 40 per cent p, ch A poor grade of lightwood may contain 15 per cent resin and yield per ton 46 gal. of total oils, including a tar with a more brownish color and less pitch, while a very rich lightwood or stumpwood with 30 per cent resin might yield 78 gal of ôil con-

4Hawley and Aiyar, Jour. Ind. & Eng. Chem. i4l 1055 (i933).

taining a tar of golden brown color and very little pitch. The increase in yields with increasing resin content is due to the much higher proportion of oils from resin than from wood, while the change in quality is due to the increasing proportion of resin products with different characteristics from the wood tar. These yield figures are computed on a basis of a yield of 7 per cent tar and oils from the wood fiber 80 per cent from the rosin and 100 per cent from the volatile oils. The "ton" is also considered as a ton of water-free wood and the resin is assumed to consist of 80 per cent rosin and 20 per cent volatile oils.

When the corresponding yields are figured on a cord basis the variation in the yields from the different qualities of wood are even greater, due to the fact that as the resin content of the wood increases its weight per cord also increases. For instance, ordinary longleaf pine-wood with 15 per cent moisture and 5 per cent resin weighs about 42.5 pounds per cu. ft. (see Table 11) and an increase of resin content to 15 per cent should increase the weight per cu. ft. by an equivalent amount X (15 — 5) = 3.7), assuming that the additional resin does not swell the volume of the wood. A cord of wood with 15 per cent resin would, therefore, weigh (42.5 -f- 3.7) 80 = 3696 lbs., and figuring in the same way a cord of wood with 30 per cent resin would weigh 4136 lbs. Since a cord of ordinary wood with 15 per cent moisture is equivalent to--=1.48 dry tons, it will yield

1.48 X 27 = 40 gals, of oil per cord. Figuring in the same way, the wood with 15 per cent resin and weighing 3696 lbs. per cord will yield 75 gal. and the wood with 30 per cent resin and weighing 4136 lbs. per cord will yield 152 gal. of total oil.

These figures are believed to represent very well the commercial yields obtained from wood with such resin contents, and at any rate they are good examples of the variation in yields due to difference in quality of the wood. They show why it might be preferable to pay a nigh price for a very rich wood rather than to distil ordinary sawmill waste which could be obtained very cheaply. The tar obtained from poor grade wood is also so near like hardwood tar in composition that it is difficult to find uses for it.

It is a common practice to report yields of oils from a particular process or apparatus without any statement as to the quality of wood used, but with the inference that the high yields are due to the process, when the quality of wood and not the process may have been responsible for the favorable^ results. Even when conscientiously testing a new process for distilling resinous wood it is very easy to be deceived by the unconscious selection of very rich wood for the trial runs. No publication has been made on the destructive distillation of resinous wood and the author has never seen a private report on this subject in which yield figures were accompanied by determinations of the resin content of the wood used.

The charcoal obtained from resinous wood has much the same composition as that from hardwood except that it may contain a certain amount of the "coke" from the distillation of the rosin in the wood. The rosin which does not flow from the wood during the first part of the distillation finishes its decomposition while retained in the wood (or charcoal) and naturally leaves its distillation residue mixed with the charcoal. This tends to make a denser charcoal than that obtained from softwood with little resin.

The gas from resinous wood distillation is also affected by the presence of "rosin gas" which gives it a much higher illuminating power than the hardwood gas. Like the hardwood gas, however, it is used only for heating purposes at the plant.

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