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plants have drawn off a part of the' products from the bottom of the retort near the end of the distillation instead of distilling everything through the condenser. This material drawn from the bottom of the retort is naturally the least volatile portion of the products, the pitch. More recently a few retorts have been operated with the heat applied mostly at the sides and top, so that some of the high boiling oils could accumulate at the bottom along with the pitch and be drawn off as a

heavy tar, thus- obtaining a certain fractionation of products from the retort.

This type of retort was developed for small plants of one or two retorts only, with the idea that such small plants could not afford to install refining apparatus and, in this way would produce partly refined products for shipment to central refining plants. Such small plants could be located near the wopd supply and could operate in one place for several years without using up the wood in the immediate vicinity. In this way the products pf distillation would be shipped instead of the wood, with a consequent saving on transportation charges. This plan has not yet been worked out on a large scale.

The condensation of the liquid products and separation of the fixed gas is carried out the same as in hardwood 'distillation. ■ The liquid

products consist of pyroligneous acid and crude tar the same as with hardwoods, but the tar, on account of the presence of resinous constituents of low gravity, floats on the pyroligneous acid. The main .valuable products are found in the crude tar and the pyroligneous acicl is usually a waste product. In only one plant has the acetate and alcohol been removed from the pyroligneous acid and this was profitable only on account of peculiar local conditions, including a cheap supply of fuel for the refining process.

After the separation of the crude tar by settling it is separated into various products by distillation and chemical treatment. In this refining process as in the distillation of the wood the processes are various. Few standardized products are made and many special products are prepared for the markets which have been developed by the individual plants. Pine tar is a fairly well standardized product and the commercial product is the residue left after various light oils have been removed from the crude tar by distillation. The crude tar is distilled in copper stills provided with closed coils and a steam jet and the distillation is carried on until the oil in the distillate reaches a certain gravity. The distilled oil with a gravity up to about 0.95 is separated for chemical treatment if refined turpentine and pine oil are to be obtained, or if refined turpentine alone, the oil up to about 0.895 is collected separately. This is treated with a solution of caustic soda for removal of some of the rosin oils and then distilled. The refined turpentine fraction sometimes includes all the oils with a gravity up to 0.87 or 0.875, or sometimes a separation is made of the light oils with boiling points below the turpentine range. The pine oil is separated so as to correspond fairly well with the standard steam distilled pine oil, except that the range of boiling points is usually wider in the destructive product. Various intermediate fractions are taken to prepare special products and a detailed typical process can not be described.

Pitch is obtained direct from the retorts in the plants where provision is made for drawing off a product from the bottom of the retorts during the last part of the distillation. This pitch needs no further treatment for market unless it is a little too hard or too soft, when it may be mixed with a little heavy oil or "boiled down" in a special pitch pan as the case may be. Pitch may also be obtained as a residue from the distillation of the tar, various heavy tar oils being obtained as a distillate in the process. This is not commonly done, however, since the process is difficult and the market for the products is not very good.

The caustic soda solution which has been used to treat the crude turpentine contains various constituents which can be reprecipitated by acidifying the solution, and this is sometimes done, using the pyroligneous acid for acidifying,

The lack of standardization in these pine tar products is in many ways a handicap to the industry, since the expansion of the markets is difficult when the same product can not be obtained from more than one or two plants. On the other hand, it is sometimes beneficial, since when the market is poor for one product it need not be separated but can be combined with other products. For instance, there might be times when the preparation of a refined turpentine would not be profitable, but the sale of the crude turpentine alone or with other fractions would save the situation.

The yields of various products can not be accurately given since there are such variations in the quality of wood used and in the

composition of the products. The yields are sometimes stated in terms of total oil obtained per cord of wood used, and it is believed that about 80 gallons per cord is a fair average for this crude product.

Steam Distillation and Extraction Process

The raw material for this process is the same as that for destructive distillation and the same discussion on selection and collection applies, except that any piece small enough to go into the mouth of the "hog" or grinder is suitable for transportation to the plant. The wood after arrival at the plant is "hogged" into large irregular chips and then shredded into smaller and more uniform sizes. The hog and shredder are standard apparatus for comminuting fibrous material such as wood. The fine chips are 'carried by conveyor to bins at the top of the extractor building.

The removal of the valuable products from the wood takes place in a battery of cells or extractors and a part of the process is similar to the ordinary intermittent countercurrent, series extraction. Before the extraction with solvent, however, the crude turpentine is mostly removed from the wood by steam distillation, and after the extraction the solvent remaining on the chips is recovered by another steam distillation. All these operations are carried out in the same, apparatus and each cell must, therefore, be provided with a steam jet, closed steam coils, inlets and outlets for the solvent, and separate condensers for the crude turpentine and the solvent. The extractors are vertical cylinders of various sizes holding from three to fourteen tons of wood and the greatest variation in design is in the discharging arrangements. In some cases the outlet for the finished chips is on n irrjh-1. ' iJ

Fig. i6.—Interior of Extractor House.

the side near the bottom of the cylinder, in other cases the entire bottom of cell is removable so that the chips may fall out without restriction. The solvent is generally a special' fraction of gasoline without the high-boiling portions which would contaminate the pine oil and make the solvent recovery difficult.

The number of extractors in a battery varies at different plants from four to sixteen, but in every case the various operations of charging the fresh chips, steam distillation, extraction with solvent, solvent recovery and discharging the extracted chips take place in this order in each extractor and.all the operations (except possibly the shortest ones of charging and discharging) are taking place in different parts of the battery at the same time. For instance, in an eight cell battery some one cell might always be charging or discharging, sometimes one and sometimes two steaming for crude turpentine,. three extracting with solvent, and sometimes two and sometimes three steaming for solvent recovery.

The details of these operations vary at different plants ; superheated steam may be used either for the solvent recovery or for both solvent recovery and turpentine distillation ; the boiling of the solvent may be done by direct steam or by live steam ; the solvent removed from the extractors during the boiling process may be returned to the cell or sent to storage; the speed and pressure at which the steam is blown into the charge of chips may vary.

In every case, however, the crude products of these operations for further separation and refining are the crude steam distilled turpentine and the solution of rosin in the solvent. The separation of the crude turpentine into refined turpentine and refined pine oil is accomplished by fractional distillation with steam, sometimes a little caustic soda being added to the crude turpentine before distillation. The distillation is controlled by the gravity of the distillate, the first fraction with gravity up to about 0.87 going into the refined turpentine, the second fraction up to about .91 being an intermediate part held over# for redistillation, and the last fraction being refined pine oil. The residue of dark colored heavy oil is very small in quantity. The intermediate fraction is either held over until enough is obtained for . a special distillation or else it is mixed with the next batch of crude turpentine.

The solution of rosin in the solvent also contains the pine oil not removed from the wood in the first steam distillation and the refining process is for the purpose of recovering the solvent for further use and separating pine oil and rosin in condition for market. This is also accomplished by fractional distillation, with closed steam coils during the first part of the process, and with live steam also during the latter part. There is a wide difference in boiling point and# in gravity between the solvent and the pine oil, so that this separation is not difficult, and if some pine oil is distilled with the solvent it is not lost because it all goes back into the process again. Great care must be taken, however, to remove the last traces of pine oil from the rosin in order that the latter may be hard and brittle. After the pine oil is all removed the molten rosin is run into wooden vats and dipped or run into barrels. , , t1

There are two points about this process which must be carefully watched to prevent trouble, solvent loss, and contamination of the turpentine with the solvent. When .working with large quantities of hot volatile solvent a few leaks or incomplete condensations result in a serious loss. It may also be noticed from the general description of the process that the same extractor is used for turpentine distillation and solution with gasoline solvent, and that a leak at the valve in the solvent connection to the extractor during the steam dis-tUlatfon wmil<i rmit in contamination of the turpentine with solvent.

At some plants there are two valves a short distance apart in the solvent connection to the extractors, and between the two is another valve opening to the air. When the solvent line is to be closed, both valves in the main line are shut, and the one between them is opened to the air.

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