A.
w. 600s AND
A. A. REITER Cliffs Dow Chemical Company, Marquette, iMieh.
T a r s and oils are obtained as byproducts of hardwood carbonization to the extent of about 12*2% of the wood carbonized. These tars and oils are complex mixtures of a wide variety o f compounds, A practical general procedure for separating such mixtures into their most important individual cornpounds i s described, and specific examples are given. Lists of compounds that can be produced from by-product tars and oils are presented. The photograph at the left is a refinery view-, showing a portion of the extraction equipment.
C
HARCOAL is the primary product of the carbonization of
hardwood, but some useful by-products are obtained and are a significant factor in this industry with a loa profit margin. The standard by-products that’ are refined and marketed are practically limited to methanol, denaturing grade methanol, methylacetone, and acetic acid. Although these are the most important by-products in point, of value, they are greatly exceeded in quantit,y by tars and oils n.hich do not have any well established general uses. Some of these tars and oils have been separated into more or less crude fractions for specific uses, but t’here has been no general separation into wood tar chemicals analogous to coal tar practice. Probably most of the tars and oils obt,ained in the industry have been burned in plant boilers. The problem has been recognized for many years, but the tars and oils are very complex and reactive mixtures so that commercially successful separation procedures have not been knoTi-n. For a number of years this laboratory has carried out an intensive research program to develop such procedures. The work is by no means complete, but a fell- products are past the pilot plant stage and ready for plant practice.
The amount and character of tars and oils obtained vary with the kind and size of wood, with carbonization conditions, and with the refining processes used, Table I gives the main products from wood Carbonization at Marquette. The yields are based on B cord of wood, defined as that amount of “chemical wood”, including bark and rot, which if dried at 105’ C. Tvould weigh 3000 pounds. The tars and oils amount to 12.2% of the wood carbonized or of the tot,al liquid organic product obtained. Although some of t,his material has been disposed of for various industrial purposes, the major part has been burned. Possibly a large part of that, now used industrially might be utilized to better advantage if compounds present in the crude mixtures could be economically separated. Many compounds found in pyroligneous liquors have been listcd (1-7). These lists include products from the destructive distillation of hard- and softwoods, cellulose, and lignin. A few were found in acetone oils resulting from pyrolysis of calcium acetate t o produce acetone, m-hich is no longer practiced. It appears probable, however, that most of these may occur in hard-
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1946
TABLE I. PRODUCTS FROM HARDWOOD CARBONIZATION Yield/Cord Charcoal (17.5% volatile) Acetic acid (including formic and propionic) Denaturing grade methanol 4methanol methylacetone Tars and oils Noncondensable gas Water of pyrolysis and loss
+
5 4 . 0 bu.
Pounds
1080
Wt. % 36.0
1 4 . 3 gal.
125.8
4.2
9 . 3 gal. 38.4 gal. 7260 cu. ft.
61.4 366 650
2.0 12.2 21.7 23.9
......
.,..
vood distillates as well as many more that have not been identified. To show the variety of compounds, these lists are supplemented by a few additional compounds found and identified a t Marquette and are classified according to the main chemical types in Table 11; no compound was placed in more than one class. This wide variety of compounds includes many types of functional groups, many indiviguals having several different active groups in the molecule. Usually a number of members of each series is present. Obviously mixtures containing all of these types and others are difficult to unscramble. Many of the individual constituents tend to condense with other compounds present, and therefore thermal and chemical treatment must be minimized. Mutual interferences of boiling points and of solubilities are the rule and even fractions having a narrow boiling range may contain three or more widely different compounds and show abnormal responses to chemical tests that make interpretation difficult. All of the liquid product from the carbonization of wood is obtained as a heterogeneous, aqueous, tarry mixture at the retorts. An elaborate refining system is required to separate properly the two main pure compounds produced, acetic acid and methanol; and this system removes practically all the other organic matter from the water and partially separates it into several tar and oil fractions. Figure 1is a flow diagram of the acetic acid and methanol refining systems, showing where the oils and tars are removed and the amounts obtained. The investigations have been directed chiefly at the utilization of acetic oil and soluble tar. These have had least commercial utilization and are obtained in large quantities. Settled tar also
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occurs in large amounts, but it has had several fairly good outlets. From it is made a gasoline gum inhibitor] a creosote fraction, hardwood pitch, and flotation oil. However, it is expected that settled tar will also be processed for individual constituents at some future time. Because the composition of the by-product oils and tars is dependent on the kind of wood, on the methods of carbonization, and on the methods of recovery of acetic acid and methanol, the detailed procedures for separating and purifying individual compounds must be worked out for each oil or tar. However, the general methods that have been successfulin isolating compounds from complex and reactive wood oils may be used to separate other oil or tax products of wood carbonization or similar byproduct mixtures. GENERAL SEPARATION METHODS
Complete isolation of every constituent of a tar or oil is impractical if not impossible. The best that may be expected is to obtain a large proportion of those present in greatest amounts, or those most readily separated, in fair states of purity. As a rule it would not be profitable t o process a tar for only one or two compounds. Manipulations required to produce one item usually also perform a partial separation on one or more others, so that the cost of producing succeeding items is reduced. Fractional distillation is the first step in working up an oil or tar. The purpose of this step is to divide the material into specially selected crude fractions suitable for further processing. These fractions withstand chemical and thermal treatments much better than the original tar or oil, probably because of the smaller number and variety of compounds present in each compared with the original material. An effective fractionation is required. The tower has thirty bubble-cap trays or the equivalent, and the kettle capacity is proportioned to the tower so that about 100 hours are required to distill off a charge when refluxing about 10 volumes to 1 withdrawn. An absolute pressure of 50 mm. of mercury is maintained at the condenser. The crude fractions are chosen by reference to the chemical and physical properties of the distillate. The chemical properties may include acidity, saponification, and bromine values, and car-
Plant and Portion of Heserve Wood Supply, Cliffs Dow Chemical Company
134
INDUSTRIAL AND ENGINEERING CHEMISTRY
Yol. 38, No. 2
sion oi primary distillations arc corribined to make a charge fur il still 01 the same size and type as nscd for thc first distillation. This conibincd charge is Eract,ionally distilled iii t,he sarriv manner as before. Chemical arid physical properties oi F O R M I C - A C E T I C 0.64 GAL/CD. GLACIAL ACETIC 13.1 I GAL/CD. the distillate are determined and PROPIONIC 0.55 G A L ~ C D . plot,ted as in Figuw 2. This chart shows thc complexity of tlic inaterial and makes evident the pwwnce ot other substances on both sidw of t hc acetol acetate region. Preceding it, ai!d 1.2 GAL/CD marked by the refractive index hurnp is methyl fury1 ketone. 13utyric avid is responsible for most of the acidity. Figure 1. IR) -Produrts of Wood Carbonization (in G ~ l l o n sper Cord) Following the acetol acetate region anti eridenced by the rise in refractivc index and brominc ralues arc methyl furottte and nicthyl lopentenone. The portion of the distillat? bonyl, hydroxyl, and methosyl datcrminations. Physical properselected for further treatment includes the beat portion of the. ties may include specific gravity and refractive index. Such ester value curve. I t is important to cut before the bromincvnluc' properties, deterrnincd on, say, 5% incrcments of distillate, show and refractive index curves begins to rise because inclusion oi' variations which are ufieful in sclecting fractions. the substances causing these rises with the acetol acetate frarCuts may be made at minimum values of a chemical property, tion makes subseqwnt purification difficult. or a t inflection points of a physical property curve, or ot'herwisc Once the cutting points havc been .established they may bp as may be dekrmined by considcration of the major compounds duplicated for plant control by specific gravity and refractive inpresent and of subsequent procesfiing steps. A crude fraction dex tests. In this case the ester fraction was selected from 25.0 usually contains one major compound along xvith a number of t o 63.1% off. Saponification and acidity determinations indicatc others less in quantity or importance. As a rule, fractions should 84.97, ester calculated as acetol acetate and 7.5y0acids calculated be chosen t o include the major compound so that preceding and following cuts do not contain it. To aid in subsequent purifias butyric acid. Because the ester saponifies so readily, the acidity is best determined in methanol solution using alcoholic socation of the major compound, it also may be important to exdium hydroxide. Saponification is determined ai room tcmperaclude a compound of a different chemical type following or preture for, if the alkaline solution is healed in the customary may, ceding it. The best cutting point may be determined from the excessive alkali is consumed and deceptive values result. chemical property curves. After the desired cutting points have been established, the To the ester fraction in a mixing tank is added volume oL' water. Sodium carbonate equivalent, to thc acid content is theii fractions can be conveniently duplicated by reference to specific gravity changes. The specific gravity of the distillate varies added slowly. When neutralization is complcte, the mixture is allowed to settle and .the aqueous salt layer is withdrawn to bo smoothly and continuously throughout a fractional distillation subsequent'ly worked for butyric acid. The oil laycr is vacuum passing through a number of maxima and minima. This affords a distilled in n simple pot still to free it from any dissolved salt, and useful operating guide for selection 01 fractions. the distillate is then fractionally distilled and yields technicalSubsequent operations on a crude fraction are aimed a t isolagrade acetol acetate. tion of the major compound, and may or may not result in parallel separation of minor constituents. Each crude fraction requires individual treatment, but as a general principle it may bc O F C O M P O U K D S FROM W O O D T.4BLs 11. CHEMICAL TYPES atated that the mixture is partly broken up in some way, usually DISTILLATIOX by chemical treatment. Treatment that alters or removes a class .kCIDS, A L I P H h T I C FuE.Lsss 9 of compounds usually eliminates some azeotropic combinations Saturated 19 Hydrofuranes 2 Unsaturated 9 HYDROCARB~KS so that further separations can be made by fractional distillation, Other 6 Paraffins 8 Chemical treatment includes neutralizat,ion, saponification, esACETALS 3 Unsaturated 2 t'erificat'ion, and hydrogenation. Steam distillation, extraction, Aromatic i AMISES and crystallization methods are also used for the same purposeAliphatic 3 Condensed-ring 3 Pyridines 3 Terpenes 8 that is, t o break up partly or alter the mixt,ure. The resultant ALCOHOLS KETONES portions are again fractionally distilled, and fractions are selected Aliphatic) 6 Aliphatic mono 9 by reference t o the property cnrves of the distillates. Such fracUiisaturatcd 3 Aliphatic di 4 tions may be sufficiently pure as obtained. If not, they are again Unsaturated mono 3 Cyclo 6 Fury1 1 Aliphatic-fury1 I treated chemically or otherwise and again distilled. ALDEHYDES Cyclic saturated 9 Aliphatic 7 Cyclic dione 1 ISOLATION OF ACETOL ACETATE Cyclo 2 Cyclic unsaturated 6 Fury1 4 MIBCELLANEOW~ The isolation of acetol acetate illustrates the general method. Unsaturated 3 Oxypyroiie 1 Neither the ester nor the acetol has been previously reported in Glucosan I ETHERS pyroligneous liquors. The ester is easy t o hydrolyze, and acetol Aliphatic 1 Cyclic ketol 1 is readily oxidized. Bot,h are water soluble. Acetol acetate is Aryl monohydroxy I Other ketol 1 9 PHENOLS found in acetic oil, the source of which is indicated in Figure 1. Aryl dihydroxy r I Mono 7 ,4rgl trihydroxy Vacuum fractional distillation is used t o separate the oil into ten Di i ESTERS selected fractions; the sixth contains the acetol acetate. This Aliphatic 8 Tri 3 fraction is obtained a t about 90" to 100" C. under an absolute Other 4 Lactones 2 pressure of 50 mm. of mercury, and amounts to about 8% of the scetic oil. c Usually a number of similar fractions from a succes-
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February, 1946
INDUSTRIAL A N D ENGINEERING CHEMISTRY
135
ISOLATION OF BUTYRIC ACID
The salt layer from the acetol acetate neutralization contains some ester in solution, amounting t o about 107,of the total available. If desired this may be recovered by extraction with ethyl a c e t a t e . Much more butyric salt solution is obtained from the crude fraction preceding acetol acetate, and these solutions together c o n s t i t u t e the sodium butyrate stock from w h i c h b u t y r i c a c i d i s recovered. IBefore acidifying the salt solus!!? tion to liberate butyric acid, the s o l u t i o n is steam-distilled t o remove any volatile impurities. Figure 2. Property Curves for Selecting the Crude Acetol Acetate Fraction Then sulfuric acid is added equivalent to the salt content, and steam distillation is continued to remove butyric acid. The distillate is extracted with ethyl Guaiacol 2-Hydroxy-3-methyl- A2-cyclopentenone acetate. The solvent is recovered by distillation, leaving the 2,6-Dimethoxyphenol ~ ~ ~ ~ l l a c t o n e anhydrous crude acid which is fractionally distilled to yield Tiglaldehyde Acetol acetate good quality butyric acid. Butyric acid Methyl isopropyl ketone Crotonic acid Methyl ethyl ketone Malt ol [3-oxy-2-methyl-(4)Methyl fury1 ketone ISOLATION OF MALTOL 4Ethylguaiacol pyrone1 Usually compounds present in large proportions are most readily isolated and purified. However, some compounds present Commercial uses for some of these substances are not known in small amounts can easily be recovered in a high state of purbut will probably develop after they are made available. One ity; maltol is an example for, although its concentration in the disadvantage in the use of chemicals that have been obtained as retort liquor probably does not exceed 0.03%, the pure subhardwood carbonization products is the definite limit t o their Maltol stance can be isolated by relatively simple procedures. production, depending both on the amount of wood carbonized is soluble in hot solvents such as water, methanol, ethyl acetate, by the industry and on the number of producers that will be able and many others, and it crystallizes readily when the solutions to process the tars and oils. Because tho equipment requires are cooled. It crystallizes out of soluble tar fractions obtained rather large expenditures and the processes require an unusually by the fractional distillation of soluble tar in the temperature high degree of chemical supervision and control, probably only a range 130’ to 150’ C. and an absolute pressure of 25 mm. of merfew of the larger charcoal producers will enter into by-product recury. The crude crystals from these fractions arb recrystallized covery. I t would be possible, however, to utilize all the tars and from methanol solution to yield pure, white crystals melting at oils produced by shipping them or crude fractions from them t o 160”C. the plants that are suitably equipped. The utility or value of a product has a bearing OB the amount The total capacity of United States hardwood distillation plants of steam, chemicals, equipment, and work that it pays to expend is now estimated to be 1377 cords per day, of which about 29% in its production. Cheap compounds now on the market may not is in the southern, 35% in the eastern, and 36% in the north cenbe worth separating. For example, both ethanol and toluene octral region. The economic position of the industry in general has cur in hardwood oils, and it is possible to isolate and to purify been somewhat precarious, and the low prices that may be exthem. However, in view of their low market value it would probpected for acetic acid and methanol when more normal conditions ably be uneconomical to separate them as pure compounds. On return makes additional revenue appear particularly desirable. the contrary, compounds that are not now available commerWe believe that this additional revenue may be obtained from cially and are difficult and expensive t o synthesize may warrant chemicals from the oils and tars, and that these by-products will considerable work and expense in isolation and purification if become an important part of the wood carbonization industry. suitable uses for the material exist or can develop. Many of the compounds in wood tars and oils appear to be of the latter type. LITERATURE CITED I t may be well to emphasize, however, that many of the individual chemicals that can be isolated by the methods described (1) Bugge, G., “Industrie der HolzdestillationsProdukte”, Theodor Steinkopff, Dresden and Leipzig, 1927. do not occur in large amounts, and it would be uneconomical to (2) Bunbury, H. M., “Destructive Distillation of Wood”, New York, produce them singly. However, if the tars and oils are worked D. Van Nostrand Co., 1926. up systematically, the average unit cost of all the items will be (3) Chorley, J. C., and Ramsay, W., J . SOC.Chern. Ind., 11, 395 (1892). low and the total production will be substantial. (4) mar, SI.,and Rule, A., “Technology of Wood Distillation”, A list of wood tar products follows for which me now have pracNew York, D. Van Nostrand Co., 1925. tical separation procedures and which can be made in quantity ( 5 ) pringsheim, H., et ai.,Ceiiuiosechem., 8, 48-66 (1927). when equipment becomes available. Until such time they are (6) Schorger, A. W., “Chemistry of Cellulose and Wood”, New York, McGraw-Hill Book Co., 1926. available only in relatively small amounts, from laboratory or (7) Schultes, H., Bw.,69B,1870-3 (1936). pilot plant operations. Many other compounds are partiallv
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