288
INDUSTRIAL AND ENGINEERING CHEMISTRY Acknowledgment
We wish to extend thanks to the Lindsay Light and Chemical Company and the Vanadium Corporation of America for donating the ammonium metavanadate for preparing catalysts, to the Reilly Tar and Chemical Company for the gift of the naphthalene and 2-methylnaphthalene, and to the Research Corporation for the design of the Cottrell precipitator used with the Downs reactor. For technical advice we are indebted to C. R. Downs, of Weiss and Downs, and t o F. E. Cislak, of the Reilly Tar and Chemical Company.
Vol, 35, No. 3
(9) Craver, U. S. Patent 1,914,558 (1933). (10) Downs, J . SOC.Chem., Ind., 45, 188T (1926). (11) Gibbs, IND.ENG.CHEW,11, 1081 (1919). (12) Green, J . Soc. Chem. I d . , 51, 123T, 147T, 159T (1932). (13) Groggins, “Unit Processes in Organic Synthesis”, New York, McGraw-Hill Book Co., 1935. (14) Ipatiev and Orlov, B ~ T .60B, , 1963-71 (1927). Ibid., 62B,693-7 (1929). Kirst, W. E., Xagle, W. M . , and Castner, J. B., Trans. Am. Inst. Chem. Enors., 36,371-91 (1940). Kraus, Farben-Ztg., 41, 111-12 (1936). Lux, thesis, Purdue Univ., 1942. Marek and EIahn, “Catalytic Oxidation of Organic Compounds in t h e Vapor Phase”, A. C. S. Monograph 61, New York, Chemical 1932. ~ .... Catalog -. ~ Co.. ~ ~ (20) Maxted, J . Soc. &em. Ind., 47, 101-5T (1928). (21) Meyer, “Analyze und Konstitutionsermittlung organisohen Verbindungen”, Berlin, Julius Springer, 1931. (22) Shriner and Fuson, “Identification of Organic Compounds”, New York, John Wiley & Sons, 1935. (23) Sundgren, Albert, Ann combustibles lipuides, 5, 35-174 (1930). (24) Ullmann, Enzyklopaedie der technischen Chemie, Vol. 10, pp. 265 ff,,Berlin, Urban und Schwarzburg, 1932. (25) Weiss, Downs, and Burns, IXD.ENG.CHEM.,15, 965 (1923). (26) Willstatter and Majima, Be?., 43, 1173 (1910). ~
Literature Cited (1) Beilstein, “Handbuch der organischen Chemie”, Vol. 7, p. 709,
Berlin, Julius Springer (1918-40). (2) Berkmann, Egloff, and Morrell, “Inorganic and Organic Catalysis”. New York. Reinhold Publishing Cora.. 1940. (3) B&l and Li& Petroleum Z . , 26, 1027-142, 1657-70 (1930). (4) Charlot, Ann. chim.,[ l l ] 2, 415-90 (1934). (6) Charlot, Bull. soc. chim., 51, 1007-14 (1932). (6) Chowdhury and Saboor, J . Indian Chem. SOC., 14, 633-7 (1937). (7) Cislak, F. E., private communication. (8) Conover and Gibbs, INDENQ.CHEM.,14,120 (1922).
CONDEKSBD from the Ph.D. thesis of Russell W. Welborn at Purdue University.
TAR ELI INATION in D o n a l d F. O t h m e r a n d Raphael Katzen POLYTECHNIC INSTITUTE, BROOKLYN, N. Y.
P
YROLIGKEOUS acid from hardwood distillation has long been a source of acetic acid of greater or lesser purity; the latter may readily be made as pure as that from any synthetic source, directly and continuously. Pyroligneous liquors contain numerous classes of chemical materials-for example, aldehydic and phenolic compounds which condense to form many types of resins or tars. Almost anything that may be done t o the liquor, either chemically or physically, causes a combination of some of the molecules. These reactions yield tars which separate in one form or another, ranging from oils to hard, firmly adhering, dense pitches of almost cokelike characteristics. Even the “green” or “settled” or “insoluble” tar which separates from the condensed liquor is not distilled from the wood in that chemical form, but is the product of the combination of various simpler compounds leaving the wood during carbonization. It is usual to speak of “soluble tars” as those which do not settle out during this step. A more correct expression would be “soluble tar-forming materials”, since they are not in tar form then or they would not be soluble. Part of these materials is separated by distilling all of the liquors away from the higher boiling residue which is formed, largely because of the increased speed of combination caused by the higher temperature. The residue is also called “boiled tar”, but some of the tar-forming constituents distill over with the “boiled liquors” and are available t o form tar a t subsequent stages. I n fact, hard cokelike scaling occurs on condenser tubes in the vapor spaces of multiple-effect evaporators used for this “primary” distillation as a result of the combination of these materials immediately after having been in the vapor state.
Removal of Tar-Forming The classical method is the neutralization of the dilute acid with lime to give calcium acetate which is treated, after evaporation to dryness, with sulfuric acid t o free the volatile acetic acid. A small amount of tar remained in the commercial calcium acetate. (“Gray acetate of lime” refers to that from pyroligneous liquor which had been first distilled and distinguishes it from the older “brown acetate of lime” made from pyroligneous liquor which had not been distilled away from the bulk of the tar.) Newer methods produce acetic acid directly from the pyroligneous acid. I n the Albin-
b b b Hundreds of organic compounds have been identified in pyroligneous acid from wood distillation; many of them condense or polymerize to form tars and cokelike pitch at every successive stage of each of the various processes which have been used to recover methanol, acetic acid, and other materials. It was desired to find a method which would remove easily, with minimum adjustment of equipment and processing, substantially all of the tar-forming materials in a single step early in the processing stages) the continuous plugging of heat transfer and other equipment with tars could then b e obviated, and pure materials could b e more easily made as finished products. It was found that treatment OF the liquors with sulfuric acid for several hours resulted in the
Courtesy, Gray Chemicdl Company
General Picture o f W o o d Distillation Plant, Taken from W o o d Yards Loaded buggies at left are ready for retorts. W o o d pile shown a t right, retort building with five stacks (ten retorts) in center, still house at left.
PYROLIGNEOUS ACID Constituents by Polymerization and Condensation Previous Work on Tar Removal
Suida process ( I ) , a high-boiling solvent extracts the acetic acid from the vapors formed by distilling the liquor, and the solvent and acetic acid are then separated by vacuum distillation. The Brewster process utilizes a low-boiling solvent which removes the acetic acid in a liquid-liquid extraction (IO). The process of removing the water by an azeotropic distillation with an appropriate organic compound was described previously (8, 9). Tar-forming materials cause difficulties in these and every other method of processing pyroligneous liquors.
In addition to clogging up machinery and equipment, tars raise havoc with heat transfer (9) and discolor the products
in the direct methods of making acetic acid as well as in those for acetate of lime (4, 6) or sodium acetate ( I S ) . Many methods have been suggested t o remove most or all of the tars, but their failure is evidenced by the fact that only the one mentioned above, the expensive primary distillation method, has been adopted to any great extent. Representative suggested processes follow: 1. Tar separators, in which vapors from the hardwood distillation retorts are scrubbed with liquid tar, are described by Klar, Bergstrom, and Wesslin ( 7 ) . Both Hawley (6) and Bunbury (6)point out that tar separators have not been widely adopted because of various operational difficulties. A related method of tar extraction with wood oils was developed by Barbet (3). The vapors from the retorts are partially condensed; the condensate (principally tar oils) is then run countercurrent to the incoming vapors to wash them free of tar. 2. Chlorinated solvents in a cold extraction were used by Othmer (11) to remove the tars. 3. Stone (IS) purifies sodium acetate formed from pyroligneous acid. The tarry impurities accompanying the dry crystals are carbonized at about 200' C. 4. Heat treatment under pressure was used by Othmer ( l a ) to remove tars by causing r l y merization or condensation o the tarry constituents and precipitating them out of solution. 5. In the Charles-Lambiotte method (6) demethanolized pyro-
ebsy removal of practically all tar-forming constituents as fluid tar oils. The processing was developed on a laboratory scale to determine the optimum conditions OF acid concentration and time of treating liquors from various steps of the refining processes, then on a pilot plant scale, and finally on a plant scale i n conjunction with an earlier process for removing methanol and acetic acid by distillation. In a plant handling 60 cords of wood daily, only a Few pounds of sulfuric acid per day and no additional equipment were required. A simple plant control method i s described; i t s application to other plant methods of processing was considered, and flow sheets for incorporating this system of detarring are indicated.
289
INDUSTRIAL AND ENGINEERING CHEMISTRY
290
Vol. 35, No. 3
Figure 1. Effect of Sulfuric Acid Concentration in Removing Tar-Forming Constituents from Settled and Demethanolized Pyroligneous Acid Figure 2. Effect of Reflux Time in Removing Tar-forming Constituents from Settled and Demethanolized Pyroligneous Acid Figure 3. Effect of Continuous Treatment of Settled and Demethanolized Pyroligneous Acid with Various Sulfuric Acid Concentrations W h i l e Vapors W e r e Removed at the Same Rate as L i q u o r w a s Fed
0
10
20
30
40
HaSO.
50
60
70
( C C/LITER)
ligneous vapors are passed through a column in which a solvent for the tars is used as a scrubbing liquid. 6. The most usual method is by evaporation of the raw liquor away from the higher boiling tars as already indicated. Some tar oils are, however, steam-distilled with the liquor; and the additional heat cost is excessive.
sired, one that was even more general in its application was preferable. Pyroligneous acid from representative northern and southern plants was used to compare results on liquors from different characteristic wood species. The liquors also came from one of three stages in the processing: (a) raw liquor which had merely been settled to remove green or settled tar already formed; ( b ) liquor which had been allowed to settle and then had been distilled to remove methanol; or ( c ) pyroligneous acid which had been settled, demethanolized, and distilled away from the insoluble tars. From a chemical standpoint, a catalyst might be found which would cause combination and removal of any potential tar-forming body to the subsequent benefit of all processing steps. Pyroligneous acid from one of PROCEDURE. the points of a process indicated above was heated in the presence of a small amount of the catalytic agent, either under a reflux condenser, during a continuous distillation, or at a constant temperature below the boiling point. Control runs were made without added catalyst. The tarry materials which formed were of threo types.: a light tar oil floating on top, a heavy semiliquid tar settling to the bottom, and a solid brittle pitch separating from the semiliquid tar on cooling. These constituents were separated by decantation. The tar oil on top of the liquor adhered to the flask and was weighed with the 80 rest of the tarry material and the flask. Treatment with strong caustic soda dissolved the tar oil and semiliquid tar, leaving behind the brittle pitch. The pitch was scraped out and weighed. The flask was then weighed empty and its weiczht subtracted from the total weight - of tars and flask to determKe the total residue. CSTALYST.Since the liquors are acidic, an acid catalyst seemed desirable to give a lower pH; a nonvolatile, highly ionized acid was indicated. Sulfuric, phosphoric, and oxalic acids
-
Development of the Method
B method was desired which, a t a minimum of cost and derangement of processing, would cause the combination of all these tar-forming materials at one time into relatively nonvolatile and readily removable substances. While a method which would fit into the steps of a process using azeotropic distillation was de-
Table 1.
Effect o f Sulfuric Concentration A c i d in 3-Hour Continuous Treatment
of Demethanolized Settled Pyroligneous A c i d (Pennsylvania) Total
Distn. KO.
HgSOa ( 6 6 O BB.),
Total
1
0
2865
Grams 6
2680
33
25 lo 50 75
2850 3190 3140
45 78 96
co.
2 3 4 5
Dist., Cc.
Resi-
due,
Pitch, Grams 0 5 15 30 46
G J L . of Dist. Appearance Residue Pitch Dist. of 1.75 0 Dark brown, colloidsl tar 1.9 Clear colorless 12.3 15.8 6.3 ~ l o u d yyellow . 24.4 9.4 Clear, light yellow 30.5 14.7 Same
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1943
29 1
were compared to determine their relative value in the tar-forming reaction. The following results were obtained on demethanolized settled liquor; 1000 cc. of pyroligneous acid were refluxed for 3 hours in each case with 50 grams of acid catalyst: Total Residue, Grams 13 8
Acid Sulfuric (66" BB.) Phosphoric (82%) Oxaha (solid)
Pitch, Qrams 4
0
0
5
Since neither methanol nor acetic and homologous acids (the only valuable materials present) would react under these conditions t o give water-insoluble materials, the best catalyst appeared to be the one which produced most residue in a given experiment; for less tar-forming material is left by the catalyst which gives the largest mass of insoluble residue. The above figures show that sulfuric acid is the most efficient material of the three catalysts; it is also the cheapest. Therefore, all further experiments were made with sulfuric acid, since there was no apparent difference either in the residues produced or in the types of tar-forming materials left in the solution. Sulfuric A c i d Concentration
To determine the effect of sulfuric acid concentration on the formation of both total residue and pitch residue, two different times of heating under reflux were used-2.5 and 20 hours; the liquor (from a Pennsylvania source) had been settled and demethanolized; the results of treating 1000 cc. are given in the following table and in Figure 1: --2.5-Hr. Hzs04 added (66' BB.), cc. 0 n
Pitch, grams
1
0
10
5
25
9 10
50 75
-.
RefluxTotal residue, grams
12
7-20-Hr. H&O4 added (66' B O , cc. 0
1 2
5 10
25 50
6
7
RefluxTotal residue, grams 2 6 8 11 15
Pitch, grams 0.02 0.5 3
2 5 4
The longer treatment results in considerably greater total residue, A higher concentration did not seem desirable for commercial operation. Time of Treatment
Figure 1 shows that the amount of residue increased with time a t a constant concentration of salfuric acid. To find a suitable time for processing, a control sample of pyroligneous acid was treated with sulfuric acid a t room temperature for 24 hours; 2 grams of total residue were produced and no pitch. The results of treating 1000 cc. of demethanolized settled pyroligneous acid (Pennsylvania) with 25 cc. of 66" B4. sulfuric acid a t different times of reflux are shown in the following table and Figure 2 : Reflux Time, Hr. 1 2 4
7.25
Total Residue, Grams 6 5 6 15
Pitch Gram's 0
2 2 1.5
From these and previous runs, it is apparent that time and concentration are interrelated, and that in any industrial processing a longer time would allow a lower concentration of acid (and vice versa) to obtain the same results. Continuous Treatment
I n most industrial processes it would be an advantage to be able to operate continuously; it seemed that the sulfuric acid might be added to a continuously operated still to which the pyroligneous acid was constantly being fed as a liquid and removed as a vapor. The nonvolatile sulfuric acid would always remain with the liquid being treated in the still, The same liquor was added continuously to a flask which was evaporating the volatile material (water, acetic acid, and methanol). The volume in the flask was one liter, the time
Courtesy, G r a y Chemical Company
Close-up OF Wood Distillation Plant Showing Loaded Buggies on W a y to Retort and Still House in Background
of initial reflux before withdrawal of vapors was 3 hours, and the boiling rate balanced the feed so as to maintain an average time in the flask of 3 hours. Several runs were made with different concentrations of sulfuric acid; the results are shown in Table I and Figure 3. The distillates from the first and third operations of the last run were treated with 25 cc. of 66" BB. sulfuric acid per liter, and refluxed for 3'/a hours to find the amount of tarforming material which had been volatilized. From the following results it is evident that only about one third as much tar-forming material was present in distillate 3 as in distillate l where no sulfuric acid treatment had been given; if distillates 4 or 5 had been used, there would probably have been much less residue: Diat. No. 1 3
Dist. Vol
Cc:' 2855 2850
HzSOa
Totel
Cc. 71
Grams 18 7
(66O
BB.), Residue,
so
Pitch Gram; 5 2
G./L of Dist. Residue Pitch 6.3 1.75 2.2 .63
The conclusion drawn was that if 2.5 per cent by volume of sulfuric acid is added to the primary (or "copper") still of the old calcium acetate process, there would be only one third as much tar-forming material in the distillate, and even less if a greater amount of sulfuric acid was always presest in the primary still. The treated distillates from these runs were tested for sulfur dioxide which would be objectionable in the distillate. This
292
INDUSTRIAL AND ENGINEERING CHEMISTRY
IO
10
8
8
6
6
4
4
= 2
2
0
sulfuric acid would be made during this distillation for acid recovery. On the other hand, in some other operations such a distillation m i g h t n o t f o l l o w dem e t h a n o li z a t ion, b u t storage of the hot liquor might be involved. Therefore, the treatment was tried with the liquor held a t about 80" C. The results for different concentrations of sulfuric acid a t a constant time of 19 hours are shown in the following table and Figure 5 ; here again, no pitch was obtained:
39 2 W
0 W v)
0
0
10
20 H,SO,
30
40
50
Vol. 35, No. 3
0
10
1C C./LITER)
40
30
20
HISO+
50
