Water-Soluble Polycar lis Acids by Oxidation of Coal N. W. FRANKEI, M.
w.
KIEBLER2, C. H. RUOF, T. R. SAVICH3,
AND
H. C. HOWARD
Coal Research Loboratory, Carnegie lnsfitute o f Technology, fiffsburgh, fa.
ONVENTIONAL combustion reactions a t elevated temperatures result in the conversion of coal largely t o low molecular weight gaseous products, carbon dioxide, and water vapor. However, if such reactions are restricted t o temperatures not higher than a few hundred degrees centigrade, a variety of intermediate oxidation products resulting from various stages of reaction can be isolated. I n the early stages of such low temperature oxidation reactions the coal is superficially little affected; oxygen is added and some oxides of carbon and water are evolved. Hydrogen is preferentially oxidized with a resulting increase in carbon-hydrogen ratios; cation exchange properties may appear, but the bulk of the coal is still insoluble and nonvolatile. Further reaction results in the degradation of the main coal structure with the formation of additional carbon dioxide and water, and along with these, soluble higher molecular weight products. These range in molecular size from the simple aliphatic acids, acetic and oxalic, ‘through the benzenecarboxylic series and water-soluble polycarboxylic aromatic acids, of nuclear size larger than the benzene ring (19), up t o the dark-colored, alkali-soluble, acid-precipitable “humic acids’’ of unknown structure. The proportion in which the various acids are formed is a function of the extent of the oxidation; a convenient measure of this is the fraction of the carbon of the coal appearing as carbon dioxide. I n liquid phase reactions, carbon dioxide formation of 20 to 30% results chiefly in humic acids; when the fraction of the carbon converted to carbon dioxide reaches SO%, the humic acids disappear and the solid reaction products consist largely of water-soluble types, a part being the definitely characterized benzenecarboxylic acids, the balance of larger condensed cyclic structure (8). Controlled low temperature oxidation reactions on coal have been carried out with a variety of oxidizing agents such as air, oxygen, ozone, nitric acid, permanganate, peroxides, persulfates, and chromates (9). I n general, reactions in acid media and a t higher temperatures result in smaller yields of carboxylic acids and larger yields of carbon dioxide. Permanganate in acid solutions results almost exclusively in carbon dioxide and water, but in alkaline solutions high yields of carboxylic acids are obtained. Nitric acid is a notable exception giving good conversions t o carboxylic acids. I n general, a liquid phase appears t o be essential for complete reaction. Alternate gassolid phase oxidations and extraction with a liquid phase have been described ( 7 ) . Moist ozone has been reported t o effect
C
Present address, Gulf Research and Development Co., Harmarville, Pa. * Present address, The Glidden Co., Cleveland, Ohio. 1 Present address, R.D. 6, Rensselaer, Ind. 1
oxidation t o water-soluble carboxylic acids (15). RIesomorphic forms of carbon, such as carbon black, give good yields of aromatic polycarboxylic acids by primary oxidation with nitric acid, followed by alkaline permanganate (11). Because of the large fraction of the carbon converted in these reactions t o carbon dioxide and consequent large consumption of oxidizing agent, only the cheapest oxidants, air, oxygen, or nitric acid, are of commercial significance. By proper control of reaction conditions any organic material can be converted t o carbonic and/or carboxylic acids. Aliphatic compounds yield carbonic and aliphatic acids exclusively; cyclic structures are converted to these plus carboxylic acids with cyclic nuclei. The coal series from peat to graphite is characterized by the fact that on oxidation all members yield carboxylic acids with cyclic nuclei. Rforeover, the yields of these acids calculated both on a weight basis and on a carbon basis increase with rank. Thus, the nature of the oxidation products, as well as such properties as carbon-hydrogen ratios and volatile matter, indicate increasing development of cyclic structure in the coalification process, and because of this it is possible t o control the relative amounts of aliphatic and cyclic acids in the oxidation product by proper choice of starting material. This is generally true for all oxidizing procedures, but some exceptions should be noted. The yield of oxalic acid appears to be markedly affected by the conditions of the oxidation-both temperature and concentration of reagent. Further, a t higher temperatures decarboxylation reactions may result in the conversion of polycarboxylic acids to those with fewer carboxyls. Also, while the oxidation of high rank mesomorphic carbon like carbon black, with chemical oxidants a t 100°C., leads to good yields of benxenehexacarboxylic acid, it has not yet been found possible to effect this
Table 1.
Oxidations of Various Carbonaceous Materials
(24 hours a t 250’ C. in aqueous KOH; 137 lb. partial pressure of 0 2 ; -200 mesh, 50-gram samples) Total Oxalate Aromatic Soluble Carbonate Carbon, Carbon, Carbon, Carbon,
Illinois No. 6 Pittsburgh High Splint Pocahontas No. 3 .4nthracite 500’ C. coke 700° C. coke High temp. coke Graphite (40-gram sample) Pitch (30-gram sample)
2 184
96
%
70
%
100.2
58.1 49.7 57.7 57.0 59.3 61.3
9.33 12.90 8.03 4.50 3.40 5.20 4.15 2.90 1.90 6.45
32.8 36.2
98.8
1c2.2 100.6
100.5 99.5 85.5 72.7
62.1 80.5
69.8
63.1 57.9 44.0
36.5
3Q.l 37.8 33.0 21.5 6.7 2.3 30.0
November 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
2785
be converted into water-soluble organic acids and t h a t with a high rank bituminous coal nearly 40% of the carbon appeared as polycarboxylic aromatic acids. It was also found t h a t potassium hydroxide could be replaced by sodium hydroxide, but with the same reaction conditions the products were darker in color and represented a less complete reaction. Attempts t o replace all the potassium hydroxide with lime were unsuccessful, but it was found t h a t a part could be so replaced. This effect was believed t o be due t o the formation of insoluble calcium salts in the early stages of the reaction which underwent metathesis t o soluble potassium salts if potassium hydroxide were present in sufficient amounts. The importance of alkali concentration is shown by the data in Figure 3. I n water alone the reaction resembles a combustion, First Experiments with Oxygen Gas and Alkali producing carbon dioxide, ash, small amounts of humic acids, Were Carried O u t in a I-Liter Autoclave and negligible amounts of the desired soluble acids. At 1.5 N a maximum yield of humic acids is reached which, on further Oxidations have been carried out in this laboratory with nitric increase in alkali concentration, is decreased by further oxidation acid ( l a ) , alkaline permanganate ( l a ) ,sulfur trioxide (14), nitric t o give additional quantities of the desired soluble acids. acid and oxygen (IO),and oxygen in aqueous alkaline solutions Of some 19 materials tested as catalysts, cobalt and copper ($0). The first work was done with the chemical oxidants, were outstanding in their effect on rate of carbon dioxide formanitric acid and alkaline permanganate. Nitric acid followed by tion, but their usefulness in increasing rate of formation of organic alkaline permanganate was found to be particularly effective in acids was doubtful. I n the light of later experience it appears the oxidation of high rank materials to mellitic acid. Further t h a t these materials accelerate decarboxylation rather than oxidastudy showed t h a t the action of oxygen gas a t elevated prestion reactions. sures and temperatures on aqueous alkaline suspensions of Attempts t o oxidize materials higher in rank than a 500' C. coal resulted in oxidation products similar t o those obtained with coke led almost exclusively t o carbon dioxide. Unsatisfactory permanganate in alkaline solutions. In view of the possible results were also obtained commercial value of such a with a coal tar pitch which process, special funds were formed a viscous liquid appropriated by a group of phase a t the reaction temthe sponsors of this laboraperature and consequently tory t o study the reaction furnished small contact for on a larger scale and t o prereaction. With coals, the pare large enough samples rate seemed t o be largely of the products for precontrolled by the gas liquid liminary commercial evaluinterface. ation. T h e f i r s t e x p e r i F u r t h e r studies were ments with oxygen gas and made in a horizontal batch alkali were carried out in reactor of the effect of t h e a I-liter n i c k e l , r o c k i n g nature of the starting mateautoclave (Figure 1). Fiftyrial on reaction rates and gram samples of through yield of organic acids. The 200-mesh coal were susdata are shown in Table 11. pended in 350 ml. of a n All runs were at 250" C. aqueous solution containing 335 grams of potassium hyand 750 pounds per square inch gage total pressure, and droxide. Oxygen was supplied a t a constant pressure the same weight of carbon through a reducing valve. (387 grams) was used in each; the actual weight of It was found necessary t o introduce a trap in the oxycoal used differed depending gen line near the autoclave on carbon content. The to prevent plugging by the ground coal was slurried with a solution of 1600 reactants. Sufficient alkali was used t o fix all the cargrams of commercial flake bon dioxide formed assumcaustic in 3.0 liters of water. ing complete oxidation, thus Five-hour reaction times ensuring a constant partial were used with all the matepressure of oxygen. D a t a rials and also shorter periods characteristic of the results in a number of instances. obtained in this apparatus The alkaline solutions of are shown in Figure 2 and r e a c t i o n p r o d u c t s were Table I. This work estabanalyzed for total carbon, lished that by liquid-phase c a r b o n a t e , a n d oxalate. oxidation with oxygen gas The difference between in an alkaline medium, total carbon and the sum about 50% of the carbon of the carbonate and oxalate of bituminous coals could was assumed t o b e i n Figure 1. One-Liter Rocking Autoclave
same reaction at higher temperatures with gaseous oxygen. Under these conditions the reaction results in carbonic acid almost exclusively. However, materials of as high rank as anthracite coal or a low temperature coke can be oxidized by gaseous oxygen with satisfactory yields of aromatic acids. Another factor which affects the choice of starting material is t h a t of reaction rate. Although higher yields of aromatic acids are obtained from higher rank materials, they are definitely less reactive and this may be a factor if air or oxygen is used as the oxidant. Concentrated nitric acid, however, at its boiling point oxidizes even the highest rank carbonaceous material, graphite, at rates satisfactory for laboratory preparations.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 11
~
with nickel turnings and the ends closed with nickel heads. Silver gaskets were used and the (250' C., 750 Ib /sq inch gage total pressure; charge. 387 grams of carbon) closure, which was of the same design as that Organic of the 1-liter vessel, was made on the ends of Soluble Carbonate Organic Acids, the nickel tubing. Alkaline "humate" solutions, Charge, Carbona. Carbon", Carbona, G./100 G. Hours Grams % Coal % % prepared by a preliminary mild oxidation with Rock Springs, m'yoming 1 622 98.1 46 0 nitric acid, m-ere circulated by a gear-pump down 100 65 6 S o . 6 Illinois g.5 576 43 8 97.6 this reactor countercurrent t o the oxygen. The 2.0 .. 98.6 52 6 5.0 .. 61 2 99.6 gear-pump a.as constructed of carbon steel and High Splint, Kentucky 511 97.3 49 5 3 .. 61 5 originally supplied with bronze sleeve bearings. 99.7 Pitrsburgh, Pennsylvania 518 96.2 48 2 These were not satisfactory and were replaced by 3 99.7 61 6 94.0 Pocahontas No. 3, K e s t 2 500 43 5 silver-plated steel. This reactor operated satisVirginia 0 .. 98.4 48 2 Anthracite, Luzerne, Pa. 5 482 97.8 63 8 up to 260" C. and total factorily a t temperatures Disco b ? 530 98.6 43 4 3 97.9 pressures of 1000 pounds per square inch gage. 51 2 98.6 500' C. Cokea 2 505 39 8 I t was demonstrated that a t 225" C. and 600 5 .. 51 2 99.2 pounds per square inch gage satisfactory rates of Per cent of carbon in charge From Pittsburgh seam coal ovidation from the humate t o the desired soluble acid stage could be attained. It was concluded that with suitable pumping equipment, slurries of coal could be circulated and the ovidation car"aromatic" acids. After acidification of the alkalinr solution ried out continuously in one stage. with sulfuric acid, the organic acids were recovered by threestage methyl ethyl ketone extractions. The higher rate of Pi'ot 'Iant Runs were Made in a 25reaction of the lower rank materials is evident from the higher Gallon, Vertical, Batch Autoclave carbonate-total carbon ratios obtained with these materials.
Tabla II.
Oxidation of Coals and Cokes in Horizontal Reactor
-
From the standpoint of yield of aromatic acids the ]OW volatile Pocahontas coal and the 500" c. coke are outstanding. On the basis of these results and because of ready availability Pocahontas coal was used in the larger scale work.
For the preparation of larger all-,ounts of the acids for chemical characterization and coinmercial evaluation, an internally stirred, batch autoclave TT-as specified because of the greater flexibility of the batch compared TTith the continuous type of operation. The necessarv pilot plant eaubmcnt ( 4 ) for recovery of - the acids was designed on the basis of bench'scale experiments. The principal components of the installation ape shown in Figures 6, 7 , 8 , and on the floor plan in Figure 9. A stationary vertical autoclave, 191/L inches I.D. and 30 inches deep with an operating capacity of approximately 25 gallons, was designed for a pressure of 1000 pounds per square inch gage a t temperatures up to 300' C. (Figure 10). I t was fabricated by Youngstown Welding and Engineering Co., Youngstown, Ohio. The shell of the autoclave was 10% nickel-clad steel l l / a inches thick: all party in contact with the alkaline reaction mixture were eithrr nickel-clad steel or nickel, Tvhich I
A Horizontal Rotary Reactor Furnished Temperature and Pressure Data for Designing a Continuous Reactor Following the work in the 1-liter pressure vessel the reaction was studied subsequently in: (1) a small rotary type pressure vessel with a capacity of about 1 pound of coal per batch; (2) a small packed-column type of continuous reactor; and finally (3) in an internally stirred 25-gallon pressure vessel with a capacity of 15pounds of coal per batch. This reactor is shown in Figure 4. I t was constructed of a piece of 6l/*-inch I.D. Shelby steel tubing n-ith i/2-inch wall. One end was closed by welding, the other by a removable gasketed head. The inside length was 28l/2 inches and the total volume about 4 gallons. The reactor was supported by, and rotated on short shafts coaxial with the principal axis of the reactor. It was fitted with internal lifting vanes for agitation, cooling coil, and thermocouple well. Oxygen and cooling water were introduced through glands a t the ends of the shafts. I t was heated by an external electric heater and rotated a t about 20 r.p.m. through a chain drive. Charges of about 1 pound of coal could be oxidized in this reactor. Approximately 100runs were carried out in this vessel a t temperatures as high as 250" C. and total pressures of 750 pounds per square inch gage. At the end of this time it was carefully inspected for evidence of caustic embrittlement, but none was found and this indicates that mild steel could be used in a n emergency for this type of reaction. A study was made of the continuous-reactor design illustrated in Figure 5. This consisted of a piece of Shelby steel tubing, 6 feet long, into which was pressed a piece of 3-inch I.D. by '/,-inch wall, nickel tubing. It was packed
_
OXALATE CARBON
225'
600 LBS. TOTAL PRESSURE 12
24
36
48
HOURS Figure 2.
Effect of Time of Oxidation on Carbon Distribution-Pittsburgh Seam Coal
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1952
2787
I
Acids
.
-- -
--------- -
-----+
I
0
1
2
3
4
5
6
-Q
7
NORMALITY OF SODIUM HYDROXIDE
Figure 3.
Effect of Alkali Concentration on Yield of Oxidation Products
Pocahontas coal,
PO
PO0 ml. soln, 250' C. lor 560 Ib./sauarr inch gage
grams.
P
hours? oxygen,
was chosen for its resistance to corrosion with high temperature alkali. Closure of the removable autoclave head onto a 3 / ~ a inch silver wire gasket was made by tightening down sixteen 2 l/n-inch diameter stud bolts through the head and a flange on the top of the autoclave. The vessel was fitted with an internal nickel coil which could be connected through a valve to a high pressure steam line for heating or through another valve to a cold water line for cooling. A nickel tube leading from the outside, through the head, down t o the bottom of the autoclave was connected t o a set of appropriate valves for charging, discharging, and oxygen introduction. Agitation of thn autoclave contents was accomplished by means of a turbostirrer driven by an electric motor with appropriate gear reduction box mounted on the head of the autoclave. A packing gland containing Garlock graphite-impregnated asbestos
Figure 4.
Horizontal Rotating Autoclave
Figure 5.
Continuous Reactor
packing and a lantern ring was used around the stirrer to ensure a pressure-tight seal. Before insertion, the packing wa8 exhaustively extracted with trichloroethylene to remove any traces of combustible organic material and then oven dried. The lantern ring was connected to a pressure-equilizing water reservoir pressured directly from the autoclave. The packing gland was cooled by t a p water passing, through a n external coil of copper tubing. Electrical heating was supplied by thirty-four 220-volt, $50watt Chromalox heaters attached t o the external wall of the autoclave which was then surrounded by rock wool insulation and an outer metal jacket. The heaters were connected in parallel in four banks, two of which contained eight heaters each, while the other two contained nine heaters. A single thermocouple immersed in a well in the reaction mixture was used t o measure and control the temperature through a Leeds and Northrup Micromax controller and relay circuit which regulated one of the heater banks; the other three heater banks were switched on and off manually as desired. A safety disk assembly using nickel disks with 1800 pounds per square inch gage ratings was first provided but blowouts occurred after about 20 runs a t about half the rated rupture values. Examination of the disks showed extensive corrosion, and, since it appeared probable t h a t solutions of ammonia formed from the nitrogen in the coal could accumulate in the cooler parts of the apparatus in appreciable concentration, the nickel disks were replaced by Inconel, which is resistant to ammonia as well as t o sodium hydroxide. The Inconel disks (Black-Sivalls and Bryson,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 11
pressure and 550 pounds per square inch gage vapor pressure of the reaction mixture. Any tendency to overheat was easily quenched by passing cold water through the internal coil and by regulating oxygen flow. No runawav reaction was observed in 323 runs. -4time-temperature chart from a typical run is shoir-n in Figure 11. I n the heating up period approximately 30% of the carbon of the coal was converted t o carbon dioxide and the remainder was soluble in alkali as the humic acid stage. The reaction conditions of 270" C. and 900 pounds per square inch gage were maintained for an additional 2 hours to convert 20% more of the coal carbon to carbon dioxide During the oxidation an approximate total of 1.63 pounds of oxygen Tvas consumed per pound of coal charged.
Two-Stage Methyl Ethyl Ketone Extraction Yielded 50 to 70% Aromatic Acids
In order to cool the products rapidly a t the end of the reaction period, cold water was passed through the coil. After cooling below 100" C. the autoclave contents n'ere forced out of the reactor by the residual pressure and into a 55-gallon drum which was equipped Lvith stirrer and heating coils to keep the sodium carbonate in solution. After removing a small sample for analysis, the 18 gallons of product were diluted v i t h water to 22 gallons and filtered through heavy cotton ta ill in a Shriver plate-andframe press to remove ash, some sodium carbonate, and a small amount of insoluble carbonaceous residue. The filtrate was then poured into about 50 pounds of 95% sulfuric acid in a Pfaudler glass-lined SO-gallon tank with cooling jacket. The acidity was adjusted to 1 X, the optimum normality for extraction. h pitchlilre material accumulated on the liquid surface and was removed manually; reoxidation of this pitch in the autoclave gave good yields of high quality aromatic acids indicating Figure 6.
Pressure Vessel for Batch Oxidation of Coal to Aromatic Acids
Inc., Kansas City, Mo,) showed no corrosion even after 50 runs. -4muffling- device ( 5 ) mas attached t o the discharge - side of the safety disk to eliminate noise and property damage from the alkaline spray in the event of disk rupture.
Reaction Conditions were Standardized at 270" C. and 900 Pounds per Square Inch Gage I n a standard run 15 pounds of Pocahontas coal pulverized to -100 mesh was stirred vigorously with 45 pounds of sodium hydroxide and 15 gallons of Rater in an external hopper. The quantity of alkali employed was just sufficient to neutralize the carbon dioxide formed when 50% of the carbon of the coal was converted t o carbon dioxide so t h a t sodium carbonate and only a negligible amount of bicarbonate resulted. The coal was thoroughly wetted to prevent the possibility of explosive reaction between dry coal particles and oxygen gas. The well-mixed allraline slurry was then pumped into the autoclave and heated t o about 160' C. by 90-pound steam in the internal nickel coil. At this point the steam was shut off, the electrical heaters turned on, and oxygen introduced. While 3 to 4 hours would be necessary to reach 270' C. with electrical heat alone, the heat liberated by the exothermal reaction of the oxygen with the coal raised t h e temperature of the system to 270' C. in less than 30 minutes where it was readily maintained by an input of 3.8 to 5.7 Irw. t o the Chromalox heaters. The oxygen flow was manually regulated to maintain the total pressure on the system a t 900 pounds per square inch gage, made up of 350 pounds per square inch gage oxygen partial
Figure 7.
Reactor in
0 orating Position in Safety Pit and Coal SErry M i x i n g Tank
November 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
t h a t i t represented a n intermediate stage in the oxidation process. The acidified solution was then extracted twice for 2 hours at,40° C. with two thirds its volume of methyl ethyl ketone in glass-lined Pfaudler extraction tanks. The tanks were equipped with stirrers geared t o operate a t 150 r.p.m. to prevent emulsion formation and with heating coils to prevent separation of sodium sulfate. The methyl ethyl ketone was practically insoluble in the saturated solution of sodium sulfate encountered here. An efficiency of over 85% was consistently obtained in the two-stage extraction. The methyl ethyl ketone was next distilled from the extract until the residue reached the consistency of 50 t o 70% aromatic acids. This sirup was then dried in a Buffalo laboratory vacuum drum dryer operated a t 28-inch vacuum and 1 pound of steam on the rolls. A trap of adsorbent carbon (6) was used t o protect the vacuum pump oil from contamination with solvent, water, and volatile acids. The dried porous friable mass of the acids was readily powdered in a ball mill. A flow diagram of the process is shown in Figure 12 and the average yields of aromatic acids and carbon balances from ten runs in Table 111. The efficiency of recovery in the various steps could undoubtedly be improved in commercial production,
2789 -
Table 111,
Pilot Plant Data on Oxidation of Pocahontas Coal (Carbon balances on 10 runs, 150 Ib. coal) CErrbon, Lb.
Fed to reactor After reaction and before filtration: Organic carbon Carbonate carbon After filtration: Organic carbon Carbonate carbon Filter cake Csrbonate carbon Organic carbon After acidification : Soluble organic carbon Pitch 1st etage extraction 2nd atage extraction Total Found in recovered acids
125 0
Weight, Yo 100
61 5
63.5 (diff .)
49 2 50 8
50 5
40 4
55.5 8 . 0 (dlff.)
11.0 (diff.)
8 8
37 7 2 7
28.7
4 4 33 1 31 8
Total acid yield, 75 lb.; 50% coal used.
The Mixture of Acids Recovered from the Process Forms a Hygroscopic, Friable, Yellowish-Brown Mass Typical analytical data for the acid mixtures recovered are shown in Table IV. The ratio of molecular weight to equivalent weight gives the number of functional groups per average molecule. The average nuclear size, N , is calculated from the relation: N = mol. wt. - funct. X 44. This corresponds to the nuclear size with the carboxyl hydrogen attached t o the nucleus. These acids are nearly completely soluble in water and in oxygenated organic solvents such as alcohols, ethers, ketones, and esters. The carbon-hydrogen ratio points t o acids of cyclic structure and the average nuclear size calculated for the mixture is greater than for the benzene ring indicating a mixture of multiring structures and benzenecarboxylic acids. Other properties lead to the same conclusion. All the acids of the benzenecarboxylic series are white, crystalline compounds which sublime in vacuum without decomposition. The mixture of acids recovered from the oxidation of coal are colored and despite the fact that they were prepared at temperatures of 270" C., under high oxygen pressures, are sensitive t o temperatures above 150" C. even in the absence of oxygen. Thus, by rapid heating up t o 350' C. in a molecular still, only 55 to 60% of the mixture distills and a coke residue and gaseous products are formed,
About O n e Third of the Oxidation Products Were Benzenecarboxylic Acids Fractionation of this mixture of acids can be effected by the solvent methods used in fractionating polymeric homologous series. By this procedure, fractions with nuclei as small as 90 and as large as 270 have been separated. Solvent fractionation, followed by esterification and fractional crystallization, has led to the isolation of the following benzenecarboxylic acids: orthoand isophthalic, trimellitic, pyromellitic, prehnitic (1,2,3,4- ), and benzenepentacarboxylic (17). Of these, the trimellitic predominates in the products from the higher temperature oxidations with gaseous oxygen, and the total amount of the benzenecarboxylic acids in the products from such reactions may be a third of the whole. Under the operating conditions used, the coal very rapidly becomes completely soluble in the alkaline solution, except for ash components, and by withdrawal of samples of the solution from the reactor through a condenser, and analysis of these, it is possible to follow the course of the reaction. For this purpose two quantities were determined-total soluble carbon and carbon as carbonate For the former, a suitable aliquot of the alkaline
Figure 8.
Reactor Safety Barrier and Control Panel
solution was reacted with a solution of chromic acid in concentrated sulfuric acid in a n apparatus similar t o t h a t used for the determination of carbon dioxide in limestone. This reagent evolves the carbon dioxide from carbonates and also oxidizes quantitatively to carbon dioxide, all other carbon present. The evolved carbon dioxide is absorbed and weighed in the usual manner. The carbonate carbon is determined in a similar type of apparatus and by a similar procedure, except that dilute hydrochlorie acid which evolves carbon dioxide from carbonates and does not oxidize organic carbon, is employed. Oxalate carbon was determined by acidification of a suitable aliquot t o about p H 3 and precipitation with calcium acetate. The precipitate was thoroughly washed, dissolved in sulfuric acid, and titrated with standard permanganate. Acetic acid was determined by acidification of a suitable aliquot
2'196
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
U Figure
wi8h sulfuric acid, distillation, and titration of the distillate with et andard alkali. Molecular weights were determined ebullioscopically in acetone using a modified hlenzies-Wright apparatus ( 1 )>and equivalent aeights, with a glass electrode and standard aqueous alkali. Distillation in a molecular still was used both for characterization and fractionation. The fraction of acids distilled up to 350' C. in 1 hour was found to be a useful index of the complexity of the sample. rill the benzenecarboxylic acids, including mellitic, distil! without decomposition under these conditions. Secondary Reactions with Alcohols W i l l Produce a Variety of Esters These are strong organic acids and undergo the usual reactions characteristic of the carboxyl group. They react readily with alcohols t o form esters ( 2 ) . With monohydric alcohols the ester8 range in properties from soft resins to viscous liquids; with polyhj dric alcohols, the esters are brittle resins. The esters can be fractioned by the solvent procedure described for the acids and also by molecular distillation. Approximately 60% of the b u t j l rsters can be so distilled before decomposition starts. A dark-colored, complex nonvolatile fraction can be separated by treating a pentane solution of butyl esters with small amounts of anhydrous stannic chloride (18). The yield of refined esters ie about 60%, and thry are of improved color and viscosity. hbnut 90% of the refined product can be molecularly distilled. Aqueous solutions of the acids can be hydrogenated to give polycarboxylic uaphthenic types. These are lighter in color and of improved thermal stability. Partial decarboxylation also occuru along with hydrogenation. Iltating aqueous solutions of the acids in the temperature rarigc 250' to 350' C. results in partial decarboxylation, and nitrogen barn, phenolic materials, and polycyclic aromatic hydi ocxbonp arc formed. The polycyclic hvdrocarbon, fluo-
Vol. 44, No. 11
9. Floor Plan of Pilot Plant
rene, has been isolated and characterizrd. The higher benscnecarboxylic acids are converted largely to isophthalic acid, and small amounts of benzoic acid have also been isolated from such reactions. The mixture of acids is very soluble in water and surh solution3 on evaporation yield viscous sirups of high concmtration Some, of the acids have the interesting property of forming JTatcrsoluble calcium, barium, and heavy metal salts.
The Polyfunctionality of Carboxyl Groups and Aromatic Character of Products Should Be Attractive to the Chemical Industry Practically all the aromatic chemlcais produced a t prwent from coal are obtained as the by-products from the coking of coals for the steel industry. K i t h steel production alrrxdv a t peak capacity and with the present high cost of b! -product roke
Table IV.
Typical Analytical Data of the Acids
Molecular weight Equivalent weight Functionality Average nuclear size Ultimate analysis Carbon Hydrogen Nitrogen Sulfur Oxygen (diff.) Free Oxalate carbon Residue on ignition Loss in weight in vacuum At 25' C . .4t 1100 c. Sublimation t o 360' C.in Sublimate Residue Gas and loes
270 82 3.3 125 Per Cent 54.4 3.0 0.1
0.4 42.1 0.26 0.17 0.33 1.2 9 1 36.7 28.1
16.2
November 1952
7
IN D U S T R I A L A N D E N G I N E:E R IN G C H E;M IS T R Y
:I Worm Gear
/ I 1
Figure 10.
General Arrangement of 25-Gallon Autoclave
oven installation, large increases in by-product chemicals do not appear probable. Therefore, any process would be attractive which could operate independently of steel operations and which could start directly with coal and convert a large portion of it t o useful chemicals. The oxidation process outlined starts directly with coal and converts about 50% of its carbon to a mixture of the watersoluble, polycarboxylic coal acids; with possible increases in efficiency of recovery, the weight of recovered acids would be more than half the weight of coal used (Table 111). The polyfunctionality of carboxyl groups and the aromatic character of the products should prove to be attractive to the chemical industry. To obtain manufacturing cost estimates and a market survey of the feasibility of commercialization of the process, the consulting services of the Scientific Design Co., New York, N. Y., were employed. The report was completed in June 1947 and included complete estimates of plant investment and production costs on scales of operation ranging from 1,000,000 to 30,000,000 pounds per year. Briefly, it was concluded that a production capacity of 1,000,000 pounds annually would be self-supporting although nonprofitable and t h a t at 10,000,000 pounds per year the total production costs per pound of coal acids would be only about half the market price of phthalic anhydride. Figure 13 shows the decrease in cost per pound with increased scale of production for three separate modifications of the process, all involving continuous rather than batch operation, as estimated by the Scientific Design Co. Process A is the one used in the pilot plant with no credit assumed for the recovery of the resultant large amounts of sodium sulfate; Process B involves a crystallization of the sodium carbonate after the reaction step, followed by treatment with lime t o recover caustic soda
2791
for recycling to the reactor; and Process E follows the crystallization of sodium carbonate with the complete recovery of the sodium ion by adding carbon dioxide and ammonia t o the mother liquor as is done in the Solvay process. Additional ramifications of the process can be suggested. Sodium carbonate could be recycled t o the reactor without conversion to caustic soda, or it could be converted with by-product carbon dioxide t o the less soluble bicarbonate which could then be charged to the reactor. Acidification of the reaction product with hydrochloric instead of sulfuric acid and recovery of the alkali by electrolysis and of the acid by recombination of t h e hydrogen and chlorine from the electrolysis is also a possibility. Air could be used in place of oxygen except that the total pressure would need to be increased t o maintain the necessary partial pressure of oxygen. I n the event t h a t esters are t h e desired product, the acids need not be recovered directly since the extraction could be carried out with the suitable alcohol-e.g., butyl alcohol for the preparation of the butyl esters. The main factors affecting the cost of the entire operation appear t o be the alkali recovery and the availability of tonnage oxygen a t low cost. Another factor would be the value of the recoverable by-products. About 50% of the original carbon of the coal would appear as carbon dioxide; another 1 to 2% as acetic acid; and perhaps 4 t o 5% as oxalic acid. The oxalic acid yield is dependent upon o p e r a t i n g conditions such as the t e m p e r a t u r e of oxidation, the rate of reaction, and the extraction efficiency which is fiensitive t o pH. Most of the nitrogen of the coal appears as ammonia. The process also affords a means of obtaining the inorganic residues of the coal Figure 11. Time-Temperature Record for Typical Oxidation without subjecting them t o high temperatures. One of the valuable by-products would be the heat evolved by the oxidation process. I n the pilot plant about 1.63 pounds of oxygen was consumed per pound of coal oxidized, whereas the total conversion of the Pocahontas coal t o carbon dioxide, water, nitric oxide, and sulfur dioxide would require 2.64 pounds of oxygen. As a rough approximation, a maximum of 9100 B.t.u. of the total of 14,760 B.t.u. per pound of Pocahontas No. 3 coal would be available.
Table V.
Plasticizing Properties of the Butyl Esters
Base Formulation 49.570 Geon 100 X 210 resin 49 5% plasticizer 1 0% stabilizer 0 125-inch thick films baked 30 minutes a t 375' F. Tensile Tear Elon- Permanent Strength, Resiatanoe, gation, Set, 1/16 Lb./Sq. Inch Lb./Inch % Inch/Inch Plasticizer Butyl esters of coal acids: Raw esters refined with 1008 224 276 4 SnCla in pentane 1117 161 367 8 Diaillation fraction 2a 1227 181 357 5 Distillation fraction 6" 1673 360 288 2 Distillation fraction loa Dioctyl phthalate 993 148 342 8 a For properties of these ester fractions cf. reference ( 2 ) .
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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jF , a$;
OXIDATION OF COAL TO AROMATIC
Vol. 44, No. 11
ACIDS
IMixina
Process ‘A*
Reactor 200-300” C 600-i200ps~
sl;::‘
Mixer
c
Ash
i
Insoluble Acids
1 ‘Ot
401t Figure 19.
Process ‘8’
Flow Sheet of Process for Oxidation of Coal to Aromatic Acids
The potential uses and fields of applications for the c o d acids are indicated by the interest which met the announcement ( 3 ) that samples for testing purposes were awilable. One outlet would be in the plasticizer field which is now dominated by the phthalic acid esters. The coal acids have been successfully esterified with met,hyl, ethyl, n-butyl, and octyl alcohols: t,hc plasticizing properties of the butyl esters have been evaluated iii a preliminary fashion and are compared with cornmerciall>available dioctyl phthalate in Table V. It is evident that the fractions of the esters of the coal acids are in a number of instances superior in tensile strength, clongation, and tear resistance. They have been found inferior from the standpoint of brittle temperature and loss of elongation on heating. These latter properties can, no doubt, be improved by proper additives. Esters of t.he same alcohol have the very great advantage over t,he o-phthalic esters of much lower volatility. They have lower plasticizing “efficiencies”--that is, more is required to produce t,he same plasticizing effect, and color would be a disadvantage for some but not all purposes. iinother possible applicat,ion of the coal acids would be in the production of glyptal type resins, although color is also objectionable here and the high functionality tends to produce brittle products. The esters prepared from the coal acids and monohydric alcohols or monohydric phenols possess properties making them valuable as lubricants either alone or as additives t o mineral oils ( 1 6 ) ; their densities are higher than those of most ester lubricants. Other interesting smaller scale uses of the acids have been suggested. The most important of these are: (1) acid solutions for metal cleaning baths for which they are apparently superior to phthalic acid; (2) replacements for maleic acid for breaking petroleum emulsions in oil fields; (3) dispersing agents in drilling muds; and (4) substitutes for other organic acids. The total potential demand for these acids largely as a replacement and supplement for phthalic acid has been estimated to be 150,000,000 pounds per year at prices not, exceeding 15 cents per pound.
Acknowledgment The authors wish t,o express t,heir appreciation to Matilda 11 Fine, H. W. McCune, Daniel T. Muth, Joseph B. Simsk, W. E. Wehn, and J. J. Wolfe for assistance in operation and control of the pilot plant, and t o J. A. Thompson and George F. Waechter for construction, assembly, and maintenance. Further, the authors are indebted for financial support t o Mellitic Associates, a group consisting of H. N. Eavenson, General Coal Co., JamiEon Coal & Coke Co., Pittsburgh Consolidation Coal Co. (and
It
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”I\ 20
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1
,
3
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20 UILL~OHS OF
Figure 13.
mums
)o
PER YEAR
Relation of Estimated Costs to Scale of Production A
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--
Manufadurin8 cost Materials
subsidiaries), Pittsburgh and Fairmont Coal Go., and the Pocahontas Fuel Co., Inc.
Literature Cited (1) B e r m a n , N., a n d H o w a r d , H. C., Anal. Chem., 21, 1200-2 (1949). (2) Zbid.. FueE, 29, 109-11 (1950). (3) Chem. Eng. News, 24, 683 (1946). (4) F r a n k e , N. W , , a n d Kiebler, M. IT., Chem. Inds., 58, 580-1 (1946). ( 5 ) Ibid., 62, 248 (1948). ( 6 ) F r a n k e , N. W., Kiebler, R.I. W., a n d W e h n , IT, E., Ibid..6 1 , 870 (1 947). (7) Friedman, W. D., a n d Kinney, C. R., ISD. ESG. C m h r . , 42, 2525-9 (1950). (8) H o w a r d , H. C., E d . , H. H . Lowry, “ C h e m i s t r y of Coal Utilizat i o n , ” pp. 346-58, New York, J o h n Wiley & Sons, Inc., 1945 (9) Ibid., pp. 358-72. (IO) H o w a r d , H . C., U. 8. P a t e n t 2,555,410 (June 5, 1951). (11) J u e t t n e r , B., J . Am. Chem. SOC.,59, 1472-4 (1937). (12) J u e t t n e r , B . , S m i t h , R. C . , a n d H o w a r d , H . C . , Ibid., 57, 2322-(i (19351. (13) Ihid., 5 9 , 236-41 (1937). (14) Kiebler, M. W . , U. S. P a t e n t 2,461,740 (Feb. 15, 1950). (15) Kinney, C . R., a n d Friedman, W.D., J . Am. Chem. Soc.. 74, 57 61 (1952). (16) Montgomery, C . W., Gilbert, W. I., a n d Kline. R. E., L7. 8. P a t e n t 2,568,965 (Sept. 25, 1951). (17) Roy, A. N . , a n d H o w a r d , H . C., J . Am. Chem. Soe., 74, 3239 (1952). (18) Ruof, C. H., a n d H o w a r d , H. C., presented before t h e Division of G a s a n d Fuel Chemistry, 118th Meeting AXERICAN SOCIETY, Chicago, Ill. CHEMICAL (19) Ruof, C. H . , Savich, T.R., a n d H o w a r d , H. C., J . Am. Chem. SOC.,73, 3873-9 (1951). (20) S m i t h , R. C.. Tomarelli, R. C . , a n d H o w a r d , H. C., I b i d . . 61, 2398-402 (1939).
RECEIVED for review August
22, 1962. ACCEPTEDSeptember 10, 1952, Presented as part of the Symposium on Chemicals f r o m Coal before the SOCIETY Division of Gas and Fuel Chemistry of the AYERICAKCHEMICAL State College, Penna., M a y 5 , 19.52.
