The Technology of Low Temperature Carbonization
Frank M. Gentry
[ Chapter 3: Low Temperature Coal Tar ]
Table of Contents
Preface & Table of Contents
Chapter I ~ Fundamentals
Chapter II ~ Low Temperature Coal Gas
Chapter III ~ Low Temperature Coal Tar
Chapter IV ~ Low Temperature Coke
Chapter V ~ Nitrogenous & Other By-Products
Chapter VI ~ Processes of Low Temperature Carbonization
Chapter VII ~ Operation, Design, & Materials of Construction
Chapter VIII ~ Economics & Conclusion
[ Note: The quality of the scanned graphics (tables & figures) are "uneven" at best despite repeated efforts to scan and tweak the images. Enough is, so here it is anyway: I've had enough and done enough for the time being. ~ R.N. ]
Low Temperature Coal Tar
Eduction of Primary Tar ~
Designation of the condensable products from low temperature carbonization, taken collectively, as a tar is most unfortunate, for they more nearly resemble oils or petroleums than they do tars. Since it has become well established by prior usage, the name tar will be retained in this volume and the term primary tar will be used synonymously with low temperature oil. This confusion in terminology has caused a great deal of misunderstanding and many persons have thereby been led to evaluate low temperature crude oil in terms of high temperature coal tar, to which it bears very little relation.
Like any industry in the early stages of its development, the products of low temperature carbonization have had no established market in the past. This applies in particular to the primary oil. The chief assured industrial uses of low temperature tar, at present, is as a crude stock for the preparation of creosote oil and as a liquid fuel. While the valuable by-products, which may be obtained by fractionation of the tar will doubtlessly find industrial favor eventually, no such outlets can at this time (1928) be guaranteed. There are but few plants operating on a large scale, and consequently no standard method of industrial analysis has been developed, despite the quality and quantity of work that has been done in the study of low temperature tar by numerous investigators. For the most part, these workers were interested more from the chemical than from the engineering standpoint, and therefore planned their investigations more from the standpoint of science than of technology.
The attitude of the tar distillers, in regard to low temperature tar at the present time (1928), appears to be that its refinement will require extensive alterations in their plants, due to the manner in which primary tar differs from the high temperature tar, which they are accustomed to handle. The absence of regularly operating large scale low temperature carbonization plants of sufficient capacity to assure them of dependable quantities of raw material has thus far rendered the necessary changes impractical. This situation is rapidly being remedied and will be overcome in proportion as sources of raw material increase and as markets for the refined products develop.
According to Parrish (117), whatever coal may be used in the same high temperature coke oven, the resultant tars resemble each other to a great extent. Primary tar, on the other hand, is much more closely allied to the coal from which it originated and variations in the composition and quantity of the tar yielded follow more closely variations in the character of the coal from which it was produced. It will be observed later that, in general, the younger the coal and the greater its oxygen content, the greater is the quantity of low temperature tar yielded per unit of weight.
The influence of the conditions of carbonization can almost wholly be attributed to the action of heat upon the tar vapors. The effect of these conditions on the low temperature tar has already been alluded to in the definition of low temperature carbonization. Aside from the bearing which the chemical nature of the coal and the conditions of retorting have upon the character of the tar, there remain certain influences, resulting from the physical properties of the coal. If the coal is a readily fusing one, the formation of a plastic layer may so retard the passage of the primary tar vapor from the carbonization chamber as to cause it to remain for a comparatively long time in contact with hot surfaces. The effect of hot surfaces in catalyzing secondary decomposition has already been demonstrated.
Low temperature tars differ materially from high temperature tars. The former are brownish black, fluid at ordinary temperatures, and more viscous than crude Pennsylvania petroleum. In general, the low temperature tars consist of hydrocarbons of the alipathic series, a few aromatics, and a high proportion of the tar acids. This tar is not greatly different from shale oil and crude petroleum, and for that reason some of the methods used in the petroleum industry may be applied to the evaluation of low temperature tar.
Table 44 gives a representative comparison of the fractions obtained from high and low temperature tars. The high temperature tar yields are the average of 6 coke oven tars of the United States, while the low temperature yields are the average of 8 British experiments at 600° C. In the high temperature product, only about 25% of the tar is volatile up to 315° C, while low temperature tar yields nearly 55% of its weight in oils. High temperature pitch is almost three-quarters of the entire yield of tar and contains a high percentage of free carbon, whereas the primary tar is characterized by less than 50% pitch of a low free carbon content. The decrease in the yield of oils at high temperatures is complementary to the high gas yield. It has been demonstrated that, in this case, the primary volatile products are cracked and decomposed at higher temperatures, with the formation of gas and the deposition of carbon.
Experiments conducted in England, in especially constructed cast-iron retorts, on 5 high class Barnsley coals whose proximate analyses were almost identical with that of Pennsylvania bituminous coal, showed that in low temperature carbonization the average yield of tar was 23.5 gallons per net ton of coal. The average of 6 Pennsylvania coals distilled at 550° C by Davis and Parry (97) gave 27.8 gallons of dehydrated tar, with a specific gravity of 1.039, per net ton of coal carbonized. This is equivalent to 12.2% of the coal by weight. The Fuel Research Board, in large-scale horizontal retorts, obtained a tar yield of from 17.9 gallons to 19.6 gallons per net ton. They recommend, as a result of their extensive investigations, that 19 gallons of tar per net ton is the average yield to be counted upon in low temperature distillation. The present author, however, believes this figure to be unduly low for other processes and for high volatile American coals. The fuel value of this primary tar is very high, the average of 8 British experiments giving 16,630 BTU/lb.
Porter and Ovitz (96) in their investigation on the volatile matter of coals of the United States, obtained from 12.4 gallons to 21.2 gallons of tar per net ton of coal in small-scale experiments, depending to a great extent on the geologic age of the coal examined. The experiments were conducted over a period of one hour in a furnace adjusted so that the interior of the charge remained at 800° C. Monett (100) observed, from a study of 11 Utah coals, that the tar yield ranged from 16.1 gallons to 29.4 gallons per net ton. It is interesting to note in Table 45 the general decrease in yield of tar accompanying the consolidation of the fuel.
The results of Table 45 have been observed also by Berry (118), who found a distinct relation between the amount of tar distilled and the geologic formation of the coal, or more specifically of the carbon-hydrogen ration of the fuel. The curve in Figure 18 is due to him. According to this curve, the maximum amount of thick tar is to be expected from fuels having a carbon-hydrogen ration ranging from 13.5 to 18.0.
Berry (118) investigated the tar-forming temperatures of American coals and found that the first trace of tar appeared, in all the coals examined by him, at about 300° C and the last traces disappeared at about 550° C, although in a few instances tar formation did not cease until over 600° C was reached. Tar was formed in quantities in the temperature range from 375° C to 475° C. he found no evident relation between the volatile matter in the coal and the amount of tar evolved. He also concluded that neither geologic age, carbon-hydrogen ration, nor percentage volatile matter in the coal had any relation to the temperature of tar evolution.
Burgess and Wheeler (57), in examining Welsh coals, found certain variations in the yield of tarry matter when the temperature of carbonization ranged form below 500° C to above 1000° C. Their data are plotted in Figure 19, where it is seen that the yield of tar per gross ton of coal rises to a maximum at about 700° C. The experiments of Burgess and Wheeler (57) are not in agreement with the figures given by Lewes (17), whose maximum yield occurred about 300° C lower. No information was given by the latter authority regarding the composition of the coal used to secure his data, so that the disagreement cannot be explained. The specific gravity of the tar gradually increases with the temperature increase. This is to be expected, since the lighter hydrocarbons are destroyed through decomposition at the higher temperatures to form fixed gas, leaving the heavier, higher-boiling hydrocarbons and the carbon freed in the cracking.
Porter and Ovitz (96) investigated the tar distilled at various temperatures from several coals of the United States. Their figures, computed from a percentage basis to gallons of tar per net ton, are plotted in Figure 20. Comparison of this illustration with Figure 19 will disclose that, qualitatively, the data are in agreement with the results obtained by Burgess and Wheeler (57) on Welsh coals. In Figure 20 it appears that, from the standpoint of securing maximum tar yield, the optimum temperature of carbonization is in the neighborhood of 800° C, although in commercial plants this figure appears somewhat high. Too much confidence cannot be placed in the quantitative value of these curves for they seem low for high volatile coals.
According to Fischer (119), the solid paraffins in primary tar represent 1% or 2% of the tar and in this respect we find one striking difference between the tar obtained from brown coals and that obtained from bituminous coals. Fischer (120) found the tar from Saxon brown coal to contain as much as 29% solid paraffins, while the tar from Rhenish brown coal yielded 13%. In other respects, the two primary tars are quite similar.
Garbe (121) distilled a Shetland peat containing 21.4% moisture and 73.8% volatile matter, at 500° C in a Tozer retort and obtained a yield per gross ton of 33.6% peat char, 4500 cu ft of 500 BTU/cu ft gas, 1.44 gallons of 0.7885 specific gravity motor spirit, 23.4 gallons of crude oil, and 13.6 lb of ammonium sulfate. The peat tar contained 35.2% moisture, 2.2% light oil up to 170 C, 19.8% medium oil in the fraction 170° C to 230° C, and 37.8% residuum above 230° C. The middle oil contained 22% tar acids.
Garvin (112) reports that, on the average, distillation of Scottish shale yields 24.5 gallon of 0.860 sp gr tar, 9800 cu ft of 270 BTU/cu ft gas, 35.7 lb of ammonium sulfate, and 1550 lb of waste spent shale from each ton of raw material. Oil shales of the United States have tested all the way from less than 7.5 gal of tar/ton, in the case of a Nevada sample, to over 60 gal/ton for a specimen from Colorado. The specific gravity of these tars varies also from 0.881 to 0.924. The average test yield of tar from 11 US shales from 6 states was 37.1 gal/ton.
Time Effect ~
The time element deserves important consideration in regard to the tar as well as with respect to the gas. If the distillation is slow and the tar vapors are removed slowly, secondary decomposition may occur with a corresponding decrease in the yield of oils and increase in the evolution of gas. But the tar yield is not the only outcome of slow carbonization. The structure of the tar is vastly changed. A tar produced in slow distillation contains a larger proportion of the aromatic derivatives and fewer members of the alipathic series. The curve of tar yield against time is somewhat similar to that of gas evolution, shown in Figure 14. We know, of course, from the differential calculus, that the derivative of this curve is the curve of the rate of yield of tar. Inspection of the slope shows that the rate of yield in gallons/minute, as plotted against time, should reach a maximum and then gradually fall off until a point may be reached where it will be uneconomical to prolong the process for the tar alone. In other words, the rate of yield curve for tar has been found by experiment to be similar to that for gas shown in Figure 17. It should not be overlooked that, although the production of tar after a certain period may not be sufficient to warrant continuation of carbonization, the gas evolved may entirely justify prolongation.
The effect of bringing the charge up to maximum temperature in 5 hours, as compared with attaining the same temperature within a fraction of an hour, is shown by the results of Table 46, after Taylor and Porter (98). These experiments were conducted on Pennsylvania coal. It is seen that in every case the yield changed from about 16.0 gal/gross ton of coal to about 20.5 gal/gross ton when the period of attaining the maximum carbonization temperature was decreased from 5 hours to a much shorter period.
Pressure & Vacuum Effect ~
Some 30 years ago, Dewar and Redwood patented the distillation of petroleum under pressure to crack the heavy distillates into low-boiling and more valuable fractions. In 1910, Burton developed the pressure cracking system and now it is a well-established industry. Attempts have been made to reduce the deposition of carbon by the introduction of catalyzers and Bergius has proposed the hydrogenation of the hydrocarbons by carbonization under pressure in an atmosphere of hydrogen. Cracking, however, in low temperature carbonization process, is ordinarily avoided in order to preserve the primary products of distillation. Hence, the retorts are usually operated at atmospheric pressure under slight vacuum, of hardly more than 2 inches of water, to overcome the resistance of the piping system. A discussion of the possibility of increasing the yield of lighter hydrocarbons, suitable as a fuel for internal combustion engines, by cracking the primary tar will be taken up under the subject of motor spirit.
A vacuum system assists in the removal of the vapors and prevents condensation of the heavy tars in the retort. It also removed the air which is present, thus preventing oxidation, and preserves the primary tar by avoiding cracking. In 1905 an 1906, Simpson obtained patents which were afterwards incorporated in the Tozer retort for distilling in partial and complete vacuum. Little work has been done by way of investigation in this field, but Taylor and Porter (98) have examined certain coals of the US when distilled at pressures below 4 cm Hg. A comparison of the tar yields from atmospheric and vacuum carbonization is instructive. The atmospheric distillation in Table 47 was carried out at 475° C. The figures, for convenience, have been converted from a percentage basis to gallons per gross ton.
The effect of temperature on the yield of vacuum tar also was investigated by the same authorities. In this case, different periods of distillation were used at different temperatures, so that the data are not strictly comparable, but they are given in Figure 21 because no other similar figures are available. The data are computed to gallons per gross ton. It will be noted that the yields in this illustration are higher than those indicated by Table 47. This is due, doubtlessly, to the systematic error pointed out above, as well as to an additional cause. In this case, the percentage tar obtained was determined by subtraction, rather than by direct observation, and consequently the cumulative errors of analyses fall entirely on the tar content. Figure 21 should be compared with Figure 20, where the corresponding results of atmospheric distillation are shown. The main difference in the characteristic temperature versus yield curves of atmospheric and vacuum carbonization lies in the absence of a maximum on the vacuum system. The yield of tar, in the latter case, quickly rises until about 450° C is reached, when little change in the quantity of tar produced is noticed during the next 150° C temperature increase. Thereafter, the yield of tar increases with temperature rise even up to 1050° C.
Atmospheric & Moisture Effect ~
Taylor and Porter (98) investigated the effect of inert atmospheres on the quantity of tar produced. The presence of coal gas or nitrogen did not exert any decided influence on the yield. Thus, over a 5-hour period of distillation, 17.1 gallons of tar per gross ton were obtained in an atmosphere of coal gas at 950° C and 16.0 gallons when pure nitrogen was introduced into the retort during carbonization at 1040° C.
Weathering of the coal before coking appears to have an effect on the quantity of tar evolved in carbonization. Thus, it was found that a Wyoming coal yielded 9.1% of its weight as tar, when distilled as received, but only 6.2% after air-drying. It is a well known fact that weathering reduces the volatile content of the coal, both through escape of the occluded gases and through oxidation. It is not surprising, therefore, to find a larger evolution of tar immediately upon reception of the sample than after the coal has been exposed to the action of the air.
The moisture content of the coal does not have any influence on the amount of tar produced, when computed upon a dry coal basis, according to Porter and Ovitz. (96). Thus the yield of tar from a dry Pennsylvania coal, containing but 1.2% moisture, as compared with another sample containing 9.6% moisture, is given in Table 48. The decrease in tar, when figured as a percentage of the coal as charged, is a fictitious decrease only, arising entirely from the selection of a base for computation which contains a smaller quantity of the tar-producing element.
Davis and Parry (97), it will be recalled, conducted experiments to determine the effect of superheated steam upon the carbonization products. It has been shown that probably the temperature is not sufficiently high in the low temperature distillation process to cause any interaction between the steam and the primary volatile products, and consequently the principal effect of steam will be in preventing secondary decomposition by assisting the quick removal of the products. With Pennsylvania coal from the Upper Freeport bed, Davis and Parry (97) found that 33.0 gallons of tar per gross ton were obtained without the use of steam, whereas the following yields were obtained when 88% steam was passed through the retort: 30.8 gal/gross ton at 475° C, 33.3 gal/gross ton at 550° C, and 50.9 gal/gross ton at 650° C. At 550° C, which is the only temperature at which experiments were made on the same coal without the use of steam, apparently there was little effect on the yield. It is evident that an enormous quantity of tar was evolved under steam distillation at 650° C. Unfortunately, no information is at hand to determine whether this large yield is due to the presence of steam or is a peculiarity of the coal.
The tar produced in steam distillation at low temperatures is a little more viscous but otherwise it does not appear essentially different from the tar obtained without the use of steam. Its fractionation, however, shows it to have a marked difference in volatile content. It contains a smaller percentage of light oils, but more of the heavier hydrocarbons. This topic will be taken up again under a discussion of the fractionation of low temperature tar.
Constitution of Primary Tar ~
Although low temperature tar obtained from lignite and brown coal may be solid at ordinary temperatures, due to a large proportion of solid paraffins, primary tar, in general, is lighter and more fluid than that from high temperature processes. The specific gravity of primary tar usually ranges from 0.95 to 1.06, as compared with 1.2 sp gr for high temperature tars, according to Fischer (119). The comparatively low density is due mainly to the presence of large proportions of liquid hydrocarbons and phenols. Low temperature tar usually has a strong odor of phenols and of hydrogen sulfide, but it never has the naphthalene odor of secondary tars. In transparent layers, its color ranges from orange to red. Fischer and Gluud (60) have pointed out another way in which low temperature tar resembles petroleum and differs from high temperature aromatic tars. They found that some of the hydrocarbons from primary tar exhibit a small but distinct optical activity. In a tar from Lohberg coal, freed of phenols, the optical rotation with sodium light through a 20 cm tube varied from +0.05 for the fraction 175° C to 185° C, to the value +0.30 for the fraction from 225° C to 235° C. Low temperature tars are usually soluble in the ordinary organic solvents, such as benzene, ether, acetone, chloroform, carbon tetrachloride, and petroleum ether, but its solution is usually accompanied by precipitation of resins.
It has been shown conclusively that distillations of the primary tar, during analytical procedures, distinctly alters its composition, especially that of the higher boiling compounds. Fischer (49) states that primary tar cannot be distilled without the part above 300° C decomposing, the viscous lubricating oils suffering the greatest decomposition and the higher phenols being affected to a lesser degree. A specific case of this for low temperature brown coal tar has been given by Fischer (85) and is reproduced in Table 49. The distillation was carried out slowly for 8 hours, yielding 85.6% oil, 4.7% gas, 9.2% coke, and a small amount of water. It will be observed that distillation so altered the constitution of the fraction from 70° C to 250° C as to increase its proportion from 18.5% to 31.7% of the original tar. The proportion of acids was simultaneously greatly decreased. Schneider (122) confirmed these results with tar from a bituminous coal.
Decomposition of primary tar during distillation is further indicated by the evolution of gas. Windel (123) found that gas first was evolved at 270° C, reaching a maximum at about 335° C. The gas contained 7% hydrogen sulfide, 4% carbon dioxide, about 75% methane and its homologues, and about 10% ethylene and acetylene hydrocarbons. Edwards (124), and also Brittain, Rowe, and Sinnatt (125) detected the evolution of sulfur dioxide at the beginning of distillation and hydrogen sulfide at 170° C, when studying tar from the Coalite process.
It is quite evident that the evaluation of low temperature tars is a difficult task and that the method of separation by solvents must be used to avoid distillation as much as possible, but even this method of approach is not without its difficulty of solvent reaction. These troubles are augmented by the fact that most low temperature tars change color and character on standing. This has been attributed by Jaeger (126) to oxidation by the atmosphere through the action of light, but the presence of bases are important factors. Parr and Olin (37) found, also, that the viscosity of the tar increased with standing, apparently the result of oxidation, for Brittain, Rowe, and Sinnatt (125) found no change when the tar was kept in darkness and out of contact with air.
As early as 1862, Williams (127) and Schorlemmer (128) seem to have made the first chemical examination of low temperature tar obtained from cannel coal. The latter of these investigators identified paraffin hydrocarbons. However, the first noteworthy research was conducted by Bornstein (95) in 1904. He fractionated primary tar from Westphalian coals, from 50° C to 450° C, and found it conspicuously different from high temperature tar, being a thin liquid with little free carbon and containing no naphthalene or anthracene, but quantities of phenols and up to 2% solid paraffins. With another Westphalian low temperature tar he found no solid paraffins, but something less than half of one percent methylanthracene.
Pictet and Bouvier (51, 129, 130 & 131), and also Pictet, Kaiser, and Labouchere (52) conducted a series of investigations, beginning in 1913, on primary tar obtained from vacuum distillation of a French bituminous coal containing about 11% ash, 70% fixed carbon, and 19% volatile matter. The fresh tar was composed of 68% unsaturated hydrocarbons, 30% saturated hydrocarbons, 2% alcohols and 0.2% bases. They found no phenols in the fresh tar, but detected some after the tar had remained standing a long time. As other investigators have found over 15% phenols in fresh tar examined by them, it appears that Pictet and Bouvier must have had an exceptionally true primary tar. They established the presence of toluidine, pentamethylphenol, tetramethylphenol, xylenol, hydroguinoline, and isoquinoline.
Jones and Wheeler (42) examined the tar obtained from a Scottish and a Durham coal, distilled at 450° C in vacuo. In the oils below 300° C, they found about 40% unsaturated hydrocarbons, about 7% aromatic compounds, probably homologues of naphthalene, about 12% phenols, chiefly cresols and xylenols, traces of pyridine bases, and a small amount of solid paraffins.
Identification of Compounds ~
Regarding particular chemical compounds that are found in low temperature tars, some quantitative determinations by Schultz and Buschmann (34, 35) and also by Fieldner (36) have been given under the chemistry of low temperature carbonization, discussed in Chapter I and compiled in Table 5 and Table 6 respectively. A qualitative list of compounds, actually identified in low temperature products, was given in Table 7, accompanied by references to the authorities who isolated them.
Among the aromatic hydrocarbons, Whitaker and Crowell (132, 134), when investigating a Pennsylvania coal, observed that benzene first formed at 500° C and toluene at 400° C, but that maximum yields were not obtained until 800° C for benzene, 700° C for toluene, and 600° C for xylene. Traces of benzene have been found by a number of observers, but most investigators have been unable to detect it in primary tars examined by them. Parr and Olin (37) found benzene, as well as toluene, xylene, and possibly mesitylene, in Illinois low temperature tar. Schultz (43) is the only authority who has reported considerable quantities of benzene hydrocarbons. He examined a tar obtained by distillation of Furst-Hardenberg coal in a rotary retort at about 480° C. Broche (53), however, examined tar from the same coal and found less that 0.4% benzene, from which he concluded that the tar examined by Schultz was abnormal. Fischer and Gluud (39) detected a very small amount of benzene in tar from one coal examined by them.
Besides Parr and Olin, toluene in minute quantities was found by Jones and Wheeler (42) and its presence was confirmed by Frank and Arnold (133). The latter observers also found three xylenes in the tar fraction from 120° C to 180° C. Klein (135) found that both toluene and xylene in the light oil from a large rotary retort, while Weissberger and Moehrle (46), and also Kruber (136) and Kaffer (137), found the higher homologues of benzene, of which pseudocumene, durene, and a few others have been identified.
Less than 1% naphthalene seems to be present in low temperature tars, as compared with 4% or 5% in aromatic tars. According to Ruhemann (138), brown coal low temperature tar contains some naphthalene. Brittain, Rowe, and Sinnatt (125), Morgan and Soule (45, 48), as well as Weissberger and Moehrle (46), found it in tar from full-scale low temperature plants, while Parrish and Rowe (139) found small quantities in tar carefully distilled from coal at 600° C. Anthracene is present in even smaller proportions than naphthalene, but it has been reported as present by Britain, Rowe and Sinnatt (125) and by Weissberger and Moehrle (46), the last of which also established the presence of naphthalene homologues such as the methylnapthalenes, confirming the observations of Fischer, Schroeder, and Zerbe (47).
Among the saturated and unsaturated hydrocarbons, Fischer and Gluud (39) found pentane, hexane, many of their homologues, and possible terpenes, among the low-boiling fractions of primary tar. Schultz, Buschmann, and Wissebach (38) established the presence of ethylene, propylene, butylenes, pentene, butadiene, and cyclopentadiene in the light oils, most of which were also found by Fieldner (36) in certain American low temperature tars, as shown in Table 6. Klein (135) also detected octylene and nonylene in large-scale primary tar.
Among the oxygenated compounds, acetone has been observed by Frank and Arnold (133), by Schultz (140), and by Broche (53). Acetaldehyde and indications of ketones were obtained by Brittain, Rowe and Sinnatt (125), while Schultz (140) also identified acetaldehyde as well as methylethylketone and acetonitrile.
Free sulfur is probably present to a small extent in low temperature tar, although its presence is undesirable, but it cannot be avoided if the raw coal is one of large sulfur content. Schultz (43) detected methylmercaptan and dimethylsulfide, in addition to the simple sulfur compounds hitherto mentioned. Brittain, Rowe and Sinnatt (125) found over 1.5% combined sulfur in the crude full-scale low temperature tars examined by them.
Tar Acids ~
Heretofore it has been pointed out that one of the outstanding peculiarities of low temperature tar is its high acid content. These acids do not consist so much of phenol itself as of homologues of the series, such as the cresols, xylenols, and more complex phenol derivatives.
There has been a great variation in the amount of acids present in the tars examined by various authorities. Thus, Jones and Wheeler (42) found about 7%; tests on the Toser retort gave 10.2%; Edwards (124) reported 10.8% to 15%; Morgan and Soule (48) determined 14.7%; Davis and Gallway (141) found from 12% to 19%; the Fuel Research Board reported 17.8%; Morgan and Meigham (142) 17.5%; Davis and Parry (97) 18.2%; tests on the Nielsen retort yielded 20%; Parr and Olin (37) 27.9%; and Church and Weiss (143) as much as 50%.
The work of Morgan and Soule was conducted on tar from the Carbocoal process; Jones and Wheeler used vacuum tar; Church examined a tar from an Illinois coal of high oxygen content; Parr and Olin used Illinois coal; Davis and Parry used tar derived from Pennsylvania coal; Edwards used English coals. The high yield obtained by Church does not seem extraordinary, when it is recalled that he used a coal of high oxygen content and that the phenolic compounds contain combined oxygen. In fact, it seems permissible to predict that, in general, coals which are highly oxygenated may be expected to give a larger quantity of the tar acids upon carbonization. The same may be said, perhaps, of coals which have been weathered through exposure to the atmosphere or retorted in the presence of a considerable amount of oxygen. Unfortunately, no experimental data are at hand to substantiate this belief.
The bulk of low temperature tar phenols consist of complex hydrogenated and alkalated derivatives or ordinary high temperature tar phenols. They are largely saturated compounds. Morgan (144) states that on account of their long side chains the primary tar phenols are less soluble in water than those derived from aromatic tars.
Phenol itself usually is present in amounts of less than 0.5% of the tar, as compared with twice that amount for high temperature tars. Its presence in small quantities has been established by Fischer and Breuer (145), by Brittain, Rowe, and Sinnatt (125), by Morgan and Soule (48), by Parrish and Rowe (139), and by Frank and Arnold (133). Schultz (43) and his coworkers (38) are the only investigators reporting as much as 1.35% phenol I the crude tar. Pictet, who examined an exceptionally true primary tar, found no phenol but up to 2% methylcyclohexanol, an unstable alcohol which decomposed into phenol upon standing.
All three cresols have been identified, but there seems to be no uniformity in the way in which they predominate. Gluud and Breuer (50) found that m-cresol predominated; Brittain, Rowe and Sinnatt (125) found about 46% of-cresol, about 37% m-cresol, and about 16% p-cresol; while Morgan and Soule (48) found 54% p-cresol, 27% of-cresol, and 19% m-cresol. Schultz, Buschmann, and Wissebach (38) have identified 4 of the zylenols, while Avenarius (147) has isolated a fifth.
Catechol was detected qualitatively by Morgan and Soule (48), by Edwards (124), and by Parrish and Rowe (139). Bornstein (95) also detected it, while Gluud (148) estimated the quantity of this compound in the tar from Lohberg coal, examined by him, to amount to 0.02% of the coal. Brittain, Rowe, and Sinnatt (125) found catechol present in tar from the Coalite process.
Among the higher-boiling phenols, Morgan and Soule (48) reported the presence of napthols, while Weindel (123) observed beta-napthol, and Edwards (124) reported both alpha- and beta-napthol. Gluud and Breuer (50) found trimethylphenols, the presence of which was confirmed by the researches of Fromm and Eckard (40) and of Pictet, Kaiser, and Labouchere (52). The latter experimenters also identified tetramehtylphenol and pentamethylphenol. Marcusso and Picard (149) recorded the presence of solid phenols in primary tar from Upper Silesian coal, but Tropsch (150), who examined the low temperature tar of a coal from the same region, was unable to confirm this.
Among the sulfonated phenols, Schultz, Buschmann, and Wissebach (38) reported appreciable quantities of mercaptans and Avenarius (147) concluded that thiophenols were present. Parrish and Rowe (139) found 0.73% sulfur in their crude phenols, while Brittain, Rowe and Sinnatt (125) estimated the sulfur content of the crude phenols from Coalite tar to be less than 1%.
Some idea of the distribution of the various constituents may be gained from a rough separation of the phenols made by Fischer and others. They found from 1% to 6% cresols, bout the same proportion of xylenols, 5% to 25% higher phenols, and 8% to 15% acid resins. The tar from the Fuel Research Board horizontal retorts yielded about half the acid content reported by Fischer, about 5% cresols, 11% xylenols, and the remainder was composed of the heavier derivatives of phenol. Parrish (117) has compiled a list, shown in Table 50, which gives the distribution of phenols among the tar acids in low temperature carbonization.
According to the results of Davis and Parry (97), introduction of steam into the retort during distillation of the coal considerably alters the quantity of tar acids produced in each fraction of the tar. Their data are given in Table 51 as percentages of the fraction. The tar examined was obtained in both cases by carbonization of Pennsylvania coal from the Freeport bed at 550° C. The yield is equivalent to 4.76 gallons of tar acids per net ton of coal, or 14.5% of the crude tar, for dry distillation and 4.04 gallons per net ton, or 12.2% of the crude tar, under steam distillation. The average yield per net ton from 5 Pennsylvania coals, tested by Davis and Parry (97) was 5.45 gallons. The specific gravities of the acids separated from the various fractions of tar were 1.021, 1.028, and 1.051 for the light, middle, and heavy oil cuts, respectively. The middle oil contained a preponderance of cresols and xylenols.
Tar from the same coal, carbonized at different temperatures, was examined by the same investigators with a view of determining the effect of temperature variation on the production of tar acids. The results are reproduced in Table 52, which shows conclusively that increase of the temperature from 475° C to 650° C does not influence the yield to any extent. It is apparent that the acids are stable up to beyond 650° C and that no decomposition occurred at that temperature. This experiment was carried out in the presence of steam, which to a great extent reduced the susceptibility to thermal decomposition.
Figure 22 shows the variation of the percentage of tar acids, as a function of the average boiling point of the fraction, for low temperature tar from Pennsylvania coal, as determined by Davis and Parry (97), when compared with the same determinations on tar from the Carbocoal process, as found by Morgan and Soule (151). In the illustration, it will be noted that the maximum yield of tar acids occurs in the fraction whose average boiling point is 200° C, in the Freeport tar, whereas in the Carbocoal tar this maximum occurs 60° C higher.
Tar Bases ~
The nitrogen bases are characterized usually by absence of the simpler members, although pyridine and the simple alipathic amines have been detected in small quantities. The tar bases consist mostly of secondary and tertiary compounds and others of unknown origin.
Davis and Parry (97) found 1.23% nitrogen bases in the low temperature tar from Pennsylvania coal; Davis and Galloway (141) found 2.8% to 5.7% in the tar distillate up to 275° C, representing 1.1% to 2.3% of the total tar; Edwards 124) found from 2.9% to 5.8% in the distillate up to 311° C, amounting to 1.3% and 2.5% respectively of the total tar; Jones and Wheeler (42) found less than 1%; Parr and Olin (31) found 0.9%; Pictet, Kaiser, and Labouchere (52) found as little as 0.2%; Fischer and Gluud (152) found 0.46%; Parrish and Rowe (139) found 2.7%; Morgan and Soule (48) found 0.6% in Carbocoal tar; and Brittain, Rowe and Sinnatt (125) found from 2.45% to 4.1% in the total tar from the Coalite process. These figures, however, can be misleading, as most of the determinations were made on fractions representing as little as 30% of the total tar and the quantity of bases present in those distillates computed back on a total tar basis, whereas it is quite apparent that a large percentage of the bases remains in the tar residuum above 300° C. Indications are that the tar bases amount to from 3% to 5% of the total tar.
Gollmer (146) found among the primary amines in low temperature tar, analine, the toluidines, and the xylidines; the primary amines all amounting to 4.5% of the tar bases. Schultz, Buschmann, and Wissebach (38) confirmed the findings of Gollmer. Other investigators, however, found no primary amines, but Morgan and Soule (48) found 20% secondary and 80% tertiary amines; Brittain, Rowe and Sinnatt (125) fund 56% secondary and 44% tertiary amines; While Parrish and Rowe (149) found 15% secondary and 85% tertiary amines.
It has been observed generally that a large proportion of unsaturated compounds are found in the tar bases. Traces of pyridine have been observed by Gollmer (146), by Brittain, Rowe and Sinnatt (125) in the tar liquor, by Fromm and Eckard (40), by Schultz, Buschmann, and Wissbach (38), and by Morgan and Soule (48). Among the homologues of pyridine, Schultz and his coworkers found methylpyridines, dimethylpyridines, and trimethylpyridines; the presence of the latter being confirmed by Gollmer (146), while Morgan and Soule (48) found ethylpyridine and its hydrogenated derivatives. Brittain and Soule (48) found ethylpyridine and its hydrogenated derivatives. Brittain, Rowe and Sinnatt (125) also found small amount of diethylamine and triethylamine in the tar liquor. Schultz (38) and his associates established the presence of quinoline and its derivative, methylquinoline, whiel Pictet and his coworkers found a number of dihydroquinolines present.
The distribution of nitrogen bases in the fractions of low temperature tar, as determined by Davis and Parry (97), is given in Figure 23, where the percentage yield is plotted against the average boiling point of the fraction. The determinations were made by washing the oil fractions, after removal of the tar acids, with 20% sulfuric acid to remove the bases. The results shown in this curve agree with the fact discovered by Morgan and Soule (48), namely, that the pitch contains a larger proportion of bases than the lighter oil fractions.
Brittain, Rowe and Sinnatt (125) fractionated two samples of Coalite tar under atmospheric and under reduced pressures and determined the percentage of nitrogen bases present in each fraction. The results are given in Table 53. It will be observed that the quantity of bases, present in the various fractions, is considerably greater than that shown in Figure 23, but the general trend to large proportion of bases as the boiling point of the fraction increases is in agreement with other experimenters. It is quite apparent that the proportion of bases in a given fraction is more than doubled by vacuum distillation.
Tar Solids, Sulfur, and Liquor ~
Many low temperature tars have been observed easily to form emulsions with the tar liquor, thereby entailing a great deal of trouble in bringing about demulsification. This tendency to emulsify has ordinarily been attributed to the low specific gravity of primary tar and to dust and other mineral matter carried over with the tar during distillation. In the course of their investigations, Brittain, Rowe and Sinnatt (125) isolated a brown amorphous powder, precipitated from the tar fractions by various organic solvents. After removal of this solid powder, which amounted to as much as 0.94% of the tar in one sample and 9.9% of the tar in another, it was observed that emulsification of the samples did not take place. It was concluded that this solid material, present in the tar either as suspended powder or partly in solution, was a contributing factor in emulsification. The existence of such a brown powder was confirmed by Edwards (124), who attributed its formation to changes in the tar vapor prior to condensation or to changes in the tar itself during storage, rather than to primary decomposition of the coal during carbonization.
Ruhemann (138) distilled low temperature tar from lignite and found sulfur in the various fractions to the amount of 3.5% in the lower cuts and 1.9% in the higher cuts. The neutral oil of the primary tar, examined by Marcusson and Picard (149), contained from 5.4% to 6.3% sulfur. Avenarius (147) and Schultz, Buschmann, and Wissebach (38) found sulfur in their tar acids. The nature of these sulfur compounds has already been discussed and need not be repeated here. Parrish (117) has compiled a table from the experiments of Brittain, Rowe and Sinnatt (125) and of Parrish and Rowe (139) to show the distribution of sulfur in low temperature tars which were examined by them. This compilation is given in Table 54. The notably high sulfur contents of the tar solids suggests that it plays an important part in these substances.
The aqueous distillate from low temperature tar is usually straw yellow in color and has an acid reaction. According to Jones and Wheeler (42), the tar liquor contains both hydrochloric acid and ammonium chloride. Brittain, Rowe and Sinnatt (125) made a careful examination of the tar liquor from distillate of Coalite tars, and established the presence of acetic acid, formic acid, acetaldehyde, and certain ketones which could not be identified. They also found traces of diethylamines, triethylamines, pyridine, and methylpyridine, while phenol, cresol and xylenols were isolated.
Primary Tar Distillation ~
Following the procedure adopted in the evaluation of petroleum, a rough separation of the constituents of low temperature tar can be obtained by preliminary fractionation. Thus, the tar obtained from the distillation of 5 high grade Barnsley coals has been separated into 3 qualities of oil, according to the boiling points. The first cut, up to 170° C, is given as light oil, the second cut, from 170° C to 245° C, is called middle oil, and the last cut, from 245° C to 315° C, is designated as heavy oil. The viscous residuum is mostly pitch. The results of the fractionation are given in Table 55. On the average 6.9% of the tar by volume is light oil, 30.5% middle oil, and 18.4% heavy oil. The proximate analyses of the coal, whose tar yield is given in Table 55, from tests by Lewes as reported by Wellington and Cooper (89), was very similar to that of Pennsylvania coal containing about 60% fixed carbon and about 32% volatile matter.
A Warwickshire coal, containing 10.3% moisture, 35.3% volatile matter, 40.8% fixed carbon, and 13.5% ash, was distilled at 600° C in vertical retorts at the Fuel Research Station. The tar form this experiment had a specific gravity at 15° C of 1.029, contained less than 1% mineral matter and 2.29% water. It was fractionated by Parrish and Rowe (139) with the results shown in Table 56. The percentage acids and bases are also shown.
In regard to experiments made upon coals of the United States, Parr and Olin (37) obtained a dark brown tar which had a specific gravity of 1.069, from Illinois bituminous coal. Crude fractionation gave the following results: 17.2% light oil up to 210° C, with a specific gravity of 0.966; 52.7% heavy oil from 210° C to 325° C, with a specific gravity of 1.032; and 30.1% pitch over 325° C, with a specific gravity of 1.270. These fractions were refined and the yield between certain points computed as a percentage of the tar with the results shown in Table 57.
Low temperature tars obtained from the Pittsburgh and Upper Kittanning beds of Pennsylvania at 550° C to 650° C have been fractionated by Davis and Parry (97). A comparison of the boiling ranges of low temperature tar, crude shale oil, and petroleum is given in Figure 24. Examination of these curves discloses that below 250 C none of the low temperature tars is as volatile as petroleum and that below 200° C shale oil contains a larger percentage of volatile constituents than the tar. Above 200° C, the percentage by volume of the tar which distills exceeds that of the shale oil and, at a slightly higher temperature, even that of petroleum. The low temperature tar obtained from the Carbocoal process contains higher-boiling hydrocarbons than the other tars, as demonstrated by the position of the curve in the illustration.
Figure 25, after Davis and Parry (97), shows the distillation range of tar from 5 Pennsylvania coals. All of these tars had about the same volatility, except Pittsburgh rooster coal, which contained more of the lighter oils with boiling points below 150° C. This particular tar gave a low percentage of phenol and unsaturated compounds than the other samples. The specific gravity of these tar fractions increased from 0.882 at 175° C to 0.992 at 275° C.
Parr and Layng (102) distilled low temperature tar from Utah coal and found that 4.02% distilled from 0° C to 170° C; 37.1% from 170° C to 300° C; 34.7% from 300° C to 3260° C, leaving 22.8% pitch and loss. The fraction from 170° C to 300° C was composed of 28.6% tar acids, 4.6% amines, and 22.4% paraffins.
Giles and Villebrandt (99) made a careful study of the low temperature carbonization products obtained at various temperatures from Farmville, NC coal. The effect of temperature upon the composition of the gas from this coal has already been seen in Figure 12. The influence of temperature of carbonization on the volatility of the tar is nicely illustrated in Table 58. It will be observed that when the temperature of distillation is advanced from 300° C to 660° C, the proportion of the lower fractions as a percentage of the entire tar undergoes a decided decrease, while the proportion of the heavier fractions have a corresponding increase. Between the distillation temperatures of 300° C and 660° C, the percentage of the fraction 70° C to 170° C is reduced at the higher temperature to almost one-fourth of its initial value; the percentage of the fraction from 170° C to 230° C is reduced to about one-third of its former amount; and the fraction 230° C to 270° C suffers a reduction to about three-fifths of its value at the lowest temperature. On the other hand, increase of the carbonization temperature from 200° C to 660° C causes the amount of the fraction from 315° C to 355° C to about double, while the residuum above 355° C is increased from 7.5% of the total tar to 54.9%. It is quite evident that the higher the temperature of carbonization, the less is the proportion of light oil and the greater is the proportion of heavy oil and pitch in the tar.
We have already seen in Table 29 the effect of the temperature of carbonization on the composition of the gas evolved from distillation of Washington lignite. Benson and Canfield (108) also fractionated the tar condensed from these experiments. The results are given in Table 59. The findings in this case agree with those of Table 58, except that here the decrease in light oil fractions with increasing temperature of carbonization is compensated by an increased proportion of coke. With Washington lignite, the specific gravity of the light oil fraction advanced from 0.807, when obtained at 250° C, to 0.821, when the coal was distilled at 600° C. The specific gravity of the middle oil fraction advanced from 0.905 to 0.955 at the same temperatures, while that of the heavy or paraffin oil changed from 0.939 to 0.985.
In Table 27 there was shown the composition of gas obtained from carbonizing peat in vertical retorts, as determined in an investigation conducted by the Fuel Research Board (106), under conditions of temperature somewhat exceeding the limits ordinarily construed to fall within the category of low temperature distillation, but which will be reproduced here, as data on peat tar are decidedly lacking in the technical literature. The results of these tests, as applied to the tar, are given in Table 60.
The tar from peat was a thick and semi-solid mass which contained a high proportion of paraffin wax and only a small amount of ammoniacal liquor. The tar from the 843° C test contained about 6.5% paraffin wax which melted at about 52° C. The first fraction contained about 13.2% bases and unsaturated compounds and 40.5% tar acids, while the second fraction was composed of about 54% bases and unsaturated hydrocarbons and 8% tar acids. The last two fractions were solid at 10° C and contained practically all the paraffin wax. A separate proportion of the 843° C tar was submitted to vacuum distillation with the result that 0.45 gallons of purified and refined light motor spirit was obtained per gross ton of peat. The total peat tar, which was produced, represented about 20 gallons per gross ton. From a study of the effects of temperature on the proportion of the tar fractions, as demonstrated in Table 58 and Table 59, it would appear that under true low temperature distillation, peat tar would fractionate to about 7% in the cut up to 170° C, about 31% in the cut from 170° C to 230° C, about 12% in the cut from 230° C to 270° C, and about 24% in the cut from 270° C to 335° C, with about 26% pitch and loss.
According to Garvin (112), to whom the data in Table 61 are due, shale oils of the USA are less volatile than Pennsylvania crude petroleum and somewhat more volatile than Scottish shale oil. Up to 270° C, nearly 53% of Pennsylvania petroleum distills over, as compared with about 48% for Indiana shale oil, 40% for Utah shale oil, and 39% for Colorado shale oil. It is well to compare the results of Table 61 with those of Figure 24, where it is quite evident that low temperature tars from bituminous coals are closely related to shale oil.
The data in Figure 26 are plotted from figures reported by Garvin (112). The illustration gives a distillation analysis of the various fractions of Scottish shale oil and two distillation analyses of American gasolines for comparison. It is seen from the illustration that light oil fractionated from crude shale oil is considerably more volatile than ordinary gasoline, while the naptha cut has a characteristic distillation curve very nearly resembling that of gasoline, except that it has a slight deficiency in oils boiling below 100° C and contains slightly more of the oils boiling between 120° C and 220° C, The lubricating oils, fractionated from shale oil, contain only about 25% oils boiling below 340° C.
Steam Distilled Tar ~
It has been pointed out that, although the presence of steam in the retort during low temperature carbonization appears to have little effect on the quantity of tar produced, there is a decided change in the constitution of the product, as indicated by its fractionation. It has also been shown that the major function of steam is the prevention of secondary reactions by rapid removal of the products of distillation from the carbonization chamber. Steam distilled tar should, therefore, contain more of the primary products of coal. The curves in Figure 27, after Davis and Parry (97), compare the distillation range of steam distilled tars at various temperatures with that obtained in atmospheric distillation at 550° C. The low temperature tar was distilled from Pennsylvania coal. The volatility curve for atmospheric tar obtained at 550° C lies entirely below the curve for steam distillation beyond fractions with a boiling point of 150° C. This indicates that steam carbonization at low temperatures yields a tar with a larger percentage of higher-boiling constituents than the ordinary product. It will be noted that under steam distillation the proportion by volume of lighter oils in the tar increases with temperature rise in carbonization, due no doubt to cracking effects.
Variations of the pressure under which fractionation is accomplished is a factor in determining the percentage of tar distilled at a given temperature, Figure 28, after Davis and Parry (97), shows the effect of fractionation under 4 cm of mercury pressure, when working with low temperature tar from Pennsylvania coal. In the first place, it will be seen that the steam tar is far more volatile than ordinary tar obtained at the same temperature. This is the reverse of the case when fractionation is carried on under atmospheric pressure, as shown in Figure 27. Furthermore, above 275° C, the proportion of the steam tar distilled increases far more rapidly than does ordinary low temperature tar. The volatility of both steam and regular low temperature tars, when fractionated in vacuo, is decidedly less than when the distillation is carried out under ordinary pressures. At first thought, this would not be anticipated, because it might be supposed that reduction of pressure would lower the boiling point and consequently increase the volume of oil distilled at a given temperature. This would, of course, be the case if the constituents of the tar existed in the same form in both the crude and the redistilled state. By comparing Figure 27 and Figure 28 it will be seen, however, that at 225° C only about 2% of the tar is distilled in vacuum fractionation, whereas about 6% is obtained at the same temperature under ordinary distillation. This behavior finds easy explanation in the fact that temperatures even as low as 225° C cause transformation of the tar constituents from primary to secondary compounds.
The unsaturated hydrocarbons which are present in the tar must be accounted part of the refining loss, judging from the present standards of the petroleum industry. Pennsylvania low temperature tar contains about 42% unsaturated material in the light oil, 45% in the middle oil, and 63% in the heavy oil. This is of importance from the standpoint of motor fuel, because the unsaturated compounds must be removed before the oil is considered satisfactory for use in internal combustion engines, according to present standards . Therefore, a low percentage of unsaturated material in the low boiling fraction will greatly increase the yield of motor spirit.
Low temperature tar is usually examined by determining the percent of each crude fraction soluble in 10% sodium hydroxide, which represents the amount of tar acids present in the cut. Then the tar is treated with 95% sulfuric acid, which removes the unsaturated hydrocarbons. The residuum consists of the saturated compounds which are present in the fraction. The Pennsylvania coals, studied by Davis and Parry (97), have been examined in this manner. Using a tar obtained at 550° C, they found for ordinary low temperature tar the following percentages of saturated compounds in the various fractions: 50.5% from 0° C to 175° C; 30% from 175° C to 225%; and 26% from 225° C to 275° C. With steam distilled tar the quantity of saturated hydrocarbons in the various fractions was 53.1%, 37%, and 30.3% respectively, showing a slight increase. The percentage of unsaturated compounds in the corresponding fractions of the same tars was 35%, 25%, and 44% respectively for the ordinary tar and 38.5%, 23%, and 43.4% respectively for the steam-distilled tar. We see that no great change occurred in the proportion of unsaturated material in the fractions of low temperature tar by virtue of the presence of steam. Under the discussion of tar acids, it was shown that the small increase in saturated compounds, noted above in the case of steam tar, is accounted form not through any change in the amount of unsaturated material present, but through a reduction in the percentage of tar acids evolved in steam distillation.
The results of Davis and Parry (97) on the examination of low temperature tar from Pennsylvania coal carbonized at 550° C showed that a larger amount of oil up to 275° C was present in ordinary low temperature tar than in tar obtained by carbonization in the presence of steam. This higher yield of oil up to 275° C is doubtlessly the result of some secondary reactions, prevented by the introduction of steam, as indicated by the presence of considerably more pitch in the ordinary tar than in the steam tar.
Full-Scale Retort Tar ~
Many additional factors enter into the situation when large-scale operations are carried out, so that only rough estimation of commercial retorts can be drawn from comparatively small laboratory experiments it is important, therefore, to study the products of large-scale carbonization processes when such data are available. In this connection, it should be pointed out that so much depends upon the coal used that it is important that some method of small-scale distillation be devised so that by the application of certain factors of proportionality a fair idea may be gained as to what might be expected in large-size retorts. A discussion of several laboratory devices for this purpose has already been given in Chapter I under the subject of assay. A more detailed examination of low temperature tars will be undertaken in the discussion of individual processes, but for the present purposes a comparison of the low temperature tars, obtained by several different processes, will suffice. This comparison is given in Table 62. The various retorts, given in the table, represent different types. All are eternally heated, except for the Nielsen retort, which is a rotary kiln wherein the coal is carbonized by the sensible heat of producer gas. The Fuel Research board and Tozer processes are horizontal and vertical static retorts, respectively. The Carbocoal system is a two-stage process involving primary carbonization in a horizontal internally agitated retort, briquetting, and finally secondary carbonization in an inclined oven.
The Fuel Research Board (114) experiments were made with Dalton Main #2 coal as the charge. It will be seen that not only is there a variation between the small- and large-scale products, but also among the large-scale processes themselves. Especial attention is called to the low yield of the cut, 270° C to pitch, obtained in the Fuel Research Board experiments, as compared with that obtained in other processes. Furthermore, the large amount of pitch obtained and the generally higher yield of lighter hydrocarbons are all indicative of cracking and secondary reactions in the Fuel Research Board horizontal retorts. The fractions of tar obtained from Dalton Main #2 coal in the Fuel Research Board horizontal retorts can be classified according to their use in the trade. The classification in Table 63 gives in gallons per gross tone the quantity of oil that may be expected in these cuts. In addition to the yields reported in the table there was a refinery loss of 4.09 gallons per gross ton, a yield of 2.54 gallons per gross ton of tar acids, and a residuum of 67.5 pounds of pitch per gross ton. The temperature of carbonization was 600° C.
Four British coals, carbonized at low temperatures by the Fuel Research Board (114) in horizontal retorts, averaged 20.7 gallons of crude oil per gross ton of dry coal retorted. This yield of crude oil was reduced to 5.8 gallons per gross ton upon washing and refining, or a loss of about 44% in the refinement process. It has been shown that low temperature tar contains 35% to 40% unsaturated hydrocarbons which must be removed from the oils to render them of commercial value according to the present standards of the petroleum industry, which is the chief competitor of low temperature carbonization as a source of liquid fuels. Thus, the unsaturated material becomes the major portion of the refining loss. Table 64 gives the original fractional yields in gallons of dry tar per gross ton, as well as the yields of washed and refined oils. The reduction in refining values varies from about 25% in the first fraction to about 50% in the last.
Low temperature tar has a high fuel value, ranging from 16,000 BTU to16,500 BTU per pound, but it has a low flash point. Attempts to remedy this fault have been made through removal of the light oils, but so far this procedure has rendered the tar too viscous for use. The question of mixing low temperature tar with mineral oils for use as a fuel has been raised. In a number of instances this has been found impractical because of their immiscibility. Thus, with American petroleums and shale oils, low temperature tar has been found to be quite immiscible in all proportions, their mixtures causing a separation of a thick black gummy fluid. Burmese oil formed a stable mixture up to 25% tar and Mexican oil did not give a separation until more than 50% low temperature tar was present. Trinidad was the only petroleum which was miscible in all proportions and then only upon heating during the mixing process. Table 65 shows the results made on this problem by the Fuel Research Board (114).
A method of overcoming the immiscibility of low temperature tar with crude petroleum has been proposed by Lessing (153). The tar is mixed with petroleum, which causes a separation of the pitch, and the mixture is then subjected to steam distillation, which gives a sharp separation of the pitch and the oil. The distillate is finally fractionated into liquor, neutral oil, tar acids, bases and pitch. The crude oil is approximately 81% of the mixture and yields on distillation: 56.1% neutral oil, 24.4% tar acids, 0.2% tar bases, and 16.9% pitch.
Tar Cracking ~
Dalton (154) apparently made the first investigation of the pyrogenic decomposition of hydrocarbons, but no systematic effort was made to study this phenomenon until Berthelot (155, 156, 157) published the results of his researches, in which he found that methane, ethane, and ethylene decomposed into acetylene and other hydrocarbons. The polymerization of acetylene to form aromatic hydrocarbons has been discussed in Chapter I under the subject of chemistry.
According to Williams-Gardner (158), the views of Berthelot, that acetylene plays an important part in the formation of cyclic compounds, are obsolete and the mechanism of the formation of aromatics must be modified to include either the momentary existence of unsaturated residuums, which may by transformed by polymerization, or broken down into carbon and hydrogen, or else the direct formation of olefins and methane, with or without the liberation of hydrogen and without the necessity of forming acetylene. The opinion of Williams-Gardner is substantiated by his own experiments, by the work of Auschutz (159), who obtained aromatic compounds, ranging from benzene to anthracene, from Russian oil residuums by condensation of the complex hydrocarbons without elimination of hydrogen, and by Haber (160), who found that n-heptane decomposed between 600° C and 700° C to form methane and the next lowest olefin, that is, amylene, without the formation of acetylene. This is also in agreement with the results of Thorpe and Young (161), who distilled solid paraffins under pressure and obtained amylene, pentane, hexylene, hexane, heptylene, heptane, octylene, octane, nonylene, nonane, undecylene, and undecane, all boiling below 200° C. The latter experimenters concluded, with others, that the decomposition gives rise to olefin and lower paraffins with or without the evolution of hydrogen.
Jones (162) observed, from his study of the influence of temperature in cracking low temperature tars, that the mechanism of the napthalenes, paraffins, and unsaturated hydrocarbons to form olefins, which condense at higher temperatures to form aromatic compounds. He found the proportion of higher olefins, which appear in the gas from cracking, reached a maximum at 350° C and almost disappeared at 750° C. At the latter temperature, naphthalene appeared in the gas and shortly afterwards there was a rapid increase in hydrogen evolution.
The effect of heat on the various constituents of primary tar was studied by Fischer (49), who concluded that the amount of phenol and its lower homologues, initially present in the tar, is augmented by decomposition of the higher members of the series at temperatures below 700° C, aromatic hydrocarbons of the benzene series being formed from the phenols above 700° C. According to Parrish (117) also, below 700° C the effect of heat upon primary tar is to cause decomposition of the higher boiling constituents into lower boiling compounds of a similar type. The higher phenols in particular are sensitive to this cracking effect. In general, the higher the temperature, the greater is the proportion of lower members of a given series of compounds. Decomposition becomes more radical above 700° C, producing aromatic compounds, especially benzene, naphthalene, anthracene, phenol, and pyridine. Most of the higher phenols are entirely cracked into phenol, cresols, and xylenols. The tar bases also decompose to form pyridine and quinoline together with their homologues and derivatives.
The light and middle oils reported in Table 70 as fractionated from Pennsylvania low temperature tar by Davis and Parry (97) were vaporized in a retort and passed through a cracking chamber which was heated to various temperatures at atmospheric pressure. The raw stock constituted the neutral fraction from 175° C to 275° C. Table 66 shows the amount of motor spirit obtained from the cracked distillate and scrubbed from the cracked gas. A maximum yield of hydrocarbons boiling below 175° C was obtained from the neutral oil at a cracking temperature of 750° C. the refining loss, due to the presence of unsaturated material and tar acids in the fraction, amounted to 56%. Analysis of the fraction showed the presence of aromatic hydrocarbons, principally in the form of benzene.
The curves in Figure 29, after Davis and Parry (97), shows that the amount of oil recovered after cracking is almost exactly inversely proportional to the temperature and that the yield of light oil and gas increased with temperature rise, but at a decreasing rate. Comparison of Table 66 and Figure 29 points out that, although the percentage of motor spirit contained in the cracked oil reaches a maximum at 750° C, the yield of motor spirit in the gas has no such maximum, but continues to increase with the temperature.
When the anthracene oil fraction from 280° C to 350° C of Table 70 was subjected to similar cracking reactions at 800° C and atmospheric pressure, about 50% of the cracked oil was recovered, yielding per net ton of coal 0.12 gallons of crude light oil boiling below 175° C. This was no increase from that obtained in the Hempel distillation of the anthracene oil before cracking, so that in cuts up to 200° C, at least, it may be said that little secondary decomposition took place in the anthracene oil fraction. About 47 cu ft of gas per gallon were evolved during the heating at 800° C and this yielded upon scrubbing 2.2% of the fraction, or 0.09 gallons of crude light oil per net ton of coal.
Curtis and Beekhuis (163) conducted a number of cracking tests on the neutral oil of low temperature tar from the McIntire process and made from West Virginia coal containing 3% moisture, 36% volatile, 50% fixed carbon, and 10.5% ash. The coal was carbonized at 620° C. Pressures varying from 100 pounds to 500 pounds per square inch were used inc racking. They found that decomposition was slow below 400° C, but that above 450° C the formation of coke, accompanied by high gas yield, was rapid. Within the range of their observations, the effect of pressure variation of the rapidity and character of the cracking was slight. By cracking the kerosene fraction of the neutral oil, they found that the yield of oil boiling below 210° C could be increased from 20% of the neutral oil initially present to about 35%.
Morrel and Egloff (164) cracked a typical low temperature tar from a West Virginia bituminous coal, which yielded about 25 gallons of crude tar per net ton. The crude tar, with a specific gravity of 1.074, was cracked at 100 pounds per square inch pressure and 452° C temperature to yield 40% cracked distillate with 0.8927 sp gr. The gasoline fractionated from the cracked distillate amounted to 18.1% of the initial crude tar, and had a specific gravity of 0.8299. The straight distillate from this low temperature tar analyzed 26.4% tar acids, 3.3% tar bases, 44.3% unsaturated hydrocarbons, 9.5% aromatic hydrocarbons, and 16.4% napthenes and paraffin hydrocarbons, while the cracked distillate from the straight distillate contained 21.3% tar acids, 4.3% tar bases, 27% unsaturated hydrocarbons, 20.3% aromatic hydrocarbons, and 27.3% napthalenes and paraffin hydrocarbons.
The neutral oil from this same low temperature tar, having a specific gravity of 0.9484, when cracked at 175 pounds per square inch and 427° C, yielded 50% cracked distillate of 0.8514 sp gr, which fractionated to give 31.9% of the initial crude tar as gasoline of 0.8090 sp gr. Analysis of the neutral oil before cracking showed it to be composed of 2.7% tar acids, 4.5% tar bases, 60.6% unsaturated hydrocarbons, 9.4% aromatic hydrocarbons, and 22.8% napthene and paraffin hydrocarbons, while the cracked distillate analyzed 4.8%, 4.8% tar bases, 38.4% unsaturated hydrocarbons, 19.4% aromatic hydrocarbons, and 32.7% napthalenes and paraffin hydrocarbons.
These data show a decided decrease upon cracking in the unsaturated compounds and a small increase in the aromatic hydrocarbons. The tar acids in the cracked distillate appear to be mainly cresols. No naphthalene was present, either in the initial crude tar or in the cracked distillate. While only a trace of anthracene was found in the original low temperature tar, considerable quantities were found after cracking. This was thought remarkable inasmuch as the temperature of cracking was only 452° C, whereas anthracene heretofore had been regarded as a high temperature product.
Egloff and Morell (165) also cracked the low temperature tar from an Ohio-Indiana bituminous coal at 100 pounds per square inch and 426° C. The crude tar has a specific gravity of 1.0794 and yielded upon fractionation 29% neutral oil, 27.5% tar acids, and 1.6% bases. When cracked, this crude low temperature tar yielded 33.9% gasoline containing 35% tar acids, or an acid-free yield of 22% gasoline. The neutral oil from this same crude stock was cracked at 200 pounds per square inch and 455° C to yield over 50% high grade antiknock motor fuel.
Shale oils from the USA, Australia, and France were cracked by Morrell and Egloff (166) under pressures ranging from 120 pounds per square inch and at temperatures ranging from 398° C to 433° C to produce over 50% of the raw stock as gasoline. This cracked gasoline was declared to be a motor fuel of high antiknock value. The results of the cracking tests on shale oil are given in Table 67.
Cracked Tar Gas ~
The fixed gas evolved during cracking is of an entirely different nature from the gas which is evolved during carbonization of the coal. The latter is the result of destructive distillation, or primary modification of the fuel constituents, while the former is the outcome of secondary decomposition induced through the medium of heat. An analytical study of the fixed gas from the neutral oil fraction from 175° C to 275° C at various cracking temperatures was made by Davis and Parry (97). This is the same oil fraction from Pennsylvania coal already referred to in Table 66 and Figure 29, and which will be further discussed in Table 70. Analyses of the fixed gas which was evolved from cracking the neutral oil at various temperatures are given in Table 68.
The permanent gas, obtained from cracking the anthracene oil fraction from 275° C to 350° C at 800° C was almost identical in composition with that liberated by the neutral oil when cracked at 820° C. The large percentage of hydrogen is indicative of much unsaturated material in the cracked oil and this was shown to be the case. The proportion of ethylene and of ethane decreased with rise in the cracking temperature, while the percentage of methane greatly increased. The small percentage of the oxides of carbon which was produced is worthy of notice, especially in comparison with the compositions of the gas from carbonization, as shown in Table 23 and in Figure 10.
If low temperature neutral oil is essentially the same as petroleum gas oil, there is a possibility of finding a commercial outlet in that market. A great deal of information is thrown on that matter by an examination of the fixed gas obtained in the cracking of petroleum gas oil. Such an experiment has been conducted by Davis and Parry (97), the results of which are given in Table 69.
The gas, evolved in cracking petroleum gas oil, has about the same thermal value as that obtained from the low temperature tar neutral oil, but a somewhat larger quantity of gas is produced. Far less hydrogen and methane is evolved from the former, but a very large percentage of nitrogen. It might be said, in general, that the two oils are not greatly different and that low temperature tar neutral oil is comparable with the petroleum gas fraction.
Upon consideration of the facts hitherto presented, it appears that primary tar normally consists of alipathic hydrocarbons and phenolic compounds. Whatever small percentage of light aromatic compounds that is found to be present in the tar must be derived from reduction of these phenolic substances. This suggests the possibility of entirely reducing the tar acids to low-boiling oils for use as a motor fuel. This problem has been studied by Fischer (167) and by Fischer and Schroeder (168), who passed elementary hydrogen together with a mixture of phenols and hydrocarbons through a tube heated to 750° C. A great deal was found to depend upon the nature of the reaction tube, but they discovered that a tinned iron tube was suitable for commercial processing and avoided the excessive deposition of carbon that sometimes plugged the tube. The deposition of carbon was apparently accelerated by iron. However, if water gas was used as a source of hydrogen for hydrogenation, the tinned coating peeled away, as a result no doubt of the action of carbon monoxide in the gas. An iron reaction tube, coated internally with iron sulfide, was finally adopted by Fischer and Zerbe (169) to overcome this last difficulty. With the improved form of hydrogenating tube, little separation of carbon occurred and the phenols were reduced to benzene homologues. With cresols and xylenols, the conversion to motor fuel was up to 99% and 72%, respectively, of the theoretical.
Whether or not hydrogenation at high pressures and high temperatures, according to the Bergius process (170, 171, 172), is applicable to low temperature tar acids has not yet been demonstrated. According to Fischer (173), at the temperature of 400° C, employed in such circumstances, hydrogenation could be successful only at hydrogen pressures exceeding the dissociation pressure of hydrogen in the compounds to be produced. At lower temperatures of 200° C, the reaction velocity would be entirely too slow. The introduction of a catalyst to accelerate the reaction rate at lower temperatures necessitates excessive and expensive purification of the reaction gases to eliminate sulfur and avoid poisoning the catalyst. Fischer and Schroeder (174) obtained the conversion of the phenolic portion of primary tar into oils under pressure at 400° C with nascent hydrogen derived from sodium formate and from carbon monoxide and steam.
Sabatier and Senderens (175) showed that naphthalene vapor mixed with and excess of hydrogen could be passed over finely divided nickel at 175° C to produce decahydronapthalene with a boiling point of 187° C, and at 200° C to from tetrahydronapthalene with a boiling point of 205° C. Fischer (176, 177) obtained the partial liquefaction of naphthalene to the extent of 35% by treating it for about half an hour at 150 lb/sq in pressure in the presence of aluminum chloride. From these experiments, it is apparent that naphthalene can be considered as a raw material for the manufacture of motor fuel. On account of the limited amount of naphthalene present in primary tar, however, it is doubtful if this process is important from the standpoint of low temperature carbonization, unless it be in the secondary treatment of cracked tar products.
For the most part, cracked motor fuel contains a high proportion of unsaturated compounds which, according to present standards, are more or less undesirable, due principally to unpleasant odors and darkening of the distillate. The method of hydrogenation, developed by Bergius (172) and commonly known as berginization, in which the distillate is cracked at the required temperature under a high pressure of elementary hydrogen, largely overcomes this difficulty and yields a distillate containing only a small percentage of unsaturates. Figure 30, after Kling (178), shows the pressure developed in berginization and in cracking the same oil. It is obvious that, as the heating period is prolonged, the pressure of the cracked vapor becomes greater and greater. During berginization, however, there is a rapid increase of pressure during initial heating but finally, when equilibrium has been reached, the pressure decreases at constant temperature, thereby indicating hydrogenation. Berginization appears to require close pressure and temperature control, as indicated by the negative experiments of Waterman and Perquin (179), who were unable to effect hydrogenation of paraffin wax at 435° C and initial pressured of 600 psi, and of Skinner and Graham (180), who were unable to secure hydrogen absorption by low temperature tar obtained at 600° C from Fifeshire cannel coal and redistilled at 450° C, when the tar was treated under 1560 psi hydrogen pressure at 411° C for 4 hours. Bergius used initial pressured of as much as 1500 psi which rose in some cases to above 3000 psi when the autoclave was heated to the required temperature.
Motor Fuel ~
Motor fuel is obtained directly from two sources in low temperature carbonization, the tar and the gas, and it can be obtained from three subordinate sources by special processing, such as cracking of the heavy tar fraction, hydrogenation of the tar acids, and liquefaction of the gas by high pressures and low temperatures.
The entire fraction up to 200° C, when freed of the tar acids, bases, and unsaturated material, may be sued as an internal combustion engine fuel. This fraction consists mostly of higher paraffins, unsaturated hydrocarbons, and in some cases a small quantity of benzene and toluene. This cut may yield anywhere from one to 3 gallons of refined motor spirit per ton of coal. Davis and Parry (97), in their work on the low temperature carbonization of Pennsylvania coals paid special attention to the yield of products valuable as motor fuel. Using the tar obtained from retorting Freeport coal at 550° C in a stationary vertical retort, they examined the light oil fraction obtained from the tar and the possibility of cracking the higher-boiling hydrocarbons. The fractionation of this tar is given in Table 70
=== Table 71[Not available] ===
The entire fraction boiling under 175° C was considered a crude light oil suitable as a motor fuel. It was found that 42% of this cut consisted of unsaturated hydrocarbons and 17% of tar acids, which must be removed in refinement before the oil could be sold to the trade. This means that the crude yield of 2.8 gallons must be reduced 59% to 1.15 gallons of refined motor spirit obtained from this fraction.
The recovery of light oils in the gas is not a simple process. It can be accomplished by condensation, scrubbing, or adsorption. Fischer and Gluud (181) recovered the motor spirit from the low temperature gas in their experiments by scrubbing with paraffin oil and subsequent rectification. They obtained a yield of 3.8 gallons to 2.0 gallons per net ton of coal carbonized, representing 1.23% and 0.67% of the coal, respectively. The Fuel Research Board (114) also carried out scrubbing experiments on the gas from low temperature carbonization in horizontal retorts, as shown in Table 71. They recovered 1.68 gallons of refined spirit per gross ton of coal. It has a specific gravity of 0.731 at 15° C. Dalton Main #2 coal, which gave 3200 cu ft of gas at the temperature of distillation, was used in the work of the Fuel Research Board. The duration of the scrubbing, as well as the rte of washing, was varied in order to dtermine the influence of these factors on the quantity of light oil recovered.
=== Table 72 [Not available]===
The refining loss of motor spirit, obtained from low temperature gas, is less than that occurring in the light fraction of the tar, Scrubbing greatly reduces the percentage of hydrocarbons present in the gas and it has been pointed out, notably in Figure 13 and elsewhere, that low temperature gas is in part characterized by a low proportion of unsaturated hydrocarbons. Consequently, it is anticipated that there should be a low refining loss in the light oils scrubbed from the gas. The rate of scrubbing with wash oil does not appear to have as great an effect on the amount of spirit recovered as the quantity of wash oil used per cubic foot of gas scrubbed.
Engelhardt (182) investigated the recovery of light oil from illuminating gas by activated charcoal, the condensed vapors being subsequently liberated by treatment with superheated steam. This same process was used by Davis and Parry (97) in the recovery of light oils from the gas evolved in the low temperature carbonization of Pennsylvania coals. They found that the efficiency of the charcoal was greatly reduced in the presence of hydrogen sulfide and water vapor. Thus, while up to 3 gallons of crude light oil per net ton of coal were recovered in ordinary distillation, only one gallon was obtained under steam distillation. By this method they recovered an average of 2.75 gallons of crude motor spirit per net ton of coal from 6 different coals. This light oil had a specific gravity of 0.740 and represented 0.838% of the coal charged. It contained 23% unsaturated hydrocarbons, so that the net yield of refined spirit was reduced to 2.23 gallons. A recent development utilizing this same principle of adsorption has been the introduction of silica gel in place of activated charcoal.
Table 72 shows the yield of light oils reported by a number of different investigators in low temperature carbonization. All of these experiments were made in externally heated retorts, except those of Gluud (23) and a few runs by Parr and Olin (37), in which cases internal heating was used. Lewis (17) used a gas coal; Runge (183) used a by-product coal; Parr and Olin (37) used Illinois coal; Davis and Berger (184) used Freeport coal; Morgan and Soule (48) used Pittsburgh Terminal coal; Davis and Parry (97) used coal from the Freeport bed; and the Fuel Research Board (114) used Dalton Main #2. Attention must be called to the fact that the yields in Table 72 are crude light oil and large reductions occur in refining, as has been pointed out, so that the yield of refined motor spirit will be considerably less. It is interesting to note the wide variation in the yields reported and caution must be sounded that some observers report their tar yields before hydrogenation and their crude tar yields are swelled by moisture content. The high yields of Parr and Olin, and also of Gluud, are due in part to the fact that they included higher boiling hydrocarbons in their light oil fraction. The figure given by Davis and Parry includes the light oil obtained from both the tar and the gas.
The explosive range of low temperature motor spirit is over 20% greater than that of petrol, while a mixture of equal parts of motor spirit and petrol has an explosive range 30% greater than petrol alone and 35% greater than benzol, according to the Fuel Research Board (114). The explosive range, expressed as cubic centimeters per cubic foot of air at 15° C, was 2.7 to 6.7 for benzol of 0.881 sp gr; 2.5 to 7.3 for petrol of 0.723 sp gr; 2.3 to 9.5 for low temperature motor spirit of 0.731 sp gr; and 2.5 to 10.5 for a mixture of equal parts of petrol and motor spirit.
The motor spirit obtained from low temperature distillation of coal is miscible in all proportions with benzol and petrol for use as a fuel in internal combustion engines. With the growing use of alcohol in the field of motor fuels, something should be said concerning the miscibility of low temperature motor spirit with alcohol. The amount of low temperature motor spirit held in solution by alcohol depends largely upon the strength of the latter; thus at 10° C the Fuel Research Board (114) found 95% alcohol held as much as 74.2% motor spirit in solution, while 90% alcohol held but 16.3% and 85% alcohol retained only 7.1%. With a mixture of benzol and 95% alcohol, in equal proportions, low temperature motor spirit is dissolved up to 60% at 10° C. Even with refined spirit, there is a slight tendency for resinous matter to deposit on standing, thus darkening the fuel, but alcohol prevents deposit by dissolving the resins which separate.
It has been pointed out that Davis and Parry (97) recovered 1.15 gallons per net ton of refined motor spirit from the light oil fraction of low temperature tar which they obtained from Pennsylvania coal and 2.23 gallons per net ton of refined motor fuel from the low temperature gas, netting 3.38 gallons per net ton which were obtained directly without additional processing. It has also been pointed out that they obtained an additional 0.66 gallons per net ton by cracking the higher-boiling tar fractions. We have already seen that other Pennsylvania coals yielded 5.45 gallons per net ton of tar acids, 56.5% of which can be converted to motor fuel by hydrogenation at an efficiency of 72% to 99% of the theoretical maximum conversion, which ranges from 64% to 87% depending on the initial and final products. On this basis, an additional 1.5 gallons to 2.5 gallons of motor fuel per net ton could be obtained. By employing these additional means, there appears the possibility of increasing the total motor fuel yield to 5.5 gallons or 7.5 gallons or even more per net ton of coal. The desirability of associating hydrogenation with low temperature carbonization, to effect an increase of motor fuel by hydrogenating the tar acids, has been stressed by Gentry (185), but Egloff and Morrell (165) see no reason at all why the tar acids should be removed from the motor fuel, as demanded by present standards.
Fuel Oil ~
Crude low temperature tar has a viscosity quite suitable for fuel oil, but according to Lander and McKay (186) its flash point is far too low. On the other hand, if sufficient low volatile constituents are removed by distillation to increase its flash point, the crude tar becomes too viscous for use as a motor fuel. However, the tar may be fractionated in such a way as to obtain a distillate which meets all the requirements of a fuel oil, both from the standpoints of flash point and of viscosity. Furthermore, there is no evident reason why the fuel oil, if too viscous, cannot be heated before firing to increase its fluidity. The immiscibility of low temperature tar with other fuel oils has already been dealt with.
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