« PreviousContinue »
Second, the matter of the purification of these cracked distillates is one of great importance. Such a large proportion of unsaturated bodies is present in the cracked gasolines that it would be out of the question to purify them by the ordinary methods employing sulphuric acid or alkali. Within the last 30 days I have been developing a process for refining the Rittman gasolines which is absolutely satisfactory, simple and cheap. So far I have not used any sulphuric acid. The refined product may not be as sweet as ordinary gasoline, but it is plenty sweet enough.
(SUBJECT TO REVISION]
DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented in person at the New York meeting, February, 1916, when an abstract of the paper will be read. If this is impossible, then discussion in writing may be sent to the Editor, American Institute of Mining Engineers, 29 West 39th Street, New York, N. Y., for presentation by the Secretary or other representative of its author. Unless special arrangement is made, the discussion of this paper will close Apri 1, 1916. Any discussion offered thereafter should preferably be in the form of a new paper.
The Effect of Aeration and "Watering Out” on the Sulphur Content
In order to discuss the subject intelligently, it will be necessary to touch briefly on the forms in which sulphur is supposed to exist in coking coal to be carbonized in beehive or byproduct ovens.
Sulphur is known to exist in coal as sulphides and sulphates, as can be determined experimentally. Then there is the so-called organic sulphur, i.e., sulphur in combination with the carbon, hydrogen, and oxygen of the coal, about which much has been written, but nothing definitely proven in an experimental way. In fact, so far as we know, there never has been developed a satisfactory and conclusive method in the laboratory for the direct determination of organic sulphur.
For most coking coals, it can be safely assumed that the preponderance of sulphur is in the form of pyrite (FeS2). When exposed to comparatively low temperatures during the coking process, it loses its sulphur according to the following chemical reaction: 7Fes, = Fe,S8 + 6S. It will be seen, therefore, that six out of the 14 atoms of sulphur (or 42.8 per cent.) are expelled, or volatilized. Fe,Sg remains in the coke as pyrrhotite, or magnetic sulphide of iron; this accounts for the highly magnetic properties of the powdered coke. A more commonly accepted reaction is FeS, = F
FeS + S, in which the straight sulphide of iron is produced with 50 per cent. volatilization of sulphur.
In the beehive methods of coke making, air is introduced in sufficient amounts to carry on the distilling and the coking processes, and the sulphur is oxidized along with the other volatile products: First, into sulphur dioxide (SO2), known by the pungent and suffocating odor emitted from the trunnel head; second, into sulphuric anhydride (SO3); and, finally, into sulphuric acid, as it comes into contact with the air and moisture. In a properly regulated draft on a beehive oven, there is never complete combustion of the gases. In other words, the coking process should be carried on in a reducing atmosphere, and a low-grade producer gas kept issuing from the trunnel head. For coals of about 30 per cent. volatile matter, the ratio of air to gas is 312 to 1. In complete combustion, the ratio is 6 to 1, producing an extremely high temperature
* Chief Chemist, H. C. Frick Coke Co.
in the crown of the oven (3,500°F.) more than enough to cause fusion of the refractories used. A good coking temperature is 2,500°F. in the crown of the oven.
It follows then that the sulphur remaining in the coking mass as iron sulphide cannot in any way be affected by the aeration or draft on the
Among beehive coke-oven operators there used to be a saying, "the hotter the oven, the more sulphur burned out." In view of the foregoing, this probably never was true unless the aeration was carried to complete combustion of not only the volatile gases but the fixed carbon itself, in which case the iron sulphide (FeS) would, of course, be oxidized to Fe2O3, and the sulphur liberated, as usual. This condition would not be productive of good results, as the percentage of coke yield would be abnormally low and the "ashes," or "braize," correspondingly high. This is evidence that so far as the pyritic sulphur is concerned, any sulphur that is volatile at all is so at comparatively low temperatures through the agencies of distillation, and no practical method of superaeration will help in the least in the further elimination of sulphur.
On the other hand, overheating promotes certain chemical reactions in which sulphur forms compounds with other bodies on which heat has no effect. If the coking process proceeds too quickly, or if the heat is irregular, the desulphurization of the coke is apt to be less complete. In these matters, much depends upon the skill and judgment of the burner.
In the byproduct oven, which is essentially a true distillation process, the heat being applied externally, and no air supplied to the oven itself, the sulphur is distilled from the pyrite as atomic sulphur, as previously explained, but, apparently, it afterward combines with the hydrogen of the coal gas, forming sulphuretted hydrogen (H2S), for it is necessary to remove this very objectionable gas before artificial gas can be used for domestic purposes. This is accomplished by the use of hydrated oxide of iron, a byproduct now obtainable from mine-water purification.
Here again appears the fallacy of superaerating beehive ovens to eliminate sulphur, for in byproduct ovens in which no air is admitted there is just as much desulphurization of the coke as in the beehive process; in fact, there is more desulphurization when the matter of increased yield is considered. For example: Given a yield of 70 per cent. coke by the beehive process and 75 per cent. by the byproduct process from coal carrying 1 per cent. sulphur, the respective problems are as follows:
Sulphur in coal.
Thus the practical volatilization of sulphur from the beehive process is only 18.3 per cent., while in the byproduct it is 23.7 per cent. In our own beehive practice we have always used 20 per cent, as the amount of sulphur eliminated from all sources, figuring from coal to coke, this being the result of numerous laboratory and practical tests on the Connelsville coal.
Attention has been directed in the preceding paragraphs to the sulphur as pyrite. Let us look into the other forms that may be present in addition to the sulphide. Any organic sulphur present remains, for the most part, in the coke. It is readily seen how it would be unaffected by aeration except in event of total destruction of the carbon itself, with which the organic sulphur is supposed to be combined.
Any sulphates present, such as CaSO4, would also be unaffected by aeration or superaeration in the beehive process; no amount of “airing" would help to eliminate the sulphur. We know of cases in which the sulphur in the resultant coke was as high, or even higher, as in the coal, from which it must be concluded that much of the sulphur was present in the coal as sulphate, or the so-called organic sulphur; or else the ash of the coal was rich in iron, lime and magnesia, for there is a dictum that it is never possible to produce a low-sulphur coke from the coal the ash of which is high in these elements. Superaeration cannot and will not ameliorate these conditions.
The Effect of Quenching on Sulphur Perhaps everyone is familiar with both the beehive and byproduct oven practice in this country, quenching the coke, by copious amounts of water. In the former case, it is customary to "water out” in the oven itself, while in the latter it is done externally by means of suitable quenching stations. In beehive practice, it takes approximately 1,000 gal. of water to quench a 5-ton charge of coke, consuming about 34 hr., either by hand watering with a hose, or with a sprinkler of the Stauft type, which is placed in the oven on top of the coke. The byproduct practice is much quicker, but the external watering produces darker coke.
Theoretically, it would seem possible to remove considerable sulphur by the quenching process. We have already indicated how iron sulphide (FeS) is formed in the coking process. The action of the water on the sulphide is as follows: FeS + H,0 = FeO + H2S, in which it is assumed that the coke is quenched externally. The odor of sulphuretted hydrogen is easily detected (but it will be appreciated that a little of this gas goes a long way), and the color of the coke, wherein rust spots appear, shows the presence of iron oxide. The practical coke burner is always suspicious of what he terms "rusty coke," as it invariably is an indication of high-sulphur coke. The same holds true in quenching in the oven itself, as in beehive practice. Sulphuretted hydrogen (H2S) is evolved, but, at the same time, the water as steam is decomposed by the carbon, H,0 + C = CO + H2, which fact probably accounts for a part of the large percentage of carbon monoxide and hydrogen gas found in the gas from trunnels of beehive ovens, the water being formed from the combustion of the gases during the coking processes, which is virtually a low-grade producer gas of the following composition:
3.0 9:0 11.0
The desulphurization of coke by water cannot be complete. The coke mass cools too quickly and its very structure prevents the rapid penetration of the water thrown on it, especially in the denser varieties. As the temperature is lowered the reaction involved becomes too slow to be of practical benefit. Data as to the exact amount of sulphur eliminated in this way are rather scarce, but our own experience, based on the general laws of volatilization set forth elsewhere, leads us to the conclusion that only an infinitesimal percentage of sulphur is thrown off during the quenching process. Laboratory and practical tests, in which the coke has been allowed to cool naturally, show but little difference in the sulphur content from those in which the coke has been quenched with water. Certainly there could be only a few hundredths of 1 per cent. in favor of the water-quenched coke, at the most, in a coke averaging 1 per cent. sulphur.
The prime object of the use of water is to lower the temperature of the coke for handling, not for desulphurization, and the quenching process should be so regarded. However, we are of the opinion that quenching by the byproduct practice will eliminate more sulphur than by the beehive method, which may partially explain the greater total volatilization of sulphur in the former, elsewhere intimated.
It may not be generally known, in connection with the subject, that addition of muriatic acid (HCI) to the water greatly facilitates the removal of sulphur during the quenching process. The action of this acid on iron sulphide is positive at all temperatures, thus, FeS + 2HCl = FeCl2 + H2S. There was, of course, a time when the cost prohibited the use of this method, but with the depletion of our low-sulphur coking coals this may, in the near future, be a factor in the elimination of sulphur.