Corrosion Of Tar Stills

This is a trouble which has a very-great bearing on the whole industry of tar distilling, for the wear and tear on stills, and particularly those of the pot still type, by corrosion is a very great financial factor.

The problem has been studied by many investigators, but no satisfactory solution of the difficulty has been found. The corrosion is caused chiefly through the tar being mixed with liquor, and if distillation could be carried out with water-free tar it would be almost eradicated.

Warnes and Davey1 give a summary of the work done on this subject and maintain that the corrosion is caused rather through chemical or mechanical than electrolytic action. It seems certain that dissociation of ammonium chloride, ammonium sulphide, ammonium hydrosulphide, and ammonium cyanide takes place at certain temperatures, and that the dissociation products act upon the iron. The action is most pronounced at parts of the still that have been subjected to mechanical strains in manufacture, and parts that are continually in contact with hot vapours rather than hot liquids. For instance, stills that have had the rivet holes punched instead of drilled show the most marked corrosion round the rivet holes, and it is very noticeable around a manlid mounting or charging pipe. Again, the sides of the still are most corroded at. the height where the tar and liquor are in violent ebullition at the beginning of the distillation when the water is coming off.

Warnes and Davey state that the process of corrosion goes on at a greater rate during the latter portion of the distilling operation, principally during the period when steam is used to assist in the distillation.

Whatever the causes may be, electrolytic action undoubtedly plays a certain part in the process, and great care should be taken in the selection of suitable plates for the still, whether they be of mild steel or iron. It should be seen that there is no sign of lamination, crystalline structure, or blisters. Indeed, in bending the plates metal hammers should not be allowed but only wooden mallets.

Fractions

The question of the number of fractions, size of fractions, etc., that are to be taken off when tar is distilled is dependent on so many factors that it is only possible to give generalisations. For instance, the kind of tar to be dealt with obviously plays a big part, while the kind of plant that is available to deal with the fractions is another, but the market value of the products is probably the most important. For if the price of any individual product should be too low for its extraction to be profitable, the fractions would probably have to be entirely altered. Generally speaking, however, they are the following: No. 1 Fraction. - In this is contained the ammoniacal liquor, and naphthas, which are mixtures of benzene, toluene, xylenes, and pyridine.

1 Warnes and Davey, J.S.C.I., 1910, 29, 657.

The boiling-point range is from 80° C. to about 140° C, and the specific gravity .870 to .950. The quantity of water is of course dependent upon the amount in the original tar, and whether it has been partially taken out before distillation. It separates easily from the naphtha, and is drawn off from the bottom, and sent direct to the ammonia works.

Great care must be taken in getting off the first fraction, as frothing is very prevalent, particularly in a tar with a high free carbon content. The point when this danger is passed can be easily noticed by the noise that is heard inside the still, known as the "rattles." When nearly all the water is off, globules of water condense on the inside of the top of the still and occasionally fall back into the hot liquid below : they are immediately turned into vapour again with almost explosive force, with the resulting rattling noise. The same phenomenon can be observed in distilling a liquid like benzole containing traces of water in the laboratory.

No. 2 Fraction. - This is known as the light oil fraction, and boils from about 140° C. to 200° C, and has a specific gravity of about .950 to 1.000. It contains the higher hydrocarbons of the benzene series such as mesitylene, cumenes, some naphthalene, also phenol, and higher homologues of pyridine. Many distillers do not separate this fraction, but mix No. 1 and 2 together ; on the other hand, in districts such as Lancashire and Yorkshire where the tar is rich in these valuable fight products, it pays to collect the two fractions.

No. 3 Fraction. - This fraction is collected purely to obtain the phenol in as concentrated a state as possible, and is consequently called the carbolic oil or middle oil fraction. It boils between 200° C. and 240° C.,| has a specific gravity of 1.000 to 1.025, and contains phenol, cresols and higher hydroxy acids, much naphthalene and creosote hydrocarbons. In the distillation great care must be taken to see that the condenser water is quite hot, so that crystallisation of the naphthalene shall not take place in the coils. The cold water should be turned off in the middle of No. 2 fraction, and if the cooling water does not get warm quickly enough, steam should be turned into the condenser.

Safety valves are supplied to all stills, and these should be looked to before each distillation, to see that they are working freely, as explosions have occurred in tar works through the jamming of the valves.

This carbolic oil fraction is not always separated, as the acid in some cases does not pay to extract, and in others the tar contains too small a quantity. Again, it is sometimes found more economical to re-distil the creosote fraction, as described later.

No. 4 Fraction. - This, known as the creosote oil fraction, is the largest of all, and contains naphthalene and heavy oils, which are aromatic hydrocarbons with a high carbon and hydrogen content, and cresols and other phenol homologues. The boiling point is about 240° C. to 280° C, and specific gravity 1.025 to 1.065.

No. 5 Fraction. - This fraction is marked by its distinctive colour, and is consequently called the green oil, yellow oil, or anthracene oil fraction. Its specific gravity is 1.065 to 1.100, and boiling point from 280° C. upwards to the end of the distillation. It contains still higher aromatic hydrocarbons, anthracene, phenanthrene, also carbazol, etc.

Numerous attempts have been made to largely increase the number of fractions taken off the tar with the idea of better isolating the products. All these have failed, as the distillates obtained are no purer, so many complex azeotropic mixtures being formed. Again, nothing is saved, as many of the fractions have to be mixed together again for treatment in subsequent processes.