The usual methods of separation of substances from the complex mixtures contained in the fractions are employed. The solids such as naphthalene and anthracene are crystallised out and freed from the other liquids. The acids such as phenol are extracted by washing with alkali, generally caustic soda, and the acid is separated out from the sodium salt by treatment with mineral acid. The bases such as pyridine are subjected to the reverse treatment, as they are converted into the sulphate by the action of sulphuric acid, and subsequently neutralised with alkali, generally ammonia. Lastly, the remaining bodies, which are mostly hydrocarbons, are separated by further distillation.
The methods of further separation and treatment of the primary fractions may be mentioned, but it must be borne in mind that many of the products to be extracted, and therefore also the processes employed, are common to several of the fractions. For instance, phenol and tar acids generally are common to all the fractions after the crude naphtha, and under certain market conditions every fraction has to be subjected to the treatment of washing out with soda. Only one typical case of each separation will therefore be described.
Nos. 1 and 2 Fractions. - These will be left for the next chapter, as the separations can only be carried out by fractional distillation, and the whole of the next chapter is devoted to this subject.
No. 3 Fraction - Naphthalene. - The oil contains fairly large quantities of naphthalene, which is sometimes allowed to cool out first, but generally after extraction of the acids, as the solubility of naphthalene in tar acids is greater than in the creosote hydrocarbons. Better separation of the naphthalene is therefore obtained if the acids are washed out first.
The crude oils are washed in washers or wrought-iron tanks of about 2000 to 5000 gallons capacity. The tanks are fitted with good agitating gear, as described in the next chapter, also they are either steam-jacketed or contain steam coils, in order to keep the oils hot while the washing is carried out, otherwise crystallisation of the naphthalene would cause trouble. Caustic soda of a strength of about 16° to 22° Tw. is the alkali used, and, after settling, the sodium phenate and cresolate, commonly known as the carbolate and cresolate, are drawn off from the bottom.
The separation of the phenol from the cresols or higher homologues is carried out direct in the washing process, and is based on the fact that phenol is more acidic than its homologues. The method is to wash the light oil first of all with the so-called crude carbolate solution. This is the middle wash out of three and is a saturated solution of sodium phenate and cresolate. The action of bringing this into contact with fresh oil is to decompose the cresolate in the crude with the phenol contained in the oil, so that an interchange of phenol from oil to aqueous solution is obtained. Thus by regulating the quantity of crude carbolate used in the wash, sodium phenate fairly free from sodium cresolate can be obtained. The light oil, now containing relatively small quantities of the phenol and large quantities of the cresols, is washed with clean caustic soda and the resulting solution is the crude carbolate for the next charge. The oils are now fairly free from phenol and contain chiefly cresols and homologues which are extracted by washing with the required quantity of clean caustic soda.
In order to obtain the acids from the solutions some mineral acid must be used. The old method was simply to neutralise with sulphuric or hydrochloric acid, in an open vessel. The method most in use at the present time is to treat the solutions in upright tanks or gassing cylinders with carbon dioxide. These tanks are run in series, so that any gas not absorbed in the first is caught up by the next. The carbon dioxide is usually obtained from a lime kiln, for, besides the gas, limestone produces lime which is required in the process. After the solutions are neutralised by the carbon dioxide, the tar acids can be easily separated from the top, and the sodium carbonate at the bottom is used to react with the lime from the kiln to make caustic soda for the washing of further quantities of light oils. The process is a continuous one and works in a cycle, viz.: 1. NaOH + C6H5OH = C6H5ONa + H20,
2. 2 C6H5ONa + C02 + H20 = 2 C6H5OH + Na2C03,
3. Na2C03 + CaO + H20 ' = 2 NaOH + CaC03.
Washing with a cream of lime instead of caustic soda can be employed, and has been worked out successfully by some distillers.
Again, washing the light oil with sodium carbonate solution and freshly burnt lime will effect the same result, the action taking place at quite low temperature, viz. :Na2C03 + CaO + 2 C6H5OH = 2 C6H5ONa + CaC03 + H20.
Both of these methods present mechanical difficulties, as objectionable emulsions are formed with consequent loss of phenol.
Refining of the Tar Acids
The acids as separated above are in the form of dark-brown oily liquids, and contain 12 to 15 per cent of water. The phenol is prepared to the specification known as 50's, 60's, or 70's crude carbolic acid. The names are derived from the result obtained by submitting the acids to the "Lowe' test, and do not denote the quantity of absolute phenol in the crude substance, although the figures, as a matter of fact, do approximate to these values. So that a 60's crude carbolic acid may be taken to contain about 60 per cent absolute phenol, 15 per cent water, and 25 per cent cresols and homo-logues.
The refining of the phenol and its separation from the cresols is carried out entirely by fractional distillation and fractional crystallisation. We may take 60's crude carbolic acid as a good example, as this is the usual form in which the material is prepared from the light oils.
The mixture is first distilled in an ordinary iron pot still with the object of separating the water, cresols, and pitchy matter from the phenol. The first fraction, coming over below 180° C, consists of water, neutral oils, and some phenol. The second fraction is collected between 180° and 205° and contains mostly phenol. The third fraction, 205° to 220°, consists of cresols with a small quantity of phenol. The residue contains practically no phenol and may be collected from successive charges and reworked for the yield of cresols or pale cresylic acid or, as it is known to the trade, "Liquid Carbolic Acid."
The fraction collected between 180° C. and 205° is run into small pans holding 25 to 50 gallons, and allowed to crystallise. These pans are arranged so that cold water can be circulated round them, or in some instances the pans are kept in a refrigerating house. After crystallisation has taken place the liquid is drained off and mixed with the fraction from 205° to 220°, which is again distilled. The crystals of phenol are dried by means of a centrifugal machine, when they show a fusing point of about 30° C. These crystals are melted and subjected to a further distillation and a further crystallisation, as above, when the fusing point should reach about 35° C. To obtain 42° crystals the same process must be repeated again. It will be seen that the process is a long and tedious one and that many fractions are obtained, but by careful observation of their boiling points many of them can be mixed together and reworked.
The distilling apparatus is sometimes supplied with a fractionating column, but often not. There can be no doubt that a column adds to the efficiency if it is of a good type ; but there is no doubt that distillers do not pay enough attention to the columns they use, and consequently are apt to conclude that they are unnecessary, while the fault really lies in the construction of the column itself.
In cases where very pure phenol is required it may be advisable to redistil once more, and to use either zinc or silver still-head pipe and condenser coil, as it has been found that iron, copper, lead, and aluminium are unsuitable.