Continuous Washing

Several attempts have been made to make the washing of naphtha or crude benzole a continuous one, and the arrangement of Blyth and Miles (Eng. Pat. 123839, 1918), as manufactured by Newton Chambers, Ltd., may be described. The invention is not only applicable to the washing of naphthas, but may be used for the purification of petroleums or the extraction of tar acids from creosotes by means of soda.

As shown in Fig. 163, a is a storage tank connected by a pipe b to a regulating tank c, provided with two outlets controlled by valves d, or similar devices, one of which controls the flow of benzole to the upper end of a coiled or convoluted pipe e, the other to a tank f, containing sulphuric acid or other reagent. The lower part of this tank f is connected by an overflow pipe G to the upper end of the pipe e. If now the valves d are opened, benzole will flow from the tank c directly into the pipe e, through the pipe h ; it will also flow through the pipe I to the tank F, and accumulate on the surface of the reagent contained therein, whereby an amount of the reagent corresponding to the weight of benzole thus admitted will be forced through the overflow pipe G into the pipe E. Thus the proportions of benzole and sulphuric acid (or other reagent) supplied to the pipe e can be determined by suitable adjustment of the valves d. If desired, the tank f may be duplicated or divided into separate compartments, so that when one tank or compartment is emptied of sulphuric acid or other reagents the other may continue in use whilst the first is recharged with the reagent. The tank f is provided with a gauge glass j.

Fig. 163. - Continuous Washing Plant.

Having passed through the coil E, in which the acid or other reagent effects the purification of the benzole flowing along in contact with it, these materials enter a receiver K, from the lower end of which the spent reagent is drawn off through an overflow pipe L. The benzole passes from the upper part of this receiver K, through a pipe M to a washing vessel n, which it enters at a point below the level of the water or purifying reagent contained therein. The foul water or spent reagent is drawn off from the bottom of this washing vessel through an overflow pipe o, and the benzole is drawn off from the top of this vessel through a pipe p. The overflow pipe o extends upwards from the bottom of the vessel N, to a sufficient height above the inlet of the pipe M. The benzole passes successively through a number of such washing vessels N1 n2, provided with overflow pipes o1 o2, and benzole pipes Pl P2 and is finally delivered to a purified benzole receiving tank Q.

In the arrangement illustrated for the treatment of benzole the washing vessels N, N2, are supplied with water from a pipe r, in quantities regulated by valves s, and the vessel n1 is supplied with caustic soda solution from a tank T, through a pipe u, under the control of a valve v.

With this apparatus, the regulating valves d having been once adjusted, it is only necessary to maintain the supply of unpurified benzole to the tank c, the supply of sulphuric acid to the tank f, the supply of caustic soda solution to the vessel Nl, and of water to the vessels N, n2, in order to obtain a continuous flow of purified benzole into the receiving tank Q.

If desired, the material treated may be passed in succession through more than one proportioning system such as that comprising the tank f, and coils e, and any number of washing vessels such as the vessel N may be provided connected one to the other in succession.

Impurities To Be Removed

The naphtha impurities that have to be extracted are phenolic bodies, pyridine bases, and numerous other complex bodies chiefly containing sulphur. The first two are valuable and therefore have to be saved.

The naphthas, before they are sent to the refinery, should have had all the phenols extracted from them (p. 384), but generally traces are still present. If the quantity is greater than about 2 per cent, further washing with soda is necessary, otherwise difficulty will arise in extracting the bases with acid. This is possibly due to the acid forming sulphonation products of the phenols which in some way prevent the solution of the pyridine in the acid.

In most cases, however, the naphtha may be directly agitated with dilute acid (sp. gr. 1.300) which extracts the pyridine bases in the form of pyridine sulphate.

The next agitation is with concentrated acid to remove the impurities, and generally this has to be repeated several times until the required purity is obtained. The quantity of acid used varies of course with the quality of the crude naphtha and the purity of the finished article that is required, but may range from 2 to 10 per cent of the spirit. Slight economy in washing may be obtained by using the pyrites brown acid (B.O.V. 140° Tw.) in the primary washings and finishing off with (C.O.V. 168° Tw.).

The alkali wash, which is generally preceded by a washing of water to remove the majority of the acid, is caustic soda of the usual 16° to 20° Tw. strength ; and sufficient is used to ensure the complete extraction of all phenolic bodies and the neutralisation of traces of acid that may still be present. Finally, the naphtha is agitated several times with water, so that it may be quite neutral before it is run into the still. The first water wash after the acid and soda should be only slightly agitated, and even so objectionable emulsions are liable to be formed. In such cases it is advisable simply to allow the water to trickle down through the naphtha.

The dilute acid wash, as has been mentioned, extracts the pyridine bases, and the recovery of these is generally carried out in conjunction with the manufacture of ammonium sulphate. The ammoniacal liquor obtained from the dehydration of tar is boiled in good contact stills or columns with excess of lime, whereby ammonia is set free. This is passed into the pyridine acid or sulphate until the solution is neutralised. After well settling, the ammonium sulphate solution is run off and concentrated until the salt crystallises. The crude dark brown pyridine is refined by two or sometimes three distillations, and dehydrated by means of lump caustic soda. The refined bases have to pass a rigid specification and are sold for denaturing alcohol. The agitation with concentrated acid produces a dark brown or red viscous mass which is known as " acid "or "vitriol " tar. This product has been the subject of innumerable researches and patents, but little of practical importance has yet been discovered. Even as a waste product there is great difficulty in disposing of it. If deposited into a river it kills all fish life, and if burnt it produces so much sulphurous fumes that generally factory inspectors object. One of the best methods of dealing with it is to boil it up with the water washes to extract all free acid, the solution then being diluted to the correct strength for subsequent washing of naphtha for pyridine. The residue, after cooling, is a pitchy substance which floats on the top and can be disposed of without further treatment by burying it in a tip.