GRAY: A SIMPLE APPARATUS FOR THE WASHING O F GASES. 279

XX.-A Simple Appai-atus f o r the Washing of Gases.

By HAROLD HEATH GRAY.

DUKING the progress of some recent work on the extraction of hydrocarbon vapours from a mixture of gases containing them, the difficulty of obtaining a suitable gas-washing apparatus caused much inconvenience and some delay. Such a determination as that of the amount of toluene or xylene present in coal gas,

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180 GRAY : A SIMPLE APPARATUS FOR THE WASHING OF QASES.

carburetted water gas, or similar gaseous mixtures usually involves the operations of:

(1) Washing the gas by means of a suitable solvent. (2) Distilling and fractionating the .solution so obtained in

accordance with a scheme of operations similar to that drawn up by Dr. H. G. Colman or by Mr. James.

Little trouble arises under heading (2) owing to the care with which the details have been worked out, but the washing of gas in such a way as t o obtain accurate results may cause trouble. Certainly the author experienced much difficulty in putting together an apparatus which would give reasonably accurate results and a t the same time not require constant attention. Even such a simple apparatus as a series of washing bottles containing (‘ green oil” is capable of causing trouble and annoyance in the shape of stoppages of the gas flow and unexpected leakages. Assuming that a satisfactory estimation requires the collected vapours from about 100 cubic feet of gas (2800 litres), the chief experimental difficul- ties liable to occur can be summarised as follows: (a) A t the slow rate required for complete extraction, the wash-

ing of 2800 litres of gas occupies a considerable time, say seventy t o one hundred hours.

( b ) The solvent may become saturated, thus causing low results. ( c ) The total depth of liquid through which the gas %as to pass

causes the development of a considerable “ back-pressure ” ; that is t o say, the gas to be washed must possess a considerable initial pressure in order that it may be forced through the liquid neces- sary for the extraction of contained vapours.

(d) The necessity for working with dilute solutions involves, as corollary, the use of large quantities of washing medium (for example, “ green oil ”).

( e ) The general complexity of apparatus makes the preparation of gas-tight joints difficult, and errors ensue owing to leaks.

Item ( c ) caused most difficulty. No washing apparatus was known t o the author which would wash quickly 2800 litres of gas without the development of a considerable ‘‘ back-pressure.” Mechanical distributing devices, either for the gas or for the oil, failed to realise the hopes placed in them, and in no way destroyed the difficulty of “ back-pressure.” Accordingly, the gas to be washed was passed through a mass of condensing vapour in the apparatus shown. As it was desired t o extract toluene from coal-gas, the first

vapour to be tried as solvent was the vapour of benzene.

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QRAY : A SIMPLE APPARATUS FOR THE WASHING OF GASES. 181

Description of Apparatus.

The gas to be treated is led through the glass tube A , which is arranged concentrically throughout the length of the inner tube of a water-cooled Liebig condenser, the end of the tube A project ing into the flask C. The flask C contains the solvent, which in the case of toluene estimations may suitably be heated benzene (of a known boiling-point range). The gas, now heavily laden with benzene (or other) vapour, passes out of the flask into the annular space in the inner tube of the Liebig condenser ; i t there deposits benzene in the form of a mist, indi- cated a t D. The benzene fog, or mist, washes out toluene and allied vapours, and the gas, now saturated with benzene vapour, passes out of the con- denser by the sidetube 3. The final mixture of toluene and benzene which remains in the flask C is subjected t o fractianation in the usual manner. The joints F and G' can be made quite satisfactorily by means of good bark corks; the mercury cup-joints shown in the figure are less subject to leakages, but are expensive and difficult to repair.

A few trials made it clear that the " vapour " washer, under suitable con- ditions, possessed many advantages over " liquid " washers, and by its use the author succeeded in eliminating, or a t least greatly reducing, each of the several drawbacks enumerated above as attaching to the use of liquid in a gas-washing apparatus.

Efjicieizcy of Extraction.-The effici-

A

ency of extraction is due mainly to t$e ideally intimate contact between the gas and the washing medium. The drenching of the gas by means of the precipitating liquid compares very favourably with the more gross and less intimate washing which occurs when gas is simply bubbled through liquid.

Another important factor is that the solvent is continually present in large excess and in a pure form.

A catch-bottle placed after a series of three vapour washers

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152 GRAY : A SIMPLE APPARAT‘ITS FOR THE WASHING OF BASES.

was found to contain pure benzene of the same boiling-point range as that originally employed. This may be taken as an indication of complete extraction.

Choice of Solvent.-The choice of benzene as solvent for toluene was based on the fact that a t the ordinary temperature the vapour tension of benzene is greater than that of toluene. I f one expresses this in another way, by saying that the benzene has a greater tendency to enter the gas than has toluene in similar circumstances, it will readily be seen that benzene is a more suitable solvent for toluene than is such a substance as (‘green oil,” since it is con- ceivable that a t certain concentrations, easily attained in a gas- washing apparatus, the toluene is more likely t o be given up to the gas than to be retained by the (‘green oil.” The danger of oversaturation of the solvent is rather remote when benzene is used to dissolve toluene. Similar considerations arise in the converse operation known as (‘ carburetting ” gases of poor calorific or illuminating value on a large scale. The examination of liquid deposits in gas mains indicates clearly that the use of hetero- geneous carburetting liquids is not always economical in practice, however cheap such liquids may be, and can seldom fail to lead to erratic results. Other conditions being equal, i f two liquids are to be examined with a view to their use as carburetting agents, that liquid with the narrower boiling-point range should give the most constant results.

Apart from the relative vapour tensions of the solvent and solute, the suitability of a solvent will be determined by the behaviour during subsequent distillation of the solution obtained, A separation is essential. The form of apparatus shown in the figure demands the use of a solvent of higher vapour tension than that possessed by the vapour to be extracted; there seems, how- ever, to be no reason why a xylene mist should not extract toluene in a suitably designed washer. The condenser would not then be of the reflux form, nor would the solutions so obtained be of any great degree of concentration.

Rate of Grcs Flow.-The velocity of gas flow with which it’ is convenient to work will depend largely on the areas of cross-section of the various portions of the apparatus; when a glass tube of 0.6 cm. internal diameter was used, it was possible to wash the gas a t the rate of 280 litres per hour, which is an improvement on the usual rate of 42 litres per hour used when washing coal-gas with oil in theliquid form. Wider tubes would allow of still more rapid washing.

A pplications.-The “ vapour ” washer is capable of general application in the examination of mixtures of permanent gases

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SOLVENT EFFECT AND BEER’S LAW. 183

with vapours, provided that for each case a suitable solvent is found. The use of vapour seems well adapted t o the determinn- tions of dust or tar fog in gases, determinations which are fre- quently required, but are really difficult t o carry out in a truly satisfactory manner. Wherever i t is desired to extract soluble vapours with maximum efficiency from a large quantity of gas in minimum time, the use of vapour would seem to promise many advantages over the usual forms of “liquid” washers, such as washing towers, spirals, worms, bead-washers, etc., all of which have the comnion drawback of interposing great resistance to the passage of gas.

The experimental work was carried ou t in the laboratories of the Birmingham Cit.y Gas Department, and the author wishes cordially to acknowledge his indebtedness to the Birmingham Gas Com- mittee for the opportunity of doing it, and to E. W. Smith, Esq., M.Sc., Chief Chemist to the Gas Department, for very kindly putting a t his disposal every facility for testing the method under the varied conditions available a t Birmingham.

LONDON. [Received, December 15th, 1916.1

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