THE CONTACT PROCESS
Sulphur Trioxide. When elements are combined in different proportions by weight, they produce different compounds. Thus, in the case of sulphur and oxygen, there are two well-known compounds, namely, sulphur dioxide and sulphur trioxide. In the former, a given weight of oxygen is combined with an equal weight of sulphur; in the latter, this same weight of sulphur is combined with 50 per cent. more oxygen. On this account, sulphur trioxide is spoken of as the higher oxide.
We can now state in general terms another method by which sulphuric acid can be built up from its elements. Sulphur, as we have seen, burns in oxygen, forming sulphur dioxide. This substance can then be made to unite with more oxygen to give sulphur trioxide, which, with water, yields sulphuric acid. There are three steps in this synthesis. The first, namely, sulphur to sulphur dioxide, has already been considered; the last, sulphur trioxide to sulphuric acid, only requires that sulphur trioxide and water shall be brought together: we can, therefore, confine our attention to the intermediate step, namely, the conversion of sulphur dioxide into trioxide.
This operation, when carried out in a chemical laboratory, is a very simple one. [Fig. 4] shows the necessary apparatus. Sulphur dioxide from a siphon of the liquefied gas and air from a gasholder are passed into the Woulff’s bottle A, containing concentrated sulphuric acid; this removes moisture from the gases. The drying process is completed in the tower B, which contains pumice stone soaked in sulphuric acid. The mixed gases then pass through the tube C, containing platinized asbestos heated to about 400° C.: the sulphur trioxide collects in the cooled receiver D.
Fig. 4. SULPHUR TRIOXIDE—THE CONTACT PROCESS
Platinized asbestos is made by soaking long-fibred asbestos in a solution of platinum chloride. The material is then dried and subjected to a gentle heat. In this way, metallic platinum in an exceedingly fine state of subdivision is deposited on the asbestos fibre, which merely serves as a convenient support.
Catalytic or Contact Action. The influence of the finely divided platinum is a very important factor in the reaction. It cannot, however, be said to cause the union of sulphur dioxide with oxygen, for the gases combine to a very slight extent when it is not present. What the platinum actually does is to influence the rate of formation to such a degree that, under favourable conditions, practically the whole of the sulphur dioxide is changed to sulphur trioxide instead of an exceedingly small fraction of it.
The most interesting, and at the same time the most perplexing, feature of the reaction is that the platinum itself does not appear to undergo any change. It is not diminished in quantity, for only a very small amount is necessary for the conversion of a very large amount of the mixed gases. Its activity lasts for a very long time, and even when it does become inactive, it can be shown that this is due to some external cause, such as the presence of dust and certain impurities in the gases.
Many other similar cases are known in which the presence of a small quantity of a third substance greatly influences the course of a chemical reaction without appearing in any other way to be necessary to the reaction. These substances, which are often metals in a very fine state of subdivision, are called catalytic or contact agents.
The Contact Process for making sulphuric acid is nothing more nor less than the simple laboratory operation which we have described above, carried out on a larger scale.
The sulphur dioxide is produced as in the lead chamber process by roasting iron pyrites in a current of air. This gas, together with the excess of air, is passed into the contact furnace, which consists of four tubes, each containing platinized asbestos, supported on perforated plates. The union of the two gases is said to be almost complete: an efficiency of 98 per cent. of the theoretical value is claimed for this process. The sulphur trioxide, or “sulphuric anhydride”[1] is either condensed in tin-lined drums or absorbed in ordinary concentrated sulphuric acid.
The proposal to manufacture sulphuric acid by this method was first made in 1831 by Peregrine Phillips, of Bristol. The early attempts were not successful, and it was not until about forty-four years later that the difficulties arising in the working of the contact process were overcome sufficiently to enable the sulphuric acid produced in this way to be sold at the same price as that made by the lead chamber process. Since 1890, the total quantity of acid made by the contact method has increased very rapidly, so that it now furnishes about one-half of the world’s supply, and seems likely in time to displace the lead chamber process altogether.
The history of the rise of the contact process is interesting because it illustrates in a striking manner the very great difference that there is between a successful laboratory process and a successful manufacturing process, though seemingly identical.
The first and possibly the most serious difficulty encountered in the working of the contact process was the frequent interruption caused by the loss of activity of the contact substance. Iron pyrites always contains arsenic which volatilizes on heating, and this quickly caused the platinum to lose its activity, or, as it was sometimes rather fancifully expressed, “poisoned the catalyst.” Dust also is inevitable, and this, carried forward mechanically with the stream of gas, settled on the contact substance and caused the action to cease.
To get over this difficulty it is necessary to purify the gases. They are first passed slowly through channels in which the coarser particles of dust settle down. Steam is injected into the mixture to wash out the finer particles of solid, and also to get rid of arsenic, and then the gases are passed through scrubbers. Before being admitted to the contact furnace, the moist gas is submitted to an optical test. It is passed through a tube, the ends of which are transparent; a bright light is placed at one end and viewed from the other through a column of gas of considerable length. If the purification process is working satisfactorily, there is a complete absence of fog. The gases are then dried by passing through concentrated sulphuric acid and admitted to the contact tubes.
In all operations carried out on a large scale, the regulation of temperature is a matter of some difficulty. In the case which we are considering, the most suitable temperature range is a rather narrow one, and the difficulty of keeping within the limits is very much increased by the large amount of heat given out when the sulphur dioxide and oxygen combine. The result of the failure to maintain the temperature at a fairly constant level was that the process worked in a very irregular manner, for as soon as it was working really well and sulphur trioxide was being formed rapidly, the heat given out by the reaction itself was also great, and consequently, the higher temperature limit was exceeded.
The method of controlling the temperature in the contact process is worth noting, because it is really ingenious. The tubes containing the platinized asbestos are surrounded by wider concentric tubes. The gases which are about to enter the contact furnace pass through the annular space between the two tubes, and are thereby heated to the required temperature, while at the same time they serve to cool the inner tubes. The most satisfactory temperature is about 400° C. The tubes are first warmed to 300° C. to start the reaction, and thereafter the heat evolved by the reaction itself is sufficient to keep it going.
The absorption of the sulphur trioxide also caused some difficulty at first. This substance reacts most violently with water, dissolving with a hissing sound like that produced when a red-hot poker is plunged into water. At the same time great heat is developed, and consequently, much of the sulphur trioxide is vaporized, and in that way lost. This difficulty was got over by using 98 per cent. sulphuric acid for the absorption, the acid being kept at this strength by the simultaneous addition of water.
The contact process has some very distinct advantages over the older lead chamber process. The plant covers a much smaller area than the bulky lead chambers. Although the preliminary purification of the gases is somewhat tedious and costly, this is in great measure compensated by the purity of the acid produced. No separate plant is required for concentration and purification, as in the older process. Finally, sulphuric acid of any concentration can be produced at will, including the fuming acid, which is required as a solvent for indigo, and in the manufacture of artificial indigo and other organic chemicals.
The lead chamber process produces what is called chamber sulphuric acid very cheaply. Although this is only a 60-70 per cent. solution and very impure, nevertheless, it is quite good enough for the heavy chemical trade, particularly for the first stage of the Leblanc soda process, and for making superphosphate. These two industries alone consume many thousands of tons of this sulphuric acid every year. Probably for some years to come the two processes will continue to exist side by side, but it may be doubted whether new works will now be installed to make sulphuric acid by the lead chamber process.
Properties of Sulphuric Acid. The pure non-fuming acid is a colourless oily liquid whose density is 1·84. It mixes with water in all proportions, yielding dilute sulphuric acid, and it also dissolves sulphur trioxide, yielding the fuming acid.
The mixing of sulphuric acid and water is accompanied by an evolution of heat and by contraction in volume. It is an operation which must be carried out with great care, the acid being always poured into the water, otherwise the water floats on the heavier acid, and so much heat is developed at the surface of separation that some of the water will be suddenly converted into steam, and this, escaping from the liquid with explosive violence, may cause the contents of the vessel to be scattered about.
Strong sulphuric acid chars most organic substances. From substances such as wood, sugar, paper, starch, it withdraws the elements of water, liberating carbon. Since it acts in the same way upon human flesh, it is clear that the concentrated acid must be handled with very great care, for it causes most painful burns. For this reason, vitriol throwing has always been regarded as a most serious and dastardly offence. A simple first-aid remedy for burns produced by sulphuric acid is the liberal application of an emulsion of linseed oil and lime water. The lime, being an alkali, neutralizes the acid, and the oil excludes air from the wound.
The readiness with which sulphuric acid combines with water is often made use of both in the laboratory and in industrial Chemistry for the purpose of drying gases. One illustration of this use has already been given in describing the contact process. Another instance which may be fairly familiar occurs in the case of liquefying air, where the gas must be thoroughly dried before being passed into the refrigerating apparatus, otherwise this would soon become blocked with ice.
The position which sulphuric acid occupies in Chemistry is due mainly to three outstanding features. In the first place, it is a strong mineral acid and displaces all other acids from their salts. Secondly, it has a high boiling point (338° C.), and consequently, the displaced acid with the lower boiling point can be distilled from the mixture. Lastly, sulphuric acid can be made very cheaply from materials which are very abundant in Nature, and, therefore, it meets all the requirements of an acid which is to be used for general purposes.