Contact Process

Although the commercial manufacture of sulphur trioxide and sulphuric acid by the catalytic process has attained success only in comparatively recent years, a patent was acquired in 1831 by P. Phillips of Bristol for the production of sulphuric acid in this way, the suggested catalyst being platinum. The commencement of the twentieth century saw the main difficulties overcome and the installation of an economical and commercial process in Germany. Since then the number of plants has increased largely and various modifications have been introduced in many countries.

The most active catalyst is platinum applied in finely divided form, for example platinised asbestos. Certain elements, especially arsenic and mercury, have a powerful effect in reducing the activity of the platinum, a quantity of arsenic equal to 0.2 per cent, of the weight of the platinum reducing the activity by 50 per cent. These "poisons," as they are termed, also include less harmful substances such as antimony, lead, bismuth, etc. The presence of small quantities of rhodium, iridium or osmium in the platinum also causes diminished yields of trioxide, but the presence of palladium or ruthenium has the opposite effect.

Sulphur or iron pyrites again serves as the source of sulphur dioxide, the former being preferable as many of the undesirable impurities are present in pyrites. When using pyrites, investigation has shown that most of the arsenious oxide is to be found in the fumes of the sulphur trioxide which is formed; steam or fine water spray is therefore injected, when the fumes form a mist of dilute sulphuric acid which encloses the arsenious oxide together with other dust, and can be condensed by passing the gas through a series of lead pipes. Subsequently the gases are dried by passing up a series of towers down which concentrated sulphuric acid is made to trickle. Tests are then made to ensure the absence of impurities from the gases and the presence of a sufficient excess of oxygen. The reaction

2SO₂ + O₂ = 2SO₃

Contact Chamber.

is accompanied by a considerable evolution of heat, but, as has already been stated, the formation of trioxide is incomplete above 450° C. For this reason the contact chambers (see fig.), containing a group of vertical tubes in which are placed small horizontal sieves to support the platinised asbestos and prevent it becoming unduly compressed, are maintained near 400° C. In order to prevent the chambers being overheated by the liberation of heat in the chemical change, the cool entering gases are first circulated round the outside of the contact tubes; by this device not only is the temperature inside the tubes maintained constant, but the gaseous mixture is also preheated to a suitable degree before meeting the contact material. Instead of asbestos, anhydrous magnesium sulphate or other material may be used as a platinum carrier; in such cases, in order to ensure the availability of the platinum, the carrier should first be saturated with alcohol, dipped into an alcoholic solution of the platinum compound, quickly dried and ignited. Useless deposition inside the carrier is thus prevented.

After the gaseous mixture has traversed one or more chambers, the sulphur trioxide is absorbed by sulphuric acid usually maintained at 97 to 99 per cent, concentration by regulated dilution with weaker acid or water. Acid of this concentration possesses the double advantage of being usable in iron pots and of absorbing the anhydride better than water, which causes fumes of sulphuric acid, and better than stronger acid, which leaves some sulphur trioxide unabsorbed. If fuming acid is required, the absorbent is generally maintained at a content of 27 to 40 per cent, of "free" anhydride, acid of this strength also leaving iron unattacked.

Should the platinum become "poisoned," that is, rendered inactive by contamination with impurities which have escaped the initial purification process, it can generally be revivified by heating in a current of a reducing gas such as hydrogen. Safety precautions for the operators are desirable during the process.

Although platinum is the most effective catalyst for this reaction, other catalysts are known, including many oxides and sulphates. Ferric oxide in the form of pyrites ash has proved capable of technical application, but on account of its lower activity, a somewhat higher temperature, usually near 550° C., is necessary. At this temperature the formation of trioxide is not complete, and therefore, after the removal of the trioxide from the iron oxide contact chamber, the issuing gases are passed through a platinum contact chamber at a lower temperature, where the reaction is completed. Such an arrangement possesses an advantage arising from the fact that the catalytic action of ferric oxide is not adversely influenced, but even improved, by the presence of arsenic; the crude combustion gases therefore no longer need careful purification, this being effected by the iron oxide itself.

Other forms of apparatus have been constructed or proposed to employ ferric oxide as the sole catalyst. In one scheme, use is made of the power of ferric oxide containing ferrous compounds to absorb sulphur dioxide. The ferric oxide impregnated with ferrous sulphate gradually descends an inclined rotating flue up which passes the usual mixture of sulphur dioxide and air. By the heat of the gaseous mixture the descending solid is maintained near 350° C. at the upper end of the flue, and at the temperature of its optimum catalytic effect, namely 550° C., at the lower end. Sulphur trioxide is produced by incomplete combination of sulphur dioxide and oxygen in the lower portion of the flue, whilst the uncombined sulphur dioxide, being largely absorbed by the cooler mass higher in the flue, is returned to the hotter region, where it becomes expelled and is thus given another. opportunity of conversion into trioxide. As no precautions are taken to exclude moisture, the vapours issuing from the top of the flue can be condensed directly to fuming sulphuric acid.

It is probable that the mode of action of platinum and possibly also that of the metallic oxides as catalysts is by surface adsorption of the sulphur dioxide, this gas being thus brought into a condition more favourable to reaction. As regards the action of the metallic oxides, however, the view that it depends on alternate oxidation and reduction has recently been developed. Thus it is assumed that in the case of iron oxide the following reactions occur:

1. 3Fe₂O₃ + SO₂ = 2Fe₃O₄ + SO₃,
2. 2Fe₃O₄ + 4SO₂ + 2O₂ = 2FeSO₄ + 2Fe₂O₃ + 2SO₃,
3. 2FeSO₄ = Fe₂O₃ + SO₃ + SO₂,
4. SO₂(nascent) + ½O₂ = SO₃.

The fact that ferrous sulphate above 605° C. decomposes to yield the products given in (iii), and that small quantities of ferrous sulphate are found in the used contact mass, are cited as evidence of its formation as an intermediate compound.

Recently a new catalyst has come into use in the form of vanadium pentoxide or a metallic vanadate. A siliceous material of the zeolite type is impregnated with the vanadium compound and ignited; or the catalyst may be mixed with a finely divided porous substance such as kieselguhr or asbestos. It is claimed that this catalyst is almost as efficient as platinum and possesses the advantage of long life, having great resistance to high temperature and being unaffected by the substances which poison the platinum. Neumann considers that the catalytic action probably depends upon the intermediate formation and reduction of vanadyl sulphate, traces of which are found in the used contact mass.

From a consideration of the details of the ordinary contact process it will readily be recognised that the catalytic method of manufacture of sulphuric acid is capable of yielding an acid of the highest degree of purity.

A promising development in the manufacture of sulphuric acid consists in the combination of the contact and chamber processes, whereby the amount of contact catalyst used is only about one-eighth of that used in the ordinary process. The gases first pass into a contact chamber and the sulphur trioxide formed is then converted into oleum, whilst unchanged sulphur dioxide passes to the lead chamber to be converted to sulphuric acid. Or, nitrosylsulphuric acid is made to act as a true surface catalyst, intimate contact between it and the gases containing sulphur dioxide and free oxygen being obtained by forcing the gases through fine films of the acid supported on perforated plates, or through a series of tubes containing mixing devices through which the acid circulates; the oxidation proceeds in the liquid phase.

Various other modifications of the sulphuric acid plant are in use, the tendency in recent years being to develop intensive processes in which the reactions leading to the formation of sulphuric acid occur at high velocity.