US2017676A - Method of and apparatus for condensing sulphuric acid - Google Patents

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Oct- 15, l935 c. a. voN GlRsEwALD ET Al. 2,07675 METHOD OF AND APPARATUS FOR CONDENSING SULPHURIC ACID Filed March 8, 1934 yl'm/ema/hs: CGNWM BARN von GBRSEWMD WOLFHART StECKE Mm womwnu.

A tt @may Patented Oct. i5, 1935 UNITED STATES PATENT ori-ice METHOD OF AND APPARATUS FOR CN- DENSING SULPHURIC ACID York `Applimnm March s, 1934, serial'No. '114,5sz In Germany March 11, 1933 6 Claims.

This invention relates to a method .of and apparatus for condensing sulphuric acid.

It is known that considerable dimculties are encountered in condensing sulphuric acid vapours, since mists are often formed which cannot be reduced to a liquid state capable of forming drops. In order to prevent such formation of mist it has been proposed to pass the gases containing S03 through boiling water and thus allow the SO: to be absorbed by the steam carried olf in the operation. This, however, would merely produce an extensively diluted acid. It has also been asserted that protracted contact between S03 and H2O within the falling range of temperature between 450 and 338 C. is necessary in order to facilitate the combination to sulphuric acid in vapour form.

The problem of condensing sulphuric acid has recently attained increased importance, processes having been proposed to catalyze SO2 in presence of an amount of steam suicient for the formation of monohydrate and then condense it directly to sulphuric acid. 'I'his process is of great importance for the production of sulphuric acid, since it can be carried out without the ex' tensive apparatus for pre-drying the gases.

In this connection investigations have been made into the conditions of condensation, and it was found to be important that the condensation should be eected Within a. range of temperature, the upper limit of which is the dew point of the gases, and the lower limit lies at about 140 C. and that the cooling within this range of temperature should be performed as uniformly as possible.

The subject of the present invention is a further improvement in the measures to be employed in the condensation of the sulphuric acid with the object of simplifying the process and reducing the cost of the requisite apparatus. Since the cooling period must not be unduly abbreviated, a certain amount of care is needed in the selection of the cooling medium. For this reason attention has hitherto been primarily directed to indirect cooling by means of air7 or direct cooling with small quantities of high-grade sulphuric acid.

Success has now been attained in developing a heat exchanger, in which indirect cooling can also be eiected with cooling liquids. In order more clearly to understand the invention, reference is made to the accompanying drawings, which illustrates dagrammatically and by way of example, one embodiment of apparatus suitable for carrying out the invention and in which;

Figs. 1 and 3 are longitudinal sections, and;

Fig. 2 is a cross section through theapparatus on line 2 2 of Fig. 1.

Said apparatus consists of an outer shell a of suitable material, preferably sheet iron having inlets and outlets m and n, and tubes B disposed therein so as to leave small intermediate spaces. 'I'hese tubes-and this is the most important feature-must be constructed of two tubes of different materials which are preferably spaced 10 apart and the space Afilled with a filling material e. The outer tube c, facing the cooling liquid, should be of very solid and stable material,` in order to be able to stand all the mechanical. stresses occurring. On the other hand, it need is not be corrosion-proof since it only comes in contact with the external cooling liquid. The inner tube d, on the other hand, must be composed of ceramic material capable of withstanding condensing and hot concentrated sulphuric 20 acid, in order tov cope with the chemical stresses that are set up. Ferro-silicon, or high-silicon steel tubes have been found'suitable for the purpose. 'I'his material must also be a poor conductor of heat in order to retard the transmis= 25 sion of heat to the outside, this being liable to occur to an undue extent when cooling liquids are employed. It is true that such vessels have already been constructed of metal with an interior lining of ceramic material, and have been 80 exposed to temperatures of up to 100 C. but such vessels have never before been employed for heat exchanging.

The combination of these two structural materials is of special importance for condensing $5 sulphuric acid.

The two materials are preferably connected together by means of a mass e in order to ensure increased stability and prevent insuiiicient heat transmission. For this purpose suitable metallic 40 alloys-such as lead-antimony alloysor an acid-proof cement which provides a certain elasticity of support for the ceramic material and exhibits a fair degree of porosity, may be used. However, powders such as powdered metals or fine sea sand, have been found substantially more suitable as iilling material, because they allow relative displacement to occur between the tubes.

It is to be observed that by means of the pres ent invention it is possible to use successfully l liquid cooling media, such as oil and water, to effect slow uniform and controlled cooling of the gases containing sulphuric acid fumes in the aforementioned critical temperature range. namely from about 340 C. to about 140 C., and 55 phuric acid fumes without the formation of mist. As more fully described hereinabove, the formation of sulphuric acid mist can be prevented if the gases containing the sulphuric acid fumes are cooled from the dew point of the fumes to a temperature of about C. at a slow, uniform and controlled rate. Heretofore, it was possible to effect a retarded cooling of the gases by means of air or of other gaseous cooling medium having low specific heat and poor heat carrying capacity. Attempts to use liquid cooling media, such as water or oil, resulted in, a. too rapid cooling of the gases and in the formation of a sulphuric acid mist, due to the higher specific heat and to the better heat exchange properties of the liquids. The use of gaseous cooling media. however, presented certain diiiculties and shortcomings, which necessitated the use of large contact cooling areas and which rendered cooling of the gases diicult to control at a uniformly slow rate. Attempts to use liquid cooling media in conjunction with a refractory material having low heat conductivity as the heat exchanging medium were not entirely successful due to the fact that the refractory materials in general possessed a low coefficient of expansion and the thermal stresses set up bythe hot gases on one side and by the cool liquid on the other side caused the refractory material to crack.

We have discovered that' controlled and slow cooling of the gases containing sulphuric acid fumes can be carried out successfully by` means of liquid cooling media, such as water or oil, if we interpose between the gases to be cooled and the cooling medium a refractory material contacting the gases, and a metal Yhaving high heat conductivity such as steel, contacting the cooling liquid. Preferably a filling material is interposed between the metallic material and the refractory material the said lling material (for instance dry powder or sand as'more fully explained hereinabove) allowing free and unrestrained expansion of the tubes. By observing the foregoing conditions we'are able to use refractory material having low heat conductivity in conjunction with more eiiicient liquid cooling media and We can provide better control of a slow and uniform rate of cooling of the hot gases to condense out the sulphuric acid fumes contained therein without the formation of sulphuric acid mists inherent to the quenching or uncontrolled rapid cooling of the gases. Furthermore, we can reduce the size of the apparatus considerably and we can increase the life thereof by being able to use successfully refractory and acid resisting material of low heat conductivity in contact with the hot gases and thel condensing sulphuric acid. Moreover, the outer metal tube which is capable of transmitting heat quickly and freely removes the desired heat from the hot condensing gases via the refractory tube. TheV filling material, such as sand. transmits the heatfrom the outer surface of the refractory tubes to the inner surface of the tubes, but due to the heat retaining properties of sand there is a tendency to preserve thermal stability in the system and to maintain thermal conditions once they have been established.

The foregoing proposal to employ ceramic material for the inner tubes and firmer material for the outer tube, is merely by way of preference, since, as inthe case of any heat exchanger the conditions may be reversed and the cooling liquid passed through the tubes, while the condensation proceeds on the outside. Such an ar- 2,017,676 vto condense eiciently substantially all the sulrangement, however, is more complicated because in such event, the walls of the heat exchanger also must be faced with acid-resisting material.

The gases may be passed through the heat exchanger either in parallel ow or counteriiow. The latter method is preferable, because it enables concentrated sulphuric acid to be drawn off when a considerable excess of steam is present. In such event, the moisture-laden gases from the contact apparatus would pass through the tube g into the collecting chamber h with an intake temperature of about 340 C. and ascend through the tubes provided with fillers. The sulphuric acid condensing in the meantime, collects in the chamber h and is drawn oi through the syphon seal i. The gases freed from sulphuric acid are led away tothe stack through the tube k. For reasons connected with the minimum possible range of temperature it is of advantage to allow the cooling medium used to become heated to high temperatures. On this account, oils of high boiling point or boiling-hot water should be used as the cooling medium. The difference between the temperatrues of the gas and the cooling water can be kept very low when the condenser is constructed to stand pressure like a steam boiler. In such case, the temperature of the cooling water can be raised to about C.

The reinforcement of the ceramic material is again the condition precedent for this methodI of operating.

The resulting possible generation of steam (about 1.5 tons per ton of H2SO4 with a pressure of up to 4 atmospheres) provides a novel means of reducing the cost of sulphuric acid manufacture.

We \claim:

l. A condensing tube for condensing sulphuric acid which comprises an inner tube of refractory material, an outer tube of a metal capable of transmitting heat freely and structurally independent of said inner tube, and a layer of loose, dry, solid material interposed between said tubes and adapted to transmit heat from said inner tube to said outer tube and to maintain thermal stability.

2. A Vcondensing tube for condensing sulphuric acid which comprises an inner tube of refractory material, an outer metallic tube capable of transmitting heat freely and structurally independent of said inner tube, and a thin layer of dry sand interposed between said tubes and adapted to transmit heat from said inner tube bility.

3. A sulphuric acid condenser which comprises an outer jacket, a plurality of elongated, narrow refractory tubes vertically disposed within said' jacket, a plurality of metallic tubes surrounding said refractory tubes, a thin layer of sand interposed between said metallic tubes and said refractory tubes, means for passing gases containing sulphuric acid fumes through said refractory tubes, and means for circulating a cooling medium within said jacket and around said metallic tubes.

4. AA sulphuric acid condenser which comprises an outer cooling jacketadapted to be filled with a liquid cooling medium, a lower condensate collecting chamber, a lower head for separating said jacket from said condensate collecting chamber, an upper gas outlet chamber, an upper head for separating said gas outlet chamber from said chamber, a plurality of narrow elongated refractory tubes vertically disposed within said chamfber and connecting said upper chamber with said lower chamber, a plurality of metallic tubes surrounding said refractory tubes, a thin layer of sand interposed between said refractory tubes and said metallic tubes, means for circulating cooling water within said cooling jacket and around said metallic tubes, means for feeding gases containing sulphuric acid fumes into said lower chamber, means for withdrawing gases substantially free from said acid fumes from said upper chamber, and means for withdrawing sulphuric acid condensate from said lower chamber.

5. A sulphuric acid condenser which comprises an outer jacket, a plurality of elongated, narrow refractory tubes vertically disposed within said jacket, a plurality of metallic tubes surrounding said refractory tubes, a thin layer of sand interposed between said metallic tubes and said refractory tubes, means for passing gases containing sulphuric acid fumes through said refractory tubes, and means for circulating a liquid coolingmedium within said jacket and around said metallic tubes counterow to the direction of said gases.

6. The process of condensing sulphuric acid which comprises -passing gases containing sulphur trioxide fumes and water through refractory tubes surrounded by a layer of sand and an outer metallic tube, and cooling the outer peripheral surface of said metallic tube with water to effect condensation of said acid fumes.

CONWAY BARON voN GIRSEWALD. WOLFHART SIECKE. MAX WOHLWILL.

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