Waste sulfuric acids and acid tars or acid sludges, are obtained from many processes used in the organic chemical industry, particularly the petrochemical industry in which the processing of oils and other materials leads to the formation of such wastes. Furthermore, during the processing of solid coal tar, as well as in the manufacture of intermediate chemical products in the synthetic chemical and dyestuff industries, sulfur-containing organic compounds are obtained as by-products or waste products. Those by-products and waste products include higher alkyl mercaptans and/or aryl mercaptans and thioether mercaptans.
Great difficulties are involved in processing, or destroying, sulfur-containing waste products in a way which is not detrimental to the environment but which permits recovery of the sulfur and utilization of the thermal energy potential in such products. Many of the difficulties are caused by the greatly varying physical and chemical characteristics of the products as, for example, their consistency, viscosity, heat of combustion and heat of decomposition or separation.
In the past, sulfur-containing by-products and waste products, usually after neutralization, have been disposed off in sewage or sanitary lines, or by discharge into rivers and coastal waters. Because of the increasing amounts of such wastes and their harmful effect on the environment, such disposal is no longer permitted in most parts of the world. However, disposal of sulfur-containing wastes by the described method continued for a long time, and not until recently has that method of disposal been stopped, even though processes of treating the waste products were known.
One method of processing sulfur-containing wastes is disclosed in German Pat. No. 626,481. That patent discloses mixing waste sulfuric acids with solid particles, such as sand, spent Fullers earth and/or granulated coke, heated to 300.degree. to 350.degree. C. in special furnaces to decompose the waste acids. Use of the solid particles, which must be recirculated in the process, is detrimental since they cannot be completely removed from the product reclaimed from the waste acids.
Another method, disclosed in German Pat. No. 960,184, sprays waste sulfuric acids alone, or mixed with acid tars, into a vertical or horizontal cylinder to decompose them. Gaseous or liquid fuels, and possibly also sulfur, are used to maintain a suitable heat balance. A disadvantage of this method is that only relatively low viscosity waste acids of uniform composition can be used. Highly viscous acid tars must first be mixed with concentrated sulfuric acid and the mixture then fed into the furnace. However, it is first necessary to separately prepare the highly concentrated sulfuric acid by evaporation of dilute waste acid.
Another previously known method is to decompose the waste acid by contacting it with a gas, which can be up to 75% water vapor, heated to about 430.degree. to 480.degree. C., and then cooling the reaction gas mixture to 15.degree. C. In this way, water as well as the non-decomposed hydrocarbon vapors were to be condensed out, leaving a gas which was to be 95% sulfur dioxide. A disadvantage of this method is that hydrocarbon dissolve in sulfur dioxide so that separation of these two components is about impossible. Further processing of the gas into 100% pure sulfur dioxide, or into sulfuric acid, encountered insurmountable difficulties.
Another process which failed involved processing acid sludge and waste sulfuric acids to obtain sulfur dioxide with a high percentage purity, or sulfuric acid. In this process, waste acids were continuously mixed with 15 to 30 times as much ground coke previously heated to 230.degree. to 300.degree. C. The coke was first heated, directly and separate from the waste acids, and transported hot by means of worm conveyors and lifting conveyors. Another disadvantage of the process was the extreme wear which occurred on the conveyors due to the strong corrosiveness of the coke.
None of the described processes was successful even though repeated substantial efforts were made in industrial process engineering to control the waste acid heating and reclaiming process and to free it of problems.
One must not overlook the fact, in developing a reclaiming process, that the carbon content of waste sulfuric acids and acid tars varies. The amount of additional heat needed for their decomposition will be higher or lower, depending on the carbon content in the waste product. However, excess heat can be produced during decomposition of the waste product and this, above all, is true when acid sludge and acid tars are decomposed.
Even when combustible materials, such as heating oil, gas or elemental sulfur, are used in the decomposition process often, due to too little heat, incomplete decomposition results and the sulfuric acid or SO.sub.3 which form first are not further decomposed. However, a surplus of carbon causes the formation of coke in the waste acids and that coke prevents complete combustion of the hydrocarbons in the separation gas. This coke only burns reluctantly and does not supply the heat required for the separation process.
Efforts have been made, as see German Pat. No. 627,327, to use two rotary furnaces in series and to effect the separation, as well as combustion of surplus coke, in separate chambers and to effect heat transfer by radiation. This method of operation was unsuccessful.
A further problem involved in reclaiming waste acids and acid tars is due to the extremely variable consistencies and viscosities of these products. It is almost impossible to spray all of them into a combustion chamber with a sufficiently fine distribution so that the fine droplets completely react in suspension or completely decompose themselves. Also, preheating of acid tars and acid sludge, and the use of high spraying pressures, does not result in a sufficiently fine distribution necessary for decomposition in suspension, even though spraying is presently an indispensible prerequisite in all common waste acid treatment processes. Consequently, the qualitative scope of the waste sulfuric acid reclaiming methods is limited to those acids which have a uniform viscosity and a uniformly high carbon content.
Another problem common to all previously known acid separating processes which use added oil or gas and operate in the range of 1000.degree. C. to 1200.degree. C., is the formation of nitrous oxides. The nitrous oxides contaminate fresh sulfuric acid formed from the released separation gases. Moreover, when sulfur dioxide reacts with nitrous oxides in a "lead chamber reaction," the formation of sulfuric acid vapors is favored. These vapors, which are hard to absorb or cannot be condensed, cause an increase in damaging emissions from sulfuric acid factories.
From the above it is believed clear that there is a need for a new or improved method, and apparatus, for reclaiming sulfuric acids, acid tars and similar waste products, containing sulfur and carbon, of varying composition, consistency and viscosity, and with varying content of carbon, sulfuric acid, and sulfur in other combined forms, together or in quick succession, in one single process which can be carried out continuously and is easily controllable. In addition, it would be highly desirable to be able to employ a multi-stage combustion furnace for the waste acid recovery and to effect therein complete decomposition of all of the sulfur trioxide which is formed early in the acid separation, complete combustion of all hydrocarbons which are converted to gases, as well as a minimum formation of nitrous oxides.