Abstract: A process for the preparation of 4-nitrotoluene-2-sulfonic acid by sulfonating 4-nitrotoluene with oleum or liquid SO.sub.3 is described. The sulfonation is carried out continuously with oleum of at least 50% concentration or liquid 100% SO.sub.3 while keeping the conversion of 4-nitro-toluene in the reaction mixture at .gtoreq.90% during the entire reaction course.The process is inherently non-polluting, and provides the product in high yield and low sulfuric acid content in a form usable for 4,4'-dinitrostilbene-2,2'-disulfonic acid production without further purification.

Abstract: The heat exchange mantle of a reaction vessel is connected with a circulating pump and a chamber provided with an electrical heating coil to form a heat exchange circulation system. To this main circulation system, a cooling circulation system comprising a circulating pump and a heat exchanger is connected by way of a continuously operable valve. The temperature of the medium circulating in the main circulation system can be very effectively and rapidly influenced by dosed addition of cooled circulation medium from the cooling circulation system into the main circulation system as well as by heating with the electrical heating coil. In the cooling circulation system, in the reaction vessel and in its mantle, temperature sensors are arranged which are linked to a control computer. The computer controls the electrical heating coil and the valve in such a manner that the temperature in the reaction vessel or in its mantle, respectively, has a desired distribution prescribed by the operational mode.

Abstract: An apparatus for determining the thermal efficiency of a chemical reaction wherein a reaction vessel is equipped with a double-wall jacket or shell which forms a heat exchanger, a heat transfer fluid medium is circulated so rapidly that the difference of its temperature at the inlet and at the outlet of the shell throughout the entire duration of the reaction, with the exception of possibly occurring momentary irregularities in the reaction kinetics, does not exceed 1.degree. C. A temperature regulator is provided which is constructed as a mixture regulator and encompasses a respective container for heat transfer fluids which are hotter and colder with respect to the temperature of the heat transfer fluid which is circulated in the circulation system and each such container is operatively connected with the circulation system as a function of the reference value deviation of the temperature of the reaction mixture.

Abstract: An apparatus for controlling the temperature of a heatexchange liquid in accordance with the mixing principle, the apparatus including a centrifugal pump connected to a drive motor for conveying or circulating the liquid in a working circuit, two containers for the liquid in a shunt circuit connected via respective shut-off devices to the working circuit, means for maintaining the temperature of the liquid in one container at a given value below the temperature in the working circuit and the temperature of the liquid in the other container at a given value above the latter temperature and a mechanical propeller-like circulating means is each container, the centrifugal pump being disposed between the two containers and having a pump shaft which extends into one of the containers and piercingly extends completely through the other container and is connected to the drive motor, said motor being arranged externally of the latter container, and the propeller-like means being mounted on the two portions of the pump