Abstract: An apparatus unit for the fractional distillation and rectification of liquid mixtures includes a plurality of horizontal superimposed tubular main sections interconnected in series by a plurality of tubular connecting sections, e.g. U-shaped, to define an elongated space along which liquids and vapors flow. Intimate contact between liquids and vapors and efficient heat and mass transfer are provided by a stirrer rotating within the main section about horizontal axes and magnetically driven from the outside. Suitable inlet and outlet ports and heating and cooling devices are provided. An arrangement for joining the adjacent sections without the use of flanges or threaded portions is provided. A number of such units are preferably associated to form a modular apparatus, the individual units being operatively connected as desired to provide series, parallel or mixed flow of the fluids.