Abstract: The present invention describes a method for measuring the concentrations of species present at one point of a separation unit functioning in simulated moving bed mode (SMB), using an immersing probe located at one point in the unit or on one of the streams entering or leaving said unit, and a thermocouple located in the vicinity of the immersing probe, in which a Raman spectrum obtained using a laser source functioning at a wavelength of 785 nm is utilized.

Abstract: The present invention describes a method for measuring the concentrations of species present at one point of a separation unit functioning in simulated moving bed mode (SMB), using an immersing probe located at one point in the unit or on one of the streams entering or leaving said unit, and a thermocouple located in the vicinity of the immersing probe, in which a Raman spectrum obtained using a laser source functioning at a wavelength of 785 nm is utilized. Application of the method to controlling and regulating a SMB xylenes separation unit.

Abstract: A monochromatic light signal of a wavelength of between 400 and 1300 nanometers is simultaneously passed by a source 1, a beam splitter 2 and optical fibers 3, 3a to at least two points of a zone 9 for the separation of a mixture of isomers and aromatic hydrocarbons having from 8 to 10 carbons atoms. A polychromatic signal is recovered corresponding to the Raman effect between 400 and 3500 cm.sup.-1 by optical receiving fibers 5, 5a connected to sensors 4 at the locations of said points. The chemical composition of the mixture is determined from the Raman spectra obtained by a spectrometer 6. The analysis sequence is repeated, the profiles with respect to the concentration of the isomers in the mixture are reconstituted and by comparison with reference concentration profiles, action is taken on at least on operational variable 11, 12 of the separation process.