Simulated moving bed xylenes separation unit, called an SMB unit, comprising two adsorbers and connected in series, i.e. with the circulation current originating from the bottom of adsorber sent to the top of adsorber and with the circulation current originating from the bottom of adsorber sent to the top of adsorber, said unit comprising a distillation column fed by the raffinate (RAF B) originating from the adsorber and said unit comprising a device for measuring concentration by analyzing a Raman spectrum, said measuring device comprising a Raman spectrometer for measuring concentrations at different sampling points, characterized in that said measuring device is constituted by two identical and independent routes (AR1, AR2), called analysis route 1 and analysis route 2, each analysis route (AR1, AR2) having two branches (MP, SB), one called main branch being configured for operating for a majority of the time, the other called the subsidiary branch being configured for operating for a minority of the time, and the sampling points being defined as follows: - for analysis route 1 connected to the internal circulation current going from adsorber to adsorber, a) internal circulation current sent from adsorber to adsorber, b) extract (EX A, EX B) originating from one or other of the adsorbers (A, B), c) raffinate (RAF A, RAF B) originating from one or other of the adsorbers (A, B), - for analysis route 2 connected to the distilled raffinate (RAF DIST) originating from distillation column, d) distilled raffinate (RAF DIST) leaving distillation column, e) internal circulation current from adsorber to adsorber, f) feed of the unit at the inlet of one or other of the adsorbers (A, B), said two branches (MP, SB) being distinct and configured for operating with mutually exclusive operation, branch 1, called the "main branch" being used for the Raman analysis and branch 2, called the "subsidiary branch" being used for sample collection, and branch 1 of each analysis route (AR1, AR2) comprising the following main components taken in the direction of the stream to be analyzed: - a shut-off valve making it possible to force the circulation into branch 2 in certain situations, - a thermocouple making it possible to take the temperature of the sample to be analyzed, - an analysis cell containing an optrode, receiving an input laser signal and sending an output signal to the Raman spectrometer, the internal volume of said analysis cell being of the order of 1 cm 3, - a second shut-off valve for isolating the analysis cell in the event of maintenance. - for analysis route 1 connected to the internal circulation current going from adsorber to adsorber, a) internal circulation current sent from adsorber to adsorber, b) extract (EX A, EX B) originating from one or other of the adsorbers (A, B), c) raffinate (RAF A, RAF B) originating from one or other of the adsorbers (A, B), a) internal circulation current sent from adsorber to adsorber, b) extract (EX A, EX B) originating from one or other of the adsorbers (A, B), c) raffinate (RAF A, RAF B) originating from one or other of the adsorbers (A, B), - for analysis route 2 connected to the distilled raffinate (RAF DIST) originating from distillation column, d) distilled raffinate (RAF DIST) leaving distillation column, e) internal circulation current from adsorber to adsorber, f) feed of the unit at the inlet of one or other of the adsorbers (A, B), said two branches (MP, SB) being distinct and configured for operating with mutually exclusive operation, branch 1, called the "main branch" being used for the Raman analysis and branch 2, called the "subsidiary branch" being used for sample collection, and branch 1 of each analysis route (AR1, AR2) comprising the following main components taken in the direction of the stream to be analyzed: d) distilled raffinate (RAF DIST) leaving distillation column, e) internal circulation current from adsorber to adsorber, f) feed of the unit at the inlet of one or other of the adsorbers (A, B), said two branches (MP, SB) being distinct and configured for operating with mutually exclusive operation, branch 1, called the "main branch" being used for the Raman analysis and branch 2, called the "subsidiary branch" being used for sample collection, and branch 1 of each analysis route (AR1, AR2) comprising the following main components taken in the direction of the stream to be analyzed: - a shut-off valve making it possible to force the circulation into branch 2 in certain situations, - a thermocouple making it possible to take the temperature of the sample to be analyzed, - an analysis cell containing an optrode, receiving an input laser signal and sending an output signal to the Raman spectrometer, the internal volume of said analysis cell being of the order of 1 cm 3, - a second shut-off valve for isolating the analysis cell in the event of maintenance.