The present invention relates to the use of an optical line detector for regulating the position of the wash front andor of the build-up front of the crystal bed of a wash column in a melt crystallization process and a corresponding regulation method. Methods and systems consistent with the present invention manage a call in real-time based on input from a user. An alignment pin having a cylindrical shape has a reduced diameter groove near the rear portion thereof which has a diameter slightly less than the transverse dimensions of the restricted passageway and forming front and rear shoulders, spaced a distance slightly greater than the length of the stop members.