This paper describes the design of a multivariable robust controller for a drying process. The plant consists of a co-current rotary dryer to evaporate moisture from a waste product, called alpeorujo or two phase cake, generated by olive-oil mills. A methodology to design a multivariable H_∞ controller is shown, including the selection of appropriate weighting functions for a mixed sensitivity control formulation. The performance obtained with this control strategy has been validated under simulation, using a very accurate nonlinear simulator developed in Dymola.