Pengendalian Proses Purifikasi Dimetil Eter pada Campuran Metanol menggunakan Multivariable Model Predictive Control (4x4) dengan Model Auto-Regressive eXogenous = Dimethyl Ether Purification from Methanol Process Control with Multivariable Model Predictive Control (4x4) and Auto-Regressive eXogenous Model