A process for maintaining the fluid-like properties of a heavy phase being withdrawn from a separation zone maintained at elevated temperature and pressure during periods in which the flow of this heavy phase is terminated due to pressure reduction valve plugging or to permit cleaning of downstream processing apparatus. When the flow of withdrawn heavy phase is interrupted, deashed coal is melted, if necessary, and introduced under pressure into the withdrawal conduit to mix with the heavy phase therein. The mixture of deashed coal and heavy phase retains the fluid-like properties exhibited by the heavy phase and flow can be resumed once the downstream apparatus is repaired. Alternatively, other high-boiling aromatic materials may be used instead of the deashed coal.