The present invention provides thermal oxidizers containing improved preheating designs and processes for improving the preheating of thermal oxidizers. The processes are practiced by preheating the matrix-bed of matrix materials in a flow path that is opposite in direction to the flow path for the processing fluids through the matrix bed. In such a process, there is a substantial reduction in the time and energy required for the preheating of the matrix bed in comparison to the prior processes using same flow direction preheating.