The invention describes a process and apparatus producing a dense coke fraction from a first particulate coke fraction having a first average density and a first average particle size distribution, stratifying the coke fraction in a density separator into at least two fractions, the at least two fractions comprising a light coke fraction and the dense coke fraction. A shoulder of the guard accommodates the shoulder of the gutter hangers. Anodic aluminum oxide membranes serve as the catalyst substrate. Preferably, the receptacle includes a piercing projection disposed in the receptacle adapted to pierce a seal on a container of tire sealant secured in the receptacle. A DAC generates a bias voltage with a small quantization step, additionally providing a fine granularity of gain control.