This disclosure relates to a processing that includes a first shell and a second shell disposed within the first shell. The second shell includes a first end, a second end, and a wall extending between the first end and the second end. The second shell also defines a cavity and a longitudinal axis extending between the first end and the second end. A cross section of the second shell transverse to the longitudinal axis includes a first arcuate inner wall portion having a first radius of curvature and a second arcuate inner wall portion having a second radius of curvature. The first radius of curvature is larger than the second radius of curvature.