How do you design a hydrotreater for deep desulfurization?

Deep desulfurization design: Kinetics - refractory sulfur species (dibenzothiophenes with alkyl substituents) much harder to remove than easy sulfur. Rate follows: -dC/dC = k*C^n*P_H2^m. n varies from 1 (easy sulfur) to 2 (refractory). Catalyst selection - CoMo for easy desulfurization, NiMo better for difficult feeds and denitrification, specialized catalysts for ultra-deep. Reactor design - often two stages: first stage removes bulk sulfur under milder conditions, second stage with fresh catalyst for deep removal. LHSV typically 0.5-2 hr^-1 for diesel, lower for ULSD. Hydrogen partial pressure critical - higher pressure improves kinetics and reduces coking. Temperature limited by product quality and catalyst stability. Bed grading - smaller particles at bottom for deeper desulfurization, larger at top for pressure drop control.