In trickled bed reactor gaseous and liquid reactants flow co-currently downward over a packed bed of solid catalyst particles. The liquid is distributed across the reactor cross section by a distributor plate. The gas enters at the top and distributed along with the liquid. The liquid flows downward by gravity and drag of the gas. For low liquid flow rates and low to moderate gas flow rates, the gas phase is continuous with liquid trickling down forming film over the solid catalyst. The thickness of the liquid film has been estimated to vary between 0.01 and 0.2 mm. This flow regime is known as a trickle flow regime. The fluid approaches plug flow leading to higher conversion than slurry reactors for the same reactor volume. Other advantages include ease of installation, minimal catalyst handling and low catalyst attrition as in packed bed reactor. Disadvantages include maldistribution of flow resulting in channeling or bypassing, possibility of non uniformity in packing, incomplete contacting or wetting and intraparticle diffusion resistance. Catalyst bed depth is limited by pressure drop, catalyst crush strength and maximum adiabatic temperature increase for stable operation. The reactor length to diameter ratio can vary between 1 and 10 depending on the allowable pressure drop. Other parameters important for trickled bed include void fraction of bed, holdup for phases, wetting efficiency (fraction of catalyst wetted by liquid), gas – liquid mass transfer coefficient, liquid–solid mass transfer coefficient, liquid and gas mixing, pressure drop and heat transfer coefficients. The wetting efficiency of the catalyst is important for reaction rate and increases with increasing liquid rate. The trickle bed reactor is most commonly used for hydrogenation and hydrodesulfurization reactions.