A reactor is provided for reacting a liquid with a treat gas in the presence of a catalyst, the reactor comprising a continuous wall enclosing a first reaction zone, wherein the first reaction zone includes a catalyst for causing a desired reaction between the liquid and the treat gas; a liquid inlet above the first reaction zone for allowing a portion of the liquid to enter the reactor; a gas inlet below the first reaction zone for allowing a portion of the treat gas to enter the reactor; a liquid outlet below the first reaction zone for allowing a reacted portion of the liquid to exit the reactor; a gas outlet above the first reaction zone for allowing a portion of the treat gas to exit the reactor; and a gas bypass device in the first reaction zone for allowing a portion of the treat gas to bypass a portion of the first reaction zone, the gas bypass device including a gas bypass regulating device for regulating the amount of treat gas which bypasses the first reaction zone. In a preferred embodiment, a plurality of reaction zones are present, and a liquid distribution tray is disposed above each reaction zone. Preferably, the gas bypass device creates a hydrostatic seal with the liquid accumulating in the distribution tray so that the timing and extent of gas bypass is self-regulating based on predefined flooding conditions.