These primarily refer to the reactor's operating temperature and pressure. Temperature is a sensitive factor affecting the reaction process; therefore, a suitable operating temperature or temperature sequence must be selected to ensure the reaction proceeds under optimized conditions. For example, for reversible exothermic reactions, a high-then-low temperature sequence should be used to balance reaction rate and equilibrium conversion (see Chemical Equilibrium).
Reactors can operate under atmospheric pressure, pressurized pressure, or negative pressure (vacuum). Pressurized reactors are mainly used for reactions involving gases. Increasing the operating pressure accelerates gas-phase reactions. For reversible gas-phase reactions with a decreasing total molar number, it can improve the equilibrium conversion rate, such as ammonia synthesis and methanol synthesis. Increasing the operating pressure can also increase the solubility of gases in liquids; therefore, many gas-liquid phase and gas-liquid-solid phase reaction processes use pressurized operation to increase the reaction rate, such as the oxidation of p-xylene.