In one embodiment, a parallel batch reactor generally includes a plurality of reactor vessels and a plurality of valves configured to allow flow into the reactor vessels when a gas is supplied to the reactor at a pressure higher than a pressure within the reactor vessels and restrict flow from the reactor vessels. A method includes inserting chemical components into the reactor vessels, supplying a first gas to the reactor vessels to pressurize the reactor vessels, stopping supply of the first gas, and supplying a second gas to the reactor at a lower pressure than a pressure within the reactor vessels. The second gas passes through the valves and into the reactor vessels when the pressure within the reactor vessels drops below the pressure of the second gas.