Hydrogen is a gas at standard temperature and pressure and is the most common element in the universe. However, free hydrogen is extremely rare on Earth because it evaporates so easily in space. Liquid hydrogen is most commonly used as a rocket fuel where it's burned with liquid oxygen and it also has applications in cryogenics as a coolant. Liquid hydrogen is also a useful means of storing and transporting hydrogen because it takes up less room than it does as a gas. Hydrogen gas can be liquefied by applying the correct combinations of pressure and cooling.
Identify the critical temperature for hydrogen as 33 degrees Kelvin. This is the maximum temperature at which hydrogen can be a liquid, no matter how great the pressure is. The process of liquefying hydrogen must therefore get it below 33 degrees Kelvin.
Determine the critical pressure for hydrogen as about 13 atmospheres (atm). This is the minimum pressure needed to keep hydrogen a liquid at its critical temperature. These critical points provide the parameters for keeping hydrogen a liquid.
Examine the regenerative cooling process. This method pressurizes gas and allows it to expand. This allows the gas to take heat from its environment, thus cooling it. The gas is then passed through a heat exchanger, which cools the gas, thereby compressing it. This process is repeated until the gas cools enough to liquefy.
Apply the regenerative cooling process to liquefy hydrogen as first performed by James Dewar in 1898. Pressurize the hydrogen to 180 atm and pre-cool it with liquid nitrogen. Allow the hydrogen to expand through a valve that is also cooled by liquid nitrogen.
Repeat Step 4 until the hydrogen liquefies. Dewar's experiment yielded about 20 cubic centimeters (CCs) of liquid hydrogen, which was about 1 percent of the hydrogen in the experiment.