Established Methods Based on Compression and Cryogenics - Hydrogen Generation, Storage, and Utilization - page 85

(a) (b)

FIGURE 5.7 The illustration of the JT inversion curve (reproduced from Barron [11]) and the JT
inversion curves for some conventional gases. Source : Reproduced with permission from Flynn [12]. (See
color insert.)

the gas is further expanded to cool down to the boiling point of hydrogen
using a JT valve or a cryogenic turbine.
The simplest liquefaction process is the Linde cycle, which is shown
schematically in Figure 5.8 along with the T - s (entropy) diagram. Makeup
gas is mixed with the uncondensed gas from the previous cycle, and the
mixture at state 1 is compressed by an isothermal compressor to state 2. The
high pressure gas is further cooled down after passing through a constant-
pressure heat exchanger (ideally) by liquid nitrogen and the uncondensed
gas from the previous cycle to state 3, and is then forced to pass through a
throttle valve to state 4, which undergoes an adiabatic expansion, producing
a saturated liquid-vapor mixture. The liquid (state 6) is collected as the
desired product, and the vapor (state 5) is routed through the heat exchanger
to cool the high pressure gas approaching the throttling valve. Finally, the
gas is mixed with fresh makeup gas, and the cycle is repeated. A simple
Linde cycle may not work for hydrogen at room temperature since its inver-
sion temperature is very low, and physically it may be impossible to transfer
enough energy in the heat exchanger to produce liquid. Therefore, modified
Linde cycles with precooled gases may need, such as precooled Linde cycle,
dual-pressure cycle, or even cascade Linde cycle [11].
The Claude cycle is a common method to liquefy high volume of hydro-
gen, as shown in Figure 5.9. The Claude cycle combines the isentropic
(Brayton cycle) and isenthalpic (Linde cycle) expansions, and both the heat
exchangers and mechanical expanders are used to cool the compressed and
precooled hydrogen below its inversion temperature. As shown in Figure 5.9,
the gas is first compressed, and passed through the first heat exchanger.