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| Liquefaction rate: 300t/day |
| Liquid hydrogen pressure : 0.11MPa |
| Para hydrogen concentration : more than 95% |
Utilization of nitrogen
For liquid nitrogen or cold nitrogen gas used in the liquefaction process as auxiliary cold, equivalent energy of nitrogen is treated as power consumption. Nitrogen condition is fixed to be 0.12MPa and 78.9K(saturated temperature), and equivalent energy is 1.44kW per g/s of liquid nitrogen and 0.39kW per g/s of saturated nitrogen gas.
Compressor
The adiabatic efficiency of compressor is assumed to be 80%.
Turboexpander
The adiabatic efficiency of the turboexpander is assumed to be 85%. The generated power by the turboexpanders is assumed to recovered, with efficiency of 90%. The application of two phase expanders(wet expanders) is not considered.
Ortho-para convertor
Mainly continuous ortho-para conversion is applied, isothermal and adiabatic conversion are also applied. Conversion efficiency is assumed to be 100%(equilibrium on that temperature).
Boil-off gas(BOG)
Evaporating ratio for a liquid hydrogen tank of 50,000m3 is assumed to be 0.1%/day and boil-off gas(BOG) is recovered to the low pressure line of the process.
Pressure drops and heat in leak
These are ignored this time.
To be compared with hydrogen, neon is approximately 10 times larger in atomic weight and has near boiling point, to that of hydrogen. To considered the application of a centrifugal compressor for both feed gas and recycle gas, larger atomic weight is preferable. Therefore, neon can be considered to be suitable for hydrogen liquefier except that the boiling temperature is slightly higher than that of hydrogen.
The Hydrogen Claude cycle studied as the base process is shown in Fig.1. The feed gas is compressed to approximately 5.07MPa, cooled-down near 80K by liquid nitrogen, and is installed at the same time, converted isothermally to 47.7% para hydrogen. A supercritical turboexpander(T4) in the feed line and its outlet pressure is limited to be minimum 1.28KPa(critical pressure). A warm temperature turboexpander(T1) and a cold temperature turboexpander(T2) is installed in recycle line. In the temperature range between 80 and 30K, ortho-para hydrogen conversion is done continuously. By a final expansion at a J.T.valve liquid hydrogen is produced at 0.11MPa, 20.4K, with para concentration 99.8%.
The Neon Brayton cycle studied as the base process is shown in Fig.2. With respect to the pressure of the feed gas and installation of T4, this cycle is the same as Hydrogen Claude cycle. But in this cycle there are middle and cold temperature turboexpanders (T2,T3) in series installed in the recycle line.
In addition to above-mentioned process, a process with a cold pump is the recycle line is shown Fig.3. The pump outlet pressure and temperature are 0.05MPa, 25K respectively.
The result is indicated in Table1 with those of Case1 and Case2. It is indicated that this process has higher efficiency than Case1.
A process in which reverse ortho-para conversion is applied in BOG recovery line is shown Fig.4.
The result is indicated in Table1. The efficiency is slightly improved. It is to be noted that the reverse conversion is effective when recovered para hydrogen flow rate is large.
Several hydrogen liquefaction processes of 300t/day have been studied. Each process has over 40% Carnot efficiency based on assumed condition.
The cold pump installation is effective to improve the process efficiency in Neon Brayton cycle and when the flow rate of BOG recovery line is relatively large, reverse ortho-para conversion will be also be effective.