LOW PURITY OXYGEN PRODUCTION PROCESS
BY UTILIZING LIQUID HYDROGEN COLD ENERGY

Y. Murata
Nippon Sanso Corp., Kojima-cho, Kawasaki-ku, Kawasaki-city 210

Abstract�|In WE-NET project, the power generation system by the combustion of hydrogen has been studied. In this system, hydrogen is transported in the liquid condition, vaporized and combusted with oxygen. In order to reduce the specific power consumption of oxygen, cryogenic air separation processes to produce required oxygen with the utilization of the hydrogen cold energy has been studied.
Considering required purity level of oxygen, which is assumed to be typically 96 vol.%, we have conducted the comparison of the specific power consumption of oxygen between the conventional double column process, the single-column process and the double-column process, both utilizing hydrogen cold energy. It is assumed that the liquid hydrogen can be used to condense the nitrogen gas. For the production of low purity oxygen at approximately atmospheric pressure, we can reduce the specific power consumption of oxygen by about 18% in the single-column process and by about 29% in the double-column process. For the production of the low purity oxygen at 1.08MPa, we can reduce the specific power consumption of oxygen by about 42% in the double column process utilizing hydrogen cold energy.

At present we depend on the fossil fuel as the energy source. In considering depletion of fossil fuel and effect to the environment, we must change the energy source in the near future. In WE-NET project, the energy supply system based on hydrogen has been investigated. In this project, hydrogen is produced by electrolysis utilizing renewable energy such as solar energy and geothermal energy. The hydrogen is liquefied, transported, stored, and used as a fuel with oxygen gas by a combustion turbine for power generation. Hydrogen can be applied not only for such massive power generation but also for distributed utilization such as fuel for transport, town gas and other fuel in smaller scale. In every system, the combination of hydrogen and oxygen is required to achieve clean energy cycle. As described above, hydrogen is transported in liquid form, of which cold energy can be effectively utilized.
In this paper, we have studied a cryogenic distillation plant to produce oxygen required in such a hydrogen system, of which required cold energy is supplied by liquid hydrogen to reduce specific power consumption of oxygen. As known[1], the specific power consumption is depend on not only the size of the cryogenic distillation plant but also the oxygen purity level. If hydrogen energy system requires not so high purity oxygen, there can be a possibility to develop an improved cryogenic distillation process utilizing the cold energy of liquid hydrogen.

In subtask 7 of WE-NET project, cold energy applications of liquid hydrogen for oxygen production has been studied. Table 1 shows required oxygen specifications for small scale hydrogen applications with output power generated by the applications. Though the cryogenic distillation process is effective for producing large amount of oxygen, applications in Table 1 do not require such a large amount of oxygen. In this paper, the cryogenic distillation process for case3, which is the largest in scale among these, has been investigated. Through this paper, the required pure oxygen flow rate shall be 1651Nm³/h and the available liquid hydrogen flow rate shall be 3302Nm³/h which is combusted by the oxygen of amount above.

A conventional cryogenic air distillation process is first studied for the comparison with the process utilizing hydrogen cold energy. The schematic diagram of this process is shown in Fig. 1. This process is based on the double-distillation column to recover oxygen from feed air to the maximum. Feed air has to be liquefied for cryogenic distillation. Because there are several thermal energy losses such as warm end loss of main heat exchanger and heat in leak to the process, cold energy has to be generated by an expansion turbine, through which a part of feed air is expanded. The oxygen has to be compressed to the required pressure, because the produced oxygen pressure is low.

In the study of utilization of liquid hydrogen cold energy, only heat transfer is considered. Other factors such as the solidification of the fluid are not considered.

1) Single-column process
A single-column process utilizing liquid hydrogen cold energy is studied. The schematic diagram is shown in Fig. 2. This process is improved with respect to the following points;
a) A part of required nitrogen reflux is produced by a heat exchange with liquid hydrogen. Remaining part of nitrogen reflux is supplied from a reboiler/condenser at the bottom of the column where nitrogen gas is condensed after recompression by a nitrogen recycle compressor.
b) To make ascending vapor flow rate and descending liquid flow rate to be optimum for the required oxygen purity( 96%O₂), a part of feed air at low pressure is directly supplied to the column. If high pressure feed air and low pressure feed air are compressed and purified independently, individual pre-purification facility is required for each feed air. Especially lots of energy is required to remove impurities such as water and carbon dioxide from the low pressure air. To avoid this, we have arranged that the purification is conducted in one pressure and the feed air is divided after the purification. One part of the feed air is expanded by an expansion turbine and remains is compressed by this expansion work.
c) As all of the cold energy supplied by liquid hydrogen is not utilized in the nitrogen condenser, the excess cold energy is available near the room temperature region. In order to utilize this excess cold energy, the nitrogen is withdrawn at low temperature from the main heat exchanger and warmed in a pre-cooler to cool the feed air to the required temperature for pre-purification. On the other hand, as the nitrogen gas leaves pre-cooler at high temperature, this can reduce required power for the regeneration of pre-purification unit.

2) Double-column process
A double-column process utilizing liquid hydrogen is shown in Fig. 3. This process is developed to reduce the specific power consumption of oxygen more than the single-column process. In the single column process, the shortage of the nitrogen reflux is compensated by the nitrogen recycling. However, as the required reflux can be supplied from the high pressure column in this process, the nitrogen recycle compressor is not necessary and therefore the specific power consumption of oxygen can further be reduced.
In the single column process, the nitrogen heated up at the compressor has to be cooled down by heat exchange with the hydrogen gas in the main heat exchanger. In this process, however, much more excess cold energy is available because the nitrogen recycle compressor is omitted as described above. In order to utilize this excess cold energy, we have studied the process where the liquid oxygen is withdrawn from the bottom of the upper column, pumped up to the required high pressure and vaporized. The schematic diagram of this process is shown in Fig. 4. As shown in Fig. 4, an additional low temperature air compressor is installed next to the cold compressor which is driven by the expansion turbine. A high pressure feed air from this low temperature air compressor is used as a warm fluid to vaporize the liquid oxygen, and the heat of the compression at the low temperature air compressor is removed by the heat exchange with hydrogen gas.

1. By the air distillation process utilizing the liquid hydrogen cold energy, to reduce the specific power consumption of low purity oxygen is possible. Especially if high pressure oxygen is required, the utilization of the cold energy is more effective.
2. Only about 50% of the hydrogen cold energy is recovered in the air distillation process. It is shown that more than 50% of the utilized hydrogen cold energy is used in the nitrogen condenser at the top of the column.