WE-NET Home Page/Summary of Annual Reports 1998

5. Subtask 5 : Development of Hydrogen Transportation and Storage Technologies

5.1 Development of Large Capacity Hydrogen Liquefaction Facilities

5.1.1 R&D Goals

Goals of studies in the Phase I are that to make a conceptual design of large-scale liquefaction facilities, and to select technical items to be developed necessary for constructing it.

5.1.2 Results in FY 1998

Based on the final trade-off study in the last year, we have chosen a hydrogen Claude cycle as a liquefaction process. To achieve the required process efficiency, adiabatic efficiencies of turboexpanders and compressors are important and especially the hydrogen compressor is the key issue not only because the required efficiency is high but also because the capacity required is far larger than those of existing hydrogen compressors.

To conclude our study of the large hydrogen liquefaction plant in WE-NET phase I, we have conducted a conceptual study of the whole plant including important auxiliary equipment to make a visual image of the plant, and have started preliminary study of a large centrifugal type hydrogen compressor.

5.1.2.1 Conceptual study of equipment

(1) Hydrogen Liquefier
The liquefaction process of 300 t/D is based on a Claude cycle with nitrogen precooling and ammonia refrigeration as auxiliary cold. The pressure conditions are the same as those of the previous study, which are 3 MPa(30 atm) for the feed hydrogen and 0.11 MPa(1.1 atm) to 4 MPa(40 atm) for the recycled hydrogen. Based on the study on the hydrogen turboexpanders and the nitrogen reliquefaction process, the process has been modified as follows:

The generated power by the turboexpander is transferred to a brake compressor associated to the turbine and part of recycled hydrogen is pressurized by these brake compressors while the previous study was based on the process where the generated power was transformed to electric power.
The nitrogen precooling is done with two different pressure liquid nitrogen based on the nitrogen process while only low pressure liquid nitrogen was used in the previous study.
All the heat exchangers are aluminum alloy plate-fin type and for the feed hydrogen stream ortho/para conversion catalyst is filled.
The revised flow is indicated in Fig.5-1-1. The cold-box of the liquefier where cryogenic equipment for the process is installed to insulate heat flow from the ambient is divided into two parts according to the temperature ; for equipment above 80 K its cold-box is perlite insulation type as those adopted in cryogenic air separation units and for equipment below this temperature multilayer vacuum insulation is applied as those helium liquefies.

(2) Hydrogen Turboexpanders
One turboexpander is installed at the temperature of 65 K with an expansion ratio of 2.4 and another at the temperature of 42 K with the expansion ratio of 2.8. Both of them adopt dynamic gas bearings that eliminate immigration of any impurities. The rotational speed is controlled by the brake compressor. The part of the recycled hydrogen is pressurized by these compressors as mentioned in section 5.1.2.1-(1).

(3) Hydrogen compressors
There are three groups of hydrogen compressors; the first group is to compress the feed hydrogen under atmospheric pressure to a suitable pressure for purification, and the second group is to compress the purified feed hydrogen to the final pressure for the liquefaction, and the third group is to compress the recycled hydrogen from 0.11 MPa(1.1 atm) to 4 MPa(40 atm). All the compressors adopt centrifugal compression. The required number of compression stages for each group is 28 for the first group, 8 for the second, and 40 for the third.

(4) Hydrogen Purification Unit
As the feed hydrogen is to be cooled and liquefied by the liquefier, if the feed hydrogen contains gaseous impurities such as water vapor, nitrogen, and oxygen, they will be solidified in the process and accumulated, which leads to performance deterioration of the heat exchangers and blockage of the flow at the worst case. Therefore these impurities have to be removed with a purification unit before the cool-down for the liquefaction. As the purification process, a PSA(Pressure Swing Adsorption) process that has been used in many hydrogen liquefaction plants was chosen this time.

Considering that the feed hydrogen is generated by water electrolysis, we have assumed the impurities and their concentration as follows:

Water vapor Saturated at the temperature

CO₂ 20 vpm

N₂ 400 vpm

O₂ 200 vpm

The operating pressure of 1.5 MPa is chosen because the highest recovery is possible around 1.5 to 2.0 MPa. Since the PSA unit has to handle such a big mass flow as 139,600 Nm³/h, which is equivalent to the liquefaction rate of 300 t/D, we decided to apply two identical PSA units. One PSA unit consists of 4 adsorption beds, each of which repeats adsorption and regeneration with a switch-over operation.

(5) Nitrogen Reliquefier
This system is installed to recover the liquid nitrogen consumed at the hydrogen liquefier and to liquefy it again. The reliquefaction process is based on a nitrogen Claude cycle where the required cold is generated with two nitrogen turboexpanders, as shown in Nitrogen Reliquefaction section of Fig. 5-1-1. Each turbine speed is controlled by its associated brake compressor that boosts the pressure of the nitrogen. The reliquefier is designed to produce the subcooled liquid nitrogen of about 76,000 Nm³/h, which is required at the hydrogen liquefier.

(6) Plant Layout
Based on the study in the previous sections, a rough size of each equipment has been studied to make a conceptual plot plan for the hydrogen liquefaction plant. The studied result is shown in Fig.5-1-2.

The feed hydrogen generated in the electrolysis system, which is not shown in the plot, is assumed to be supplied from the left side of the plot. As shown, the nitrogen reliquefier is separated from the hydrogen equipment. The liquid hydrogen storage is not included in the plot because it will be included in the liquid hydrogen storage and transfer system area.

5.1.2.2 Element Technology Development of Hydrogen Compressor

Due to a small molecular weight of hydrogen gas, it is difficult to rise the pressure of hydrogen gas with centrifugal compression. Though the design technology of impellers of centrifugal compressors for heavier molecular weight gases such as air has been established, if this existing technology is applied to the design of the hydrogen compressor, many compression stages are required. A suitable compressor for the large hydrogen liquefaction plant has to be equipped with a large compression ratio per stage and a high adiabatic compression efficiency. In order to develop such a compressor, it is required to develop aerodynamic design technology based on the understanding of fluid dynamics in the compressor. As one of the means to get the large compression ratio, to design the impeller with a small backward angle can be adopted. However if the impellers are designed only as having the small backward angle , the compression efficiency tends to be low. Therefore it is important to optimize the shape of the impellers and to accumulate experimental data that will establish the design technology for this type of the impellers. We have made an impeller of diameter of 340 mm with the backward angle of 25 deg and conducted an experiment to measure its performance and to examine the flow condition inside. The experiment was done with air but the similar results to those with hydrogen can be obtained when the circumferential speed is adjusted to give the same Mach number. The inside flow conditions were observed with a LDV (Laser Doppler Velocimeter).

The obtained result is shown in Fig.5-1-3 where the pressure ratio is indicated as a function of the flow rate. The design pressure ratio has been confirmed. Fig.5-1-4 shows the efficiency obtained at the same experiment. As shown, the design high efficiency is demonstrated.

These results will be effective as data base for the hydrogen compressor development.

5.1.3 Research Plan for FY of 1999 and Phase II

The hydrogen purification unit was studied on the assumed conditions. Though the hydrogen recovery ratio was set as high as about 80%, a large amount of the purge gas from the unit has to be processed. Since the purification system depends on the feed hydrogen condition such as impurities, their concentrations, and pressure as well, the more study will be necessary for the total optimization after these conditions are fixed.

Though we have prosperous results in the preliminary compressor experiment, it will be necessary for the accumulation of the experimental results. If we consider that we still need large numbers of compression stage for the hydrogen compressor, it will be necessary to study dynamic behave.