Hydrogen Purification System

Hydrogen is a promising clean energy carrier that can be used for various applications, such as fuel cells, power generation, and transportation. However, hydrogen production often involves impurities that can affect its quality and performance. Therefore, hydrogen purification is an essential step to ensure the efficiency and safety of hydrogen utilization.

Pressure swing adsorption
Pressure swing adsorption (PSA) is a widely used method for hydrogen purification that relies on the selective adsorption of impurities on porous materials, such as activated carbon or zeolites, under high pressure. The adsorbed impurities are then released by reducing the pressure and flushing the adsorbent with a purge gas. PSA can achieve high purity and recovery of hydrogen, but it also requires high energy consumption, large equipment size, and periodic regeneration of the adsorbent.

Membrane separation
Membrane separation is another common method for hydrogen purification that uses thin and permeable materials, such as polymers, metals, or ceramics, to separate hydrogen from other gases based on their molecular size, shape, or affinity. Membrane separation can operate at low or ambient pressure and temperature, which reduces the energy and capital costs. However, membrane separation also faces challenges such as membrane fouling, degradation, and selectivity.

Cryogenic distillation
Cryogenic distillation is a method for hydrogen purification that exploits the different boiling points of hydrogen and other gases. By cooling the gas mixture to extremely low temperatures, hydrogen can be separated as a vapor while the impurities are condensed as liquids. Cryogenic distillation can achieve very high purity and recovery of hydrogen, especially for removing inert gases such as nitrogen and helium. However, cryogenic distillation also involves high energy consumption, complex equipment, and safety risks.

Palladium diffusion
Palladium diffusion is a method for hydrogen purification that utilizes the unique property of palladium metal, which can absorb and diffuse hydrogen atoms through its lattice structure. By applying a pressure or temperature gradient across a thin palladium membrane, hydrogen can be selectively transported from one side to the other, leaving behind the impurities. Palladium diffusion can achieve ultra-high purity and recovery of hydrogen, but it also suffers from high material cost, limited availability, and susceptibility to poisoning and embrittlement.

Biological methods
Biological methods are emerging methods for hydrogen purification that employ microorganisms, such as bacteria, algae, or fungi, to convert or remove impurities from hydrogen gas. For example, some bacteria can use carbon monoxide, a common impurity in hydrogen production, as a substrate for growth and produce carbon dioxide and water as by-products. Biological methods can offer low energy consumption, environmental benefits, and potential value-added products. However, biological methods also face challenges such as low efficiency, scalability, and stability.

For the first time, researchers have recovered 98.8 percent of the hydrogen from a conventional water-cooled water gas shift reactor's exit stream, which is the highest value ever recorded.

In traditional hydrogen separation methods, a water gas shift reactor is used, which necessitates an additional step. In the water gas shift reactor, carbon monoxide is first converted to carbon dioxide, and then the hydrogen and carbon dioxide are separated using an absorption process. A compressor is used to pressurize the purified hydrogen for immediate use or storage.

The use of high-temperature proton-selective polymer electrolyte membranes, or PEMs, is required to quickly and economically separate hydrogen from other gas molecules such as carbon dioxide and carbon monoxide. It can also operate at higher temperatures than other high-temperature PEM-type electrochemical pumps, enhancing its ability to separate hydrogen from other gases.

Hydrogen purification process
To achieve the separation, the team used an electrode "sandwich," in which electrodes with opposing charges serve as the "bread" and a membrane serves as the "deli meat." The electrode ionomer binder materials are designed to hold the electrodes together, similar to how gluten holds bread together.

The bread slice, or positively charged electrode, in the pump releases protons and electrons from the hydrogen. While protons travel through the membrane, electrons travel through the pump via a wire that touches a positively charged electrode. After passing through the membrane and arriving at the negatively charged electrode, the protons and electrons combine to form hydrogen once more.
Because the PEM only allows protons to pass through, carbon monoxide, carbon dioxide, methane, and nitrogen gas cannot pass through. The team created an adhesive phosphonic acid ionomer binder to keep electrode particles in the hydrogen pump together so they could function properly.

Researchers will use their approach and tools to investigate hydrogen purification in natural gas pipelines. Although this method of transporting and storing hydrogen has yet to be put into practice, it holds a lot of promise. Hydrogen could be used to support solar and wind energy systems, as well as a variety of other environmentally friendly applications, by using a fuel cell or turbine generator.

Industrial gas contains a large number of waste gases with various hydrogen. The separation and purification of hydrogen is also one of the earliest industrialized fields of PSA technology.

The principle of PSA separation of gas mixture is that the adsorption capacity of adsorbent for different gas components changes with the change of pressure. The impurity components in the inlet gas are removed by high-pressure adsorption, and these impurities are desorbed by pressure reduction and temperature rise. The purpose of removing impurities and extracting pure components is achieved through pressure and temperature changes.

PSA hydrogen production uses JZ-512H molecular sieve adsorbent to separate the rich hydrogen to produce hydrogen, which is completed through the pressure change of the adsorption bed. Because hydrogen is very difficult to adsorb, other gases (which can be called impurities) are easy or easy to be adsorbed, so hydrogen rich gas will be produced when it is close to the inlet pressure of the treated gas. Impurities are released during desorption (regeneration), and the pressure gradually decreases to desorption pressure
The adsorption tower alternately carries out the process of adsorption, pressure. equalization and desorption to achieve continuous hydrogen production. Rich hydrogen enters the system under a certain pressure. The rich hydrogen passes through the adsorption tower filled with special adsorbent from bottom to top. Co / CH4 / N2 is retained on the surface of the adsorbent as a strong adsorption component, and H2 penetrates the bed as an adsorption component. The product hydrogen collected from the top of the adsorption tower is output outside the boundary. When the adsorbent in the bed is saturated with CO / CH4 / N2, the rich hydrogen is switched to other adsorption towers. In the process of adsorption desorption, a certain pressure of product hydrogen is still left in the adsorbed tower.

This part of pure hydrogen is used to equalize and flush the other pressure equalizing towers just desorbed. This not only makes use of the remaining hydrogen in the adsorption tower, but also slows down the pressure rise speed in the adsorption tower, slows down the fatigue degree in the adsorption tower, and effectively achieves the purpose of hydrogen separation. 

Hydrogen gas is produced from a variety of different processes and is typically produced in an impure form. Typical processes include chemical synthesis by methane steam reforming, off-gassing from styrene or ethylene plants where hydrogen gas is produced as a by-product, and petrochemical applications such as hydrocracking or desulphurization. To use the hydrogen, a purification process is necessary to create purified hydrogen gas. Hydrogen pressure swing adsorption (H2PSA) is a process that capitalizes on the volatility of hydrogen and its overall lack of polarity and affinity for zeolites to purify contaminated gas streams.

Hydrogen generation typically involves the production of contaminants or side products that need to be removed. It includes compounds such as carbon monoxide, carbon dioxide, nitrogen, water and unreacted hydrocarbons. Hydrogen PSA takes advantage of the preferential adsorption of these components, eliminating them from the hydrogen stream to yield purified hydrogen.

Traditionally, Hydrogen PSA takes advantage of multiple sieve beds and is comprised of four phases: an adsorption phase, a depressurization phase, a regeneration phase and a repressurization phase. In the process, the impure stream of hydrogen is passed into the sieve bed where impurities are selectively adsorbed onto the molecular sieve under pressure. After the adsorption step is completed, regeneration is accomplished by depressurizing the bed which decreases the affinity of the impurities allowing them to be discarded.

Further purification of the bed is achieved by purging with pure hydrogen to remove any remaining contaminants. The bed is again pressurized to repeat the adsorption process. The beds run in sync to allow continuous hydrogen generation. 

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