To achieve a given degree of hydrogen concentration and remove impurities, the process uses numerous adsorption layers. In the short-node adsorption unit, the converted hydrogen-containing gas is purified from impurities of methane and carbon oxides by adsorption of contaminants on the adsorbent at high pressure and desorption at low pressure.
The raw material for hydrogen production is hydrogen-containing gas with a hydrogen concentration of about 75-80% vol. The products are hydrogen with a concentration above 99.5% vol., as well as stripping gas with a hydrogen content of 10-40% vol.
Application
- Oil refining processes
Extraction and concentration of hydrogen in oil refining cycles. Concentration of hydrogen from exhaust gases of hydrotreating, hydrocracking, isomerization, catalytic reforming, reducing the concentration of undesirable impurities to the required level.
- Chemical and petrochemical processes
Extraction and concentration of hydrogen from pyrolysis exhaust gases, dehydrogenation of alkanes and alkenes, methanol production exhaust gases and other process streams.
- Ammonia production
Separation of hydrogen from nitrogen, methane and argon in ammonia purge gas streams.
- Synthesis gas production
Production of high-purity hydrogen from synthesis gas with almost complete removal of CO and CO2.
Part of the equipment
- Pressure vessels
• combined fine separator;
• product hydrogen filter (optional).
• Adsorber;
• residual gas tank with internal devices.
- Compressor system:
• residual gas compressor (optional). Screw compressor for supplying residual gas to the fuel ring;
• screw compressor on the assembly slipway;
• assembled screw compressor unit.
- Valve assembly with instrumentation and pipelines
- Safety valves to protect the installation from overpressure
- Control system
- Control program
- Ladders and service platforms, piping outside the valve assembly
- Specially selected adsorbent
Advantages
- Extensive experience of our specialists in the design and commissioning of PSA plants (more than 1000 units)
- High product recovery
- Automatic start from “zero” pressure. All preliminary start-up operations occur automatically
- Decommissioning of the installation in automatic mode without shutting down while maintaining the purity of the product and the degree of its recovery
Specially selected switching valves with a high number of actuation cycles
- Specially selected adsorbent with a service life of 15-20 years
- For reliable operation in winter, all valves are equipped with thermally insulating casings with electrical heating. At the Customer's request, the entire valve assembly can be placed in a shelter with heating and ventilation
Fundamentals of the short-node adsorption process
The pressure swing adsorption process is based on physical adsorption phenomena, where highly volatile compounds with low polarity (hydrogen or helium) are practically not adsorbed compared to molecules of CO2, CO, N2 and hydrocarbons. Consequently, most impurities in a hydrogen-containing stream can be selectively adsorbed to produce high-purity product hydrogen.
PRESSURE SWING ADSORPTION unit is designed for continuous purification of water containing gas. Although the process appears to be continuous on the outside, it is internally discontinuous, consisting of many sequences running in parallel. In general, each adsorber performs a special cycle of short-cycle adsorption, periodically repeated in a cyclic mode. One cycle of pressure swing adsorption consists of two main phases: adsorption and regeneration.
The control program ensures the sequence of technological operations and switching of the operating adsorber to maintain product purity. Therefore, sequential operation ensures that before the adsorption capacity of the operating adsorber is exhausted, another adsorber will undergo regeneration and be pressurized to take over the adsorption function.
The figure below explains the pressure swing adsorption process. It shows adsorption isotherms that describe the relationship between the partial pressure of a component and its equilibrium loading into the adsorbent for constant temperatures.
Adsorption is carried out under high pressure (usually in the range of 10 - 40 kg/cm2), increasing the corresponding partial pressure and therefore the equilibrium loading of impurities into the adsorbent.
Desorption or regeneration takes place at low pressure (usually slightly above atmospheric pressure), reducing the equilibrium loading of impurities into the adsorbent.
The amount of impurities adsorbed in one cycle corresponds to the difference between the adsorption and desorption loads (residual load).
ADSORPTION
The feed gas passes through the adsorber in an upward direction from the bottom to the top and impurities such as CO2, hydrocarbons and CO are selectively adsorbed on the surface of the adsorbent. Purified product hydrogen gas exits the top of the adsorber and is supplied to the product manifold.
The purity of product hydrogen remains constant throughout the adsorption cycle. At the end of the adsorption cycle, impurities begin to accumulate. This indicates that the adsorber is loaded with impurities and must be regenerated.
After loading through adsorption sequences, the adsorber is regenerated in four main steps:
- the pressure in the adsorber is reduced to a low level parallel to the supply flow. Parallel depressurization uses the adsorber's hydrogen to pressurize or purge other adsorbers;
- the pressure in the adsorber is reduced in the direction countercurrent to the pressure of the residual gas (dump sequence) to remove impurities from the adsorbent;
- the adsorber is purged under residual gas pressure with hydrogen obtained in the adsorbers during the purge sequence or with pure hydrogen gas from a hydrogen collector to remove residual impurities from the adsorbent;
- The adsorber is placed under pressure in steps up to the pressure of hydrogen adsorption of the adsorbers.
Source gas composition
For a continuous process, impurities adsorbed during the adsorption phase must be removed from the adsorbent bed during the regeneration phase. This means that the adhesion forces between the impurity component and the adsorbent must be high enough for high pressure adsorption, but low enough for low pressure desorption. To continuously remove all impurities, pressure swing adsorption units typically have a layered bed, a lower layer (with the lowest adhesive forces) to remove the "heaviest" components, and a top layer to remove the "light" components:
If, for example, due to a difference in the composition of the source gas compared to the calculated one, a heavier component is transferred to the upper layer, then this component will be completely adsorbed by this layer, but desorbed poorly and will therefore continuously deactivate this layer.
Roughly, impurities can be divided into 2 groups.
Group 1:
CO, N2, C1, C2, O2, He
Impurities that cannot lead to irreversible overload of the adsorption material
Operating a PSA unit with a concentration of these impurities that is too high (compared to the design concentration) can lead to the penetration of impurities into the product hydrogen. Typically, such product contamination can be detected by an in-line analyzer and the operator can respond by reducing the PSA inlet flow and/or USE RATIO. The adsorption materials can then be completely regenerated if the amount of incoming impurities is reduced.
Group 2:
All components not listed in group 1, especially: CO2, C3, C4+, H2O(steam)
Impurities that can lead to irreversible overload of the adsorption material. This is possible in the following cases:
- a) Impurities in the source gas that are not specified in the plant design framework.
In this case, there cannot be any corresponding layer for unspecified components in the adsorber container.
- b) The content of critical components in the source gas is higher than calculated OR the consumption of raw materials for pressure swing adsorption exceeds its actual maximum productivity.
The performance of the dedicated bed for these components may not be sufficient even without immediate breakthrough of the light components into the product, which can be detected by the in-line analyzer.
In all the above cases, critical components will come into contact with the adsorbent layers, from which they are poorly or not desorbed at all. This results in ongoing decontamination that cannot be immediately detected by product gas analysis. If, after a certain period of time, breakthrough of light components occurs due to decontamination of the lower layers in the bed, this decontamination will most likely be irreversible under pressure swing adsorption conditions and will lead to permanent deterioration of its performance.
Therefore, overloading the pressure swing adsorption unit with GROUP 2 impurities should be avoided in any case.
LIQUIDS should never enter the gaseous phase of swing adsorption because they will deactivate the adsorbent layers in any case.
The following diagram shows the configuration of the swing adsorption plant
