Deep analysis of Pressure Swing Adsorption (PSA) Gas Separation and Purification Technology
1. Process
The PSA technology utilizes the differences in adsorption characteristics of gas components on adsorbents and the variation of adsorption capacity with pressure to achieve gas separation through periodic pressure changes. Adsorbents have different adsorption characteristics for different gases, and by selecting appropriate adsorbents, the mixed gas can be effectively separated and purified.
The adsorption capacity of the same adsorbent for the same gas varies with the adsorption pressure and temperature: higher pressure leads to higher adsorption capacity; higher temperature leads to lower adsorption capacity. Utilizing this characteristic, adsorption can be carried out at high pressure or low temperature, and then the gas on the adsorbent can be desorbed by reducing pressure or raising temperature to achieve adsorbent regeneration and recycling. The process of adsorption or regeneration using temperature changes is called variable temperature adsorption, while the process using pressure changes for adsorption or regeneration is called pressure swing adsorption.
2. Characteristics
As a representative of non-low-temperature gas separation technologies, PSA technology has rapidly developed in the industrial application field and is gradually moving towards large-scale. Compared with other gas separation technologies, PSA technology has the following significant characteristics:
(1) Low energy consumption: The PSA process requires lower pressure, and some pressurized gas sources can be saved from additional pressure addition, and it operates at room temperature without additional heating or cooling energy consumption.
(2) High product purity and flexible adjustment: For example, in PSA hydrogen production, the product purity can reach 99.999%, and the hydrogen purity can be flexibly adjusted according to process requirements, with minimal impact on the operation of the device after adjustment.
(3) Simple process flow: It can achieve the separation of various gases, has strong tolerance to water, sulfides, ammonia, hydrocarbons and other impurities, and does not require complex pretreatment processes.
(4) High automation level of the device: Computer control, convenient operation, rapid start-up and shutdown, and qualified products can be produced within 0.5 hours.
(5) Strong regulation capability of the device: The operating flexibility is large, it can operate stably within a load range of 30% to 120%, and the process adjustment range is small.
(6) Low investment and low operating costs: Simple maintenance, long service life.
(7) Good environmental benefits: Apart from the characteristics of the raw gas, the PSA device operation does not produce new environmental pollution, and there is almost no “three-waste” discharge.
3. Research Progress
The wide application of PSA technology has promoted its rapid development. In recent years, the progress of PSA technology has mainly been reflected in the following aspects:
(1) Continuous improvement of PSA process: The vacuum process is adopted to significantly improve the product recovery rate; the multi-bed and multi-adsorbent filling method is used to simplify the pretreatment and post-treatment processes of certain gas sources and reduce investment and consumption.
(2) More widely applicable gas sources for PSA: PSA technology can handle dozens of gas sources. Previously, gas sources with low product component content or difficult-to-desorb impurities could now be recycled through PSA technology. Currently, more than 200 PSA devices for purifying hydrogen from industrial waste gas have been put into use in China.
(3) Product recovery rate gradually increases: The hydrogen production recovery rate of PSA can reach over 95%.
(4) Adsorbent performance continuously improves: Adsorption capacity, separation coefficient increase, the adsorption front of impurity components decreases, regeneration is easier, adsorbent strength is enhanced, providing sufficient space for the improvement of the PSA process.
(5) Improvement of program control valves: Program control valves are the key equipment for the normal operation of the PSA device, and their performance requirements are higher than those of ordinary valves.
(6) Application of Computer Expert Diagnosis System: The PSA device has numerous control valves and frequent on-off operations, with high automation requirements. After the application of this system, it can promptly and accurately determine the cause, scope and degree of the fault, quickly isolate the faulty part, utilize the remaining equipment to maintain the system operation, and carry out maintenance. After the fault is handled, it can automatically or manually restore the original process, significantly enhancing the anti-interference ability, operational reliability and safety of the PSA device.
4. Current Application Status in China
The PSA technology in China started relatively late but has developed rapidly. Currently， it is already at the international leading level in many aspects. The application fields include：
(1) Separation and purification of hydrogen
Due to the variety and complexity of raw gas types used for hydrogen production， there are multiple purification processes. In the early days of the metallurgical industry， electrolysis was used to produce hydrogen， but the rich hydrogen sources in steel mills (such as coke oven gas) can be hydrogenated through the PSA method， with an electricity consumption of approximately 0.5 kW·h/m³， much lower than 6-7 kW·h/m³ of the electrolysis method. Currently， several major steel enterprises in China have widely adopted PSA technology to replace electrolytic hydrogen production. In the petroleum refining process， a large amount of hydrogen is required， and a large amount of hydrogen-containing tail gas (such as hydrogenation cracking tail gas， catalytic reforming by-product gas， etc.) can be recycled and utilized through the PSA technology.
(2) De-carbonization of shift gas
PSA can remove CO₂ from the raw gas. Currently， the maximum processing capacity of PSA de-carbonization devices is over 50，000 m³/h， and there are nearly 70 various-sized PSA de-carbonization devices in China.
(3) Separation and purification of CO
Carbon monoxide is a basic raw gas for C1 chemistry， but the purification methods are limited. Previously， domestic methods used distillation or COSORB， with complex pre-treatment systems， many equipment， and high investment， resulting in unsatisfactory effects. Yellow phosphorus tail gas， blast furnace gas， and high-temperature gas from blast furnaces， which are rich in CO， are ideal gas sources for PSA purification and can also be used to increase the calorific value of blast furnace gas for industrial gas.
(4) PSA air separation for oxygen and nitrogen production
PSA air separation for oxygen and nitrogen production has a trend of replacing traditional cryogenic air separation devices in medium and small-scale fields due to its low investment， low energy consumption， and convenient operation. The purity of PSA oxygen production can reach 99.95%， and the energy consumption has decreased year by year. For PSA oxygen production devices with a purity of 93%-95.5% and a production capacity of 1000 m³/h， the unit electricity consumption is 0.42 kW·h/m³. PSA nitrogen production can use zeolite molecular sieves and carbon molecular sieves as adsorbents. The former can obtain high-purity nitrogen above 99.99%， and the latter has a simple process and low energy consumption， obtaining 99.5% nitrogen in one step.
(5) Separation and purification of CO₂
PSA separation and purification of CO₂ technology was industrialized in 1986 and can separate and purify CO₂ from various CO₂-containing gas sources， meeting various industrial applications.
(6) Application of PSA in Other Fields
PSA can be used for natural gas purification to remove 0.5%-3% of hydrocarbon impurities， reducing the content to below 100×10⁻⁶， which is an ideal purification method. PSA can also be used for concentrating methane from coal mine gas， increasing its calorific value to the level of urban gas， and achieving the transformation of waste into treasure. In addition， PSA is widely used in ethylene concentration， tail gas purification， gas drying， and desulfurization and denitrification in various fields.
