Comparison between chemical absorption and PSA pressure swing adsorption
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1 Basic principles – Chemical absorption method: Under the action of a catalyst, an absorption solution is used; through acid-base neutralization reactions, acidic substances such as carbon dioxide and hydrogen sulfide in biogas are absorbed, while gases that are soluble in water such as ammonia can also be absorbed. The gas containing various impurities has its impurities and gases removed through a solution self-purification system and a gas regeneration system, allowing the solution to be reused. The regenerated gas contains 95% carbon dioxide, which can be further purified for sale as a product. This method requires approximately 5% of the biogas to be used to generate steam for production purposes. PSA pressure swing adsorption method: This method involves passing gas through an adsorption bed; by taking advantage of the different adsorption capacities of the adsorbent for various gases, most of one (or several) gases in the mixed gas is adsorbed on the bed, while only a small portion passes through ； Allow most of the other gas(s) in the mixed gas to flow out, with only a small portion being adsorbed on the bed ； Thereby achieving the goal of increasing the concentration of the outlet gas. This method is suitable when the adsorption capacities of the various components of the target gas to be separated by the adsorbent differ greatly. This depends on the extent of differences in properties such as molecular weight and molecular size among the various components of the substance to be separated. The greater these differences, the easier it is to separate them, the lower the separation cost, and the higher the yield. Conversely, the smaller the difference, the more difficult it is to separate them, the higher the separation cost, and the lower the yield.
2 Selective comparison: Chemical absorption method: The chemical absorption method exhibits high selectivity for the separation of carbon dioxide and methane; it reacts rapidly with acidic gases such as carbon dioxide but does not react with methane. While purifying biogas, it is also desirable to remove various other acidic substances simultaneously. Therefore, the chemical absorption method is particularly suitable for the engineering of methane purification from biogas. PSA pressure swing adsorption method: As can be seen from the principle behind it, the selectivity of the adsorbent for gases depends primarily on the properties of the gas molecules. For the separation of gases such as hydrogen, carbon monoxide, and carbon dioxide, this method offers excellent selectivity, high yields, and low consumption. This is why PSA pressure swing adsorption is currently widely used in the fields of hydrogen production, as well as in the separation and purification of water gas, semi-water gas, and coke oven gas. However, for the separation of carbon dioxide and methane, since the adsorption capacity of the adsorbent for these two gases is very similar, they are in fact two substances that are recognized in the pressure swing adsorption industry as being difficult to separate economically. It is because the adsorbent has poor selectivity for these two gases
3 Yield comparison: Chemical absorption method: Due to its high selectivity for the separation of carbon dioxide and methane, the solution reacts rapidly with acidic gases such as carbon dioxide, but almost does not react with methane. This determines its high yield. Almost all of the methane is recovered. PSA pressure swing adsorption method: As can be seen from the principle, the selectivity of the adsorbent for gases depends primarily on the properties of the gas molecules. In the case of separating carbon dioxide from methane, since the adsorbent has a similar adsorption capacity for these two gases, increasing the concentration of one of them often requires a significant reduction in the yield as a trade-off. Based on current technology, to achieve a product methane concentration of 95%, the yield is below 50%.
4 Comparison of product concentrations: Chemical absorption method: The chemical absorption method determines the outlet concentration of the gas based on the vapor partial pressure at the surface of the solution. Therefore, by appropriately adjusting the residual carbon dioxide level in the solution during its regeneration, it is possible to adjust the carbon dioxide vapor pressure at the surface of the solution during absorption, thereby achieving the goal of regulating the carbon dioxide concentration. If inert gases such as nitrogen and oxygen are absent from biogas, the methane concentration can reach over 99%. PSA pressure swing adsorption method: As previously explained, for the separation of carbon dioxide and methane, since the adsorbent has a similar adsorption capacity for these two gases, increasing one of their concentrations often requires a significant reduction in yield as a trade-off. Based on current technology, to achieve a product methane concentration of 95%, the yield is already below 50%. Reaching over 98% is practically impossible with current technology.
5. Consumption comparison (assuming both methods produce bottled natural gas at 20 MPa and 95% purity): Chemical absorption method: The main energy costs in this method include electricity for compression, electricity for circulation pumps, a small amount of water used for supplementation, and steam consumption. PSA pressure swing adsorption method: The main energy costs for this method are electricity for compression, electricity for programmable valves, and a small amount of water used for supplementation. On the surface, it seems that the PSA pressure swing adsorption method does not require steam; in reality, the steam needed by the chemical absorption method can be met by reducing the yield of biogas by 5% to be used for steam production. In this way, the methane yield can be as high as 94–95%, whereas the yield using PSA pressure swing adsorption is only around 45%. Due to differences in gas yield, the electrical energy consumption for compressing the same volume of compressed natural gas varies significantly. The PSA pressure swing adsorption method requires 2 to 3 times more compression of the gas to achieve the same output as the chemical adsorption method, and it generates a large amount of waste gas. From an investment perspective, for investments of the same scale, the cost of chemical absorption methods is less than one-third that of PSA pressure swing adsorption methods. A comprehensive comparison of the energy consumption of the two methods shows that the chemical absorption method consumes between one-fifth and one-third of what the PSA pressure swing adsorption method does.
6 Degree of comprehensive utilization of raw materials: Chemical absorption method: As explained earlier, the chemical absorption method exhibits high selectivity in the separation of carbon dioxide and methane; as a result, a high yield of methane is obtained. The regenerated gas contains no methane, and the purity of carbon dioxide is high, allowing for further processing – it can be processed into pure carbon dioxide products for sale. In this way, chemical absorption separation for purifying biogas can utilize almost all of the feed gas. PSA pressure swing adsorption method: For the separation of carbon dioxide and methane, since the adsorbent has a similar adsorption capacity for these two gases, increasing one of their concentrations often requires a significant reduction in yield as a trade-off. As mentioned earlier, to achieve a product methane concentration of 95%, the yield is already below 50%. Reaching over 98% is practically impossible with current technology. In this case, to produce bottled natural gas with 95% methane content, 75% of the total volume of the feed gas is emitted as waste gas, and this emitted gas contains a large amount of methane, resulting in a low purity of carbon dioxide as well, rendering it unsuitable for recovery. Therefore, PSA pressure swing adsorption is not suitable for the separation of methane and carbon dioxide, as it results in high costs, low yields, poor quality, and low efficiency in this application.