Is the principle of CO removal by liquid nitrogen elution absorption or condensation?

I think condensation is more likely, because the melting point of hydrogen is -259.14°C, while that of carbon dioxide is -199°C.

In my opinion, it involves absorption; this process makes use of the selective permeability of molecular sieves to selectively absorb carbon monoxide at around -190 degrees Celsius (since carbon monoxide is in a liquid state at this temperature, while hydrogen is not). The hydrogen that has not been absorbed passes through the molecular sieve to the next stage, while carbon monoxide gets absorbed on the surface of the molecular sieve. When the molecular sieve reaches a certain level of saturation, it needs to be regenerated. Therefore, in the liquid nitrogen washing stage, the molecular sieves are used in pairs, one in operation and one as a backup, with them switching roles among each other The liquid nitrogen used for absorption actually constitutes only a small portion; it contains carbon monoxide dissolved in it, which can be used as fuel gas for Claus sulfur recovery (or sent to a flare for combustion), while the majority of the liquid nitrogen is used to provide cooling! The above are merely personal opinions for reference only! !

It has little to do with molecular sieves; in the liquid nitrogen washing process, molecular sieves are primarily used for dehydration, to prevent a small amount of water from condensing at low temperatures. Of course, they also absorb some CO during this process. The principle of liquid nitrogen washing relies on the fact that CO and methane have boiling points higher than those of liquid nitrogen; they come into contact with liquid nitrogen in the washing tower and are condensed. At the same time, a small amount of liquid nitrogen evaporates. Since the concentrations of CO and CH4 in the syngas are very low, the heat released during their condensation has a negligible impact on the washing process. Therefore, the liquid nitrogen washing process can be regarded as an absorption operation under constant temperature and pressure.

The guy upstairs, your understanding is completely wrong. The molecular sieve is used before the liquid nitrogen washing step; its role is to remove methanol and a small amount of carbon dioxide from the gas that has been purified through low-temperature methanol washing. It has nothing to do with removing carbon monoxide. The process of liquid nitrogen washing goes as follows: the gas that has been purified through low-temperature methanol washing passes through the molecular sieve, and then enters the washing tower where it is washed using high-pressure liquid nitrogen derived from air separation. Inside the washing tower, the temperature drops further due to the evaporation of the liquid nitrogen; this liquid nitrogen becomes less and less as it moves from the top of the tower to the bottom, and carbon monoxide is also present in it. Once this liquid nitrogen is warmed up in a cooling box, it can be disposed of. I checked, and the temperature in the washing tower is generally a little over 190 degrees, not reaching 200 degrees. So I think dissolution does still occur; my previous understanding was incorrect. It seems that polar molecules dissolve easily in liquid nitrogen, while non-polar molecules have difficulty dissolving in it. I hope those who are capable of performing liquid nitrogen cleaning will participate and give everyone a definite answer.

It was my mistake; you thought it was a friend of 3#. I made an error in my operations just now. Please understand! ! ! ! !

The principle of removing CO by liquid nitrogen elution is absorption.

The basic principles of the nitrogen washing section include the adsorption principle, the mixed refrigeration principle, and the liquid nitrogen washing principle: (1) Adsorption principle – Adsorption is a physical phenomenon in which no chemical changes occur. Due to intermolecular attractive forces, a surface force is generated on the surface of the adsorbent. As the fluid flows through, it comes into full contact with the adsorbent. Some molecules, due to their random movements, collide with the surface of the adsorbent; they may then be attracted by surface forces and adsorbed onto the solid surface, thereby reducing the concentration of such molecules in the fluid and achieving the purpose of purification. The adsorption force of molecular sieves on polar molecules is much greater than that on non-polar molecules; therefore, in the gas obtained after methanol washing, CO2 and CH3OH are selectively adsorbed by the molecular sieves due to their higher polarity compared to H2. Since H2 is a non-polar molecule, it is relatively difficult for molecular sieves to adsorb H2. ⑵ Principle of mixed refrigeration: It is well known that cooling can be achieved by throttling and expanding a gas. Scientific experiments have shown that mixing one gas with another under sufficiently high pressure can also be used for cooling. This is because, with the total system pressure remaining constant, the partial pressure of a gas in a mixture decreases. To achieve this accurately, the boiling points of the main components of the gases mixed together should differ by at least 33°C on average, and ideally by 57°C. This principle is applied in the design of the nitrogen washing process: the product from the scrubber tower is used in the heat exchanger to cool nitrogen and feed gas. During this washing process, the feed gas and liquid nitrogen come into counterflow contact; as a result, substances such as CO, CH4, and Ar in the feed gas are removed, while some nitrogen is also introduced. However, this amount of nitrogen is not sufficient to achieve a H2/N2 ratio of 3:1 in the gas leaving the tower. Therefore, another method of nitrogen addition to the gas leaving the tower was designed; this process takes place within a heat exchanger, enabling the H2/N2 ratio to reach 3:1. At the same time, most of the cooling required by the system is obtained during the mixing of nitrogen with the feed gas. ⑶ Principle of liquid nitrogen washing: Liquid nitrogen washing is similar to multi-component distillation, yet it differs from it. It takes advantage of the large difference in boiling points between hydrogen and CO, Ar, and CH4 to dissolve CO, CH4, and Ar from the gas phase into liquid nitrogen, thereby removing impurities such as CO, CH4, and Ar. The table below shows the physical property constants of some gases. Physical constants of some gases: Gas, Boiling point °C, Heat of vaporization at atmospheric pressure Kcal/Kg, Critical temperature °C, Critical pressure atm. CH4: -161.4, 58.4, -82, 45.8; Ar: -185.8, 37.6, -122.1, 466; CO: -191.5, 51.6, -140.2, 434.53; N2: -195.67, 47.7, -147.1, 33.6; H2: -252.81, 109.0, -139.9, 13.2. As can be seen from the table above, the critical temperatures of these substances are relatively low; nitrogen’s critical temperature is -147.1°C, which is why liquid nitrogen washing requires low temperatures to be carried out. Based on the boiling point data of various components, it can be seen that the boiling point of H2 is much lower than that of N2 and other components. In other words, during the low-temperature liquid nitrogen washing process, CH4, Ar, and CO dissolve more easily in liquid nitrogen, whereas raw hydrogen does not dissolve easily in it, thereby achieving the purpose of liquid nitrogen washing. Note: The principle behind removing carbon monoxide at low temperatures is based on the dissolution of carbon monoxide in liquid nitrogen, that is, the vapor-liquid equilibrium between carbon monoxide and nitrogen. Because to reduce carbon monoxide to below 5 ppm, even at a total pressure of 70 bar, its condensation temperature will be below 68 K. Therefore, relying solely on complete condensation, under the corresponding conditions (nitrogen purging conditions), the purification requirements cannot be met; a lower temperature is required. So “absorb” is more accurate. This post was last edited by Tianrun Fertilizer-Hou on 2008-4-28 15:28.]

In fact, both condensation and wash absorption should be employed; at lower operating temperatures, the vapor pressure of CO is lower, making it easier to be washed away. However, unlike normal absorption, the fractions of CO in the gas and liquid phases are almost equal, following Henry’s law. In fact, if the CO concentration is high enough, it is entirely possible for most of it to condense first; once the CO concentration becomes low enough, absorption takes over as the dominant process. In a sense, the process of physical dissolution of a gas in a liquid can also be regarded as a process in which the gas first condenses and then mixes; this is why the heat of dissolution and the heat of condensation are sometimes quite similar.

In fact, when CO dissolves in liquid nitrogen, it is mainly through condensation; the heat released during the dissolution process is also due to condensation. I’ll calculate it later; if the heat of condensation and the heat of dissolution are the same, that will confirm it further.

The process of removing impurities such as CO and CH4 using liquid nitrogen is a physical process with no chemical reactions involved. However, it differs from ordinary chemical unit operations; it is similar to distillation but not the same as multi-component distillation. Since impurities such as CO and CH4 in the feed gas are removed in the nitrogen scrubber, when these gases come into contact with liquid nitrogen and are condensed, part of the liquid nitrogen evaporates. The extent to which liquid nitrogen washing can be effective is determined by the gas equilibrium conditions under operating conditions