Principle of liquid nitrogen washing

This post was last edited by Yilong Pengfei on 2012-8-16 at 15:51. The basic principles of the liquid nitrogen washing process 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 the adsorbent, it comes into full contact with the adsorbent. Some molecules collide with the surface of the adsorbent due to random movements, and may be attracted by surface forces and adsorbed onto the solid surface, thereby reducing the amount 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 coming from the low-temperature methanol washing process, 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 more difficult for the molecular sieves to adsorb H2. When the number of molecules adsorbed on the surface of the adsorbent reaches a certain level, adsorption equilibrium is attained and the adsorbent becomes saturated. At this point, the amount of substance that can be adsorbed per kilogram of adsorbent reaches its maximum value; this is known as the static adsorption capacity (or equilibrium adsorption capacity). During the adsorption process, due to the influence of the fluid flow rate and requirements such as the purity of the gas at the outlet, not all of the adsorbent can reach adsorption equilibrium; a portion of the adsorbent remains unsaturated. The adsorption capacity at this point is the average adsorption capacity per unit of adsorbent, and it is referred to as the dynamic adsorption capacity. Under normal conditions, the dynamic adsorption capacity is only 0.4 to 0.6 times that of the static adsorption capacity. The timing for switching the adsorbent bed is determined based on the dynamic adsorption capacity of the adsorbent under certain operating conditions. If the switch is not made in time when the designated time arrives, the impurity content in the outlet gas will exceed the allowable limits; therefore, it is necessary to regenerate the adsorber by switching it at regular intervals as specified in the design requirements. 2) The principle of mixed refrigeration is well known: under certain conditions, compressing a refrigerant to a certain pressure and then subjecting it to throttling expansion to induce the Joule-Thomson effect allows for cooling. Scientific practice has proven that: \"By mixing one gas with another at a high enough pressure, this second gas can also be used for cooling.\" This is because, with the total system pressure remaining constant, the partial pressure of the gas decreases after it is incorporated into the mixture. The boiling points of the main components of these mixed gases (such as H2 and N2, CO, CH4, Ar, etc.) differ by at least 33°C on average, and ideally by 57°C; this facilitates the purification of the low-boiling-point component H2 as well as the separation of the low- and high-boiling-point components, while also resulting in lower consumption. The liquid nitrogen washing process utilizes the aforementioned principle. The heat exchangers (E1104, E1105, E1106) are used to purge with product nitrogen from the nitrogen washing tower, cooling the high-pressure nitrogen entering this process as well as the purified gas from the low-temperature methanol washing process ； In the nitrogen washing tower, the purified gas and liquid nitrogen are brought into countercurrent contact ； During this process, not only are substances such as CO, CH4, and Ar in the purified gas washed away, but some nitrogen is also added. However, this amount of nitrogen is not sufficient to achieve a H2/N2 ratio of 3:1 in the product gas exiting the nitrogen washing tower; therefore, there is another method for adding nitrogen (this nitrogen addition process takes place between the heat exchangers E1105 and E1106, so as to ultimately achieve a H2/N2 ratio of 3:1) ； Meanwhile, throughout the mixing process of nitrogen and the purge gas, PN2=5.9 MPaG is introduced into the purge gas, causing its partial pressure to drop to PN2=1.3 MPaG; the J-T effect then generates most of the cooling capacity required for the liquid nitrogen washing process. 3) Principle of liquid nitrogen washing: Liquid nitrogen washing is similar to multi-component distillation. It takes advantage of the large differences 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. This process takes place in the core equipment for the liquid nitrogen washing step, namely the nitrogen washing tower. Since the vaporization latent heats of nitrogen and carbon monoxide are very similar, the liquid nitrogen washing process can be considered essentially an isothermal process. The table below shows the relevant physical property parameters of the gases involved in the liquid nitrogen washing process. Relevant physical property parameters of gases: Gas, Name, Boiling point at 1 atm (°C), Heat of vaporization at 1 atm (kJ/kg), Critical temperature (°C), Critical pressure (atm). CH4: -161.45, 509.74, -82.45, 45.79; Ar: -185.86, 164.09, -122.45, 47.98; CO: -191.50, 215.83, -140.20, 34.52; N2: -195.80, 199.25, -147.10, 33.50; H2: -252.77, 446.65, -240.20, 12.76. As can be seen from the table above, the critical temperatures of these various components are relatively low; nitrogen’s critical temperature is -147.1°C (the values for other components are shown in the table). This fact determines that liquid nitrogen washing must be carried out at low temperatures. 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 the other components. In other words, during the low-temperature liquid nitrogen washing process, CH4, Ar, and CO dissolve easily in liquid nitrogen, whereas hydrogen in the feed gas does not dissolve easily in it, thereby achieving the goal of purifying the feed gas of CH4, Ar, and CO through liquid nitrogen washing.