Why does the high-pressure nitrogen cooler not frost up when the cryogenic tank is cooled by liquid nitrogen?

Principle of liquid nitrogen washing: Liquid nitrogen washing is similar to multi-component distillation. 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. This process takes place in the nitrogen washing tower used in the liquid nitrogen washing step. Since the vaporization latent heats of nitrogen and carbon monoxide are very similar, the liquid nitrogen washing process can be considered essentially an isenthalpic process. The table below shows the relevant physical property parameters of the gases involved in the liquid nitrogen washing process. Gas Name, Boiling Point at Atmospheric Pressure (°C), Heat of Vaporization at Atmospheric Pressure (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, the critical temperatures of these various components are relatively low; nitrogen’s critical temperature is –147.1°C (the values for other components are listed 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 liquid nitrogen, thereby achieving the purpose of purifying the feed gas of CH4, Ar, and CO through liquid nitrogen washing.    Since the selected absorbent is liquid nitrogen, and nitrogen can be liquefied only under pressure and low temperature – conditions that also increase the solubility of gases – this liquid nitrogen washing process is carried out at 5.2 MPaG and -194°C.