Problems with atmospheric distillation and nitrogen sealing

A nitrogen-sealed atmospheric distillation system is used to prevent air and oxygen in the reflux tank from entering the distillation tower, thereby avoiding the risk of explosive reactions involving flammable gases. Adding a nitrogen blanket can effectively reduce the oxygen content in the distillation tower and improve safety. In a normal-pressure distillation system with nitrogen sealing, the operation of the nitrogen supply valve and the nitrogen release valve can control the nitrogen pressure in the reflux tank. Theoretically, nitrogen sealing has no direct effect on the condensation process, as condensation occurs based on the principle of temperature difference and is not directly related to gas pressure. In practical operation, it is necessary to properly control the opening degrees of the nitrogen supply valve and the nitrogen release valve in order to maintain a certain pressure of nitrogen inside the reflux tank. Generally, maintaining a nitrogen pressure of over 3 KPa is sufficient to effectively prevent air from penetrating. When considering the issue of nitrogen sealing, the following points should be taken into account: 1. Ensure a stable supply of nitrogen to avoid excessive fluctuations in nitrogen pressure ； 2. Control the opening degrees of the nitrogen supply valve and the nitrogen release valve to maintain the nitrogen pressure in the reflux tank within an appropriate range ； 3. Regularly check the sealing condition of the nitrogen supply pipelines and valves to ensure no gas leakage occurs. In summary, adding a nitrogen seal to an atmospheric distillation system is intended to improve safety, and it has no direct effect on the distillation condensation process. However, reasonable operations and management are required based on the actual conditions to ensure a stable nitrogen supply and good sealing. .

The purpose is to increase safety; it’s not known whether a large amount of nitrogen will be required during operation. We use nitrogen in tube chain conveying, and it consumes a lot of nitrogen

Generally, a nitrogen seal can be used in distillation processes, but overall changes in nitrogen pressure will affect the distillation temperature. Therefore, the precision requirements for nitrogen seal valves are very high; otherwise, pressure fluctuations will cause fluctuations in the distillation temperature. The simplest way is to use a nitrogen seal plus a water seal.

Adding a nitrogen seal to an atmospheric distillation system can have some effects on the distillation condensation, but these issues can be resolved through proper design. After adding nitrogen sealing, the gas in the reflux tank will be a mixed gas composed of material vapor and supplemental nitrogen. This leads to an increase in the condensation temperature within the condenser, and as the heat transfer resistance of the mixed gas increases, the efficiency of the condenser is reduced. A decline in the efficiency of the condenser may lead to incomplete reflux of the distillate fractions in the upper part of the distillation column, thereby affecting the separation performance of the system. Theoretically, by adding a nitrogen seal, the reflux ratio will increase, that is, the amount of reflux liquid will increase, which means more liquid is returned to the tower. An increase in the amount of liquid in the tower may lead to insufficient capacity of the condenser and separator, resulting in impure fractions or an inability to separate them. To address this issue, the following strategies can be adopted during the design process: Adjusting the reflux ratio – by increasing the flow rate of the condensing liquid, it is possible to lower the condensation temperature and improve the efficiency of the condenser. It enables it to handle the increased liquid return flow. Optimize the condenser design: The heat exchange area of the condenser can be increased, or a more efficient condenser design can be adopted to enhance its condensation capacity. Optimizing tower design: During the design process, it is necessary to consider increasing the number of trays or optimizing the liquid distribution between them in order to improve separation efficiency and accommodate the increased amount of liquid returning to the tower. Conduct simulation and experimental verification: After the design is completed, simulation and experimental verification can be carried out to determine the system’s performance, with optimizations made based on the actual conditions. In summary, the addition of a nitrogen seal has a certain impact on distillation condensation, but these issues can be effectively addressed through proper design and optimization to ensure system stability and separation efficiency

Thank you. So, how is it working in actual use?:)

This project in the enterprise has met the expectations set at the initial stage; the installation of nitrogen seals has not had any impact on the equipment’s normal production capacity. It is necessary to carry out technical modifications to the relevant equipment before the renovation

In other words, usually this impact is relatively small in actual production?

This post was last edited by Shengzhen丶 on 2023-7-3 11:41. Agree with the view from the fifth floor. Is it feasible for the vent line of the reflux tank to run separately, without going through the condenser? In this way, these adjustments specific to the tower can be omitted

The gas phases of the condenser and the reflux tank are not connected, so the back pressures may differ, which could have an adverse effect on the flow of the liquid phase downward

Yes, the pressure difference is calculated based on the height of the water seal and the minimum nitrogen supply pressure required by the nitrogen seal valve (it’s best if this difference remains relatively constant; values in the range of several dozen to a few hundred pascals are sufficient). Using tables or calculations, the boiling point and volatility of the distillation material are determined within this pressure difference, and the number of tray levels is increased appropriately so that even slight changes in pressure remain within acceptable limits. 1. Generally, before feeding material into the new tower or after cleaning it, nitrogen is used to purge the bottom of the tower until the oxygen content detected in the exhaust gas is within acceptable levels. 2. Then feeding follows distillation; the temperature increase is a process of expansion, and generally air does not enter the tower. 3. The distillation process is a steady-state process; batch distillation hardly involves any gas exchange, while continuous distillation is also a process of gas emission with little gas absorption. Any minor amount of gas exchange occurs due to slight changes in the temperature of the cooling medium and the flow rate at the top of the tower, at which point the nitrogen seal valve supplies a small amount of nitrogen. 4. Once distillation is complete and the cooling process begins, the gas inside the tower starts to contract; heating must be stopped immediately, and the exhaust valve should be closed to prevent air from entering during this contraction process. Therefore, the heating valve needs to be interlocked with the exhaust valve, and an additional pressure relief valve should be installed to prevent danger caused by leaks from the heating medium valve. 5. During the cooling process, which is also a contraction process within the tower, the nitrogen seal valve serves to supply additional nitrogen. 6. Therefore, the distillation tower has two nitrogen supply routes: one at the bottom of the tower and the other in the exhaust pipeline.