【Weekly Topic】Week 27, 2011——How should the opening degree of the valve for liquid nitrogen reflux to the lower column be set?

This post was last edited by Huo Pusa on 2011-7-3 at 10:45. By opening this valve, the volume of liquid on the main cooling nitrogen side decreases, which increases the amount of air that can enter. The opening degree of this valve serves to reduce the space occupied by liquid nitrogen, thereby providing more space for air and increasing the surface area for heat exchange between air and liquid oxygen. The opening degree of this valve has a significant impact on the overall system; it should be adjusted gradually, step by step, after stabilization, and care should be taken not to act too hastily, as this could disrupt the entire operation and affect the exhaust pressure of the air compressor

The liquid nitrogen reflux valve should be adjusted according to the resistance in the lower column, and the adjustment should be done slowly. Increasing the liquid nitrogen reflux valve opening raises the gas flow rate into the tower, which causes the pressure at the air compressor inlet to drop. When no other valves are adjusted, increasing the liquid nitrogen reflux valve opening results in an increased reflux ratio at the bottom of the tower.

The main thing is to monitor the tower resistance and the pressure at the compressor outlet. However, adjustments must be made slowly, as this valve has certain limits; once it is opened to a certain extent, it no longer has much impact on the air intake volume. This needs to be determined through actual operation

Under normal circumstances, we keep this valve fully open here. . . It just starts slowly, and the air compressor operator is informed to maintain the outlet pressure, while the air separation unit operator adjusts the gas flow into the tower to full capacity. . Here we use internal compression air separation; when the load is reduced, the amount of air fed into the plate heat exchanger is adjusted, thereby gradually decreasing the load on the air compressor and slowing down the speed of the liquid oxygen pump. . Oxygen production decreases, while the amount of nitrogen removed is also reduced. . . Furthermore, it is best to adjust the load gradually. However, in practice, oxygen is usually adjusted in large steps, from a low level to a higher one; first, the amount of air intake and the amount of liquid oxygen pumped out as well as the amount of nitrogen pumped out need to be adjusted. But it is important to ensure the purity of nitrogen first. The purity of oxygen may decrease, but only temporarily – once more air starts to be introduced, the oxygen purity will improve again. . . .

In some external compression processes, V11 (the valve for liquid nitrogen reflux to the tower) is kept fully open, and the amount of gas being supplied is adjusted by gradually changing the flow rate at the compressor outlet； It is relatively easy to adjust the load; when the air compressor flow rate changes, the flow inside the tower adjusts accordingly. There are also systems that use V11 valves to control the amount of gas entering the tower; in such cases, adjustments need to be made taking the air compressor into account. When adjusting the load, it is common to reduce the setting of V11, adjust the pressure of the air compressor, lower the oxygen production volume, and adjust the temperatures of the high- and low-pressure plate exchangers. Basically, whenever the air volume changes, the settings in the tower need to be adjusted again.

Each set of equipment has its own characteristics; especially for valves, the linear performance can vary, and PID control can work fast or slowly. Therefore, the issue of matching the valve opening to the operating conditions requires gradual exploration through actual use

When operating it, should one also pay attention to the temperature difference in heat exchange with the main cooler?

Some air separation units do not have this valve; only pipes are present. This shows the function of this valve at full load. I believe the main function of this valve is to work together with the liquid nitrogen throttle valve to adjust the purity of the liquid air. If the load is to be adjusted, it should be by adjusting the amount of product gas, that is, the gas output from the tower. When this valve is operated, it indeed reflects changes in the air volume, but stabilization is still required by adjusting the operating conditions.

We need to adjust this valve many times a day; using it properly allows for good control of the operating conditions. Our pressure air comes from the system, so we don’t need to consider the air compressor system. When the valve is closed, the amount of liquid flowing downstream decreases, and less air gets liquefied, which naturally reduces the volume. However, it’s necessary to adjust the amount of product taken out as well as the liquid nitrogen throttle valve in the tower accordingly. If there is only oxygen and nitrogen, then the valve can be opened and closed more quickly (this requires some experimentation), but if the equipment uses argon, then the process must be done more slowly, as argon has a significant impact on the system!

It is possible to understand well the function of this valve; only by adjusting it can one handle it with ease. Early air separation units did not have a valve for liquid nitrogen reflux at the lower column; the load regulation of such units was achieved by controlling the flow of liquid nitrogen in order to adjust the opening degree of the valve at the upper column, thereby regulating the pressures in both the upper and lower columns. The purpose of setting the valve for liquid nitrogen reflux to the lower tower is to adjust the level of liquid nitrogen in the main cooler by controlling the opening degree of the valve, thereby adjusting the heat exchange area on the nitrogen side of the main cooler. This provides an additional means of adjustment, making it easier to control the operating conditions of the equipment. For example, closing all the liquid nitrogen reflux valves during the equipment startup phase facilitates the accumulation of liquid in the equipment, thereby effectively reducing the equipment startup time.