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In air rectification, reflux ratio generally refers to the ratio of the liquid volume in the tower and the steam volume in the tower, which is also called the vapor-liquid ratio.
The purity of the rectifying product depends on the size of the reflux ratio under the condition that the number of tower plates is certain. When the reflux ratio is large, the purity of the gas phase nitrogen obtained is high, while the purity of the oxygen in the liquid phase is low. This is because: after the higher temperature rising gas and lower temperature downstream liquid are mixed on the tower plate for heat exchange, their temperatures can tend to be consistent under ideal conditions, that is to reach the same temperature, the outside temperature is between the original gas-liquid temperature. If the reflux ratio is large, that is, when there are more cold liquid in the downstream or less rising steam, the gas-liquid mixing temperature must be on the side of the low temperature liquid, so the temperature drop of rising steam is large, steam condenses more. Because oxygen is a volatile component, so the condensation of oxygen component is relatively more, so the nitrogen concentration of the rising gas leaving the tower plate also increases quickly. This is the case for every Tower plate, so the purity of the gas obtained on the top of the Tower is high. On the other hand, because the temperature of gas-liquid mixture is on one side of the low temperature liquid, the temperature rise of the downstream liquid is small, and the liquid evaporation is also less, so the nitrogen component in the liquid evaporates correspondingly less, in this way, the oxygen concentration of the downstream liquid leaving the tower plate increases slowly. This is the case for every Tower plate, so the oxygen concentration of the liquid obtained at the bottom of the tower is low.
When the reflux ratio is small, it is contrary to the above situation and no longer repeated. The adjustment of rectification working condition is actually mainly to change the reflux ratio of each part in the tower.
