How to adjust the cold reflux in a fractionation tower

Isn’t the top temperature used to control the dry point of crude gasoline?

The top temperature of the fractionation tower is controlled by the top circulation flow rate and the temperature of the material returning to the tower; when the heat absorption load of the top circulation reaches its upper limit, crude gasoline is used as a cold reflux to adjust the dry point of the crude gasoline. For the control of the dry point of crude gasoline, I personally prefer to adjust it across various stages in order to achieve optimal operation. After all, using cold reflux increases the energy consumption of the facility and raises the pressure drop across the trays. However, in terms of operation, adjusting the dry point of gasoline with cold reflux results in a rapid response with minimal lag

When considering the thermal balance and heat removal in the fractionation tower from a bottom-up perspective, it is necessary to take into account the degree of superheat removal alongside the amount of oil slurry returning to the tower; the temperature of the oil returning to the middle section of the tower and the temperature at which diesel is extracted must be such that the diesel distillate meets the required standards. Using cold reflux to adjust the tower temperature increases the load on the tower and raises the likelihood of salt formation at the top of the tower; therefore, top circulation should be utilized as much as possible to control the temperature at the top of the tower

The top temperature serves only as a reference point for the dry point of crude gasoline; when the top temperature remains constant, changes in the vapor pressure of the oil and gas result in corresponding changes in the dry point of crude gasoline.

First, it is necessary to clarify the purpose behind the designer’s use of cold reflux. In my opinion, it serves two main functions: 1. At the beginning of operation or in the event of an accident, when reflux cannot be effectively established on the trays, cold reflux is used to control the temperature at the top of the tower. 2. During normal production, when the temperature at the top of the tower is high and the top circulation is not sufficient to meet the temperature control requirements, cold reflux can be used as a supplementary reflux. The purpose of cold reflux, whether under normal or abnormal conditions, is to control the top temperature. Now let’s focus on the second scenario. In my opinion, during normal operation, the cold reflux should be turned off, for the following reasons: 1. When the heat distribution throughout the tower is uneven, it becomes difficult to control the top temperature; in such cases, top circulation should be used as a means of adjustment first. If there is no more room for adjustment, then it is necessary to consider whether the amount of heat taken up by the other reflows in the tower is appropriate and make corresponding adjustments. 2. Due to its low temperature, cold reflux can easily cause salt buildup on the tray surfaces over time, leading to blockages in those trays. 3. Increase the load on the rear cooler. The above are my personal views; I welcome corrections and criticism from fellow sailors

You can still adjust the vapor-liquid partial pressures! Impressive, but how is it adjusted to control the reaction pressure by adjusting the dryness level?