Sometimes feeds going to a distillation column are subcooled. One reason a feed may be subcooled is because during energy optimization of the plant the feed stream going to the column was identified as a source of heat that could be cross-exchanged with colder stream. Another reason is it may be advantageous to subcool the overhead product of a column at the condenser instead of using an additional product cooler. The biggest problem is that these factors are often not taken into account when sizing column internals. This may lead to a premature flooding, loss of column efficiency, and reduced capacity.
Excessive sub-cooling of a reflux feed to a distillation column can lead to a variety of operational problems. Excessive subcooling of a reflux feed condenses some of the internal vapor traffic. This, in turn, increases the liquid traffic in the affected area of the column (Figure 1). Often, the operations department will try to counter this by cutting back on the amount of reflux being introduced into the column or by adjusting the condenser duty. Reductions in reflux being introduced will have an effect on the effectiveness of the column.
Sub-cooling also reduces overall column efficiency. The subcooled reflux or feed shifts some of the internal equipment from a mass-transfer service to a heat-transfer service.
Subcooled Liquid Feed Mechanism
Sub-cooled liquid feed is at a temperature below its column-pressure bubble point. The effect of a subcooled feed or reflux can be estimated by:
LF change in liquid flow at the feed stage
F total moles of feed (reflux)
H molar enthalpy of liquid feed at conditions to the column
h* molar enthalpy of liquid feed at the column pressure boiling point
Heq molar enthalpy of vapor which would exist in equilibrium with the feed if the liquid
feed were at the column pressure boiling point.
Referring to Figure 1, we see that LF equals L2-L1. When a sub-cooled liquid feed is used, the increase in liquid molar flow at the feed stage is greater than the liquid molar feed rate alone. Vapor rising to the feed stage is condensed in order to raise the feed conditions to the bubble point temperature. The condensing vapor increases the liquid flow leaving the feed stage, flooding the column (Figure 2.)
After a revamp of a commercial petrochemical column the column was started up and lined out. Operations brought the column up to the new design-operating rate. Before the column reached the new design-operating rate the column started to experience a loss of efficiency. The capacity of the column fell five to ten percent short of the design capacity. The column was gamma scanned and the scan revealed that the top five to six trays had an extremely high liquid level on the tray active area. The down comer also had an extremely high clear liquid back up. All of these conditions were consistent with a premature flooding condition.
A test run was performed to evaluate the column’s performance and to verify the design. The data collected from the test run was used to evaluate the model.
While reevaluating the model, it was discovered that 100°F (55ºC) subcooled reflux was being introduced into the column. The use of sub cooled reflux was missed during the design phase of the column revamp project. The simulation was rerun using the subcooled reflux conditions.
The results of the simulation indicated that the liquid traffic in the rectification section of the column was dramatically higher than previously used in the design. The internals were re-rated with the loadings from the simulation. It was determined that the column would get about five percent less capacity than planned. This was consistent with the results seen from the simulation and the gamma scan.
Sub-cooling the reflux was a normal part of the operation of the plant. The reflux was subcooled to make the plant more energy efficient. This practice could be abandoned if necessary. The operations group agreed to increase the temperature of the reflux in order to determine if this was the problem with the column. Once the reflux was introduced to the column at bubble point temperature, the capacity and efficiency of the column increased. The column was able to handle the new design-operating rate and the efficiency of the column was within design specifications.
The introduction of subcooled feed into a column may cause operational problems and lead to premature flooding. If a subcooled reflux feed is going to be used, the effects of this stream must be accounted for in the design of the column.