Circulating reflux

Normally, all of the heat is removed from the fractionator by three or more circulating reflux streams. The proportion of gas and naphtha in the cracked products is much higher than in crude oil, so it is seldom possible to reduce the diameter of the tower top as in atmospheric pipe still design. Due to the low operating pressure, it is necessary to provide expensive compression capacity to permit recovery of these light hydrocarbons in subsequent equipment. 

Often, we remove heat from a tower, at an intermediate point, by use of apumparound or circulating reflux. Figure 7.11 is a sketch of such a pumparound. In many towers, the liquid flows in the pumparound section are greater than in the other sections, which are used for fractionation. That is why we are often short of capacity and initiate flooding in the pumparound or heat-removal section of a column. 

There are two ways to remove heat from a distillation tower top reflux and circulating reflux. In this chapter, we call a circulating-reflux stream a pumparound. 

Figure 12.1 shows an alternate method, called circulating reflux, or pumparound, to remove heat from a tower. Hot liquid, at 500°F, is... 

Frequent pressure surges, such as those occurring when slugs of water are introduced into a hot column or into circulating reflux streams. 

Figure 17.1 shows an alternate method, called circulating reflux or pumparound, to remove heat from a tower. Hot liquid, at 500°F, is drawn from tray 10, which is called the pumparound draw tray. The liquid pumparound is cooled to 400°F. The cooled liquid is returned to the tower at a higher elevation onto tray 9. It appears from Fig. 17.1 that the cold 400°F pumparound return liquid is entering the downcomer from tray 8. This is often good design practice. Tray 9 is called the pumparound return tray. 

My conclusion is that the preheat exchanger (E-1), the circulating reflux loop (E-2), and trays 33-40 serve no real chemical engineering purpose. But don t take my word for this. Try it on your own unit, as I did in Mumbai, India last week. 

Circulating reflux differs from the foregoing because it is not vaporized. It is able to remove only the sensible heat quantity that is represented by its change in temperature as it circulates. This reflux is withdrawn from the tower as a liquid at a high temperature and is returned to the tower after having been cooled. This type of reflux may be conveniently used to remove heat at points below the top of the tower side reflux). If used in this manner, it tends to decrease the volume of vapor that the tower must handle. This is illustrated in Fig. 16-1 by the dotted lines. Other means or combinations are occasionally employed, and each manner of withdrawing side-reflux heat requires a different tower diameter. 

The fact that the quantity of internal reflux flowing from the top plate of the tower is always the same, regardless of the type of external reflux, used, is confusing. Vapor arises from the plate below the top and is condensed by the reflux on the top plate. If cold reflux is used, it will take about 2 lb of vapor to heat and vaporize 1 lb of cold reflux. Thus about 2 lb of internal reflux flow from the top plate for each pound of cold reflux that is admitted to the top of the tower. Similarly, 1 lb of vapor may deliver enough heat, as it condenses, to heat more than 2 lb of circulating reflux. In this ca.se, the amount of internal reflux is less than the amount of circulating reflux. When hot reflux is used, the amount of reflux is about the same as the amount of internal reflux. 

Example 16-2. Quantity of Reflux. A tower fractionating system is such that 2,000,000 Btu per hr of reflux heat must be removed. Example 16-1 illustrates the method of determining the reflux heat. How many pounds and gallons of (1) hot, (2) cold, and (3) circulating reflux are required ... 

Circulating Reflux. Assume the reflux is cooled from 300 to 200T. 

Example 16<-3. Calculation of Top Temperature. See Fig. 16-2 and Example 16-1. The top temperatures for hot, cold, and circulating reflux will be computed. This system was actually operated with hot reflux and with circulating reflux, and hence a comparison with the actual temperatures is possible. Tower pressure 780 mm at the top. 

Circulating Reflux. When circulating reflux was used, the top temperature was 244°F. The reflux circulated between 264 and 166°F. The circulating reflux does not vaporize, and hence only the gasoline vapor and the steam need be considered. Although the reflux heat is increased by the lower top temperature of 244 F, it does not affect the computations because no reflux passes overhead. 

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