Circulating-reflux stream

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. 

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. 

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

Heavy cycle oil, heavy naphtha, and other circulating side pumparound reflux streams are used to remove heat from the fractionator. They supply reboil heat to the gas plant and generate steam. The amount of heat removed at any pumparound point is set to distribute vapor and liquid loads evenly throughout the column and to provide the necessary internal reflux. 

Description Hydrocarbon feed is preheated with hot circulating solvent and fed at a midpoint into the extractive distillation column (EDC). Lean solvent is fed at an upper point to selectively extract the aromatics into the column bottoms in a vapor/liquid distillation operation. Nonaromatic hydrocarbons exit the column top and pass through a condenser. A portion of the overhead stream is returned to the column top as reflux to wash out any entrained solvent. The balance of the overhead stream is the raffinate product, requiring no further treatment. 

The schemes with natural reflux are operated based on the principle of natural convection (i.e. circulation based on density difference between cold and heated fluid streams) to create a thermo-siphon. The hotter stream has the lower density. In natural reflux, the heated feed passes to the top of the heat exchanger and then to the bottom of tower by itself while the colder stream drains from the tower bottom to the heater. Forced reflux is based on using pumps for the circulation. 

Operation. Typical operating conditions for butadiene recovery plants have been described by Buell and Boatright (I). These data were used as a guide for setting the conditions used in this study. At the start of a run, the column was preheated electrically to the desired column temperature profile. The C4 feed was pumped into the column until the pressure reached 30-35 psig. Solvent (preheated to 130°F) was then circulated through the column, and the reboiler temperature increased to about 275°F. The hydrocarbon feed was pumped in at a rate which gave the desired solvent-to-hydrocarbon feed ratio. A portion of the butene stream was returned to the column to provide reflux. Temperature of the solvent feed tray was used to control the amount of reflux. Only that portion of C4 s which dissolved in the solvent could be carried down the column. The excess C4 s were revaporized to the hydrocarbon trays. 

Liquid from the column base is split Splitter S2 in Fig. 13.3) between the bottoms and a circulating stream that flows through a HeatX model used for the reboiler. The bottoms flow rate is set equal to that foimd in the base case (720.7 kmol/h). Note that the feed and bottoms flow rates are set to same values as the base case. Therefore, the distillate flow rate must also be the same. Using the same reflux flow rate should yield exactly the same tray and product compositions, which is indeed true (99.9 mol% purities of both product streams). 

Feed from the Toluene tower is preheated (1) by the MX tower distillate product and then enters the MX tower (2) at approximately the middle tray. The MX tower overhead is totally condensed using heated water firom the gas fired reboiler (3). Steam is produced in the condenser (4) and dehv-ered to a utility header. Flow of water into the shell side of the condenser is manipulated by a level controller. The condensed MX tower overhead material is collected in an accumulator (5). A level controller manipulates the reflux to the tower to maintain the accumulator level. The overhead product flow is set externally, either by an advanced controller or manually. At the bottom of the MX tower the flow is divided into two streams. One of the streams is circulated through the gas fired reboiler, where it is partially vaporized and then returned to the MX tower. The vaporization rate is set... 

Process requirement Provide reflux Ibr a distillation column. Circulate cooling water to various heat exchangersv Inject chemical into a process stream. 

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