5 replies to this topic

[bluewater]

Someone please help me understrand these lines.... plz explain in simple language


"The energy efficiency of the distillation operation is also improved by using
pump-around reflux. If sufficient reflux were produced in the overhead condenser
to provide for all sidestream drawoffs as well as the required reflux, all of the
heat energy would be exchanged at the bubble-point temperature of the overhead
stream. By using pump-around reflux at lower points in the column, the heat
transfer temperatures are higher and a higher fraction of the heat energy can be
recovered by preheating the feed".

[Zauberberg]

It means that, by using circulating reflux (pumparound) from lower points in the column, you are drawing a stream that is at a higher temperature than the column overhead stream. Since it is at a higher temperature, you have more driving force for heat transfer than if you were just to use the overhead stream for the same purpose.

That is exactly the configuration of real crude distillation towers, where you can find up to 4 circulating reflux streams - Heavy Naphtha, Kero, Diesel, and AGO. All these streams are used for preheating the Crude oil feed, and at the same time you are removing heat from the column at several locations instead of using only top reflux. Using only the top reflux would not allow you to preheat the Crude higher than the column overhead temperature is, and by using pumparounds you can heat the crude up to 250 degC or even higher, depending on the column/unit configuration.

[bluewater]

It means that, by using circulating reflux (pumparound) from lower points in the column, you are drawing a stream that is at a higher temperature than the column overhead stream. Since it is at a higher temperature, you have more driving force for heat transfer than if you were just to use the overhead stream for the same purpose.

That is exactly the configuration of real crude distillation towers, where you can find up to 4 circulating reflux streams - Heavy Naphtha, Kero, Diesel, and AGO. All these streams are used for preheating the Crude oil feed, and at the same time you are removing heat from the column at several locations instead of using only top reflux. Using only the top reflux would not allow you to preheat the Crude higher than the column overhead temperature is, and by using pumparounds you can heat the crude up to 250 degC or even higher, depending on the column/unit configuration.

thankyou...for the ans.
can u explain how does these pump-arounds help in controlling column temperature at the point they enters..

[Padmakar Katre]

thankyou...for the ans.
can u explain how does these pump-arounds help in controlling column temperature at the point they enters..

Hi,
The pump around liquid after exchanging heat with the crude feed stream will return at lower temperature at a certain return tray say Tray#8. Now the vapors flowing to tray #8 will partially condense thus lowering the heat load on the overhead condenser. This is the normal heat integration in refineries as Zauberber as said in his post. In Crude Distillation Unit the typical vaprization of crude feed at the outlet of crude furnace or the main fractionator flash zone @ 60% with temperature in the range of 650 to 700 deg F it means that the stream contains lots of heat. Now in a typical distillation column you have a bottom reboiler to add heat and a overhead condeser to control the column top temperature. If we assume in CDU fractionator, we have just overhead condenser to control top temperature. The size of the condenser will be too large which will result the high capex and high opex (high cooling water circulation or air cooler power consumption). So instead to put the entire heat load in ovhd condenser, the liquid at higher temperatures is withdrawn and returned back to column at lower temperature after exchanging heat with cold streams available in the unit. This not only helps in removal of the heat load but also help in lowering costs associated with heating of the cold streams.

[arun261085]

Is pump around is used only for heat recovery?? Since it is a circulating reflux how it is going to affect the product purity?? Because u r taking a draw from tray say 8th recovering its heat and putting it back in some top tray say 5 by this u r dumping more less volatile compounds in top 5 th tray..How this action going to help in mass transfer???

[ramlalithravi]

The concept behind this pumparound reflux is to preheat the feed stream (Low temperature inlet stream to column) by utilizing Internal column reflux stream (Hot temperature stream at column bottom tray), which ultimately has advantage over utilization of heat at an extent as it will reduce the condenser duty as well as preheat the inlet stream.

Ditillation column actually functions based on two criterias, Temp. diff and Conc. diff, both can be enhanced usually with column reflux, additionally internal reflux stream has significance over it. As generally towards bottom of distillation column has heavier components in the stream and while refluxing this stream back at lower temperature to top tray leads to more condensation of heavier components, this will results in concentration diff. of heavier component between top and bottom tray, conversely mass transfer rate will be stimulated and enhances the effective separation process. And so on heavier condensate stream will be revaporized and continue to rise through the column again, continuously unless required specification is achieved in the product.