Energy Requirements of Crude Distillation | Energy Conservation
Typical crude distillation unit, atmospheric and vacuum distillation units, uses very high quantity of high grade energy. Besides, substantial quantity of steam for the main atmospheric distillation column side streams/products stripping to achieve desired product quality specifications and for the vacuum distillation tower to aid feed vaporization and produce vacuum.
In the atmospheric distillation unit, the feed is usually heated to a sufficiently high temperature above 600°F, such that the volume of crude vaporized at the flash zone inlet of the atmospheric distillation column is approximately equal to the total volume of distillate products. Some additional vaporization of the crude is necessary to provide what is called “overflash”, which normally lees than 7% by volume.
The purpose of the “overflash” is to prevent coking and the carryover of coke to the bottom side-stream product. In conjunction of the bottom stripping steam, the overflash provides fractionation between the bottom side-stream and the tower bottoms. A minimum overflash of 3%LV, on crude, is used for atmospheric distillation tower and 2% is used for vacuum distillation tower. If better bottoms/lower side-stream separation is desired, higher overflash rates are normally used. This is accomplished via raising the flash zone temperature and/or bottoms stripping steam rates within the constraints of the maximum allowable flash zone temperature and/or stripper hydraulics.
The steam used for side-stream stripping is used to remove the traces of lighter products that are dissolved in the main tower withdrawal streams. If steam rates are very low, fractionation suffers. As steam is added, fractionation improves up to a certain point. Above such point, any extra steam has no significant stripping effect but overloads the tower and consequently the overhead condenser, too. Only part of the heat in the tower is removed by the overhead condenser.
It is possible to remove all the heat from the top. However, it is more economical to use Pumparounds and remove heat at several points in the column to help heating up the feed to the tower. It is a general practice now in many crude distillation units to use top reflux to condense only the upper one or two side-streams.
The reflux to condense the lower side-streams is obtained by installing two or more pumparounds reflux systems. The economic factors that affect that choice include: reduction in top reflux permits a reduction in the tower diameter in the section above the pumparounds, thereby reducing capital; use of the high temperature available downward in the column to heat up the incoming feed and reduce load on the fired heater and cooling water cost is reduced as well as the reduction of the total surface area due to integration among these streams and the feed to the tower. All of that is going to warrant more plates in the main column to allow the installation of the two or more pumparounds.
In certain situations, it may be economical to condense the overhead in two stages, reflux in the first one and distillate product in the second one.