Distillation Columns – Distillation is one of the most energy intensive operations in the petroleum refinery. Distillation is used throughout the refinery to separate process products, either from the CDU/VDU or from conversion processes.
Distillation Columns | Crude Distillation Unit (CDU) Energy Components and Guidelines
The incoming flow is heated, after which the products are separated on the basis of boiling points. Heat is provided by process heaters and/or by steam Energy efficiency opportunities exist in the heating side and by optimizing the distillation column.
The optimization of the reflux ratio of the distillation column can produce significant energy savings. The efficiency of a distillation column is determined by the characteristics of the feed. If the characteristics of the feed have changed over time or compared to the design conditions, operational efficiency can be improved.
If operational conditions have changed, calculations to derive new optimal operational procedures should be done. The design reflux should be compared with the actual ratios controlled by each shift operator. Steam and/or fuel intensity can be compared to the reflux ratio, product purity, etc. and compared with calculated and design performance on a daily basis to improve the efficiency.
Check Product Purity
Many companies tend to excessively purify products and sometimes with good reason. However, purifying to 98% when 95% is acceptable is not necessary. In this case, the reflux rate should be decreased in small increments until the desired purity is obtained. This will decrease the reboiler duties. This change will require no or very low.
Seasonal Operating Pressure Adjustments
For plants that are in locations that experience winter climates, the operating pressure can be reduced according to a decrease in cooling water temperatures. However, this may not apply to the VDU or other separation processes operating under vacuum. These operational changes will generally not require any investment.
Reducing Reboiler Duty
Reboiler(s) consume a large part of total refinery energy use as part of the distillation process. By using chilled water, the reboiler duty, in principal, can be lowered by reducing the overhead condenser temperature. A study of using chilled water in a 100,000 bbl/day CDU has led to an estimated fuel saving of (12.2 million Btu/hr) for a 5% increase in cooling duty 2.5 million Btu/hr, assuming the use of chilled water with a temperature of 10°C (50°F). The payback period is normally at 1 to 2 years, however, excluding the investments to change the tray design in the distillation tower. This technology is not yet proven in a commercial application. This technology can also be applied in other distillation processes.
Upgrading Column Internals
Damaged or worn internals can result in increased operation costs. As the internals become damaged, efficiency decreases and pressure drops rise. This causes the column to run at a higher reflux rate over time. With an increased reflux rate, energy costs will increase accordingly. Replacing the trays with new ones or adding a high performance packing can have the column operating like the day it was brought online. If operating conditions have seriously deviated from designed operating conditions, the investment may have a relative short payback.
New tray designs are marketed and developed for many different applications. When replacing the trays, it will often be worthwhile to consider new efficient tray designs. New tray designs can result in enhanced separation efficiency and decrease pressure drop. This will result in reduced energy consumption. When considering new tray designs, the number of trays should be optimized.
Steam is injected into the process stream in strippers. Steam strippers are used in various processes, and especially the crude distillation unit (CDU) is a large user. The strip steam temperature can be too high, and the strip steam use may be too high. Optimization of these parameters can reduce energy use considerably. This optimization can be part of a process integration for the particular unit.
The concept of dividing-wall columns originates from 1949, but it was not until recently that practical and commercial designs became possible. Process integration studies and other development work since the early 1990’s have resulted in the first commercial application of the dividing-wall distillation column. A dividing-wall column integrates two conventional distillation columns into one column, increasing heat transfer.
Dividing-wall columns can save up to 30% in energy costs, while providing lower capital costs, compared to conventional columns. Current divided wall columns applications are limited to benzene removal from gasoline or the separation of lighter fractions in gasoline production. Further development for the major distillation processes in the petroleum refining industry is proving to be warranted.
Energy utilized in fractionation as mentioned above is the major thermal energy consumption component not only in crude refining but also in oil and gas industry.
A typical light ends recovery process area uses considerable amount of low grade heat. Normally, the energy quantity is in hundreds of millions of energy units (Btu/hr). Such energy if not met by waste heat from other process areas it will be satisfied by steam. In many plants steam is usually used most of the time to avoid inter-unit integration for process reliability reasons.
The right ends application in oil and gas industry (oil refining) is picked here in our discussion about distillation columns since it is a clear cut opportunity for energy conservation in fractionation/distillation design and operation.
The principles for distillation columns design and operation for energy conservation is well established for many years. Such principles are well explained using the oil and gas industry light ends fraction process area mentioned above. These principles are the degrees of freedom in the design and operation as well as the optimization of the distillation columns. Such degrees of freedom can be listed as follows:
- Minimize reflux ratio/increase number of trays to decrease utilities consumption
- Optimize feed tray location
- Optimize side stream draw-off location
- Use inter-heaters and inter-coolers and optimize its flowrate and temperature draw and return
- Maximize heat integration with the rest of the process areas
In case of the existing units, it may be possible to reduce utilities by retrofitting the tower internals to increase the number of theoretical trays. For instance, you can replace the trays with more efficient ones or the packing with high efficient ones. Since the old trays/packing may not be working at their full potential efficiency. With regard to the feed tray location or side streams draw, badly-positioned feed or side stream location can result in using of excessive reflux.
In cases where we have multiple-feed and/or multi-product draw offs, studying the optimal locations will be warranted. The optimum feed location tray is the one that creates minimum reflux requirements and hence minimum heating and cooling loads on both reboiler and condenser.
Nowadays, due to the rising cost of energy components worldwide, new unconventional ways for energy conservation are kicking in for application especially in energy intensive process such as oil and gas industry including oil refining and petrochemical plants.
For instance, new sequences of fractionation columns are arising now not only in light ends applications but also in oil refining processes.
In such sequences towers are integrated among themselves and/or the rest of the process. Among themselves different operating pressure schemes for such columns can be utilized to make possible the thermal integration of a condensation (condenser) in one column with vaporization (reboiler) of another. The heat pump applications are also emerging through vapor compression of the overhead of one column to be used as a heating medium for the reboiler of another column or the same column if the column temperature range is not long and the difference between the condensation and vaporization temperature sis not high. A third unconventional energy saving design feature in distillation column is the one that started to be widely used today not only in crude fractionation but other chemical and petrochemical applications.
It is the application that is addressing not only the energy quantity conservation but also the energy quality conservation. Pumparounds in crude fractionation enable the tower to get its heating from several points along the tower length. It is the concept of minimizing Energy loss via distributing the heat supply and/or heat rejection along the length of the column instead of putting it at one end and rejecting it at the other one.
In crude fractionation it enabled us better integration of the column with the rest of the process. In other applications you can use lower quality energy in fractionation heating purposes and save the high quality energy/steam for instance and extract it in another place where a combined heat and power system can use steam turbine to letdown the high pressure steam and get work from it before sending the exhaust steam for the tower heating application required. It is an excellent practice we all know that the use of inter-coolers and inter-heaters will enhance the energy efficiency but on the expense of the number of trays that need to be increased and the hydraulics/traffic of vapor and liquid inside the tower that is essential to be scrutinized.
In the rest of this part again about fractionation an industrial example in oil refining will be discussed.