Three steps to optimize performance of fractionator with plate technology
By: Jay Jeong, Refinery Market Manager, Global Technology, Business Division Energy, Alfa Laval Lund AB
The refinery industry has been dealing with various challenges throughout the history of the industry, however, the pressure onto the industry regarding to energy optimization, emission reduction, debottlenecking/capacity increase, product yield & quality improvement, off-gas reduction, minimized cooling water requirement, and reliability & uptime improvement is literally unprecedented. Various efforts are being made to save energy while optimizing the process from all players involved in the industry – ranging from refiners, licensors, technology providers to EPC contractors and system builders.
Since crude distillation process is the largest energy consumer, a lot of effort has been made onto optimizing preheat trains in ADU and VDU where Alfa Laval has been supporting customers with more than 1000 welded plate heat exchangers to ADU and VDU. Hydrotreaters also take significant share of energy consumption, more and more refiners are looking at optimizing combined feed exchangers (CFEs) targeting energy optimization and molecule management through securing stability in the furnace and reactor until the EOR, and Alfa Laval plate technology has been making this effort successful in several recent naphtha hydrotreater projects.
Though energy optimization in ADU/VDU preheat train and CFE in hydrotreater have been getting focus, less effort has been given to optimizing simpler columns such as fractionator and stabilizer/stripper while such an activity would bring various benefits including energy saving, installation cost saving, improved molecule management, off-gas reduction, minimized cooling water requirement and so on.
In the sections below, three steps for fractionator optimization will be described with benefits from each step.
Optimize the feed-bottoms exchanger
The first position which would come to your mind when you think about fractionator optimization would probably be the feed-bottoms exchanger.
The basic principle is to recover maximum level of energy from the bottoms and preheat the feed to the fractionator, as the bottoms is to be cooled down while the feed needs to be heated as much as possible. Maximizing energy recovery from the bottoms to the feed lowers the burden to the bottoms cooler and lowers the burden to the column reboiler (Fig. 2) or the amount of steam injected. The question left is what the limiting factor is in recovering maximum level of energy from the bottoms to the feed within the range the feed remains stable, and that is fully dependent on heat exchanger technology you are using.
If you still consider conventional technology, such as shell and tube exchanger, you are not reaching the potential maximum level because the conventional technology itself will be the obvious limiting factor. With shell and tube heat exchanger, it is not possible to achieve tight enough temperature approach and it also is not possible to have good level of temperature cross even with several units in series. So, the number of units in series easily becomes unrealistically high if you want to achieve temperature cross needed for maximum energy recovery. Besides, poor wall shear stress, even on tube side, makes the shell and tube exchangers very vulnerable to fouling which makes it necessary to have standby units in parallel.
However, with fully welded plate technology with corrugated pattern and full mechanical cleaning possibility, such as Alfa Laval Compabloc heat exchanger, above listed limitation is not an issue any longer. High level of turbulence promoted by corrugated pattern brings very high heat transfer efficiency which makes it possible to achieve much tighter temperature approach compared to conventional technology. Also, since the Compabloc heat exchanger can have full countercurrent flow, it also is possible to have significant temperature cross within single unit which will save plot space needed. High turbulence secures very high wall shear stress which makes it possible to have much lower fouling tendency.
Optimize the overhead condenser
Among all the challenges you are having in dealing with fractionator overhead condenser, corrosion control and managing pressure drop are the two distinctive challenges. It can be said that corrosion control is easier than managing pressure drop because upgrading material of construction to higher grade alloy which would prevent corrosion, though the cost is usually a hurdle to overcome.
Usually, there are several heat exchangers installed in parallel as overhead condenser to manage the pressure drop as low as possible. Common approach is to use multiple bundles of air coolers in parallel or several shell and tube exchangers in parallel, however, there always is a limitation in keeping pressure drop below certain level because of the footprint on the structure or weight of condenser itself.
If it is used as condenser, Alfa Laval Compabloc heat exchanger can make it possible to overcome the barriers which conventional technologies have as a default. Thanks to multiple channels with short travel length on vapor side, it possible to reach much lower pressure drop compared to conventional technologies. On top of that, free condensate flow paths in plate pattern design prevents pressure drop increase related to stacked condensate, which means pressure drop is kept low for long time.
The first direct benefit of lower pressure drop in the overhead condenser is possibility to have lower column operating pressure because the actual column operating pressure is decided by the design operating pressure and additional pressure needed in the overhead condenser (Fig. 3-1). Therefore, by lowering the pressure drop in the overhead condenser, you have column operating pressure which brings improved separation between fractions in the column. Consequently, series of indirect benefits become visible including energy saving in reboiler or direct steam injection by having lowered boiling temperature and better separation in the fractionator thanks to improved separation dynamics.
The second direct benefit of having lower pressure drop in the overhead condenser is increased recovery of valuable molecules at the outlet of the condenser (Fig. 3-2). Lower pressure drop in the overhead condenser makes the outlet pressure closer to the inlet pressure, which means the outlet pressure is maintained high. Consequently, the vapor fraction at the outlet gets lower while the liquid fraction gets higher, which means more valuable molecules recovered. So, in the end, the light end product flow (or reflux flow) is maximized while the off-gas flow is minimized.
Alfa Laval has been providing overhead condenser optimization solutions with Compabloc heat exchanger in various processes ranging from ADU overhead condenser till much simpler stripper overhead condensers. Upgrading to higher grade material in Compabloc does not necessarily mean huge cost increase thanks to much lighter weight of heat transfer plates because Compabloc usually needs much less heat transfer area and also the plate itself is thinner than that of tube.
Recover and reuse low-grade energy
There are several streams which are cooled down with cooling water or ambient air simply because it is easier to simply waste those low-grade energy than recover them because it would be too costly to recover them with conventional shell and tube technology. Product coolers, rundown coolers and even condensers are using considerable amount of cooling water and adding burden to the cooling water supply system.
However, this can be completely changed if heat exchangers with significant temperature cross can be used and generate hot water or preheat boiler feed water instead of returning warm cooling water back to cooling water supply system. Recovering waste heat in this way would turn those heat exchangers from cost generator to profit generator (Fig. 4).
There are many refiners who have selected Alfa Laval solution for waste heat recovery from low-grade energy source around fractionator column for decades. For example, air cooler or shell and tube condensers have been changed to compact condenser with temperature cross which recovers energy from vapor to hot water, simple shell and tube coolers with cooling water have been changed to coolers with temperature cross and tight temperature approach which recovers energy from bottom product or rundown stream and generate hot water.
Recovered energy are reused in various applications, such as, boiler feed water preheating, fresh water generation, waste water evaporation, district heating and so forth, which brings significant OPEX saving at very small CAPEX.
Conclusion
While energy saving, emission control and molecule management are getting focus, optimizing the performance of fractionator beyond the limit of conventional technology is fully in line with such a trend. You can save energy through optimizing the feed-bottoms exchanger and the overhead condenser, recover valuable molecule through optimizing the overhead condenser, and you can also recover energy by optimizing coolers which use cooling water. Optimizing coolers also will save cooling water usage. Of course, alternatively, the energy saved can be used for increasing capacity in the plant instead of reducing energy cost.
Alfa Laval has been providing optimization solutions to refinery customers around the world and more than 3000 working references are preaching to the world that now is the time to act.