Three steps to optimise fractionator performance with plate technology

The refining industry has dealt with various challenges throughout its history, but the current pressure from a range of factors, such as energy optimisation, emissions reduction, debottlenecking/capacity increase, product yield and quality improvement, off-gas reduction, minimised cooling water requirement, and reliability and uptime improvement, is unprecedented.

Jay Jeong
Alfa laval

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Article Summary

All players involved in the industry, from refiners, licensors, and technology providers to EPC contractors and system builders, are working to save energy while optimising the process.

Since the crude distillation process is the largest energy consumer, much effort has been made to optimise preheat trains in both atmospheric and vacuum distillation units (ADU and VDU), where Alfa Laval has been supporting customers with more than 1000 welded plate heat exchangers. Hydrotreaters also take a significant share of energy consumption, and more and more refiners are looking at optimising combined feed exchangers (CFEs), targeting energy optimisation and molecule management through securing stability in the furnace and reactor until the end of run (EOR). Alfa Laval plate technology has been successfully used in several recent naphtha hydrotreater projects.

Although energy optimisation in the ADU/VDU preheat train and CFE in the hydrotreater have been getting attention, less effort has been given to optimising simpler columns such as fractionator and stabiliser/stripper columns (see Figure 1). Such an activity would bring various benefits, including energy savings, installation cost savings, improved molecule management, off-gas reduction, and minimised cooling water requirements.

In the sections below, three steps for fractionator optimisation will be described, with the benefits from each step.

1 - Optimise the feed bottoms exchanger
The first position that comes to mind when considering fractionator optimisation would probably be the feed bottoms exchanger.
The basic principle is to recover the maximum level of energy from the bottoms and preheat the feed to the fractionator, as the bottoms are to be cooled down while the feed needs to be heated. Maximising energy recovery from the bottoms to the feed lowers the burden to the bottoms cooler and lowers the burden to the column reboiler (see Figure 2) or the amount of steam injected. The question left is, what is the limiting factor in recovering the maximum level of energy from the bottoms to the feed within the range the feed remains stable? The answer is fully dependent on the heat exchanger technology you are using.

Considering conventional technology, such as shell and tube exchangers, it is  not possible to reach the maximum potential level because the technology itself will be the limiting factor. With shell and tube heat exchangers, you cannot achieve a tight enough temperature approach and a good level of temperature cross, even with several units in series. So the number of units in series quickly becomes unrealistically high if you want to achieve the temperature cross needed for maximum energy recovery. Besides, poor wall shear stress, even on the tube side, makes shell and tube exchangers very vulnerable to fouling, so it is necessary to have standby units in parallel.

However, with fully welded plate technology with a corrugated pattern and the possibility of full mechanical cleaning, such as the Alfa Laval Compabloc heat exchanger, the limitation discussed above is no longer an issue. The high level of turbulence promoted by the corrugated pattern brings very high heat transfer efficiency, making it possible to achieve a much tighter temperature approach than with conventional technology. Also, since the Compabloc heat exchanger can have full countercurrent flow, it is possible to have a significant temperature cross within a single unit, saving the plot space needed. High turbulence secures very high wall shear stress, which makes it possible to have a much lower fouling tendency. 

2 - Optimise the overhead condenser
Among all the challenges in dealing with the fractionator overhead condenser, corrosion control and managing pressure drop are the two most distinctive challenges. It can be said that corrosion control is easier than managing pressure drop because it is possible to upgrade the construction material to a higher grade, corrosion-resistant alloy, although the additional cost is usually a hurdle to overcome. Generally, there are several heat exchangers installed in parallel as overhead condensers to maintain the pressure drop as low as possible. A common approach is to use multiple bundles of air coolers in parallel or several shell and tube exchangers in parallel. However, there is always a limitation in keeping the pressure drop below a certain level because of the footprint of the structure or the weight of the condenser itself.

If used as a condenser, the Compabloc heat exchanger can overcome the barriers encountered by default with conventional technologies. Thanks to multiple channels with short travel lengths on the vapour side, it is possible to achieve a much lower pressure drop compared to conventional technologies. On top of that, free condensate flow paths in the plate pattern design prevent pressure drop increases related to stacked condensate, which means a lower pressure drop is maintained for longer.

The first direct benefit of a lower pressure drop in the overhead condenser is the possibility of having a lower column operating pressure because the actual column operating pressure is determined by the design operating pressure and the additional pressure needed in the overhead condenser (see Figure 3a). Lowering the pressure drop in the overhead condenser creates a column operating pressure that improves the separation between fractions in the column. Consequently, a series of indirect benefits become apparent, including energy saving in the reboiler or direct steam injection due to a lower boiling temperature and better separation in the fractionator due to improved separation dynamics.

The second direct benefit of a lower pressure drop in the overhead condenser is increased recovery of valuable molecules at the outlet of the condenser (see Figure 3b). The lower pressure drop in the overhead condenser brings the outlet pressure closer to the inlet pressure, which means the outlet pressure is maintained high. Consequently, the vapour fraction at the outlet is reduced while the liquid fraction is increased, which means more valuable molecules are recovered. So, in the end, the light end product flow (or reflux flow) is maximised while the off-gas flow is minimised.

Alfa Laval provides overhead condenser optimisation solutions with Compabloc heat exchangers in various processes ranging from ADU overhead condensers to much simpler stripper overhead condensers. Upgrading to higher grade material in Compabloc does not necessarily result in a huge cost increase thanks to the much lighter weight of heat transfer plates because Compabloc usually needs much less heat transfer area and the plate itself is thinner than a tube.

Recover and reuse low-grade energy
Several streams cooled with cooling water or ambient air waste low-grade energy because when using conventional shell and tube technology it was easier than the more expensive option of recovering the energy. Product coolers, rundown coolers, and even condensers use a considerable amount of cooling water and add a burden to the cooling water supply system. However, this can be completely changed if heat exchangers with significant temperature cross can be used. This can generate hot water or preheat boiler feed water instead of returning warm water to the water-cooling system. Recovering waste heat in this way could change those heat exchangers from cost generators to profit generators (see Figure 4).

For decades, many refiners have selected Alfa Laval solutions for waste heat recovery from low-grade energy sources around the fractionator column. For example, air coolers or shell and tube condensers have been changed to compact condensers with temperature cross  to recover energy from vapour to hot water. Similarly, simple shell and tube coolers with cooling water have been changed to coolers with temperature cross and a tight temperature approach to recover energy from the bottom product or rundown stream and generate hot water.

Recovered energy is reused in various applications, such as boiler feed water preheating, freshwater generation, wastewater evaporation, and district heating, which brings significant Opex saving at very small Capex.

While energy saving, emission control, and molecule management are gaining focus, optimising the performance of the fractionator beyond the limit of conventional technology is fully in line with such a trend. This will save energy by optimising the feed bottoms exchanger and the overhead condenser and recovering valuable molecules through the overhead condenser. Energy can be recovered by optimising coolers that use cooling water, which will also reduce cooling water usage. Of course, alternatively, the energy saved can be used for increasing capacity in the plant instead of reducing energy costs.

This short article appeared in the 2022 Refining India Newspaper, which you can view HERE

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