Refining India 2022 Newspaper

refining india 2022

Three steps to optimise fractionator performance with plate technology

Jay Jeong Alfa laval

Introduction The refining industry has dealt with vari- ous challenges throughout its history, but the current pressure from a range of fac- tors, such as energy optimisation, emis- sions reduction, debottlenecking/capacity increase, product yield and quality improve- ment, off-gas reduction, minimised cool- ing water requirement, and reliability and uptime improvement, is unprecedented. All players involved in the industry – from refiners, licensors, and technology provid- ers to EPC contractors and system build- ers – 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 distilla- tion units (ADU and VDU), where Alfa Laval has been supporting customers with more than 1000 welded plate heat exchang- ers. Hydrotreaters also take a significant share of energy consumption, and more and more refiners are looking at optimis- ing combined feed exchangers (CFEs), tar- geting 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 ben- efits, including energy savings, installation cost savings, improved molecule manage- ment, 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.  Optimise the feed bottoms exchanger The first position that comes to mind

Energy consumption in refinery

P min

Process

Consumption (MW)

% 33 15

Atm. distillation Vacuum Dist. Visbreaking Delayed coking Hydrocracking Hydrotreating FCC

217.3 100.6 0.9 23.5 73.1 18.3 15.5 2.5 7.3 2.8 150.5 50.6 662.9

25˚C

min

Ogas

0 4 11

dP min 30˚C

dP min

Fractionator

max

Light end

Reex drum

Reforming Alkylation

8 2 0

Ethers

lsomerisation

R/D

Lube oil

P min

Total

100

Based on capacity 150,000 bpd / reference EIA, USA

Feed

Table 1

when considering fractionator optimisa- tion would probably be the feed bottoms exchanger. The basic principle is to recover the maximum level of energy from the bot- toms and preheat the feed to the frac- tionator, as the bottoms are to be cooled down while the feed needs to be heated.

Bottom product

Figure 3a First direct benefit of lower pressure drop in the condenser

Figure 3b Second direct benefit of lower pressure drop in the condenser

However, with fully welded plate technol- ogy with a corrugated pattern and the pos- sibility 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 pro- moted by the corrugated pattern brings very high heat transfer efficiency, making it possible to achieve a much tighter tem- perature approach than with conventional technology. Also, since the Compabloc heat exchanger can have full countercur- rent flow, it is possible to have a significant temperature cross within a single unit, sav- ing the plot space needed. High turbulence secures very high wall shear stress, which makes it possible to have a much lower fouling tendency.  Optimise the overhead condenser Among all the challenges in dealing with the fractionator overhead condenser, cor- rosion 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 mate-

rial to a higher grade, corrosion-resistant alloy, although the additional cost is usu- ally a hurdle to overcome. Generally, there are several heat exchangers installed in parallel as overhead condensers to main- tain the pressure drop as low as possible. A common approach is to use multiple bun- dles 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 conden- sate 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 pres- sure drop in the overhead condenser is the possibility of having a lower column oper- ating pressure because the actual column operating pressure is determined by the design operating pressure and the addi- tional 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 frac- tions 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 boil- ing temperature and better separation in the fractionator due to improved separa- tion dynamics. The second direct benefit of a lower pressure drop in the overhead condenser is increased recovery of valuable mole- cules at the outlet of the condenser (see Figure 3b ). The lower pressure drop in

Did you know Alfa Laval plate technology has been successfully used in several recent naphtha hydrotreater projects?

Maximising energy recovery from the bot- toms 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 recov- ering 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 poten- tial 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 unre- alistically 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.

25˚C

Ogas

Fractionator

Light end

Reex drum

Light end

Reex drum

T max

R/D

Feed

Q min

Reboiler

Feed

Feed bottoms exchanger

Q max

Bottom product

Bottoms cooler

Figure 2 Optimising the feed bottoms exchanger

Figure 1 Fractionator