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Dewaxing Paraffinic Feedstocks
Catalytic solutions for cold flow property improvement.
Many refiners are seeking more effective and cost-efficient ways to improve the cold flow properties of paraffinic feedstocks.
At low temperatures, products that contain waxy components start to crystallise, which has an adverse effect on the flow characteristics of the final diesel product. Three main cold flow properties characterise a diesel fuel: cloud point, pour point and cold filter plugging point. However, the industry has techniques ranging from additivation and kerosene blending to advanced catalytic dewaxing to help ensure that products meet low temperature flow properties.
Increasingly, the industry is turning to catalytic dewaxing to minimise the use of cold flow additives, reduce kerosene blending requirements, upgrade heavier feedstocks to have higher cloud and/or pour points, and so create more room in the blending pool for heavier feeds.
Renata Szynkarczuk, Senior Technical Specialist, Criterion Catalysts & Technologies, sets out the background: “Many catalytic dewaxing options are available to refiners. Applying speciality catalysts and processes can be cost-effective ways to improve the low-temperature properties of ultra-low-sulphur diesel and can help refineries to expand their market share and increase overall profitability.”
“Flow improvers can reduce the cold filter plugging point and the pour point,” Szynkarczuk says, “but the cloud point is difficult to reduce by additivation or cost-effective dilution using kerosene. This limitation becomes even more apparent when the feeds become more paraffinic and contain longer and, consequently, higher-cloud-point linear alkanes.”
Cloud point improvement using additives is typically a maximum of 3–4°C. Blending with hydrotreated kerosene will generally enable refiners to achieve a cloud point improvement (reduction) of about 1°C for every 10% of kerosene added. If the refiner is hoping for a cloud point improvement of more than 6–8°C, then catalytic dewaxing is usually the most commercially viable solution.
Catalytic dewaxing typically involves a combination of selective cracking and isomerisation reactions, as Szynkarczuk explains: “The objective is to improve the cold flow properties either by selective cracking to lighter alkanes and isoalkanes or by isomerisation of alkanes to isoalkanes of similar molecular weight, all with better cold flow properties.”
Shell Global Solutions and Criterion have developed innovative selective cracking (SDD-800) and isomerisation (SDD‑821) dewaxing catalysts for distillate applications. These speciality dewaxing catalysts use medium-pore zeolites as the active acidic catalyst component in combination with a binder to selectively crack or isomerise the linear and slightly branched alkanes in the feedstock.
The surface of the catalyst is crucial, according to Szynkarczuk. “Highly selective dewaxing can be achieved if there is no acidic function on the surfaces of the zeolite pores and channels because the reactivity of feedstock molecules is based on their shape. Linear and branched molecules react within the pore structure of the zeolite whereas larger molecules are unaffected, as they cannot gain access. Dewaxing catalysts with passive pore and channel surfaces therefore provide the best option for dewaxing by selective cracking and/or isomerisation.”
Depending on a unit’s capability, targets and dewaxing catalyst choice, Shell dewaxing technology can be applied in a first- or a second-stage configuration. In first-stage configuration, the dewaxing bed is part of the hydrotreating section in a so‑called drop-in solution. The second-stage configuration requires a minimum of two reactors with a stripper between them. The feedstock is hydrotreated in the first reactor; hydrogen sulphide and ammonia are removed in the stripper; and then the feedstock is dewaxed in a dedicated clean second reactor.
Selective dewaxing in action
A refinery operator planned to apply a deep catalytic dewaxing solution to an existing unit while minimising investment and maintaining the same depth of hydrodesulphurisation. With targeted pour and cloud point improvements of 10 to 45°C and starting from a feedstock containing between 20 and 30% of heavy paraffinic components, only a first-stage dewaxing solution was possible.
The solution involved smart adjustment of the dewaxing catalyst bed’s location and using SDD-800 dewaxing catalyst and the right combination of hydrodesulphurisation/ hydrodenitrogenation hydrotreating catalysts. This delivered a system compatible with the existing unit and offered cycles of up to five years without rejuvenating, regenerating or changing the dewaxing catalyst. The refiner found that it was possible to obtain deep to very deep dewaxing (a 20– 5°C improvement) while maintaining dewaxed diesel yields between 90 and 86 wt% on feed by combining selective cracking and isomerisation on the SDD-800 catalyst.
The dewaxing challenge becomes more significant when very paraffinic feedstocks (up to 90–95 wt%) are involved. The first step is to assess whether first- stage dewaxing is possible. In some cases, this requires temporary adjustments that deliver very deep dewaxing at an acceptable yield loss.
The option considered for a different refinery was modified first-stage dewaxing with bed-to-bed temperature control on the dewaxing catalyst for stable operation and to control the dewaxing to prevent any unnecessary yield losses. This achieved very deep dewaxing, up to 60°C, while maintaining stable unit operation and a sufficiently high yield of good-quality, deeply dewaxed diesel and gasoline.
The other option for highly paraffinic feedstocks (up to 90–95 wt% paraffinic components) is a complete revamp into a two-stage dewaxing application. Pilot plant studies have shown that, when starting from paraffinic feedstocks with pour points as high as 40–45°C, it is possible to improve the cloud point and/or pour point by 70–75°C. One of the key challenges is the presence of sulphur and nitrogen poisons that can hamper the activity and selectivity of the noble metal dewaxing catalysts.
Testing for over more than 15,000 hours provided a detailed understanding of this challenging operation by considering various paraffinic feedstock poisoning levels from under the detection limit to 19-ppm sulphur. The results showed that the same high-quality product could be produced in the presence of limited sulphur concentrations, though this required a higher temperature. The study showed that a dewaxed yield of 80–87 wt% of feed could be achieved, even at the deepest dewaxing.
“Catalytic dewaxing provides an alternative method for cold flow improvement of diesel and lubricant oil that cannot be met properly by more conventional methods,” affirms Szynkarczuk. “We have tailored the latest generation of dewaxing catalysts specifically to the application and feedstock type by utilising shape-selective zeolites to preserve maximum distillate yields. Our proprietary, outer-surface passivation method provides this dewaxing catalyst selectivity. With the current increase of heavy paraffinic material on the market, refiners have to adapt their processing facilities to meet these new challenges and we can help them.”