Dewaxing challenging paraffinic feeds
Catalytic dewaxing using recent developments in dewaxing catalysts provides an alternative method for cold flow improvement in diesel and lube oil.
RENATA SZYNKARCZUK, Criterion Catalysts & Technologies
MICHELLE ROBINSON and LAURENT HUVE, Shell Global Solutions International
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Improving the cold flow properties of paraffinic feedstocks in a selective way has become a hot topic during the last decade as refiners search for more effective and cost-efficient ways to achieve improvements in cold flow properties. The growing trend is to use catalytic dewaxing to limit the use of cold flow additives, reduce kerosene blending requirements, upgrade heavier feedstocks with higher cloud and/or pour points and, consequently, to create more room in the blending pool for heavier feeds.
Increasingly stringent specifications, the rise in new types of crude from different origins or process routes, and the desire to sell products that meet cold flow property specifications result in the need to process more challenging types of feedstocks — some of them being heavier, some lighter but of different compositions, and some being significantly more paraffinic.
This article gives an overview of the possible catalytic dewaxing solutions that can be offered to solve different challenges in improving cold flow properties. Advances in catalyst and process development by Shell Global Solutions and Criterion enable not only a better understanding of what is achievable in a prescribed set of conditions and constraints but are also leading to the development of innovative solutions in association with customers.
Examples of research and development carried out in understanding and processing paraffinic feedstocks, as well as an illustration of commercial applications of Shell Global Solutions’ and Criterion’s catalytic dewaxing for some challenging feedstocks, are highlighted here.
Cold flow improvement via catalytic dewaxing
At low temperatures, products with ‘waxy’ components start to crystallise and affect the flow characteristics of the final product. To avoid problems and to ensure that products meet low temperature flow properties, different techniques have been and are being used in the industry, from the use of additives and/or kerosene blending to advanced catalytic dewaxing.
Three main cold flow properties are typically used to characterise a diesel fuel: cloud point, the most stringent property; pour point; and cold filter plugging point. Standard industrial analytical methods are prescribed for each of these properties.
Flow improvers modify the wax crystallisation process, by reducing the crystal size and/or the lattice formation of the solid phases, and reduce both the cold filter plugging point and the pour point. However the cloud point, a property related to individual component characteristics and driven by the heaviest molecules within the feedstock boiling range, is also the most thermodynamically driven property. Consequently it is also the most difficult to effectively reduce by additives or by cost-effective dilution with kerosene. This becomes a greater challenge when feeds are becoming more paraffinic in nature, with the presence of longer and consequently higher cloud, linear alkanes.
Cold flow improvers can significantly reduce cold filter plugging point and pour point. Cloud point improvement using additives is typically within a couple of degrees, up to a maximum of 3-4°C (5-7°F). With hydrotreated kerosene blending, a cloud point reduction of ~1°C (1.8°F) is typically achieved for every 10% of kerosene added. If a cloud point improvement of more than 6-8°C (11-14°F) is desired, then catalytic dewaxing is usually a more economical long-term solution than any alternative method (additives and/or kerosene blending and/or feedstock boiling range adjustment).
Improving the cold flow properties of any feedstock requires mainly modifying or removing linear alkanes (usually referred to as paraffins). This can be achieved either by a physical separation method (extraction) or by different selective chemical reactions (catalytic dewaxing). This article focuses on the latter.1
Conversion of linear and/or slightly branched alkanes during catalytic dewaxing is typically carried out by a combination of selective cracking and isomerisation reactions (see Figure 1), the objective being to reduce cold flow properties (represented here by melting point), either by selective cracking to lighter alkanes and iso-alkanes with lower cold flow properties or by isomerisation of alkanes to iso-alkanes with similar molecular weights but lower cold flow properties.
Influence of molecular structure on cold flow properties
A significant amount of research, including the use of modern and recently developed analytical tools, has been carried out in the last five to 15 years, resulting in a large database for investigating and capturing the influence of molecular structure on cold flow properties.
These studies focused on understanding what type of isomerisation and/or selective cracking of linear alkanes had the greatest effect on cold flow improvement, what were the governing parameters, and how this knowledge can be used to design catalysts and catalytic systems of superior performance. This enables the development of solutions to a number of key issues such as achieving deep dewaxing (by several tens of degrees) of diesels or base oils with limited yield loss and limited gas formation while keeping other properties within agreed specifications or better.
Branching of linear alkanes by a single methyl group already has a significant impact on cold flow properties (represented here by melting point, as found in the literature,2 or measured on a pure sample).
For example, while nonadecane, the linear alkane with 19 carbon atoms, boiling at 329.7°C (625.5°F) within the diesel boiling range, has a melting point of +32.1°C (89.8°F), any of its single methyl-branched isomers has a significantly lower melting point, the highest being for 2-methyl-octadecane with a melting point of +13°C (55.4°F), the lowest for the isomer with a methyl group located in the middle of the chain, 9-methyl-octadecane, with a melting point of -16.5°C (2.3°F, see Table 1).
While positioning a methyl group in the middle of the chain shows the largest fall in melting point, it is also known that a single methyl group is not sufficient when chain length increases; this is illustrated by a comparison of the melting points of 9-methyl octadecane, 10-methyl eicosane and 13-methyl hexacosane (see Table 2, left). Therefore, more branching is required to further decrease the melting point, as illustrated by n-triacontane and 2, 6, 10, 15, 19, 23 - hexamethyl tetracosane (see Table 2, right).
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