How can we improve the cold flow properties in the ULSD from our gasoil hydrotreater?Mar-2022
Rainer Rakoczy, Clariant Catalysts, Rainer.Rakoczy@clariant.com
Optimised feed management of opportunity crudes combined with co-processing of biogenic triglycerides, such as used cooking oil, fats from sewage, or vegetable oils, calls for appropriate dewaxing solutions. Clariant and its partners offer an optimised catalytic dewaxing solution with the Hydex Catalyst portfolio to follow any cold flow requirements for jet fuel, on-road diesel, and beyond. In addition, Clariant offers various additives such as DodiFlow for the final cold flow adjustment.
Eelko Brevoord, Catalyst Intelligence Sarl, Brevoord@catalyst-intelligence.com
ULSD cold flow properties are determined by the distillation curve and product composition. Especially normal paraffins create pour cold flow properties. With the co-processing of vegetable oils in mind, converted to normal paraffins, cold flow properties are of some interest again. Cold flow properties can be improved by:
1. Processing fewer paraffin feeds
2. Reducing cutpoint. As a consequence, considerable yield loss is experienced.
3. Adding flow improver additives or blending the diesel with kerosine. Both options may lead to substantial costs.
4. Cracking dewaxing with a shape-selective zeolite, converting the paraffins in the diesel fraction to LPG and gasoline. Consequently, some diesel yield is also lost, but not as much compared to reducing cutpoint. By replacing part of the hydrotreating catalyst with dewaxing catalyst, cold flow properties can be improved. The unit can be operated in winter and summer modes by changing the reactor temperature, improving the cold flow properties only when required.
5. Isomerisation dewaxing, converting the normal paraffins to branched paraffins. Yield loss is minimised. As this operation needs usually a sweet environment, free of H2S and NH3, a special unit design is required, while the previous options can be drop-in solutions.
Chad Perrott, Albemarle Corporation, email@example.com
Cold flow properties such as cloud point, pour point, and cold filter plugging point are a function of the diesel paraffin content. Longer carbon chain n-paraffins exhibit poor cold flow properties, and the concentration must be reduced via one or a combination of several solutions to meet desired product specifications.
One avenue is to dilute the n-paraffin content with a lower cloud point material, such as blending kerosene into the diesel pool. Although this is a relatively simple solution, it has the disadvantage of downgrading kerosene to diesel and may impact other diesel properties, such as end point.
Another solution is to adjust the feedstock quality by reducing the end point and removing the high carbon number n-paraffins. This solution has the benefit of potentially improving the performance of the hydrotreating catalyst; however, it also downgrades diesel feed to vacuum gasoil and lowers the yield of diesel products.
Finally, cracking or isomerisation dewaxing catalysts enable improvement of cold flow properties without requiring any feed adjustments or blending of other streams. Cracking dewaxing catalysts work by cracking n-paraffins to shorter carbon numbers with lower cold flow properties. Isomerisation dewaxing catalysts isomerise n-paraffins to iso-paraffins, thereby improving cold flow properties while maintaining molecules in the diesel boiling range. When only moderate cold flow improvement is required in trim dewaxing, either catalytic solution can provide comparable performance. However, when high severity cold flow improvement is required, such as the production of winter or arctic diesel, an isomerisation catalyst is the preferred route because cracking catalysts result in high yield loss with the production of naphtha and light ends.
Regardless of the technology selected for catalytic dewaxing, a reduction in treating catalyst volume to make room for dewaxing catalyst is typical. This change can result in higher space velocities and operating severity to ensure dewaxing activity. Higher severities can lead to unit cycle length constraints. The treating section can be rebalanced by using bulk metal catalysts with increased volumetric activity in HDN/HDA reactions to mitigate cycle length concerns when initiating dewaxing operations.
Danny Verboekend, Zeopore Technologies, firstname.lastname@example.org
Paraffinic components in standard diesel feedstocks tend to crystallise when temperatures drop below 10ᵒC. Improving the cold flow properties implies substantially lowering the so-called cloud point (CP) – the threshold temperature at which a specific fuel starts to become waxy, opaque, and more viscous. Dewaxing of diesel and lubricants may be necessary in colder operating environments or during winter. Dewaxed products ensure optimal performance, reduce emissions, and prevent malfunction or damage to diesel engines and other machinery.
The cold flow properties of a feedstock can be improved by either catalytic dewaxing or using additives or blending with (valuable) other finished products. Additives and blending are often expensive routes and offer little flexibility in managing opportunity crudes. From an economic standpoint, the preferred approach is catalytic dewaxing.
In catalytic dewaxing, the aim is to selectively convert the fraction with the highest cloud or melting point (linear paraffins) to products with significantly lower CPs. In selective cracking, it is targeted to specifically crack the large linear paraffins into molecules of smaller carbon number, whereas in isomerisation, one targets to selectively isomerise these waxy components into branched paraffins (see Figure 1). In both catalytic dewaxing scenarios, the technical and economic challenge is to control the cracking degree, as undesired (over)cracking of molecules leads to expensive diesel yield loss and the formation of undesired streams of lights/gases.
In catalytic dewaxing, the highest selectivity to an ULSD product with much improved cold flow properties, hence the lowest degree of undesired (over)cracking, is often achieved using isomerisation-selective zeolite-based bi-functional catalysts. The narrow unidirectional micropore structure of today’s isomerisation zeolite catalysts forms the highest industry standard of dewaxing improvement based on a stable and tunable CP improvement. Nevertheless, the zeolite’s narrow micropores also provoke undesired access and diffusion limitations, resulting in significant losses through overcracking.
Mesoporous zeolites add a secondary level of larger mesopores to conventional zeolites, thereby increasing their catalytic efficiency by improving access to the active sites located in the zeolites’ micropores (principle visualised in related Zeopore article: https://bit.ly/3qIHRsE). The superior performance of mesoporous zeolites in selective isomerisation is firmly established. For example, Zeopore scientists have, together with world-renowned isomerisation expert Prof. Johan Martens (University of Leuven, Belgium), pioneered the superior catalytic performance of mesoporous zeolites almost a decade ago [Refs DOI: 10.1002/cssc.201200888 and DOI:10.1016/j.cattod.2013.03.041]. However, the big challenge remains to manufacture such superior mesoporous zeolites at an industrially affordable price tag.
Recently, to convince the global dewaxing industry, Zeopore has tested a noble-metal-containing unidirectional mesoporised dewaxing zeolite on a waxy ULSD in a parallel state-of-the-art 16-fold high-throughput testing unit at hte GmbH in Heidelberg, Germany (related Zeopore article: https://bit.ly/3AaD7jH). The test has been conducted in broad ranges of industrially relevant pressures, temperature, and space velocities. The results (also summarised in related Zeopore article: https://bit.ly/3AaD7jH) demonstrate a breakthrough in catalytic dewaxing: at fixed CP improvement, the mesoporised Zeopore dewaxing zeolite enables a five-fold lower diesel yield loss, reaching below 0.06 wt% loss per degree of CP improvement. This achievement becomes particularly attractive when considering that Zeopore is in the position to couple these catalytic benefits with an economical and tunable zeolite manufacturing process.
Per Zeuthen, Haldor Topsoe, email@example.com
The cold flow properties of a given diesel are dictated by the wax components or the amount of normal paraffins in the stream. The more carbon atoms there are in the normal paraffins, the higher the melting point and the poorer cold flow properties of a given stream. To lower the cold flow properties of the produced ULSD from a given gasoil hydrotreater will thus require a modification of the wax components. Such a required modification or manipulation of the wax components is not possible by a conventional hydrotreating catalyst; we have to use a catalyst that is able to impact the normal paraffin molecules. This can be done catalytically by shortening the length, means of carbon atoms numbers, of the wax molecules by a conventional cracking reaction using some of the more old-fashioned catalyst types. This will provide a cold flow improvement and a significant and costly yield loss into smaller LPG fragments. Alternatively and much more preferred, the latest generation dewaxing catalysts, which perform the skeleton isomerisation without any yield loss into useless gas products, can also be used. This is a very economically attractive route, improving the cold flow properties of any diesel stream. Both catalyst methods will avoid the use of costly cold flow improving additives.
Using the catalytic dewaxing route will certainly take up some reactor volume for this type of catalyst. However, using the latest generation and most active ULSD hydrotreating catalysts will overcome this potential issue, as the latest generation ULSD catalysts are often 10-20% more active than previous generation catalysts and can operate at a higher LHSV. In other words, the higher activity of the ULSD catalysts will provide room for the dewaxing catalysts without cycle shortage. Furthermore, it is important to ensure that the dedicated dewaxing catalysts have active metals to increase stability and provide a significant hydrotreating activity. Using the right combination of ULSD and dewaxing catalysts will ensure a long and stable cycle and provide significant economic benefits.
Topsoe has numerous references of dewaxing catalysts in ULSD units significantly improving diesel cold-flow properties.