For cold areas of the globe it is essential that during the winter season the flow properties - Cloud Point, Pour Point and Cold Filter Plugging Point - of diesel for automotive and heating oil for domestic and industrial applications are optimised.
Catalytic dewaxing is used to improve cold flow properties of diesel fuels by selective hydroisomerization and hydrocracking of normal and slightly branched-paraffins. Normal paraffins have the most detrimental effects on the low-temperature properties of diesel fuels.
Even with lower end boiling points as expected from forthcoming specifications in Western Europe, remaining paraffins in diesel fuels may still be a problem in reaching acceptable cold flow properties without expensive additivation. This problem becomes even more important in countries where additives package are not commercially an attractive option and where local weather conditions force the use of extreme winter gas oils grades.
The Shell Catalytic Dewaxing processes reduce the amount and/or chain-length of normal and slightly-branched paraffins, the cold flow properties of diesel fuels strongly improve (Figure 1).
About the technology
In the last decade improving the cold flow properties of distillates in a selective and cost efficient way has gained increasing importance. In addition to providing a product in the specified cold flow property range, refineries apply cold flow improvement processes to avoid using cold flow additives, to reduce their kerosene blending requirements, to upgrade heavier feedstocks with higher cloud and/or pour points, and to create room in the distillate blending pool for heavier feeds.
The growing interest in cold flow improvement technologies stems from increasingly stringent engine specifications, processing of more paraffinic crudes, and selling the products at markets with demanding cold flow property specifications.
Catalytic dewaxing is an established and reliable process for improving the cold flow properties of diesel. During the process “wax” is selectively removed from the feed by conversion (as opposed to solvent dewaxing, which extracts waxes) into isomerized and cracked molecules. With specifically developed proprietary catalyst formulations issued from our Research & Testing Facilities across the globe (Figure 2), Shell Dewaxing Technologies offer the best diesel yields, significantly reduced naphtha production and an unbeaten on-stream stability combined with operating flexibility.
Catalytic dewaxing is typically employed in either a single stage or a two stage configuration (Figure 3).
In Single Stage Dewaxing the dewaxing bed is part of the main hydrotreating section, the dewaxing catalyst, an acidic catalyst, is exposed to the total amount of nitrogen in the feed either as unconverted organic nitrogen or as ammonia resulting from the HDN reaction. Both forms are capable of strongly adsorbing on an acidic site, inhibiting its catalytic function and possibly blocking access to other sites.
In a generally contaminated environment within existing or revamped diesel hydrotreaters, Shell Dewaxing Technology provides a low cost and flexible solution that allows proper control of summer and winter mode generally resulting in some distillate product to be lost through partial cracking during the main dewaxing period but with limited yield loss in any due to quench utilisation during summer period.
SDD-800 is the catalyst of choice for single stage applications. Its proprietary formulation minimises distillate yield loss in dewaxing mode (Figure 4), decreases by half the naphtha production compared to conventional dewaxing catalyst and maximizes cycle length between regeneration (up to 7 year cycle has been recorded).
This catalyst employs a base metal that is suitable for operating under high concentrations of H2S and NH3.
In Two Stage Dewaxing the feedstock is subject to hydrotreating in the first stage of the process after which it is separated from the gas phase containing H2S and ammonia, and treated with the dewaxing catalyst in a second stage. By using a dedicated second stage reactor, downstream of the hydrotreating reactor, the low levels of inhibitors allow there the use of high-activity noble metal catalysts and milder process conditions.
The investment and process costs are higher than a single stage process, but product qualities (cold flow properties, cetane index) and distillate yields are maximised. For a two stage application, the unique formulation of SDD-821 makes it the most suitable.
This catalyst uses a noble metal for the activation of paraffins necessary for the isomerization reaction and, therefore, it works best under conditions where only very limited amounts of H2S and NH3 are present in the reactor.
The difference between the single stage and two stage operations is highly dependent on the type of feedstock and the depth of dewaxing (the target cloud or pour point improvement). With increasing cold flow improvement the distillate yield differences between the two types of operation will be, in general, larger. The yield differences in single stage and two stage catalytic dewaxing are illustrated in Figure 5 and Table 1.
Shell Global Solutions Catalytic Dewaxing can provide systematic and significant cold flow improvements for high value diesel and lube oil production to meet specifications, resulting in high yield, high product quality and differentiated operating strategies with long cycles.
We validate our innovations rapidly in our own facilities; this facilitates proven, reliable and up-to-date technology.
Kremenchug refinery (Ukraine)
Kremenchug refinery (Ukraine) has selected Dewaxing technology from Shell Global Solutions for a new hydrodesulphurization/ dewaxing catalysts. The capacity of the hydrodesulphurisation/ dewaxing unit is 25000 bbl/d. The Dewaxing technology permits the refinery to produce Ultra low Sulphur Diesel that meets the cold flow properties required (Euro4 quality with up to 28oC CP reduction winter and arctic grades).
GS Caltex Corporation (South Korea)
GS Caltex chose Shell Global Solutions HDS and Catalytic Dewaxing catalysts and technology to modify the Diesel HDS unit in their Yeosu Refinery (South Korea). Our technology has enabled the refinery to produce Ultra low Sulphur Diesel meeting all the stringent diesel specifications and the required cold flow properties for the winter and summer Diesel grade. The feed stock to be processed contains, apart from VGO, also light cycle oil.
The capacity of the hydrodesulphurization / dewaxing unit is 70000 bbl/d.
North American refinery revamp
A North American refinery desired to revamp an existing two-reactor HDS/Dewaxing unit.
The unit had a history of rapid dewaxing catalyst deactivation resulting in high temperature operation and diesel yield loss. In the revamp, the refinery changed the service of the lead reactor to another application, leaving only one reactor for HDS and dewaxing. The main objectives of the new operation were to achieve the sulphur specification along with a minimum cloud point improvement of 25oF during the winter mode operation (6 to 7 months/year).
Shell Dewaxing Technology and process line-up using SDD-800 combined with state-of-the Art Criterion HDS catalyst was used to match refiner specifications. Six weeks after the unit start up, a performance test run in winter mode was successfully conducted at even 12% higher feed rate than design. At the end of the planned cycle, based on the good performance and minimal deactivation of the SDD-800, the refiner re-used the SDD-800 without regeneration in the subsequent unit cycle.