Catalytic dewaxing

Refiners that sell diesel in regions that experience severe winters must ensure that it meets demanding cold flow specifications for cloud point, pour point and cold filter plugging point. This can be achieved in various ways. One approach is to add kerosene, but when kerosene commands a relatively high price, this can detract from the bottom line. Cold flow additives? That can be an ineffective and high-cost solution. Instead, many refiners are using catalytic dewaxing, which can often be implemented at a low cost for a large margin uplift.

In recent years, catalytic dewaxing, which changes the product properties in order to prevent the wax compounds from crystallising out in diesel at low temperatures and causing engine problems, has emerged as a powerful process for improving the cold flow properties of diesel and lubricating oils. More than 90 refineries worldwide apply or have applied this technique, and many more could enhance their margins by using it.

Whether kerosene blending, additivation or catalytic dewaxing offers the most profitable solution for your operation to produce winter diesel depends on your specific circumstances and constraints. There is no definitive answer, as it also depends on local factors such as the price of naphtha, gasoline and kerosene.

If the cold flow plugging or pour point is the blocking factor, additivation may be appropriate: these properties can improve significantly with small amounts of additives.

However, additivation cannot achieve large improvements in cloud point: it only improves the cloud point by 2–4°C. Kerosene blending is also of limited effectiveness here; it requires the addition of about 10% of kerosene to improve the cloud point by 1°C, though it also improves the pour point by about 3°C.

In general, catalytic dewaxing tends to be the most-economic solution if the cloud point needs improving by more than 5–7°C.

A low-cost, flexible solution

Catalytic dewaxing technology continues to improve. For example, Criterion Catalysts & Technologies (Criterion) has recently introduced more selective catalysts that enable deeper dewaxing, improved yields and longer cycle lengths, and Shell Global Solutions has improved the heat integration and reliability of the process.

Potentially, it is an extremely valuable opportunity for many refiners. Nevertheless, it may not be the most economic option for all refiners, so they should carefully evaluate the benefits against other cold flow improvement techniques.

Customer case study

NIS A.D.

NIS a.d. in Serbia recently implemented catalytic dewaxing by revamping a redundant hydrodesulphurisation unit. This low-cost project was implemented quickly and delivered a high rate of return, writes Sergey Pervyakov, Technology Sector Manager, NIS a.d.

“In recent years, we have invested heavily in modernising and upgrading our assets, and we are now seen as a leading player in the region. Our biggest investment was in a combined mild hydrocracker–distillate hydrotreater (MHC–DHT) at the Pančevo refinery, which produces Euro 5 specification diesel.

“After the unit started up in 2012, we experienced issues in meeting winter diesel specifications. There were no issues during the summer, but during the winter we had to decrease the unit’s ASTM D86 95% point from 360 to 330°C to achieve the necessary cloud point of –10°C or less.

“Consequently, heavy-end diesel components remained in the hydrocracker’s unconverted oil, which is fed to a fluidised catalytic cracking unit. We were losing margin because of the price differential between gasoline from fluidised catalytic cracking and diesel.

“As well as lowering the ASTM D86 95% point, we were also blending up to 20 wt% kerosene into the diesel in winter. This achieved the required cloud point, but lowered the diesel yield by about 15% compared with summer operations. We were also using additives to help meet the cold filter plugging point specification.

“To resolve this, we worked with Shell Global Solutions and Criterion, and the solution that we went on to devise was particularly simple – and inexpensive. When the new MHC– DHT unit came on stream, it replaced a hydrodesulphurisation unit and made it redundant. So we revamped this unit into a first-stage dewaxing unit (see Process options above). This involved adding dewaxing catalysts, installing new Shell reactor internals to enhance the gas–liquid distribution and fitting a line to connect it to the MHC–DHT unit.

“Essentially, this dewaxing reactor treats a proportion of the hydrotreated diesel coming from the MHC–DHT unit and then blends it with untreated diesel. In this way, we can achieve the cold flow properties for winter diesel for the complete diesel pool. We no longer have to blend kerosene and have reduced our use of cold filter plugging point additives by 40%.

“The required capital expenditure for this project was, therefore, extremely low and the payback period was less than two years. In addition, the timeline was short, which was key as we wanted to implement the project quickly to achieve the benefits as soon as possible.

“Since startup, the dewaxing unit has run without problems, although we only operate it during winter when there is a requirement for diesel cold flow property improvements (five to six months a year). We then shut the unit down, clean out the hydrocarbons and keep it under a nitrogen atmosphere until its next startup.

“During a performance test in 2015, the unit achieved a throughput of 42 t/h with diesel yield greater than 86 wt%. The cloud point improvements it achieved during its last cycle are shown in Figure 2.”