MIDW technology as a drop-in â€¨catalyst solution
Benefits of upgrading to a highly isomerisation-selective distillate â€¨dewaxing catalyst.
Kathryn Peretti, Tim Hilbert, Chad Perrott, Louis Burns and Ruben Melendez, ExxonMobil
Jerry Lockie, Silver Eagle Refining Inc
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Although diesel specifications on cold flow properties vary depending on the region and time of year, the majority of diesel products require improvements to cold flow properties. Using catalysts to improve cold flow properties can be a higher value approach because it maximises utilisation of existing equipment and generates a more profitable product distribution.
During a planned turnaround in September 2015, Silver Eagle Refining, Inc. refinery in Woods Cross, UT implemented ExxonMobil’s MIDW™ isomerisation dewaxing catalyst technology resulting in a net increase in liquid production of up to 15 vol% due to a significant reduction in light ends production and an increase in diesel yield of up to 40 vol%. The catalyst changeout reduced operating cost and improved the quality of the #2 ULSD as evidenced by cetane numbers above 62. This improvement required only minor hardware upgrades and loading more selective isomerisation dewaxing catalyst in place of the former cracking dewaxing catalyst resulting in a six month simple payback for the catalyst changeout costs.
Opportunities for increased profitability through diesel technology
Several market forces are creating an opportunity for diesel producers to increase their profitability by optimising refinery operations and technology around their diesel units. In order to take advantage of these opportunities, the diesel technology that refineries are currently using may need to be optimised, particularly around meeting cold flow property specifications such as cloud point (CP) and cold filter plug point (CFPP). Those opportunities include:
1. Demand growth in both diesel and jet fuel through 2040. As jet fuel demand increases, the use of kerosene as a blend stock into diesel to reduce cold flow properties is less desirable because it can reduce refinery profitability.
2. Diesel specifications continue to become more stringent worldwide. The low sulphur specifications that are already enforced in Europe and North America are soon to be required in the rest of the world. These specifications are only met with additional hydrotreating. The increased hydrotreating in turn reduces aromatics, which have a solubilising impact on normal paraffins that cause high CP and CFPP. As a result, meeting CP and CFPP targets is more challenging with increased hydrotreating needs.
3. As European refineries close, the demand for diesel exports into Europe will increase. The diesel exported will be required to meet Euro V specifications and may have tighter CP and CFPP specifications than where the diesel is produced today.
4. Use of “opportunity crudes”, such as tight oils, increase the margin of product coming out of a refinery. However, these crudes present different challenges than traditional crude sources.
One challenge is that the diesel range of the crude could have higher paraffin content (and cold flow properties) than the crude it is replacing. This will cause challenges in blending the distillate pool, or lead to additional undercutting of diesel, thus reducing refinery profitability.
In general, refineries built around gasoline production, may be running sub-optimally to boost diesel production and meet tightening diesel specifications. There is a tendency to address only the constraints of existing processes; however, it may be more beneficial to re-evaluate whether the existing processes used to meet diesel specifications are still appropriate given the shifting landscape.
Cause of cold flow problems in diesel
Cold flow properties were highlighted as one of the major challenges in meeting diesel specifications and optimising profitability when diesel demand grows. To address the problem, the cause must first be understood. What causes high cloud point and CFPP in diesel? The answer is normal paraffins.
Normal paraffins are straight chains of aliphatic hydrocarbons. As the number of carbons in that chain increases, so does the corresponding melting point, as seen in Figure 1. Many of the n-paraffins in the diesel range (C9-C25) will crystallise above 0°C. Paraffin crystals result in a cloudy appearance of the diesel product and when the crystals agglomerate, they will eventually plug the engine filter.
Options for reducing diesel cloud point
There are many options for reducing diesel cloud point to reach specification limits and the optimal solution will depend on the refinery. General approaches to improve cold flow properties include:
- Restricting feed cloud
- Blending finished products
- Cold flow improvement additives
- Catalytic dewaxing
Poor cold flow properties come from crystallisation of paraffinic hydrocarbons in the feed. These molecules crystallise at low temperature and agglomerate together, causing cloudy product as measured by cloud point, filter plugging as measured by cold filter plug point (CFPP), and ultimately the inability to flow as measured by pour point. Potential solutions for improving cold flow properties all have advantages and disadvantages that must be weighed on a case-by-case basis.
One solution is to cut out the “problem” molecules entirely by selecting feeds with low cloud points by limiting the feed to a lighter distillation cut. However, this likely requires more expensive feeds and downgrades a large portion of what could be diesel fuel to lower value products.
Another option is to blend the finished diesel product either a low cloud point product such as kerosene. However, kerosene can be sold as jet fuel, which is a higher value product than diesel in some markets. Therefore, blending kerosene into the diesel pool can reduce the overall profitability of the refinery.
Flow improvement additives called middle distillate flow improvers (MDFI), may also be used to reduce pour point. Their use can be expensive and has diminishing effectiveness at increasing concentrations. Furthermore, since MDFIs are tailored to the specific blend of products they are added to, reliance on them can make feed flexibility cumbersome. Finally, MDFIs do little to improve cloud point. Therefore, if cloud point is a specification, MDFIs are not a viable option.
Catalytic dewaxing is another alternative. Catalysts are used to facilitate reactions in which the paraffinic hydrocarbons are either cracked to lighter products (cracking-dewaxing) or isomerised into products with lower cloud points (isomerisation-dewaxing). The type of catalyst will determine the balance between cracking and isomerising the paraffins.
There are several advantages to catalytic dewaxing. Kerosene may be pulled out of the diesel blend and instead be sold as jet, obtaining higher margins. Higher cloud feeds are accessible for upgrading, such as heavier cuts or “opportunity” crudes such as tight oils. Catalytic dewaxing lowers cloud point, pour point and CFPP at the same time, and can affect very large changes in these characteristics.
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