Refiners suffer penalties when overtreating diesel

Aug. 6, 2007
Continuously operating distillate hydrotreaters at a finished ultralow-sulfur diesel (ULSD) product target just a few ppm below the requirement for refinery specifications to maintain a certain comfort level is uneconomic and will cost the refinery a significant loss of catalyst activity.

Continuously operating distillate hydrotreaters at a finished ultralow-sulfur diesel (ULSD) product target just a few ppm below the requirement for refinery specifications to maintain a certain comfort level is uneconomic and will cost the refinery a significant loss of catalyst activity. Operating with finished ULSD product several ppm below the refinery specification may, however, be necessary for a short period of time to correct an off-spec tank.

The first year of ULSD operations has gone well. As usual, actual available feed sources seldom match the design cases. Attractive economic margins drove some refiners to push units beyond the original intention of the catalyst design, with good return. Although the run cycles were shortened, the cost of catalyst activity loss was adequately covered by increased revenue.


The switch from low-sulfur diesel to ULSD for highway use has been an obstacle for the refining sector. A large investment in capital was required in the last 2 years in order to stay in business.

As with all new equipment, there is a period of time in which the operator wants to learn just what the unit can do and how the process can be controlled to maximize refinery profits. These units have been up and running for almost a year and there has been a full array of experience.

It is not unusual for these operations to process at higher-than-design rates or heavier-than-design feedstocks to take advantage of favorable price spreads. Although these decisions will reduce run lengths, it is clear there is economic incentive that overrides the higher rate of catalyst deactivation.

Although catalyst manufactures may wish to protect the integrity of their product, it is more important that the operator understand the effect of process decisions so it can be compared with the economic benefit, such as processing more material.


All hydrotreaters are controlled by reaction kinetics. Although some catalyst systems may have more or less activity, all will respond according to these principles. Equation 1 (see equation box) shows ideal plug flow.

Reaction orders for typical refinery feeds range between 1.0 for naphtha and 1.65 for vacuum gas oil. For most ULSD operations, reaction order is about 1.4. Feed sulfur concentration will vary from one refinery to another and perhaps from one crude cargo to another, depending on the crude type and unit configuration of each individual location.

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The kinetic effect of temperature is defined in the Arrhenius equation (Equation 2).

In order to find the temperature increase requirement, one must calculate a reference reaction rate using Equation 3.

Combining Equations 2 and 3 results in a method to “normalize” a unit that is operating at different throughput (space velocity) or feed and product sulfur content (Equation 4). This normalized temperature represents the weighted average bed temperature (WABT) of the reactor.

This is a lengthy way to explain that there is a fairly simple relationship between reactor temperature and operating conditions. Furthermore, this is based on kinetics and is independent of any specific type or grade of catalyst. There is really no need for each refinery to develop software for this, because the catalyst supplier should have a spreadsheet available for its clients.

Predicting how sulfur concentrations change as the feed quality and end point changes is a much more complex issue. Typically, higher end points mean higher sulfur concentrations and more difficult-to-desulfurize molecular structures.

Run length projections

The relationship between throughput and reactor temperature, or concentrations and reactor temperatures, is now established. But is that all the refiner needs to know about monitoring and predicting ULSD units? Not quite.

To properly predict run length it is important to describe how the catalyst deactivates over time. Deactivation is very much a function of the average bed temperature. Higher temperatures mean higher deactivation rates.

Temperature is not the only indicator, however. Available hydrogen as well as contaminants also have a big influence on catalyst activity maintenance and run length. Refining companies and catalyst suppliers strive to develop more rigorous models to do this, but the details are confidential for the most part.

Because the models are developed based both on basic kinetics, as well as with empirical pilot plant data, a considerable amount of investment must be made to perfect their detailed accuracy. The one thing that is true about all models is they are never better than the accuracy of the input data.

For purposes of comparison, we simulated the design and operation of a distillate hydrotreater using the Albemarle model (Table 1).

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ULSD Unit 1 processes 14,000 b/d of distillate and contains $1.3 million of catalyst, which provides a 1.0 liquid hourly space velocity (LHSV). The feed sulfur is 0.7 wt % and the product is 7 ppm at the battery limits. The unit was designed to operate at 800 psig with good quality hydrogen in sufficient quantities.

The projection indicates it will take 23 months of smooth operation to reach a 700° F. reactor inlet temperature, which is considered end of run.

Just about the time the unit was built and started up, highway diesel prices shot up and margins for this product were $5/bbl vs. the next best option for the feedstock. Fortunately, there was an extra 5% of this feedstock available and increasing feed was not limited by pumps or furnace duty. The question is: Should the refinery process this extra material?

Obviously, increasing the flow to the new unit will increase the LHSV and reduce the overall run length. The extra revenue, however, offsets the loss in catalyst activity before the end of run.

Certainly there are more costs that must be considered, including increased turnaround expenses. It is also not always possible to increase the unit charge without changing the feed composition of the incremental barrel. If the extra barrel is from increasing the FCC light cycle oil end point, the deactivation will be greater.

Conversely, moving barrels from FCC slurry (one of the lowest valued products in the refinery) to ULSD (at times the highest valued product) will have a big incentive.

Each refiner must determine the economics that fit their units and will also have to make accommodations for different change-out schedules.


Contrary to the economic benefits of increased throughput or even higher end point feedstock, overtreating ULSD product has a devastating effect with almost no real economic value. There is variability in the testing and no unit runs perfect all the time, but overcompensating for this costs money.

We ran another case in which the feed rate does not change; we only lowered the finished product sulfur by 10%, to 6.3 ppm from 7 ppm, to quantitatively compare the results.

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Table 2 shows that the decline in run length for Case 3 is the same as running the extra barrels in Case 2.

The problems with finished product blending should not be oversimplified. Nothing can fix a ULSD shipment that is just a little bit high on sulfur spec unless the unit is run harder. There is certainly no room for error due to a bad line fill, leaking valve, or line up mistake. Allowing a couple of ppm here for dirty bottles and a couple ppm there for laboratory precision, however, will wind up costing the refinery a great deal in the long run.

One of the best solutions is to have an on-stream product sulfur analyzer (Fig. 1). Even this is far from perfect; but with time, operators will be able to develop a level of confidence that will allow them to make the proper corrections for inevitable process swings.

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Installation and care of these on-stream analyzers cost money, but considering the risks for not staying on top of the ULSD product, the extra investment is probably worth it.

Refiners should compare daily operating sulfur levels with finished tank results to determine if they are leaving any money on the table.

The authors

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Bob Macomber ([email protected]) is a senior technical service representative with Albemarle Catalyst, Houston. He has more than 20 years’ experience as an operations and product blending manager with Total Petroleum, Kerr McGee Corp., and Murphy Oil Co. He holds a degree from Texas A&M University.

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Oscar Brown ([email protected]) is a technical service representative for Albemarle Catalyst. His experience includes monitoring the deactivation of catalysts at many refineries, mainly for ultralow-sulfur diesel. He began working for Akzo Nobel (now Albemarle) during his last few years of college, where his training focused on kinetic modeling development. He holds a BS in chemical engineering from the University of Houston.