Dec-2017

Meeting low sulphur diesel challenges

Liquid stream hydroprocessing technology enabled a refiner to meet fast-changing diesel sulphur specifications and process a wide variety of feedstocks.

TIMOTHY CARMANN
DuPont Clean Technologies

Article Summary

Diesel is the predominant fuel used for shipping goods and moving freight across the country and around the world. Increasing global demand for cleaner burning transportation fuel (particularly diesel fuel) is driving refiners to look for ways to increase low sulphur diesel production capacity. At the same time, refiners are looking to process larger amounts of low cost sour and heavy feedstocks to increase profitability. With increasingly strict fuel quality regulations being rolled out across the world, meeting this increased demand for cleaner burning diesel fuel is a significant challenge.

To meet increased demand for cleaner burning fuel while simultaneously maintaining or increasing profitability, refiners need to increase their hydroprocessing capacity and capabilities to produce more diesel and remove more sulphur while minimising capital investment and operating costs. Hydroprocessing involves chemically treating a petroleum stream with hydrogen in the presence of a catalyst at elevated temperatures and pressures. Hydro-processing units allow refiners to improve product quality, comply with government regulations, and increase profitability by converting 
low valued streams into high margin and high quality products. Refiners can increase hydroprocessing capacity by constructing 
new grassroots hydroprocessing units or debottlenecking and revamping existing units.

IsoTherming technology
Hydroprocessing has been part of the typical refinery configuration since the mid-20th century, and it has essentially been done the same way since that time – using conventional, two-phase trickle bed reactor technology.

Approximately 10 years ago, a disruptive hydroprocessing innovation, IsoTherming technology, was finally brought to the marketplace, challenging the norms associated with conventional trickle bed technology. The core of IsoTherming technology is the ability to provide the hydrogen necessary for hydroprocessing reactions using a liquid stream, rather than a recycle gas stream. To satisfy hydrogen requirements within the reactor, fresh reactor feed is saturated with hydrogen. Additional hydrogen can be added to the feed by means of a saturated product recycle stream (using a reactor recycle pump) and by hydrogen injection to resaturate the liquid within or between catalyst beds. These different hydrogen addition options eliminate the need for a recycle gas compressor. Figure 1 illustrates these different hydrogen delivery options for an IsoTherming reactor.

The technology also employs a novel reactor system with a single liquid phase that uses hydrogen and catalyst more efficiently. Regardless of the method of hydrogen delivery, all of the hydrogen needed for the hydroprocessing chemical reactions is dissolved in a single liquid phase. Applications in which the technology has been commercially deployed include kerosene and diesel hydrotreating, fluid catalytic cracking (FCC) pre-
treating, and mild hydrocracking. IsoTherming technology has commercially processed a wide range of straight run and cracked feedstocks, including 100% light cycle oil (LCO), at capacities ranging from 2000 b/d to 78 500 b/d (13 m3/hr to 520 m3/hr).

The technology has other commercially proven advantages compared to conventional trickle bed technology. They include the following.

Increased catalyst performance and improved yields
A conventional trickle bed reactor depends on near perfect feedstock distribution throughout the catalyst bed to maximise reaction efficiency and to avoid overheating and coking. In addition, the quench stages in a trickle bed reactor are meant to manage temperature rise and require the injection of large volumes of additional hydrogen into the reactor. With an IsoTherming liquid full reactor, the catalyst is completely wetted. This draws the heat of reaction away from the catalyst surface and eliminates local hot spots that would otherwise 
promote coking and catalyst deactivation. All commercially operating IsoTherming gasoil hydrotreating units have experienced catalyst life in excess of four years, demonstrating the technology’s ability to achieve lower catalyst deactivation rates than conventional trickle bed technology. In addition, uniform liquid flow throughout the catalyst bed in an IsoTherming reactor results in a uniform radial temperature profile and acts as a heat sink for exothermic chemical reactions. This results in a lower temperature rise across the reactor and minimises light ends generation. 

Reduced operating expenses and capital cost savings
IsoTherming technology’s approach to hydrogen delivery eliminates the need for a recycle gas compressor (and associated high pressure separation equipment). Figure 2 shows a visual comparison of the equipment differences between IsoTherming technology and conventional trickle bed technology. IsoTherming commercial units have noticeably less equipment and smaller plot space requirements compared to conventional trickle bed units.

Electricity usage is lower for IsoTherming technology due to the use of a lower power reactor recycle pump for delivering hydrogen instead of a higher power recycle gas compressor. In addition, the direct heat transfer resulting from the recycle of hot reactor product to the inlet of the reactor reduces the required fired heater duty needed to achieve the target reactor temperature. For units with low hydrogen requirements, the ability to supply hydrogen using only feed saturation (and no saturated product recycle) means even fewer pieces of high pressure equipment are required. The technology has been shown to have 40-60% utility savings and up to 30% capital cost savings compared to conventional trickle bed technology in evaluated units.

Robustness, reliability, and safety
One unique feature of IsoTherming technology is the reactor recycle pump. As mentioned previously, the reactor recycle pump recycles a portion of the reactor product to the inlet of the reactor. This additional liquid volume is then saturated with hydrogen to ensure that sufficient levels of hydrogen are delivered to the reactor for all required hydroprocessing chemical reactions. This particular service requires low head and high flow to pump high pressure and high temperature liquid. Conventional pumps with mechanical seals do not offer the necessary safety and reliability, so a canned motor pump is the only viable type of pump to use for this service. Canned motor pumps are often an anomaly in the refining industry, but are very commonplace in the chemical industry. They are used because the high suction pressure and temperature make a conventional shaft sealing system unreliable, and the design guarantees there are no leaky seals. DuPont engaged rotating equipment experts to develop a complete specification for the IsoTherming reactor recycle pump, which has resulted in robust reactor recycle pump designs well known for their commercial reliability.

The technology is also an inherently safer hydroprocessing technology. Elimination of the recycle gas compressor and associated treating equipment not only removes a large amount of high pressure equipment from the system, but also significantly reduces the hydrogen inventory in the unit. There is also no potential for reactor runaway (and associated catalyst deactivation) in the IsoTherming reactor. This is illustrated in Figure 3, which shows operating data from a commercial unit that experienced a four hour power failure.