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Oct-2011

Diesel hydrotreating and mild hydrocracking

Increased hydroprocessing capability must provide an acceptable long-term return on capital, while producing the desired product yields and qualities

Andrew Tyas, DuPont Sustainable Solutions Clean Technologies
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Article Summary
Today’s refiners are facing an unprecedented number of challenges: producing gasoline and diesel fuels to lower sulphur and increased combustion quality specifications; more stringent environmental controls on emissions; the need to process the lowest cost feedstocks such as heavier, sour crudes; and cost escalation in capital projects

One result of these challenges is the need for petroleum refiners to make significant investments to increase their hydroprocessing capabilities, either through unit debottlenecks or new unit construction. However, these investments must provide an acceptable long-term return on capital, while producing the desired product yields and qualities from cost-advantaged feedstocks such as FCC cycle oils, coker gas oils, shale oil and tar sands.

The IsoTherming process from DuPont Sustainable Solutions is a commercially proven technology that enables refiners to meet their increased hydroprocessing objectives at significantly reduced capital and operating costs over conventional technologies. The process was initially developed by Process Dynamics Inc and was acquired by DuPont in August 2007. The first commercial unit was successfully commissioned at the Giant Industries refinery in Gallup, New Mexico, in 2003. Since this time, the technology has been utilised commercially in a number of other ultra-low-sulphur diesel (ULSD <10 ppm sulphur) and mild hydrocracking (MHC) applications. A large vacuum gas oil hydrotreating unit commissioned in Q4 2010 and a mild hydrocracker designed for upgrading 100% LCO started up in China in Q2 2011.

This article summarises the basis of IsoTherming technology and its operational and capital advantages over conventional hydroprocessing designs. It also includes examples of where this technology has been used to address particular refinery hydroprocessing needs with respect to ULSD and MHC services.

IsoTherming technology
Hydroprocessing has been practised by the refining industry for several decades. For hydrocarbon feedstocks in the heavy naphtha/light distillate boiling ranges, hydroprocessing units normally utilise a two-phase trickle-bed reactor configuration.

In a trickle-bed reactor, combined hydrogen make-up and recycle gases and liquid feed are mixed, heated to the desired reactor temperature and passed to a reactor fitted with an internal distribution device on top of the catalyst bed. The distributor serves two purposes. First, it provides even distribution of the two-phase mixture across the catalyst bed. Second, it promotes further mixing to ensure saturation of the feed liquid with hydrogen prior to entering the catalyst bed.

This reactor design has several fundamental shortcomings. As the reaction occurs at the catalyst surface between the dissolved hydrogen and the reactive species in the feed, the hydrogen is depleted from the liquid. In most refinery applications, the amount of hydrogen required for the reactions is greater than the solubility of the hydrogen in the fresh feed alone. Thus, for the reaction to continue to completion, additional hydrogen must be replenished from the vapour phase. This, in effect, makes the rate of diffusion of hydrogen into the liquid phase a factor in the overall kinetics of the process.

To overcome this diffusion limitation, excess hydrogen, often several times the quantity required for the reactions, is recycled back to the reactor to maximise the hydrogen partial pressure throughout the system. Hydrogenation reactions are highly exothermic. The mass flow of hydrocarbon feed and recycle hydrogen through the reactor has a limited capacity to adsorb the liberated energy. Therefore, in applications that require large chemical hydrogen consumption, the rise in reactor temperature can be quite significant. The larger the temperature rise, the more undesirable reactions take place, such as cracking to light products (fuel gas, LPG and light naphtha). Cracking results in a loss of valuable 
products and increased catalyst coking. Inter-bed hydrogen quenches, although providing supplemental hydrogen, can do little to minimise this temperature increase due to the limited mass flow of hydrogen that can be added to the system economically.

Two-phase flow through a heterogeneous catalyst is prone to maldistribution. Any slight variations in pressure drop through the bed, even an unlevel catalyst bed, result in liquid/vapour maldistribution. This leads to inefficient use of the catalyst and, in worse cases, regions deficient in hydrogen, resulting in hot spots or coke formation. Ultimately, a premature unit outage and/or a catalyst dump/screen may be necessary.

IsoTherming was developed to address these inherent shortcomings in the conventional hydroprocessing trickle-bed design. In this process, the hydrogen required for the reaction is dissolved in the liquid before entering the catalyst beds. The supply of hydrogen is accomplished by saturating a combined feed and recycle stream of previously hydrotreated liquid prior to entering the reactor. The liquid recycle rate is set so that the amount of hydrogen dissolved in the combined (fresh and product liquid) feed is much greater than the reaction requirements. This design ensures a sufficient availability of hydrogen for the reactions at all points within the reactor.

With all the necessary hydrogen in the liquid, the overall reaction is now controlled by the intrinsic reaction rate (the combination of the effectiveness of the catalyst and the actual reaction rate). In addition, the heat capacity of the total reactor feed is considerably greater than that of the combined raw feed and the hydrogen recycle stream in conventional hydroprocessing technology. As a result, the temperature rise across the reactor bed is a lot less than that of a conventional design — hence the name IsoTherming.1 This lower temperature rise eliminates the need for inter-bed cooling and the additional complexity it brings to the reactor’s operation and design.

For particularly severe services, such as hydrotreating cracked stocks, including coker gas oils or light cycle oils, or during MHC, the much lower temperature rise helps to minimise undesirable side reactions. Hence, in the same service, an IsoTherming reaction system has an increased yield of desired products over a conventional counterpart.
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