Optimising middle distillate production in a hydrocracker

How to maximise middle distillate production and assure product quality from a range of hydrocracking catalysts and processing schemes.

Haldor Topsoe

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Article Summary

Hydrocracking is one of the key processes of a modern refinery, and a hydrocracking unit is one of the most profitable units in a refinery. Hydrocracking will continue to be a very important process to meet the increased demand for clean transportation fuels. In view of this, new developments in hydrocracking catalysis and process technologies are continuously taking place.

The processing requirements will determine the optimum design of a hydrocracking process and the optimum catalyst selection. This will include the feed rate and feed quality as well as the make-up gas quality and availability. Typical hydrocracker feedstocks are straight-run gas oils, vacuum gas oils (VGO), coker gas oils, fluid catalytic cracking oils, and decant oils as well as mixtures of those. The desired product(s), which may range from liquefied petroleum gas (LPG), naphtha, kerosene, and diesel to unconverted oil (UCO) as well as the required product properties and yield, determine the conversion requirements.

Nowadays, practically all hydrocrackers operate with a pretreatment catalyst for removal of nitrogen compounds in front of the hydrocracking catalyst. Recent developments within NiMo hydrotreating catalyst technologies like the HyBRIM technology invented by Topsoe resulted in a new hydrocracking pretreatment catalyst named TK-611 HyBRIM.1 This catalyst lifted the activity by 25% compared to the previous generation of commercially available state-of-the-art pretreatment catalysts like the Topsoe TK-609 HyBRIM catalyst. The use of such a catalyst in hydrocracking service adds additional flexibility to the operation of a hydrocracker like longer catalyst cycles, more throughput, better product qualities, or even the ability to process more severe feedstocks. It also allows the refiner to cut back on pretreatment catalyst volume and to install less active hydrocracking catalysts, resulting in an improved middle distillate selectivity and/or product quality, if this is economically feasible.

Last but not least, optimum operation of a hydrocracker not only requires high performance state-of-the-art pretreatment and hydrocracking catalysts but also well functioning reactor internals and catalyst grading systems. Furthermore, an optimal process scheme should be used. The company developed a full range of proprietary high performance reactor internals. For full conversion hydrocrackers, the build-up of heavy polynuclear aromatic compounds (HPNA) has to be controlled to ensure seamless operation. For hydrocrackers, where HPNA management is needed, Topsoe developed a new technology named HPNA Trim.

Selection of hydrocracking catalysts

When it comes to the selection of the best hydrocracking catalyst system for a given application, many different factors have to be taken into consideration. This is far more complicated than selecting a hydrocracking pretreatment catalyst, where the use of a high activity hydrocracking pretreatment catalyst would be the preferred choice, as this gives additional flexibility in hydrocracking catalyst selection and operation as mentioned above. In the selection of a hydrocracking catalyst system, a very important factor would be the optimisation of the catalyst loading to maximise the profitability of the unit. The profitability of the unit is related to the values of the different products from the hydrocracker. For each product, the value is related to the quality of the product. Some hydrocrackers produce mainly naphtha, whereas other units produce mainly middle distillates. For a number of hydrocrackers, the production of UCO for either lube production or for steam cracker feedstock is very valuable. Other hydrocrackers benefit from their ability to shift between the production of mainly naphtha and the production of mainly middle distillates dependent on a change in seasonal demands and/or in the value of products. In some areas, the cold flow properties of the diesel product are very important. For hydrocrackers aiming at production of high quality UCO for use as feedstock for lube dewaxing, the viscosity index of the UCO is a crucial factor. In essence, this means that hydrocrackers have very different processing objectives, and that all hydrocrackers are unique in typical feedstock composition and main objectives.

To meet different processing objectives, a broad portfolio of hydrocracking catalysts for different applications is a must. The portfolio of base metal hydrocracking catalysts offered by Topsoe is shown in Figure 1. It is well known that low activity hydrocracking catalysts have a high selectivity towards middle distillate production, whereas high activity hydrocracking catalysts exhibit a lower middle distillate selectivity. Topsoe offers three series of hydrocracking catalysts, which cover a wide activity range. The red series is for applications where maximum product hydrogenation is required. The blue series is for applications where the improvement of cold flow properties of diesel is important. The catalyst belonging to the D-sel series is used to maximise the middle distillate yield. In many cases, the use of more than one hydrocracking catalyst in a given unit is the optimal choice. It is possible to combine, for example, a hydrocracking catalyst from the red series with a hydrocracking catalyst from the blue series to tune the properties of the produced diesel, so that good cold flow properties and an excellent cetane number are obtained at the same time. This loading solution was successfully used by a handful of commercial units.

In the following, three cases will be discussed, which illustrate how the selection of catalysts and the use of different process schemes can result in a maximisation of middle distillate production and of the product quality of diesel.
Catalysts for maximising middle distillate production in single stage and two stage units
Traditionally, hydrocracking process configurations are classified by four basic process schemes: single stage hydrocracking with recycle, once-through hydrocracking, two stage hydrocracking, and separate hydrotreat hydrocracking. The most common design is the single stage system.

A simplified process scheme for single stage and two stage hydrocracking is shown in Figure 2. In single stage hydrocracking, the fresh feedstock and UCO are treated with surplus hydrogen over a hydrocracking pretreatment catalyst followed directly by a hydrocracking catalyst system. In many cases, two reactors are used, one for the hydrocracking pretreatment catalyst including catalyst grading and one for the hydrocracking catalyst system. The product is fractionated into light ends, naphtha, kerosene, diesel, and UCO. The UCO can be recycled back to the reactor. It is an option to let the UCO recycle pass a HPNA Trim unit for removal and management of HPNA compounds before it is recycled back to the reactor. Once-through hydrocracking is without liquid recycling. For simplicity’s sake, the high pressure separator as well as the recycle compressor were omitted from the process scheme shown in Figure 2. In the two stage process, the hydrocracking pretreatment of the feedstock and part of the hydrocracking are carried out in the same way as during the single stage process. The product from the first cracking stage is fractionated in the same fractionation system as mentioned above, and the UCO is passed to an additional reactor loaded with a hydrocracking catalyst system. As mentioned before, it is in this case also an option to let the UCO recycle pass a HPNA Trim unit. In the two stage process, the hydrocracking in the second stage can take place in the absence of ammonia and in a sulphur-free atmosphere (sweet operation), or in a sulphur containing atmosphere (sour operation). In case a sweet operation is desired in the second stage, this can be carried out with two separate gas recycle loops or with a common recycle gas that has been amine treated. In the single stage process, hydrocracking is carried out in the presence of ammonia and hydrogen sulphide (sour operation). The activity and selectivity of a hydrocracking catalyst are very dependent on the amount of ammonia present in the treat gas. The presence of ammonia passivates the acidic sites typically provided by the silica alumina and/or zeolite compounds present in the hydrocracking catalyst support. A small bleed of the recycle oil is normally taken out for HPNA management. The HPNA Trim technology is a versatile tool for minimising the bleed rate.

For many hydrocrackers, the main objective is maximisation of middle distillate production. In this case, a hydrocracking catalyst system with a high middle distillate selectivity should be selected. As can be seen in Figure 1, the natural choice would be to use the Topsoe catalyst TK-939 D-sel. Due to the lower activity of TK-921, TK-925, and TK-926, these catalysts would be less attractive to use in existing single stage hydrocrackers. In the absence of ammonia it was demonstrated in dedicated pilot testing that TK-921 can have a better middle distillate selectivity than TK-939 D-sel in two stage hydrocracking. From a process point of view, the lower activity of TK-921 is not an issue in this sweet environment, as the required reactor temperature will be significantly lower than 400°C. When the ammonia concentration in the treat gas of the second stage becomes higher, it becomes attractive to use the more active TK-939 D-sel catalyst.

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