June 1, 2009
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Home / Issue Archive / 2009 / April #4 / Improvement of the Technology for Diesel Oil Fractions Hydrotreating

№ 4 (April 2009)

Improvement of the Technology for Diesel Oil Fractions Hydrotreating

Hydrotreating of the diesel oil fractions is one of the most common catalytic processes in the layouts of petroleum refineries.

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Key elements in the development of this technology include the following:

Determination of the technological parameters in order to achieve the lowest initial process temperature.

   The reactor is the main device determining the efficiency of this process and the depth of the raw stock conversion; its technical design should ensure the specified productivity, have the necessary reaction capacity, develop the contact area of interacting phases required for the process, and maintain the necessary heat exchange during the process and the level of the catalyst activity. This design should have the minimal hydraulic resistance and ensure the uniform distribution of the gas-liquid stock flow over the whole reaction volume. Usually, reactors with axial infeed of the gas-liquid stock mixture are used at the domestic units for diesel oil hydrotreating.

   Practical experience of use of diesel oil fractions hydrotreating indicates that performance of the reactor with axial movement of the stock flow is characterized by non-uniform distribution of the gas-liquid stock mixture over the cross-section of the device and increasing hydraulic resistance of the catalyst layer, especially during long operation. Increase of differential pressure in the reactor results in decrease of its productivity and higher power consumption by the unit. To suppress the rise of differential pressure during the operating cycle, the following actions are usually undertaken: the top layer of the catalyst is periodically removed from the reactor; so-called filtering baskets, inert spherical materials (porcelain balls of variable diameter); combined protective catalyst layers, distribution devices are used. The listed methods make it possible to protect the main catalyst layer, but cannot solve the problem of the high hydraulic resistance.

   It is possible to reduce differential pressure in the hydrotreating reactor without the catalyst performance degradation by changing the scheme of the stock infeed in the reactor from axial to axial-radial (Table 1).
Non-uniformity of the reaction behavior is conditioned generally by the gradient of flow distribution over the layer cross-section and catalyst volume. The design of the inner components of the axial-radial reactor develops the cross movement of the reacting gas and liquid flows in the catalyst layer. At the same time, the hydrogenous gas is redistributed along the layer height, uniform catalyst layer performance and absence of dead zone for gas slippage are achieved; lower hydraulic resistance to the flow when it passes through the catalyst layer; improved distribution of the gas-liquid stock flow over the cross-section of the unit. The mentioned factors ensure (as practical experience proves) lower differential pressure in the axial-radial reactor during its long-time operation.

   In Russia, the first axial-radial reactors were installed at the second module of the standard double-flow hydrotreating unit L-24/6 of Salavatnefteorgsyntez. During the four-year service cycle of kerosene-gasoil fractions processing, molybdenia-cobalt-alumina catalyst was used in the first reactor with the gas-liquid stock flow, and molybdenia-nickel-alumina catalyst was used in the second one; this scheme ensured the required product quality (residual sulfur content is 0.05-0.15 mass percent) at low hydraulic resistance (0.03-0.04 MPa) of the catalyst layer and lower power consumption.

   Since 2002, production of diesel fuel with the reduced sulfur content has been started at the diesel oil hydrotreating unit L-24-6 of Angarsk refinery of Angarsk Petrochemical Company (APC); this unit was fabricated by the technology of VNII NP (All-Russian Research Institute of Petroleum Industry) and according to the design of Lengyprogas Institute. Catalyst AGKD-205A with high hydrating and strength properties and catalyst of the protective layer FOR-1 (both produced by the Angarsk plant of catalysts and organic synthesis – APC&OS) were charged into the reactors. Experience of hydrotreating unit operation showed that during the four-year service cycle, AGKD-205A steadily provided production of the diesel fuel with residual sulfur content 0.04-0.10 mass percent from the stock mixture at weight hour space velocity 4-5 h-1. The required amount of diesel fuel was achieved at the total pressure at the reactor inlet 3.3-3.7 MPa, the initial cycle temperature 335 С, and the final one – 355 С.
To organize production of low-sulfur diesel fuel at APC, a decision was made to upgrade the operating hydrotreating unit L-24/6. The basic design was developed by Angarskneftechimproyekt based on the initial data of VNII NP. The hydrotreating process improvement conception consisted in implementation of the following technical solutions:

   Taking into account the characteristic of the stock to be processed, and also in order to prevent increase of hydraulic resistance in the reactors, maintain the productivity of the unit at the same level and ensure two-year service cycle, a scheme was selected, when the reactor of the axial-radial type comes first along the gas-liquid stock flow, and the axial radiator comes second. To reduce the exothermic effect of the reaction and increase the duration of the reaction cycle, possibility of cold hydrogenous gas supply into the cross-flow between the reactors is provided. Increase of the hydrogen partial pressure in the second reactor will have positive effect on the stock treatment depth and catalyst performance stability.

   Engineering designs of the reactors were developed by VNIIneftemash. In 2007 these reactors were brought to the construction site of the L-24/6 unit. Molybdenia-nickel-alumina catalyst (AGKD-400-BN) and molybdenia-cobalt-alumina catalyst (AGKD-400-BK) of the Angarsk plant of catalysts and organic synthesis (APC&OS) were selected to be charged into the reactors of both flows. In the catalyst system, to prevent clogging of the main catalyst layer by the products of corrosion and coke, improve distribution of gas-stock mixture over the reactor cross-section, catalysts of the protective layer FOR-2 and FOR-1were used, these catalysts formed as hollow cylinders. After the catalysts were prepared for operation (hydrogenous gas drying, sulfurization by a sulfiding chemical), the hydrotreating unit was placed in operation late in 2007.

    Improvement of the process flowsheet for production of diesel fuel at the L-24/6 unit of the Angarsk refinery of APC resulted in the possibility of stable production of diesel fuel containing less than 350 ppm of sulfur and less than 7 mass percent of multiring aromatic hydrocarbons (MAH) at low hydraulic resistance in the catalyst layer.

   To evaluate the potential of the catalytic system of the first flow of the hydrotreating unit, a pre-determined run was performed in January of 2008. By the results of this run, it was ascertained that during processing of the mixed stock containing 38 mass percent of secondary components, it is possible to achieve 28-45 ppm of sulfur and MAH content not exceeding 5.5 mass percent at the first reactor inlet temperature 335-340 С.

   By end of March of 2009, the unit has been in continuous operation for 14 months without the catalyst recovery. At present, at the temperature 305-310 С, residual sulfur content is less than 500 ppm, and less than 50 ppm at 330-335 С. During the operating period, differential pressure increased from 0.04 to 0.07 MPa.

   The main performance data of L-24/6 unit operated at APC are given in Table 2.
Analysis of these data enables us to state the following: upgrading of the reactor assembly with application of new catalyst systems made it possible to ensure the optimal technological parameters for use of the domestic desulfurization catalysts and organize production of the ecological diesel fuel having sulfur and MAH content in compliance with the requirements of Еurо-3 and Еurо-4 Standards.

   Diesel fuel having less than 10 ppm of sulfur (Euro-5) can be obtained by changing the technological parameters: reduction of the share of the secondary raw stock involved in the process to 15 percent, rise of the partial pressure in the reactor and increase of the temperature to 345-350 С.

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