Diesel Hydrotreating Process Overview – Diesel hydro-treating is a unique refining process. It is the only hydro process in which the feed can change from mostly liquid to mostly vapor over the course of the catalyst cycle. The feedstock boiling range also varies considerably as the end point specification on the product diesel fuel varies considerably throughout the world (Generally, the feed is in the 400-700ºF range).
The quality is dependent on whether kerosene is included and the severity of upgrading units such as FCC or cokers which provide diesel boiling range products. The operating conditions also vary widely as some designers utilize very low space velocities. High pressure operations approaching 900 psig can be used for low sulfur requirements operations.
Diesel Hydrotreating Process Overview | Energy Efficiency Guidelines in Refinery
Mandated diesel sulfur contents of less than 500 ppm and higher cetane quality requires considerable attention. High performance catalyst has developed to achieve the product objective. Marginal cetane improvement or smoke point improvement can be achieved by the catalyst selection too.
Aromatic saturation to achieve mandated aromatic levels in diesel fuels can be achieved through key development of DHT process technology such as:
Selection of a reactor unit which would ensure the maximal contact of the catalyst with the initial raw stock and its uniform distribution;
Selection of a catalyst having stable hydro-desulfurization activity and high strength properties;
Selection of the optimal catalyst system in order to reduce the pressure differential;
Catalyst activation completeness;
Identification of technological parameters to achieve the lowest initial process temperature is an important design task.
DHT reactor is the main device determining the efficiency of this process and the depth of the raw stock conversion. Its 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. The 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 in feed of the gas-liquid stock mixture are used at the domestic units for diesel oil hydro-treating.
experience of use of diesel oil fractions hydro-treating 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. The 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; combined protective catalyst layers, distribution devices are used.
It is possible to reduce differential pressure in the hydro-treating reactor without the catalyst performance degradation by changing the scheme of the stock in feed in the reactor from axial to axial-radial. 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.
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 design shall be selected, where 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.
Continuous improvement of the process for production of diesel fuel has resulted in the possibility of stable production of diesel fuel containing less than 350 ppm of sulfur and less than 7 mass percent of multi-ring aromatic hydrocarbons (MAH) at low hydraulic resistance in the catalyst layer.
New retrofit projects in DHT plants can render better diesel fuel; having less than 10 ppm of sulfur (Euro-5), via changing the technological parameters: reduction of the share of the secondary raw stock involved in the process to 15 percent, raising the partial pressure in the reactor and increasing the temperature to 345-350ºС.
The first step in the DHT process targeting is to draw a boundary around the plant where there in and out, directly, and/or indirectly through steam consumption and/or generation, water production and/or generation, power consumption and/or generation are among the plant’s boundary are all identified.
Diesel hydrotreating plants energy integration is very similar to NHT plants energy integration. Integration between feed and product is common; integration between reaction and separation section and in-separation sections are recommended to achieve lower energy consummation targets. Complete pinch technology application for DHT plants will be included in later versions of this manual.