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Technical Paper About Refinery Hydroprocessing Technology

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The paper outlines the systamatic approach to selecting a refinery hydroprocessing technology for the cost effective production of clean fuels.

Text of Technical Paper About Refinery Hydroprocessing Technology

Choosing a hydroprocessing scheme

Hydroprocessing technologies are well established in the refining industry for the production of clean fuels. However, increased competition within the industry mandates a greater focus on awareness of the right tech-nology and catalysts to achieve the products and performance needed in the market.

For refiners to sustain their profit margins, economical access to state-of-the-art technology is a must. Refinery management needs to plan for the future to maintain long- term growth, maximise asset performance, formulate an effective response to changing environ-mental legislation and incorporate sufficient flexibility to withstand business cycles while crude supplies are becoming increasingly heavy and sour. Increased operational excellence is a priority for refineries, which leads refiners to look at more innovative ways of maintaining reasonable margins in new projects, to quickly recover the investments they have made and to justify additional investment to cope with a changing market.

Hydrotreating, the workhorse of the refinery, serves to meet several significant product quality specifi-cations. Increasingly stringent regulations for fuel (for instance, 1015 ppm sulphur in diesel and gasoline), the processing of lower-quality, higher-sulphur crudes, tightening site emissions standards (SOx and NOx reduction), and rising gasoline and diesel consumption are all factors that make significant demands of a hydroprocessing unit in a refinery. In addition, a hydro-processing unit helps refiners to

A systematic approach to selecting hydroprocessing technology meets process objectives with optimal operating and capital costs

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reduce nitrogen and aromatic content, and enhance cetane number, API gravity and smoke point. Hydroprocessing of middle distil-lates also plays a key role in improving cold flow properties such as pour point, cloud point and cold filter plug point. This enables refiners to meet the stringent product specifi-cations determined by regulatory bodies.

The established refinery configur-ation includes a minimum of three or four hydroprocessing units for upgrading light, middle and heavy

distillates. Upgrading light distillate involves the use of proven technology for the desulphurisation of FCC naphtha with minimum octane loss, as this stream contri-butes significantly to the refinery gasoline pool. Upgrading middle distillate (kerosene and diesel) focuses on managing hydrogen and energy consumption, while produc-ing ultra-low sulphur products. The gas oils and residue upgrading technology, such as hydrocracking and residual oil desulphurisation

(RDS), should be flexible enough to process a wide range of feed qualities, of diverse origin, at different conversion levels.

Basis of technology evaluationAll technologies work well within a specific context and under certain conditions. The total investment costs for a hydroprocessing unit increase with unit size, feedstock sulphur, nitrogen and quantity of cracked stocks. The evaluation of new technology should be based on detailed technical and economic analysis.

The total on-site capital cost estimate for a new hydrotreater unit varies, depending on the licensed and proprietary technology. The overall system can be broadly classified in three parts: a reactor system, hydrogen make-up/recycle gas compressor and other separation equipment. The cost of the reactor system and compressor depends on the percentage of cracked stock present in the hydrotreater feed. The cost of the separation equipment is a function of unit capacity. The basic difference in the capital costs of a unit at a given capacity level is the result of variations in the fractions of the different types of feed; for example, straight-run vs cracked stock and the sulphur level of the feed as well as the catalyst.

The major items of focus during the evaluation of hydroprocessing technology are process configur-ation, reactor operating conditions, number and size of high-pressure items, quantity and type of catalyst used, catalyst deactivation rate, make-up hydrogen purity and design pressure level, depending

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The total investment costs for a hydroprocessing unit increase with unit size, feedstock sulphur, nitrogen and quantity of cracked stocks

process severity/variablesA key factor to be considered in establishing an effective hydro-processing technology is the level of conversion required for achieving the desired objective. This level of conversion effectively sets the level of process severity required, from mild hydrofinishing for removing contaminants such as sulphur and nitrogen containing compounds, to complex molecular reconstructions associated with hydrocracking and aromatic saturation reactions. The operating conditions of a hydro-processing unit are a function of feedstock characteristics based on origin. The operating and capital cost of the unit increases with the severity of the unit. The proper combination of process parameters should be in accordance with the optimal use of hydrogen and the available utilities, such as fuel, cooling water and steam. The key process variables are liquid hourly space velocity (LHSV), hydrogen partial pressure, temperature and gas-to-oil ratio.

liquid hourly space velocity LHSV is a measure of the residence time in the reactor. The lower the LHSV, the higher the residence time. The lower the LHSV, the bigger the reactor and the higher the capital cost. Typically, the LHSV require-ment depends on the boiling range of the hydrocarbons. A heavy feed contains higher amounts of sulphur and nitrogen impurities with a complex ring structure. The removal of such compounds requires more residence time in a reactor and therefore lower LHSV. LHSV can be adjusted by either reducing the feed throughput, which is not economical, or by the addition of a new reactor or more catalyst in the same reactor, which requires substantial capital investment. An optimal design is usually one that takes advantage of a higher-activity, commercially proven catalyst to set reactor catalyst volumes and pressure levels for a target run length. Figure 1 shows the effect of a decrease in LHSV on polyaromatic saturation levels.

Hydrogen partial pressureThe type of feed to be processed,

on the product quality requirements. In determining the compatibility of the licensors technology with existing facilities it is essential to check its capability with regard to variations in feed qualities and the effect of product slate for blending.

The key process objectives to define in order to establish a transparent and consistent evalu-ation methodology are: Desired function of the hydro-processing unit in the refinery, such as hydrodesulphurisation (HDS), hydrodenitrification (HDN), olefin saturation, aromatic saturation and metals removal Feed and product specifications Minimum catalyst cycle length Hydrogen utilisation Availability of the unit (on-stream operating factor per year) Unit turndown capacity.

Criteria for selecting technology No single process technology solution can be applied to all refineries because of their widely different configurations and objec-tives. A comprehensive, site-specific study is needed to identify the most suitable process scheme under given scenarios. The evaluation should be based on the criteria developed and approved by project management and the client during the planning phase.

Technical evaluation of a licensors technology is of prime importance when it comes to customising its unique features, amplifying its reliability, flexibility and operational performance, and so meet current needs and future requirements. The key points that significantly influence the hydroprocessing units process design follow.

Feed characterisation Good feedstock characterisation, including off-design variations, is essential for the proper selection of catalyst, reaction conditions and process configuration. A study of feedstock at the micro level provides a thorough understanding of feedstock reactivity and the subsequent processing conditions needed to meet process objectives. The distribution and nature of

sulphur and nitrogen compounds in a feed depends on the feedstocks boiling range, prior processing history (whether thermally or catalytically cracked) and the crude oil type from which it is derived. The olefin content associated with cracked stock gives an idea of anticipated exotherms, the configuration of efficient quenching, heat recovery and the separation system. It also enables a refiner to choose a reactor catalyst bed arrangement. The aromatic content of a feed and its saturation requirements fixes the partial pressure of a distillate hydrotreating unit, which plays an important role in the operating and capital cost of the unit. The prediction of

unit dynamics caused by feed quality changes is of prime importance during the design phase of the unit.

process chemistry An understanding of the chemistry involved in the removal of sulphur and nitrogen compounds is essential when defining the operating severity, based on varying relative rates of reactions of different com-pounds. Desulphurisation, denitrifi-cation and olefin saturation are kinetically controlled reactions. Increasing the process severity, such as raising the temperature, usually allows these reactions to approach near complete conversion. However, the aromatic saturation reaction is thermodynamically limited, so a careful balancing of kinetic and thermodynamic equilibrium is required when deciding on pre