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Technical Paper About Refinery Hydroprocessing Technology Selection.Pdf


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

Technical Paper About Refinery Hydroprocessing Technology Selection.Pdf

  1. 1. Choosing a hydroprocessing scheme A systematic approach to selecting hydroprocessing technology meets process objectives with optimal operating and capital costs AlpesH GurjAr Fluor Daniel India H ydroprocessing technologies reduce nitrogen and aromatic (RDS), should be flexible enough to are well established in the content, and enhance cetane number, process a wide range of feed refining industry for the API gravity and smoke point. qualities, of diverse origin, at production of clean fuels. However, Hydroprocessing of middle distil- different conversion levels. increased competition within the lates also plays a key role in industry mandates a greater focus improving cold flow properties such Basis of technology evaluation on awareness of the right tech- as pour point, cloud point and cold All technologies work well within a nology and catalysts to achieve the filter plug point. This enables specific context and under certain products and performance needed refiners to meet the stringent conditions. The total investment in the market. product specifi-cations determined costs for a hydroprocessing unit For refiners to sustain their profit by regulatory bodies. increase with unit size, feedstock margins, economical access to state- The established refinery configur- sulphur, nitrogen and quantity of of-the-art technology is a must. ation includes a minimum of three cracked stocks. The evaluation of Refinery management needs to plan or four hydroprocessing units for new technology should be based on for the future to maintain long- upgrading light, middle and heavy detailed technical and economic term growth, maximise asset analysis. performance, formulate an effective The total on-site capital cost response to changing environ- The total investment estimate for a new hydrotreater unit mental legislation and incorporate varies, depending on the licensed sufficient flexibility to withstand costs for a and proprietary technology. The business cycles while crude supplies overall system can be broadly are becoming increasingly heavy hydroprocessing classified in three parts: a reactor and sour. Increased operational unit increase with system, hydrogen make-up/recycle excellence is a priority for refineries, gas compressor and other separation which leads refiners to look at more unit size, feedstock equipment. The cost of the reactor innovative ways of maintaining system and compressor depends on reasonable margins in new projects, sulphur, nitrogen and the percentage of cracked stock to quickly recover the investments present in the hydrotreater feed. The they have made and to justify quantity of cracked cost of the separation equipment is additional investment to cope with a function of unit capacity. The basic a changing market. stocks difference in the capital costs of a Hydrotreating, the workhorse of unit at a given capacity level is the refinery, serves to meet several distillates. Upgrading light distillate the result of variations in the significant product quality specifi- involves the use of proven fractions of the different types of cations. Increasingly stringent technology for the desulphurisation feed; for example, straight-run vs regulations for fuel (for instance, of FCC naphtha with minimum cracked stock and the sulphur level 10–15 ppm sulphur in diesel and octane loss, as this stream contri- of the feed as well as the catalyst. gasoline), the processing of lower- butes significantly to the refinery The major items of focus during quality, higher-sulphur crudes, gasoline pool. Upgrading middle the evaluation of hydroprocessing tightening site emissions standards distillate (kerosene and diesel) technology are process configur- (SOx and NOx reduction), and rising focuses on managing hydrogen and ation, reactor operating conditions, gasoline and diesel consumption are energy consumption, while produc- number and size of high-pressure all factors that make significant ing ultra-low sulphur products. The items, quantity and type of catalyst demands of a hydroprocessing unit gas oils and residue upgrading used, catalyst deactivation rate, in a refinery. In addition, a hydro- technology, such as hydrocracking make-up hydrogen purity and processing unit helps refiners to and residual oil desulphurisation design pressure level, depending PTQ Q1 2010 71
  2. 2. on the product quality requirements. sulphur and nitrogen compounds in process severity/variables In determining the compatibility of a feed depends on the feedstock’s A key factor to be considered in the licensor’s technology with boiling range, prior processing establishing an effective hydro- existing facilities it is essential to history (whether thermally or processing technology is the level of check its capability with regard catalytically cracked) and the crude conversion required for achieving to variations in feed qualities and oil type from which it is derived. the desired objective. This level of the effect of product slate for The olefin content associated with conversion effectively sets the level blending. cracked stock gives an idea of of process severity required, from The key process objectives to anticipated exotherms, the mild hydrofinishing for removing define in order to establish a configuration of efficient quenching, contaminants such as sulphur and transparent and consistent evalu- heat recovery and the separation nitrogen containing compounds, to ation methodology are: system. It also enables a refiner to complex molecular reconstructions • Desired function of the hydro- choose a reactor catalyst bed associated with hydrocracking and processing unit in the refinery, such arrangement. The aromatic content aromatic saturation reactions. The as hydrodesulphurisation (HDS), of a feed and its saturation operating conditions of a hydro- hydrodenitrification (HDN), olefin requirements fixes the partial processing unit are a function of saturation, aromatic saturation and pressure of a distillate hydrotreating feedstock characteristics based on metals removal unit, which plays an important origin. The operating and capital • Feed and product specifications role in the operating and capital cost of the unit increases with the • Minimum catalyst cycle length cost of the unit. The prediction of severity of the unit. The proper • Hydrogen utilisation combination of process parameters • Availability of the unit (on-stream should be in accordance with the operating factor per year) The prediction of optimal use of hydrogen and the • Unit turndown capacity. available utilities, such as fuel, unit dynamics caused cooling water and steam. The key Criteria for selecting technology process variables are liquid hourly No single process technology by feed quality space velocity (LHSV), hydrogen solution can be applied to all partial pressure, temperature and refineries because of their widely changes is of prime gas-to-oil ratio. different configurations and objec- tives. A comprehensive, site-specific importance during liquid hourly space velocity study is needed to identify the most suitable process scheme under the design phase of LHSV is a measure of the residence time in the reactor. The lower the given scenarios. The evaluation the unit LHSV, the higher the residence time. should be based on the criteria The lower the LHSV, the bigger the developed and approved by project reactor and the higher the capital management and the client during unit dynamics caused by feed cost. Typically, the LHSV require- the planning phase. quality changes is of prime ment depends on the boiling range Technical evaluation of a licensor’s importance during the design phase of the hydrocarbons. A heavy feed technology is of prime importance of the unit. contains higher amounts of sulphur when it comes to customising its and nitrogen impurities with a unique features, amplifying its process chemistry complex ring structure. The removal reliability, flexibility and operational An understanding of the chemistry of such compounds requires more performance, and so meet current involved in the removal of sulphur residence time in a reactor and needs and future requirements. and nitrogen compounds is essential therefore lower LHSV. LHSV can be The key points that significantly when defining the operating adjusted by either reducing the feed influence the hydroprocessing unit’s severity, based on varying relative throughput, which is not economical, process design follow. rates of reactions of different com- or by the addition of a new reactor pounds. Desulphurisation, denitrifi- or more catalyst in the same reactor, Feed characterisation cation and olefin saturation are which requires substantial capital Good feedstock characterisation, kinetically controlled reactions. investment. An optimal design is including off-design variations, is Increasing the process severity, such usually one that takes advantage essential for the proper selection of as raising the temperature, usually of a higher-activity, commercially catalyst, reaction conditions and allows these reactions to approach proven catalyst to set reactor catalyst process configuration. A study of near complete conversion. However, volumes and pressure levels for a feedstock at the micro level provides the aromatic saturation reaction is target run length. Figure 1 shows a thorough understanding of thermodynamically limited, so a the effect of a decrease in LHSV on feedstock reactivity and the careful balancing of kinetic and polyaromatic saturation levels. subsequent processing conditions thermodynamic equilibrium is needed to meet process objectives. required when deciding on pressure Hydrogen partial pressure The distribution and nature of level and catalyst volume. The type of feed to be processed, 72 PTQ Q1 2010
  3. 3. product quality requirements, yield span, and so will benefit from a activity. There is a minor boost in and the amount of conversion for a longer run, increased unit hydrogen partial pressure as well specific catalyst cycle life determine throughput or the ability to process with increasing gas circulation rates. the hydrogen partial pressure a challenging feed mix. The However, above a certain gas rate, required for the operation of a operating temperature should be the increase in hydrogen partial hydroprocessing unit. The hydrogen high enough to facilitate faster pressure will be relatively small and partial pressure must be high kinetic reaction rates, but not so incur extra heating and cooling enough to accomplish the desired high as to promote undesirable side costs. level of denitrification and partial reactions or to exceed the metal- In addition to affecting hydrogen saturation of heavy aromatic lurgical limits of high-pressure partial pressure, the gas rate is molecules. At higher partial vessels. important because it acts to strip pressures, the desulphurisation and As the end point of hydrocarbon volatile products from the reactor denitrification process is “easier”; feed increases, there is an increase liquids, and thus affects the however, the unit becomes more in the concentration of recalcitrant concentration of various components expensive because of the need for sulphur and nitrogen species in the in the reactive liquid phase. It also thicker-walled reactors. The form of dibenzothiophenes, which maintains proper mass velocity in minimum pressure required necessitates a higher SOR temper- the catalyst bed, thus reducing the typically rises with the required ature. For grassroots units, a higher- possibilities of channelling in the severity of the unit. A higher hydrogen partial pressure decreases catalyst deactivation and, therefore, increases the predicted cycle length Thermodynamic effects for a fixed quantity of catalyst. The importance of maintaining adequate hydrogen partial pressure increases as sulphur levels approach the sub-ppm level, because the primary HDS reaction shifts from a predominantly non-reversible, Increasing pressure single-step reaction to a reversible, Decreasing equilibrium-limited, two-step reac- LHSV tion. As the unit approaches a higher LHSV effects operating temperature at end-of-run (EOR) conditions, the lower-pressure unit may struggle to meet 10 wppm sulphur requirements because of the effects of thermodynamic equilibrium. At lower sulphur levels, the remaining species behave like polyaromatics and, therefore, the Figure 1 The impact of process conditions on polyaromatic saturation chemistry of their removal obeys similar rules to the saturation of activity catalyst may be employed bed, and carries the reaction heat. It polyaromatics. Figure 1 shows that to optimise the unit design temper- is prudent to maintain a healthy an increase in total pressure/ ature and pressure requirements in hydrogen-to-oil ratio to prevent hydrogen partial pressure increases order to save capital investment. coking and subsequent deactivation the absolute level of polyaromatic of the catalyst. As a guide, the saturation. Hydrogen/hydrocarbon ratio and available hydrogen at the top of the recycle gas rate reactor should be two-and-a-half to Temperature (WABT) The choice of recycle gas rate is three times the chemical hydrogen For most hydrotreating units, the governed by economic consider- consumption for easier feedstocks only parameter that typically varies ations. Recycle hydrogen is used to and three to four times the chemical once the unit is built is the start-of- enable flow distribution and uniform hydrogen consumption for cracked run (SOR) temperature, depending physical contact of the hydrogen feedstocks. on catalyst activity. Since the EOR with oil-soaked catalyst to ensure temperature is usually fixed, based adequate conversion and removal Catalyst selection on the specification or unit of impurities, while minimising Hydrotreating catalysts consist of a hardware constraints, a higher- carbon deposition. For high-activity hydrogenation component dis- activity catalyst will help to start the hydrotreating catalysts, there may persed on a porous, fairly inert reaction at a colder temperature, be a minimum treat gas circulation material. For hydrotreating, catalysts thereby increasing the temperature requirement to preserve catalyst with weak acidity are used, since 74 PTQ Q1 2010
  4. 4. hydrogenation and cracking capa- HDS/HDN bilities, its size, shape and pore reactor with silica guard structure are very important, as they Di-olefin reactor govern the pressure drop, surface- to-volume ratio and diffusion rate. Start-up heater Different shapes of catalyst are often used to take advantage of their high surface-to-volume ratio, while still maintaining a reasonable reactor pressure drop. The reaction kinetics are usually diffusion limited; a small catalyst with a high surface-to- volume ratio has better diffusion for the relatively heavy feed. The pore diameter for the residuum hydrotreating catalyst needs to be quite large relative to a catalyst for light feed. The increase in pore size decreases the surface area and the Figure 2 Naphtha hydrotreater unit co-processing cracked stocks catalyst activity. To overcome the limitations of small vs large pore cracking and the associated is high, a NiMo catalyst system may trade-off, catalysts are layered to production of light ends and lighter be the right choice. Recent advances increase activity; for example, large liquid product(s) are usually include staging or stacking both pore-sized catalyst in the top section undesirable. A combination of base CoMo and NiMo catalysts within a of the reactor, followed by smaller metals, such as NiMo and CoMo, is single fixed-bed reactor. pore-sized catalyst. Typical pore used to achieve deep HDS and HDN Hydrocracking catalysts serve sizes of 75–85 Å for light/heavy gas activity. Feed composition and dual functions, containing both oil feed and 150–250 Å for residue product quality requirements define hydrogenation and cracking sites. feed can be used. the required chemical composition The cracking sites are usually the and quantity of the catalyst. result of using a porous support of Feed filtration Typically, a CoMo catalyst may be an acidic nature, such as amorphous Feed filtration is important to the right choice where the feed is silica-alumina and crystalline mitigate exchanger and reactor straight-run distillate with very little aluminosilicates or zeolites. The best plugging. An appropriate feed nitrogen content and the operating choice of catalyst for a specific filtration system can reduce the pressure is low to moderate. On the objective requires a particular build of pressure drop in the other hand, if the feed contains a balance between the cracking and reaction section of the unit, which high percentage of cracked stock hydrogenation functions. results in a significant reduction in that has a significant nitrogen In addition to the chemical nature operating costs. A cartridge or content and the operating pressure of the catalyst, which dictates its wedge wire backwash filter with 25 micron retention is typical for this application. Cracked feeds should feed the hydrotreater hot from the upstream facilities or from inert gas blanketed storage. The use of steam- stripped feed that contains a significant amount of water requires the installation of a feed coaleaser upstream of the feed filter. However, Co-current or counter-current traces of water can be removed by AroSat reactor using a horizontal feed surge drum with associated water boot, eliminating the need for a coaleaser. Stripper Feed heating section Hydrotreating This section comprises a series of reactor heat exchangers followed by a charge heater. Hydroprocessing reactions are exothermic in nature. The reactor feed effluent exchanger Figure 3 ULSD unit with AroSat option must recover as much heat as is PTQ Q1 2010 75
  5. 5. economically practical to minimise the heat input in the charge furnace to typically less than 15–20% for heat balance and emergency operation. Off-design cases such as cold start-up or loss of feed results in a high design duty and thus a CHPS higher capital cost. The proper selection of exchangers enables a maximum recovery of heat of reaction, minimises the probability of leakage and increases the unit’s reliability. If the feed to the charge heater is a two-phase stream, to Figure 4 For straight-run feed (naphtha hydrotreater) avoid coking/hotspot in the charge heater tube passes, it is recommended the second reactor, with a stripper to have pass balancing to maintain in between, may be the preferred processed using moving-bed and/ equal flow distribution. Bypassing configuration (see Figure 3). For or ebullated-bed reactors. Recent the heat exchanger train should be hydrocrackers, depending on pilot work includes slurry-based considered in the case of an capacity, conversion and product reactors for deep-conversion residue uncontrollable increase in the reactor specification targets, it may be best hydrocracking. Multireactor systems bed temperature when processing to investigate two-stage reactor consisting of moving- and/or large amounts of cracked stocks. systems that allow staging of the ebullated-bed reactors integrated HDS/HDN reactions in a sour, with fixed-bed reactors can be used reactor configuration ammoniacal environment and HDA to process difficult feeds. The number of reactors and their reactions in a sweet, colder configuration depends on factors environment to capitalise on kinetic reactor internals such as catalyst volume, mass and reaction rates. Reactor internals are exceptionally volumetric flux, reactor pressure The reactors selected for light important for the safe, reliable and drop, reactor dimensions and feeds differ markedly from those profitable operation of a hydro- materials of construction. A typical selected for heavy feeds. Fixed-bed processing unit and may have a pressure drop of about 0.7–1.5 psi/ reactors have been traditionally major effect on reactor performance ft of catalyst bed (SOR to EOR) is used for light feeds. High asphaltene in terms of catalyst utilisation desirable to promote uniform flow and high metal content feeds, such efficiency and unit availability. For through the catalyst bed to have a as vacuum residue, are successfully a gas-phase reaction such as naphtha uniform radial temperature profile. An excessive pressure drop will increase the recycle gas compressor power consumption and could challenge the mechanical integrity of the reactor catalyst support trays. Other factors are flexibility in fabrication and transportation from CHPS workshop to refinery site. In special cases, the refinery’s ground conditions may also preclude the installation of a single heavy weight reactor. If the feed contains a high level of HHPS olefins, to avoid fouling in the heat exchangers, in the heater and in the top catalyst bed it is advisable to saturate the olefins at a lower reactor temperature. A separate olefin Intermediate pressure saturation reactor may be added break-up drum upstream of the HDS reactor (see Figure 2). For a unit where deep aromatic saturation is required, a two-stage reactor approach using HDS/HDN catalyst in the first reactor and a noble metal catalyst in Figure 5 For cracked stock feed (high-severity ULSD unit) PTQ Q1 2010 77
  6. 6. CHPS HHPS CLPS HLPS Figure 6 Classic four-drum separation system (hydrocracker unit) hydrotreating, a distribution tray is product cooling and separation schemes ensures the efficient not necessary, but for a trickle-bed A typical configuration in any recovery of hydrogen from hydro- reactor, processing middle/heavy hydrotreater includes a feed/ carbon, reduces the relief load of distillates, specially designed gas effluent heat exchanger train, a large the system and facilitates a lower liquid distributors to achieve small- air-cooled heat exchanger and one, design pressure for the downstream scale contacting of process gas and two or more flash drums, depending columns. The configuration of the liquid are mandatory. An ideal on unit heat balance and hydrogen separation system depends on the liquid distributor should have a recovery requirements. economic balance between operating high distribution element density, A straight-run feed with alumina- and capital cost, in addition to feed low pressure drop, large spray based catalyst produces fewer light quality and hydrogen availability. angle, turndown flexibility and be ends, so a single, cold, high-pressure easy to clean. This enables full separator (CHPS) may be adequate High-pressure amine absorption catalyst utilisation and thermal for smaller-capacity units (see Feed with a higher sulphur content uniformity across the catalyst bed, Figure 4). If the feed contains a results in the accumulation of H2S in so that the lowest possible SOR large percentage of cracked stock, the recycle gas loop. H2S inhibits temperature is achieved in which generates large exotherms in HDS reactions and lowers the purity conjunction with minimisation of a reactor catalyst bed, the use of the recycle gas and thus the the catalyst deactivation rate. of a hot, high-pressure separator partial pressure of hydrogen. A high The use of robust reactor internals (HHPS), with a gas component that H2S concentration in the recycle gas improves radial temperature distrib- will be routed to a CHPS via an air (typically >2–3 vol%) will influence ution and catalyst utilisation, which cooler/trim cooler, and a liquid catalyst selectivity in an undesirable ultimately translates into a better component routed to the stripping way. To compensate, a higher unit yield, longer catalyst life and more section, may be justified (see Figure pressure may be needed. A high efficient use of limited hydrogen 5). This facilitates enhanced heat sulphur content in the feed and resources. Some of the best-in- integration and lower air cooler ultra-low sulphur products may class reactor internals help to duty, but increases the hydrogen require an amine scrubbing system achieve less than 5°C radial loss and results in a higher capital in a recycle gas loop to prevent H2S temperature spread at the bottom of cost of the unit. build-up and improve catalyst deep catalyst beds. Additionally, Limited resources of hydrogen activity. The high-pressure amine tightly designed high-capacity necessitate further separation of absorber increases the partial trays and quench systems help to HHPS liquid to a hot, low-pressure pressure of hydrogen, which, in reduce reactor heights in multibed separator (HLPS) followed by a turns, results in lower operating and reactors. CLPS (see Figure 6). This process capital costs for the unit. 74 PTQ Q1 2010
  7. 7. Additionally, it increases catalyst Hydrogen management life, reduces hydrogen losses Lower-purity (<85%) hydrogen through purge and requires lower make-up gas increases the design power consumption in the recycle pressure of the unit, and increases gas compressor because of the both the capital and operating costs increased purity of the gas. of the unit. A supply of high-purity hydrogen increases the partial Gas compression system pressure of the hydrogen in the The choice of recycle and make-up reactor in conjunction with a lower gas compressor depends on gas total operating pressure, and so purity. The make-up gas flow is results in lower operating and typically 25–40% of the recycle gas capital costs. Additionally, it flow. High-purity make-up gas increases catalyst selectivity, stability compression requires more stages. and overall cycle length. Optimis- The multistage reciprocating com- ation of the hydrogen system may pressor works well for this service. provide additional H2 availability, A sparing arrangement is necessary while avoiding capital investment. to ensure reliability and availability. Pressure swing adsorption or semi- The lower purity of make-up gas permeable membrane technology increases both the capital and could be considered for the operating cost of the compressor. purification of the H2 purge and The high purity of recycle gas make-up hydrogen streams from requires a lower gas circulation rate, the catalytic reforming unit. but the lower molecular weight of The cascading of H2 purge streams gas requires more head and for use as H2 make-up streams to potentially more compression other HDT units increases the purge stages. The recycle gas acts as a rate for higher recycle gas purity major heat sink in a reactor and and is more economical. Quench avoids excursion probability. hydrogen rates between the catalyst Typically, a centrifugal compressor beds should be minimised consistent is used for this service in view of its with safe operation, the ability to higher reliability and efficiency over maintain the required hydrogen a reciprocating compressor. partial pressure and desired catalyst life. stripping section The stripping section of a middle summary distillate hydroprocessing unit Hydroprocessing plays an increas- mainly removes H2S and light ingly important role in oil refining hydrocarbons from the hydrotreated and is key to the production of clean product and stabilises the product transportation fuels. Selection of the to meet the flashpoint specification. right technologies, the right combin- If lighter components are present in ation of high-activity catalysts, the feed, to offload the light appropriate reactor system arrange- products, distillation can be carried ment, operating conditions and out in two steps: H2S steam stripping advanced reactor internals provides at a higher pressure, followed by refiners with a range of advantages. lower-pressure stabilising. This will These include low operating and increase operational flexibility and capital costs, improved hydrogen enable the use of a smaller column utilisation, greater flexibility, more size with a low design pressure. scalability and high reliability for Use of heavy feed and less the processing of a range of cracking in the reactor means a feedstocks. standalone, low-pressure stripper Alpesh Gurjar is an Associate Process column can be employed. It may be Specialist at Fluor Daniel India Pvt Ltd. He one of two types: attached fired has six years’ experience in process design, reboiler or steam stripping. For technical services and operation of various kerosene and diesel boiling-range refinery hydroprocessing units. He has worked hydrocarbon, a fired heater reboiler with Essar Oil refinery and has a degree in is more suitable, as it is hard to chemical engineering from M S University of achieve a higher temperature Baroda, India. with steam. Email: PTQ Q1 2010 77