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Chapter 6c -_ht_design_consideration_-_latest

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Chapter 6c -_ht_design_consideration_-_latest

  2. 2. INTRODUCTION • Issue arise when designing a hydrotreating facility to produce diesel fuel with very low levels of total sulphur. • Middle distillates contain various types of sulphur species: - mercaptans - sulphides - tiophenes - aromatic sulphur compounds
  3. 3. INTRODUCTION • Sterically hindered (steric resistance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules) dibensothiophenes are a group of aromatic sulphur compounds – that the most difficult to remove when hydrotreating to very low sulphur levels. • Particularly for diesel fuels that contain significant quantities of cracked stocks, such as FCC light cycle oil (LCO) – which contains a large concentration of aromatic sulphur compounds.
  4. 4. CONSIDERATION • Effective removal of aromatic sulphur compounds requires: - tailored/ designed catalyst - tailored process conditions - other factors, such as feed nitrogen content and aromatics equilibrium OBJECTIVE – Clean diesel hydrotreating
  5. 5. DESIGN CONSIDERATION • Numerous issues to be addressed in the design of a hydrotreater: 1. Feed characteristics and variability 2. Product quality requirements – cetane index 3. Catalyst selection 4. Optimization of reactor process variables 5. Equipment design requirements
  6. 6. DESIGN CONSIDERATION 6. Reliability 7. Minimizing product contamination 8. Handling of off-spec diesel product All factors should be carefully considered during the front end process design is a process that takes an idea and turns in into a design. It consists of an input (an idea that would change the current product), output (what the final design would look like), and a process (how the idea form to design).
  7. 7. PROCESS VARIABLES • The principal operating variables - temperature - Hydrogen partial pressure - Space velocity • Temperature increases, hydrogen partial pressure increases – sulphur and nitrogen removal & hydrogen consumption increases. • Pressure increases – hydrogen saturation increases – reduces coke formation • Space velocity increases– reduces conversion, hydrogen consumption & coke formation. • Excessive temperatures must be avoided – can cause the increased coke formation.
  8. 8. TYPICAL PROCESS VARIABLES IN HT Temperature 270 – 340 oC Pressure 690 – 20700 kPa g Hydrogen per unit of feed Recycle 360 m3/m3 Consumption 36 – 142 m3/m3 Space velocity (LHSV (liquid hourly space velocity) = the ratio of the hourly volume of oil processed to the volume of catalyst. 1.5 – 8.0 v/v/hr LHSV is simply an approximate way of estimating the amount of catalyst needed to purchase for a given feed capacity and product yield
  9. 9. FEED & PRODUCT SPEC • Sulphur, nitrogen and aromatics content are the most important feed characteristics, that impact the process design of HT facilities. • Nitrogen content – significant impact on required operating pressure for a new design. • Nitrogen has to be removed at same level as sulphur to reach the ultra –low target. • Catalyst employed & hydrogen partial pressure must be consistent with a high nitrogen removal operation.
  10. 10. FEED & PRODUCT SPEC • Bulk of feed nitrogen is contained in light coker gas oil and FCC LCO (light cycle oil). • Aromatic content of feed will govern the chemical hydrogen consumption at low space velocities and high hydrogen partial pressures required for very low sulphur diesel production.
  11. 11. FEED & PRODUCT SPEC • Cracked stocks can be included in feed up to the level limited by the product cetane index or gravity without having a significant impact on Hydrotreater design. • Small increase in cetane index during HT reaction. • If significant improvement in cetane is required, a multi-stage design using aromatics saturation catalyst in second stage of HT – is more economical option.
  12. 12. FEED & PRODUCT SPEC • Feed is pratically mandatory to confirm reaction process condition. • Testing variations in feed spec, FCC LCO and coker light gas oil should be considered…to avoid poor separation achieved in the products fractionators. • If not, increase in the content of the most difficult-to-treat sulphur compounds in the hydrotreater feed and requires an increase in reactor temperature. • Moreover, this will increase the catalyst deactivation rate and reduce cycle length.
  13. 13. REACTION PROCESS VARIABLES • The key reaction process variables are: - Space velocity - Hydrogen partial pressure - Make-up hydrogen purity - Ratio of total hydrogen to reactor/ chemical hydrogen consumption - Cycle length - Reactor temperature
  14. 14. REACTION PROCESS VARIABLES • Feeds with significant aromatics and/or nitrogen content, a Ni/Mo or Ni promoted Co/Mo catalyst will be used, along with an appropriate selection of graded catalyst in the top of the bed to reduce reactor pressure build-up. • For a given cycle length and treating severity reactor space velocity, hydrogen treat gas quantity and hydrogen partial pressure are the variables that are optimised during the process design along with reactor temperature.
  15. 15. REACTION PROCESS VARIABLES • From a practical viewpoint, one should be aware of the limiting pressure of the alloy piping flanges in the reactor section when setting the hydrogen partial pressure and total operating pressure. • This includes piping from the combined feed exchangers to the feed heater, from the heater to the reactor, and from the reactor to the combined feed exchangers (especially when the feed contains significant quantities of cracked stocks).
  16. 16. REACTION PROCESS VARIABLES • This piping will be a 300 series stainless steel, and for 600 psi ANSI (American Standard) flanges this corresponds to a maximum operating pressure of around 800 psig at the reactor inlet when using 321 SS and 880 psig when using 347 SS.
  17. 17. REACTION PROCESS VARIABLES • Make-up hydrogen purity impacts the hydrogen partial pressure for a fixed reactor operating pressure. • Lower purity make-up hydrogen requires higher hydrogen circulation rates to maintain the target hydrogen partial pressure and may even require a purge stream from the cold separator. • For a improved design, increased make-up hydrogen purity is the most effective means of increasing the hydrogen partial pressure.
  18. 18. REACTION PROCESS VARIABLES • Hydrogen partial pressure has a major impact on cycle length from a catalyst activity standpoint. • For a fixed space velocity, the cycle length increases with hydrogen partial pressure. • Hydrogen partial pressure must be greater that the hydrocarbon partial pressure, to improve the removal of sulfur and nitrogen compounds and reduce coke formation.
  19. 19. REACTION PROCESS VARIABLES • Maximum reactor outlet temperature at end-of- cycle catalyst conditions is generally set at 725– 750°F to avoid aromatics saturation equilibrium constraints. • This is also influenced by the quantity of cracked stocks in the feed and the crude source. • Hydrotreating catalyst performance correlations for reactor temperature are usually based on the weighted average bed temperature (WABT). • WABT is calculated as the reactor inlet temperature plus two-thirds of the reactor temperature rise.