1. Chapter 6d
HYDROTREATING
DESIGN CONSIDERATIONS

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.