The document discusses the selection of hydroprocessing technologies for refining, emphasizing the need for a systematic approach to maximize performance while minimizing capital and operating costs. Key considerations include feedstock characteristics, operational conditions, catalyst selection, and compliance with regulatory standards for product specifications. It highlights the importance of understanding the chemistry involved in hydroprocessing techniques and adapting to economic and environmental challenges in the refining industry.

4. 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
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5. 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,
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6. 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
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7. 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
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8. 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)
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9. 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.
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10. 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: alpesh_gurjar@yahoo.co.in
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