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1
Introduction
Mechanism of Catalytic Hydrogenation
Hydrogen Production from HC
Factors Affecting Hydrogenation
PSA Technology for Hydrogen Purification
Hydrogenation in Refining Processes
Hydrogenation in Gas Processes
Ch.E-305 Muhammad Asif Akhtar 2
Hydrogenation is the addition of hydrogen to a
chemical compound.
Generally, the process involves elevated
temperature and relatively high pressure in the
presence of a catalyst.
Ch.E-305 Muhammad Asif Akhtar 3
Hydrogenation may be either destructive or non-
destructive.
In the former case, hydrocarbon chains are ruptured
(cracked) and hydrogen is added where the breaks have
occurred.
In the latter, hydrogen is added to a molecule that is
unsaturated with respect to hydrogen. In either case, the
resulting molecules are highly stable.
Ch.E-305 Muhammad Asif Akhtar 4
1. Besides saturating double bonds, hydrogenation
can be used to eliminate other elements from a
molecule. These elements include:
Oxygen
Nitrogen
Halogens
Sulfur
Ch.E-305 Muhammad Asif Akhtar 5
APPLICATION
2. Cracking (thermal decomposition) in the presence of hydrogen is
particularly effective in desulfurizing high-boiling petroleum fractions,
thereby producing lower-boiling and higher-quality products
REACTION TYPE ILLUSTRATION ∆HR
kJ per standard cubic meter of consumed H2
† R = alkyl
M = Fe, Ni
A = metals-adsorbing material
Ch.E-305 Muhammad Asif Akhtar 6
3. Oils have been
hydrogenated for
many decades, to
prolong their shelf life
and make the oils
more stable.
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Saturation of olefins is irreversible and the saturation
of aromatics is reversible.
Hydrogenation is generally carried out in the presence of a catalyst and under elevated temperature and pressure. Noble metals, nickel, copper, and various metal oxide combinations are the common catalysts.
The catalyst binds both the H2 and the unsaturated substrate and facilitates their union.
Pd and Pt are poisoned by sulfur and can only be used in low-H2S environments
Gaseous hydrogen is the usual hydrogenating agent.
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H H C C
A
B
X
Y H H
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H
C C A
B
X
Y
H H H
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H
H H H
C C
A
B
X Y
Ch.E-305 Muhammad Asif Akhtar 15
H
H H H
C C
A
B
X Y
Ch.E-305 Muhammad Asif Akhtar 16
H H
H
C C
A
B
X Y
H
Ch.E-305 Muhammad Asif Akhtar 17
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Hydrogen use has become more widespread in refineries,
hydrogen production has moved from the status of a high-
technology specialty operation to an integral feature of
most refineries. This has been made necessary by the
increase in hydrotreating and hydrocracking, including the
treatment of progressively heavier feedstocks.
Steam reforming is the dominant method for hydrogen
production. This is usually combined with pressure-swing
adsorption (PSA) to purify the hydrogen to greater than 99
vol %.
Ch.E-305 Muhammad Asif Akhtar 19
The best feedstocks for steam reforming are light, saturated, and low in sulfur; this includes natural gas, refinery gas, LPG, and light naphtha. These feeds can be converted to hydrogen at high thermal efficiency and low capital cost.
Many recent refinery hydrogen plants have multiple feedstock flexibility, either in terms of backup or alternative or mixed feed.
Automatic feedstock change-over has also successfully been applied by Technip in several modern plants with multiple feedstock flexibility.
Ch.E-305 Muhammad Asif Akhtar 20
Natural gas is the most common hydrogen plant feed, since it meets all
the requirements for reformer feed and is low in cost.
A typical pipeline natural gas contains over 90 percent C1 and C2, with
only a few percent of C3 and heavier hydrocarbons.
It may contain traces of CO2, with often significant amounts of N2.
The N2 will affect the purity of the product hydrogen: It can be
removed in the PSA unit if required, but at increased cost. Purification
of natural gas, before reforming, is usually relatively simple.
Traces of sulfur must be removed to avoid poisoning the reformer
catalyst, but the sulfur content is low and generally consists of H2S
plus some mercaptans.
Zinc oxide, often in combination with hydrogenation, is usually
adequate.
Ch.E-305 Muhammad Asif Akhtar 21
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Refinery gas, containing a substantial amount of hydrogen, can be an attractive steam reformer feedstock.
Since it is produced as a by-product, it may be available at low cost.
Processing of refinery gas will depend on its composition, particularly the levels of olefins and of propane and heavier hydrocarbons.
Olefins can cause problems by forming coke in the reformer. They are converted to saturated compounds in the hydrogenator, giving off heat. This can be a problem if the olefin concentration is higher than about 5 percent, since the hydrogenator will overheat.
Ch.E-305 Muhammad Asif Akhtar 23
Liquid feeds, either LPG or naphtha, can be
attractive feedstocks where prices are
favorable. Liquid feeds can also provide backup feed, if there is a risk of
natural gas curtailments.
Ch.E-305 Muhammad Asif Akhtar 24
The generic flowsheet consists of
Ch.E-305 Muhammad Asif Akhtar 25
Feed Pre-treatment
Pre reforming (Optional)
Steam-HC Reforming
Shift Conversion and
Hydrogen Purification By Pressure Swing Adsorption (PSA).
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Hydrogen Production By Steam
Reforming/PSA.
Ch.E-305 Muhammad Asif Akhtar 30
Steam Reforming/Wet Scrubbing
Feed pre-treatment normally involves removal of sulfur,
chlorine and other catalyst poisons after preheating to
350 – 400°C.
Ch.E-305 Muhammad Asif Akhtar 31
The treated feed gas mixed with process steam is
reformed in a fired reformer (with adiadatic pre-
reformer upstream, if used) after necessary super-
heating. The net reforming reactions are strongly
endothermic.
Heat is supplied by combusting PSA purge gas,
supplemented by makeup fuel in multiple burners in a
top-fired furnace.
Ch.E-305 Muhammad Asif Akhtar 32
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Reforming severity is optimized for each specific case.
Waste heat from reformed gas is recovered through steam
generation before the water-gas shift conversion.
Most of the carbon monoxide (CO) is further converted
to hydrogen. Process condensate resulting from heat
recovery and cooling is separated and generally reused in
the steam system after necessary treatment.
The gas flows to the PSA unit that provides high-purity
hydrogen product (up to < 1 ppm CO) at near inlet
pressures.
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Typical utility requirements for a 50 million SCFD hydrogen
plant feeding natural gas are as follows (no compression is
required).
Technip has been involved in over 240 hydrogen plants
worldwide.
Licensor: Technip.
Ch.E-305 Muhammad Asif Akhtar 39
Previous Lecture Review
Ch.E-305 Muhammad Asif Akhtar 40
41
Absorption is not adsorption
absorption: accumulation within (not on) a solid
Ch.E-305 Muhammad Asif Akhtar 42
Profitability for hydrogen and ammonia plants depends heavily on the efficiency and reliability of carbon dioxide (CO2) removal from process gas.
Over the last 30 years, several innovations have evolved regarding CO2-removal units. New methods have dramatically increased
• Absorption efficiency
• Reduced CO2 slip to a few parts per million by volume (ppmv),
• Lowered energy requirements for CO2 regeneration and mitigated corrosion of plant equipment.
Ch.E-305 Muhammad Asif Akhtar 43
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K2CO3 + CO2 + H2O = 2KHCO3
K2CO3 + H2S = KHS + KHCO3
Reactions:
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52
PSA
Technology
for Hydrogen
Purification
Ch.E-305 Muhammad Asif Akhtar
The PSA process produces a hydrogen stream of four- nines (99.99%) purity.
It separates carbon monoxide, carbon dioxide and unconverted hydrocarbons.
A bank of adsorbers operates in a cycle where the adsorbers are rotated through a higher-pressure adsorption portion, followed by a pressure reduction, which allows the contaminants to be released from the adsorber.
The hydrogen gas passes through the adsorber as almost-pure hydrogen
Ch.E-305 Muhammad Asif Akhtar 53
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A PSA installation consists of four major parts:
Adsorber vessels made from carbon steel and filled
with adsorbent
Valves and instrumentation
Control system which is normally located in a remote
control room
Mixing drum
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A complete pressure-swing cycle consists of the
following five basic steps:
1. Adsorption
2. Cocurrent depressurisation
3. Countercurrent depressurisation
4. Purge at low pressure
5. Repressurisation
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Production of any purity hydrogen, typically 90% to +99.9999 mole%.
Impurities efficiently removed include:
N2, CO, CH4, CO2, H2O, Ar, O2, C2–C8+, CH3OH, NH3, H2S and organic sulfur compounds.
The technology can also be used to:
Purify CH4, CO2, He, N2 and Cl;
Remove CO2;
Adjust synthesis gas stream composition ratios and separate nitrogen from hydrocarbons.
Ch.E-305 Muhammad Asif Akhtar 80
Steam reformer (at any point after the reformer),
Catalytic reformer net gas
Refinery purge streams
Gasification offgases
Ammonia plant purge gases (before or after the NH3 waterwash)
Ethylene plant offgases
Partial oxidation gases
Styrene plant offgases
Ethanol plant purge gases
Coke-oven gas
Cryogenic purification offgases or other H2 sources.
Feed pressures up to 1,000 psig have been commercially demonstrated.
Ch.E-305 Muhammad Asif Akhtar 81
Recovery of H2 varies between 60% and 90%,
depending on composition, pressure levels and
product requirements.
Typical temperatures are 60°F to 120°F.
Purity can be +99.9999 mole%.
Ch.E-305 Muhammad Asif Akhtar 82
Purification is based on advanced pressure swing adsorption (PSA) technology.
Purified H2 is delivered at essentially feed pressure, and impurities are removed at a lower pressure.
Polybed PSA units contain 4 to 16 adsorber vessels. One or more vessels are on the adsorption step, while the others are in various stages of regeneration.
Single-train Polybed PSA units can have product capacities over 200 million scfd.
Ch.E-305 Muhammad Asif Akhtar 83
Operation is automatic with pushbutton startup and
shutdown.
After startup, the unit will produce H2 in two to four hours.
Onstream factors in excess of 99.8% relative to unplanned
shutdowns are typical.
Turndown capability is typically 50% but can be even
lower where required.
The units are built compactly with plot plans ranging from
12 ft x 25 ft to 60 ft x 120 ft.
Ch.E-305 Muhammad Asif Akhtar 85
Units are skid-mounted and modular to minimize installation costs.
Material for piping and vessels is carbon steel.
Control can be via a local or remote-mounted control panel or by integration into the refinery’s computer control system.
Units are designed for outdoor, unattended operation and require no utilities other than small quantities of instrument air and power for instrumentation.
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More than 700 units are in operation or under construction,
including the world’s first 16-bed system, and the world’s
largest single-train system.
Licensor
UOP LLC.
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FEED: Natural gas, refinery off gases, LPG, naphtha or mixtures.
PRODUCT:
High-purity H2 (typically >99.9%), carbon monoxide
(CO), carbon dioxide (CO2), high-pressure steam and/or
electricity may be produced as separate creditable by-
product.
Ch.E-305 Muhammad Asif Akhtar 91
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The feed is desulfurized (1), mixed with steam and
converted to synthesis gas in steam reformer (2) over a
nickel-containing catalyst at 20 – 40 bar pressure and
outlet temperatures of 800 –900°C.
The Uhde steam reformer features a well-proven top-fired
design with tubes made of centrifugally cast alloy steel and
a unique proprietary “cold” outlet manifold system for
enhanced reliability.
Ch.E-305 Muhammad Asif Akhtar 93
A further speciality of Uhde’s H2 plant design is an optional bi-sectional steam system for the environmentally friendly full recovery of process condensate and production of high-pressure export steam (3) with a proven process gas cooler design.
The Uhde steam reformer concept also includes a fully pre-fabricated and modularized convection bank design to further enhance the plant quality and minimize construction risks.
The final process stages are the adiabatic CO shift (4) and a pressure swing adsorption unit (5) to obtain high-purity H2.
Ch.E-305 Muhammad Asif Akhtar 94
Process options include:
Feed evaporation
Adiabatic feed pre-reforming and/or
HT/LT shift to process, for example, heavier feeds
and/or to optimize feed/fuel consumption and steam
production.
Uhde’s design allows combining maximized process heat
recovery and optimized energy efficiency with
operational safety and reliability.
Ch.E-305 Muhammad Asif Akhtar 95
The Uhde reformer design is particularly advantageous for
the construction and reliable operation of large-scale
reformers with H2 capacities up to 220,000 Nm3/ h (197
MMscfd) in single-train configurations.
Uhde offers either standard or tailor-made designs and
applies either customer or own design standards.
Ch.E-305 Muhammad Asif Akhtar 96
Depending on the individual plant concept, the typical
consumption figure for natural gas-based plants (feed + fuel
– steam) can be as low as 3.13 Gcal /1,000 Nm3 (333
MMBtu/MMscf) or 3.09 (329) with pre-reforming.
Ch.E-305 Muhammad Asif Akhtar 97
Recently, Uhde has successfully commissioned large-scale H2 plants for
SINCOR C.A., Venezuela (2 x 100,000 Nm3/ h or 2 x 90 MMscfd)
Shell Canada Ltd., Canada (2 x 130,000 Nm3/ h or 2 x 115 MMscfd)
and is presently executing four H2 projects, including H2 plants for
Neste Oil Oyj (formerly Fortum Oil Oy)
Finland (1 x 155,000 Nm³/ h or 140 MMscfd) and
Shell Canada Ltd., Canada, (1 x 150,000 Nm³/ h or 135 MMscfd).
More than 60 Uhde reformers have been designed and constructed worldwide.
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To produce methanol from natural or associated gas
feedstocks using two-step reforming followed by low-
pressure synthesis. This technology is well suited for world-
scale plants to modify ammonia capacity into methanol
production.
Ch.E-305 Muhammad Asif Akhtar 102
Methanol or methyl alcohol (CH3OH) is a colourless
liquid with a boiling point of 65oC.
Methanol will mix with a wide variety of organic
liquids as well as with with water and accordingly it is
often used as a solvent for domestic and industrial
applications.
Methanol is the raw material for many chemicals,
formaldehyde, dimethyl terephphalate, methylamines
and methyl halides, methyl methacrylate, acetic acid,
gasoline etc.
Ch.E-305 Muhammad Asif Akhtar 103
In recent years methanol has also been used for other
markets such as production of DME (Di-methyl-ether)
and olefins by the so-called methanol-to-olefins process
(MTO) or as blendstock for motor fuels.
The annual production of methanol exceeds 40 million
tons and continues to grow by 4% per year.
Ch.E-305 Muhammad Asif Akhtar 104
The production of methanol from coal is increasing in locations where natural gas is not available or expensive such as in China. However, most methanol is produced from natural gas.
Several new plants have been constructed in areas where natural gas is available and cheap such as in the Middle East.
There is little doubt that (cheap) natural gas will remain the predominant feed for methanol production for many years to come.
Ch.E-305 Muhammad Asif Akhtar 105
The capacity of methanol plants has increased
significantly only during the last decade. In 1996 a
world scale methanol plant with a capacity of 2500
MTPD was started up in Norway. Today several
plants are in operation with the double of this
capacity.
Ch.E-305 Muhammad Asif Akhtar 106
All commercial methanol technologies feature three
process sections and a utility section as listed below:
Synthesis gas preparation (reforming)
Methanol synthesis
Methanol purification
Utilities
Ch.E-305 Muhammad Asif Akhtar 107
In the design of a methanol plant the three process sections may be considered independently, and the technology may be selected and optimized separately for each section.
The normal criteria for the selection of technology are capital cost and plant efficiency.
The synthesis gas preparation and compression typically accounts for about 60% of the investment, and almost all energy is consumed in this process section.
Therefore, the selection of reforming technology is of paramount importance.
Ch.E-305 Muhammad Asif Akhtar 108
Important properties of the synthesis gas are the CO to
CO2 ratio and the concentration of inerts.
A high CO to CO2 ratio will increase the reaction rate
and the achievable per pass conversion. In addition, the
formation of water will decrease, reducing the catalyst
deactivation rate.
High concentration of inerts will lower the partial
pressure of the active reactants. Inerts in the methanol
synthesis are typically methane, argon and nitrogen.
Ch.E-305 Muhammad Asif Akhtar 109
Several reforming technologies are available for
producing synthesis gas:
One-step reforming with fired tubular reforming
Two-step reforming
Autothermal reforming (ATR)
Ch.E-305 Muhammad Asif Akhtar 110
The synthesis gas is produced by tubular steam
reforming alone (without the use of oxygen). This
concept was traditionally dominating.
Today it is mainly considered for up to 2,500 MTPD
plants and for cases where CO2 is contained in the
natural gas or available at low cost from other
sources.
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The synthesis gas produced by one-step reforming
will typically contain a surplus of hydrogen of about
40%. This hydrogen is carried unreacted through the
synthesis section only to be purged and used as
reformer fuel.
The addition of CO2 permits optimization of the
synthesis gas composition for methanol production.
CO2 constitutes a less expensive feedstock, and
CO2 emission to the environment is reduced.
Ch.E-305 Muhammad Asif Akhtar 112
The application of CO2 reforming results in a very
energy efficient plant.
The energy consumption is 5–10% less than that of a
conventional plant . A 3,030 MTPD methanol plant
based on CO2 reforming was started up in Iran in
2004.
Ch.E-305 Muhammad Asif Akhtar 113
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Methanol — Steam-methane Reforming
Licensor: Haldor Topsøe A/S.
This process features a combination of fired tubular reforming (primary reforming) followed by oxygen-fired adiabatic reforming (secondary reforming).
The secondary reformer requires that the primary reformer is operated with a significant leakage of unconverted methane (methane slip).
Typically 35 to 45% of the reforming reaction occurs in the tubular reformer, the rest in the oxygen-fired reformer.
As a consequence the tubular reformer is operated at low S/C ratio, low temperature and high pressure.
Ch.E-305 Muhammad Asif Akhtar 115
These conditions lead to a reduction in the
transferred duty by about 60% and in the reformer
tube weight by 75 to 80% compared to one-step
reforming.
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Methanol— Two-step Reforming
Licensor: Haldor Topsøe A/S.
The gas feedstock is compressed (if required), desulfurized(1) and sent to a saturator (2) where process steam is generated. All process condensate is reused in the saturator resulting in a lower water requirement.
The mixture of natural gas and steam is preheated and sent to the primary reformer (3). Exit gas from the primary reformer goes directly to an oxygen-blown secondary reformer (4).
The oxygen amount and the balance between primary and secondary reformer are adjusted so that an almost stoichiometric synthesis gas with a low inert content is obtained. The primary reformer is relatively small and the reforming section operates at about 35 kg/cm2g.
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The flue gas’ heat content preheats reformer feed.
Likewise, the heat content of the process gas is used to produce superheated high-pressure steam (5), boiler feedwater preheating, preheating process condensate going to the saturator and reboiling in the distillation section (6).
After final cooling by air or cooling water, the synthesis gas is compressed in a one-stage compressor (7) and sent to the synthesis loop (8), comprised of three adiabatic reactors with heat exchangers between the reactors. Reaction heat from the loop is used to heat saturator water.
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Steam provides additional heat for the saturator system.
Effluent from the last reactor is cooled by preheating feed
to the first reactor, by air or water cooling.
Raw methanol is separated and sent directly to the
distillation (6), featuring a very efficient three-column
layout.
Recycle gas is sent to the recirculator compressor (9) after
a small purge to remove inert compound buildup.
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Topsøe supplies a complete range of catalysts that can be used
in the methanol plant. Total energy consumption for this
process scheme is about 7.0 Gcal/ton including energy for
oxygen production.
Ch.E-305 Muhammad Asif Akhtar 121
Total investments, including an oxygen plant, are
approximately 10% lower for large plants than for a
conventional plant based on straight steam reforming.
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The two-step reforming lay-out was first used in a
2400 MTPD methanol plant in Norway. This plant
was started up in 1997. A 5000 MTPD plant based
on similar technology was started up in Saudi Arabia
in 2008.
Licensor:
Haldor Topsøe A/S.
Ch.E-305 Muhammad Asif Akhtar 123
ATR features a stand-alone, oxygen-fired reformer.
The autothermal reformer design features a burner, a
combustion zone, and a catalyst bed in a refractory
lined pressure vessel .
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The burner provides mixing of the feed and the oxidant.
In the combustion zone, the feed and oxygen react.
The catalyst bed brings the steam reforming and shift conversion reactions to equilibrium in the synthesis gas and makes the operation of the ATR soot-free.
The catalyst loading is optimized with respect to activity and particle shape and size to ensure low pressure drop and compact reactor design.
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Methanol synthesis gas is characterised by the
stoichiometric ratio (H2 – CO2) / (CO + CO2), often
referred to as the module M. A module of 2 defines a
stoichiometric synthesis gas for formation of
methanol.
The synthesis gas produced by autothermal reforming
is rich in carbon monoxide, resulting in high reactivity
of the gas. The synthesis gas has a module of 1.7 to 1.8
and is thus deficient in hydrogen.
Ch.E-305 Muhammad Asif Akhtar 127
The module must be adjusted to a value of about 2 before the synthesis gas is suitable for methanol production. The adjustment can be done either
By removing carbon dioxide from the synthesis gas or By recovering hydrogen from the synthesis loop purge gas and recycling the recovered hydrogen to the synthesis gas .
Adjustment by hydrogen recovery can be done either by a membrane or a PSA unit. Both concepts are well proven in the industry.
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Methanol production by ATR at low S/C ratio. Adjustment
of synthesis gas composition by hydrogen recovery and
recycle.
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Natural gas is preheated and desulfurized.
After desulfurization, the gas is saturated with a mixture of preheated process water from the distillation section and process condensate in the saturator.
The gas is further preheated and mixed with steam as required for the pre-reforming process.
In the pre-reformer, the gas is converted to H2, CO2 and CH4.
Final preheating of the gas is achieved in the fired heater.
In the autothermal reformer, the gas is reformed with steam and O2.
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The product gas contains H2, CO, CO2 and a small amount
of unconverted CH4 and inerts together with under
composed steam.
The reformed gas leaving the autothermal reformer
represents a considerable amount of heat, which is
recovered as HP steam for preheating energy and energy for
providing heat for the reboilers in the distillation section.
The reformed gas is mixed with hydrogen from the pressure
swing adsorption (PSA) unit to adjust the synthesis gas
composition.
Ch.E-305 Muhammad Asif Akhtar 132
Synthesis gas is pressurized to 5 –10 MPa by a single-casing synthesis gas compressor and is mixed with recycle gas from the synthesis loop
This gas mixture is preheated in the heater in the gas-cooled methanol reactor.
In the Lurgi water-cooled methanol reactor, the catalyst is fixed in vertical tubes surrounded by boiling water.
The reaction occurs under almost isothermal condition, which ensures a high conversion and eliminates the danger of catalyst damage from excessive temperature.
Exact reaction temperature control is done by pressure control of the steam drum generating HP steam
Ch.E-305 Muhammad Asif Akhtar 133
The “preconverted” gas is routed to the shell side of the gas
cooled methanol reactor, which is filled with catalyst.
The final conversion to methanol is achieved at reduced
temperatures along the optimum reaction route. The reactor
outlet gas is cooled to about 40°C to separate methanol and
water from the gases by preheating BFW and recycle gas.
Condensed raw methanol is separated from the unreacted
gas and routed to the distillation unit.
Ch.E-305 Muhammad Asif Akhtar 134
The major portion of the gas is recycled back to the
synthesis reactors to achieve a high overall conversion. The
excellent performance of the Lurgi combined converter
(LCC) methanol synthesis reduces the recycle ratio to about
2.
A small portion of the recycle gas is withdrawn as purge
gas to lessen inerts accumulation in the loop.
Ch.E-305 Muhammad Asif Akhtar 135
In the energy-saving three-column distillation section,
low-boiling and high-boiling byproducts are removed.
Pure methanol is routed to the tank farm, and the
process water is preheated in the fired heater and used
as makeup water for the saturator.
Ch.E-305 Muhammad Asif Akhtar 136
Energy consumption for a stand-alone plant, including
utilities and oxygen plant, is about 30 GJ/metric ton of
methanol.
Total installed cost for a 5,000-mtpd plant including
utilities and oxygen plant is about US$350 million,
depending on location.
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Thirty-five methanol plants have been built using
Lurgi’s Low-Pressure methanol technology.
One Mega Methanol plant is in operation, two are
under construction and three Mega Methanol contracts
have been awarded with capacities up to 6,750 mtpd of
methanol.
Licensor
Lurgi AG.
Ch.E-305 Muhammad Asif Akhtar 138
In the methanol synthesis conversion of synthesis gas into
raw methanol takes place. Raw methanol is a mixture of
methanol, a small amount of water, dissolved gases, and
traces of by-products.
Typical byproducts include DME, higher alcohols, and
minor amounts of acids and aldehydes.
The methanol synthesis catalyst and process are highly
selective. A selectivity of 99.9% is not uncommon.
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The conversion of hydrogen and carbon oxides to
methanol is described by the following reactions:
The methanol synthesis is exothermic and the
maximum conversion is obtained at low temperature
and high pressure.
A challenge in the design of a methanol synthesis is to
remove the heat of reaction efficiently and
economically - i.e. at high temperature - and at the
same time to equilibrate the synthesis reaction at low
temperature, ensuring high conversion per pass.
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Different designs have been used:
Quench reactor
Adiabatic reactors in series
Boiling water reactors (BWR)
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It consists of a number of adiabatic catalyst beds
installed in series in one pressure shell. In practice, up
to five catalyst beds have been used. The reactor feed is
split into several fractions and distributed to the
synthesis reactor between the individual catalyst beds.
The quench reactor design is today considered
obsolete and not suitable for large capacity plants.
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A synthesis loop with normally comprises a number (2-
4) of fixed bed reactors placed in series with cooling
between the reactors. The cooling may be by preheat of
high pressure boiler feed water, generation of medium
pressure steam, and/or by preheat of feed to the first
reactor.
The adiabatic reactor system features good economy of
scale. Mechanical simplicity contributes to low
investment cost. The design can be scaled up to single-
line capacities of 10,000 MTPD or more.
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The BWR is in principle a shell and tube heat exchanger
with catalyst on the tube side.
Cooling of the reactor is provided by circulating boiling
water on the shell side.
By controlling the pressure of the circulating boiling
water the reaction temperature is controlled and
optimized. The steam produced may be used as process
steam, either direct or via a falling film saturator.
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The isothermal nature of the BWR gives a high
conversion compared to the amount of catalyst installed.
However, to ensure a proper reaction rate the reactor will
operate at intermediate temperatures - say between
240ºC and 260ºC - and consequently the recycle ratio
may still be significant.
Complex mechanical design of the BWR results in
relatively high investment cost and limits the maximum
size of the reactors.
Thus, for very large scale plants several boiling water
reactors must be installed in parallel.
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It is the last section of the plant. The design of this unit
depends on the desired end product. Grade AA methanol
requires removal of essentially all water and by products
while the requirements for fuel grade methanol are more
relaxed.
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HYDROGENATION IN REFINNING
AND GAS PROCESSES
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REFINERY LAYOUT
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Hydrocracking is a two-stage process combining catalytic cracking and hydrogenation, wherein heavier feedstock are cracked in the presence of hydrogen to produce more desirable products.
The process employs high pressure, high temperature, a catalyst, and hydrogen.
Hydrocracking is used for feedstock that are difficult to process by either catalytic cracking or reforming, since these feedstock are characterized usually by a high polycyclic aromatic content and/or high concentrations of the two principal catalyst poisons, sulfur and nitrogen compounds.
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Demand for gasoline and diesel is increasing, while the
demand for heavy-oils, such as fuel-oil is declining.
Refiners are therefore taking more steps to convert
heavy oils into lighter distillates.
Hydrocracking can significantly improve refining
margins by upgrading low-value products into higher-
value, high-demand products.
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Typical hydrocracking feedstocks include heavy atmospheric and vacuum gas oils, and catalytically or thermally cracked gas oils.
These products are converted to lower molecular weight products, primarly naphtha or distillates.
Sulphur, nitrogen and oxygen removal and olefin saturation occur simultaneously with the hydrocracking reaction. Typical reactor operating conditions are
280 – 475 °C
35 – 215 bar
depending on the feedstock and final products desired. The reactions consume hydrogen and are highly exothermic.
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Single stage, once through
hydrocracker
Single stage hydrocracker
with recycle
Two stage hydrocracker
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This configuration uses only one reactor and any uncracked
residual hydrocarbon oil from the bottom of the reaction
product fractionation (distillation) tower is not recycled for
further cracking.
For single stage hydrocracking, either the feedstock must first
be hydrotreated to remove ammonia and hydrogen sulfide or
the catalyst used in the single reactor must be capable of
both hydrotreating and hydrocracking.
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This is the most commonly used configuration.
The uncracked residual hydrocarbon oil from the bottom
of reaction product fractionation tower is recycled back
into the single reactor for further cracking.
Again, for single stage hydrocracking, either the
feedstock must first be hydrotreated to remove ammonia
and hydrogen sulfide or the catalyst used in the single
reactor must be capable of both hydrotreating and
hydrocracking.
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This configuration uses two reactors and the residual
hydrocarbon oil from the bottom of reaction product
fractionation tower is recycled back into the second reactor for
further cracking.
The first stage reactor accomplishes both hydrotreating and
hydrocracking, the second stage reactor feed is virtually free
of ammonia and hydrogen sulfide. This permits the use of
high performance noble metal (palladium, platinum) catalysts
which are susceptible to poisoning by sulfur or nitrogen
compounds.
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Basically, catalytic hydrocracking involves three primary chemical processes:
Cracking of high-boiling, high molecular weight hydrocarbons found in petroleum crude oil into lower-boiling, lower molecular weight hydrocarbons.
Hydrogenating unsaturated hydrocarbons (whether present in the original feedstock or formed during the cracking of the high-boiling, high molecular weight feedstock hydrocarbons) to obtain saturated hydrocarbons usually referred to as paraffins or alkanes.
Hydrogenating any sulfur, nitrogen or oxygen compounds in the original feedstock into gaseous hydrogen sulfide, ammonia and water.
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Reaction 1:
Addition of hydrogen to aromatics converts them into hydrogenated rings.
These are then readily cracked using acid catalysts.
Reaction 2:
Acid catalyst cracking opens paraffinic rings, breaks larger paraffins into
smaller pieces and creates double bonds.
Reaction 3:
Addition of hydrogen to olefinic double bonds to obtain paraffins.
Reaction 4:
Isomerization of branched and straight-chain paraffins.
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Hydrocracking catalysts consist of active metals on solid, acidic supports and have a dual function, specifically a cracking function and a hydrogenation function.
The cracking function is provided by the acid catalyst support and the hydrogenation function is provided by the metals.
The solid acidic support consists of amorphous oxides such as silica-alumina, crystalline zeolite or a mixture of amorphous oxides and crystalline zeolite.
Cracking and isomerization reactions (reactions 2 and 4 above) take place on the acidic support. Metals provide the hydrogenation reactions (reactions 1 and 3 above).
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The metals that provide the hydrogenation
functions can be the noble
metals palladium and platinum or the base metals
(i.e.,non-noble
metals) molybdenum, tungsten, cobalt or nickel.
Catalyst cycle life has a major impact on the
economics of hydrocracking. Cycles can be as
short as 1 year or as long as 5 years. Two years are
typical.
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A versatile family of premium distillates technologies
is used to meet all current and possible future premium
diesel upgrading requirements.
Ultra-deep hydrodesulfurization (UDHDS) process can
produce distillate products with sulfur levels below 10
wppm from a wide range of distillate feedstocks.
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High volume yield of ultra-low-sulfur distillate is
produced. Cetane and API gravity uplift, together
with the reduction of polyaromatics to less than 6
wt% or as low as 2 wt%, can be economically
achieved.
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The UDHDS reactor and catalyst technology is offered through Akzo
Nobel Catalysts bv.
A single-stage, single-reactor process incorporates proprietary high-
performance distribution and quench internals.
Feed and combined recycle and makeup gas are preheated and contact
the catalyst in a downflow, cocurrent fixed-bed reactor.
The reactor effluent is flashed in a high- and a low-pressure separator.
An amine-absorber tower is used to remove H2S from the recycle gas. In
the example shown, a steam stripper is used for final product recovery.
The UDHDS technology is equally applicable to revamp and
grassroots applications.
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Over 60 distillate upgrading units have applied the
Akzo Nobel ultra-deep HDS technology.
Twenty-five of these applications produce, or will
produce, <10ppm sulfur, using UDHDS technology.
Licensor: Akzo Nobel Catalysts bv.
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Reduction of the sulfur, nitrogen and metals content
of naphthas, kerosines, diesel or gas oil streams.
Products
Low-sulfur products for sale or additional processing.
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Single or multibed catalytic treatment of hydrocarbon liquids in the presence of hydrogen converts organic sulfur to hydrogen sulfide and organic nitrogen to ammonia.
Naphtha treating normally occurs in the vapor phase, and heavier oils usually operate in mixed-phase. Multiple beds may be placed in a single reactor shell for purposes of redistribution and/or inter bed quenching for heat removal.
Hydrogen rich gas is usually recycled to the reactor(s) (1) to maintain adequate hyrogen- to-feed ratio. Depending on the sulfur level in the feed, H2S may be scrubbed from the recycle gas.
Product stripping is done with either a reboiler or with steam. Catalysts are cobalt-molybdenum, nickel-molybdenum, nickel-tungsten or a combination of the three.
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550°F to 750°F and
400 psig to 1,500 psig reactor conditions.
Yields:
Depend on feed characteristics and product
specifications. Recovery of desired product almost
always exceeds 98.5 wt% and usually exceeds 99%.
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Licensor: CB&I Howe-Baker Process and Technology.
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Topsøe hydrotreating technology has a wide range of
applications,including the purification of naphtha,
distillates and residue, as well as the deep desulfurization
and color improvement of diesel fuel and pretreatment of
FCC and hydrocracker feedstocks.
Products:
Ultra-low-sulfur diesel fuel, and clean feedstocks for FCC
and hydrocracker units.
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Topsøe’s hydrotreating process design incorporates our
industrially proven high-activity TK catalysts with
optimized graded-bed loading and high-performance,
patented reactor internals.
The combination of these features and custom design of
grassroots and revamp hydrotreating units result in
process solutions that meet the refiner’s objectives in
the most economic way.
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Typical operating pressures range from
20 to 80 barg (300 to 1,200 psig)
and typical operating temperatures range from
320°C to 400°C (600°F to 750°F).
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More than 40 Topsøe hydrotreating units for the
various applications above are in operation or in the
design phase.
Licensor:
Haldor Topsøe A/S.
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