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By. Stephen Harrison, Linde
HiQ
In
the 21st century, the term “scraping the bottom of the barrel” has become quite
literal to the refining industry and hydrogen comes to the fore as a means to
do just this
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Only
a few decades ago, the thick, heavy crudes being utilised today would not have
even been a consideration for the production of mainstream products and were
used mainly as bunker fuels. Thirty years ago crude quality was a good match
with what was being demanded by the market, but today’s refiners are being
compelled to dig deeply into the dregs of the remaining resources and must
upgrade these crudes to reduce sulphur content and to keep up with market
demand and environmental regulations. Hydrogen is therefore absolutely critical
to convert this poor quality crude oil into modern-day products, and to comply
with strict environmental mandates.
Although
these heavy crudes are actually cheaper, refineries are faced with the
additional expense of upgrading to sophisticated processes to refine them to
the required standards and product slate meeting demand. The alternative is to
pay a premium for the lighter crudes. This awkward choice has already impacted
many refineries, notably on the east coast of the US , where refineries
originally built to process light and sweet crudes have had to shut down
because they could not fund the technology upgrade necessary to process heavier
crudes. The cost of hydrogen is part of the premium that the refiners must pay
to process cheaper crudes economically.
The
challenge is made more complex by the fact that no two refineries are alike and
that the naturally occurring hydrocarbon distribution in crude does not always
match customer demand. Various additional processing steps are required to
re-adjust the molecules, reshape them and remove contaminants to ensure the
refinery products meets the requirements for end-use and the product demand
profile, as well as environmental performance.
Hydrogen
is a key enabler allowing refineries to comply with the latest product
specifications and environmental requirements for fuel production being
mandated by market and governments and helping to reduce the carbon footprint
of their plants.
Margins
are tight in the highly competitive refinery business, a situation exacerbated
by the costs of creating low sulphur fuels from heavier, more sour crude, as
the world’s crude oil resources dwindle. The sulphur content of the world’s
diminishing crude oil resources is higher than ever before as oil companies are
forced to tap into a cheaper but lower quality of crude that requires more
refining to meet tightening environmental standards and while maximising
margins. Product sulphur levels are lower than ever before — for instance, 30
parts per million (ppm) in gasoline and 15 ppm in diesel fuels.
Growth in demand
From
a global perspective hydrogen is demonstrating significant growth. Large heavy
crude oil reserves, still under development, may increase the hydrogen demand
ever further. Two examples are the extra heavy crude oil in the Orinoco Belt in
southern of Venezuela and the Canadian Oil Sands. While there are many refinery
configurations, all refineries harness large quantities of hydrogen across a
spectrum of operations. Hydrogen is utilised in several refining processes, all
aiming at obtaining better product qualities. The main processes include
hydrotreating of various refinery streams and hydrocracking of heavy products.
While
the lighter, sweet crudes require less processing, the heavier, sour crudes
contain higher levels of sulphur, other contaminants and fractions. Processing
them typically begins with the same distillation process as for the sweet
crudes to produce intermediate products, but additional steps are necessary.
Hydrotreating
is one such process, introduced to remove sulphur, a downstream pollutant, and
other undesirable compounds, such as unsaturated hydrocarbons and nitrogen from
the process stream. Hydrogen is added to the hydrocarbon stream over a bed of
catalyst that contains molybdenum with nickel or cobalt at intermediate
temperature, pressure and other operating conditions. This process causes
sulphur compounds to react with hydrogen to form hydrogen sulphide, while
nitrogen compounds form ammonia. Aromatics and olefins are saturated by the
hydrogen and lighter products are created. The final product of the
hydrotreating process is typically the original feedstock free of sulphur and
other contaminants. Single or multiple product streams (fractionated) are
possible, depending on the process configuration.
The
hydrocracking process is a much more severe operation to produce lighter
molecules with higher value for diesel, aviation and petrol fuel. Heavy gas
oils, heavy residues or similar boiling-range heavy distillates react with
hydrogen in the presence of a catalyst at high temperature and pressure. The
heavy feedstocks are converted (cracked) into light distillates — for example,
naphtha, kerosene and diesel — or base stocks for lubricants. The hydrocracker
unit is the top hydrogen consumer in the refinery. Hydrogen is the key enabler
of the hydrocracking to reduce the product boiling range appreciably by
converting the majority of the feed to lower-boiling products. Hydrogen also
enables hydrotreating reactions in the hydrocracking process; the final
fractionated products are free of sulphur and other contaminants. Other
refinery processes including isomerisation, alkylation and tail gas treatment
also consume small amounts of hydrogen.
Critical decision
Considering
that the cost of a refinery expansion can be in the order of US$1billion, with
hydrogen supply representing in some cases about 10% of this investment, the
decision concerning the optimum way to source this hydrogen has become a
critical one. In many cases, refinery operators see the investment into
hydrogen supply as a defensive outlay necessary to remain competitive in the
market.
Hydrogen
is required in large volumes –
typically
10-200 000 Nm3/hr on a refinery, but is needed for a variety of applications in
several different scales of supply. Due to hydrogen representing a significant
percentage of a refinery’s total investment, a pivotal decision confronting
operators is the supply method. There are essentially three options for large
scale hydrogen supply.
Firstly,
the refiner can build an on-site hydrogen production plant, which it owns and
operates using its own personnel. An advantage of this option is that hydrogen
production becomes fully integrated with the other refinery processes. While
this enables the refinery to keep control of its own hydrogen supply, this
option requires more capital and demands skilled attention from the refinery
labour force for efficient operation, maintenance and repair. If the in-house
team is unable to operate the plant efficiently, the refinery will incur
financial losses, including increased consumption of natural gas and even other
more costly raw materials such as naphtha, water and power. Loss of
hydrotreated products attributed to poor reliability may also be a concern.
