Hydrocarbon Processing - February 2022 - 39

Bio-Based
Processing
P. LE GRANGE, K. TEKEBAYEV, L. GOETTLER
and J. KIEBERT, Sulphur Experts, Katwijk aan Zee,
Netherlands; and M. SHEILAN, Amine Experts,
Calgary, Alberta, Canada
Impact of biofeed retrofits, co-processing
on refinery amine units, SWSs and SRUs-Part 2
Producing diesel with a portion of biologically
sourced carbon is being done at an
increasing number of conventional crude
oil refineries. This is commonly achieved
by co-processing biofeedstocks in refinery
hydrotreaters and FCCUs or through the
installation of a dedicated biofeedstock
hydrotreater to produce commercial biodiesel
products. Generally, refineries are
looking to technologies that allow them to
easily incorporate biofeedstocks into their
existing infrastructure. Part 1 of this article,
published in the January issue of Hydrocarbon
Processing, described the impacts that
this incorporation has on a refinery's existing
amine and SWS units. Part 2 will detail
the impacts on the SRU, operational and
design options available to manage those
impacts, and a specific case study.
Impacts on the SRU. The amine acid
gas (AAG) from an amine unit and sour
water acid gas from an SWS unit are fed
into one or more SRUs. FIG. 8 shows the
typical process flow diagram for a twostage
SRU. In the reaction furnace, a portion
of the H2
S is partially oxidized to
SO2, which then reacts with the remaining
H2S to form elemental sulfur over the
thermal and catalytic stages of the process.
A detailed description of the process
and chemistry is provided in literature.18
The co-processing of biofeedstock will
change the amount and gas composition
to the SRU. This will be mostly an
increase of CO2
and a reduction of H2
S.
Processing the changed feed is unlikely to
be a problem since most SRUs can handle
changes in composition and load to the
FIG. 8. Process flow diagram of a two-stage modified Claus unit.
unit. While this means that the existing
equipment may be adequate, it does require
careful review before committing to
biofeedstock processing.
The impact of the extra CO2
on the
SRU is limited to the thermal stage (reaction
furnace). The thermal stage not only
oxidizes H2
S and thermally converts it
into elemental sulfur, but it also helps to
destroy feed contaminants such as hydrocarbons,
benzene, toluene, ethylbenzene
and xylene (BTEX) and ammonia. If not
destroyed, these contaminants can cause
operational challenges such as catalyst
damage and plugging in the downstream
equipment due to salt formation (FIG. 9).
The additional CO2
FIG. 9. Catalyst deactivation by contaminants
(left) and by plugging in equipment (right).
will cool the bulk
gas temperature in the reaction furnace,
which could reduce contaminant destruction.
With only hydrocarbons and BTEX
in the feed, the minimum target temperature
is 1,050°C. When sour water acid gas
(SWAG) is one of the feeds, the key requirement
is to ensure good destruction
of ammonia, meaning that the process
gas temperature in the furnace is recommended
to be at least 1,250°C.19
The ability to achieve the target temperature
will be reviewed for the two most
common thermal stage configurations
(FIGS. 10A and 10B): the straight-through
process (single burner, single chamber)
and the split-flow process (single burner,
two chambers). The advantages and disadvantages
of both configurations have
been well documented in literature.19
Straight-through furnace. In the
Hydrocarbon Processing | FEBRUARY 2022 39
Hydrocarbon Processing - February 2022

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