S. Falzone, G. Licandro, F. Laganà, M. Tarantino, I. Arces
and R. Grillo, Raffineria di Milazzo, Messina, Italy; and P. Chiantella and C.
Albanese, Eni, Rome, Italy
Based on new environmental regulations focusing on
volatile organic compounds (VOCs) at the emissions point of a refinery’s sulfur
recovery unit (SRU), Raffineria di Milazzo—a JV between Eni and Kuwait Petroleum
Italia—carried out troubleshooting activities to identify and implement
suitable actions to minimize the concentration of VOCs at the emissions point
of the SRU complex.
Many definitions of VOCs exist in scientific
literature and technical references. In this article, VOCs refer to the sum of
the contributions of methane and non-methane VOCs (C1 and C1+). The
technological arrangement of Raffineria di Milazzo’s SRU complex ensures high
sulfur recovery performance.
This article shares a troubleshooting case study
(analysis and related solutions) as a support reference when facing similar
As described in Part 1 of this work
(published in the March issue of Hydrocarbon Processing), Raffineria di Milazzo’s
sulfur complex arrangement consists of three SRUs. Each SRU includes a Claus
section (with two catalytic stages), a tail gas treatment (TGT) section
[utilizing the Shell Claus Offgas Treating (SCOT) process or a derivative
technology] and a final conversion section (incinerators). The incinerators’
tail gas outlets are combined into a common stack.
Theoretical analyses, combined with analytical
activities on each SRU section, enabled refinery personnel to identify the key
factors of VOCs present in the complex. The first step consisted of identifying inlet streams with a relevant impact
on VOC content at the emissions point. The removal of VOCs from these streams enabled
a reduction of VOCs at the stack by an order of magnitude.
A second step consisted of identifying and eliminating the VOC
contribution in the SRUs’ internal process streams. In SRU3, a carryover of the
sweetening amine solution in the tail gas outlet led to a significant
contribution of VOCs (due to thermal degradation of the amine solution itself
in the incinerator section). A new design of sweetening column internals has
allowed the minimization of amine solution entrainment and, eventually, a
further reduction of the VOCs at the stack.
A third (and final) step investigated possible VOC generation due to
chemical reactions in the SRUs. A methanation reaction in the TGT unit’s (TGTU’s)
reduction reactor was identified, illustrating that the presence of carbonyl
sulfide (COS) and carbon disulfide (CS2) in a reducing environment (excess
of hydrogen) can generate methane. By minimizing the precursors involved in the
reaction, it has been possible to reduce the content of “methane VOC” at the
stack to a very low level. The synergy of these actions led to an overall VOC reduction
at the SRU complex stack by two orders of magnitude vs. the original value
(target achieved VOCs < 5 mg/Nm3).
Oil and gas products [e.g., naphtha,
gasoil, liquefied petroleum gas (LPG)] must be desulfurized to minimize sulfur
oxide (SOx) emissions of internal combustion engines. Over
the past several decades, different refinery processes (including sweeting and
hydrotreating) have been developed to achieve more restrictive limits of
residual sulfur in refined products. The extracted sulfur from products must be
recovered as elemental sulfur, and this is possible through the Claus process
(and associated tail gas treatment processes), where 99.9% of sulfur is retrieved
in liquid form to be utilized in the chemicals industry (e.g., sulfuric acid
production, the vulcanization of tires) or farming (especially as fertilizer).
Sulfur recovery at Raffineria di Milazzo. Three
different SRUs at Raffineria di Milazzo are dedicated to sulfur recovery. Each
unit comprises a Claus section, a TGTU section and a final conversion section (FIG. 10). Every Claus
section has two catalytic Claus reactors. The TGTU sections utilize the SCOT
process (SRU1 and SRU2) and a high Claus ratio (HCR) (SRU3). The final
conversion section includes catalytic incinerators for SRU2 and SRU3, along
with a thermal incinerator for SRU1. Note: For process flow diagrams of these
sections, please refer to Part 1 of this article.
According to European Commission (EC) Directive
1999/13/EC (Solvent Emissions Directive), VOCs are functionally defined as
organic compounds having, at 293.15 K (20°C), a vapor pressure of 0.01 kPa or
more, or having a corresponding volatility under conditions of use. The aim of this work is to identify the sources of
VOCs at Raffineria di Milazzo and to minimize their concentrations in the SRU
complex’s stack to adhere to new environmental constraints (i.e., 20 mgCeq/Nm3).
Step 2: Identifying VOC sources in SRU3. Part 1 of this article covered theoretical
considerations, a description of the analytical setup for VOC detection, the investigation’s
starting point and Step 1. TABLE 4 details the configuration of Raffineria di Milazzo’s
SRU complex at the beginning of the deeper investigation into SRU3.
Comparing the configuration of the
starting point and Step 1, the configuration of Step 2 was the following:
11 shows the configuration of SRU3 in Step 2. Some considerations can be
deduced by the analytical overview shown in FIG. 11:
A check on the burning condition in the tail gas preheater of the final
conversion section was carried out. First, the burner was replaced with a new
one, followed by several tests (TABLE 5).
The burning in hot standby condition and
in different unit loads seemed to be different. Conversely, the VOC
concentration at the final conversion outlet, with a bypass of TGTU3 (the tail
gas from the Claus section was sent directly to the final conversion section),
was less than 5 mgCeq/Nm3.
During the different tests, the
composition of VOCs did not change with different fuels: CH4 content
in the refinery’s fuel gas was 30 vol% and it was higher than 85 vol% of natural
gas. If a burning issue occurred using a different fuel, a different
composition of VOCs in the final conversion section’s outlet stream had to be
detected. The tests suggested that the source of VOCs was not identified, but
it could be found in TGTU3 (the VOC concentration went to very low values by bypassing
To determine the source of VOCs in TGTU3,
a review of the analytical setup was performed. It was discovered that all
analyses were carried out using the H2S neutralization step (with a caustic
solution). In this condition, if a liquid carryover had occurred from the amine
absorber, it could not have been detected (the liquid would have remained in
the caustic solution). For this reason, the neutralization step was removed,
and a direct sampling was carried out from the amine absorber tail gas outlet
A substantial increase in the concentration of the
VOCs in the amine absorbing tail gas was measured after the analytical setup
modification (FIG. 12). Lab tests were performed to verify that the flame
ionization detector (FID) could detect the amine in terms of the VOC output. To
simulate an amine solution carryover, a stream of gas (air/nitrogen) was
injected into the amine solution sample at different temperatures (FIG. 13).
14 shows the VOC concentrations detected by the FID vs. amine solution
temperature. The graph indicates that the VOC concentration detected by the FID
increased with the temperature due to the increase in methyl diethanolamine
(MDEA) vapor pressure with the temperature. Another result was that the methane
concentration was higher than 85% within the VOC concentration. Finally, lab
tests confirmed the VOC measurements at the amine absorber output stream. An amine
solution is very likely to be entrained, and its thermal degradation occurred
at the incinerator or FID itself. Therefore, a deeper analysis of the vapor-liquid
separation performance at the head section of the absorber was required.
The design of SRU3’s absorber column is different from that of the SRU1
and SRU2 absorber columns, especially in the head separation section where SRU1
and SRU2 have a gas-liquid separator installed, while, at the SRU3 absorber, there
is a demister inside the column immediately before the gas outlet (FIG. 15). Therefore,
in SRU1 and SRU2, the additional gas-liquid separation step minimized the
possible amine solution carryover. A VOC contamination source from the amine
absorbing column was not detected in these units.
To minimize the amine carryover phenomena
at SRU3’s amine absorbing column, a revamping of internals was proposed by a
leading internal design company (FIG. 16). The project consisted of the following:
After revamping the amine absorber internals, new measurements with the
new analytical setup were carried out. The VOC concentration at the outlet of
the amine absorber had decreased substantially (FIG. 17). Conversely, a slight increase
in the VOC concentration from the reduction outlet reactor was detected. In
addition, a different composition of VOCs at the outlet of the final conversion
section was discovered (only CH4 was detected). This led the authors
to think that a new source of VOCs had been identified: CH4 generation
in the reduction reactor.
In the reduction reactor, the Claus tail gas sulfur compounds (SO2,
CS2 and COS) reacted with hydrogen to produce H2S through
a CoMo catalyst action. When the CS2 concentration increased in the
tail gas, the following reactions (Eq. 1 and 2) took place:
CS2 + 3H2 → CH3SH (1)
CH3SH + H2 → CH4 + H2S (2)
These reactions showed that hydrogen partial pressure plays a role—if
the hydrogen concentration is higher in the reactor, then the CH4
concentration will also be higher at the reactor’s outlet stream.
The first catalytic Claus reactor did not contain a titanium dioxide (TiO2)
layer to maximize the COS and CS2 hydrolysis. Therefore, the
COS and CS2 hydrolysis had to be maximized at the thermal reactor—the
oxygen enrichment configuration was employed to maximize both temperature and
residence time in the thermal reactor.1 Simultaneously, the hydrogen rate to the
reduction reactor had been minimized to reduce the hydrogen partial pressure in
Following these actions, the concentration of VOCs in SRU3 was measured,
which is shown in FIG.
18. After identifying the VOC sources and taking proper actions to
minimize their concentration, very low values were measured at the SRU3 outlet
Results. At the conclusion of these troubleshooting
activities, all possible sources of VOCs had been investigated. The most
relevant sources detected were:
The final SRU complex configuration after troubleshooting activities is
detailed in TABLE 6.
FIG. 19 shows that the VOC concentration at the
outlet of each SRU was lower than 5 mgCeq/Nm3. It is
worth noting that the VOC concentration that was detected in SRU2 at the end of
the troubleshooting activities was even lower than the concentration measured
after the first troubleshooting step. This was because the hydrogen rate to the
reduction reactor was optimized to inhibit the methanation reaction in TGTU2.
After the sources of VOCs were identified and actions were taken to minimize
them, VOC concentration was reduced by two orders of magnitude at the SRUs’
Takeaways. Troubleshooting was performed to reduce
VOCs at the SRU complex. A theoretical analysis to identify the sources of VOCs
in each SRU was conducted, and a series of measurements at different SRU streams
was carried out to detect the most relevant ones. These analyses were carried
out through a proper analytical setup based on FID.
Initially, the VOC concentration was 480 mgCeq/Nm3,
with different contributions of each SRU. The highest VOC concentration was detected
at SRU2’s outlet stream (1,100 mgCeq/Nm3)—a hydrogen
stream from the catalytic reforming unit was fed to the reduction reactors of TGTU1
and TGTU2. VOCs entered the system due to the low hydrogen purity of this
stream (58 mol%–82 mol%). The existing stream was replaced with a high hydrogen-purity
stream (> 99.5 mol%), which led to the reduction of VOCs at the outlet
streams of SRU1 and SRU2 (< 5 mgCeq/Nm3 and < 10
mgCeq/Nm3, respectively) and, in turn, at the stack (40
An investigation for the sources of VOCs was carried out at SRU3, where the
concentration of VOCs at the outlet stream was 60 mgCeq/Nm3–80
mgCeq/Nm3. A slight modification of the analytical setup
was necessary to identify that a carryover of amine solution occurred at the
TGTU3 absorbing column. The amine degradation occurred at incinerator
temperatures (300°C–320°C), and the VOC concentration was measured at the SRU3
outlet stream. A revamping of the column internals was essential to minimize
the amine solution carryover, which led to a VOC reduction at SRU3’s outlet
stream (10 mgCeq/Nm3–15 mgCeq/Nm3).
The final step consisted of an investigation into the possible chemical
reaction for VOC generation, as a methanation reaction in the TGTU reduction
reactor had been identified. The presence of COS and CS2 in a
reducing environment (excess of hydrogen) can create CH4. By
minimizing the precursors involved in the reaction, it was possible to reduce
the content of CH4 VOCs at the stack to a very low level.
The synergy of these actions led to an overall reduction of VOCs at the
SRU complex by two orders of magnitude vs. the original value (target achieved
VOCs < 5 mgCeq/Nm3). HP
SALVATORE FALZONE is a Process Engineer at Raffineria di Milazzo.
GIANFRANCESCO LICANDRO is the Operations Manager at Raffineria di Milazzo.
FORTUNATO LAGANA’ is
the Technical Director at Raffineria di Milazzo.
MARCELLO TARANTINO is the General Director
at Raffineria di Milazzo
IGNAZIO ARCES is a Chief Executive Officer and a
Member of the Board at Raffineria di Milazzo.
ROBERTO GRILLIO is a Chief Executive Officer and a
Member of the Board at Raffineria di Milazzo.
PAOLO CHIANTELLA is the Operations Performance and Deputy Managing
Director at Eni.
CLAUDIO ALBANESE is the Head of
Industrial Technology and Licensing Management at Eni.