S. Falzone, G.
Licandro, F. Lagan, 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 environmental topics.
Raffineria di Milazzo’s SRU
complex. 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 section [utilizing the Shell Offgas Treating (SCOT) process or a derivatives
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. Part 1 of this article details troubleshooting pathways for reducing
VOCs. This includes identifying inlet streams that have a relevant impact on VOC
content at the emissions point. The removal of VOCs from these streams has reduced
VOC content by an order of magnitude.
Part 2 of this article (to be published in the April issue) will provide
the steps taken to reach this order of magnitude reduction in VOCs (the VOC
target achieved from these troubleshooting techniques was < 5 mg/Nm3).
Products from oil and gas processing (such as naphtha, gasoil and LPG)
must be desulfurized to minimize sulfur oxide (SOx) emissions
from internal combustion engines. Different refinery processes (including sweeting
and hydrotreating, among others) have been developed to achieve more
restrictive limits on residual sulfur in refined products. The extracted sulfur
from refined products must be recovered as elemental sulfur. This is possible
through the Claus process (and the tail gas treatment processes associated with
it), where 99.9% of the sulfur is retrieved in liquid form, which is then used
in the chemical industry (e.g., sulfuric acid production and the vulcanization
of tires) or farming (especially as fertilizer).
The sulfur recovery
process. The Claus process consists of partially combusting the hydrogen sulfide
(H2S)-rich gas stream (with one-third of the stoichiometric quantity
of air) and then reacting the resulting sulfur dioxide (SO2) and unburnt
H2S in the presence of an activated alumina catalyst to produce
elemental sulfur. As shown in FIG. 1, the Claus unit consists of a reaction furnace,
followed by a series of converters and condensers, where:
Side reaction effects also occur that
produce carbonyl sulfide (COS) and carbon disulfide (CS2), which can
create problems in the operations of Claus plants, since they cannot be easily
converted to elemental sulfur and carbon dioxide (CO2).1
The high water content and temperatures involved should be adequate to
hydrolyze both COS and CS2; however, these slow reactions are
generally quenched in a waste heat exchanger before completion—higher
temperatures and longer residence times aid the hydrolysis of these species. In
split-flow plants, maximizing the bypass fraction (toward the absolute maximum
of two-thirds) will provide improvements in both residence time and
temperature. The same effect can be obtained with the use of oxygen enrichment technologies
(e.g., with the OxyClaus process) or by reducing the feedrate to the plant. However,
it is possible to provide the first Claus catalytic converter with a layer of
catalyst [based on titanium dioxide (TiO2)] to maximize the
hydrolysis of COS and CS2, as shown in the following reactions:
COS + H2O --> CO2 + H2S
CS2 + 2H2O --> CO2 + 2H2S
Since these hydrolysis reactions only
occur to a sufficient degree at temperature above 315°C (600°F) and the only
catalyst bed that can be operated at such a high temperature is the first, this
is, therefore, the only chance to hydrolyze the COS and CS2.2
The SRU is characterized by its sulfur
recovery efficiency, which is calculated as the fraction of sulfur in the feed that
is recovered as a liquid sulfur stream and routed to the sulfur collection
The sulfur recovery efficiency of an SRU
closely depends on the number of Claus reactors that are operated in series in
the SRU chain, as shown in TABLE 1. As illustrated in this table, the sulfur
recovery efficiency can reach 98% through the Claus process. To reach a higher sulfur
recovery efficiency (theoretically to 99.99%), a tail gas treatment unit (TGTU)
or cleanup procedure must be associated with the Claus process. Many processes
for TGTUs have been developed to enhance the recovery of sulfur compounds from
natural gas and/or refinery sources. According to the principles applied, the
most frequently operated TGTU processes can be broadly divided into the
following four categories:
In this article, the authors will focus on
the SCOT process and its derivative HCR process, since these are the only two
technologies associated with the Claus units at the Raffineria di Milazzo.
As shown in FIG. 2, the concepts underlying the H2S
scrubbing processes in the SCOT unit are:
The clean tail gas (outlet of the amine
adsorber) reaches an incinerator section (as illustrated in FIGS. 1 and 2) where the residual
H2S is converted to SO2 before the emissions point. The
incinerator section can be catalytic or thermal. In other words, the catalytic
incinerator consists of a heater [to heat the clean tail gas to the proper
reaction temperature of 280°C–350°C (536°F–662°F)] and a fixed-bed reactor
where H2S is selectively oxidated into SO2. In a thermal
incinerator, the tail gas is heated to a very high temperature [> 720°C (>
1,328°F)] in oxidant conditions (proper air excess) to guarantee a complete
conversion of the residual H2S to SO2.
recovery at Raffineria di Milazzo. Three different SRUs are dedicated to sulfur
recovery at Raffineria di Milazzo. Each unit comprises a Claus section, a TGTU section
and a final conversion section (FIG. 3). Every Claus section has two catalytic Claus
reactors. The TGTU sections utilize the SCOT process (SRU1 and SRU2) and HCR
(SRU3). The final conversion section includes catalytic incinerators for SRU2
and SRU3, along with a thermal incinerator for SRU1. FIGS. 4, 5 and 6 show the process
flow diagrams of SRU1, SRU2 and SRU3, respectively.
The main differences between the three
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
However, it is important to specify that the
design of each SRU plant aims to maximize the sulfur recovery yield (see the “Sulfur
recovery process” section in this
article). In the design of the SRU complex, minimizing the concentration
of VOCs at the outlets of the SRUs was not considered. For this reason, a preliminary
theoretical analysis to identify all possible sources of VOCs was carried out,
followed by several analytical activities.
considerations. The first step was to identify all possible sources of VOCs in the refinery’s
SRU complex. Analysis considerations included:
troubleshooting activities. The theoretical analysis conducted identified the primary possible sources
of VOCs, which had to be investigated through a field measurement. The following
is a brief description of the analytical setup used, along with the
of the analytical setup for the detection of VOCs. UNI EN 12619:2013 was
used to measure the concentration of VOCs (as total organic carbon) in the
streams of Raffineria di Milazzo’s SRU complex. This method determines the mass
concentration of total gaseous organic carbon through a flame ionization
detector (FID). The measuring principle of the FID is summarized in FIG. 7.
The hydrogen flame burns out of a metal nozzle, which simultaneously
represents the negative electrode of an ionization chamber. The positive counter-electrode
is fixed above the flame. Direct voltage is applied between the two electrodes.
The ion current is measured as a voltage drop above the resistor. The measuring
gas is added to the burning gas shortly before entering the burner nozzle. The
air required for combustion flows in through a ring slot around the burner
For stable measuring conditions, it is essential that all gases—combustion
gas, combustion air and measuring gas—are conducted into the flame in a constant
volume flow. For this, all gas flows are conducted via capillaries. Constant pressure
before the capillaries ensures a constant flow. Sensitive pressure regulators
for gas combustion and air are used to achieve this fine-tuning. The measuring
gas is pumped past the capillary in the bypass in a high-volume flow. Pressure is
kept constant by the backpressure regulator so that a constant partial flow
reaches the flame via the capillary. To avoid condensation of the hydrocarbons
to be measured, all instruments must be heated to 150°C–200°C (65.5°F–93°F).
Heating includes the particle filter and the measuring gas pump. In most cases,
particularly with cold exhaust gases, a heated sampling line is also used for
measuring gas sampling to the measuring instrument.
Hydrocarbon compounds are oxidized in the flame, with ions being formed
as an intermediate product. In a certain range of the accelerating voltage, the
strength of the ionization current is in first approximation directly
proportional to the amount of carbon atoms of the burned substance. Therefore,
an FID basically responds to all hydrocarbons and measures their total sum.
Corresponding to the number of carbon atoms, larger molecules with many carbon
atoms produce a higher signal than smaller molecules with a small number of carbon
atoms. Ionization energy does not only stem from the flame’s energy, but primarily
from the oxidation energy of the carbon. Accordingly, partially oxidized
hydrocarbons provide a weak detector signal, and completely oxidized
hydrocarbons provide no signal at all (e.g., CO and CO2 are not
The FID analyzer employed for this work was equipped with an oxidation
catalyst to oxidate all hydrocarbon species, excluding methane. In this way, it
was possible to switch the gas sample to the catalyst to also include a methane
concentration measurement. It is worth noting that the H2S
concentration was not negligible in many of the analyzed streams. In these
cases, an additional caustic solution neutralization step was necessary to
avoid possible instrumentation damage.
troubleshooting: Starting point. During troubleshooting, several VOC
concentration measurements were carried out through the analytical setup
described above. First, a VOC concentration was measured at the outlet of the
SRUs and the stack to determine if some differences could be detected. Second, the
refinery’s SRU complex was divided into clusters, and the VOC concentrations were
measured at the inlet and outlet of the clusters to identify the possible VOC
sources. For each source identified, a proper action was implemented to minimize
or eliminate the source.
FIG. 8 reports VOC concentration as mgCeq/Nm3
at the outlet of each final conversion section and at the stack before starting
troubleshooting activities. The measurement of the concentration of VOCs was corrected
to the reference condition of 3% O2, according to UNI EN 12619:2013.
TABLE 2 summarizes
the configuration of the refinery’s SRU complex at this starting point.
Some considerations can be deducted from this starting point. These
included the following:
Ultimately, the VOC concentration in any
outlet stream of the SRUs does not reach the expected target of < 20 mgCeq/Nm3,
and the VOC concentration at the stack is well above it.
troubleshooting Step 1: Contamination of the hydrogen stream in the TGTUs. In
the “Theoretical considerations” section, the contamination due to the hydrogen
stream feed at the inlet of the reduction reactor was highlighted. For TGTU1
and TGTU2, the hydrogen stream comes from the catalytic reforming unit. The
composition of this stream varies with the catalyst’s lifecycle (TABLE 3). In particular,
the hydrogen concentration decreases during the catalyst’s lifecycle, while the
hydrocarbon concentration increases. Consequently, the hydrogen stream to TGTU1
and TGTU2 must increase during the reforming catalyst’s lifecycle to ensure a
constant hydrogen concentration at the reduction reactor. The hydrogen stream
is a definite source of VOCs, but it also increases with the reforming catalyst’s
Considering the catalytic reforming hydrogen composition at the starting
point and its rate to TGTU2 (15 kg/hr–20 kg/hr), it is easy to calculate the
theoretical hydrocarbon contamination at SRU2 due to this source: 12 kg/hr–16 kg/hr.
Neither the reduction TGTU2 reactor nor the catalytic incinerator can guarantee
hydrocarbon oxidation. Therefore, most of the hydrocarbons entering with the
hydrogen reduction stream will reach the outlet of SRU2. Considering that the
SRU2 outlet volumetric rate at the starting point was 10,000 Nm3/hr,
it is easy to estimate the expected VOC concentration at the SRU2 outlet (Eq.
XVOCs,SRU2 = (12/10,000) × 106 (mg/Nm3) ≈ 1,200 (mg/Nm3) (1)
of magnitude obtained from the estimation through Eq. 1 is the same as the
value measured with the FID (FIG. 8). Consequently, a new system to feed a high-purity
hydrogen stream (99.5% mol) was built in place of the catalytic reforming
hydrogen stream for SRU1 and SRU2. In this way, the refinery’s SRUs employ a
high-purity hydrogen reduction stream for the TGTUs. FIG. 9 shows the VOC concentration after Step 1 was completed.
A reduction of one order of magnitude of the VOC concentration at the
stack was achieved. However, it did not represent an acceptable value (the target
was < 20 mgCeq/Nm3). For this reason, further investigation
into SRU3 was necessary. Although this unit has the lowest number of
theoretical VOC sources, its outlet stream had the highest VOC concentration.
Takeaways. Troubleshooting was performed to reduce
VOCs at Raffineria di Milazzo’s SRU complex’s stack. A theoretical analysis to
identify VOC sources in each SRU was conducted, and a series of measurements at
different SRU streams was carried out to detect the most relevant ones. This
analysis was carried out through a proper analytical setup based on FID.
Initially, the stack’s VOC concentration was 480 mgCeq/Nm3,
with different contributions by each SRU. The highest VOC concentration was detected
at the SRU2 outlet stream (1,100 mgCeq/Nm3)—a hydrogen
stream from the catalytic reforming unit was fed to TGTU1’s and TGTU2’s
reduction reactors. A relevant VOC rate entered the system due to the low
hydrogen purity of this stream (58 mol%–82 mol%). The existing stream was
replaced with a high-purity hydrogen stream (> 99.5 mol%), which led to a
reduction in VOC concentration by an order of magnitude of one at the outlet
streams of SRU1 and SRU2 (< 5 mgCeq/Nm3 and < 10
mgCeq/Nm3, respectively), and, in turn, at the stack (40
Part 2 of this article will appear in the April issue of Hydrocarbon
SALVATORE FALZONE is a Process Engineer at Raffineria di
LICANDRO is the Operations Manager at Raffineria di Milazzo.
LAGAN 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.
CHIANTELLA is the Operations Performance and Deputy Managing Director at Eni.
CLAUDIO ALBANESE is the Head of Industrial Technology and Licensing
Management at Eni.