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Dynamics >>23
SCOT
inline heater combustion and mixing
Mike Henneke and Joseph
Smith, CD-adapco
John Petersen and John McDonald, Zeeco, USA
/ Dave Wilson, Marathon Ashland Petroleum, USA
Refineries
processing high sulfur crude oils produce significant quantities
of by-product hydrogen sulfide (H2S), also called acid gas. This
gas is often processed in a Claus Sulfur Recovery Unit (SRU). The
Claus process converts acid gas (H2S) into elemental sulfur in an
oxygen-deficient combustion process and then liquid sulfur from
the condenser runs through a seal leg into a covered pit from which
it is pumped to trucks or railcars for shipment to end users. Approximately
65 to 70 percent of the sulfur is recovered. The SCOT Process (Shell
Claus Off-gas Treating Process) was developed by Shell, and introduced
in the early seventies as an attractive process for improving the
efficiency of a Claus sulfur recovery unit (see Figure 1). The process
consists of four combustion processes (as well as catalytic reactors
which are not discussed here):
Fig.
1: SCOT (Shell Claus Off-gas Treating) process Fig.
2: Transparent surface view showing location of gas gun and spin
vanes
Fig.
3: Base case temperature (°F) contours of on centerline of burner
and
vessel
1. Reaction furnace
2. Inline reheater
3. Reducing
gas generator
4. Tail gas incinerator
The CFD analysis
discussed in this article considers only the second process, the
inline reheater. The inline reheater heats the acid gas by mixing
it with hot reducing products of combustion. An important design
consideration is that the combustion products being mixed are under
a reducing atmosphere. If O2 slip (uncombusted O2) is available
to mix with the acid gas, the H2S can be oxidized to undesirable
compounds (e.g., SO3, SO4, H2SO4) that can attack refractories and
damage the environment.
Fig.
4: Wet O2 mole fraction (contours from 0-2%) shown 12”, 24”, 36”,
and 48”downstream of fuel discharge. This figure shows the fuel/air
mixing and indicates that O2 carry over does not occur. Fig.
5: Log10 of C2H2 mole fraction in the mixing zone between the products
of combustion and the SRU tail gas. Note that only the mixing zone
where the Claus gas enters has been analyzed.
CFD analysis
The purpose of this CFD analysis was to determine if the proposed burner design for the inline reheater would perform as required. In particular, the client was concerned regarding the following issues:
1.
O2 slip
2. Soot formation in the reactor
3. Flame length
4.
Swirl number of the combustion air
5. Uniformity (mixedness)
of SRU tailgas and combustion products leaving reheater
These
issues were analyzed using CFD at several operating conditions,
but only the maximum liberation case is discussed in this article.
Figure 2 shows the geometry of the CFD model as well as the flow
inlets and outlets considered.
Chemistry
The
chemistry has been approximated using the eddy break-up model. This
model assumes mixing-limited chemistry, which is appropriate for
most hydrocarbon combustion reactions. The chemical reactions considered
are:
H2 + 1/2 O2 H2O (1)
CH4 + 3/2 O2 CO + 2 H2O (2)
CO
+ 1/2 O2 CO2 (3)
H2S + 3/2 O2 SO2 + H2O (4)
Figures
3 and 4 indicate that the thermal mixing between the SRU tailgas
and the products of combustion is sufficient and that the exhaust
is well-mixed. The figures also show that the near-burner mixing
is very thorough so O2 slip into the SRU tailgas does not occur.
Fig.
6: Predicted gas temperature (°F) Figure
7: Predicted Acetylene Concentration Profiles (Log10)
for
Cases 1, 4, and 5 for
Cases 1, 4, and 5
Soot formation potential
The
model as formulated does not directly compute the formation of soot
particles in the reheater. However, the model does do a good job
of computing the major species profiles and temperatures. To estimate
sooting potential in the mixing zone between the SRU tail gas and
the products of combustion, we used the equilibrium program CET89
to compute the equilibrium gas composition at locations in the centerplane
of the reactor. To do this, we used nine species concentrations
(H2, CH4, CO, CO2, H2O, H2S, O2, N2) and the gas temperature and
pressure at about 2500 cells in a constant temperature and pressure
equilibrium calculation. Figure 5 shows the equilibrium-predicted
C2H2 mole fractions in these cells. These calculations predicted
extremely low equilibrium levels of C2H2. While equilibrium calculations
are known to underpredict acetylene concentrations, we believe that
these predictions indicate that the mixing between the combustion
products and the SRU tailgas will form negligible amounts of soot.
Table
1. Flow rates for five performance cases used to characterize SCOT
unit
Table 2. Comparison of predicted/measured pressure drop through reactor for selected cases
Performance
testing:
Additional cases and comparison to experimental
data
Besides the base case, five additional cases were used
to demonstrate the capabilities of the current SCOT Burner design.
Process conditions for these cases (see Table 1) included high Hydrogen
flow rates at two stoichiometric conditions (Cases 1-3), refinery
fuel (Case 4), and 100% fuel gas (Case 5). To evaluate the model’s
ability to predict soot formation potential and performance, a test
rig was built and operated at ZEECO. Comparisons between predictions
and measurements for Cases 1, 4, and 5 are shown in Table 2. Results
show (see Figures 6 and 7) essentially zero soot formation at the
stack, which agreed with visual observations. Comparison between
predicted and measured pressure drop through the reactor also show
good agreement. Based on these comparisons, the proposed design
was constructed and is being installed at Ashland-Marathon.
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