Chemical Engineering June 2012 - 65

a. Case I
b. Case II
c. Case III
d. Case IV
FIGURE 2. The
infl uence of the
wind direction
and velocity can
be seen in the
temperature path
lines for the four
cases discussed
in the text
different ambient conditions involving
two wind directions and two wind
speeds to find out the maximum temperature
of the structures under fullload
operation of flares. Steady-state
heat-transfer analyses were carried
out using a general-purpose, commercial
CFD code. To take care of the effects
of convective and radiative heat
transfer from flares to the structures,
combustion and radiation were also
modeled. All of the combustible components
of the different gases were
converted to equivalent methane, and
a single-step methane-oxidation reaction
was modeled to limit the number
of species present in the domain. The
maximum temperature of the flames
was predicted to be around 1,900°C.
Also, it will be shown that even at
full load operation and for the most
adverse ambient conditions, temperature
for the support structures and
the connecting bridges would be well
within the maximum allowable limit
of structural steel.
Analysis approach
In the current study, CFD analyses
are carried out for an offshore oiland-gas
process complex having three
flares. These flares are disposed to atmosphere
through tripods. The 3D domain
are comprised of the two flare tripods
and the interconnecting bridges.
Steady-state heat-transfer analyses
were carried out using a generalpurpose,
commercial CFD code considering
a rectangular computational
domain. Although the three flares are
supposed to burn different gases, all
the combustible components of the different
gases were converted to equivalent
methane and a single-step methane-combustion
reaction (CH4 + 2O2 =
CO2 + 2H2O) was modeled using eddy
dissipation model. Radiation was modeled
using the P1 model. The ideal gas
law is used to determine density as a
function of temperature. Heat transfer
from the structural members to
the ambient is modeled by providing
a wall heat-transfer coefficient and
ambient temperature. Four different
cases were studied involving two wind
directions and two wind speeds.
Mathematical model
Gas phase equations. The steadystate
continuity and momentum equation
of the gas phase are given as
Equations (1) and (2). The source term,
Sp, results from combustion. The component
of velocity in coordinate direction
x is given in Equation (2), which
includes pressure, gravitational force
(buoyancy effects), and the generalized
source term. Equations for the y
and z components are similar.
∂
∂x ()=
i
∂xj
∂ ()=− ++ ++ρgF Sii p
p
ρuu
ij
∂
∂
∂τ
∂
xci
ij
j
(2)
Model for turbulence. The model
employed in the present simulation
is the standard k- model proposed
by Launder and Spalding. This emρuS
ip
(1)
∂x
()=−
ρνih
∂
i
∂
∂xi


Γh
∂
∂
h
x
i


+ Sh
(6)
Owing to a higher temperature of the
flame, radiation is the predominant
mode of heat transfer from the flame
to the structures. In the current modeling,
radiation has been modeled using
a commercial code. In the commercial
radiation model, radiation flux (qr) is
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2012 61
ploys two partial differential equations
to estimate the velocity length
scales of turbulence:
∂
∂
t
()+ρk
=− +ρρε
P
∂
∂
t
(ρε) +
= Cε1
ρ ε
P
k
∂x ( uj
∂
ρ ε
j
− Cε2
ρε ∂
∂
2
+
k
xj




(µl + µt ) ⋅
σk
∂ε
∂xj




(4)
In the above two equations, P represents
the production term given by
Equation (5).
P ν= t




∂
∂
u
x
i
j
+
∂
∂
u
x
j
i
−
2
3
⋅
∂U δ
∂x
m ij
m




∂
∂
u
x
j
j
− k
2
3
δ
ij
(5)
The energy equation used to solve for
enthalpy is given by Equation (6). The
source term, Sh, in the energy equation
includes combustion and radiation
heat-transfer rates:
∂x ()
∂
∂ j
x
j
ρuk
j
∂ ()+




µµ
σ
)
lt
k
⋅
∂xj
∂k




(3)
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