Chemical Engineering July 2012 - 37

NOMENCLATURE
A, Af, ALM tube areas, ft2
Ao
CaO
CH4
Cpco
Hv
tube outside area, ft2/linear ft
carbon monoxide initial
concentration, lb-moles/ft3
methane feed to the HYCO
reactor
heat capacity of carbon
monoxide product
CpH2 ht capacity of hydrogen product
Cpflue heat capacity of the flue gas
Cpmeoh heat capacity of methanol product
Cpstm heat capacity of steam
CV
valve flow coefficient,
dimensionless
DEA deaerator
Di
Do
fco
inside tube diameter, in.
outside tube diameter, in.
dhwgs heat of HYCO gas reaction
corrected to reaction conditions
carbon monoxide feed to
methanol reactor
feedwtr feed water flow to the deaerator
Fa
tube bank arrangement factor,
dimensionless
Fao
Fcarb
Fm
carbon monoxide initial feedrate,
lb-moles/h
steam to carbon ratio, dimensionless
methanol
produced in reactor,
lb/h
Hg saturated vapor enthalpy, Btu/lb
Hf
saturated liquid enthalpy, Btu/lb
Hhv fuel high heating value, Btu/lb
Hid
Hod
hrxnp
hrxnr
Heat of reaction of HYCO gas
products
Heat of reaction of HYCO gas
reactants
tube inside heat transfer coefficient,
btu/h-ft2°F
tube outside heat transfer coefficient,
btu/h-ft2°F
kair
kgas
kh2O
liquid heat of vaporization,
Btu/lb
thermal conductivity of combustion
air, Btu/h-ft-°F
thermal conductivity of fluegas,
Btu/h-ft-°F
thermal conductivity of the feed
water, Btu/h-ft-°F
K, K(1), K(2), Kp equilibrium constants,
dimensionless
kl
kw
kv
liquid thermal conductivity,
Btu/h- ft2-°F/ft
thermal conductivity for carbon
steel, Btu/h- ft2-°F/ft
steam thermal conductivity,
Btu/h- ft-°F
Ltube length of tube, ft
Lhv fuel low heating value, Btu/lb
Mu
liquid viscosity, centipoise
ntube number of tubes
Opn
P, dp
model methane valve percent
open, %
drum pressure and pressure
differential
PIDout model controller output, %
Pr
qfr
heat generated in furnace
combustion, Btu/h
R[dx/KALM], Rtube thermal resistance to
heat transfer, hr-°F/Btu
Re Reynolds number, dimensionless
Rf
saturated liquid specific volume,
ft3/lb
rxn HYCO gas reaction rate, h-1
stdh standard heat of reaction
ta, dta
economizer gas-bank outlet temperature
and temperature
differential
Q heat added to liquid, Btu/h
q
Prandtl number, dimensionless
heat lost by fluegas, Btu/h
tavg
tb
tdea
tf
tj, dtj
t0, dt0
average temperature across
economizer, (inlet + outlet)/2
saturated liquid boiling temp
liquid temperature in the
deaerator
fuel flame temperature
methanol jacket outlet temp and
temperature differential
water drum gas-bank outlet
temperature and temperature
differential
tsat, dtsat saturated steam temperture
and temperature differential
tw, dtw economizer liquid outlet temperature
and temperature differential
tw2, dtw2 steam superheater outlet
temperature and temperature
differential
ts
ty, dty
superheater gas-bank inlet temperature
economizer
gas-bank inlet
temperature and temperature
differential
U, Ub, Ucg, Uo
Unit heat transfer
coefficients, Btu/h-ft2-°F
ugas fluegas viscosity, centipoise
uvap steam viscosity, centipoise
V reactor volume, ft3
wflue fluegas mass flow
wfw
feedwater flow from the deaerator
to the economizer
wstm, dw steam generation flow and
steam mass differential
xc
xh2
fuel carbon content, wt. %
fuel hydrogen content, wt. %
xo2 fuel oxygen content, wt. %
xn2 fuel nitrogen content, wt. %
x, xm unit conversions, mole %
xrh2o fuel moisture content, wt. %
xsul fuel sulfur content, wt. %
Note: All simulation temperatures are expressed in °F, pressures are in psi and liquid flow is in lb/h. Heat capacity is expressed in
Btu/lb-°F, and heats of reaction are shown in Btu/lb-mole units.
they will be structured and how they
will interact in the simulation. Figure
2 shows the methodology.
Next, assign inlet and outlet numbers
or letters to the furnace banks
for the estimates of model gas temperature
so that model heat flow can
be developed in the proper direction.
The smaller arrows in Figure 2 depict
information flow between the code
segments.
The red arrows within the Figure
2 process module represent combustion
gas temperature and information
flow, while the blue arrows
provide feed water condition information
progression from the economizer
to the steam drum and steam
to the super heater (SUPH). The
arrow color for fluegas decreases in
intensity (from darker red to lighter
red), to depict the corresponding loss
of heat energy, in the data passed,
as the temperature of the fluegas
reduces through this series of coded
segments. The magenta arrows in
Figure 2 show the HYCO gas information
progression from the methane
preheat units to the methanol
reactor segment. The model can be
set up so that a particular segment's
calculated information appears in a
realtime graphical display.
Implementing the model code
The user interface of Figure 2 is a
module made up of one or more 'C'
code segments. Segments in the example
module, because of their relatively
small size, comprise the entire model
user interface shown.
However, users working with larger
furnace simulations - for instance,
those with reheat and additional superheating
components - may find it
expedient to dedicate an entire module
to one process function alone (that
is, all code segments in that module
are dedicated to a single task).
The selected simulation user interface
should be able to contain multiple
user blocks with individual discrete
segments. Multiple user blocks
must be able to pass information to
other user blocks, just as if they were
segments within the same module.
The modules must be linkable to
your control strategy in the chosen environment.
Once
the model is structured and
ready for coding, initialize all variables
used in that segment before developing
detailed computations, as shown
in the furnace segment example for
updating fuel composition.
One can develop the model for enCHEMICAL
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Chemical Engineering July 2012

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