Chemical Engineering July 2012 - 35

Trend ptrs
Graphic
interface
Control
strategy
controllers
Final control
elements
with filter
Ref ptrs
Process
variables
Module
User interface
Common 'n'
Ref ptrs
Ref ptrs
Methane
Furnace
Com ptrs
Gas bank
temps
Superheater
Drum
Ref ptrs
Economizer
Deaerator
Stack
MeOH
reactor
Module
Variable
trending
Notes:
ptrs = Pointers
Ref = Reference
memory
Com = Common
memory
ECONOMIZER SEGMENT
/*......................................Economizer Segment....................................*/
/* Source code as it should appear in the 'C' editor */
Re = 4 * Wfw/(*(Di/12)*Mu*ntube);
Pr = cph2o*Mu/Kl;
/* Liquid Reynold's No */
/* Prandtl No. */
tsat = a+dp*(b+dp*(c+dp*(d)));
tavg = (tw + tdea)/2;
kw = 345 -.077*tavg;
Re = wflue/Af*(Di/12)/ugas;
Pr = cpflue * ugas/kgas;
/* Saturated temp at drum pres */
/* Avg temp across economizer */
/* TC data - .23 C.S. , 1972 B&W */
/* STEAM, p. 4-2, Fig. 1 */
Rtube = Do/12 * log(Do/Di)/(2*kw); /* Tube therm resistance */
/* from 1972 STEAM p.4-9 */
/* Gas Reynold's No, STEAM, Chap 4 */
/* Where: wflue/Af is the bulk gas velocity */
/* Gas Prandtl No. */
Hid = 0.023*Kl/(Di/12)*(4*wfw/(*(Di/12)* uliq * ntube))^.8 *Pr^.4;
kair = (a + b*dty)/12;
/*Regress fit of air thermal cond */
/* Where: 'dty' is the differential gas bamk temp entering the economizer */
kgas = xrh2o* kh2o +(1-xrh2o)*kair; /* flue gas therm Cond */
Ucg = a1*kgas*Re^.61*Pr^.33;
U = 1/((Do/Di)* 1/Hid + Rtube + Ucg; /* Overall Heat Trans coeff */
q = wflue * cpflue *(ty - ta);
area = ntubes * Ltube * Ao;
dtw = tdea - tw +q/(area*U);
tw = tw + dtw;
cph2o = a + b* tavg + c * tavg^2;
q = wfw * cph2o*(tw - tdea);
t a v g = ( t w + t d e a ) / 2 ;
cph2o = a + b * tavg + c * tavg^2;
q = wfw * cph2o*(tw - tdea);
dta = ta - ty -q/(area * U);
ta = ta + dta;
specifications to develop the model
profile. In our example, solid fuel pellets
are conveyed from a feed bin onto
a furnace-traveling grate. Calcium
carbonate is injected above the combustion
area as an SO2 absorbent.
Methane feed enters the first of two
/* Gas ht-trans, 1972 STEAM, Chap 4 */
/* Heat added by flue gas */
/* Gas outside tube area A0=ft2/ft */
/* T(i+1) = F(n) T(i) / F(n') T(i) */
/* Integrate Economizer Outlet temp */
/* Economizer ht-capacity */
/* Heat added by the flue gas */
/* Economizer ht-capacity */
/* T(i+1) = F(n) T(i) / F(n') T(i) */
/* New Econ Bank outlet temp
preheaters at 100+°F and is heated
successively in two stages from
950°F to 1,150°F. The hot methane
feed enters a HYCO reformer where
600-psi steam is added. The HYCO
gas - which is comprised of carbon
monoxide, hydrogen, and unreacted
The model
Once you have established a good process
definition, the next step is to begin
to develop an outline of the model 'C'
coding, adding external references and
variables, as needed. 'Pseudo code' is
preferred at this stage. Be certain that
supporting definitions and library
functions are loaded.
When the pseudo code is complete,
formalize and code the general and library
routine statements into a main
process module. Add separate segments,
as needed, and name them according
to their intended function (for
example,the economizer segment, and
so on).
Library routines containing other
supporting functions must be called
with an #include statement. For instance:
/*
MAIN
SEGMENT */
#include 
/*
math subroutine library */
float Di, Do, Q, V, ts, tg, tsat; /* local
floating pt single */
float Hg, Hid, Hod, Lhv, Hhv, Mu,
Ntube; /* precision variables
....
extern double c[9], w[9];
/*
global double precision */
(Note: All terms are defined in the
Nomenclature box on p. 37.)
Entering comments or reminders
into the code language for future reference
is highly recommended. For
CHEMICAL ENGINEERING WWW.CHE.COM JULY 2012 35
*/
New 'C' code
module
.....
fcarb=c[9];
c[4]=feedwtr;
ch4=c[1];
.....
Reformer
Separator
Process
output
Com ptrs
Com ptrs
Ref ptrs
Ref ptrs
Alarms
FIGURE 2. This
igure shows the
dynamic model
environment,
identifying dedicated
segments
and interconnections
within the
'User' Interface.
The simulated
process information
moves between
segments
steam - passes from the reformer to
a water separator. Steam condenses
out in the separator and the remaining
dry gas flows from the separator
tank to a catalyzed reactor that operates
at 100°F and 200 psi to produce
methanol.
Makeup water at 77°F cools the
methanol reactor and is pumped to
the separator tank, where it is mixed
with hot condensate from the HYCO
reformer and sent on to the deaerator.
Oxygen is steam stripped from the
feed water in the deaerator at 298°F
before entering the economizer.
The model unit is designed to operate
at 1,500°F with 18% excess air and
negligible air preheating.
Com ptrs
http://WWW.CHE.COM
Chemical Engineering July 2012

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