19 Low Temperature Process for T N T Manufacture Part 3. Computer Simulation
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R. W . H U T C H I N S O N
and D . G O L D S T E I N
Picatinny Arsenal, Dover, N.J. 07801
Purpose During the process of designing the TNT p i l o t plant, questions arose concerning plant start-up. The coupling of the two ends of the process by the recycle stream made i t very d i f f i c u l t to "think through" proposed start-up schemes. It was f e l t that a dynamic model would be an ideal way to answer questions regarding start-up and to verify the IIT Research Institute (IITRI) design. Development of the model during the f i n a l design stage seemed well timed since the design would provide a basis for the model, while, at the same time, the model could be used to investigate design changes which, if shown to be advantageous, could be readily incorporated. The purposes of the dynamic simulation were, therefore, to (1) verify the IITRI design and test i t under start-up and normal run conditions, (2) test alternate start-up procedures in order to determine the best way to handle the recycle and c r y s t a l l i z a tion operations, and (3) determine dynamics of the p i l o t plant in order to estimate acid sampling periods and c r i t i c a l sampling and control points. In addition, i t was felt that the model would provide a useful format for data logging and correlation once the p i l o t plant was in operation. Longer-range plans included the use of the model in on-line, high-level, control strategies. Approach A simplistic, building-block approach was used in developing the model. For each vessel, a dynamic material balance was written. The resulting system of first-order differential equations was coupled through the common inlet and outlet streams. Equations 1 and 2 are the general mass balances applying to all vessels, assuming perfect mixing.
290
In Industrial and Laboratory Nitrations; Albright, Lyle F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
19.
HUTCHISON AND GOLDSTEIN
F
I
i
R
" C
±
i"
F
O
i
=
V
0
L
(
TNT
d
c
i
/
d
t
Manufacture:
Computer
Simulation
(
^
1
= ¥0 /q
291
)
(2)
±
where FI
• inlet
i
flow o f component i ,
lb/hr.
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= r e a c t i o n r a t e o f component i , F0
» o u t l e t flow o f component i ,
i
lb/hr. lb/hr.
3 VOL - v e s s e l volume, f t .
3 C±
= c o n c e n t r a t i o n o f component i ,
t
» time, h r .
q
• v o l u m e t r i c flow, f t /hr.
lb/ft .
3 Then i
(3)
2
d C / d t - d(FO /q)/dt
= (l/q )(dqF0 /dt-F0 dq/dt)
j[
i
i
Combining Equations 1 and 3 r e s u l t s i n Equation 4 , which when i n t e g r a t e d f o r each component and each v e s s e l gives the flow o f each component from each v e s s e l . (4)
dFO ,/dt = (q/VOL) ( F I - R - F O ) + (FO /q) (dq/dt) j
1
1
±
±
The l a s t term i n Equation 4 can be considered as a c o r r e c t i o n term t o account f o r changing v o l u m e t r i c flow r a t e s w i t h time. In t h i s system the term i s very s m a l l , although n o t zero, s i n c e d e n s i t y changes do occur i n the v e s s e l s whenever r e a c t i o n takes place. The r e a c t i o n rate terms, R^, are based on k i n e t i c data. The d i n i t r a t i o n of toluene appears t o occur i n s t a n t a n e o u s l y i n the anhydrous a c i d s used i n the process: Thus,
R
R
t o l u e n e " DNT
F I
~ toluene
The rate o f t r i n i t r a t i o n
i s much slower than t h a t of
d i n i t r a t i o n , and a more complicated, k i n e t i c a l l y dependent model i s r e q u i r e d . The s t o i c h i o m e t r i c equation Η CO
DNT + HN0
1 4 > TNT + H 0
3
2
f o r t r i n i t r a t i o n i s represented k i n e t i c a l l y as the b i m o l e c u l a r rate process: D
T O T
° dt
-
K
2 W
C
3
DNT
" «TMT
In Industrial and Laboratory Nitrations; Albright, Lyle F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
(
5
)
292
INDUSTRIAL A N D LABORATORY
NITRATIONS
3 f
where the C s are molar concentrations, moles/ft . The bimolecular r a t e constant, k , i s given by: ^N0£ ?
k
2 " 2 k
C
where C Q +
3
( 6 )
i s the molar concentration of the a c t i v e n i t r a t i n g
N
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HN0
s p e c i e s , the nitronium i o n [JJ . In order to u t i l i z e the above r a t e expression i n the m a t e r i a l balance of Equation 1, the s p e c i f i c molar r a t e , R, must be expressed as the bulk mass r a t e :
«TNT " *TNT
(MW
(
TNT>
V
0
L
)
(
7
)
Combining Equations
5 , 6 and 7 gives the r a t e expression:
« n i "
I— - — L HNO,
k
Î
^TNT
V
0
L
C
I
C
HN0
C 3
DNT
(
8
)
_
The i n f l u e n c e of temperature upon r e a c t i o n r a t e i s taken i n t o account by assuming that the constant, k^ , e x h i b i t s an Arrhenius dependence:
where krpjjçj,
=
frequency f a c t o r
TNT =
(124.5 hr
a c t i v a t i o n energy (9300 Kcal/lb-mole)*
Τ
=
temperature (°K)
R
=
U n i v e r s a l gas constant
(*These values a r e based on f i t t i n g the model to data taken from the continuous TNT l i n e s at Radford Army Ammunition Plant.) S u b s t i t u t i o n of Equation 9 i n t o Equation 8 gives the f i n a l expression:
In Industrial and Laboratory Nitrations; Albright, Lyle F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
HUTCHISON A N D GOLDSTEIN
19.
TNT Manufacture:
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= k TNT TNT
TNT RT
exp
Simulation
293
χ
E
R
Computer
MW
7rT1
TNT
VOL
, C
C
HN0 DNT
(
1
0
3
C
HNO,
Use o f E q u a t i o n 10 a s a m o d e l f o r t r i n i t r a t i o n depends upon t h e a b i l i t y t o compute t h e n i t r o n i u m i o n c o n c e n t r a t i o n from t h e a c i d e q u i l i b r i u m r e a c t i o n s which occur. For the anhydrous a c i d mixtures used i n the p r o c e s s , t h r e e separate a c i d c o n c e n t r a t i o n r e g i o n s , each e x h i b i t i n g d i f f e r e n t e q u i l i b r i u m , are defined. In the f i r s t region ( n i t r i c l i m i t e d ) , p y r o s u l f u r i c a c i d ( H S 0 ) i s i n large excess o f the n i t r i c a c i d to the extent that: ' ?
9
7
[HN0 ]
