29 Computer-Aided Development of the Cyclohexane Oxidation Process J. A . DE LEEUW D E N B O U T E R , L. L. VAN D I E R E N D O N C K , and W . O . B R Y A N
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C R O / D S M , P.O. Box 18, Geleen, The Netherlands
1. INTRODUCTION Within the scope of its program aimed at process improvements, the Corporate Department for Research and Patents of DSM is now seeking ways of optimizing the cyclohexane oxidation route towards cyclohexanone (anone) and cyclohexanol (anol). These products are intermediates in the preparation of caprolactam. Caprolactam is polymerised to nylon-6. As DSM has a great interest in the world production of caprolactam, much effort is put into research and optimization of the various process steps. In the oxidation of cyclohexane, the efficiency is relatively low, which is the reason why research into and optimization of this process step was taken in hand. The flowsheet of the oxidation section is shown in fig. 1. Fresh cyclohexane is mixed with the recycled cyclohexane from the top of the cyclohexane distillation column. The mixture is fed to the condensation section to exchange heat with the vapour stream from the reactors. Next, the cyclohexane is oxidized with air in a cascade of stirred reactors. The oxidate leaving the reactors undergoes an after-treatment in a decomposition reactor, while the acids formed as byproduct in the oxidation section are neutralized in a mixer-settler unit. In the cyclohexane distilation section non-converted cyclohexane is separated from cyclohexanol, cyclohexanone and byproducts. DSM studied the catalysed process in the years before 1960. Steeman et a l . (1) on the basis of experimental work, calculated the optimum degree of cyclohexane conversion to be 7-8 % at an efficiency of 75 %. Alagy e t a l . ( 2 ) , i n t r o d u c i n g b o r i c a c i d (metaborate) i n t o the o x i d a t i o n s e c t i o n , achieved e f f i c i e n c y f i g u r e s i n the range 85-90 %. The metaborate used by these authors r e a c t s w i t h cyclohexanol (anol) t o form an e s t e r , thereby p r e v e n t i n g o x i d a t i o n o f anol to anone, and, as a consequence, suppressing the anone c o n c e n t r a t i o n . Since the bulk of the byproducts i s formed v i a anone, the use o f b o r i c a c i d t h e r e f o r e brings i n t h i s © 0-8412-0401-2/78/47-065-348$05.00/0
Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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29.
DE L E E U W
ETAL.
Cyclohexane Oxidation Process
349
case an improvement i n e f f i c i e n c y . The new process v a r i a n t i n v e s t i g a t e d a t DSM i s a l s o based on suppression o f the anol and anone concentrations, which i s achieved by l e a v i n g out the c a t a l y s t . A p p l i c a t i o n o f appropriately chosen c o n d i t i o n s counteracts decomposition o f the c y c l o h e x y l hydroperoxide intermediate (PER), thus lowering the anol and anone concentrations and as a consequence improving the e f f i c i e n c y . Depending on the degree o f cyclohexane conversion, e f f i c i e n c i e s of 85-95 % can now be obtained. Of course, the degree to which the o v e r a l l e f f i c i e n c y i s increased a f t e r the decomposition o f the peroxide depends on the s e l e c t i v i t y o f the peroxide conversion step, but t h i s w i l l not be considered here. The process improvement has been thoroughly evaluated i n bench-scale experiments and afterwards been t r i e d out i n a commercial p l a n t process. A l l operations i n v o l v e d were conducted i n gear with computer model c a l c u l a t i o n s . On the b a s i s o f the l a b o r a t o r y experiments a k i n e t i c model was developed on an analog computer. Next, a r e a c t o r model based on the k i n e t i c model was worked out on a d i g i t a l computer. In the f o l l o w i n g step, a model o f the new process v a r i a n t was set up by means o f the DSM flowsheet s i m u l a t i o n system TISFLO. F i n a l l y , the process model was used f o r o p t i m i z i n g the process. The various stages o f the i n v e s t i g a t i o n s and the supporting a c t i v i t i e s are s c h e m a t i c a l l y shown i n Table I. Table I. Survey o f the work done on improvement o f the CHo x i d a t i o n process Experimental
Model c a l c u l a t i o n s
Bench s c a l e experiments
Analog s i m u l a t i o n o f the reaction kinetics. D i g i t a l s i m u l a t i o n of a g a s - l i q u i d r e a c t o r . Simulation o f an actual plant for test-run planning
Test runs i n an e x i s t i n g p l a n t
Correction of several q u a n t i t i e s i n the model a f t e r the t e s t runs. Optimization of the design.
The present paper b r i e f l y o u t l i n e s the development of the computer models and some o f the r e s u l t s achieved with these. 2. KINETIC MODEL In bench-scale batch experiments, cyclohexane was o x i d i z e d with a i r i n a s t i r r e d autoclave. To d e s c r i b e the change i n the concentrations o f the p r i n c i p a l o x i d a t i o n products, we p o s t u l a t e d the f o l l o w i n g r e a c t i o n scheme:
Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CHEMICAL
350 k
i
k
2
R + 0
REACTION
ENGINEERING—HOUSTON
— PER
(2.1)
ANOL + J 0
PER k
(2.2)
2
3
PER
ANONE +
(2.3)
k 2 - * ANOL + ANONE + H O + J Ο
2 PER
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ANOL + J 0
(2.4)
—ANONE + H O
2
(2.5)
ANONE + 3/2 0 1—"acids" (2.6) This scheme i s p r a c t i c a l l y analogous to those put forward by Steeman e t a l . (JL), Alagy e t a l . (2) and Vasey e t a l . ( 3 ) . The d i v e r s i t y o f byproducts, such as a c i d s , e s t e r s , heavies, l i g h t s , CO, C 0 and alkanes, a r e included under " a c i d s " . Reaction (2.1) i s regarded as the sum o f two r a d i c a l r e a c t i o n s : 2
2
R* + Ο
— ROO* k — » PER + R*
2
ROO' + R
(2.7) (2.8)
The c y c l o h e x y l r a d i c a l s a r e generated during the breakdown o f the PER according t o the r e a c t i o n s (2.2), (2.3) and (2.4). Along the same l i n e s as followed by Emanuel ( 4 ) , i t can be d e r i v e d that the r a t e o f t h i s a u t o - o x i d a t i o n system i s p r o p o r t i o n a l t o the square root o f the r a d i c a l - g e n e r a t i n g components. S i m p l i f i c a t i o n o f the r a d i c a l r e a c t i o n equations y i e l d s the f o l l o w i n g expression f o r the formation r a t e s o f the various components : (
) =
" 4 t ^
k
l*[°2]- f 1 [ (
PER
+
P E R
1
· [ANON])^
On the b a s i s of the above reasoning, equations can be s e t up: ^dt^
=
k
x
. [ 0 ] . ([PER]
^jjf^ = k
2
d[ANONE] ^
4
the f o l l o w i n g d i f f e r e n t i a l
+ [PER] . [ANONE])^ - ( k + kg) .
2
. [PER] - k
(2.9)
£
. [ P E R ] . [ ANONE]
(2.10)
. [ P E R ] + k . [ P E R ] .[ANONE] - k ^ A N O L ] . [θ ] (2.11) 4
fc ] ER
+
K
4
2
. [
P
E
R
] . [ A N O N E ]
+ k .[ANOL].[0 ] 6
2
- k .[AN0NE].[0 ] ?
2
Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
(2.12)
29.
d C
DELEEUW
";;
l d 8
"
]
Cyclohexane Oxidation Process
ET AL.
= k .[ANONE]{0 ] 7
(2.13)
2
d[0 ]
~~dtT
ι
" l ' [°2] f
=
k
i . k
351
,(
PER
3
+
C
PER
]
· C ANONE]) 5
. [ANOL].[0 ] - l j . k .[ANONE].[0 ] + J.k. [PER]
5
2
g
+
2
+ J.k .[PER] . [ANONE]
(2.14) s
t t e c
For the 0 -consuming r e a c t i o n s the product o f k -[Og] * f i *. Equation (2.14) allowed us to c a l c u l a t e the s t o i c h i o m e t r i c oxygen need o f the r e a c t i o n s . This need can be r e l a t e d to an independent supply o f a i r to the r e a c t o r s . The f a c t o r F i s defined as
Downloaded by FUDAN UNIV on January 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch029
2
Q
2
F
0
supply
Z
0
2
0
2
consumed i n the s t o i c h i o m e t r i c
conversion
The k-values were f i t t e d f o r those experiments i n which F^ > 1. For t h i s case a zero-order d e s c r i p t i o n i n Ο i s v a l i d . 2 When F Q < 1, a l l 0 -consuming r e a c t i o n s were m u l t i p l i e d by the f a c t o r F Q 2 . This means that i n t h i s case a f i r s t - o r d e r r e a c t i o n i n 0^ 2 i s assumed. The example shown i n F i g . 2 serves to i l l u s t r a t e to what extent the l a b o r a t o r y experiments are covered by the model. 2
3. MODEL OF THE GAS-LIQUID REACTOR With regard t o the g a s - l i q u i d r e a c t o r the f o l l o w i n g assumptions have been made : - the l i q u i d phase and the gas phase are p e r f e c t l y mixed, - the i s s u i n g gas stream i s i n p h y s i c a l e q u i l i b r i u m w i t h the i s s u i n g l i q u i d stream, - the r e a c t o r operates a d i a b a t i c a l l y , - the compositions, f l o w r a t e s , temperatures and pressures o f the feed streams are known. In c a l c u l a t i n g the p h y s i c a l e q u i l i b r i u m , we made use o f gasl i q u i d e q u i l i b r i u m constants, which are defined as
