43 Dynamic Studies of Acetylene Hydrogenation on Nickel Catalysts M . R. B I L I M O R I A and J. E . B A I L E Y
Downloaded by UNIV LAVAL on October 12, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch043
Department of Chemical Engineering, University of Houston, Houston, TX 77004
Several benefits including improved selectivity, increased conversion, and reduced sensitivity may in some cases be realized by intentional periodic operation of chemical reactors [1-6]. These alterations in reactor performances arise because, in dy namic operation, the governing rate processes may assume different relationships than can occur in the more constrained steady state case. Thus, in addition to revealing promising new operational policies for chemical reactors, unsteady-state reaction studies may provide valuable insights into the fundamental properties of the reaction system. This study focuses on the vapor-phase catalytic hydrogenation of acetylene to ethylene and ethane: C H + H --> C H C H + H --> C H 2
2
2
2
4
2
4
2
2
6
Bond [7], Komiyama and Inoue [8], and several others have studied this reaction sequence using nickel catalysts, and Bond et al. [9] have reported that the ethylene s e l e c t i v i t y S varies with hy drogen p a r t i a l pressure p according to h2
S = 1 - Κpnh where Κ = a constant and 2
0 t
ρ
(6)
where tp i s d e f i n e d as the s t a r t - u p time r e q u i r e d to reach p e r i o d i c c o n d i t i o n s . The o s c i l l a t i n g e f f l u e n t concentrations may be damped by a surge tank (see Figure 1) to o b t a i n a t i m e - i n v a r i a n t process e f f l u e n t which may be d i r e c t l y compared to the products o f s t e a d y - s t a t e r e a c t o r o p e r a t i o n . The time-average e f f l u e n t concentrations may a l s o be c a l c u l a t e d using 1 ft +τ x. = — Ρ x.(t)dt ι τ tn
(7)
1
and sets o f instantaneous c o n c e n t r a t i o n data which span the p e r i od. These computed values were c o n s i s t e n t l y i n good agreement w i t h d i r e c t surge-tank measurements. Results and D i s c u s s i o n The experimental r e s u l t s are presented i n Figures 2 through 8. Figure 2 i s a comparison o f steady-state compositions w i t h time-average values obtained by c y c l i n g w i t h d i f f e r e n t periods f o r C a t a l y s t I . This c a t a l y s t showed n e g l i g i b l e f o u l i n g w i t h almost constant a c t i v i t y , a r e s u l t c o n s i s t e n t w i t h conventional B.E.T. s u r f a c e - a r e a measurements which gave values o f -125 129 m /g before and a f t e r r e a c t i o n . The data show that p e r i o d i c o p e r a t i o n r e s u l t s i n i n c r e a s e d conversion f o r τ=60 s, τ=180 s and τ=300 s, w i t h the maximum e f f e c t obtained f o r the longest p e r i o d . In a d d i t i o n , the data f o r τ=300 s show a s i g n i f i c a n t improvement i n both ethylene and ethane y i e l d s . This dependence on c y c l i n g p e r i o d i s q u i t e d i f f e r e n t from L e e s p r e d i c t i o n s (optimum p e r i o d ~ 1/2 mean residence time = 33 s) and suggests that the c a t a l y s t phase dynamics cannot be neglected f o r t h i s system. The instantaneous concentrâtion-time p r o f i l e s obtained dur i n g p e r i o d i c o p e r a t i o n w i t h C a t a l y s t I f o r τ=300 s are shown i n Figure 3. A l l these curves were obtained by making 15 d i s c r e t e 2
1
In Chemical Reaction Engineering—Houston; Weekman, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV LAVAL on October 12, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch043
BiLiMORA AND BAILEY
Acetylene Hydrogénation
on Nickel Catalysts
INTEGRATOR
Figure 1.
Experimental reaction system highlighting cyclic feed makeup apparatus and effluent analysis equipment
Δ ACETYLENE ο ETHYLENE • ETHANE
ζ Lu 3
ο
