Chemical Reaction Engineering-Houston

6 Methanation in a Parallel Passage Reactor

Downloaded by UNIV OF TENNESSEE KNOXVILLE on November 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch006

E . W . DE B R U I J N , W . A . DE JONG, and C . J. VAN DER S P I E G E L Laboratory of Chemical Technology, Delft University of Technology, Delft, The Netherlands

One of the steps i n the process of making SNG via coal g a s i f i cation, the methanation of carbon oxides, has received considerable attention i n recent years. Owing to the highly exothermal nature of the reaction, the temperature control of methanation reactors is difficult. Among the solutions proposed are the application of p a r a l l e l plate (1) and coated tube (2) reactors. I t i s also possi ble to apply recirculation of cold product gas, but when this i s done with conventional fixed-bed reactors the resulting high pres sure drop i s a disadvantage. The parallel passage reactor (PPR) recently described i n connection with S h e l l ' s Flue Gas Desulphurization Process (3) does not have this drawback because it contains shallow beds of solid reactant separated from narrow channels by wire screens, the gaseous reactants flowing through the channels with a r e l a t i v e l y low pressure drop. Such reactors could, i n p r i n c i p l e , be applied i n any process i n which large volumes of gas must be treated at minimum pressure drop, provided that sufficient capacity for absorption of the heat of reaction i s available. Ex amples of such processes are oxidation reactions, Fischer Tropsch synthesis and, as outlined above, carbon oxide methanation. This paper describes preliminary results of a study on this reactor using the methanation of carbon dioxide i n hydrogen at atmospheric pressure as the test reaction. Moreover, a mathematical model was formulated and used to compare computed conversions with experimental data. The objective of the first phase of this work is to obtain a rough estimate of the a p p l i c a b i l i t y of the PPR for methanation purposes. Experimental The test reaction^ C0 + 4 H CH, + 2 H 0 (ΔΗ° = -164.7 kJ/mole C0 ) 2 2 4 2 r,s 2 has been studied extensively and r e l i a b l e k i n e t i c data are a v a i l able (4_) . Work on the use of this reaction i n studying the tran sient behaviour of an adiabatic methanator indicates that i t can be applied as a test reaction between 200 ° and 280 °C, with good o

0

o

© 0-8412-0401-2/78/47-065-063$05.00/0

In Chemical Reaction Engineering—Houston; Weekman, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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64

CHEMICAL REACTION ENGINEERING—HOUSTON

r e s u l t s (5). The k i n e t i c equation used i n the present work i s given i n t a b l e 1, along w i t h the experimental c o n d i t i o n s of the i n i t i a l phase of the work. The set of c o n d i t i o n s being covered i n current work i s a l s o given i n the t a b l e , as w e l l as i n f o r m a t i o n on the i n d u s t r i a l methanation c a t a l y s t a p p l i e d . Table I The r e a c t i o n r a t e i s given by: K exp.(-E /RT)p Downloaded by UNIV OF TENNESSEE KNOXVILLE on November 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch006

œ

r

a

=

C Q

^

C02

mol.h

2

1

+

K

C0 pC0 2

.g

2

Experimental c o n d i t i o n s

f i r s t phase

current work

Temperature Concentration Flow T o t a l pressure Reactant

208, 224, 242 0,19 - 2,56 0,05 - 0,45 1 H , C0

190 - 240 0,19-4 0,05 - 7 1 - 1 2 H , CO, C0 ,

°C vol% Nm /h atm. 3

2

2

2

C a t a l y s t : G i r d l e r G-65 Ni/AUO.; NiO/AUO. = 3 : 3 s i z e = 0,35 - 0,42 mm; S = 42,4 m /g; 2

B E T

2

w/w;

H0 2

particle = 6,6 m /g 2

The equipment used i s s i m i l a r to that of r e f . 4 , except f o r the p a r a l l e l passage r e a c t o r and the gas throug>ut, which i s between 0,1 and 0,5 Nm^/h. I t c o n s i s t s of a feed p r e p a r a t i o n s e c t i o n f o r metering and c o n t r o l l i n g the reactant mixture, the PPR immersed i n a f l u i d i z e d bed thermostat and a s e c t i o n f o r o n - l i n e a n a l y s i s of feed and product gases by gas chromatography. Figure 1 shows a b l o c k diagram. The dimensions of the r e a c t o r are shown i n f i g u r e 2. The two c a t a l y s t beds are f i l l e d w i t h p a r t i c l e s of 0.35-0.42 mm; the bottom and top p a r t s of the beds c o n t a i n i n e r t m a t e r i a l of the same dimensions to ensure that the flow regime i n the channel i s completely e s t a b l i s h e d when the gas reaches the c a t a l y s t beds. Reactor Model Let ζ be the coordinate i n l o n g i t u d i n a l d i r e c t i o n and y i n l a t e r a l d i r e c t i o n and assume that the c o n c e n t r a t i o n changes caused by r e a c t i o n and mass t r a n s p o r t to the c a t a l y s t bed are s i m i l a r to that of f i g u r e 3. I f the flow regime i n the channel i s laminar and the r e a c t o r i s o t h e r m a l , and supposing that mass t r a n s p o r t i n channel screen and c a t a l y s t bed are e n t i r e l y due to d i f f u s i o n , the mass balance f o r the channel reads: ν JC y 9z d 2 y The equation contains the assumption that the d i f f u s i o n can be represented by F i c k ' s law, i n other words that flow due to the volume change by chemical r e a c t i o n can be neglected. Furthermore, a x i a l d i f f u s i o n i s not taken i n t o account. A l s o , the channel i s taken to be wide enough to consider i t as being bounded by two = ] D


( 1 )

6.

BRuijN E T A L .

Methanation in a Parallel Passage Reactor

65

vent


COo

reactor gas conditioning

H * I GC I gas chromatograph carrier gas p

2

recorder / digital integration Figure 1.

6

5

6

Block diagram of a parallel passage reactor

mm lateral cross section

wire screen

gas channel

inert longitudinal cross section catalyst

inert Figure 2.

Dimensions of a parallel passage reactor


66

CHEMICAL REACTION

ENGINEERING—HOUSTON

i n f i n i t e l y wide p a r a l l e l p l a t e s . The boundary c o n d i t i o n s

are:

f| = 0 a t y = 0 ( 2 ) ; C = C a t ζ = 0 ( 3 ) ; D-|f - » - | f ^ q


B

=

f

(

a t

y

=

y

( 5 )

a n d

y-ia (4);

8c

V

=

1

a

6

e f f ^ V k y " (y^ > The d i f f u s i o n c o e f f i c i e n t i n the w i r e screen QD i n eq.4) i s taken to be equal t o the product of the screen p o r o s i l y and the d i f f u s i o n c o e f f i c i e n t i n the gas channel. Boundary c o n d i t i o n (5) must be found by i n t e g r a t i n g the mass balance over the c a t a l y s t bed, which f o r a volume w i t h thickness dy i n the x-z plane reads: eff· f £ - C < V b e d 3y 2 In t h i s equation, r ^ represents the r a t e equation of t a b l e I . The e f f e c t i v e d i f f u s i o n c o e f f i c i e n t i n the c a t a l y s t bed i s equal to the d i f f u s i o n c o e f f i c i e n t i n the channel, corrected f o r the bed p o r o s i t y and a f a c t o r f o r the t o r t u o s i t y of the d i f f u s i o n path. Both f a c t o r s are assumed t o be 3 according t o r e f . 6^. Assumptions made i n formulating the mass balance over the c a t a l y s t bed are that pore d i f f u s i o n l i m i t a t i o n i n the c a t a l y s t p a r t i c l e s can be neglected over the e n t i r e c o n c e n t r a t i o n range of e x i s t i n g i n the bed ( 5 ) , that the bed i s isothermal and homoge neous, and that mass t r a n s p o r t i n the bed i s e n t i r e l y due t o d i f f u s i o n . The boundary c o n d i t i o n s are: B

C = C

at y = y

k

r

k

p

(8) and | | = 0 at y = y

( 7 )

w

(9)

Numerical s o l u t i o n o f the mass balance i n the c a t a l y s t l a y e r i s r e l a t i v e l y simple. However, a complete s o l u t i o n i s superfluous since we are p r i m a r i l y i n t e r e s t e d i n mass transport at Ύ Ύ^Ι t h i s transport can be c a l c u l a t e d i f the f i r s t d e r i v a t i v e of the con c e n t r a t i o n i n the y - d i r e c t i o n i s known. I t i s p o s s i b l e t o o b t a i n t h i s d e r i v a t i v e a n a l y t i c a l l y from equation ( 8 ) ; i f t h i s i s done the r e s u l t i s : 3C 2A — = (—Ô(BC - I n (1 + B C ) ) + i n t e g r a t i o n constant ay 2 The i n t e g r a t i o n constant can be determined w i t h boundary c o n d i t i o n (9); i f C i s the c o n c e n t r a t i o n of reactant at the r e a c t o r w a l l one f i n d s : ~ = (^y(BC - I n (1 + BC) - BC + I n (1 + BC ) ) * (10)

Chemical Reaction Engineering-Houston

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