2019-反应工程基础(程易)chpt14-fixedbedreactor-r2-206808519-PPT文档资料-文档资料

ddPz0 PP0 TT0 FFTT0
ρb= bulk density ρc= solid catalyst density = porosity (void fraction)
W 1 A czc
yP/ P0

2 0
Acc1P0
dy T FT
Effective temperature control of large fixed beds can be difficult because such systems are characterized by a low heat conductivity. If operations are to be restricted within a narrow temperature range, then the fluidized bed is favored.
14.2 Pressure Drop in the Packed Bed Reactor
In liquid phase, effect of pressure drop can be ignored.
In gas phase, for an ideal gas, the concentration of reacting species i is
dW 2yT0 FT0
17
Pressure drop in reactor
dy T FT
dW 2yT0 FT0
This Eqn. is for multiple reactions and reaction in a membrane reactor.
For single reaction:

rA'
Rate law, rA' k'CA2
Stoichiometry,
CA
CA0
1X
1X
P P0
Combine,
d d W Xk F C A A 2 0 0 1 1 X X 2 P P 0 2k C 0 A 0 1 1 X X 2 P P 0 2
3. For large values of the Thiele modulus for an n order reaction,

21/2
n1
3
21/23
n n1 R
D kn eCA (s-1n)/2
4. For internal diffusion control, the me reaction order is related to the measured reaction order by
Ci F i F A00(( 1 i vX iX ))P P 0T T0
where
i

Fi0 FA0
,
=yA0δ
δ=C hangesintotalnum berofm oles M oleofA reacted
For isothermal operation:
Ci CA01ivXiXPP0
5
14.1 Types of the fixed bed reactor
6
7
Fixed-bed reactors for methanol synthesis
多段冷激式反应器
列管式合成反应器
径向反应器
8
Fluidized bed reactor
9
10
Selection of the reactor type
G u i s t h e s u p e r f i c i a lm a s s v e l o c i t y 16
Ergun Equation
Giv 0 e G 0g n 1 cD P 3 15 1 D P 0 1 .7G 5
4
Contents
Introduction of fixed-bed reactor Adiabatic fixed bed reactor Fixed-bed reactor with heating or cooling Model of the fixed-bed reactor
Fixed beds cannot use very small sizes of catalyst because of plugging and high-pressure drop, whereas fluidized beds are well able to use small-size particles.
The Weisz-Prater criterion dictates that:
If CWP<<1, no internal diffusion limitations present If CWP>>1, internal diffusion limitations present
6. Mears Criteria for Neglecting External Diffusion and Heat Transfer
ntrue 2nappare1nt
The true and apparent activation energies are related by
Etrue2Eapp
2
Summary (cont’d)
5. The Weisz-Prater Parameter
CWP12 rA D ' oe,C bRsA2sc
Characters of the fixed-bed reactor
In passing through fixed beds, gases approximate plug flow. If efficient contacting in a reactor is of primary importance, then the fixed bed is favored.
0 X W(1W)
k'CA0 (1X)
2
21
Solving for conversion gives:
k 'C A 0 (1 W )
X
1
0
k 'C A 0W
2
(1 W
where 1 is the Thiele modulus. For a first-order
reaction

2 1

k1R 2 De
2. The internal effectiveness factor and overall
effectiveness factor.
1
Summary (cont’d)
dy dW 2 y
y

P
1
(1W)2
P0
18
Reaction with pressure drop
0 PP0(1W)1/2 P (a)
W
X
(d) W
CA
(b) W
No P P
No P P
1.0
P CA CA0(1X) P0
rA
(c)
W
P
No P P
There is no catalyst attrition in the fixed-bed reactor.
12
Temperature profile of the fixed bed reactor Hot spot
Exothermic reaction
13Biblioteka See Section 4.5
14
Analyze the following second order gas phase reaction that occurs isothermally in a PBR
2A → B+C
Mole balance (must use differential form),
FA0
dX dW
0
PT0 P0 T

FT0 FT
d d P z 0 g G C D P 1 3 11 D 5 P0 1 .7 G 5 P P 0 T T 0 F F T T 0
Need to find (P/P0) as a function of W (or V if you have a PFR).
15
Pressure drop in a packed bed
P/P0 = f(Volume, V) or f(catalyst weight, W) Majority gas phase reactions happened in a packed
rA' Rbn 0.15
kcCA b
HrxnrA ' bRE0.15 hR gTb2 3
Chapter 14 Fixed-Bed Reactor
Department of Chemical Engineering
Tiefeng Wang

Reactor type preferred as a function of on stream time of the catalyst between two regenerations (van Swaaij et al., Chem. Eng. J., 2019, 90: 25–45) 11
FT 1 X
FT 0
NTf NT0
NT0

20
Acc 1P0
Changeintotalnum berofm olesforcom pleteconversion Total m oles fed