reacting solids i- overview prof. p. canu university of padova - italy university of liège -...
DESCRIPTION
Reactions Tentative classification Wen, IEC, 1968 to be updated with subsequent process (MEMS,..) P. Canu – Reacting Solids Liège, Sept. 2011TRANSCRIPT
Reacting SolidsI- Overview
Prof. P. CanuUniversity of Padova - Italy
University of Liège - Laboratoire de Génie chimiqueSeptember 2011
Occurrence
More often than expected → need for a unique approach
P. Canu – Reacting Solids Liège, Sept. 2011
L.D. Schmidt, The engineering of chemical reactions
ReactionsTentative classification
Wen, IEC, 1968to be updated with subsequent process (MEMS,..)
P. Canu – Reacting Solids Liège, Sept. 2011
ReactionsA) Solid → Fluid
• pyrolysis of carbonaceous materials• combustion of double-base propellants• thermal decomposition (explosions) of some organic or inorganic
compound, especially explosives, e.g.
Solid can decompose gradually from the outer surface to its center, giving off fluid products.
At T>>Tdecomp → reaction may occur at the surface as well as inside the solid.
P. Canu – Reacting Solids Liège, Sept. 2011
Reactions B) Solid → Fluid and Solids
Typical: pyrolysis and thermal decomposition of organic and inorganic solid materials.
• pyrolysis of carbonaceous materials• calcination of carbonates• dehydration of hydroxides and hydrates• removal of crystalline water from crystalline compounds
P. Canu – Reacting Solids Liège, Sept. 2011
Reactions C) Fluid and Solid → Fluids
• combustions and gasifications of carbonaceous materials • oxidation of other solid compounds• solids (metals) and ions in aqueous solutions• reaction in ion-exchange resins
P. Canu – Reacting Solids Liège, Sept. 2011
ReactionsD) Fluid and Solid → Solids
• nitrogenation of calcium carbide to produce cyanamide:
• rusting reaction of metals, e.g.
• chemisorptions of gas or liquid on solid adsorbents
P. Canu – Reacting Solids Liège, Sept. 2011
ReactionsE) Fluid and Solid → Fluid and Solid
Quite general:
• Calcination of sulfides to make oxides• reductions of metal oxides• steam-iron process to produce hydrogen
P. Canu – Reacting Solids Liège, Sept. 2011
Reactionsin general
a F1 + b S1 → d F2 + e S2
Each species can be present or not
Stoichiometry is relevant (also for volumetric effects)
P. Canu – Reacting Solids Liège, Sept. 2011
KineticsMechanism of local interactions between fluids and solids
SiH4(g) + Si(s) → 2H2(g) + Si(s) + Si(b)
g = in the gas, in front of the surfaces = adsorbedb = in the bulk of the solid
P. Canu – Reacting Solids Liège, Sept. 2011
Mass (& heat) transfer
additional processes, before and after reaction(heterogeneous + homogeneous reactions)
P. Canu – Reacting Solids Liège, Sept. 2011
Mass transfer
Species:
• in the fluid, far away from the surface (bulk)
• in the fluid, ‘in front’ of the surface
• on the surface (adsorbed)
• in the solid (bulk)
P. Canu – Reacting Solids Liège, Sept. 2011
KineticsSimplified view – lev. 0
A(g/l) + b B (s) → ….
1. one global reaction
2. irreversible
3. no adsorption (or Henry type)
R” = superficial reaction rate = k”(T) CB” CA≈ k’’’(T) CA
“ = per unit surface (e.g. 1/cm2)
CA = volumetric concentration in the fluid, in front of the surfaceP. Canu – Reacting Solids Liège, Sept.
2011
KineticsSimplified view – lev. 1
A(g/l) + b B (s) → ….
1. one global reaction
2. irreversible
3. adsorption equilibrium (Hinshelwood type, single specie ads.)
R” = superficial reaction rate ≈
Not pseudo-1st order anymore!
AA
A
CKCk
1'''
P. Canu – Reacting Solids Liège, Sept. 2011
KineticsDetailed approach - adsorption mechanisms
AsH3(g) + Ga(s) → AsH3(s) + Ga(b) AsH3(g) + O(s) → AsH3(s)
Atomic Site Open Site the reaction conserves sites reaction conserves sites and
elements
P. Canu – Reacting Solids Liège, Sept. 2011
Kinetics Detailed approach - surface-reaction mechanisms
It can be complex, if detailed
P. Canu – Reacting Solids Liège, Sept. 2011
Solids geometryClassification
1. Irregular (grit, crystals, flocs, …)
2. Films/slabs
3. Particles
4. Cylinders, pillars, extrudates,..
Approximation to the simplest, more regular shape
P. Canu – Reacting Solids Liège, Sept. 2011
Solids geometryParticles?
1. Shape
2. Size
Size and/or Shape distributions(→ need for Population Balance Equations)
P. Canu – Reacting Solids Liège, Sept. 2011
Solids geometryEvolution in size
1. Dissolving (→) / growing (←) film l(t)
2. Dissolving (→) / growing (←) particle r(t)
Not all the reacting solids change their size (→ r(t) )P. Canu – Reacting Solids Liège, Sept.
2011
Solids geometryInternal structure (porosity) – spherical particles
S1 (black) transforms in S2 (colorless)
• sharp reaction front?• r1 and r2 are equal?
P. Canu – Reacting Solids Liège, Sept. 2011
Solids geometryInternal structure (porosity)
Reaction takes place within the porous solid
P. Canu – Reacting Solids Liège, Sept. 2011
Solids geometryInternal structure (porosity) - film
S1 (black) transforms in S2 (colorless)
Reaction front? Changes in volume?
P. Canu – Reacting Solids Liège, Sept. 2011
Solids porosity
impervious ← actual solids (porous) → perfectly permeable
easy difficult easy
P. Canu – Reacting Solids Liège, Sept. 2011
MicrostructureGrain model
Solids are composites (with internal grains)
Concentrations vary within each grain and across the grains composite
→ particle model
P. Canu – Reacting Solids Liège, Sept. 2011
ReactorsContacting mode
For each phase:
1. mixing/segregation?2. in-/outflow?
P. Canu – Reacting Solids Liège, Sept. 2011
Conclusions
1. Reactive solids are pervasive and growing
2. The field is even broader than expected,
3. A unified approach is sought
4. Porosity (internal structure) the keypoint
P. Canu – Reacting Solids Liège, Sept. 2011
References
1. L.D. Schmidt, The Engineering of Chemical Reactions, Oxford Univ. Press, 2° Ed.
2. O. Levenspiel, Chemical Reaction Engineering, 3° Ed, Wiley, 1999
3. J. Szekely, J. W. Evans, and Hong Yong Sohn, Gas-solid reactions Academic Press, 1976.
4. Wen, C. Y., Ind. Eng. Chem., 60 (9), 34 (1968).
P. Canu – Reacting Solids Liège, Sept. 2011
Reacting SolidsII – quantitative analysis
Prof. P. CanuUniversity of Padova - Italy
University of Liège - Laboratoire de Génie chimiqueSeptember 2011
Reacting SolidsIII – application of SCM
Prof. P. CanuUniversity of Padova - Italy
University of Liège - Laboratoire de Génie chimiqueSeptember 2011
The application chemistry
Direct reduction
3 heterogeneous reactions like
a F1 + b S1 → d F2 + e S2
F1= H2 and/or CO F2= H2O and/or CO2
P. Canu – Reacting Solids Liège, Sept. 2011
The application chemistry
Homogeneous reactions can occur
H2O + CO = CO2 + H2 WGS
CH4 + H2O = CO + 3H2 SR/Methanation
CH4 = Cs + 2H2 Cracking
P. Canu – Reacting Solids Liège, Sept. 2011
The application pellet model
4 domains, 3 interfaces → SCM(frequently reduced to 3 of even 2 domains )
P. Canu – Reacting Solids Liège, Sept. 2011
The application pellet model
Assumptions (critical) in SCM approach• Hematite is impervious
• Same diffusion rate in each solid phase, constant in time
• Constant porosity in each layer
Advantages of SCM• At any time the state of the pellet
is summarized by the interface coordinates
4 domains, 3 interfaces → SCM(frequently reduced to 3 of even 2 domains )
P. Canu – Reacting Solids Liège, Sept. 2011
The application Reactor model
A. Gas is always flowing through a packed bed of solids
B. Solids can be:• At rest (batch) → test apparatus• ‘Flowing ‘ → Shaft
P. Canu – Reacting Solids Liège, Sept. 2011
The application Test apparatus
A basket suspended on loading cells(reaction looses weight significantly)
Approx dimensions D = 6 cm, L = 10 cmpellet diameter: dp = 13 mm (=0.42)
rs = 3.4 t/m3sol
P. Canu – Reacting Solids Liège, Sept. 2011
The application Test apparatus
porous bed at rest (Brinkman-type momentum equations)
u = surface (or apparent) velocity = bed porosityQ = (gas) mass production (H2 → H2O CO → CO2)k = permeability (from Ergun eq. - viscous and inertial terms)
P. Canu – Reacting Solids Liège, Sept. 2011
The application Test apparatus
Maxwell-Stefan & T diffusion in (conservative) MBi (i=H2, CO, H2O, CO2, CH4, N2)
iiT
k kkkikii rw
TTD
ppwxxDw
tw
rrr u
DT [x 107 m2/s] == [-37.2 28.1 0.4 7.8 0.8]
@ T =100K and w=w°
2
4
2
2
2
24222
24
0.25.14.28.16.67.15.20.23.6
9.15.18.53.21.8
5.6
10
NCHCO
OHCOH
NCHCOOHCOH
smikD
P. Canu – Reacting Solids Liège, Sept. 2011
The application Test apparatus
Initial contitions (t=0):Solids
T=800°Cc°= pure, dry, Hematite
Gas inside T=800°Cx°= H2 CO H2O CO2 CH4 N2
= [70 20 2 0.6 0 7]%
Gas INT=825°C+f(t)x°, v°
P. Canu – Reacting Solids Liège, Sept. 2011
The application test apparatus:
Weight loss
Some tuning of the kinetics is required
0 50 100 150
-500
-450
-400
-350
-300
-250
-200
-150
-100
-50
0
DP_exp
DP_calc
t (min)
Wei
ght l
oss (
g)
0 50 100 150 200 250 300 350 400 450 5000
10
20
30
40
50
60
70
80
|WL(g)|
Met
alliz
zatio
n %
P. Canu – Reacting Solids Liège, Sept. 2011
The application test apparatus:
Temperature along the bed
Only qualitative agreement (but TIN was varying)
P. Canu – Reacting Solids Liège, Sept. 2011
The application test apparatus: gas phase composition
Beginning: H2 reactivity largely under predicted (see also H2O)CO reactivity quite under predicted (see also CO2)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 50 100 150 200
Perc
entu
ale
volu
met
rica
(mol
are)
t (min)
H2
CO
H2O
H2_CFD
CO_CFD
H2O_CFD
H2
CO
H2O
P. Canu – Reacting Solids Liège, Sept. 2011
The application test apparatus: gas phase composition
• CO2 instantaneous production well described; long term reactivity overestimated
• no CH4 prediction (lack of methanation reaction )
0%
2%
4%
6%
8%
10%
12%
14%
0 50 100 150 200
Perc
entu
ale
volu
met
rica
(mol
are)
N2
CH4
CO2
N2_CFD
CH4_CFD
CO2_CFD
N2
CO2
CH4
P. Canu – Reacting Solids Liège, Sept. 2011
The application test apparatus
Convenient set-up for tuning kinetics
P. Canu – Reacting Solids Liège, Sept. 2011
The application shaft (industrial)
Two critical issues:
1. Solids flow2. Solids reactivity
SOLIDS
P. Canu – Reacting Solids Liège, Sept. 2011
The application Solids flow
How does a dense bed of particles move?Quite scarce theories/models!
Our pseudo-thermal (Tg) model
1. solids in a drum
2. flow down the shaft
P. Canu – Reacting Solids Liège, Sept. 2011
The application Solids flow
Steady-state solids flow and porosity in the shaft(Artoni, Santomaso, Canu, PRE &CES)
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas configuration -1
SOLIDS
REDUCING GAS
SOLIDS
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas velocity in the bed
• Compares well with experimental average in the upper part
• Stagnation in the bottom
EXP
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas composition (mass. frac.)
0 - 0.128
0 - 0.494
H2
CO CH4
0 - 0.108
H2O
0.019 - 0.570
CO2
0.212 - 0.445P. Canu – Reacting Solids Liège, Sept.
2011
The application Gas composition
Compares well with expected results
Species x_calc (%) x_exp (%)
H2 51 48
CO 14 15
H2O 19
CO2 9
CH4 5
N2 2
P. Canu – Reacting Solids Liège, Sept. 2011
The application Solids composition (kmol/m3
sol)
Hem
0 - 33
Wus
0 - 4418
Fe
0 - 50
C(s)
0 - 4 0 – 75 %
metallization
P. Canu – Reacting Solids Liège, Sept. 2011
The application Temperature (K)
On the axis:model lacks cooling in the bottom
Tgas Tsolid
300 - 1350
700
300
0 5 10 15 20 25 30 35 40800
850
900
950
1000
1050
1100
1150
1200
1250 TS exp
TS calc
Heigth (m)
T so
lid (K
)
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas configuration – 2
Cooling gas from bottom
SOLIDS
REDUCING GAS
SOLIDSCOOLING GAS
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas velocity in the bed
Non more stagnation in the bottom
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas composition (mass. frac.)
0 - 0.127
0 - 0.487
H2
CO CH40 - 0.585
H2O0 - 0.662
CO20- 0.327
P. Canu – Reacting Solids Liège, Sept. 2011
The application Gas composition
Similarly to case 1, compares well with expected results
Specie x_calc (%) x_exp (%)
H2 50 48
CO 14 15
H2O 19
CO2 9
CH4 5
N2 3
P. Canu – Reacting Solids Liège, Sept. 2011
The application Solids composition (kmol/m3
sol)
metallizationHem
0 - 33
Wus
0 - 4412
Fe
0 - 56
C(s)
0 - 7 0 – 84 %
P. Canu – Reacting Solids Liège, Sept. 2011
The application Temperature (K)
On the axis:evident cooling in the bottom
0 5 10 15 20 25 30 35 40800
850
900
950
1000
1050
1100
1150
1200
1250 TS expTS calc
Height (m)
T so
lid (K
)
300 - 1350
310
770
300
740
P. Canu – Reacting Solids Liège, Sept. 2011
Tgas Tsolid
Conclusions
1. SCM allows simulating complex configuration
2. It allows interfacing with a CFD code (scalars=interface positions, need to be tracked)
3. Though instrinsically approximated/wrong, it can be tuned to experimental data
4. Need for more realistic pellet models
P. Canu – Reacting Solids Liège, Sept. 2011
Reacting SolidsIV –reacting porous solids
Prof. P. CanuUniversity of Padova - Italy
University of Liège - Laboratoire de Génie chimiqueSeptember 2011
The physical picturesolid conversion
Issues• Reaction across the solid• Diffused interface• Variable volume
P. Canu – Reacting Solids Liège, Sept. 2011
The physical pictureApproches
→ Volumetric reaction models
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel eqs.
Gas (c)
PSSA and equimolarity (or large volum. flow rate) reduce it to
Solid (c’)
u accounts for shrinking/enlargment
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel eqs.
Effective diffusivities Di,eff = f(Di , )
Local variation of porosityai = aio xi
bi
ai = surface of i-solid/volume
bi = sintering exponentSome information from BET measurements
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Coupled PDEs
Unknown functions: c(t,r) c’(t,r)
• MOL (discretize on r, integrate on t)• (othogonal) collocations• Finite differences• …
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Discretization in space (100 grid points) - Integration in time
Profiles(t) at literature parameters
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Average mass fraction in pellet
0 500 1000 1500 2000 2500 3000 35000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
t (s)
X
Fe2O3
Fe3O4
FeOFe
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Hematite → Magnetite
Fast transients in time; never sharp in space
SCM?
00.1
0.20.3
0.40.5
0.6
0500
10001500
20002500
30003500
0
0.005
0.01
0.015
0.02
0.025
0.03
r (cm)
t (s)
c Hem
at (m
ol/c
m3)
0
0.2
0.4
0.6
0.8
0 5001000 1500 2000
2500 3000 35004000
0
0.005
0.01
0.015
r (cm)
t (s)
c Mag
n (mol
/cm
3)
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Wustite → Iron
Slow kinetics; even smoother in space
SCM?
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0500
10001500
20002500
30003500
4000
-0.01
0
0.01
0.02
0.03
0.04
0.05
r (cm)
t (s)
c Fe (m
ol/c
m3)
0
0.2
0.4
0.6
0.8
0 500 1000 1500 2000 2500 3000 3500 4000
-0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
r (cm)
t (s)
c Wus
(mol
/cm
3)
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Hematite → Magnetite
Distributed reaction SCM?
0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r (cm)
X Hem
at
0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
0.1
0.2
0.3
0.4
0.5
0.6
0.7
r (cm)X M
agn
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Wustite → Iron
even smoother in space
SCM?
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
r (cm)
X Wus
t
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
r (cm)X Iro
n
P. Canu – Reacting Solids Liège, Sept. 2011
The diffused reactionModel solution
Step-like profiles require much larger reaction/diffusion rates
P. Canu – Reacting Solids Liège, Sept. 2011
Conclusions
1. Diffused Reaction in a solids pellet allows for• Diffused reaction region (instead of sharp interfaces)• Simultaneous diffusion and reaction (instead of sequential)• Local sintering • Size reduction/enlargement• Any reaction rate expression
2. Easily applicable for solids of • a known displacement law• in a constant fluid environment
3. Sintering laws are quite uncertain and difficult to investigate experimentally
P. Canu – Reacting Solids Liège, Sept. 2011
References
1. S.P. Trushenski, K. Li, W.O. Philboork, Metallurgical Transaction , 5, 1149, (1974)
2. Ishida M, and Wen, C. Y., Chem. Eng. Sci., 26, 1031 (1971).
3. O. Levenspiel, Chemical Reaction Engineering, 3° Ed, Wiley, 1999
4. J. Szekely, J. W. Evans, and Hong Yong Sohn, Gas-solid reactions Academic Press, 1976.
5. Wen, C. Y., Ind. Eng. Chem., 60 (9), 34 (1968).
P. Canu – Reacting Solids Liège, Sept. 2011