Modelling Water Scrubbing Biogas Based Technology
William Nock, Sonia Heaven, Charles Banks
Faculty of Engineering and the Environment, University of Southampton
Promotion of Biogas Upgrading and Bottling in India and European Union
Promotion of Biogas Upgrading and Bottling in India and European Union
Why do we need a model?• Description of real life situations• Tool to help design and operate a system• Provide predictions on how a system will
behave under different conditions• “Everything should be made as simple as
possible, but not simpler”- Albert Einstein
Promotion of Biogas Upgrading and Bottling in India and European Union
Modelling Approach
EquilibriumBased Models
RateBased Models
• 1D, 2D or 3D• Steady state or dynamic model• CFD
• Empirical design•HETP and HTU/NTU
Promotion of Biogas Upgrading and Bottling in India and European Union
•Well mixed system with negligible directional concentration gradients• Ideal Gas Law and ideal liquid flow• Isothermal reactor•Chemical reaction•Which species should mass transfer consider•Mass transfer liquid or gas side dominant?
Modelling Assumptions
Promotion of Biogas Upgrading and Bottling in India and European Union
Model DerivationLIN
LOUT
GOUT
GIN
Δz
QG, Z
cG, Z
QG, Z+ΔZ
cG, Z+ΔZ
QL, Z+ΔZ
cL, Z+ΔZ
QL, Z
cL, Z
QG, Z+ΔZ
cG, Z+ΔZ
cL, Z+ΔZ
QL, Z
cL, Z
• Mass Balance:
• No axial mixing• Perfect radial mixing
Promotion of Biogas Upgrading and Bottling in India and European Union
Plug Flow Reactor
Δz
QG, Z
cG, Z
QL, Z+ΔZ
QG, Z+ΔZ
cG, Z+ΔZ
cL, Z+ΔZ
QL, Z
cL, Z
z+Δz
zAccumulation = input – output – reaction
𝜕𝑐𝐴
𝑑𝑡 =−𝜕 (𝑢𝑐𝐴 )𝜕 𝑧 +𝑟 𝐴
Promotion of Biogas Upgrading and Bottling in India and European Union
Axial Dispersion Model
Δz
QG, Z
cG, Z
QL, Z+ΔZ
QG, Z+ΔZ
cG, Z+ΔZ
cL, Z+ΔZ
QL, Z
cL, Z• Mass Balance:
• Considers back mixing• Peclet number
Accumulation = input – output – reaction
Promotion of Biogas Upgrading and Bottling in India and European Union
Boundary ConditionsClosed - ClosedOpen -
Open
Promotion of Biogas Upgrading and Bottling in India and European Union
Mass Transfer Theory• Two – Film Theory
• Film thickness• Proportional to
diffusion coefficient
Interface
BulkGas
Bulk Liquid
Liquid Film
Gas Film
cGcGi
cLi
cL
Promotion of Biogas Upgrading and Bottling in India and European Union
Mass Transfer Theory• Surface Renewal
Theory
• Contact time ‘packets’ in contact with interface
• Proportional to square root of diffusion coefficient
cG
Interface
BulkGas (cG)
Bulk Liquid(cL)
Liquid Film
Gas Film
cL
Promotion of Biogas Upgrading and Bottling in India and European Union
Empirical Correlations• Henry’s Law• Mass transfer coefficients• Interfacial area• Pressure drop• Gas/ liquid holdup
Promotion of Biogas Upgrading and Bottling in India and European Union
Finite Difference• Numerical solution to ode and pde’s.• Explicit or implicit schemes• Discretisation techniques• Upwind scheme:
• Multiphase counter-current flow• Gauss Seidel iteration
Promotion of Biogas Upgrading and Bottling in India and European Union
Finite Difference
Promotion of Biogas Upgrading and Bottling in India and European Union
Summary• Simple models often out perform more
detailed models• Assumptions need to be taken with care
Promotion of Biogas Upgrading and Bottling in India and European Union
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