modelling water scrubbing biogas based technology

Modelling Water Scrubbing Biogas Based Technology

William Nock, Sonia Heaven, Charles Banks

Faculty of Engineering and the Environment, University of Southampton

w.j.nock@soton.ac.uk

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

Thank you for listening