Chemical Process Mass Transfer Principles Explained
Chemical Process Mass Transfer Principles
Mass transfer in chemical technology processes can be categorized:
- With exchange of several different components.
Absorption Equilibrium State
In the theoretical absorption equilibrium state between contacted liquid and gas phases:
- The same mass streams of substance(s) are transported to and from each of the phases.
System Dynamics
A system arranged to be in a nonequilibrium state naturally strives for:
- Reaching the equilibrium state corresponding to present conditions.
Theoretical Diffusion and Convection
A pure theoretical diffusion process:
- Can be met in a forced laminar flow mode.
Convection processes in mass transfer occur when:
- The fluid is artificially agitated, pumped in turbulent regime, or mixed.
Unsteady-State Diffusion
Unsteady-state diffusion is a phenomenon characterized by:
- Continuous stochastic changes in concentration space.
Kinematic Diffusivity and Gas Phase Transfer
The kinematic diffusivity parameter value:
- Depends on the process thermal conditions.
Mass transfer in a gas phase is considered to be a combined process, coupling:
- Transport of the component from the bulk of the gas phase to the boundary laminar layer by convection.
- Further transport through this layer to the interface by diffusion.
Mass Transfer Models and Theories
The theoretical mass transfer model based on the assumption that turbulent eddies formed within the fluid are so intensive that they prevent the formation of a stagnant boundary film layer at the interface is termed the surface renewal model theory (Danckwerts approach).
Gas bubbling through a liquid layer in chemical technology processes is an example of:
- Interphase mass transfer in unforced flow mode.
Resistance and Flooding
The mass transfer resistance in one phase during chemical technology processes design may be neglected when:
- Pure component is provided on one side of the interface.
The hydraulic phenomenon called “flooding of packed tower” is a consequence of:
- Matching of a too large stream of liquid in respect to a fixed value of gas stream.
Multicomponent Mixtures and Driving Forces
In case of a multicomponent gas mixture in a given volume of 1 dm³ of homogenous composition, stagnant, mass transfer intensity will be: 0.
When the concentration of some component A in the gas phase is higher than the equilibrium concentration (in respect to liquid phase concentration), one can expect:
- Mass transfer to the liquid phase.
Packing introduced into the mass exchanger is responsible for:
- Increase in turbulence in both phases, thus premixing.
In absorption processes, the equilibrium state is defined as the composition in respect to both contacting phases which is self-established when the system is in:
- A dynamic state where both mass streams are equal, thus their net effect is 0.
The models which assume that turbulent eddies formed inside the fluid are so intensive that they prevent the formation of a stagnant film at the interface can be represented by:
- Penetration models.
The driving forces of the interphase mass transfer which occur between both phases can be estimated using:
- The operating line together with the equilibrium curve.
The mass transfer models in unforced flow do not cover:
- Turbulent forced flow.
The driving force along the mass exchanger is:
- Usually with a decreasing tendency.