1. Importance Of Transport Phenomena?
Ans. To understand the importance of transport phenomena in chemical engineering we
need to understand what is transport phenomena first.
What is transport phenomena
The subject of transport phenomena deals with the transport of mass, momentum, energy, and other
entities. These various phenomena can be examined at three different levels:
the molecular level, where one describes the viscosity, thermal conductivity, and diffusivities
of macroscopic materials in terms of models of the constituent molecules.
the continuum level, where one ignores the molecular motions and focuses on the partial
differential equations (the "equations of change") which describe the profiles of velocity,
temperature, and concentration.
the equipment level, in which one is concerned mainly with relations among input and
output quantities for some piece of equipment or a portion thereof.
Importance of transport phenomena in a Chemical Engineering
These three approaches are intertwined inexorably, and the chemical engineer can be called upon to
use results from all three.
In order to put the role of transport phenomena in chemical engineering into perspective, consider
the diagram.
The chemical reactor is shown as a focal point of the field of chemical engineering. Surrounding it
are the various engineering subjects which are connected intimately with reactor engineering:
separation and mixing processes, heat transfer operations, catalysis, fluid and particle dynamics,
instrumentation and control, and materials of construction. These subjects are clearly essential to
the design and operation of a chemical reactor.
In the next layer of subjects we list the "engineering sciences" which are needed in various ways for
understanding and further developing the core engineering subjects: thermodynamics, chemical
kinetics, electrochemical phenomena, and transport phenomena. These engineering sciences, which
are themselves interrelated, form the basis for the analytical and numerical description of the
chemical reactor and its peripheral equipment. For example, the subject of transport phenomena can
be used to analyze diffusion-controlled reactions, separation schemes, transient processes in
reactors, thermal processes, flow patterns in reacting systems, corrosion, diffusion in porous media,
and other problems connected with reactor engineering.
Transport Phenomena in other fields
Transport phenomena occur in many other fields: acoustics, zoology, micrometeorology, plasma
physics, combustion, nuclear engineering, fermentation, biomedical engineering, electrochemistry,
soil physics, ocean engineering, atmospheric pollution, pharmacology, and polymer processing. In
each of these fields the equations of change (i.e., the equations of continuity, motion, and energy)
can form the starting point for the description, organization, and systematization of substantial parts
of the subject material. The knowledge which a chemical engineering student acquires about the
solution of problems in transport phenomena is thus easily transferable to other fields.
It can, of course, be argued that the subject of transport phenomena is really nothing more than a
grouping together of three well-known subjects: fluid dynamics, heat transfer, and diffusion.
Bringing the subjects together into one subject is, however, advantageous for several reasons:
In nature, in biological systems, and in the chemical industry, the three phenomena often
occur simultaneously.
The mathematical descriptions of the three phenomena are related closely and hence
considerable use may be made of analogies among the various phenomena.
There are also important differences among the three fields, and these can be emphasized
when the subject material is juxtaposed. A person who has studied transport phenomena
from this unified viewpoint is in an excellent position to proceed to the study of special
treatises and advanced texts on fluid dynamics, heat transfer, and diffusion as well as
rheology, electrochemistry, acoustics, combustion, turbulence, boundary layer theory, and a
host of other related fields.
2. Analogies Between Momentum, Heat & mass Transfer?
Ans. 3 different laws define mass , heat and momentum analogies .fick's law for mass
transfer, fourier's law, and Newton's law of viscosity
According to Newton's law shear stress is directly proportional to velocity gradient in direction
perpendicular to the motion of fluid likewise according to fick's law ,rate of mass flux is directly
proportional to concentration gradient and heat transfer flux is directly proportional to temperature
gradient by fourier's law .In all these law , mass heat and momentum is transferred in terms of flux
and proportional to respective gradients
shear stress = u* ( dv/dy)
mass flux rate J = D*(dc/dy )
heat transfer flux Q= k (dt/dy)
.
where u=viscosity of fluid , D =diffusivity,k= thermal conductivity representing here constants of
respective law .
for momentum,heat,mass transfer there are few dimensionless no describes nature of flow
like , Reynold's no,, nusselt no, prandtle no , schimdt no, stanton no , sherwood no which are
anologous .
Sherwood no for mass transfer quantity is analogous to nusselt no (for momentum and
heat ) .
schimdt no (for mass transfer ) is analogous to prandtle no ( for momentum and heat )
