World Library  
Flag as Inappropriate
Email this Article
 

Damköhler numbers

The Damköhler numbers (Da) are dimensionless numbers used in chemical engineering to relate the chemical reaction timescale (reaction rate) to the transport phenomena rate occurring in a system. It is named after German chemist Gerhard Damköhler.

In its most commonly used form, the Damköhler number relates the reaction timescale to the convection times scale, flow rate, through the reactor for continuous or semibatch chemical processes:

\mathrm{Da} = \frac{ \text{reaction rate} }{ \text{convective mass transport rate} }

In reacting systems that include interphase mass transport, the second Damköhler number (DaII) is defined as the ratio of the chemical reaction rate to the mass transfer rate

\mathrm{Da}_{\mathrm{II}} = \frac{ \text{reaction rate} }{ \text{diffusive mass transfer rate} }

It is also defined as the ratio of the characteristic fluidic and chemical time scales:

\mathrm{Da} = \frac{ \text{flow time scale} }{ \text{chemical time scale} }


Since the reaction timescale is determined by the reaction rate, the exact formula for the Damköhler number varies according to the raw law equation. For a general chemical reaction A → B of nth order, the Damköhler number for a convective flow system is defined as:

\mathrm{Da} = k C_0^{\ n-1}\tau

where:

On the other hand, the second Damköhler number is defined as:

\mathrm{Da}_{\mathrm{II}} = \frac{k C_0^{n-1}}{k_g a}

where

  • kg is the global mass transport coefficient
  • a is the interfacial area

The value of Da provides a quick estimate of the degree of conversion that can be achieved. As a rule of thumb, when Da is less than 0.1 a conversion of less than 10% is achieved,and when Da is greater than 10 a conversion of more than 90% is expected.[1]

The Karlovitz number is related to the Damköhler number by Da=1/Ka.


References

  1. ^ Fogler, Scott (2006). Elements of Chemical Reaction Engineering (4th ed.). Upper Saddle River, NJ: Pearson Education.  
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.