Diffusivity of Water in Air at 1 atmosphere and 20C. A = and B = Air.

Diffusivity equation. Assuming nonpolar and nonreacting molecules

Need and , From Table K.2 of the appendix, values are obtained for water and Air

Water: and

Air: and

Now I need , which can be found on Table K.1. To find , need to calculate :

T = 20 + 273 = 293 K

P = 1 atm

For Table K. 1, need the inverse

Must interpolate:

Now the diffusivity equation and diffusion of water vapor in air at 20C and 1 atm

Compare to the experimental value from Table J.1 of book

at T = 298 K

Need to find the experimental diffusivity value at pressure of 1 atm and 298 K

Now, we will use an equation to predict the experimental diffusivity at 20C. The following equation can be used for pressures below 25 atmospheres

Need

No need to interpolate Table K.1 since 1.60

Diffusion coefficient prediction

Percent difference

Percent Difference =

There is nice agreement between calculated and predicted experimental water in air diffusivity values. The final diffusivity values for water in air at 20C and 1 atm are

and

**Summary**

Nonpolar Hirschfelder, Bird, and Spotz Equation

Experimental Value from Table J.1

Predicted Value

**References:**

Welty, James R.; Wicks, Charles E.; Wilson, Robert E. (1984) Fundamentals of Momentum, Heat, and Mass Transfer, third edition. New York: John Wiley & Sons.

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thanks a lot , i was looking for the water diffusivity to calculatr the flux across membrane distillation

Hello,

First, thanks for the comment.

Yeah, I rely upon books for most of my examples.

I read a journal article about the evaporation rate of sarin equaling water, but the divergent boiling points of each caused me to disagree. As such, I wanted to do a molar flux calculation that compared sarin to water. Although not a perfect calculation, the molar flux calculation verified the vapor pressure ratio, and each suggested that there was a difference in evaporation rates.

As you know, chemical engineering is quite complicated, and I graduated in 1999! 🙂 As such, my ability to do complex calculations is dependent upon much review. In this case, I had to do a basic review of diffusivity to eventually perform a molar flux calculation.

Have a nice day.