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Homework Help: Langmuir kinetics problem

  1. Feb 10, 2014 #1
    1. The problem statement, all variables and given/known data
    Adsorption of many solutes is governed by Langmuir kinetics given below.

    Equation 1
    dq/dt=ka (qm−q )c−kdq

    If a batch of adsorbent pellets of mass M is added to a solution of adsorbate, at an initial concentration ofc0 and volume V, the mass balance yields:

    Equation 2
    c= co−(M/V)q

    Substituting (2) into (1) gives:

    Equation 3
    dq/dt = aq2 + bq + f

    a = ka (M/V)

    b = -(ka(M/V)qm + kd + kaco)

    f = kaqm

    Subject to the initial condition that q (t=0 )=0 Solve Equation (3) by simple quadrature (integration). To do this, first separate the variables q and c and integrate analytically.

    The long time solution of Equation (1) should give the Langmuir isotherm.
    q*= (qm(ka/kd)c) / ( 1 + ( ka/kd)c )

    3. The attempt at a solution

    So after plugging in all the data, to isolate q and c I subtracted kaqm and divided by q yielding:

    (-kaqm / q) = (ka(M/V)q) - ka(M/V)qm - kd - kaco

    Then I added kd to both sides in the hopes to simplify some of the data into the variable c. Not really sure what to do next or if I'm even headed in the right direction. Any thoughts or help would be really appreciated
  2. jcsd
  3. Feb 10, 2014 #2
    Seems confusing indeed, since apparently c and q are functions of each other... are you sure you're not asked to separate q and t? because they appear to be the only variables and then equation 3 is integrated easily:
    dq/(aq^2 + bq + f) = dt,
    then complete the square on the left and change variable, integrate both sides. You can plug back the constants at the end if you like...
  4. Feb 10, 2014 #3
    I appreciate the help, I'm going to run with what you've presented since my professor really doesn't have it together

    So as far as completing the square I've got it to (q+(bq/4a))^2 = -f (bq/4a)^2 and it looks like it's about to get ugly, so I feel I'm on the wrong track. Does it look right so far? It's been ages since I've done any calculus, so my next question would be with what do I change the variable with?
  5. Feb 10, 2014 #4
    If you factor out the "a" and take it to the right, you get:
    dq/[(q + b/2a)^2 – (b/2a)^2 + f/a] = a•dt
    So with x = q + b/2a, dx = dq and g^2 = f/a – (b/2a)^2
    your equation has the form: dx/(x^2 + g^2) = a•dt,
    which you might find familiar. if you don't, just look up the integral (the right one is trivial of course), it's a very common identity. You may even look up the initial one, i don't know if you're supposed to carry the whole integration...
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