udpolo15
Well-known member
Suppose I have a influent stream with PCE (isotherm constant 400) and TCE (isotherm constant 100), the carbon will continue to absorb PCE even after the TCE breaks through, right?
Activated Carbon
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Do you have LaGrega? (Hazardous Waste Management) The answer can probably be found in that book.
Sorry, it's not really my field and I'm too busy (or lazy) to crack open LaGrega right now. I did ask a co-worker who has lots of GW remediation experience, and his answer was so full of "depends" that I don't even know what to put down here. I think that he said the activated carbon would continue to absorb the other material, but "it depends" on the concentration in the influent. ?????
Maybe someone else with some experience can chime in here.
Sorry, it's not really my field and I'm too busy (or lazy) to crack open LaGrega right now. I did ask a co-worker who has lots of GW remediation experience, and his answer was so full of "depends" that I don't even know what to put down here. I think that he said the activated carbon would continue to absorb the other material, but "it depends" on the concentration in the influent. ?????
Maybe someone else with some experience can chime in here.
FLBuff PE
Multi-disciplinary engineer
I checked LaGrega on Wednesday, and didn't find much helpful in terms of this problem. The closest that I found was that in anaerobic degradation, PCE degrades to TCE, which then degrades to DCE, which then degrades to vinyl chloride, then to Ethylene (pg. 565). Not exactly helpful here. Sorry I can't be more helpful.
klk
Well-known member
I'm a bit confused what you mean by "isotherm constant". I thought an adsorption isotherm is a line of adsorption density versus concentration at a constant temperature. Do you mean the adsorption coefficient K_ads?
The way I remember it from school, the amount of carbon required to remove a contaminant is based on how much it can adsorb to the particular material (in this case the carbon) AND the concentration of the contaminant in the liquid stream. I found the following equation in Water Chemistry by Mark Benjamin (pg 565) that describes the relationship of competive adsorption, when two compounds, A and B are trying to adsorb into media at the same time:
q_A = ((K_ads,A*{A})/(1+K_ads,A*{A}+K_ads,B*{B}))*q_max
q_A=Adsorption density for A (moles of A adsorbed per gram of adsorbent)
K_ads,A = Adsorption coefficient for A
{A} = activity of A (assuming the reactivity is 1, then {A} is equal to the concentration)
K_ads,B = Adsorption coefficient for B
{B} = activity of B
q_max = maximum adsorption density of the media
Basically, lets assume you are trying to determine how much of A will adsorb to the carbon. If the concentration of B is high and the tendency for B to sorb (K_ads) to the carbon is higher than that for A, the presence of contaminant B will have a larger effect on the removal of contaminant A.
But, if I were to assume that the concentrations of the PCE and TCE were equal, and that the "isotherm constants" you describe above indicate the adsorption capacity of the carbon for those compounds (K_ads), yes, I would agree, that the carbon would probably continue to remove the PCE once you saw breakthrough of the TCE.
Also, you didn't mention whether this was an air stream or liquid stream. I also remember from school that we used partician coefficients to determine removal of compounds in liquid streams. I don't have a good book on this type of material (our "textbook" in the class was a notes packet from the teacher) and all of my class notes are somewhere in my attic not readily accessible.
DISCLAIMER: I don't do this type of work for a living and it's been many, many years since I've looked at this type of problem, so I could be wrong in my 5-min interpretation of my textbook. I didn't study any of this for the PE exam.
The way I remember it from school, the amount of carbon required to remove a contaminant is based on how much it can adsorb to the particular material (in this case the carbon) AND the concentration of the contaminant in the liquid stream. I found the following equation in Water Chemistry by Mark Benjamin (pg 565) that describes the relationship of competive adsorption, when two compounds, A and B are trying to adsorb into media at the same time:
q_A = ((K_ads,A*{A})/(1+K_ads,A*{A}+K_ads,B*{B}))*q_max
q_A=Adsorption density for A (moles of A adsorbed per gram of adsorbent)
K_ads,A = Adsorption coefficient for A
{A} = activity of A (assuming the reactivity is 1, then {A} is equal to the concentration)
K_ads,B = Adsorption coefficient for B
{B} = activity of B
q_max = maximum adsorption density of the media
Basically, lets assume you are trying to determine how much of A will adsorb to the carbon. If the concentration of B is high and the tendency for B to sorb (K_ads) to the carbon is higher than that for A, the presence of contaminant B will have a larger effect on the removal of contaminant A.
But, if I were to assume that the concentrations of the PCE and TCE were equal, and that the "isotherm constants" you describe above indicate the adsorption capacity of the carbon for those compounds (K_ads), yes, I would agree, that the carbon would probably continue to remove the PCE once you saw breakthrough of the TCE.
Also, you didn't mention whether this was an air stream or liquid stream. I also remember from school that we used partician coefficients to determine removal of compounds in liquid streams. I don't have a good book on this type of material (our "textbook" in the class was a notes packet from the teacher) and all of my class notes are somewhere in my attic not readily accessible.
DISCLAIMER: I don't do this type of work for a living and it's been many, many years since I've looked at this type of problem, so I could be wrong in my 5-min interpretation of my textbook. I didn't study any of this for the PE exam.
G
Guest
That's a pretty complicated question ... and you have to consider a few fundamentals about the absorption theory, namely that the adsorption is assumed to be in equilibrium and that the temperature and pressure across the carbon bed affects the absorption based on each volatile species. Generally speaking, the adsorption increases with increasing VOC partial pressure and decreases with increasing temperature.
Since your problem statement doesn't allow one to determine loading, I must agree with the other posters in that you can't extrapolate the mass loading rate based on the information.
If you are looking for a pretty decent document that discusses the theory, check out http://www.epa.gov/ttn/catc/dir1/cs3-1ch1.pdf.
If you have a particular question, please feel free to ask.
JR
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