4 replies to this topic

[peaston]

Hi there, I am currently in the process of designing a chemical reactor for an experiment to be used in the lab, which I am hoping, could be scaled up for large-scale production.

The picture attached demonstrated a batch reactor containing a fixed volume of Reverse Osmosis Retentate (highly concentrated waste seawater).

I have an input stream at the bottom linked to a pressurised canister of carbon dioxide gas and a flow meter of some sort.

The CO2 gas is going to be fed into the batch reactor vessel which will be continuously stirred with the use of an agitator and hopefully micro bubbles of gas will pass up through the liquid and react with the RO retentate - obviously I want the bubbles to be as small as possible so that more gas can react with the solution. Any excess gas should exit the top of the tank and possibly be incorporated back into the cycle if applicable.

The aim of experiment is to try and precipitate MgCO3 through this reaction at the exit stream. the liquid effluent and solid carbonates shall be produced.

Mg^2+(aq) + CO3^2-(aq) = MgCO_3 solids

So the main question is what parameters should I examine - I have already thought of a few:

• The volume of the tank should be kept constant, maybe design the experiment with 1 litre of RO retentate present in the tank initially?
• How can i get the CO2 to form tiny bubbles, through the use of a nozzle perhaps?
• I will need to measure the initial concentration and examine any changes that happens in the reaction.
• Type of flow meter to be attached, and how will I even get the CO2 gas stream to attached to it and then to the bottom of the vessel?
• I know what seawater usually has a pH of around 8.5/8.6 - alkaline. By adding CO2 gas, it will form carbonic acid which in turn will lower the pH, is this worth analysing?
• Gas will need to exit from the top so that there will not be an increase in pressure
• What experimental apparatus would be reasonable to use to precipitate MgCO3 from the product stream?

I have looked into fair bit of the process chemistry involved in the precipitation of magnesium carbonate from reacting RO retentate with CO2 gas.

• The carbon capturing technique is carbon sequestration, correct?
• Henry's law appears to be an important parameter to examine especially with CO2(g) -> CO2(aq). meaning that: Yco2 x P = Xco2 x H. whereby, y is the Y is the mole fraction in the vapour phase, X is the mole fraction in the liquid phase, P is absolute pressure and H is henrys constant, which for CO2 at 25degrees is 1,670 H/bar ( taken from Introduction to Chemical Engineering Thermodynamics by Smith, Van Ness and Abbott. - How would i determine X and Y for this reaction?
• Looking at Henrys law could further suggest looking into dissociation of CO2,equations of state, fugacity, equilibrium relationships etc - is this all relevant?

Any help or suggestion on whether I am going about this the right way, or suggestions of useful literature to examine would be greatly appreciated! I am current under a lot of stress so any help to ease this street would be fantastic.

I thank you all in advance and look forward to hearing from you!

Andrew

Attached Files

•  IMG_1632.jpg   2.36MB   15 downloads

Edited by peaston, 16 December 2011 - 12:07 PM.

[kkala]

Situation is much clearer now with the patient description provided. Nevertheless it seems to need further elaboration, unless I miss something. Not having extended knowledge on the subject, contribution from other members would be welcomed.
Α. Relevant Chemistry. Let's call retentate sea water (SW), although it is more concentrated.
A1. SW also contains CaCO3, much less soluble than MgCO3. So CaCo3 will precipitate first (or coprecipitate), in case of carbonates precipitation (http://www.ktf-split...en/abc/kpt.html).
A2. Majority of SW carbonates are in the form of HCO3^- at the natural pH of sea water. Introduction of CO2 (g) will lower pH increasing HCO3^-even more. Ca(HCO3)2 and Mg(HCO3)2 are soluble. How can they precipitate?
A3. Letting SW be saturated in bicarbonates seems rejectable, due to scaling. Ca(HCO3)2 solubility is about 17.1 g / 100 g H2O (ambient temperature, ~30 oC); Mg(HCO3)2 solubility expected to be much higher (http://en.wikipedia....olubility_table).
A4. If pH of SW gets higher (more alcaline), CaCO3 and MgCO3 would precipitate, probably their hydroxides too, but this cannot be realized through CO2.
A5. You may need to study and apply solubility product and ionic equilibrium principles, to clarify what can be expected from the experiment. Or even change it to obtain a result. You may think again, then decide on the chemistry and describe it, if possible. I might have forgotten some essential things long unused. Let us try to find out under what conditions carbonates could precipitate.
B1. Carbon sequestration can mean collecting CO2 (http://en.wikipedia....n_sequestration).
B2. For Henry's law concerning CO2, see http://www.cheresour...-carbon-dioxide. Yes, ionic equilibrium etc should be considered (as well as solubility products).
Γ. Despite limited lab experience, I think experimental apparatus needs to be simpler (to be feasible).
Γ1. Continuous passing of CO2 from retentate will take some liquid away as vapor, besides CO2 will need measurement.
Γ2. Blocked CO2 flow can create overpressure in the bottle.
Γ3. Mounted agitator is not an easy solution for lab, especially when sealing is needed.
Γ4. Probably you can discuss a simpler arrangement, like that indicated in attached "lab.doc" with your instructor. Certainly the latter can make improvements, knowing lab possibilities.
But first priority is to clarify Chemistry basics, at least to my understanding. Reactor configuration will be affected by the results.
Hope of success and of additional help by others.

Note: Always be cautious and wear glasses (usual eye glasses) in the lab.

Editing note 12 Dec 11: Web reference for Henry's law corrected

Attached Files

•  Lab.doc   26KB   21 downloads

Edited by kkala, 18 December 2011 - 02:47 PM.

[peaston]

Thanks for your reply it has been very informative! ...with regards to the lab set up, the drawing i attached it similar to the one provided by my instructor. if i had a magnetic stirrer at the bottom of the tank, this would become interfered by the CO2 gas entering, which we decided would come from the centre of the bottom of the tank. i was told to take the agitator type into consideration - whether it be a propeller or impeller and taking blade width etc into consideration.

I believe it has to be a sealed tank so that excess CO2 cannot escape but there will be some sort of hole at the top of the tank possibly leading to CO2 recycling or regarded as waste - i can't be too sure yet but id imagine that would be the scenario as CO2 would be hazardous.

Does anyone know how I could create microbubbles of CO2 gas to react with the retentate? or what kind of flow meter i could use? i red to make a list of apparatus that i can have ordered to the lab

thanks in advance

andrew

[kkala]

A. General comments.
Sea water can contain Ca²⁺=346 mg/l, Mg²⁺=1132 mg/l, HCO3^-=229 mg/l (see post by kkala in "Removal of carbonates from concentrated sea water"). Assuming that retentate contains these concentrations 50% higher, Ca+Mg cations are 82.85 mmol/l and HCO3^- = 3.75 mmol/l. Apparently there anions, Cl^-, SO4^-2 , to balance the electric charge of cations.
The question (not yet clarified ?) is how to precipitate Ca+Mg cations and how much of the 82.85-3.75/2=81 mmol of them can be precipitated by CO2. Probably 0, but let us assume that all of them can precipitate as Ca(CO3)2 and Mg(CO3)2 (we have not specified how). In this case, stoichiometric need of CO2 is 2*81 mmol / l, that is 3.6 Nm3 CO2 / l of retentate. So probably stoichiometric need of CO2 is 0 - 3.6 Nm3 for 1 l of retentate, and experiment can last 2-3 hours. But first priority is to clarify chemistry basics, then estimate flows, then order equipment for the experiment. Or have an experiment with simplest available equipment possible to see the result. Then one could proceed to more expensive lab equipment.
B. Anyway, I may miss something. Let us look at the specific points.
B1. It is not easy to introduce gas from bottom; much easier from top cover, through a glass tube approaching bottom and having a distribution device at end. Magnetic stirrer could be below it, touching the bottom (and it is probably available at lab).
B2. This distribution device is a porous diaphragm of glass (or porcelain) letting small gas bubbles passing to the liquid. Said diaphragm is the upper area of a glass disk, of approx 2-3 cm diameter, at the end of the inserting tube (approaching the bottom of the glass). The total bottle is called barboter ("barboteur" in French). Since this apparatus is used for gas analysis, the shop selling lab equipment must know it. Goggling "barboteur" has not shown the distribution device, but shows lab equipment for this purpose (I have used it in 1979, it was simpler, only bottle and inserting tube with distribution device, 4 bottles in series).
B3. Sealed glass tank would be risky, since CO2 in the "bottle" has pressure of probably 60 barg (http://www.cheresour...dioxide-supply/), so a blocked outlet can cause accident. Exit of CO2 to atmosphere should be never plugged. Connecting tubes can be of thin soft rubber to burst first, avoiding bursting of (thick) glass container (where the CO2 reacts).
B4. Concerning gas continuous sampling from stacks, I had used common natural gas meters. I doubt they can satisfy such a small flow rate though. A common rotameter? These rotameters could be suitable, although not used by me. You will not have indication of totalized flow.
Besides check whether CO2 shall freeze when pressure-reduced from 60 to 0 Barg. The latter would create trouble, but I assume that lab has already found a practical way out (heat tracing?). You seem to need a small flow rate.

Edited by kkala, 20 December 2011 - 10:14 AM.

[peaston]

HI there, been a while since I have posted anything on here, but looking for some further guidance...

I have majority of the experiment set up, still waiting to get a CO2 cylinder regulator fitted and the cylinder to be secured next to the fume hood, where my experiment will take place.

I currently have a 2000 ml beaker of 13cm diameter to perform the experiment in.

The impeller is a radial flow turbine impeller, which can be found here:

https://uk.vwr.com/a...number=441-1228

It has a 5 cm diameter.

the beaker size was chosen as i believe diameter of the agitator divided by diameter of the tank should lie in the range 0.3 to 0.6. with 13cm diameter tank and 5 cm agitator, this comes out at 0.38 approx which lies in the range.


the impeller was chosen by my instructor who claims that when introducing flow of carbon dioxide gas from the centre base of the beaker, the gas would rise up and collect underneath the impeller forming a large gas bubble to react with the retentate solution.

I have currently been testing the experiment with 1800ml of water and with a flow of air coming in from the bottom but problems are arising... at 1800ml the liquid level height is 14cm from the base and assuming impeller positioning to be 14/6 = 2.33cm from the base.

for the case of the gas flow tube, i am not sure how i could secure it to the base of the beaker... the lab technician suggest using silicon, but unsure if this would create problems or not...

also without the gas flow i have tested speeds of the impeller unto 800rpm which is the point where the vortex formed touches the top of the impeller... how much of a vortex can be allowed if any? as soon as the gas tube enters the solution, i can reach far greater speeds without vortex formation but without it being secured, i cannot test it very fast...

the speed of the impeller rotation is something which i cannot determine until i have the rest of the kit assembled or is there somewhere i can look for information on that? if it needs to be higher, then i would need to install baffles - but complication arise from the method in which they are secured...


Also what should I be testing for? Im assuming to judge whether the CO2 is reacting with the retentate, I should measure the pH as the formation of carbonic acid will lower the pH of the solution. Should the effects be tested at different temperatures? and should the conductivity be noted? surely it would decrease as the ions in water would form carbonates...


Any further suggest would be greatly advised, thanks for everyones help in advance!