Caustic Concentrator Design 1

[Okie2]

I am investigating a process that includes concentrating 5,000 gallons of a 25% NaOH, 75% water mixture to 40% NaOH, 60% water mixture. I will have 24 hours to process a batch. It will have to be done with no operator in attendance. The system will be monitored as needed remotely. Maintenance will be available on call. I am more chemist than engineer and have no experience with this type of process equipment.

My questions are;
1. What would be the preferred system to accomplish this concentration?
2. What materials of construction?
3. What parameters can be monitored to determine the % Concentration?
4. Would I need mixing if nothing precipitates?
5. Assuming natural gas as the energy source, how would you heat the liquid?

I'm sure I am overlooking some obvious points. Thank you for any ideas you might offer.

 

[MJCronin]

Does this sound waaaaaay too much like a homework problem ?

If its not a homework problem, what genius manager assigns something like this to a chemist ?

(Hint, its going to be be Big, PPG has designed these and the MOC in the hot parts will be Nickel 200 !!!)

My opinion only

 

[Okie2]

Thanks MJCronin for your reply. I apologize for the question, but I can assure you it is not a homework problem and I have not been assigned the project by anyone. I only have myself to blame.

There is an inorganic dissolved in the solution, but it is not germane to the problem. I am aware of vacuum evaporation and by keeping the temperature below 90 deg C, I understand that 304 SS can be used. Alternatively, I have wondered if I could use an alcohol to extract the water and benefit from a lower energy consumption. As I stated, I don't have experience with these types of systems and was hoping someone could steer me in the right direction, especially if there is a better system available.

If I had to start with what I know now, I would build the vessel of 304L SS, apply a water jacket and use a gas fired water heater with the temperature controlled to 85 deg C. I would use a liquid ring vacuum pump with a cooler on the vapor from the vessel. I purposely didn't go into this description as I didn't want to lead someone into a particular type of equipment when there might be a better system.

Again, I apologize for my request. I can assure you my intent was to define the optimal system based on the readers experiences and not to come off sounding as I did. Thanks again for your comments. I will contact PPG to get their input on this process.

 

[mbt22]

Okie22,

I hope that you are getting someone with experience of process equipment to help you if this goes anywhere beyond a back of the envelope sketch.

A good starting point in the US is the OSHA process safety management standard, UK would be COMAH. They might not strictly apply to this, but the principles are good.

22 te/day or so of hot caustic is dangerous. The first step has to be looking for an inherently safe design - can you minimise the volume of hot caustic, perhaps by continuous processing rather than batch; what happens if each piece of equipment stops working; what happens if you get a leak; what are you going to do with the contaminated waste water stream; what happens when the temperature goes high or the batch goes wrong?

Sorry if I sound patronising, but things like the recent T2 chemicals explosion (link attached) show that chemical process design is not a good field to teach yourself. Safety is part of the design, not trips, relief valves and procedures you put on once you've finished.

Matt

 

[Okie2]

mbt22,

Thank you for the safety reminder. I have been involved in water purification systems for 35 years now and outside of electrical issues, the main issue of concern has been wet feet. I was unaware of OSHA's program. I will certainly study that and keep safety at the front of the design.

As far as having someone to perform the actual design of the system, I had hoped to define the process before taking on the costs related to the design. I am, as you might guess, on a limited budget. I also know that I will have to put money out for the needed resources, I just don't want to waste it.

Thanks again, I appreciate your insight.

 

[mbt22]

Okie2,

The first hit on google for "caustic concentration" is someone selling a caustic concentrator close enough to your scale to give you all the information you need. Sulzer have already done the hard work optimising the process for you, and as MJCronin rightly says there's a lot of nickel 200. The link's attached below.

If you're interested in working through this as a learning exercise prior to engaging the relevant professionals, I would seriously consider continuous processing rather than trying to heat the batch in a tank. Coulson and Richardson's Chemical Engineering volume 6 is a fairly accessible for anyone with a reasonable physics and chemistry education to start to get an idea of what the issues might be.

3, monitoring concentration. It depends on what your inorganic impurity is and at what concentration, as it might interfere with all of these. You could directly measure pH, though pH electrodes have a limited lifespan. Density measurement would be more robust, using a level gauge (guided wave radar or float) and a differential pressure over the tank.

4, mixing. If you still want to use a big tank for some reason, not mixing would give you concentration and temperature gradients. The average might be 40%, but parts of the tank might see much higher concentrations. Heating and cooling becomes very, very slow if you rely on natural convection, especially if you are limiting your water jacket temperature to only a little above the process temp for metallurgy.

Short answer, you can probably buy or license a design for what you want to do rather than paying to reinvent the wheel. If you decide it's worthwhile, experienced process/mechanical/(I/E) engineers and operations people will be needed.

Matt

 

[Okie2]

Matt,

Thanks for the information. I will get a copy of Coulson and Richardson's book on order tomorrow. I had initially thought a continuous concentrator would be too complex, but now I look forward to learning more of the details.

I appreciate your patience with me on this request.

Lew