Pump Sizing and Selection of Fluid with 10cP Viscosity with Solids. 1

TiCl4 said:

A few things come to mind:

First, and most importantly, this idea will not work. Since you have an weir overflow system, recirculating from Tank 2 back to Tank 1 will not serve to increase residence time. Your residence time is controlled solely via your wastewater inlet flow to Tank 1. If your pumping system will also shut off the WW flow to Tank 1, then the system, as proposed, will work. If you begin to recirculate material from T1 to T2 without slowing incoming flow, though, your overall residence time does not change, and you will accomplish nothing. [highlight #EDD400]Yes the waste water pumping stops when the pump is recirculating, that how we will write the interlock.[/highlight]

Recirculating back to your original WW storage tanks instead of back to Tank 1 would increase residence time, but I don't know if those are mixed tanks that will not have issues with settling. Your call. [highlight #EDD400]Yeah I thought about it too. I will take the call after talking to the Plant Manager.[/highlight]

Second, what is your current residence time through the whole system? [highlight #EDD400]I donot know. I will calculate soon.[/highlight] Your neutralization system looks like it is set up for a regular caustic (NaOH) mixing system instead of Mg(OH)2. [highlight #EDD400]Yes the system is restricted by the low residence time of the system and the slow reaction of Mg(OH)2.[/highlight] Tank 1 usually is the "rough cut" tank, targeting a pH of perhaps 3.5-4.5, with Tank 2 being the final control tank with NaOH also being injected there, targeting the final pH. [highlight #EDD400]Yes that's how it is being done till now. They did not want to use NaOH as the PH can easily jump to 14. Max PH of Mg(OH)2 we use is 12. The city's limit is 12 , we just lowered the spec for our purpose to 9 to be compliant. In fact Tank 1 is called Coarse Tank and Tank 2 is called Fines Tank.[/highlight]

Third Is your Mg(OH)2 solution a slurry, or is all the magnesium dissolved? [highlight #EDD400]Mg(OH)2 is a thin slurry not fully dissolved and these particles carry forward to Tank 2 (Fines Tank). Hence my questions on sizing of the pump for slurry with 10 cP viscosity.[/highlight] It doesn't take much to hit 9.5 PH. [highlight #EDD400]I think the max PH of Mg(OH2 slurry we use is 12. I will check. The upper limit is 12. We actually lowered the spec for our purpose to have PH below 9.0.[/highlight] If there are remaining undissolved Mg(OH)2 particles (which is likely), then agitation speed and your Mg(OH)2 particle size are extremely important to reaction rates due to the low solubility of Mg(OH)2 in water. You can improve reaction time by ordering a finer particle size slurry or by running the slurry through a grinding unit before adding. [highlight #EDD400]Yes that's a very helpful suggestion. We will decrease the particle size when we order or increase the agitation rate to help increase the reaction rate.[/highlight]

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georgeverghese said:

Strange why you intend to use Mg(OH)2 instead of NaOH. If there are even small amounts of bicarbonates or carbonates in the feed waste water, the resulting Mg(HCO3)2 or MgCO3 will precipitate and form a hard coat over almost everything it touches. Lines will get blocked, pump seals will fail; difficult to remove.

[highlight #EDD400]Our plant management decided not o use NaOH as the PH would easily jump to 14 with our current system ( our limit is 12). We plan to flush the lines with steam and later blow with air/ N2 to keep the lines empty when the pump is not operating to prevent any solids from depositing[/highlight]

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pierreick said:

Agree with George's comments, better to use caustic soda solution.
I've experience with the treatment of brine to remove Ca2+and Mg2+ using sodium carbonate and milk of lime in a cascade of reactors (3) to generate residence time. The issues are the settlement of solid at the bottom of the tanks and the coating of the wall which required periodic high-pressure cleaning (shut down). As George said recirculation will not work long due to blockage of the lines and pump failure. BTW you will need to find a proper way to isolate the pump for maintenance without shutting down your operation, not mentioned on your sketch. Operation and maintenance costs will be high. For this type of application with solid, I've been using pig tail pump.
Be prepared to have a mess around the operation due to leaks, blockages.
Notes:
Continuous monitoring of the Ph is a "joke", probe coated. Operators are required to take regular samples and perform analysis.
I don't understand the usefulness of the 6"pipe (plunger), you should have a direct pipe to transfer from reactor to reactor.
Not in your scope but you should assess the quality of the mixing (type of agitators, number of agitators, baffles) within the reactors before adding a recirculation line + pump.

I do believe caustic solution is the way to go.
My views only.

[highlight #EDD400]We are going to have a spare pump which will be operated when the first pump is on maintenance. Also we plan to flush clean the lines with Steam and blow all the slurry to the tanks when the pump is not operating.

Yes we are monitoring PH continuously. We do swap out probes to clean out frequently. When we want to swap the coated PH probe with a clean one we will shut-off the pump.

The 6" pipe into the tank is the dip pipe that is intended to make sure all the Mg(OH)2 add the top of the tank is mixed
all the way to the bottom and taken out from the bottom, instead of overflowing from the top.

We are planning to increase the agitation rate and will explore the option to decrease the particle size of undissolved Mg(OH)2 particles in water in the future.[/highlight]

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shvet said:

Why do you believe viscosity equals 10cP? Did you measured it? [highlight #EDD400]I guessed it by looking at the slurry consistency. I am going to measure next week using a Vibrational Viscometer that we have at our lab.[/highlight]

What is solid fraction in slurry? [highlight #EDD400]I will get a sample and check today. It is a thin slurry though.[/highlight]

What are particles size and hardness? [highlight #EDD400]I do not know. I will assume the same as Mg(OH)2 because the solid content is due unreacted Mg(OH)2. I will get the specs for the Mg(OH)2 solution supplier today. [/highlight]

Describe in details what way you are intending to measure pH in slurry. [highlight #EDD400]We are using a PH electrode that has a membrane glass that measures the electrochemical potential and is converted to the PH of the solution.[/highlight]

How are you planning to provide min velocity in pH-analyzer bypass to avoid solids settling? [highlight #EDD400]The PH probe will an inline instrument that will be inserted into a 4" Line through threaded connection. The recirc line is 2". I will calculate the minimum velocity required to prevent settling before I decide the line size.[/highlight]

Is sewer able to transport slurry? [highlight #EDD400]Yes we have water sprays in the sewer that dilutes the slurry.[/highlight]

Return to waste tank seems gravity driven line - how will sufficient static head be provided? [highlight #EDD400]The return line is on the discharge line of the pump and the pump will be sized for the maximum head requirements.[/highlight]

[highlight #EDD400]I plant to use 3K method to calculate the required head for the flow rate that will be recirculating at. Also I will read through the material that Mr. Pierre provided. I was planning to use the viscosity correction per ANSI/HI 9.6.7 - 2010 guideline. Please let me know if this is how it is done for slurry and viscous liquid.[/highlight]


for info
Metso's Slurry Pump Basic - highly recommend to start with
Abulnaga's Slurry Systems Handbook
Jacobs' Design of Slurry Transport Systems
GIW's Slurry Pipeline Design Manual

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LittleInch said:

Have you considered changing this to be a batch system where you fill each tank in turn and then let it just do it's job over a longer period of time before emptying it after testing it? you could dose as you go along or just wait for the tank to fill up, test it, perhaps work out over time how much MG(OH)2 to add as a single volume and then agitate it to keep everything in suspension.

[highlight #EDD400]The liquid level in the tank is always at the overflow line so any waste water and Mg(OH)2 we add mixes and overflows into Tank 2 ( the Fines Tank). We do not want to drain all the treated water as it will overload the sewer with high TSS. So we will have to stick the recirculation system option.[/highlight]

Seems much simpler than this pumped arrangement and online testing.

If you're going the pump option then you probably need some sort of PD pump (maybe progressive cavity) to give you a fixed volume flow as your friction losses will be small, but difficult to get right and any small error could make a lot of difference to a centrifugal pump.

[highlight #EDD400]I will consider using the Progressive Cavity pump. The Mg(OH)2 solution pump that we use is a actually a Progressive Cavity pump. But the treated water in Fines Tank is a Thin slurry. I will check the viscosity and solids % as soon as I get a sample today. To size this pump, I will still need to do the frictional head loss calculation as we do for Newtonian fluids albeit with higher viscosity correct?.[/highlight]

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