How to Increase Turbulence in a Chemical Reactor

I'm sure people (myself included may ask more details.

Such as the viscosity of this polymer? (if it is a polymer)

Is the product Newtonian/non-newtonian?

What are the temperatures?

Is shear a problem?

I don't know if I can help, as far as my experience being in the food industries, I have experience with reactors and cookers, there are different methods that can be applied.

But I never heard of a gurney screw before????

Viscosity ranges from 1 cps to 50 cps.

It´s an aqueous suspension: initially water + monomer, at the end of the process: water + polymer.

The suspension is newtonian.

Temperature during the process is 60° C

I do not get what you mean when you ask if shear is a problem.

Thks!

sorry about the shear, having worked in the dairy industry shear can be an issue due to the fat content of the product.

I actually did not understand the meaning of this word in this context (still don´t do): nothing to be sorry about!

When you mentioned polymers, this always puts a flag up for me....... If you didn't I still would have asked. Gets to be a habit for me.

Either way, I'm surprised your expert didn't bring it up.

There are (4) ways to apply Shear mechanically.

If I remember,

1. Axial
2. Linear
3. Rotational
4. Telescopic

These I believe are pretty self -explanatory, I can come up with some graphics

When I talked to some of the Food Scientists at USDA in Alexandria, VA basically their response was, Shear is Shear, no matter how its mechanically applied....... basically. :-/

It has to deal with rheology of the product, And having designed process equipment in the dairy industry, particularly cheese, shear and heat played a big part.

Thanks for sharing the info! I actually knew the word in my mother tongue (spanish) not in EN.

ahhh,... 'Hola'?..... that's about it for me, thanks to my niece who majored in Spanish and International Business.

well, being able to say Hola is a good start!

shear forces occur where the high velocity impelled fluid meets the slower or static bulk of it

too high a shear can cause cavitation or I guess other undesirable effects ( e.g. in dairy stuff )

I would look for a better informed guess by experimenting with smaller batches, say 4 litres

how about an impeller shaft that traverses up and down through the mix , maybe diagonally ?

yeap, it´s a very good idea to break the flow between the tandem impellers. As soon as we can I will have a motor installed to do that.

High shear can potentially break down polymers. It can be a problem with rheological thickening agents such as Carbopols in the thickened state. It is basically a function of the impeller tip speed (as this is the fastest moving point) vs. the liquid speed around it. The more viscous a product the higher the shear rate. Homogenizers tend to use a rotor/ stator combo to create the shear conditions in less viscous or non-Newtonian fluids, and usually operate at 3600 rpm or more.

In suspension polymerization you probably don't want shear. Your combination of viscosity and speeds is probably still below critical levels, and most one piece impeller choices will not change that much. Saw tooth disperser blades are one of the higher shear types but are not likely what you're looking for.

For suspension polymerization, high shear is not necessary, concentration and type of surfactant are. Although the above recommendations are all good remember that in suspension polymerization the individual beads function as miniature reactors. Temperature and water purity in the suspension are critical.

Thanks for sharing your knowledge! Yes, a strict control of the temperature of the jacket during the process (heat to start it up and adequate cooling once the process started) are crucial. We also care a lot for the quality of the water we use.

I'm not your expert, however, it sounds as if your expert was referring to the pitch of the propeller blade. I was always fascinated at how the agitation motors were outfitted with what looked like an outboard motor propeller. Knowing that different propellers are available for a given outboard engine, then you may have some options available. There will likely be trade-offs you will have to consider in terms of performance.

I can't help but wonder if you wouldn't have more latitude if you used a variable speed drive. Perhaps all you need is the ability to regulate the speed of the motor.

Sounds like a Lightnin Mixer, I think is owned by SPX now.

For the OP, I do have an excel spread sheet for calculations that I had used for...... I used for mixing (been so long, I think for turbulence flow because of heating)

Its somewhat user friendly..... somewhat I said....

.... if your interested, PM me.

How about increasing RPM?

there is an optimal RPM # for each type of impeller. In this case, increasng RPM will cause cavitation and decrease of turbulence

Try installing a tandem propeller instead of one per shaft put 2 sets(like blade paddles)- the same RPM twice the mixing effect.

The reactor has two tandem propellers on the shaft (´ believe having mentioned it in my OP). Nevertheless I will try to find out why (50 years ago) someone decided to go for a propeller like impeller instead of for some other shape, also if another shape was tested and if the propeller gave best results

Check the amp draw now (worst case), If you have reserve (below FLA) you can mess around with pitch and major diameter. By your OP, "offset" to mean gearbox?

I am re-writing this post, as the previous one vanished (sorry if a duplicate reply shows up later). The Amps drawn by the motor are closely monitored during each process (can´t recall the value right now), being well below FLA: this (agreeing with you) allows us to mess around with pitch and diameter :-). We probably will work a bit on the first variable modifying the pitch. As I was told, there is also the option of slightly bending the tip of the blades upwards making a little "winglet" -as seen in modern airplanes- The electric motor is not in the center of the vessel: this prevents formation of swirls and undesired sticking of the polymer to the walls of the reactor. The displacement is roughly @ ¼ of the diameter.

Google "Adjustable mixer propellers" and watch you full load current.

You mean to say --adjustable pitch propellers , like marine propulsion - that would be applicable to increase rpm and avoid cavitation at increased speed.

Propellers must be fixed and cannot have any additional mechanical components (like the cones seen on airplane propellers for changing the pitch), as (under certain circumstances) the polymer can harden and deteriorate those parts.

My 2 cents worth...

First, please specify if the 1100 rpm speed is motor speed or agitator speed; second, please specify vessel and agitator dimensions and if there are any internal baffles.

Also, are you looking for increased flow (pumping speed from the agitator, relatively linear) or turbulence (intermixing action)? Single piece agitators tend to be relatively poor at creating turbulence within a vessel at low viscosities, as their major effect is a pumping action. Turbulence can be increased simply by changing one impeller to a reverse pitch, which then changes the mixing action to a double toroid shape.

The transmission from the motor to the shaft is direct (meaning there is no gearbox in-between). The RPM´s are measured on the shaft. I´m at home right now, but the diameter of the reactor is more or less 1,2 meters (1,200mm) = 4 Ft and the width of the impeller is of about 300mm = 1 Ft. The reactor has no internal baffles as the process requires a smooth surface inside the vessel in order to increase heat exchange (heating and cooling) as well as for preventing the undesired polymerisation of the product on the baffles (slow speed areas). We actually look for a increased intermixing speed. Our impeller is a tandem type (large impeller at the end of the shaft and a smaller one with reversed pitch @ mid-shaft)

Sounds like you've already got that type of setup then. Could you verify the direction of rotation- that the bottom impeller pumps upwards and the top down?

If that is so and you have reserve power, another choice could be to add a "C" type impeller near the mid point of the other two. These are very effective at moving fluid outwards when infed by others, and will speed up the turnover rates and reduce stagnation where the flow patterns coincide near the shaft.

The current mixer might be just causing the material to rotate within the mixing chamber since all rotation is in the same direction.

Maybe adding baffles that slow the rotation would then provide added mixing potential since the relative speed of the mixer blades to the liquid would be increased.

The rotating blades you currently describe would provide good exposure to the outside surface for heat exchange or such, but not necessarily good optimum mixing process.

Hi! as I said before, the motor / shaft / impeller are eccentric in order to prevent what you mention. Baffles are an issue to, cause (despite very useful in other kind of products) they create areas of slow motion in the mixture, leading to a unmanageable spontaneous polymerisation which can ruin the internal walls of the reactor. Thanks for sharing your thoughts!

Typically in this type of mixer both blades are the same pitch and pull the liquid in the same direction. The result is a circular motion with liquid flowing down on the shaft side of the mixer and up the unobstructed side. In steady state the flow is almost laminar and does not create much turbulence. If you reverse the pitch of the upper impeller the resulting flow is two circular motions, one clockwise and the second anti-clockwise with the liquid being pulled towards the shaft near the center. This should increase the turbulence without having much effect on the power input. You will need to experiment with the blades at different distances apart to optimize for the best results. If you already have opposing pitch impellers, add a second 45° shaft agitator fixed in the side of the vessel. Its purpose is merely to break up the laminar flow and transfer some liquid from one circular motion into the other one, so you don't need much power or speed (200-400rpm, 12"deep pitch blades, 1.5-3HP) With luck you already have an access port with a flat lid to mount it on.

You know? it´s an excellent idea to add a second impeller instead of changing what we already have got! There is a inspecion port on the top of the reactor I will use to attach the second motor to. If it works, we´ll make a decent modification of the reactor.

Since I posted my reply I have been for my daily two mile walk and pondered your problem. For a very quick cheap fix depending on how much turbulence you want to create, add a chevron shaped baffle with the sharp end pointing into the flow. As the liquid passes the baffle it will shed a vortex alternately to the left and right of the baffle. The swirling motion of the each vortex may be sufficient to solve your problem. A piece of sheet metal, bent, fixed to a stand to hold it off the bottom of the tank, drop it in and test the results. If it works engineer it properly and fix it in place. Just a random thought on a beautiful spring morning while strolling in the sunshine.

I think we should get back to what you are looking to achieve here.

What is it?

Are you looking for a turbulent flow for better heat transfer?

Nope... I´m looking for more turbulence, in order to improve the intermixing of components inside the reactor

hydrofoil is a good general impeller for high turbulence. Obviously increasing the rpms and/or impeller diameter just lower than the point of cavitation will increase turbulence, so long as you have the access HP and the shaft is robust enough to not snap. If you need dispersion, a cowels blade is a good place to start.

Todd

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Hey Doc,

As I mentioned earlier, I have an excel program for mixing as well as thermals. it's rudimentary, but it could help.

I have a few files that I have to look through, I know I have something on Agitator design (Multiple agitator, counter-rotating and such. It gives a reference point, you use empirical information/data what you have and you can put in correction factors.

if your interest PM me.

Here's a clip

I´m sending you a PM in some minutes. Thank you!

Perhaps you can improve mixing turbulence by circulating the batch through a side pump with a static mixer in line with the pump output tubing. Use a magnetic-coupled pump - no leaks.

Can't you just call your "expert" and ask him what the he was talking about because you can find no information on what he recommended. He could probably explain further, if not he is not the "expert" you need to consult.

I actually contacted him in order to get more info.... but he said: you hire me and you'll get it! As his fees are quite high, we will try to figure it out by ourselves... and if nothing works we know there is a "B" Plan at hand! ;-)