hell all,

i was reading on the oilybits website that given the proper pressure the liquid going through the mixer will be fully mixed as it exits.

so what is the pressure?

if pressure is increased my limiting the flow then how high can the pressure be set to; i am using a gear pump.

thanks

evan

evan
-did you sneek away from the renovation how do we measure pressure from a static mixer. A bio friend of mine who operates a commercial plant using only a prop mixer, processes at .5 bar or 14.77 in Hg. I'm not sure why this works, but am about to give it a try! Tom

Evan,
Pressure is not the critical factor for a SM. Velocity of the fluid through the mixer is what matters most. There is a small pressure increase when flowing through a sm. Perhaps that is to what you are referring. High shear mixers are the kind we want for BD, not the helical type elements. HTH

quote:

High shear mixers are the kind we want for BD, not the helical type elements.

Have you seen tests showing a comparison? The helical type seem to work very well for me.

Apologies first. I didn't mean to go on so long about mixers in general. I really only meant to post a short message about my static mixer experience, but it got away from me. Sorry, but here it is:

A few years ago I did some BD experiments using static mixers designed for mixing epoxy. I had three of them in series, totalling about 36 elements as I recall. These created surprising back pressure. A critical diffference to most processors was my use of two metering pumps, injecting the correct proportions of oil and methoxide. Without the correct proportions (as in most batch processors when liquid is drawn from the bottom), any mixing action is limited in its effectiveness.

A high-shear mixer looks sort of like a circular-saw blade, spinning at high speed in the bottom of a batch tank. As the liquid passes through the blades it goes through a zone of intense turbulence, with each little blade cutting and folding the liquid, which presents more opportunities for chemical reaction sites. Since there is no housing or boundary, the mixing action relies on high speed to mix efficiently. Since the blade is sitting in the open body of liquid, circulation patterns determine how long it must run before all of the liquid has passed through the blades. High-shear mixers must be sized to the tank and the kind of liquid being mixed. Too small won't get it all. Too big generates a destructive vortex.

Prop mixers are sort of a "low-shear" high-shear mixer. Most of their mixing action is elsewhere in the tank, rather than at the blades. They keep things stirred up so the reaction has more sites than pump mixing usually creates. Prop mixers are very energy efficient for the amount of mixing done, but it's easier to build a pump mixer.

A static mixer constrains the liquid, cutting and folding the stream in slow motion, relative to a high-shear mixer. Because the elements are in a tube, all of the liquid passes through the entire series. Each element cuts the liquid in half, turns the halves 180 degrees, then recombines them. This is more energy efficient than a high-shear mixer, but slower (unless a very large array of elements is used). The energy per unit of fluid is not as concentrated in a static mixer, which may be critical for some applications. A high-shear mixer can add substantial heat to a mix. I never noticed heat from my static mixer string.

There is some controversy in some industries whether static mixers work best at laminar flow velocities, or in turbulent flow velocities. I doubt that my experiments were in the turbulent flow regime, and they worked well. If turbulent flow is desired, then high pressure is a good thing.

Most pumps, including gear pumps, do substantial internal mixing. Clear water pumps in particular are efficient "blenders", behaving very much like high-shear mixers. Since the liquid is constrained inside the pump at the time, they are pretty efficient mixers, IF the correct proportions are present. BD brewers typically run their pumps long enough to ensure the correct proportions eventually pass through the pump. Adding a static mixer only improves the mixing of the liquid passing through the pump. It doesn't improve the homogenization of the liquid in the tank. If there is little velocity of liquid in the tank (as happens when too small a pump is used for the size of tank), then the methoxide will float on top of the oil. When this happens, the reaction only takes place where the two liquids meet, which is a pretty small area. Stirring it up increases the area, which speeds things up. That's the goal of the various mixer strategies being discusses - to increase the surface area, and to bring the partially reacted floating liquids into contact with the partially reacted sinking liquids.

OK, here's the conclusion as it relates to static mixer + psi: if the back pressure is high enough to slow down the liquid passing through the static mixer, then there will be less circulation in the tank. This could lead to stagnation, with partially reacted liquid floating on the top layer, and partially reacted liquid sinking to the bottom. Since most processors use a single pump, then some caution is in order to insure velocities in the tank stay high enough to keep all of the liquid circulating.

Cheers,
JohnO

They may be talking about the pressure drop from entry to exit of the mixer. This could be used as a measure of the mixing forces acting on the oil as it passes through the mixer. The greater the mixing force the better the mixing especialy in the case of high sheer mixing.

JohnO,

quote:

OK, here's the conclusion as it relates to static mixer + psi: if the back pressure is high enough to slow down the liquid passing through the static mixer, then there will be less circulation in the tank. This could lead to stagnation, with partially reacted liquid floating on the top layer, and partially reacted liquid sinking to the bottom. Since most processors use a single pump, then some caution is in order to insure velocities in the tank stay high enough to keep all of the liquid circulating.

I agree completely. To me, it helps to understand that the pressure on the pump side of the mixer represents the work being done by the mixers. The loss of flow is the trade off. On my modified appleseed, I installed upper and lower inputs to the pump. The idea was that this would keep the upper and lower layers mixed. On this setup, I could pass 3/27 in less than 1 hr with 18% methanol single stage. Also, I could put all the methoxide in quickly - less than 5 min. vs the usual 20+ min. on a normal appleseed.