7 replies to this topic

[curious_cat]

We have a glass lined batch reactor where a reaction between two immisscible liquid phases takes place. Current levels of agitation were deemed insufficient for sufficient droplet formation so I was exploring options to improve mixing. Tank holds ~10,000 L and has diameter 2500 mm and height ~3800 mm

 

The liquids are not very viscous (~10 cP) and their densities are 900 and 1200 kg/m3. The agitator has a 25 hP motor but that motor seems grossly underutilized based on current measurements. 

 

The fluids are fast coalascing. Coalascance time is ~30 secs in lab tests. No danger of emulsion formation.

 

Since droplet formation is an important goal increasing the motor rpm would be one option but that seems hard to do without major changes in stuffing box, gear box etc. (currently ~100 rpm) Other goal would be to avoid layering due to density gradients by providing sufficient axial circulation. 

 

I wanted opinions on several ideas I had:

 

(a) Currently it has only one top inserted beaver tail baffle. Increasing this to two or three baffles. (Spare nozzles are available) Would help? Or not much? I understand that 4-baffles is assumed to be a "fully baffled" condition but OTOH guys using glass lined reactors (where integral / welded baffles are hard to fit ) often work with single baffles. 

 

( b )Currently both impellers are pitched blade turbines. Would changing one to a more high-shear oriented impeller help? Being glass lined equipment options are limited but vendor offers a retreat / curved blade impeller. Which I suppose would be higher shear than a PBT?

 

( c ) Impeller dia. is currently 1200 mm. Based on handbook correlations this seems ok for a 2500 mm dia tank. 

 

(d) Would adding a third level of impellers on that same shaft make sense (assuming motor is ok to supply the power)? Or are three impellers too many for this height of tank i.e. ~3800 mm? If I did go for three impellers and had to choose freely between  pitched blade turbines and Curved blade turbines what combination would be optimal? I was thinking a pitched blade in the  middle to provide circulation (and to avoid strong axial flow too close to surface & air entrainment) and a high shear CBT at top and bottom. Does this make sense?

 

( e) If I do go for three impellers, what's the minimum depth from liquid surface an impeller can safely be on the shaft to not have vortexing etc. 

 

 

Any other ideas? Or would having an external loop with a static mixer help? Not sure if static mixers are a recommended option for immiscible liquid reactions. 

Edited by curious_cat, 26 July 2013 - 01:46 AM.

[breizh]

Curious_cat ,

 

I will ask help from Agitators Suppliers first , they may be able to perform simulation (CFD). I 've similar experience and asked Chemineer to support .

 

Breizh 

[curious_cat]

 

Thanks Breizh!

 

I agree that would be ideal. Unfortunately they are a smaller vendor and not as good as Chemineer (I wish!). I already spoke to them and they don't have a lot of good correlations let alone CFD capabilities.

 

I personally think Management did us a disfavor by skimping on costs and not going with a large vendor with good in-house support. But can't change that unfortunately. 

 

To be fair, the quality of construction, delivery, etc. is top-notch even for the small vendor. It's just that they lack the solid technical backup for deeper optimizations. 

[fseipel]

I take it the agitator is glass-lined steel from either Pfaudler, DeDietrich, TYCO, or Taiji?  I have personally never seen a glass lined reactor with more than one baffle; typically one baffle is fitted in standard g/l reactors, to prevent vortexing and promote axial mixing.  In a standard vessel configuration, there aren't even sufficient nozzles on top head for additional baffles.  Can you purchase larger diameter impeller blades or is the size restricted by the cover ring diameter?  Your message didn't state whether reactor had a clamped head or not.  Depending upon material compatibility, the entire shaft could be replaced with a stainless steel, Hastelloy, or Titanium shaft with bolted-on blades to allow larger diameter impellers & increased loading on motor.  If it's DeDietrich you can press in replacement blades with increased diameter so long as shaft, bearings, etc can take it.  You indicate motor is not fully loaded.  If the stuffing box can take it and load is low, putting motor on a frequency drive & speeding up may be viable.  Even increasing to 70 Hz may make a big difference if approved by vendor.  I've done this routinely with Pfaudlers for years of operation at 70 Hz & no problems though this was with double mechanical seals.  Setting up a lab reactor with identical geometric ratios as plant reactor, may also prove helpful for lab testing various configurations.  Does reactor have a sight glass & if so, when you stop agitator, what does it look like & what does it look like while mixing?  Is baffle oriented correctly (angle relative to agitator)?  Is lowest impeller fully submerged in the heavy phase? When you sample, say, 5 gallons from bottom valve & another 5 gal do you see two phases from bottom valve or just one when examining as pulling sample?

 

Has a test been executed in the lab with a specialized, high-speed agitator, to determine even semi-qualitatively, at extremely high shear rates, what the reaction rate/% conversion vs time, ooks like vs same test at say, 1/2, and 1/4 that speed?  A dremel tool or high speed variable speed drill can be used for a crude lab test if funds are prohibitive for such a test.  Would it make sense to consider a packed column that continually re-disperses one phase as an alternative to CSTR?  Can you add an emulsifier or surfactant to aid phase contacting or does this cause downstream problems with settling or removal of said agent?  I ask this because one of the phases may be aqueous?

Edited by fseipel, 28 July 2013 - 02:14 PM.

[curious_cat]

@fseipel:

 

Thanks for those very useful inputs! Very helpful indeed. 

 

To try and answer some of your questions first:

 

(1) It is surprisingly neither Pfaudler, DeDietrich, TYCO, or Taiji. It's a smaller vendor. Their models do look very similar to Pfaudler's. 

 

(2) I'd never seen more than one baffle in a glass lined reactor either. But fundamentally, there seems no reason why not, right? A fully baffled config. for a conventional non-glass-lined vessel is considered 4-baffles. Looking at power consumption curves adding a baffle to a single baffled config. ought to draw in 20% more power at least, I feel. This seemed the easiest change to me. Surprisingly  we have ample nozzles on the top head. 

 

(3) It is an integral, welded head. Not a clamp on / flanged head. In current config agitator blade dia is restricted by what can fit the top opening. 

 

(4) Changing entire shaft is an option. But an option of last resort due to cost. Modifying the impeller type is cheaper. Current shaft is solid, glass-lined with dia of ~120 mm

 

(5) VFD & scale up tests are both excellent ideas. Thanks again! I'm in the process of exploring those. 

 

(6) Yes, currently, lowest impeller is indeed entirely submerged in the heavy phase. 

 

(7) Emulsifiers are a great idea. Sadly, we haven't found anything that works yet. Suggestions? It has to be stable in acids and T of ~80 C. 

[breizh]

http://www.dedietric...lined-reactors/

 

Consider this resource , you may find ideas . Why not consulting the big names ( Pfaudler , De dietrich,...)

 

 

 

Breizh 

[fseipel]

I agree, looking at information from the g/l vendors is worthwhile, DeDietrich in particular has some good data.  CFD may also be worthwhile but may be best to determine in lab how much rate can be increased at max agitation.  Doing a lab test on geometrically similar tank with 1, 2, and 4 baffles monitoring current draw and % yield vs time may also be interesting.  I think the dremel or high speed drill lab test would provide much insight, rx rate @ max agitation.  This test will also tell you whether you can successfully emulsify the material with an extremely high tip speed homogenizing pump or mixer which I would think would be a useful data point, because you will be able to operate at several thousand RPM (at these speeds, try with cowles blade style impeller rather than an agitator designed for any flow).

 

Agents to consider for emulsifying might include asphaltenes, resins, waxes, detergents.  Stability may or may not be an issue; it could be metered in continuously at a very low rate similar to how antifoam is added to a batch reactor if the emulsifying agent breaks down over time -- especially if it is effective even when dilute.  Some surfactants have an acidic moiety e.g. linear alkyl benzene sulfonic acid & may be worth a try. 

Edited by fseipel, 29 July 2013 - 05:18 PM.

[curious_cat]

 

Why not consulting the big names ( Pfaudler , De dietrich,...)

 

Past experience has been they were too expensive. 

 

 

Thanks for all the tips. I'm working on those, especially lab scale up & emulsifiers. CFD is out of reach unless I find a good free software package. We don't have any of the big ones in house.

 

I'm tending towards this config:  Two PBT's with a Retreat blade on the bottom of the shaft. Twin top inserted baffles. 

Edited by curious_cat, 31 July 2013 - 07:09 AM.