L/L separation equipment

[petepsingpy]

It's been awhile for me; I have an application where it's necessary to separate 2 immiscible liquids with a contact time of < 5-10 minutes. A centrifuge comes to mind, but would there be any other technology out there worth looking into, such as coalescer's? A decanter requires too much residence time.

Here's some info on the application:
Heavy Liquid sg: 1.15, viscosity ~ 5 cP
Light Liq sg: 1, viscosity ~ 15 cP

Vol ratio Heavy/Light: 4:1
Total vol flowrate: 10 gal/min

Any tips appreciated
Thanks,
Pete

 

[25362]

I'd suggest reading Perry VI chapter 21 on liquid-liquid separation, or Perry VII, if available. This book is a good tutorial on sedimentation, coalescing, hydrocylones, helping accesories, etc., that may lead to a solution of your problem.

 

[Nosey]

Hi Pete,

25362 sets you on the right lines by directing you to Perry's. From the info you have given I'd think closely about hydrocyclones, though the delivery pressure will be a crucial factor, as will the extent of mixing between the two fluids.

You'll need to ask yourself the following questions if you are thinking about applying a hydrocyclone.
[ul][li]Are they in the form of an emulsion?[/li][/ul][ul][li]Are they likely to form an emulsion?[/li][/ul][ul][li]What is the delivery pressure into the hydrocyclone?[/li][/ul][ul][li]What is the required pressure out of the hydrocyclone?[/li][/ul][ul][li]Which is your product stream?[/li][/ul]If you need to pump the feed up (generally you require a feed pressure of >3barg) then you could end up shearing the fluids and emulsifying them even further. The key to successful hydrocyclone installation and operation is carefully controlling the pressure drop ratios (dpr) around the unit. The vendor will advise.

If you do have a relatively small droplet size (i.e. <10microns) then you may want to consider a fibrous coalescing media upstream of the hydrocyclone (

http://www.owtc.com

or

http://www.cyclotech.com).

Hydrocyclones (

http://www.mozley.co.uk)

will separate the feed into two streams; light product and reject. The light product should be relatively clean while the reject will comprise the heavy fluid plus some light fluid. The total reject flow may reach up to 5% (by vol) of the inlet flow, and up to 80% of the reject may be light fluid.

I hope that this helps in some way.

Nosey

 

[jmw]

You may find that the temperature will affect the separation. If you have any data on temperature effects it is worth a look. I know that in the pharmaceutical industry it is common to chill the solvent water mix as the mix flows into a separation vessel (obviously both are low viscosity). The low density product flows over the wier and the high density is drawn out at the bottom. As prginally set up a sight glass in the discharge was used to detect the interface and then the operator shut off or regulated the bottom discharge. In this one installation they later installed a tube density meter vertically (like a manometer) and controlled the interface within the separation tank by regulating the bottom discharge flow automatically.
How relevant this is to you, I couldn't say. I suspect that your viscosities may be against this method.
Centrifuging may be a better option (GEA Westphalia, ALfa Laval for example). In the marine industry they use the centrifuges to seprate water from heavy fuel oil. This is usually at 98degC with typical oil density 981kg/m3 and viscosity 35cst; water, well 1000kg/m3 and 1 cst, near as makes no difference.

 

[nbucska]

I think the more important question is what is your object:
1) to recover Heavy or Light (H/L ) or both component(s) ?
How clean ?
2.) To assure that H or L is NOT contaminated with
the other component ( specify !)


<nbucska@pcperipherals.com>

 

[petepsingpy]

Thanks for the responses. Here's some other info on the application:

Both Light and Heavy are products that will be recovered.

The 2 phase mixture is not emulsified; it separates easily into 2 clear phases in about a minute or less in a 500 ml size bottle.

Currently, the stream of the 2 liquids flows by gravity. A pump would be required to generate pressure to go into a hydroclone.

 

[TD2K]

Why not just use gravity separation?

You said the mixture easily separates into 2 phases in a minute or less. At 10 gpm, 10 minutes residence time wouldn't mean a very large piece of equipment would be needed. From a cost point of view, centrifuges or hydrocyclones are going to be a lot more money.

 

[bchoate]

bchoate:
A variety of means have been listed to separate these two liquids. I would like to comment on two already mentioned and perhaps add a third.
Coalescer: A coalescer can handle this separation. If emulsions arise, coalescer elements can often break the emulsions as well as separate the phases. Pall has liquid/liquid coalescing elements and housings for this service. The feed will have to be pumped into the coalescer to develop pressure drop across the element. Both liquids will contain the other liquid at its solubility at the separation temperature. If low levels of cross contamination are desired, pre-cooling the feed is advantageous. A pre-filter is necessary for any coalescer. An issue will be chemical compatibility of the liquids with the filter media.
Centrifugal separator: CINC (Nevada) has a centrifugal separator that will make the phase separation. This is a more costly approach. The feed should be pumped into the CINC. Product streams can gravity flow out.
Decanter: Simple, horizontal decanter with heavy phase boot. Level control the heavy phase out (or detect interface with magnetorestrictive gage with float that floats on heavy phase) and overflow the light phase. Feed pumped in subsurface in the light layer.

 

[Nosey]

I agree with TD2K, I would use gravity separation in this case. Since the two fluids separate easily I don't see a problem with this. You may want to enhance the separation using an electrostatic coalescer (if one of the compounds is polar), but at that flowrate it really is not that viable. Centrifuges will do the job, but they are very energy intensive, you sacrifice power consumption for footprint.

Gravity separation is the way to go.

 

[25362]

To petepsyinpy, at first site all above conclusions seem logical since the static phase separation is quick and easy in the 500 mL glass-made bottle.

It is, however, worthy to try and analyse the bottle experimental results on whether they are indeed reproducible in a plant-size prototype metal-made continuous flow settler.

Besides, are the liquids totally immiscible or do they partially dissolve in one another ? Do they need further treatment for their purification ? How do the given viscosities, surface tension and densities relatively change with temperature ?

Good luck.

 

[petepsingpy]

Sorry if this is a double post, my first one vanished.

Gravity will work for this. In the past, two decanters have been tested on this stream. One was vertical with 1 hr res time for heavy, 30 min res for light. The other was horizontal, with more res time than the vertical.

The problem is that an undesireable solid byproduct forms from contact between the phases, and comes out with the Heavy of either decanter. The horizontal decanter, with more contact area and res time, made more bydroduct than the vertical.

For this reason, I want to minimize contact time and area between the phases. I know in lab tests with a sep funnel that if I can separate the phases after a few minutes, I do not get any byproduct. Unfortunately I do not know the time limit for a batch separation to avoid byproduct formation.

It might be possible to use a smaller decanter to avoid making the byproduct and still get a good L/L split.

I know I could use a centrifuge to get a good sep'n with a few minutes contact time.

 

[bchoate]

bchoate,
If interfacial solids formation limits the use of a gravity decanter, the CINC centrifugal separator is perhaps a good method. Phase contact time and surface area are small.

http://www.cit-ind.com

Bill Choate

 

[25362]

Some thoughts.

1. Gravity separation in horizontal decanters can be enhanced with the help of smoothening vanes or gratings, thus the vessel can be of smaller dimensions and the time of contact at the interphase may be shortened. Some provisions to continuously remove the &quot;band&quot; near the interphase may be needed.

2. Is there a possibility of using a third immiscible chemically inert liquid with an intermediate density to act as a physical separating wall between the process fluids ?

3. Or can a third &quot;inert&quot; liquid be dissolved in the heavier phase to increase its density and reduce its viscosity to make the phase separation quicker ? This &quot;new&quot; fluid would have to be easily removable as needed at a cost.

4. Can you use a higher temperature to make separations easier and quicker without increasing the phases' chemical reactivity ?

Good luck.

 

[erpeterson]

One other thought.

If your solid forming reaction is strongly temperature dependent, even though lowering the temperature of the fluid may increase viscosity and increase separation time, you may want to run decanter colder in order to stop reaction from occuring. If solid is something that you have to filter out or decreases your useful product volume, you might save this expense.

 

[petepsingpy]

Good discussion, thanks for the responses.

The side reaction to form solids is slowed by reducing temperature, so this might be a viable approach, but I'll have to put some cooling in which will add cost.

Some observations from the lab; if I use a sep funnel and separate batchwise with about 10 minutes holing time of the phases, I don't seem to form any solids. However, if I run continuously with a decanter at average res time of around 10 min, or even less, solids still seem to form. My theoretical explanation for this is that it must be due to a residence time distribution phenomena in the continuous system. i.e. in the continuous system, some fluid from each phase will be in contact for longer than the avg res time.

 

[KWHUI]

Try to investigate the feasibility of using a hydrocyclone to separate this mixture!

 

[bchoate]

bchoate

The difference between the lab sep funnel model and the process decanter with the same residence time is probably due to the difference in interfacial area. The sep funnel model probably has a very low area. The reaction is occuring by mass transfer of some species from one phase to the other with a product that is somewhat insoluble in either phase. The transfer flux is proportional to the concentration of the reactant in whatever phase it is in and the interfacial area. More solids are formed at a fixed residence time in the system having the higher interfacial area.
Several viable options have been offered by contributors to this forum. The reaction slows as the temperature is dropped. Phase separation is slowed as the temperature drops requiring higher residence times. A gravity decanter, to optimize the result desired, would have to be a tall, vertical tank to minimize area and provide required residence time for separation. In addition the feed would have to be cooled to slow reaction.
If centrifugal force is used, the separation may be more easily managed.

 

[petepsingpy]

I should have been clearer; I was comparing a lab batch sep funnel to a lab scale continuous decanter. The continuous decanter in the lab actually has a little less interfacial area than the sep funnel.

Comparing the lab continuous decanter to the plant continuous decanter, they generate roughly the same wt% solids in the heavy stream at the same avg res time. The interfacial surface area/volume is roughly the same between the lab and plant scale.